Investigation of surface treatments for galling protection of titanium alloys

A galling test has been developed in which two nominally identical crossed cylinders are slid over each other under a steadily increasing contact load. The test has been applied to samples of the titanium alloy Ti6Al4V with modified surfaces. Loads up to 200 N were applied to cylinders of 6.35 mm in diameter. Six different treatments were applied: (1–2) two variants of a novel commercial vapour phase treatment (IonSlip); (3) a PACVD diamond-like carbon (DLC) coating; (4–5) two different forms of PVD TiN coatings, one arc-evaporated and the other electron beam-evaporated; and (6) a thermal oxidation treatment which produced a rutile (TiO2) surface layer. Specimens of the un-coated titanium alloy were also examined as controls. The carbon-based coatings, (DLC and standard IonSlip), provided the best performance, with both giving low friction coefficients and galling protection at very high contact loads. Thermal oxidation also provided excellent protection against galling but with a friction coefficient not so low as the carbon-based coatings. Electron-beam evaporated TiN provided low friction and adequate galling protection at low loads but failed to protect at high contact loads.