Character and chemical-wear response of high alloy austenitic stainless steel (Ortron 90) surface engineered with magnetron sputtered Cr–B–N ternary alloy coatings

Abstract

Many transition metal borides, in particular CrB2, show high inherent hardness, corrosion resistance and chemical stability. Appropriate additions of nitrogen to the Cr–B material system could in principle allow the synthesis of multi-phase materials containing hard, corrosion resistant (Cr–B, Cr–N) and lubricious (h-BN) phases providing a combination of good corrosion–wear resistance and in-situ lubrication. This factor has stimulated interest in the development of Cr–B–(N) magnetron sputtered thin tribological coatings. In the present reported work, several Cr–B–(N) coatings were produced by reactively sputtering a CrB2 target in an argon atmosphere containing increasing amounts of nitrogen. They were mostly applied to Ortron 90 (Fe–20Cr–10Ni–2Mo–0.4N) austenitic stainless steel substrates. The coating structure changed from crystalline to largely amorphous with increasing nitrogen content, this was accompanied by a fall in nano-indentation hardness from 40 GPa at 0 at.% to 18 GPa at 21 at.%. N. The corrosion–wear performance of Cr–B–(N) coated Ortron 90 stainless steel when sliding against aluminium oxide in 0.9% saline solution, was worse than uncoated Ortron 90, due to their propensity to undergo chemical (dissolution) based wear. In these tests, no friction reduction was noted for the Cr–B–N coated materials. However, the CrB1.87 coated Ortron 90 displayed superior corrosion–wear resistance and a slightly lower friction coefficient.