Microstructural development of multielement Cr, Mo, W doped TiAlN coatings by hybrid DC-HIPIMS magnetron sputtering

This study examines the composition, microstructure, and indentation hardness of TiAlN-based coatings deposited by DC pulsed magnetron sputtering, which were alloyed with group VIB metals (Mo, Mo+Cr, and Mo+W) using an additional HiPIMS source. The resulting deposited ternary TiAlMoN and quaternary TiAlMoCrN, TiAlMoWN coating series were analyzed as a function of N₂ mass flow during the deposition, using optical emission spectroscopy, x-ray diffraction, electron microscopies and indentation hardness. At low N₂ mass flows, the coatings exhibit multi-metal NaCl-type solid-solution structures with low crystallinity. Increasing the N₂ mass flow promotes higher crystalline order and a pronounced (111) preferred orientation. The diffraction peaks of the quaternary coatings showed an increased full width at half maximum values compared to ternary TiAlMoN, due to multi-element solid-solution effects. This phenomenon is slightly more significant in W-containing coatings, attributed to the larger atomic radius of tungsten.The mechanical response, assessed via indentation hardness, correlates with microstructural evolution. Grain-refined structures formed at low N₂ mass flows yield high hardness values, whereas higher nitrogen contents promote the development of coarser columnar morphologies and a corresponding reduction in hardness, especially in the TiAlMoN and TiAlMoCrN series. In contrast, the TiAlMoWN coating series maintained a grain-refined microstucture which led to a more stable high hardnesses along the N2 mass flow series. This is likely attributed to the influence of energetic W+ ion bombardment, as produced by the HIPIMS sputtering, which inhibited the development of coarse columnar growth. No definitive evidence was found for the formation of secondary phases such as δ-MoN or w-AlN under the investigated deposition parameters.