Microstructure formed by suspension plasma spraying: From YSZ splat to coating

In the current work, by means of suspension plasma spraying (SPS), yttria stabilized zirconia splats and coatings were deposited onto mirror-polished and rough substrates, respectively. The splats were characterized by scanning and transmission electron microscopes (TEM). The influence of substrate temperature on splat morphology was analyzed via comparing the splats deposited onto substrates held at different temperatures, namely room temperature (RT), 300 °C, 500 °C, 700 °C and 900 °C. As the substrates were heated from RT to 300 °C, the feature of splats changed rapidly. However, when the substrate temperature further increased, the variation of morphology was found less conspicuous. Integrated splats were peeled off from the substrate using a “plating and etching” approach for TEM examination. The observation on morphological and crystallographic features of splats revealed that the cooling rate during impacting and spreading of SPS splats was lower than that of splats deposited by conventional atmospheric plasma spraying. Based on the characteristics of splats, a modified mechanism consisting changing of droplets trajectory and shadowing effect was proposed to explain the formation of the microstructure of as-sprayed SPS coating. In addition, the coatings were also examined by X-ray diffraction and nano-indenter to determine the phase composition and microhardness, respectively.