Wear of some F.C.C. metals during unlubricated sliding part IV: Effects of atmospheric pressure on wear

Wear of some F.C.C. metals during unlubricated sliding part IV: Effects of atmospheric pressure on wear

165 Wear, 43 (1977) 165 - 174 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands WEAR OF SOME F.C.C. METALS DURING UNLUBRICATED SLIDING ...

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165

Wear, 43 (1977) 165 - 174 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

WEAR OF SOME F.C.C. METALS DURING UNLUBRICATED SLIDING PART IV: EFFECTS OF ATMOSPHERIC PRESSURE ON WEAR

N. SODA The Institute

of Physical and Chemical Research,

Wak.6, Saitama (>aian)

Y. KIMURA and A. TANAKA Institute

of Space and Aeronautical

Science,

The University

of Tokyo,

Tokyo

(Japan)

(Received May 26, 1976; in final form July 15, 1976)

Summary

The dependence of wear on atmospheric pressure was studied in relation to the fatigue of materials. Ni, Cu and Au specimens were fatigued at various atmospheric pressures and the results were compared with their wear behaviours. Similar curves were obtained for each material when the reciprocal of the fatigue life and the number of wear fragments were plotted against the atmospheric pressure. It was concluded that changes in the number of wear fragments or in the amount of wear are governed by the resistance of the materials to fatigue. On the basis of the findings obtained, mechanisms of wear are discussed.

1. Introduction

Throughout this study on wear, a simple sliding system has been analyzed in its various aspects. Changes in wear with variations in the normal load, the sliding velocity and the atmospheric pressure were determined for the unlubricated sliding of nickel, copper and gold [l] . These wear behaviours were then interpreted in terms of the volume and the number of wear fragments [ 21. It was revealed that the change in wear with a variation of the normal load or the sliding velocity was predominantly due to changes in the volume of individual wear fragments. This is seemingly inconsistent with the common understanding based on the adhesion theory. Further investigation showed that the normal load or the sliding velocity governs the magnitude of the actual forces working on the sliding surfaces and that these forces determine the thickness of the plastically deformed substrate layer and accordingly the volume of each fragment [3]. In contrast, the atmospheric pressure has little effect on the volume of individual fragments but it affects the wear because it changes the number of

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fragments [Z] . It appears that current adhesion theories can be adopted to explain wear behaviour, since a change of wear is related to the probability of the formation of wear fragments. However, adhesion theories do not give any clear picture of the process by which loose fragments are formed. The experimental results obtained in this study suggested a potential mechanism in which the forces working on the surfaces cause substrate damage that leads to the formation of wear fragments. It seems reasonable, therefore, to analyze the dependence of the number of wear fragments on the atmosphe~c pressure. In the present paper, the effects of atmospheric pressure on wear were studied in comparison with those on the fatigue lives of the materials. Being the final report of this wear study, this paper includes some general discussions on the mechanisms of the unlubricated wear of metals.

2. Wear and fatigue As wear is a fracture phenomenon of sliding surfaces, it should be discussed in relation to the mechanical properties of the materials but such treatment is rare. For example, hardness is the only material factor appearing in Archard’s formula [4]. However, it merely determines the area of real contact and it does not represent the resistance of materials to the fracture which gives rise to loose fragments. Among various mechanical properties, fatigue resistance of materials seems to afford a promising clue to the understanding of wear mechanisms when the repetitive action of forces in sliding contact is considered. Here, the term fatigue is used in its wider sense. This term would usually be understood to mean a type of fracture caused by repeated loading cycles of stresses that are lower than the elastic limit of the material. When severe plastic deformation of the surfaces is considered under adhesive wear conditions, it might seem not to be the case. However, low cycle fatigue has been found to be another type of fatigue fracture which is characterized by fracture caused by repetitive plastic deformation. Fatigue is characterized by a process where damage accumulates in the material leading to eventual failure. Fatigue fracture of materials exhibits different features from those of fracture by a single action of forces. In the case of fracture by a single action of forces, the atmospheric pressure may affect the formation of the oxide film on the surface of the material, which blocks the escape of dislocations, but its effect on the ultimate strength is slight [ 51. In contrast, it has been established that the atmospheric pressure exercises profound effects on the fatigue behaviour of materials [6 - 81; reducing the atmospheric pressure generally increases the fatigue life. It was observed previously [ 1, 31 that wear fragments were liberated from the bulk material as a result of the repetitive action of normal and frictional forces. As the friction input was almost constant when the atmosphe~c pressure was varied, the dependence of wear on atmospheric

