The damage tolerance of S-phase coated biomedical grade stainless steel

There is an increasing demand to manufacture total joint replacements (TJRs) with damage tolerant bearing surfaces that minimise the creation of wear debris in vivo. To this end there is potential for exploiting hard coatings. The work reported here investigates the potential of using S-phase coatings as scratch resistant bearing surfaces. A series of these coatings with nitrogen contents [N] ranging from 8 to 29 at.% were synthesised by magnetron sputtering and applied to polished medical grade austenitic stainless steel (Ortron 90). The load invariant hardness (LIH) was proportional to [N]. Damage tolerance of the coated and uncoated Ortron 90 was assessed using both Rockwell C hardness indentation and scratch testing. Nine modes of scratch response were identified which fell into three broad groupings: (i) tough; (ii) intermediate; (iii) brittle. These were subsequently presented in the form of a scratch mode map. The toughest coating contained 8 at.% N with an LIH ∼ 900 kg/mm2, and did not crack, even after scratching with a 5 kg load or being indented by Rockwell C hardness testing using a 150 kg load.