1

SYNTHESIS AND

CHARACTERIZATION

OF THE CUBIC FORM OF

TANTALUM NITRIDE

Student: Angel Alfonso Lopez

Project Supervisor: Mats Johnsson

Department of Materials and Environmental chemistry

(Stockholm University)

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INDEX 1 – Abstract ............................................................. Page 4

2 – Nitrides

2.1 - Tantalum Nitride (TaN) ............................ Page 5

2.2 - Titanium nitride (TiN) .............................. Page 9

2.3 - Silicon nitride (Si3N4) .............................. Page 12

3 – Carbides

3.1 - Tantalum Carbide (TaC) .......................... Page 17

3.2 Titanium Carbide (TiC) ............................ Page 19

3.3 - Tungsten Carbide (WC) ........................... Page 22

4 – Solid Solutions ................................................ Page 26

5 – Grinding Mill .................................................... Page 28

6 – Spark Plasma Sintering (SPS) ....................... Page 31 7 – Archimedes Principle …………………………. Page 34 8 – X-Ray Diffraction ………………………………. Page 36 9 – Scanning Electron Microscopy (SEM) …….. Page 43 10 – Experimental procedure

10.1 – Mixture preparation ………………… Page 48

10.2 - Powder mixing ……………………….. Page 50

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10.3 - Spark Plasma Sintering (SPS) ……… Page 51

10.4 - Powder X-Ray analysis ……………… Page 55

10.5 – Theoretical Density calculation …….. Page 55

10.6 – Archimedes method ………………… Page 57

11 – Results and discussion 11.1 SPS sintering results ………………….. Page 59

11.2 SPS sintering discussion ……………… Page 62

11.3 - Powder X-Ray diffraction results …. Page 63

11.4 - X-Ray results discussion ……………. Page 81

11.5 - Density measurements results ……… Page 82

11.6 - Density measurements discussion ….. Page 85

11.7 - SEM images …………………………. Page 86

11.8 - SEM discussion …………………….. Page 103

12 – Conclusions …………………………………. Page 105

13 – Bibliography …………………………………. Page 107

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1- Abstract

The aim of this study is to optimize a method to turn the typical lattice of TaN at

normal conditions (hexagonal) into its cubic counterpart using Spark Plasma Sintering (SPS).

The effect of some parameters such as synthesis temperature and pressure on the cubic

conversion is studied, as well as the role that plays the addition of some dopants (TaC, TiN) in

low quantities. On the other hand, some physical parameters such as density and compactness

were assessed for each of the samples, in an attempt to relate them to the presence of cubic

and hexagonal lattice. To carry out the study, different characterization techniques such as X-

Ray diffraction were used to determine the hexagonal and cubic present lattices, as well as

density measurements. Some SEM images were also obtained providing information about

the formation of the lattices, their physical properties and the grain sizes of the different

samples.

2 - Nitrides

Nitrides are compounds consisting of Nitrogen and another element which is less

electronegative than Nitrogen, in which N has a -3 valence in the bonding. Since they cover a

large variety of compounds, their properties, applications and synthesis methods are different

for each compound. Apart from the ordinary nitrides, there are other compounds formed with

nitrogen ions such as pernitrides (N2-2

) and azides (N3-)

Uses of Nitrides

Nowadays, nitrides are most widely used for the following applications:

1) As abrasives ( Cubic Boron Nitride)

2) As lubricants ( Hexagonal Boron Nitride)

3) As refractory materials ( like Silicon Nitride )

4) For the fabrication of cutting tools ( Silicon Nitride)

5) For metallic coatings ( Titanium Nitride )

6) For hydrogen storage, because of their high reactivity with H2 (Lithium Nitride),

creating the species LiNH2 when the hydrogen is stored

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2.1 - Tantalum Nitride (TaN)

Properties

Tantalum Nitride (TaN) is an inorganic compound consisting of crystals whose tonality

is within a range from brown to black and has a Co-Sn structure type. In fact, its colour is

dependent on the lattice arrangement, which can be either hexagonal or cubic when the moles

ratio is 1:1.

Although naturally TaN presents itself in an hexagonal lattice, the cubic phase can also

be formed when applied high pressures and temperatures to the synthesis process. TaN is also

insoluble in water and possesses a high melting point (around 3360 ºC) as well as the fact that

it is a refractory material capable of retaining its strength at high temperatures.

Some of its main physical properties are presented in the following table

Molecular Weight (g/mol) 195

Density for the Hexagonal form (g/cm3) 13.7

Color Brown to Black

Melting Point 3360ºC

Appearance Crystalline Solid

Synthesis

Various methods can be used to synthesize solid TaN

1) Chemical Vapor Deposition (CVD)

CVD is a set of chemical reactions that turn gaseous molecules (called

precursors) into a solid material, creating either a powder mixture or a thin film on the surface

of the substrate. When choosing the precursor different considerations must be taken into

account, such as its bonds strength (which will affect the operating temperature and the purity

of the product), its thermal stability and reactivity. For example, tantalum hydrides (TaH,

TaH2) and tantalum halides (TaCl4, TaCl5) could be used as a precursor.

6

Energy can be applied as thermal energy (from furnaces and lamps) or as photovoltaic energy

(from UV and laser) to the reactor. Apart from the solid, also gaseous products are formed,

which means they must be treated carefully if hazardous. In the process, the precursor is

densified by a thermically enhanced nitrating gas made up of oxygen, nitrogen and ammonia.

A metal nitride layer is subsequently formed and densified thanks to the nitrating gas action,

even though sometimes a metal layer must be deposited over the substrate prior to the gas

exposure. To create semiconductor devices such as electrodes, by depositing dielectric layers

between the tantalum nitride layers hence creating a stack.

Solidification process in CVD

2) Atomic layer deposition (ALD)

ALD is a technique to create thin films out of gas reactants by a chemical process.

This process allows the creation of metal nitrides layers with ease. Mostly 2 different

precursors are used. The reaction takes place between the precursors and the substrate surface

(one precursor at a time) in a sequential way. It differs from CVD in the fact that ALD breaks

the CVD into two half reactions, meaning that each precursor is used in a different half

reaction. Since the precursor amount in each reaction cycle is set, the reaction is self-limiting

which makes it easy to control and so is the film thickness, which can be as low as 0.1Å per

monolayer. The amount of precursor is controlled by means of a purged gas (nitrogen or

argon) which removes the remaining gas from the former reaction stage.

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The process consists of four major steps which are repeated subsequently

1) Application of the 1st precursor

2) Purge of the precursors and the gaseous products of the 1st stage

3) Application of the 2nd

precursor

4) Purge of the precursors and the gaseous products of the 2nd

stage

The main advantage of ALD over CVD is its ease to control film thickness only by setting the

number of cycles, and also that the mixture needn’t to be homogeneous which simplifies the

process because it allows larger contact area and higher reproducibility. The only drawback to

this technique is its slowliness since only a layer is deposited at a time.

3) Magnetron Sputtering deposition technique.

This is a type of physical vapor deposition (PVD). In this technique material from a

source (which is called target) is resputtered or reejected into the substrate where the thin

films are created by means of a sputtering gas which provides the atoms of the target with

the necessary energy. The sputtering gas has a wide energy distribution which goes up to

10eV. Some of the sputtering gas atoms can cause the atoms from the target to be ionized

and head for the substrate and impact energetically to create the thin film. This effect is

called ―resputtering‖ and is the main effect which produces the film formation.

The sputtering gas must be inert (Argon for example) so as not to react with the

substrate. Its energy can be controlled by setting the pressure conditions of the sputtering

gas. The closer the atomic weights of the sputtering gas and that of the substrate are, the

better the momentum is transferred to the atoms on the substrate. That’s why neon is more

likely to be used when spluttering light elements and krypton is usually used for heavier

elements. There are a lot of parameters to take into account in this process which make it

complex. However, its complexity also allows a high control over features such as film

growth and microstructure if the parameters are well controlled. All its features make it

suitable for fields such as electronics, (especially when depositing thin films of various

materials to create integrated circuits) or coating.

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The main advantage over other techniques is that it can even sputter high

melting point substrates if the sputtering gas beam is energetic enough, the fact that the

film composition is very close to the former substrate, and better adhesion of the films. Its

main disadvantage is that the sputtered atoms can’t be directed exactly to the substrate,

which may lead to chamber contamination.

Magnetron Sputtering deposition technique.

Applications

Tantalum Nitride (TaN) has its main applications in the electronics field. Some of its most

usual applications are listed below:

1) Barrier Layers

TaN is used in the electronics industry to create thin barriers which are placed between

copper and silicon-based components such as interlevel dielectrics (ILDs) made of silicon

dioxide, in integrated circuits. TaN based layers are effective when it comes to avoiding

copper or silicon diffusion between the layers. This non-diffusive properties are kept even at

high electric fields and voltages when TaN is used. This barrier also presents a good adhesion

between the metallic copper layer and the silicon-based ILDs.

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2) Resistors

A resistor is a device that produces a voltage between two electronic components. This

voltage is directly proportional to electric current according to Ohm’s law.

Together with nickel-chrome, tantalum nitride is used to manufacture resistors

for precission applications due to the reduction of thermal and electronic noise that this

material provides in the shape of thin films, being the most widely used material for that

purpose. Unlike its nickel-chrome counterparts, tantalum nitride films are resistant to

humidity even at high application potentials. This occurs thanks to a tantalum oxide layer

formed espontaneously on the surface which prevents it from corroding, process known as

passivation.

2.2 - Titanium nitride (TiN)

Properties

It’s a ceramic and hard material stable at room temperature (oxidizes at 800ºC) and reactive to

strong acids. It also has good IR properties in the range of gold (Au) which give it a golden

colour. Most commonly it presents the Na-Cl type crystal structure when the stechiometry is

1:1 that is a cubic lattice. Some other stechiometrys also present stable structures for titanium

nitride.Its good properties make it suitable for applications such as coating or superinsulators

among others.

Some other of its properties are

Vickers hardness = 18-21 GPa

Thermal conductivity = 19.1W/(m* ºC)Modulus of elasticity 251 GPa

Modulus of elasticity = 251GPa

Thermal expansion coefficient = 9.35*10-6

K-1

Superconducting transition temperature = 5.6 K

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Synthesis

TiN films are usually synthesized by physical vapor deposition (PVD)

techniques such as sputtering, electron beam heating or cathodic deposition as well as by

chemical vapor deposition (CVD). Titanium reacts with highly energetic nytrogen in vacuum

conditions to create titanium nitride. As explained, PVD is mostly used when TiN is

combined with higher melting point materials like stainless steel or to create titanium alloys.

TiN can also be obtained by packing powdered titanium by compression in a nitrogen

atmosphere (nitrogen or ammonia) at high temperature (1200ºC) . The heat is provided by the

exothermal reaction between titanium and nitrogen, having as a product a compact and hard

material. TiN coatings can also be obtained by thermal spraying.

Applications

Electronics

Thin films of TiN are used for semiconductors. They’re used in coper based

chips as a conductive barrier placed between the silicon parts and the metallic ones. Its role is

to block copper diffusion and allow electrons to flow through at the same time, acting as a

metal diffusion blocker. It’s also used to improve transistor performances. In fact, when

combined with gate dielectrics such as HfSiO posseses better performance than pure SiO2

providing them with a better threshold voltage and a lower leakage

Coatings

TiN is used to avoid corrosion, specially on tools such as drills milling cutters

and even improving their lifetime at least trice. It’s also used to coat another metals such as

chromium or nickel on plumbing and hardware applications. Since it’s not toxic, can also be

applied to medical tools such as scalpels or orthopedic saw blades which provide the

necessary cutting sharpness for medical operations. In fact, TiN can also be used to make up

prothesis.It is also used for jewellery and decorative motifs because of its golden colour.

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As a constituent in steel making

Titanium is added intentionally to some steel alloys to form TiN within the steel

alloy. It’s produced at high temperatures in the form of micrometric cubic particles and has a

low solubility in steel. The titanium helps to stabilise the steel by hindering possible

corrosion, since titanium has a higher affinity for carbon, oxygen and nitrogen rather than

chromium, thus producing stable carbides, oxides and nitrides in the steel formation process.

All these species could have formed detrimental compounds if they had not reacted with

titanium, which could lead to reduced mechanical properties. However, this kind of steels are

prone to cracks formation after welding.

