Tribology International Vol. 30, No. 12, pp. 881–888, 1997  1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301–679X/97/$17.00 + 0.00 PII: S0301–679X(97)00075–3 Adsorption and surface chemistry in tribology Christopher McFadden*, Cristian Soto† and Nicholas D. Spencer†‡ Adsorption on sliding surfaces and the chemical changes occurring within a few nanometers of the surface are key to the performance of lubricants and lubricant additives in the boundary lubrication regime. By means of the methods of modern surface science, these phenomena are beginning to be elucidated on a molecular level. This knowledge will be essential, both for the development of higher-performance lubricants, capable of high- temperature operation, as well as for the design of environmentally benign alternatives to the lubricant systems in current use.  1998 Elsevier Science Ltd. All rights reserved. Keywords: boundary lubrication, additives, surface analysis Introduction Real, lubricated tribological systems, operating under conditions of high load and low speed, frequently run in the boundary lubrication regime. Under these conditions, the chemistry occurring within a few nano- meters of the surface becomes a critical determining factor with respect to properties such as frictional forces and wear, as well as transient effects such as stick–slip behavior. In order to control the chemistry in this important interfacial region, lubricant manufac- turers blend additives into oils. 1,2 These additives either adsorb onto the sliding surfaces upon contact (friction modifiers) 3,4 or react with the surfaces under extreme conditions (extreme-pressure, EP, additives) 5,6 , and in both cases form protective layers of low shear modulus, which protect the underlying solid surfaces. Despite the obvious importance of the interfacial mol- ecular behavior during boundary lubrication, our spe- cific knowledge of the chemistry that is occurring is remarkably sketchy. Nevertheless, empirical studies have led to the development of many, highly effective lubricant additives, such as zinc dialkyl dithiophosphate (ZnDTP), which are present in almost every automobile engine, for example 5 . The tools of surface science, *Swiss Federal Institute for Materials Testing and Research, Physical Metallurgy/Surface Analysis, 8600 Du ¨ bendorf, Switzerland ²Laboratory for Surface Science and Technology, Department of Materials, ETH-Zu ¨ rich, 8092 Zu ¨ rich, Switzerland ‡Corresponding author: Laboratory for Surface Science and Tech- nology, Department of Materials, ETH-Zentrum, NO H64, CH-8092 Zu ¨ rich, Switzerland. Tel: 41 1 632 5850; Fax: 41 1 633 1027; Email: nspencer@surface.mat.ethz.ch Received 30 November 1996; accepted 30 October 1997 Tribology International Volume 30 Number 12 1997 881 which have already contributed greatly to our under- standing of other complex interfacial phenomena, such as catalysis and corrosion, are now starting to be applied in a systematic way to additive chemistry. This development is timely, since many of the empirically developed additives are either toxic or environmentally undesirable: many additives have historically consisted of chlorinated hydrocarbons, which are already out- lawed in many countries, or metal- or phosphorus- containing molecules, whose negative environmental effects are well known. It is only with a clear grasp of the mechanisms by which these molecules protect sliding surfaces that new, more environmentally benign alternatives can be designed. Further driving forces for the development of new additives are both economic and technological: more effective wear-reduction addi- tives translate into lower replacement costs and less downtime, and higher-temperature operation (turbines, high-speed machining) means that the traditional addi- tives can no longer be employed and must be replaced by other molecules. In this article, we have endeavored to summarize selec- ted significant results in two main areas where surface science is contributing to our knowledge of the chemis- try of boundary lubrication: model tribological studies of unreactive molecules interacting with surfaces under controlled conditions, and the reactions of extreme- pressure additives with surfaces of tribological interest. These two approaches are both important and comp- lementary.