Eur. Phys. J. B 76, 347-352 (2010)
https://doi.org/10.1140/epjb/e2010-00193-3

Mechanical properties of zinc and calcium phosphates

Structural insights and relevance to anti-wear functionality

¹ Department of Materials Science, Universität des Saarlandes, 66123 Saarbrücken, Germany
² Department of Chemistry, University of Western Ontario, 1151 Richmond Street North, London, Ontario, N6A 5B7, Canada
³ Arcelor Mittal-Dofasco Research and Development, 1390 Burlington Street East, Hamilton, Ontario, L8N 3J5, Canada

Corresponding author: ^a martin.mueser@mx.uni-saarland.de

Received: 2 April 2010
Revised: 19 May 2010
Published online: 24 June 2010

Abstract

Recent studies on a variety of metal phosphates (MP) have revealed that MPs tend to be soft at ambient pressure if the coordination on the metal cation is low and the degree of hydration or hydrogenation is high, while they are stiff otherwise. In addition, the softer MPs were found to stiffen dramatically more quickly with increasing pressure than the stiffer MPs. Here we review these findings and support their relevance with new results on the mechanical properties of tribofilms aged in air of relative humidity, which were produced from commercial, zinc phosphate-containing lubricant packages via heating and rubbing. We find that the films can soften quite substantially after having been exposed to humidity, as to be expected from the studies of bulk MPs. Moreover, when the hydrated films are exposed to high loads, the force-distance withdrawal curve becomes identical to that of unaged, non-hydrated films. A straightforward explanation of this observation is that large pressure reverses the hydration of the tribofilms.