Characterization of Nonmelted Particles in Plasma-Sprayed Al2O3 Coatings by Scanning Electron Microscopy, Laser Scanning Microscopy, Raman Analysis and x-Ray Diffraction Analysis

Plasma-sprayed ceramic coatings are deposited by flattened splats together with some nonmelted particles. Compared to typical flattened splats, the nonmelted particles present an irregullar morphology and may influence coating properties. In this study, the nonmelted particles in plasma-sprayed alumina coating were examined by scanning electron microscopy, laser scanning microscopy, Raman analysis and x-ray diffraction analysis to aim at the quantitative evaluation of coating microstructure. Results showed that the quantitative characterization of the nonmelted particles in the coating was limited by the weak diffraction of alumina. Since nonmelted particles in α phase is harder than quenched splats in γ phase, nonmelted particles can present an empaistic morphology by controlling the sample preparation process. Based on the confirmation of phase structure of the nonmelted particles by confocal laser Raman analysis, the distribution and the fraction of nonmelted particles can be quantitatively characterized by scanning electron microscopy and confocal laser scaning microscopy.