Fabrication of Ultrananocrystalline Diamond/Nonhydrogenated Amorphous Carbon Composite Films for Hard Coating by Coaxial Arc Plasma Deposition

Ultrananocrystalline diamond (UNCD)/nonhydrogenated amorphous carbon (a-C) composite (UNCD/a-C) films were deposited on cemented carbide (WC-Co) substrates by coaxial arc plasma deposition (CAPD). To suppress the graphitization induced by Co in the WC-Co, the film deposition was carried out on unheated substrates. The hardness and Young’s modulus were 51.3 GPa and 520.2 GPa, respectively. These values are comparable or rather larger than those of UNCD/a-C films deposited on other substrates such as Si, which implies that the graphitization of UNCD/a-C hardly occurs. Surprisingly, UNCD/a-C films could be deposited at the maximum film thickness of approximately 3 μm in spite of the room temperature growth. The internal compress-stress of the film is approximately 4.5 GPa, which is evidently smaller than that of comparably hard a-C films. The existence of a large number of grain boundaries in the films, which is structural specific to UNCD/aC, might play an important role in the release of an internal stress in the film. It was experimentally demonstrated that UNCD/a-C films prepared by CAPD are potential hard coating materials for WCCo.