The Running-in Process in Tribology. Proceeding of the 8th Leed–Lyon Symposium on Tribology, Held in the Institute National des Science Appliquées de Lyon, France, 8–11 September 1981

242 Pages·1982·10.28 MB·English

by  D. Dowson

THE RUNNING-IN PROCESS
IN TRIBOLOGY
edited by
D.Dowson, C.M.Taylor, M.Godet and D.Berthe
Proceedings of the 8th Leeds-Lyon Symposium on Tribology
held in the Institut National des Sciences Appliquées de Lyon,
France
8-11 September 1981
•Β
Butterworths
for the Institute of Tribology, Leeds University
and
The Institut National des Sciences Appliquées de Lyon
Published by Butterworths, PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 5BH,
England.
Copyright © Butterworth & Co (Publishers) Ltd 1982
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ISBN 0 408 01226 9
Printed in Great Britain
Introduction
The eighth Leeds-Lyon Symposium was held at the Institut National des from Leeds led by Professor Dowson and Dr Taylor.
Sciences Appliquées de Lyon from 8 to 11 September, 1981. This is the fourth The official banquet was held on the evening of Tuesday, 8 September, the
time in this series that the Symposium has been held in Lyon. opening day of the Conference, at the Château St. Bernard, 30 km north of
The subject chosen for this year's meeting was 'The running-in process in Lyon.
tribology'. This is in line with previous Symposia held in Leeds and Lyon which Festivities included an evening trip to the medieval city of Perrouges on
covered topics such as elastohydrodynamics, roughness and thermal effects in Thursday, 10 September, and an excursion on Saturday which took delegates
tribology. Twenty eight papers were presented, covering, amongst other to visit the Roman aqueduct at Pont du Gard, the medieval city of Aigues-
things, basic thermodynamics, mechanics of continuous solids, metallurgy, Mortes, the city of Aries and the village of Les Baux before returning to Lyon
polymers, profilometry and surface physics, all of which showed that running- late in the evening.
in is of primary interest in a large number of scientific and industrial problems. We wish to acknowledge the help we received from all members of the
Some 87 delegates from 13 countries attended the meeting, thus conserv- Laboratoire de Mécanique des Contacts, who solved the innumerable prob-
ing its international character. Such an attendance confirms our belief that lems which always occur before and during such events. Thanks are also due to:
conferences with one topic, in which both basic and applied research is DGRST, the Association Universitaire de Mécanique, Compagnie Française
presented, are still needed. We were particularly gratified to welcome a sub- de Raffinage, ELF France, CNRS and the Ministère des Universités, for the
stantial contingent from the USA and Japan which included loyal attendees to financial help which allowed us to receive delegates in greater comfort.
the conference such as Dr Saibel, Dr Wilcox and Professor Winer. The largest
contingent came from the UK amongst which we particularly noted Professor M. Godet
Barwell, Dr Lancaster, Dr Hamilton, Dr Greenwood and of course the group D. Berthe
Some of the delegates attending the Eighth Leeds-Lyon Symposium.
vii
Session I
Interactions in Tribology
Chairman: Professor D. Dowson
PAPER l(i) Chemostress effect in tribology
PAPER l(ii) The running-in of concentrated steel
contacts: a system orientated approach
Paper l(i)
Chemostress effect in tribology
Mikael Ciftan and Edward Saibel
We present a new theory of chemo-mechanical interaction that explains in one coherent framework a
number of so far inexplicable and seemingly disparate phenomena in the field of tribology on a quan-
titative basis starting from a first principles formulation.
The most important new quantity in this theory is the variation of the Gibbs chemical potential
with mechanical stress, a quantity we have formulated and calculated explicitly using statistical
thermodynamics and many-body theory. We call this quantity "the chemostress coefficient11 and the
related effect "the chemostress effect."
The theory gives the basis for quantitative explanation of stress-corrosion cracking, pitting
corrosion, fretting corrosion, the Rebinder effect, and enhanced chemical activity on crystal sur-
faces. In particular it suggests methods of arresting corrosion by controlling the charge distribu-
tion of electrolytes near the surfaces under consideration and explains observed correlations between
the zero of the zeta potential, microhardness and drilling rates. The connection of the chemostress
effect to the running-in process in tribology will be discussed.
