CHAPTER 2

LITERATURE REVIEW

2.1 Preamble

This chapter describes the literature survey o f the broad topic o f interest

regarding various coating techniques with a special reference to plasma spraying. It

also presents a review o f plasma sprayed molybdenum coating, effect o f particle size,

coating thickness and porosity on dry sliding wear. This chapter also includes

literature review regarding the post treatments o f plasma sprayed coating and wear.

2.2 Surface Engineering

The importance o f surface coating is increasing because higher energy

efficiency and longer service life are expected more strongly for various plants and

industries. Thermal spraying holds a unique position in the spectra o f surface

modification technologies as it can provide thick coatings over -lOOum (Fauchais and

Vardelle, 2000; Niranatlumpong and Koiprasert, 2010a). Surface engineering can be

defined as “treatment o f the surface and near surface regions o f a material to allow the

surface to perform functions that are distinct from those fiinctions demanded from the

bulk o f the material” . Engineers have always been faced with difficult decision when

selecting materials for the structural components in the modem high technology field

such as nuclear, space power, oil exploration and so on. The parts which are operating

in the extremely hostile environment o f temperature, gas flow, pressure and corrosion

media requires the surface coating. The properties desired at the surface are different

from those at the bulk o f the components. This is the major reason for the use o f

surface coating.

Wear, corrosion, erosion, fatigue and creep can cause environmental

degradation o f the surface over time. Surface engineering involves altering the

properties o f the surface in order to reduce the degradation over time. This is

accomplished by making the surface robust to the environment in which it will be

used (Hogmark et al., 2000; Stolarski and Tobe, 2001). Surface Engineering is the

name o f the discipline - surface modification technique is the philosophy behind it.

Surface treatment for engineering material may be necessary to:

1. Improve resistance to wear, erosion, and indentation (wear surfaces o f machinery,

and shafts, rolls, cams, and gears slideways in machine tools).

2. Control friction (sliding surfaces on tools, dies, bearings, and machine ways).

3. Reduce adhesion (electrical contacts).

4. Improve thermal insulation.

2.3 Techniques of Surface Modification

Today a large number o f commercially available technologies are present in the

industrial scenario. An overview o f such technologies is presented in Figure 2.1.

C»A'

N o s4 4«tattk$

A _____

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------ 1-Po jTBMr Q is s Cm m ie

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------ ^------HvdFaeiBSVtfwu I^osttioB

_______

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— Fkm*— Electric Arc— FIu b u Ak

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Oosm nssui

Eltcteo-Chwnic i l Deposition— Spfaj-on -

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jS^ny&Fusc Low !>lns SUM Fknau

---- Dvtonaiion Gun- Elactric Arc■M. PhuHBIS

D»fonnalionOtSfnstoaBnziBg

L «s «r

Figure 2.1 Summarized plot o f surface modification techniques (Mishra et al., 2000).

2.4 Thermal Spraying

In the eariy 1900s, M.U. Schoop was the first scientist to explore the possibility

that a stream o f metallic particles formed from molten metal might be used to produce

coatings. But the thermal spraying technologies expanded in the 1970s due to

development o f thermal plasmas, resulting in the increasing demand o f high

temperature and w ear resistant materials and coating systems (Knotek, 1975).

In the 1990’s thermal spraying was demandingly available and had become a

standard tool for improving surfaces in about all industries. Thermal spraying is the

application o f a material (consumable) to a substrate by melting into droplets and

impinging the softened or molten droplets on a substrate to form a continuous/pulsed

coating. Thermal spray consumables can be metallic, ceramic, alloys or polymeric

substances. Any material can be sprayed as long as it can be melted by the heat source

employed and does not undergo degradation during heating (Pfender, 1988; Pierlot et

al., 2008; Zimmer, 2010). The spray techniques that have been used to deposit

coatings for protection against hostile atmosphere are enlisted below, as summarised

by Chatha et al., (2012):

• Flame spraying with a powder particle or wire

• Electric arc wire spraying

• Plasma spraying

• High Velocity Oxy-Fuel (HVOF) spraying

• Spray and fiise

These processes are basically differentiated from each other on the basis o f

particle speed, flame temperature and spray atmosphere (Singh et al., 2007). Thermal

spray technology is uniquely important to an ever increasing engineering community,

for its (i) improved spray footprint definition versus wide spray beam; (ii) high

throughput versus competitive techniques; (iii) significantly improved process

control; (iv) lower cost per mass o f applied material, together with overall competitive

economics.

10

Thermal spray coatings have been produced for at least 50 years but the last

decade has seen a virtual revolution in the capability o f the technology to produce

truly high performance coatings o f a great range o f materials on many different

substrates (Talib et al., 2003).

The various advantages o f thermal spraying technology are listed as:

• Choice o f wide variety coating materials; metals, alloys, ceramics, cermets

and carbides.

• Thick coatings can be applied at high deposition rates.

• Coatings are mechanically bonded to the substrate can often spray coating

materials which are metallurgically incompatible with the substrate, e.g.

materials with high melting point than the substrate.

• Parts can be rebuilt quickly and at low cost and usually at a fraction o f price o f

a replacement.

• By using a premium material for the thermal spray coating, coated

components can outlive new parts.

• Thermal spray coatings can be applied both manually and automatically.

• The main principle behind thermal spraying is to melt material feedstock (wire

or powder) to accelerate the melt to impact on a substrate where rapid

solidification and deposit build up occur. Thus, a heat source and a means o f

accelerating the material are required. This is pictured schematically in Figure

2 .2 .

11

(a) (b)

Spray gun

; • : i : • \' spray •

/ jet \

/ \Incoming A j i U . ' Q ; molten ^ O ',

Deposited • a ‘ a w ▲ splats — ^

Substrate

r I ;

Convection heat transfer from coating to gas {h^)

Splats with initial temperature {Tn^)

Heat conduct tjetween coating and substrate (Ki,

Convection heat transfer from substrate to gas (ha\)

Figureure 2.2 (a) Schematic diagram showing plasma spraying (b) Schematic

physical thermo mechanical description o f plasma spray process (Chen et al., 2010).

The nature o f bonding at the coating substrate interface depends on mechanical

or metallurgical bonding. This is one extremely significant feature o f thermal

spraying. Another aspect o f thermal spraying is that the substrate surface temperature

seldom exceeds 2000°C. Stress related distortion problems are also not so significant.

The spraying action is achieved by the rapid energy transfer o f combustion gases to

the molten droplets or by a separate supply o f compressed air. There are two basic

ways o f generating heat required for melting the consumables. They are (i)

combustion o f a fiiel gas and (ii) high energy arc processes (Heimann, 1996),

categorize in Table 2.1.

12

Processes available for thermal spraying have been developed specifically for a

purpose and fall into two categories-high and low energy processes. The key

processes and their energy sources are summarized in Table 2.2 (Heimarm and

Lehmarm, 2008).

