Wear characteristics of one plastic against another vary widely, even among those materials with good natural lubricity. When an application calls for plastic-to-plastic bearings, shafts, gears and other parts, the combination must be chosen carefully.
Because plastics are not rigid bodies, they do not behave according the natural laws of friction. It is these deviations which cause some of the unexpected results when plastics are run against metal. Frictional forces are not proportional to load, friction increases with increasing speed and the static coefficient of friction is lower than the dynamic. When two viscoelastic, low modulus materials are run against each other, additional inconsistencies result. Despite these inconsistencies, one trend remains clear: the wear factor generated when the same thermoplastic is run against itself is extremely high unless the operating temperature and pressure are extremely low. In applications requiring all plastic components, the wear rate can be reduced (if crystalline reins are being used) by running dissimilar plastics against each other. If amorphous resins are involved or if environmental or manufacturing procedures require that only a single compound be used, that compound should contain an internal lubricant such a PTFE at loadings of 15-20%.
Wear is often greater on the moving surface when dissimilar neat resins are paired, and similar behavior occurs with pairs consisting of lubricated (unreinforced) resins running against themselves or against dissimilar lubricated (unreinforced) resins. The addition of a reinforcing fiber generally produces increased wear in a mating unreinforced resin or composite. The addition of a reinforcing fiber to both surfaces may result in decreased wear, compared to unreinforced resins.
Wear factors of glass fiber containing composites are lower than those of carbon fiber reinforced composites when run against the carbon reinforced materials because glass fiber are much harder than carbon fibers.
Lubrication of composites with PTFE dramatically reduces wear factors in both-similar or dissimilar mating resins. During the initial break-in period, a film of PTFE is transferred to the mating surface, creating a PTFE-to-PTFE bearing condition. This condition lowers the wear factors for both moving and stationary surfaces. The addition of PTFE lubricant to the mating material reduces the detrimental effects of glass fiber (with respect to wear) on the opposing surface.
The addition of silicone fluid to thermoplastic composites results in compounds with reduced wear factors when compared to the neat resin, but wear factors are greater than those of PTFE-lubricated composites. In plastic-on-plastic wear applications, composite pairs having similar wear factors when run against each other are preferred to pairs having large differences in wear factor provided the total wear in the system is acceptably low.
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