YSZ thermal barrier coating for automotive turbocharger turbine application

Materials and Design 109 (2016) 47–56

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Microstructural evaluation of a slurry based Ni/YSZ thermal barrier coating for automotive turbocharger turbine application Muhammad Rabiu Abbas a,b,c, Uday M. B. a,b, Alias Mohd Noor a,b,⁎, Norhayati Ahmad b, Srithar Rajoo a,b a UTM Centre for Low Carbon Transport in cooperation with Imperial College London, Institute for Vehicle System and Engineering, Universiti Teknologi Malaysia, 81310, UTM Skudai, Johor Bahru, Malaysia b Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia c Hassan Usman Katsina Polytechnic, P.M.B 2052, Katsina, Katsina State, Nigeria

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Slurry based system was used to fabricate effective functionally graded thermal barrier coating for high temperature. • Increase in the percentage of Yttria Stabilized Zirconia gives good thermal resistance of the coating. • Appropriate percentage of Nickel powder in the composition reduced spallation of the coating. • Bonding strength of the coating with substrate increases with an increase in the sintering cycles and number of layers.

a r t i c l e

i n f o

Article history: Received 10 April 2016 Received in revised form 12 July 2016 Accepted 13 July 2016 Available online 15 July 2016 Keywords: Thermal barrier coating Microstructure Automotive turbocharger Ceramics Metal Slurry

a b s t r a c t In the present study, the method of slurry coating process was used to fabricate functionally graded thermal barrier coating (FG-TBC) for turbocharger application. An automatic film applicator was used for the coating in an attempt to minimize the production cost of ceramic thermal barrier coatings. Yttria stabilized zirconia ceramic and nickel metal powders were mixed in appropriate proportion and produced a functionally graded material mixture. A slurry based coating method was used for the fabrication of the FG-TBC. The coating compositions of 30 wt% Yttria Stabilized Zirconia and 70 wt% Nickel, 55 wt% Yttria Stabilized Zirconia and 45 wt% Nickel as well as 75 wt% Yttria Stabilized Zirconia and 25 wt% Nickel were used in depositing the first, second and third layers of the functionally graded thermal barrier coating on the substrate respectively. Field Emission Scanning Electron Microscopy, Scanning Electron Microscope and X-ray Diffraction were used to study and evaluate the integrity and reliability of the coating produced. The results have indicated the sustainability and suitability of the method adopted in the study with regards to the production of good and quality functionally graded thermal barrier coating with no spallation problem and having good adherence to the substrate. © 2016 Published by Elsevier Ltd.

1. Introduction ⁎ Corresponding author at: UTM Centre for Low Carbon Transport in cooperation with Imperial College London, Institute for Vehicle System and Engineering, Universiti Teknologi Malaysia, 81310, UTM Skudai, Johor Bahru, Malaysia. E-mail address: [email protected] (A.M. Noor).

http://dx.doi.org/10.1016/j.matdes.2016.07.070 0264-1275/© 2016 Published by Elsevier Ltd.

Turbocharging is a common technique use to increase the power density of internal combustion engine. Turbocharging reduce fuel consumption and pollutant emission [1–3]. Turbocharger used the

