The antiwear and antifriction (lubricating) properties of polyorganosiloxanes of various kinds and of their mixtures with hydrocarbons are studied. It is shown that in mixtures with hydrocarbons, polyorganosiloxanes may display a high activity under boundary friction. The conditions under which this activity is brought out are determined. A change in the oxidative activity of the gas phase acts on the lubricity of polyorganosiloxanes in the same direction as on that of mineral oils; this action is weaker as the stability of the polyorganosiloxanes to thermal oxidation increases. Additives highly active in preventing steel seizure in hydrocarbon lubricants are either utterly inactive in polyorganosiloxanes (polymethylphenylsiloxanes) or are much weaker (polyethylsiloxanes). A hypothesis is suggested, attributing this to the difficulty in polyorganosiloxanes of processes occurring which involve chain reactions initiated by free radicals. The mechanism of action of E.P. additives in lubricating media is discussed.

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