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CHAPTER 8

ANALYSIS OF MICROSTRUCTURE AND

COMPOSITION OF THE COATING

8.1 INTRODUCTION

The performance of the coating depends on the microstructure,

percentage of porosity, the composition of the coating and the residual

stresses arising during deposition of the coating. This chapter studies the

micro structural aspects, the porosity distribution and the compositional

details of the coating. Samples of the deposits were characterized in several

ways. The methods employed were the evaluation of the microstructure,

elemental analysis, phase analysis and porosity of the coatings, using Optical

microscope, SEM (Scanning electron microscope), EDS (Energy dispersive

spectroscopy) and XRD (X-ray Diffractometer). From literature reports,

Al2O3 and YSZ coatings manufactured by atmospheric plasma spraying onto

Ni alloy substrates had a typical splat quenched microstructure which

contained various types of defects, including incompletely filled pores, inter-

splat pores and intra-splat microcracks.

The mechanical behavior of a material is dependent on its

microstructure which depends in turn on the processing technique. This

concept applies to TBC’s also, which have complex microstructures. The

microstructure related mechanical failure in plasma sprayed TBC’s arise due

to the following:

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1. High density of micro-cracks.

2. Isolated large pores.

3. Weak interfaces between splats.

4. Anisotropy in microstructure and mechanical properties.

Prystay et al (2001) showed that changing in-flight particle

temperature and velocity can change the distribution of micro cracks and

hence affect the mechanical behavior of the coating. Cracks can propagate

more easily in the plane parallel to the coating-substrate interface than in the

perpendicular plane (Luo et al 2003) .Cracks in TBC’s are of two types,

horizontal and vertical. Horizontal cracks parallel to the coating substrate

interface and vertical cracks perpendicular to the coating –substrate interface

perform different roles. Horizontal cracks, located at splat boundaries, are

considered non-detrimental to the coating, and helpful to reduce the out of

plane heat transfer in it, making the TBC more effective. However, these

cracks can grow during thermal cycling, link together, and cause coating

spallation (Basu et al 2005). Vertical cracks, which propagate through the

coating thickness (then often referred to as segmentation cracks) can increase

the coating compliance and extend its lifetime (Basu et al 2005).Thermal

cycling induces failure in plasma sprayed TBC’s which is a complex process

involving interplay between several general phenomena listed below:

(i) thermal expansion mismatch stress ( residual stress); (ii) growth

of thermally grown oxide (TGO) at the interface; (iii) cyclic creep of the bond

coat; (iv) depletion of Al in the bond coat leading to the formation of brittle

oxides other than - Al2O3; (v) sintering of the porous TBC and the attendant

deterioration of strain tolerance and thermal resistivity; (vi) degradation of the

metal ceramic interface toughness; (vii) delamination and cracking; (viii)

crack coalesces. The TBC failure mechanisms are highly system and

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application specific, where one or more of the above phenomena dominate

(Schlichting et al 2003). For example, in thick thermal barrier coatings (for

diesel engine applications), service temperature is not high enough for TGO

formation, top coat sintering or cyclic creep of the bond coat. Thermal

stresses are the most important factors in this application (Hamed and Coyle

2006). This applies to petrol engines also. This study pertains to the effect of

thermal stresses on the microstructure of plasma sprayed duplex mullite

coatings.

The failure mechanism can be due to the thermal stresses alone in

diesel and petrol engines. Coating failures in diesel engines are known to

occur due to either loss of cohesion in the ceramic layer or loss of adhesion at

the coating/bond coat or the bond coat/substrate interface. Loss of adhesion

may occur at high service temperatures due to the growth of an oxide layer

between the bond coat and top coat, known as a thermally grown oxide

(TGO) layer. This mechanism is not significant in water-cooled diesel

engines, as the maximum service temperature does not exceed 1000°C. At the

lower service temperatures, thermo-mechanical fatigue and residual stresses

play a more important role in coating failure (Ramaswamy et al 2000).

Thermal stresses generated by the difference in the coefficient of

thermal expansion between the substrate and coating are one of the major

factors contributing to failure in plasma sprayed coatings (Kuroda and Clyne

1991). The residual stresses which are induced in the fabrication process of

the TBCs are associated with many mechanical failures of the coating. For

example, delamination may occur along the interface of the pre-tensioned

coatings (Kokini et al 1997) while compressive residual stress may cause

spalling inside the coating (Bartlett and Dalmaschio 1995).

