Characteristics of Friction in Small Contact Surface

To determine the friction characteristics of small contact surfaces, the friction coefficient between steel balls of 0.5 to 5 mm radii and block gauges were measured while varying the friction speed, normal load, sliding distance etc. A high friction coefficient was found under a low normal load such as 1 mN, particularly for larger steel balls. However, the friction coefficient under a low normal load increased with sliding distance. Observations by AFM (Atomic Force Microscope) showed that deposited material thought to be wear particles accumulated on the steel ball with sliding distance. The adhesion force between a steel ball of 2 mm radii and a block gauge was measured under friction in order to quantify the adhesion force which affects the friction force under low normal load. The results show that the adhesion force varies greatly, and is sufficiently large to influence the friction coefficient under low normal load.