DISCUSSION It is therefore not surprising that the
possibility of a chemical potential to carry
It was indeed a gigantic step by J.W. Gibbs detailed microscopic information on the state of
(réf. 1) to introduce the "chemical potential", a stress in a solid or on the surface of a solid,
result of his deep appreciation that somehow thereby elucidating the coupling between chemi-
chemistry had to be injected into the mechanistic cal and mechanical interactions at the atomic-
thermodynamics that had developed. Thus he added molecular level was not considered until very
to the equation that expresses the first and recently (ref. 2-7), particularly in situations
second laws of thermodynamics where chemical reactions occur in heterogeneous
systems such as at a gas-solid interface. We
dE = TdS - PdV call this coupling between the chemical and me-
the term chanical interaction the "chemostress Effect"
which is represented by the dependence of the
Σ yidNi chemical potential μ on stress σ via the deriva-
i tive r\
9μ
where Ε, T, S, Ρ, V are the internal energy, tem-
9σ
perature, entropy, pressure and volume of the
system; the new quantity μ. was the chemical po- that we have called the "chemostress coeffi-
tential of the i-th component and N. denoted the cient . "
number of atoms or molecules in i-tft component of Even if this quantity had been conceived of
the system. in its atomistic detail previously, calculation
Gibbs1 insight becomes even more notable of its value would have been fraught with many
when we remember that at that time quantum me- technical difficulties as the references 2,3,4
chanics wasnft yet developed, let alone quantum amply demonstrate not only because theoretical
statistical mechanics which one needs to calcu- physics had not advanced enough but also because
late the chemical potential from first principlea of the absence of experimental data on several
However, his feat was not a singular event; of the microscopic qualities such as excitation
most of thermodynamics was created by such giant energies, polarizabilities, phonon spectra, par-
steps in the absence of detailed microscopic ticularly as these change under pressure.
physics. It is also not surprising therefore to Some phenomenological attempts can be found
find that thermodynamics was a "mean field" type in the literature particularly in relation to
of a theory, a phenomenological theory, and a diffusion in solids, but again the fundamental
very successful one at that. role that the chemostress coefficient plays has
Putting oneself back into the last century, not been clearly brought out. What has been
one could still perceive that this "chemical po- done is more in trying to relate this quantity
tential" would be a function of pressure and tem- to other mean-field thermodynamic quantities.
perature for a mixture of gases even though one In a series of papers (ref. 2-7) we have
would not be able to calculate its value for rea- shown and calculated the stress dependence of
sons that we shall see. In fact one would have the chemical potential in three levels of re-
to wait for Ursell's pioneering work in the early finement: 1) statistical thermodynamic, 2) Max-
part of this century to obtain an equation of well Equations coupled to quantum statistical
state from frist principles! mechanics, 3) many-body Green's function theo-
3
retic. We have then (ref. 5-7) applied these re- interaction of repulsive Coulomb and attractive
sults to such phenomena as stress-corrosion, wear van der Waals forces the zeta potential of the
and the Rebinder effect. diffuse Gouy layer can be changed by adding cer-
Why is the chemical potential so important tain "impurities" into the electrolyte. Here it
in tribology? The answer is that whenever the is essential to understand the role of the in-
fundamental processes of physisorption, chemi- verse square force law, the Coulomb force, for
sorption, and diffusion and chemical reaction be- which there exists a theorem that we use. To
come involved in a given phenomenon then the explain the theorem the analogy to gravitational
fundamental quantity that drives the process is force can be used. It is the same theorem that
the chemical potential of the species involved. is responsible for the simple harmonic motion of
More exactly, in a given reaction it is the alge- a moving massive particle in and out of a tunnel
braic sum of the chemical potentials of the through the earth into which the particle is
species multiplied by their stoichiometric fac- dropped. The theorem states that a particle at
tors, this sum being called "afinity". We note any distance R away from the center of the earth
that we have included not only chemisorption but experience only the force from that portion of
also physisorption in which charge transfer does the total mass of the earth which is enclosed in
not take place. Again in the case of diffusion, a sphere of radius R and no force, from the mat-
the fundamental law is not Fick's law but the one ter outside of that sphere.
