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Steel-to-steel friction over a very wide range of sliding speeds
338
WEAR
STEEL-TO-STEEL SPEEDS
FRICTION
OVER
G. V. VINOGRADOV,
1. V. KOKEPOVA
AND
A VERY
YU.
YA.
WIDE
RANGE
OF SLIDING
PODOLSKY
Instituteof Petrochemical Synthesis of the Academy of Sciences of the USSR, Moscm (USSR) (Received
November
28, 1966)
SUMMARY
This paper presents the results of investigations of hardened steel-to-steel friction in the presence of the nonpolar (napththeno-paraffin) fraction of bright stock, individual hydrocarbons and polyorganosiloxanes. The range of relative velocities of the friction bodies studied embraces nine orders of magnitude. It is shown that at constant loads and bulk temperatures of the Iubricating medium both raising and lowering the sliding speed may result in a rise in the friction coefficient. At high speeds the sharp rise in the friction coefficients involves a thermal shock in the friction zone and a considerable increase in the rate of wear, this phenomenon being known as hot seizure. At low and super-low speeds the friction coefficients increase to a certain limit value more or less slowly and this increase is not accompanied by a rise in temperature. It is suggested that this process be termed “cold seizure”. Its course depends on the nature of the lubricating medium. The assumption is put forward that cold seizure is related to intensive development of plastic deformation of the metal throughout the entire friction zone, i.e., is due to its athermal plasticity. Six specific features of cold seizure are given.
Heavy boundary friction duties (when high contact pressures come into play) depend to a considerable extent on the speed of relative movement of the rubbing bodies. In most papers devoted to friction research it is accepted that the effect of the sliding speed is due primarily to the thermal effects resulting from intensification of heat liberated at the contact when the speed is increased. This assumption has formed the basis of several methods of estimating the E.P. characteristics of lubricating materials rs2and methods of modelling friction processes under static conditions3-7. The correctness of this conception has been questioned in the literatures. In connection with the discussion of the results of investigating the mechanism of action of certain E.P. additive@, it was assumed that the sliding speed is important from the point of view of the kinetics of interaction of lubricants with the microareas of the virgin metal surface bared in the process of friction: raising the speed reduces the time during which a protective layer of oxides or inorganic salts of the metal, preventing development of microseizure, can form. In connection with this, when the speed is increased, WW’,
IO (1967) 338-352
STEEL-TO-STEEL
FRICTION
339
the number of single acts of microseizure and the probability of occurrence of macroseizure rise sharply. Further research into steel-to-steel friction on variation of the sliding speed over a range of g orders of magnitudei@12 revealed another characteristic feature of its influence on processes taking place at high specific loads in the presence of lubricants. It was established that, at low sliding speeds, the dependence of friction on the load deviates considerably from direct proportionality. In the papers quoted a new phenomenon was described, accompanying friction processes at these duties. It was called “cold” seizure to distinguish it from “hot” seizure which is characteristic of friction at medium and high sliding speeds. Since the study of friction at low and superlow sliding speeds is not only of theoretical, but of great practical interest as well (sliding guides, friction units in instruments, etc.), the principle features of cold seizure are discussed in this paper and this phenomenon is compared with hot seizure. EQUIPMENT
AND OBJECTS OF STUDY
The tribometer The work was carried out with a four-ball tribometer which has already been described very brieflyis. The fundamental part of the instrument is a friction unit constituting a pyramid of four steel balls (I/Z in. in diameter, grade LUX = 6 steel, hardness HR,=60-62). Friction is caused by rotating the ball at the peak of the pyramid. The three bottom balls are fixed in a cup filled with the lubricating medium and connected with a rigid dynamometer, the latter being a steel plate with tensometric wire pick-ups glued to it. During the friction process the dynamometer readings are recorded continuously on the tape of an electronic potentiometer. A hydraulic loading system is employed. Its essential advantage is that it ensures damping of the vibrations arising in the friction unit. Realisation of a sufficiently extensive range of loads (from 1-800 kg), is obtained by varying the ratio between the plunger areas in the hydraulic press. For this purpose one of them is made in the form of two concentric cylinders which may be separated or connected rigidly. In the latter case the cylinders become an integral whole and therefore a large diameter piston. A special dynamometric insert between the plunger and the friction unit, made in the form of a system of tensometric beams, serves for measuring and recording the axial stress during friction. An important and complicated problem involved in studies of friction over an extensive range of low sliding speeds is that of ensuring uniform movement of the friction bodies at the contact. This problem has been solved by using as the drive a combination of a hydraulic pump and a system of hydraulic motors, each with a worm reducer to reduce its rev./min. The hydraulic motors were employed in the range of speeds where the rev./min practically does not depend on their shaft moment. In a worm engagement the motion is transmitted with the worm continuously contacting the worm wheel. Therefore the use of such a drive ensures uniformity of rotation of the tribometer shaft. By interconnecting the five worm reducers in series by means of claw clutches the speed of rotation can be varied between 2,500 and 8.10-6 rev./min, which corresponds to a change in sliding speed at the theoretical point of contact between the balls of between IOO and 0.3.10-6 cm/set. Within the Wear, IO
(1967) 338-352
G.
340
V. VINOGRADOV,
I. V. KOREPOVA,
1-U. YA.
PODOLSKY
limits of each of the five steps of speed variation it can be increased or decreased smoothly in time according to a linear law. Such programming of the velocity is achieved by changing the volume capacity of the pump with the aid of a special servomechanism. The tribometer enables tests to be carried out not only at room temperature, but at elevated temperatures as well. Two types of heaters are used for this purpose. The first is a jacket with the heat carrier from a thermostat circulating through it, which is put directly on the friction unit and is used in the temperature range of zo-70% This method of thermostatic control ensures high accuracy in sustaining the bulk temperature of the friction unit at a preset level, but has a serious disadvantage in that the flexibility of the rubber hose causes a substantial error when measuring friction forces. This disadvantage has been eliminated in the heater of the second type, which is a radiation furnace made in the form of a low resistance element and used for creating temperatures between 50 and 600°C. To concentrate the heat flux entirely on the object of thermostatic control the element is mounted inside a cylindrical screen made of polished metal, encompassing the friction unit but not touching it. The hot joint of a thermocouple situated in the direct vicinity of the heater coil registers the intensity of heat flow from the heater to the friction unit and may serve as a pick-up in an electronic circuit designed to programme the increase in the bulk temperature of the four-ball pyramid. The working cycles of the tribometer are controlled automatically. The circuit for timing the tribometer operation is based on a multicircuit time relay. This is a system of disks rotated by a s~lnchro~ous motor. As the disks rotate, stops fixed to them close the contacts of independent electric circuits. The length of the time interval elapsing between the moments of making of each of the circuits can be varied at the experimentor’s will from o to 20 min. The circuit used enables not only automatic control of a number of operations, but multiple repetition of these operations as well.
Friction
and wea testing The friction study was carried out as a series of tests. Each series was run at a single sliding speed. It began with a test at minimum load and proceeded without changing or turning the ball and without replacing or adding lubricant. The load was increased from test to test. Each series ended at the seizure load or at even higher loads. In all cases the friction unit was loaded before setting the require(1 speed. Balls and lubricant were replaced before each series of tests. The initial load in each series of tests and the load increment from test to test (degree of loading) depended on the sliding speed. Steady friction conditions at sliding speeds above z.3.10-” cmjsec were reached in less than I sec.. At lower speeds the durati~)n of the tests had to be prolonged, and the more so the lower the speed. In this study only the occurrence of seizure was registered, without going into the features of its development. Accordingly, at high sliding speeds the duration of the test was 30 sec. Table I lists the conditions under which the tests were made for clifferent duties. Wear was assessed by the diameter of the wear spots on the lower bail which were measured with a tool microscope. The main object of study was the nonpolar naphtheno-paraffin fraction of bright stock (viscosity ~~0= 57.5 cSt). In addition, low molecular weight individual Wear, 10 (NV)
338-352
STEEL-TO-STEEL
TABLE
341
FRICTION
I
FRICTION
DUTIES
Sliding speed
Initial drgvee (kg)
(cm/set)
IOO-58 58hJ.23 2.3.10-2-2.3
load and
of loading
3.75 7.5 30.0
10-e
of test
Dwation
Intevval between tests
(win)
(min)
0.5 0.5 6.0-60
3 3 5-10
hydrocarbons (cumene and n-heptane, see Table I, ref. IO for description) polydimethylsiloxane liquids of different viscosities were studied. RESULTS
AND
and two
DISCUSSION
Our friction
study covering
a wide range of velocities
showed that there exist
three ranges of sliding speeds, each characterized by a definite change of friction in time z at various loads, W. Figure I shows the friction vs. time diagrams obtained for sliding of steel in the presence of the naphtheno-paraffin fraction of bright stock.
