The development of a functionally graded TiC–Ti multilayer hard coating

A functionally graded transition zone between a hard TiC coating and a WC–Co substrate, e.g. a cutting tool, can be formed over the range composition of the titanium carbide phase that extends from Ti2C to TiC. The transition zone is formed by sputter deposition of a multilayer stack of nanometric TiC and Ti layers. The composition gradient within the carbide layer is generated by varying the relative thickness of the as-deposited Ti and TiC layers. A subsequent short diffusion treatment eliminates the interfaces between the adjacent layers, yet maintains an overall carbon composition gradient across the thickness of the coating. The relative thickness of the individual sputtered layers is adjusted to yield a low carbon composition in the coating close to the substrate and a stoichiometric composition near the external surface. The composition profile within the coating can be further optimized with respect to residual stresses that arise during cooling after the diffusion anneal. In the present work, different graded Ti–TiC profiles were deposited on WC–Co substrates in two steps. First, a 0.5-μm graded Ti–TiC multilayer was deposited followed by a 2.5-μm outer layer stoichiometric TiC. Scratch tests, carried out on the as-deposited and on the annealed coatings showed that the critical load and the wear resistance depended on the concentration profile within the transition layer.