Wear behaviour of thermally sprayed ceramic oxide coatings

The wear resistance of plasma-sprayed ceramic coatings (Al2O3, Al2O3-13%TiO2, Cr2O3) has been investigated through pin-on-disk and dry sand-steel wheel tests, has been correlated to microstructural and micromechanical characteristics (microhardness, fracture toughness) and has been compared to well-known platings (such as Cr electroplating and electroless Ni) and HVOF-sprayed cermets (WC-17%Co, WC-10%Co-4%Cr). Plasma-sprayed ceramics are hard but brittle: dry particles abrasion occurs through splats detachment. The toughest coating (Al2O3) displays the highest wear resistance, which in fact overcomes HVOF-sprayed cermets and Cr electroplating, when a low number of wheel revolutions are considered. In pin-on-disk tests, no coating undergoes wear loss against the 100Cr6 ball, that possess lower hardness. Against the alumina ball, Al2O3 and Al2O3-TiO2 coatings show high wear rates and friction coefficients (due to chemical affinity), while Cr2O3 possesses better wear resistance, lower friction coefficient and inflicts less wear on the counterpart. Cr2O3 wear scar consists in plastically deformed splats and debris forming a quite adherent protective tribofilm.