JTTEE5 25:213–221 DOI: 10.1007/s11666-015-0345-9 1059-9630/$19.00 Ó ASM International

Tao Liu, Shu-Wei Yao, Li-Shuang Wang, Guan-Jun Yang, Cheng-Xin Li, and Chang-Jiu Li (Submitted May 23, 2015; in revised form September 30, 2015) The infiltration of molten CMAS in thermal barrier coatings (TBCs) at high temperature is significantly affected by the microstructure of the ceramic coating. Enhancing the bonding ratio between splats can reduce the interconnected pores and suppress the infiltration of the molten CMAS into the coating. In this study, a dual-layered (DL) TBC with the dense 8YSZ on the top of the conventional porous 8YSZ was proposed to enhance CMAS corrosion of atmospheric plasma-sprayed YSZ. The dense YSZ coating with improved lamellar bonding was deposited at a higher deposition temperature. The microstructure of the coatings before and after CMAS attack test was characterized by scanning electron microscopy. It was clearly revealed that by adjusting the microstructure and applying a dense ceramic layer with the improved interface bonding on the top of porous TBC, the infiltration of CMAS into porous YSZ coating can be effectively suppressed. Moreover, by designing DL TBCs, the thermal conductivity of the TBC system exhibits a limited increase. Thus with the design of DL structure, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved.

Keywords

atmospheric plasma spraying, CMAS, corrosion resistance, interface bonding

1. Introduction Thermal barrier coatings (TBCs) are widely applied to protect the metal substrate of the hot section components of gas turbine. TBC system is usually composed of a ceramic top coat as thermal insulation layer with low thermal conductivity, a metallic bond coat (BC, MCrAlY, M=Ni/Co). Moreover, the ceramic top coat is usually fabricated by either electron beam physical deposition (EB-PVD) or atmospheric plasma spraying (APS). Currently, zirconia containing 6-8 wt.% of yttria (8YSZ), which exhibits low thermal conductivity and excellent mechanical properties, is used as standard material of ceramic top coat of TBCs (Ref 1-3). Generally, the performance of TBCs includes two important aspects: thermal insulation ability and life duration during thermal cycle. TBCs are designed to reduce the temperature of the metallic substrate. Therefore, low This article is an invited paper selected from presentations at the 2015 International Thermal Spray Conference, held May 11-14, 2015, in Long Beach, California, USA, and has been expanded from the original presentation. Tao Liu, Shu-Wei Yao, Li-Shuang Wang, Guan-Jun Yang, Cheng-Xin Li, and Chang-Jiu Li, State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, XiÕan Jiaotong University, XiÕan, China. Contact e-mail: [email protected].

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thermal conductivity is essential for TBCs. For certain ceramic materials, by choosing different deposition approaches and adjusting the deposition parameters, the coatings with different microstructures and consequently different properties including the thermal conductivity could be achieved (Ref 1-3). According to previous studies, APS ceramic coatings with a lamellar structure exhibit very low thermal conductivity, which is almost less than 1/2 of that of the corresponding bulk material (Ref 4-6). However, the factors influencing the failure of the TBCs are complicated (Ref 7, 8). One of the failure mechanisms is attributed to fine sand particles deposition on the coating surface as molten calcium-magnesium-alumina-silicate (CMAS) glass, penetrating into the ceramic coatings through the connected pores and subsequently resulting in the loss of strain tolerance of the coatings and premature failure of TBCs. Moreover, CMAS reacts with ceramic matrix, which remarkably changes both the composition and the structure of ceramic coating (Ref 9, 10). The penetration of CMAS increases reaction area, and as a result, significantly accelerates the reaction process. Nowadays, increasing the inlet gas temperature is an effective approach to increase the efficiency of gas turbine. However, under higher temperature condition over 1200 °C, the deposition and penetration of molten CMAS become a severe problem that affects the duration of TBCs significantly (Ref 11). Therefore, in order to fabricate TBCs with high performance, the penetration of CMAS must be suppressed. Many investigations have attempted to enhance the hot corrosion resistance of TBCs (Ref 12-16). One of the promising methods is to fabricate a dense layer at the surface of TBCs. This dense layer physically suppresses the infiltration of CMAS (Ref 14-16). In this study, dense ceramic layer was deposited through APS. According to

