OF TIN BRONZES
B. LUNN AI.5 Nordiske
(Received March 23,1965)
The use of tin bronzes as anti-wear materials has been known for several thousand years. The present use of alloys is based upon tradition. Their use is confined within narrow limits by standard specifications and traditional uses. Very little knowledge exists on the wear resistance of special alloys and on the wear mechanism of lubricated copper alloys. There is a wide field for future research.
In a recent publication on bearing metals FORRESTER~ mentions “that the widespread use of cast and wrought bronzes for bearings is a good example of the empiricism of bearing practice. There has been little investigation of&--or even published discussion on-the merits of these alloys as bearings”. As bronze has been known from the oldest times and probably has been used as bearing material for at least 3,000 years this is not very surprising. The pure scientist will no doubt abstain from applied research, and the technicians are not very keen on research on problems which are heavily based on tradition and experience and where the economical aspect is rather limited. As a result the few new facts about bearing bronze which appear in published papers are overlooked and very little ground is gained. THE BRONZES
The generally accepted opinions of the bronze alloys are as follows. Tin bronze mostly with IO to 14% is the preferred bearing material. Phosphorus, especially in tin bronzes, is an advantage. Contents up to 1% are being used. Lead-tin bronzes with a lead content up to 20% possess good anti-wear properties. Zinc in tin bronzes is generally omitted in spite of the fact that in small amounts it is one of the best components for ensuring a sound casting. Contents of the order 2-5o/o are sometimes permitted. High zinc alloys as straight brasses with 30-407; zinc are regarded as unsatisfactory bearing metals in the cast state but good bearing properties might be found in the cold-worked condition, especially of the more complex brasses containing iron, aluminium and the like. Aluminium bronzes with S-12:4 Al are remarkable for their wear-resistant properties under heavily stressed conditions. They will not be discussed in this paper. Wear, 8 (1965) 401-406
Cast vs. cold-worked
It is a further general experience that the good bearing properties of the tin bronzes are to be found in the cast condition. Annealing to a hamogene~~usstructure is detrimental to the bearing properties while a cold-working might restore the properties as for the brasses. HISTORICAI~ BACKGROUND
A close study of the accepted experience shows that it is mostly based upon the technical development in the industrial countries in the nineteenth century. The result of this development is still to be found in the standard practice followed in these countries which is recorded in their standard specifications. For this reason an analysis of the historical background is of interest.
The oldest known copper alloy is the tin bronze and it is included in all the standard specifications on bearing bronzes. In the U.S.A. and England a phosphorus content is specified while the German standards until January 1963 considered phosphorus a tolerated impurity. The new German specification from x963 is more in accordance with the British and United States specifications. One of the British standards specifies a minimum phosphorus content of 0.5% while the corresponding United States standards permit a maximum of 0.25% I?. Phosphor
Phosphorus was first added to tin bronzes in France as experiments around 1850 were carried out in order to improve the tin bronze then used for guns. The normal composition of gun bronzes was seven parts of copper and one part of tin, i.e. 124% tin bronze*. It was found that an addition of phosphorus made the gun bronze more finegrained and more resistant to blows. It should be borne in mind that at that time mechanical testing methods were not used. The properties of the materials were mainly judged by the appearance of the fracture and other simple technological tests. It was considered that wear consisted in a plucking out of single crystals which led to the assumption that a fine-grained material was most wear-resisting. This introduced the application of phosphorised tin bronzes for bearing purposes. Phos;bhorized
The influence which phosphorus has on the bearing properties is not confined to tin-bronzes. A German standard specification issued in 19482 and based upon an Austrian development during the war contained a phosphor-brass for bearing purposes. It was withdrawn in 1954 owing to difficulties in the manufacture, but it is an interesting phenomenon as brass, especially in cast condition, is generally regarded as a very bad bearing material. * Seven was in former times il holy number-the composition for guns was 7 Cu, I Sn, for churchbells 7 Cu. z Sn and in antiquity for mirrors 7 Cu, 3 Sn. These proportions go back to pre-historic times of superstition but it must be admitted that all the compositions are well suited. The bell bronze is still considered to be the best composition for this specific use. Wear, 8 (rg65) 40x-406
WEAR RESISTANCEOF TIN BRONZES
A publication by the author3 containing brass and it was found that properties of the ordinary gun metals phosphorus content rather than that
403 confirmed the good behaviour of phosphoruseven under lubricated conditions the anti-wear with varying tin content varied mainly with the of tin or the hardness of the alloy.
plotted against phosphorus content.
