Effects of Synchronous Bias Mode and Duty Cycle on Microstructure and Mechanical Properties of AlTiN Coatings Deposited via HiPIMS

The good mechanical properties of metal nitrides make them ideal surface coatings for cutting tools and mold components. Conventional TiN coatings have largely been replaced by AlTiN due to their superior mechanical properties and resistance to high temperatures. In this study, we investigated the application of bias voltage to the substrate to enhance ion bombardment during the synthesis of protective AlTiN coatings using high-power impulse magnetron sputtering (HiPIMS) with synchronous trigger-direct current (ST-DC) bias voltage. The ST-DC parameters included the duty cycle duration (3%, 6%, 12%, 18%) and turn-on time, which included synchronous (TD0) or a trigger delay of 50 mu s (TD50). Scanning electron microscope images revealed that the highest deposition rate (22.1 nm/min) was achieved using TD50 with a duty cycle of 3%. The results obtained using an electron probe microanalyzer and X-ray diffractometer revealed the formation of an h-AlN structure when the Al/Ti ratio was between 0.71 and 0.74. Transmission electron microscopy and nanoindentation results revealed that transforming DC bias into synchronous bias to boost the bias output time (i.e., increasing the duty cycle) increased AlTiN grain refinement (from similar to 100 nm to similar to 55 nm) with a corresponding increase in hardness (from 22.7 GPa to 24.7 GPa) as well as an increase in residual stress within the AlTiN coating (from 0.16 GPa to 51 GPa). The excellent adhesion performance of the coatings provided further evidence indicating the importance of duty cycle and trigger delay when using pulsed-DC bias in HiPIMS.