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2.3 - Silicon nitride (Si3N4)

Properties

Silicon Nitride is a hard ceramic material which is resistant over a broad range of

temperatures. It also has a low thermal expansion coefficient, moderate thermal conductivity,

a high elastic modullus and quite a high fracture toughness (unlike most ceramics). All these

properties provide Si3N4 with the capabilities of supporting high loads succesfully and with

excellent thermal shock resistance, which make it suitable for applications such as

manufacturing devices which operate at high temperatures (turbines, car engines ..) as well as

cutting tools and metal working. Silicon nitride films are also used in the electronics industry.

Synthesis

There are three different routes to obtain Silicon Nitride (Si3N4)

1) By reaction between silicon and nitrogen at high temperatures (1300 —1400ºC)

3 Si(s) + 2 N2(g) → Si3N4(s)

2) By diimide synthesis:

SiCl4 + 6 NH3 (g) → Si (NH)2(s) + 4 NH4Cl(s) at 0 °C

3 Si(NH)2(s) → Si3N4(s) + N2(g) + 3 H2(g) at 1000 °C

3) By carbothermal reduction in nitrogen atmosphere at 1400–1450 °C:

3 SiO2(s) + 6 C(s) + 2 N2(g) → Si3N4(s) + 6 CO(g)

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The simple nitridation of silicon was formerly the main large scale production method.

However, silicates are also formed and contaminate the process.

The diimide synthesis produces amorphous silicon nitride which needs further annealing with

nitrogen at high temperatures to make it crystalline. The carbothermal reduction method and

presents high cost and S3N4 purity performances

Si3N4 films can be formed by chemical vapor deposition CVD,being one of its variants,

Plasma enhanced chemical vapor deposition (PECVD) the most effective method.The process

complies with the following reactions

3 SiH4(g) + 4 NH3(g) → Si3N4(s) + 12 H2(g)

3 SiCl4(g) + 4 NH3(g) → Si3N4(s) + 12 HCl(g)

3 SiCl2H2(g) + 4 NH3(g) → Si3N4(s) + 6 HCl(g) + 6 H2(g)

If what is needed is to place films between semiconductor substrates (made of silicon), also

Low pressure chemical vapour deposition (LPCVD) can be used. In this process, deposition

parameters are important since if not controlled correctly, it could lead to stress appearance,

due to the fact that Si2N4 and pure silicon don’t have the same lattice parameters.

Applications

The main drawback when using Si3N4 is not its technical performance but rather

its price. Since it has become cheaper, new horizons have opened to this promising material,

being the most important

1) Car industry

One of the major applications of sintered silicon nitride is in automobile

industry as a material for engine parts. Those include, in Diesel engines, glowplugs for faster

start-up; precombustion chambers (swirl chambers) for lower emissions, faster start-up and

lower noise; turbocharger for reduced engine lag and emissions.

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In spark-ignition engines, silicon nitride is used for rocker arm pads for lower

wear, turbocharger for lower inertia and less engine lag, and in exhaust gas control valves for

increased acceleration. As examples of production levels, there is an estimated more than

300,000 sintered silicon nitride turbochargers made annually.

2) Bearings

Different types of bearings

Si3N4 bearings can be either pure or ceramic hybrids altogether with steel. The

silicon nitride’s good shock resistance makes it perfect for such an application. Since Silicon

Nitride is harder than metal, when such a material is used for bearings the contact with the

bearing track is radically reduced. This leads to lower friction (80% lower than when its metal

counterpart is used), longer lifetime, higher operation temperature and to a reduction in

corrosion.

3) High-temperature material

Due to its excellent behaviour at high temperatures, shown by its resistance to high

thermal shocks and thermal gradients, this material can play an important role when used in

devices which operate at high temperatures such as rocket engines.

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4) Metal working

Silicon Nitride is perfect to play roles such as milling, grinding and piercing of metals.

In fact, is used for cutting and abrasive tools because of its excellent thermal and mechanical

performance. S3N4 can cut through cast iron and high steel at a speed 25 times higher than

when conventional materials are used. In fact, Silicon Nitride has had a great importance to

assemble well-finished metal products.

5) Electronics

S3N4 is used in electronics as both an insulator and a chemical barrier for

integrated circuits and to electrically isolate circuit parts. As a chemical barrier is better than

SiO when blocking species such as water and sodium ions which can cause corrosion and

instability. It can also be used as dielectric between polysilicon layers in capacitors.

3 - Carbides

A carbide is a binary compound made of carbon and a more electropositive

element. Carbides can be classified into different groups, and their properties, synthesis and

applications are dependent on the group to which they belong to. The simplest classification

is:

1) Salt-Like carbides: They are compounds formed by carbon and the compounds from

groups I,II A and III B (except Actinium).depending on the lattice arrangement, these

carbides can be represented by the anions C4-

, C34-

,C22-

2) Covalent carbides: These are silicon (SiC) and boron carbides (B4C). Both materials

are hard and refractory, which makes them important industrially speaking.

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3) Insterstitial carbides: These group includes the transition metals from the groups IV,

V and VI B (except chromium). They all are refractory and possess metallic properties.

Some of them (Titanium carbide, tantalum carbide and tungsten carbide) are used to coat

cutting tools.

4) Molecular carbides: These are metal complexes containing carbon. There are

molecules with carbon fragments or most commonly clusters in a metal-centered body.

One example of a molecular carbide is the compound Fe5C(CO)15

Fe5C(CO)15 molecular carbide

.

Uses of Carbides

1) Abrasives and cutting tools ( Silicon carbide)

2) Steelmaking (Calcium Carbide)

3) Coatings on Iron and Steels ( Titanium Carbide)

4) High temperature applications (Tantalum Carbide)

5) Machine tools and military purposes (Tungsten Carbide)

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3.1 - Tantalum Carbide (TaC)

Properties

TaC is a hard and refractoryceramic material (with a hardness around 9 – 10 mohs).

It’s one of the hardest materials, and only diamond has a higher hardness.This makes its use

suitable for cutting tools and for the manufacturing of ceramic—metallic materials among

others. It appears to be a brownish powder and its crystal arrangement is cubic and equal to

that of NaCl. In fact, is the stechiometrically binary compound (1:1) with the highest melting

point, only excedeed by its counterpart TaC0.89, with a melting point of 4000ºC. It’s also a

flammable compound when it’s presented in the form of a dust, because it can form

flammable gases.

Some other of its physical properties are exposed in the following table:

Molecular weight 192.96 g/mol

Density 13.9 g/cm3

Melting Point 3880ºC

Boiling Point 5500ºC

Appearance Solid brownish powder

Solubility Not soluble in water

Crystal Structure Cubic

Risks None

Synthesis

There are various routes to synthesize TaC:

1) By mixing tantalum pentoxide (Ta2O5) with tantalite [(Fe, Mn) (Ta,Nb)2O6] in the p

resencepresence of carbon ( C ) and sodium carbonate (Na2CO3) at 1500ºC, leaving 6% of

djdjdjdunreacted carbon.

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2) By means of a simple reaction among metallic magnesium (Mg), sodium carbonate

(Na2CO3) and tantalum pentachloride (TaCl5) at 600ºC, thus obtaining good tantalum

carbide with good thermal stability and oxidation resistance when used below 450ºC

in an air environment.

3) By the reaction of metallic Tantalum powder (Ta) with carbon (C) at 2000ºC in a

graphite furnace. This method leaves around 6% of unreacted carbon.

4) Carbidization of tantalum powder and tantalum oxides in the presence of carbon and a

hydrogen stream at 1600ºC

Applications

1) Oxidation and thermal protection coating:

TaC layers can be deposited on materials such as carbon, refractory metals and alloys to

protect them from thermal impacts and oxidation with a very good performance. The

advantages of this carbide for this use are the possibility to coat the material at a low

temperature (10% of the melting point of the carbide), the lack of porosity in the coating and

its adaptation to difficult shapes and surfaces. TaC offers great protection in very corrosive

and high temperature environments.

2) Cutting tools

When TaC is used to manufacture cutting tools it provides very good mechanical

properties such as high hardness, low tool deformation even at high temperatures, good wear

resistance and low brittleness. All of these properties are paramount when cutting at high

speeds. In fact, tantalum carbide coated tools cut three times faster than high-speed steels.

Tantalum carbide is often used with a mixture of other carbides to obtain the necessary

mechanical properties. It’s specially used to manufacture high-speed machine tool bits,

mining equipment, cutting tools and teeth for construction.

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3) Cermet material (metallic-ceramic) composites

This composite can be created with the ceramic material Tantalum Carbide. The main

reason is to combine its properties with those of metals, such as malleability. For this

application the most interesting properties provided by TaN are its resistance to high

temperatures and its good wear resistance.

4) Tantalum carbide/graphite composite material

This hybride is one of the hardest materials ever manufactured by man, and has a

meltng point of 3738ºC. This material combines the best properties of each material, being the

best properties of one of the material the worst of the other, thus neutralising them. In other

words, carbon has very good ablation and thermal shock resistance, but erodes easily.

Tantalum carbide resists erosion perfectly, but its ablation and thermal shock resistance is

poor. In the output composite materials, all of these properties have been improved when

compared to the former materials. This improvement opens a new horizon of applications

where excellent thermal and mechanical properties are required, such as bearings, turbines or

rocket nozzles.

3.2 Titanium Carbide (TiC)

Properties

Titanium carbide is a refractory ceramic material, with a hardness around 9.5 in the

Mohs scale. It’s a crystalline black powder with a cubic centered structure like NaCl. This

material is resistant to wear and corrosion, and preserves its hardness at high temperatures.

It’s not flammable in the solid state, although grinded dust can be flammable and also cause

skin and eyes irritation. It’s also a chemically stable compound which only dissolves in acids

or alkalis. All this properties make it useful for applications such as cermets assembly, cutting

tools and alloying.

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Some of its properties are listed below:

Molecular weight 59.9 g/mol

Density 4.93 g/cm3

Melting Point 3160 ºC

Boiling Point 4820 ºC

Appearance Black powder

Solubility Not soluble in water

Crystal Structure Cubic

Risks Can cause irritation

Synthesis

Different methods can be used to synthesise TiC, among which the following were remarked:

1) Self-propagating High-temperature Synthesis (SHS)

This synthesis takes place inside of a reactor and can be easily reproduced industrially. In this

process, titanium powder (Ti), carbon black (C), and an inert diluent such as titanium carbide

(TiC) form the starting mixture. The TiC diluent plays an important role: it reduces the

combustion rate, decreases the necessary synthesis temperature around 200K and enables the

obtention of TiC in a powder form. Different gases are formed during the synthesis due to the

impurities present in the starting powders, such as TiO2 layers that may have been formed,

which need to be purged to avoid product contamination.

2) Mechanochemical process

By means of mechanical grinding, the reaction between black carbon and titanium in powder

form can take place at ambient temperature. This process is relatively cheap and fast, and

provides ultrafine powder that is perfect to manufacture cutting tools due to its improved

fracture toughness.

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3) Chemical Vapor Deposition (CVD)

In this synthesis, the used precursors are TiCl4 gaseous molecules. These solidify to form

solid TiC thin films over a quartz substrate after the reaction with toluene (the carbon source)

takes place. The depositing temperature must be set over 900ºC and nickel must be present to

play an important role as a catalyst. Otherwise, only amorphous carbon would be deposited.

The reaction to obtain TiC is the following:

2

º900.),(

3564 10877 ClHClTiCCHHCTiClCTcatNi

Applications

1) Tool bits and cutting tools

Titanium carbide is widely used for cutting tools and tool bits because of its good

mechanical properties. When it is added to hard metals it provides excellent wear resistance

and hardness (since it is one of the hardest carbides) and oxidation stability, being usually

combined with a mixture of other carbides to improve the mechanical properties. Titanium

carbide is used for cutting tools such as grooves, cutting teeth and milling slots. This material

is also used for surgery tools as coating.

2) Titanium carbide – nickel-cobalt Cermets (ceramic-metallic composite)

These cermets can be prepared with a wide range of metals (nickel, cobalt, chromium,

molybdenum, stainless steel …). Their main characteristic is their high toughness and

hardness although they’re dependent on the material base. And so then, nickel based cermets

with titanium carbide are thermally and abrasive wear resistant, making them suitable for

engineering applications such as turbine blades and electrical brushes. Chromium-Titanium

carbide cermets possess high temperature erosion resistance, which makes them suitable for

metal coating.

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3) Coatings

By coating different metals such as steel and iron with Titanium Carbide their

wear and corrosion resistances are improved, as well as their hardness. It also acts as a heat

barrier which allows titanium carbide to be used for applications in which the thermal shock is

important such as aerospacial components, ball bearings, compressor blades and metal cutting

tools coatings. Its high hardness also makes it suitable for scratch-proof coating applications

such as clinical dentistry and watches.