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pressure can be compared with that of the fatigue resistance of the materials under constant mechanical input. Changes in wear [l] resemble the fatigue behaviour obtained by Stegman and Shahinian [8] and by Wadsworth and Hutchings [ 61, as observed earlier [l] . This correlation between wear and fatigue behaviour acquired a clearer physical significance from earlier findings [2], namely that the changes in wear with atmospheric pressure were due to changes in the number of wear fragments. Wear behaviour must therefore be governed by the crack growth rate, as is fatigue behaviour.

3. Fatigue experiment To study the correlation between wear and fatigue in more detail, the fatigue of the same materials used for wear experiments was studied in reversed bending as a function of the atmospheric pressure. The metals used were Ni, Cu and Au; the Ni specimens were 99.7% pure and the others were 99.99% pure. Sheet specimens were machined to the dimensions shown in Fig. 1. A modified Schenck machine was used and the test portion was enclosed in a vacuum chamber (Fig. 2) which allowed experiments to be carried out at pressures between 760 and 8 X 10M6 Ton at room temperature. Specimens were subjected to reverse bending at a frequency of 16.7 Hz and at a constant amplitude. When preliminary experiments were made at 760 Torr under an initial maximum stress of 16.3 kg mme2, fatigue lives ranged from 2.64 X lo4 cycles with Au to 1.01 X lo6 cycles with Ni. The initial maximum stress was controlled so that the fatigue lives were of the same order of magnitude for all the materials; the initial maximum bending stresses with Ni, Cu and Au were 16.3 kg mmP2 (strain 0.077%), 12.0 kg mme2 (strain 0.091%) and 8.0 kg mmm2 (strain O.lOO%), respectively. A predetermined pressure was established in the chamber by admitting Driving

shaft

Specimen

Vacuum

Fixed

Fig. 1. The sheet specimen Fig. 2. A test portion

used for fatigue

in a modified

Schenck

experiments. machine.

shaft

chamber

0

m-5

10-a

Atmospheric

lo-’

xl’

pressure

1

llorr

10-5

10-3

Atmospheric

t4J’

6’

pressure

103

(Tow)

Fig. 3. Effect of atmospheric pressure on the fatigue life of Ni: frequency 16.7 Hz; initial maximum bending strain 0.077%. Fig. 4. Effect of atmospheric pressure on the fatigue life of Cu: frequency 16.7 Hz; initial maximum bending strain 0.091%.

Atmospheric

pressure

(Ton

I

Fig. 5. Effect of atmospheric pressure on the fatigue life of Au: frequency 16.7 Hz; initial maximum bending strain 0.100%.

decontaminated air through a variable leak valve after evacuation by an oil diffusion pump and a rotary pump. Figures 3 - 5 show the experimental results. With Ni and Cu, the fatigue lives are seriously affected by the atmospheric pressure. The curves are step shaped with little or no dependence of the lives on the pressure at low and high pressures; the major change takes place at intermediate pressures where the fatigue lives increase with reducing pressure by a factor of 5 with Ni and 4 with Cu. In contrast, the fatigue lives with Au are substantially unaffected

Fig. 6. Fatigued specimens of Ni: frequen_cy 16.7 Hz; initial maximum bending strain Torr, (b) 760 Torr. 0.077%; atmospheric pressure (a) 1 X 10

by the atmospheric pressure. These relations have common features with those appearing in the literature [ 6 - 8 ] . Severe ~mpling of the surface was observed around the fatigue cracks of Ni and Cu specimens after they had been fatigued at low pressures but not after they had been fatigued at high pressures (Fig. 6). Although specimens undergo a greater number of cycles at low pressures, this difference must be due to en~ronmen~ effects on the deformation of the materials, as suggested by Snowden and Greenwood [9].

