Effect of Ultrasonic Surface Rolling Process on the Fretting Wear Behavior of Ti6Al7Nb Alloy in Air and Bovine Serum

Artificial hip joints are often subjected to cyclic alternating or impact loads during long-term service in the human body, and the friction interface is inevitably subjected to sustained multi axial composite stresses such as torsion, slip and compression, resulting in a large number of fretting wear problems. In order to improve the fretting wear resistance of artificial implants, ultrasonic surface rolling process (USRP) was used to strengthen the Ti6Al7Nb alloy. On this basis, the fretting wear behavior and damage mechanism of the modified material in air and bovine serum were studied. Before conducting the experiment, the material hardness improvement was characterized, and the experiment was conducted on a indigenously developed fretting wear testing machine, which could provide real-time feedback on the dynamic response of the contact interface. After completing the experiment, SEM with EDS function and white-light interferometer was used to analyze the damage mechanism of the worn area. The results indicated that a grain refinement layer of about 33 μm was formed on the USRP treated surface. Through SEM analysis, it could be observed that the microstructure of the substrate after USRP surface treatment exhibited grain refinement and plastic flow, and the surface hardness measured by micro-hardness tester increased by about 28.1%. The fretting wear resistance performance of USRP samples had been improved to a certain extent, but the fretting running regime of the USRP treated surface had no significant changes. The failure mechanism were mainly adhesion wear and slight fatigue wear in fretting partial slip regime, abrasive wear, fatigue wear and oxidation wear in fretting slip regime, and the mixed regime was dominated by fatigue wear and oxidation wear. It was worth noting that in partial slip regime, bovine serum might exacerbate the wear degree. But in the mixed zone and slip zone, due to the boundary lubrication effect of the bovine serum, the wear degree was reduced compared with air condition. Besides, the damage degree of delamination and wear in the fretting contact area was reduced due to the grain boundary orientation of the surface material was changed via USRP, but the oxidative wear was promoted by the increase of grain boundary channels. In addition, the results under different fretting conditions showed that the USRP treated surface had a good wear reduction effect. According to the results of SEM and white-light interferometer, it could be concluded that the combined effect of untreated debris and calf serum was the most effective in reducing wear damage. Therefore, the wear degree of USRP samples in bovine serum was significantly reduced by about 28 % compared with the substrate. Based on the above conclusions, a wear mechanism diagram was drawn. The research results provided theoretical data and application references for the surface engineering protection of artificial joint materials.