Friction-Induced Change of Boron Hybridization in Lubricant Additives

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and overbased detergent calcium borate (OCB) lubricant additives have been investigated. A Cameron-Plint friction machine was used to generate large tribofilm areas in mild tribological conditions. Two analytical techniques, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES), have been employed to characterize the chemical species in tribofilms. XANES spectroscopy at the P K and L-edges, S K-edge, Mo L and M-edges and B K-edge was carried out in order to investigate phosphate, sulfide and borate species in tribofilms. The XANES spectra were recorded in both the total electron yield (TEY), surface sensitive and fluorescent yield (FY), bulk sensitive mode. This was completed with XPS analyses in the same location in the tribofilm. The ternary system Zndtp + Modtc + OCB provides a low wear rate comparable to the binary system Modtc + OCB. XANES analyses have shown that adding OCB to the binary system Zndtp + Modtc increases the formation of MoS₂ and reduce the formation of ZnS. It has been seen that few quantity of boron is consumed during the tests, meaning that boron does not react with other additives, we only observe a formation of calcium phosphate. A special attention has been paid on the hybridization of boron (trigonal sp2 and tetrahedral sp3). All tribofilms show a higher proportion of trigonal borate than the additives alone. However, this proportion is the highest in the case of tribofilms produce with the ternary mixture OCB + Zndtp + Modtc. Borate has a planar conformation and could be friction-oriented in the sliding interface. The oriented borate planar molecule might be at the origin of the MoS₂ sheets orientation in the ternary system.