U=23 cm/set
II=0.23 cm/see
Fig. I. Friction coefficient non-polar fraction of bright diagrams (in kg).
vs. time diagrams obtained for steel-to-steel friction in presence of stock. Sliding speeds are indicated at the right; loads, directly on the
The sliding speeds at which the tests for each of the rows of diagrams were performed are shown at the right and the loads directly on the diagrams (in kg). The bulk temperature of the oil in these tests was ZO’C. The time is indicated in minutes. In the range of high sliding speeds (23 cm/set and more) at small loads (diagram
I)
the friction
force, f, reaches
a stable value in a very short interval Wear,
IO
of time
(1967) 338-352
342
G. V. VINOGRADOV,
I. V. KOREPOVA,
YU. YA. PODOLSKY
commensurable with the frequency characteristic of the recording instrument, and subsequently does not change with time. At the moment the drive is stopped, marked on this and the subsequent diagrams by an arrow, a more or less short-lived increase in friction is observed, which has already been discussed 15.The nature of the variation of friction with time corresponding to diagram I is retained at all loads right up to the load at which seizure develops (at 165 kg in the case under discussion, diagram 2). Besides a sharp increase in friction, seizure is accompanied by a sharp rise in wear and in the temperature of the rubbing bodies. It is the considerable heat effects accompanying this process that justify the name of hot seizure. The severity of hot seizure may differ depending on the conditions of friction, and the chemical activity of the lubricant and the gas phase. The most severe forms of seizure may end in welding of the friction surfaces. On the other hand, seizure may be degenerate in naturel4315, when only a brief insignificant rise in friction and wear are observed. Typical of all kinds of hot seizure are more or less profound structural changes in the surface layer of the metal under the influence of the thermal shock and adhesion of the heated friction surfaces. In the second speed range (between about 2.3 and 2.3 - 10-2 cm/set, diagrams 3 and 4, Fig. I) the friction coefficient vs. time diagrams are similar to diagram I. The peculiarity of friction in this sliding speed range is the absence of seizure throughout the entire load range used (up to 500 kg). At low and superlow sliding speeds (from 2.3.10-2 cmjsec and lower, diagrams 5 and 6, Fig. I) and sufficiently high loads the friction force is observed to increase slowly with time, and the greater the load, the longer it takes to reach a steady friction value. This is undoubtedly related to the formation of a definite state (structure) in the friction zone. Diagrams 5 and 6, Fig. I, are presented in idealized form in the sense that more or less smooth fluctuations of the friction forces are usually observed. A distinct difference between the duties of boundary friction at high and low sliding speeds can be traced by the load dependencies of the steady friction values at constant speed. The corresponding curves obtained at three different speeds for the same naphtheno-paraffin fraction of bright stock are shown in Fig. z(a). Were curves 1-3 correspond to the speeds 23, 2.3 and 2.3+x0-* cm/set. The tests were run at a bulk temperature of the friction unit of zo°C. For comparison Fig. z(b) shows the load dependencies of steady friction values at different speeds for the case of dry friction. Now let us first consider
Fig. 2(a).
In the range of high and medium sliding
speeds the F(W) dependencies fit well on straight lines passing through the coordinate origin. At a speed of 23 cmjsec there is a sharp break in the line at the point corresponding to the seizure load. The line corresponding to a speed of 2.3 cmjsec is straight over the entire range of loads studied. As the sliding speed decreases the steepness of the lines expressing the dependence of F on W increases and, accordingly, so do the friction coefficients, f. In the case of very low sliding speeds (curve 3) the friction increases rapidly with the load. However, in the region of high loads growth of the friction slows down. For the ease of the lowest speeds, considered here, curve 3(a) shows the load dependence of the effective friction coefficients calculated as the ratio of the friction force to the load. As curve 3(a) shows, with growing loads under conditions of low sliding speeds, friction coefficients are reached which are of the same order as those observed under hot seizure duties when the loads are not very Wenu, 10 (194~)33%35Z
STEEL-TO-STEEL
FRICTION
343
much higher than the critical seizure loads. It is this circumstance that makes it possible to regard the rise in friction at low speeds as a seizure process also but one of cold seizure in this case, since the rise in F in the range of low sliding speeds is not accompanied by any perceptible thermal effect. The slow-down in the friction increase with increase of the load in the high-load region must be related to plastic
(6)
Fig. 2. (a) Load us. friction and load vs. friction coefficient curves in presence of non-polar fraction of bright stock. (b) Load vs. friction and load vs. friction coefficient curves, dry friction.
deformation of the rubbing bodies, i.e. is due to flowing of the metal in the zone of contact between the balls. Naturally, this process is of basic importance at very low speeds and may require a considerable length of time for its development (diagrams 3 and 6, Fig. I). This is supported by Fig. a(b). Indeed, in the case of dry friction the dependencies of the friction and the friction coefficient on the load at low sliding speeds is similar in nature to that observed in the presence of lubricating media. This may be regarded as evidence of the fact that at low and very low sliding speeds, when the process of cold seizure is developed in the presence of lubricating media, its mechanism is close to that of plastic flow of the metal at low speeds and high contact stresses. This is exactly the process described by the top parts of the F(W) curves and the downward branches of the f(W) curves. Taking into account that under conditions of intensive plastic flow Coulomb’s law is inapplicable, the lowering of the effective friction coefficient in the range of high loads is a fictitious mollification of the friction duty. At the same time, the extreme nature of the load dependence of the friction coefficient is good evidence of the change in the friction duty. Thus, slow attainment of a stationary friction duty and weakening of the load dependence of friction at high values of load are the first two specific features of friction at low and superlow speeds. Wear,
IO
(1967)
338-352
344
G. V. VINOGRADOV,
The third specific
feature
of cold-seizure
I. V. KOREPOVA,
friction
YU.
YA.
PODOLSKY
is that it involves
no con-
siderable wear despite high contact stresses. This is illustrated by the data of Table II which shows the wear values obtained in a number of tests run with the naphthenoparaffin fraction of bright stock at a bulk temperature of the friction unit of ZO’C.
.-I veva,~r dianzeter
o,fZWY spot at end of seriesof tests
i Wl)
77 58 46 2.3 2.3 2.3’ 10-l 2.3.10-2 2.3 10-a 2.3.10-a 2.3 10-a
37.5 4s 67.5 150 405 360
270 450 4So 4’0
0.30
I.59 1.67
0.31
0.36 0.40 0.64
2.25 0.7'
0.67 0.67 0.56 0.05 0.62 0.02
0.02
0.50 0.67 0.68
0.64
Fig. 3. Effect of sliding speed on results of dry friction deformation area diameter (after Hertz).
wear tests;
a, load dependence
of elastic
Hot seizure (lines 1-4 of Table II) occurs at relatively not-very-high loads, but is accompanied by considerable wear of the steel surfaces. In the case of “cold” seizure (lines 7-10) the wear spot diameters differ little from the diameters of elastic deformation areas calculated according to the Hertz theory. “Cold” seizure leaves the friction surfaces smooth and without tears or excrescences. However, the most surprising are the wear values measured for dry friction Weau, IO (1~67) 338-352
STEEL-TO-STEEL FRICTION
345
of hardened steel. They are presented in log-log coordinates in Fig. 3, where the axial loads are plotted as the abscissa and the average diameters of the wear spots on the lower balls, as the ordinate. In these tests, the loads were changed, as usual, from low to high values. The time conditions of the tests corresponded to the data of Table I. After each test at a given load the four-ball unit was dismantled and the wear measured, after which the next test was performed without altering the position of the three balls fixed in the cup. It is evident from the data, of Fig. 3 that, at very low speeds, the wear values are insignificant even for dry friction, though they are slightly higher in this case than for friction in the presence of lubricant. One may ask whether comparison of wear scar diameters produced at greatly differing speeds and different friction paths is quite legitimate. It is evident from the data of Table II that the wear spots produced at speeds differing by a factor of 105 (and friction paths by 103) are of the same size. Besides, two tests were run with the oil indicated above at 2.3.10-3 cmjsec which corresponded to cold seizure duty. The conditions of these two tests were absolutely identical (load 270 kg) except for their duration, that of the first being 6, and of the second, 540 min. The results recorded in both cases were practically the same (wear scar diameter 0.54 mm). Thus, a go-fold increase of the friction path had no effect on the wear value under the conditions of the tests. This shows that comparison of the data of Table II is justifiable. The two main types of seizure, viz., “cold” and “hot”, observed during heavy boundary friction duties are analogues of the processes of adhesion of the first and second kind according to the classification of types of wear of metals during dry friction, given by KOSTETSKY~~. The presence of a lubricant substantially changes the course of seizure processes compared with the adhesion processes indicated, These differences are manifested primarily in the fact that, unlike adhesion of the second kind, on dry friction “hot” seizure may become degenerate in the presence of a lubricating material14J5. Besides, as indicated above, when “cold” seizure occurs, the intensive wear characteristic of adhesion of the first kind is absent. However, in our tests and on dry friction under conditions of low speeds that very intensive wear which characterizes adhesion of the first kind according to KOSTETSKY, is absent. The reason why our tests disagree with the data of KOSTETSKY is not clear. The fourth peculiarity of cold seizure is its “reversibility”. Tests to illustrate this were run with the lubricant indicated above and a bulk temperature of the friction unit of 2o“C. They were started at a speed of 2.3 cm/set, raising the axial load on the four-ball pyramid in steps up to 360 kg (as indicated in Table I). After this, the load was left constant and the speed was lowered in steps (reducing it tenfold). When the sliding speed is reduced to a certain critical value the effective friction coefficient was observed to increase in time. After a constant value of the latter was reached the speed was changed to a lower one. After a constant value of the effective friction coefficient was attained at a speed of 2.3-10-5 cmjsec the speed was raised in steps in the same way as it was previously lowered. Thus, friction was always along the same wear scar. The result of the tests considered here is shown in Fig. 4, where the arrows indicate the direction of switching from speed to speed. It is evident from the data presented in Fig. 4 that, irrespective of the direction in which the speed is changed, the occurrence and disappearance of cold seizure is confined to the same range of sliding speeds. Under the test conditions in question Wear, IO (1967) 338-352
G. v.
346
VINOGRADOV,
I. V. KOREPOVA,
YU.
