Bio-tribocorrosion resistance of CoB–Co₂B and Co₂B layers on CoCrMo alloy

Cobalt-based alloys, such as cobalt-chromium-molybdenum (CoCrMo), are known for their high mechanical strength and find extensive applications in the biomedical field such as manufacturing of tools, dental components, and orthopedic implants. The longevity of the CoCrMo alloy in service is intricately linked to its resistance to corrosion and wear. Specifically, tribocorrosion can contribute to material loosening; therefore, it is essential to explore surface treatments for cobalt-based alloys as a means to enhance their wear resistance, ensuring the prolonged durability of the material. This study provides novel insights into the bio-tribocorrosion resistance of the borided CoCrMo alloy when immersed in calf serum, emulating the synovial fluid. Two distinct microstructures of boride layers were examined in this research: (1) a CoB–Co₂B layer formed through powder-pack boriding and (2) the borided surface underwent diffusion annealing to completely dissolve the CoB, resulting in a monophasic layer (Co₂B). Following the ASTM G119-09 procedure, the total material loss (T), encompassing both material loss due to wear (W_C) and corrosion (C_W), was determined using a linear reciprocating ball-on-flat tribometer equipped with an electrochemical cell. Test results indicated that the presence of CoB–Co₂B and Co₂B layers on the CoCrMo alloy increased bio-tribocorrosion resistance approximately 2.4 times and 1.3 times, respectively, compared to the non-treated CoCrMo alloy. A dominant wear regime was observed for the borided surface exposed to diffusion annealing and the non-treated CoCrMo alloy, whereas the borided CoCrMo alloy exhibited a corrosion-wear regime. Clearly, these findings highlight the capability of the cobalt boride layer to improve the performance and extend the service life of the CoCrMo alloy in biomedical applications.