Investigation of friction on hard homogeneous coatings during reciprocating tests at micro-Newton normal forces

Adhesion is an important surface phenomenon that controls many physical events in nature and technology. The use of miniature devices like comb drives, and contact MEMS is still limited mostly due to adhesive interactions of free standing structures and friction, respectively. Moreover, adhesion is an important friction mechanism at low normal forces. In this study, the interplay between surface roughness and adhesion force was studied and consequently their influence on the friction force at low normal forces in the range of μNs was experimentally investigated. Friction studies were carried out through a series of bi-directional sliding tests on commonly used engineering surfaces like silicon wafer, TiN, and DLC coatings using a high precision modular microtribometer in a ball-on-plane configuration. This study illustrates how friction operates between rough multiasperity contacts under contact stresses below 125 MPa. Force volume imaging was performed using an AFM on surfaces with different surface roughness. For rough surfaces, a large fluctuation in the local adhesion force was observed instead of an overall decrease in adhesion as reported in the literature. As the normal force decreases, the influence of the adhesion component on friction increases resulting in a high coefficient of friction especially on silicon and DLC surfaces. Hydrogenated DLC surfaces can exhibit low friction only when the surface is rough. On the other hand, TiN coatings are less sensitive to changes in surface roughness, and exhibit a practically constant coefficient of friction due to a low adhesive component. The adhesion component of normal force could be estimated using JKR adhesion model. There is a clear dependence of sliding friction at low normal forces in the μN range on adhesion events observed at the nano-N range.