Effects of titanium and boron alloying with non-equimolar AlCrNbSiTi high entropy alloy nitride coatings

Among various fabrication technologies for high entropy alloy coatings, high power impulse magnetron sputtering (HiPIMS) technologies have been utilized to synthesize dense microstructure and high hardness coatings with enhanced mechanical and chemical properties. Mid-frequency (MF) pulses have been introduced in the off-time of HiPIMS pulses of the superimposed HiPIMS-MF system to improve the deposition rate. In this study, AlCrNbSiTiBN coatings were deposited using a superimposed HiPIMS-MF system connected to an Al₄Cr₂NbSiTi₂ high entropy alloy target and a direct current (DC) power source for TiB₂ target in a reactive Ar and N₂ gas mixture. The DC power of the TiB₂ target was varied to increase the titanium and boron contents. With increasing Ti and B contents, the microstructure transforms from loose granular and crystalline structure to denser and refined nanocrystalline phases. A residual stress-free coating with a B content of 4.2 at.%. and a hardness of 16.2 GPa was achieved without intentional heating and substrate bias. The poor corrosion resistance of the coatings with lower B contents can be enhanced with a further increase of B content up to 6.4 at.% and reaching a maximum of 10.3 times improvement than that of AISI 304 stainless steel.