Paper VII (iv) Rolls Formation with Triboceramics: Which Parameters and What Mechanisms?

Abstract

In the field of polymer tribology, rolling pins wear particles (called often rolls) have been described for a long time. Concerning the case of triboceramic materials, this velocity accommodation mechanism has been recently recognised both with covalent (Sic, Si 3 N 4 ,…) and with ionic (Al 3 O 3 ) ceramic surfaces. Rolls have been especially studied when environmental factors are present, such as humidity, gases, temperature… As rolls formation is always associated with friction and sometime wear reduction, there is a need for a survey of rolls formation with solid-lubricated ceramics for predicting future practical situations. First the morphology (size, length) of rolls is shown to obey to some basic principals whatever the situation, the length/diameter ratio is relatively large (above 10) and can reach the whole contact area diameter. Basically rolls are aligned nearly parcelled to each other and perpendicularly to the sliding direction. Second, the nature of these rolls can be very different from the initial frictional pair materials. They are often composed of oxides or hydroxides from silicon and aluminium based materials (Sic, Si 3 N 4 , Al 3 O 3 ), but in some case there are found to have the same composition (BN). A characteristic feature is the amorphous structure of roll material. Lastly, the mechanisms of rolls formation are discussed in the light of the data available. Two mechanisms emerges: the first one involves the existence of a germination process and the formation of the rolls by a “snow ball” effect with contamination and minute oxide films (screen) or fragment, the second one corresponds to roll out of rupture films or crystal split. When a transparent sapphire plate is used, the formation of rolls can be directly visualised “in situ” by a CCD camera. The reduction of friction is explained by the fact that rolls can act as miniature roller bearings but this velocity accommodation mechanism needs special boundary conditions in terms of sliding and adhesion, and the production of very smooth surfaces.