XPS Study of Co28Cr6Mo Biocompatible Alloy

CoCrMo alloys are widely used for the production of such medical devices as orthopedic and dental implants. Of particular relevance is the possibility to customize biomedical implants through additive manufacturing (AM) to better fit the patient's anatomy. Moreover, in the case of Co-base alloys, AM allows overcoming some manufacturing limits typical of conventional processes. In biomedical applications, the surface of the implant interacts with biological environments; therefore, its properties are of crucial importance to guarantee patient safety. X-ray photoelectron spectroscopy (XPS) analyses have been carried out on the surface of the Co28Cr6Mo alloy produced by selective laser melting (SLM) in as-built condition and after a heat treatment consisting of a heating–cooling cycle from room temperature up to 800°C. The as-built material is in nonequilibrium conditions and undergoes microstructural and surface modifications as a consequence of the heat treatment. Heating promotes the transformation of part of the metastable γ phase into ε phase, a coarsening of microstructure, and surface segregation of Mo and Cr that form an oxide layer. Surface segregation of Mo occurs in all the samples while Cr segregation is only observed after the heat treatment. Segregation is driven by the energy reduction related to the formation of Mo and Cr oxides. In as-built samples this effect is low because long-range diffusion is hindered by the high cooling rate and thermal gradient involved in AM processes. During heat treatment long-range and surface diffusion can take place; therefore, more Mo and Cr atoms reach the surface. The higher amount of Mo and Cr at the surface promotes the formation of Mo and Cr stable oxides that improve surface passivation.