Elastic Modulus and Hardness of Cr--Nb Nano-Multilayers

Cr--Nb nano-multilayered films with various modulation wavelengths Λ are prepared by e-gun evaporation and their mechanical properties are investigated. Cr and Nb both have bcc structures with large differences in lattice constants and Young's modulus, which are supposed to favour modulus enhancement. Nevertheless, nano-indention measurements show no enhancement for the modulus and a slight decrease for the hardness with decreasing Λ down to 6nm. This is mainly due to counter-contribution to modulus from adjacent layers subjected to reverse strains, in agreement with recent theoretical study, while the decrease of hardness arises from grain boundary sliding. Interestingly, at Λ= 3nm, the hardness of the film has an
increase of 44% relative to the value of a rule of mixture, owing to the emergence of a new phase for reconciling the structure difference at the interfaces.