Amontons’ laws and the friction in miniature elements

Abstract

This research demonstrates that Amontons’ laws, which state that the friction force is proportional to the (normal) applied load and is independent of the apparent contact area, begin to break down when the sliding solids have dimensions of several millimetres and loads of tens of millinewtons. The method used to test the hypothesis involved sliding a cylinder into another cylinder. The results indicated that the static friction coefficient as a function of the load exhibited hyperbolic behavior at low loads; the friction force was not proportional to the load. While the static friction coefficient as a function of the apparent contact area exhibited linear behavior, the friction force was dependent on the apparent contact area. The tests were conducted without lubrication and the materials used were steel on SAE 40 bronze and steel on polytetrafluroethylene.