Chapter 2

Theory and Literature Review

2.1 Thermal spraying

Thermal spray processing is a well establish means of forming coating of

thicknesses greater than about 50 microns or we can call thick coating. A wide rang

of material can be thermal sprayed for a variety of applications, number of

applications for thermal spray runs in to the thousands throughout the aerospace,

industrial gas turbines, automotive and industrial market sectors. Thermal spray

coatings have been produced for at least 40 years, but the last decade has seen a vitual

revolution in the capability of the technology to produced truly high performance

coatings of great range of materials on many different substrates. This enhancement

of the technology has been achived largely through the introduction of new spray

techniques, the enhacnement of spray process controls, by employing state of the art

methods of feedstock materials production, and althrogh the used of modern

techniques of quality assurance. Although the use of advanced thermal spray coating

method has larglely occured within the aircraft industry, newer, extended applications

of the technique have demonstrated its versatility [15]. Application includes

protections from wear, high temperatures, chemical attack, and the more mundane

uses of environmental corrosion protection in infrastructure maintenance engineering.

From many applications, thermal spraying technique also has the advantages that the

industrial market required which are following [16].

1. An extremely wide variety of substrate and coating material selections can be

made. Coating can applied over large areas. Virtually any material that melts

without decomposing can be sprayed.

2. The coatings are inexpensive and relatively simple to produce compared to

chrome plating and many of them provide excellent wear and corrosion

resistane.

3. The coating is applied without significantly heating the substrate which below

1500 at a fast deposition rate. Thus, materials with very high melting points

6

can be applied to finely machined, fully heat-treated parts without changing

the properties and thermal distortion of the parts.

4. Lower maintenance cost, extension of service life, better product quality, and

higher production capacity can be achieved.

5. It is possible, in some cases, to strip with high-pressure water jets or other

methods and recoat worn or damaged coatings without changing the properties

or dimensions of the part.

6. Thickness of modified layer can be in order of mm.

7. It is an appropriate process for the surface molidification of Al alloys and for

improvement in the functionality of mechanical equipment by imparting

necessary properties to their surface alone.

8. Unlike chrome plaing, it is an environmentally friendly process.

9. In some case, it is possible to form near net and free standing shaped parts.

10. It can be applied to welded surface to provide additional protection to the

componets.

2.1.1 Thermal spraying systems

Thermal spray coating processes differ mostly in the manner by which the

droplets of the material to be deposited are heated and accelerated. In most cases, the

sprayed surfaces should be cleaned and degreased, masked, and roughened by grit

blasting prior to spraying to ensure adequate bond strength between the coating and

the substrate. The various thermal spray processes are distinguished on the basis of

the feedstock characteristics (wire/powder) and the heat source employed for melting

[17]. In the following are described the various thermal spray processes in use today.

2.1.1.1 Frame spraying

Flame spraying, the oldest form of thermal spraying, has been used since the

late 1800's. Flame spraying technique may be used to apply a wide variety of

feedstock materials including metal wires, ceramic rods, and metallic and nonmetallic

powders. Flame spraying use the heat of combustion of a fuel gases and oxygen

mixture to melt the feed stock material, common fuel gases are acetylene, propane,

and methyl acetylene-propadiene. They are mixed either with oxygen or air and

7

undergoes combustion external to the nozzle, however flame temperature and

characteristics are function of the oxygen and fuel gase ratio (O2:fuel gase) as shows

in Table 1. Feed stock material is fed directly into the flame by a stream of

compressed air or innert gas such as argon (Ar2) or nitrogen (N2). The coating is

heated to near or above its melting point after that the melted coating material is

atomized by combustion of the coating material and propelled to the surface

workpiece to form the coating [18].

The main advantages of these processes are low capital investment costs and

ease of operation. Because of the relatively small size of the equipment and the ease

of operation, the process is field-portable, and there is little restriction to the size and

complexity of components that can be coated. However, flame spraying is probably

the simplest of all the spray processes beside that flame spray guns are lighter and

smaller than other types of thermal spray guns [19]. The two consumable types give

to the two process including powder flame spraying and wire flame spraying.

2.1.1.1.1 Powder flame spraying.

A thermal spray process in which the material to be sprayed is in powder form.

In the flame powder process as shown in Figure 2.1, powdered feedstock is aspirated

into the oxyfuel flame, melted, and carried by the flame and air jets to the workpiece.

Particle speed is relatively low less than100 m/s, and bond strength of the deposits is

generally lower than the higher velocity processes. Porosity can be high and cohesive

strength is also generally lower. Spray rates are usually in the 0.5 to 9 kg/h range for

all but the lower melting point materials, which spray at significantly higher rates.

Substrate surface temperatures can run quite high because of flame impingement [20].

2.1.1.12 Wire flame spraying

Figure 2.2 shows wire flame spraying, a spray process in which the feed stock is in

wire or rod form. Wire flame operates in much the same way as powder flame sprays

except that a wire flame spray gun consists of a drive unit with a motor and drive

rollers for feeding the wire. In wire flame spraying, the primary function of the flame

is to melt the wire feedstock material. A stream of air then atomizes the molten

material and propels it toward the work piece. Spray rates for materials such as

8

stainless steel are in the range of 0.5 to 9 kg/h. Again, lower melting point materials

such as Zn and Zn alloys spray at much higher rates. Substrate temperatures often

range from 95 to 205 0C because of the excess energy input required for flame

melting. Compared with arc spraying, wire flame spraying is generally slower and

more costly because of the relatively high cost of the oxygen-fuel gas mixture

compared with the cost of electricity [21].

2.1.1.2 Electric Arc Spraying

An Electric arc spraying is a thermal spray process in which an arc is struck

between two consumable electrodes of a coating material. Compressed gas is used to

atomize and propel the material to the substrate. This process in Figure 2.3 has been

used for spraying Al or Zn on steel structures to protect them in marine environment.

For best performance the coating is usually sealed with organic compounds [22]. Arc

wire thermal sprayed coatings bear a similarity to flame wire coatings in that both are

applied by completely melting and atomizing the coating material before projecting

the droplets onto the work surface. Rather than using an oxy-fuel flame to liquefy the

metal, an arc wire system uses the heat of an electric arc. An electrical current

generate thermal energy that is used to melt materials [23]. A principal is that the

wire arc process uses two conductive metal wires as the feedstock. These two metal

wires act as two electrodes that are continuously consumed as the tips melt due to an

electrical arc that is struck between them. In order for this to happen, two wires are

electrically charged, one positive and one negative, and moved forward on an angle so

the distance between the wires is gradually reduced. A potential difference of between

15 to 50 Volts is applied, and then before the wires actually would touch, the arc is

struck. The heat that is generated melts the tips of the wires, and an atomizing gas

shears off these molten droplets and propels them towards the substrate [24]. The

atomizing gas is usually compressed air but it can also be done with an inert gas such

as nitrogen or argon. By using compressed air, part of the sprayed metals or alloys is

oxidized resulting in a large amount of metal oxides in the coating. Because of this,

the coating is harder and more difficult to machine than the material the coating

originated from. This can be a disadvantage because some coatings have to be ground.

However, the increased hardness can also be beneficial with respect to improved wear

9

resistance [25]. The temperature of the arc by far exceeds the melting temperatures of

the sprayed materials and results in superheated particles. These extremely high

particle temperatures can in some cases result in localized metallurgical interactions

or diffusion zones. Due to these microscopic processes, good cohesive and adhesive

strengths can be achieved using the wire arc process. The spray rate is dependent on

the applied current and can be as high as 40 lbs [26].

2.1.1.3 The High Velocity Oxygen-Fuel (HVOF)

The High Velocity Oxygen-Fuel (HVOF) Thermal spray process belongs to

those characterized by high energy. By accelerating the coating particles to supersonic

speed, the process achieves a remarkably high degree of bond strength at the substrate

interface, and a very limited level of porosity. In the early 1980's Browning and

Witfield, using rocket engine technologies, introduced a unique method of spraying

metal powders. The technique was referred to as High Velocity Oxy-Fuel (HVOF).

HVOF spraying differs from conventional flame spraying in that the combustion

process is internal, and the gas flow fates and delivery pressures are much higher than

those in the atmospheric burning flame spraying processes [27]. In HVOF ,fuel

usually hydrogen, propane, propylene, hydrogen and even kerosene, is mixed with

oxygen and continuously burned in a combustion, which created a suppersonic

velocity of about 1800 m/s, with a characteristic multiple shock diamond pattern that

is visible in the flame. Flame speeds of 2000 m/s and particle velocities of 600 to 800

m/s are claimed by some HVOF equibment suppliers. Powder is introduced mostly

axially into the nozzel with a powder feed rate of 20 to 80 g/min as suspension in the

carrier gas and its heated and accelerated by the hot gas flow forward to the substrate

to form a coating. The HVOF process in Figure 2.4 produces exceptionally high

quality cermet coatings such as WC-Co coatings for fretting wear resistance on

aeroengine turbine components, but it is now also used to produce coatings of metals,

alloys and ceramics. The porosity of HVOF sprayed coating is lower than one

percent, typically thickness is in the range of 100-300 µm [28].

