Effects of Contact Pressure on the Coefficient of Friction in Friction Tests

The effect of sample thickness on the coefficient of friction (COF) determined by the Draw Bead Simulator (DBS) tests was observed, i.e., the COF of heavy gauge samples was lower than that of thin gauge samples in the same surface condition. To better understand this experimental anomaly, DBS tests were performed on galvannealed and cold rolled steels of different gauges. Twist compression (TC) tests were also performed because it offers the controllable contact pressure between the sample and the tool. The TC test results indicated a strong contact pressure influence on the COF at pressure range tested, which invalidates the assumption of Coulomb's friction law in the friction tests. It is concluded that the different sample gauge results in different contact pressure in the DBS, and in turn, different COF because of the breakdown of Coulomb's law. The decrease in COF with increasing pressure was explained in terms of mixed-film lubrication and the modified Stribeck curve.