Wear 252 (2002) 1–8 A mechanism of high friction in dry sliding bearings M. Mosleh a,∗ , N. Saka b , N.P. Suh b a Department of Mechanical Engineering, Howard University, Washington, DC 20059, USA b Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 19 October 2000; received in revised form 26 March 2001; accepted 2 April 2001 Abstract The conventional definition of friction coefficient as the ratio of the frictional force to the initial normal load is misleading when complete mechanical interlocking between two surfaces occurs. In that case, even after the initial normal load is released, the frictional resistance of surfaces to relative motion remains high and the magnitude of friction coefficient, by the conventional definition, is infinity. It is shown in this paper that the entrapment of wear particles at the sliding interface of a geometrically constrained bearing operating under dry conditions leads to increased normal load. The higher normal load in turn results in a rapid increase in the frictional torque of the bearing and can cause seizure. A model is developed to predict the normal load increase. The model is based on the assumption that the entrapped wear particles are compacted at the initial contact point of the bearing. The model predicts that the large increase in the frictional torque of bearings experiencing seizure is mainly due to the increased localized normal load and not necessarily due to higher friction coefficient. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sliding bearings; Wear particles; Seizure 1. Introduction The role of wear particles on the tribological behavior of sliding surfaces has been investigated by numerous investi- gators [1]. Much of the literature suggests adverse tribolog- ical and environmental effects caused by the wear debris. With regards to friction specifically, it has been shown that wear particles augment plowing of surfaces leading to higher coefficients of friction [2,3]. Several authors have developed models that predict the contribution of wear de- bris to the coefficient of friction [2–4]. These models have accounted for the size and number of wear particles at the interface, and other geometrical characteristics. The exper- imental evidence for agglomeration of wear particles was also presented by several researchers [5–7]. Recently, Oktay and Suh [8,9] have conducted experiments focusing on the agglomeration of wear particle in dry sliding. The experi- mental results have shown that wear particle agglomeration results in higher friction. The wear agglomerate is consisted of individual wear particles that are compacted under the normal load. It reaches a maximum size beyond which it collapses and a new cycle of particle build up starts over. ∗ Corresponding author. Tel.: +1-202-806-6622; fax: +1-202-783-1396. E-mail address: mmosleh@fac.howard.edu (M. Mosleh). The majority of works on the formation and the role of wear particles, however, have been performed in “geomet- rically unconstrained” or “open” sliding systems. The slid- ing surfaces in these systems can displace from each other to accommodate the entrapped wear particles as shown in Fig. 1a. Thus, the normal load on the slider remains the same as the initial normal load. By contrast, in a “geomet- rically constrained” or “closed” sliding system as depicted in Fig. 1b, the high stiffness of the system prevents appre- ciable separation of surfaces. The entrapment and growth of wear particles in a closed system can thus cause significant increase in the frictional resistance [10,11]. The seizure of sliding surfaces, especially the geomet- rically constrained ones, in engineering tribosystems is of great importance. It occurs when the mechanism suddenly comes to a stop due to a large increase in the resistance of surfaces to relative motion. This phenomenon was first ex- plained in terms of an increase in the real area of contact at the interface, followed by cold welding [12]. Another expla- nation was based on the extensive deformation of rubbing surfaces at the interface which results in interlocking of the moving bodies in contact [13]. The role of wear particles in seizure, especially in closed sliding system, was addressed later [14,15]. In a solution presented for galling seizure, it was proposed that the clearance of the system must be larger than the maximum size of the generated wear particles [16]. 0043-1648/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0043-1648(01)00583-X