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extracted energy of the engine exhaust gas by expanding it through the turbine which in turn drives the compressor by a shaft and compress ambient air into the engine cylinder via the intake manifold. Studies have shown that thermal energy transfer from the turbocharger turbine seriously affects the turbine power and in turn affects the overall turbocharger performance [4–6]. However, experimental data from several studies have shown that insulating the turbocharger turbine can significantly improve the non-adiabatic performance of the turbocharger and hence the overall engine efficiency [4,7,8]. There are several research efforts which were concentrated towards the development and fabrication of thermal barrier coatings (TBC) for the thermal insulation of components operating in severe temperature conditions [9]. Thermal barrier coating is a thin layer of material(s) having high insulating properties which are bonded to a metal substrate to insulate and protect it from temperature excursion or damage by foreign object. Thermal barrier coating materials selection is constrained by requirements like lower thermal conductivity, high melting point, good adhesion to substrate, chemical inertness, lower thermal expansion mismatch with the substrate as well as absence of phase transformation between operation and room temperatures [10]. The application of thermal barrier coating can significantly improve the operating temperature capability, improve the efficiency, reliability and durability of a number of engineering components operating under elevated temperature environments [11]. Many applications including but not restricted to automotive industry, gas turbine, nuclear industries, aerospace and heavy-duty utilities such as diesel trucks have benefited from one of these techniques [9,11–20]. Hitherto, only a few ceramic materials had been found to basically satisfy the basic requirements of thermal barrier coating, and among these materials Yttria Stabilized Zirconia (YSZ) is the most generally used and studied material for thermal barrier coating application due to its excellent performance capabilities in applications operating under severe temperature condition like gas turbine and diesel engines [10,21–24]. Furthermore, research findings have indicated that Nickel powder effectively reduce residual thermal stresses between the ceramic coating and the substrate through reducing the thermal expansion mismatch between them [25,26]. Furthermore, as reported by Polanco, Miranzo and Osendi [27] the inclusion of Nickel within the ceramic coating does not lead to phase variations within the thermal barrier coating material. Functionally graded thermal barrier coating (FG-TBC) is a new thermal barrier coating technique consisting of non-homogeneous materials whose composition and microstructure are varied according to a predetermined profile in order to enhance its thermo-mechanical properties and reduce spallation problems occurring in thermal barrier coated engineering materials [25,28,29]. Two or more different materials powder in most cases ceramic and metal are being mixed and used depending on the objective, applications and the nature of the substrate material. Recently, studies have shown that FG-TBC technique had received a lot of attention from various applications in the field of science and engineering and had been classified as heat-resistance, electronic, biological and chemical engineering FG-TBCs which of course all depend on the application [25]. The aim of the present research was to evaluate the quality, integrity and reliability of a functionally graded thermal barrier coating (FG-TBC) produced using automatic film applicator machine through conducting analysis like Scanning electron microscopy (SEM), X-ray diffraction (XRD), SEM-line scan EDX and mapping as well as Field emission scanning electron microscopy (FESEM) respectively.

0.001MgO) from Sigma-Aldrich, UK, having average particle size of 0.38 μm with commercially purity of 99.9% and Nickel powder (95.8Ni, 3.69Fe, 0.20Co, 0.123Ti, 0.081Zr, 0.025Cr2, 0.025Mo, 0.019C, 0.018Cu2, 0.011Mn, 0.004Pb, 0.004 Mg,) from Sigma-Aldrich, USA, having average particle size of 1.24 μm with commercial purity of 99.99%. Other coating materials used included Polyvinyl alcohol (PVA) from Sigma-Aldrich, USA, with commercial purity 99% serving as the binder, and distilled water as the solvent. Also a commercially obtained Nickel alloy (95.49Ni, 4.17Co, 0.148Ti, 0.078Zr, 0.029Cr2, 0.025Mo, 0.019C, 0.018Cu2, 0.013Mn, 0.0047Pb, 0.004 Mg, 0.002P) from Baoji Tianbang Ti & Ni Co., China, was used in this study as the substrate material for the coatings.

2. Materials and methods

Experiments were conducted with FG-TBC coated samples and uncoated sample subjected to the heat flux supplied by oxy-acetylene flame using a rectangular combustion chamber (200 mm × 200 mm × 300 mm). The oxy-acetylene flame supplied a temperature of 1000 °C to the combustion chamber environment. The combustion chamber environment temperature was measured using infrared thermometer (Amprobe, USA) and later confirmed with a highly customised k-type thermocouple (CAHO

2.1. Materials The raw materials used in this study were commercially obtained Yttria stabilized zirconia (YSZ) powder (91.02ZrO2, 7.11Y2O3, 1.58HfO2, 0.091MnO, 0.085Fe2O3, 0.079SiO2, 0.03TiO2, 0.004CaO,