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Plasma-Sprayed Thermal Barrier Coatings with Enhanced Splat Bonding for CMAS and Corrosion Protection

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recent studies, the interface bonding between splats during APS is significantly affected by the substrate temperature in terms of deposition temperature before the molten droplets impact on Ref 17-19. By increasing the deposition temperature during plasma spraying, Zhang et al. deposited a dense Scandia-stabilized zirconium (ScSZ) coating, which exhibited a low gas leakage rate and was successfully applied to solid fuel oxide cell (SOFC) as the electrolyte (Ref 20). Compared with the transportation of gas phase, the penetration of molten CMAS in the coatings is more difficult. Therefore, to deposit a dense layer by increasing the deposition temperature is a promising approach to enhance the corrosion resistance of TBCs against CMAS droplets. In addition, it is a convenient approach to deposit a dense layer through APS without any post-spray treatment. Therefore, in this study, a traditional porous 8YSZ coating and a dense 8YSZ coating were deposited by APS to construct a dual layer TBC. The microstructure of the coatings before and after thermal exposure with CMAS at 1250 °C was characterized to examine the effectiveness of the introduction of a dense top layer for reducing CMAS corrosion. Moreover, thermal conductivity of dual layer TBC was also examined.

2. Experimental Procedure 2.1 Sample Preparation According to the previous study, the shape and size of spray powders influence the molten degree of spray particles in a plasma jet, and subsequently affect significantly both microstructure and properties of the coatings (Ref 21). In this study, a fuse-crushed YSZ powder (AMPERIT 825, H.C. Starck, Germany) was used to deposit the dense 8YSZ coating with fully molten droplets. The porous 8YSZ coating was deposited by a conventional plasma spray routine using a hollow spherical powder (Metco 204B-NS, Sulzer Metco Inc., New York, USA) to retain a fraction of semi-molten particles in the coatings and thus increase the porosity and reduce the thermal conductivity. Figure 1 shows the microstructure of the spray powders. It is usually difficult to deposit ceramic coatings with splats fully bonded by APS when the deposition temperature is kept at near ambient temperature. According to the previous study (Ref 22), for certain ceramic droplet, there exists a critical temperature over which the splats can be completely bonded together. When the deposition temperature exceeds over 600 °C during thermal spraying such as at a temperature of 700 °C, YSZ splat can be perfectly bonded to YSZ substrate (Ref 22), and thus, the lamellar bonding ratio of APS 8YSZ coatings is significantly increased and a dense ceramic coating could be achieved. Therefore, during preparation of dense 8YSZ coating, the surface of the coating during deposition was kept at about 650 °C with flame heating the back surface of the substrate. A pyrometer (RAYR312ML3U, Raytec, Santa Cruz, CA) was used to monitor the temperature during spraying. Plasma spray deposition was carried out by a commercial plasma spray system (GP-80, Jiujiang, China, 80 kW class). The spray parameters are shown in Table 1.

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Fig. 1 Microstructure of the 8YSZ powders: (a) fuse-crushed powder, (b) spherical powder

Table 1 Spray parameters Parameter

Unit

Value

Plasma arc power Flow rate of the primary gas Pressure of the primary gas Flow rate of the secondary gas Pressure of the secondary gas Spray distance Traverse speed

kW slpm MPa slpm MPa mm mm/s

39 50 0.8 7 0.4 80 350

Free-standing 8YSZ coatings with a thickness about 1 mm were also prepared for the measurement of thermal conductivity. The coatings were firstly deposited on stainless steel substrate and then the free-standing 8YSZ coating was obtained by dissolving the substrate in the hydrochloric acid.

