Wear behaviour of thermally sprayed ceramic oxide coatings

The wear resistance of plasma-sprayed ceramic coatings (Al 2O 3, Al 2O 3–13%TiO 2, Cr 2O 3) 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 (Al 2O 3) 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, Al 2O 3 and Al 2O 3–TiO 2 coatings show high wear rates and friction coefficients (due to chemical affinity), while Cr 2O 3 possesses better wear resistance, lower friction coefficient and inflicts less wear on the counterpart. Cr 2O 3 wear scar consists in plastically deformed splats and debris forming a quite adherent protective tribofilm.