2.2. The coating process The Nickel alloy substrate was first cut into pieces in the dimensions of 60 mm × 30 mm × 4 mm, all the dimensionally cut substrates were then sand blasted using GW-18 model of sand blasting machine (Growell Manufacturing, Thailand). The sand blasted substrates were washed with detergent then they were washed with acetone for 15 min using an using ultrasonic machine by immersing them into a beaker containing acetone so as to remove dirt and any other conterminants. The ceramic powder together with the nickel powder were appropriately mixed inside the ball milling jar containing the solvent (distil water) in the presence of binder and dispersant after which it was ball milled using ball milling machine from Nidec-Shimpo Corporation, Japan, for 3 h in the presence of 5 mm diameter zirconia balls using the ratio of balls to powder of 10:1. An automatic film applicator (Sheen Instruments, UK) was used for coating the nickel substrate with the FG-TBC material. The machine was equipped with different wire-wound bars with very small gaps of 150 μm, 200 μm and 300 μm which was based on the user requirements in order to have the required coating thickness frequently used for thermal barrier coating applications. The automatic film applicator was then switched-on from the main supply and a wire-wound bar of 150 μm, 200 μm and 300 μm were used for the coating of the first layer, second layer and third layer of the FG-TBC respectively on the substrate at different occasion on the machine. The substrate was placed directly under the wirewound bar and the whole set up was adjusted until when there was a grip between the substrate and the wire-wound bar material. A required amount of the prepared slurry was then poured on to the substrate and at a draw-down speed of 50 mm/s the wire-wound bar mounted on the carriage was ‘draw down’ across the entire surface of the substrate and as a result it provided a smooth and uniform coating on the substrate due to the good control of the shear rate of the wirewound bar during coating. The slurry composition of the FG-TBC for the first, second and third layers were 30 wt% YSZ-70 wt% Nickel, 55 wt% YSZ-45 wt% Nickel and 75 wt% YSZ-25 wt% Nickel respectively. The coated samples were then allowed to dry under atmospheric condition for 24 h after which it was further dried in an oven at a temperature of 100 °C for 4 h before proceeding to the sintering stage. A programmable KSL-1800× high temperature muffle furnace from MTI Corporation, USA, was used for this purpose in the presence of Argon air. The samples were effectively sintered at 1200 °C for 2 h to ensure effective sintering of the nickel powder contained in the FG-TBC composition. Fig. 1 shows the pictorial image of the different coated FG-TBC samples produced. 2.3. Coating test on high temperature rig

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Fig. 1. The pictorial image of the different coated FG-TBC samples produced (a) One layer, (b) Two layers, (c) Three layers.

Shiuan Rong, Taiwan). A Pico T-08 model of temperature data logger (Pico Technology Limited, UK) was used for the recording of the temperatures measured from the test rig via the thermocouples. All the tests were conducted under the initial temperature conditions of 30 °C and standard atmospheric pressure of 1.01325 bar. The readings of the rear surface temperature of the coated samples and uncoated sample were recorded with two k-type thermocouples situated at different positions on the sample up to the time when the combustion chamber temperature attained almost a steady state condition. One layer, two layers, three layers FG-TBC coated samples and the uncoated sample were tested for the duration of 90 min each on the test rig, several tests on the samples (both coated and uncoated) were conducted from which their average rear surface temperatures were determined. Coated sample micro-examinations were conducted before and after the high temperature test in order to evaluate the durability and reliability of the FG-TBC produced. The uncoated sample was also tested on the test rig so as to find out the amount of heat the substrate is able to resist.

from AT-A DeFelsko Corporation, USA, was used in this study in determining the adhesion strength of the different coated layers of the FGTBC samples. A 20 mm dolly size was chosen and used for the test based on the sample sizes as well as the manufacturer's specifications. The dolly head was carefully coupled into the actuator assembly and ensured that it has been correctly and firmly connected into the assembly (Fig. 2 a). After all the necessary settings were done and checked, the test was conducted until failure occurred, that is by pulling away the coated area from the substrate and the readings were recorded (Fig. 2 b). Seven samples each from the different coated layers of the FG-TBC

