A New Experimental Approach to the Study of Boundary Lubrication

A description is given of apparatus for measuring the relative ability of rubbed-down monolayers of polar lubricants to maintain low friction under test conditions which do not permit replacement of the lubricant. This quality of a lubricant is called its ``durability.'' The clean polished rim of a slowly rotating steel wheel rubs on the monolayer deposited on a polished steel flat. An account is given of the preparation of the surfaces and films. It was not found possible to prepare reproducible test surface-monolayer combinations, hence it was necessary to compare each lubricant with a standard deposited on a separate area of the same surface. Significant differences are found in the relative durabilities of a number of polar compounds. For the monocarboxylic acids tested, the values are found to increase with the number of carbon atoms per molecule. An investigation of the stability of rubbed-down monolayers on polished stainless steel shows that aging in a desiccator causes a continuous decrease in durability, and also a decrease in molecular orientation as shown by electron diffraction. Similar reduction of durability and orientation results when the film is flushed with a fine stream of benzene. On chromium also the orientation is destroyed by the solvent, but the durability is not changed.