Ž . Wear 225–229 1999 975–982 Inorganic fullerene-like material as additives to lubricants: structure–function relationship L. Rapoport a, ) , Y. Feldman b , M. Homyonfer b , H. Cohen c , J. Sloan d , J.L. Hutchison d , R. Tenne b a Department of Science, Center for Technological Education, Holon, PO Box 305, Holon 58102, Israel b Department of Materials and Interfaces, Weizmann Institute, RehoÕot 76100, Israel c Chemical SerÕices Unit, Weizmann Institute, RehoÕot 76100, Israel d Department of Materials, The UniÕersity of Oxford, South Parks Road, OX1 3PH, UK Abstract Ž . Ž . Recently, inorganic fullerene-like IF supramolecules of metal dichalcogenide MX M sMo, W, etc.; X sS, Se , materials with 2 Ž . structures closely related to nested carbon fullerenes and nanotubes have been synthesized. The main goal of the present work was to study the tribological properties of IF–WS in comparison to 2H–WS and MoS platelets over a wide range of loads and sliding 2 2 2 Ž . velocities. The size and shape distributions of the nanoparticles were studied by transmission electron microscopy TEM . The average size of the IF–WS particles was 120 nm, while that of 2H–WS and 2H–MoS was 0.5 mm and 4 mm. The chemical reactivity of the 2 2 2 different powders in an oxygenated atmosphere was verified by heating the powders in ambient atmosphere. The friction experiments Ž . were performed in laboratory atmosphere humidity ;50% using a ring-block tester. Complementary information on the state of wear of the powders in the lubricating fluid and on the metal surfaces of the specimens was obtained using a combination of TEM, scanning Ž . Ž . electron microscopy SEM and X-ray photoelectron spectroscopy XPS . It has been established that IF–WS nanoparticles appear to 2 have excellent tribological properties in definite loading range in comparison to typical metal dichalcogenides. The oxidation of the IF Ž . particles and the wear track was essentially less than with solid lubricants made of platelets of the same chemical compound WS . The 2 main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Fullerenes; Friction and wear; Lubricant; Surface 1. Introduction Layered materials such as graphite, MoS and WS 2 2 Ž . platelets of the 2H polytype are used both as solid w x w x lubricants 1,2 and as additives in liquid lubricants 3,4 . Minimum tangential resistance is commonly associated Ž with shearing of the weak inter-layer typically van der Ž .. w x Waals vdW bonds in these materials 5,6 . Unfortu- nately, the 2H platelets tend to stick to the mating metal pieces through the reactive dangling bonds on the pris- Ž . matic edges 0110 , which leads to their rapid annihilation through burnishing and oxidation. Recently, the tribological properties of C and C 60 70 wx fullerenes were described 7 . It was speculated that the nearly spherical fullerenes may behave as nanoscale ball ) Corresponding author. Tel.: q972-3-5026616; fax: q972-3-5026619; e-mail: rapoport@barley.cteh.ac.il wx bearings. Experiments by Campbell et al. 8 with C 60 molecules dissolved in dry toluene tend to substantiate this hypothesis, but comparison of this result with macroscopic friction measurements is not obvious. Further work of this group has demonstrated that, whereas the adhesion energy of smooth C films is very low, the friction coefficient is 60 rather high in this case. The tendency of the fullerene powders to clump and compress into a high shear strength layer was demonstrated to be a main cause of the high wx friction coefficient 9 . Intuitively, the fullerene molecules are thought to be too small to separate between asperities of the mating metal surfaces and therefore they tend to enter into crevices or valleys. Ž . In the past few years, inorganic fullerene-like IF Ž supramolecules of metal dichalcogenide MX M s Mo, 2 . W, etc.; X s S, Se , materials with structures closely re- Ž . lated to nested carbon fullerenes and nanotubes have w x been synthesized 10,11 . Recent experiments showed that 0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. Ž . PII: S0043-1648 99 00040-X