Process of Boundary Film Formation from Fatty Acid Solution

Abstract

Ultrathin-film interferometry was used to investigate the developing process of boundary films in a steel-on-silica contact lubricated by a solution of n-hexadecanoic acid dissolved in n-hexadecane. Immersion of the surfaces in the solution is not sufficient to initiate boundary film formation. However, upon a nominally pure rolling contact, boundary film formation is initiated, and, once initiated, the boundary film develops spontaneously. It is believed that the initial monolayer film adsorbed strongly on the solid surface induces subsequent molecular stacking from the solution. The developed boundary film can be partially destroyed by the rolling contact, although the nominally identical rolling contact acts as the trigger of boundary film formation. These findings reveal that the immobile boundary films to reduce friction of solids under high pressure result from the equilibrium of two competing processes: spontaneous development by molecular stacking and mechanical destruction by relative motion of the contact surfaces.