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

• CrB1.87 and a series of Cr–B–N coatings (~ 5 µm thick) were applied to a series of Ortron 90 (Fe–20Cr–10Ni–2Mo–0.4N austenitic stainless steel) substrates. The former was crystalline whilst the latter were amorphous. No crystalline BN was detected.
• Increasing the N content of the Cr–B–N coatings caused hardness and reduced elastic modulus to fall, whilst a slight increase in resistance to fracture was observed.
• Static corrosion tests showed all the coated materials to be more resistant to pitting and crevice corrosion in saline solution compared to uncoated Ortron 90.
• Dynamic sliding contact corrosion–wear testing in saline solution showed the Cr–B–N coated Orton 90 to displayed higher friction levels and total material losses compared to CrB1.87 coated and uncoated Ortron 90.

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.