Selective strategy of reactive hysteresis loop for coatings on alloy substrates with different moduli

Abstract

The structure and properties of nitride films, such as titanium nitride (TiN), depend on the reactive gas (N2) flow rates, which are normally selected according to the reactive hysteresis loops. Film-substrate adhesion depends on the properties of the films and substrates. A selective strategy for the reactive gas flow rate within the hysteresis loop was investigated by characterizing the structure, properties, and failure mechanisms of TiN films on Ti6Al4 V titanium alloy (TC4) and 4Cr5MoSiV1 hot-work die steel (H13). The hysteresis loop of the titanium (Ti) target potential as a function of the N2 flow rate was measured, and flow rates in different sputtering modes were used to prepare TiN films using plasma-enhanced magnetron sputtering. As the N2 flow rate increased from 5 cm3/min, 10 cm3/min, 15 cm3/min to 20 cm3/min, from the metallic mode to the compound mode, the morphologies of the films changed from loose to dense, the phase structures changed from TiN0.3 (002) to TiN (111), (200), and (220), and the nano-hardness and elastic moduli increased. Applying a Rockwell normal load, asymmetric circular cracks appeared and became significant for TiN/TC4 as the N2 flow rate increased to 15–20 cm3/min; cracks were only observed in TiN/H13 at an N2 flow rate of 20 cm3/min. Applying normal and shear scratch stresses, the TiN films peeled off from the TC4, except for TiN, with an N2 flow rate of 10 cm3/min, indicating that the adhesion between TiN and TC4 was weak. No peel-off chips were observed in the scratch morphologies of TiN/H13, indicating excellent adhesion between the films and H13 substrate. Circular cracks appeared in the scratch morphology of TiN0.3, indicating that cohesion had broken within the film. The possible failure mechanism was the large difference in the elastic moduli and hardness of TiN and TC4, which led to TC4 elastic and plastic deformation much earlier than in TiN films. According to numerical simulation, the interfacial tensile stress of TiN/TC4 under a normal load was higher, and the interfacial strain near the indentation edges was larger than that of TiN/H13. Considering the comprehensive properties, a reactive flow rate near the critical point such as 15 cm3/min for TiN/TC4 should be used for the nitride film on a low-hardness and low-modulus substrate; in the compound mode stage, 20 cm3/min for TiN/H13 should be used for the nitride film on a high-hardness and high-modulus substrate.