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
166
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
167
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
171
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.
172
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.
173
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.
174
It may be stated that the mechanical factor affects the volume while the
chemical factor is responsible for the number of wear fragments.
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