22 Sep Improving Performance in Wear and Friction
Rotating parts, regardless of whether the part rotates in one direction or oscillates back and forth, will eventually fail. The longevity of one material against another in rotation, also known as sliding, is governed by many variables. Some of these include the contact pressure, the sliding speed, the ambient temperature, external lubrication, and of course, the nature of the two materials. Material selection for wear and friction applications is a complicated science seasoned with a lot of art.
When selecting materials for a sliding or rotating application, we begin with the axiomatic recommendation to choose two dissimilar materials, and we go on to advise against ‘like on like’ pairs. As with most ‘rules’ in engineering plastics design, the book of exceptions is quite thick. Like on like sliding pairs can be (and are) very successful with long service life, but that’s because the contact pressure and sliding speed are carefully considered. Ultimately, all sliding pairs fail because either one of the sliding surfaces overheats and melts, or one of the sliding surfaces is literally worn away. Too much material is lost, and the device cannot function. Whether the pair is metal-metal, metal-plastic, or plastic–plastic, the sliding speed, contact pressure, and heat generated from friction always cause the demise of the pair.
Excessive heat generation in a sliding pair is the manifestation of friction and the primary cause of failure. To manage frictional heat, we add specialized lubricants, such as PTFE, to one of the materials in the pair to lower the coefficient of friction. Lowering the COF directly affects and lowers the temperature at the interface. As an example, POM against POM in a pairing works well as long as the sliding speed or the contact pressure is kept low. When the service speed or the contact pressure is greater than what POM–POM can manage, adding PTFE to one of the POM components dramatically lowers the frictional heat buildup and increases the service performance of the pair.
Material loss or abrasive wear results when a sliding pair encounters a foreign material in the sliding interface, such as the ubiquitous dirt, grit, or sand. This foreign substance gets embedded in one or both of the sliding surfaces, and the COF of the sliding pair is altered. The adulterated surface now behaves like sandpaper, scoring the other surface. To enhance resistance to foreign material, additives such as aramid fiber or boron nitride are added to one of the components. For example, commercial farming equipment exists in a perpetually dirty environment, which can be quickly damaged when particulate soil gets into the interface of a sliding pair. Hermetically sealing the interface is not always possible.
There is plenty of data in the literature for metal–plastic wear pairs, not so much for plastic–plastic pairings. Consequently, PolySource has invested heavily in collecting data on plastic – plastic pairings, carefully studying the effects of additives in plastics, with the objective to find optimal pairings for materials across the continuum of sliding speeds and contact pressures. Testing takes time. We have the data. You achieve success faster when working with us. Contact our team today to find the optimal material pairing for your application.