Effect of normal load on microscale friction measurements

Abstract

Friction force microscopy (FFM) was used to study microscale friction between a sharp tip and various samples. Effect of normal load and tip material on the coefficient of friction has been studied. Friction force as a function of normal load of virgin silicon with a thin film of native oxide and dry-oxidized SiO2 coating showed two distinct slopes. The coefficient of friction in the low load region of less than about 15 mN is lower than that in the high load region. The critical load at which the coefficient of friction starts to increase corresponds to the specimen hardness. Ploughing at high loads is believed to be responsible for high values of the coefficient of friction. The coefficient of friction of polished natural diamond remains virtually independent of normal load because no ploughing occurs. The coefficient of friction on a macroscale is higher than that on a microscale for comparable contact stresses. When measured for the small apparent area of contact and very small loads used in microscale measurements, the indentation hardness and modulus of elasticity on a microscale are higher than that at the macroscale. This reduces the degree of wear at the microscale. In addition, small apparent areas of contact in microscale measurements reduces the number of particles trapped at the interface and thus minimizes the ploughing contribution to the friction force. Based on this study, it is concluded that measured values of the coefficient of friction on a microscale are a strong function of normal load and the apparent area of contact. Ultralow values of the coefficient of friction and near-zero wear can be achieved with microscale components at very light loads in the absence of significant ploughing.