ABSTRACT
Presently, thin diamond films are under development for many applications due to their excellent hardness, strength, thermal conductivity, chemical stability and opti cal transmission. However, there is still some problem in growing diamond films on non-diamond substrates due to poor nucleation density and poor adhesion. It has been well recognized that a pre-treatment of the substrate or use of a transition layer before CVD diamond growth can significantly improve diamond nucleation as well as adhesion on non-diamond substrates [1]. Joffeau et al. [2] reported that during the CVD growth, there were carbide layers between the carbide-forming elements and the diamond films. This means that the transition layer may play an important role during diamond growth on these non-diamond substrates. Since the adhesion is the major obstacle in diamond film application, the intermediate layer applied prior to diamond deposition might finally prove to be one of the best possible solutions to enhance the diamond adhesion [3]. For ferrous metal substrates, it is very difficult to grow diamond films on them because first, carbon can diffuse into these metals with a relatively high diffusion rate at high temperature, and the CVD diamond nu cleation density is very low. Second, ferrous metal elements, such as iron or cobalt, have a catalytic effect on the growth of graphite and nano-crystalline carbon. Third, the thermal expansion coefficients between the diamond and a ferrous metal are not
compatible and this mismatch usually causes high residual stress and poor adhesion [4]. A common way to grow a diamond film on a ferrous metal is to deposit an in termediate layer serving as a carbon diffusion barrier as well as adhesion enhancer. Spinnewyn et al. [5] used a refractory metal, such as W or Mo, as an intermediate layer. Their calculation indicated that different materials of intermediate layers needed different minimun thicknesses to restrain carbon diffusion. For example, Si and Ti required thicker layers than W, Cr and Ta. They did not compare the adhe sion strength of diamond films on these different intermediate layers in their paper. The intermediate layer has to be bonded to both the diamond film and the substrate. One must consider at least these two requirements: first, diamond film must be grown on the intermediate layer without graphite formation, and second, the inter mediate layer should have good adhesion strength to both the substrate and the diamond film. In some cases, the intermediate layer may relax the intrinsic stress induced by the differences in lattice parameters and thermal expansion coefficients of the diamond film and the substrate. Poor adhesion is still an impediment for the application of diamond films on ferrous substrates. Thus, how to choose and how to grow these intermediate layers is very important for diamond film growth on fer rous substrates. On the other hand, how to evaluate film adhesion strength to a sub strate is also very important. The Rockwell-C indentation test [6 ] appears to be a simple and convenient method for evaluating the adhesion strength of diamond films; however, this method cannot give a quantitative result and is strongly af fected both by the substrate as well as the applied load. The tensile test is an easy way to judge the film adhesion strength quantitatively. Because of high surface en ergy and complete covalent bond structure, diamond has poor adhesion to most materials [7]. It is very difficult to attain enough glue (adhesive) strength between the diamond film and the tensile bar. There was no success for diamond film adhe sion measurement using a tensile test before [8 ]. In this paper, we sputtered Cu/TiC double layers on Si and steel substrates, and then grew diamond films on them by hot filament CVD method (HFCVD). Characterizations by X-ray, SEM and Raman spectroscopy were carried out to check film quality. The tensile test as well as in dentation tests were performed to evaluate diamond film adhesion strength.
