I've read that it's not a good idea to rub two surfaces of the same material against each other, e.g., plastic against plastic, metal against metal. Plastic against metal is preferred. Why is this the case? Is it because that the coefficient of friction is higher for same materials?

According to this site, steel-steel gives 0.5, plastic-plastic gives 0.3, but plastic-metal gives 0.25.

Suppose two rollers rotate and rub against each other. Will it be better for one to be made of metal and one of plastic?

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    $\begingroup$ Dissimilar materials will likely wear at different rates, therefore some designs may include a softer component that is more easily replaceable. $\endgroup$ – welf Feb 1 '16 at 12:47

Friction is probably one of the primary reasons; as friction also correlates to wear. Friction is much more complicated than the equation we learned back in physics. Published friction coefficients are very much ball park figures because the friction greatly depends on the micro surface texture. This causes of friction link does a good job explaining some of the mechanics involved.

One of the main factors influencing lower friction for dislike materials is that there is a lower likeliness that there will be adhesion between them (atomically and macroscopically). A materials professor I had once lectured that if you were to polish two pure copper surfaces perfectly smooth in a vacuum and touch the surfaces together they would immediately become one piece of metal (cold welding). Similar adhesion occurs for plastic; perhaps even more so in the standard environment.

Another consideration is that plastic on plastic can not dissipate heat nearly as fast as metal on plastic, which could lead to additional adhesion wear or failure.

Sometimes the erosion of softer material is used to "dry" lubricate a surface such as oil impregnated brass bushings or graphite motor brushes.

Also, the harder material can stay smooth and true, reducing the friction and wear of the system in the long term. And like welf mentioned in the comments, the softer surface is usually a consumable (bushing, brush, etc) that is more easily replaceable.

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When material A of coefficient of friction $\mu_A$ and material B of $\mu_B$.

If material A is being rubbed with material B. The requirement for abrasion and heat generated would affect both parts. Hence if the application was for grinding, the tool itself would get affected. So it would make sense to use a harder material to smooth the softer material. For a steel part, you could use diamond, or even steel itself, but here the point of contact is different.

Also a lot of other factors come into play, if you have rollers, and a part going in might be hot rolled, hence there is temperature difference or material property difference which helps roller material stay in its own geometry. Also rollers have to be designed so as to take care of camber.

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