The load-dependency of the friction coefficient under a contact load in the order of sub-micronewtons was revealed. The frictional force was measured experimentally with a diamond tip of 0.1 μm radius on silicon-oxide surfaces in a laboratory atmosphere. The frictional force was obtained even under a certain amount of negative external force. The contact load was regarded as the sum of the external force and the attractive force between two surfaces, the latter being estimated from an elastic contact theory which took into account the van-der-Waals force. The friction coefficient, as the ratio of the frictional force to the contact load (not to the external force), was plotted against the contact load. Below 0.4 μN in the contact load, the friction coefficient increased in inverse proportion to the decrease in contact load. Above 0.4 μN in the contact load, the friction coefficient was stable at about 0.1. We discussed this load-dependency, and derived a hypothesis that the real area of contact should include some part of the attractive region around the repulsive region at the contact point when the contribution of the attractive force is not negligible to the contact load.

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