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CHAPTER 7

STUDY OF THE THERMAL PROPERTIES OF

MULLITE COATINGS

7.1 INTRODUCTION

Ceramics are selected for many design applications requiring high

temperature exposure of parts and assemblies, due to their high heat capacity,

low thermal conductivity and coefficient of thermal expansion and high

thermal shock resistance. These properties are generally better in the case of

ceramics than in metals. In this chapter, thermal properties, namely the

thermal conductivity and thermal shock resistance of mullite coated cast

aluminum A 356.0 are measured and comparison with the literature values of

other commercially available ceramic materials is made. The following

thermal properties of ceramic materials are important in various design

considerations ( Dmitri Kopeliovich 2010):

1. Thermal conductivity

2. Thermal expansion

3. Heat capacity

4. Thermal shock resistance

5. Maximum service temperature

Comparison with the thermal shock resistance property of duplex

coated and functionally graded YSZ coatings and with aluminum oxide

coatings, have been made.

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Comparison of the thermal conductivity of mullite coatings, YSZ

coatings, alumina PEO coatings and mullite rich PEO coatings from literature

reports has been done.

7.1.1 Thermal Conductivity

Thermal Conductivity (k) is amount of heat passing in unit time

through unit surface in a direction normal to this surface when this transfer is

driven by unit temperature gradient under steady state conditions. Thermal

conductivity may be expressed and calculated from the Fourier’s law.

Q/ t = k*a * T/ x (7.1)

where, Q -heat, passing through the surface A; t - change in time;

k - Thermal conductivity; a - Surface area, normal to the heat transfer direction;

T/ x- temperature gradient along x – direction of the heat transfer.

Fourier’s law is analogous to Fick’s first law, describing diffusion

in steady state. Ceramics have low thermal conductivity compared to metals,

due to Ionic-Covalent bonding in ceramics which does not form free

electrons. For example, k of alumina = 6.3 W/m*K and k of aluminum

=231W/m*K.

7.1.2 Coefficient of Thermal Expansion

Thermal Expansion (Coefficient of Thermal Expansion) is defined

as the relative increase in length per unit temperature rise.

= L/ (Lo* T) (7.2)

where, -coefficient of thermal expansion (CTE); L – length increase;

Lo – initial length; T – temperature rise. Thermal expansion of ceramic

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materials is generally lower, than that of metals. For example, CTE of

SiC = 4.0ºC ¹, CTE of Al = 23ºC ¹.

7.1.3 Specific Heat Capacity

Heat capacity is amount of heat required to raise material

temperature by one unit. Specific heat capacity is amount of heat required to

raise temperature of unit mass of material by one unit.

S= Q/ (m * T) (7.3)

where, S -specific heat capacity; Q – amount of heat; m – Material mass; T

– temperature rise. Specific heat capacity of ceramic materials is higher, than

that of metals. For example, “S” of alumina = 850 J/ kg*K. “S” of steel =

481 J/ kg*K.

7.1.4 Thermal Shock Resistance

Thermal Shock Resistance is an ability of material to withstand

sharp changes in temperature. If a ceramic material is rapidly cooled, its

surface reaches the temperature of cooling environment and tends to contract

(thermal contraction). Since the interior regions of the material are still hot,

thermal contraction of the skin surface is impossible. This leads to formation

of tensile stress (thermal stress) in the skin. Such thermal stresses may cause

cracks and consequent failure, due to the brittle nature of ceramics. Thermal

shock resistance of a material may be estimated in accordance to the formula:

Rs = (k* F)/ ( *E) (7.4)

where, Rs – Thermal shock resistance; k - Thermal conductivity; F – flexural

strength; -coefficient of thermal expansion (CTE); E – Modulus of

elasticity.

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Sensitivity of ceramic materials to thermal shock may be also

determined by experimental method (Hasselmann Method) for bulk materials.

