Behaviour of Plastics and Elastomers

The relationship between wear and surface hardness obtained for metals would predict a comparatively poor behavior for polymers. However, their special structural features give rise to properties that can play a special role in wear.

The viscoelastic deformation behavior is characterized by time-, temperature- and velocity-dependent deformation processes. Relatively low levels of hardness and strength, high plasticity, low thermal conductivity, and high thermal expansion are effects of the weak secondary bonding forces between the macromolecules and their coiled structures. In particular, the low tendency to adhesion gives polymers their good slip characteristics with steels as the sliding partners — in the absence of additional abrasive particles — because of the low frictional forces involved, and the slip system is characterized by additional emergency running properties. Polyamide and PTFE occupy the prime positions here as they possess good cohesive linkage properties compared with other unreinforced polymers.

If abrasive sliding stress is present, the dependence on hardness known for metals cannot really be depicted in the same way. It has been demonstrated that polymers exhibit a good relationship between wear resistance and crack propagation energy, or even between wear and the product of tensile strength and fracture strain.

Due to their material properties, polymers have proved successful where streams of small particles cause impact stress in addition to sliding wear, i.e., with abrasive impact wear and with erosive attack. Although polymers generally have poor resistance to abrasive sliding attack, their ductility, especially of elastomers, leads to a behavior superior to that of metals when the impacting component is dominant. Their behavior therefore differs significantly depending on the angle of impact. The material becomes heated due to internal friction, which can lead to complete failure at high jet intensities.

The preferred elastomers include the polyurethanes and synthetic rubbers because of their outstanding resistance to wear. In polyurethanes, greater resistance is found in the hardness range 70 – 95 Shore A, whereas normal grades of rubber reach their optimum between 50 and 70 Shore A. It is not possible to separate the influencing factors systematically with respect to tribological behavior because of the large number of additives, types of rubber, and applications.

If, for rubber and C 60 H steel, the amount of wear relative to St 37 steel is plotted versus the impact angle and the hardness of the jet material then, it is possible to show the very different wear behavior of these two materials.