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Mechanism of reaction layer formation in boundary lubrication
Wear, 100 (1984)
301
301 - 313
MECHANISM OF REACTION LAYER FORMATION IN BOUNDARY LUBRICATION H. UETZ,
M. A. KHOSRAWI
and J. F6HL
Staatliche Materialpriifungsanstalt, Stuttgart 80 (F.R.G.)
Universitiit
Stuttgart,
Pfaffenwaldring
32,
7000
Summary Highly loaded moving machine parts under hertzian contact conditions such as gears and roller bearings require lubricants which enable the formation of tribochemical reaction layers to reduce the adhesion component in the interface. The most critical conditions have to be referred to the sliding (slip) portion of such systems which cause microseizure and subsequent high wear. Sliding tests with a pin-on-disc machine were carried out to explore the efficiency of different lubricants with organic and organometallic additives. Two modes of action govern the formation of reaction layers and lead to distinct differences in friction and wear behaviour. Organometallic (e.g. with zinc and lead) dialkyldithiophosphates were found to be the most effective additives compared with organic phosphorus and sulphur compounds even with chemically inert steels, where other lubricants fail at relatively low load levels.
1. Introduction Constant-velocity joints of motor vehicles (Fig. 1) are heavily stressed machine parts for which elastohydrodynamic conditions cannot be sustained. High hertzian pressure, low relative velocity and high temperature lead to boundary lubrication conditions and require materials with a low tendency for adhesion and high resistance against fatigue as well as lubricants which are able to generate reaction layers in the local contact areas to reduce further the adhesion component [ 1,2]. In the case of steady material loss from the surface, the joint fails because of an increase in the play (Fig. 2); however, at an extremely low wear rate level, failure can occur as a result of fatigue, cracking and breaking of the balls, since scratches on the surface caused by local seizure (resulting from load peaks) act as microstress risers. The materials used at present (ballbearing steel for the ball and hardfaced low alloy steel for the inner and outer rings), resist the applied hertzian loading conditions. However, the lubricant 0043-1648/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
302
Fig. 1. Exploded view of a constant-velocity
joint used for motor vehicles.
overload sofetv
i I failure through continuaus /material rcmovat
Load Fig. 2. Operational limits affected by the system parameters load, material and Lubricant.
has to be improved with the aim of allowing a very small amount of wear in order to remove any kind of disturbances and to smooth the surface again. In the past, extensive ~vestigations have been carried out to investigate the mechanisms of reaction layer formation with extreme pressure (EP) additives and the effectiveness of frictian and wear reduction [2 - 71. However, the interaction of additives, materials and loading conditions is so complicated that present knowledge about the basic mechanisms is not yet sufficient for current efforts to optimize the performance of higbly stressed lubricated systems to be ~oroughly carried out. A variety of lubricants with different additives were investigated to explore wear and friction behaviour, the wear mechanism and the mechanism of reaction layer formation under tribologieal conditions.
303
2. Test equipment Although the kinetics in the constant-velocity joint are rolling and superimposed sliding, the dominant loading situation results from the sliding component. To simulate the most critical situation, laboratory tests were performed with a pin-on-disc machine, only the sliding component being taken into account. The lubricant was circulated at a low flow rate and the wear of the pin was measured continuously by using radioactive pins and applying a radionuelide technique (Fig. 3).
-FN --
FN measurmg chamber
-
I
I
Fig. 3. Test arrangement for performing laboratory tests under boundary lubrication conditions with application of the radioisotope technique for wear measurements.
3. Materials, lubricants
and test parameters
The materials used are listed in Table 1. The materials 20 MnCr 5 EH (face hardened) and 100 Cr 6 H (hardened) are actually used for the original drive shaft, whereas the hardened chromium steel X 40 Cr 13 H was used for comparison as a more inert material with respect to chemical reactions.
