High temperature fretting wear behavior of WC–25Co coatings prepared by D-gun spraying on Ti–Al–Zr titanium alloy

Detonation gun (D-gun) spraying is one of the most promising spraying techniques for producing wear-resistance coatings. A thick layer (about 0.3 mm thickness) of WC–25Co with high hardness was covered on Ti–Al–Zr titanium alloy by D-gun spraying and the fretting wear behavior of WC–25Co coatings was studied experimentally on a high precision hydraulic fretting wear test rig. An experimental layout was designed to perform fretting wear tests at elevated temperatures from room temperature (25 °C) to 400 °C in ambient air. In the tests, a sphere (Si3N4 ceramic ball) was designed to rub against a plane (Ti–Al–Zr titanium alloy with or without WC–25Co coatings). It was found that the fretting running regimes of WC–25Co coatings were obviously different from those of Ti–Al–Zr titanium alloy. The mixed fretting regime disappeared in WC–25Co coatings, and the boundaries in the running condition fretting map (RCFM) showed hardly any change as temperature increased. The worn scars were examined using a laser confocal scanning microscope (LCSM), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the coefficients of friction (COF) of WC–25Co coatings at elevated temperatures were nearly constant in the partial slip regime and very low in the steady state. The fretting damage of the coatings was very slight. In the slip regime, the WC–25Co coatings exhibited a good wear resistance, and the wear volume of the coatings obviously decreased with increasing tested temperature. The fretting wear mechanisms of WC–25Co coatings were delamination, abrasive wear and oxidation wear at elevated temperature. The oxide debris layer formed at higher temperature was denser and thicker on top of WC–25Co coatings, thus providing more surface protection against fretting wear, which played an important role in the low fretting wear of the coatings.