Archard

Archard J F - Mechanical Polishing of Metals

Thursday, March 17, 2005 4:48 PM

Reprinted from Phys. Bull 36(1985) ; IOP

MECHANICAL POLISHING OF METALS- A scientific argument of long standing J F Archard Theories of mechanical polishing are of long standing; a full history of the subject would include many of the great names of classical physics, for instance Hooke and Newton. In preparing a critical review of the subject for an Institute of Physics meeting last year, I noted a second interesting feature: this is that differences of opinion which occurred a century or more ago still exist in essentially the same form today, Consider the following quotations; '. . . the process of polishing is in fact nothing more than the grinding down of large asperities into small ones by gritty powders which . . . are yet vast masses in comparison with the ultimate molecules of matter. A surface polished artificially must bear somewhat the same kind of relation to the surface of a liquid, or a crystal, that a ploughed field does to the most deliberately polished mirror, the work of human hands.' (Herschel, 1830) '. . . it seems probable that no pits are formed by the breaking out of fragments but that the material is worn away ... almost molecularly . . . one is inevitably led to the conclusion that no coherent fragments containing a large number of molecules are broken out. If this be so there would be much less difference than Herschel thought between the surfaces of a polished solid and a liquid.' (Lord Rayleigh, 1901) 'Firstly, polished surfaces always consist of fine grooves. Secondly material is removed at a significant rate during polishing.

Thirdly, a plastically deformed layer is produced . . . Polishing differs from abrasion only in degree on all these counts.' (Samuels, 1967) 'The concept of polishing by removal of material on a molecular scale is in accord with theory and experiment.' (Rabinowicz, 1970) 'Plus รงa change, plus c'est la mime chose.' (A Karr, 1849).

Between the times represented by the above quotations, ideas about the mechanism of polishing have ebbed and flowed and have incorporated a number of different theories. The elements of this history which I regard as most crucial are summarized in Table 1.

Table 1 - Brief History of theories of polishing Originator

Theory

Hooke

Fine Abrasion

Newton

Elaboration of Hooke's Ideas

Herschel

Fine Abrasion of asperities (see quotation)

Lord Raleigh (1901)

Abrasion carried to finest limit, i.e. molecular removal (see quotation)

Beilby (1921)

Surface flow by plastic deformation

Bowden & Hughes (1937)

Beilby process with flow by surface melting (see Bowden & Tabor, 1950)

Samuels (1967)

Metallographic polishing as fine scale abrasion (see quotation)

Rabinowicz (1968)

Removal of material on a molecular scale (see quotation)

Figure 1 The Beilby theory of polishing: a) abraded surface - flow of material in polishing shown; b) surface after polishing -Beilby layer shown shaded; c) surface after subsequent etching - reappearance of scratches

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The Beilby layer The contributions to this debate of Sir George Beilby in 1921 cannot be ignored. His ideas have become woven into the consciousness of most technologists and engineers; indeed, L E Samuels, author in 1967 of one of the most careful and detailed studies of polishing, later (1979) expressed surprise at the tenacity with which some details of his ideas have persisted in the face of more recent work. (This comment was in the context of a wider debate by Kuhn, Ziman and others upon the social and sociological aspects of the development and, acceptance of scientific ideas - 'Myths die hard'.)

Beilby's view of polishing was that the surface layers, and in particular the high spots, were subjected to strong mechanical deformation and, as a result, two separate processes occurred. First, the valleys became filled with this worked material from the peaks. Secondly, as a result of this continued working, these surface layers acquired a special structure, the so-called 'Beilby layer'. This layer, it was argued, was a special form of matter, structureless and akin in character to a liquid.

One key element of experimental evidence for Beilby's theory was his explanation of the way polished metallic surfaces behaved after chemical etching. Scratch marks, assumed to be those arising from the preceding, coarser stages of surface preparation, were revealed. The processes assumed to occur during polishing and subsequent etching are shown in Figure 1. The reappearance of the scratch marks was considered to have been brought about by the preferential etching of the highly worked material of the Beilby layer. However, as we shall see, an alternative, more recent, explanation of this etching behavior, if accepted, would remove this support for Beilby's theory. Such recent work confirms that the surface layers of mechanically polished metals are, indeed, highly worked to a greater or lesser extent and to greater or lesser depths, depending upon the nature of the methods used. Modern physical techniques, such as electron microscopy, have explored the structure of these surface layers. The resultant disagreements about the distinction between the concept of a Beilby layer and a description in terms of surface layers having a range of deformed crystal structures seems, at times, to be an exercise in semantics rather than physics!

