Friction force lubrication properties

In 1988 a modified surface forces apparatus (SFA) was introduced [470,471] to analyze friction. The principle of operation of the SFA has already been introduced in Section 6.4. The modified version allowed a relative shearing of the two mica surfaces. In the SFA, the substrate has to have an atomically flat, transparent surface. In most cases mica is used to fulfill these requirements. Although there is a strong limitation in the choice of materials, due to the high resolution in the vertical direction, the SFA has become an important tool to study the friction and lubrication properties of molecularly thin films. 

This chapter and the two that follow are introduced at this time to illustrate some of the many extensive areas in which there are important applications of surface chemistry. Friction and lubrication as topics properly deserve mention in a textbook on surface chemistiy, partly because these subjects do involve surfaces directly and partly because many aspects of lubrication depend on the properties of surface films. The subject of adhesion is treated briefly in this chapter mainly because it, too, depends greatly on the behavior of surface films at a solid interface and also because friction and adhesion have some interrelations. Studies of the interaction between two solid surfaces, with or without an intervening liquid phase, have been stimulated in recent years by the development of equipment capable of the direct measurement of the forces between macroscopic bodies. 

Because fiber frictional properties are so important in the conversion of staple yams to spun yams, ASTM D2612 has been designed to measure the cohesive force encountered in the drafting or fiber alignment of sHver and top under static conditions. This frictional force is affected by surface lubrication, linear density, surface configuration, fiber length, and fiber crimp. 

In order to investigate the friction properties of lubricant film in TFL, an apparatus with a floating device was developed by Shen et al. as shown in Fig. 17 [48]. The steel ball is fixed so that it does not roll in the experiment and a pure sliding has been kept. The measuring system of micro-friction force is composed of a straining force sensor with a resolution of 5 yu,N, a dynamic electric resistance strain gage, an AD data-collecting card, and a computer. 

When the water film is squeezed out, the thick water layer is removed and the surfaces are separated by lubricant film of only molecular dimensions. Under these conditions, which are referred to as BL conditions, the very thin film of water is bonded to the substrate by very strong molecular adhesion forces and it has obviously lost its bulk fluid properties. The bulk viscosity of the water plays little or no part in the frictional behavior, which is influenced by the nature of the underlying surface. By comparing with the friction force of an elastomer sliding on a rigid surface in a dry state, Moore was able to conclude that for an elastomer sliding on a rigid surface under BL conditions, one can expect ... 

However, as RH further increases, tablet strength decreases for most tested tablets, and it was suggested that condensed water on the solid surface at high RH weakens intermolecular attraction forces between particles in the tablets and the further softened particles and solid bonds cause the tensile strength to decrease. " In addition, at high RH, water may form multilayers on the solid surface, which can act as a lubricant and reduce the frictional forces between particles, thus decreasing tensile strength as well. Ultimately, the effects of adsorbed moisture on particle surfaces are very complex, affected by many factors, especially the properties of the tablet excipients. 

A device, developed in our laboratory, which measured the coefficient of friction and the scuff load was employed to evaluate various lubricant systems. A strain gauge bridge was employed to measure the frictional force. The device used for the study is capable of resolving small differences in the lubricant properties. 

To characterize the lubricating properties of water for a variety of tribopairs, load- and speed-dependent frictional data have been measured. Frictional forces were measured as a function of load at a fixed sliding speed (at 0.005 m/s unless otherwise mentioned), and p, (p,=Flw, where F is friction and w is load) was measured as a function of speed (from 0.00025 to 0.1 m/s) at a fixed load (IN unless otherwise mentioned). For both types of measurement. 

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