A Basic View of Reactions between Additives and Metal Surfaces

A BASIC VIEW OF REACTIONS BETWEEN ADDITIVES AND METAL SURFACES 

Two basic factors should be considered in assessing the role of chemical reactivity in additive action (a) the nature of the additive— X.e. its structure and the reactive elements therein (b) the reactivity of the rubbing surfaces, which includes their state of activation as well as the intrinsic reactivity of the constituent materials. In tribological processes, additive reactions occur at surfaces that are being rubbed or have just been rubbed. For the most part the sequence of events is so complex that the reactivity of the surfaces cannot even be estimated, let alone be specified precisely. Nevertheless it is valuable background to know how surfaces of known composition and properties react generally with chemical groups typical of additive substances as well as with specific substances of established utility. 

Films of metal in a highly reactive condition can be prepared by flash evaporation in vacuum from a hot filament onto a receiving surface. The gaseous substance whose reaction is to be studied is then leaked into the system at low pressures. Morecroft [1], on admitting n-octadecane at 0.1 Pa (7.5 X 10 torr) to a film of evaporated iron at room temperature, recovered methane, hydrogen and carbon monoxide in the ratio 

Roberts and Kemball [6] interpreted the interaction between methyl mercaptan and an evaporated film of nickel in the following terms. At temperatures in the range 193 to 293 K (-80 to + 20 C) the formation of methane was ascribed to the reaction  

Tungsten holds methyl mercaptan tenaciously in the temperature range 

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