Damage tolerance of high entropy nitride coatings: Monolithic versus multilayered designs via in situ tensile and scratch testing

High entropy alloy nitrides (HEN) are an emerging material class for wear-resistant coatings due to their superior hardness and high-temperature stability. High damage tolerance is of particular interest for hard coatings. This work compares the HEN systems (AlCrTaTiZr)N, (AlCrNbSiTiV)N and (HfNbTaTiVZr)N as well as the multilayers (AlCrNbSiTiV)N/TiN and (HfNbTaTiVZr)N/TiN with (AlTi)N regarding mechanical properties and damage tolerance. The coatings were deposited by means of cathodic arc evaporation, enabling highest deposition rates. Different loading conditions were applied for testing: in situ tensile test and scratch tests with small and large indenters. The results show that the resistance to mechanical failure can be significantly increased with HEN coatings, but there is no universal damage tolerance which adequately describes a coating-substrate compound for various applications. Instead of using intrinsic coating characteristics, such as fracture toughness or hardness-to-modulus ratio, the composite behavior must be analyzed in the context of the respective load scenario. The (HfNbTaTiVZr)N coating is the most promising in this study. (AlCrTaTiZr)N also appears interesting due to its highest hardness (37.7 GPa). The multilayer design mainly leads to a reduction in residual compressive stresses, which is beneficial when scratching with a small sphere radius and detrimental with a large sphere radius.