In Fig. 1(a), the XRD patterns of as-deposited 18YSH coating reveal that as-sprayed coating is composed of the cubic HfO2 phase (c-HfO2). Raman results shown in Fig. 1(b) confirms this c-HfO2 as the sole phase present in the 18YSH coating. A small amount of monoclinic HfO2 in ceramic powders might have been stabilized into the cubic phase due to Y2O3 redistribution upon passing through the plasma flame due to its extremely high temperature. The surface of 18YSH is constructed as a 3D digital image in Fig. 1(c). It can be seen that the coating is characterized by a rough surface (Ra = 6.54 μm). Many wholly or partially unmolten 18YSH particles have survived the high-energy flame due to high melting point of HfO2 (~2800 ◦C). Compared with flattened molten splats, partially molten or unmoltem particles yield a lower fraction of
inelastic deformation on the surface, thus contributing to a coarse surface of the 18YSH coating. Fig. 1(d) is a cross-section micrograph of the plasma-sprayed 18YSH coating. The average thickness of the 18YSH coating is estimated to be ~220 μm and adhesion of the coating to the bond coat appears to be excellent.
Fig. 1. (a)–(b) XRD patterns and Raman spectra of 18YSH coating before and after thermal cycling test; (c) as-deposited coating surface reconstructed by 3D laser scanning microscope; (d) SEM image of as-sprayed 18YSH coating.
SEM images (Fig. 2(a)-(b)) provide a closer view of the coating microstructure. A porous microstructure can be observed and the porosity is measured to be 12.9% by image method. There are three different types of pores present in as-sprayed 18YSH coating, three-dimension globular pores, intersplat gaps and intrasplat cracks. The fine globular pores are mainly attributed to unmolten or partially molten particles and/or to gas entrapment during plasma spraying. The intersplat gaps are mainly induced by imperfect adhesion of lamellae splats as confirmed by fractured morphology in Fig. 2(c). Intrasplat cracks are observed inside splats as well relaxing quenching stress. This porous structure not only makes the ceramic topcoat an excellent thermal barrier but also alleviates stress arising from the thermo-mechanical mismatch between the coating and underlying substrate.
Fig. 2. (a)–(b) Cross-sectional micrographs of the 18YSH ceramic coating in high magnification to characterize the microstructure; (d) fracture morphology of the coating.
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