Friction and wear of polymers

Friction and wear of polymers

74 BOOK REVIEWS The book concludes with lists of surfaces available commercially and of equipment suppliers. Only brief details of surfaces are give...

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The book concludes with lists of surfaces available commercially and of equipment suppliers. Only brief details of surfaces are given and these lists probably date too rapidly for such a book to be their ideal place; the list of test equipment suppliers is far from exhaustive. Returning to the question of for whom the book is intended; it is a technical book and much of it will not appeal to players, sports hall managers, etc. The chemistry and technology content will be known to technical people working in the industry. Probably, the book will be of greatest value to technical 'students' of artificial surfaces, particularly those without a polymers background. These remarks in no way detract from the fact that for the first time a comprehensive, objective account of all aspects of polymeric surfaces has been brought together and it will be a useful reference book for all of us concerned with the performance of these products. R. P. BROWN

Friction and Wear of Polymers, G. M. Bartenev and V. V. Lavrentev. Translated by D. B. Payne and edited by Lieng-Huang Lee and K. C. Ludema. Published by Elsevier, Amsterdam, (1982). 320 pp. Price: £42.60 (US $ 76.75). This book, number six in Elsevier's Tribology Series, is a translation of the well-known Russian book published by the Leningrad Division of Khimiya Press in 1972. The English version is reproduced by offset lithography from unjustified typescript which, while easy to read, gives the book an unfinished appearance. The original Russian book is divided into sections each with subheadings in bold type. The subheadings in the English version are in lower case typescript and merge with the text so that it is difficult to locate particular sections. As would be expected, the authors describe mainly Russian work covering the last 30 years or so, but the influence of Western writers is acknowledged and the relationship of their work to that of the Russians is clearly expounded. The book is almost entirely theoretical and is likely to appeal most to those engaged in academic research. In the preface the editors state that, probably because of differences in basic approach, most Western readers have difficulty comprehending Russian papers. Western authors construct models using only those quantities that are known to be relevant and which have dimensional compatibility; Russian writers have a more empirical approach and are ready to build mathematical models with great dispatch, invoking all possible physical and chemical quantities, whether defined or not. The editors obviously had considerable difficulty with the symbols and did their best to order these and maintain consistency

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throughout the book, thus making it easier for Western readers to check the many equations. Unfortunately this has, in places, rendered a simple Russian equation more cumbersome. One example will suffice. Normal load is a term universally known and understood and the entry 'effect of normal load' appears three times as a subtitle in different chapters. In the editor's preface to the English book the symbol N is used for normal load as in, for example, equation 2.15 in the Russian, written F = ~N, and where the Russians have retained N--a letter which does not appear in the Cyrillic alphabet. In the translation equation 2.15 appears as Ff = /~P--here P is used for load whereas in the list of symbols p is given as pressure. The text on p. 38 contains a mixture of P and p, leaving the reader wishing for N, which unfortunately is now reserved for intermolecular attraction. Sometimes confusion arises because the editors use partially the Russian symbols and partially their own. For example, equation 4.68 appears in both English and Russian texts as 1//u = A + Bp, where A and B are constants. The Russian text then continues with equation 4.69 given as & = ap/(1 ×bp), where a = 1 / A c and b = B/A. The English text gives equation 4.69 as q~ = dp/ (1 + ap), where tr = 1/Ac, b = B/A and B = 1/c. Clearly these two versions are totally incompatible. In fairness, it must be said that the translator appears to be working from a different edition of the Russian text from that available at R A P R A since the English section from equation 4.69 up to, and including, Figure 4.35 does not appear in the RAPRA copy of the Russian book. Incidentally, Thirion's formula, referred to as equation 6.49 on p. 159 is clearly a reference to equation 4.69. It is unfortunate that the editors in a creditable attempt to reduce confusion by systematically organising the symbols have allowed their efforts to be overshadowed by careless proofreading. The book consists of seven chapters; the first chapter usefully reviews the four states in which polymers can exist--crystalline, glassy, rubbery and viscous flow. The work of Ferry et al. is acknowledged but it is interesting to note that the Russians point out that Aleksandrov and Lazurkin were the first to use the frequency method for the investigation of relaxation effects in 1939. The second chapter, entitled 'Friction of Metals', covers basic friction theory such as the relationship of frictional force to the real area of contact, normal load, and so on. More interesting, perhaps, are the discussions of the effect of stationary contact time, an important concept when static friction is considered and the influence of normally oriented free contact vibrations accompanying friction. Chapter 3 covers polymer friction in the glassy and crystalline states. The earlier part of the chapter is based on the work of Shooter & Tabor and King & Tabor and, fortunately, references to 'polytehylene' and 'polycapolactam' in Figure 3.2 cause no difficulty! Readers will probably be aware that the results