In this method a specimen (flexural test specimen) is heated to a specified

temperature and then quenched. The specimen cools rapidly by temperature

T (the difference between the specimen temperature before and after

cooling). After quenching the flexural strength of the quenched material is

measured by standard flexure (bending) test. The test results are plotted on the

graph Strength vs T. When T reaches a certain value the specimen strength

falls sharply. This value of T is a parameter indicating thermal shock

resistance of the material. Test pieces (in the simplest case these are bending

bars) are quenched to drop them from a temperature T1 to a temperature T2.

The strength of the samples is measured after the quenching. The curve of

strength against the temperature difference, T = T1- T2, has the shape shown

on figure 7.1. Up to a temperature difference of Tc the strength does not

alter. The strength then drops sharply within a narrow range, T. Up to Tc’

this reduced length then remains constant, falling away again at higher

temperature differences.

Figure 7.1 The strength of thermally shocked bending samplesaccording to Hasselman

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Test procedures for determining the resistance to temperature

change are specified in DIN V ENV 820-3 standards. Some ceramic materials

have very low coefficient of thermal expansion therefore their resistance to

thermal shock is very high despite of low ductility (e.g. fused silica): Rs of

fused silica / Rs of soda-lime-silica glass = 45.

Glass and some ceramic objects are particularly vulnerable to this

form of failure, due to their low toughness, low tensile strength, low thermal

conductivity, and high thermal expansion coefficients. However, they are

used in many high temperature applications due to their high melting point.

Thermal shock occurs when a thermal gradient causes different parts of an

object to expand by different amounts. This differential expansion can be

understood in terms of stress or of strain, equivalently. At some point, this

stress overcomes the strength of the material, causing a crack to form. If

nothing stops this crack from propagating through the material, it will cause

the object's structure to fail. The problem then becomes how to prevent

thermal shock while still maintaining the temperature extremes required by

the process.

When materials must be tested for their ability to withstand

temperature extremes, they are tested inside a thermal shock chamber. Within

the chamber they are exposed to rapid cycling of extreme hot and cold

temperatures, to determine the temperatures at which the tensile strength of

the material is overcome. This type of testing is used in a very broad range of

industries, including land, air, and spacecraft development, as well as

industrial manufacturing.

Whereas high local thermal stresses in metals merely lead to a

slight local plastic deformation, they can lead to the propagation of cracks in

ceramic materials. For this reason sudden, large changes of temperature

should be avoided whenever possible.

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7.1.5 Maximum Service Temperature

Ceramic materials retain their properties at elevated temperatures

due to the strong ionic-covalent bonding. Ceramics working at high

temperature are called refractory ceramic materials. Some Borides, carbides

and nitrides, having melting temperature above 3040 ºC, are used in high

temperature applications up to 1800 ºC to 3000 ºC. For example, scaling

(oxidation) temperature of refractory stainless steel AISI 310 is 1150ºC.

7.2 EXPERIMENTAL WORK ON THERMAL CONDUCTIVITY

7.2.1 Introduction

Plasma sprayed coatings are built up by successive accumulation of

molten or semi-molten splats on the substrate surface, forming thin lamellae

upon solidification. The thermal contact between these lamellae is not perfect

and is limited by the presence of pores and cracks at the interface between the

lamellae. The micrographs show these defects. Hence, the thermal properties

of plasma sprayed coatings are quite different from those of the bulk

materials. Thus, measuring thermal conductivity is important for thermally

sprayed coatings and especially for thermal barrier coatings where thermal

properties are of great importance. The thermal conductivity is given by the

following expression:

k= S (7.5)

where, k is thermal conductivity, is thermal diffusivity, is the density, and

S is the specific heat capacity of a material. In this study, the thermal

conductivity of the coated specimens is determined experimentally. Generally

ceramic materials are poor conductors of heat and are extensively used for

thermal barrier applications. In the present section, an attempt is made to

determine the thermal conductivity of mullite coated specimens in order to

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investigate the potential application of this material for IC engines. Hamed

Samadi (2009) in his work measured the thermal diffusivity and thermal

conductivity of free standing coatings of three ceramic materials, mullite,

forsterite and spinel during both the heating and cooling cycle and found that

the variation of both thermal diffusivity and conductivity is less and the

thermal diffusivity /temperature and the thermal conductivity/ temperature

curves are flatter in the case of mullite and also the values are less compared

to the other ceramics owing to its good thermal barrier properties. The tests

were conducted upto 1000°C. Based on this research study, since the thermal

conductivity did not vary with temperature for mullite, a lower test

temperature of 100°C was chosen for the experiments to determine the

thermal conductivity of duplex coated mullite specimens.

