An empirical relation for critical load of DLC coatings prepared on silicon substrates

The adhesion, of a coating to the underlying substrate, in terms of critical load (using scratch test) is complex and is a function of many parameters such as coating thickness, interfacial stress, coefficient of friction, intermixing, hardness, structural parameters, etc. The present study examines the relationship of critical load to other properties such as film thickness, compressive stress, coefficient of friction, etc. of diamond-like carbon coatings prepared under floating condition (no substrate bias), on silicon substrates. The results show that the compressive stress increases with increasing thickness. However, the adhesive strength in terms of critical load increases with increasing the film thickness up to a certain value and then it decreases. The decrease in critical load with thickness of the films is due to the dominant effect of compressive stress. In order to obtain the parameters that affect the adhesion, a factor analysis has been carried out on the experimental data using SPSS - a statistical software package. Thickness, stress, and critical load emerged as one of the factors and hence it suggests that thickness and stress are two primary parameters that affect adhesion. None of the parameters of Raman study were emerged in the factor analysis along with critical load and thickness. This suggests that the variation of the sp3-bonded carbon with thickness is relatively small and that may not have any direct impact on the adhesion. In this case, stress shows a strong linear relationship with thickness. Moreover, it is not an operational parameter. Hence inclusion of stress in the model becomes unnecessary. Therefore, an empirical relation has been obtained for critical load as a function of thickness.