Characteristics of impact friction and wear of polymeric materials

Abstract

The fundamental differences between friction and wear under impact and friction and wear under sliding are the pressure (i.e. the load responsible for friction) and the friction coefficient. The dynamic equivalents of both factors are determined for impact effects by specific parameters; these parameters are the particle velocity ν 0 and the angle α 0 of attack. The pressure is given by π = ρν 0 2 sin 2 α 0 and the friction coefficient by f = ν 0 cosα 0 − ν f cosα f ν 0 sinα 0 + ν f sinα f In the range of small and medium values of α 0 the impact friction coefficient for elastomers decreases with an increase in the angle of attack and increases with increases in the angle of attack for plastics. For large values of α 0 the friction coefficient of all materials decreases and approaches zero at normal impact. The friction coefficient under impact is of fundamental importance to wear by a flow of particles because it defines the fatigue strength of the body being worn. To increase wear resistance under impact this friction coefficient should be lowered; this is similar to the case of sliding friction and wear. For microcutting by a colliding particle, in contrast with sliding, it is advantageous to have an increased coefficient of friction under impact. When the similarity between the pressures p and Π is taken into account, the expressions for sliding wear and impact wear are analogous. Because the initial process of failure is the same in both cases, the relationship between the wear resistance and the properties of the material being worn (strength, deformability, endurance and friction coefficient) is the same in both cases. In the microcutting of plastics an increased hardness is important. In the microcutting of vulcanizates by flying particles an increased modulus of elasticity is important.