Table 2.1 Basic way to generate heat for melting spray powder

Oxy-fuel/wireThermal Spray Gas combustion process Oxy-fiiel/powder

Detonation gunProcess HVOF

Arc process Electric ArcPlasma Arc

T able 2.2 Thermal spraying processes.

Processes

Energy

sources Different Nomenclature

Low energy

process

Flame spraying chemical

Oxy-fiiel gas powder spraying

Oxy-fiiel gas wire sprayingMetalizing

Arc spraying ElectricalElectric arc spraying

Twin-wire arc sprayingMetalizing

High energy

process

Plasma spraying Electrical

Atmospheric plasma spraying

Vacuum plasma sprayingLow pressure plasma

sprayingW ater stabilized plasma

spraying

Inductive plasma spraying

Detonation flame spraying

Chemical D-gun

High velocity Oxy-fiiel spraying

Chemical

HVOF sprayingHigh velocity oxygen fiiel

sprayingHigh velocity flame

sprayingHigh velocity air fuel

13

2.5 Plasma Spraying

Plasma spraying is one o f the most widely used thermal spraying technique

which finds a lot o f applications due to its versatility o f spraying a wide range o f

materials from metallic to non metallic and hence more suitable for spraying o f high

melting point materials like refractory ceramics material, cermets (Fauchais and

Vardelle, 2000; Vaxevanidis et al., 2004) . A schematic diagram o f plasma spray

process is shown in Figure 2.3. This process is part o f thermal spraying in which

finely divided metallic and non-metallic materials are deposited in a molten or semi-

molten state on a prepared substrate (Selvan et al., 2011).

In the late fifties and early sixties, the first attempts were reported using plasma

torches for spraying o f primarily refractory materials. Almost any material can be

used for plasma spraying on any type o f substrate. This flexibility is probably one o f

the major reasons for the rapid development o f this technology (Pfender, 1988). The

high temperature enables the use o f coating materials with very high melting points

such as alloys, ceramics, cermets and refi-actory. Materials can be processed as long as

there is a temperature difference o f at least 300K between the melting temperature

and decomposition or evaporation temperature (Fauchais et al., 1991).

Among other key features o f plasma spraying are the formation o f

microstructures with fine, non-columnar and equiaxed grains. It has the ability to

produce homogeneous coatings that do not change in composition with thickness and

length o f deposition time, the ability to process materials in virtually any environment

(e.g., air, reduced pressure inert gas, high pressure, under water) (Fauchais et al.,

1996).

Applications o f plasma spraying include corrosion, erosion, temperature and

abrasion resistant coatings and production o f monolithic and near net shapes which14

are at the same time take advantage o f the rapid solidification process. Powder o f

glassy metals can be plasma sprayed without changing their amorphous

characteristics. High temperature superconductive materials have also been deposited

by the plasma spray technique.

Cooling Water .

Plasma Gas

Electrode

Spray Deposite I

Spray Steam

Insulation Nozzle Substrate

Figure 2.3 Conventional plasma Spray process (Wierzchon, 1999)

In this technique an arc is created between tungsten cathode and copper anode.

Thus generated plasma gas is forced to pass through the annular space between the

electrodes. The gas undergoes ionization in the high temperature environment

resulting in plasma while passing through the arc. Temperature in the plasma arc can

be o f very as high temperature (as shown in Figure 2.4) and is capable o f melting

anything.

15

calttod* powd«r pofi

I - l b132000 3 M 0 0 25000 20000 K

t - t— s ym m e try axisf) plostno cone

^ 6 T~ .--------- --------- --------- symmetry axis

I f i0 0 o \ l ? 0 0 0 10000 5000 2000 K pow der con«

orM>dft p lasm o (lan>»

1________ I________ I_____0 10 20 30 £0 50 mm

Figure 2.4 Temperature Distribution and geometry o f the plasma jet (Mishra et

al., 2006).

The ionization is achieved by collisions o f electrons o f the arc with the neutral

molecules o f the gas. The plasma protrudes out o f the electrode encasement in the

form o f a flame. Electrodes are water cooled. The raw coating material in the

powdered form is poured into the flame with required feed rate. The powders melt

immediately by gain o f plasma energy and momentum and then rush towards the

target to form a thin deposited layer. In this way the coating builds up layer by layer

(Shaw et al., 2000). Elaborate cooling arrangement is required to protect the spray

system from excess heating. The equipment consists o f the following modules

(Venkatramani, 2002) such as:

• The plasmatron: It is the device which houses the electrodes and in which the

plasma reaction takes place. It has the shape o f a gun and it is connected to the

water cooled power supply cables, powder supply hose and gas supply hose.

• The power supply unit: Normally plasma arc works in a low voltage (30-60

Volts) and high current (300-700 Amps), DC ambient. The available AC power

o f 3 Phase, 440Volts must be transformed and rectified to suit the reactor. This

is taken care o f by the power supply unit.

16

• The powder feeder: The powder is kept inside a hopper. A separate gas line

directs the career gas that fluidizes the powder and carries it to the plasma arc.

The flow rate o f the powder can be controlled precisely.

• The coolant and water supply unit: It circulates water into the plasmatron,

power supply unit and the power cables. Units capable o f supplying refrigerated

water are also available.

• The control unit: Important functions (current control, gas flow rate control

etc.) are performed by the control unit. It also consists o f relays and solenoid

valves and other interlocking arrangements essential for safe running o f the

equipment. For example an arc can only be started if the coolant supply is on

and water pressure and flow rate is adequate.

2.5.1 Advantages o f Plasma Spraying

The major advantages o f the plasma spraying process have been presented as

follows (Sidhu and Prakash, 2006):

1. Very flexible in coating material selection and optimization for specific

resistance to corrosive environments and particle abrasion/erosion.

2. Coating systems (multi-layer or functionally graded) can be used.

3. Unique alloys and microstructures can be obtained with thermal spraying which are

not possible with a wrought material. These include continuously graded composites

and corrosion resistant amorphous phases.

4. Costs o f the coating solution are normally significantly lower than those o f a highly

alloyed bulk material; thermal spray coatings are especially interesting for their

cost/performance ratio.

17

5. Thermal spray coatings additionally offer the possibilities o f on-site application and

repair o f components, given a sufficient accessibility for the sprayer and his

equipment.

6. Forms microstructure with fine, equiaxed grains and without columnar boundaries.

7. Produces deposits that do not change in composition with thickness (length o f

deposition time).

8. Can change from depositing a metal to a continuously varying mixture o f metals

ceramics (i.e. fiinctionally graded materials).

9. High deposition rates (>4kg/h).

10. Process materials in virtually any environment e.g. air, reduced pressure inert gas,

high pressure etc.

