Lamellar lubrication in vivo and vitro: Friction testing of hexagonal boron nitride

Abstract

Phospholipid molecules (PLs) in vivo and graphite, molybdenum disulfide, tungsten disulfide and hexagonal boron nitride (h-BN) in vitro are good examples of frictionless lubricants. PLs and solid materials have the ability to form multi-bilayer or layered structures similar to lamellate solid. It has been confirmed experimentally that PLs as lamellar lubricants protect the surface of joints against wear while acting as frictionless lubricant. An experimental study has been conducted on the friction properties of h-boron nitride on porous non-full journal bearings. The porous non-full journal bearings were a mixture of 97.5 wt.% Fe and 2.5 wt.% Cu powder, and compressed to a density of 5.9 g/cm³. The porosity of non-full journal bearings were15.5 and 27.8 wt.% and were impregnated with vaseline and vaseline + 5 wt.% h-BN. Additionally, the two additives SFR NLGI #2 (or SFR 2522) grease and graphite grease were used for comparison to h-BN. The tribological tests were performed on a four-ball machine under load of 49 daN, and a friction tribotester. The above experiment strongly suggested that h-BN has the ability to lubricate under load with very low friction coefficient comparable to phospholipids. Relatively low surface energy and low adhesion between the crystallites are giving the additives low friction coefficient. The results of the experimental studies showed that h-BN as an additive in vaseline possesses friction reducing properties, and excellent anti-wear properties.

Introduction

Lamellar structures are composed of fine, alternating layers of different materials in the form of lamellae. Lamella is a term for a platelike structure appearing in multiples that occur in various situations, such as biology or material sciences, it implies a thin layer (or layers) (Hills, 1989, Hills, 1992, Hills, 2002). The ability of a solid to function as a lubricant is determined by the degree of the attraction of its molecules to each other and the sliding surfaces. As a rule, solid lubricant films are more superior to liquid film and provide better surface coverage (Rabinovicz, 1995).

Phospholipids are natural compounds in living systems. The selective properties of phospholipids in biological interactions are formation of bilayers and membranes. When in articular cartilage the two multi-bilayers rub against each other as opposing hydrophilic charged surfaces with the electric double-layers resulting in repulsive electrostatic forces, and in the presence of water and macromolecules, e.g., lubricin, are capable of lubricating with low friction forces (Israelachvili and Wennerstrom, 1996).

Articular cartilage is a highly structured biological tissue and its fluid-saturated pororelastic nature has been severally discussed. Research over the last decade has indicated the presence of a surface amorphous layer (SAL) overlaying the articular surface of the healthy articular cartilage in the normal joint (Forster and Fisher, 1999, Graindorge et al., 2005, Graindorge et al., 2006, Jurvelin et al., 1996, Kobayashi et al., 1996). This amorphous surface layer is significantly softer than the underlying bulk cartilage. The surface amorphous layer has variously been claimed to comprise glycolproteins, glycos-aminoglycans, proteoglycans, hyaluronic acid and phospholipids (Hills, 1989, Guerra et al., 1996). The tendency of phospholipids to form multilamellar structures, i.e., the three-bilayer membrane on articular surfaces is an important factor in enabling frictionless work under load (Hills, 1989, Mow et al., 1984). The friction coefficient (f) determined for the articular surface is consistent with f = 0.06 that characterizes “lamellated solid lubrication” such as that found in graphite and MoS₂. The lamellar structure multi-bilayer is similar to graphite, molybdenum disulfide and hexagonal boron nitride (Deacon and Goodman, 1958, Martin et al., 1992, Ladaviere et al., 2003). A lamellar solid material has a layered structure in which the bonds between the layers are much weaker than the interlayer bonding. The individual platelike crystallites, consisting of several thousands of atomic layers, align themselves parallel to the direction of relative motion and slide over one another. Boron compounds and other lamellar solid lubricants such as MoS₂, WS₂, graphite, and h-BN when used in lubricating oils and greases achieve low friction and wear (Erdemir, 1991, Pawlak et al., submitted for publication, Rapoport et al., 2002). Recent experiments with porous bearings, in which boron nitride (h-BN) was used as an additive, showed very low friction coefficient and slow wear rate. It is well-known, that h-BN is a softer phase bonded through localized graphite-like sp² hybridization in hexagonal plane and delocalized weak π-orbital (Mosuang and Lowther, 2002). The structure of h-BN is constructed from layers consisting of a flat or nearly flat network of B₃N₃ hexagons and the layers stacked one over the other along the [0 0 1] direction.

In this paper, we investigated the friction forces of hexagonal boron nitride (h-BN) as the lamellar additive in vaseline using four-ball machine and friction tester. The porous non-full journal bearing was under PV = 3.84 MPa m/s (pressure–velocity) parameter lubricated by h-BN additive is tested for frictional and wear characteristics. In this study, the friction force was measured under various conditions of the porous-bearings using for comparison two well-known additives, namely, graphite grease and SFR NLGI #2 grease. The main part of this work was to evaluate and compare the lamellar friction due to contact between biological and engineered surfaces.