Sliding wear of a self-mated thermally sprayed chromium oxide coating in a simulated PWR water environment

Bearing surfaces in the primary circuit of pressurised water reactors (PWR) are prone to damage due to aggressive chemical and tribological conditions under which they operate, and a wide range of materials have been examined in this regard. One of the most promising candidates is chromium oxide in the form of a thermally spayed coating, and in this work, the behaviour of a commercially available Cr₂O₃ coating in self-mated sliding was considered. Tests consisted of a number of start-stop cycles of sliding between a crowned pin and a rotating disc in a water environment in an autoclave in an attempt to simulate the most aggressive phase of bearing run-up and run-down. Wear and damage mechanisms were examined at temperatures from ambient up to 250 °C (a representative PWR environment). Samples were characterised before and after wear testing using mass measurements, profilometry, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS).

Across the temperature range, wear was mild, with no evidence of coating delamination. A five-fold increase in wear was observed between 80 °C and 250 °C (with wear depths of generally less than 8 µm being observed on the disc samples even at the higher temperature), despite there being only very small changes in hardness of the coating over the same temperature range. Debris was observed on the wear tracks following testing, with the evidence together suggesting that this debris was a very fine-grained mixture of Cr₂O₃ and amorphous γ-CrOOH, a corrosion product of Cr₂O₃.