High-Performance Plastics for Friction and Wear Applications

Including plastic components in machinery systems that experience friction and wear results in a number of performance advantages when compared with traditional all-metal designs. These include:

Low wear on mating metal parts
No requirement for liquid lubricants such as grease or oil
Reduced weight and low friction, which improve energy efficiency and reduce strain on drive motors
Reduced noise during operation
Extended service life, which results in less downtime for maintenance

Selecting plastics for friction and wear applications can be challenging, especially given the wide range of polymer materials available to part designers. Material selection is further complicated by the additives, reinforcements and fillers that may need to be included in the plastic formulation to maximize performance.

The purpose of this article is to provide an overview of polymer tribology (the science of friction and wear) so that machinery designers will be able to make more informed decisions when specifying plastic materials for friction and wear components.

Mechanisms of Wear

“Wear” refers to the gradual removal of material due to repeated contact on the surface of a part. It is important to recognize that there are a number of distinct mechanisms of wear and more than one type of wear may be operating on a particular machinery system. In this article, we will explore four mechanisms of wear: sliding wear, abrasive wear, rolling contact fatigue and impact fatigue.

Sliding wear refers to part surfaces that are in relative motion while in direct contact. The black polyethylene plates on the luggage conveyor shown in Figure 1 experience this type of wear. In this application, the plastic plates slide against the mating steel conveyor frame. Relatively soft plastics such as polyethylene and filled grades of PTFE tend to have low friction and low wear rates when sliding against metals due to their ability to deposit microscopic plastic films onto the surfaces of mating metal parts.

That being said, both polyethylene and PTFE have limited strength and modulus and relatively poor creep characteristics. For these reasons, they are generally limited to applications that involve modest mechanical loads.

Polyethylene luggage conveyor plates experience sliding wear against the steel conveyor frame.

Other plastics such as nylon, acetal, PEEK, PET and PBT are often specified for applications that require the ability to withstand higher mechanical loads. The sliding friction and wear characteristics of these harder polymers rubbing against metals can be greatly enhanced by including low-friction additives such as a PTFE or graphite in their formulations.

For example, adding 15% PTFE to hard plastics such as nylon, PET or PEEK can reduce the coefficient of friction of the system by half and increase the wear life by an order of magnitude (Mens, 1991). Other additives such as molybdenum disulfide, carbon powder, oils and waxes can also reduce friction and wear under certain circumstances.

When specifying plastics for sliding wear applications, it is important to review both the limiting PV (pressure velocity) ratings and the compressive creep characteristics of the polymers being considered. Limiting PV refers to the combination of pressure (mechanical load) and velocity (speed of the sliding surfaces) that the plastic part will experience during use.

If the combination of pressure and velocity creates excessive frictional heat, the polymer material will soften, resulting in failure. The limiting PV value for a given plastic material will depend on both the thermal softening characteristics of the base polymer as well as additives in the formulation that reduce frictional heat generation.

Plastics are viscoelastic materials and their responses to mechanical loads are dependent on strain rate. Compressive creep strain refers to the deformation of a plastic part under a compressive load over a long period of time. Creep strain can happen at much lower loads than the strength values reported on material properties sheets since these reflect the results of testing done at the comparatively high strain rates specified in ASTM and ISO testing methods.

It is important to review creep data prior to specifying a plastic material for a sliding wear application to prevent creep failures from overloaded plastic components. In these cases, the plastic parts may deform under load due to creep prior to experiencing significant sliding wear.

Tribological variables such as coefficient of friction and wear rate are “system properties” rather than material properties. The performance of a plastic in sliding wear can vary greatly depending on the chemistry, hardness and surface finish of the mating metal part (Rigney, 1993; Weilbla, 2007; Yousif, 2010).

Sliding plastics against harder metals generally results in superior tribological performance compared with plastics wearing against soft metals such as 304 stainless steel. Metal components with overly rough surfaces can abrade plastic materials. Metal parts with highly polished surfaces may inhibit the deposition of a transfer film of the plastic to the surface of the metal, detracting from friction and wear performance.

The optimal surface finish for friction and wear will vary depending on the hardness of the plastic material, with softer plastics performing better on somewhat smoother polished surfaces and harder plastics performing well on slightly rougher machined surfaces (Quaglini, 2009). In general, superior wear performance can be achieved by pairing plastic parts with relatively hard metals that have optimized surface finishes.

Figure 2 shows an example of abrasive wear operating on a snack food manufacturing conveyor. Abrasive wear is typically associated with applications that involve abrasive particles such as sand, coffee, wood chips or coal. Certain plastics including UHMW-PE (ultra-high molecular weight polyethylene) and hard cast polyurethanes tend to have outstanding resistance to abrasive wear (Budinski, 1997).

Figure 3 illustrates rolling contact fatigue, which is typical for plastic components such as wheels or conveyor rollers where the parts experience repeated cyclic stresses during rotation and Hertzian contact with mating surfaces. Certain plastics such as acetal and PEEK tend to have good resistance to material loss from rolling contact fatigue, resulting in long service life for wheels and rollers (Stolarski, 1993).

Conveyor belt rollers — These conveyor belt rollers experience rolling contact fatigue during operation.

The plastic shaker rods on the coffee harvesting machine shown in Figure 4 dislodge beans from coffee shrubs by repeatedly impacting the plants at high speed. Wear that involves repeated impacts is referred to as impact fatigue.