10

2.1.1.4 Plasma Spraying

Plasma spraying is a thermal spray process in which a nontransterred arc is a

source of heat that ionizes a gas which melts the coating material and propels it to the

workpiece. The plasma spray process (Figure 2.5) uses a so-called plasma gun or

torch to generate an arc, which creates the plasma. Plasma spraying involves the

introduction of a flow of gas between a water-cooled copper anode and a tungsten

cathode [29]. Typical plasma forming gases include nitrogen, hydrogen, helium

argon and argon based such as argon-hydrogen, argon-helium, nitrogen-hydrogen.

The arc is struck between a water cooled copper anode and a tungsten cathode.

Injected into this arc is a continuous flow of argon gas. The electrical arc ionizes the

Argon and the plasma is formed. Plasma is a conductive gas and is sometimes

referred to as the "fourth-state-of-matter" (the other states being solids, liquids, and

gasses). The internal working temperatures of plasmas are extremely high (around

10,000°C), but little heat is transferred thereby keeping the part relatively cool. For

example, the process temperature of a 20-lb. part will stay around 200°F (100°C).

However, due to the high internal operating temperature, this process is ideally suited

to spray materials with high melting temperatures such as ceramics and refractory

metals. The plasma process is generally operated at energies in the neighborhood of

40-100 kWatts [30]. The plasma heats the powder coating to a molten state.

Compressed gas propels the material to the workpiece at high speeds. Materials

suitable for plasma spraying include Zn, Al, Cu alloys, Pb, Mo, some steels, and

numerous ceramic materials. Platters can use plasma spraying to achieve thicknesses

from 0.3 to 6 millimeters depending on the coating and substrate materials.

Companies can use plasma spraying to deposit molybdenum and chromium on piston

rings, cobalt alloy on jet engine combustion chambers, tungsten carbide on the blades

of electric knives, and wear coatings on computer parts (Kirk-Othmer 1987) [31].

2.1.1.5 Detonation gun

In the early 1950's Gfeller and Baiker, employees of Union Carbide

Corporation, Linde Division, developed concepts of using explosions in a unique

manner. Their concept was to introduce powdered materials into detonation or shock

waves. The "waves" are produced by igniting a mixture of acetylene and oxygen into

11

the detonation chamber which is opened to a one meter long tube two and one-half

centimeters in diameter. The system in Figure 2.6 is complex. In operation, a

mixture of spray material, acetylene and oxygen is injected into the detonation

chamber. Combustion gases can be neutral, reducing or oxidizing and can have their

temperature controlled by the addition of an inert gas, for cooling, or hydrogen to heat

it. The procedure is initiated by a gas/powder metering system that measures and

delivers the mixture to the chamber where it is ignited. After ignition, a detonation

wave accelerates and heats the entrained powder particles. The resulting shock wave

accelerates the powder particles to over 731 m/sec and produces temperatures in

excess of 4,000°C. The obtained particle velocities are high. Consequently, the

coatings are dense and exhibit high bond strengths. Pressures from the detonation

close the controlling valves until the chamber pressure is equalized. When this occurs

the cycle may be repeated either 4 or 8 times per second. There is a nitrogen purge

between cycles. The process produces noise levels that can exceed 140 decibels and

requires special sound and explosion proof rooms. The coatings produced through this

process are of excellent quality. The drawback is that the process is relatively

expensive to operate. Detonation coatings are designed for applying hard materials,

especially carbides, on surfaces subject to aggressive wear [32-33].

2.1.1.6 Cold spray

Cold spray utilizes kinetic energy to project particles at a prepared surface.

The extreme velocities cause plastic deformation of the particles on impact, which in

turn creates very dense coatings. The cold Spray process as shown in Figure 2.7

basically uses the energy stored in high pressure compressed gas to propel fine

powder particles at very high velocities (500 - 1500 m/s). Compressed gas (usually

helium) is fed via a heating unit to the gun where the gas exits through a specially

designed nozzle at very high velocity. Compressed gas is also fed via a high pressure

powder feeder to introduce powder material into the high velocity gas jet [34]. The

powder particles are accelerated and moderately heated to a certain velocity and

temperature where on impact with a substrate they deform and bond to form a coating.

As with the other processes a fine balance between particle size, density, temperature

and velocity are important criteria to achieve the desired coating. The particles

12

remain in the solid state and are relatively cold, so the bulk reaction on impact is solid

state only. The process imparts little to no oxidation to the spray material, so

surfaces stay clean which aids bonding. No melting and relatively low temperatures

result in very low shrinkage on cooling, plus with the high strain induced on impact,

the coatings tend to be stressed in compression and not in tension like liquid/solid

state reactions of most of the other thermal spray processes. Low temperatures also

aid in retaining the original powder chemistry and phases in the coating, with only

changes due deformation and cold working. The advantage of cold spray versus the

"hot" spray processes, which melt or soften the feedstock, is a significantly reduced

level of coating oxidation. Electrical conductivity of cold sprayed Cu has been

reported at about 90% of wrought material and a significant increase over the <50%

typical for other sprayed Cu deposits [35]. Cold spray coatings also exhibit improved

adhesion, reduced material loss by vaporization, low gas entrapment, insignificant

grain growth and recrystallization, low residual stress, phase and compositional

stability, reduced masking requirements and improved surface finishes. Applications

for cold spray coatings include corrosion protection, where the absence of process-

induced oxidation may offer improved performance, deposition of electrical

conductors and solders; and, the application of metallic coatings to ceramic and glass

substrates [36].

2.1.2 Coating build-up

Thermal spraying technique is a group of processes which a spray material is

heated to molten or semi-molten state and propelled as in-flight particles or droplets

onto a grinding surface substrate. The thermal spray gun generates the necessary heat

source by using combustion, electric or plasma energy. As the deposited materials are

heated, they are changed to a plastic or molten state and are accelerated by a

compressed air/gas, subsequently, atomizes the material into in-flight particle and

carries it at sufficient speed to allow it to deposit onto a grit blasted substrate [37].

The in-flight particles strike the prepared substrate, flatten, and form thin splats that

conform and adhere to the irregularities of the substrate and to each other. As the

sprayed particles impinge upon the surface, solidificate and build up coating, splat by

splat, into a laminar structure forming the thermal spray coating. Figure 2.8

13

illustrates a typical coating cross section of the lamella structure of oxides and

inclusions. The coating that is formed is not homogenous and typically contains a

certain degree of porosity and in the case of sprayed metals, the coating will contain

oxides of the metal. Feedstock material may be any substance that can be melted,

including metals, metallic compounds, cements, oxides, glasses, and polymers [38].

Feedstock materials can be sprayed as powders, wires, or rods as shown in Figure 2.9.

The bond between the substrate and the coating may be mechanical, chemical, or

metallurgical or a combination of these. Thermal spray coatings are intended to

enhance a substrate’s performance by improving its ability to resist wear (material

degradation), oxidation, corrosion, and/or to protect it from high temperatures. The

coating is also cost effective ways to enhance the performance of the component’s

surface. The process and the selection of the process and material are determined by

specific industrial application and properties of the applied coating are dependent on

the feedstock material, the thermal spray applications.

2.1.3 In-flight behaviour

The chemical process of thermal spray coating such as oxidation and

vaporisation which take place in any kind of thermal spray process. When the hot

flame exit the thermal spray torch, cold ambient air is entrained in the flame spray.

This can trigger either oxidation or chemical reaction of vaporised material with

elements in the air. In plasma spraying where the highest temperature are reached,

evaporation is more likely to happen [49]. Different thermal spray technique produced

differnt temperature of energy source that effected on fully melting feeding material.

In process, where the velocity is high such as HVOF produced small in-flight particles

and dwell time will be short and so degredation of essentail elements in in-fligth or

interaction with the sourrouding atmosphere will be less, oxidation only have a few

time to take place at the outer shell of in-fligth particels and thus the oxide level in

coating will be low [40]. In arc spray or low velocity technique in-flight particle stay

longer in the air, so oxidation and/or degradation of in-flight material will be higher.

Even for solid in-flight particle, the oxide will be greater because of the high

reactivity of ionise oxygen entrained from the asmoshere. Additionally, as an in-

flight particle can fully melt in high temperature process, covection process inside the

14

particle then control oxidation, resulting in high oxide content in the coatings [41]. In

the same time that in-flight oxidation proceeds, oxidation of the previously deposited

layer take place. Oxidation of layer depends on the time that the hot steam remains

over a specific area but it increase as the substrate temperature increase and especially

if the spray distace is close to the substrate. As you have seen in the case before, the

spraying distance plays an imporant role in thermal spraying. Thermal exchanged will

be greater with longer spraying distance because the dwell time of in-fligth particle

inside the hot steam will be greater. In-flights particle behavior with in spray steam

has a significant effect on the thermal spray lamella sturcture. From that reason, the

good coating microstructure also can controll through the factor which is effected to

in-flight particle characteristic, such as temperature of heat energy source where as

effected on the melted in-flight particle of the coating materail. However, the in-

flight particle size distribution of spray material must be requiredd to avoid non-

melted in-flight particles in thermal spray coating [42].

