Performance of low-friction coatings in helium environments

Abstract

The aim of this research work was to investigate the possibility of replacing soft PTFE‐based coatings on components operating in helium atmosphere. Focus was on maintaining low friction and improving surface wear resistance. Therefore two commercial DLC coatings (a-C:H and Me-C:H), CrN coating and reference PTFE‐based Rulon and Xylan coatings were included in this investigation. Coatings were deposited on hardened 100Cr6‐bearing steel discs and tested against uncoated steel balls in pin-on-disc contact configuration under dry reciprocating and unidirectional sliding in helium atmosphere. Investigation was concentrated on the effect of running-in, contact pressure, sliding speed and counter-material type and surface treatment on the tribological behaviour of hydrogenated DLC coatings when running‐in in helium.Results show that for PTFE-based self-lubricating coatings low friction is reached through coating removal and formation of thick layer of transferred coating material on the steel counter-surface. As long as coating is not worn through stable coefficient of friction of about 0.2 is maintained. In a similar way DLC coatings can provide even lower friction in helium, at the same time almost eliminating wear of the coated part. However, at the same time hard DLC coatings prolong running-in and provoke wear of the steel counter-surface, which greatly depends on the coating type, counter material and contact conditions. The best tribological behaviour in helium was thus achieved when pairing softer metal doped W-DLC coating with nitrided steel surface.