Lesson 6 2014. Lesson 6 2014 Our goal is, that after this lesson, students are able to recognize the...

Lesson 62014

Lesson 62014

Our goal is, that after this lesson, students are able to recognize the key criteria for selecting ceramics and are able to use this knowledge to support the systematic material selection process.

What are ceramics for engineering applications?

?

Typical ceramics used in engineering applicationsAluminium oxide (Alumina) Al2O3

Aluminium nitride AlNSilicon carbide SiCSilicon nitride Si3N4

SiAlON (Si-Al-O-N)Zirconium oxide (Zirconia) ZrO2

Boron carbide B4CBoron nitride BN

Some general properties of ceramicsDensity

In general the density of ceramics is between metals and polymers.

Most light weight ceramics are Boron compounds and Silicon compounds.

For ceramics two types of density values are used: Density to describe weight Density to discribe the functional porosity (e.g. in

filters porosity could be 40-80% of total volume) Functional density can be tuned according to

requirements.

Melting pointThe melting point of ceramics is remarkably

higher compared to metals.Heat conductivity

Heat conductivity of ceramics is between metals and polymers.

Can be tuned according to requirements.Heat expansion

Depends a lot about the compound. Can be tuned according to requirements.

Modulus of elasticityThe modulus of elasticity is near or above the

values of metals.Modulus of elasticity can be tuned by utilizing

composite compounds: E.g. WC+Co: E= 600GN/ mm2, Al2O3+SiO2-particles added + Al: E= 200 GN/mm2

Al: E=70 GN/mm2

Strength Brittle behaviour is typical for ceramics.Instead of yeld strength only ultimate

tensile/compression strengths are given.Compression strength is even 10-times higher

than tensile strength. Porosity affects greatly the strength.Ceramics have higher strength in extreme

(high) temperatures compared to metals, but due to so-called “glass deformation phase” also the strength of ceramics usually decreases in elevated temperatures (could be avoided with reaction sintering).

HardnessThe highest values of hardness are found

among the ceramics (Boron compounds).High hardness remains also in high

temperatures (even at 1000 °C).The final hardness is achieved during the

sintering process.

Electrical propertiesCeramics can function as electric insulators

(the most typical option), semi-conductors , conductors or even superconducters

Also piezo-electric properties can be producedSome ceramics are good electric insulators and

have also good heat and corrosion resistance.Magnetic properties

Possible to produce permanent magnets.

DETAILED SELECTION OF

CERAMICS

SINTERING METHOD

SINTERING METHOD

LEVEL OF PURITY

LEVEL OF PURITY

POROSITYPOROSITY

ALLOYINGALLOYING

GRAIN SIZEGRAIN SIZE

DIRECTION OF COMPRESSION

DUCTILITY EFFECT (ZIRCONIUM)

PROPETIES OF THE RAW MATERIALPROPETIES OF

THE COMPOUND

ASPECTS OF THE POWDER

METALLURGICAL PROCESS

MANUFACTURE OF THE MOLD AND THE

COMPRESSION TOOLS

MANUFACTURING (GRINDING) PROCESSES OF

THE POWDER

MIXING PROCESSES OF THE POWDER

SHAPING OF THE POWDER

SINTERING PROCESS

FINISHING PROCESSES

ATOMIZATION

REDUCTION PROCESSES

ELECTROLYSIS

MgO, CeO2, Al2O3, SiO2, Y2O3, ZrO2, CaO, MoSi2

Pressureless sinteringNitride bonding

Reaction bondingLiquid-phase sintering

Recrystallization Hot isostatic pressing

Hot pressing

POWDER METALLURGIC MANUFACTURING PROCESS

During the properly made systematic material selection process it is necessary to recognize::Limitations and possibilities of powder metallurgical manufacturing process

Guidelines of designing the suitable geometry of the product for the powder metallurgical manufacturing process

The optional materials (typically constructional ceramics) which can be applied to powder metallurgical manufacturing process

Temperature TPressure p

Time t

Density

Compression pressure

The most important process parameters in powder metallurgical process

OPTIONAL MANUFACTURING TECHNOLOGY 1

Component made of Silicon carbide and nitridewith Gelcasting.

Powder layer to be sintered

Motion path of the sintering laser beam

Product made of sintered powder layers

Utilized space for powder mass

Laser beam

OPTIONAL MANUFACTURING TECHNOLOGY 2

Example of expressing the purity grade

Ceramics(alloying)

Bending strength (N/mm²)

Al2O3 350-380

Al2O3 + ZrO2 350-550

ZrO2 + MgO 650-800

ZrO2 + Y2O3 1000-1500

How alloying affects the bending strength?

