Functionally graded Ti-TiC multilayers: the effect of a graded profile on adhesion to substrate

Thin Ti-TiC multilayer coatings on silicon substrates, when sputtered under bias, are in a state of compressive stress due to the incorporation of argon into the deposited layer. Under certain conditions, the compressive stresses lead to spontaneous delamination of the coating. The delamination process takes place by the propagation of a front of concentrated stress that produces local interface separation. The delamination process is characterized by outward buckling of the coating that propagates with relatively well-defined wavelengths of the order of 30 to 70 nm. Ultimately the delamination grows into a complex wrinkling pattern of the coating. The Ti-TiC graded multilayers were deposited with pre-designed composition profiles that affect the state of stress within the coating. The profile reflects the variation of the TiC/Ti ratio along the z-direction, perpendicular to the coating. Elementary composition profiles are characterized by one single parameter, k, that defines whether the TiC/Ti ratio is concave (for k<1) or convex (k>1) with respect to z. Graded multilayers with different values of k display delamination patterns that vary both by their kinetics and by the final morphology of the coating. In particular, graded multilayers with k≥3, display long term stability. The results illustrate the potential of graded coatings to reduce intrinsic stresses leading to delamination and failure.