Asperity Tips Research Articles

Wear characteristics of some metals in relation to surface temperature

Tests on En1A and En3B steels and Nimonic 75 alloy, in dry sliding on a pin-disc test rig under conditions giving rise to high temperatures, show changes in wear behaviour which may be determined from the properties of the metal and its oxides at the total temperature of the asperities. These changes, which are also dependent on load and sliding speed, follow similar patterns for each material, the main feature being a reversal in the effect of speed changes on wear rate when the temperature, at which the asperity tips begin to soften, is exceeded.

The Effect of Road Surface Texture on Tire Wear

Abstract (1) This experimental work has shown that there is a large variation in the degree of tire wear on different surfaces, the wear on the rough, harsh surface being approximately three times as severe as that on the rough polished surface. (2) Measurement of weight loss by the method described provides a sensitive measurement of tire wear. (3) Qualitative analysis has shown that it is the harshness of the surface which is of major importance in tire wear, with surface roughness as a slightly modifying factor. (4) Quantitative analysis of various parameters indicating surface characteristics has shown that the shape of the asperity tips and the low-speed, wet coefficient of friction are major factors. These can be used to predict wear to a high degree of significance within the range of values covered by these measurements, although a large sample of surfaces would be desirable to check this relationship further. (5) A decrease in the severity of wear of six times involving a drop in speed from 50 km/h to 25 km/h has not altered the relative wear ratings of the three surfaces which have been compared and it seems reasonable to assume that the relative wear rates, although not the absolute values, would be the same in normal conditions of use. Consequently, the increased use of rough road surfaces is not expected materially to affect tire wear. (6) Significant increase in tire life from a wear standpoint can be expected from the use of radial ply tires instead of cross ply tires.

A Comparison of Surface Temperature Theories and Experimental Resultsfor High Speed Dry Sliding

Tests are described in which an SAE 1113 steel pin is rubbed on a similar disc at sliding speeds up to 100 m/s and loads up to 7 kgf. Measured pin sub-surface temperatures are used to predict total surface temperatures by considering the heat flow through an idealised single asperity tip, and from the existing theories of Jaeger, Archard and Barber. The frictional coefficient and transitions in the sliding process are shown to be related to the predicted total temperatures. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference held in Cincinnati, Ohio, October 13–15, 1970

Electrical resistance of silver-silver contacts under vibrational loads

Abstract Accelerated life tests on silver-silver contacts vibrated at 500 Hz were conducted. When, for a period of 12 h, the applied force was 10 + 5 sin (2 π 500 t ) N, the electrical contact resistance decreased, but the reverse occurred when the applied force was 1+0.5 sin (2 π 500 t ) N. These phenomena are attributed respectively to increasing compliance and work hardening of the asperity tips. After sufficiently long periods ( ~100 h) the resistances of all the tested contacts between these soft surfaces decreased compared with their initial values, and the eventual static load-unload curves obtained after completion of the vibration exhibited no hystereses. No changes in the c.l.a. surface roughnesses of the contacting surfaces were detected even though large changes in the electrical contact resistance occurred.

切削初期現象に及ぼす工具切れ刃先端半径と被削材表面層の影響について

A phenomenon in the beginning of cutting is classified rubbing region and ploughing region. In the rubbing region the deformation of bulk is elastic though tips of surface asperities are deformed plastically. It is obtained that the phenomenon is influenced by many foctors, especially by the tool-edge roundness.In this study, in order to find the factors influencing the beginning phenomenon of cutting, an experiment was carried out on low speed orthogonal-cutting with a gradual increasing depth of cut.As the results, there is a critical normal force which has a linear relation to the tool-edge roundness at the stage when the rubbing phenomenon changes into ploughing phenomenon, and the rubbing phenomenon is influenced by the machined surface layer of workpiece. Then, from the distribution of shear-stress, it is suggested that the transition to ploughing occures because the yielding of work-material developes in neighbourhood of the boundary region (the bulk and the surface layer).