YA.
PODOLSKY
this occurs in the sliding speed interval 2.3.10~%?.3.10-3 cmjsec. The reversibility of cold seizure is evidence of the reversibility of the states of contact between the steel balls when the friction duties are changed. -4s indicated, the formation of a definite state of contact takes place in the pre-stationary stage (see diagrams 5 and 6 in Fig. I), which is not reflected in Fig. 4 where steady friction duties are represented.
Fig. 4. Effect of sliding speed on steel-to-steel of bright stock.
friction
coefficient
in presence of non-poiar fraction
The fifth specific feature of cold seizure is that the steady conditions at which it occurs are not affected by certain agents causing chemical and chemi-sorptional modification of the steel surface, preventing the occurrence of hot seizure or resulting in degeneration and break-off of the latter. The development of cold seizure is approximately the same whether it occurs in air or in a vacuum of the order of 10-5 torr, nor does the addition of such organosulphur compounds as dibenzyldisulphide to the lubricating oil prevent its appearance. This means that molecular oxygen and organosulphur compounds of medium activity, which usually affect hot seizure similarly and very strongly, have no substantial effect on friction duties at low sliding speeds. The same may be said with respect to the action of stearic acid type surfaceactive substances in the air. At the same time organophosphorus additives to lubricating oils, such as (CCl~)PO(OC&I~)~, prevent cold seizure. This is evidence of the specific influence of the chemical composition of lubricating media on cold seizure, This question will be discussed further below. The sixth specific feature of cold seizure is the peculiar effect of temperature on its occurrence. With rising bulk temperature of the metal the occurrence of cold seizure is facilitated in the sense that it may be observed at a higher speed. In the case of hot seizure, increasing the bulk temperature of the friction unit lowers the value of the critical speed at which seizure occurs in a definite load intervalIT. This is illustrated by the results of the tests with the naphtheno-paratfin fraction of bright stock at 25,75, and 1z5Y. These results are presented in Fig. 5 in diagramatic form by a shaded band denoting the sliding speed region in which both types of seizure discussed here are observed. Increasing the bulk temperature of the friction unit from ~5 to 75°C brings down the lower speed boundary at which hot seizure can be registered in the range ~f’,ear, IO (1967)
338-352
STEEL-TO-STEEL FRICTION
347
of loads investigated, by approximately one order of magnitude. On the other hand, the range of sliding speeds corresponding to cold seizure expands with growing temperature toward higher speed values. In the case of 125°C the speed boundaries at which hot and cold seizure are displayed, are not shown in Fig. 5 because rapid development of hot seizure at a sliding speed of 2.3.10-s cm/set evidently masks the rather slow rise of the friction coefficient on cold seizure. Thus, a rise in temperature results in a narrowing of the speed range within which friction is characterized by the absence of both types of seizure within the load interval used.
Fig. 5. Diagram of cold and hot seizure boundaries at various temperatures of bright stock. Friction unit bulk temperatures are indicated at left.
for non-polar fraction
In studying the friction of steel at high sliding speeds it was found that the composition of mineral lub~cating oils substantially influences their antiwear and antifriction characteristicsls. Subsequently a very great difference was detected between the antiwear and antifriction properties of low-molecular weight hydrocarbons depending on their nature, which was found to be related to their different oxidizabilities and capacities for carrying active oxygen to the steel surface in the friction zonelo. Investigation of the friction and wear of steel in the presence of Iow-molecular weight hydrocarbons at low sliding speeds and comparison of their behaviour with what is observed for lubricating oils, reveals how the nature of the lubricating media influenced the development, not only of hot, but of cold seizure as well. The nature of the behaviour of low-molecular weight hydrocarbons at the speeds and loads at which both hot and cold seizure are possible is illustrated vividly by the data of Figs. 6 and 7 and Table III. As in the cases considered above, each series of tests at a given sliding speed was carried out without turning the balls and with the wear spot successiveIy increasing. The shaded vertical bands in Figs. 6 and 7 characterize the friction fluctuations at constant speed and load values. They were sharp in the case of hot seizure and smooth with cold seizure. The height of the shaded bands characterizes the amplitude of the friction fluctuations. If only the load dependence of the friction (steady or pseudo-steady values) are considered, the type of seizure cannot be determined. It is essential to make use of wear data (Table III) and to take into account the nature of the friction ?rs. time diagrams at constant speeds and loads. Weear,IO (x967) 338-352
G. X7.VINOGRADOV, I. V. KOREPOVA, TU. YA. PODOLSKT
348
In the case of n.-heptane (Fig. 6) at a speed of 2.3 cm/see, hot seizure occurs directly after a load of 15 kg. It is manifested in more or less considerable (sharp) fIuctuations of the friction a.nd in elevated wear. Lowering the speed to 2.3*10-i and 23.x0-2 cmjsec raises the critical seizure loads to 4s and 120 kg respectively.
Load,
W/kg)
Fig. 6. I.oad us. friction curves in presence of n-heptane. 30 cl -2.3
Fig. 7. Load us. friction curves in presence of cumenc.
Kowever, further decreases of the speed to 2.3.10-3 cmjsec results in cold seizure. This causes an increase in the steepness of the initial part of the F(W) dependence, so that cold seizure at 2.3 *1~1-3cm/see and hot seizure at a speed of 2.3 cmjsec are described by practically the same curve. However, no sharp friction fluctuations are observed at cold seizure and the wear is small. Wcav, IO ($967) 338-352
STEEL-TO-STEEL
349
FRICTION
UBLE III WEAR
OF
Substance
Cunlello
LOWER
BALLS
IN PRESENCE
Sliding speed (cmjsec)
2.3
2.3.10~’ 2.3.10-z 2.3 n-Heptane
10-a
2.3 *.3.10-'
2.3.10-Z
OF
Load (kg)
AND
1Z-HEPTANE
.4 uerage WeaY SCaY diametev at end of test series
Diametev ofelastic deformation awas (after Hertz)
(mm)
(mm)
300 315
0.64 0.58
0.58
180
0.50
225
c.55 0.58
0.49 0.53 0.58
0.38
0.27
I.02 0.32
0.53 0.31
0.36
0.34
0.43 0.49
0.44 0.49
0.43 0.54
0.43 0.49
315 30 225 45 60 I35 180
2.3.10-3
CUMENE
I20 180
0.59
n-Heptane is distinguished by low lubricity. Therefore hot seizure in its presence can easily develop even at low speeds. However, even in this case a considerable decrease in the speed results in cold seizure. Cumene behaves entirely differently (Fig. 7). In lubricity (in air) it resembles the non-polar fraction of bright stock (see Fig. z(a)). At speeds of 2.3 and 2.3.10-l cmjsec no seizure occurs up to loads as high as 300 kg. At the same time, cumene differs from the non-polar fraction of bright stock in friction fluctuation. The behaviour of cumene is due to its ready oxidizability into peroxide compounds which promotes rapid formation of protective oxide layers on the friction surfaces. When the sliding speed is lowered to 2.3.10-z cm/set the steepness of the load ZIS.friction dependence is observed to increase, this being characteristic of cold seizure. The friction fluctuations observed in this case were smooth. Attention is drawn to the major decrease in the rate of growth of the friction with increasing loads in the range of their high values, this also being characteristic of cold seizure. An especially convenient and vivid way of comparing the behaviour of various lubricating media is by taking advantage of their load VS. friction coefficient dependencies. Such a comparison has been made in Fig. 8 which also contains data for dimethylpolysiloxanes (with viscosities of 6 and 2.104 CP at zo’C). The data given in this figure are for a sliding speed of 2.3 * 10-2 cm/set. Besides, it must be remembered that, in the case of n-heptane and cumene, the down branches of thef(lV) dependence coincide for all speeds at which cold seizure is observed; with dimethylpolysiloxanes these branches coincide for all speeds from 2.3 to 2.3.10-3 cm/set. Since wear data are of great importance in assessing the nature of the friction, Fig. 9 shows the load vs. wear dependence at various speeds for a dimethylpolysiloxane with a viscosity of z * 104 cP. Similar results were obtained for a low viscosity dimethylpolysiloxane. Now let us turn to Fig. 8. From the point of view of the hypothesis put forward above, the up branches of the f( W) dependence curve characterize the development of contacting between the friction bodies, and the downward branches correspond to plastic flow of the hardened steel.