that starts from the gradient of the chemical po- Because of this theorem a test charge at a
tential: distance D away from the Helholtz layer on a
3μ colloid will experience only a limited and un-
9x balanced (nonneutral) part of the total charge
distribution which itself will be affected dra-
and therefore it is apparent that in a stressed
matically by the interdispersed "impurity ions".
environment we have
The repulsive Coulomb forces can thus be lowered
d\i do to such an extent that the weak van der Waals
do 9x ' forces can take over. The impurities thus drop
also other dependences may be involved in a par- the zeta potential to zero and thereafter speci-
ticular phenomenon: fic ions will diffuse via the chemical potential
gradient to the surface, now due to the van der
dv_ _9T _9μ dE j)y_ _9H
Waals physisorptive forces (the repulsive effec-
9T 3χ' dE Βχ' dH 3x'
tive net Coulomb forces having been lowered or
where Ε and Η are electric and magnetic fields. almost eliminated). Of course the stronger
Let us now mention briefly how the chemo- chemisorptive forces can also play an even
stress coefficient comes into stress corrosion stronger role once through physisorption atoms,
and the Rebinder effect before discussing its ions, etc. come close enough to the surface.
relation to "running-in" in tribology. What we have also shown is that the force
Stresses do come in either intrinsically as of attraction that lead to adsorption itself can
in stress-corrosion or crack propogation via im- change when there are stresses near the surface
perfections at surfaces such as crystalline of the solid. This is the effect of the chemo-
ledges, dislocations and crack tips, or they can stress coefficient and can be, for example, man-
be due to externally applied forces as in tests ifested at crack tips, dislocations coming to
for the Rebinder effect, microhardness and dril- the surface, surface steps, etc., where rather
ling which stresses in turn may generate defect high levels of stress exist compared to their
structures. In the case of "running-in" also, larger surrounding environment. Here the gra-
although there are several mechanisms involved at dient of chemostress coefficient, the driving
the macroscopic level, such as tearing off of as- force, gets a boost from the coupling of the
perities, melting and resolidification, there is chemostress coefficient to the stress gradient.
also no doubt that stresses are involved; that At this level of microscopic detail diffusion
is the obvious part. What has not been obvious and adsorption become intertwined and it is rea-
prior to recent work (ref. 2-7) is that in all of sonable to think that an oxygen ion driven into
these phenomena the separation of internal energy the crack tip may even pass into the "bulk"
into mutually exclusive two parts, a mechanical solid near the surface, cause volumetric changes,
component and a chemical (electron associated) thereby affecting further stress gradient and
component is not always strictly valid and that which in turn cause a stronger chemostress
if to within a reasonable approximation such a effect, and so on, causing an avalanche process.
separation is made, then there can be a left-over At this level of detail the phenomenological
chemomechanical term which shows precisely to macroscopic differences between stress-corrosion
what extent the mechanical components of the and Rebinder effect fade away but new and de-
energy of a cluster of atoms are coupled to each tailed specificity that is truly dependent on
other. The answer to the question of when such a the choice of materials, ions, etc. come up. We
separation is valid lies in a rigorous many-body begin to see clues as to which materials may de-
quantum mechanical treatment of these problems. crease or increase the zeta potential, the dril-
However, precisely because such a detailed treat- ling rate, microhardness, chemical reactivity,
ment becomes extremely difficult, if not impos- etc.; such specificity could not be understood
sible with present day tools of theoretical before. Of course, precise quantitative values
physics, it makes good sense to use a phenomeno- for the stronger chemisorptive cases can be ob-
logical approach to these problems and bring into tained only with detailed self-consistent many-
the theory as much of the experimentally well body theoretic calculations at a quantum mechan-
established principles and results of physics and ical level—a program that we are carrying out.
chemistry as possible. It is to this end that We have also outlined a series of experi-
our theory which explains the Rebinder effect and ments that need be performed to determine values
stress-corrosion uses the findings of Gouy and of several parameters that enter the theory so
Helmholtz on the effect of "impurities" on the that we need not await for the full quantum
"zeta potential" which we have explained in de- theoretical results before further progress can
tail (ref. 6,7). We have explained why due to be achieved.