TABLE 1 Materials investigated in pin-on-disc tests Material
Tempering conditions
Hardness (HV 10)
20 MnCr 5 EH I.00 Cr 6 H X40Cr13H
Face hardened Oil hardened Oil hardened
750 820 650
304 TABLE 2 Content of reactive elements in the lubricants Lubricant
and additive
package
Element
in the oil (wt.%)
concentrations
P
S
Paraffin oil (pure)
-
-
Base oil SAE 80 (plain)
-
0.5
-
-
-
Hypoid gear oil
0.16
2.4
0.034
-
-
Extreme pressure gear oil (ZnDTP; O~S~MO~(DTP)~; S-P compound)
0.23
2.7
0.01
0.13
-
Oil I (ZnDTP; PbDTP; OZS~MO~(DTP)~) Oil II
0.1
2.1
0.08
0.27
1.4
0.3
1.8
0.21
0.1
-
Zn
MO
Pb
-
-
(ZnDTP; S-P compound)
(ZnDTP; OzSzMoz(DTP)2; S-P compound)
Lubricants without additives and with different additive packages (dialkyldithiophosphates) were investigated (Table 2). Since the complex structure of the commercial lubricants was not known in detail, some lubricants were mixed in the laboratory, the additive being added to a plain SAE 80 base oil (Table 3).
TABLE 3 Content of reactive elements in the additive concentrates Additive Description and reactive elements
Symbol
Tricresylphosphate (P) S-P compound (S, P) Zn dialkyldithiophosphate (Zn, S, P) Pb dialkyldithiophosphate (Pb, S, P) MO compound in dithiophosphate (Mo, S, P)
Element concentrations concentmte (wt.%)
in the additive
P
s
Zn
MO
Pb
Al
8.2
-
-
-
-
A3
1.7
31.5
-
-
-
A5
1.2
1
0.5
-
-
-46
2
3.5
-
-
6
AI
2.5
7
-
5.5
-
305
Tests were carried out at 1 m s-i under different loading conditions (Fig. 4) as follows. (1) Approximate information can be obtained from a test where the load is increased continuously up to the failure or limiting load of the machine, which is at about 5000 N. (2) In a second step the load is first increased and than held constant at a certain level in some cases for up to 50 h to investigate the running-in behaviour and behaviour under steady state conditions.
1 m/s
/--U
-0
TI
B
8
/ time
increasing failure
load up to
or load limit
of machine
j/--f time
running time at constant load up to 50 h
time
stepwise
increasing
up to failure
load
or
limit of machine
(5000 N 2 400 N/mm’)
Fig. 4. Loading pattern for studying reaction layer formation under different conditions.
(3) The third loading pattern is a step-like loading with a hold time to investigate both the running-in processes and the failure load developing from steady state conditions.
4. Test results Two examples demonstrate the general test procedure and the information which can be obtained from pin-on-disc tests with two basically different commercial oils I and II. Oil I (Fig. 5) exhibits a high friction and temperature peak after each transition from one load to the other; however, each peak is only of short duration. At low and medium load, wear occurs only in that transition regime and declines when the coefficient of friction decreases to its previous level.
306
0
M
20
IO
LO
50
60
10
80
h
90
time
Fig. 5. Typical results of friction, 100 Cr 6 H lubricated with oil I.
wear
and
pin temperature
for
ball-bearing
steel
For oil II (Fig. 6) the transition conditions persisted over longer periods of time, leading to a higher wear rate, and finally the system became instable at a load of about 3000 N, whereas oil I still performed well even at a load of 5000 N.
70
M
WI
50
60
70
W
h
90
time
Fig. 6. Typical results of friction, 100 Cr 6 H lubricated with oil II.
wear
and
pin temperature
for
ball-bearing
steel
307
Test runs with continuously increasing load using different oils and materials (Fig. 7) indicate that two different mechanisms are acting. 20 MnCr 5 EH and ball-bearing steel 100 Cr 6 H do not come to failure with any of the three lubricants, but reach the limit load of the machine. Therefore no differentiation between these oils was achieved. When the pin diameter was reduced from 4 to 3 mm to increase the maximum pressure level the different quality of the lubricants became obvious, but still no failure occurred with oil I. The tests with hardened chromium steel X 40 Cr 13 H confirmed that drastic differences exist between oil I and the two other lubricants. Oil I leads to a high load-carrying capacity whereas extreme pressure gear oil and commercial oil II were not effective on X 40 Cr 13 H steel.