EFFECT OF ALLOYING ON HEAT CONDUCTIVITY

SiC (BeO alloying)

SiC (B4C alloying)

TEMPERATURE [°C]

HE

AT

CO

ND

UC

TIV

ITY

[W

/mK

]

CeramicsE

[GPa]Hot pressed silicon nitride

Alloying with 8% Y2O3 335

Alloying with 1% MgO 325

Alloying with 10% CeO2 327

Alloying with 4% Y2O3 + 3% Al2O3 305

Alloying with 4% Y2O3 + SiO2 305

How alloying affects the modulus of elasticity?

10% improvement

Zirconium oxide itself suffers from unbalanced change of length and volume expansion between the tetragonal and monoclinic phase. This causes huge internal stresses and pure Zirconium oxide can not be used for constructional purposes.

The phase change can be fully stabilized by Calcium oxide (CaO) alloying, but the strength and heat resistance properties will be so poor that neither the fully stabilized Zirconium oxide can’t be used in mechanical constructions.

How Zirconium oxide (ZrO2) alloying affects the mechanical properties of other ceramic compounds?

MOLTEN

CUBIC PHASE

TETRAGONAL PHASE

MONOCLINIC PHASE

CRYSTAL STRUCTURE

Partially stabilized Zirconium oxide (PSZ =Partially Stabilized Zirkonia) can be manufactured by alloying Yttrium oxide (Y2O3) or cerium oxide (CeO2). This decreases the risk of failure due to internal stresses and increases the strength and ductility.

Partially stabilized Zirconium oxide can be utilized as an alloying component in other ceramic compounds to improve their ductility (e.g Zirkonia Toughened Alumina, ZTA).

Different powder particles(Compound)

Different sizes

Same powder particlesDifferent sizes mixed

Same powder particlesOptional (same) sizes selected

Ceramics(commercial grades) 

Strength Grain size Grain sizeGrain size

ratioDensity

Modulus of elasticity

MPa min μm max μm max/min g/cm³ GPa

Sintered Silicon carbide

General Electric β-SiC 439 0,5-2 100 50 - 200 3,04 376

Carborundum α-SiC 325 2-5 15-18 3 – 9 3,09 428

Kyocera α-SiC 386 1,5-5 10 2 – 7 3,14 403

When the grain size ratio decreases, the strength decreases and the modulus of elasticity increases. However, the selected sintering process gives a new viewpoint to this conclusion!

How grain size affects strength and modulus of elasticity?

Same powder particlesDifferent sizes mixed

MPa

STRENGTH

STRENGTH OF SILICON CARBIDES / GRAIN SIZE

Note!Selected sinteringprocess affects together with the grain size!

Note!Smaller grainsize referresto higherstrength

Same powder particlesOptional (same) sizes selected

GPa

MODULUS OF ELASTICITY

MODULUS OF ELASTICITY OF SILICON CARBIDES / GRAIN SIZE

Note!Smaller grainsize referresto lower modulus ofelasticity

Note!Selected sinteringprocess affects together with the grain size!

Same powder particlesOptional (same) sizes selected

Powder metallurgy/Focus of review articles

Changes of the grain structure due to sintering and compression.

What happens during the compression and sintering?

Examples of the overall shrinkage of some ceramics during the powder metallurgical manufacturing process.

Ceramics Shrinkage %

Silicon carbide 18-20

Aluminium oxide 17-20

Zirconium oxide 25-32

SILICON CARBIDE

SiC LPSIC(Liquid-phase sintered)

SSIC(Pressureless sintered)

HPSIC(Hot pressed)

HIPSIC(Hot isostatic pressed)

SSIC(Pressure sintered)

NSIC(Nitride bonded)

RSSIC(Reaction-bonded)

RCSIC(Recrystallized)

SINTERING PROCESSES / SILICON CARBIDE

BONDING REACTION

PRESSING T

ECHNOLOGY

SILICON NITRIDE

Si3N4

HPSN (Hot pressed)RBSN

(Reaction-bonded)

HIPSN(Hot isostatic pressed)

SSN(Pressure sintered)

SINTERING PROCESSES / SILICON NITRIDE

Examples of typical sintering temperatures

Ceramics Sintering

temperature

[°C]

Al2O3 1800

SiC 2500

Si3N4 1700

Ceramics andsintering process

Bending strength (N/mm²)

Reaction sinteredSiC

200-450

SinteredSiC

350-550

Reaction sinteredSi3N4

200-400

SinteredSi3N4

500-750

How sintering process affects bending strength?

