The Role of Tin in the Boundary Lubrication of Bronzes

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Abstract
The friction-temperature characteristics of a series of lubricated copper-tin solid solutions are investigated on a modified four-ball machine. Heats of adsorption of the polar component of the lubricant onto the alloy surfaces are measured by a critical temperature method. These are not found to be a simple function of the bulk tin content of the alloys as would be expected intuitively. Electron probe microanalysis of the wear scars of the alloys reveals that tin atoms migrate towards the worn surfaces during the friction tests, most probably by an interaction with dislocations. The average tin contents (within the depth penetrated by the probe) appear to control the values of the heats of adsorption. It is suggested that, in many cases, bulk compositions should not be expected to hold near worn surfaces.