2.4. Microstructure observation Samples were cut into rectangular bars to the dimensions of 30 mm × 10 mm × 4 mm by using a PICO155 diamond precision cutting machine (Buehler Isomet 400, Germany). The cut samples were then resin mounted before embarking on the grinding process. All the coated samples were surface grinded with Mecapol P260 grinding/polishing machine (Wei Pu Precision Hardware, China) prior to the polishing operation of the coated sample surfaces. The grinding processes were performed using Struers water proof silicon carbide (SiC) abrasive foils of grit sizes between 400 and 1200. The final stage in the sample preparation for the microstructural analysis is the polishing of the grinded samples. The samples were polished using 2 μm alumina paste and then cleaned with aceton bath. Field Emission Scanning Electron Microscope (FESEM, VP35 Zeiss Supra, Germany) equipped with Energy Dispersive X-ray spectroscope (EDX), Scanning Electron Microscope (SEM S3400 N, Hitachi, Japan) for microstructure observation, Variable Pressure Scanning Electron Microscope (VP-SEM JSM-IT300, Joel, Japan) for the line-scan EDX and the mapping, and X-ray Diffraction spectroscopy (XRD, Siemens-D5000, Germany) were used for the observations of the microstructure of the raw materials and all the FG-TBC samples. 2.5. Mechanical characterization In order to evaluate the adhesion strength of the coating on to the substrate, a pull-off adhesion testing method which conforms to the ASTM standards with designation numbers of ASTM D4541 and D7234 was employed. A PosiTest automatic adhesion test equipment

Fig. 2. Adhesion test of FG-TBC (a) test set up, (b) after test showing broken coated layers.

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were used for measuring their respective adhesive strength, and average was taken from the results obtained for each respective layer to get the overall adhesive strength. The fracture image of the broken layers were analyzed using Scanning Electron Microscope (SEM S3400N, Hitachi, Japan). Chung and Chaudhury [30] combined both Griffith's equation for strength of materials and linear elastic fracture mechanics (LEFM) phenomena to obtain the pull-off adhesion critical stress σc using the assumptions of Hooke's law UE. UE ¼

1 F2 1 2 ¼ δ k 2 k 2

ð1Þ

where F is the tensile force, k is the stiffness of the system and δ is the tensile displacement. Thus, using Griffith's fracture principles the strain energy release rate Gc is obtained as shown in Eq. (2). Gc ¼


∂U E

−F 2c ∂ð1=kÞ
Fc ¼ 2 ∂A ∂A


ð2Þ

where A is area of contact and Fc is critical pull-off force. But for systems which dissipate heat, the thermodynamic adhesion work W is equivalent to the strain energy release rate Gc. The critical pull-off stress σc is obtained by rearranging Eq. (2). σc ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi    ∂ð1=kÞ −4W= πa3 ∂a

ð3Þ

To assess the adhesion using the PosiTest pull-off test method, the thermal stress arising from cooling of the FG-TBC/substrate system can propagate an initial crack along the interface of the FG-TBC/substrate. At a critical temperature, Tc, the debonding occurs as a result of the stress concentration close to the tip of the crack. The pull-off critical stress, σc for debonding of the FG-TBC can be obtained using Eq. (4): σC ¼