G.
350
V. VIiXOGRADOV,
I. V. KOREPOVA,
YU.
YA.
PODOLSKY
An important difference between the cases of friction in the presence of various lubricating media is that the maximum for the f(W) dependencies corresponds to different loads, i.e. the transition from quicker to slower increase of the friction with
050-
0- Dry
\
l-
too
zoo
Load,
300
I
40
50
60
on nature
of load
I
I
150 Load,
medium
70 80 90 ?OO load,
Fig. 9. Effect polysiloxanes (after Hertz).
/
W /?fg)
Fig. 8. Effect of nature of lubricating and friction coefficient (right).
%
O.fO’$
/SO
200
Friction
Bright
-Stow
1
,
200 Wfkgl
dcpcndcncc
of friction
(left)
I
a50 300
Wikg)
of sliding speed on results (viscosity at ZOT, 2.104 cP)
of wear
; a,,load
tests during friction in presence dependence of elastic deformation
of dimethylarea diameter
STEEL-TO-STEEL
FRICTION
351
rising loads occurs at different load values. This means that, depending on the nature of the lubricating medium, the transition to the friction duty at which plastic flow of the metal is of determinative importance supposedly throughout the entire friction zone, occurs at different loads. It is also essential that the friction coefficients for different lubricating media may differ in the region of the down branches of the f(W) dependence. This is evidence that the effect of lubricating media may be manifested even under conditions of intensive adhesion and plastic flow of metals. This becomes evident upon comparing the down branches of dry steel friction and friction of steel in the presence of the non-polar fraction of bright stock, low-molecular weight hydrocarbons and dimethylpolysiloxanes. The question as to the effect of surface-active substances on the occurrence and course of cold seizure remains open, because the above-mentioned tests with stearic acid in mineral oil show convincingly the difference between this process and hot seizure, but are in themselves by no means sufficient for drawing conclusions as to the ineffectiveness of surface-active substances and adsorptional modification. CONCLUSIONS
The experimental results discussed above are evidence that, at high contact stresses, the influence of the sliding speed on the nature of boundary friction of hardened steel is not confined to thermal effects. When the sliding speeds are varied over a wide range the change in the nature of interaction of individual microprojections the contact between the friction bodies, with each other and with the lubricant gaseous media is hardly of less importance.
at or
In the region of high sliding speeds, seizure or welding of the friction surfaces occur when bridges of intensive adhesion interaction form simultaneously between the majority of the microareas of the virgin metallic surface bared during friction. Such a situation arises if the time necessary for interaction of the pure metal with the lubricant (and with the substances dissolved in it) become greater than the average time of run of a single microprojection between two consecutive contacts with the projection of the mated surface. The average time of run indicated depends on two factors, viz., the density of the contact points and their relative velocities. In its turn, the density of contact points depends on the load and, to a lesser extent, on the viscosity and structure of the lubricating liquid. Naturally, exceeding certain definite speed and load values for the system in question (solid-lubricant-gaseous medium) should result in seizure of the friction surfaces. The influence of the temperature is much more complex. On the one hand, a rise in temperature will increase the contact-point density owing to desorption of the oriented surface layers and lowering of the viscosity of the lubricating liquid. On the other hand, an increase in temperature results in higher rates of chemical interaction between the metal and the lubricating medium, i.e., reduces the time needed to screen the virgin areas of the surface with chemi-sorption layers. Depending on the ratio of these influences, the occurrence of seizure with rising temperature may be either facilitated or retarded, although, as a rule, the former prevails, because in processes of chemical interaction during friction the slowest stage is evidently diffusion of the active compounds to the metal surfaces. Thus, in the region of high sliding speeds, increasing each of the main friction Wear,
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G. 1’. VINOGRADOV,
352
I. V. KOREPOVA,
YU.
YA.
PODOLSKY
parameters (speed, load, temperature) may result in seizure of the steel surfaces. However, the influence of each of these parameters on the nature of the process is specific and cannot be attributed to the influence of any of the others. It is this, we think, that makes it incorrect to attempt to model heavy friction duties by excessively increasing one of the parameters while keeping the others at a relatively low level. By decreasing the sliding speed by 2-3 orders of magnitude a situation can be reached where the rate of chemical interaction between the metal and the lubricating or gaseous media will always be higher than the rate of attrition of the inorganic surface layers. This means that the processes of chemical i~~teraction gradually loose the paramount importance they have on friction at high speed. The absence of seizure over a rather wide range of speeds, even on sharp decrease of the concentration of such chemically active components of the gaseous medium as oxygen, in passing from tests in air to tests in a vacuum of the order of IO-+ torrl*,ll is convincing evidence of this. As the sliding speed drops, the rheological properties of the contacting solids begin to play a more and more important part. The high contact stresses in combination with the low speeds of relative movement set up favourable conditions for the development of cold flow even at the contact between samples made of very hard tempered steel. It is evidently the plastic deformation of the metal at the contact between the friction surfaces that is responsibIe for the process of coId seizure. At the same time, this study has demonstrated that the nature of the lubricating medium influences cold seizure considerably.
H.
BLOCX.
Mechanical R.
~isc~~ssianm Lz&ication mu8 L~~bvicmts, Vol. 2, Institution of 3937, p. 14. Te~~~e~~~~f~e~~etko~ of Est~~z~t~~zgthe Limiting Lzkb~icity oj I
in CofE. GemraE
Engineers,
31. MATVEYEVSKY,
London,
Oils, USSR hcad. Sci. Press, Moscow, 1956 (in Russian). 1. E. VINOGRADOVA AND E. A. ALEXEYEVA, Khim. i Tekhnol. Tofiliv i Masel, 6 (7) (1961) 50. I. E. VINOGRADOVA AND E:.A. ALEXEYEVA, Zh. Prikl. Khim., 3s (1962) 176. I. B. VINOGRADOVA AND I<. 1-i.KHALIKOV, in Call. Theovy oj Lubvzcity and Nrw Matevials. Nauka Publishers, Moscow, 1965, p. 41. IX.. T. BARCROFT, Wear, 3 (1960) 440. Uu. S. ZASLAVSKY, G. I. SHOR, B. V. YEVSTICNEYEV AP(D F. R. LEBEDEVA, ii1ColE. Theory oj I.zthvicify mzd New M&vials, Nauka Publishers, Moscow, 1965, p. 45. (;. V. VINOGRADOV. Izv. Akad. A-auk SSSR, Otd. Tekhn. Na%k, 12/lekharz. i JPushinostr., (19.58) (41 138, C;. 1;. VIXOGRADOV, N. T. PAVLOVSKAYA AND Yu. Y.4. PODOLSKY, Wear, 6 (1963) 202. G. '\'. VINOGRADOV, I. V. KOREPOVA, Yu. YA. PODOLSKV AND N. T. PAVLOVSKAYA, Tvam. ,45;1f1?, .%r. D, 87 (1965) 74'. Yu. YA. PODOLSKY, I. V. KOREPOVA AND G. V. VINOGRADOV, Mashinowd., (5) (1965) 109. ‘U’U. l7A. ~oDoLsIiY, I. V. IiOREPOvA AND G. V. VINOGRnDOv, In Coil.?%ovy ofLltbviCity n*zd
New Mntemzls, Xauka Publishers, Moscow, 1965, p. 23. I.V. KOREPOVAAND V. A.R~~sTAFBYEv,~~C~EI. Theory ofFrictionand Weav,NaukaPublishers, Moscow, 1905, p. 293. C. V. VINOGRADOV, V. V. ARKHAROVA AND A. A. PETROV, ~~'enr,J (1961) 274. G. V. VINOGRADOV. LIAXG GO-LIN AND N. T. PAVLOVSKAYA. in Call. Frictzon and Wrav ~$1 Machines, No. 15, CSSR Xcad. Sci. Press, 1962, p. 432. 13. 1. KOSTETSKY. Resistance of Mmhine Elements to Wear, hIash&, Moscow, Kiev, 1959 (in Russian). G. V. VINOCRADOV, M. M. Kussn~av, &I. D. BEZBORODKO, iu.T. L'AVLOVSI(AYA, V. D. HELENSKY. S. B. KRE~N APJD 31. S. ROROVAYA, Khim. i Tekhnol. Topliua, I (1956) 61. G. \'.VINOGRADOV _WU S. E. IcREW, in C&l. Composition a& ‘Propertiesof the WigA-Molemlar Pal? of Petroleum, (Collection of Papers on the Study of the Composition and Properties of Mineral Oils and Oil Products), USSR Acad. Sci. Press, Moscotv, 1958, p. 167. W&U’,
IO
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338-352
WEAR
STEEL-TO-STEEL SPEEDS
FRICTION
OVER
G. V. VINOGRADOV,
1. V. KOKEPOVA
AND
A VERY
YU.
