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A recent study published in Tribology Letters shows evidence that acids, produced by base oil oxidation, can contribute to reduced scuffing in a bench test. The investigation was conducted by researchers at the Tribology Laboratory in Perth, Australia using a ball-on-disc apparatus at slow sliding speeds.
The lubricant used in the study was a mineral base oil with a viscosity of 94 cSt @ 40 degrees C. The samples were oxidized at 120 degrees C in the presence of an iron catalyst over a period of hours. Oxidation was confirmed using infrared spectroscopy (FTIR) by measuring spectral changes known to be influenced by the presence of carboxylic acids, ketones, alcohols, and aldehydes (i.e., products of oxidation).
In their study, scuffing tendency is shown to be inversely proportional to the failure load achieved on the ball-on-disc apparatus. Restated, the higher the failure load (in kilograms) the lower the scuffing tendency. The specific influence of carboxylic acids (from oxidation) on scuffing resistance was further confirmed by doping the base oil with stearic acid and then challenging the mixture to the same test conditions. Both the oxidized oil and the stearic acid oil plots are shown in Figure 1. Is it possible that the activity of the acids that contribute to lower scuffing is similar to that from anti-scuff additives (EP & AW) commonly used in hydraulic, bearing, and gear oils? Because oxidation contributes to an increase in oil viscosity, the researchers investigated its influence as an independent variable. The same base oil was blended with small amounts of high-visc brightstock to show the contribution of viscosity on failure-load from 94 to 144 cSt (40 degrees C). Figure 2 shows that, while increased viscosity can have a mitigating influence on scuffing, its influence is considerably less than that from carboxylic acids formed from oxidation.
The researchers concluded that oil oxidation contributes to lower likelihood of scuffing. It is believed to be due to the surface absorbing nature of the carboxylic acids. They go on to state, however, that the presence of these acids may increase corrosion, accelerate additive attrition, and form high-molecular-weight (e.g., sludge) oxidation products.
In oil analysis there is practical application for this new information. In sum, it is helpful to know that one should not take comfort with negative-going wear metal trends, especially in aged oils. The false sense of security could lead to a risky decision not to order a badly needed oil change.
Ref: Bowman, W. F. and G. W. Stachowiak, "The effect of base oil oxidation on scuffing," Tribology Letters 4, 1998, 59-66.