On the Mechanism of Lubrication by Tricresylphosphate (TCP)—The Coefficient of Friction as a Function of Temperature for TCP on M-50 Steel

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Abstract
A pin-on-disk tribometer was used to study the coefficient of friction as a function of temperature for tricresylphosphate (TCP) on CVM M-50 tool steel under the following conditions: TCP was present in a liquid reservoir (bulk lubrication), and TCP was applied as a liquid layer directly to the disk (limited lubrication). Under limited lubrication conditions, experiments were performed in dry (< 100 ppm H2O) air, dry (< 20 ppm H2O) nitrogen, dry nitrogen with the disks heated to 700°C, then cooled to room temperature before the TCP was applied and the measurements made (preheated disks), and moist nitrogen using preheated disks. The coefficient of friction decreased at a characteristic temperature, Tr Tr values observed were: 265°C for bulk lubrication conditions in dry air, 225°C for limited lubrication conditions in dry air, and 215°C for limited lubrication conditions in dry nitrogen. The use of preheated disks produced a sharp failure temperature at 218°C which was taken as the temperature about which the behavior of TCP should be judged. X-ray photoelectron spectroscopy confirmed the presence of phosphate on the surface of TCP-lubricated iron pins. Depth profile studies support the suggestion that a chemical reaction occurs between the TCP and the metal surface at Tr.