7.2.2 Experimental Set-up

Thermal conductivity was measured by the Lee’s Disc apparatus

shown in figure7.2. The apparatus consists of a steam boiler sufficiently filled

with water and its mouth is closed by a rubber stopper. There is an outlet

through which steam/water vapor escapes. The steam boiler is placed on a hot

plate. The hot plate distributes heat evenly to the steam boiler. A metallic disc

suspended from a stand, has a hole through which a thermometer T2 is

inserted. The insulator specimen whose thermal conductivity is to be

measured is placed over the metallic disc. The diameter of the bad conductor

is the same as that of the metallic disc. A metallic disc is placed on the bad

conductor. A hole is provided through which a thermometer T1 is inserted.

A hollow steam chamber is placed on the metallic disc. It has an outlet and

inlet. Steam is made to pass from the outlet of the boiler to the inlet of the

steam chamber through a rubber tube.

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Figure 7.2 Lee’s disc apparatus

The following measurements are made before conducting the

thermal conductivity study.

1. The thickness of the bad conductor ‘d’ and metallic disc ‘h’.

2. The mass of the metallic disc, M.

3. The radius of the metallic disc ‘r’.

Then the hot plate is switched on. The steady state temperatures 1

and 2 are noted from the thermometers T1 and T2 . Now the bad conductor is

removed and 2 increases. It is allowed to increase till 2 + 10°C, after which

the steam chamber is also removed. The temperature drops and for every 2°C

fall in temperature the time is noted till 2 – 10°C. Now a graph is drawn with

time ‘t’ along x- axis and temperature along y- axis. The slope d /dt is found.

The time taken for the temperature to drop each degree from 80°C to 70°C

was measured. Slope is found in-between the values of 2 + 1°C and 2 -

1°C.The specific heat of the metallic disc is taken as S= 385 J/kg/K (ASM

handbook, Vol.II).

The coated specimen is shown in the Figure 7.3.

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Figure 7.3 Coated specimens for thermal conductivity test

The thermal conductivity of the bad conductor is determined from

the formula

K/m/Wh2r2r

)h2r(dddMS

k21

2t

1

(7.6)

where , M is the mass of the metallic disc :

850x10-3 kgs

S1 is the specific heat capacity of the disc : 385 J/kg/K

d / dt is the rate of cooling at :

0.028 °C /s

d is the thickness of the bad conductor : 6x 10-3 m

r is the radius of the metallic disc : 55 x 10-3 m

h is the thickness of the metallic disc 10 x 10-3 m

1 is the steady temperature of the steam chamber : 98 °C

2 is the steady temperature of the bad conductor : 75 °C

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The measurements were conducted on the following types of samples

1. T6 treated cast aluminum A 353.0 without bond coat and coated

with 100 µm thick mullite (single layer mullite coating).

2. T6 treated cast aluminum A 353.0 without bond coat and coated

with 150 µm thick mullite (single layer mullite coating).

3. T6 treated cast aluminum A 353.0 with bond coat of nickel

chrome (100 µm thick) and with 100 µm thick mullite (duplex

coating).

4. T6 treated cast aluminum A 353.0 with bond coat of nickel

chrome (150 µm thick) and with 150 µm thick mullite (duplex

coating).

Note: The coating thickness varied from 100 µm to 150 µm

for the single layer coating and 200 to 350 µm for the duplex

coating. Nominal values have been used.

7.2.3 Results and Discussion

Five specimens each were tested and the average results are shown

in Table 7.1.

Table 7.1 Results of the thermal conductivity measurements

S.No. Descriptionof specimen

Thermal conductivity inw/m/k

1 Type 1 0.192

2 Type 2 0.181

3 Type 3 0.170

4 Type 4 0.151

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An average value of 0.151 Wm-1K-1 @ RT( room temperature) may

be taken as the thermal conductivity of the coated specimens of this study.

