Microscale Friction Behaviour of Single Crystal Si (111) When Rubbed with Diamond Spherical and Conical Styli

Abstract

An experimental study is described of the friction behaviour of samples of single crystal Si (111) when they were rubbed with diamond spherical (radius of 0.18 mm) and conical styli of included angles of 120° and 136°. Several techniques were used including (1) a reciprocating friction machine, (2) Raman spectroscopy and (3) environmental scanning electron microscopy. Friction force was measured for a constant relative speed of 0.22 mm s−1 between the rubbing stylus and the substrate. The normal load on the stylus was varied in the range 100–500 mN. In the case of the spherical stylus, the coefficient of friction was in the range 0.03–0.08, whereas for the conical styli, it was up to ten times higher, but it varied with the number of friction cycles. Raman spectroscopy revealed that in all cases, structural phase transitions of the silicon had occurred in the friction tracks and both amorphous silicon and Si-III phases were found. Environmental scanning electron microscope showed no debris in the friction tracks produced by the spherical stylus, whereas a considerable amount of debris was found within the friction tracks produced by the conical styli. This debris has been shown to be responsible for the gradual decrease in the coefficient of friction with increasing number of friction cycles. Calculations of the frictional heating between the spherical stylus and the Si (111) substrate have shown that only a negligible interfacial temperature rise would occur. It has been suggested that, in the absence of ploughing, the coefficient of friction between the spherical stylus and the Si (111) substrate is controlled by the adhesion between the two surfaces, whereas the very initial coefficient of friction between the conical styli and the Si (111) substrate is controlled by the ploughing process.