Plastic 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.
Plastic 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 well-designed 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-on metal 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 Plastic 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 wear resistance. 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 wear resistant plastic 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. It is important to prototype or do actual application testing whenever wear is a concern.
Calculating Wear Rates
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 wear-resistant the plastic. 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.
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