 Ef  α S −α f T C −T ref 1−γ

ð4Þ

where E is the elastic modulus, α is the coefficient of thermal expansion, and Tref is a reference temperature where the FG-TBC is assumed to be in a stress-free state and use to be the Tg in most cases. The subscripts “f” and “s” denote the FG-TBC and substrate respectively. 3. Results and discussions 3.1. Temperature reduction of different coating layers From the experiments conducted with the FG-TBC, it was found that the one layer FG-TBC has the capability of reducing the heat lost to the

environment by conduction through reducing the temperature gradient by about 40 °C then followed by the two layer FG-TBC which was found that it reduces the temperature gradient by about 130 °C and lastly the three layer FG-TBC which was found in reducing the temperature gradient by about 250 °C. First, high temperature tests were conducted with the uncoated samples using the same condition mentioned in Section 2.5, the average rear temperature was found as 893.68 °C, this shows that the uncoated sample provide a heat protection of 106.32 °C (i.e. 1000 °C–106.32 °C). The one layer FG-TBC samples were tested using the same condition as that for the uncoated samples. The average rear temperature was found as 853.68 °C, which indicates that it provides heat protection of 40 °C when the heat protection provided by the uncoated sample is considered (i.e. 1000 °C–106.32 °C–853.68 °C). After that, two layers FG-TBC samples were tested using the same condition for the uncoated samples and the average rear temperature of 763.68 °C was found, it therefore indicates that two layers FG-TBC provides a heat protection of 130 °C putting into consideration the heat protection provided by the uncoated sample. Finally, three layers FGTBC were tested using the same condition for the uncoated samples and the average rear temperature of 643.68 °C was found, thus it indicates that three layers FG-TBC provides a heat protection of 250 °C when the heat protection provided by the uncoated sample is considered. It can be seen therefore that the FG-TBC has a good heat protection capability for an effective thermal barrier application considering the results of other researchers on the heat protection provided by thermal barrier coatings in literature [31–34]. It can be observed from the results, that the temperature reduction ability of the FG-TBC increases with an increase in the amount of YSZ content in the coating compositions as well as the thickness of the coating. This can be attributed to the heat insulation capability of the YSZ. 3.2. Microstructure obtained by scanning electron microscope (SEM) Fig. 3 shows the scanning electron microscopy (SEM) of the different coated layer samples of the FG-TBC produced before the high temperature sample testing. It was observed that the presence of nickel metal in each layer varies. As can be seen from the figure, the nickel content keep decreasing as the number of layers increases which is due to the fact that the nickel content presence in the slurry composition of each layer during preparation and deposition of the FG-TBC slurry also differs from one another. It can be recalled that the first layer of the FG-TBC is the nickel rich layer (30 wt% YSZ–70 wt% Ni), then followed by the second layer (55 wt% YSZ–45 wt% Ni) and finally the ceramic rich layer which is the third layer (75 wt% YSZ–25 wt% Ni) as explained previously. Since the nickel rich layer is the one directly on the substrate, the arrangement was able to reduce the thermal expansion mismatch between the substrate and the first layer as they almost have similar properties. In subsequent layers the content of the nickel was kept on reducing and that of YSZ increasing so as to exploit the inherent thermal

Fig. 3. SEM micrographs showing different layer coatings before high temperature test (a) One layer, (b) Two layers, (c) Three layers.

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Fig. 4. SEM micrographs showing different coating layers after high temperature test (a) One layer, (b) Two layers, (c) Three layers.

properties of the ceramics since the aim is to have an effective insulation of the component. At the same time ensuring the elimination or reduction of the thermal expansion mismatch which seriously affect the durability, integrity and reliability of all thermal barrier coatings. There was no crack observed within the nickel rich areas or within the ceramic (YSZ) rich areas or at the interface between them in all the different FG-TBC layers which demonstrated their integrity and reliability. It can be understood that, the reason for having FG-TBC with no crack formation can be attributed to the close resemblance of the coefficient of thermal expansion (CTE) of first layer coating to that of the substrate. Also, the absence of crack formation between layers and within can be attributed to the same reason stated above. For example, the CTE of one, two and three layers FG-TBC were estimated as 11.3 × 10−6 K−1, 10.6 × 10−6 K−1 and 10.0 × 10−6 K−1 respectively. While the CTE for the substrate was found as 13.0 × 10−6 K−1. On the other hand, Fig. 4 shows the scanning electron microscopy (SEM) of the different coated layer samples of the FG-TBC produced after the high temperature sample testing. Also in the figures it was observed that the content of the nickel in each layer decreases as the number of the layers increases. This can be attributed to the fact that the percentage composition of the nickel presence during the FG-TBC slurry preparation decreases with increase in the number of layer(s) as explained earlier. Interestingly, in all the figures, no crack was observed within the FG-TBC coating either in the YSZ rich areas or the nickel rich areas or within the interface in all the different layers FG-TBC tested samples. There is a presence of nickel oxide that was observed in the result. This can be attributed to the fact that the nickel being in sub-micron size has reacted with the oxygen present in the oxy-acetylene flame as well as with the oxygen within the environment. This result is a clear demonstration of the integrity, durability and reliability of the FG-TBC produced. Furthermore, the result have clearly indicated the suitability of FG-TBC produced to be used as automotive turbocharger turbine volute casing material.

