A proposal for an unusually stiff and moderately ductile hard coating material: Mo₂BC

The elastic properties of Mo₂BC were studied using ab initio calculations. The calculated bulk modulus of 324 GPa is by 45% larger than that of Ti_0.25Al_0.75N and 14% smaller than that of c-BN indicating a highly stiff material. The bulk modulus (B) to shear modulus (G) ratio is with 1.72 at the transition from brittle to ductile behavior. This, in combination with a positive Cauchy pressure (c12-c44), suggests moderate ductility. When compared to a typical hard protective coating such as Ti_0.25Al_0.75N (B = 178 GPa; B/G = 1.44; negative Cauchy pressure) Mo₂BC displays great potential as protective coating for metal cutting applications. In order to test this proposal, Mo₂BC thin films were synthesized using DC magnetron sputtering from three plasma sources on Al₂O₃(0001) at a substrate temperature of ~900°C. The calculated lattice parameters are in good agreement with values determined from xray diffraction. Measured Young’s modulus values of ~460±21 GPa are in excellent agreement with the 470 GPa value obtained by calculations. Scanning probe microscopy imaging of the residual indent revealed no evidence for crack formation as well as significant pile-up, which is consistent with the moderate plasticity predicted. The apparent contradiction between moderate ductility on one hand and indentation hardness values of 29 GPa can be understood by considering the electronic structure particularly the extreme anisotropy. The presence of stiff Mo-C and Mo-B layers with metallic interlayer bonding enables this intriguing and unexpected property combination.