4
As to the "running-in" processes, at the
microscopic level there is also no doubt that ul-
timately bond breaking is involved or, at the
minimum, bond rearrangement is involved. It is
here again that our extension of the concept of
chemical potential plays the role of the funda-
mental quantity particularly as it connects the
mechanical and chemical aspects of the processes
involved.
References
(1) GIBBS, J.W., Collected Works, Yale Univ.
Press, New Haven, Vol. 1(1957).
(2) CIFTAN, M. and SAIBEL, Ε., Solid State Com-
munications, Z7, 435(1978).
(3) CIFTAN, M., RUCK, V., and SAIBEL, Ε., Solid
State Communications, 27, 439(1978).
(4) CIFTAN, M. and RUCK, V., Physica Status
Solidi (b) 95, 237(1979).
(5) CIFTAN, M. and SAIBEL, Ε., Int. J. Engng
Sei, Ð_> 175(1979).
(6) CIFTAN, M. and SAIBEL, E., Wear, 53, 201
(1979).
(7) CIFTAN, M. and SAIBEL, Ε. , Wear, .56, 69
(1979).
5
Paper l(ii)
The running-in of concentrated steel contacts:
a system orientated approach
G. Salomon and A.W.J, de Gee
Transitions in the lubrication condition of sliding concentrated steel contacts are discussed with
special reference to running-in effects. It is shown that the persistence of high friction
transients, which precede stable low friction periods, depends on the chemistry of the lubricant and
on oxidation. Some examples of programmed, two-step running-in procedures are given as well. It is
concluded that the transition diagram may serve as a base for interlaboratory comparisons. Its use
should also considerably reduce (though not entirely eliminate) the need of (expensive) component
testing.
1 INTRODUCTION standard deviation in the
Km
distribution of roughness
Running-in is an ill defined engineering term, heights (r.m.s. value) (ym)
indicating the need of special operations in the
r radius of curvature of wear scar (mm)
commissioning of machinery. Despite of such pre-
cautions, seagoing vessels sometimes limp home r* radius of curvature of wear scar,
from an abandoned maiden trip, because the power formed during programmed running-
transmission system failed to run-in properly. in (mm)
Aircrafts in a corresponding situation might,
Ô temperature (°C)
unfortunately, reach the point of no return.
Persistence of such major engineering hazards t* running-in time (s)
has many causes, a frequent one being the tran-
sient occurrence of mechanical instabilities in cc duration of high friction period (s)
boundary friction regimes. As component testing V speed of sliding (ms"1)
is expensive and time consuming it seems justi-
fied to trace the origin of such phenomena with running-in speed (ms-1)
rapid, cheap, but highly reproducible simulating vm speed of sliding at which F^
tests. Patterns of running-in processes will be occurs (ms-1)
outlined first, next appropriate items from the
thin-film-lubrication-failure programme are t test speed (ms-1)
selected, amplified by additional experimental AV* volume wear during programmed
evidence, and finally the potential significance running-in f 3 Λ
of such data for the engineer will be briefly (mm )
discussed.
2 PATTERNS OF RUNNING-IN PROCESSES
1.1 Notation
2.1 Instantaneous friction transients
d* running-in distance (m)
On sudden loading, the initially low friction
F^ normal force (N) of a certain contraformal contact system may
increase instantaneously, but temporarily. This
FN* running-in force (N) was named by us the first primary friction
transition force (load carrying transition. After a high friction running-in
capacity) (N) period, the friction force comes down to a much
lower level. This, in our terminology, is the
FN maximum load carrying capacity secondary transition. A discussion of this
of (partial) EHD film (N)
running-in effect will be the principal topic
FT friction force (N) of the present contribution.
