ORIGINAL PAPER Structural Changes in Tribo-Stressed Zinc Polyphosphates Sophia Berkani • Fabrice Dassenoy • Clotilde Minfray • Jean-Michel Martin • Herve Cardon • Gilles Montagnac • Bruno Reynard Received: 3 April 2013 / Accepted: 9 July 2013 / Published online: 19 July 2013 Ó Springer Science+Business Media New York 2013 Abstract The influences of pressure, shear stress and temperature on the structure of zinc orthophosphate and zinc metaphosphate was investigated. Tribological tests were performed to study the combined effect of pressure and shear stress at two temperatures. Friction tests were carried out in the boundary lubrication regime from dis- persions of zinc polyphosphates in base oil. The effects of pressure alone were investigated using a diamond anvil cell in order to separate it from those of shearing. Raman spectroscopy was used to follow in situ or ex situ the structural changes of the zinc polyphosphate powders and the tribo-stressed areas. Tribofilms obtained with both polyphosphates display a partial and full depolymerization of the zinc metaphosphate at ambient and high (120 °C) temperature, respectively. The large stress and strain con- ditions of the tribological tests are necessary to induce a tribochemical reaction between zinc metaphosphate and iron oxide leading to a depolymerization of the phosphate in the tribofilm. The tribochemical reaction and antiwear tribofilm formation are significantly enhanced by the modest temperature increase from ambient to 120 °C. Pressure alone induces only disordering in the structure of zinc polyphosphates, with only minor changes of the chain length in phosphates and does not contribute significantly to the observed structural changes in tribofilms. Keywords Antiwear additives Á Tribochemistry Á Boundary lubrication Á Zinc polyphosphates Á Raman 1 Introduction Zinc dialkyldithiophosphate (ZDDP) has been recognized as a multifunctional additive in engine oils for over 70 years. It is widely used in formulating lubricants to enhance their antioxidant, antiwear and extreme pressure properties. When tribo-stressed in a lubricated tribological contact under severe friction conditions, the ZDDP mole- cule is thermally degraded and eventually forms a thin antiwear solid-like protective film (called a tribofilm) about one hundred nanometers thick on the metal rubbing sur- faces. This film is known to protect the surfaces from adhesion and abrasion [1]. It presents a heterogeneous morphology, including pads and valleys of irregular sizes [1]. Chemical composition varies gradually from the top of the film to the substrate [2]. It is well established that the tribofilm is mostly composed of amorphous zinc/iron polyphosphates with variable chain lengths, with inclusions of ZnS and FeS [2–4]. Mostly orthophosphates (with mixed Zn and Fe) are found near the tribofilm contact with the steel surface while polyphosphates (mostly zinc meta- phosphate composition) are located at the top surface of the tribofilm. Several mechanisms have been proposed in the literature to explain the formation, the antiwear action of ZDDP tribofilm [3–11], as well as the formation of the gradient of phosphate chain length (or polymerization) in the film thickness. Martin [12] proposed a mechanism derived from the Chemical Hardness model of Pearson [13]. Firstly, the ZDDP undergoes a thermo-oxidative decomposition in the bulk of the lubricant. At temperatures above 100 °C, a S. Berkani Á F. Dassenoy (&) Á C. Minfray Á J.-M. Martin LTDS, Ecole Centrale de Lyon, UMR 5513, 36 Avenue Guy de Collongue, 69134 Ecully, France e-mail: fabrice.dassenoy@ec-lyon.fr S. Berkani Á H. Cardon Á G. Montagnac Á B. Reynard LGL, Ecole Normale Supe ´rieure de Lyon, Universite ´ Claude Bernard Lyon1, CNRS, 15 Parvis Rene ´ Descartes, 69364 Lyon, France 123 Tribol Lett (2013) 51:489–498 DOI 10.1007/s11249-013-0188-9