Smart Materials Behaviour in Phosphates: Role of Hydroxyl Groups and Relevance to Antiwear Films

The elastic properties of materials under high pressure are relevant to the understanding and performance of many systems of current interest, for example, in geology and tribology. Of particular interest is the origin of the dramatic increase in modulus with increasing pressure, a response which is also called ”smart materials behaviour”. In this context, simple phosphate-containing materials have been studied experimentally and theoretically, and the origins of this behaviour have been associated with factors such as coordination of the cations, and changes in the degree of polymerization and hydrogenation of the phosphate units. In the present paper we extend the former analysis on simple metal phosphates model compounds to so-called thermal films, an intermediate stage in the formation of effective antiwear films. Specifically, we report the equations of state and IR spectroscopy of a complex material for pressures up to 25 GPa. The material was produced by heating a commercial zinc dialkyldithiophosphate (ZDDP, a common antiwear additive in lubricating oils) in poly-α-olefin base oil solutions to 150C, a process known to produce the thermal films, Its structure was studied by means of X-ray diffraction and IR synchrotron radiation techniques during compression/decompression in a diamond-anvil cell. As is the case for the simple metal phosphates, we find that the thermal films are relatively soft at low pressures, but stiffen rapidly and ultimately amorphize irreversibly at high pressure. However, in addition to phase transformations involving cation sites occurring in the metal phosphates, thermal films undergo displacive transitions associated with instabilities of the hydroxyl groups. These results may imply that ZDDPs ligands and those of the transformed materials not only affect ZDDPs decomposition rate in engines, but also the mechanical properties of the resulting anti-wear films.