2.1.4 Single splat formation

Thermal sprayed deposite are comprised of cohesively bonded, splat which

result form high rate impact and rapid solidification of a high flux of flame melted

particles. The physical properties and behaviour of such a deposite will be expected

to depend on the cohesive strenghts among the splat, the size and morphology of the

porosity, the occurrence of cracks and defects and, finally, on the ultra fine grained

microstructure within the splats themselves [43]. The microstructures of the

thermally sprayed deposits are ultimately based on the solidification of many

individual melten droplets. An individual single splat results when a droplet of

molten material, tens of micrometers in diameter, melted in the flame, strike a surface

substrate, then flattens out and finally solidifies as shown in Figure 2.10. First of all,

the melted particle or in-fligh particle is propelled out from the thermal spray torch in

the form of shere, second of all, its impact the substrate suface which is creates a

shock wave inside itself and in the substrate. After that, the in-flight particle expands

in a redial direction which is producing the different form of a single splat shape [44].

A pancake shape of splat associated with moderated particle velocity and moderate

heat content of splat. A flower splat shape is connected with an elevated velocity of

15

the particle and elevated heat content. A disk shape of splat is also found when the

solidification process takes place. Shemeatic of three different types of splat are

shown in Figure 2.11. The collection of many individual splats forms the deposit.

There are numerous considerations relative to the dynamics of deposit evolution

during thermal spraying. The mechanics or physical aspects of splat formation deals

with the spreading of the molten droplet interact with the substrate. These

charactersistic are affected by the temperature of the splat, the splat viscosity, surface

tension, as well as other considerations. Splat morphology will depend on a variety of

the things, the most important of which are particle velocity, temperature, diameter

and substrate surface profile. Further consideration involve the physical properties of

the splat, which deal with cooling rate, solidification criteria, nucleation and growth

of the crystals, phase formation. The above aspect of splat formation and

solidification are complex and interrelated [45].

2.1.5 Splat shape

Splat shape influences the packing of structure. It can also influence the

porosity and oxide content. The shape of the splat is usually decribe in term of the

shape factor, which provides some measure of the deviation form an idealized

geometry. For cylindrical symmetry, the most commonway of representing the shape

factor is in the terms of aspect ratio.

2

4. . SS FPπ

= ....................... (2.1)

Where S is the surface of the splat, and P is its perimeter. For aspect ratio less than 1,

resulted in the oblate spheroids and if spat shape factor increase to 1, then elongated

ellipsoids was created [46]. However, different kind of splashing phenomina were

observed. For more understanding of spashing phenomina, the shape factor of splat

on a plannar section was also observeb, which can be presented in equation (2) to (4).

( ). . 4 /E D A π= × ............(2.2)

16

2

. .4

LE FA

π= × ......................(2.3)

21. .4

PD SAπ

= × ...................... (2.4)

E.D., defined as the equivalent diameter of splat with the same area as the selected

feature. An elongation shape factor of coating (E.F.) this is presented the planar

shape of splat. D.S. or a degree of splashing caculated form measuring the peripheral

projection of material at the impack. Where A is the splat area (mm2), L is the longest

dimesion and P is the perimeter of splat (mm).

A schematically of different type of splat shape factor are shown in Figure

2.12. As thermal spray coating produce solid deposits by impigement and

solidification of molten metal droplets onto a substrate. When a liquid droplet hits a

solid surface it may, depending on impact conditions, spash and disintergrate into

many small satellite droplets. Droplet splashing is undesirable in most applications

since it not only results in wastage of material but also produces pores in the deposite

and reduces its strength. Because of its practical importance, many studies have been

devoted to investigate molten metal droplet spashing [47, 48]. Droplet splashing is a

commonly observed phenomenal in many metallurgical processes, which involve the

impact of molten droplets on a substrate or pre-deposite materials, such as splat

quenching. A fundamental understanding of splashing mechanism is not only of

scienctific interest, but also believe that flattening process of thermal spray droplet is

generally detrimental to coating quality. The flattening process of a spray droplet

impacting on a substrate is one of the most important fundamental processes during

the formation of thermal spray coating. The structure of splat will determine the

structure of the coating [49]. The ratio of the splat diameter to the initial droplet

diameter represents a mathematicaly quntity refered to as the” flattening ratio” and is

indicative of the level of viscous dissipation and surface tension encountered during

the spreading process. Flatteing ration in the flattening process is given by

methematically equation as below [50].

17

Dd

ξ = ............................ (2.5)

Where D is the splat diameter and d is the initial droplet diameter.

2.2 Coating Characterization

As indicated above, thermal spray deposition involves the quasi-continuous

rapid deposition of molten droplets. This process results in highly defected

microstrucures, including porosity, thus coating testing a characterization of themal

spray coating are required [51].

2.2.1 Thickness

Coating thickness is an important variable that plays a role in product quality,

process control and cost control. Measurement of coating thickness can be done with

many different instruments. Such as R. Ghafouri-Azar examined the coating thickness

of stainless steel and tungsten carbide cobalt onto stainless steel using a high velocity

oxy-fuel (HVOF) by scanning electron microscope, so did J. Stokes. Beside that

standard point to point measurement are often done with magnetic gages or eddy

current also found [52]. This method is convenience and easily, so we used this

method to measure our coating thickness. Understanding the equipment that is

available for coating thickness measurement and how to use it is useful to every

coating operation.

The thickness of spray coating can be determined simply from the effect of

liftoff on impedance of eddy current technique (Figure2. 13). The coating serves as a

spacer between the probe and the conductive surface. As the distance between the

conductive base metal increases, the eddy current field strength decreases because less

of the probe’s magnetic field can interact with the base metal. As with a magnetic

induction probe, the eddy current method also contains a coil. In this case the coil

has the dual function of excitation and measurement. This probe coil is driven by a

high-frequency oscillator to generate an alternating high-frequency field. When near

a metallic conductor, eddy currents are generated in the conductive material. This

causes an impedance change in the probe coil. The distance between the probe coil

18

and the conductive substrate material determines the amont of impedance change,

which can be measured, correlated to a coating thickness and displayed in the form of

a digital reading [53]. Fairly precise measurement can be made with a standard eddy

current flaw detector and a calibration specimen as shown in Figure 2.14. The

referrence standard should be of the same materail that use as substrate, or if it is not

possible, it shold be of material that has the same electrical conductivity and meganeti

permeability. Specialized eddy current coating thickness detectors are also availible

and are often pocket size with the probe resemblin a small pencil [57]. They are

usually operated by a small bettery and provide a digital read out in the appropiate

units.

2.2.2 Roughness

Surface roughness is of great importance in many areas of industrial

metrology. Tribology, contact mechanics, fluid flow, conting technology, obtics,

electronics and bio-engineering are areas, where roughness is of importance and

highly affects the function of a surface [54]. Therefore, surface roughness becomes

one of the importance qualities that we should considered. Generally, surface

roughness measurement has been performed by using a stylus instrument, where as a

stylus is drawn along the surface and the vertical movement of the stylus is used to

calculate surface roughness parameters such as Ra [55-57]. The principle instruments

used to study surface shape are the scaning electron microscope (SEM) and the

profiles analyzer. The SEM can provide micrographs with sufficient resolution to

reveal individual detail and has a large enough features can be seen in the practical

triboloy. However, SEM can not qualify roughness, thus profilometer is important to

describe surface roughness of material. Surface roughness is best understood in term

of a surface profile, which is the counter of surface in plane perpendicular to the

surface. Figure 2.15 shows a surface roughness profile in parallel plane. The sliding

and measuring the height is calling a diamond stylus-type profilometer. It is capable

of producing a first signal proportional to the height of the couter relative to

referrence surface and a second signal proportion to the length traveled along the

surface that can. A surface profile is taken a long given straight line path on a surface.

In gerneral, a characteristic of surface roughness is one line will be statiscally similar

19

to those of another parallel line given a uniform surface production process. Some

case, a profile has little chance of actually passing throught the peak of the asperities,

what is seen in the profile is a traverse across the sides of the peak ingerneral [58].

The average roughness parameter, Ra is used most frequently and is defined as a

arithmetic average of the abosute values of measured roughness heigh deviations,

y(x), from the mean within an assesment length, L:

0

1 ( )L

aR Y x dyL

= ∫ .................... (2.6)

When calculated from digital data, Ra is usually estimated by a trapezoidal rule.

1

1 Na n

RN =

= ∑ nr .................... (2.7)

Furthermore, if a profile is taken a long a line in diferrence direction, the surface

roughness results may be diferent, if the surface producing method has direction

characteristics [59]. Thus one must be decided in what direction the roughness in

importance.