YA.
WIDE
RANGE
OF SLIDING
PODOLSKY
Instituteof Petrochemical Synthesis of the Academy of Sciences of the USSR, Moscm (USSR) (Received
November
28, 1966)
SUMMARY
This paper presents the results of investigations of hardened steel-to-steel friction in the presence of the nonpolar (napththeno-paraffin) fraction of bright stock, individual hydrocarbons and polyorganosiloxanes. The range of relative velocities of the friction bodies studied embraces nine orders of magnitude. It is shown that at constant loads and bulk temperatures of the Iubricating medium both raising and lowering the sliding speed may result in a rise in the friction coefficient. At high speeds the sharp rise in the friction coefficients involves a thermal shock in the friction zone and a considerable increase in the rate of wear, this phenomenon being known as hot seizure. At low and super-low speeds the friction coefficients increase to a certain limit value more or less slowly and this increase is not accompanied by a rise in temperature. It is suggested that this process be termed “cold seizure”. Its course depends on the nature of the lubricating medium. The assumption is put forward that cold seizure is related to intensive development of plastic deformation of the metal throughout the entire friction zone, i.e., is due to its athermal plasticity. Six specific features of cold seizure are given.
Heavy boundary friction duties (when high contact pressures come into play) depend to a considerable extent on the speed of relative movement of the rubbing bodies. In most papers devoted to friction research it is accepted that the effect of the sliding speed is due primarily to the thermal effects resulting from intensification of heat liberated at the contact when the speed is increased. This assumption has formed the basis of several methods of estimating the E.P. characteristics of lubricating materials rs2and methods of modelling friction processes under static conditions3-7. The correctness of this conception has been questioned in the literatures. In connection with the discussion of the results of investigating the mechanism of action of certain E.P. additive@, it was assumed that the sliding speed is important from the point of view of the kinetics of interaction of lubricants with the microareas of the virgin metal surface bared in the process of friction: raising the speed reduces the time during which a protective layer of oxides or inorganic salts of the metal, preventing development of microseizure, can form. In connection with this, when the speed is increased, WW’,
IO (1967) 338-352
STEEL-TO-STEEL
FRICTION
339
the number of single acts of microseizure and the probability of occurrence of macroseizure rise sharply. Further research into steel-to-steel friction on variation of the sliding speed over a range of g orders of magnitudei@12 revealed another characteristic feature of its influence on processes taking place at high specific loads in the presence of lubricants. It was established that, at low sliding speeds, the dependence of friction on the load deviates considerably from direct proportionality. In the papers quoted a new phenomenon was described, accompanying friction processes at these duties. It was called “cold” seizure to distinguish it from “hot” seizure which is characteristic of friction at medium and high sliding speeds. Since the study of friction at low and superlow sliding speeds is not only of theoretical, but of great practical interest as well (sliding guides, friction units in instruments, etc.), the principle features of cold seizure are discussed in this paper and this phenomenon is compared with hot seizure. EQUIPMENT
AND OBJECTS OF STUDY
The tribometer The work was carried out with a four-ball tribometer which has already been described very brieflyis. The fundamental part of the instrument is a friction unit constituting a pyramid of four steel balls (I/Z in. in diameter, grade LUX = 6 steel, hardness HR,=60-62). Friction is caused by rotating the ball at the peak of the pyramid. The three bottom balls are fixed in a cup filled with the lubricating medium and connected with a rigid dynamometer, the latter being a steel plate with tensometric wire pick-ups glued to it. During the friction process the dynamometer readings are recorded continuously on the tape of an electronic potentiometer. A hydraulic loading system is employed. Its essential advantage is that it ensures damping of the vibrations arising in the friction unit. Realisation of a sufficiently extensive range of loads (from 1-800 kg), is obtained by varying the ratio between the plunger areas in the hydraulic press. For this purpose one of them is made in the form of two concentric cylinders which may be separated or connected rigidly. In the latter case the cylinders become an integral whole and therefore a large diameter piston. A special dynamometric insert between the plunger and the friction unit, made in the form of a system of tensometric beams, serves for measuring and recording the axial stress during friction. An important and complicated problem involved in studies of friction over an extensive range of low sliding speeds is that of ensuring uniform movement of the friction bodies at the contact. This problem has been solved by using as the drive a combination of a hydraulic pump and a system of hydraulic motors, each with a worm reducer to reduce its rev./min. The hydraulic motors were employed in the range of speeds where the rev./min practically does not depend on their shaft moment. In a worm engagement the motion is transmitted with the worm continuously contacting the worm wheel. Therefore the use of such a drive ensures uniformity of rotation of the tribometer shaft. By interconnecting the five worm reducers in series by means of claw clutches the speed of rotation can be varied between 2,500 and 8.10-6 rev./min, which corresponds to a change in sliding speed at the theoretical point of contact between the balls of between IOO and 0.3.10-6 cm/set. Within the Wear, IO
(1967) 338-352
G.
340
V. VINOGRADOV,
I. V. KOREPOVA,
1-U. YA.
PODOLSKY
limits of each of the five steps of speed variation it can be increased or decreased smoothly in time according to a linear law. Such programming of the velocity is achieved by changing the volume capacity of the pump with the aid of a special servomechanism. The tribometer enables tests to be carried out not only at room temperature, but at elevated temperatures as well. Two types of heaters are used for this purpose. The first is a jacket with the heat carrier from a thermostat circulating through it, which is put directly on the friction unit and is used in the temperature range of zo-70% This method of thermostatic control ensures high accuracy in sustaining the bulk temperature of the friction unit at a preset level, but has a serious disadvantage in that the flexibility of the rubber hose causes a substantial error when measuring friction forces. This disadvantage has been eliminated in the heater of the second type, which is a radiation furnace made in the form of a low resistance element and used for creating temperatures between 50 and 600°C. To concentrate the heat flux entirely on the object of thermostatic control the element is mounted inside a cylindrical screen made of polished metal, encompassing the friction unit but not touching it. The hot joint of a thermocouple situated in the direct vicinity of the heater coil registers the intensity of heat flow from the heater to the friction unit and may serve as a pick-up in an electronic circuit designed to programme the increase in the bulk temperature of the four-ball pyramid. The working cycles of the tribometer are controlled automatically. The circuit for timing the tribometer operation is based on a multicircuit time relay. This is a system of disks rotated by a s~lnchro~ous motor. As the disks rotate, stops fixed to them close the contacts of independent electric circuits. The length of the time interval elapsing between the moments of making of each of the circuits can be varied at the experimentor’s will from o to 20 min. The circuit used enables not only automatic control of a number of operations, but multiple repetition of these operations as well.
Friction
and wea testing The friction study was carried out as a series of tests. Each series was run at a single sliding speed. It began with a test at minimum load and proceeded without changing or turning the ball and without replacing or adding lubricant. The load was increased from test to test. Each series ended at the seizure load or at even higher loads. In all cases the friction unit was loaded before setting the require(1 speed. Balls and lubricant were replaced before each series of tests. The initial load in each series of tests and the load increment from test to test (degree of loading) depended on the sliding speed. Steady friction conditions at sliding speeds above z.3.10-” cmjsec were reached in less than I sec.. At lower speeds the durati~)n of the tests had to be prolonged, and the more so the lower the speed. In this study only the occurrence of seizure was registered, without going into the features of its development. Accordingly, at high sliding speeds the duration of the test was 30 sec. Table I lists the conditions under which the tests were made for clifferent duties. Wear was assessed by the diameter of the wear spots on the lower bail which were measured with a tool microscope. The main object of study was the nonpolar naphtheno-paraffin fraction of bright stock (viscosity ~~0= 57.5 cSt). In addition, low molecular weight individual Wear, 10 (NV)
338-352
STEEL-TO-STEEL
TABLE
341
FRICTION
I
FRICTION
DUTIES
Sliding speed
Initial drgvee (kg)
(cm/set)
IOO-58 58hJ.23 2.3.10-2-2.3
load and
of loading
3.75 7.5 30.0
10-e
of test
Dwation
Intevval between tests
(win)
(min)
0.5 0.5 6.0-60
3 3 5-10
hydrocarbons (cumene and n-heptane, see Table I, ref. IO for description) polydimethylsiloxane liquids of different viscosities were studied. RESULTS
AND
and two
DISCUSSION
Our friction
study covering
a wide range of velocities
showed that there exist
three ranges of sliding speeds, each characterized by a definite change of friction in time z at various loads, W. Figure I shows the friction vs. time diagrams obtained for sliding of steel in the presence of the naphtheno-paraffin fraction of bright stock.