Behaviour of multilayer stacks of thin layers under dynamic loads

Although considerable attention has been given to solids in static contact, very little information is available concerning vibrated contacts. Stacks of thin metallic layers were subjected to high and low dynamic loads (⩽ 156 + 1290 sin 2 (2800 πt) N, t being in min). Freshly assembled stacks show an initial fall in resistance, (which was not found for run-in stacks), the rate of fall being greater the harder the material. For run-in stacks under high dynamic loads, the asperity tips tended to shear, resulting in a resistance rise, while under low dynamic loads the asperities tended to deform so that the resistance decreased. Also under low dynamic loads, an exponential rate of fall of the electrical resistance of stacks of thin layers of soft material has been observed.

Solids in static contact

The present review emphasizes that the load index in the relationship between load and area of contact may be independent of whether the deformation is elastic or plastic. The deformation process may be shown to be a function of the surface topography and the strength characteristics of the surfaces considered. Criteria for the transition from elastic to plastic deformation of the asperity tips are discussed. A review is presented of optical, stylus, electrical and thermal methods for obtaining a measure of the real area of contact. The Wiedemann-Franz “law” relating electrical and thermal resistance does not apply for metal-metal contacts.

Lubricated Friction of Rubber. III. Components of Friction in Mating Surfaces

Abstract Friction is usually discussed in terms of contributions from two processes: deformation and adhesion. This terminology has been retained in discussions of lubricated friction of rubber although there is not apparent any clear mechanism by which adhesion might occur. Our study of wet friction leads us to suggest that the components making up frictional resistance to sliding might more appropriately be referred to as deformation and abrasive components. On this basis, there is an obvious mechanism for both processes, and some of the disagreements about interpretation of certain experimental results may be resolved. The basis for our interpretation is implicit in the literature but was originally expressed somewhat tentatively and has not been restated in recent discussions. Greenwood and Tabor pointed out that hysteretic losses make an important contribution to friction of rubber sliding on a lubricated surface and that this contribution is independent of asperity size or distribution so long as only the asperities support the load and the radius of curvature at the asperity tip is above a critical limit which is remarkably low. Further, the experiments of Sabey showed that it was difficult to obtain coefficients higher than 0.4 with spherical sliders. Mean average local pressures high enough to give high friction coefficients could be obtained only with sharp asperities. In the laboratory experiment these were cones. Greenwood and Tabor found that with these damage to the rubber becomes visible at just the semiangle at which the coefficient began to rise abruptly (Figure 1). It is entirely reasonable to call the process causing this damage abrasive friction.

A theory of viscous hydroplaning

The traction developed in the contact area of a pneumatic tyre rolling and accelerating on a wet road surface is simulated by a block of rubber slipping on a two-dimensional sinusoidal surface covered with a thin water film. The resulting pressure-wedge formation on individual asperities of the surface provides a net uplift which tends to force tyre-tread and surface apart, thus promoting the onset of skidding or viscous hydroplaning. The important factor in minimizing skidding hazards is the provision of an adequate micro-roughness at asperity tips which contributes an adhesional mechanism to the overall friction coefficient. It is shown that in comparison with the adhesional contribution to friction, the contribution of hysteresis and viscous shear of the film are insignificant at low slip speeds.

Thermal aspects of galling of dry metallic surfaces in sliding contact

The phenomenon of galling or seizing of metals is believed to be, in general, a function of the thermal and mechanical conditions under which the metallic surfaces are rubbed together. For polished surfaces, under ideal dynamic conditions, i.e., conditions where the surfaces are devoid of appreciable oscillations in the direction normal to the surfaces, thermal aspects of galling predominate. Galling due to thermal conditions is viewed as that condition where the tips of surface asperities weld together and are then sheared apart according to the weld-junction or adhesion theories. Welding may be achieved when the melting temperature is reached; also when surface asperities, which are under a high degree of strain, are in sliding contact. Welding may be achieved by recrystallization recovery. The latter process is taken as the governing condition for thermal galling since the recrystallization temperature is only a fraction of the melting temperature. The galling criterion, relating normal load, velocity, and time of rubbing, is established theoretically. The result compares favorably with experience.