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obtained by Tabor and his co-workers did not agree with the theory. It is comforting to read later on that in the case of rigid polymers the Russian authors consider that there is no consensus of opinion on the relation to load of either the friction force or the friction coefficient. The authors point out that, because of the rate dependence of plastics, velocity and temperature have a profound influence and Bowden's theory takes no account of this. As in the case of metals, contact time is also important in its effect on friction under stationary contact. The Williams-Landel-Ferry transform is briefly discussed on p. 93 but no reference is given and neither Williams nor Landel are listed in the index. In this the translators have been faithful to the Russian. The authors continue with an interesting discussion of the many factors which influence friction in practice, for example, transfer of material from one surface to the other, and the chapter concludes with a discussion on the choice of materials and lubricants. Friction in the rubbery state follows on naturally in Chapter 4, and here the importance of time and temperature have been recognised. Static friction, however, presents a special difficulty. Bartenev's theory of rubber friction, based on molecular kinetic concepts is discussed in some detail and the relationship of Grosch's experimental results to this theory is examined. The different approaches of Bulgin, Savkoor and SchaUamach are then discussed. Sections are then devoted to the real area of contact and the friction force, discussing the influence of factors such as normal load, contact time, temperature, etc. The chapter concludes with advice on the choice of materials and a discussion of lubrication. Chapter 5 deals with the friction of polymers in the viscous state--a field studied very little because of the experimental difficulties. The authors begin by pointing out that for polymers in the viscous state pure external friction often does not develop, as shear takes place in the polymer itself. Tolstoy's theory is given in some detail and related work by Rebinder, Volarovich, Deryagin, Mooney and others is discussed. Three types of dispersed systems are distingu i s h e d - t h o s e that depend on sliding velocity and pressure, those that depend on velocity alone and those that depend on pressure alone, the most prevalent being the first. The chapter ends with a brief discussion on the significance of boundary sliding on processing. This concludes the section on friction. The next chapter is entitled 'Wear of Polymers' and begins by setting out the criteria for assessing wear. The Bartenev-Lavrentev theory is given and the relation between friction and wear is discussed. Kragelsky's work on fatigue wear follows and the relationship of other work by Babichev, Scallamach and Ratner is discussed. Rise in temperature at the surface and oxidative effects play a dominant role while the effect of coefficient of friction and sliding velocity are harder to isolate. The importance of the conditions of testing and the nature and thermal conductivity of the contacting surface are noted. Wear

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by roll formation is a special case and the work of Reznikovsky and Brodsky is discussed. Western readers will have a certain sympathy with their Russian colleagues who end by saying that they cannot offer a general prescription for improving wear resistance. Nevertheless they do offer some very useful advice. Readers of Polymer Testing will probably be most interested in the final chapter which deals with instruments and methods for friction and wear research. Although the Russian authors state that they are concerned only with scientific tests, one Russian State Standard is mentioned. Following a useful discussion on the choice of test method and the factors which affect measurement the authors go on to compare direct mechanical devices with transducers. In particular, the influence of dynamometer stiffness on the measured frictional force is stressed. The section on transducers ends with a nostalgic reference to a valve operated bridge circuit. A brief discussion on temperature measurement, velocity determination and measurement of load follows. Over 20 Russian instruments for friction and wear testing are described in some detail. Reciprocating machines, rotating machines, crossed filaments, pin and plate, four ball, all these are described, together with special instruments for measuring real contact area, friction at high pressures, high sliding velocity, frictional vibrations and friction in a vacuum. A few of the machines are designed to measure wear as well as friction. One of these, the MIR-1 machine, bears a strong resemblance to the DIN Abrasion Tester. The original Russian drawing of this machine was very poor and the American artist has considerably improved the perspective---engineers will be amused to see that the absence of a worm drive makes it appear as a machine for wearing gears but that is also a fault in the original diagram. Each chapter ends with a set of references and these are cross-referenced in the Author Index at the end of the book. Additionally there is a Subject Index. Internal evidence suggests that the references have not been checked, nor have references to English translations of Russian articles been given where these are known to exist. The editors ask the reader's indulgence with regard to the spelling of many names. This is understandable but it should not be used as an excuse for carelessness or inconsistency. For example, Bulgin is rendered as Buldin on p. 140; Melnikova as Melmikova on p. 205; Farberova as Farberove on p. 217; Mme Kuritsyna has degenerated to Kuritsyn on p. 275 and to Kurityn on p. 276; Matveevskii appears on pages 296 and 297 but is given as Matveevsky in the Author Index. Incidentally the preference for the ending -sky rather than -skii is strange since I. V. Kragelskii is listed on the Editorial Board of Elsevier, but appears as I. V. Kragelsky elsewhere in the book. Variation in the spelling of names can be extremely confusing to librarians, particularly where translations of Russian papers have already been published in English with different spellings of the names. For example, reference 16 on p. 185,