B 2
l!L time
EP extreme pressure I
oil1
II
0ilU
gear
oil
1 1000
0 EP
I
II
EP
I
II
EP
I
II
Lubricant Fig. 7. Comparison of lubrication efficiency of different oils in combination with different materials under boundary lubrication conditions with a pin-on-disc machine.
The same tendency was obtained when the load was increased step like in such a way that steady state conditions were reached during each load step (Fig. 8). The hypoid gear oil behaves similarly to the high pressure gear oil and the commercial oil II. These three lubricants are very effective on low alloy carbon steels but fail in combination with the chromium steel. Commercial oil I, however, has the same efficiency on all three materials and does not fail within the loading capacity of the machine.
308 500
1
pin/disc, v=lm/c
I
-
I
--
oil1 oil
II
time
Sr 3"I
I
--hypoidgearoil
o, 250-
-4
-extreme pressure gearoil
5
I
2
-.,i_ ZOMoCr5EH L---
5
XbOCrtJH
0
~ 0
I
I
I
I 3000 load
IO00 al00
N
5000 0
1000
2000
3000 load
N
5000
Fig. 8. Wear behaviour of different materials as a function of load under boundary conditions with different lubricants.
The explanation of this phenomenon may be found in the reactivity of the additive in connection with the material. Tests with ball-bearing steel using SAE 80 base oil with different additives (Fig. 9) led to a more detailed understanding of the mechanisms that occurred. Obviously the lead dithiophosphate is able to form a reaction layer without participation of the bulk
pin/disc1OOCr6H, base oii SAE 60, v=lm/s 150,
I
I
50
1000
2000 load
N
3000
Fig. 9. Effect of oil additives on wear behaviour of hardened ball-bearing steel under boundary lubrication conditions (pin-on-disc machine; 100 Cr 6 H; base oil SAE 80; u = 1 m s-l).
309
metal and therefore the optimum wear behaviour is independent of the reactivity of the material. As soon as a certain peak temperature is reached, a lead compound is being precipitated on the sliding surface, filling up the grooves and causing smoothing of the surface so that on the one hand hydrodynamic and elastohydrodynamic lubrication is supported and on the other hand adhesion processes are avoided through the separating reaction layer (Fig. 10). These reaction products adhere extremely well to the bulk material.
T: 9
i
time
(a)
(cl
sliding direction
@I
(d)
Fig. 10. Reaction layer formation on a hardened ball-bearing steel pin formed under boundary lubrication conditions with oil I (pin-on-disc machine; 100 Cr 6 H; u = 1 m s-l; 3000 N): (a) pin; (b) region A in (a); (c) region B in (b); (d) region C in (c).
Microanalyses of the surface using energy-dispersive X-ray (EDX) analysis indicate that the grooves are substantially filled with a lead component. Phosphorus as well as sulphur play only a minor role (Fig. 11). When the molybdenum sulphur dithiophosphate was used, no significant reaction layer was found on 100 Cr 6 H using EDX analysis. However, from the surface appearance and wear behaviour it is assumed that the smooth
310
time
(b)
Fig. 11. Reaction layer formation on hardened ball-bearing steel lubricated with oil I (tribological conditions equivalent to those shown in Fig. 10) and corresponding lead and phosphorus distribution: (a) pin; (b) enlarged view of part of the pin (a); (c) lead distribution for the region shown in (b); (d) phosphorus distribution for the region shown in (b).
surface results from a thin reaction layer which does not cause microseizure (Fig. 12). As a more sensitive analysis method, Auger spectroscopy was applied to this surface. In the outermost layer the presence of reaction products could be demonstrated (Fig. 13). The amount of reaction products decreases with increasing sputter time so that the thickness of the layer could roughly be estimated to be about 10 nm. From a variety of test runs with different oils and additive mixtures pin surfaces were investigated using EDX or microprobe analysis. From these investigations two groups of additives could be differentiated in general (Tables 4 and 5).
