Smart materials behavior in phosphates: Role of hydroxyl groups and relevance to antiwear films

Abstract

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 behavior." In this context, simple phosphate-containing materials have been studied experimentally and theoretically, and the origins of this behavior 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 phosphate model compounds to so-called thermal films, an intermediate stage in the formation of effective antiwear films. The material was produced by heating a commercial zinc dialkyldithiophosphate (ZDDP), a common antiwear additive in lubricating oils, in poly-alpha-olefin base oil solutions to 150 degrees C, a process known to produce the thermal films. Its structure and equation of state were studied by means of x-ray diffraction and IR synchrotron radiation techniques during compression up to 25 GPa in a diamond anvil cell as well as during the subsequent decompression. 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 studied previously, thermal films undergo displacive transitions associated with instabilities of the hydroxyl groups. These results may imply that ZDDP ligands and those of the transformed materials not only affect ZDDP decomposition rate in engines but also the mechanical properties of the resulting antiwear films.

Phosphate Esters, Thiophosphate Esters and Metal Thiophosphates as Lubricant Additives

“…A schematic representation of the structure of the films is shown in Figure 4. A particularly interesting property of the films is the increase in the modulus with an increase in pressure, a property referred to as smart material behavior [43].…”

Excellent melt lubrication of alkali metal polyphosphate glass for high temperature applications

“…ZDDP. [8][9][10][11] Remarkably, this tribolm shows an excellent adaptive tribology behaviour, which is intelligently responsive to the tribo-stressed conditioins. 10,11 Moreover, it can protect the substrate against further oxidation.…”

“…[8][9][10][11] Remarkably, this tribolm shows an excellent adaptive tribology behaviour, which is intelligently responsive to the tribo-stressed conditioins. 10,11 Moreover, it can protect the substrate against further oxidation. Alkali metal phosphates glasses, a class of thermoplastic polymers similar to epoxy resin based thermoset, 12,13 have been implemented in the formulation of ceramic restorative cements and more generally of bioactive functional materials, employed as active elements in laser engineering owing to itself useful optical properties, [14][15][16][17] owing to its exceptionally thermal stability and inability to oxidize, as well as high coefficient of thermal expansion.…”

“…28,29 On the other hand, an acid-base reaction occurs between alkali metal polyphosphate and iron oxide nanoscales forming the polyphosphate tribolm along the rubbing steel surface, this tribolm have "smart materials behaviour", thus enhancing anti-wear ability to a certain extent. 10 In addition, polyphosphates of alkaline metals preferentially bonds with the iron/iron oxide surface enables a good adherence of this tribolm, and this melting lm plays the role of the barrier to further oxidize.…”

“…10,11 Especially polyphosphate tribolms show the adaptive tribological behaviour in response to different extreme range of conditions, i.e. high loads, high speeds and temperatures, which is primarily related to the preferential alignment of the tribolm in structure and composition: a layered structure with a gradient composition from surface to the depth.…”