Characterization of Nonmelted Particles and Molten Splats in Plasma-Sprayed Al2O3 Coatings by a Combination of Scanning Electron Microscopy, X-ray Diffraction Analysis, and Confocal Raman Analysis

Plasma-sprayed ceramic coatings are deposited by flattened splats together with some nonmelted particles. In this study, both nonmelted particles and molten splats in plasma-sprayed alumina coatings were examined by a combination of scanning electron microscopy, x-ray diffraction analysis, and confocal Raman analysis to aim at the comprehensive understanding and quantitative evaluation of coating microstructure. Results showed that the nonmelted particles can be clearly identified from the polished cross section due to the raised pattern morphology (with convexes) resulting from the higher hardness of the nonmelted particles than molten splats. Raman analysis revealed that there was little alpha-Al2O3 phase in the flattened splat region, which confirmed that alpha-Al2O3 phase in the coating originated only from nonmelted particles. Moreover, it is proposed that attention should be paid to the weak bonding of the nonmelted particles to surrounding flattened splats during the preparation of samples for the quantitative characterization of coating microstructure.