Insights into the mechanisms of friction and wear reduction for surface-modified implants in bovine serum protein solution

In this paper, a series of NbN-Cu composite coatings were prepared using non-equilibrium magnetron sputtering. The composition, microstructure, mechanical properties, chemical stability, and tribological performance of the NbN-Cu coatings in bovine serum albumin (BSA) solution were systematically investigated using EDS, XRD, TEM, nanoindentation, a friction tester, and Raman spectroscopy. The results show that the NbN-Cu composite coatings consist of fcc-NbN, hcp-NbN, and fcc-Cu phases. As the Cu content increases, both the hardness and elastic modulus first increase and then decrease. At a Cu content of 0.89 at.%, the NbN-Cu composite coatings exhibit the highest hardness and elastic modulus, reaching 24.5 GPa and 312.6 GPa, respectively. Additionally, at the same Cu content, the NbN-Cu composite coatings demonstrate the best chemical stability, with the lowest ion release and the highest corrosion resistance, reflected by a corrosion current of 8.2 × 10 −7 A/cm 2 and a corrosion potential of −0.15 V. During friction in bovine serum protein (BSA) solution, the friction coefficient initially decreases and then stabilizes, with a minimum value of 0.155. The wear rate also first decreases and then increases, reaching a minimum of 3.75 × 10 −8 mm 3 N −1 mm −1 at a Cu content of 0.89 at.%. The tribological mechanisms of the NbN-Cu/BSA system were explored from the perspectives of lubrication state, adsorption behavior, and tribochemical products. The findings indicate that: (1) the NbN-Cu/BSA system operates under mixed lubrication conditions. (2) the introduction of Cu facilitates the formation of a robust physical and chemical adsorption layer on the surface of the NbN-Cu composite coatings, effectively separating the contact surfaces. (3) under the friction-induced catalytic action of Cu within the coatings, BSA molecules degrade, forming high molecular weight hydrocarbons with lubricating effect, with the catalytic effect becoming more pronounced as the Cu content increases.