2.2.3 Porosity and Oxide

The easy way to observe porosity and oxide in coating is an optical

microscope, which is usually used to characterize basic structure information of

coating, this is also inexpensive technique. Fundamental information from optical

microscope for thermal spray coating usually adhesion between substrate and coating,

deformation of substrate, coating porosity, thickness, shape and size in case in-flight

or splat [60]. The size distribution of in-flight or splat can be measured by using an

optical microscope. The optical components of a microscope are its two imaging

lenes (eyepiece and objective) and a condenser lens. The eyepiece and objective are

responsible for magnifying the image of the specimen and projecting it onto the

viewer’s retina or onto the film plane in a camera. The job of the condenser lens is to

focus a cone of incident light onto the specimen. To provide the incident light or may

direct external natural or artificial light towards the condenser lens. It can also

20

provide means for enhancing the contrast and detail seen in the image. Finally, there

is a moveable stage which holds the specimen in the optical path and allows the

specimen to be moved in and out of the focal plane and even left, right and rotated

about the optic axis. To complete the instrument the microscope may include other

attachments such as camera, a viewing screen [61]. An optical microscopy is

beneficial is a quantification of the metallographical study with cooperated of image

analysis. An image analysis system usually has image analysis software and

capturing devices. Captured images can be transferred from any of device such as

optical microscopes in this case, scanning electron microscope (SEM) or computer

scanners. After completing the image processing, the primary mode of feature

detection is gray level thesholdeing. Several procedures, ranging from automatic to

manual thesholding, may be available for feature detection. For optimum detection

accuacy, the sample feature of interest must be treated to have as narrow a contrast

(gray-level) range as possible. The light source in the microscope must then be

aligned for even illumination. Most image analyzers have circuits to provide shading

correction to even out variations across the screen. Through the production and

reprocessing this gray image a binary image is quantitively generated manually or

automatically by segmenting the regions of interest. The operator then selects which

features from the binary representation are important to measure. After completing

the image processing, the image was store in exel format and analysed for obtained

the porosity or oxide content in the case of thermal spray coating [62-64].

2.2.4 Hardness

The metals handbook defines hardness as "resistance of metal to plastic

deformation, usually by indentation. However, the term may also refer to stiffness or

temper or to resistance to scratching, abrasion, or cutting. It is the property of a metal,

which gives it the ability to resist being permanently, deformed (bent, broken, or have

its shape changed), when a load is applied. The greater the hardness of the metal, the

greater resistance it has to deformation [65]. Hardness measurement can be defined

as macro-, micro-scale according to the forces applied and displacements obtained.

Measurement of the macro-hardness of materials is a quick and simple method of

obtaining mechanical property data for the bulk material from a small sample. It is

21

also widely used for the quality control of surface treatments processes. However,

when concerned with coatings and surface properties of importance to friction and

wear processes for instance, the macro-indentation depth would be too large relative

to the surface scale features. Where materials have a fine microstructure, are multi-

phase, non-homogeneous or prone to cracking, macro-hardness measurements will be

highly variable and will not identify individual surface features. It is here that micro-

hardness measurements are appropriate [66, 67]. Micro hardness is the hardness of a

material as determined by forcing an indenter such as a vickers or knoop indenter into

the surface of the material under 15 to 1000 gf load; usually, the indentations are so

small that they must be measured with a microscope. Capable of determining

hardness of different microconstituents within a structure, or measuring steep

hardness gradients such as those encountered in casehardening. Conversions from

microhardness values to tensile strength and other hardness scales are available for

many metals and alloys [68, 69]. There are three types of tests used with accuracy by

the metals industry; they are the brinell hardness test, the rockwell hardness test, and

the vickers hardness test. Since the definitions of metallurgic ultimate strength and

hardness are rather similar, it can generally be assumed that a strong metal is also a

hard metal. The way the three of these hardness tests measure a metal's hardness is to

determine the metal's resistance to the penetration of a non-deformable ball or cone.

The tests determine the depth which such a ball or cone will sink into the metal, under

a given load, within a specific period of time. The followings is the most common

hardness test methods used in thermal spray coatings in today`s technology: In the

vicker’s microhardness test, the indenter is a pyramidal shape diamond with an

included angle of 1360 between opposite faces and the resulting impression is

observed under a microscope and the diagonal lenght were measured as shown in

Figure 2.16. The indentation is applied into the surface of the material with loads

usaully ranging from 1 to 1000 g under the action of a static load for 10 to 15 seconds

[70, 71]. The hardness is quoted in vickers’s pyramid hardness humber; Hd.

22

2

2 sin 2d

FH

d

θ= .................. (2.8)

Where F is applied force, θ is angle between opposite pyramid faces, and d is mean

lenght of indentation diagonals. For a standard pyramid angle of 1360

2

1.854d

FHd

= ................ (2.9)

The diagonal length of impression is measured to determind the vicker hardness, thus

careful specimen surface preparation such as grinding and polishing may be necessary

to ensure dwell defined indentation that may be accurately measured. The vickers

test offers advantages such as all indentation made are geometrically similar and the

hardness value obtained is independent of the value indenting force applied, also

because there is no plastic deformation of the diamond, this test may be used for very

hard metals[72].

2.2.5 Scanning electron microscope (SEM)

Scanning electron microscope or SEM is a very popular technique used in

material science and engineering as well as in industry for microsctructure analysis at

high magnification. Beside that SEM has a large depth of field, which allows a large

amount of the sample to be in focus at one time. The SEM also provides morphology

and chemical composition of sample at high resolution. Peparation of the samples is

relatively easy since most SEMs only require the sample to be conductive. The

combination of higher magnification, larger depth of focus, greater resolution, and

ease of sample observation makes the SEM one of the most heavily used instruments

in research area today. Especially coating technology researched [73]. The SEM uses

an electron beam produced by a tungsten-hairpin filament, a LaB6 tip or a field

emission gun. Electrons from the electron source are accelerated to high energies

and focused though a system of electromagnetic lenes onto the sample. The energetic

electrons in the microscope strike the sample and various reactions can occur as

shown in Figure 2.17. In the SEM, the signals of interesting signals are the

secondsary and backscattered electrons, sicne these vary according to difference in

23

surface topograhphy as the electron beam sweeps across the specimen. Other signals

may be use to used to measure different properties such as auger electrons or

characteristic x-ray is normally used to determine the chemical composition of sample

[74]. The SEM is useful to study the surface, or near surface structure of bulk

specimens. It employs a beam of electrons directed at the specimen and show image

of the specimen surface on the cathode ray tube (CRT). The SEM composed of the

electron gun, condenser lenes, vaccum system, and the cathode ray tube. An electron

gun usually of the tungsten filament thermionic emission type, produces electron, and

accelerates them to an energy between 1 Kev and 30 Kev. Two or three condenser

lenes then demagnify the electron beam until, as it may have a diameter of only 2-10

nm. The fine beam of electron is scanned across the specimen by the scan coils, while

a detector counts the number of low energy secondary electrons, or other radiation,

given off from each point on the surface. At the same time, the spot of a cathode ray

tube is scanned across the screen, while the brightness of the spot is modulated by an

amplified current from the detector. The electron beam and the CRT spot are both

scanned in similar way to the television reciver that is, in a rectangular set of straight

lines known as a raster. The mechanism by which the image is magnified is then

extreamly simple and involves no lenes at all. The raster scanned by the electron

beam on the specimen is made smaller than the raster displayed on the CRT. The

linear magnification is then the side lenght of the CRT devided by the side lenght of

the raster on the specimen. For example, if the electron beam is made to scan a raster

10µm x 10µm on the specimen, and the image is displayed on a CRT screen 100nm x

100nm, the linear magnification will be 10,000x. Alternatively, or sometimes

simultaneously, on a seperate waveform monitor, the microscope can display the

variation of signal with beam position for the carrent raster line. Some SEM

instruments have the very valuable additional feature of providing an electron analysis

of sample composition. When a sample in the microscope is bombarded with high

energy electrons, many things can happen, including the generation of X-rays. The

X-rays are characteristic emission spectra of element. By scanning either the

wavelength (wavelenght dispersive, WD) or the energy (energy dispersive, ED) of the

emitted X-rays, it is possible to identify the elements presented in the samples with

suitable calibration, quantitative elemental analysis may be made.

24

2.3 Wear

Wear is surface damage or removal of material from one or both of two solid

surfaces in a sliding, rolling, or impact motion relative to one another. In most case,

wear occurs through surface interactions at asperities. During relative motion, first,

material on the contacting surface may be displaced so that properties of the solid

body, at least at or near the surface, are altered, but little or no material is actually lost.

Then, material may be removed from a surface and result in the transfer to the mating

surface or may break loose as a wear particle. In the case of transfer from one surface

to another, mass loss of the interface is zero, although one of the surfaces is worn.

Wear damage precedes actual loss of material, and it may also occur independently.

Definition of wear is usually based on loss of material, but it should be emphasized

that damage due to material displacement on a given body with no net change in

weight or volume, also constitutes wear [75]. Wear can be either good or bad.

Examples of wear are writing with a pencil, machining, polishing, and shaving, which

require controlled wear. Wear is undesirable in almost all machine applications such

as gears, bearings, and seals. Components may need replacement after a relatively

small amount of material has been removd or if the surface is unduly roughened [76].

In many cased, wear is initiated by one mechanism and it may followed by other wear

mechanism. Wear can be divided into six principal including Adhesive wear,

Abrasive wear, Fatigue wear, Fretting wear, Erosive wear, and Corrosive wear.

2.3.1 Adhesive wear

Adhesive wear is wear by transference of material from one surface or another

during relative motion due to a process of solid-phase welding or wear due to

localized bonding between contacting solid surfaces leading to material transfer

between two surfaces or loss from either surface as shown in Figure 2.18 Adhesive

wear processes are initiated by the interfacial adhesive junctioins that form if solid

materials are in contact on an atomic scale. As a normal load is applied, local

pressure at the asperities becomes extremely high. In some cases, the yield stress is

exceeded and the asperities deform plastically until the real area of contact has

increased sufficiently to support the applied load. In the absence of surface flims, the

25

surfaces would adhere together, but very small amounts of contaminant minimize or

even prevent adhesion under purely normal loading. However, relative tangential

motion at the interface acts to disperse the contaminant films at the point of at the

point of contact and cold welding of the junctions can take place. Continued sliding

happen causes the junctions to be sheared and new junctions to be formed. The chain

of events that leads to the generation of wear particles includes the adhesion and

fracture of the mating surfaces. Since both adhesion and fracture are influenced by

surface contaminants and the evironment, it is quite difficult to relate the adhesive

wear process only with the bulk properties of solid surface [77].

