Relating Molecular Structure to Tribological Chemistry: Borate Esters on Copper

The surface and tribological chemistry of 2-propanol and borate esters, isopropoxy tetramethyl dioxaborolane (ITDB), and tetramethyl dioxaborolane (TDB) as gas-phase lubricants for copper surfaces are compared by measuring the friction coefficient and contact resistance in an ultrahigh vacuum tribometer. 2-propanol reacts on copper below room temperature to desorb primarily acetone and causes no friction reduction when used as a gas-phase lubricant. TDB decomposes to deposit BO_x species on the surface and produces an initial decrease in friction coefficient that is not sustained. ITDB, which reacts at room temperature to form a tetramethyl dioxaborolide species on the surface, results in an immediate and sustained decrease in friction coefficient when used as a gas-phase lubricant for copper. This indicates that the surface chemistry of the borate esters can be correlated with their tribological properties and illustrates the effect of replacing a hydrogen atom in TDB with a 2-propoxy group in ITDB on both the surface and tribological chemistry. Analysis of the species formed in the wear track after lubrication with ITDB reveals the presence of graphitic or diamond-like carbon and oxygen, and these elements are also found in the subsurface region of the sample. Rubbing the sample in vacuo after reacting with ITDB shows the removal of a boundary film where the coefficient remains constant and the subsequent removal of the subsurface layer in which the contact resistance rises to its original value.