As discussed earlier, plasma-sprayed TBCs fail by spallation of the

ceramic coating. For TBC thickness <250 µm, spallation occurs close to the

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bond coat but within the ceramic coating. The driving force for the

spallation is the combination of cyclic thermal strain due to CTE

mismatch, continued oxidation of the bond coat leading to TGO growth,

and externally imposed cyclic mechanical strain. The failure sequence of the

TBC may be described as follows:

Formation of subcritical cracks in the ceramic.

Progressive link-up of adjacent subcritical cracks.

In-plane crack within the ceramic but close to the bond

coat-ceramic interface (De Masi et al 1989).

The separation of ceramic at the dominant crack leading to

failure.

The failure mode is laminar compressive failure.

For thickness > 250µm, failure generally occurs within the ceramic

layer, away from the interface. In thick coatings, additional thickness and low

thermal conductivity result in a higher thermal resistance. Kokini et al (1996)

showed that the stress relaxation process occurring in thick TBC systems at

high temperature is a cause for crack initiation and propagation. Failure mode

is normally tensile cracking.

For many applications, thick (~1000µm) TBCs are required. For

air plasma sprayed TBCs, experimental data generated in cyclic burner rig

testing show that the failure life decreases with increased TBC thickness

(Bose and De Masi-Marcin 1997). It should be kept in mind, however, that

in the field; thicker TBCs reduce substrate temperature by a larger amount

and, therefore, prolong substrate life.

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Generally it is desirable for the particles to be melted and spreadonto the substrate or previously deposited splats. An excessive amount ofunmelted or semi-melted particles can significantly decrease the coatingadhesion and cohesive strength and increase the porosity. There are differentways one can determine if most of the particles have been melted. One way isto study the SEM micrographs of coating cross-sections to observe theunmelted particles.

8.2 EXPERIMENTAL WORK ON MICROSTRUCTURE OFMULLITE COATINGS

The typical microstructure of plasma sprayed duplex coatingdeposited on cast aluminum is shown in Figure 8.1. The microstructure of thecoatings was studied by putting polished cross sections of the coating sampleunder a microscope (Neomate) equipped with a CCD ( Charge coupled

device) camera (JVC, TK 870E). This system is used to obtain a digitizedimage of the object. The digitized image is transmitted to a computerequipped with Quantimet image technology software.

8.2.1 Results and Discussion

The plasma sprayed layers are seen consisting of the nickel chromebond coat and mullite top coat. The microstructure is crystalline with someamorphous content, coating discontinuities, pores, micro cracks, cavities,which is the nature of thermal barrier coatings. The volume fraction of pores

has been calculated as 6 % to 21 % after many measurements taken at variouslocations of the coating on six coated specimens. The microstructure isgenerally crystalline in nature, as evident from the sharp peaks in the XRDplots with some amorphous content as evident from the hump in the patternfrom 15 to 30 degrees 2 theta scale. The characteristic features of the coatingmicrostructure are summarized as follows:

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The microstructure shows the splat morphology (molten

particles deformed on impact into a pancake shape, enclosingtransverse micro cracks, porosity, and occasional unmeltedparticles), of the nickel chrome and mullite layers, which hasan enormous and dominant effect on the properties of the coating.The splat morphology shows the coating discontinuities present.

The microstructure of the final coatings by image analysis showedthe porosity distribution across the coating (Figure 8.2). Theceramic layer contains 6 to 21 % porosity by volume.Typically, finer powder particle size and closer spray distanceresult in lower porosity.

The interface between the ceramic and the bond coat is rough.

Figure 8.1 (a-o) Microstructure of mullite coatings (Polished cross sections)

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Figure 8.1 (Continued)

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Figure 8.1 (Continued)

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Figure 8.1 (Continued)

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Figure 8.1 (Continued)

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Figure 8.1 (Continued)

The plasma sprayed mullite coatings exhibited the same

features as any metal coating except that some particles did

not flow at the bond coat surface. Internal porosity was

distinguished in some particles by spot-like features below the

surface. The bond coat of nickel chrome also did not flow at

the substrate surface and discontinuities were noticed. .

In general a large number of fine particles were observed on

the surface of ceramic coatings and these were probably the

lower size fraction of the powders. Sites can be distinguished

between particles where continuous, very fine inter lamellar

porosity would be prevalent (formed by imperfect bonding

between adjacent lamellae) in contrast to voids within

particles arising from gas evolution (internal porosity).

