Friction behaviour of TiN, CrN and (TiAl)N coatings

Abstract

A series of experiments have been carried out, using a ball-on-disc testing machine, to investigate the sliding friction behaviour of physically vapour-deposited TiN, CrN and (TiAl)N coatings against steel under both dry and lubricated conditions, employing various applied loads and sliding speeds. All the friction coefficient vs. sliding distance curves were characterised by an initial transient state where the friction coefficient either increased (dry condition) or decreased (lubricated condition) rapidly, followed by a steady state. Experimental results revealed that ploughing actions of the hard asperities of the coating surface and material transfer from the steel counterface to the coating surface played an important role in determining the friction behaviour during the initial transient state and steady state respectively. Among all the coatings tested, the (TiAl)N coating, which possessed the highest hardness and surface roughness, exhibited the highest friction coefficient under both dry and lubricated conditions. Lubrication significantly reduced the friction coefficient of all coatings investigated.