**3.4 X-ray diffraction analysis of coatings**

X-ray diffraction (XRD) patterns for the top surfaces of plasma sprayed Al2O3 and ZrO25CaO coatings are shown in Fig.6. The XRD patterns of Al2O3 coatings show the presence of γ-Al2O3 as a principal phase and α-Al2O3 as minor phase. It shows that oxidation has occurred during spraying by converting hard phase of Al2O3 into soft phase of γ-Al2O3. ZrO25CaO coatings possess tetragonal ZrO2 as a principal phase and CaZrO3 as minor phase.

XRD patterns suggest that Al2O3 particles are not completely transferred into soften γ-Al2O3 phase after the plasma spray process. This is a good result for tribological behavior of coatings where the hardness plays an important role in wear resistance due to abrasion and erosion. The hardness of Al2O3 coating is lower than that of bulk alumina (HV-2045) which is mainly due to the intrinsically lower hardness of γ-Al2O3 than α-Al2O3. The indentation response of a plasma sprayed material is governed not only by the intrinsic hardness of the material, but also by the lamellar microstructure, with splat boundaries giving off under load to facilitate the indenter accommodation [25].

Fig. 6. X-Ray Diffraction Diagrams of Al2O3 and (b) ZrO25CaO Coatings


Table 4. Thicknesses, Porosity and Average Surface Roughness of Coatings
