Micro/nanoscale friction and wear mechanisms of thin films using atomic force and friction force microscopy

Abstract

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales ranging from atomic and molecular to microscales. At most solid–solid interfaces of technological relevance, contact occurs at numerous asperities; a sharp AFM/FFM tip sliding on a surface simulates just one such contact. However, asperities come in all shapes and sizes. To study the effect of tip radius, experiments are conducted using an AFM tip with radii ranging from about 50 nm to 14.5 μm. The effect of the radius on adhesive forces and coefficient of friction at different relative humidities is measured. It is found that adhesive forces at low humidities do not change with tip radius whereas these increase with tip radius at high humidities. Coefficient of friction increases with the tip radius at all humidities. Samples coated with perfluoropolyether lubricant, which are hydrophobic in nature, are less sensitive to the environment and the tip radius. It appears that hydrophobicity of liquid films can be studied using large radii tips. Another objective of this study is to understand material removal mechanisms on microscale. Silicon surfaces are micromachined using a sharp diamond tip in an AFM. It is found that wear rate as well as the coefficient of friction is negligible up to a certain load and these increase rapidly above this load. The critical load corresponds to the local hardness of the silicon implying that the wear rate is negligible and the coefficient of friction is very low if the deformation is primarily elastic. The range of scanning velocity used in this study has little effect on the wear rate. SEM studies show that at light loads used in AFM, material is removed by ploughing and fine particulate debris is observed. At higher loads, cutting type including ribbon-like debris is observed. SEM and TEM studies of the wear region suggest that the material is removed in a brittle manner or chipping without major dislocation activity and crack formation. Evolution of wear of thin films also has been studied. Wear is found to be initiated at nano-scratches. These fundamental studies have provided insight to molecular origins of friction and wear mechanisms.