Ž . Thin Solid Films 324 1998 190–197 The tribological behavior of type II textured MX 2 ž / M s Mo, W; X s S, Se films S.R. Cohen a, ) , L. Rapoport b , E.A. Ponomarev c , H. Cohen a , T. Tsirlina d , R. Tenne d , C. Levy-Clement c ´ ´ a Chemical SerÕices Unit, Weizmann Institute, RehoÕot 76100, Israel b Department of Mechanics and Control, Center for Technological Education-Holon, PO Box 305, Holon 58102, Israel c LPSB, CNRS, UPR 1332, 1-Place Aristide Briand, Meudon 92195, France d Department of Materials and Interfaces, Weizmann Institute, RehoÕot 76100, Israel Received 23 September 1997; accepted 4 December 1997 Abstract Ž The tribological properties of textured WS , MoS and WSe films, which were prepared using an ultra-thin interlayer of NirCr van 2 2 2 . der Waals rheotaxy technique on quartz substrate, were determined in ambient conditions. Using scanning force microscope adapted for Ž . tribological measurements, very low 0.04 and below friction coefficients and little wear were measured on flat areas of the films. Ž . Macroscopic engineering tribological measurements, using the reciprocating ball on flat tribometer, exhibit somewhat higher friction coefficients. Compactization of the films under the load and little wear were observed for the films even after a few hundred cycles. X-ray photoelectron spectroscopy of the WS film after the wear experiment confirmed that some oxidation took place within the wear track, 2 but the overall integrity of the film was preserved. These measurements indicate that highly textured films of this kind are promising candidates for tribological coatings, where oil-free lubrication is required. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Type II textured MX films; Tribological properties; Wear experiment 2 1. Introduction Layered metal dichalcogenides are used extensively as tribological coatings, where protection of moving surfaces against friction and wear is necessary. In the classic Bow- wx den–Tabor friction model 1 , the frictional force is given Ž. as a sum of two terms: I the product of the contact area Ž . and the shear strength and II a plowing term, associated with the translation of a hard, penetrating contact through the softer material. For thin solid lubricant coating, the wx plowing term is negligible 2 . Many solid lubricants pos- sess a layered structure, e.g., graphite and MoS , in which 2 case the shear strength is commonly associated with shear- w x ing against the weak inter-layer van der Waals forces 2,3 . Ž The tribological properties of MX M s W, Mo; X s S, 2 . Se films were shown to be very sensitive to the atmo- w x sphere 2,4,5 . The friction coefficient m of MoS films 2 was shown to increase from a value of 0.001 in vacuum to wx 0.15 in ambient conditions 6 . The endurance of sputtered ) Corresponding author. MoS films under wear conditions was found to be better 2 wx wx in dry Ar gas 7 or vacuum 8 than in dry air. The likely chemical reaction in air is the oxidation of MoS into 2 MoO or more complex metal oxides. The dissipation of 3 frictional energy leads to heating of the contact area between the surfaces sliding across each other, which facilitates the chemical reaction of the film with oxygen wx Ž . and water 9 . The reactive prismatic edges on the 1120 face are more prone to chemical attack during wear tests Ž . Ž . than the inert 0002 van der Waals vdW basal plane w x 10 . The adsorbed platelets are burnished and consumed after a few minutes work, but the remaining powder is very effective in protecting the sliding metal parts against w x wear 11 . Therefore, type-II oriented films, for which the c-axis of the crystallites is oriented perpendicular to the substrate are preferred for tribological applications of MX 2 films. In this texture, the low energy and chemically inert Ž . w x 0002 face is exposed to the ambient 12 . A transfer of Ž . thin films from the MoS particles coatings onto the past 2 sliding metal surfaces is a leading explanation for the low w x friction coefficient in the presence of such platelets 11,12 . 0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.