Tribochemical reaction, polymer wear

V. Korshak, I. Gribova and A. Krasnov. Tribochemical reactions and wear resistance of polymers of the polyphenylene type. Intern. Conf. Wear of Materials, Houston, Texas, 1987, Vol. 1, No. 1, pp. 435-438. 

Analogous cycles are observed in the polyacetal-steel and polyolefin-steel friction pairs. Physical-chemical processes can exert a favorable effect on the metal-polymer friction pair as well. For instance, metal-containing compounds of copper stearate and oleate types are formed in the contact zone as a result of tribochemical reactions when CFD copolymer rubs against copper. These compounds retard thermally destruction processes in CFD, serving as wear inhibitors (WI) for this system. 

As can be seen, the problem of improving resistance of the metal-polymer friction pair to mechanochemical wearing is determined by the governing cycle of tribochemical reactions that intensifies wear and elimination of conditions stimulating their generation. 

Chemical Wear. Chemical wear, which can be operative in both abrasive and adhesive wear types depending upon the deformational and thermal conditions, is the mechanism of wear debris formation generated as a result of some chemical reaction between the interacting materials and their environment. Such kinds of chemical reactions are also known as tribochemical reaction as they are promoted only in a tribological interaction. In the case of polymers sliding against metal surfaces, there are four important mechanisms by which a tribochemical reaction can be promoted (2). The first one is due to the interfacial heating... 

The primaiy role of sulfur species in the tribochemical processes is passivation by sulfur species on nascent surfaces caused by a severe wear process (extreme pressure conditions) Fe2+ + S2" - FeS, the heat of formation AHf (FeS) = -1.04 eV. The sulfide can prevent adhesion and also the attack of oxygen species, AHf (FeO) = - 2.82 eV (Mori, 1995). There is enough sulfur in engine oil to initiate acid-base reactions in the formation of zinc sulfide (Martin, 1999). Organic sulfur species other than in thiophosphate form can react with the ZnO produced by the phosphate reactions according to the following ZnO + S2 - ZnS + O2. The zinc sulfide can also be directly produced if the polyphosphate contains sulfur atoms in the polymer chain (thiophosphate), for example ... 

We know two major competing directions of tribodestruction of heterocyclic polymers i) decomposition of a heterocycle (quinoxaline, imide, oxa-diazole or others) as a result of the destruction of weak links of the backbone chain leading to a drop in the polymer molecular mass until the formation of low-molecular products and ii) the reactions resulting in formation of a branched and cross-linked polymer structure [97, 104-106]. Low-molecular products of macromolecular destruction (both gaseous and oligomer) may serve as tribochemical lubricants that reduce the friction coefficient and function as wear inhibitors. 

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