Influence of surface roughness and coating type on the galling properties of coated forming tool steel

The aim of the present work is to elucidate the influence of surface roughness on the galling properties of coated forming tool steel. The tribological evaluation included TiN, TiB2, TaC and WC/C coatings deposited on cold work tool steel. Representing a material difficult to form, austenitic stainless steel was used as a counter-material. A special test configuration made it possible to gradually increase the normal load during forward sliding strokes, and to correspondingly decrease the load during reversed ones. In this investigation, the load was varied between 100 and 1300 N, corresponding to a contact pressure between 2 and 5 GPa. The main observation is that the galling and anti-sticking properties of the tool surface dramatically improve by reducing the surface topography. Consequently, reduced substrate roughness or polishing of the contact surface after coating is highly recommended. However, selection of a carbon-based low-friction coating leads to reduced probability of worked material adhesion even at high surface roughness values and under starved lubrication.