Improving the wear properties of Stellite 21 alloy by plasma surface alloying with carbon and nitrogen

This research was directed at improving the wear resistance of Stellite 21 alloy by developing novel surface engineering technologies. To this end, a series of plasma surface alloying (PSA) treatments with both carbon and nitrogen were conducted using a DC plasma unit with C-N bearing gas mixture at temperatures ranging from 400 to 550 °C. The surface alloyed layers were analyzed by XRD, GDS, SEM/EDX, TEM and a microhardness tester. Detailed tribological characterization was carried out by using a reciprocating tribometer in air and in 3.5% NaCl solution. Experimental results have demonstrated that C and N supersaturated metastable phase, i.e. S-phase, can be formed by the PSA treatments with both carbon and nitrogen but at a high temperature of 550 °C, chromium nitrides precipitated in the surface treated layer. The surface hardness of Stellite 21 alloy can be significantly increased by 2-3 times following the PSA treatments. The wear factor of Stellite 21 alloy in air and in 3.5% NaCl solution can be reduced by 99% and 96%, respectively.