A comparison of the thermal conductivity of other materials is given in

Table 7.2. Literature reports of the thermal conductivity measurements are

also shown.

Table 7.2 Comparison of thermal conductivity of materials

S.No. Material Thermal Conductivityin Wm-1K-1 @RT

1 Copper 3852 Aluminum 2013 Brass 1104 Lead 355 Card board 0.046 Cork 0.057 Glass 1.08 Rubber 0.159 Wood 0.15

10 YSZ coatings 1.25 to 1.4511 Mullite coatings (by others) 1.512 Alumina PEO coatings 1.513 Mullite rich PEO coatings 0.514 Mullite, bulk 6.06

Note : Reference Carl L. Yaws

Thermal conductivity(TC) of mullite coatings differed by a factor of 10

from other coatings reported due to the thickness of the coated layer (350

µm), the microstructure and the processing technique used. Plasma sprayed

coatings result in a splat structure with discontinuities and porosity of 20%

maximum and hence are good thermal barriers. One coating has used an

advanced technique like PEO (Plasma electrolytic oxidation), (Curran et al,

2007) with coating thickness of 200 µm and the microstructure seems to

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contain less pores and defects. The TC reported was 1.5 W/m/K for alumina

rich coating and 0.5 W/m/K for mullite rich PEO coatings. Another coating

with thickness of 1 mm (Hamed, 2009), using the air plasma technique

reported a TC of 1.5 W/m/K with 14 % porosity. Also the measuring

apparatus used was a “Lee’s Disc apparatus” in this study, against other

techniques like laser flash method used by others. Factors affecting the TC are

a. Thickness of the coating.

b. Composition of the coating.

c. Microstructure of the coating.

d. Defects like pores, discontinuities, voids and cracks in the

coating.

The TC of bulk mullite is reported as 6.06 W/m/k at room

temperature and 0% porosity (CRC Material Science & Engg. Handbook,

page 287, 3rd edition).Coated mullite will have a lower TC due to the defects

present. Figures 7.4 and 7.5 show the splat structure of the coating, the

micrographs with defects in the coating and the SEM image of a PEO coating.

(a)Figure 7.4 (Continued)

Splatstructureofcoating

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(b)

(c)

Figure 7.4 (a) and (b) SEM images showing the splat structure of thecoating, the pores and discontinuities, (c) Optical imageshowing coating discontinuities

Figure 7.5 SEM micrograph (SE mode) of a polished section through a200 m thick PEO coating (Curran et al, 2007)

Mullitetop coat

20 µ m

Coatingdiscontinuities

Pores

Voids &Discont-inuities

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For ceramics, the effects of crystalline nature, lattice imperfections,

and mixtures, factors such as internal porosity, grain boundaries and

impurities can affect this property. Higher or lower levels of thermal

conductivity can be attained in fine ceramic materials by controlling these

factors. In thermal barrier coatings and other ceramic oxides, heat is

conducted by lattice waves. The conductivity is composed of contributions

from a spectrum of waves, interaction between lattice waves (intrinsic

processes), scattering by atomic scale point defects and scattering by extended

imperfections such as grain boundaries (Klemens and Gell 1998).

Heat is transported in solid materials by both lattice vibration

waves (phonons) and free electrons. A thermal conductivity is associated with

each of these mechanisms, and the total conductivity is the sum of the two

contributions, or

k = kl + ke (7.7)

Where kl and ke represent the lattice vibration and electron thermal

conductivities, respectively. The thermal energy associated with phonons or

lattice waves is transported in the direction of their motion. The kl

contribution results from a net movement of phonons from high to low-

temperature regions of a body across which a temperature gradient exists.

Free or conducting electrons participate in electronic thermal conduction. To

the free electrons in a hot region of the specimen is imparted a gain in kinetic

energy. They then migrate to colder areas, where some of this kinetic energy

is transferred to the atoms themselves (as vibrational energy) as a

consequence of collisions with phonons or other imperfections in the crystal.

