Mechanical and tribological characterisation of nanostructured Ti/TiB2 multilayer films

Abstract

Ti/TiB2 nanomultilayer thin films with different bilayer thicknesses λ were deposited onto unheated Si(100) wafers (for mechanical analyses) and AISI M42 tool steels (for tribological measurements) by unbalanced dc magnetron sputtering. The effects of different λ values on mechanical and tribological properties were investigated. These films were characterised and analysed in terms of their hardness by microindentation measurements, their surface root mean square roughness by AFM, their stress by an optical interference method, and their friction and wear behaviors by Rockwell-C testing, nano-/micro-scratch testing, dynamic impact testing and pin on disc tribometer. It was found that the mechanical and tribological properties of multilayer films (typically 1˙58±0˙10 µm in thickness) were closely related to λ (varied from 1˙1 to 9˙8 nm). For the best multilayer film with λ51˙9 nm, a maximum hardness of ∼32˙5 GPa was achieved and the best cohesive and adhesive strength was evidenced in terms of critical load values of LC1 (∼26 N) and LC2 (∼62 N). Moreover, by dynamic impact testing this multilayer film could endure impact cycles up to 4 × 105 without adhesive failure. However, when the λ was further decreased to 1˙1 nm, the hardness, cohesive and adhesive strength were decreased due to a high level of intermixing and lack of a layered structure. It was also found that the nanoscratch test under single pass and constant load conditions showed that the frictional coefficients decreased with λ and increased with normal load due to the ploughing effect. The enhanced hardness in the multilayer films with small λ values improved the wear resistance and lowered the frictional coefficients. These adhesive properties and wear performance are also discussed on the basis of mechanical properties and wear mechanisms.