However, thermal spraying in the work shop is preferred whenever possible, to

achieve optimum results. Among the thermal spray coating processes, plasma

spraying is reported to be a versatile technology that has been successful as a reliable

cost effective solution for many industrial problems Fauchais et al., (1996). Plasma

spraying is the most flexible thermal spray process with respect to the sprayed

materials. The high temperatures o f plasma spray processes permit the deposition o f

coatings for applications in areas o f liquid and high temperature corrosion and wear

protection and also special applications for thermal, electrical and biomedical

purposes. Plasma sprayed metallic coatings are used in high temperature applications

e.g. in diesel engines, aircraft engines and land based gas turbines to protect the

component from oxidation and corrosion (Karoonboonyanan et al., 2007).

2.5.2 Requirements for Plasma Spraying

• Roughness of the substrate surface: Surface roughness o f substrate provides

anchorages for better mechanical interlocking- Better the surface roughness better is

18

the adhesion strength. A rough surface is generally created by grit blasting technique.

The grits are kept inside a hopper and compressed air is supplied at the bottom o f the

hopper. The grits are taken a float by the compressed air stream into a hose and

ultimately directed to an object kept in front o f the exit nozzle o f the hose. The grits

used for this purpose are irregular in shape, highly angular in nature and made up o f

hard material like alumina, silicon carbide and so on. Upon impact they create small

craters on the surface by localized plastic deformation, and finally yield a very rough

and highly worked surface.

• The roughness obtained is determined by shot blasting parameters i.e., shot

size, shape and material, air pressure, angle o f impact, stand-off-distance between

nozzle and the job, substrate material etc (Grigorescu et al., 1991). The effect o f shot

blasting parameters on the adhesion o f plasma sprayed alumina has been studied

(Celik and Demirkiran, 1999). M ild steel serves as the substrate material. The

adhesion increases proportionally with surface roughness and the parameters listed

above are o f importance. A significant time lapse between shot blasting and plasma

spraying causes a marked decrease in bond strength (Kang and Ng, 2006).

• Cleanliness o f the substrate surfaces: The substrate to be sprayed must be

free from any dirt or grease or any other material that might prevent intimate contact

o f the splat and the substrate. For this purpose the substrate must be thoroughly

cleaned by acetone (ultrasonically, if possible) before spraying. Spraying must be

conducted immediately after grit blasting and cleaning. Otherwise on the nascent

surfaces, oxide layers tend to grow quickly and moisture may also affect the surface.

These factors deteriorate the coating quality drastically (Kang and Ng, 2006).

• Bond coat: Materials like ceramic cannot be sprayed directly onto metals,

owing to a large difference between their thermal expansion coefficients (a).19

Ceramics have a much lower value o f “a ” and hence undergo much less shrinkage as

compared to the metallic base to form a surface in compression. If the compressive

stress exceeds a certain limit, the coating gets peeled off. To alleviate this problem a

suitable material, usually metallic o f intermediate a value is plasma sprayed onto the

substrate followed by the plasma spraying o f ceramics. Bond coat may render itself

useful for metallic topcoats as well. Molybdenum is a classic example o f bond coat

for metallic topcoats. Molybdenum adheres very well to the steel substrate and

develops rough top surface ideal for the topcoat spraying. The choice o f bond coats

depends upon the application. For example, in wear application, an alumina and Ni-Al

top and bond coating combination can be used (Steinhauser et al., 2000). In thermal

barrier application, CoCrAlY or Ni-Al bond coat (Lee et al., 1996) and zirconia top

coat are popular. Ceramic coatings when subjected to hertzian loading deform

elastically and the metallic substrate deforms plastically. During unloading, elastic

recovery o f the coating takes place whereas for the metallic substrate a permanent set

has already taken place. Owing to this elasto-plastic mismatch the coating tends to

spall o ff at the interface. A bond coat can reduce this mismatch as well.

• Cooling water: Distilled water was used for cooling purpose. Normally a

small volume o f distilled water is re-circulated into the gun. It is cooled by an external

water supply from a large tank. Sometime water from a large external tank is pumped

directly into the gun.

2.5.3 Process Parameters in Plasma Spraying

In plasma spraying parameters are interrelated with each other, which determine

the degree o f particle melting, adhesion strength and deposition efficiency o f the

powder. Deposition efficiency is the ratio o f amount o f powder deposited on substrate

20

to the amount fed to the gun as raw material. An elaborate listing o f these parameters

and their effects are reported in the literature (Gurusamy and Akira, 2005). Some

important parameters and their roles are listed below;

Arc power: It is the electrical power drawn by the consumable/non-consumable

arc.

The power is injected into the plasma gas which in turn gains energy from plasma

stream. Part o f the energy o f power is dissipated as radiation and also by the gun

cooling water. Arc power determines the mass flow rate o f a given powder that can be

effectively melted by the arc with an appropriate contact time. Deposition efficiency

improves to a certain extent with an increase in arc power, since it is associated with

an enhanced particle melting (Kanta et al., 2011).However, increasing power beyond

a certain limit may not cause a significant improvement. On the contrary, once a

complete particle melting is achieved, a higher gas temperature may prove to be

harmful. Because, at some point vaporization may take place which to lowering the

deposition efficiency.

Plasma gas: Generally Nitrogen or Argon doped with about 10% Hydrogen or

Helium is used as a plasma gas. The major constituent o f the gas mixture is known as

primary gas and the minor is known as the secondary gas.

The neutral molecules are subjected to the electron bombardment resulting in

their ionization. Both temperature and enthalpy o f the gas increases as it absorbs

energy. Since nitrogen and hydrogen are diatomic gases, they first undergo

dissociation followed by ionization. Thus, they need higher energy input to enter the

plasma state. This extra energy increases the enthalpy o f the plasma. On the other

hand, the mono-atomic plasma gases, i.e. Argon or helium, approach a much higher

temperature in the normal enthalpy range. Good heating ability is expected from them

21

for such high temperature (Heimann and Lehmann, 2008). In addition, hydrogen

followed by helium has a very high specific heat, and therefore is capable o f acquiring

very high enthalpy. When argon is doped with helium the spray cone becomes quite

narrow which is especially useftil for spraying on small targets.

C a r r ie r gas: Usually the primary gas itself is used as a carrier gas. The flow

rate o f the carrier gas is an important factor. If the flow rate is very high, then the

powders might escape the hottest region o f the je t and a very low flow rate cannot

convey the powder effectively to the plasma je t (as shown in Figure 2.5) .There is an

optimum flow rate for each powder at which the fraction o f unmelted powder is

minimum and hence the deposition efficiency is maximum.

