Shear characteristics of a boundary film for a paper-based wet friction material: friction and real contact area measurement

A paper-based wet friction material lubricated with automatic transmission fluids (ATFs) produces plateau and positive slope regions in the friction–sliding velocity diagram. This feature observed at less than 1 mm/s of sliding velocity is discussed in this paper. The relationship between the friction and contact pressures of less than 5 MPa was determined by a reciprocating friction test rig lubricated with mineral oil including phosphoric acid ester, one of the additives for ATFs. The real contact area for dry condition was also determined by optical interferometry. The consistency of the real contact pressure dependency of the shear strength of the boundary film was indicated at pressures of less than 1 GPa obtained by steel-on-steel boundary friction (oiliness) test. This suggests that the generation of the positive slope region is due to thin film lubrication. To the plateau region, possibility of application of the phase transition model is considered.