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Eingeladener Vortrag
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After having checked in a previous study the influence of stroke and normal load in laboratory fretting tests accomplished with thin steel roping wires, in this paper the effect of the contact pressure (test configuration), environmental conditions (relative humidity) and reduction of the friction coefficient (lubricants) on the wear behaviour were analysed. The wires were 0.45 mm in diameter and the material was a cold-drawn eutectoid carbon steel (0.8% C) with a Tensile Strength over 2800 MPa. The tests were performed on an oscillating test rig and the volumetric wear of both specimens of the tribocouple was determined separately from three-dimensional topographies of the wear scars acquired by means of a confocal profiler, and a calculation algorithm developed in using MATLAB®. The strong effect of humidity and lubrication on wear behaviour was proved, whereas a slight effect of contact pressure was derived.
The tribological performance of a hydrogenated amorphous carbon coating on Ti6Al4V against ceramic balls was checked by simple reciprocating sliding tests and by more complicated tests in a hip joint simulator. 3 of 4 joints failed during hip simulator testing already after approximately 100 × 10³ cycles and possibly due to coating failures caused by insufficient polishing of the metallic acetabular cups prior to coating deposition. Since such failures occurred only occasionally, the problem was not revealed by reciprocating sliding tests which are stressing a much smaller surface area compared to the tests in the hip joint simulator. Another type of failure, pits of 12 µm in diameter distributed randomly at the coating surface, was frequently observed by Scanning Electron Microscopy and analyzed comprehensively by Focused Ion Beam technique in combination with Transmission Electron Microscopy in one case. Although not necessarily affecting wear, such small failures might cause long-term problems in vivo, by providing access of body fluid to the substratecoating interface making it susceptible to corrosion.
Three different contact geometries, ball on disk and two different settings with crossed cylinders have been investigated with regard to the agreement of the results of the tribological quantities determined by each method. The material combination was α-alumina as upper specimen and steel 100Cr6 as lower specimen. For each of the contact geometries 5 independent measurements were carried out and evaluated independently. The agreement between the single results of coefficient of friction and the coefficient of system wear as well as between the different groups and conditions are good. Largest scatter of the results were found in the dry air regime where both friction and wear depend strongly on the relative humidity. However, comparison of the absolute values of the coefficient of wear between the ball on disk contact geometry on one side and the 2 crossed cylinder contact geometries on the other side reveals a significant difference in that the ball on disk results are lower by about a factor of two. The results document also a high degree of repeatability of single wear tests.