Combined in situ (ATR FT-IR) and ex situ (XPS) study of the ZnDTP-iron surface interaction Federica M. Piras a , Antonella Rossi a,b and Nicholas D. Spencer a a Laboratory for Surface Science and Technology, Department of Materials, Swiss Federal Institute of Technology, ETH Zu ¨rich, CH-8092 Zu ¨rich, Switzerland b Department of Inorganic and Analytical Chemistry, University of Cagliari, INSTM, Cagliari, Italy Received 15 September 2002; accepted 23 February 2003 Attenuated total reflection infrared (ATR FT-IR) and X-ray photoelectron spectroscopy (XPS) have been used for the in situ and ex situ characterization of thermal and tribological films formed on iron from a commercial zinc dialkyldithiophosphate (ZnDTP). From in situ ATR FT-IR analysis, information on the chemical changes occurring at the iron/lubricant additive interface was obtained during heating and sliding at high temperatures. Different mechanisms and chemical compositions have been found for the thermal and tribochemical reactions between the ZnDTP and the iron surface under the experimental conditions used in this work. Both the ATR FT-IR and the XPS results show the decomposition of ZnDTP with the formation of polyphosphates following thermal testing at 150 8C. However, after tribological testing at the same temperature an inorganic phosphate film has been detected on the iron surface instead. KEY WORDS: tribochemistry, boundary lubrication, ZnDTP, attenuated total reflection, FT-IR, small-area X-ray photoelectron spectroscopy 1. Introduction Zinc dialkyldithiophosphates (ZnDTPs) have been the most widely used lubricant additives in engine and industrial oil formulations since the 1940s, due to their multifunctional performance [1]. The ZnDTP family comprises primary and secondary aliphatic dithiophos- phates, with chain lengths from C3 to C12, and alkylated phenoldithiophosphates. ZnDTPs were added first as antioxidants, but it was soon recognized that they can reduce or even prevent both mild and severe wear, acting as antiwear and extreme-pressure additives, respectively [2,3]. It is generally accepted that the antiwear and extreme-pressure performance of ZnDTP results from the formation of tribofilms, which show a shear strength that is sufficiently low to ensure that the shear plane is located within the protective tribofilm, while being sufficiently high to maintain its integrity [4,5]. In addition, the rate of formation of the ZnDTP tribofilms is low enough to avoid the corrosive wear of the substrate and sufficiently high to avoid its complete removal during sliding. ZnDTP has been described as a ‘‘smart’’ material [4] due to its ability to act, under different tribological conditions, as both antiwear and extreme-pressure additive, and to form films with different mechanical properties under differ- ent conditions. The structure and the chemical composition of ZnDTP antiwear tribofilms have been mainly studied ex situ, with surface analytical techniques such as X-ray photoelectron spectroscopy (XPS) [6,7], Auger electron spectroscopy (AES) [8,9], and X-ray absorption near- edge spectroscopy (XANES) [10,11]. It has been found that the film composition depends on the temperature and tribological test conditions. Some authors have suggested that the films consist of inorganic amorphous phosphates, mainly orthophosphate ðPO 3 4 Þ and pyr- ophosphate ðP 2 O 4 7 Þ associated with zinc and other metals, such as Ca 2þ ; derived from overbased detergent additives [12,13]. Other authors have proposed the formation of a mixture of short- and long-chain polyphosphates on the basis of XANES results obtained comparing spectra collected on tribostressed samples with those from sodium phosphate compounds of known chain length [14]. In addition, the presence of sulfides and oxides in the inner part of the tribofilm has been indicated [11] by nanoindentation measurements [15] and depth-profiling results obtained by SIMS [16]. Martin et al. recently proposed a two-layer structure of the ZnDTP tribofilms, where a thin, ð 10 nmÞ long- chain zinc poly(thio)phosphate film is superimposed on a thicker ð 100 nmÞ short-chain mixed Fe/Zn polypho- sphate film, containing some embedded nanocrystallites of ZnO and ZnS [17]. A model for elucidating the mechanism of the tribochemical reaction of zinc dithiophosphate with a steel substrate has been based on the hard and soft acids and bases (HSAB) theory, developed by Pearson [18,19] and applied to the ZnDTP by Jones and Coy [20] and by Martin [21]. Despite the large number of investigations conducted on the formation, structure, and chemical composition of the ZnDTP tribofilms, further studies need to be Tribology Letters, Vol. 15, No. 3, October 2003 (# 2003) 181 1023-8883/03/1000–0181/0 # 2003 Plenum Publishing Corporation