The structure of nickel-chromium and nickel aluminum

composite coatings have been investigated extensively by

many researchers because of their unique property of good

adhesion to a surface which may not have been grit blasted. It

has also been established that ceramic coatings adhere more

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strongly to these composite coatings than to the grit blasted

substrate, so that it is common thermal spraying practice to

use such a metal interlayer during the preparation of a coating.

The interlayer is termed a ‘bond coating’ and most suitable

materials are based on Ni-Al, Ni-Cr and Mo. It is important to

note that Ni-Al, Ni-Cr powders are not mixtures of the

individual constituents but are composite powders consisting

of a core material encased in the other component (Longo

1966, Dittrich and Sheppard 1969, Houben and Zaat 1974b).

The adhesion mechanism of bond coatings is by metallurgical

bonding (chemical reaction) with the substrate. The high

temperature prerequisite for alloying at the substrate surface

arises from the chemical reaction of the powder constituents

and this behavior has been verified (Dittrich 1965, Longo

1966) by observing an increase in the particle brightness

during its time of flight. Gases are liberated during this

reaction and may form the internal porosity that was observed

within the coatings.

The microstructure of hypoeutectic cast aluminum silicon

alloy is also shown at different magnifications with the

characteristic dendritic structure of aluminum with precipitates

of silicon in the grain boundaries.

8.3 EXPERIMENTAL WORK ON POROSITY

The porosity of a plasma sprayed coating may be measured by

several techniques: image analysis, mercury intrusion porosimetry, gas

adsorption, Archimedean displacement, and small angle neutron scattering.

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Measurement of porosity, in this study is done using the image analysis

technique. Image analysis is based on measuring the area fraction of pores in

a micrograph of a coating cross section. At higher magnifications, finer scale

porosity is visible. The porosity of the coatings was measured by putting

polished cross sections of the coating sample under a microscope (Neomate)

equipped with a CCD camera (JVC, TK 870E). This system is used to obtain

a digitized image of the object. The digitized image is transmitted to a

computer equipped with Quantimet image technology software. The total area

captured by the objective of the microscope or a fraction thereof can be

accurately measured by the software. Hence the total area and the area

covered by the pores are separately measured and the porosity of the surface

under examination is determined.

8.3.1 Results and Discussion

The volume fraction of pores has been calculated as 6 % to 21 %

after many measurements taken at various locations of the coating on six

coated specimens. The report is enclosed in below. The porosity level is well

within the limit for a good thermal barrier coating. Higher the porosity, lower

the mechanical strength and adhesion strength, but better the thermal barrier

property. The levels seen will not affect the mechanical strength. In

conventional thermal barrier coatings, porosity levels of 20 % are generally

observed. The coating porosity influences the wear in two ways. Firstly, it

reduces the material strength against plastic deformation or chipping since the

material at the edge of a void lacks mechanical support. Secondly, pores can

impair strength by acting as stress concentrators and/or decreasing the load-

bearing surface. The report on porosity measurements on the duplex coated

specimens is shown in Figure 8.2 below.

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(a)

Figure 8.2 (a) and (b) Porosity reports

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(b)

Figure 8.2 (Continued)

8.4 EVALUATION OF COATING DEPOSITION EFFICIENCY

Deposition efficiency is defined as the ratio of the weight of coatingdeposited on the substrate to the weight of the expended feedstock. Weighingmethod is accepted widely to measure this. Each specimen is weighed beforeand after coating deposition. The difference is the weight (Gc) of coating

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deposited on the substrate. From the powder feed rate and time of depositionthe weight of expended feed stock (Gp) is determined. The depositionefficiency ( ) is then calculated using the following equation (8.1).

= (Gc / Gp) X 100 % (8.1)

Weighing of samples is done using a precision electronic balance

with + 0.1 mg accuracy. Deposition efficiency of 90 % has been observed inmany coating trials. The measurement for one run is shown below.

Weight of coated material = 15 grams

Weight of material expended in the spraying system = spray rate x spray time= 19kgs/hr x 3 secs = 16 grams.

Therefore deposition efficiency = 15/16 = 90 % ( approx.)

