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 TiAISiN coatings deposited on M42 tool steel substrates at 500 degrees 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 Xray 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 TiAISiN 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 similar to 180 to similar to 60 nm. There was also a structural transition from a mixed fcc TiN + fcc AIN phases into a single fcc TiA1N 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-Fetch effects are believed to be responsible for the hardness improvement. (C) 2017 Elsevier B.V. All rights reserved.