The Influence of Alkali Metal Polyphosphate on the Tribological Properties of Heavily Loaded Steel on Steel Contacts at Elevated Temperatures

Polyphosphate inorganic polymer is evaluated as a high-performance lubricant for steel/steel contacts at 600, 700, and 800 degrees C. In situ thermal tests of this lubricant indicate that liquid lubrication at the rubbing interface is occurring. Tribological testing indicates the molten polyphosphate is effective as a lubricant and, reduces friction and wear of the sliding steel/steel tribo pair substantially. The lubricated steel/steel pair shows desirable tribological performance that correlates closely with the temperature-dependent hierarchical structure of the tribo-interface under the combination of pressure, shear, and temperature. Morphological observation of worn surfaces and interfaces of steel discs is achieved using scanning electron microscopy and transmission electron microscopy (TEM). Compositional analysis and elemental distribution are performed using X-ray photoelectron spectroscopy and energy dispersive X-ray on a scanning TEM. These enable the tribochemical reactions at the rubbing surface and the formation mechanism of the hierarchical tribo-interface to be understood.