2.3.2 Abrasive wear

Abrasive wear in Figure 2.19, occurs when asperities of a rough, hard surface

or hard particles slide on a softer surface and damage the interface by plastic

deformation or fracture. Other terms for abrasive wear also loosely used are

scratching, scoring or gouging depending on the degree of severity. In the case of the

ductile materials with high fracture toughness such as metals and alloys hard

asperities or hard particles result in the plastic flow of the softer material. Most

metallic and ceramic surfaces during sliding show clear evidence of plastic flow, even

some for ceramic brittle materials. Contact asperities of metal deform plastically even

at the lightest loads [78]. In the case of brittle materials with low fracture toughness,

wear occurs by brittle fracture. In these cases, the zone consists of significant

cracking. There are two general situations in which this type of wear occurs. In the

first case, also called two-body abrasion, the hard surface is the harder of two rubbing

surfaces for example in mechanical operations, such as girnding, cutting, and

mechining . Second cases or three body abrasion, the hard surface is a third body,

generally a small particle of grit or abrasive, caught between the two other surfaces

and sufficiently harder that it is able to abrade either one or both of the mating

surfaces for example in free abrasive lapping and polishing. In many cases, the wear

mechanism at the start is adhesive, which generates wear particles that get trapped at

the interface, resulting in a three body abrasive wear [79].

26

2.3.3 Fatigue wear

Fatigue wear is the removal of particles detached by fatigue arising from

cyclic stress variations, or wear of a solid surface caused by fracture arising from

material fatigue. The repeated loading and unloading cycles to which the material are

exposed may induce the formation of subsurface or surface cracks, which eventually,

after a critical number of cycles, will result in the breaking up of the surface with the

formation of large fragments, leaving large pits in the surface,also called as pitting.

Before this critical point happened which may be hundreds, thousands, or millions of

cycles, negligible wear takes place, which is in marked contrast to the wear caused by

an adhesive or abrasive mechanism, where wear causes a gradual deterioaration form

the start of running [80]. Most common in ceramics, chemically enchanced crack

growth is commonly referred to as static fatigue. Fatigue wear is directly related to

load, normally under rolling contact. Beside that it is common found at roller

bearings and the pitch line of gears. High load means short fatigue life. Fatigue

wears results when the high shear stresses from rolling contact cause subsurface

microcracking. These microscopic cracks begin under the surface of the roller or race

or gear tooth. The cracks later become interconnected and then intersect the surface.

Eventually the particles get released to the oil, leaving behind a delamination or spall

defect [81]. Schematic of fatique wear process is shown in Figure 2.20.

2.3.4 Fretting wear

When components are subjected to very small relative vibratory movements at

high frequency, an interactive form of wear, called fretting which is a small amplitude

oscillatory motion, usually tangential, between two solid surfaces in contact. Fretting

wear can occur whenever low amplitude vibratiory sliding takes place between two

surfaces, which vibration is a major cause of fretting wear. Basically fretting wear is

a form of adhesive or abrasive wear, where the normal load causes adhesion between

asperities and oscillatory movement casues tupture, resulting is wear debris, which is

trapped in the contact area and tends to dam the lubricant away from this area. This

wear debris is heavily work hardening and oxidized a condition which also results in

three body abrasion [82]. Fretting wear can be serious with many operations, for

examples of vulnerable components are shrink fits, bolted parts, and splines. The

27

contacts between hubs, shrink, press fits, and bearing housings on loaded rotating

sharfts or axles are particularly prone to fretting damage. Flexible couplings and

splines, particularly where they form a connection between two shafts and are

designed to accommudate some misaligment, can suffer fretting wear. Most

commonly, fretting wear is combined with corrosion and in that case the wear mode is

known as fretting corrosion [83]. For examples, in the case of steel paritcles, the

freshly worn nescent surface oxidiz to Fe2O3 and the characteristic fine reddish brown

powder is produced, known as cocoa. These oxide particles are abrasive, because of

the close fit of the surfaces and the oscillatory small amplitude motion, the surface

are never brought out of contact, and therefore, there is little opportunity for the

products of the action to escape. Further oscillatory motion casues abrasive wear,

oxidation and so on. Therefore, the amount of wear per unit sliding distance due to the

fretting may be large than that from adhesive and abrasive wear. The oscillatory

movement is usually the results in early initiation, but in many cases it is the

consequence of one of the members of the contact being subjected to a cyclic stress

which results in early initiation of fatigue cracks and results in a usually more

damaging aspect of fretting, known as fretting fatigue.

2.3.5 Erosive wear

Erosive wear can be caused by either solid particles or liquid dropelts striking

the surface at high velocities as shows in Figure 2.21. When the angle of

impingement is small, the wear produced is closely to a form of abrasion that is

gernerally treated rather differently because the contact stress arises from the kinetic

energy of particles flowing in an air or liquid stream as it encounters a surface.

Erosive wear is normal occure to the surface material by plastic deformation and/ or

brittle fracture, dependent upon materail being enroded away and operating

parameters. Plastic deformation usually occurs in ductile material which material

removed by the displacing or cutting action of the enrode particle. In a brittle material,

erosive wear will be removed by the formation and intersection of cracks that radiate

out from the point of impact of the enroded particle. The shape of the abrasive

particles affects that pattern of plastic deformation around each indentation,

conseuently the proportion of the material displaced from each impact. For brittle

28

material, sharper particles would lead to more localized deformation and consequently

wear as compared to the rounded particles [84]. The severity of the erosive wear is

also strongly dependent on the velocity and hardness of the particles as well as the

angle of impact. Erosion wear is a problem in machinery such as ingested sand

particles in gas turbine, helicopter and air plane propellers, windshields of airplanes,

nozzles for sand blasters, coal turbines, hydrolic turbines and centrifugal pumps used

for coal slurry pipelines [85].

2.3.6 Corrosion

Corrosion is the degradation of a material’s properties or mass over time due

to environment effects. It is the natural tendency of a material’s compositional

elements to return to their most thermodynamically stable state. For most metallic

materials this means the formation of oxides or sulfides or other basic matallic

compounds generally considered to be ores. Fortunately, the rate at which most of

these processes progress is slows enough to provide useful building materials. Only

inert atmosphere and vacuums can be considered free of corrosion for most matallic

materials [86].

2.3.6.1 Electrochemical corrosion

Electrochemical corrosion occurs in metal corrosion due to liquids or

electrolytes. Most liquids such as ordinaray water, seawater, acids and other

chemicals are good conductors of electricity. Electrochemical theory is the accepted

theory and it is actually quite simple. There are two basic requirements: (1) anodes

and cathodes must be present to form a cell and (2) direct current must flow. The

anodes and cathodes may be very close together or they may be far apart. The current

may be self-induced or it may be pressed on the system form an outside source. The

anode is the area where corrosion occurs and where current leaves the metal and

enters the solution. The cathode is the area where no corrosion occurs and where

current enters the metal from the solution. Anodes and cathodes can form on a single

piece of metal because of local differences in the metal or in the environment. The

metal at the anode dissolves and becomes an ion. It is oxidized and loses electrons.

For example, iron dissolves and loses two elecrons to become the ferrous ion Fe++.

29

A schematic picture of this situation is shown in Figure 2.22. The iron atom detaches

itself and gose into solution as a ferrous ion [87].

The electrons (e) are left on the metal and travel through it to the cathode is in

Figure 2.23 and Figure 2.24. These electrons are accepted at the cothode area.

Immersion in an electrolyte or conducting fluid is required to complete the circuit or

to carry the current from the anode or anodic area to the cathode or cathodic area.

For example, high resistance water is used in certain applications to keep corrosion at

a very low rate. However, most waters such as tap water and seawater are not pure

and are good conductors. Examples of metal corrosion are following by [88].

Fe(s) + H2O (l) + O2 (g) Fe2+ (aq) + OH- (aq)......... (2.11)

Oxidation reaction of iron

Fe (s) Fe2+ (aq) + 2e-.......... (2.12)

Reduction reaction of iron, water and oxygen in ambient asmoshpere are taken

an electron from oxidation of iron.

2H2O (l) + O2 (g) + 4e- 4OH-(aq)............. (2.13)

Redox reaction is a combination of oxidation reaction and reduction reaction.

2Fe +2H2O + O2 2Fe2+ + 4OH-............. (2.14)

Afterthat, 2Fe2+ and 4OH- are mixed together resulted in ferrous hydroxite.

Fe2+ + 2OH- Fe (OH) 2............... (2.15)

Ferrous hydroxite is contact with surrounding atmosphere which created rust.

