The Wear of Metals under Unlubricated Conditions

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  • The Wear of Metals under Unlubricated ConditionsAuthor(s): J. F. Archard and W. HirstSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 236, No. 1206 (Aug. 2, 1956), pp. 397-410Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/99967 .Accessed: 05/05/2014 07:44

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  • The wear of metals under unlubricated conditions

    BY J. F. ARCHARD AND W. HIRST

    Research Laboratory, Associated Electrical Industries Limited, Aldermaston, Berks

    (Communicated by T. E. Allibone, F.R.S.-Received 25 February 1956- Revised 3 April 1956)

    [Plate 16]

    The wear of a wide range of material combinations has been studied in unlubricated con- ditions. Loads of 50 g to 10 Kg and speeds of 2 to 660 cm/s have been used. A representative selection of the results is given. As a broad classification two contrasting mechanisms of wear have been observed. In nearly all experiments, and for all types of wear mechanism, once equilibrium surface conditions are established the wear rate is independent of the apparent area of contact. The wear rate is accurately proportional to the load for only a limited number of combinations but there are many other examples for which the relation between wear rate and load shows only a small deviation from direct proportionality. It is suggested that with the same surface conditions the wear rate is proportional to the load; in practice this simple relation is modified because the surface conditions depend on the load. These rules of wear may be derived, on a priori grounds, from the experimental results, or from more detailed theoretical calculations.

    1. INTRODUCTION

    Attempts have recently been made to develop a theoretical basis to explain the wear of materials, the ideas used being analogous to those on which the generally accepted theory of friction is founded (Bowden & Tabor I950). It is supposed that the real area of contact between two objects is far less than the apparent area and is determined by the extent of the deformation of the touching surface asperities under the applied load. There will be a number of local regions of true contact whose location will change during sliding and, in consequence, sliding gives rise to local encounters between small regions of the rubbing surfaces. The basis of these theories of wear is to relate the wear to the number and nature of the local en- counters but in some theories the unit event leading to the production of a wear particle is regarded as an encounter between two atoms, one on each of the opposing surfaces (Holm I946, I953; Burwell & Strang I 92a, b; Burwell I953), whereas in another theory it is the encounter between two surface asperities (Archard I952, I953 a, b). If the surface asperities deform plastically under the applied load and it is assumed that at each unit event there is a definite probability that a wear particle will be produced, the theories lead to the relation

    W = KPlpm, (1)

    where W is the worn volume, s is the sliding distance, P is the applied load, Pm is the flow pressure of the softer material, and K is a constant related to the prob- ability per unit encounter of production of a wear particle. It will be seen that these theories predict that there should be two simple rules of wear, i.e. that the wear rate is independent of the apparent area of contact and that it is directly proportional to the applied load.

    26 [ 397 ] Vo1. 236. A.

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  • 398 J. F. Archard and W. Hirst

    These rules are analogous to Amonton's laws of friction, and there is now a certain amount of evidence that they are obeyed in simplified conditions of rubbing. Thus, Burwell & Strang (I 952 a, b) have measured the wear of steels and some other metals at slow speeds using cetane as lubricant. The dependence of wear rate upon load and pressure was determined and they concluded that the wear rate is proportional to the load and independent of pressure unless the mean surface stress exceeds a value equivalent to one-third of the hardness of the material. One of the authors (Archard I953 ) has also reported that the wear rates of some materials vary linearly with the applied load and are independent of pressure over a wide range. Because both K and p, may be expected to depend upon the material, the theory makes no direct prediction about the variation of wear rate with hardness. However, Hughes & Spurr (I955) have observed an inverse relationship between wear rate and hardness-which is predicted when K is a constant-when measuring the wear rate of a wax whose hardness they altered by changing its temperature. Krushchov & Babichev (I953) have measured the wear of a range of metals when rubbing against emery cloth, special precautions being taken to ensure reproducibility of the results. They find that, under these conditions, the wear rate of different materials is inversely proportional to their hardness, an exception to this general rule being found for heat-treated steels.

    These simple results are in marked constrast to those of the majority of wear experiments reported in the literature; these suggest that wear is dependent on a large number of variables and there is no general agreement about how the wear depends on such quantities as the load, speed and apparent area of contact. It may be that some of these results can be discounted because the experiments were carried out under ill-defined conditions designed primarily to simulate the operating conditions of a particular piece of machinery. Moreover, the majority of experi- ments have been carried out using lubricated conditions where it is generally found that the wear rate decreases with time. This is usually because the condition of hydrodynamic lubrication is being approached so that in the limit the surfaces separate and wear, which is due to rubbing, ceases (Kenyon I946). However, even after considering these factors, one still gains the impression from the literature that rules of wear will not apply generally.

    The present work was planned to extend the experimental work mentioned earlier. The experiments were made in unlubricated conditions and a wide range of materials, chosen largely at random, was examined to find out to what extent the same rules of wear apply to all materials. In this paper it is possible to quote only a limited selection of the results; these have been chosen to illustrate the conclusions drawn from several hundred experiments using different combinations of materials, loads and speeds.

    It was found as a fairly general rule that the wear rate was independent of the apparent area of contact, and usually reasonable explanations could be advanced when this did not occur. Also, although the wear rate often increased roughly proportionally with the load, this was accurately true for only a limited number of cases. These results are discussed and an attempt made to define general rules of wear. These rules may be derived, on a priori grounds, from the experimental results,

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  • Wear of metals 399

    or from more detailed theoretical considerations. The first derivation requires few assumptions, whereas the latter involves theoretical postulates which require experimental verification.

    2. APPARATUS

    Two pin-and-ring machines have been used. The essential details, common to both machines, are shown in figure 1 (a). A ring B of us in. (2 38 cm) diameter was mounted on a shaft A and in the majority of the experiments it rotated at approxi- mately 1500 rev/min, the surface speed being about 180 cm/s (370 ft./min). A flat- ended 4 in. (0.635 cm) diameter pin C was pressed under a load P against the cir- cumference of the ring. Three such pins and rings were carried on each machine. On one machine it was possible to measure the frictional force and thus determine the coefficient of friction.

    iP

    (b)

    FIGup.E 1. Schematic diagram of two arrangements of pin and ring wear machine. A, rotating shaft; B, ring 15 in. diam.; C, W in. diam. pin pressed under load P against ring.

    The surfaces of the rings were usually prepared on a universal grinding machine; when a finer surface finish was required they were subsequently honed by the Superfinishing process. The ends of the pins were lapped with 240 Carborundum on a cast-iron lap and finished with 320 Carborundum on a lead lap. The pins and rings were cleaned