Synthesis and antiwear behaviour of ashless alkylthioperoxydiphosphates

A series of ashless alkylthioperoxydiphosphates antiwear lubricant additives were synthesised via oxidation of O,O'-dialkyldithiophosphoric acids under mild condition. The compounds were characterised using NMR, FT-IR and mass spectrometry, and their antiwear performance was systematically evaluated using a high frequency reciprocating ball (HFRB). Tribofilms generated by alkylthioperoxydiphosphates were investigated by scanning electron microscopy, optical profilometry and X-ray absorption near edge structure (XANES) spectroscopy. Formulations with alkylthioperoxydiphosphates in base oil at 0·1% (w/w) phosphorus level exhibit significantly better antiwear performance compared to zinc dialkyldithiophosphate (ZDDP), in particular for compounds with medium or long alkyl chains. The XANES analysis of thermal films and tribofilms indicate that thermal films are made up of short chain zinc phosphates when ZDDP is used and short to medium chain iron phosphates when the alkylthioperoxydiphosphates are used. Tribofilms from ZDDP have Zn₃(PO₄)₂ while tribofilms from alkylthioperoxydiphosphates have Fe₄(P₂O₇)₃ near the surface with both tribofilms having sulphates near the surface of the tribofilms.