Chemical Homogeneity And Morphology Of Coatings

In the literature it is generally accepted that the concentration of yttria in zirconia is critical. The most spall-resistant zirconia composition is attained with 6.5-8 wt% yttria in solid solution homogeneously distributed in the powder particles. The second requirement is to control the residual stresses and porosity during deposition. The required porosity corresponds to a coating density of 5.2 g/cm . The porosity in zirconia should be increased from the inside of the zirconia to the outer surface. The third requirement in producing high quality zirconia coatings is the quality control of the powder production methods. This achievement has brought more improvements to the coating quality than any of the other factors. When a bond coat was deposited by low pressure plasma spray (LPPS), rather than air plasma spray (APS) the resistance to spallation was doubled. The bond coat improvement from LPP results from reduced oxidation of the metallic bond coat during the deposition and operation In comparison, there do not seem to be any significant differences in the spall resistance of an yttria stabilized zirconia deposited by either APS or LPPS.

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