Interaction of ZDDP with Borated Dispersant Using XANES and XPS

Abstract

The thermochemical reaction and tribochemical reaction of zinc dialkyldithiophosphate (ZDDP), a borated dispersant, and the mixture of ZDDP and borated dispersant on steel surfaces were investigated. Both pin-on-disk and ball-on-disk were used to generate tribofilms. The chemical state of nitrogen, boron, phosphorus, and sulfur in heated oil solutions, thermal films, and tribofilms were analyzed by X-ray absorption near edge structure (XANES) spectroscopy to obtain the chemical nature of species on the surface and in the bulk of the films. High-resolution X-ray photoelectron spectroscopy (XPS) has also been used to analyze boron (B) in tribofilms.The borated dispersant in base oil by itself yields good anti-wear behavior. This can be attributed to the presence of boron in the dispersant. The wear scar widths (WSW) for ZDDP alone, and in combination with the dispersant, yield similar results within the experimental error. It was found that the borated dispersant facilitates the decomposition of ZDDP and the formation of phosphate in tribofilms and thermal films. B K-edge XANES shows that boron has a trigonal coordination in the untreated additive, but the coordination changes partially to a tetrahedral coordination in the tribofilm upon rubbing. No BN was detected in the film analyzed by B K-edge or N K-edge. Boron 1s XPS also did not show the presence of BN in the film.