2.2 CMAS Attack Test A laboratory-synthesized CMAS with the chemical compositions of 22CaO-19MgO-14AlO1.5-45SiO2 in mole percent was used for corrosion test. The chemical composition of CMAS was determined based on that of the deposits on vane blade in aircraft engines. The CMAS was prepared by a same method as that reported in the literature (Ref 23). The 8YSZ coatings with CMAS deposit at a loading of about 25 mg/cm2 were put in the muffle fur-

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nace in air at 1250 °C for 20 h to examine the corrosion behavior of the coatings.

2.3 Microstructure Characterization The microstructure of the coatings before and after CMAS attack test was characterized with a scanning electron microscopy (SEM, TESCAN MIRA 3). Before sectioning for metallographic examination, in order to preserve the microstructure of the coatings, all the coatings were impregnated by epoxy resin in an evacuated condition. Then, the cross section of the coatings was ground and then polished. Energy-dispersive spectrum analysis (EDS) was employed to characterize the Si distribution in the coatings after CMAS attack test. The porosity of the coatings was determined by image analyzing method using the back scattered electron images (BEI) of the cross section of polished samples at a magnification of 91000. At least five images were used to estimate the apparent porosity for each coating sample. The phase of the coatings was characterized by x-ray diffraction analysis (XRD, Rigaku D/max-2400) using Cu Ka1 radiation at a scanning rate of 5º/min for 2h. To clearly identify the diffraction peaks for the cubic phase and tetragonal phase in YSZ coatings, the detailed XRD patterns were obtained at a scanning rate of 1º/min for diffraction angle ranges of 27º-32º and 72º-76º. The phase content of the coating was estimated based on the relative intensity method. The content of monoclinic (M) phase was calculated using following equation which has been used for the determination of phase contents in APS YSZ coatings by different investigators (Ref 24): CM ¼

IM ð11 1Þ þ IM ð111Þ : IM ð11 1Þ þ IM ð111Þ þ ITC ð111Þ

ðEq 1Þ

2.4 Thermal Conductivity Measurement Thermal diffusivity of 8YSZ coatings was measured by laser flash method and then thermal conductivity was calculated by the following formula (Ref 25): k ¼ a  Cp  q;

ðEq 2Þ

where a is the thermal diffusivity, Cp is the specific heat capacity, q is the density, and k is the thermal conductivity. A laser flash analyzer 457 (Netzsch, Germany) was used to measure the diffusivity of the coating systems up to 1000 °C in ambient atmosphere. The heat capacity of the different coatings was measured by differential scanning calorimeters 404 (Netzsch, Germany). The density of the coatings was calculated through dividing the mass of the sample by its volume.

3. Results and Discussion Figure 2 shows the fracture morphology of the conventional 8YSZ coating and dense 8YSZ coating deposited using different powders and different deposition

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Fig. 2 Fracture morphology of the coatings: (a) conventional porous 8YSZ coating deposited by HOSP 8YSZ powders at ambient deposition temperature with air jet cooling, (b) dense 8YSZ coating deposited by fuse-crushed 8YSZ powders at a deposition temperature of 650 °C

temperatures. It can be observed from Fig. 2(a) that the conventional 8YSZ coating presents a typical lamellar structure with many inter-lamellar unbonded interfaces. However, by preheating the substrate during plasma spraying, the dense 8YSZ coating with some long throughlamellar columnar grain was formed as can be observed in Fig. 2(b). It is clear that most of the splats effectively were bonded together, and the inter-lamellar bonding was significantly increased.

3.1 Microstructure and Properties of the AsSprayed Coatings To further study the porosity and pore structure of these two types of coatings, the polished cross sections were observed. Figure 3 shows the microstructure of the conventional 8YSZ coating and dense 8YSZ coating. Two types of YSZ coatings present different porosity level and pore structure. The conventional 8YSZ coating presents a porous structure and both the inter-lamellar pore and intrasplat cracks exhibit a relatively large width compared with that in the dense 8YSZ coating. Molten CMAS can penetrate into the APS ceramic coating rapidly though the pores which are connected via intrasplat cracks and unbonded inter-lamellar interface

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gaps. In addition, the increase of the contact area between the infiltrated CMAS and ceramic matrix accelerates reaction process. Therefore, the porosity of the coating has remarkable influence on the corrosion resistance of the coating. The estimation of the porosity of two 8YSZ coatings by image analyzing yielded about 12.5% for the conventional 8YSZ coating and 3% for the dense 8YSZ coating deposited by the fuse-crushed powder. In addition, it was found that almost all the pores in the porous coating were filled with epoxy resin, which penetrated through the coating via the connected pores. Therefore, in the present study, the conventional 8YSZ coating is also referred to as porous 8YSZ coating.

