Model formation of ZDDP tribofilm from a mixture of zinc metaphosphate and goethite

The influence of various iron-containing compounds on ZDDP tribofilm formation was investigated. Friction tests were carried out under boundary lubrication using two sliding materials: steel and sapphire. Lubricants were composed of dispersions of zinc metaphosphate in base oil. The tribofilms were characterized using Raman spectroscopy. In the presence of steel, zinc metaphosphate depolymerizes to form a tribofilm mainly composed of zinc orthophosphate. No depolymerization of zinc metaphosphate was observed with sapphire. In subsequent tests with sapphire, iron oxide particles of different natures were added to the dispersion to simulate the iron oxide species present at the surface of the steel substrate. Only the addition of FeOOH (goethite) micro-particles in the dispersion leads to the formation of a depolymerized zinc-iron phosphate tribofilm. (C) 2014 Elsevier Ltd. All rights reserved.