Abstract
Introduction
Frictional behavior and, therefore, the coefficient of friction (CoF) play an important role in the evolution of fretting wear. Several studies investigated fretting at the ball head-taper junction with a remarkable variation in the CoF (0.15 to 0.55). This may be due to different material couplings, surface topographies or macro-geometries. Since the results of Finite Element (FE) models are strongly dependent on the choice of CoF it is crucial to determine the correct CoF for a speci?c system. Therefore, this study aimed to determine the CoF for the interface between ceramic ball heads and metal tapers.
Materials and Methods
Three groups of taper-ball head couplings were investigated (n=18 titanium (Ti), n=18 cobalt chromium (CoCr), n=18 steel tapers (SS)). Line profiles of the taper surfaces were measured and tapers and ball heads were assembled using different loads (2, 4, 6 kN). Tapers were disassembled from ball heads by using liquid nitrogen, surface topography was remeasured and the effective contact area was determined. Another set of measurements was conducted (n=5 tapers per taper material) to measure the contact pressure. Here, pressure sensitive films were placed between tapers and ball heads during assembly. Using the effective contact area and contact pressure the CoF was calculated.
Results
Effective contact area increased logarithmically with increasing assembly load with maximum values around 100 mm² for SS and Ti tapers at 6 kN. Contact pressure also increased with increasing load. Maximum contact pressures were found at the proximal end of the tapers and decreased linearly towards the distal end. Highest values were found for SS and CoCr (138 and 126 MPa). CoF increased with increasing load and varied from 0.44 to 0.68, while a decrease of the CoF between 4 and 6 kN for SS tapers was found. Largest values were found for Ti and CoCr tapers.
Discussion
Since contact pressure increases with increasing load it seems plausible that the CoF also increases. Absolute pressure values are within the range of literature data (25–280 MPa). At first sight, the CoF seem to be independent from the material coupling, but looking at taper subsidence there are distinct differences between materials with SS showing the lowest and Ti the highest subsidence. Therefore, the different deformation behavior of the materials and, thus, the different evolution of effective contact area have also an effect on CoF. Ti shows largest deformation and SS shows lowest deformation. This effect seems plausible since Ti has the lowest Young's Modulus of the three taper materials examined. Some of the CoF determined here are larger than literature values. This may be due to different surface specifications and geometrical parameters (e.g. angular mismatch), different material couplings and loading conditions. Here surface roughness and angular mismatch was kept constant for all couplings tested. The results of this study will be used to develop friction laws to be implemented in FE models examining fretting and wear processes.