311
time
(cl Fig. plex 20% gion
(d) 12. Reaction layer formation on hardened ball-bearing steel lubricated with a commolybdenum dithiophosphate in base oil SAE 80 (pin-on-disc machine; 100 Cr 6 H; O~S~MO~(DTP)~; u = 1 m s-l): (a) pin; (b) enlarged view of part of the pin; (c) reA in (b); (d) region B in (b).
(1) Lubricants with additives mainly based on sulphur and phosphorus generate a reaction layer with a high content of iron. This is the common mode of action and is widely described in the literature. This type of reaction can only occur on materials with appropriate reaction kinetics as these are the low alloy steels. The removal of the reaction layer due to the tribological stressing implements the loss of material and results in an equilibrium between reaction layer formation and removal. (2) Lubricants with metal dithiophosphates (e.g. lead and zinc) create a reaction layer that contains almost no iron, i.e. effectiveness of these additives is not dependent on the reactivity of the material and the wear process of the reaction layer does not necessarily implement the loss of bulk material.
17.
&utter t,A
*in
6
Fig. 13. Element distribution (Auger spectroscopy) across the thickness (corresponding sputter time) of a reaction layer formed on hardened ball-bearing steel lubricated with molybdenum dithiophosphate in base oil SAE 80. TABLE 4 Composition
of reaction product@ on 100 Cr 6 H at 3000 N
Lubricant
Element concentrations
Hypoid gear oil Extreme pressure gear oil Oil I Oil II
(wt.%)
Fe
P
s
Zn
MO
Pb
56 50 2 61
19 6 1 4
3 12 15 32
22 1 1 1
31 1 2
80 -
aDetermined by EDX analysis. TABLE
5
Composition
of reaction productsa on 100 Cr 6 H at 2000 N with base oil SAE 80
Additive
5% S-P compound fP> S) 100% Zn dialkyldithio-
in the reaction layer (wt.%)
Element concentrations
I?;
P
58
38
9
Zn
MO
Pb
4
_
-
-
21
1
69
-
-
3
5
23
-
-
69
18
5
13
-
-
64
18
5
13
-
S
phosphate (P, S, Zn) 20% Pb dialkyldithiophosphate (P, S, Pb) 20% MO compound in dithiophosphate (P, S, Pb) 20% Pb dialkyldithiophosphate aDetermined by EDX analysis.
64
313
The amount of iron in the reaction layer probably results from a chemical reaction of the additives on organic sulphur and phosphorus compounds since the oil contained a combination of both types of additives.
5. Conclusions With respect to highly stressed machine parts which operate in the regime of boundary lub~cation under high hertzian pressure, sliding tests with a pin-on-disc machine were carried out to explore the tribological behaviour with the focus being on reaction layer formation. To summarize the results it can be stated that two groups of additives, on the one hand organic sulphur and phosphorus compounds and on the other hand organometallic dithiophosphates, can be differentiated according to their mode of action. With additives based on sulphur and phosphorus, iron of the bulk material does participate in reaction layer formation which results in a low but controlled loss of material from the partners. The reactivity of the bulk material influences the effectiveness of the lubricant. With additives based on organometalhc dithiophosphates, with metals such as lead and zinc, the bulk material does not necessarily participate in the reaction layer formation and therefore the tribological process runs almost in a state of zero wear. For several applications this is not always desirable. In opposition to the commercial additives based on sulphur and phosphorus, organometallic dithiophosphates are suitable as lubricants also for relatively inert materials, e.g. high chromium alloyed steels. To render the running-in processes possible and to keep the wear under steady state conditions at a low level, the development of lubricants must probably be aimed at a combination of both types of additives.
