By Kevin S. Marshall
Edited by Jean Hoffman
The concepts of friction and wear are easy to understand, yet they can interrelate in complex ways within a tribological system (science concerned with interacting surfaces in relative motion including friction, wear, and lubrication). This is especially true for designs where plastic parts mate. Successful wear-resistant applications rely not only on the right compound choice, but also on the proper individual part and overall system design.
FRICTION AND WEAR
Friction is a natural resistance to sliding motion between two surfaces. It is quantitatively described by dynamic and static coefficients. These provide an estimate of energy requirements for moving a part. The lower the coefficient of friction, the easier the two surfaces slide over each other. Work done overcoming friction in bearings and other mechanical components dissipates as heat so reducing friction leads to better efficiency.
Friction is relatively easy to measure but no simple model predicts or calculates friction coefficients for a given pair of materials. Frictional values, for example, may rise during an initial break-in period, then gradually drop to a steady state. So they must be derived through actual testing.
Frictional properties of thermoplastics differ from those of metals. Because thermoplastics have a lower modulus (more flexibility) and are softer than metals, they do not follow the classic laws of friction as applied to metals.
Unlike metals, thermoplastics usually have a static coefficient of friction (related to starting motion) that is less than the dynamic coefficient (related to maintaining motion). This gives a "slip/stick" or intermittent sliding motion when plastic moves against metal or another plastic. Adhesion of plastic to metal and deformation of the plastic surface characterize plastic-to-metal surface interaction. Such effects result in frictional forces that are proportional to speed rather than load.
Friction generates heat where surfaces touch. The temperature can greatly increase when the contact pressure (P) and the sliding speed (V ) become large. The combination of P and V is referred to as the PV capability for a bearing application. Frictional heating can make a thermoplastic soften or deform if it has a low glass-transition temperature (Tg) or melting point (Tm). High-temperature polymers are affected less by frictional heating and, as such, are good candidates for wear and friction applications.
Wear, like friction, is a complex phenomenon. It takes place as two surfaces slide or roll against each other and the forces of relative motion gradually remove material. Two common wear mechanisms are adhesion and abrasion. Adhesive wear occurs when mating surfaces slide against each other and fragments of one surface dislodge and adhere to the other. In a lubricated material, the resulting debris forms a fine powder on the mating surface. This is the primary wear mechanism for thermoplastics in rubbing contact.
Abrasive wear, on the other hand, occurs when the harder surface scrapes or abrades its mate. This type of wear is characterized by grooves or gouges cut into the part surface. Dislodged particles such as glass fibers may roll between surfaces causing severe abrasion. Polymers with inherent toughness help reduce abrasive wear.
Wear can lead to unwanted freedom of movement or loss of precision or both. Even the loss of relatively small amounts of material can cause system failure. While even a welldesigned tribological system can't completely eliminate the removal of material, it may reduce wear to an insignificant level.
Wear qualities of lubricious thermoplastics differ greatly. Designs employing plastic-onmetal perform best. But designs requiring plastic-on-plastic can be made to perform well by using dissimilar polymers with one or more wear-resistant additives such as PTFE.
DESIGNING FOR WEAR
Once the system design is in place, the engineer needs to determine if "significant wear" is likely. If so, the wear rate must be adjusted to "acceptable" levels.
The system wear rate is determined by the interaction of mostly controllable variables. For example, structural variables include materials in relative motion and their surface finishes, as well as interfacial materials such as lubricants and abrasive particles. Another factor is the type of motion — reciprocating versus continuous or geometrical motion (i.e., sliding, rolling) between components. Operating conditions such as speed, load, and temperature also can have an impact.
Often selection of materials for bearings, bushings, seals, and gears hinges on factors that have little or nothing to do with tribology. Attributes such as cost, weight, chemical resistance, or thermal and mechanical properties may drive these designs. Nevertheless, it is still possible to get good friction and wear qualities even with limited material options.
When a thermoplastic compound is not performing properly, engineers may consider altering additive levels or introducing new ones. They may also select a different polymer or change the mating surface material or both to boost performance.
The real cost of wear is not the purchase price of the compound, but rather the hidden costs of not using the correct thermoplastic in the first place. Standardized tests such as ASTM D-3702 give an indication of relative wear rates. But it's still essential to prototype or do actual application testing.
Thermoplastic compounds, including unfilled or neat polymers, make good tribological materials. The compounds are self-lubricating, resist wear and corrosion, and emit little noise. Additionally, they are lightweight, damp vibrations, and economical. Unlike many metals, self-lubricating thermoplastics can work in unlubricated environments and can be colored. Thermoplastics also tolerate some abrasive particles at the mating surface because sharp particles generally will embed in the compound.
Thermoplastic compounds may require additional considerations. Their thermal conductivity is lower than that of metals. Wear additives with enhanced thermal transfer properties can compensate for this. Reinforcements can boost heat-deflection temperature. Appropriate polymers can work at high temperatures, provide chemical resistance, or minimize moisture absorption.
Of the large range of polymers available, only a few are selected specifically for friction and wear. Acetal and nylon may be used for low PV applications. Likewise, PPS and PEEK may handle high temperature and high PV applications. And PC is an option when dimensional stability is needed.
Polymers for tribological uses are classified into two major categories: semicrystalline and amorphous. Most thermoplastics used for wear application are semicrystalline. Semicrystalline polymers have a highly ordered molecular structure with sharp melt points. They do not gradually soften with a temperature increase but remain solid until a given quantity of heat is absorbed. Then they rapidly change to a low-viscosity state.
