Design of functionally graded carbon coatings against contact damage

Three different functionally graded amorphous carbon (a-C) thin films were deposited on to aluminium substrates using a closed-field unbalanced magnetron sputtering ion plating method. The closed-field configuration prohibits the loss of secondary electrons and consequently enhances the plasma density significantly. The functional gradient of the a-C films was achieved by varying the bias voltage linearly during deposition. Three graded a-C systems possessing different variations in Young's modulus were deposited with the highest Young's modulus at the (i) top surface, (ii) interface or (iii) middle of the film. Of the three systems investigated, the one with the highest Young's modulus at the middle of the film thickness was found to exhibit significantly lower levels of cracking at higher indentation depths. Finite element models that included an embedded ring crack controlled by cohesive zone elements were developed to clarify the effect of ring cracks on the deformation of the films. This study provides guidance for the design of functionally graded coatings against contact damage.