3.3 - Tungsten Carbide (WC)

Properties

Tungsten carbide is an inorganic compound made up of Tungsten (W) and

Carbon (C) in a molar ratio 1:1. It has got the appearance of a grey powder which is usually

pressurized into different shapes for industrial purposes. WC has very high strength as shown

by its high stiffness (three times stiffer than steel) and hardness. Only few compounds among

(SiC or diamond among others) can polish it. Its impact resistance performance is good even

at low temperatures and also possesses high thermal and electrical conductivities. Its hardness

is maintained up to 760ºC and has got a low coefficient of friction. This material is also more

corrosion resistant than noble metals and more wear resistant than steel as well as resistant to

heat and oxidation. At high temperatures it decomposes to tungsten (W) and carbon (C) and is

oxidized over 600ºC. The most normal crystal structure is the hexagonal (α-WC) although at

high temperatures a cubic structure can be formed (β-WC)

Some of the WC properties are the following:

Molecular weight 195.9 g/mol

Density 15.8 g/cm3

Melting Point 2870ºC

Boiling Point 6000ºC

Appearance Grey Solid

Solubility Insoluble in water

Crystal Structure Hexagonal

Risks Can cause irritation, inhalation may cause

lung damage (fibrosis)

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Synthesis

Tungsten carbide can be prepared in many different ways.

1) Reaction of tungsten metal and carbon at high temperatures:

)()()( º20001400 sWCsCsW C

2) A fluid bed process which entails the reaction of tungsten metal or blue tungsten oxide with

carbon oxides and hydrogen at high temperatures in a patented fluid bed

OHsWCgHgCOCOsW C

2

º1200900

22 )()()(/)(

3) By Chemical vapour deposition (CVD) WC can be synthesized in two different reactions:

3.1) Tungsten hexachloride reacts with methane in the presence of a reducing agent (H2)

HClsWCgHCHsWCl C6)()()(

º670

246

3.2) Tungsten hexafluoride reacts with methanol in the presence of H2

OHHFsWCgHOHCHsWF C

2

º350

236 6)()()(

24

Applications

Tungsten Carbide is used in a wide range of applications. Those most important

are described below

1) Machine tools

Tungsten carbide is used in machinery parts, cutting tools, drills and abrasives.

When manufacturing these tools, the used tungsten carbide has been bound specially with

cobalt, which is known as cemented carbide and belongs to the group of cermet materials

(ceramic – metal matrices). For example, WC is used to manufacture both huge and tiny drills

with a wide range of applications. Tungsten Carbide is the hardest material besides diamond

and the second most efficient when it comes to cutting. In fact, a tungsten-copper-steel alloy

is used to bind millimetric diamond pieces that would not be of further industrial use as a

grinding tool, thus increasing the diamond lifetime twice. When applied to rotatory cutting

tools, it cuts five times faster and also has a much longer lifetime than a steel-made one.

Tungsten Carbide tools can also be used at higher temperatures and produce better finish to

products than their steel counterparts. WC drills hold a great significance when exploiting

earth resources such as gas and petroleum because of its great hardness, which allows piercing

deep layers of soil made up of different materials.

Generally speaking, tungsten Carbide cutting tools are important when operating

on hard materials such as steel or large material quantities, where most of the tools would

wear out. This wearing out resistance is what makes it suitable for applications such as

bearings and machinery parts.

2) Cemented Carbide synthesis

Cemented carbide is a cermet material (a composite made of a ceramic and a

metal) obtained by binding the ceramic material (WC) with a metallic compound, mostly with

cobalt (Co) and sometimes with nickel (Ni).

25

This material combines the hardness and temperature resistance of the tungsten carbide with

the malleability of the metal which allows stress application without breaking. In other words,

the metal addition reduces brittleness. This composite can be easily synthesized and also

provides magnificent properties. During the sintering, tungsten carbide is soluble in cobalt at

high temperatures, and the used cobalt-based liquid precursor also spreads out properly

throughout tungsten carbide. This leads to a dense and pore free product cermet which

possesses high hardness, toughness and strength at the same time.

3) Military

Both its high density and hardness make it a perfect material (especially when

uranium is not available) for military machines parts, piercing ammunition and projectiles due

to its high penetration power. In fact, tungsten carbide is especially used in the cores of

ammunition made up of two different parts in which the two of them come off on impact. WC

is also widely used in the nuclear weapons industry due to its capability to reflect neutrons,

which allows nuclear chain reactions.

4) Sports

The tungsten carbide hardness allows its use in poles which are meant to hit

hard surfaces and poles used in trekking and skiing simulation in hard surfaces. This carbide

allows the traction and duration that such poles need. This material is even used in tires studs

to provide a better adherence on ice, being its performance better than that of steel studs.

5) Jewellery

Tungsten carbide’s remarkable properties make it perfect for jewellery. This

material provides jewellery with contrast and bright mirror-like finishes with an hematite-like

colour, apart from improving the jewels´ resistance to marks and scratches. However, due to

its high hardness they can barely be removed. In fact, only specialised tools can cut through it

without jeopardizing the hand’s wellbeing.

26

4 - Solid Solutions

Introduction

A solid solution is a mixture of two solid compounds that form a single

crystalline solid. There are various methods to produce solid solutions. Therefore, they can be

formed by melting two different solids, mixing them and cooling down the mixture or by

dissolving both solutes in a common solvent and letting it dry up. Vapor deposition over a

substrate also leads to solid solutions in the shape of a thin film.

Solid solutions are most likely to be produced when the solutes are close to each

other in the periodic table (especially for metals). In fact, solids and liquids behave alike in

solubility. That is, depending on their chemical properties and crystalline structure, they have

different degrees of mutual solubility which determine the positions of the mixed atoms in the

final crystal.

Types of solutions

These type of solutions can be formed either by a substitutional mechanism (in

which the atoms of one species replace some from the other) or interstitial (in which the

atoms occupy free gaps in the solvent lattice). The solubility of the species is dependent on

their concentrations in the mixture and in the solids nature. Different properties of both

compounds regarding chemical composition and crystal lattice must be taken into account to

determine whether a solid solution will be formed, some of which are listed below.

1) Both must have similar radii with less than 15% difference.

2) Their valency must be alike

3) The electronegativities of the compounds must be close

4) Their lattice arrangement must be similar.

27

When the solid solution conditions are not met, the solid solution is reversed and two separate

phases make their appearance. This process is called exsolution.

Examples

Some examples of such solutions are minerals created naturally at high

temperatures and pressures like the solid solution composed of two olivines, fayalite

(Fe2SiO4) and forsterite (Mg2SiO4). Both species form mutual solid solutions with a wide

range of different iron and magnesium concentration which provides the mixture with slightly

different physical properties than that of the starting materials.

Therefore, solid solutions can provide if used with the proper concentrations

better properties than that of the former materials which leads to potential commercial and

industrial applications such as metal alloying. Most of the alloys such as brass (Co-Zn) are

considered solid solutions formed by a substitutional mechanism, whereas in steel carbon is

interstitially forming a solid solution in an iron matrix.

Solid solutions are also a great asset in the semiconductors industry because

they allow the selection of intermediate electrical properties between those of the former

materials which make them suitable for optical and electronical applications. An example is

the band gap of the solid solution formed by InAs and GaAs which can be set somewhere

between the values for InAs (0.36eV) and GaAs (1.4eV).

28

5 - Grinding Mill

A grinding mill is a mechanical device that breaks the fed solid into smaller particles, thus

reducing the particle size of the solid which was introduced. In the mill, the machine’s

mechanical forces try to surpass those of the internal bonds of the matter itself, in order to

eliminate such bonds. After this, the grain size and their shapes are totally modified, as well as

their arrangement.

The grinding purpose may be to extract valuable constituents in different ore materials or to

prepare the raw materials for an industrial process, such as grinding coal for furnaces or clay

for pottery. In engineering, it helps to increase the surface area of a solid and also to create

products with the desired grain size.Various kinds of grinding mills can be found. In the very

beginning they were operated by manual rotation (like mortars), animal force, and the action

of wind and water. Later on electrical ones were introduced.

The main types of grinding mills are listed below according to their operation principle:

Ball mill

It consists of a tilt or horizontal cylinder which rotates around its horizontal axis, filled up

with balls, either metallic or made of stone. By impact, friction and abrasion these balls grind

the fed solid material to a certain degree of grain size. These ball mills are used in industry

(for example to manufacture cement) in a large scale. There can be also found little versions

used in research laboratories which are used to grind sample material in order to ensure the

quality of the sample.

Laboratory ball mill

29

Rod mill

They’re also usually referred to as ―slitting mills‖. In this type of mills a rotating vessel helps

to create the necessary mechanical friction between steel rods and the solid particles. Its rods

are usually made of iron or some other metal.

Laboratory Rod Mill

SAG mill

SAG or Semi-Autogenous Grinding are comprised by mills that use for grinding a mixture of

both steel balls and big rocks. Its ball charge ranges from 6 to 15%. In this mill, both the balls

and the rocks are thrown down at the solid and this action causes the break down into smaleer

pieces of the solid samples by impact and compression while the drum is rotating. These

drums are usually large in diameter and short in length. There are lifts installed on the inside

to lift the solid particles up and around the mill, after which it falls off the plates boosting

particle movement throughout the mill. Some of its industrial applications are the grinding of

precious metals such as gold, platinum and silver among others.

30

Principle of SAG Mill operation SAG Mill

Autogenous mill

In this system large rocks are thrown down thus breaking down the solid

particles by impact and compression. In fact, it is very similar to the SAG system, apart from

the fact that it does not use metal balls. It can be used when contamination by metals is to be

avoided.

High pressure grinding rolls mill

The solid meant to be grinded is put between two rollers located very close to

each other. These rolls rotate in opposite directions thus crushing the solid material.In fact,

while it is spinning, the solid fits into a tiny gap in between, and it reaches pressures so high

that are capable of breaking it down into finer particles, even fracturing the material at the

grain size level.

Operation diagram of a high pressure grinding rolls mill

31

6 - Spark Plasma Sintering (SPS)

Definition

SPS (Spark Plasma Sintering) is a sintering process that can assemble compact

samples from ceramics and metal powders at a different range of temperatures and pressures.

The technique is especially interesting due to its capability of doing so at low temperatures. It

is yet to be discovered which are the mechanisms taking place in the densification and grain

growth processes as no direct observations of a plasma have been made. In this process

uniaxial pressure is applied while heating. In order to heat, a pulsed direct current goes

through the electrically conducting plates and sometimes also through the sample, unlike

other heating systems. This means that the sample is heated from both the inside and the

outside.

SPS Sample Holder

Mechanism

When operating, the SPS applies pulsed direct current. When the voltage is

applied to the powdered material the energy is transferred and dispersed homogenously all

over the powder, thus presenting highly energy efficient sintering conditions. This method

focuses the energy on the intergranular bondings which leads to synthesis improvements

when compared to other methods such as hot-press sintering. As explained, because of the

spark discharges between the particles of the material, localized and high temperatures are

achieved. This leads to both vaporization and melting of the powder which causes plastic

transformations of the material.

32

In the end, a dense tablet of over 99% compactness is obtained. As a matter of

fact, since only the surface of the particles is heated a better control over the grain growth is

held, and a rapid and precise sintering is possible.

SPS instrumentation

Features

What makes this process special is the fact that fast heating rates in short periods of

time can be used to produce quite dense samples. There are three main factors which

contribute to this purpose:

1) The mechanical pressure: It helps the powder to diffuse therefore avoiding porosity

2) The heat transfer: The equipment presents incredible heat transfer efficiency from

the plates to the sample. In fact, the plates act as a heat source themselves.

3) The use of direct current: The presence of an electrical field generates spark

discharges and plasma appearance within the powder. This helps to dispose of

adsorbed substances such as CO2 and H2O in the first place. Afterwards, both the lack

of contaminants and the spark discharges boost material diffusion all over the sample

increasing its density.

33

However, since this is a rapid densification process the grain growth is difficult to

control, and that is the reason why the conditions must be set appropriately. In fact, conditions

like holding time, applied pressure, heating rate and initial and final temperature influence the

grain growth and the mechanical properties of the sample such as brittleness and fracture

toughness.

Laboratory SPS sintering equipment

Main Advantages

The advantages offered by SPS over other sintering methods are:

• Tight control over temperature, pressure and cooling down.

