摘要:

In order to determine the shear strength of boundary lubricants it is essential to use a substrate which is smooth on a molecular scale. An apparatus is described in which it is possible to apply normal and tangential loads to sheets of mica covered with monomolecular layers of boundary lubricant. The shear strength obtained from these experiments explains in part why frictional force and pickup are not reduced proportionately, in the presence of a boundary lubricant. The remaining difference is probably due to elastic hysteresis losses in the sliding solids. The surface energy of mica in air has also been measured by determining the force necessary to propagate a crack in the material. Cycles of opening and closing the crack have been performed and the difference in energy which is observed is attributed to the adsorption and migration of an interfacial film of air or water vapor. An electronmicrographic study of the structure of monolayers deposited by retraction from nonpolar solution is also described. This indicates that the area covered by the monolayer is only about ⅓ the total surface area. This poor coverage arises from incorporation of solvent molecules in the monolayer, which later evaporate.

ISSN:0021-8979    

DOI:10.1063/1.1728370

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

This paper is a review of some of the recent work which has been done in the Research Laboratory for the Physics and Chemistry of Solids, University of Cambridge. Three main topics are discussed: (a) The friction and wear of materials at very high rates of sliding. When a ball, spinning at high velocity, is brought into contact with a flat surface of a material such as bismuth, very rapid wear occurs as a result of melting on a large scale. For this to occur, the material should have a low melting point and a low thermal conductivity, as these properties together influence the rate at which the melted zone penetrates the solid. (b) Elastic hysteresis losses and rolling friction. A cylinder rolling over the surface of rubber causes the material under the roller to be subjected to a complex deformation cycle, partly torsion and partly tension. Experiments to investigate the hysteresis losses in such complex cycles are described and used in the interpretation of rolling friction results. (c) The effect of combined stresses and contamination on the growth of junctions between metal surfaces. The simple theory of friction treats the mean yield pressure and the maximum shear stress as independent strength properties. Plasticity theory suggests that the yielding of a junction should occur as the result of their combined action. The theory has been confirmed by experiment and used to explain the fact that small traces of contamination can reduce the very high values of friction observed with outgassed metals to normal values of 1 or 2.

ISSN:0021-8979    

DOI:10.1063/1.1728371

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The analysis of wear experiments suggests that most of the events which occur in rubbing are contacts between protuberances which are deformed elastically and which separate without damage; an asperity encounter with damage is a relatively rare event. Apparatus for the study of isolated single contacts is described. A single contact which is deformed elastically does not obey Amontons's law, but an assembly of such contacts (multiple contact conditions) should do so. It is shown that under multiple contact conditions the load which can be borne by elastic deformation of the protuberances may be as much as a million times larger than that which can be borne by each individual asperity contact. Reflection electron microscopy shows that many irregularities on worn surfaces must bear their share of the load without plastic flow. Recent experiments suggest that, although a worn particle is produced very infrequently, it is nevertheless the direct consequence of the many preceding encounters which occurred without apparent damage.

ISSN:0021-8979    

DOI:10.1063/1.1728372

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The results of rolling friction studies of several thermoplastics provide additional evidence for the correlation of such friction with dynamic mechanical losses in polymeric materials. Among the polymers discussed are polymethyl methacrylate, polytetrafluoroethylene, polyethylene, nylon, polyvinyl chloride, polyvinyl acetate, and polystyrene. The effects of spin and other deviations from pure rolling are demonstrated for polymethyl methacrylate, polytetrafluoroethylene, and nylon by varing the experimental parameters. In essentially pure rolling, the dependence of friction on temperature illustrates the importance of the extent of crystallinity in polytetrafluoroethylene, the amount of branching in polyethylene, and the concentration of plasticizer in polyvinyl chloride. In the latter effect, it is found that the temperature at which the rolling friction goes through a maximum varies linearly with plasticizer content.

ISSN:0021-8979    

DOI:10.1063/1.1728373

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A number of experiments were conducted on the metallic friction in reciprocating sliding with a pendulum. It was shown that the reciprocating sliding gives quite different results from those of single‐traverse or unidirectional repeated sliding, which were emphasized in both the coefficient of friction and the electric contact resistance, surface damage due to sliding, and the low friction phenomenon.

ISSN:0021-8979    

DOI:10.1063/1.1728374

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

A number of friction and wear phenomena are explicable in terms of the surface energy of adhesion of the contacting materials. In the friction field, it is found qualitatively that high friction coefficients are found for sliding materials with high surface energy/hardness ratios and conversely. Unfortunately, it is not easy to test this relationship quantitatively because the derived expression contains parameters which cannot be independently controlled. However, in the wear field, it has been found possible to derive an expression for the size of loose wear particles which can be readily tested; namely, that the average size of loose wear particles is proportional to the surface energy/hardness ratio, the nondimensional constant of proportionality being 60 000. Experiments with 15 different materials show the validity of this expression. Another phenomenon, adhesion, which also seems to be governed by surface energy considerations, is discussed in qualitative terms.

ISSN:0021-8979    

DOI:10.1063/1.1728375

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Previous work on the lubrication of polymers is reviewed and the explanations which have been advanced are examined and are shown to be inadequate. An alternative mechanism of polymer lubrication is proposed which is based on the hypothesis that when a lubricant is applied to a polymer it is possible for the lubricant molecules to penetrate the polymer and alter its mechanical properties. An investigation into the lubrication behavior of textile polymers is described and an explanation of the results of this and other investigations is offered in terms of the proposed mechanism.

ISSN:0021-8979    

DOI:10.1063/1.1728376

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Simple experiments made on a hard bitumen support the adhesion theory of friction. The static friction of the bitumen depends upon its rheological properties, and for hemispherical specimens, &mgr;sis determined by the shear strength and flow pressure of the bitumen, the latter being obtained from identation measurements. For nominally flat specimens, the time and temperature dependence of &mgr;scan be determined from indentation measurements. At low sliding speeds the dynamic friction can be related to the static friction, and to the rheological properties of the bitumen; at higher speeds frictional heating complicates interpretation of results.

ISSN:0021-8979    

DOI:10.1063/1.1728377

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

Several physical properties of graphite whiskers have been measured in ultra‐high vacuum and other controlled atmospheres. The properties which have been studied include elasticity, electrical resistance, and cohesion between whiskers. The atmosphere dependence of these properties suggests that surface adsorption of contaminants is the most important factor in producing changes in graphite friction. There is some evidence that the adsorbed gases reduce surface roughness on an atomic scale. This ability of the adsorbed film to smooth out surface asperities may be used to explain some of the friction and wear characteristics of graphite.

ISSN:0021-8979    

DOI:10.1063/1.1728378

出版商:AIP    

年代:1961

数据来源: AIP

摘要:

The electron spin resonance of one of the defects produced by fast neutron irradiation of crystalline quartz is analyzed. Thegtensor and hyperfine tensors deduced from these results imply that the defect electron is in a nonbonding tetrahedral hybrid orbital on a silicon. It is suggested that the instability of the lattice at high doses results in part from the presence of these defects.

ISSN:0021-8979    

DOI:10.1063/1.1728379

出版商:AIP    

年代:1961

数据来源: AIP