Scratch deformation response of thin CNx coatings at ultra-low loads

The scratch test is often used for assessing the properties of coated systems since it readily allows the differing dominance of various deformation mechanisms - and the friction response - to be measured as a function of contact severity. Further, the scratch test can be used to determine a critical load at which failure of the coating-substrate interface occurs and thus give a semi-quantitative measure of the adhesion of the coating. Usually, micro- and macro-scratch tests are performed in the load range between 0.05N to 100N but here, to explore the response of a much thinner coating (300nm of CNx on silicon), we report on scratches made with loads between I and 20mN. We have found a load dependent transition in the scratch and friction responses. Below 4mN, the scratches exhibited a completely elastic response with no evidence of failure. At 5mN, the scratches showed some completely elastic areas and some areas that were typical of a buckle failure of the coating and at the same time, the coefficient of friction rose from similar to 0.1 to similar to 0.5. Above 5mN, the scratches showed a continuous plastic groove and buckle-type failure. We discuss these results in I elation to the scratch response and tribology of ultra-thin films.