4Fe (OH) 2 + 2H2O + O2 4Fe (OH) 3......... (2.16)

4Fe (OH) 3 2Fe2O3 + 6 H2O.................. (2.17)

30

2.3.6.2 Forms of corrosion

The effect of corrosion on a metallic surface can take many forms which are

shown in Figure 2.25. Identifying these forms can assist in understandig the corrosion

process and offer insight into its control.

Uniform corrosion

The simplest form of corrosion is unifrom or general corrosion. Uniform

corrosion is an even rate of metal loss over the exposed surface. It is one of the most

easily measured and predictable forms of corrosion. Many references exist which

report average or typical rates of corrosion for various metals in common media.

Since the corrosion is so uniform, corrosion rates for materials are often expressed in

terms of metal thickness loss per unit time. One common expression is mils per year

or somtimes, milimeters per year. Because of its predictability, low rates of corrosion

are often tolerated and catastrophic failures are rate if planed inspection and

monitoring is implemented. For most chemical process and structure, general

corrosion rates of less than 3 mils per year (mpy) are considered acceptble. Rates

between 2 and 20 mpy are routinely considered useful engineering materials for the

given environment. In severe environments, material exhibiting high general

corrosion ratess of between 20 to 50 mpy might be economically justifiable. Material

which exhibit rates of general corrosion beyond this are usually unacceptable. It

should be remembered that not only does the metal loss need to be considered but

where that metal is going must also be considered. Contamination of product even at

low concentrations can be more costly than the replacement of the corroded

component. Uniform corrosion is generally thought of in terms of metal loss due to

chemical attact or dissolution of the metallic component onto metallic ions. In high

temperature situations, uniform metal loss is more commonly preceded by its

combination with another element rather than its oxidation to a metallic ion.

Combination with oxygen to form metalic oxides or scale results in the loss of the

material in its useful engineering form which ultimately flakes off to return to nature

[89].

31

Galvanic corrosion

When two different metallic materials are electrically connected and placed in

a conductive solution, an electrical potential will exist. This potential difference will

provide a stronger driving force for the dissolution of the less noble (more electrically

negative) material. It will also reduce the tendency for the more noble material to

dissolve. While the relative differences in potential will change from one

environment to another, they remain fundamentally the same since the potential is

related to the energy required to oxidize them to metal ions in the given

enviroment.The significance of this becomes more apparent when a variety of

materials are listed in order of their electrical potentail in a familiar environment.

Notice that the precious metals of gold and platinum are at high potential end of the

series, which zinc and megnesium are at the low potential end. A practical

implication of this concept constantly surronds us. It is this principle that forms the

scientific basis for using a material such as zinc to sacrificailly protects a stailess steel

drive shaft on a plesure boat. It supplies the logic for the use of galvanized steel.

Galvanic corrosion is often experienced by owners of older homes where more

modern copper water tubing is connected to the older existing carbon steel water lines

[90].

Pitting corrosion

Pitting corrosion is in itself a corrosion mechanism but it is also a form of

corrosion often associated with other types of corrosion mechanisms. It is

characterized by a highly localized loss of metal. In the extreme case, it appears as a

deep, tiny hole in an otherwise unaffected surface [91]. The initiation of a pit is

associated with the breakdown of the protective film on the metal surface. In case

where pit depths increase rapidly, the enviroment is usually such that no repair of

repassivation of the protective layer can be accomplished. For other instances where

many shallow pits form, the environment is usually one where repassivation of the

damaged film can be made but initiation of new series is occuring on a regular basis.

The localized nature of pitting attack can be associated with component geometry, the

mechanics of corrosion process or with imperfections in the material itself. The

32

growth of pits, once initiated, is closely related to another corrosion mechanism such

as crevice corrosion [92].

Crevice corrosion

Crevice corrosion occurs in some environments becasue the nature of the

environment within the crevice will become more aggresive with time. Movement of

the corrodent within a crevice is slow or nonexistent. Over time, small changes due to

minor localized corrosion may become magnified because they are not constantly

being replenished or minimized by the bulk solution. As a result of a slow initial rate

of corrosion the pH of the crevice environment may become more acidic or

detrimental ion species may concentrate. As a result of the low flow condition the

crevice region may become depleted of oxygen or precluded the replacement of reactd

inhibitors [93].

Selective leaching

Selective leaching is the process whereby a specific element is removed form

an alloy due to an electrochemical interaction with the environment. Dezincification

of brass alloy is the most familiar example of this type of corrosion. It occurs most

commonly when exposed to soft waters and can be accelerated by high carbon

dioxide concentrations and the presence of chloride ions. The result of this corrosion

is that of leaving a porous and usually brittle shadow of the original component. Other

alloy systems are susceptible to this form of corrosion. Examples include the

selective loss of aluminium in aluminium-copper alloy and the loss of iron in cast

iron-carbon steels [94].

Stress corrosion cracking

The mechanism of stress corrosion cracking is specific to certain alloys in

specific environments. It is characterized by one or more crack fronts which have

developed as a result of a combination of the particular corrodent and tensile stresses.

Depending on the alloy system and corrodent combination, the cracking can be

intergranular or transgranular. The rate of crack propogation can very greatly and is

affected by stress levels, temperature, and concentration of the co rodent. In some

33

severe conditions such as type 304 stainless steel in a boiling magnesium chloride

solution, extensive cracking can be generated in a matter of hours. Fortunately, in

most industrial applications the progress of stress corrosion cracking is much slower

[95]. However, becasue of the nature of cracking it is difficult to detect until extensive

corrosion has already developed, which can lead to unexpected catastrophic failure.

Such an example of this crack mechanism is shown in Figure 2.25. Alloy system and

corrodent combinations which are known to exhibit stress corrosion cracking are

fairly well documented and should be considered in intitial design stages.

Apart from stress corrosion cracking mechanism, stress can assist in other corrosion

processes. Since this stress assisted corrosion is related to tensile stresses it is logical

to expect that it will also accelerate the simple mechanical fatigue process. Corrosion

fatigue is often difficult to differentiate from simple mechanical fatigue but is

recognized as a factor when the environment has been judged to have accelerated the

normal fatigue process. Such systems can also have the effect of lowering the

endurance limit such that fatigue will take place at the stress level at which, without

the environment effect, fatigue failures would not be expected [96].

Intergranular corrosion

As the name suggests this type of corrosion mechanism attacks those sites

where individual grains within a metallic material touch each other. These boundaries

are natural regions of higher energy due to the greater frequency of dislocations of

atoms from the natural order of the material’s structure. Figure 2.25 is an example of

intergranular attack caused by high temperature sulfidation in a nickle base alloy.

In addition, these regions also tend to act as sites for the formation of secondary phase,

which are essentially small islands within the matrix that have a chemical composition

different from the alloy itself. Depending on the corrodent and the alloy system,

corrosion attack may initiate at these locations due to preferential attack of the

secondary phase itself or attack the surounding matrix whihc was locally dealloyed in

forming the secondary phase. Either mechanism will result in the metallic surface

being etched along the grain boundaries. As the attack progresses, individual grains

are seperated form the matrix and the surface layer becomes porous. In severe cases

the surface texture will be grainy or powdery leading to more rapid metal loss [97].

34

2.3.7 Wear testing

Information on the wear resistant properties of coatings is urgently required by

suppliers and users to provide confidence in the performance of engineering coatings.

Many different methods are used in the quantitative and qualitative assessment of

friction and wear. This situation exists because wear damage can take several

different forms and because the amount of wear also influences the selection of a

particular measurement period. Inevitably, questions also arise on how the different

methods relate or how a particular wear test is detailed in specifications. While no

publication can address all questions relating to the wide breadth of friction and wear

tests, this volume collects several key specifications and reference articles under one

cover for convenient reference. The contents are reprinted from selected portions of

five annual books of ASTM Standards. The focus is on test methods but because wear

test methods depend on the type of wear modes, we should select the one that

properly for your requirment [98].

2.3.7.1 Sliding wear test

The sliding wear test of material is performed on an experimental set khown as pin-

on-disk apparatus, which is schematically represented in Figure 2.26. This apparatus

is a unit to determine the wear of material during sliding which is according to ASTM

G99-90 [Appendix C]. Prior the sliding wear test, all the samples were grounded and

polished to obtained more accurate results. Materials in this test are required in pair,

one is a WC sphere was used as counterbody against testing sample and obviously no

abrasive particles were fed into the contact area. During the operation, the WC is

pressed against the rotated disk at a specific load by mean of an arm level and

controlled weights. Since, the WC counterbody rotates freely on the sample’s surface,

in this type of the test, the wear of counterpart caused by its own wear is negligible,

and the wear volume of the sample an easily be evaluated using the following

geometrical equation by Rutherford Hutchings as follow [99].

35

4

64dVR

π= .............. (2.18)

Where R is the WC sphere radious

d, the wear crater diameter which was measured by an optical microscope

After that, the exact volume loss was obtained by divinding the volumetric wear loss

by sliding distane and the contact load.

