Tribological behavior of a Ta-based coating on a Co–Cr–Mo alloy

The cobalt–chromium–molybdenum (Co–Cr–Mo) alloys are widely used for the total substitution of the human hip and knee joints, because of their good mechanical properties. However, they induce human tissue reactions and inflammation, because of wear and corrosion phenomena; the debris, arising from metal wear and related ions, are toxic and hazardous for the patient health and reduce implantʹs life. s work, a high carbon content Co–Cr–Mo alloy has been coated to improve both wear resistance and biocompatibility. The coating, formed on the alloy surface by means of a thermal treatment in molten salts, consists of a multilayer of tantalum carbides (TaC and Ta2C) or a single carbide layer (TaC). Structural, morphological, mechanical and wear properties of the untreated and coated materials were studied. Furthermore, the effect of a surface pre-treatment with deposition of a carbon layer has been investigated. As for the wear properties, tribological tests were carried out on samples reproducing three of the typical coupling systems of artificial joints, i.e. metal on polyethylene (MoP), ceramic on metal (CoM) and metal on metal (MoM). The friction coefficient and wear behavior are influenced by the presence of the coatings; the multilayer coating exhibits higher wear resistance and adhesion to the substrate compared to the single carbide layer. The pre-treatment with carbon enhances surface reactivity and adhesion to the substrate.