The chemical characterization of the reaction films formed by lubricant additives on metallic surfaces under both thermal and tribological conditions in the boundary-lubrication regime is of crucial importance for understanding their mechanism of action. Although challenging, the development of in situ analytical methods has been demonstrated to provide valuable insights into the reactions taking place under steady-state conditions.
In the present work, an in situ tribological test system has been developed on the basis of the previous version of the instrument built in our laboratories by merging an ATR/FT-IR spectrometer with a reciprocating tribometer. By periodically acquiring ATR/FT-IR spectra, the chemical changes due to thermal and/or tribochemical reactions occurring at the metal/oil interface could be monitored and correlated with friction data.
The reaction layers formed on air-oxidized iron-coated germanium surfaces in the presence of 1.82 wt.% solutions of an environmentally friendly anti-wear additive, triphenyl phosphorothionate (TPPT), in poly-alpha-olefin (PAO) heated at 423 K and during tribological tests carried out at 423 K using the same lubricant composition with a cylinder-on-flat configuration applying a load of 6.9 N, were characterized in this work. These conditions correspond to the boundary-lubrication regime.
The ATR/FT-IR spectra acquired in situ under purely thermal conditions indicated that the TPPT molecule reacted at the air-oxidized iron surface at 423 K to yield pyrophosphates, organo-phosphates and sulphates together with carbonates and carboxylates. The subsequent ex situ angle-resolved X-ray photoelectron spectroscopy (AR-XPS) analysis, besides substantiating the formation of short-chain polyphosphates, organo-phosphates and sulphates on the air-oxidized iron surface, indicated the presence of oxygenated organic compounds in the outermost part of the reaction layer. Modeling the ATR/FT-IR system allowed the correlation of the changes observed in the experimental spectra with the reflectivity alterations caused by the formation of reaction products on the iron-coated germanium ATR crystal.
The in situ ATR/FT-IR tribological results suggested that the TPPT molecule is not a friction modifier additive and it reacts at the air-oxidized iron surface upon tribotesting at 423 K to form a tribofilm consisting of pyrophosphates, organo-phosphates and sulphates, together with carbonates and carboxylates. The ATR/FT-IR results also indicated that the rate of tribofilm formation was lower than that of tribofilm removal. The subsequent ex situ XPS analysis corroborated the formation of iron polyphosphates, organo-phosphates and iron sulphates, together with carbonates and carboxylates. Sulphoxides were also generated in the boundary lubricant layers formed by TPPT at 423 K, as indicated by the XPS results.
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