Friction polymers are often observed in used oil samples from gear boxes and highly-loaded rolling element bearings. For many years the nature of how friction polymers are formed has been shrouded in mystery. One theory suggests that the friction polymers are tough, nylon-like compounds that form in the recesses and fissures of contact surfaces. As such, they are thought to have load-bearing abilities, along with asperities.

 

Friction Polymers on Filtergram
Courtesy Oleotec

Recently, a paper was presented at the joint ASME/STLE Conference in Toronto, Canada that offers new insight into the formation of friction polymers. A unique chemical reaction is believed to occur between mating surfaces under load. The critical element needed to initiate the reaction is thought to be mechanical energy, not just temperature. The researchers refer to this as "tribochemical reaction," adding that the term is not clearly defined.

The formation of friction polymers is believed to be one of the products of tribochemical reactions. Conversely, normal oxidation reactions are known to produce high molecular weight materials generally called sludge or varnish, which can lead to poor lubrication. The distinction between friction polymers and sludge is still indefinite.

In order to validate the "mechanical" contribution to the formation of friction polymers an investigation was conducted to define the influence of anti-oxidants. In common stressing conditions that traditionally lead to oxidation, the concentration and effectiveness of the anti-oxidants (hindered phenols & sulfur containing inhibitors) define the period of time the oil is able to resist the onset of oxidation (induction period). However, in the case of tribo-mechanical stressing conditions, the formation of high-molecular weight products (presumed to be friction polymers) proceeds in advance of anti-oxidant depletion and oxidation.

The study employed the use of a four-ball test rig supplying the tribo-mechanical energy needed to incite the reaction. The balls and race were carefully rinsed before the analysis was done to extract any imbedded friction polymers. The concentration of high molecular weight products was measured using gel permeation chromatography (GPC). The plot in Figure 1 shows how, in one test, the formation of friction polymers from the tribo experiment occurred well in advance of base oil oxidation.

Although this study did not reveal the actual reaction mechanism that formed the polymers, it is assumed that hot spots and nascent surfaces provide the reaction field and energy. Viewing it from a condition monitoring perspective, should friction polymers suddenly appear where they were uncommon before, one might conclude there has been:
1. A change in mechanical condition (increased load/pressure, speed reduction, misalignment, etc.), or
2. A loss of film strength due to a breakdown of the lubricant, the wrong lubricant being used, or the lubricant being contaminated, or
3. Inadequate lubrication, lubricant starvation, etc.

Ref: Sukirno and M. Masuko, "Oxidative Degradation of Mineral Oil under Trobcontact and Ineffectiveness of Inhibitors," Proceedings ASME/STLE Tribology Conference, Toronto, Canada, October, 1998.