22

(a) Powder Injector

i

Plasma G es-

Camode

Plasma Jat

(b)Powder Injector

i

Ptasm a Qaa

Powder

Substrata

Coa^

CatfKJde _____

7= ^ '

Ptasma Jet

(C) Powder Injector

i

PoMNlar

Substrata

Coaling

Plasma Oaa-

Cattiode

Plasma JM Siibsbate

Figure 2.5 Carrier Gas Flow Rate a) Too low b) Correct

c) Too high (Venkatramani, 2002)

The mass flow ra te of the pow der: Ideal mass flow rate for each powder has

to be determined. Spraying with a lower mass flow rate, keeping all other conditions

constant results in under utilization and slow coating build up. On the other hand, a

very high mass flow rate may give rise to an incomplete melting resulting in a high

amount o f porosity in the coating. The unmelted powders may bounce o ff from the

substrate surface as well keeping the deposition efficiency low (Fauchais et al., 2006).

23

Pow der related variables: These variables are powder size, shape and size

distribution, phase composition, processing history and so on. They constitute a set o f

extremely important parameters. For example, in a given situation if the powder size

is too small it might get vaporized. On the other hand a very large particle may not

melt substantially and therefore will not deposit. The shape o f the powder is also quite

important. A spherical powder will not have the same characteristics as the angular

ones, and hence both could not be sprayed' using the same set o f parameters (Morsi et

a l . ,2012).

S tand-off-distance (Spray Distance): It is the distance between the tip o f the

gun and the substrate surface. A long distance may result in freezing o f the melted

particles before they reach the target, whereas a short standoff distance may not

provide sufficient time for the particles in flight to melt and may erode the substrate

surface (Guessasma et al., 2004a; Morsi et al., 2012). A larger fraction o f the

unmelted particles go in the coating owing to an increase in stand-off-distance.

Spraying angle: Angle is one o f an important factor during spraying. It is

responsible for splat formation. Different angle is chosen by considering different

material (ductile/ brittle substrate). The influence o f spraying angle on the cohesive

strength o f chromia, zirconia 8wt% yittria and molybdenum has been investigated,

and it has been found that the spraying angle does not have much influence on the

cohesive strength o f the coatings (Hennaut et al., 1990). SEM examination o f splat

morphologies obtained from impacting droplets at substrate with different angle o f (a)

0° (b) 10° (c) 20° (d) 30° (e) 40° (0 50° (g) 60° are shown in Figure 2.6.

24

Figure 2.6 SEM examinations o f splat morphologies obtained from impacting

droplets at substrate inclinations o f (a) 0° (b) 10° (c) 20° (d) 30° (e) 40° (f) 50°

(g) 60° (Kang and Ng, 2006)

S ubstra te cooling: During a continuous spraying, the substrate might get

heated up and may develop thermal stresses related distortion accompanied by a

coating peel-off. This is especially true in situations where thick deposits are to be

applied. To reduce the substrate temperature, it is kept cool by an auxiliary air supply

system. In addition, the cooling air je t removes the unmelted particles from the coated

surface and helps to reduce the porosity.

Angle of pow er injection: Coating Powders can be injected perpendicularly,

coaxially or obliquely in to the plasma jet. The residence time o f the powders material

will vary with injection angle for a given carrier gas flow rate. The residence time will

influence the degree o f melting o f a given powder. For example, to melt high melting

point materials a long residence time needed and hence oblique injection may prove

25

to be better. The angle o f injection is found to influence the cohesive strength and

adhesion strength o f the coatings (Afzal et al., 2014; Kang and Ng, 2006).

2.5.4 Mechanism of Coating Formation in Plasma Spraying Process

Plasma spray is formed by the impact o f a stream o f particles from nozzle

striking the substrate surface, the major controlling factors which influenced by the

structure o f a particular coating are the velocity, temperature and size distribution o f

the incident particles. Ideally all the surface striking particles would be completely

molten. Unmelted particles may bounce off reducing the deposition efficiency and

partly melted particles are incorporated within the deposit modifying its

microstructure and properties (McPherson, 1989).

Coatings are formed by the build up o f successive layers o f molten droplets

which flatten and solidify on impact to give lamellar microstructure. W hen a liquid

droplets strikes the surface at low velocity, it flattens to a disc (shown in Figure

2.7)(Chen et al., 2010) which then come to the equilibrium shape o f spherical cap to

form a cone and spreads again to the final equilibrium shape determined by the static

surface tension forces (shown in Figure 2.8). At high impact velocities the thin sheet

o f liquid becomes unstable and disintegrates at the edge into many small droplets i.e.

splashing occurs. Its cooling rate then rapidly increases by conduction from molten

particle to surface o f the substrate. The cooling rates achieved are o f the order o f 106-

107Ksec ‘ (McPherson, 1981).

Fauchais et al., (1996) investigated on coating generation and predicted a model

for calculating the splat quenching rate. It was observed that some metastable phases

are formed during cooling, like y-alumina rather than a-alumina which was explained

on the basis o f nucleation kinetics, i.e. y- alumina was easily nucleated because o f

lower interfacial energy between crystal and the liquid and at sufficiently rapid

26

cooling rates, the metastable form is retained at room temperature. Mechanical

behaviour o f the coatings is limited to the degree o f contact between the lamellae

within the coatings (cohesion strength) and between the lamellae and substrate

(adhesion strength) rather than the nature o f bonds in regions o f good contact. This

study was made on alumina coatings. The low apparent area o f contact may be due to

entrapped gases and other asperities between the impinging droplet and the

substrate(McPherson and Shafer, 1982). So surface grit blasting and cleaning o f the

substrates is necessary for better bonding between coating and the substrate (Safai and

Herman, 1977).

Acnul Idtaliation

Spcrxktj X**'

locommgJ

sutKmr

Solidi5nl

sobstraie

(») (b)

Figure 2.7 Schematic o f the (a) physical plasma spray process and (b) its idealization

for modelling (Kang and Ng, 2006).

S plal fe m m i a f te r iropjict

Spr:t}1ngd irrc tion S phrricu l p u rtic lf

b ffo rr inipsirt

Figure 2.8 Splat formations after the impact o f the spherical powder during spraying.

27

2.6 Wear

W ear may be defined as damage to the solid surface caused by the removal or

displacement o f material by the mechanical action o f a contacting solid, liquid or gas.

W ear occurs as natural consequence when two surfaces are in relative motion

interacting with each other. W idely varied wearing conditions causes wear o f

materials.

It may be due to surface damage or removal o f material from one or both the

solid surfaces in a sliding, rolling or impact motion relative to each another. Different

types o f wear have been investigated by different researcher by taking different

materials (Aouadi et al., 2004; Masanta et al., 2011; M istry et al., 2011; Singh et al.,

2011; Wei et al., 2012; Wong et al., 2012). In these investigations various wear

theories are taken in which physico-mechanical characteristics o f the materials and

the physical conditions (i.e. the resistance o f the rubbing body and the stress state at

the contact area) are taken in to consideration. W ear o f metals depends on many

variables for which wear investigation programs must be planned systematically. It

should be understood that the real area o f contact between two solid surfaces

compared with the apparent area o f contact is invariably very small, being limited to

points o f contact between surface asperities. The load applied to the surfaces will be

transferred through these points o f contact and the localized forces can be very large.

