Aims: The aim of this research activity is the synthesis and characterization of surface modified cobalt alloys, in order to obtain a biocompatible material presenting at the same time good wear performances, low metal ion release and low toxicity. It is of interest for hip and knee joints with metal-on-metal or metal-on-UHMWPE contacts. In the first case the main aim is to enhance the biocompatibily of the surface and to reduce the amount and the toxicity of the metal debris. In the second case the main aim is to reduce the amount of polymeric debris. The surface chemical composition of several implant cobalt alloys (as cast and forged alloys, low and high carbon alloys) was modified, producing relevant tantalum enrichment. Tantalum was chosen because of its low toxicity and high corrosion resistance. The employed process is of interest because it is low-cost and it does not involve the formation of a brittle ceramic coating.

Methods: The chemical composition and morphology (SEM-EDS), roughness (prophilometry), crystallographic structure (x-ray diffraction analysis) and wettability (contact angle) of the surface were characterized. The mechanical behavior of the modified materials was investigated by nanoindentations, scratch tests (Revetest), friction measurements and wear tests (pin-on-disc).

Results: A surface enrichment with tantalum was obtained through a thickness of several microns and the interface between the substrate and the modified layer is continuous and crack free. The maximum tantalum content on the surface is about 90 wt%. Scratch tests showed that the modified layer is well adherent to the substrate through a diffusion layer and its first detachment appeared at 55 N without any brittle or catastrophic event. The modified layer presented a higher wettability than the untreated one, with a contact angle of about 48° respect to 82°. The elastic modulus of the treated surface layer (nanoindentation tests) is 352 GPa, while it is 281 GPa in the untreated alloy and its hardness is 17 GPa while it is 8.6 GPa in the untreated one. The surface presents also a better wear resistance (0.7·10⁻⁶ respect to 5.7 10⁻⁶ mm³/ Nm) and lower friction coefficients (0.24 respect to 0.34) in the Me-on-Me contact, according to its higher hardness and wettability.

Conclusions: A surface with high tantalum content can be obtained by a low-cost process. It shows good mechanical properties (high elastic modulus, high hardness value, low friction coefficient, high wear resistance) and chemical properties (high wettability, good corrosion resistance). So it is of interest for joints presenting good biocompatibility and great longevity.