Surface Layer Modifications in Co-Cr-Mo Biomedical Alloy from Cryogenic Burnishing

Severe plastic deformation (SPD) processes have been used to modify the surface integrity properties of many materials by generating ultrafine or even nanometer-sized grains in the surface and subsurface region. These fine grained materials created by SPD and dynamic recrystallization (DRX) usually have higher hardness and frequently exhibit enhanced mechanical properties (wear resistance, corrosion resistance, fatigue life, etc.). A novel burnishing SPD process, cryogenic burnishing, was used to improve the surface integrity of a Co-Cr-Mo biomedical alloy. Results show that grain refinement in the surface region was achieved through burnishing-induced SPD and DRX. The SPD layer thickness was shown to be increased by over 170% with the application of liquid nitrogen compared to the equivalent dry burnished workpiece. Microstructure refinement with a final grain size of 300 nm was obtained from cryogenic burnishing, which reflects a reduction from an initial grain size of about 80 μm. Microhardness measurements indicate that the hardness in the small grained SPD layer created by cryogenic burnishing was increased by 87% relative to the bulk value. Current results show that cryogenic burnishing could be an effective processing method to modify the surface integrity of Co-Cr-Mo biomedical alloy.