Effect of film thickness and interlayer on the adhesion strength of diamond like carbon films on different substrates

Abstract

The addition of an interlayer is often used to increase the adhesion strength in thin film coating. For diamond like carbon (DLC) films, titanium and chromium are two common interlayer materials to enhance adhesion, especially for metal substrates. In an attempt to explore interlayer effect on nonmetal substrates, plasma enhanced chemical vapor deposition (PECVD) was utilized to deposit DLC with methane on silicon and glass substrates with interlayer titanium and chromium. The film structure and adhesion strength were studied by Raman spectroscopy, optical microscope, nanoindentation and nanoscratch. For DLC on silicon substrates without interlayer, the results show as the film thickness increases, the ratio of I(D)/I(G) increases and the hardness decreases. For DLC on silicon substrate with interlayer, both interlayers do not enhance the adhesion strength. For glass substrate, the chromium showed improved adhesion strength only in small film thickness (200 nm). As the film thickness increases, the peeling of the DLC thin film deteriorated for both interlayers. These phenomena were examined by the bonding structure, hardness, nanoscratch and residual stress. As the film thickness increases, the I(D)/I(G) ratio increases and the fraction of sp 3 decreases. This indicates as the film thickness increases, the bonding is towards graphite structure and reducing hardness. The high sp 2 fraction and low hardness explain the poor adhesion of large film thickness. The reasons for chromium acting as a better interlayer on glass than on silicon are attributed to the low residual stress and preferable carbide formation. The similarity in the magnitude of coefficient of thermal expansion between chromium and glass results in less residual stress. Another possibility is the carbide formation on chromium interface which should enhance the adhesion. This hypothesis is currently under investigation.