A similar experience can be found in DANIEL AND GRAHAM’S publication on the wear of gear wheels made from tin bronzes 4. This publication differs from most of the other publications on the wear of bronzes by being almost complete, containing precise information, not only on the strengths and compositions of a series of alloys, but also on the micro-structures, impurity contents and the oil used. The conclusion of DANIEL AND GRAHAM is that a fine-grained tin-bronze is more wear resistant than a coarse-grained. There is some reason for this conclusion, but from the presented material it is possible to extract another conclusion, viz. that the wear is independent of the tin content, but decreases with increasing phosphorus content (Fig. I). Further with constant phosphorus content, increased content of iron shows increased wear (Fig. 2). It is known from tin based Babbitt-metals and also from aluminium alloys
Fe (%) Fig. 2. Wear plotted against iron content at constant phosphorus content. Wear, 8 (1965) 401-406
that iron can be an injurious ingredient if it is found in a solid solution in the alloyh. The influence of phosphorus ought to be studied much more. As yet no explanation has been found for the improvement in the wear qualities of lubricated bronzes brought about by phosphorus additions. Leaded tin bronzes
These bronzes have been used and specified in the U.S.A. for many years. The early use of leaded bronzes in American specifications resulted from large-scale experiments6 made at the Pennsylvania Railroad by DUDLEY in I875-ISgo. The experiments were conducted on actual bearings in large series placed in locomotives and cars, and it was found that an alloy with 77% Cu, 8% Sn and 15% Pb was more wear-resisting than phosphorised lead-tin bronze with 10% Sn, 9*% Pb and 0.8% P which, again, was better than the classical gun bronze. The experiments by DUDLEY were later followed by a series of laboratory experiments made by CLAMER~,1903. These early publications are still valid. Zinc in tin bronzes (gun-metals,
red brasses) It is noteworthy that the American standards to a high degree permit the presence of zinc in leaded-tin bronzes, which is the preferred bearing material, while in England and, until recently, Germany some reservation is shown on this point. The ordinary argument is that, when cast brass containing 3o-35% Zn is a bad bearing material, a tin bronze with I/IO of this zinc content cannot be good either. This argument falls down and FRENCHANDSTABLE+ show that 4% zinc does not influence the wear resistance of tin bronzes and leaded tin bronzes. FRENCHAND STABLES’report was recorded in an unusual way by HANSONAND PELL WALPOLE~.They concluded that 4% zinc is dangerous to tin bronze, even though the condensed report on the experiments of FRENCHAND STABLESshowed the contrary.
