Effect of film thickness on the stress and adhesion of diamond-like carbon coatings

The important properties of thin films that are used for any applications, especially tribological, are lower internal stress and extremely good adhesive properties. The present investigation examines the effect of film thickness on the compressive stress, adhesive strength (critical load), microstructure and coefficient of friction of diamond-like carbon films prepared under floating conditions (no substrate bias), on silicon substrates. The results show that the compressive stress increases rapidly with thickness in the lower thickness range of up to 50 nm, followed by a slight decrease and thereafter it increases at a slower rate before it delaminates at a thickness of approximately 190 nm. The adhesive strength in terms of critical load shows that the critical load increases with increasing the film thickness up to approximately 110 nm and thereafter it decreases with thickness. The decrease in critical load with thickness, for the films having thickness above 110 nm, is due to the dominant effect of internal stress. The tribological characterisation reveals that the substrate influence on the coefficient of friction is relatively high for the thinnest films (~<40 nm). The paper also discusses the results of UV and visible Raman spectroscopy as a function of thickness. It is suggested from the characterisation results that appropriate thickness range of the film (prepared under floating condition), is ~40-110 nm.