W ear is not an intrinsic material property but characteristics o f the engineering

system which depend on load, temperature, speed, hardness, the environmental

conditions and presence o f foreign material (Bolelli et al., 2006; Lim and Ashby,

1987). During relative motion, material on contacting surface may be removed from

a surface, may result in the transfer to the mating surface, or may break loose as a

wear particle. The wear resistance o f materials is related to its microstructural28

characteristics may take place during the w ear process and hence it seems that in

wear research emphasis is placed on microstructure (Viljus and Letunovit'', 2006;

W angeta l., 1990).

2.7 Types of Wear

In most basic wear studies on dry friction has been investigated where the

problems o f wear have been a primary concern to avoid the influences o f fluid

lubricants. Dry friction is defined as friction under not intentionally lubricated

conditions but it is well known that it is friction under lubrication by atmospheric

gases, especially by oxygen (Soda et al., 1977). A fundamental scheme to classify

wear was outlined by Burwell, (1957) , include five distinct types o f wear, namely

(a) Abrasive (b) Adhesive (c) Erosive (d) Surface fatigue (e) Corrosive.

2.7.1 Abrasive Wear

Abrasive wear or abrasion is account for most failures in any industrial

equipment which is originated from the two rubbing surfaces.

It can be define as the wear that is caused by the displacement o f material from

a solid surface due to hard particles or protuberances sliding along the surface and

cutting grooves on the softer surfaces. In sliding mechanisms, abrasion can arise from

the existing asperities on one surface ( if it is harder than the other), from the

generation o f wear fragments which are repeatedly deformed. Hence it is work

hardened for oxidized until they became harder than either or both o f the sliding

surfaces, or from the adventitious entry o f hard particles, such as dirt from outside the

system.

Two body abrasive wear occurs when one o f the harder surface cuts material

away from the second o f less hard. Abrasives can act as in grinding where the

abrasive is fixed relative to one surface or as in lapping where the abrasive tumbles29

producing a series o f indentations as opposed to a scratch. According to the recent

tribological survey, abrasive wear is responsible for the largest amount o f material

loss in industrial practice (Stewart et al., 1999; Yilmaz, 2009).

2.7.2 Adhesive W ear

Adhesive wear can be defined as wear due to localized bonding between

contacting solid surfaces leading to material transfer between the two surfaces or the

loss from either surface (Burwell, 1957). In this wear it is necessary for the surfaces

to be in intimate contact with each other. To obstruct for adhesion wear it is

necessary for two surfaces to hold apart by lubricating films, oxide films etc. Fretting

wear is also this type o f wear mechanism.

2 .7 3 Erosive W ear

Erosive wear can be defined as the progressive loss o f original material from a

solid surface due to mechanical interaction between that surface and a fluid, a m ulti-

component fluid, or impinging liquid or solid particles. When the angle o f

impingement is small, the wear produced is closely similar to abrasion. When the

angle o f impact is normal to the surface, more loss o f material occurs. The erosion

mechanism depends on the material (brittle/ductile). So erosion has been divided into

brittle and ductile erosion.

Ductile materials fail as a result o f impacting particles causing localized plastic

flow that exceeds the critical strain to failure in the local areas. When the erodent

particles in either gas or liquid carrier fluid strikes the surface o f a ductile material,

they initially extrude thin micro-platelets o f the base material fi-om craters which are

formed at the sites o f impacts and then the platelets are then further flattened. After a

small number o f particles have impacted the same localized area, the extruded

30

platelets would have been strained to their critical strain and fracture o f portions o f

the platelet will occur (Wang et al., 1992).

The mechanism o f erosion o f brittle materials (i.e. ceramic type materials) is

considerably different. Brittle materials are removed by a cracking and chipping

mechanism. Here erosion occurs by the propagation and intersection o f cracks

produced by the impacting particles. The dense, columnar grain, outer scale cracks

are chipped away, while the small equiaxed grains o f the inner scale initially form

hertzian cone cracks or ring cracks. Subsequently, at latter times, increased loading

leads to increasing number o f ring cracks leading to chipping away o f the inner

scales(W ang et al., 1992).

2.7.4 Surface Fatigue W ear

W ear arises from material fatigue is called as surface Fatigue Wear. This wear

became dominant, when a solid is subjected to cyclic loading involving tension and

compression above a certain critical stress. Repeated loading causes the generation o f

micro cracks at the site o f a pre-existing point o f weakness and leads to join o f micro

void and form the crack. When crack reaches the critical size, it changes its direction

to emerge at the surface, and thus flat sheet like particles is detached during wearing.

The number o f stress cycles required to cause such failure decreases as the

corresponding magnitude o f stress increases (Akdogan et al., 2002; Nieminen, 1997).

2.7.5 Corrosive W ear

Thermodynamically unstable metals react with oxygen to form an oxide and

gradually develop scales or layers on the surface. These layers are very weak or

unprotected which allows gradual degradation o f metal on the surface. Deterioration

also cause by the effects o f the atmosphere, acids, gases, alkalis, etc. This type o f

31

wear creates pits and perforations and may eventually dissolve metal parts (Abd-El-

Kader and El-Raghy, 1986).

2.8 Symptoms of Wear

W ear is a characteristic o f the system & its surrounding and is influenced by

many parameters. So it is necessary to understand the wear mechanism to protect the

metal. In Laboratory scale investigations, individuals o f tribo-systems are carefully

control and study the effects o f different variables on the wear behaviour o f the

coating. The data generated through such research under controlled conditions may

help in correct interpretation o f the results. A summary o f the appearance and

symptoms o f different wear mechanism is indicated in Table 2.3 (Ko, 1987) and the

same is a systematic approach to diagnose the wear mechanisms.

T ab le 2.3 Different wear mechanism, symptoms and surface appearance.

Type o f wear Symptoms Appearance o f worn out Surface

Abrasive Presence o f chip-out o f surface Grooves and tom -out surfaces

Adhesive Metal transfer s prime symptoms Seizure, catering rough and tom -out surfaces

Erosion Presence o f abrasives in the fast moving fluid and short abrasion furrows

Waves & Troughs

Fatigue Presence o f surface or subsurface cracks accompanied by pits and spalls

Sharp and angular edges

Corrosion Rough pits and

Presence o f metal corrosion products Rough pits and depressions

Delamination Presence o f subsurface cracks parallel to the surface with semi-dislodge or

loose

Loose, long and thin sheet like particle flakes

Impacts Surface fatigue, small sub-micron particles or formation o f spalls

Fragmentation, peeling ,peeling and pitting

Fretting Production o f voluminous amount o f loose debris

Roughening, seizure and

development o f oxide ridges

32

2.9 Recent Trends in Material Wear Research

Most o f wear research carried out in the 1940’s and 1950’s were conducted by

metallurgical and mechanical engineers to generate data for the protective structural

materials o f different motor drive, trains, bearings, brakes, bushings and other types o f

moving mechanical assemblies (Eyre, 1978).