• Fast and uniform synthesis

• Obtention of highly dense tablets with low porosity

• Efficient method, which leads to low costs.

• Easy to operate.

• Elimination of present contamination

• Low grain growth

• Low effect of the sintering on the material microstructure.

Besides, it also allows the attachment of some accessories such as automation systems and

robotic interfaces.

34

7 - Archimedes Principle

This principle allows the volume determination of an object when it is immersed

in a liquid media. Therefore it can be used to determine densities.

Such as shown by the density formula, the density is the relation between a mass

and the volume it occupies ( = m/V). The mass can be easily weighed by a set of scales,

whereas the volume determination cannot be performed accurately most of the times,

especially when the objects to be measured present irregular shapes. It is in such cases when

the Archimedes principle becomes useful.

The volume of an object or a liquid can be easily calculated applying the

principle of buoyancy force, which simply states that an object in a liquid is subjected to

forces from all directions, caused by hydrostatic pressure. However, the value of the vertical

forces caused by the liquid, which point towards both directions (up and down) do not usually

cancel each other out. The reason is that the gravitational force of the water must also be

taken into account in the vertical axis, being directly proportional to the depth at which the

object is located in water.

Vertical forces caused by hydrostatic pressure in an immersed object

35

In fact, the resulting force (Buoyancy force) in the vertical axis (F2 – F1) is

proportional to the gravitational force applied by the mass of liquid displaced to the object as

follows:

gVFgVF loB

or

llB

Where: Vl = Volume of liquid displaced by the object.

Vo = Volume of the immersed object.

Eventually, since the volume of the object is the same as that of displaced water

(Vl ) the value of the object’s volume can be easily worked out from the former formula. Once

both values (weight and volume) are known the density value can be easily assessed.

36

8 - X-Ray Diffraction

Introduction

In this technique, X rays interact with matter thus preceding interferences (which can

either be positive or negative). The interaction of different wavelength rays are plotted and

later used for analysis. The software measures all the atom positions in the crystal. Once the

cell parameters are known it’s possible to determine the matter structure and the

corresponding angles.

X-ray diffractometry are physical techniques used for the identification of substances,

and for other types of analysis, principally for crystalline materials in the solid state. In these

techniques, a monochromatic beam of X-rays is directed onto a polycrystalline (powder)

specimen, producing a diffraction pattern that is recorded on film or with a diffractometer.

This X-ray pattern is a fundamental and unique property resulting from the atomic

arrangement of the diffracting substance. Different substances have different atomic

arrangements or crystal structures, and hence no two chemically distinct substances give

identical diffraction patterns. Identification may be made by comparing the pattern of the

unknown substance with patterns of known substances.

.

In modern age, X-ray diffractography is undoubtedly one of the largely used

techniques because the experimental measurement is relatively short in time and allows

finding out the compounds and phases present in an unknown sample and also all sort of data

regarding its structure and even the processes the phases have gone through in the past. It

must be taken into account that even though in powder diffraction the material is not forming

a simple crystal in its own right, the analysis is still possible since this powder is still capable

of diffraction thanks to the degree of crystallinity it possesses.

37

Lattice geometry

The atoms in crystalline substances are arranged in a symmetrical three-dimensional

pattern: some atomic arrangement is repeated by the symmetry of the crystal along straight

lines throughout the crystal. The smallest group of atoms which has the symmetry of the

entire pattern is called the unit cell. The traces of the various lattice planes (normal to the

drawing) are indicated by heavy lines. To identify the planes, one must count the number of

planes crossed from one lattice point to the next along a, then repeat the procedure along b

and c are known as the Miller indices of that set of planes, and assignment of indices to each

line is called indexing. The spacings d between the planes are related to the Miller indices and

the unit cell dimensions.

In crystals of the cubic system (such as cubic TaN), the crystallographic axes are

normal to each other and have the same length, a = b = c; and spacing d is given by Bragg’s

law:

38

Bragg’s law:

Sindn 2

Where λ is the wavelength of the incident X-Rays.

The angle between the two spacing is θ.

2θ is the angle between impact and detection line.

Thus the conditions for x-ray reflection are very restrictive because these is only one

angle θ at which the X-rays of a given wavelength are reflected by a particular set of atomic

planes of spacing d.

Characteristics of powder patterns

Many materials are not available in the form of large single crystals, and moreover it

is impractical to obtain all the X-ray reflections from single crystals for identification

purposes.

If the sample does not already exist in polycrystalline form, it may be pulverized.

When a fine grained powder consisting of thousands of small, randomly oriented crystallites

is exposed to the X-ray beam, all the possible reflections from the various sets of atomic

planes can occur simultaneously.

Pieces of information can be derived from the experimental pattern:

(1) The 2θ value from which the d-spacing can be calculated;

(2) The absolute intensity, from which relative intensities can be calculated;

(3) The peak width;

(4) The form of the background.

The complexity of the pattern is determined primarily by the symmetry of the

substance rather than by its chemical composition.

39

If the crystallites do not have a completely random orientation, the line shapes and

relative intensities will change accordingly. Comparison of the random and oriented X-ray

patterns shows the degree of orientation in the sample. Since the two types of patterns are so

different, the method is ideally suited to distinguish between crystalline and amorphous

substances and to determine the degree of crystallinity of substances between the two

extremes.

There are also many smaller changes in the X-ray pattern which may reveal important

information. In substitutional solid solutions, for example, atoms of different elements may

substitute for one another and occupy the same relative positions as in the pure metals. The

substitutions of solute atoms occur on the same lattice sites occupied by the solvent atoms, but

are randomly distributed. If the atoms are of different size, the average unit cell size will

change accordingly. In simple cases, it is possible to determine the chemical composition of

intermediate members by measuring the unit cell dimensions because there is often a nearly

linear relationship between the two. In interstitial solid solutions, atoms are added to the

empty spaces in the structure and there are little, if any, changes in the dimensions.

Diffraction databases

There are a number of databases available for X-ray diffraction work. The majority of these

databases are designed and maintained for the single-crystal community rather than for the

powder community. However, some Powder Diffraction File (PDF) are calculated from single

crystal data of the type contained in the other databases.

The Powder Diffraction Files (PDF) are collections of single-phase x-ray powder diffraction

patterns in the form of tables of the interplanar spacings (d) and relative intensities

characteristic of the compound. These databases have proven their usefulness in a wide range

of applications because every crystalline material gives, at least in principle, a unique x-ray

diffraction pattern, study of diffraction patterns from unknown phases offers a powerful

means of qualitative identification by comparing an X-ray pattern from the material to be

analyzed with a file of single phase reference patterns.

40

The ability to recognize a reference pattern in an unknown material strongly depends

on the quality of the d’s and I’s in both the reference material and the unknown sample. One

of the major problems in the identification of materials by comparison of an experimental

pattern with reference patterns is the variability in the quality of the data. As the quality of

both reference and experimental patterns improves, the problem of pattern recognition

becomes easier.

Instrumentation

There are many types of powder diffractometer available ranging from simple

laboratory instruments to versatile and complex instruments using a synchrotron source.

Completely automated equipment for X-ray analysis is available. Most laboratory instruments

consist of a high-voltage generator which provides stabilized voltage for the X-ray tube, so

that the X-ray source intensity varies by less than 1%. X-rays are produced using a

molybdenum source.

Electronic circuits use an X-ray detector to convert the diffracted X-ray photons to

measurable voltage pulses, and to record the diffraction data. The number of elements useful

for X-ray tube targets is limited to a few, of which tungsten is the most commonly used.

During the tests, either the X-ray tube or the detector move, depending on the settings, thus

covering all the 2θ range

X-ray diffractometer

41

Optical Instrumentation in a X-Ray diffractometer

Signal Instrumentation in a X-Ray diffractometer

42

The Rietveld Refinement

This analysis technique is used to analyze the presence of different phases in

powder X-Ray diffractograms. In fact, powder diffraction is used for the analysis of the great

majority of materials since they cannot be synthesized in the form of big crystals. However,

one of its main drawbacks is the overlapping of data, which makes difficult a proper

determination of the material’s structure.

These difficulties can be overcome by using the Rietveld refinement, which

indeed separates the overlapping data, therefore making it possible to determine the structure

accurately. Its accuracy relies on the fact that it analyses the overall diffractogram as a whole,

and takes into account the height, widths and positions of the reflections by means of a least

squares approach. This approach tries to reproduce the data profile mathematically and as

approximately as possible.

Ever since the application of this technique to analyze powder diffraction huge

improvements like never before have been made in the reliability of the so commonly

overlapped data. This method is so trustworthy that nowadays structures can be determined

almost as well by powder diffraction as they are in its single crystal counterpart. Some other

of its applications are the determination of different components in mixturesl, also known as

phase analysis and therefore find some industrial uses in fields such as the oil or cement

industry.

Figure showing a Rietveld calculated profile (in blue), its divergence with the diffractogram

data (black line) and the determined structure (red lattice)

43

9 - Scanning Electron Microscopy (SEM)

Definition

The Scanning Electron Microscope technique is an instrument that allows the study of

organic and inorganic materials, providing morphological information. It’s used to observe

surfaces and analyze their relief, texture, grain sizes, grain shapes and chemical composition

(EDS) from biological and mineral simples. That is why it is a technique with a wide range of

applications in different fields concerning science and technology.

It allows the carrying out of qualitative and quantitative analysis by using the Energy

Dispersive X-ray Spectroscopy (EDS) technique. Results regarding the atomic mass

percentage and percentages from the different elements present in the sample are obtained.

Theoretical basis

In SEM, the image appears when the electrons beam goes down through the

column until the sample. A scanning generator is responsible for the beam movement. When

the electrons come into contact with the sample, some signals are emitted, which are then

picked up by their corresponding detectors (each signal has its own detectors). The detector

turns the received signal into an electronic one. Afterwards the latter is sent out to a Cathode

Ray Tube (CRT), which allows either the formation of the image or the chemical analysis.

Interaction Incident Beam – SEM sample

When the beam comes into contact with the sample, interactions between the

electrons and the sample atoms take place. From this interaction signals such as secondary

electrons, retrodispersed electrons, Auger electrons and X-rays are obtained, each of which is

captured by a different detector. For example, the secondary electrons detector detects signals

at low voltages (50eV) because those signals are emitted due to ionizations caused by

inelastic interactions.

44

Scanning Electron Microscope Signals

Main characteristics

The SEM’s main characteristics are its high resolution (~100 Å), its great field’s

depth which provides 3D images and the sample’s preparation simplicity. This equipment also

allows images obtention in a wide range of operation pressures (from vacuum to high pressure

conditions). Good images can also be obtained at low potentials, which sometimes can spare

sample pretreatments.

SEM’s Instrumentation

.

A Scanning Electron Microscope consists of different parts:

An optical-electronical unit, which generates the electrons beam.

A sample holder, with different movement degrees.

A signal detection unit with an attached amplification system.

An images display system

A cathode Ray Tube (CRT)

A vacuum system, a refrigeration system and an electrical power system, similar to

those of MET

A photographic, magnetic and video recording system

An image processing system (optional)

Different types of detectors depending on what to detect (secondary electrons, back-

scattered electrons, X-Rays, Auger electrons, transmitted Electrodes and light

cathodoluminescence)

45

Applications

The Scanning Electron Microscope allows the main following applications:

1. Observation at a high magnification rate

The image resolution is much more higher than that of its optical counterpart, since electrons (

with lower wavelength) rather than light are used.

2. Fractographic studies:

Due to its great field depth, fracture surfaces can be observed at high magnifications. .

3. Chemical analyses of small areas, such as intermetallic phases, precipitates, pollutant

particles …

4. Morphological and analytical surface characterization of a wide range of materials

5. Diffusion, segregation, quality analysis and irregularity studies processes.

Energy Dispersive X-ray Spectroscopy (EDS).

In EDS the microanalysis is carried out by measuring the energy, intensities and

signals distribution obtained from X-ray signals generated by the electrons beam. This allows

to find out the material’s chemical composition from areas down to 1mm2. Nevertheless, to

carry out the analysis, the signals intensities must be compared with that of a pattern, which

requires the sample’s surface to be flat and well polished.

In the EDS analyzer different analyses are carried out, which are later on plotted

in histograms and graphs that show the chemical elements distribution in the sample. The

EDS spectrum is obtained by using a software which collects photons per minute emitted by

the sample and classifies them according to energy. In the spectrum Energy in KeV (X axis)

versus intensity (Y axis) is plotted. Then the software automatically identifies and analyzes

the elements both quantitatively and qualitatively from the peaks of the histogram.