Deposition efficiency of any coating is a characteristic, which notonly rates the effectiveness of the spraying method but also is a measure ofthe coatability of the material under study. Particle deposition i.e. the coating

thickness is influenced mainly by the input power to the plasma torch. Withincrease in power level, the plasma density increases leading to a rise in enthalpyand thereby, the particle temperature. Hence more number of particles get meltedduring in-flight traverse through plasma jet. When these molten species hit thesubstrate, they get flattened and adhere to the surface. The deposition of layersis favoured with availability of more number of molten / semi molten particleswhich is enhanced by increasing the torch input power. This increases thecoating thickness. But, beyond certain limit of operating power level;

fragmentation and vaporization of sprayed particles do occur simultaneouslyand for these two mechanisms, some (powder) particles fly off during sprayingrestricting further increase in coating thickness. Hence in this study, optimumpower has been used which has resulted in high deposition efficiency.

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8.5 SEM (SCANNING ELECTRON MICROSCOPE) STUDIES

The finer micro structural features of the coated specimens were

studied using SEM. Standard metallurgical procedures were followed for the

study. The metallographically polished specimens were observed in a Jeol

JSM T100 scanning electron microscope at suitable magnifications. The

specimens used for SEM/EDS analysis is shown in Figure 8.3.

Figure 8.3 Polished specimens prepared for SEM/EDS analysis

Plasma sprayed coated specimens and plasma processed powders

were studied by scanning electron microscope mostly using the secondary

electron imaging. The surfaces as well as the interface morphology of all

coatings were seen in the microscope. Small specimens are sliced from the

coated samples and were mounted using thermosetting molding powders.

Coating cross-sections are polished in three stages using SiC abrasive papers

of reducing grit sizes and then with diamond pastes on a wheel for coating

interface analysis in SEM. These specimens are also utilized for the micro

hardness measurement.

8.5.1 Results and Discussion

The interface adhesion of the coatings depends on the coating

morphology and interparticle bonding of the sprayed powders. SEM images

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of the coatings deposited using optimized process parameters is shown in

Figure 8.4. From the figure it is found that, coatings show uniform

distribution of molten/semi molten particles. The images show the as-sprayed

splat morphology of the coating, the good blend between the coating and the

substrate and suggest a good adherence of the coating to the substrate.

The coating substrate interface plays an important role on the

adhesion of the coating. From the images, the lamellar microstructure

confirms the solidification of molten particles to form splats during coating

deposition; the coating is homogenous throughout and hence has produced

higher adhesion strength.

The coating with bond coat is a functionally graded duplex coating

with a crystalline layer of mullite plasma sprayed as the top coat. The ceramic

layer acts as a composite by virtue of its porosity. Surface analysis by EDS

confirmed the presence of all coating elements- Al, Si, Ni, Cr in the coating

with bond coat ( refer EDS spectra).

Figure 8.4 SEM images of the as-sprayed cross section of duplex coating

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8.6 XRD (X-RAY DIFFRACTION) STUDIES

The process of coating formation involves extremely fast

quenching of molten particles, and due to this reason air plasma sprayed

mullite exhibits a non equilibrium microstructure. The phases present in

the coating, therefore, do not conform to the equilibrium phase diagram.

The actual phases and their content in the ceramic layer of TBC depend

strongly on the process parameters, the characteristics of the spray powder

used, and the thermal exposure history. Phase content is generally determined

by the use of x-ray diffraction (XRD).

Micro-hardness test shows different hardness values on different

optically distinct regions on the coating cross-sections. Therefore, to ascertain

the chemical composition and phases present such as oxides or carbides and

phase changes / transformation taking place during plasma spraying, the

X-ray diffractograms are taken on the raw material and on some selected

coatings using a Philips X ray diffractometer with Ni- filtered Cu-K

radiation ( = 1.5418 Å). The characteristic d-spacing of all possible values

were taken from JCPDS (Joint Committee on Powder Diffraction Standards)

cards and were compared with d-values obtained from XRD patterns to

identify the various X -ray peaks obtained.

8.6.1 Results and Discussion

The XRD results are shown in Figure 8.5 for the coated specimens

and Figure 8.6 for the powder. The JCPDS report of mullite, aluminum oxide

and silica is shown in Appendix 1.The XRD of the feed material shows the

presence of aluminum oxide and silica powders; some traces of - quartz is

present. Alternate peaks shown are that of mullite, Al2O3 and SiO2..

