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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.
E. Rabinowicz (Tue,) studied this question.