Sliding Wear of TiN coated 321 stainless steel in CO2 at 300 and 500°C

Advanced carbon dioxide-cooled nuclear reactor environments can be tribologically hostile, with steel components sliding in contact at high temperatures. Although wearprotective oxides can reduce damage in the steady-state conditions, severe wear can occur in the early stages, particularly at temperatures of 300 to 500 "C, where oxidation rates may be low. In this paper, results are presented of the effects of thin titanium nitride coatings in protecting 321 stainless steel against sliding damage in carbon dioxide at 300 and 500 "C. At 500 "C, the coating prevented the severe wear which occured in the early stages for uncoated specimens and there was a smooth transition from protection by the ceramic coating to protection by the oxide on the substrate, ensuring maintenance of low wear rates and steady friction profiles. At 300 "C, breakdown of the coating was followed by metal-metal contact and a sharp increase in friction and wear rate. This was consistent with the behaviour of uncoated steel where wear-protective oxides were unable to develop under the present conditions at 300 "C. However, pre-oxidation of both coated and uncoated specimens for long periods at higher temperatures improved considerably the wear performance at thus temperature, due to the increased availability of oxide to develop the wear-protective surfaces.