It became apparent during the survey that wear o f metals was a prominent topic

in a large number o f the responses regarding some fiiture priorities for research in

tribology. Much o f the wear research conducted over more than past 50 years is in

ceramics, polymers, composite materials and coatings (Blau, 1997). Now-a-days this

type o f research is in rapid progress in different country in different part o f the world.

2.10 Wear Resistant Coatings

Today a variety o f materials, e.g., carbides, oxides, metallic and so on are

available commercially for protecting the metal surface. The wear resistant coatings

can be classified into the following categories:

(i) Carbides: WC, TiC, ZrC, Cr2C3 ,SiC and so on.

(ii) Oxides; Cr203, AI2O3, Ti02 , Zr02 and so on.

(iii) Metallic: NiCrAlY, Triballoy and so on.

(iv) Diamond

The choice o f a material depends on the application. However, the ceramic

coatings are very hard and hence can provide more abrasive resistance than their

metallic counterparts.

33

2.10.1 Carbide Coatings

For wear and corrosion applications WC is very popular among all carbides

(Cadenas et al., 1997). The WC powders are clad with a cobalt layer. During spraying

the cobalt layer undergoes melting and upon solidification form a metallic matrix in

which the hard WC particles remain embedded.

Spraying o f WC-Co involves a close control o f the process parameters such that

only the cobalt phase melts without degrading the WC particles.

Such degradation may occur in two ways: one is Oxidation o f WC leading to the

formation o f C0W O4 and WC2 (Nolan et al., 1998) and another one is dissolution o f

WC in the cobalt matrix leading to a formation o f brittle phases like C0W 3C which

embrittles the coating (Naerheim et al., 1995). An increase in the spraying distance

and associated increase o f time in flight leads to a loss o f carbon and a pickup o f

oxygen. As a result, the hardness o f the coating decreases. An increase in plasma gas

flow rate reduces the dwell time and hence can control the oxidation to some extent.

However, it increases the possibility o f cobalt dissolution in the matrix (Knotek,

1975). The other option to improve the quality o f such coating is to conduct the

spraying procedure in vacuum (Naerheim et al., 1995). Some carbide like TiC, TaC

and NbC are provided along with W C in the cermet to improve upon the oxidation

resistance, hot strength, and hardness. A coating o f CrsC2 with Ni-Cr alloy cladding is

known for its excellent sliding wear resistance, superior oxidation and erosion

resistance but its hardness is lower than that o f WC. After spraying in air, Cr3C2 loses

carbon and transforms to CvjC^. Such transformation generally improves hardness and

erosion resistance o f the coating (Mohanty et al., 1996).

34

2.10.2 Oxide Coatings

Metallic coatings and metal containing carbide coatings sometime are not

suitable in high temperature environments in both wear and corrosion applications

due to formation o f oxidation or decarburization. In such case, the material o f choice

can be an oxide ceramic coating, e.g., Al203,Cr203, Zr02 , TiOa or their combinations

(Liu and Chen, 2009). However, a high wear resistance, and chemical and thermal

stability o f these materials are counterbalanced by the disadvantages o f low values o f

thermal expansion coefficient, thermal conductivity, mechanical strength, fracture

toughness and somewhat weaker adhesion to substrate material. The thickness o f

these coatings is also limited by the residual stress that grows with thickness.

Therefore, to obtain a good quality coating it is essential to exercise proper choice o f

bond coat, spray parameters and reinforcing additives.

2.10.3 Metallic Coatings

Metallic coatings can be easily applied by flame/plasma spraying or welding

techniques making the process very economical. Metallic wear resistant materials are

classified into three categories: (i) Cobalt based alloys (ii) Nickel based alloys (iii)

Iron based alloys.

The common alloying elements in a cobalt-based alloy are Cr, Mo, W and Si.

The microstructure is constituted by dispersed carbides o f M7C3 type in a cobalt rich

FCC matrix.

The carbides provide the necessary abrasion resistance and corrosion resistance.

Hardness at elevated temperatures is retained by the matrix (Atamert and Bhadeshia,

1990). The principal alloying elements in Ni based alloys are Si, B, C and Cr. The

35

abrasion resistance can be attributed to the formation o f extremely hard chromium

borides. Iron based alloys are classified into pearlitic steels, austenitic steels,

martensitic steels and high alloy irons. The principal alloying elements used are Mo,

Ni, Cr and C. The softer materials, e.g., ferrite are for rebuilding purpose. The harder

materials, e.g., martensitic, on the other hand provide wear resistance. Such alloys do

not posses much corrosion, oxidation or creep resistance. Nickel aluminide is another

example o f coating material for wear purpose.

The pre-alloyed Ni-Al powders, when sprayed react exothermically to form

nickel aluminide. This reaction improves the coating substrate adhesion. In addition to

wear application, it is also used as bond coat for ceramic materials. NiCoCrAlY is an

example o f plasma sprayable super alloy. It shows an excellent high temperature

corrosion resistance and hence finds application in gas turbine blades. In addition, it

serves as a bond coat for zirconia based thermal barrier coatings.

2.10.4 Diamond Coatings

In some industrial application diamond films are commonly produced by CVD,

plasma assisted CVD, laser ablation technique and ion beam deposition. Such

coatings are used in electronic devices and ultra wear resistant overlays. The

limitation o f the aforesaid methods is their slow deposition rates. The DIA-JET

process involving DC Ar/H2 plasma with methane gas supplied at the plasma je t is

capable o f depositing diamond films at a high rate. However, the process is extremely

sensitive to the process parameters. Deposition o f diamond film is also possible using

an oxy-acetylene torch (Zhu et al., 1993). One significant limitation o f a diamond

coating is that it cannot be rubbed against ferrous materials, owing to a phase

transformation leading to the formation o f other carbon allotropes (Hollman et al.,36

1994). Diamond films are tested for the sliding wear against abrasive papers where

wear progresses by micro fracturing o f protruding diamond grits. The process

continues till the surfaces become flat and thereafter wear progresses by an interfacial

spalling. Therefore, the life o f the coating is limited by its thickness (Alahelisten,

1995).