3.2 Thermal Conductivity of the As-Sprayed 8YSZ Coatings The measurement yielded the thermal conductivity of 0.8 W/m/K for the porous 8YSZ coating, which is about 35% of the corresponding bulk material (Ref 26). This value is consistent with that of APS 8YSZ coatings reported in many previous publications (Ref 27-30). The low thermal conductivity of the conventional 8YSZ coating is ascribed to high porosity and substantial inter-lamellar unbonded interfaces especially, since the unbonded lamellar interfaces are perpendicular to the coating through-thickness direction, which cut off the direct heat conduction. However, for the dense 8YSZ coating, due to the reduced porosity and improved bonding at the interfaces between splats, the thermal conductivity was increased to about 1.6 W/m/K.

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3.3 Effect of Dual-Layered (DL) Coating Design on Thermal Conductivity and Resistance Against CMAS Infiltration Depositing a dense layer is an effective approach to enhance the corrosion resistance against CMAS of TBCs. However, as the porosity of the ceramic coating decreases, the thermal conductivity and stiffness significantly increase (Ref 18, 31). As a result, the thermal insulation performance of the TBCs is reduced and thermal lifetime is decreased (Ref 32). However, the multilayer-structured ceramic coating design for TBCs can effectively utilize features of both dense coating and porous coating. Through the introduction of lamellae well-bonded dense YSZ layer between the conventional porous YSZ and the superalloy bond coat, it was found that the thermal cyclic lifetime of TBCs can be significantly increased (Ref 31). To prevent the molten CMAS penetration, in the present study, a dense YSZ layer is introduced over the top of the conventional porous 8YSZ. According to the previous study, a multilayered ceramic coating can be easily deposited through control of the deposition temperature with APS (Ref 31). Figure 4 shows the typical microstructure of the TBC with DL-8YSZ. A conventional porous 8YSZ coating was firstly deposited at a deposition temperature of ambient temperature. High porosity constituted by inter-lamellar pores and intrasplat cracks is present in this porous coating, which is applied as the thermal barrier layer. Then, a dense ceramic layer in a thickness of about 60 lm was deposited on the top of the porous YSZ layer at the

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R¼

d1 d2 þ k1 k2

ðEq 3Þ

Fig. 4 Microstructure of DL-8YSZ coating deposited by APS

k¼

5k1  k2 4k1 þ k2

ðEq 4Þ

where R is the total thermal resistance of the DL-8YSZ, d1, and k1 are the thickness and thermal conductivity of the dense 8YSZ coating, d2, and k2 are the thickness and thermal conductivity of the porous 8YSZ coating, and k is the equivalent thermal conductivity of the whole DL8YSZ coating. Then, the thermal conductivity of the DL8YSZ coating system can be calculated. The thermal conductivity of the DL coating depends on the thickness of dense layer and porous layer. For example, for the DL coating composed of 50-lm dense layer and 250-lm porous layer, the calculation yielded a thermal conductivity of about 0.89 W/m/K that is only about 10% larger than conventional porous 8YSZ. Therefore, by designing multilayer-structured TBCs composed of a porous layer and well-bonded layer, the TBCs with high CMAS corrosion resistance and low thermal conductivity are likely to be achieved through APS. Figure 5 shows the microstructure of the porous 8YSZ coating after CMAS attack test at 1250 °C for 20 h. It can be found that Molten CMAS has penetrated throughout all the porous coating. The coating can be divided into two distinct layers after CMAS corrosion test based on the microstructure change shown in Fig. 5(a). The top layer exhibits a rather porous microstructure due to the severe reaction between CMAS and 8YSZ (Fig. 5b). Beneath this region, the other part of the coating was filled with CMAS.

