Residual stress evaluation using hole-drilling method on a Co-Cr-Mo alloy after a nanocrystallisation surface treatment

Abstract : Cobalt-Chromium-Molybdenum alloy, used for joint replacements in the human body, hip and knees was subjected to a nanocrystallisation surface treatment called SMAT (Surface Mechanical Attrition Treatment). This treatment is known to improve the mechanical properties of materials. Previous work revealed an increase of more than 100HV10 hardness and a martensitic transformation at the surface of Cobalt Chromium alloy subjected to SMAT. The microstructure of the SMATed samples was characterised using electron microscopy (SEM, TEM) and EBSD that show a drastic reduction of the grain size down to the micrometer scale. Besides, in-depth residual stresses were analysed performing incremental hole-drilling measurements before and after SMAT. High compressive stresses were found to be present in a layer as thick as 600 µm below the surface after SMAT.