Wear and mechanical properties, and cell viability of gas-nitrided beta-type Ti-Nb-Ta-Zr system alloy for biomedical applications

Frictional wear resistance is one of the important properties of metallic biomaterials. Surface hardening treatments such as oxidizing, nitriding and ion implantation tend to be applied for improving the wear resistance of titanium and its alloys. The simple gas nitriding process is expected to further improve the wear resistance of newly developed beta-type Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) for biomedical applications. However, there is a possibility for the mechanical properties such as tensile and fatigue strength of TNTZ to be degraded through gas nitriding process. Therefore, the gas nitriding process was carried out in this study to improve the wear resistance of TNTZ and alpha+beta-type Ti-6A1-4V ELI alloy (Ti64), which is one of the representative titanium alloys practically applied for biomedical applications, in simulated body fluid (Ringer's solution). Their tensile and fatigue properties and cell viability was also investigated in order to confirm the reliability as biomedical materials. The Vickers hardness near the specimen surface of nitrided TNTZ and Ti64, where TiN and Ti₂N forms, increases significantly as compared to that of their matrices. The wear resistances of TNTZ and Ti64 are improved significantly in Ringer's solution by nitriding process as compared to those of as-solutionized TNTZ (TNTZ_ST) and Ti64 (Ti64_ST). The tensile strength of nitrided TNTZ increases by around 90 MPa as compared to that of TNTZ _ST- The tensile strength of nitrided Ti64 does not change significantly at all nitriding temperatures. On the other hand, the elongation decreases with increasing the nitriding temperature. The run out (plain fatigue limit) of TNTZ subjected to a nitriding process at 1123 K, which has relatively good balance between wear resistance and tensile properties, is around 300MPa, and is nearly equal to that of Ti64 subjected to a nitriding process at 1123 K, although the tensile strength of the nitrided TNTZ is around 200 MPa smaller than that of the nitrided Ti64. The cell viabilities of nitrided TNTZ and Ti64 range from 1.4 to 1.6 against that of control (cell disc), and are a little higher than that of TNTZ_ST and Ti64_ST- The cell viabilities of nitrided TNTZ and Ti64 after removing the oxide layer on their surfaces are similar to that of control and are not significantly degraded.