Effects of different additives on microstructure and high-temperature tribological properties of plasma-sprayed Cr2O3 ceramic coatings

The microstructure and high-temperature tribological properties of low-pressure plasma-sprayed Cr2O3 ceramic coatings undoped or doped with different additives, such as CaF2, Ag2O and ZrO2 were studied. Under the identical plasma-spray condition, the composite coatings doped with additives exhibited a larger thickness and distinctly lower hardness than pure Cr2O3 coating. Within the as-sprayed composites, ZrO2 and Cr2O3 constituents were observed as fine lamellar structures and Ag2O as isolated particles with different gray levels in the back-scattered electron (BSE) mode, while CaF2 appeared as a darker region with a great volume fraction. Some resolidified structure of ZrO2 crystals featured by fine equiaxed or dendritic grains were clearly observed in the ZrO2 lamellae on the cross-section or the polished surface. Friction coefficients of the composite coatings doped with additives were quite high at room temperature and gradually decreased with increasing of temperature. The situation was reversed for the purr Cr2O3 coating. Among the three kinds of Cr2O3-based coatings, Cr2O3-CaF2-Ag2O (CFA) composite coating exhibited the lowest friction and wear characteristics at above 500 degreesC and reached a minimum value of friction coefficient at 800 degreesC. The Ag2O additive was beneficial to improve the adherence and wettability of the CaF2 transfer film to the counterface, while the ZrO2 additive was detrimental to the formation of a complete CaF2 transfer film because of crushing and microfracture debris of ZrO2 crystals on the worn surface at elevated temperature. (C) 2001 Elsevier Science B.V. All rights reserved.