2.11 Molybdenum as Wear Resistant Coating

Vaidya et al., (2005); Hwang et al., (2005); Laribi et al., (2003); Stolarski and

Tobe, (2001); Zhiqiang Liu and Hua, (2000) deposited a molybdenum on stain less

steel and carbon steel substrate using plasma spray technique. Thus, material such as

molybdenum is widely sprayed on steel substrate in order to exploit their superficial

characteristics such as wear and scuff resistance. Along with these characteristics it

has better values o f thermal expansion coefficient, thermal conductivity, mechanical

strength, fracture toughness and good adhesion to substrate material. Plasma sprayed

Molybdenum coatings are preferred mostly for automotive, aerospace, pulp and paper

industries due to their exceptional wear resistance properties. It also finds applications

in industrial components such as seals, bearings and shafts as an overlay coating to

prevent surface degradation and also to reduce the coefficient o f fiiction in

components that are in sliding contact. However, the fiinctional properties o f plasma

sprayed molybdenum coating rely on its structure, phase composition, uniformity o f

microstructure and porosity. The uniformity o f microstructure and porosity in plasma

sprayed coating greatly affected by size o f the particles and coating thickness.

2.12 Effect of particle size on dry sliding wear resistance

The microstructure o f a plasma-sprayed coating is greatly influenced by the

deformation behaviour o f the particles impinging on the substrate and by the37

characteristics o f the underlying surface. Experimental and theoretical evidence

suggests that the spreading and cooling o f molten particles on the surface depends on

the particle size, velocity, and molten state prior to impact. The velocity and

temperature o f each in-flight particle are two o f the primary parameters influencing

the microstructure o f coatings because they dramatically influence the flattening

behaviour o f the particles and ultimately the microstructure o f the coating (Bai et al.,

2013; Zhao e ta l., 2013).

The velocity and temperature o f in-flight particles increased with decreasing the

particle size (Niu et al., 2012) and similar results were observed by

Shanmugavelayutham et al.,(2006). The velocity and surface temperature o f the in­

flight particles are directly proportional to the reciprocal o f the porosity and to the

structural uniformity o f the coatings (Bai et al., 2013).

Fukanuma, (1994) proposed a physical and mathematical model for the

production o f porosity by considering a deformation o f molten particles during the

process. The model shows that the impinging velocity and particle diameter contribute

to producing porosity. Pores often exist in many materials, generated during material

manufacturing processes. The presence o f pores usually results in detrimental

influence on the performance o f materials. There have been many studies showing

negative influences o f porosity on material properties such as reduction in elastic

moduli, decrease in both strength and wear resistance, conductivity and magnetic

properties. The presence o f porosity can be very detrimental to the wear resistance of

materials. Thus, the influence o f porosity on the wear behaviour o f materials depends

on their microstructures (Chen et al., 2009; Toshio Nakamura, G.Qian, 2000;

Venkataraman et al., 2007; Westergard and Hogmark, 2004a).

38

2.13 Effect of coating thickness on dry sliding wear resistance

The plasma spray technique produces a heterogeneous layered structure o f

thermal sprayed coatings consisting o f Inter-lamella or volumetric pores and weak

interface between splats (Yin et al., 2008).

Due to the high velocity and temperature gradients in the plume, the small

changes in the parameters can cause in momentous changes in the particle properties

and thus change in the microstructure feature. Among these feature, porosity level is

a key parameter describing the anisotropy o f sprayed coatings(Zhao and Ye, 2013),

(Guessasma et al., 2004b).

Kesler et al., (1998) studied the measurement o f residual stress in plasma

sprayed metallic, ceramic and composite coatings. They reported that enhanced

thickness will increase the amount o f residual stress and thus affect the final

properties o f coating. Yin et al., (2010) investigated the influence o f thickness

parameter on porosity, hardness, surface roughness and corrosion resistance o f the

coatings. They reported that the porosity o f plasma sprayed coating was increased

with increasing thickness, which in turn resulted in the decrease o f micro hardness.

Sarikaya, (2005) has investigated the effect o f spraying distance, substrate

temperature, coating thickness and surface roughness on the properties o f the sprayed

coating. He found that the increase o f coating thickness were lowered the hardness

and enhanced the porosity.

39

Holmberg et al., (1998) analysed the material deformation and the influence o f

coating thickness and elastic modulus by three dimensional finite element method.

They concluded that the thicker coating has higher stress concentration compared to

the thin coatings. Higher stress concentration in coating leads to the formation o f

cracks, delamination and surface failure. W ang et al.,(2003) discussed the effect o f

coating thickness on the wear particle generation. They found that wear particle

generation generally increases with increasing the coating thickness.

Thus, there is a correlation between coating thickness parameter and porosity,

hardness and wear resistance o f coating.

2.14 Sealing to improve wear resistance

The porosity is widely considered as the main cause to the low mechanical and

wear performance o f sprayed coatings as compared to the corresponding dense,

sintered materials (Guessasma and Coddet, 2005;Iordanova et al., 1995; Yilmaz,

2009). Hence, a lot o f attention has been paid to reduce the porosity by a variety o f

methods, commonly by optimisation o f the deposition parameters. However, it is

almost impossible to spray perfectly dense, crack free coatings. Therefore, a number

o f post treatments have been used to reduce the porosity. One being laser remelting o f

the coating and other methods used to fill the pores and cracks are immersion o f the

coating in plastic resins, molten metal or aluminium phosphate.

Li et al., (2001) has applied a mixed resin consisting mainly o f poly methyl

silicon and vinyl methyl silicon as a sealant to diminish the effects o f the pores and

micro cracks on the corrosion properties o f the plasma-sprayed Cr203/NiCr dilayer

40

coatings. They reported that this treatment decreased the porosity both in the ceramic

and in NiCr delayer.

Leivo et al., (1997) discussed wear and corrosion properties o f plasma sprayed

AI2O3 and Cr203 coatings sealed by aluminium phosphates. They found that

aluminium phosphate sealing significantly improved the abrasive and erosive wear

resistance o f A I2O3 and Cr203 coatings. An alternative to these methods is to

electrochemically seal the porosity with metal. Westergard and Hogmark, (2004)

investigated tribological properties o f sprayed alumina coatings sealed by electro­

deposition o f Cu.

They reported that Cu was probably not the best choice when reducing wear in

sliding contact. W estergard and Hogmark, (2004) studied to improve the wear

properties o f plasma sprayed alumina by electro-deposited Ni. They concluded that

electro-deposition o f Ni is a promising sealing treatment for sprayed alumina coatings

intended for tribological applications.

2.15 Laser remelting to improve wear resistance

Plasma spraying is commonly used to obtain wear resistant coatings. However,

there are two major problems with plasma spraying. The primary problem is the poor

bonding strength between the coating and the substrate, which causes the sprayed

material to peel o ff under high bending stress or heavy load. The secondary problem

is the high porosity in the as-sprayed coatings, which reduces the wear performance

(Yuanzheng et al., 2000).

In order to improve the tribological behaviour o f plasma sprayed coatings, post

treatment by laser has been developed. Laser treatment may lead to the elimination o f

porosity, enhancement o f the coating strength and chemical homogeneity and the

development o f metallurgical bonding at the coating substrate interface producing41

strengthened coating adhesion. Laser processing can also offer greater flexibility such

as a short processing time, less thermal distortion and less microstructural changes to

the substrate (.Femhdez and Cuetos, 1996; Guo et al., 1995; Mateos et al., 2000).