The relative contribution of ke to the total thermal conductivity increases with

increasing free electron concentrations, since more electrons are available to

participate in this heat transference process.

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Metals are extremely good conductors of heat because relatively

large numbers of free electrons exist that participate in thermal conduction.

The thermal conductivities of several of the common metals generally range

between about 20 and 400 W/m-K. Nonmetallic materials like ceramics are

thermal insulators inasmuch as they lack large numbers of free electrons.

Thus the phonons are primarily responsible for thermal conduction: ke is

much smaller than kl. Again, the phonons are not as effective as free electrons

in the transport of heat energy as a result of the very efficient phonon

scattering by lattice imperfections. Room-temperature thermal conductivities

range between approximately 2 and 50 W/m-K. Glass and other amorphous

ceramics have lower conductivities than crystalline ceramics, since the

phonon scattering is much more effective when the atomic structure is highly

disordered and irregular. Porosity in ceramic materials may have a dramatic

influence on thermal conductivity; increasing the pore volume will, under

most circumstances, result in a reduction of the thermal conductivity. In fact,

many ceramics that are used for thermal insulation are porous. Heat transfer

across pores is ordinarily slow and inefficient. Internal pores normally contain

still air, which has an extremely low thermal conductivity—approximately

0.02 W/m-K (Callister, Jr. and Rethwisch, 2010)

7.3 THERMAL SHOCK TEST (TST)

7.3.1 Introduction

Thermal Shock is performed to determine the resistance of the part

to sudden changes in temperature. The parts undergo a specified number of

cycles, which start at ambient temperature. The parts are then exposed to an

extremely low (or high) temperature and, within a short period of time,

exposed to an extremely high (or low) temperature, before going back to

ambient temperature. Burner rig durability testing is performed to study the

effect of cyclic thermal gradients on coatings. In this test, a heating period

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(e.g., 4 min) followed by a cooling period (e.g., 6 min) is applied to the top

coat. Various heating sources have been employed, including a burner and

laser. After the final cycle, external visual examination of the specimens is

performed at 10 X to 20 X. An illegible mark and/or any evidence of damage

to the specimen after the stress test shall be considered a failure. Failure

acceleration due to thermal shock and temperature cycling depends on the

following factors: 1) the difference between the high and low temperatures

used 2) the transfer time between the two temperatures and 3) the dwell times

at the extreme temperatures. For reliability testing or qualification of new

devices, 1000 temp cycles are usually performed, with interim visual

inspection at 200X and 500X.

7.3.2 Experimental Work

Thermal cycling was carried out for the duplex coated T6 treated

samples by placing the specimens in a muffle furnace maintained at 550°C for

a holding time of 10 minutes and blowing compressed air on the specimens

after removing from the furnace, to cool the samples quickly to room

temperature. Repeated runs were carried out to notice any sign of apparent

failure. The test is repeated to check the number of cycles before spallation.

The specimen geometry is shown in figure in chapter 3.

7.3.3 Results and Discussion

During the thermal shock process, there are two main factors

destroying the coatings. One is that the oxidation at high temperatures affects

the combining state of interfaces. The other is that there is a difference of

thermal expansion coefficient between the coating and the substrate materials.

With the temperature up or down, the volume of the two materials changes

differently and this leads to interior stress. The greater the difference of

thermal expansion coefficient, the higher the stress. During the cycling, the

stress keeps rising and when local stress exceeds the strength limitation of

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coating material, the cracks form and grow continuously until the coatings

spall. In this study six specimens were tested for 100 cycles, without any

noticeable degradation or spallation of the coating. The test was stopped after

100 cycles. The reason for this good performance is due to the high thermal

shock resistance of mullite and its high recrystallization temperature of

1000°C. Figure 7.4 shows the comparison of mullite coating with aluminum

oxide coating. The performance was comparable to functionally graded

coatings reported in literature and presented in Table 7.3.

