Abrasion response and abrasion–corrosion interactions for a coated biomedical stainless steel

A simple screening test has been developed to assess the suitability of surface coatings to protect the femoral articular surfaces of Ortron 90 total hip replacement (THR) prostheses from abrasion damage. A detailed comparison was conducted of the dimensions of scratches on explanted Charnley (Ortron 90) femoral heads and those of scratches produced on polished Ortron 90 barstock (of similar hardness) in the laboratory using a Rockwell C indenter and a range of static loads. From this comparison a load of 0.5 kg was found to give similar scratch depths (D) and lip heights (H) to the upper limits of scratches on the explants. Additionally a “multi-scratch” grid of two sets of five intersecting scratches (giving 25 scratch intersections) was used to simulate the multi-directional nature of in vivo abrasion damage. Commercial titanium nitride (TiN) and diamond-like carbon (DLC) coated Ortron 90 samples were subjected to this multi-scratch test in addition to single scratch tests. Adhesive and cohesive coating failures were observed at some of the scratch intersections for all of the coatings except the magnetron sputter deposited DLC coating which showed only through-coating fracture. These adhesive and cohesive failures were also observed in single scratches but at higher loads. Corrosion testing of the coated samples showed that the multi-scratch grid introduced additional sites at which substrate corrosion could take place.