small traces of oxygen and yttria observed within the region in question, and this can be concluded that these elements are present as a result of the fact that the raw material composition of the YSZ contain such elements. In the intermetallic region (from the 82 μm to 122 μm range), a high nickel presence was observed and probably that is what gave the FGTBC the properties of good adhesion to the nickel metal substrate. In addition, there are small traces of zirconia and oxygen observed in the intermetallic layer [35]. Thus, it can be concluded that this happened as a result of diffusion of the elements during the sintering process. Finally, in the substrate region (from 122 μm to 136 μm range) a pure nickel with high intensity was observed, this is due to the fact that the substrate material is a nickel alloy with more than 94% composition of pure nickel. However, note that, there are some traces of carbon observed in the FG-TBC layer from point a to some few microns away (from 0 μm to 10 μm range). It is possible that the small amount of carbon present in the polyvinyl alcohol (PVA) has reacted with the nickel

3.3. EDX line scan analysis of different layers Fig. 5 shows the EDX line scan of the FG-TBC before the high temperature test. The line scan started at point a and ends at point b cutting across the FG-TBC layer, the intermetallic layer and the substrate. From the result the presence of high intensity zirconia was observed in the FG-TBC layer (from 10 μm to 80 μm range). The presence of the zirconia in the FG-TBC can be attributed to the raw material composition and of course that was what gave the thermal insulation properties of the FG-TBC. Also in the same region, the presence of nickel was observed but its presence is not as high as that of zirconia. The presence of the nickel also can be attributed to the composition of the raw material and of course its presence help in reducing the thermal expansion mismatch between the FG-TBC and the nickel substrate, which is one of the primary benefit of the FG-TBC concepts. Likewise there are

Fig. 5. EDX line scan of three coating layers before high temperature test.

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and form nickel carbide. Possibly, the reaction took place due to the fact that the sintering process was done in Argon (inert) gas environment. Therefore, there was limited oxygen available in the furnace for the carbon to react in order to form carbon (IV) oxide gas which will facilitate the formation of more pores within the FG-TBC. Fig. 6 shows the line scan EDX of the FG-TBC after the high temperature test. The scan started from point a and cut across the FG-TBC layer, the intermetallic layer and end at point b inside the substrate. High intensity of carbon and zirconia were observed inside the coating (from 0 to 45 μm range). The presence of zirconia in the FG-TBC layer can be attributed to the raw material composition of the coating slurry. The high presence of the carbon inside the black spot area of the FG-TBC layer indicates that most of the nickel have changed to nickel carbide during the high temperature testing possibly as a result of reaction between the nickel and carbon presence in the acetylene gas and atmosphere. Therefore, from the result it can be concluded that a carburizing flame category of the oxy-acetylene was used during the high temperature test. In the intermetallic region (from 50 μm to 90 μm range), relatively high presence of nickel and oxygen were observed and this may be attributed to the diffusion process of such elements into the layer. There is an increase of oxygen in the intermetallic layer after the high temperature test when compared with the result of the line scan of the FG-TBC before the high temperature test, this indicates that the layer have changed to nickel oxide. The traces of oxygen may have come from the oxygen in the oxy-acetylene flame and atmosphere, which then diffuses through the intermetallic layer. The zirconia traces in the intermetallic layer may be attributed to the diffusion process that takes place. The high traces of the nickel in the substrate region (from 90 μm to 150 μm) can be attributed to the substrate material composition. In all the figures (i.e. Figs. 5–6), the presence of nickel carbide and zirconia in the coating are indications of the high thermal resistance of the FG-TBC produced. 3.4. X-ray diffraction (XRD) analysis of different layers Fig. 7 shows the XRD pattern of the different FG-TBC layers on the substrate before the high temperature testing. As can be seen from the