Fig. 5 Microstructure of the conventional porous 8YSZ coating after thermal exposure at 1250 °C for 20 h with the CMAS on the top surface of the coating, showing the significant reaction between CMAS and YSZ across much thickness of the coating and full penetration of CMAS throughout the coating

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deposition temperature of 650 °C. This dense layer is used as corrosion-resistant layer. The equivalent thermal conductivity can be estimated simply by one-dimensional heat conduction model, since the coating thickness is much less than the other two dimensions involved in TBCs. By neglecting the interface effect between the two 8YSZ ceramic layers, the total thermal resistance and thus thermal conductivity of the 8YSZ TBC could be calculated though the following equations.

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Figure 6 shows the typical microstructure of the DL-TBC after thermal exposure at 1250 °C for 20 h with the CMAS on the top surface of the coating. First, it was found that there still exists residual CMAS on the surface of the coating after

thermal exposure test (Fig. 6b). The thickness of the reaction zone is less than the thickness of the top dense 8YSZ layer. As a result, most of the porous layer in the double layer TBC remained its original porous structure feature.

Fig. 6 Microstructure of the DL-8YSZ after thermal exposure at 1250 °C for 20 h with the CMAS on the top surface of the coating, showing the reaction of CMAS to a limited thickness and the porous YSZ layer near the top dense layer keep a porous structure

Fig. 7 Microstructure and Si distribution of the DL YSZ coating in comparison with porous YSZ coating after CMAS attack: (a) microstructure of the DL coating; (b) Microstructure of porous YSZ coating; (c) Si distribution in the DL YSZ coating; (d) Si distribution in the porous YSZ coating

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where L is the penetration depth, x is the pore fraction open to fluid, Dc is the capillary diameter, Kt is a tortuosity factor, g is the viscosity of the molten CMAS, and rLV is the surface tension of molten CMAS. The terms within the first parentheses in Eq 6 are only dependent on the geometry of the system, and the terms within the second parentheses are only dependent on the molten CMAS. Since the CMAS attack test was performed in the isothermal furnace and the chemical composition of both the CMAS and the ceramic matrix are the same, therefore, the difference of the penetration behavior of the molten CMAS in the 8YSZ coatings is only resulted from the different porosity and pore structure. Therefore, in this study, by fabricating a dense 8YSZ coating, the connected porosity and the size of the pore are significantly reduced. As a result, the infiltration of CMAS was suppressed.

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Figure 7 shows microstructure and Si distribution of the DL YSZ coating in comparison with porous YSZ coating after CMAS attack. For the DL 8YSZ coating, a CMAS layer can be observed on the surface of the coating after CMAS attack. Si was mainly distributed in the area closed to the surface with trace of Si within the coating. This fact indicates that CMAS was effectively blocked out by the dense layer. However, for the porous coating, Si was detected throughout the coating besides the reaction area. For ceramic coating, according to KramerÕ study (Ref 9), the penetration depth of the molten CMAS can be expressed as follows: !1=2   8Dc x2 rLV t; ðEq 6Þ L¼ g ð1  xÞ2 kt

Fig. 8 XRD pattern of the as-sprayed 8YSZ coatings

3.4 Comparison of Phase Composition of Two TBCs Before and After CMAS Attack Test Molten CMAS reacts with 8YSZ matrix. The reaction leads to phase transformation from T/T0 phase to M phase (Ref 9). Based on the fact that the content of M phase in the 8YSZ coating which experiences thermal exposure up to 100 h at 1300 °C is limited (Ref 33), in this study, the content of M phase in the coatings before and after CMAS attack test could be used to estimate the reaction between CMAS and 8YSZ. Figure 8 shows the XRD patterns of the as-sprayed coatings. It was found that both the porous 8YSZ coating and DL-8YSZ coating were mainly composed of T/T0 phase. For the coatings after CMAS attack test, before XRD test, the residual CMAS was carefully removed by polishing and then the x-ray diffraction was carried out on the surface of the coating. Therefore, in this study, the XRD result presents actually the phase structure in the tested depth of the coatings near the surface. As can be observed in Fig. 9, some reaction product appears and this result is consistent with that reported by Li et al. (Ref 13). In addition, it can be found that the peak intensity of M phase in the coatings significantly increased after CMAS attack test. This means that M phase appeared in the