Notes: Type 1: Al2O3 oxide coating on carbon steel substrates tested at 800°C.Type 2: Mullite coating

Figure 7.6 Thermal shock resistance of mullite coating

Table 7.3 Comparison of mullite coatings with FGC’s

S.No. Description

Yttria stabilizedzirconia/

NiCoCrAlY Hydroxyapatite/Ti-6Al-4V FGC

Fe3Al/Al2O3FGC

Mullitecoatedcast Al(DuplexCoating)

Duplexcoating FGC

1. Bond strengthin MPa 9.3 17.8 38 51 20

2. No.of thermalcycles 15 90 - 119 100

The test temperature used was 550°C only, for an IC engine

application and the recrystallization temperature of mullite being 1000°C no

Coating type

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apparent damage was expected in the specimens during thermal cycling. As

anticipated, there were no changes in the coated structure seen visually and

also through SEM images. Figure 7.6 below shows the SEM images.

( a)

(b)Figure 7.7 (Continued)

Splatstructureofcoating

Topsurfaceofcoating

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(c)

(d)

(e)

Figure 7.7 SEM images showing the specimens before testing : (a), (b),

(c) and after testing : (d) and (e)

Topsurfaceofcoating

Topsurfaceofcoating

Topsurfaceofcoating

Machiningmarks

Machiningmarks

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7.4 STATISTICAL ANALYSIS OF THERMAL

CONDUCTIVITY TEST

The experimental data for the thermal conductivity test is presented

in the Table 7.4.

Table 7.4 Thermal conductivity from experiments in W/m/K

Type of material( Coating thickness is nominal)

Total ofobservations yi

Averages yi

Cast aluminum A 356.0 (T6 treated) withsingle layer mullite coating(100 µm coating thickness)

1.03 0.206

Cast aluminum A 356.0 (T6 treated) withsingle layer mullite coating(150 µm coating thickness)

0.937 0.187

Cast aluminum A 356.0 (T6 treated) withduplex coating (200 µm coatingthickness)

0.85 0.17

Cast aluminum A 356.0 (T6 treated) withduplex coating (350 µm coating thickness) 0.738 0.148

yn = 3.555 yn = 0.178

Since four different material types and in each case five specimens

were tested, there are four levels (or treatments) and five observations at each

level. The objective is to test the appropriate hypothesis about the treatment

and to estimate them. The results of the statistical analysis for the thermal

conductivity of the specimens are presented in Table 7.5.

Table 7.5 Analysis of variance of the thermal conductivity data

Source ofvariation

Sum ofsquares

DOF Meansquare

Ratio of SampleVariance, F0

P-Value

Material type 0.0093 3 3.1x10-3 99.2 < 0.05Error 0.0005 16 3.125 x 10-5

0.0098 19

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It is seen from the above tables that between treatment mean

squares are many times larger than within treatment or error mean square. The

F ratios for the thermal conductivity is computed as F0 = 3.1x10-3/ 3.125 x

10-5 = 99.2 and compared with an appropriate upper tail percentage point of

F 3,16 distribution. For 5% level of significance (risk), from statistical table

(Douglas C.Montegomery 1997), F 0.05,3,16 = 3.24. Since F0 for the thermal

conductivity is greater than 3.24, the difference in material type (single layer,

duplex layer coating), significantly affects the mean thermal conductivity and

it can be concluded that an upper bound for the P-value is 0.05; that is

P < 0.05.

7.5 CONCLUSIONS

The thermal shock resistance properties of the T6 treated duplex

coating /substrate system is good and it has withstood 100 thermal cycles,

when exposed to an ambient temperature of 550°C in the furnace without

delamination of the coating taking place, due to its high melting point and

creep resistance. The strain rates are low and crack initiation and propagation

in the coating do not take place for operating temperatures of around 600°C,

normally found in IC engines. The thermal cycling test showed the enhanced

thermal behavior of the coated samples. Hence this coating can be used for IC

engine applications. The thermal conductivity of the T6 treated duplex coated

specimens is found to be low and suitable for thermal barrier applications. An

average value of 0.151 Wm-1K-1 was measured. Statistical analysis and

analysis of variance (ANOVA) confirmed the variation in the thermal

conductivity measurements of the specimens and also showed the lower

values for the duplex coated specimens of 350 µm thickness.