Tribochemical behavior of phosphate compounds at an elevated temperature

The tribological behaviors of three types of sodium phosphates (sodium ortho-, pyro-, and metaphosphate) with different chain lengths are evaluated using the ball-on-disk tribometer at 800 °C. The tribological results show that all three sodium phosphates can reduce friction by approximately 45−65% and wear rate by 80−90%, and, among these, shorter-chainlength phosphate shows a lower friction and wear. XPS spectra indicate a phosphate glassy film consisting of Na, Fe, P, and O elements formed on the tribostressed area. The tribochemistry of tribosurface investigated by XPS and XANES reveals that NaPO3 and Na4P2O7 react with oxide wear debris to produce mixed short-chain Na/Fe polyphosphate in the tribostressed area, leading to a severe depolymerization for NaPO3 and a slight polymerization for Na4P2O7. However, little polymerization/ depolymerization was found for crystalline sodium orthophosphate.