Surface melting During the post-war period, in which the modern science of tribology was established, Bowden and Hughes (Bowden and Tabor, 1950) proposed a modification of the Beilby process as the mechanism of polishing. They showed that during rubbing the true areas of contact, which are generally limited to the highest parts of the surfaces, could be subjected to very high transient temperatures (generally called flash temperatures) as a result of the dissipation of frictional energy over these relatively small areas. In this way polishing could occur by a modification of the Beilby mechanism (Figure 1) in which flow by surface melting replaced flow by mechanical deformation. The problem with the surface melting theory is that the existence of high flash temperatures has been demonstrated experimentally only when the dimensions of the true areas" of contact have been relatively large, say 10-4 m, whereas an essential feature of polishing is that it must be a fine scale process. The areas of contact between the surfaces are thus much smaller — for example, a typical grain size of a particle used in polishing can be of the order of 10-6 m or less. In a recent review of polishing I deduced the maximum possible values of the flash temperature for such conditions, using well established theory. Assuming a rubbing speed of 1 ms-1 and a contact radius of 10-6 m, the results shown in table 2 are obtained, from which the conclusion must be that the surface melting mechanism is much less likely than has generally been assumed in the past. Indeed, it would seem to be possible only with materials of low thermal conductivity and/or low melting point.

Abrasion The mechanisms of both abrasion and its allied subject, grinding, have been the subject of much study; the scale of size of the individual events is usually significantly larger than in polishing. The essential feature of abrasive grits is that they have sharp corners and thus indent the abraded surface. It is known that when the grits become worn or clogged the abrasive becomes ineffective (figures 2 a and b). In an ideal situation - if the process were 100% efficient and all the grits were removing material - one can calculate the rate at which material should be removed from a knowledge of the grit geometry, the indentation hardness of the workpiece and the applied load. However, many studies have shown that, even with freshly prepared abrasives, the rate at which material is removed is less than the calculated value by an order of magnitude or more.

The most detailed studies of abrasion were made during the late 1950s and early 1960s by Samuels and co-workers in Australia, whose main concern was to obtain an understanding of the preparation of metallic surfaces for metallography - the first stages of this involve the use of abrasive papers. Their work consisted of a coordinated series of experiments which included the following elements: measurements of rates of abrasion; detailed studies of the shape and orientation of the grits; model experiments, in which different shapes and orientations of grits were represented by scratching experiments with indentors scaled up in size; and careful metallographic examination of the subsurface regions of abraded surfaces to discover the extent of deformation produced during abrasion.

The first important conclusion to be drawn from this work is that the low efficiency of abrasion arises from the fact that only about 10% of the grits are cutting and remove material. These cutting grits are those with a suitable 'attack angle' - the orientation of the cutting face to the work material. If sections of unused abrasive papers are examined, it can be seen that about 10% of the grits have attack angles which tally with the range shown by model experiments to be necessary for cutting.

This provides excellent agreement with the efficiency of abrasion as derived from the measured rates of material removal.

The second important conclusion derived from this work is that the depth of deformation arising from abrasion is much greater than had been previously suggested. The results drawn from metallography using etchants capable of detecting various degrees of deformation are shown in figure 3.

Polishing as abrasion Samuels' studies of abrasion were extended to the next stages of metallographic polishing, in which diamond abrasives of decreasing particle size are used. This work shows, quite clearly, that polishing with diamond pastes involves processes which are essentially similar to those involved in abrasion with abrasive papers, yet on a significantly smaller scale. In particular, using etchants designed to detect a range of degrees of deformation as before (illustrated in figure 3), the same regions of differing degrees of deformation, whose depths bear the same relationship to the depths of the scratches, were again revealed.

Significant conclusions arise from this work if polishing is carried out in the traditional manner, each stage of the polishing being pursued until all visual evidence of scratches from the preceding stage is lost, (This is sometimes achieved by polishing at right angles to the direction of the previous stage.) What emerges from this procedure, as is readily evident from figure 3, is that the severe subsurface deformation arising from earlier stages remains below the surface and is then revealed by subsequent etching, as shown in figure 4, This is the promised alternative explanation of one of the main experimental props of the Beilby theory of polishing.

A second important result of this work is that it provides new criteria for metallographic polishing procedures. It turns out that

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you need to carry out each polishing stage for significantly longer periods than were assumed previously to produce polished surfaces which are largely free from deformed material. When this is done the reappearance of scratch marks under etching (a cornerstone of the Beilby theory) no longer occurs!

