The roles of Ag layers in regulating strengthening-toughening behavior and tribochemistry of the Ag/TaC nano-multilayer films

Abstract Though the high hardness nature of transition-metal carbides or nitrides TMC(N) has been extensively exploited in protective films for various tools and wear-resistant films for the machine parts, improving toughness and reducing friction continue to be immensely challenging. Herein, we prepared the Ag/TaC nano-multilayers with various Ag thickness (lAg) ranging from 2 nm to 14 nm by magnetron sputtering, successfully achieving enhanced hardness, toughness and wear-resistance ability, together with reduced friction at lAg = 2 nm. In addition, lAg-dominated growth, strengthening-toughening behavior, and tribochemistry of the Ag/TaC nano-multilayers were investigated. These studies revealed that as very thin Ag layers (=2 nm) are alternatively inserted into TaC layers, discontinuous Ag/TaC nano-multilayer structure constructs in term of poor wetting induced the growth of discontinuous Ag layers, remarkably enhancing the hardness to ∼42 GPa associated with the Orowan mechanism and improving toughness and wear-resistance ability. Together with all these remarkable properties, the coefficient of friction (CoF) rapidly drops to ∼0.227, because of the formation of lubricious Ag nanoclusters and AgTaO3 phase on the contact surface during sliding. However, at thicker lAg, continuous soft Ag layers would form, and these are easy to activate the slip in soft Ag layers, thereby decreasing the hardness and toughness as well as dramatically decreasing the wear-resistance, but accompanying with further decline in the CoF owing to the increment of lubricious silver tantalate (AgTaO3) and Ag nanoclusters. Obviously, the incorporation of very thin Ag layers into the TMC(N) layers to construct nano-multilayer structure may provide an effective strategy to boost the comprehensive properties including hardness, toughness, friction, and wear-resistance.