Tribological behaviors of a Ni-free Zr-based bulk metallic glass in simulated physiological environments

Abstract

Ni-free Zr-based bulk metallic glass (BMG) generally exhibits excellent combination of biocompatibility and mechanical properties, making it a potential candidate for biomedical implants. However, the in-vitro tribological behaviors and wear resistance of Zr-based BMGs still remain less understood. In this study, the wear process of a Ni-free biocompatible Zr_60.14Cu_22.31Al_9.7Fe_4.85Ag₃ BMG in 3 simulated physiological environments, i.e., deionized (DI) water, 0.9 wt% NaCl solution, and phosphate buffer saline (PBS) solution, are studied using Si₃N₄ ceramic as the counter-material. The results indicate that Zr_60.14Cu_22.31Al_9.7Fe_4.85Ag₃ BMG show a specific wear rate less than 1/3 of Ti6Al4V alloy in all the 3 simulated physiological environments, which also surpasses currently reported wear resistance of Zr-based BMGs in the same environments. Specifically, the wear process of Zr_60.14Cu_22.31Al_9.7Fe_4.85Ag₃ BMG shows a 3-staged character, i.e., the running stage, the transition stage, and the dynamic stable stage, with the main wear mechanism transiting from adhesive wear to coexistence of adhesive wear and oxidative wear, and eventually to oxidative wear. During the transition of wear mechanism, the formation of oxide layer on the worn surface plays the key role, which provides protection against wear and leads to better wear resistance. Notably, the relatively higher wear rate of Zr_60.14Cu_22.31Al_9.7Fe_4.85Ag₃ BMG in 0.9 wt% NaCl solution and PBS solution than that in DI water is attributed to the corrosivity of wear environments, which weakens the adhesion between oxide layer and BMG substrate thus promoting spalling of oxide layer and enhancing wear degradation. These results indicate the synergistic effect of corrosion and wear in Zr-based BMG in simulated physiological environments. Our work provides insights in developing wear-resistant Zr-based BMGs for implantable biomaterials.