Relating hardness-curve shapes with deformation mechanisms in TiAlN thin films enduring indentation

In this paper, we report the effect of structure and morphology on the depth-sensing hardness of titanium aluminium nitride (TiAlN) films. Nanocomposite Al-rich films, develop refined equiaxed structures and their hardness revealed constant behaviour throughout indentation before substrate effect appears. Transmission electron microscopy (TEM) from the cross section of indentations showed that nanocomposite TiAlN yields significant cracking. The formation of the hex-AlN and grain diameter lower than 10 nm in the nanocomposite films confer greater capacity for plastic deformation inhibiting crack propagation while film hardness remain constant under berkovitch indenter load. Ti0.6Al0.4N samples crystallize in solid solution fcc-TiAlN and develop columnar structures which show moderate resistance to crack nucleation and propagation. Network of wavy-form cracks in the indenter impression showed to propagate along column boundaries leading to gradual loss of the effect of residual stress in the film hardness. Columnar type of growth films with various residual stress exhibit similar curves of hardness plotted versus relative indentation depth (RID). Procedure to estimate hardness values without contribution of residual stress are suggested. Crack propagation mechanisms of samples are correlated to the underlying grain morphology and residual stress in the film.