4. Correlation between wear and fatigue results It seems reasonable that a comparison be made between the rate of formation of wear fragments reported earlier [ 1,2] and the reciprocal of the number of cycles to fatigue fracture. The former must be proportional to the growth rate of the crack that leads to the separation of a wear fragment from the bulk material, since the volume of the fragment was scarcely affected by the atmospheric pressure [2] ; the latter is also proportional to the fatiguecrack growth rate. The results are shown in Figs. 7 - 9. Irrespective of the metal type, the rate of fragment formation and the reciprocal of the fatigue life behave in exactly similar manners when the pressure is varied. With Ni and Cu, both decrease fairly sharply with a reduction in the atmospheric pressure at intermediate pressures. Changing the pressure at low and at high pressures,has little or no effect on either. In contrast, with Au both the rate of fragment formation and the reciprocal of the fatigue life are unaffected by the atmospheric pressure. The current understanding of the effect of the environment on fatigue behaviour may be summarized as follows. Reducing the atmospheric pressure changes the number of oxygen molecules striking and being chemisorbed at

Atmosphenc

pressure

ITorrJ

Fig. 7. Dependence of the rate of wear fragment formation and the reciprocal of the fatigue life of Ni o_nlthe atmospheric pressure: n rate of wear fragment formation, sliding velocity 16.8 cm s normal load 260 g, sliding distance 50 m; 0 reciprocal of the fatigue life, frequency 16.7 k, initial maximum bending strain 0.077%. Fig. 8. Dependence of the rate of wear fragment formation and the reciprocal of the fatigue life of Cu on the atmospheric pressure: l rate of wear fragment formation, sliding velocity 16.8 cm s-l, normal load 260 g, sliding distance 50 m; 0 reciprocal of the fatigue life, frequency 16.7 Hz, initial maximum bending strain 0.091%.

10-5

10-3 Atmospheric

to-’ pressure

a’

103

I Torr1

Fig. 9. Dependence of the rate of wear fragment formation and the reciprocal of the fatigue life of Au o~uthe atmospheric pressure.. A rate of wear fragment formation, sliding normal load 260 g, sliding distance 50 m; * reciprocal of the fatigue velocity 16.8 cm s life, frequency 16.7 kz, initial maximum bending strain 0.100%.

the freshly exposed surfaces near the tip of a crack during the tension part of the cycle for which the crack is open [ 71. At high pressures where an excess of oxygen molecules is available, the chemisorbed film on the crack surfaces stabilizes the crack by preventing healing of the crack during the compression

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part of the cycle; the crack growth rate is then independent of atmospheric pressure. When the pressure is reduced below some critical value pErit, the number of oxygen molecules reaching the crack tip becomes insufficient to cover completely the fresh surfaces with even a monolayer during the tension part of the cycle. Partial healing of the crack becomes possible and the crack growth rate begins to decrease. A sudden increase in the number of cycles to fracture then takes place, Thereafter, the fatigue life increases as less and less oxygen reaches the crack tip until at low pressure it becomes substantially independent of the atmospheric pressure again, though the extent of the increase of the fatigue life is beyond present knowledge. With chemically inactive Au, on which practically no chemisorbed oxygen film is formed, the extent of partial healing of the crack tip is expected to remain constant; a constant fatigue life thus results which is independent of atmospheric pressure. To a first order approximation, the critical pressure patit can be theoretically predicted by comparing the number of freshly exposed metal atoms at the crack tip with the number of oxygen molecules striking a unit area while the crack is open, as calculated from simple kinetic theory. When it is assumed that the oxygen pressure is one-fifth of the atmosphe~c pressure and that one oxygen molecule covers two metal atoms, this approach yields the expression (Zt/$l’)

