Influence of substrate bias on microstructural evolution and mechanical properties of TiAlSiN thin films deposited by pulsed-DC magnetron sputtering

Substrate bias is one of the many factors, influencing the microstructure and, thus, properties of physical vapor deposition coatings. The aim of this study is to investigate the effect of varying substrate bias on the microstructure and mechanical properties of TiAlSiN coatings deposited on M42 tool steel substrates at 500 °C by a pulsed-DC close-field unbalanced magnetron sputtering system (CFUBMS). The microstructure of the as-deposited coatings was characterized by a range of techniques, including transmission electron microscopy, glancing angle X-ray diffraction and X-ray photoelectron spectroscopy. In addition, nanoindentation measurements were conducted to evaluate the mechanical properties of these coatings. It was found that an increase in substrate bias imposed minimal effects on the composition of the as-deposited TiAlSiN coatings, but had a significant influence on both the phase composition and microstructure of the coatings. As the substrate bias voltage increased from −40 to −80 V, a transition from zone-2 type structure to zone-3 type structure was observed, together with a reduction of the width of columnar grain from ~180 to ~60 nm. There was also a structural transition from a mixed fcc TiN + fcc AlN phases into a single fcc TiAlN phase as the negative substrate bias was increased above 60 V. Hardness and modulus were observed to increase with increasing bias voltage. Solid solution hardening and Hall-Petch effects are believed to be responsible for the hardness improvement.