2.3.7.2 Dry sand rubber wheel

One of the most popular wear tests is the ASTM G-65 Dry Sand Rubber

Wheel Abrasion Test [Appendix C]. This test measures the weight or volume loss in

a very controlled environment and simulates what is commonly referred to as low

stress abrasion or scratching abrasion. Scratching abrasion is principally

characterized by the lack of any fracturing of the abrading material. In other words,

the sand particles in the G-65 test retain their shape and size throughout the test

procedure. This characteristic is unlike its cousin abrasion wear mechanism, high

stress abrasion or grinding abrasion where the sand particles are actually fractured

into smaller pieces. The newly created particle is very sharp and angular and coupled

with the high degree of stress, imparts an entirely different type of wear. However,

back to scratching abrasive. Scratching abrasion can be easily illustrated by sand

sliding on a dump truck bed liner. In Figure 2.27 shows the illustration of dry sand

rubber wheel instrument, which is three bodies abrasive is the wear mechanism of this

test. A rubber wheel was used to carry the load and the abrasive particles such as SiC

or Al2O3 particle to the sample contact region. While the rubber wheel rotated against

the surface of the sample to be tested. Abrasive particles were continously released

into contact region by gravitation. Abrasive wear calculated by weight loss of testing

material divinding by sliding distance [100]. The estimate wear rate could be find

from the two forms of the Archard equation, which given by expression (2.19) and

(2.20)

WQ KH

= .................. (2.19)

Q kW= .................... (2.20)

36

Where Q is the wear rate

W; the applied load

H, the hardness of wear specimen

K, the dimensionless wear coefficient

and k is the dimesional wear coefficient

2.3.7.3 Salt spray test

The salt spray test is one of theoldest corrosion tests still use and highly useful

as an accelerated laboratory corrosion test that simulates the effects of marine

environments on different metallic materials with or without protective coatings.

ASTM B117 is the standard for salt fog spray [Appendix C]. The salt spray test

utilizes a box of suitable size, from about 2 m3 to walk in size, into which a 5 % NaCl

solution is aspirated with air. The pH of the salt solution shall be such that when

atomized at 35 οC the collected solution will be in the pH ranges from 6.5-7.2. The

specimen of salt spray test should be suitably cleaned. The cleaning method shall be

optionall depending on the nature of the surface and the contaminants. Care shall be

taken that specimens are not recontaminated after cleaning by excessive or careless

handing and specimens for evaluation of paints. Other organic coatings shall be

prepared in accordance with applicable specifications for the material being exposed

or as agree upon between the purchaser and the supplier. Unless otherwise specified,

the cut edges of plated, coated or duplex materail and area containing identification

marks or in contact with the racks or supports shall be protected with a suitable

coating stable under the condition of practice. A common test time is 72 hours

although exposure duration can vary considerably. The test is commonly of a day to

serveral months. This test commonly used for zinc coatings [101-102]. For corrosion

tests of gas turbine components salt susch as Na2SO4 and NaCl are added in air.

2.4 Previous work

In 1999, Kyeong Ho Baik and Patrick S. Grant studied the microstructure

evaluation of monolithic and continuous fibre reinforced Al-12wt%Si produced by

low pressure plasma spraying. The microstructure of low pressure plasma sprayed

(LPPS) Al-12wt%Si coating produced by Al-12Si powder with diameter of 5 to 50

37

μm deposited on 25.4 mm diameter, 5 mm thick stainless steel substrate, varying

number of spraying passes to control deposit thickness, temperature also investigated

and spray/wind Al-12Si composites reinforced with continuous fibres have been

investigate by optical microscope, scanning electron microscope and XRD analysis.

The LPPS monolithic Al-12Si deposit sprayed in a single pass was characterized by

splat quenched microstructure. A lamellar structure consisting of individual splats

and partially or wholly unmelted feedstock powder particles. A reduction in Al lattice

parameter for splats was attributed to the supersaturation of Al with Si. Where as

multi-pass spraying of thicker deposits promoted precipitation and growth of Si

solubility extension corresponding increase of substrate deposit temperature during

LPPS. Four-layer Al-12Si composites reinforced with stainless steel wire and SiC

fibre were successfully manufactured by the spray/wind process without fibre

breakage and exhibited low porosity and well controlled fiber spacing [103].

A friction and wear characteristic of hard coatings was studied by Malcolm K.

Stanford and Vinod K. Jain in 2001. The friction and wear behavior of four hard

coatings including Metco, Diamalloy, Stellite, and Zn–SiC were determined using a

pin-on-disk machine. The coatings were thermal sprayed on cast iron disks. The

coating compositions were Ni–17Cr–2.5Fe–2.5Si–2.5B–0.15C (Metco), Fe–30Mo–

2C (Diamalloy), Co–30Cr–12W–2.4C (Stellite), and Zn–50SiC (Zn–SiC). Sliding

was performed between cylindrical pins machined from non-asbestos organic (NAO)

brake lining and the coated and uncoated disks. The lining, consisting of resin, aramid

pulp, zirconia, graphite, calcium fluoride, rubber and barium sulfate, were developed

as a material for automotive brake pads. The coatings were characterized by

measuring their hardness, porosity, and corrosion resistance. The Metco and Stellite

coatings had a uniform morphology in all directions. The Diamalloy coating had a

lamellar microstructure whereas the Zn–SiC coating was very porous. The corrosion

resistance of the coatings was tested with exposure to 5% NaCl for 168 h. The Stellite

coating had the best corrosion resistance. The friction and wear tests were conducted

at contact pressures of 1.72, 3.45 and 6.89MPa and sliding speeds of 1 and 3m/s. The

wear of the lining material was lowest when it slides against the Stellite coated disks

and the highest coefficient of friction was observed for the Metco coated disks [104].

38

Several fabrication routes are under development to produce nanodstructured

materials, usually referring to a grain size smaller than 100 nm. Ji Gang, Jean-Paul

Morniroli and Thierry Grosdidier had been studied the nanostructures in thermal

spray coatings in 2003, the nature of the nanograins formed by high velocity oxy-fuel

thermal spraying (HVOF) of FeAl milled powder has been using transmission

electron microscopy (TEM) on cross sectional thin foils. Scanning electron

microscope (SEM) of coating also investigated. Scanning electron micrograph shows

the general aspect of deposit microstructure, some particles, that did not fully melt

while the fully melted ones flowed extensively on impact to form more conventional

elongated splats. In the backscattered electron micrograph, the dark grey and back

contrasts are due to fine light grey contrast layers and to porosity, respectively. Some

light grey contrast layer was identified as Fe3Al that formed as a result of Al

evaporation during spraying. TEM analysis clearly reveals two major type of

morphology corresponding to the so called equiaxed (3D) and rod shape (2D)

nanograins. The structure of equiaxed 3D nanometer crytallites were retained within

the unmelted powder particles, this were formed by recrystallization within the highly

deformed matrix. Elongated 2D nanogrians produced by rapid solidification within

the fully molten splats. The latter nanograins can thus be obtained by spraying

conventional “microcrystalline” powders [105].

Processing by rapid Processing by rapid solidification can affect alloy

properties microstructure refinement, extension of solid or the formation of a non-

equilibrium phase. This may improve alloy strength and enhance both fatigue and

wear resistance. In 2004 , C.J. Kong, P.D. Brown, A. Horlock, S.J. Harris, and D.G.

McCartney have been studied to describe the microstructure of an Al–12 wt.% Sn–1

wt.% Cu alloy thermally sprayed onto steel substrates by the high velocity oxy-liquid

fuel (HVOLF) technique. The original powder and the as-sprayed coating were

examined using back scattered electron (BSE) imaging and by conventional

diffraction contrast analysis using a transmission electron microscope (TEM). The

results show microstructure of most of the larger gas-atomised Al–12 wt. % Sn–1 wt.

% Cu alloy powder particles (≥10_m in size) contain Al dendrites surrounded by

interdendritic Sn phase. Conversely, the microstructure exhibited by small powder

particles (typically ≤10_m) involves nanoscale Sn particles embedded in Al. Thus,

39

powder particle size and by implication cooling rate, strongly influences the resultant

microstructure. The microstructure of as-sprayed HVOLF coating shows several

levels of complexity due to the fully molten, semi molten or unmelted states of the

powder prior to impact during thermal spraying. Molten regions are found to form

distributions of nanoscale Sn due to rapid solidification. Large scale Sn distributions

arise from insufficient mixing of liquid Sn with liquid/solid Al [106].

In 2005. High velocity oxy-fuel thermal spraying has been used to produce

deposits of an Al–20 wt. %Sn–3 wt. %Si alloy approximately 300µm thick on a steel

substrate. The microstructures of the as-sprayed and annealed deposits were

investigated by X-ray diffraction, scanning and transmission electron microscopy.

This result shows that an Al–20 wt. %Sn–3 wt. %Si alloy can be sprayed by the

liquid-fuel HVOF process to achieve a low porosity deposit. The microstructures

formed following spraying comprises, principally, splats which are fully molten at the

point of impact, along with a small proportion of splats which are semi solid just prior

to impact. The main phases present are α-Al and β-Sn, the latter has a bimodal

distribution comprising near spherical dispersed particles 20–100 nm in size within

fully melted splats as well as coarser micron sized particles associated with semisolid

splats. The formation of the Sn particle dispersion in this rapidly solidified material is

satisfactorily explained by the presence of a metastable miscibility gap in the Al–Sn

phase diagram. In the as sprayed condition, most of the Si is retained in solid solution

in the α-Al phase although there is a small fraction of nanometer sized Si precipitates.

The effect on the deposit of a short, elevated temperature annealing treatment at 300 0C is to coarsen the dispersed Sn particles and to precipitate, from the α-Al solid

solution, Si particles with an approximate size range 50–200 nm [107].