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Fig. 9 XRD patterns of the 8YSZ coatings after CMAS attack test

coating after the test. The estimation of M phase content in the coatings before and after CMAS attack based on the XRD result shown in Fig. 10 yielded the results shown in Table 2. The content M phase in the as-sprayed coatings is limited, and the difference between porous 8YSZ and DL-8YSZ is small. However, after CMAS attack test,

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cyclic lifetime of TBC may be affected, because the sintering of top ceramic layer, leading to densification and contract stress, degrades the thermal cyclic life performance. However, application of the top dense layer could reduce the sintering effect at high temperature and thus its effect on thermal cyclic performance may be different. Therefore, as the future work, the microstructure evolution of the dense layer including the effect of the thickness of the dense layer on the microstructure evolution during thermal cycle will be systematically studied.

4. Conclusions

Fig. 10 XRD patterns of the 8YSZ coatings with scan angle (2h) range from 27°-32° before (a) and after (b) CMAS attack test

Table 2 Comparison of the M phase content of in two types of 8YSZ coatings after CMAS attack test with that of the as-sprayed coatings Type of coatings As-sprayed, % After CMAS attack, %

Porous 8YSZ

DL-8YSZ

2.9 65.2

2.3 34.8

In this study, the hot corrosion behavior of dual-layered structure 8YSZ coating exposed to molten CMAS at 1250 °C was investigated by comparing with that of the conventional one. Based on the microstructure characterization and XRD analysis of the coatings after thermal exposure with CMAS, it was found that the dense 8YSZ coating deposited at about 650 °C with lower porosity exhibited much higher corrosion resistance against molten CMAS compared with the conventional one. The high corrosion resistance of the 8YSZ coating deposited at higher temperature was ascribed to the improved bonding between splats, which resulted in the decrease of the interconnected porosity in the top coating layer. In addition, the interaction area between CMAS and 8YSZ matrix was also reduced. Since it becomes easy to deposit dense YSZ coating through control of deposition temperature, the multilayer design with different microstructures is an effective approach to improve the performance of TBCs.

Acknowledgments The present project is supported by the National Basic Research Program (Grant No. 2012CB625100) and National Natural Science Foundation (Grant No. 51171144). The authors are grateful for the support of the European Program Marie Curie IPACTS (No. 268696).

References for the porous 8YSZ coating, it contained 65.2% M phase, while for DL-8YSZ, it only contained 34.8% M phase. The difference of M phase in these two coatings was mainly attributed to the different reaction area between CMAS and ceramic matrix. This result further confirmed that dense layer fabricated by APS can effectively suppress the infiltration of CMAS. Therefore, it is clear that by designing TBC with dual layer APS YSZ coatings, the penetration of molten CMAS can be effectively suppressed, and thus, CMAS corrosion resistance is significantly enhanced. However, the introduction of the dense layer will increase the stiffness of top layer and thus may change the strain distribution. Consequently, it may be considered that thermal

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1. N.P. Padture, M. Gell, and E.H. Jordan, Thermal Barrier Coatings for Gas-Turbine Engine Applications, Science, 2002, 296, p 280-284 2. C.U. Hardwickre and Y.C. Lau, Advances in Thermal Spray Coatings for Gas Turbines and Energy Generation: A Review, J. Therm. Spray Technol., 2013, 22(5), p 564-576 3. A. Feuerstein, J. Knapp, T. Taylor, A. Ashary, A. Bolcavage, and N. Hitchman, Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and EBPVD, A Review, J. Therm. Spray Technol., 2008, 17(2), p 199-213 4. R. McPherson, A Model for the Thermal Conductivity of PlasmaSprayed Ceramic Coatings, Thin Solid Films, 1984, 112, p 89-95 5. R. McPherson, A Review on Microstructure and Properties of Plasma Sprayed Ceramic Coatings, Surf. Coat. Technol., 1989, 39-40, p 173-181

