Study of the in¯uence of adsorbed water on AFM friction measurements on molybdenum trioxide thin ®lms W. Gulbinski a , D. Pailharey b, * , T. Suszko a , Y. Mathey b a Department of Physics, Faculty of Mechanical Engineering, Technical University of Koszalin, 75-620 Koszalin, Poland b Groupe de Physique des Etats Condens es UMR 6631 CNRS, Case 901, Universit e de la M editerran ee, F-13288 Marseille Cedex 9, France Received 23 June 2000; accepted for publication 6 December 2000 Abstract Thin ®lms of layered oxide a-MoO 3 have been prepared by reactive magnetron sputtering. On these ®lms we have measured friction forces and adhesive forces at the microscopic scale with an AFM, under ambient and humidity controlled conditions. We show that surface crystallinity and orientation induce local values of the microscopic friction coecient. The correlation between water adsorption, adhesion and friction is discussed. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Adhesion; Friction; Molybdenum oxides 1. Introduction The atomic force microscope AFM), a power- ful tool for surface imaging down to the atomic resolution [1,2], was shown to be helpful for un- derstanding friction and adhesion phenomenon at the microscopic level [3]. At this scale the role of water, among other adsorbed surface pollutants is fundamental. Absolute measurements of forces normal loads and friction forces) are highly dependent on can- tilevers manufacturing and delicate calibration procedures are necessary [4,5]. Nevertheless, it is possible to plot the friction signal as a function of the load. When surfaces are unaltered during ex- periments the obtained curve contains a linear part. By analogy to the macroscopic tribological experiments, the slope of this linear part is usually interpreted as related to the microscopic friction coecient. In this case AmontonsÕ law is applica- ble for this range of forces at the microscopic level. In a previous experiment carried out on single crystals of dierent lamellar materials [6], this linear behaviour has been observed for the h0 k 0i plane of a-MoO 3 . Molybdenum trioxide, among other lamellar oxides, is a good candidate for applications as high temperature self-lubricating coatings after nec- essary modi®cations at the molecular level by substitution, intercalation or implantation. Such modi®cations should induce desired changes in electronic conductivity and/or interlamellar inter- action energy remaining in relation with lubricat- ing properties. Intrinsic friction properties at every Surface Science 475 2001) 149±158 www.elsevier.nl/locate/susc * Corresponding author. Fax: +33-4-91-82-91-76. E-mail address: pailha@gpec.univ-mrs.fr D. Pailharey). 0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0039-602800)01101-8