Tribochemistry dependent tribological behavior of superhard TaC/SiC multilayer films

TaC/SiC multilayer films with different thicknesses of SiC layer (t(SiC)) were deposited by alternating sputtering of TaC and SiC composite targets in the discharged Ar gas. The SiC layers were forced to crystallize to form a coherent interface with TaC layers at t(SiC) <= 0.8 nm, resulting in an enhancement in both hardness and fracture toughness. When t(SiC) reached 0.8 nm, the film exhibited the highest hardness (46 +/- 2 GPa) and best toughness (4.2 +/- 0.2 MPa m(1/2)). With further increase in t(SiC), SiC lost crystallinity and transformed into amorphous state. The deterioration in coherent TaC/SiC interface resulted in the reduction of both hardness and toughness. The wear rate was also dependent on hardness and fracture toughness of multilayers films. The film with the lowest wear rate at t(SiC) = 0.8 nm was obtained. In addition, the alternating insertion of SiC layers into TaC layers tailored the optimization of film's tribological performance. Lubricious Ta silicate (TaSiOx) and amorphous carbon (a-C) on the surface of wear tracks were identified by SEM and XPS, and wear debris was identified by TEM. Consequently, it was believed that coefficient of friction (CoF) in multilayers was reduced when compared to the individual monolayers because of the synergistic effect of lubricious a-C and TaSiOx. Furthermore, steel and Al2O3 balls were also used as counterparts, and the same variation tendency was observed in both CoF and wear rate as when WC balls were used. Thus, improvement of wear-resistant ability and at the same time reduction of CoF can be simultaneously achieved in superhard TaC/SiC multilayers.