Carbon, a promising material in endoprosthetics. Part 2: tribological properties

Abstract

In a previous paper three types of carbon materials were presented. A basic study of their mechanical properties showed that two of these materials might be applicable for sliding parts of artificial joints such as ball and acetabular cup of a hip joint endoprosthesis. The materials are: 1. (1) a high strength isotropic carbon and 2. (2) a silicon carbide/ carbon composite material. The results of relevant tribological investigations are given in this paper. Experimental methods applied consisted of ring-on-disc and ball-in-socket tests. Ring-on-disc tests were performed with specific loads of 10, 20 and 30 MN/m 2. In the ball-in-socket tests the gap width was 0.075 and 0.1 mm but the load was adjusted in such a way that the compression stress in the contact area was constant at 6.6 MN/m 2. The latter was experimentally measured and theoretically calculated with good accordance. Ringer's solution was used as a lubricant. In ring-on-disc tests the ring was made of the SiC/C and the disc of the isotropic carbon. In ball-in-socket tests balls of SiC/C and sockets of isotropic carbon were chosen. For comparison the combination CoCrMo alloy or AI 2O 3 as ball, and HDPE as socket, were tested under corresponding conditions. It is shown, that the special carbon materials can withstand the extreme tribological conditions of high specific load and low sliding velocity. Neither material suffers from fatigue wear. The coefficient of friction is below 0.1 even at a specific load of 30 MN/m 2. The wear rate as determined with ring-on-disc test is lower than those of all known and proposed material combinations. The data with the carbon materials combination indicate a penetration of the ball into the acetabular cup of only 1 μm per year.