Ernest Rabinowicz

The Nature of the Static and Kinetic Coefficients of Friction

Experiments have been carried out to determine the transition between static and kinetic conditions when stationary metal surfaces are set into motion, a simple method being used which measures the energy that has to be given to one of the bodies to start it moving. The method is confined to cases in which the static coefficient exceeds the kinetic. Using a load of 1 kg and metal surfaces of various kinds, it is found that the static coefficient persists for distances of the order of 10−4 cm, and then gradually falls off to values corresponding to the kinetic coefficient. This behavior is shown to be consistent with a simple model based on the assumption that the friction force is needed to shear metallic junctions formed between the metal surfaces. The action of boundary lubricants is discussed, and it is shown that they can act either by diminishing the metallic interaction directly, or by preventing its increase during the sliding process.

Effect of abrasive particle size on wear

Abstract Previous experiments have shown that, for any system, there exists a minimum abrasive particle size which allows maximum abrasive action. It is suggested that this critical abrasive size corresponds to the size of adhesive wear fragments of the material being abraded. Experiments with lubricated copper and nylon surfaces show reasonably good agreement between the postulated and measured critical abrasive sizes. Disturbing effects, such as severe adhesive wear and take-up of abrasive during sliding, are discussed.

Influence of Surface Energy on Friction and Wear Phenomena

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.

The Determination of the Compatibility of Metals through Static Friction Tests

Static friction coefficients were measured for pairs consisting of 20 elemental metals. The 210 friction values were compared against the metallurgical compatibility ratings of the corresponding two phase alloy systems. Soft metals gave higher friction than harder metals, and at the same hardness level there was a steady trend to lower friction in going from like, to compatible, to intermediate, and then to incompatible combinations. The results are interpreted in terms of the surface energy model of friction, and used to derive values for the surface energy of adhesion of metal combinations in terms of their compatibility rating. Presented as an American Society of Lubrication Engineers paper at the ASME/ASLE Lubrication Conference held in Cincinnati, Ohio, October 13–15, 1970

The least wear

Abstract Of the four principal types of wear, adhesive wear is the only one which can never be eliminated. The three types of adhesive wear, namely severe wear, moderate wear and burnishing, are described, and the transitions between them are discussed. Burnishing, or material removal on a molecular scale, represents the least possible amount of adhesive wear, but we know little regarding the magnitude of the wear rate and methods of ensuring that a sliding system will operate in the burnishing regime. This is unfortunate because, for many sliding systems, especially those using unlubricated surfaces, there is no likelihood of achieving an acceptable life unless operation of the sliding surfaces in the burnishing regime can be assured.

Lubrication of Metal Surfaces by Oxide Films

Experimental data obtained with sixteen metals suggest that hexagonal structured metals have low friction, while other metals have high friction unless a lubricating oxide layer is farmed. In order for an oxide to lubricate, it must not be much harder than the substrate, and the oxide thickness must be adequate. For pin-an-desk tests at loads of about 1 kg, the critical oxide thickness is 10−6 cm, and the metals must be heated until this thickness is reached. Other solid lubricant films appear to obey the same criteria of not 100 hard, not too thin.

The Nature of the Coefficient of Friction

The modern theory of the friction between dry metal surfaces ascribes it to local minute welds or adhesions between the surfaces and suggests that for a given pair of surfaces the friction force is uniquely defined by the normal load alone. Herein it is demonstrated that this cannot in general be true and that some further condition of operation must also be defined. Experiments are reported indicating that one such possible condition is the sliding speed so that the friction force is actually a function of the normal load and the sliding speed. It is pointed out that the speed can influence the friction force in two ways—one, by the resulting shear strain rate in the vicinity of the welded junction, and the other by the length of time taken for a junction of full strength to form.

Variation of Friction and Wear of Solid Lubricant Films with Film Thickness

For solid lubricant films such as oxides, soft metals and molybdenum disulfide in a resin low friction and low wear have been found when the film thickness lies between 10−6 and 10−2 cm, and high friction and high wear outside these limits. At the 10−3−cm limit, the rise in friction occurs because the load is carried by the film rather than by the substrate, while the rise in wear is caused by the formation of large wear particles. The experimental data appear to agree reasonably well with theoretical predictions based on the concepts of characteristic junction size and wear particle size. Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Minneapolis, Minnesota, October 18–20, 1966.

Practical uses of the surface energy criterion

Abstract The ratio of surface energy to hardness (the W p ratio) of sliding surfaces has the dimensions of length, and many length parameters of sliding systems, including the diameters of loose and of adherent wear particles, the equilibrium peak-to-trough surface roughness, the diameter of junctions, and the size of the smallest abrasive particles which give full abrasive action, are shown to be proportional to W p . At very low loads, when the real area of contact has a diameter smaller than that of the wear particles for that system, new effects are encountered, leading to low wear rates and the production of smooth surfaces.

Friction seizure and galling seizure

Abstract Two different factors, not clearly distinguished in the literature, cause sliding surfaces to seize. Friction seizure occurs when the driving member provides insufficient force to overcome frictional resistance, and no permanent surface damage occurs. Galling seizure occurs when, during sliding, the surface roughening and the formation of wear particles eliminates the clearance within a sliding component and brings on press-fit conditions. This type of seizure is catastrophic and irreversible. Practical experience indicates that galling seizure can be prevented if the clearance is at least three times the average diameter of wear particles formed during sliding.