X 6.2 X 1021 5

= 4.0 x 1o-22 x n/hat where 2’ is the absolute temperature, h is the interatomic spacing and t is half the time for one cycle. By substituting values from the present experiment of T= 300 K, h = 3.5 X lo-‘* cm and t = 3 X 10m2 s,pitit becomes 1.9 X lo-* Ton, When this calculated value is compared with the observed critical pressures (about 1 Torr with Ni and 20 Torr with Cu) it is found that the calculated value is lower by a factor of about lo* - 105; this discrepancy is of the same order as that in Snowden’s investigation [ 7f . Though considerable efforts have been devoted to modify the theory so that this discrepancy may be reasonably explained [lo - 111, it appears that no theory has yet been generally accepted. The same theory is now applied to the wear behaviour. The time a crack is open tWis assumed to be equal to the duration for which an asperity on a surface is in interaction with that on the mating surface. Then the critical pressure in the case of wear prtit can be written as Pcrit w = 4.0 x 1o-22 x fl h2b/v

where b is the mean length of a real contact point and u is the sliding velocity.

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With Ni, b is of the order of 0.05 mm under the nominal load when an asperity is characterized as a quadrangular pyramid, as described earlier [ 31. The critical pressure is then obtained by substituting values of T = 300 K, h = 3.5 X lo-’ cm, and u = 16.8 cm s-l: w 2 _Dcrit = 1.9 X loTO~JY This calculated pressure is lower by a factor of about lo3 than the observed value, which is about 20 Torr. Apparently, this result suggests that the discrepancy between the calculated and the observed pressure is smaller in the wear experiment than in the fatigue experiment. When the accuracy of the values of Pbrit,pr& and tW determined experimentally is considered, however, it may be doubtful whether this difference has any significance. It can be inferred that, if the above mentioned healing mechanism explains the pressure effects on the fatigue behaviour, the same explanation applies to the pressure effects on wear, i.e. the basic mechanism is common to both. The comparison between the wear and fatigue experiments thus leads to the conclusion that the change in the rate of wear fragment formation, i.e. the wear amount, with atmospheric pressure is caused by a change of the resistance of the materials to fatigue, a mechanical property of the materials, with the pressure. At higher pressures, Au showed minimum wear in the present experiment but the resistance of Au to fatigue was lower than that of Ni if they were compared under the same stress level. Prediction of the relative wear based on the fatigue resistance of the material is not possible with different metals. This may be due to a number of factors other than those analyzed in the present study, for instance, the difference in the actual stress levels, different extents of healing of the cracks and different deformation properties. The possible role of these and other factors must be determined in the future.

5. Discussion on mechanisms of wear and conclusion As this is the final report on this series of wear studies, mechanisms of the wear of metals are discussed. When the unlubricated wear of metals is investigated, experimental results are commonly interpreted in terms of adhesion theories. According to the adhesion theory formulated by Archard [4] the amount of wear is directly proportional to the normal load and to the sliding distance and it is inversely proportional to the flow pressure of the softer material. It is often observed that experimental results approximately satisfy this relationship, as with the present results [l] . However, the theory does not give any clear picture of the process through which experimental variables such as the sliding velocity, the environment etc. affect wear. That is, the theory is not concerned with the mechanisms of fracture which lead to loose fragment formation despite the fact that the wear is due to fracture in sliding contact. This deficiency must be remedied for a comprehensive understanding of wear.