46

Energy Dispersive X-Ray spectrogram

Comparison between SEM and Optical microscope

The optical and the electronic microscope are almost identical. Both allow to

magnify details invisible to the naked eye. The main difference is the illumination source.

Therefore, the Optical Microscope uses a light beam whose wavelength is within the visible

range, whereas the SEM emits an electrons beam of very short wavelength which allows a

higher resolution to be obtained.

The main advantages that the electronic microscope has over its optical counterpart are:

• Higher field depth: The field depth is the capability of focusing on two different points at a

different height. This allows the analysis of fracture surfaces at high magnification, which

can’t be done using optical microscopy.

• Higher image resolution: The resolution boundary in an optical microscope is located

around a wavelength of 2000 Å because of the visible light nature.

47

In the electronic microscope the electrons possess a wavelength lower than 0.5 Å, which

makes it possible to reach magnifications up to 800.000x. However, due to instrumental

parameters, it only reaches up to 75.000x.

• Energy Dispersive X-ray Spectroscopy (EDS): Unlike the optical microscope, the

electronical microscope has also an EDS detector which allows to carry out chemical analyses

of the sample both qualitatively and quantitatively.

Scanning Electron Microscope Optical Microscope

48

10 – Experimental procedure

10.1 – Mixture preparation

In order to transform the hexagonal form of TaN and also to stabilize the cubic

form, solid mixtures with little amounts of similar compounds that already show a cubic form

lattice (known as solid solutions) must be carried out.

According to this, two different types of mixtures were prepared by mixing TaN

(hexagonal) with little amounts of TaC (cubic) or TiN (cubic).

1) TaN + TaC TaCxN1-x

2) TaN + TiN Ta1-x TixN

These mixtures were prepared by using different molar rates of the compounds (x = 0.05, x =

0.03). The necessary amounts that needed to be weighted are the following:

A) Mixture of TaCxN1-x with molar ratio x = 0.05:

If 30g of TaN are needed in the mixture, so as to have enough powder for preparing

different samples:

The moles in 30g TaN: TaNofmolmolg

gn 154.0

/95.194

30

If this amount entails 95% in moles of the sample (1-x), the remaining 5% is due to the

presence of TaC. This quantity is:

TaCofmolmol

n 310*1.895.0

05,0*154.0

49

The weight in 8.1 *10-3

mol TaC:

mol

gTaCPm 95.192)( TaCofg

mol

gmolm 564.195.192*10*1.8 3

B) Mixture of Ta1-x TixN with molar ratio x = 0.05

If 30g of TaN are needed in the mixture, so as to have enough powder for preparing

different samples:

The moles in 30g TaN: TaNofmolmolg

gn 154.0

/95.194

30

If this amount entails 95% of all moles in the sample (1-x), the remaining 5% is due to the

presence of TiN. This quantity is:

TiNofmolmol

n 310*1.895.0

05.0*154.0

The weight in 0.154 mol TiN:

mol

gTaCPm 867.61)( TiNofg

mol

gmolm 501.0867.61*10*1.8 3

50

10.2- Powder mixing

Once known which amount to take from each of the compounds, the mixtures

can be prepared. One example of prepared mixtures is that corresponding to Ta1-xTixN with x

= 0.05, prepared by weighing the following amounts:

m (TaN) = 30,0074 g

m (TiN) = 0,5018 g

Afterwards, the mixture was dispersed in 2-Propanol (addition of 75g) and

stirred up in a grinding mill during 30min. at a speed of 300 rpm. To help the mixing of the

compounds, balls made up of ZrO2 were added during the grinding process.

Once this is over, the suspension was dried in an oven at a temperature of around 80°C

overnight. The ZrO2 balls must also be cleaned up in the mill in water and dried before used

again.

Lab weighing scales Lab oven used to dry the solid solution

51

10.3 - Spark Plasma Sintering (SPS)

This process was run to turn the powder into a compact solid state.

1) Sample preparation

The sample has to be in a little container that is able to resist high pressures and

temperatures and does not allow the sample to leak. The following method fulfils these

objectives:

- A 39 mm carbon layer was assembled in the shape of a hollow cylinder and

introduced within the die, a graphite-based cylindrical container which had a

12mm hole in the shape of a cylinder. Afterwards, a tiny graphite cap was

put on the bottom of the cylinder. Moreover, 4g of the mixture were poured

within the die and another tiny graphite cap put at the top of the powder.

- After that, both parts of the hole are covered with graphite punches with

pressurize to make the powder more compact. Finally, an insulating surface

is attached to the outer part of the die with some threads. An image showing

the final assembly is displayed:

SPS Sample Holder

52

2) SPS sintering

The sample was introduced within the experiment chamber and fixed from the

top to the bottom with the help of some carbon plates. The experiment was run in vacuum

conditions. Different samples were run for each mixture, with different concentrations,

pressure and temperature conditions for each one. The starting temperature is set at 600ºC

because at lower temperatures it would be easier to control some of the SPS parameters. This

is because at lower temperatures the difference of the obtained signal’s wavelength from the

sample (due to the radiation emission at different temperatures) is lower than at higher

temperatures and thus more difficult to control by the SPS’s detector.

The following parameters were set on the SPS:

- Starting temperature: 600°C

- Heating rate: 100°C/ min.

- Final temperatures: 1550ºC, or 1600°C, or 1650ºC, or 1700°C, or 1750 °C or 1800°C

- The final temperatures were kept for 3 min, after which the samples were cooled down until

1500 °C and later on the equipment was turned off.

- Operation pressure: Either 50 or 75 MPa

When the final temperatures are reached, the sintering process is kept at these

temperatures for 3min., after which the sintering is complete and the temperature is slowly

decreased until 1500°C in order to reduce the building up of thermal tensions that leads to

brittleness in the sample. Afterwards the equipment can be safely disconnected and let cool

down. A graph plotting the change in the operating temperature for some final temperatures is

displayed:

53

SPS sintering conditions

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Time (min.)

Tem

pera

ture

(°C

)

Final T = 1600°C

Final T = 1700°C

Final T = 1800°C

Heating ratio = 100°C/min

Graph plotting the temperature sintering conditions

Once the experiment has been run, the different parameters have been plotted in

the computer software. The broad peaks that are drawn by the displacement parameter

correspond to the sample sintering. The sample must be held into the chamber until it cools

down to approximately 500°C.

3) Sample conditioning for Powder X-Ray Measurements

For X-ray characterization, only the material corresponding to the sample can be

present, thus being necessary to remove the graphite layer from it. Therefore the samples were

polished and sawed so as to use only a part of it and leave the rest intact for further tests and

measurements. Eventually, the fragment of the solid sample which was to be used for the

upcoming X-ray test was turned into powder for that purpose by smashing it with a

sledgehammer and stored safely afterwards. Once the powder is obtained, it must be grinded

down to the micrometric scale using a mortar and spread out over a thin silicon plate which

does not reflect the X-ray beams. Now the X-ray test can be carried out.

54

Laboratory polishing equipment

Laboratory Saw blade

Laboratory Sledgehammer and mortar

55

10.4 - Powder X-Ray analysis

Once the 2θ range of measurement was set from 20 to 120 and the test run for

30min. approximately. When the test is over, an X-ray diffractogram is displayed. After

selecting the most important peaks, these must be compared with the database to carry out a

so called ―phase analysis‖. This process will provide the phases present in the material and

further characterization.

All the samples were analyzed via X-Ray diffraction to study mainly the

presence of both cubic and hexagonal lattice in the sample. To do so the different peaks from

the diffractogram were classified as belonging to either the cubic or the hexagonal form by

comparing them when TaN cubic and hexagonal databases. Once the peaks were identified,

the percentages could be found accurately from the distribution of the peaks by carrying out

Rietveld Analysis, which takes into account the whole diffractogram. The results from the

Rietveld Analysis are presented in the tables of the next chapter.

10.5 – Theoretical density calculation

The theoretical density values have been looked up in different articles. Since different

density values were found for the same compound, it was decided to calculate an average

value of them all for the different compounds.

Hexagonal Tantalum Nitride (TaN)

Title 1 of its Authors Density (g/cm3) Average (g/cm

3)

Ta N, eine neue

Hochdruckform von

Tantalnitrid

Brauer, G.P 14.31

14.31 Die Nitride des

Tantals Zapp, K.H. 14.29

An X-ray study of

the tantalum-nitrogen

system

Schoenberg, N. 14.34

56

Cubic Tantalum Nitride (TaN)

Title 1 of its

Authors

Density

(g/cm3)

Average

(g/cm3)

Das kubische Tantalmononitrid(B1-

Typ) und seine Mischbarkeit mit den

isotypen Uebergangsmetallnitriden

und -carbiden

Gatterer, J. 15.93

15.79

The crystal structures of new

superconducting materials obtained

by high pressure treatment

Popova, S.V. 15.65

Cubic Tantalum Carbide (TaC)

Title 1 of its Authors Density (g/cm3) Average (g/cm

3)

Formation of cubic

solid solutions in the

Mo-Nb-C and

Mo-Ta-C systems

by the carbonization of

oxides in the plasma

arc

Matsumoto, O. 14.5

14.49

Hochschmelzende

Systeme mit

Hafniumkarbid

und -nitrid

Nowotny, H. 14.5

Thermal expansions of

some carbides and

tessellated stresses in

steel

Stuart, H. 14.5

The pseudo-binary

systems of uranium

carbide with zirconium

carbide, tantalum

carbide, and niobium

carbide

Brownlee, L.D. 14.5

57

Cubic Titanium Nitride (TiN)

Title 1 of its Authors Density (g/cm3) Average (g/cm

3)

The preparation of Na

Cl-type Til-x Alx N

solid solution

Inamura, S. 5.32

5.38

Non-stoichiometry of

titaniumnitride plates

prepared by chemical

vapour deposition

Takashi Goto 5,39

X ray diffraction study

of dynamic

characteristics of

crystal lattices of some

interstitial phases.

Samsonov, G.V. 5.39

Coefficients of thermal

expansion of titanium

carbonitrides

Alyamovskii, S.I 5.4

10.6 – Archimedes method

This method consists of weighing with a set of scales the weight of the sample

when it is both dry and wet. To determine the density, a relation between the weight of the dry

sample and that of the sample when it is immersed in water is used:

weightwetweightdry

weightdrycmgdcalc )/( 3

.

Once the experimental density is known, it can be compared with the theoretical value to

obtain a degree of compactness of the sample.

DensitylTheoretica

densityCalculatedsCompactnes

58

In order to measure the compactness, the proportions of each compound used to prepare the

mixtures must be taken into account when it comes to calculate the theoretical density of each

tablet.

Set of scales used for the Archimedes Method

To measure the theoretical densities, the density values from cubic and

hexagonal TaN must be used as well as the values of the dopant which is supposed to solve

within the TaN cubic lattice. Therefore, the hexagonal phase is supposed to be formed of pure

TaN whereas the cubic presents a 5% amount of the cubic dopant (either TaC or TiN). The

percentages corresponding to the hexagonal and the cubic lattices were obtained from X-Ray

diffraction by using Rietveld Refinement and exposed in the X-Ray diffraction tables.

Therefore, a formula to calculate the theoretical density for each of the samples is displayed

below:

..... %***)1(%* CubdopantTaNCubHexTaNHexTheor dxdxdd

59

11 - Results and discussion

In this chapter all the obtained results are shown as well as their conclusion

which could be drawn from them. These results are presented in the form of graphs, tables

and images for the different techniques applied to the samples (SPS sintering, X-Ray

diffraction and Scanning Electron microscopy).

11.1 SPS sintering results

In the following graphs both the sample displacement (shrinkage or expansion) and the

sintering temperature are plotted against time for different processes which go up to different

final temperatures as shown in the following plots taken as examples of such a process:

SPS sintering ABCR TaN at 75 MPa and 1800 °C

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20

Time (min.)

Tem

pera

ture

(°C

)

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Temperature

Displacement

Sample synthesized from the starting powder ABCR TaN at 75MPa and a final temperature of

1800 ° C

60

SPS sintering ALFA TaC0.05N0.95 at 75MPa and 1800°C

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20

Time (min.)

Tem

pera

ture

(°C

)

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Temperature

Displacement

Sample synthesized from the starting powder ALFA TaN mixed with TaC with a 5% molar

ratio, at 75MPa and a final temperature of 1800 ° C

SPS sintering ALFA TaC0.05N0.95 at 75 Mpa and 1500°C

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14

Time (min.)

