Reasons for inferior performance of ceramic on a-C:H-coated Ti–6Al–4V in hip simulator testing compared to ball on flat reciprocating sliding tests

The tribological performance of a hydrogenated amorphous carbon coating on Ti–6Al–4V 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 1–2 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 substrate–coating interface making it susceptible to corrosion.