Fig. 6. EDX line scan of three coating layers after high temperature test.

Fig. 7. XRD patterns of FG-TBC layers before high temperature test (a) First layer, (b) Second layer, (c) Third layer.

figure, the peaks of the YSZ increases from the first layer to the third layer while that of the nickel kept on decreasing from the first layer to the third layer. This can be attributed to the differences in the percentage composition of the YSZ and nickel present in each layer's slurry mixture. The percentage composition of YSZ increases from the first layer towards the third layer in order to exploit the full advantage of its insulation and heat resistance properties while on the other hand the percentage composition of the nickel increases from third layer towards first layer so as to exploit the full advantage of its bonding and thermal expansion match with the substrate material which is also nickel alloy. There is a peak of nickel oxide that was observed in the result. It may be as a result of the reaction between nickel and the oxygen present in the air during the slurry preparation, coating deposition and room temperature drying. Extremely small size (e.g. sub-micron and nano sizes) materials have high surface area per unit mass and it enable them to react with other element around them easily [36,37]. Therefore the lower the particle size of a material the higher the number of atoms on its surface and the more reactive it becomes with elements around it [38–40]. The XRD pattern of the result is also in agreement with other findings on YSZ-Ni mixture XRD peaks pattern available in literature [41]. Fig. 8 shows the XRD pattern of the different FG-TBC layers on the substrate after the high temperature testing. It was observed that the nickel has changed to nickel oxide. The reason can be attributed to the fact that the nickel being in extremely small size (i.e. towards sub-micron) has reacted with the oxygen present in the oxy-acetylene flame as well as with the oxygen within the environment. It is a known fact

Fig. 8. XRD patterns of different FG-TBC layers after high temperature test (a) First layer, (b) Second layer, (c) Third layer.

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that sub-micron and nano materials have high surface area which therefore made them to react with elements around them easily. 3.5. SEM-EDS mapping analysis The distribution of the elements on the coated sample was characterized by EDS elemental mapping. Fig. 9 shows the magnification SEM image and the corresponding Ni, Zr, O, C and Y elemental maps of the YSZ-Ni coating before the high temperature testing. The distribution of zirconium and nickel in the coating region varies from one another. The presence of the carbon in the coating can be attributed to the presence of binder (polyvinyl alcohol) in the coating composition. The nickel selectively attacked the carbon present in the binder and form nickel carbide. The fact that the nickel has reacted with the carbon and form nickel carbide is desirable due to the high heat resistance capability of the latter. It can be noted that from the EDS mapping analysis

53

that there were formations of aggregate compound of Ni, Zr, O, Y and little C. Fig. 10 shows the EDS mapping of the FG-TBC after the high temperature test. The mapping covered the entire FG-TBC coated layer, inter metallic layer and the substrate. The figure shows the magnification of the SEM image and the corresponding Ni, Zr, O, C and Y elemental mapping of YSZ-Ni coating after the high temperature testing. High intensity of carbon and zirconia were observed inside the coating, the carbon presence can be attributed to the oxy-acytelene flame. The presence of zirconia in the FG-TBC layer can be attributed to the raw material composition of the coating slurry. The high presence of the Ni and O inside the intermetallic area indicates that most of the nickel have changed to nickel oxide during the high temperature testing possibly as a result of reaction between the nickel and oxygen presence in the acetylene gas and atmosphere. Therefore, from the result it can be concluded that a carburizing flame

Fig. 9. SEM-EDS Mapping of three coating layers before high temperature test.