The arguments for abrasion (rather than the Beilby process) as the polishing mechanism may be summarized as follows. Material is removed from the surface at each stage, losses in weight being recorded. Metallographic evidence then reveals deformation effects similar in character to those found in abrasion, differing only in the scale. An alternative explanation to that given by Beilby for the reappearance of scratch marks after etching has been provided; moreover, this particular evidence simply does not present itself if the finer stages of polishing are continued for sufficiently long periods.

Figure 2 Abrasive grits as revealed by SEM: a) freshly prepared grits (magnification ~ x645) b) a typical worn grit (magnification ~ x345)

Table 2

Maximum calculated flash temperatures in polishing

Polished Material

Flash Temperature (0C)

Melting or Softening Points(0C)

Perspex

140

120

Quartz

291

1400

Glass

1190

800

Steel (normalized)

17.9

1500

Steel (hardened)

71.6

1500

Copper

0.9

1083

Silver

0.4

960

Assumed conditions: sliding velocity 1 ms-1, coefficient of friction 0.5, circular area of contact of radius 10-6 m. For the metals listed here the flash temperatures are proportional to the speed and to the radius of contact about the range of conditions considered

Figure 3 Typical deformed regions produced in abrasion: a surface; b (shaded) heavily deformed layer; c deformed layer >5% compression level; d elastic/plastic boundary

Critique of fine scale polishing So, as we have seen, the evidence for abrasion as the mechanism of metallographic polishing with diamond and some other abrasive materials seems unassailable. However, the argument seems largely unproved as a general rule for all fine scale polishing processes. Indeed, Samuels goes to some lengths to suggest that the finishing processes which he recommends do not

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suffer from the defects of subsurface deformation (albeit of decreasing dimensions) which occur in the earlier stages. It is worth considering this aspect of polishing against the background of research on the wider subject of wear.

Wear represents the most difficult and intractable aspect of tribology: it was recognized early on that it was not a process governed by a single mechanism - new research tends to show increased complexity and multiplicity of mechanisms. It is all the more surprising, then, to discover that the tendency to expect polishing to be limited to a single physical process continues.

Viewed against this background, the later stages of some metallographic and other forms of polishing can be seen as being akin to a controlled process of corrosive wear. In corrosive wear (or, in air, oxidative wear) the material which is removed is generally assumed to consist of the films formed by chemical reaction between the worn surfaces and their environment. In engineering practice this is usually regarded as a benign process in which the surface films prevent the severe consequences of clean metal to metal contact, but in which the chemical reactions are insufficiently aggressive to create severe and high rates of chemical corrosion. An obvious feature of some finishing processes used in metallographic polishing is the presence of materials which have the potential for mild chemical reaction with the polished surface. It seems curious that the post-war generation of researchers who established the modern science of tribology must have included many who had spent some time polishing brass buttons with conventional metal polish; their noses might have been expected to suggest the existence of chemical factors in the mechanism of polishing!

Figure 4 Reappearance of scratches: the currently accepted true interpretation, a abraded surface with its heavily deformed layer (see figure 3) - the broken tine (arrow) shows level of material removed to erase scratches; b surface after polishing with remanent deformed material; c surface after subsequent etching with 'reappearance of scratches' In recent years the requirements of laser technology have brought about a revival of interest in polishing optical components.

Although it has not produced a single set of coherent investigations comparable to that of Samuels and co-workers, this work, scattered as it is, has produced some features of interest. First, it recognizes the existence of a number of polishing mechanisms, including chemical effects. Secondly, it seems to be largely ignorant of the parallel work, on metals and other materials we have considered! The symposium on polishing organized by the Institute's Tribology Group in March 1984 can therefore be regarded as significant, since it included contributions from both of these traditions. Polishing, like wear, is clearly not limited to a single physical mechanism.

Acknowledgments The review of polishing on which this article is based was commissioned by the National Centre of Tribology, Risley, to whom my thanks are due. It also formed the basis of the opening paper at the Institute's Tribology Group meeting mentioned above.

Further reading Archard J F 'The polishing of metals' Tribology International To be published Beilby, Sir George 1921 Aggregation and Flow of Solids (London: MacMilllan) Bowden F P and Tabor D 1950 The Friction and Lubrication of Solids Part I (London: Oxford University Press) Rabinowicz E 1968 'Polishing' Scientific American 218 91 Rayleigh, Lord 1901 'Polish* Nature 64 385 Samuels L E 1967 Metallographic Polishing by Mechanical Methods (London: Pitman). See also 1979 'Myths die hard' in Physics of Materials: A Festschrift for Dr Walter Boas (Melbourne: Griffin-Press) J F Archard is a consultant and author who lives in Sheringham, Norfolk, UK

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