Mohsen Mosleh

Tribology of polyethylene homocomposites

Abstract The generation of wear particles of ultrahigh molecular weight polyethylene (UHMWPE) in knee-joint prostheses results in severe plowing which increases friction and wear, and elicits an environmental response in biological systems. The large plastic deformation of the polyethylene surface layer under cyclic frictional loading, which is the origin of crack nucleation and propagation and ultimately that of wear particle generation, can be reduced by introducing high-strength fibers in the material perpendicular to the sliding direction. A polyethylene ‘homocomposite’ consisting of an ultrahigh molecular weight polyethylene matrix and an ultrahigh molecular weight polyethylene reinforcing elements has been introduced to reduce friction and wear. Since the homocomposite is UHMWPE-based, both fiber and matrix of this type of composite are biocompatible. The homocomposite has shown a friction coefficient of about 0.1 in dry sliding and its wear is an order of magnitude less than that of ultrahigh molecular weight polyethylene.

Manufacture and properties of a polyethylene homocomposite

A conventional composite consists of a matrix and a reinforcing phase differing in chemical composition, with the latter having superior mechanical properties. This paper presents the manufacture and mechanical properties of a homocomposite consisting of an ultra-high molecular weight polyethylene matrix and an ultra-high molecular weight polyethylene reinforcing phase. While the chemical compositions of the bulk and fibers are the same, the fibers have a higher strength and a higher melting temperature due to their high degree of molecular orientation. The homocomposite was manufactured by alternatively laying up plys of an ultra-high molecular weight polyethylene fabric or randomly oriented fibers and ultra-high molecular weight resin in a mold followed by compression molding. The molding temperature was chosen to be slightly higher than the melting temperature of the matrix, but below that of the fabric or fibers. The fibers were also crosslinked using gamma irradiation so as to retain their molecular orientation during compression molding. Mechanical properties of the homocomposite such as the elastic modulus, tensile strength, and hardness are improved in a direction parallel to the fiber orientation, when compared with the properties of ultra-high molecular weight polyethylene. The homocomposite also exhibits a lower friction coefficient and higher wear resistance, which are desirable properties in bearing applications. In addition, owing to the biocompatibility of polyethylene, the homocomposite may be acceptable as a bearing material in joint prostheses.

A mechanism of high friction in dry sliding bearings

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.

A Surface Texturing Approach for Cleaner Disc Brakes

Brake dust generated in vehicle brakes causes discoloration of wheels and, more importantly, the emission of particles suspected of health hazard in the environment. Laboratory testing of brake pad materials against cast iron discs revealed that the majority of wear particles are submicrometer in size. Wear particles with a size of 350 nm had the highest percentage in the particle size distribution plots, regardless of the magnitude of the nominal contact pressure and the sliding speed. Due to their predominantly submicrometer size, a significant amount of brake dust particles may be inhalable in environmental and occupational exposure situations. In order to prevent the dispersion of particles into surrounding environments, a surface topography design with radial microgrooves on the disc was utilized to trap wear particles immediately after their formation. The particles moved radially outward in the microgrooves due to the centrifugal force and were collected in a reservoir. The design yielded a cleaner pad/disc test setup, a smoother friction coefficient plot, and a reduced wear volume. The steady-state friction coefficient remained the same as the one obtained from the control samples with a non-modified design. Presented at the ASME/STLE Tribology Conference in Logn Beach, California October 24-27, 2004 Review led by Roger Melley

Friction of Undulated Surfaces Coated with MoS2 by Pulsed Laser Deposition

Solid lubricants such as molybdenum disulfide can provide very low friction, but their effectiveness especially in the geometrically constrained sliding pairs is limited by plowing of coated surfaces by wear particles. Even in the presence of solid lubricants wear particles cause higher friction by plowing the interface. To minimize plowing, undulated surfaces with microgrooves perpendicular to the sliding direction can be used to trap wear particles. Smooth and undulated stainless steel surfaces were coated with molybdenum disulfide by pulsed laser deposition (PLD) and friction tested. Under identical test conditions, the friction coefficient of coated undulated surfaces is between 20–40 percent lower than that of coated smooth surfaces. The friction coefficient of undulated uncoated surfaces is about 100–350 percent less than that of smooth uncoated surfaces. Moreover, the entrapment of wear particles at the interfaces of geometrically constrained bearings may lead to seizure even when bearing surfaces ...

Deagglomeration of Transfer Film in Metal Contacts Using Nanolubricants

Experimental results presented in this article reveal the ability of nanoparticle-dispersed engine oil (nanolubricant) to deagglomerate metallic transfer films under extreme pressures. Specifically, metallic transfer films were formed and accumulated on the upper AISI 52100 ball in a four-ball tester, which resulted in negative wear—that is, weight gain—under Hertzian contact pressures ranging from 3.4 to 5.5 GPa. When molybdenum disulfide nanoparticles were dispersed in the engine oil, the AISI 52100 upper balls exhibited wear as expected. Scanning electron microscope examination of worn surfaces indicated large patches of metallic transfer films on the ring-shaped wear track of AISI 52100 upper balls tested with the engine oil. Such large patches of transfer films could not be found on the surfaces tested with the nanoparticle-modified engine oil. The experimental results and the mechanism of transfer film deagglomeration are discussed.

Solid Lubricant Nanoparticles as Wear-Reducing Additives in Sheet Metal Forming Fluids

Dispersed nanoparticles of solid lubricants in sheet metal forming fluids are studied for enhanced lubrication that can lead to improved product surface quality and reduced tool wear. Molybdenum disulfide (MoS2 ) and hexagonal boron nitride (hBN) nanoparticles of varying size and concentrations have shown marked reduction in wear of steel counterfaces representing the tool and in scoring of titanium sheet surfaces. The most effective particle concentration and size ranges were 0.25–4% and 70–100 nm. The counterface wear was reduced by 50–75% while the friction coefficient only marginally improved.Copyright