The effects of high-temperature oxidation on the friction and wear of titanium-nitride-coated steel

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Abstract
An investigation has been made into the friction and wear of titanium-nitride-coated 321 stainless steel during like-on-like sliding contact in carbon dioxide environments at temperatures to 500 °C, with emphasis on the synergistic interactions between oxidation and mechanical wear. Tests have been made on both as-received surfaces and those present after long-term oxidation in CO2/1%CO at 450–700 °C. During sliding at 20–300 °C, the presence of a thin (2-3μm) titanium nitride coating on the stainless steel surfaces resulted in much lower coefficients of friction and wear rates than were recorded for similar uncoated specimens. In the latter case, metal-metal contact led to extensive deformation and ploughing of the surfaces and a high wear rate, while the presence of the coating eliminated such contact, at least in the early stages. However, the extent of wear was sufficient to cause thinning of the coating until, eventually, it was penetrated and the coefficient of friction and wear rate increased considerably. At 500 °C, oxidation of the stainless steel surfaces resulted in the development of oxide ‘glaze’ on the uncoated specimens and much reduced friction and wear rates. Similarly, for the coated specimens, oxidation of both the titanium nitride coating and any exposed steel substrate ensured that low friction and wear were maintained for extended periods. Long-term oxidation of both coated and uncoated specimens at 550 and 700 °C enabled a wear-protective oxide to develop rapidly on subsequent sliding at 300 °C, giving reduced friction and wear. This is accounted for in terms of the beneficial effects of oxides in protecting surfaces under sliding wear conditions.

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