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Recent investigation of the mechanisms of wear can be classified into two categories. One is a modification of the adhesion theory where certain new concepts are introduced into the constant of proportionality, or the probability factor [ 12 - 141; the other is the analysis of the wear process from entirely different viewpoints [ 15 - 171. The present study belongs to the latter category and the results can be summarized as follows. From the observation of vertical sections of worn specimens and wear fragments, an understanding of the process of wear fragment formation was obtained. As a result of the repetitive action of normal and frictional forces, plastic flow of material occurs in the substrate and then cracks extend along the flow which eventually liberate wear fragments. Such a process leads to the hypothesis that wear is caused by fatigue fracture in the sliding surfaces; fatigue fracture is defined as the fracture of the materials by repetition of stresses. When the dependence of the amount of wear on the experimental variables was compared with that of both the volume and the number of wear fragments, it became clear that not only the number but also the volume of wear fragments can be the critical factor which determines the change in the amount of wear. Some variables affect the wear predominantly through changing the volume of individual wear fragments while others govern the wear mainly by changing the number of wear fragments. The volume of fragments is determined by the depth at which cracks occur; the depth depends on the thickness of the pl~ti~~ly deformed layer and also on the actual magnitude of the normal and frictional forces working on the sliding surfaces. The number of wear fragments is governed both by the distance that a crack traverses to liberate a wear fragment from the bulk material, i.e. the volume of the wear fragment, and by the crack growth rate. If the volume of individual wear fragments is constant, the number of fragments will be proportional to the crack growth rate alone; the growth rate is related to the resistance of the materials to fatigue. This correlation between wear and fatigue is also supported by some supplementary findings. With Ni and Cu the high friction stage was preceded by a low friction stage caused by a surface film that was formed during the preparation [ 11. The duration of the latter stage was longer in sliding at lower atmospheric pressures. This could be attributed to the increased resistance to fatigue, which delayed the detachment of the first fragment carrying the surface film. Moreover, as shown in Fig. 6, severe rumpling of the surface of Ni fatigue specimens was observed only when they were fatigued at lower pressures. This may be related to the atmospheric pressure effects on the thickness of the deformed substrate layer in the wear specimens, though this effect is not marked [l, 31. In this experiment, the normal load and the sliding velocity changed the wear through varying the actual magnitude of the forces working on the surfaces and then the volume of wear fragments. In contrast the change of the wear with atmospheric pressure was due to a change in the number of wear fragments which was governed by the fatigue resistance of the materials.

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It may be stated that the mechanical factor affects the volume while the chemical factor is responsible for the number of wear fragments.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

N. Soda, Y. Kimura and A. Tanaka, Wear, 33 (1975) 1. N. Soda, Y. Kimura and A. Tanaka, Wear, 35 (1975) 331. N. Soda, Y. Kimura and A. Tanaka, Wear, 40 (1976) 23. J. F. Archard, J. Appl. Phys., 24 (1953) 981. H. Shen, S. E. Podlaseck and I. R. Kramer, Trans. Metall. Sot. AIME, 233 (1965) 1933. N. J. Wadsworth and J. Hutchings, Philos. Mag., Ser. 8, 3 (1958) 1154. K. U. Snowden, Acta Metall., 12 (1964) 295. R. L. Stegman and P. Shahinian, Am. Sot. Test. Mater. ASTM STP 459,1968, p. 42. K. U. Snowden and J. N. Greenwood, Trans. Metall. Sot. AIME, 212 (1958) 626. M. J. Hordon, Acta Metall., 14 (1966) 1173. F. J. Bradshaw, Scripta Metall., 1 (1967) 41. N. Soda and T. Sasada, Bull. Aeronautical Research Inst., Univ. of Tokyo, 4 (1964) 49. C. N. Rowe, ASLE Trans., 9 (1966) 101. E. F. Finkin, Wear, 21 (1972) 103. R. G. Bayer, W. C. Clinton, C. W. Nelson and R. A. Schumacher, Wear, 5 (1962) 378. R. G. Bayer and R. A. Schumacher, Wear, 12 (1968) 173. N. P. Suh, Wear, 25 (1973) 111.