Plastic shaker rods on coffee harvesting machine — The plastic shaker rods on this coffee harvesting machine experience impact fatigue as they repeatedly strike coffee shrubs to dislodge the beans.

Interestingly, some plastics such as polycarbonate that have high impact strength when tested using notched Izod or Charpy test methods, have relatively poor impact fatigue characteristics. Toughened plastic materials such as rubber toughened polyamide and ABS (acrylonitrile butadiene styrene) where the rubbery phase absorbs energy and slows fatigue crack propagation tend to have good resistance to damage from impact fatigue (Adams, 1983); Adams, 1987).

As previously mentioned, multiple mechanisms of wear may be operating in a given application. In these cases, material selection involves specifying a plastic that has good resistance against the various types of wear operating in the system. For example, the rollers on the coal conveyor shown in Figure 5 experience both rolling contact fatigue as well as abrasive wear from coal debris. The plastic material selected for this application must perform well with both of these mechanisms of wear.

Coal conveyor rollers — The rollers on this coal conveyor will experience both rolling contact fatigue and abrasion from coal debris during operation.

The Operating Environment

The operating environment, including the presence of water or other chemicals, the temperature range, exposure to UV light or other radiation, and dry or vacuum conditions can all change tribological properties such as coefficient of friction and wear rate. For example, graphite is frequently used as an additive in plastic parts to improve tribological performance.

However, graphite tends to become abrasive in dry or vacuum conditions. Molybdenum disulfide can be included in the formulations for plastic parts that experience sliding wear in vacuum since unlike graphite, this additive enhances friction and wear performance in vacuum conditions (Buckley, 1966).

The Importance of Quality Material

Much attention is given to selecting plastic materials to maximize wear performance. However, even given an appropriate plastic formulation, the manufacturing methods used to create the finished part (extrusion, injection molding, machining, etc.) can have a profound influence on friction and wear behavior. Processing conditions including heating cycles, the use of reprocessed plastic scrap, cooling temperatures and cooling rates and machining methods can affect critical properties such as the molecular weight, crystallinity and residual stress in a plastic part.

All of these variables can influence friction and wear performance. Choosing a high-quality processor to manufacture plastic parts is as important as selecting an appropriate plastic material for maximizing tribological performance.

Material Selection

The following steps are suggested when selecting plastic materials for friction and wear applications in machinery:

Identify the mechanism or mechanisms of wear that are operating on the system: sliding wear, abrasive wear, rolling contact fatigue and/or impact fatigue.
Consider environmental variables such as the operating temperature range and exposure to water, chemicals, vacuum or outdoor conditions.
For sliding wear applications that involve a plastic part sliding against metal, consider the chemistry, hardness and surface finish of the metal that will optimize wear performance.
Identify base polymers that could potentially perform in the applications based on the mechanisms of wear, the mechanical loads and the operating environment.
Include additives to the plastic formulation that are likely to improve tribological performance such as adding PTFE to improve sliding wear characteristics or including rubber tougheners to enhance impact fatigue performance.
Identify a high-quality processor who will use best practices to manufacture the parts.
Do empirical testing to evaluate performance in use.

Closing Thoughts

The purpose of this article has been to provide machinery designers with a framework for selecting plastic materials for friction and wear applications. Readers who wish to take a deeper dive into this topic are encouraged to explore the articles in the reference section as well as the many books on polymer tribology available in engineering literature.

References

Adams, G. and Wu, T., (1983). Fatigue of polymers by instrumented impact testing. Annual Technical Conference (ANTEC 1983), Society of Plastics Engineers, pages 541 to 543.
Adams, G. (1987). Impact Fatigue of Polymers Using an Instrumented Drop Tower Device, Instrumented Impact Testing of Plastics and Composite Materials, ASTMSTP 936, S. L. Kessler, G. C. Adams, S. B. Driscoll, and D. R. Ireland, Eds., American Society for Testing and Materials, Philadelphia, 1987, pp. 281-301.
Buckley, D. (1966). Friction and wear characteristics of polyimide and filled polyimide compositions in vacuum 10-10 mm Hg. NASA Technical Note D-3261. National Aeronautics and Space Administration. Washington D.C., February, 1966.
Budinski, K. (1997). Resistance to particle abrasion of selected plastics. Wear 203-204, pages 302 to 309.
Mens, J. and DeGee, A., (1991). Friction and wear behavior of 18 polymers in contact with steel in environments of air and water. Wear, 149, pages 255 to 268.
Quaglini, V. (2009). Influence of counterface roughness on friction properties of engineering plastics for bearing applications. Materials and Design, volume 30, pages 1650 to 1658.
Rigney, D. (1993). The role of hardness in the sliding behavior of materials. Wear, 175, pages 63 to 65.
Stolarski, T., (1993). Rolling contact fatigue of polymers and polymer composites. Advances in Composite Tribology, volume 8, 1st edition, edited by K. Friedrich. Pages 629 to 667.
Wieleba, W., (2007). The mechanism of tribological wear of thermoplastic materials. Archives of Civil and Mechanical Engineering, Volume VII, number 4, pages 185 to 199.Yousif, B., Alsofyani, I., and Yusaf, T., (2010). Adhesive wear and frictional characteristics of UHMWPE and HDPE sliding against different counterfaces under dry contact condition. Tribology, volume 4, number 2, pages 78 to 85.