Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) with lamellar structure exhibit low thermal conductivity and low stiffness. However, high temperature exposure for certain long duration causes the sintering which heals two-dimensional (2D) inter-lamellar pores and intrasplat pores. Such sintering effect increases the stiffness and thermal conductivity of the coatings and consequently reduces the stability and durability of TBCs. It can be expected that large 2D pores with a wide opening is difficult to be eliminated. In this study, inter-lamellar 2D pores with large opening width were fabricated in the La 2 Zr 2 O 7 (LZO) coatings through spraying LZO+SrO coatings and removing the SrO splats in the water. Then, the conventional LZO coating and the porous LZO coating were subjected to high temperature exposure in the air at 1300 °C for different durations. The microstructure evolution especially in terms of the shape and density of inter-lamellar 2D pores was examined. In addition, the change of thermo-physic properties and the mechanical properties of the coatings with increasing exposure duration were studied. Results show that the 2D pores in LZO coating created by those SrO splats inherit primarily large opening width from 200nm to about 1 µm which endows the LZO coating with high sustainability at high temperature environment. Under thermal exposure at 1300°C, it was found that 2D pores resulting from SrO splats are free from healing while conventional 2D inter-lamellar pores with small opening width formed during splat cooling became healed rapidly. Thus, thermal conductivity and Young's modulus of the conventional LZO coating increased rapidly, while unhealed 2D pores in the highly porous LZO coatings contributed to the low Young's modulus and low thermal conductivity of LZO coating with remarkably high stability. With addition of 30% SrO in spray powder, a LZO coating with a thermal conductivity of about 0.39 W.m -1 .K -1 in the as-prepared state was obtained. The coating maintained a thermal conductivity of 0.57 W.m -1 .K -1 even after 100 hours exposure at 1300°C. The present results indicated that high sintering-resistant thermal barrier coating can be fabricated though designing inter-lamellar 2D pores with large opening width in the coating by the present novel approach.

The corrosion resistance of thermal barrier coatings against CMAS deposit at high temperature is significantly affected by the microstructure of the coatings. Enhancing the bonding ratio between splats can reduce the inter-connected pores and then obstructs the penetration of the molten CMAS into the coatings. In this study, atmospheric plasma sprayed ZrO 2 contains 8 wt. % Y 2 O 3 (8YSZ) coating with improved lamellar bonding ratios was deposited with full-molten droplets at an enhanced deposition temperature. The microstructure of the dense 8YSZ coating and conventional 8YSZ coating before and after thermal exposure with CMAS were characterized. It was clearly revealed that by adjusting the microstructure and designing a ceramic layer with high bonding ratio, the corrosion resistance of the thermal barrier coating could be enhanced. Moreover, by designing double-ceramic-layer (DCL) TBCs composed of a porous ceramic layer and well-bonded ceramic layer, the TBCs with high CMAS corrosion resistance and low thermal conductivity can be achieved.

The evaporation of both CeO 2 and La 2 O 3 in high temperature plasma arc leads to deviation of plasma sprayed La 2 Ce 2 O 7 coating composition from the starting powder particle. Such change results in significantly inhomogeneity of compositions within a coating which influences the performance of calcium-magnesium-alumina-silicate (CMAS) corrosion. In this study, the pellets with different Ce / La ratio were interacted with molten CMAS glass. The penetration of molten CMAS on different pellets was characterized by scanning electron microscopy and energy dispersive spectrum (EDS) and the phase of different powder mixtures treated at 1250 °C was characterized by X-ray diffraction ( XRD ). The effects of pellets with different Ce / La ratio on the CMAS corrosion after thermal exposure at 1250 °C for 50 h were investigated. The result demonstrated that pellets with ratio of Ce / La greater than or equal to 1.0 were completely dissolved into the molten CMAS, at the same time the others were also damaged, forming diffusion layer and reacted layer, respectively. The difference of the ratio of Ce / La indeed affected their performance against CMAS attack.

LCO (La 2 Ce 2 O 7 ), a solid solution of La 2 O 3 in CeO 2 , is a promising top coating in TBCs. However, it is generally necessary to use LCO as a top coat on YSZ coating to construct a multilayer TBC due to poor mechanical properties of LCO. Therefore, the thermal and chemical stability of LCO/YSZ at high temperature environment become important issue. In this paper, the 50LCO-50YSZ composite coating was deposited using blend powders of YSZ and LCO. The LCO powders used have a nominal particle size range from 10 to 44μm. The LCO/YSZ deposits were exposed at 1300 ¢XC for different durations. The microstructure evolution at the LCO/YSZ interface was investigated by quasi-in-situ SEM and EDS examination. At an exposure temperature of 1300°C, it was observed that some LCO splats in contact with YSZ splats experienced the grain morphology change from columnar one to quasi-axial grains with interface healing and however some grains tended to disappear with thermal exposure. Results indicated that LCO-YSZ composite coating is not phase stable at 1300 °C. Diffusion of La element from LCO splat towards the adjacent YSZ splat occurred during sintering, leading to the formation of La 2 Zr 2 O 7 within YSZ splat.

La 2 Ce 2 O 7 (LCO) is one of the promising candidates of high temperature thermal barrier coating materials. However, during plasma spraying, the evaporation of both CeO 2 and La 2 O 3 in high temperature plasma jet leads to deviation of coating composition from the starting powder particle. Such change results in significant inhomogeneity of compositions within a coating which influences the performance of resultant coating. In this study, an agglomerated La 2 Ce 2 O 7 powder with Ce / La ratio of 1.5 and in a size range of 5-60 μm was used to deposit the splats and coatings under different plasma spray conditions. The compositions of La 2 Ce 2 O 7 splats in different sizes were characterized by scanning electron microscopy and energy dispersive spectrum (EDS), and splat dimensions were characterized by 3D laser microscopy. The relationship between droplet size and splat composition was examined. Results showed that the Ce / La ratio in splats changes significantly with the splats diameter in a range from 1.5 to 0.7. There is a big difference in the composition of APS coatings deposited by coarse and fine powders. The mechanism causing splat composition change was examined based on the selective elemental loss during plasma heating of LCO particles in APS, aiming to control LCO coating composition.