Mateos et al., (2000) studied the tribological behaviour o f plasma-sprayed WC

coatings with and without laser remelting. The results demonstrate that laser remelting

o f plasma sprayed W C -C o coatings improve their microstructure, microhardness and

coating substrate adherence. Laser remelted coatings are more wear resistant under

dry contact condition.

Femhdez et al.,(1996) investigated the effects o f laser treatment on the wear

behaviour o f plasma sprayed AI2O3 coatings. They reported that better w ear resistance

for laser treated ceramic coating. Fu et al., (1997) studied the effect o f laser remelting

on the wear behaviour o f plasma sprayed ZrOi and ceramic coatings. They concluded

that the porosity and roughness o f the coatings were reduced significantly after laser

treatment and the wear resistance were improved. Liang et al., (2000) studied wear

resistance o f plasma sprayed and laser remelted coatings on aluminium alloy. The

experimental results showed that the laser treated plasma sprayed samples

demonstrated good wear resistance.

Belmondo et al., (1979) analysed wear resistant coatings by laser processing.

They reported that use o f laser processing in the production o f wear resistant coatings

enhances tribological performance and strong bonds to the base metal.

Iwaszko, (2006) studied the surface remelting treatment o f plasma-sprayed

A ^O a+B w t. % Ti02 coatings. The study revealed that reduction in porosity, chemical

composition homogenisation, surface smoothing and increase in the structure

dispersion. Further investigations provided evidence that the coatings improved by

remelting possess enhanced mechanical properties. Yuanzheng et al., (2000)

42

investigated the laser remelting o f plasma sprayed AI2O3 ceramic coatings and

subsequent wear resistance. They reported that the melting layer becomes much

denser and hence, its hardness has been greatly developed despite the existence o f

some cracks. Furthermore, the increase in hardness o f the melting layers is

independent o f coating type and generally increases with the laser energy density.

Laser remelting can greatly improve the wear resistance o f the coating.

2.16 Summary

Thermal spraying holds a novel position in the spectra o f surface modification

technologies because it can provide thick coatings over a large area at a very high

application rate compared with other coating processes such as PVD, CVD and

electroplating.

Plasma spraying is extensively used thermal spraying technique which finds a

lot o f applications due to its versatility o f spraying a wide range o f materials from

metallic to non-metallic. Hence more suitable for spraying o f high melting point

materials like refractory, ceramics and cermet etc. Other key features o f plasma

spraying are the formation o f microstructures with fine, non-columnar and equiaxed

grains, the ability to produce homogeneous coatings that do not change in

composition with thickness and length o f deposition time, the ability to process

materials in virtually any environment. Hence, plasma spraying technique is used to

produce corrosion, erosion, temperature and abrasion resistant coatings.

In plasma spraying, parameters are interrelated with each other which determine

the degree o f particle melting, adhesion strength and deposition efficiency o f the

powder. Plasma spray is formed by the impact o f a stream o f particles from nozzle

striking the substrate surface.

43

The structure o f a particular coating is influenced by velocity, temperature and

size distribution o f the incident particles. The literature presented in the earlier

sections o f this chapter clearly reveals that considerable amount o f research has been

carried out on thermal spray techniques and coating materials.

Today a variety o f materials, e.g., carbides, oxides, metallic, etc. are available

commercially for protecting the metal surface. Material such as molybdenum is also

largely sprayed on steel substrate in order to make use o f their superficial

characteristics such as wear and scuff resistance. Beside with these features it has

better values o f thermal expansion coefficient, thermal conductivity, mechanical

strength, fracture toughness and good adhesion to substrate material. However, the

broad literature survey reveals that the fimctional properties o f plasma sprayed

molybdenum coating depend on its uniformity o f microstructure, phase composition,

and porosity.

The literature survey suggests that the velocity and temperature o f in-flight

particles increased with decreasing the particle size. The velocity and temperature o f

each in-flight particle are primary parameters influencing the microstructure o f

coatings because they noticeably influence the flattening behaviour o f the particles.

The velocity and surface temperature o f the in-flight particles are directly proportional

to the reciprocal o f the porosity. The presence o f porosity can be detrimental to the

wear resistance o f materials. From the literature it can also find that the porosity o f

plasma sprayed coating is increases with increasing thickness which in turn results in

the decrease o f micro hardness and wear resistance o f coating.

The porosity is considered as the major cause for low mechanical and wear

performance o f sprayed coatings as compared to the corresponding dense, sintered

44

materials. The porosity o f plasma sprayed coating can be reduced by a variety o f

methods, one being laser remelting o f the coating and other methods are electro

plating and immersion o f the coating in plastic resins, molten metal or aluminium

phosphate.

From the literature studies it can be made out that not much focused work has

been carried out in understanding the effect o f powder particle size and coating

thickness on wear behaviour o f plasma sprayed molybdenum coating.

The plasma sprayed molybdenum coating show high potential for automotive,

aerospace, pulp and paper industries due to their outstanding wear resistance property.

Throughout the literature we can find microstructural studies, variation in hardness

and evaluating wear property has been considered for characterization o f plasma

sprayed molybdenum coating. From the literature available it can also find that very

little work is reported on sealing and laser remelting o f plasma sprayed molybdenum

coating.

2.17 Objective of Research Work

Based on the literature study, scope for research in the arena o f plasma spray

molybdenum coating is proposed as below:

• To investigate the effect o f particle size and coating thickness on dry sliding

wear behaviour o f plasma sprayed Mo coating.

• To investigate the effects o f post treatments such as sealing and laser

remelting on dry sliding wear behaviour.

• To establish the correlation between wear volume loss and dry sliding wear

parameters.

• To analyze the obtained results in order to determine the significant

parameters influence on wear volume loss.45

2.18 Problem statement

In view o f the scope defined above, the present research work is defined as

‘'Investigations on Wear Behavior, Surface and Subsurface Alterations on Coated

M aterials'\

2.19 Research Methodology

To accomplish the research objective, the research is conducted through the

following approach:

1. Based on the literature survey and an experimentation o f prior experimental

studies, a methodology was developed to conduct the experiments.

2. Experimental dry sliding wear tests were employed to investigate the effect o f

particle size and coating thickness on wear resistance.

3. Dry sliding wear tests were conducted to investigate effect o f post treatments

on wear resistance o f sprayed coatings.

4. The effect o f dry sliding wear process parameters on the wear volume loss o f

sprayed coating was studied.

5. The worn surface o f coating was investigated through SEM and Xfd analysis.

6. Statistical analysis was carried out to determine the significant influencing

factors and their interactions on the response.

7. The regression models were presented and validated.

The present study would help to understand the effect o f process variables on

wear resistance and wear mechanism. The study also offers scientific method to

predict the w ear volume loss in dry sliding wear o f plasma sprayed coatings.

46