Mechanical and tribological behavior of nanocomposite multilayered Cr/ a-C thin films

Abstract

The mechanical and wear behavior of Cr/ a-C multilayered nanocomposite films was studied. Two groups of multilayers were synthesized by e-beam physical vapor deposition. In the first group, the Cr and a-C layers had the same thickness ranging from 40 to 20 nm. In the second group, the layer thickness of the a-C phase was constant at 40 nm, while that of the Cr phase varied from 40 nm to 10 nm. The microstructure of the multilayers was characterized by transmission electron microscopy. The Cr layers were found to be of a columnar bcc structure and the a-C layers amorphous. The hardness and elastic modulus of the multilayers were studied by conducting nanoindentation and Knoop microhardness experiments. The hardness response can be described as a function of the metal layer thickness by a Hall–Petch relationship. Hardness was found to increase down to the smallest modulation period tested (40 nm). Fracture toughness was assessed by Vickers indentation at a threshold load below which crack would not occur. It was found that the fracture toughness improved by increasing the Cr layer thickness. The total residual stress in all Cr/ a-C multilayers was significantly lower than the average value of the two monolithic components and found to decrease with reducing metal layer thickness. Friction coefficients were measured using a pin-on-disk tribometer. All multilayers had a low friction coefficient around 0.1, and both the Cr and a-C components were found to affect the wear behavior.