Tem

pera

ture

(°C

)

-0.2

0

0.2

0.4

0.6

0.8

1

Temperature

Displacement

Sample synthesized from the starting powder ALFA TaN mixed with TaC with a 5% molar

ratio, at 75MPa and a final temperature of 1500 ° C

61

SPS sintering ALFA TaN at 75 Mpa and 1500°C

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14

Time (min.)

Tem

pera

ture

(°C

)

-0.4

-0.2

0

0.2

0.4

0.6

0.8

Temperature

Displacement

Sample synthesized from the starting powder ALFA TaN mixed at 75MPa and a final

temperature of 1500 ° C

SPS sintering ALFA TaC0.05N0.95 at 75 Mpa and 1550°C

0

200

400

600

800

1000

1200

1400

1600

1800

0 2 4 6 8 10 12 14 16

Time (min.)

Tem

pera

ture

(°C

)

-0.5

0

0.5

1

1.5

2

2.5

Temperature

Displacement

Sample synthesized from the starting powder ALFA TaN mixed with TaC with a 5% molar

ratio, at 75MPa and a final temperature of 1550 ° C

62

SPS sintering ABCR Ta0.95Ti0.05N at 75MPa and 1550°C

0

200

400

600

800

1000

1200

1400

1600

1800

0 2 4 6 8 10 12 14 16

Time (min.)

Tem

pera

ture

(°C

)

-0.5

0

0.5

1

1.5

2

Temperature

Displacement

Sample synthesized from the starting powder ABCR TaN mixed with TiN with a 5% molar

ratio, at 75MPa and a final temperature of 1550 ° C

11.2 SPS sintering discussion

The displacement plotted in the different graphs can give an overview of the

processes taking place to the sintering powder at each of the stages and therefore this

parameter will be taken into account in this discussion. In the very beginning of the sintering,

it can be seen that in some graphs there is a negative slope which is thought to be related to

the graphite expansion of the dyes used in the SPS equipment. This tendency is seen in all

graphs except for number 5.

On the other hand, the steep slope detected within the time gap from 6 minutes

to 10 minutes in most graphs is due to the shrinkage of the powder when it is sintered as a

compact solid tablet. There are also sudden ups and downs which actually make the slope

change wit no apparent reason. These are present all over the plotting and appear only

because the pressure was adjusted manually at some stages (both increased and decreased) so

as not to put a lot of strain in the equipment and carry out the sintering properly.

63

To complete the assessment, it can also be seen that the sintering is over at

temperatures close to 1550ºC, as can be seen from the graphs which reach up to this final

temperature. At this stage there is no change in the displacement and therefore a horizontal

tendency is observed in the displacement when the temperature is stabilized at 1550ºC. There

can also be seen some steep decreases at the final stage in the displacement due to the fact that

the equipment was turned off at this very moment making the displacement value drop

radically.

11.3 - Powder X-Ray diffraction results

1) Starting powders

The different starting TaN powders from both companies (ABCR and ALFA)

and the used dopants (TaC and TiN) were analysed with X-Ray diffraction and from the

corresponding diffractograms the cubic lattice percentage of the TaN powders could be found

out by Rietveld Refinement, since it is logical to know which is the cubic lattice presence in

the raw material when the main objective is to assess its synthesis. The diffractograms and

corresponding results are the following.

Sample Cubic

TaN (%)

Hexagonal

TaN (%) Ta2N

Starting TaN powder (ABCR) 27.3 72.7 0

Starting TaN powder (ALFA_1) 100 0 0

Starting TaN powder (ALFA_2) 10.4 69.8 19.8

Percentages of the different phases present in the TaN starting powders from ABCR and

ALFA companies

64

X-Ray diffractograms carried out to the powders of the pure Tantalum Nitrides and dopants

used to create the samples

Two different powder bottles of 50g each were bought from the ALFA

company. It can be seen from the percentage results that the first powder bought from that

company (labelled as ALFA 1) showed better properties since it presented no cubic phase

whatsoever and therefore the cubic phase could be sintered completely from the hexagonal

TaN phase. However, the second bottle (called ALFA 2) presented some cubic phase and

what is more it presented a vast amount of a different phase (Ta2N) whose behaviour in the

sintering was totally unknown. Therefore it was decided that using this powder to synthesize

samples should be avoided, and the samples were synthesized using both ABCR and ALFA

powder (the first bottle). However, not as many samples as wanted could be synthesized with

this ALFA powder because there were only 50 grams and as seen with the second bottle it

was difficult to find another powder with the same phases from the same company ( ALFA 1

and ALFA 2 presented different phases even though they were acquired from the same

manufacturer).

65

2) SPS sintered samples

The following are some tables with the cubic and hexagonal percentages of the TaN phases

detected by X-Ray diffraction and assessed using the Rietveld method. From these tables,

different graphs were also plotted to compare different variables that influence the cubic

transformation of the hexagonal TaN which allow some conclusions to be drawn. Eventually,

it was also considered relevant to plot diffractograms for each of the sintered series at

different temperatures and for the starting materials as follows.

ABCR POWDER (TixTa1-xN)

Sample Cubic % Hexagonal % Ti

TixTa1-xN

(x =0.05, 1600°C, 50MPa)

(ABCR TaN)

37.3 59.5 3.2

TixTa1-xN

(x =0.05, 1700°C, 50MPa)

(ABCR TaN)

93.9 6.1 0

TixTa1-xN

(x =0.05, 1800°C, 50MPa)

(ABCR TaN)

97.1 2.9 0

TixTa1-xN

(x =0.05, 1550°C, 75MPa)

(ABCR TaN)

26.6 73.4 0

TixTa1-xN

(x =0.05, 1600°C, 75MPa)

(ABCR TaN)

59.5 37.3 3.2

TixTa1-xN

(x =0.05, 1650°C, 75MPa)

(ABCR TaN)

86.6 12.3 1.1

66

TixTa1-xN

(x =0.05, 1700°C, 75MPa)

(ABCR TaN)

97.5 2.5 0

TixTa1-xN

(x =0.05, 1800°C, 75MPa)

(ABCR TaN)

98.2 1.8 0

TixTa1-xN

(x =0.03, 1600°C, 75MPa)

(ABCR TaN)

29.4 70.6 0

TixTa1-xN

(x =0.03, 1700°C, 75MPa)

(ABCR TaN)

96.2 3.8 0

TixTa1-xN

(x =0.03, 1800°C, 75MPa)

(ABCR TaN)

100 0 0

ABCR POWDER (TaCxN1-x)

Sample Cubic % Hexagonal % TaC %

TaCxN1-x

(x =0.05, 1600°C, 50MPa)

(ABCR TaN)

57.2 40.8 2

TaCxN1-x

(x =0.05, 1700°C, 50MPa)

(ABCR TaN)

95.9 2.2 1.9

TaCxN1-x

(x =0.05, 1800°C, 50MPa)

(ABCR TaN)

97.1 0 2.8

TaCxN1-x

(x =0.05, 1550°C, 75MPa)

(ABCR TaN)

57.6 42.4 0

67

TaCxN1-x

(x =0.05, 1600°C, 75MPa)

(ABCR TaN)

44.2 55.2 0.6

TaCxN1-x

(x =0.05, 1650°C, 75MPa)

(ABCR TaN)

51.9 48.1 0

TaCxN1-x

(x =0.05, 1700°C, 75MPa)

(ABCR TaN)

98 2 0

TaCxN1-x

(x =0.05, 1800°C, 75MPa)

(ABCR TaN)

96.3 0.1 3.6

PURE ABCR POWDER

Sample Cubic % Hexagonal %

TaN (ABCR) starting powder 27.3 72.7

Pure TaN (ABCR)

(1600°C, 75Mpa) 32.2 67.8

Pure TaN (ABCR)

(1700°C, 75Mpa) 97.1 2.9

Pure TaN (ABCR)

(1800°C, 75Mpa) 100 0

68

ALFA 1 POWDER (TaCxN1-x)

Sample Cubic % Hexagonal % TaC % Ta

TaCxN1-x

(x =0.05, 1500°C, 75MPa)

(ALFA TaN)

23.2 76.8 0 0

TaCxN1-x

(x =0.05, 1550°C, 75MPa)

(ALFA TaN)

30 69.1 0 0.9

TaCxN1-x

(x =0.05, 1600°C, 75MPa)

(ALFA TaN)

60.6 38.5 0 0.9

TaCxN1-x

(x =0.05, 1700°C, 75MPa)

(ALFA TaN)

96.6 3.4 0 0

TaCxN1-x

(x =0.05, 1800°C, 75MPa)

(ALFA TaN)

100 0 0 0

PURE ALFA 1 POWDER

Sample Cubic % Hexagonal %

Pure TaN (Alfa) 0 100

TaN Alfa

(1500°C, 75Mpa) 8.1 91.9

TaN Alfa

(1600°C, 75Mpa) 12.1 87.9

TaN Alfa

(1700°C, 75Mpa) 93.4 6.6

TaN Alfa

(1750°C, 75Mpa) 98 2

69

3) Plotted graphics with observations

GRAPH 1

Cubic percentages of Ta0.95Ti0.05N at different

pressures (with ABCR TaN)

0

10

20

30

40

50

60

70

80

90

100

1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase (

%)

TaTiN at 75MPa

TaTiN at 50 MPa

Observations 1:

There is a trend which shows that the higher the temperature the higher the

conversion into the cubic phase. Moreover, this effect seems to be more significant at lower

temperatures due to the fact that at higher temperatures both the sample temperature and

exposition time have been higher. That is why from 1700 ºC forward there seem to be no

significant effects on the cubic percentage.

There also seems that a higher pressure helps considerably to boost the cubic conversion,

especially at lower temperatures for the reasons mentioned before.

70

GRAPH 2

Cubic percentages of TaC0.05N0.95 at different

pressures (with ABCR TaN)

0

10

20

30

40

50

60

70

80

90

100

1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase (

%)

TaCN at 75MPa

TaCN at 50 MPa

TaCN(1550ºC,75MPa)

Observations 2:

A higher synthesis temperature always entails a higher cubic percentage in the

sample. It must be pointed out that the samples taken at 75MPa at 1600ºC and 1650ºC

respectively belonged to a different mixture than the remaining three from the same series at

1550ºC, 1700ºC and 1800ºC.

On the one hand maybe the value of 1550 ºC at 75 MPa should be discarded

because it seems to be wrong. On the other hand, it seems like the input pressure is not an

important asset over 1700 ºC and comparison at 1600 ºC of both pressures just does not add

up since it should be the other way round (75MPa higher than 50MPa). However, the trend

was perfect with the TiTaN series (graphic 1), and not much more information can be

extracted from this graph.

71

GRAPH 3

Cubic percentages of TaxTi1-xN at 75MPa with

different dopant concentrations x

(with ABCR TaN)

0

10

20

30

40

50

60

70

80

90

100

1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

as

e (

%)

x = 0.05

x = 0.03

Observations 3)

A higher synthesis temperature always entails a higher cubic percentage in the

sample. Nevertheless, no considerable changes occur at temperatures over 1700 ºC.

The dopant (TiN with a cubic arrangement) seems to help the TaN adopt a

cubic arrangement, as can be seen in the graph, especially at the low temperature of 1600C,

where the fact that both the temperature and the reaction time have been considerably low

allow the dopant concentration to show its effect on the cubic transformation. Therefore, there

seems to be a trend which exposes that the higher the dopant concentration the higher the

cubic percentage in the sample.

72

GRAPH 4

Comparison of the Cubic percentages of

TaC0.05N0.95 and Ta0.95Ti0.05N at 50MPa

(with ABCR TaN)

0

10

20

30

40

50

60

70

80

90

100

1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase (

%)

TaCN at 50MPa

TaTiN at 50 MPa

Observations 4:

A higher synthesis temperature always entails a higher cubic percentage in the

sample. Nevertheless, no considerable changes occur at temperatures over 1700 ºC.

It can be assessed from the graph that the type of dopant (TaC or TiN) is not

significant when assessing the cubic percentage. The reason for that seems to be both present

a cubic arrangement, and, in the same concentrations on the sample have the same effect on

the final cubic percentage.

73

GRAPH 5

Comparison of the Cubic percentages of TaC0.05N0.95

,Ta0.95Ti0.05N and pure TaN powder at 75MPa (with

ABCR TaN)

0

10

20

30

40

50

60

70

80

90

100

1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase (

%)

TaCN at 75 MPa

TaTiN at 75 MPa

Pure TaN at 75MPa

TaCN(1550ºC,75MPa)

Observations 5:

A higher synthesis temperature always entails a higher cubic percentage in the

sample, apart from the value at 1500ºC which could be wrong. Nevertheless, no considerable

changes occur at temperatures over 1700 ºC.

