Microstructural characterization of plasma-sprayed oxide ceramics.

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Microstructural and microchemical characterization has been carried out on plasma-sprayed zircon (ZrSiO4) and alumina-chromia (Al2O3–5wt%Cr2O3) by transmission electron microscopy and energy dispersive X–ray analysis. The as–sprayed zircon coatings consist of zirconia and silica glass as a result of phase separation during rapid cooling in the plasma–spraying process. Zirconia shows a tetragonal structure for small particles and a monoclinic for large particles. The zircon coatings annealed at 1 300°C for 96 h in air show a mixture of zircon and monoclinic zirconia phases. Alumina–chromia coatings plasma–sprayed with N2 gas consist mainly of a polycrystalline alumina–chromia solid solution with a typical grain size of 0.4 μm. Fine precipitates are observed frequently along the grain boundaries in the coatings sprayed with Ar–H2 gas mixture or N2–H2 gas mixture. The fine precipitates are a metallic Cr–rich Cr–Al alloy formed by reduction of Cr2O3 and Al2O3 during the plasma-spraying process. These precipitates disappear during annealing at temperatures higher than 1 300°C for 96 h in air. The precipitates affect the wear resistance of the coatings.