Abstract
A copper microscratch test was carried out with a spherical indenter under constant normal load of 150 mN. The effect of sample tilt on the measurement of friction coefficient was investigated by rotating the sample with surface inclination between successive tests so that the indenter could experience different surface height slopes. It was found that the experimentally measured friction coefficients linearly depended on surface height slope (or surface tilt angle) under small angle tilt and became larger when the indenter climbed over the sample surface, which was explained by a geometrical intersection model. Accurate determination of the friction coefficient requires either two scratch tests with reversed sliding directions or 180° rotation of the sample with the average value being used as the coefficient of friction under nontilting condition. The true friction angle for the condition without surface inclination can be obtained by summing the experimentally measured friction angle and surface tilt angle.