Phase effects on the hardness and elastic modulus of atmospheric plasma sprayed (APS) alumina coatings were studied experimentally and analytically in this paper. For experimental work, the hardness and elastic modulus of APS alumina coatings were investigated by nanoindentation. Large scattering was observed in the results. For analytical work, the laminated APS alumina coating was taken to be consisted as alternating layers of α and γ alumina phases. This model was used to derive an approximate closed-form solution for nanoindentation on APS alumina coatings. The analytical solutions revealed that the phase combination played an important role in the material’s behavior under nanoindentation. The model verified the experimentally observed scattering in modulus distribution of APS alumina coating was mainly due to the phase composition. In addition, it is possible to determine the ratio of α and γ phases in APS alumina coatings by comparing the experimental nano-indentation results with the analytical solutions.

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