Enhancing the hardness and damage-tolerance of CoCrNiAlTi coatings through dual-phase and nanotwinned structures

Abstract

A series of CoCrNiAlTi coatings were deposited, via direct current (DC) magnetron sputtering, onto M2 steel substrates employing a range of substrate bias voltages (−20 V ~ -120 V). Dual phase (fcc and hcp) microstructures, with high compositional homogeneity, were observed for all as-deposited coatings. Highly refined columnar grains, with a high density of twin boundaries that were oriented perpendicular to the coating growth direction, were identified. As the bias voltage increased, elements in the high entropy alloy exhibited different sputtering yields that led to different degrees of re-sputtering in the deposited films. This, in turn, played a critical role in determining the coating composition and hence stacking fault energy. Accordingly, the ratio between the hcp and fcc phases varied, with a maximum fraction of the hcp phase observed at a bias voltage of −80 V. The fraction of the hcp phase then decreased as the voltage increased to −120 V. An exceptional hardness of value of ~9.5 GPa, along with appreciable damage-tolerance, was exhibited in the coating deposited at −80 V. It is suggested that the presence of nanotwins, as well as the dual-phase microstructure, contributed to this excellent strength-ductility unity.