Influence of substrate bias on the scratch, wear and indentation response of TiSiN nanocomposite coatings

TiSiN coatings were synthesised onto AISI M42 tool steel substrates via closed field unbalanced magnetron sputtering ion plating, using bias voltages of −40 and −50 V. The aim of this study is to investigate the underlying deformation mechanisms of TiSiN coatings, prepared at two different substrate bias voltages, following scratching, wear and indentation tests. A graded columnar microstructure evolved in these coatings. A hardness value of ~30.2 GPa was determined in the coating prepared at the lower bias voltage (i.e., −40 V), which was correlated to the fine nanocomposite structure and the presence of a high compressive residual stress. Of note, for the coating deposited at −40 V enhanced scratch adhesion strength, i.e., higher critical loads (L_c1 and L_c2) against cohesive and adhesive failure were determined with the higher H/E_r and H³/E_r² values. The improved scratch resistance was ascribed to the hierarchical structure that hindered crack propagation in the TiSiN coatings during progressive loading. An approximately 21% decrease in wear rate was obtained at the lower bias voltage, which was attributed to the slightly lower Si concentration (~8.3 at.%) and, in turn, higher hardness. Deformation behaviour under indentation loading was dominated by shear sliding along the columnar grain boundaries.