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XRD patterns of two of the four as-coated specimens tested shown

in Figure 8.5 A, B, has sharp and intense peaks of mullite, the major phase

and also the presence of Al2O3, the minor phase and traces of SiO2 in the

coating. Peaks pertaining to silica (quartz or crystalline) were not seen in the

as –sprayed coating, and this absence of the XRD signature of silica may be

due to low relative proportion in the sample. A few small peaks of low

intensity were unidentifiable. The fairly sharp peaks in the XRD pattern

clearly indicate that the duplex coating has a crystalline microstructure, unlike

many plasma sprayed mullite coatings which are to a large extent amorphous,

which has resulted in coating spallation during thermal cycling.

(a)

(b)

Figure 8.5 (a) and (b) XRD pattern of coated specimens

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Figure 8.6 XRD pattern of mullite ceramic powder

The microstructure is generally crystalline with some amorphous

content due to the presence of some traces of silica ( the hump in the pattern

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from 15 to 30 degrees 2 theta scale).The other coatings are more amorphous

due to higher silica content of 30 %. Mullite is made up of alumina and silica.

On rapid quenching, as in plasma spraying, silica (glass) forms an amorphous

structure and alumina is generally a crystalline ceramic and hence a structure

which is both crystalline and amorphous has been formed (hump in XRD and

the sharp peaks).The amorphous structure can be removed by subsequent heat

treatment and made crystalline. Also slow cooling during plasma spraying can

result in a crystalline structure. Crystalline structure is suitable for working on

the ceramic; meanwhile an amorphous structure is brittle and hard.

8.7 EDS (ENERGY-DISPERSIVE X-RAY SPECTROSCOPY)

STUDIES

Energy-dispersive X-ray spectroscopy (EDS or EDX) is an

analytical technique used for the elemental analysis or chemical

characterization of a sample. It is one of the variants of X-ray fluorescence

spectroscopy which relies on the investigation of a sample through

interactions between electromagnetic radiation and matter, analyzing X-rays

emitted by the matter in response to being hit with charged particles. Its

characterization capabilities are due in large part to the fundamental principle

that each element has a unique atomic structure allowing X-rays that are

characteristic of an element's atomic structure to be identified uniquely from

one another.

To stimulate the emission of characteristic X-rays from a specimen,

a high-energy beam of charged particles such as electrons or protons, or a

beam of X-rays, is focused into the sample being studied. At rest, an atom

within the sample contains ground state (or unexcited) electrons in discrete

energy levels or electron shells bound to the nucleus. The incident beam may

excite an electron in an inner shell, ejecting it from the shell while creating an

electron hole where the electron was. An electron from an outer, higher-

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energy shell then fills the hole, and the difference in energy between the

higher-energy shell and the lower energy shell may be released in the form of

an X-ray. The number and energy of the X-rays emitted from a specimen can

be measured by an energy-dispersive spectrometer. As the energy of the X-

rays are characteristic of the difference in energy between the two shells, and

of the atomic structure of the element from which they were emitted, this

allows the elemental composition of the specimen to be measured.

8.7.1 Results and Discussion

Surface analysis and quantitative chemical analysis by EDS

confirmed the presence of all coating elements- aluminum, silicon, nickel,

chromium and oxygen in the duplex coating. The spectra shown are that of

four specimens tested. Figures 8.6 (a-d) show the EDS spectra.

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Figure 8.7 (a-d) EDS spectra of the duplex coating

Element Mass% Atom%Al K 73.51 74.43Si K 26.07 25.36Cr K 0.16 0.08Ni K 0.27 0.13Total 100 100

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Figure 8.7 (Continued)

Element Mass% Atom%O K 26.19 37.56Al K 68.3 58.09Si K 5.12 4.18Cr K 0.24 0.1Ni K 0.16 0.06Total 100 100

Element Mass% Atom%Al K 94.64 96.04Si K 2.75 2.68Cr K 1.06 0.56Ni K 1.55 0.72Total 100 100

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Figure 8.7 (Continued)

8.8 CONCLUSION

The optical micrographs and SEM images show the splat

morphology of the plasma sprayed coating and a good blending of the coated

layers with the substrate. The micrographs show an even distribution of

porosity with a maximum level of 16 to 20 %. The ceramic layer acts as a

composite by virtue of its porosity. The EDS spectra show the elements

present in the coating and XRD patterns show the presence of oxides of

aluminum and silicon, contributing to the enhanced performance of the

coating. The sharp peaks in the XRD patterns confirm the crystalline structure

of the duplex coating.

Element Mass% Atom%O K 12.97 20.25Al K 73.08 67.65Si K 13.24 11.78Cr K 0.24 0.11Ni K 0.48 0.2Total 100 100