Interfaces and properties of TIC/α-C:H nanocomposite coatings

In this paper we deal with interfaces involved in TiC/a-C:H nanocomposite coatings, at three different length scales namely coating interface adjacent to the substrate (micron scale), boundary of columnar structures (sub-micron scale) and phase interface between nanocrystallite and amorphous matrix (nanometer scale), and their influences on the properties of the coatings. The coating interface has to be engineered to optimize the adhesion of the coatings, which is a primary requisite for their function. Columnar boundaries (CBs) are harmful as preferential cracking path, resulting in low fracture toughness. Therefore, our efforts are devoted to eliminate CBs by adjusting the deposition parameters. The amorphous carbon matrix is intrinsically brittle as a common characteristic of amorphous materials. Introducing nanocrystalline ceramic particles into an amorphous matrix generates a high density of interphase interfaces that assist in crack path delocalization and termination of crack propagation. Especially, the localization of shear in amorphous matrix is hampered by the particles, leading to a spread of the cracks that enhances the toughness provided the size of TiC particles becomes of the same size as the separation between the crystalline particles. Based on the experimental results, guidelines for the development and optimization of highly adherent and tough nanocomposite coatings are provided.