A STUDY OF CHEMICAL REACTIONS IN BOUNDARY LUBRICATION

Super-refined mineral oils have been used to study the chemical reactions involved in boundary lubrication conditions. It is believed that, in pure mineral oil, the most important reactions are the oxidation of the mineral oil which produces primary oxidation products and the subsequent reaction of the primary oxidation product with the metal surface.; These reactions were studied using the microoxidation test and a four-ball wear tester. The former is a static oxidation test which studies the chemical properties of lubricants without interference from physical movement, and the latter is a dynamic test which simulates conditions that occur in actually loaded bearings. The oxidation products were analyzed using gel permeation chromatography and atomic absorption spectroscopy. The chemical reactions which take place in the microoxidation test are similar to the reactions which take place in the boundary contacts of a four-ball wear test as evidenced by their products.; Kinetic models were formulated for both systems. The modeling of the microoxidation test allows the estimation of the rate of the reaction between oxygen and mineral oil. It has been found that the temperature of the concentrated contact area is probably between 250(DEGREES)C and 350(DEGREES)C. The rate constant under boundary lubrication conditions can be estimated using the Arhenius equation developed from the microoxidation test. Applying the rate constant to the model developed for concentrated contacts, the thickness of the fluid film between two sliding surfaces can be calculated based on the conversion of oxygen to an oxygen-containing organometallic product. The estimated thickness of the fluid film agrees well with results of other studies.; Zinc dithiophosphates (ZDP) have been shown to reduce the rate of formation of organometallic compounds, in both the microoxidation test and the four-ball wear test. In the four-ball wear test, the relative effectiveness of aryl ZDP versus alkyl ZDP changes as the bulk temperature changes suggesting that thermal decomposition is important to the antiwear mechanism of ZDP.