Note!If reaction sinteringis used, the strengthof ceramics will NOTdecrease in elevated temperatures!

Ceramics andSintering process 

Modulus of

elasticity

GPa

Hot pressed Silicon carbide

450

Sintered Silicon carbide

400

Reaction sintered Silicon Carbide

360

How sintering process affects modulus of elasticity?

Sintering process of Silicon nitride

Hardness

Pressure sintered 1400-1800 HV

Hot pressed 1500-1800 HV

Reaction bonded 400-700 HV

How sintering process affects hardness of ceramics?

MPa

STRENGTH

STRENGTH OF SILICON CARBIDES / SINTERING PROCESSES

Note!Selected sinteringprocess affects together with the grain size!

GPa

MODULUS OF ELASTICITY

MODULUS OF ELASTICITY OF SILICON CARBIDES / SINTERING PROCESSES

Note!Selected sinteringprocess affects together with the grain size!

COMPRESSION STRENGTH / COMPRESSION DIRECTION

Note!Bonding reactionaffects together with the compressionangle!

BENDING STRENGTH / COMPRESSION DIRECTION

Note!Bonding reactionaffects together with the compressionangle!

THERMAL EXPANDING / COMPRESSION DIRECTION

Note!Bonding reactionaffects together with the compressionangle!

Strength!

Heatresistance!

Compressionangle

BO, CA, XP!

Justification of ceramic applicationsCeramics The most important property for industrial applications

ALUMINIUM OXIDES Cost-effectiveness compared to other ceramics with Good chemical resistance. 

ALUMINIUM NITRIDES Excellent thermal conductor but at the same time excellent electric insulator.

SILICON CARBIDES Good heat resistance.

SILICON NITRIDES Good heat resistance combined with excellent resistance against heat shocks.

Si-Al-O-N (one type of silicon nittide)

Mechanical properties close Silicon nitride combined with chemical resistance close to properties of Aluminium oxide. 

ZIRCONIUM OXIDE Could be utilized to improve the toughness/ ductility of other ceramic materials. Use in oxygen sensors.

BORON CARBIDE Extremely hard (place 4. in the list of constructional materials)

BORON NITRIDE Extremely hard (place 3. in the list of constructional materials)

A ball valve made of aluminium oxide.

Applications of Aluminium oxide

Aluminium nitride is used in waveguide amplifiers and angular waveguides (MW-mechanics or high power electronics applications).

Applications of Aluminium nitride

Turbine (blades) made of Silicon carbide

Applications of Silicon carbide

Chemically resistant seals made of Silicon carbide

Applications of Silicon nitride

Applications of Si-Al-O-NSi-Al-O-N based cutting tools Si-Al-O-N based seals and sliding bearings

Ceramic foam filtersUnit porosity(percentage ): 80…90 %Density (g /cm3): 1.0Approximate use temperature 1700 °C.Thermal shock resistance: in 1110° C above 7 times

Applications of Zirconium oxide

Ceramic Zirconia based oxygen sensors

Applications of Boron Carbide

Applications of Silicon nitrideThe boron-nitride coatings combine the

strength and durability with the lubrication and anti-frictional properties e.g. of pistons and cylinders in an engine.

HARDNESS

HEAT RESISTANCE

(COMPRESSION)STRENGTH

ELECTRICAL OR

HEAT CONDUCTIVITY

FOUR-FIELD ANALYSIS FOR MECHANICAL/ELECTRICAL ENGINEERING

HEAT RESISTANCE

COSTS

FUNCTIONALPOROSITY

CHEMICALSTABILITY

FOUR-FIELD ANALYSIS FOR PROCESS ENGINEERING

How to name properly the selected ceramics?In addition to the ceramics type (e.g.

Al2O3, SiC…) the following data is required:Purity level [%]Alloying [%], at least Zirconium content

if it usedGrain size [either grain size ratio or

grain size limit]Porosity level [vol-% and/or density]Sintering method [HPSN, RSSC…]Direction of compression

Remember the ”problems” with commercial names of different grades

Hot pressed Silicon carbides

Norton NC-203

Ceradyne 146A

Ceradyne 146I

Sintered Silicon carbides

General Electric β-SiC

Carborundum α-SiC

Kyocera α-SiC

Reaction sinteredSilicon carbides

Norton NC-435

Norton NC-430

UKAEA BNF Refel

Coors SC-1