Semicrystalline materials are anisotropic — shrinking mostly in the transverse direction of flow. Reinforcement additives boost heat-deflection temperatures and help polymers retain the strength and stiffness well beyond their Tg. Semicrystalline materials are also good candidates for chemical and highfatigue applications.
The most important semicrystalline polymers for friction and wear include:
Acetal (POM) which is rigid and strong with good creep resistance. It has a low coefficient of friction, remains stable at high temperatures, and offers good resistance to hot water.
Nylon (PA) absorbs more moisture than most polymers, affecting processability, dimensional stability, and physical properties. However, nylon's impact strength and general energy absorbing qualities improve as it absorbs moisture. Nylons also have a low coefficient of friction, good electrical properties, and resist chemicals.
High-temperature nylon (HTN) and polyphthalamide (PPA) extend the nylon family through improved temperature resistance and lower moisture absorption. They also bridge the gap between traditional and high temperature polymers and have good chemical resistance.
Polyetheretherketone (PEEK) is a hightemperature thermoplastic with good chemical and flame retardance plus high strength.
Polyphenylene sulfide (PPS) offers a balance of properties including chemical and high-temperature resistance, flame retardance, flowability, dimensional stability, and electrical properties.
Polybutylene terephthalate (PBT) crystallizes rapidly, so mold cycles are short. And PBT molding temperatures are lower than those of other engineering plastics. It's dimensionally stable and has high heat and chemical resistance with good electrical properties.
Thermoplastic polyimide (TPI) is the most heat-resistant thermoplastic. It's inherently flame retardant with good physical, chemical, and wear-resistance properties.
Amorphous polymers, in contrast to semicrystalline, have a randomly ordered molecular structure which doesn't result in a sharp melt point. Instead, these polymers soften gradually as the temperature rises. They change viscosity when heated but seldom flow as easy as semicrystalline materials. They are also isotropic, shrinking uniformly in the direction of flow and transverse to it. As a result, they typically shrink less in a mold and tend to warp less than their semicrystalline counterparts.
Although amorphous polymers lose strength quickly above their Tg they are good candidates for precision parts in lowspeed/low-load applications.
The most important polymers for friction and wear include polycarbonate and polyetherimide.
Polycarbonate (PC) has good impact strength, high heat resistance, and good dimensional stability. PC also has good electrical properties and is stable in water and mineral or organic acids.
Polyetherimide (PEI) offers strength and rigidity at elevated temperatures. It also has good long-term heat resistance, dimensional stability, inherent flame retardance, and resistance to hydrocarbons, alcohols, and halogenated solvents.
Additives to the polymer can provide an appropriate balance of mechanical, self-lubricating, wear resistant, and operating temperature properties.
Polytetrafluoroethylene (PTFE) gives the lowest coefficient of friction of any internal lubricant, forming a lubricious film on part surfaces. PTFE modifies the mating surface after initial break-in period and improves wear rates in both similar and dissimilar polymer systems. PTFE also boosts dynamic load bearing capabilities of a part.
Perfluoropolyether (PFPE) is a synthetic oil commonly known as DuPont Fluoroguard. It imparts good wear and low friction properties while maintaining a compound's physical properties. It also minimizes or eliminates "plate-out" associated with PTFE.
Silicone acts as a boundary lubricant because it migrates to the plastic's surface over time. As a partial alloying material with the base resin, some of it remains in the compound over its service lifetime. This continuous migration reduces start-up wear for lowpressure and high-speed applications.
Molybdenum disulfide (MoS2 or "Moly") creates a harder and more wear-resistant surface in semicrystalline materials. It is ideal for applications against metals, as it fills the metal's microscopic pores and creates a smoother metal surface.
Graphite molecules slide easily over one another with little friction. This is especially true in aqueous environments thus making graphite one of the best lubricants for many underwater applications.
Aramid fiber commonly known as Kevlar is softer and less abrasive than carbon or glass fiber. It decreases friction coefficients, increases wear resistance, and improves mechanical performance.
Carbon fiber improves mechanical and thermal performance of a compound and may lower the friction coefficients as compared to the base resin. Carbon is softer and less abrasive than glass fiber and will dissipate static electricity.
Glass fiber improves mechanical and thermal performance of a compound. It may increase the coefficient of friction and can be aggressive to mating surfaces. However, internal lubrication helps decrease the fiber's impact on these properties.
Wear can be quantitatively measured as the specific wear rate, which is the volumetric loss of material over a unit of time. Wear is proportional to the load on the specimen multiplied by the distance the specimen travels. The wear factor comes from the following relationship
W = K•F•V•T
where k = wear factor (in.3 min/ft/lb/hr) 10-10, W = wear volume (in.3), F = force (lb), V = velocity (ft/min), T = elapsed time (hr). The lower the k, the more resistance to wear. However, k should only be used as a relative performance measure when comparing thermoplastic alternatives.
Both the contact pressure (P) and the sliding speed (V ) strongly influence material wear rates. The PV capability of a bearing material is expressed as the product of P and V. Each material has a PV limit. Above this limit, a material will fail. The PV limit, however, is more conceptual than practical. Higher PV values indicate an ability to operate under heavier loads and faster surface velocities. An increase in pressure increases the wear rate and decreases friction, whereas higher sliding speed increases both wear and friction.