It can be seen that the values of the cubic percentages for TaN to which dopant

has been added are higher than those of the pure TaN, which shows the dopant has actually an

effect on the conversion especially at low temperatures. Moreover, for all samples but that at

1550ºC, whose value maybe is wrong, it seems that the cubic percentage is higher for those

samples doped with TiN instead of TaC, but no conclusions could be drawn up from this fact

since as mentioned before two different mixtures were used at different times to prepare the

TaCN series and maybe those could just be experimental errors.

74

GRAPH 6

Comparison of the Cubic percentages of the two

TaN powders (ABCR and ALFA) at 75MPa and their

pure forms with no SPS sintering.

0

10

20

30

40

50

60

70

80

90

100

1450 1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase (

%)

ABCR TaN powder

ALFA TaN powder

ALFA TaN with no

SPS sintering

ABCR TaN with no

SPS sintering

Observations 6:

On the one hand, it was determined by X-Ray diffraction that the starting TaN

powder from the ABCR company had considerable cubic content (around 27%) whereas the

TaN from the ALFA company did not present any cubic phase whatsoever. Therefore, it is

only logical that at low temperatures (where the effect of temperature and reaction time is not

that significant) the cubic percentage is higher for the ABCR powder, as can be seen for both

powders at 1600C. At higher temperatures the values of the percentages become stable at high

values and this former presence of a cubic phase in the ABCR powder does not seem so

significant. It must also be remarked that without the help of any dopant both powders

reached very high values in cubic percentage, even the ALFA powder with no cubic phase in

the very beginning. Therefore it seems logical to say that temperature and overall reaction

time seem to be the most significant assets for the synthesis of the cubic form of TaN.

75

GRAPH 7

Comparison of the Cubic percentages of TaC0.05N0.95 at

75MPa with ABCR and ALFA TaN

0

10

20

30

40

50

60

70

80

90

100

1450 1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase

(%

)

ABCR TaCN

ALFA TaCN

TaCN (1550ºC ,75MPa)

Observations 7:

Both series show again very similar values at temperatures over 1700C showing

temperature is a considerable asset. Unexpectedly, it seems that the cubic percentage seems to

be higher for the ALFA series than for its ABCR counterpart ( see samples at 1600C), which

just does not add up with the premonitions since ABCR had a higher cubic content in the

beginning and that tendency should be charted throughout the synthesis, especially at low

temperatures. However, it is unclear and filling up with more samples would be necessary to

make such a statement.

76

GRAPH 8

Comparison of the Cubic percentages of TaC0.05N0.95

and pure TaN powder at 75MPa (with ALFA TaN)

0

10

20

30

40

50

60

70

80

90

100

1450 1500 1550 1600 1650 1700 1750 1800

Temperature (°C)

Cu

bic

ph

ase (

%)

ALFA TaCN

ALFA TaN

Observations 8:

In this graph the effect of the dopant in the sintering can be seen, being more

significant at low temperatures. In fact, at the lowest temperature in the graph (1500C) the

doped TaN has already transformed more of its hexagonal lattice into cubic than the pure

TaN. Moreover, it can be seen that when temperature is increased (up to 1600C) the

increasement of the cubic percentage is radical for the doped TaN whereas its pure form

remains still stable. The data plotting seems relevant and therefore it must be emphasized that

the dopant is a considerable asset for the transformation of the hexagonal lattice into cubic

within a considerably low range of temperatures. This effect is especially considerable taking

into account that the starting TaN powder (Alfa) had no present cubic lattice whatsoever.

77

X-Ray diffractograms from the series TaC0,05N0,95 synthesized at 50MPa with ABCR TaN

X-Ray diffractograms from the series TaC0,05N0,95 synthesized at 75MPa with ABCR TaN

78

X-Ray diffractograms from the series Ta0,95Ti0,05N synthesized at 50MPa with ABCR TaN

X-Ray diffractograms from the series Ta0.95Ti0.05N synthesized at 75 MPa with ABCR TaN

79

X-Ray diffractograms from the series Ta0.97Ti0.03N synthesized at 75 MPa with ABCR TaN

X-Ray diffractograms from the series of pure ABCR TaN synthesized at 75MPa

80

X-Ray diffractograms from the series TaC0.05N0.95 synthesized at 75 MPa with ALFA TaN

X-Ray diffractograms from the series of pure ALFA TaN synthesized at 75 MPa

81

11.4 - X-Ray results discussion

1) The reaction temperature is the main asset, since all the cubic percentages go up

radically at temperures over 1700°C and higher, whereas there is usually a

considerable mixture of both phases (cubic and hexagonal) when the reaction takes

place at temperatures close to 1600°C. Another reason is that at higher temperatures

the overall exposure time of the sample to high pressures and temperatures is

increased, thus boosting the transformation.

2) It was also noticed there was a correlation between the pressure and the cubic

percentage (the higher the pressure, the higher the percentage). However this

parameter only had an important effect on the transformation at low temperatures

around 1600 °C. and therefore should be considered less important than parameters

such as temperature and exposure time.

3) The added dopants (TaC, TiN) were supposed to boost the synthesis into the cubic

form of TaN in all the range of temperatures. However, like pressure, they had a low

effect on the cubic percentages at temperatures over 1700°C and higher, and only were

significant at lower temperatures around 1600°C. There were questions as to whether

the reason for this was that the starting powder presented a significant cubic

percentage before the synthesis which would allow the cubic phase to grow without

the need of a dopant with a cubic lattice arrangement such as TaC or TiN. However,

this was clearly not the case since those samples synthesized with the starting ALFA

powder (which presented no cubic phase at all) behaved likewise.

4) This SPS sintering method seems reliable since all the samples had a considerably

high cubic percentage as shown in the X-Ray diffractrograms after the synthesis. It

should also be remarked that special consideration must be taken at the final

temperature reached in the sintering because it seems to be the main asset in the

transformation along with exposure time.

82

11.5 - Density measurements results

These measurements were carried out using the Archimedes method. The dry and wet weight

were obtained by weighing the sample in an air environment and drawn in water respectively..

The different types of densities along with the compactness were obtained by application of

the formulas exposed in the experimental chapter (Archimedes method).

ABCR POWDER (TixTa1-xN)

Sample name

Dry

weight

( g )

Wet

weight

( g )

Calculated

Density

( g/cm3 )

Cubic

TaN

( % )

Hexagonal

TaN

( % )

Theoretical

Density

( g/cm3 )

Compactness

( % )

Ta0.95Ti0.05N

(50MPa,1600°C)

(ABCR TaN)

1.3827 1.2731 12.61 39.7 60.3 14.90 84.6

Ta0.95Ti0.05N

(50MPa,1700°C)

(ABCR TaN)

2.7072 2.4977 12.92 93.9 6.1 15.21 84.9

Ta0.95Ti0.05N

(50MPa,1800°C)

(ABCR TaN)

0.9060 0.8368 13.09 97.1 2.9 15.24 85.9

Ta0.95Ti0.05N

(75MPa,1550°C)

(ABCR TaN)

2.2220 2.0493 12.87 26.6 73.4 14.70 87.5

Ta0.95Ti0.05N

(75MPa,1600°C)

(ABCR TaN)

1.5311 1.4127 12.93 61.4 38.6 14.90 86.8

Ta0.95Ti0.05N

(75MPa,1650°C)

(ABCR TaN)

1.8847 1.7393 12.96 87.6 12.4 15.61 83.1

Ta0.95Ti0.05N

(75MPa,1700°C)

(ABCR TaN)

1.1185 1.0325 13.01 97.5 2.5 15.25 85.3

Ta0.95Ti0.05N

(75MPa,1800°C

(ABCR TaN))

1.4425 1.3304 12.87 98.2 1.8 15.25 84.4

Ta0.97Ti0.03N

(75MPa,1600°C)

(ABCR TaN)

1.3356 1.2339 13.13 29.4 70.6 14.65 89.6

Ta0.97Ti0.03N

(75MPa,1700°C)

(ABCR TaN)

1.3632 1.2595 13.15 96.2 3.8 15.43 85.2

Ta0.97Ti0.03N

(75MPa,1800°C)

(ABCR TaN)

2.1380 1.9742 13.05 100 0 15.48 84.3

83

ABCR POWDER (TaCxN1-x)

Sample name

Dry

weight

( g )

Wet

weight

( g )

Calculated

Density

( g/cm3 )

Cubic

TaN

( % )

Hexagonal

TaN

( % )

Theoretical

Density

( g/cm3 )

Compactness

( % )

TaC0.05 N0.95

(50MPa,1600°C)

(ABCR TaN)

2.5442 2.3524 13.26 57.2 40.8 14.83 89.4

TaC0.05 N0.95

(50MPa,1700°C)

(ABCR TaN)

2.5955 2.4001 13.28 95.9 2.2 15.40 86.3

TaC0.05 N0.95

(50MPa,1800°C)

(ABCR TaN)

2.3946 2.2150 13.33 97.1 0 15.27 87.3

TaC0.05 N0.95

(75MPa,1550°C)

(ABCR TaN)

2.0093 1.8588 13.35 57.6 42.4 14.86 89.8

TaC0.05 N0.95

(75MPa,1600°C)

(ABCR TaN)

1.9590 1.812 13.33 44.2 55.2 14.85 89.8

TaC0.05 N0.95

(75MPa,1650°C)

(ABCR TaN)

1.6966 1.5698 13.38 51.9 48.1 15.08 88.7

TaC0.05 N0.95

(75MPa,1700°C)

(ABCR TaN)

1.624 1.5024 13.36 98 2 15.70 85.1

TaC0.05 N0.95

(75MPa,1800°C)

(ABCR TaN)

1.5781 1.4598 13.33 96.3 0.1 15.16 87.3

PURE ABCR POWDER

Sample name

Dry

weight

( g )

Wet

weight

( g )

Calculated

Density

( g/cm3 )

Cubic

TaN

( % )

Hexagonal

TaN

( % )

Theoretical

Density

( g/cm3 )

Compactness

( % )

Ta N

(75MPa,1600°C)

(ABCR TaN)

1.7658 1.6392 13.95 34.5 59.4 13.95 100.0

TaN

(75MPa,1700°C)

(ABCR TaN)

1.7870 1.6528 13.32 90.4 2.9 14.69 90.7

TaN

(75MPa,1800°C)

(ABCR TaN)

0.9889 0.9143 13.26 100 0 15.79 84.0

84

ALFA 1 POWDER (TaCxN1-x)

PURE ALFA 1 POWDER

Sample name

Dry

weight

( g )

Wet

weight

( g )

Calculated

Density

( g/cm3 )

Cubic

TaN

( % )

Hexagonal

TaN

( % )

Theoretical

Density

( g/cm3 )

Compactness

( % )

TaN

(75MPa,1500°C)

(ALFA 1 TaN)

1.6853 1.5553 12.96 8.1 91.9 14.39 90.1

TaN

(75MPa,1600°C)

(ALFA 1 TaN)

2.3062 2.1410 13.96 12.1 87.9 14.43 96.8

TaN

(75MPa,1700°C)

(ALFA 1 TaN)

1.5992 1.4862 14.15 93.4 6.6 15.21 93.1

TaN

(75MPa,1750°C)

(ALFA 1 TaN)

1.0132 0.9413 14.09 98 2 15.25 92.4

Sample name

Dry

weight

( g )

Wet

weight

( g )

Calculated

Density

( g/cm3 )

Cubic

TaN

( % )

Hexagonal

TaN

( % )

Theoretical

Density

( g/cm3 )

Compactness

( % )

TaC0.05 N0.95

(75MPa,1500°C)

(ALFA 1 TaN)

1.4533 1.3489 13.92 23.2 76.8 14.53 95.8

TaC0.05 N0.95

(75MPa,1550°C)

(ALFA 1 TaN)

1.8047 1.6752 13.93 30.2 69.8 14.60 95.4

TaC0.05 N0.95

(75MPa,1600°C)

(ALFA 1 TaN)

1.8450 1.7147 14.15 61 38.7 14.85 95.3

TaC0.05 N0.95

(75MPa,1700°C)

(ALFA 1 TaN)

1.3512 1.2552 14.08 96.6 3.4 15.24 92.4

TaC0.05 N0.95

(75MPa,1800°C)

(ALFA 1 TaN)

2.0982 1.9495 14.11 100 0 15.27 92.4

85

11.6 - Density measurements discussion

These density measurements were mainly done to assess whether the

compactness of the samples was high enough to allow the carrying out of some mechanical

tests to the samples such as hardness measurements. In fact, this test needs a sample

compactness of 95% or higher, otherwise the cracks will stop at the pores thus providing

wrong values for fracture toughness.