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20. S.L. Zhang, C.X. Li, C.-J. Li, Z.H. Han, and G.J. Yang, ScandiaStabilized Zirconia Electrolyte with Improved Interlamellar Bonding by High-Velocity Plasma Spraying for High Performance Solid Oxide Fuel Cells, J. Power. Sour., 2013, 232, p 123-131 21. A. Kulkarni, Z. Wang, T. Nakamura, S. Sampath, A. Goland, H. Herman, J. Allen, J. Ilavsky, G. Long, J. Frahm, and R.W. Steinbrech, Comprehensive Microstructural Characterization and Predictive Property Modeling of Plasma-Sprayed Zirconia Coatings, Acta Mater., 2003, 51, p 2457-2475 22. G.J. Yang, C.X. Li, S. Hao, Y.Z. Xing, E.J. Yang, and C.-J. Li, Critical Bonding Temperature for the Splat Bonding Formation during Plasma Spraying of Ceramic Materials, Surf. Coat. Technol., 2013, 235, p 841-847 23. J. Wu, H.B. Guo, Y.Z. Gao, and S.K. Gong, Microstructure and Thermo-Physical Properties of Yttria Stabilized Zirconia Coatings with CMAS Deposits, J. Eur. Ceram. Soc., 2011, 31, p 1881-1888 24. R.A. Miller, J.L. Smialek, and P.G. Garlick, Phase Stability in Plasma-Sprayed, Partially Stabilized Zirconia-Yttria, Adv. Ceram., 1981, 3, p 241-253 25. W.J. Parker, R.J. Jenkins, C.P. Butler, and G.L. Abbott, Flash Method of Determining Thermal Diffusivity, Heat Capacity and Thermal Conductivity, J. Appl. Phys., 1961, 32, p 1679-1684 26. X.Q. Cao, R. Vassen, and D. Stoever, Ceramic Materials for Thermal Barrier Coatings, J. Eur. Ceram. Soc., 2004, 24, p 1-10 27. N. Markocsan, P. Nylen, J. Wigren, X.H. Li, and A. Tricoire, Effect of Thermal Aging on Microstructure and Functional Properties of Zirconia-Base Thermal Barrier Coatings, J. Therm. Spray Technol., 2009, 18(2), p 160-169 28. R. Bolot, J.H. Qiao, G. Bertrand, P. Bertrand, and C. Coddet, Effect of Thermal Treatment on the Effective Thermal Conductivity of YPSZ Coatings, Surf. Coat. Technol., 2010, 205, p 1034-1038 29. R. Dutton, R. Wheeler, K.S. Ravichandran, and K. An, Effect of Heat Treatment on the Thermal Conductivity of Plasma-Sprayed Thermal Barrier Coatings, J. Therm. Spray Technol., 1999, 9(2), p 204-209 30. G. Bertrand, P. Bertrand, P. Roy, C. Rio, and R. Mevrel, Low Conductivity Plasma Sprayed Thermal Barrier Coating Using Hollow psz Spheres: Correlation Between Thermophysical Properties and Microstructure, Surf. Coat. Technol., 2008, 202, p 1994-2001 31. C.-J. Li, Y. Li, G.J. Yang, and C.X. Li, A Novel Plasma-Sprayed Durable Thermal Barrier Coating with a Well-Bonded YSZ Interlayer Between Porous YSZ and Bond Coat, J. Therm. Spray Technol., 2012, 21, p 383-390 32. E. Bakan, D.E. Mack, G. Mauer, R. Mucke, and R. Vassen, Porosity-Property Relationships of Plasma-Sprayed Gd2Zr2O7/ YSZ Thermal Barrier Coatings, J. Am. Ceram. Soc., 2015. doi:10.1111/jace.13611 33. G.J. Yang, Z.L. Chen, C.X. Li, and C.J. Li, Microstructural and Mechanical Property Evolutions of Plasma-Sprayed YSZ Coating During High-Temperature Exposure: Comparison Study Between 8YSZ and 20YSZ, J. Therm. Spray Technol., 2013, 22(8), p 1294-1302

