Tribological Coatings for Improving Cutting Tool Performance

With an increasing usage of advanced high-strength steels, there is an escalating need for improvements in cutting tool performance for these alloys. In this study, the tribological performance of superfinishing, and of CrN_x, WC/a-C:H, and Ti-MoS₂ coatings on AISI M2 steel was examined. Engineered surfaces were evaluated using dry pin-on-disk testing, Charpy impact testing, and a custom impact tribometer designed to replicate the interaction between cutting tools and high-strength steel sheet. Charpy testing of surface treated specimens showed no statistical change in impact toughness from the untreated steel. Pin-on-disk testing was performed to quantify the friction and wear of untreated and surface-treated tool steel.  These tests showed that Ti-MoS₂ coated tool steel had the lowest friction coefficient, and CrN_x, the lowest wear rate. Impact tribometer testing showed that the Ti-MoS₂ coating yielded an increase of 500% in the number of cycles to failure over untreated specimens, while the WC/a-C:H coating was found to decrease the number of cycles to failure over untreated specimens.  With an impact stress greater than the yield strength of the AISI M2 steel, the fatigue life of the surface treatments scaled approximately as E²/H³, the inverse of the resistance to plastic deformation.