U=23 cm/set
II=0.23 cm/see
Fig. I. Friction coefficient non-polar fraction of bright diagrams (in kg).
vs. time diagrams obtained for steel-to-steel friction in presence of stock. Sliding speeds are indicated at the right; loads, directly on the
The sliding speeds at which the tests for each of the rows of diagrams were performed are shown at the right and the loads directly on the diagrams (in kg). The bulk temperature of the oil in these tests was ZO’C. The time is indicated in minutes. In the range of high sliding speeds (23 cm/set and more) at small loads (diagram
I)
the friction
force, f, reaches
a stable value in a very short interval Wear,
IO
of time
(1967) 338-352
342
G. V. VINOGRADOV,
I. V. KOREPOVA,
YU. YA. PODOLSKY
commensurable with the frequency characteristic of the recording instrument, and subsequently does not change with time. At the moment the drive is stopped, marked on this and the subsequent diagrams by an arrow, a more or less short-lived increase in friction is observed, which has already been discussed 15.The nature of the variation of friction with time corresponding to diagram I is retained at all loads right up to the load at which seizure develops (at 165 kg in the case under discussion, diagram 2). Besides a sharp increase in friction, seizure is accompanied by a sharp rise in wear and in the temperature of the rubbing bodies. It is the considerable heat effects accompanying this process that justify the name of hot seizure. The severity of hot seizure may differ depending on the conditions of friction, and the chemical activity of the lubricant and the gas phase. The most severe forms of seizure may end in welding of the friction surfaces. On the other hand, seizure may be degenerate in naturel4315, when only a brief insignificant rise in friction and wear are observed. Typical of all kinds of hot seizure are more or less profound structural changes in the surface layer of the metal under the influence of the thermal shock and adhesion of the heated friction surfaces. In the second speed range (between about 2.3 and 2.3 - 10-2 cm/set, diagrams 3 and 4, Fig. I) the friction coefficient vs. time diagrams are similar to diagram I. The peculiarity of friction in this sliding speed range is the absence of seizure throughout the entire load range used (up to 500 kg). At low and superlow sliding speeds (from 2.3.10-2 cmjsec and lower, diagrams 5 and 6, Fig. I) and sufficiently high loads the friction force is observed to increase slowly with time, and the greater the load, the longer it takes to reach a steady friction value. This is undoubtedly related to the formation of a definite state (structure) in the friction zone. Diagrams 5 and 6, Fig. I, are presented in idealized form in the sense that more or less smooth fluctuations of the friction forces are usually observed. A distinct difference between the duties of boundary friction at high and low sliding speeds can be traced by the load dependencies of the steady friction values at constant speed. The corresponding curves obtained at three different speeds for the same naphtheno-paraffin fraction of bright stock are shown in Fig. z(a). Were curves 1-3 correspond to the speeds 23, 2.3 and 2.3+x0-* cm/set. The tests were run at a bulk temperature of the friction unit of zo°C. For comparison Fig. z(b) shows the load dependencies of steady friction values at different speeds for the case of dry friction. Now let us first consider
Fig. 2(a).
In the range of high and medium sliding
speeds the F(W) dependencies fit well on straight lines passing through the coordinate origin. At a speed of 23 cmjsec there is a sharp break in the line at the point corresponding to the seizure load. The line corresponding to a speed of 2.3 cmjsec is straight over the entire range of loads studied. As the sliding speed decreases the steepness of the lines expressing the dependence of F on W increases and, accordingly, so do the friction coefficients, f. In the case of very low sliding speeds (curve 3) the friction increases rapidly with the load. However, in the region of high loads growth of the friction slows down. For the ease of the lowest speeds, considered here, curve 3(a) shows the load dependence of the effective friction coefficients calculated as the ratio of the friction force to the load. As curve 3(a) shows, with growing loads under conditions of low sliding speeds, friction coefficients are reached which are of the same order as those observed under hot seizure duties when the loads are not very Wenu, 10 (194~)33%35Z
STEEL-TO-STEEL
FRICTION
343
much higher than the critical seizure loads. It is this circumstance that makes it possible to regard the rise in friction at low speeds as a seizure process also but one of cold seizure in this case, since the rise in F in the range of low sliding speeds is not accompanied by any perceptible thermal effect. The slow-down in the friction increase with increase of the load in the high-load region must be related to plastic
(6)
Fig. 2. (a) Load us. friction and load vs. friction coefficient curves in presence of non-polar fraction of bright stock. (b) Load vs. friction and load vs. friction coefficient curves, dry friction.
deformation of the rubbing bodies, i.e. is due to flowing of the metal in the zone of contact between the balls. Naturally, this process is of basic importance at very low speeds and may require a considerable length of time for its development (diagrams 3 and 6, Fig. I). This is supported by Fig. a(b). Indeed, in the case of dry friction the dependencies of the friction and the friction coefficient on the load at low sliding speeds is similar in nature to that observed in the presence of lubricating media. This may be regarded as evidence of the fact that at low and very low sliding speeds, when the process of cold seizure is developed in the presence of lubricating media, its mechanism is close to that of plastic flow of the metal at low speeds and high contact stresses. This is exactly the process described by the top parts of the F(W) curves and the downward branches of the f(W) curves. Taking into account that under conditions of intensive plastic flow Coulomb’s law is inapplicable, the lowering of the effective friction coefficient in the range of high loads is a fictitious mollification of the friction duty. At the same time, the extreme nature of the load dependence of the friction coefficient is good evidence of the change in the friction duty. Thus, slow attainment of a stationary friction duty and weakening of the load dependence of friction at high values of load are the first two specific features of friction at low and superlow speeds. Wear,
IO
(1967)
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344
G. V. VINOGRADOV,
The third specific
feature
of cold-seizure
I. V. KOREPOVA,
friction
YU.
YA.
PODOLSKY
is that it involves
no con-
siderable wear despite high contact stresses. This is illustrated by the data of Table II which shows the wear values obtained in a number of tests run with the naphthenoparaffin fraction of bright stock at a bulk temperature of the friction unit of ZO’C.
.-I veva,~r dianzeter
o,fZWY spot at end of seriesof tests
i Wl)
77 58 46 2.3 2.3 2.3’ 10-l 2.3.10-2 2.3 10-a 2.3.10-a 2.3 10-a
37.5 4s 67.5 150 405 360
270 450 4So 4’0
0.30
I.59 1.67
0.31
0.36 0.40 0.64
2.25 0.7'
0.67 0.67 0.56 0.05 0.62 0.02
0.02
0.50 0.67 0.68
0.64
Fig. 3. Effect of sliding speed on results of dry friction deformation area diameter (after Hertz).
wear tests;
a, load dependence
of elastic
Hot seizure (lines 1-4 of Table II) occurs at relatively not-very-high loads, but is accompanied by considerable wear of the steel surfaces. In the case of “cold” seizure (lines 7-10) the wear spot diameters differ little from the diameters of elastic deformation areas calculated according to the Hertz theory. “Cold” seizure leaves the friction surfaces smooth and without tears or excrescences. However, the most surprising are the wear values measured for dry friction Weau, IO (1~67) 338-352
STEEL-TO-STEEL FRICTION
345
of hardened steel. They are presented in log-log coordinates in Fig. 3, where the axial loads are plotted as the abscissa and the average diameters of the wear spots on the lower balls, as the ordinate. In these tests, the loads were changed, as usual, from low to high values. The time conditions of the tests corresponded to the data of Table I. After each test at a given load the four-ball unit was dismantled and the wear measured, after which the next test was performed without altering the position of the three balls fixed in the cup. It is evident from the data, of Fig. 3 that, at very low speeds, the wear values are insignificant even for dry friction, though they are slightly higher in this case than for friction in the presence of lubricant. One may ask whether comparison of wear scar diameters produced at greatly differing speeds and different friction paths is quite legitimate. It is evident from the data of Table II that the wear spots produced at speeds differing by a factor of 105 (and friction paths by 103) are of the same size. Besides, two tests were run with the oil indicated above at 2.3.10-3 cmjsec which corresponded to cold seizure duty. The conditions of these two tests were absolutely identical (load 270 kg) except for their duration, that of the first being 6, and of the second, 540 min. The results recorded in both cases were practically the same (wear scar diameter 0.54 mm). Thus, a go-fold increase of the friction path had no effect on the wear value under the conditions of the tests. This shows that comparison of the data of Table II is justifiable. The two main types of seizure, viz., “cold” and “hot”, observed during heavy boundary friction duties are analogues of the processes of adhesion of the first and second kind according to the classification of types of wear of metals during dry friction, given by KOSTETSKY~~. The presence of a lubricant substantially changes the course of seizure processes compared with the adhesion processes indicated, These differences are manifested primarily in the fact that, unlike adhesion of the second kind, on dry friction “hot” seizure may become degenerate in the presence of a lubricating material14J5. Besides, as indicated above, when “cold” seizure occurs, the intensive wear characteristic of adhesion of the first kind is absent. However, in our tests and on dry friction under conditions of low speeds that very intensive wear which characterizes adhesion of the first kind according to KOSTETSKY, is absent. The reason why our tests disagree with the data of KOSTETSKY is not clear. The fourth peculiarity of cold seizure is its “reversibility”. Tests to illustrate this were run with the lubricant indicated above and a bulk temperature of the friction unit of 2o“C. They were started at a speed of 2.3 cm/set, raising the axial load on the four-ball pyramid in steps up to 360 kg (as indicated in Table I). After this, the load was left constant and the speed was lowered in steps (reducing it tenfold). When the sliding speed is reduced to a certain critical value the effective friction coefficient was observed to increase in time. After a constant value of the latter was reached the speed was changed to a lower one. After a constant value of the effective friction coefficient was attained at a speed of 2.3-10-5 cmjsec the speed was raised in steps in the same way as it was previously lowered. Thus, friction was always along the same wear scar. The result of the tests considered here is shown in Fig. 4, where the arrows indicate the direction of switching from speed to speed. It is evident from the data presented in Fig. 4 that, irrespective of the direction in which the speed is changed, the occurrence and disappearance of cold seizure is confined to the same range of sliding speeds. Under the test conditions in question Wear, IO (1967) 338-352
G. v.
346
VINOGRADOV,
I. V. KOREPOVA,
YU.