Volume 25(1-2) January 2016—221

Peer Reviewed

6. R. McPherson, The Relationship Between the Mechanism of Formation, Microstructure and Properties of Plasma-Sprayed Coatings, Thin Solid Films, 1981, 83, p 297-310 7. A.G. Evans, M.Y. He, and J.W. Hutchinson, Mechanics-Based Scaling Laws for the Durability of Thermal Barrier Coatings, Prog. Mater. Sci., 2001, 46, p 249-271 8. A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, and F.S. Pettit, Mechanisms Controlling the Durability of Thermal Barrier Coatings, Prog. Mater. Sci., 2001, 46, p 505-553 9. S. Kramer, J. Yang, and C. Levi, Thermochemical Interaction of Thermal Barrier Coatings with Molten CaO-MgO-Al2O3-SiO2 (CMAS) Deposits, J. Am. Ceram. Soc., 2006, 89, p 3167-3175 10. C. Mercer, S. Faulhaber, A.G. Evans, and R. Darolia, A Delamination Mechanism for Thermal Barrier Coatings Subject to Calcium-Magnesium-Alumino-Silicate (CMAS) Infiltration, Acta Mater., 2005, 53, p 1029-1039 11. L.H. Gao, H.B. Guo, S.K. Gong, and H.B. Xu, Plasma-Sprayed La2Ce2O7 Thermal Barrier Coatings Against Calcium-Magnesium-Alumina-Silicate Penetration, J. Eur. Ceram. Soc., 2014, 34, p 2553-2561 12. B.S. Senturk, H.F. Garces, A.L. Ortiz, G. Dwivedi, S. Sampath, and N.P. Padture, CMAS-Resistant Plasma Sprayed Thermal Barrier Coatings Based on Y2O3-Stabilized ZrO2 with Al3+ and Ti4+ Solute Additions, J. Therm. Spray Technol., 2014, 23(4), p 708-715 13. W.H. Li, H.Y. Zhao, X.H. Zhong, L. Wang, and S.Y. Tao, Air Plasma-Sprayed Yttria and Yttria Stabilized Zirconia Thermal Barrier Coatings Subjected to Calcium-Magnesium-AluminoSilicate (CMAS), J. Therm. Spray Technol., 2014, 23(6), p 975-983 14. R.A. Pidani, R.S. Razavi, R. Mozafarinia, and H. Jamali, Improving the Hot Corrosion Resistance of Plasma Sprayed Ceria-Yttria Stabilized Zirconia Thermal Barrier Coatings by Laser Surface Treatment, Mater. Des., 2014, 57, p 336-341 15. R. Ghasemi, R.S. Razavi, R. Mozafarinia, H. Jamali, M.H. Oghaz, and R.A. Pidani, The Influence of Laser Treatment on Hot Corrosion Behavior of Plasma-Sprayed Nanostructured Yttria Stabilized Zirconia Thermal Barrier Coatings, J. Eur. Ceram. Soc., 2014, 34, p 2013-2021 16. P. Mohan, B. Yao, T. Patterson, and Y.H. Sohn, Electrophoretically Deposited Alumina as Protective Overlay for Thermal Barrier Coatings Against CMAS Degradation, Surf. Coat. Technol., 2009, 204, p 797-801 17. T. Liu, E.J. Yang, C.X. Li, and C.-J. Li, Splat Interface Bonding Formation During Plasma Spraying of LZO Coating, Mater. Res. Innov., 2014, 18, p 973-978 18. S. Hao, C.-J. Li, and G.J. Yang, Influence of Deposition Temperature on the Microstructures and Properties of PlasmaSprayed Al2O3 Coatings, J. Therm. Spray Technol., 2010, 20, p 160-169 19. Y.Z. Xing, C.-J. Li, Q. Zhang, C.X. Li, and G.J. Yang, Influence of Microstructure on the Ionic Conductivity of Plasma-Sprayed Yttria-Stabilized Zirconia Deposits, J. Am. Ceram. Soc., 2008, 91, p 3931-3936