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Fig. 10. SEM-EDS Mapping of three coating layers after high temperature test.

category of the oxy-acetylene was used during the high temperature test. In the substrate region, relatively small traces of Y and Zr were observed and this may be attributed to the diffusion process of such elements into the substrate. 3.6. Mechanical characterization The quality of thermal barrier coating strongly depends on the adhesion strength between the substrate and the coating as well as on the cohesion between coated layers [42]. In order to determine the appropriate adhesion strength of the FG-TBC, seven samples each from the different coated layers of the FG-TBC were used for testing their respective adhesive strength. An average was taken from the test results obtained for each respective layer to get the overall adhesive strength.

Fig. 11 shows the results of the average adhesion strength of the coating before and after the high temperature test. It was observed that the adhesion strength of the coating increases with an increase in the number of coated layers, this can be attributed to the increase in the surface roughness from the substrate towards the first and second layers. In a multilayered thermal barrier coating, cohesive failure of the coating usually occur within the layers and at the interface. But it should be noted that it can also occur in a more general way, for example a case in which it starts in a particular region and then expands to other region [42]. Fig. 12 shows the SEM image of the broken surfaces of the three layers FG-TBC after the adhesion test was conducted. From the image, the general cohesive failure can be clearly seen in which the failure started from one region (a specific layer) and then expand to other regions (across several) layers.

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Fig. 11. The adhesion strength comparison of different coating layers before and after the high temperature test.

4. Conclusions Functionally graded thermal barrier coating was successfully fabricated on a nickel alloy using simple and cheaper thermal barrier coating technique. It was observed that the FG-TBC deposited on the nickel alloy has greatly improved its mechanical properties. Yttria stabilized zirconia mixed with nickel metal powder based on proportions of 30 wt% YSZ & 70 wt% Ni for first layer coating, 55 wt% YSZ & 45 wt% Ni for second layer coating and 75 wt% YSZ & 25 wt% Ni for the third layer coating was found to be a good and suitable functionally graded thermal barrier coating (FG-TBC) material. These compositions were found to give the best result with respect to the thermal insulation and the adhesion strength of the FG-TBC. The microstructural analysis of the coating have demonstrated the quality nature the type of FG-TBC produced in this way. There were no cracks observed in all the FG-TBC samples for both before and after the high temperature testings. The distribution of the ceramic-nickel materials in the respective layers as observed from the FG-TBC microstructural images conforms with their percentage compositions during the slurry preparation as revealed in the linescan EDX and SEM-EDS mapping analysis. The phases in which the raw YSZ and nickel powder exist was revealed from the microstructure analysis. It was observed that both the YSZ and nickel powders exist in poly crystalline phases. The adhesion strength of the FG-TBC for both

before and after high temperature testing were found to increase with an increase in the number of layers. From the analysis, the adhesion strength of one layer, two layer and three layer FG-TBC before the high temperature testings were found to be 2.63 MPa, 3.08 MPa and 3.59 MPa respectively. Similarly, adhesion strength of the FG-TBC after the high temperature testings were found to be 2.88 MPa, 3.07 MPa and 3.44 MPa for the one layer, two layer and three layer FG-TBC respectively. After the high temperature testings, a slight increase in the coating strength was observed for the one layer FG-TBC while in the case of three layer FG-TBC a slight decrease in the strength was noticed. In all the cases, the increase in the strength as the number of layers increase can be attributed to the increase in the surface roughness of FG-TBC layers. Acknowledgement The authors would like to thanks the UTM-Centre for Low Carbon Transport in cooperation with Imperial College London, Faculty of Mechanical Engineering and Universiti Teknologi Malaysia (UTM) for providing the research facilities. This research work has been supported by the UTM Research University Grant (01G51) and the grant (4F445) from Ministry of Education Malaysia (MOE). References

Fig. 12. SEM image of three layers FG-TBC after adhesion test showing failure across the different layers.

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