Once the structure of a simulating contact
f coefficient of friction (")
system is understood, its reactions to a change
k specific wear rate (mm3 N"1 m"1 ) in physical or chemical parameters can be in-
terpreted and used for comparative ranking of
ç number of revolutions (-)
materials. For example the high friction (but
transition value of Hertzian mild wear) running-in period can last from less
contact pressure (ΝðÃ2) than a minute to many hours (Figs. 5 and 6): an
information useful in the evaluation of running-
radius of curvature of
Rl in aids. A run-in cycle is completed when very
stationary specimen (mm)
low friction, in some cases perhaps even EHD,
R radius of curvature of is restored. A second cycle can then be super-
2
rotating specimen (mm) imposed, leading to a higher load carrying
6
capacity. But this has been achieved, at least found from v upwards, is supposed to be due to
m
with fully hardened steels, by an enlargement of the effect of frictional heating in the contact
the local area of conformity, while the critical zone, causing an appreciable decrease in ef-
pressure is likely to remain the same. This fective lubricant viscosity (order of magnitude
testing sequence has been extensively used by of temperatures: 25-200°C; cf. ref. 10).
Sakurai et al. (1). The following mechanism is supposed to be
responsible for the first primary transition
2.2 Programmed, step-wise running-in (curve A1-S-A3 in Fig. 2):
Already at values F^ far below the transi-
Heavy machinery is frequently subjected to step- tion value FJJ , occasional asperity contacts
c
wise running-in cycles of increasing severity. occur. At higher loads the number of contacts
This is done to reduce the danger of scuffing, increases exponentially. (An approximately
due to localized overloading in new components. normal Gaussian distribution of roughness
This practical experience is reflected in the heights applies to most technical surfaces). As
many steps prescribed for the FZG testing rig. long as the load remains below the critical
Each step being based on a standardized sequence value (Ffl < FJJ ) , rapid deformation and polish-
c
of loading and length of sliding periods. It is, ing of damaged asperities prevents scuffing and
by now, well-known that deviation from standard subsequent film collapse, but at = F^ the
c
programmes will lead to different results in influx of asperities into the contact zone be-
comparative studies. comes so large that this running-in process can-
Two widely differing programmed running-in not longer keep abreast of junction formation
cycles were performed on the TNO test rig. and 1incipient scuffing1 (region II) or full-
Certain aspects are reported below. scale scuffing (region III) develop. This model
accounts adequately for the effects of rough-
3 THE TRANSITION DIAGRAM ness, oxygen concentration and chemical react-
ivity of the lubricants as has been reported
By now the applicability of the F~v-T transition elsewhere (4, 5).
N
diagram for lubricated sliding concentrated In contrast to the above, the second
(counterformal) steel contacts has been firmly primary transition (i.e. curve A2-S in Fig. 2)
established (2, 3, 4, 5, 6). The diagram defines is supposed to be triggered by a metallurgical
the lubrication condition as a function of transformation in the steel. This is in line
normal force F^, sliding speed ν and oil bath with the fact that just prior to transition,
temperature T. It applies equally well to ball- the contact temperatures may easily reach 500°C
against-ball (3), ball-against-cylinder (4) and (c.f. ref. 10).
crossed cylinder (6) contacts and it is supposed
to simulate, in a first approximation, the be- 4 RUNNING-IN PROCESS ON INSTANT LOADING
haviour of components, as, for example, cams,
tappets, or gears. Such a similarity should be Two experiments with the system, shown in Fig.l,
of practical significance in the evaluation of typical for the construction of Fig.2, will now
novel materials, surface treatments and lubri- be discussed in detail. The inputs are: the
cants . normal load F^ and the sliding speed v. As in
A cross section at constant Τ for a com- any simulating system there is no use-output.