YA.
PODOLSKY
this occurs in the sliding speed interval 2.3.10~%?.3.10-3 cmjsec. The reversibility of cold seizure is evidence of the reversibility of the states of contact between the steel balls when the friction duties are changed. -4s indicated, the formation of a definite state of contact takes place in the pre-stationary stage (see diagrams 5 and 6 in Fig. I), which is not reflected in Fig. 4 where steady friction duties are represented.
Fig. 4. Effect of sliding speed on steel-to-steel of bright stock.
friction
coefficient
in presence of non-poiar fraction
The fifth specific feature of cold seizure is that the steady conditions at which it occurs are not affected by certain agents causing chemical and chemi-sorptional modification of the steel surface, preventing the occurrence of hot seizure or resulting in degeneration and break-off of the latter. The development of cold seizure is approximately the same whether it occurs in air or in a vacuum of the order of 10-5 torr, nor does the addition of such organosulphur compounds as dibenzyldisulphide to the lubricating oil prevent its appearance. This means that molecular oxygen and organosulphur compounds of medium activity, which usually affect hot seizure similarly and very strongly, have no substantial effect on friction duties at low sliding speeds. The same may be said with respect to the action of stearic acid type surfaceactive substances in the air. At the same time organophosphorus additives to lubricating oils, such as (CCl~)PO(OC&I~)~, prevent cold seizure. This is evidence of the specific influence of the chemical composition of lubricating media on cold seizure, This question will be discussed further below. The sixth specific feature of cold seizure is the peculiar effect of temperature on its occurrence. With rising bulk temperature of the metal the occurrence of cold seizure is facilitated in the sense that it may be observed at a higher speed. In the case of hot seizure, increasing the bulk temperature of the friction unit lowers the value of the critical speed at which seizure occurs in a definite load intervalIT. This is illustrated by the results of the tests with the naphtheno-paratfin fraction of bright stock at 25,75, and 1z5Y. These results are presented in Fig. 5 in diagramatic form by a shaded band denoting the sliding speed region in which both types of seizure discussed here are observed. Increasing the bulk temperature of the friction unit from ~5 to 75°C brings down the lower speed boundary at which hot seizure can be registered in the range ~f’,ear, IO (1967)
338-352
STEEL-TO-STEEL FRICTION
347
of loads investigated, by approximately one order of magnitude. On the other hand, the range of sliding speeds corresponding to cold seizure expands with growing temperature toward higher speed values. In the case of 125°C the speed boundaries at which hot and cold seizure are displayed, are not shown in Fig. 5 because rapid development of hot seizure at a sliding speed of 2.3.10-s cm/set evidently masks the rather slow rise of the friction coefficient on cold seizure. Thus, a rise in temperature results in a narrowing of the speed range within which friction is characterized by the absence of both types of seizure within the load interval used.
Fig. 5. Diagram of cold and hot seizure boundaries at various temperatures of bright stock. Friction unit bulk temperatures are indicated at left.
for non-polar fraction
In studying the friction of steel at high sliding speeds it was found that the composition of mineral lub~cating oils substantially influences their antiwear and antifriction characteristicsls. Subsequently a very great difference was detected between the antiwear and antifriction properties of low-molecular weight hydrocarbons depending on their nature, which was found to be related to their different oxidizabilities and capacities for carrying active oxygen to the steel surface in the friction zonelo. Investigation of the friction and wear of steel in the presence of Iow-molecular weight hydrocarbons at low sliding speeds and comparison of their behaviour with what is observed for lubricating oils, reveals how the nature of the lubricating media influenced the development, not only of hot, but of cold seizure as well. The nature of the behaviour of low-molecular weight hydrocarbons at the speeds and loads at which both hot and cold seizure are possible is illustrated vividly by the data of Figs. 6 and 7 and Table III. As in the cases considered above, each series of tests at a given sliding speed was carried out without turning the balls and with the wear spot successiveIy increasing. The shaded vertical bands in Figs. 6 and 7 characterize the friction fluctuations at constant speed and load values. They were sharp in the case of hot seizure and smooth with cold seizure. The height of the shaded bands characterizes the amplitude of the friction fluctuations. If only the load dependence of the friction (steady or pseudo-steady values) are considered, the type of seizure cannot be determined. It is essential to make use of wear data (Table III) and to take into account the nature of the friction ?rs. time diagrams at constant speeds and loads. Weear,IO (x967) 338-352
G. X7.VINOGRADOV, I. V. KOREPOVA, TU. YA. PODOLSKT
348
In the case of n.-heptane (Fig. 6) at a speed of 2.3 cm/see, hot seizure occurs directly after a load of 15 kg. It is manifested in more or less considerable (sharp) fIuctuations of the friction a.nd in elevated wear. Lowering the speed to 2.3*10-i and 23.x0-2 cmjsec raises the critical seizure loads to 4s and 120 kg respectively.
Load,
W/kg)
Fig. 6. I.oad us. friction curves in presence of n-heptane. 30 cl -2.3
Fig. 7. Load us. friction curves in presence of cumenc.
Kowever, further decreases of the speed to 2.3.10-3 cmjsec results in cold seizure. This causes an increase in the steepness of the initial part of the F(W) dependence, so that cold seizure at 2.3 *1~1-3cm/see and hot seizure at a speed of 2.3 cmjsec are described by practically the same curve. However, no sharp friction fluctuations are observed at cold seizure and the wear is small. Wcav, IO ($967) 338-352
STEEL-TO-STEEL
349
FRICTION
UBLE III WEAR
OF
Substance
Cunlello
LOWER
BALLS
IN PRESENCE
Sliding speed (cmjsec)
2.3
2.3.10~’ 2.3.10-z 2.3 n-Heptane
10-a
2.3 *.3.10-'
2.3.10-Z
OF
Load (kg)
AND
1Z-HEPTANE
.4 uerage WeaY SCaY diametev at end of test series
Diametev ofelastic deformation awas (after Hertz)
(mm)
(mm)
300 315
0.64 0.58
0.58
180
0.50
225
c.55 0.58
0.49 0.53 0.58
0.38
0.27
I.02 0.32
0.53 0.31
0.36
0.34
0.43 0.49
0.44 0.49
0.43 0.54
0.43 0.49
315 30 225 45 60 I35 180
2.3.10-3
CUMENE
I20 180
0.59
n-Heptane is distinguished by low lubricity. Therefore hot seizure in its presence can easily develop even at low speeds. However, even in this case a considerable decrease in the speed results in cold seizure. Cumene behaves entirely differently (Fig. 7). In lubricity (in air) it resembles the non-polar fraction of bright stock (see Fig. z(a)). At speeds of 2.3 and 2.3.10-l cmjsec no seizure occurs up to loads as high as 300 kg. At the same time, cumene differs from the non-polar fraction of bright stock in friction fluctuation. The behaviour of cumene is due to its ready oxidizability into peroxide compounds which promotes rapid formation of protective oxide layers on the friction surfaces. When the sliding speed is lowered to 2.3.10-z cm/set the steepness of the load ZIS.friction dependence is observed to increase, this being characteristic of cold seizure. The friction fluctuations observed in this case were smooth. Attention is drawn to the major decrease in the rate of growth of the friction with increasing loads in the range of their high values, this also being characteristic of cold seizure. An especially convenient and vivid way of comparing the behaviour of various lubricating media is by taking advantage of their load VS. friction coefficient dependencies. Such a comparison has been made in Fig. 8 which also contains data for dimethylpolysiloxanes (with viscosities of 6 and 2.104 CP at zo’C). The data given in this figure are for a sliding speed of 2.3 * 10-2 cm/set. Besides, it must be remembered that, in the case of n-heptane and cumene, the down branches of thef(lV) dependence coincide for all speeds at which cold seizure is observed; with dimethylpolysiloxanes these branches coincide for all speeds from 2.3 to 2.3.10-3 cm/set. Since wear data are of great importance in assessing the nature of the friction, Fig. 9 shows the load vs. wear dependence at various speeds for a dimethylpolysiloxane with a viscosity of z * 104 cP. Similar results were obtained for a low viscosity dimethylpolysiloxane. Now let us turn to Fig. 8. From the point of view of the hypothesis put forward above, the up branches of the f( W) dependence curve characterize the development of contacting between the friction bodies, and the downward branches correspond to plastic flow of the hardened steel.
