Chapter 12

Which polymers have good abrasion resistance?

Analysing the reduction of friction and wear in plastics

Friction causes substantial economic damage and can lead to machinery and equipment break downs. Plastics can solve, or at least reduce, friction-related problems. Specifically modified plastics are suitable for making low friction bearings, guideways and gears with very good resistance to wear.

Friction is the resistance which two bodies in contact with each other develop to oppose change in their relative positions.

The coefficient of friction µ = FF/FN describes the intensity of friction, where FF is the frictional force and FN the force acting vertically between the surfaces. When the two bodies move relative to each other, we refer to kinetic or sliding friction.

If there is no movement of the bodies relative to each other, we speak of static friction, µ0. It is usually higher than sliding friction.

Sliding friction is always linked with an undesirable change, of varying degree, in the contact surfaces; this is termed abrasion.

Friction and abrasion performance are not material attributes but properties of a complete system, called a tribological system.

It includes not only the materials but also their conditions of use, including, for example, surface quality and loading, relative speed and contact pressure per unit area, as well as ambient temperature and the temperature rise due to friction.

Abrasion testing

As for all system-dependent properties, tests for determining abrasion are most significant if carried out under real-life conditions. A number of test procedures provide indications of abrasion behaviour, among which the Taber abrasion test as specified in DIN 53754 or ISO 9352 is the most important.

Graph 1: Plotting surface pressure p against sliding velocity v (using log scale) produces an experimentally determined boundary line showing the useful area for p�v values; p�v lines for constant abrasion rates lie within this area.

In this test, two rollers coated with abrasive paper abrade the surface of a rotating test specimen. The friction causes abrasion: its magnitude depends on the size of the abrasive particles and the downforce of the wheels on the test specimen, and is given in milligrams per rotation.

When using plastics as the mating medium in bearings and guideways, the surface’s load-bearing capacity may be described by the so-called p·v value. This term’s meaning is evident from the terms employed: tribological stress grows with surface pressure p in the bearing and sliding velocity v.

In a p·v diagram, an experimentally determined boundary line shows the p·v values above which abrasion rises disproportionately.

Within the useful range, the abrasion rate remains proportional to the p·v value; at low speeds this range is limited through the static load limit and at high speeds by friction-generated heat.

Abrasion behaviour

In general, the abrasion behaviour of plastics is good. In the case of sliding surfaces it is an advantage that foreign particles, as well as particles generated through abrasion, are pressed into the surface and thus lose their abrasive effect. This is why plastics mostly withstand sliding abrasion, grain abrasion and frictional abrasion better than metallic materials.

Precisely in the case of engineering plastics, there are various methods to improve their abrasion resistance further.

Friction can be reduced by incorporating finely powdered internal lubricants such as Teflon® PTFE, molybdenum sulphide or graphite in the base resin matrix.

By adding such internal lubricants tribological systems with maintenance-free, dry-running parts can be made that need no external lubrication. Glass, carbon or aramid fibres can be added to lower the rate of abrasion.

In the case of abrasive stress, it may also be necessary to add hard ceramic particles as fillers. Thus, the more accurately the overall operational stresses of a tribological system are known, the more successfully its frictional and abrasional behaviour can be optimised.

Tribological systems

Figure 1: Delrin® acetal resin has the high abrasion resistance, stiffness and toughness needed for making worm and gear drives; modification with internal lubricants enables maintenance-free gears to be made from it.

Plastics have proved their worth in many applications involving tribological stress. Making gearwheels of DuPont™ Delrin® acetal resin counts as a classic.

The high surface hardness of this material is decisive for low abrasion and, in addition, it has the high stiffness, fatigue resistance and toughness needed for gear-trains.

A particularly silent running gear-train results if DuPont™ Hytrel® thermoplastic polyester is used for one of the mating gearwheels.

Conveyors, in which the individual segments are made of modified acetal, possess high abrasion resistance; moreover, they need no lubrication, thus eliminating any risk that goods on the conveyor will pick up oil or grease.

Sports articles, such as skateboards, inline skates or ice-skates, often have to withstand abrasion. In such situations DuPont™ Zytel® ST super-tough nylon is the material of choice.

In addition to its abrasion resistance, it offers outstanding toughness even at low temperatures. Even deep scratches do not generate local stress concentrations; this feature also helps to extend the life of sports goods.

DuPont™ Vespel® SP meets the most demanding tribological requirements. This material is used to make the highly abrasion-resistant thrust washers in a stepless automatic gearbox.

Figure 2: Highly abrasion-resistant thrust washers made of DuPont™ Vespel® polyimide take up the axial forces in this gear-train and can easily withstand the temperatures generated thereby.

It allows low friction relative motion between rotating and stationary parts even at high loadings; in this way, it makes a decisive contribution to the reliable operation of low-maintenance gearboxes.

In the starting phase, the thrust washer of Vespel® SP takes up the axial forces of the helical gearwheel, where relative rotational speeds can reach 3,000rpm.

Oil recesses on both mating faces of the washers help to stabilise the lubricating film and flow channels carry the oil through to lubricate neighbouring bearings and to provide improved heat transfer. These design steps boost even further the already high p·v value, to which the thrust washers are exposed without significant wear.

DuPont™ Vespel® SP polyimides can be used from very low to very high temperatures: continuous exposure up to around 290ºC and for short periods up to about 480ºC. Vespel® types SP-21 and SP-22 are preferred for thrust washers and for various types of sealing rings.

DuPont fabricates these materials into high-precision finished parts with a direct forming technique like that used in powder-metallurgy. Direct formed Vespel® parts generally need no further finishing operations but can be machined like brass, if necessary.

DuPont™ Teflon® polytetrafluoroethylene occupies a special place with respect to friction and abrasion. It has the lowest friction coefficient of all solids and is at the same time the most chemically-resistant material. Moreover, its static and kinetic friction coefficients are identical, so no slip-stick effects appear.

As a consequence, its areas of application are very broad. They reach from internal lubricants (see above) to highly stressed coatings. In many cases, its virtually universal chemical resistance and its thermal capability up to 260ºC are further reasons for the choice of this high-performance plastic.

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The examples in this series of articles are intended to illustrate underlying principles and to explain the main influencing factors.

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