On the other hand, unexpectedly the compactness is significantly low for most of the samples.

The SPS sintering is theoretically supposed to provide with samples of at least 95%

compactness but as assessed most of the samples could barely reach high enough

compactness. This fact has no relevance on neither the phase analysis of the samples by X-

Ray diffraction nor the visualization of the grain sizes from the images obtained by SEM.

However, special considerations must be taken into account when selecting the samples if

hardness measurements are to be carried out, since most of the samples are not eligible

because they do not meet the necessary conditions in terms of compactness.

86

11.7 - Scanning Electron Microscopy (SEM) images

Different samples were visualized by SEM. The TaN powders from the

chemical companies ABCR and ALFA were observed along with different samples

created from these powders trying to cover the maximum possible range of different cubic

percentages. The sintered samples were previously fractured, being this fracture surface

the one visualized with the microscope.

Therefore the following samples ordered by cubic content percentage were tested:

Sample Cubic

%

Starting powder TaN (ABCR) 27,3

Starting powder TaN (ALFA) 100

TaN

(x =0.05, 1500°C, 75MPa)

(ALFA TaN)

8.1

TixTa1-xN

(x =0.03, 1600°C, 75MPa)

(ABCR TaN)

29.4

TaCxN1-x

(x =0.05, 1600°C, 75MPa)

(ABCR TaN)

44.2

TaCxN1-x

(x =0.05, 1600°C, 75MPa)

(ALFA TaN)

60.6

TixTa1-xN

(x =0.05, 1800°C, 75MPa)

(ABCR TaN)

61.4

TaCxN1-x

(x =0.05, 1800°C, 75MPa)

(ALFA TaN)

100

87

The following images were obtained from the SEM:

ABCR TaN POWDER

SEM Figure 1. ABCR TaN Starting powder 500X Backscattered electrons Image

SEM Figure 2. ABCR TaN 3000X (1) Secondary Electrons Image

88

SEM Figure 3. ABCR TaN 3000X BackScattered Electrons Image

SEM Figure 4. ABCR TaN 30000X Backscattered Electrons Image

89

ALFA TaN Powder

SEM Figure 5. Alfa TaN 500X Backscattered Electrons Image

SEM Figure 6. Alfa TaN 3000X Backscattered Electrons Image

90

SEM Figure 7. Alfa TaN 30000X Backscattered Electrons Image

ABCR TaC0.05 N0.95 , 1600C,75MPa

SEM Figure 8. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 500X, BackScattered

Electrons Image

91

SEM Figure 9. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 3000X, Secondary Electrons

Image

SEM Figure 10. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 3000X, BackScattered

Electrons Image

92

SEM Figure 11. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 30000X, Secondary

Electrons Image

SEM Figure 12. ABCR TaC0.05 N0.95 (1600 °C, 75MPa) 30000X, BackScattered

Electrons Image

93

SEM Figure 13. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 500X, Backscattered

Electrons Image

SEM Figure 14. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 3000X, Secondary

Electrons Image

94

SEM Figure 15. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 3000X, Backscattered

Electrons Image

SEM Figure 16. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 30000X, Secondary

Electrons Image

95

SEM Figure 17. ABCR Ti 0.03Ta0.97N (1600°C, 75Mpa) 30000X, Backscattered

Electrons Image

ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa)

SEM Figure 18. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 500X, Secondary Electrons

Image

96

SEM Figure 19. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 500X, Backscattered

Electrons Image

SEM Figure 20. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 3000X, Backscattered

Electrons Image

97

SEM Figure 21. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 3000X, Backscattered

Electrons Image

SEM Figure 22. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 10000X, Secondary Electrons

Image

98

SEM Figure 23. ABCR Ti 0.05Ta0.95N (1800°C, 75Mpa), 10000X, Backscattered

Electrons Image

ALFA TaN (1500°C, 75Mpa)

SEM Figure 24. ALFA TaN (1500°C, 75Mpa), 150X, Backscattered Electrons Image

99

SEM Figure 25. ALFA TaN (1500°C, 75Mpa), 500X, Backscattered Electrons Image

SEM Figure 26. ALFA TaN (1500°C, 75Mpa), 3000X, Secondary Electrons Image

100

SEM Figure 27. ALFA TaN (1500°C, 75Mpa), 3000X, Secondary Electrons Image

ALFA TaC0.05 N0.95 (1600°C, 75Mpa)

SEM Figure 28. ALFA TaC0.05 N0.95 (1600°C, 75Mpa), 150X, Backscattered Electrons

Image

101

SEM Figure 29. ALFA TaC0.05 N0.95 (1600°C, 75Mpa), 500X, Backscattered Electrons

Image

SEM Figure 30. ALFA TaC0.05 N0.95 (1600°C, 75Mpa), 3000X, Secondary Electrons

Image

102

ALFA TaC0.05 N0.95 (1800°C, 75Mpa)

SEM Figure 31. ALFA TaC0.05 N0.95 (1800°C, 75Mpa), 500X, Secondary Electrons

Image

SEM Figure 32. ALFA TaC0.05 N0.95 (1800°C, 75Mpa), 1000X, Secondary Electrons

Image

103

SEM Figure 33. ALFA TaC0.05 N0.95 (1800°C, 75Mpa), 3000X, Secondary

Electrons Image

11.8 - SEM Discussion

Images for both the starting powders and different sintered samples have been

taken at different magnifications in order to assess features such as the grain size, the fracture

surface and their composition.

It must be remarked that the grain size of both starting powders (ALFA and

ABCR) seems to be around 1 micrometer, although it can easily gather up to create bigger

conglomerates as can be seen from the pictures. Taking a closer look at the images of the

synthesized samples, it can be seen that their grain size has grown when compared to the

starting powders. Therefore, it seems that both factors pressure and temperature enhance the

growth of the grains. However, when comparing the different samples it seems these

parameters are not directly proportional to the grain growth. In other words, it seems that the

lattice content (whether hexagonal or cubic) is the major asset to determine the grain size.

104

As it can be seen from the images, it is in those samples that present mostly one

phase (either cubic or hexagonal) that the grain size is bigger, being at the same time bigger

when the phase is mostly hexagonal rather than cubic. In fact, the grain size seems to be

bigger mainly for the samples created with ALFA TaN at 1500 °C and 1800 °C (with 8.1 %

and 100% cubic content respectively). When comparing them to one another, the grain size of

the former one seems to be around 8 micrometers whereas it is around 5 micrometers for the

latter. The main reason for such a change seems to be that the hexagonal lattice occupies a

higher volume than the cubic. On the other hand, the reason why those samples with almost

only one phase possess bigger grain size seems to be that when there is a phase mixture one

phase hinders the other from growing. In our case, what happens is that as the reaction goes

by, the hexagonal lattice turns into cubic and when both phases are in considerable

proportions the hexagonal phase hinders somehow the cubic from growing. As time goes by

and the cubic lattice develops, the hexagonal one becomes less significant and no other phase

hinders the cubic from growing.

When assessing the fracture surface, it must be said that both types of fractures

took place in the samples: fracture through the grains and fractures through the boundaries.

An example in which both mechanisms can be seen is the SEM Figure 27. Another interesting

asset that can be seen from the fracture surface is the presence of porosity in the sample. The

porosity distribution observed in the images seems to fit the results obtained by the density

measurements. A good example is the sample synthesized at 1500°C with ALFA TaN powder

whose compactness was assessed to be close to 100%. In fact, there seems to be very low

porosity for this sample (see SEM Figure 24). However, there does not seem to be a relation

between the sintering conditions and the porosity of the samples.

On the other hand, those images obtained by backscattered electrons give an

overview of the sample’s composition with tonalities of grey and black colours. As expected,

those images present mainly bright grains due to the fact that Ta is a heavy element and is

displayed quite brightly. The EDS analysis confirmed these grains consist of TaN. It can also

be expected there are traces of unreacted dopant in the samples (TaC or TiN). This seems to

be the case at least for the samples with TiN as a dopant, since such samples present a great

deal of dark-coloured spots, which could easily be unreacted TiN (see SEM Figures 21 and

23). Therefore, it seems the samples consist of TaN with some unreacted dopant impurities.

105

12 - Conclusions

Different mixtures to which different amounts of dopants (TaC and TiN) were

added have been used to synthesize samples at different pressures and temperatures to

cover a broad range of sintering conditions, thus being able to predict the effect of the

variables on the cubic transformation.

After sample conditioning for powder X-Ray diffraction it seems that

temperature and reaction time were the main assets which boosted the cubic

transformation. In fact, the sample’s cubic lattice content was 90% or higher for all

samples from 1700°C forward, whereas the pressure and the amount of added cubic

dopant only had an effect at lower temperatures, being quite significant at temperatures

close to 1600°C and lower. Moreover, both dopants seemed to play a very similar role.

The SEM visualization also confirmed the grain growth boosted by the SPS

sintering which turned a nanometric starting powder into a tablet of bigger grain size, thus

creating a compact. It was also confirmed by EDS that most of the tablet content was

TaCN or TiTaN as expected, and some of them also presented traces of unreacted dopant,

which could also be noticed by SEM in the case of TiN.

However, one of the most considerable encountered difficulties was faced when

trying to control the porosity of the sintered samples. Actually, there seemed to be no

connection between the applied sintering conditions and the compact samples, and what’s

more, in some cases the compactness turned out to be lower than expected theoretically by

SPS sintering (samples should possess a compactness of 95% or higher).

Although this asset entails no problems for the assessment of the cubic

transformation and the phase analysis, it could be problematic when carrying out

foreseeable mechanical tests to the samples. For instance, if fracture toughness tests were

to be realized to samples with porosity, the cracks would stop at the pores and wrong

values for hardness would be provided.

106

Therefore hardness tests could only be carried out to few of these samples, those

which possess very low porosity as assessed by density measurements. Actually, such

tests are scheduled to be carried out in the foreseeable future.

On the other hand, while cutting the samples to tear them asunder (one part was

saved for SEM imaging and further hardness measurements whereas the other was

pulverized to carry out X-Ray diffraction) it could be noticed that those samples

containing mainly one type of phase (either cubic or hexagonal) seemed much easier to

cut than those in which there was a mixture of both phases. Therefore, it seems that the

presence of two phases truly reinforces the material’s fracture toughness although no

experimental values to back up such a statement were recorded.

Summing up, the SPS sintering technique turns out to be a reliable method since

the synthesis parameters can be easily controlled and the sample’s results were quite

promising in final cubic content. In fact, the hexagonal lattice was easily turned into cubic

with quite ease of operation, making the application of such a process quite feasible to

carry out such a transformation.

107

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Webpages:

www.substech.com (Boron Nitride)

www.azom.com (Silicon Nitride), (Tungsten Carbide)

www.accuratus.com ( Silicon Nitride), (Boron Nitride)

www.brycoat.com (Titanium Nitride)

www.reade.com (Tantalum Nitride)

www.americanelements.com (Carbides)

www.britannica.com (Carbides)

www.transtutors.com (Carbides)

www.ultramet.com (Carbides)

www.streetdirectory.com (Carbides)

www.tungstenchina.com ( Tungsten Carbide)

www.hydrocarbide.com (Tungsten Carbide)

www.panalytical.com (X-Ray diffraction)

http://www.mrl.ucsb.edu/ (X-Ray diffraction)

http://www.icdd.com/ (X-Ray Diffraction)

http://imr.chem.binghamton.edu/ (X-Ray diffraction)

http://serc.carleton.edu/ (X-Ray diffraction)

http://home.planet.nl/~rietv025/ (Rietveld Method)

http://epswww.unm.edu/xrd/xrdclass/09-Quant-intro.pdf (Rietveld Method)

http://www.mos.org (Scanning Electron Microscopy)

http://www.unl.edu (Scanning Electron Microscopy)

http://www.purdue.edu (Scanning Electron Microscopy)

http://mee-inc.com (Scanning Electron Microscopy)

http://www.sdm.buffalo.edu (Scanning Electron Microscopy)

www.askmehelpdesk.com (Scanning Electron Microscopy)

www.answers.com (Grinding Mill)

www.britannica.com (Archimedes Principle)

www.physicsprinciples.tripod.com (Archimedes Principle)