G.
350
V. VIiXOGRADOV,
I. V. KOREPOVA,
YU.
YA.
PODOLSKY
An important difference between the cases of friction in the presence of various lubricating media is that the maximum for the f(W) dependencies corresponds to different loads, i.e. the transition from quicker to slower increase of the friction with
050-
0- Dry
\
l-
too
zoo
Load,
300
I
40
50
60
on nature
of load
I
I
150 Load,
medium
70 80 90 ?OO load,
Fig. 9. Effect polysiloxanes (after Hertz).
/
W /?fg)
Fig. 8. Effect of nature of lubricating and friction coefficient (right).
%
O.fO’$
/SO
200
Friction
Bright
-Stow
1
,
200 Wfkgl
dcpcndcncc
of friction
(left)
I
a50 300
Wikg)
of sliding speed on results (viscosity at ZOT, 2.104 cP)
of wear
; a,,load
tests during friction in presence dependence of elastic deformation
of dimethylarea diameter
STEEL-TO-STEEL
FRICTION
351
rising loads occurs at different load values. This means that, depending on the nature of the lubricating medium, the transition to the friction duty at which plastic flow of the metal is of determinative importance supposedly throughout the entire friction zone, occurs at different loads. It is also essential that the friction coefficients for different lubricating media may differ in the region of the down branches of the f(W) dependence. This is evidence that the effect of lubricating media may be manifested even under conditions of intensive adhesion and plastic flow of metals. This becomes evident upon comparing the down branches of dry steel friction and friction of steel in the presence of the non-polar fraction of bright stock, low-molecular weight hydrocarbons and dimethylpolysiloxanes. The question as to the effect of surface-active substances on the occurrence and course of cold seizure remains open, because the above-mentioned tests with stearic acid in mineral oil show convincingly the difference between this process and hot seizure, but are in themselves by no means sufficient for drawing conclusions as to the ineffectiveness of surface-active substances and adsorptional modification. CONCLUSIONS
The experimental results discussed above are evidence that, at high contact stresses, the influence of the sliding speed on the nature of boundary friction of hardened steel is not confined to thermal effects. When the sliding speeds are varied over a wide range the change in the nature of interaction of individual microprojections the contact between the friction bodies, with each other and with the lubricant gaseous media is hardly of less importance.
at or
In the region of high sliding speeds, seizure or welding of the friction surfaces occur when bridges of intensive adhesion interaction form simultaneously between the majority of the microareas of the virgin metallic surface bared during friction. Such a situation arises if the time necessary for interaction of the pure metal with the lubricant (and with the substances dissolved in it) become greater than the average time of run of a single microprojection between two consecutive contacts with the projection of the mated surface. The average time of run indicated depends on two factors, viz., the density of the contact points and their relative velocities. In its turn, the density of contact points depends on the load and, to a lesser extent, on the viscosity and structure of the lubricating liquid. Naturally, exceeding certain definite speed and load values for the system in question (solid-lubricant-gaseous medium) should result in seizure of the friction surfaces. The influence of the temperature is much more complex. On the one hand, a rise in temperature will increase the contact-point density owing to desorption of the oriented surface layers and lowering of the viscosity of the lubricating liquid. On the other hand, an increase in temperature results in higher rates of chemical interaction between the metal and the lubricating medium, i.e., reduces the time needed to screen the virgin areas of the surface with chemi-sorption layers. Depending on the ratio of these influences, the occurrence of seizure with rising temperature may be either facilitated or retarded, although, as a rule, the former prevails, because in processes of chemical interaction during friction the slowest stage is evidently diffusion of the active compounds to the metal surfaces. Thus, in the region of high sliding speeds, increasing each of the main friction Wear,
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G. 1’. VINOGRADOV,
352
I. V. KOREPOVA,
YU.
YA.
PODOLSKY
parameters (speed, load, temperature) may result in seizure of the steel surfaces. However, the influence of each of these parameters on the nature of the process is specific and cannot be attributed to the influence of any of the others. It is this, we think, that makes it incorrect to attempt to model heavy friction duties by excessively increasing one of the parameters while keeping the others at a relatively low level. By decreasing the sliding speed by 2-3 orders of magnitude a situation can be reached where the rate of chemical interaction between the metal and the lubricating or gaseous media will always be higher than the rate of attrition of the inorganic surface layers. This means that the processes of chemical i~~teraction gradually loose the paramount importance they have on friction at high speed. The absence of seizure over a rather wide range of speeds, even on sharp decrease of the concentration of such chemically active components of the gaseous medium as oxygen, in passing from tests in air to tests in a vacuum of the order of IO-+ torrl*,ll is convincing evidence of this. As the sliding speed drops, the rheological properties of the contacting solids begin to play a more and more important part. The high contact stresses in combination with the low speeds of relative movement set up favourable conditions for the development of cold flow even at the contact between samples made of very hard tempered steel. It is evidently the plastic deformation of the metal at the contact between the friction surfaces that is responsibIe for the process of coId seizure. At the same time, this study has demonstrated that the nature of the lubricating medium influences cold seizure considerably.
H.
BLOCX.
Mechanical R.
~isc~~ssianm Lz&ication mu8 L~~bvicmts, Vol. 2, Institution of 3937, p. 14. Te~~~e~~~~f~e~~etko~ of Est~~z~t~~zgthe Limiting Lzkb~icity oj I
in CofE. GemraE
Engineers,
31. MATVEYEVSKY,
London,
Oils, USSR hcad. Sci. Press, Moscow, 1956 (in Russian). 1. E. VINOGRADOVA AND E. A. ALEXEYEVA, Khim. i Tekhnol. Tofiliv i Masel, 6 (7) (1961) 50. I. E. VINOGRADOVA AND E:.A. ALEXEYEVA, Zh. Prikl. Khim., 3s (1962) 176. I. B. VINOGRADOVA AND I<. 1-i.KHALIKOV, in Call. Theovy oj Lubvzcity and Nrw Matevials. Nauka Publishers, Moscow, 1965, p. 41. IX.. T. BARCROFT, Wear, 3 (1960) 440. Uu. S. ZASLAVSKY, G. I. SHOR, B. V. YEVSTICNEYEV AP(D F. R. LEBEDEVA, ii1ColE. Theory oj I.zthvicify mzd New M&vials, Nauka Publishers, Moscow, 1965, p. 45. (;. V. VINOGRADOV. Izv. Akad. A-auk SSSR, Otd. Tekhn. Na%k, 12/lekharz. i JPushinostr., (19.58) (41 138, C;. 1;. VIXOGRADOV, N. T. PAVLOVSKAYA AND Yu. Y.4. PODOLSKY, Wear, 6 (1963) 202. G. '\'. VINOGRADOV, I. V. KOREPOVA, Yu. YA. PODOLSKV AND N. T. PAVLOVSKAYA, Tvam. ,45;1f1?, .%r. D, 87 (1965) 74'. Yu. YA. PODOLSKY, I. V. KOREPOVA AND G. V. VINOGRADOV, Mashinowd., (5) (1965) 109. ‘U’U. l7A. ~oDoLsIiY, I. V. IiOREPOvA AND G. V. VINOGRnDOv, In Coil.?%ovy ofLltbviCity n*zd
New Mntemzls, Xauka Publishers, Moscow, 1965, p. 23. I.V. KOREPOVAAND V. A.R~~sTAFBYEv,~~C~EI. Theory ofFrictionand Weav,NaukaPublishers, Moscow, 1905, p. 293. C. V. VINOGRADOV, V. V. ARKHAROVA AND A. A. PETROV, ~~'enr,J (1961) 274. G. V. VINOGRADOV. LIAXG GO-LIN AND N. T. PAVLOVSKAYA. in Call. Frictzon and Wrav ~$1 Machines, No. 15, CSSR Xcad. Sci. Press, 1962, p. 432. 13. 1. KOSTETSKY. Resistance of Mmhine Elements to Wear, hIash&, Moscow, Kiev, 1959 (in Russian). G. V. VINOCRADOV, M. M. Kussn~av, &I. D. BEZBORODKO, iu.T. L'AVLOVSI(AYA, V. D. HELENSKY. S. B. KRE~N APJD 31. S. ROROVAYA, Khim. i Tekhnol. Topliua, I (1956) 61. G. \'.VINOGRADOV _WU S. E. IcREW, in C&l. Composition a& ‘Propertiesof the WigA-Molemlar Pal? of Petroleum, (Collection of Papers on the Study of the Composition and Properties of Mineral Oils and Oil Products), USSR Acad. Sci. Press, Moscotv, 1958, p. 167. W&U’,
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