Metal Transfer and Wear

Abstract

These thoughts are offered as a reminder that Tribology is not all about the normal contact of fractal surfaces, and indeed, not all about elastic contact of rubber and polymers, or even about dry contact. Machines do still contain metal surfaces sliding past each other, hopefully separated by an oil film; and sometimes, when tolerances have been pushed too far, or running with starved lubrication when the oil or grease supply is inadequate, with some metal to metal contact. Fortunately this is not always disastrous: surfaces do often run-in, so that after running with contact and a contribution of dry-contact friction, there is steady wear and contacts no longer occur. The traditional design criterion for gears and ball races was, and still is, the Λ − ratio: the ratio of the predicted film thickness for smooth surfaces to the rms roughness. Certainly a Λ − ratio of 3 or more¹ usually leads to full-film lubrication: but to anyone with the slightest background in surface roughness this is an absurd rule. Assuming, as is usually done, that the predicted smooth-surface film thickness refers to the distance between the mean planes of the roughness, the rms roughness says nothing about the how much contact there will be. And if running-in is successful, and the high points of the surface wear away, the rms (and the Λ − ratio) may hardly change, but there will be successful operation. But when will running-in be successful? What determines when instead of running-in there will be scuffing, and disaster? The traditional picture of the “mixed friction” regime is that when the local film thickness falls to zero, additives (or perhaps happy accidents) provide a boundary lubricant in the oil: some form of long chain polymer, which has a reactive end which attaches itself to the metal, and carries the load on its free ends: with low friction but, more importantly, preventing metal to metal contact. The Blok scuffing criterion was that the maximum surface temperature must be below a specific value: and there was the problem, what should it be? In Bowden and Tabor's laboratory experiments, using a known, pure, organic compound, clear links with the known properties could be found; but in engineering practice perhaps all that can be done is to ensure that the calculated maximum temperature in a new application is no more than in an existing application: the ISO guide concentrates on the temperature calculation, not on the temperature found. But what happens when boundary lubrication fails? Fortunately it seems that we do not move completely into the dry wear scenario. The failure will usually be local, and the dry wear process interrupted. An earlier work (Sakmann et al., 1944) reported that in a pin on disc experiment, flooding the surface with a plain mineral oil halved the transfer at a light load, but produced only a small reduction at a heavier load. But flooding with oleic acid largely eliminated transfer. Here it seems desirable to review what has been learnt about dry wear, and perhaps, forgotten. The obvious starting point is the “Archard” wear equation. This was predicted by Holm in 1938 [Holm (1938)], by postulating that for every encounter of a pair of atoms (within the contact area found as W/p_m) there was a fixed probability of one being pulled out of its parent surface. Detailed experimental confirmation was provided by Burwell and Strang (1952), but from electron micrographs of transfer particles they argue that the unit event is the encounter of two asperities. Both models predict that the volume of wear V is proportional to the distance slid L and the load W, and inversely proportional to the hardness p_m: V = k · L · (W/p_m). Archard's contribution was to show that it is not necessary to assume that the average size of the contact areas or wear particles is constant, and to calculate the probabilities k implied by the results of all the available experimental wear combinations … and to go on to contribute to the great wear research program of Hirst's group at AEI (Associated Electrical Industries) Aldermaston (see Archard, 1953; Kerridge, 1955, etc.). The natural meaning of the term “wear” is the weight, or volume, lost from the device concerned: and early researchers merely noted that this could become either transfer particles attached to the “wrong” partner, or loose wear debris. The important distinction between transfer and wear was first made when Kerridge (1955) found that when a (radioactive) steel pin was loaded against a rotating hard steel ring (“hollow drum” perhaps conveys the picture), a radioactive transfer layer built up on the ring, but the radioactivity (and therefore the amount of transfer) then became constant. When the active pin was replaced by an inactive one, the activity reduced, mirroring the path of the increase: it was not that the transfer layer had a maximum size, and could build up no further, but that a steady state had been reached where the transfer to the ring equaled the rate of loss from the ring. At this point the pin wear rate fell to the steady rate required by the wear law. The wear fragments were carefully collected and monitored, little radioactivity being found at first, but ultimately matching the wear rate of the pin: and consisting of relatively large, oxidized, particles. The detachment of the transfer layer, and so presumably its oxidation rate, was the rate-determining process. Experiments in air at 10⁻³ mm mercury found the wear rate reduced to a tenth (or lower at low loads) of the atmospheric value, confirming this. Thus, for this combination, wear is a...