SLIDING WEAR OF TIN-COATED 321 STAINLESS-STEEL IN CO2 AT 300-DEGREES-C AND 500-DEGREES-C

Advanced carbon dioxide-cooled nuclear reactor environments can be tri bologically hostile, with steel components sliding in contact at high temperatures. Although wear-protective oxides can reduce damage in the steady-state conditions, severe wear can occur in the early stages, p articularly at temperatures of 300 to 500-degrees-C, where oxidation r ates may be low. In this paper, results are presented of the effects o f thin titanium nitride coatings in protecting 321 stainless steel aga inst sliding damage in carbon dioxide at 300 and 500-degrees-C. At 500 -degrees-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 frict ion profiles. At 300-degrees-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-p rotective oxides were unable to develop under the present conditions a t 300-degrees-C. However, pre-oxidation of both coated and uncoated sp ecimens for long periods at higher temperatures improved considerably the wear performance at thus temperature, due to the increased availab ility of oxide to develop the wear-protective surfaces.