Characterization of friction and wear behavior of friction modifiers used in wheel-rail contacts

Reliable traction between wheel and rail is an important issue in the railway industry. To reduce variations in the coefficient of friction, so-called “friction modifiers” (carrier with particles) are used. Twin-disk tests were done with three commercial friction modifiers, based on different compositions of carrier and particles, to characterize their friction and wear behavior. It is shown experimentally that the influence of the carrier cannot be neglected just after application and very low (0.01-0.05) frictional values are observed in a fully flooded situation. However, starvation occurs quickly and friction values will become relatively stable at an intermediate level around μ=0.2 until the friction modifier is consumed and a new dose is required. After the carrier is pushed out of the running track the particles in the contact dominate the tribological performance. The level of friction is a function of total rolling distance, effective sliding length and sum velocity. The most dominant factor depends on the friction modifier and the working mechanism for friction stabilization. It is also shown that the wear rates during tests do not depend significantly on slip, which makes it possible to predict wear behavior. Wear rates are dependent on the type of friction modifier used.