pletely oil-submerged contact (see Fig. 1 ) shows The closed system is therefore left free to cope
three regions (see Fig. 2), i.e. region I, in with the inputs and minimize or maximize loss-
which friction is low and wear is nihil or very outputs, i.e. friction and wear, until a steady
low (see below), region III, where severe wear input/output ratio is reached. The friction
and scuffing occur and an intermediate region II, force is recorded continuously. The electrical
characterized by a transient high friction, but contact resistance can also be measured on the
only marginal, mild wear. It is assumed that in operating system. Changes in the appearance of
the three regions the following lubrication the friction track are followed visually. Wear
mechanisms apply: marks or scars, however, can be measured only
after termination of the experiment. Transitions
region I : (partial) elasto-hydrodynamic on the third primary (high wear) level - avoided
lubrication in the experiments presently to be discussed -
region II : boundary lubrication are accompanied by a large increase in noise
region III : unlubricated contact (although the level.
contact is still fully submerged in
Figs. 3 and 4 show friction force F versus
the lubricant). T
η (number of revolutions) diagrams, recorded
The transitions from I to II, II to III and I to during tests with virginal, newly assembled
III are termed first second and third primary specimens. The test conditions, given in the
3
transition, respectively. The lower curve Al-S- caption of Fig.2, apply. Fig.3 shows F^-n
A3 is believed to be continuous, point S being curves, measured at vt = 0.7 m/s, under normal
merely the intersection between curves A1-S-A3 forces FN of, respectively, 300 Ν and 350 N.
and A2-S. Early work on the subject (7, 8) as Clearly a transition in frictional behaviour
well as recent research (9) shows that the occurs between 300 Ν and 350 N. In fact under
location of the curve A1-S-A3 depends on vis- Fjq = 300 Ν the system runs in lubrication re-
cosity, this being the main reason for assuming gime I and under Fjq = 350 Ν the system runs in
that in region I a thin elastohydrodynamic lubrication regime II (c.f. Fig.2). Similar in-
lubricant film keeps the surfaces apart. formation for vt = 0.07 m/s can be found in
The load carrying capacity F^j reaches a Fig.4. The transition from region I to region II
maximum F^jm at a low speed vm (order of magni- now occurs at application of a normal force FN
tude: 1 mms~l). At lower speeds (v < v) hydro- between 500 Ν and 600 N. Figs. 3 and 4 show that
m
dynamic wedge effects are supposed to predomi- at Fjq < F^c (lower parts of Figs. 3 and 4), at
nate. The pronounced decrease in load carrying both values of test speed vt, a disturbance1
capacity at increasing speed of sliding, that is occurs at the beginning of the test (i.e.
7
directly upon application of the normal force). other experimental conditions are given in the
In both cases the duration of this high friction caption of Fig.5.
period corresponds to approx. 5 revolutions In Figs. 5 and 6, t denotes the duration of
c
(1.2m sliding) and a maximum coefficient of the high friction period. The figures show that
friction f .2 0.2 is reached. Additional results, the nature and composition of the lubricant have
presented elsewhere (9), show that a similar a pronounced influence on the duration of the
disturbance (n » 5 rev.; f x ~ 0.2) is found at high friction period t . It follows from Table 1
ma c
lower speeds (measured down to ν = 0.0007 m/s) that these differences are not related to dif-
as well. Undoubtedly this high friction period ferences in viscosity, as might be expected for
is due to the fact that, initially, the surfaces systems operating in the boundary lubrication
of ball and cylinder are not completely separated regime.
