Wear behavior and wear debris distribution of UHMWPE against Si3N4 ball in bi-directional sliding

Four kinds of wear tracks possibly occurred in the wear of artificial hip joints, including uni-directional sliding, oval curvilinear sliding, double-elliptical sliding, triple-elliptical sliding, were simulated on the UMT wear tester by using of a Si3N4 ball sliding on the UHMWPE disc under bi-directional sliding motion. The wear behavior and wear particle distribution of UHMWPE in plasma solution lubrication were studied for these sliding motions. The experimental results indicate that the wear mass loss in uni-directional reciprocating sliding is much smaller than those in bi-directional sliding modes. The wear rates of UHMWPE in bi-directional sliding modes are linearly inverse proportional to the defined frequency factor, as agreed with the cross-shear theory. This result suggests that cross-shear movement with larger intersection angles is a significant factor influencing the wear rate of UHMWPE, and the bi-directional sliding path at direction reversals will play an important role on the increasing of UHMWPE wear compared to uni-directional sliding motion. In bi-directional sliding modes, the wear particle distribution range decreases when direction reversal path increases in the sliding motions. So, the complex wear tracks are harmful to the implant joint due to the higher wear and more active wear particles. The particles sizes follow a lognormal distribution. The central size and the peak accumulation of UHMWPE particles decreases and increases against the frequency ratio, respectively, besides the uni-directional reciprocating sliding. These suggest that the intersection angle increasing on sliding path will contribute to the size decreasing of UHMWPE wear particle. Also, cross-points on sliding track will produce wear particles in smaller size. The radius of curvature of the curvilinear paths may be dictating the size reducing of UHMWPE wear particles. The main wear mechanisms are ploughing in uni-directional reciprocation, while plastic deformation, adhesion and fatigue in the bi-directional sliding modes.