Mechanical and tribological properties of titanium–aluminium–nitride films deposited by reactive close-field unbalanced magnetron sputtering

Abstract

Thin films of titanium–aluminium–nitride (Ti 1− x Al x N), with 0 ≤ x ≤ 1.0, were deposited onto Si(1 0 0) and hardened M42 tool steel substrates at room temperature by reactive close-field unbalanced magnetron sputtering at a bias voltage of −50 V in an Ar–N 2 gas mixture. These films were characterized and analyzed using X-ray photoelectron spectroscopy (XPS), optical interference method, nanoindentation measurements, scanning electron spectroscopy (SEM), scratch tester, cyclic impact tester, reciprocating wear tester, and Raman spectroscopy. It was found that the films (typically 1.0 μm in thickness) deposited under aluminium-free conditions (TiN) had a high compressive stress of ∼3.0 GPa. The compressive stress in Ti 1− x Al x N films decreased significantly with an increase in the Al content ( x) and reached its lowest value of ∼0.3 GPa at x = 0.41. Passing this point, the compression of films rose again. These results can be ascribed to the decrease in the lattice parameter caused by incorporating small Al atoms in the lattice sites and structural changes (the development of an amorphous AlN network in the range of about x = 0.3–0.41) in Ti 1− x Al x N films as the incorporation of aluminium was increased. By scratch and dynamic impact tests, it was also found that the films at x = 0.41 (hardness of ∼31 GPa and elastic modulus of ∼312 GPa) exhibited the best adhesion and cohesive strength. For the wear evaluation of the films, three distinct processes have been evidenced. (i) The TiN films ( x = 0) exhibited a mild wear behaviour, resulting in lower friction coefficients (∼0.3–0.5) and wear rates (∼3 × 10 −6 mm 3/Nm); (ii) the films at x = 0.09 showed the presence of micro-scratches penetrating the substrate; and (iii) the films with higher x values (>0.09) revealed the presence of the interfacial layer in the wear track. Raman scattering results showed that the wear debris in TiN films composed of a mixture of nanocrystalline anatase and rutile TiO 2 at applied loads of 2 and 5 N, whereas nanocrystalline rutile TiO 2 was observed at an applied load of 20 N. For films with x > 0.09, only the rutile TiO 2 was observed. These different adhesive properties and wear performance for TiN, Ti–Al–N, and AlN films are explained on the basis of microstructure, mechanical properties and tribochemical wear mechanisms.