and contact between the higher asperities occurs. Much larger than the influence of individu-
However, after a little while (i.e. at η ~5 rev. ), al lubricants is the effect of oxygen. In an at-
such asperities have been removed by deformation mosphere of argon with 0.1% oxygen the high
and polishing and complete separation of the friction period is drawn out over several de-
surfaces is obtained. This is confirmed by cades, as evident from the different time scales
electrical contact resistance measurements over used in Figs. 5 and 6 and from the t values
c
the contact, which show the presence of an compiled in Table 1. The maximum friction
undisturbed liquid film. values, expressed as f, are much higher in Fig.6
At Fjy > F^ (i.e. when the system is running than under air cover. (At this moderate speed no
c
in lubrication regime II), a much more pronounced scuffing occurs despite of the high friction
'disturbance* is observed (upper parts of Figs. 3 peaks). Further, under argon cover the friction
and 4, respectively). At v = 0.7 m/s the force versus time curves show intermediate
t
duration of this high friction period amounts to friction levels at f = 0.3, particularly marked
approx. 200 rev. (48 m sliding); at v = 0.07 m/s with lubricants A and B. Such step-wise changes
t
it takes some 40 revolutions (9.6 m sliding) for during high friction are not observed under air
the friction force to reach a low constant level, cover. Inspection of the friction surfaces at
corresponding with f a 0.11. At both test speeds intervals, during the tests with argon cover,
f reaches a maximum value of 0.31-0.33. This reveals changes in the appearance of the wear
time electrical contact resistance shows that tracks. During the intermediate period the ring
the surfaces of ball and cylinder are in intimate surface changes to a dull brown track, while the
contact all the time, i.e. when operating in pin is still brightly metallic in appearance.
lubrication regime II, fluid film separation of Later, when the run-in is completed, both pin
the surfaces is never obtained during the test and ring have a dull brown appearance. Compari-
duration of 440 s. Instead the low values of f son with pairs, oxidized during friction in air,
that are eventually reached (i.e. f = 0.10 at makes probable that even at the very low oxygen
v = 0.7 m/s and f = 0.12 at v = 0.07 m/s) are concentration under argon + 0.1% oxygen cover,
t t
ascribed to boundary lubrication, connected the friction surfaces become oxidized (FeO).
closely with oxidation of the contact area. These observations clearly show that the
To revert from the boundary lubrication running-in process under conditions of boundary
regime II, reached in Figs. 3 and 4 by a slight lubrication is governed by tribo-chemical re-
increase in load, back to the EHD condition of actions. This conclusion if further supported by
regime I, only one option is open to the system: data compiled in Table 1. In air even the
Pressure has to be reduced by increasing the paraffinic oil (B) promotes run-in within 3
local conformity at the tip of the pin. As the minutes to completion, while 200 minutes are
wear rate is extremely low (see further) and the needed under argon cover. The higher reactivity
surface roughness, formed during running under of oils C and D is presumably due to the content
boundary conditions, is rather high (i.e. R ~ of aromatics and the additive package.
m
1 ym) , this transition usually occurs only after Obviously a certain amount of 'run-in' wear
many hours of running in regime II. takes place during the high friction period. A
measure for this is the wear scar at the hemi-
spherically tipped pins (column 4 in Table 1).
5 TRIBOCHEMICAL PARAMETERS
The longer high friction periods in the argon
covered system lead to slightly larger contact
The dominating effect of surface oxidation on
areas and the faster processes in the more re-
transition phenomena will be discussed now.
active oils cause less run-in wear.
However, as other reactions (with additives) or
tribopolymer formation might take place, mostly
5.2 Wear rates
on a minor scale, an all-embracing heading has
been chosen for this section. Comparison with results obtained in other labor-
atories and also with industrial experience is
5.1 The running-in cycle facilitated by transforming information as pre-
sented above into wear rates. The amount of wear
Tests in the first primary transition region
is calculated from the size of the scar on the
were performed with four different lubricants
spherical pin tip; dividing by the length of the
under air (Fig.5) and argon + 0.1% O2 (Fig.6)
high friction period t , the speed ν and the
cover, respectively (11). The figures show c
normal force F^ yields an average value of the
friction force F - time t curves, obtained in
T specific wear rate k (volume of material, re-
lubrication regime II. The four oils were:
moved per unit of sliding distance, per unit of
A : SAE 10 W base oil normal force). Results are summarized in Table 2.
Β : medicinal white oil It can be seen that the k-values are of the
C : HD SAE 20 W crankcase oil order of 1-10 units if the system runs in the
D : proprietary hydraulic mineral oil with transient high friction periods of regimes I or
a heavy additive package. II (note, however, that the duration of this
period is much smaller in regime I than in
Their viscosities are recorded in Table 1. The
regime II; c.f. Figs. 3 and 4). In the stable
8