Other alloying ingredients
Recently KRUSCHOVet al .lO have investigated the anti-scoring properties of a series of copper alloys some of which are of a rather unorthodox composition e.g. 8% Sb, 2% Ni, balance Cu or Si 3.3%, P 0.5%, balance Cu. The investigation has been carried out both with and without lubricant, and the conclusion is that phosphorus, antimony, tin and silicon improve the anti-scoring properties of the bronzes while the other components, especially zinc, have no or a negative influence. A more systematic and exhaustive investigation of the alloying elements mentioned ought to be carried out. THE INFLUENCE OF LUBRICANTS In the majority of publications on bearing metals most consideration is paid to the mechanical properties of the metals and the load, speed and geometry of the testing equipment. Very little attention is paid to the lubricant and the surrounding atmosphere which we know have a considerable influence on the anti-wear properties of the bearing materials. The exhaustive practical test by DUDLEYpays no attention to the lubricants, but the test by CLAMERrefers to an oil called “Galena Coach oil, fed by cotton waste”, Wear, 8 (1965)
whkh might possibly be a compounded mineral oil which, until recently, has been the preferred lubricant for railway bearings*. &RWEL~, MXLNE AND -WEBBERI1 showed. by exammation of an oscillating l.-earhgmade of leaded tiri bronze fxr”,;t Sn, 6yQ Pb, 02~~ P, balance Cu), that the bearing with a compounded mineral oil could withstand a so% greater pressure than with a corresponding pure mineral oil. When sliding surfaces are properly lubricated the wear is negliible as can be seen from the pu~l~~at~o~ by &UQI?I, AND CRAEAX a. 3% &tai~ls. real wear resutts FRENCH AND STABLESTcarried out their tests under dry conditions, and [email protected]
demonstrated with dry sliding a hyperbolic dependence between the coefficient of friction and the hardness. ~npub~~hed results by BWCHW.~~D*+>show that, urder lnbricakd conditions, a leaded zinc-containing tin bronze (83% Cu, 7 “J;, Sn, 7% Pb, 3% Zn) gives a better oil film formation than the much harder tin bronzes with Ia-rd% tin. Lubricated with a very thin lubricant or ~~~-~~~~~~a~~~ the harder alloys show more wear resistance than the softer l~ad~ontai~ing alloy. Several investigators, notably VINOGRADOV~2, suggest that the atmosphere is of considerable importance. &Xuch remains to be done to complete the pi&Ire in rrrder Q explain wbkh alloy components are advantageous or injurious in copper alloys and what infhIence the structure and the components have on the sliding properties of lubricated and unlubricated copper allays. While there are some publications on cast alloys very few results are published on wear tests for rAled and drawn a.&xp. The observation of HOAR~~ that the zinc in cold deformed brass has a tendency to migrate to the surface, so that this will be more zinc-rich even at normal temperatures, might change the ability to react which in turn might improve the possibility of obtaining a hydradynamic effect from the lubricant. CONCLUSION As uur ~stab~sh~ ~~uwIe~e of the cop~~-ba~~ beaning materials is bzsed on experience and traditions much remains to be investigated to understand the anti-wear properties of the bronzes. To all investigators it should be emphasised that not only composition and mechanical properties are of interest, but also impurity con&z&s, rn~~r~~~~~e~ l~bri~nts* s~~oun~~~ a~ospheTe* temperatures surface properties and corresponding figures for the steel surfaces have to be recorded. The complexity of the wear process needs full information on all integrating factors. It is to be hoped that it should be possible to investigate wear on copper alloys more thoroughly in the future.
4 S. C;.DANIEL AND I<.GRAHAM, Some factors affecting the wear of bronze, I’Po<. First Wo?,/d Metallzqical Congress, XSM, Cleveland, 1952, p. 6133631. 5 13. LUNN, Evaluating bearing materials under boundary lubrication, Luhricatiorr~ Erlfi., 11 (‘955) 255. 6 C. B. DUDLEY, 7 G. H. GAMER, 8 9 ro 11 12 13
Bearing metal alloys, J. Franklin Inst., 13 (1892) 81-93, 161-17~. A study of alloys suitable for bearing purposes, ,J. F;ranklin Imt.,
49- 77. H. 1. FRENCH AND E. M. STABLES. Bearing bronzes with and without zinc. 1. Hrs.. Satl. Bzrv. Std.: 2 (1929) 1017-1038. D. HANSON AND W. T. PELL-WALPOLE, Chill Cast Tin Bronzes, Edward ilrnold, London, 1951, p. 261. AI. M. KRUSCHOV et al., A study of the friction of anti-friction bronzes and brasses with and without lubrication, Friction Wear in Machines, No. ‘7 (1962) 36-70. 1:. T. BARWELL, A. A. MILNE AND J. S. WEBBER, Some experiments on oscillating bearings, Trans. Inst. Engrs. Shipbuilders S&land, (1955) 267-326. _ G. 1’. VINOGRADOV et al., Lubrication under heavy friction, Wear, 6 (3) (1963) 202-225. T. 1’. HOAR, Influence of surface treatments on the chemical behaviour of metals. In Effect of Surface on the Behaviour of Metals, Illife and Sons Ltd. for The Institute of Metallurgists, London, 1958. pp. 29-40.
8 (1965) 401~406