Lowering friction coefficient under low loads by minimizing effects of adhesion force and viscous resistance

Conditions (normal load, sliding speed, ambient conditions, and material) to obtain the lower friction coefficient were studied by measuring the friction and pull-off forces between a metal pin (copper or gold) and a plate (steel or single crystal silicon). First, a pin was rubbed against a plate under a normal load between −12 and 870 μN at a sliding speed between 0.012 and 9.6 μm/s. The friction force was measured during reciprocating sliding motion. The pull-off force was measured before and after each friction force measurement. All the force measurements were taken in high vacuum at 10−5 Pa, dry argon at 1 atm, and ambient humid air of 38 and 60% relative humidity. Then, the friction coefficient was calculated by dividing friction force by the sum of normal load and pull-off force. In high vacuum, when a copper pin was rubbed against either a silicon or steel plate, the friction coefficient decreased to less than 0.05 with decreasing sliding speed. The effect of sliding speed on the friction coefficient suggests that under a low normal load the viscous resistance of liquid contributed to the friction force. When a gold pin was rubbed against a silicon plate, the friction coefficient was not affected by sliding speed.