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

A functionally graded transition zone between a hard TiC coating and a WC-C o 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 T iC and Ti layers. The composition gradient within the carbide layer is gene rated by varying the relative thickness of the as-deposited Ti and TiC laye rs. A subsequent short diffusion treatment eliminates the interfaces betwee n the adjacent layers, yet maintains an overall carbon composition gradient across the thickness of the coating. The relative thickness of the individ ual sputtered layers is adjusted to yield a low carbon composition in the c oating close to the substrate and a stoichiometric composition near the ext ernal surface. The composition profile within the coating can be further op timized with respect to residual stresses that arise during cooling after t he diffusion anneal. In the present work, different graded Ti-TiC profiles were deposited on WC-Co substrates in two steps. First, a 0.5-mum graded Ti -TiC multilayer was deposited followed by a 2.5-mum outer layer stoichiomet ric 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. (C) 2001 Elsevier S cience B.V. All rights reserved.