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given as Bartenev and Yelkin, has been published in Polymer Science USSR, 7, (6), 1965, as Bartenev and El'kin. Some mistakes have been transferred directly from the Russian where the well known American author Dannenberg appears as Dannenber, Vogt's initials are given as V and not W. W., Grosch is misspelt as Grosh in reference 107, p. 260, and Hurry and Prock are given as Harry and Prock, both in the text, p. 112, and in the references, p. 185. For anyone used to the meticulous standards of J. R. Scott the proofreading and editing leave much to be desired. In places the translation is also confusing. For example, line 7, p. 164, 'by the first approach' should read, 'as a first approximation'; the paragraph beginning, 'These similar cases', p. 162, should read, 'These and other similar factors'. Some mistakes have been transferred directly from the Russian--Figure 4.22, p. 136, although redrawn is clearly recognisable as Figure 5 of Grosch's 1963 paper (reference 24), and both here and in the Russian is incorrectly attributed to Lavrentev (reference 23); the abrasion axis on Figure 6.9 was also incompletely labelled in the original Russian book. Carelessness abounds--what appears as cost (equation 2.25, p. 42) should be const.; on p. 140 the references given as Figures 4.10 and 4.25 are, in Russian, referred to as Figures 4.19 and 4.25; in Figure 6.4 the rate of wear is clearly given in the Russian a s 10 - 6 mm/min and not a s 10 - 6 mm 3 10 mm/min as appears in English; on p. 217/z/km in Figure 6.7 should have been written /~rn/km; 'cases' in Table 6.3 should be 'classes'; there is an incomplete phrase in Table 6.6 (equivalent to neoprene); divided is misspelt on p. 261 and, more seriously, unsteady motion is described as V = const, instead of V ~ const, as is given in the Russian; Figure 7.1, p. 264, is not labelled in the same manner as the Russian and the key given beneath the diagram is consequently completely wrong---e should be b, b should be c, f should be d, c should be e, g should be f, d should be g (a and h are correct)--the captions then should read a--d and g, cylinder on cylinder, e, end friction of cylinder on cylinder, f, h, friction of cylinder ends on a plate; on p. 281, there is reference to Figure 7.1d--this refers to the Russian '~', rendered 'e' in the English alphabetical order (or 'c' as presented in the book under review)!; in Table 7.1 what appears as CrY6 should be CRYO. Other mistakes have arisen because of confusion of Russian letters with English or Greek. For example, r/ in Figure 2.6, p. 44, is a rendering of a Russian 'h', but should, in any case, have been changed by the editors to hd to correspond with the text. On p. 162 the symbol SH appears in Figure 4.37--the 'H' here is a Russian 'N' and should have been rendered as S, (according to the convention adopted by the editors). One suspects that the equation F = t~o(P + No) is inconsistent since the Russian is F = l~o(N + No) and certainly the Russian S(N) and F(N) (line 10) should have been changed to

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S(P) and F(P), as

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in line 16. A similar mistake appears on p. 203 where SH appears again in equation 6.1. A third type of error concerns the rendering into English, by phonetic spelling, of words with an English origin. One is surprised that the editors did not identify 'Yuroprene', p. 196 and elsewhere, as Europrene, a trade name of Anic SpA of Italy, or Khaipalon, p. 183, as Du Pont's Hypalon. Some bad translation has undoubtedly arisen from an over reliance on Callaham's Russian-English Technical and Chemical Dictionary. For example, 'chimney soot', Table 6.4, should be rendered as 'furnace black' and 'powdered silica gel' as 'silica filler'. Item 9 in Figure 7.7 is not a 'propeller' but a 'screw', and the 'micrometric propeller' in line 13 is a 'micrometer screw'. The 'interchangeable gears' in Figure 7.11(6) are 'interchangeable weights' and should also be rendered as such in line 3. Finally, lack of familiarity with Russian rubbers has rendered the Tables less useful than they might otherwise be. One example will suffice. In Table 6.6, p. 252, the Russian rubbers SKS-30-1, SKS-30M, and SKS-25-MPV-5 are all described as butadiene-styrene rubbers. Lambert's Russian-English Dictionary gives these as SKS-30-1, a carboxylated styrene-butadiene, SKS-30M, styrenebutadiene extended with mineral oil, SKS-25-MPV-5, styrene-butadiene 2methyl-5-vinyl pyridine (25 : 70 : 5) terpolymer rubber. Although there are many errors the RAPRA translators consider the English translation to be a fairly faithful rendering of the original Russian book, giving a good insight into Russian thinking in the matters of friction and abrasion of rubbers and plastics. That the book lacks contact with industry because there are no practical data on tyres, belts, seals or any product at all is a criticism of the original Russian rather than the English version. Nevertheless the gathering together of so much information on friction and wear is a considerable achievement and the book will probably become a standard reference work in Technological Universities. D. I. JAMES