Friction-wear properties of nitrogen-ion-implanted nickel-free Co-Cr-Mo alloy

Nitrogen ions were implanted in a Co-29 mass%Cr-6 mass%Mo alloy (ASTM F799-95) with reducing the amount of nickel in order to improve the friction-wear properties. The friction-wear properties of unimplanted and nitrogen-ion-implanted Co-Cr-Mo alloys were evaluated using a pin-on-flat-type reciprocating friction tester in air and phosphate-buffered saline, PBS(-), as a quasi-biological environment under applied stress of 3.54 MPa to understand the performance of this alloy for metal-on-metal-type artificial hip joints. As a result, the wear loss in PBS(-) was much smaller than that in air and increased with the increase of applied stress in both air and PBS(-), while the friction coefficient in a steady state during the test was larger in PBS(-) than in air, according to the lubrication behaviors of wear debris and PBS(-). Nitrogen-ion implantation is effective to decrease the friction coefficient in PBS(-) at the sliding interface between mutual Co-Cr-Mo alloys as well as to decrease the wear debris in PBS(-). Both in air and PBS(-), the surface of a N ²⁺-implanted flat specimen was smoother than that of an unimplanted flat specimen. Cobalt phosphate was precipitated as a corrosion product on the pin during the friction-wear test.