The effect of normal loading on the static friction coefficient between smooth metallic surfaces is experimentally investigated. Normal loads in the range of 10−3N to 0.3N were applied to small diameter samples made of three different aluminum alloys and contacting a nickel coated surface. The tests were done under controlled humidity and clean air conditions. A dramatic increase in the static friction coefficient was observed as the normal load was reduced to its lower level. This behavior is attributed to the role played by adhesion forces which are more pronounced at small normal loads and smooth surfaces and is in agreement with recent theoretical analyses.

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