In this research in 2006, the Influence of bond coat on shear adhesion strength

of erosion and thermal resistant coating for carbon fiber reinforced thermosetting

polyimide was determined by Aiguo Liu ., Mianhuan Guo, Jiashuang Gao, and

Minhai Zhao. Arc sprayed Al, Zn, and plasma-sprayed Al, Zn, Ni3Al and Cu were

deposited on carbon fiber reinforced polyimide substrate as bond coats for erosion and

thermal resistant coating. Shear adhesion strength of different materials was tested,

and microstructures of bond coats were analyzed with scanning electron microscope.

The results showed that the substrate was thermally damaged when Ni3Al or Cu was

40

deposited as bond coat and the bond coat was delaminated from the substrate. It could

be concluded that, materials with high melting point like Ni or Cu, were not suitable

for bond coat of erosion and thermal resistant coating on carbon fiber reinforced

polyimide. Arc sprayed and plasma-sprayed Al and Zn could be used as bond coat

materials. For Zn as bond coat material, depositing method had little influence on

shear adhesion strength. While for Al as bond coat material, plasma spray was

superior to arc spray. Preheating could improve shear adhesion strength with plasma

sprayed Al as bond coat. The maximum shear adhesion strength obtained in this paper

was 14.15 MPa with plasma-sprayed Al as bond coat and the preheating temperature

was 250 °C [108].

Corrosion protection assessment of sacrificial coating systems as a function of

exposure time in a marine environment was studied by D.P. Schmidt, B.A. Shaw, E.

Sikora , W.W. Shaw, and L.H. Laliberte in 2006. The present investigation assessed

the corrosion protection performance of 17 different Zn and Al sacrificial coating

system configurations during marine atmospheric exposure at Kure Beach, NC. The

coating systems incorporated several conversion coating layers, primers and organic

topcoats. Visual observations and electrochemical measurements (including

electrochemical impedance spectroscopy, EIS) were made on six different occasions

throughout the 20-month exposure time. Milled scribes on each of the coating

specimens allowed for defect protection as well as barrier protection to be

investigated. A novel corrosion analysis technique utilizing a specialized conducting

agar (SCAR) cell enabled impedance measurements to be made on both intact and

defect areas. Visual observations, Eoc’s, and EIS as a function of atmospheric

exposure time provided complementary results. Impedance results were found to be

useful in determining a coating’s barrier protection and scribe damage analysis

accurately represented defect protection [109].

Twin wire arc spray is known to be one of the less expensive thermal spraying

processes with an ability to produce dense coatings with a wide rate of material

deposition. M.P. Planche, H. Liao, and C. Coddet have been studied the splats of

particles are formed on flat substrate surfaces, localized at the same spraying distance

where in-flight particle characteristics were measured in 2004. However, the coating

41

properties were analyzed in terms of hardness, porosity and oxide contents. Different

working conditions were tested to point out correlations among spray parameters, in-

flight particle characteristics, splat morphologies and coating microstructures. The

result has been shown that atomizing gas flow rate clearly has a main influence on

particle characteristics in the studied range of parameters: the higher the flow rate, the

smaller and the quicker the particles. At farther spraying distances, velocity and

temperature of particles decrease but size is maintained constant. Other spray

parameters have a lower influence. Finally, particle characteristics have been

connected to coating properties and strong tendencies have been noticed. A main

interaction exists between gas flow rate and coating properties. For example, while

increasing gas flow rate, particle velocities increase as their diameters decrease, the

viscous force and the tension force increase leading together to an increase of

hardness of the deposit. Concerning the oxide content, this value increases with the

gas flow rate which is consistent with what could generally be observed. The specific

surface of the particles in contact with oxygen increases and causes an increase of the

oxide content. It has been noted simultaneously a decrease of the porosity due to a

greater flattening of the particles and a better accommodation of the particles between

themselves [110].

Furthermore, H.L. Liao (2005) investigated particle properties of individual electrodes

of the arc spray process by separating particles from the anode and the cathode wires.

A particle size distribution was found for particles from each wire. This particle size

distribution is probably due to the periodical fluctuation of the arc length and the arc

voltage leading to a varying particles size. In addition, after atomization the difference

average diameter particle between from the anode and the cathode becomes quite

small and the difference can be reduced by increasing atomizing gas pressure.

Microstructure analysis also indicates that the closed nozzle system with a converging

diverging orifice (CD/CL) nozzle tends to produce coating with finer microstructure,

lower porosity and higher oxide content than the open nozzle which leads to very

coarse coating microstructure with high porosity but low oxide content because of its

poorer atomizing performance. Analysis of the gas dynamic performance of the

different nozzles and numerical prediction of the splat diameter distribution were also

conducted. They show that one of the major drawbacks of an open nozzle system is its

42

relatively longer distance between the wires intercept point and the nozzle exit where

the gas velocity combined with the experimental particle size distribution in the

coating sprayed by using different nozzle. Results show a smaller average splat

diameter and a narrower splat diameter distribution for CD/CL nozzle that is in close

agreements with the microstructure analysis. It was found that a major disadvantage

of the open nozzle system is its relatively longer distance between the wires

interception point and the nozzle exit where the gas velocity attains its maximum

value [111].

43

Table 2.1 Flame temperature is function of the oxygen and fuel gase ratio.

Fuel : Oxygen Temperature ( 0C )

Propane-Oxygen

2530

Natural gas (Propane+Butane)-Oxygen

2540

Hydrogen-Oxygen

2660

Propylene-Oxygen

2840

Methylacetylene/propadiene-Oxygen

2930

Acetylene-Oxygen

3090

44

Figure 2.1 Schematics of powder flame spray. [19]

Powder and Carrier Gas

Oxygen and Fuel Gas

Flame

Spray Stream

Nozzel Coating

Substrate

Figure 2.2 Schematics of wire flame spray. [21]

45

Figure 2.3 Schematatic of electric arc spray [26]

Substrate

Coating Nozzel Oxygen and

Fuel Gas

Powder and Carrier Gas

Shock Compressed Air Spray

Stream Diamonds

Figure 2.4 Schematic of High Velocity Oxy-Fuel process or HVOF process [28]

46

Figure 2.5 Schematic of plasma thermal spraying. [31]

Figure 2.6 Schematic of Detonation gun. [32]

Substrate

Coating

Plasma gas

Powder

Cathode

Anode

47

Power feeder Substrate

Coating

High pressure gas supply

Sprayed Particles

Gun Gas

heater

Figure 2.7 Schematic of cold spray process. [35]

Figure 2.8 A typical coating cross section of the laminar structure of oxides and inclusions. [35]

48

Material Melting Zone Spray Stream Coating

Figure 2.9 Schematic of thermal spray process. [35]

Figure 2.10 Schematic diagram of spherical impinged onto a flat substrate. [45]

Process * Plasma * HVOF Wire or Powder

Metals Ceramics Carbides

* Flame * Arc

Substrate

Spray Distance

Workpiece temperature 700-1300 C. (1600-2700 F.)

49

(a) (c) (b)

Figure 2.11 The sketch of two principal morphological forms of lamella splashed on

the substrate: (a) pancake, (b) flower shape and (c) disk shape. The sketch shows the

top view and cross-sections, (1) corrona; (2) crack; (3) deformed substrate. [15]

Equivalent diameter

Absolute number

Elongation factor

Ratio >1

Figure 2.12 A schematically of equivalent diameter, elongation factor, and degree of

splashing of splat. [43]

Degree of splashing

Ratio > 1

50

Measurement signal

Ferrite core

Nonferrous metal substsrate

Excitation current

Eddy currents

Coating thickness

Figure 2.13 The effect of liftoff on impedance to eddy current technique. [52] Measured part Reference Figure 2.14 Schematic of standard calibration [52]

51

Y

Root mean square roughness, Ra

Peak-to-valley Roughness.

R maxx X

Mean surface L

Figure 2.15 Shows a surface roughness profile. [58]

1360 between opposite faces

Figure 2.16 Show a pyramidal-shape dimond with an icluded angle of 1360 between opposite faces and the dimond impression. [66]

52

Figure 2.17 Schematic illustration of the incident electron beam interact in the specimens. [74]

Figure 2.18 Schematic of adhesive wear process. [75]

Figure 2.19 Schematic of abrasive wear process. [75]

53

Fig. 2.20 Schematic of fatigue wears process. [76]

Figure 2.21 Schematic of Erosive wear process. [83]

54

Figure 2.22 Formation of ferrous (Fe++) ions in the corrosion of iron. [85]

H2

H+

H+

Figure 2.23 Hydrogen ions accept electrons at the cathode and form gydrogen gas. [85]

Figure 2.24 Basic diagram showing requirements for corrosion of metals. [86]

55

Pitting corrosion Galvanic corrosion

Pitting corrosion

Crevice corrosion

Stress corrosion cracking

Selective reaching

Intergranular corrosion

Figure 2.25 Schematic summary of the variuos forms of corrosion. [94]

56

Figure 2.26 Schematic of Pin-on-disk apparatus [98].

Abrasive

particles Rubber

wheel

Figure 2.27 Schematic view of the dry sand abrasion test [100]

Weight

Specimen