Relationship between microstructure and erosive wear resistance of plasma sprayed alumina coatings

Abstract

Pure alumina coatings deposited on metallic substrates by plasma spraying in air (APS) and in low vacuum (VPS) were subjected to erosive wear by silica sand particles at 50 m s⁻¹. The AP S coatings were porous and exhibited poor resistance to erosion. The VP S coatings produced at high substrate temperatures and lower chamber pressures exhibited low porosity and good erosion resistance, while those deposited at lower substrate temperatures and higher chamber pressures were more porous and eroded more rapidly, in some cases with erosion rates higher than those of the AP S coatings. A pronounced lamellar structure was found to be the most important factor in reducing the erosion resistance. The extent of the lamellar structure and the coating porosity depended on the spraying conditions. For VPS coatings the effects of residual stresses arising from differential thermal contraction were studied by using substrates with different thermal expansion coefficients and by controlling the substrate temperature during spraying. Microstructural effects in these experiments obscured any clear influence of residual stress on erosion behaviour, although there were indications that high residual stresses may lead to microcracking which in turn may promote more rapid erosion.