References 1 J. Fohl, Untersuchung von Triboprozessen in der Grenzfllche von Metallpaarungen bei Mischreibung, insbesondere im Hinblick auf Werkstoffiibertragung, Reaktionsschtichtbildung und Verschleiss, Dissertation, Universitiit Stuttgart, 1976. 2 M. A. Khosrawi, Untersuchung iiber die Reaktion~h~chtbildung bei hoehbelastbaren Schmieriilen, ~i~e~tation, Univ~si~t Suttgart, 1983. 3 E. S. Forbes, Antiwear and extreme pressure additives for lubricants, Tnbology, (1970) 255 - 265. 4 K. Wagner, Untersuchung zur Wirkung von Schmierstoffadditiven auf Sehichtbildung, Reibung und Verschleiss, Schmierungstechnik, 11 (1980) 330 - 336. 5 A. G. Papay, Gear oils today and tomorrow, ASLE Trans., 30 (1974) 446 - 454. 6 J. Hickmann and H. Giilsing, Druckbelastbarkeit und Verschleissschutz von Schmierstoffen mit neuartigen Additiven, MineraZGZtechnik, 20 (3) (1975) 1 - 28. 7 T. Sakamoto, H. Uetz, J. FShl and M. A. Khosrawi, The reaction layer formed on steel by additives based on sulphur and phosphorus compounds under conditions of boundary lubrication, Weor, 77 (1982) 139 - 157.
301
301 - 313
MECHANISM OF REACTION LAYER FORMATION IN BOUNDARY LUBRICATION H. UETZ,
M. A. KHOSRAWI
and J. F6HL
Staatliche Materialpriifungsanstalt, Stuttgart 80 (F.R.G.)
Universitiit
Stuttgart,
Pfaffenwaldring
32,
7000
Summary Highly loaded moving machine parts under hertzian contact conditions such as gears and roller bearings require lubricants which enable the formation of tribochemical reaction layers to reduce the adhesion component in the interface. The most critical conditions have to be referred to the sliding (slip) portion of such systems which cause microseizure and subsequent high wear. Sliding tests with a pin-on-disc machine were carried out to explore the efficiency of different lubricants with organic and organometallic additives. Two modes of action govern the formation of reaction layers and lead to distinct differences in friction and wear behaviour. Organometallic (e.g. with zinc and lead) dialkyldithiophosphates were found to be the most effective additives compared with organic phosphorus and sulphur compounds even with chemically inert steels, where other lubricants fail at relatively low load levels.
1. Introduction Constant-velocity joints of motor vehicles (Fig. 1) are heavily stressed machine parts for which elastohydrodynamic conditions cannot be sustained. High hertzian pressure, low relative velocity and high temperature lead to boundary lubrication conditions and require materials with a low tendency for adhesion and high resistance against fatigue as well as lubricants which are able to generate reaction layers in the local contact areas to reduce further the adhesion component [ 1,2]. In the case of steady material loss from the surface, the joint fails because of an increase in the play (Fig. 2); however, at an extremely low wear rate level, failure can occur as a result of fatigue, cracking and breaking of the balls, since scratches on the surface caused by local seizure (resulting from load peaks) act as microstress risers. The materials used at present (ballbearing steel for the ball and hardfaced low alloy steel for the inner and outer rings), resist the applied hertzian loading conditions. However, the lubricant 0043-1648/84/$3.00
0 Elsevier Sequoia/Printed
in The Netherlands
302
Fig. 1. Exploded view of a constant-velocity
joint used for motor vehicles.
overload sofetv
i I failure through continuaus /material rcmovat
Load Fig. 2. Operational limits affected by the system parameters load, material and Lubricant.
has to be improved with the aim of allowing a very small amount of wear in order to remove any kind of disturbances and to smooth the surface again. In the past, extensive ~vestigations have been carried out to investigate the mechanisms of reaction layer formation with extreme pressure (EP) additives and the effectiveness of frictian and wear reduction [2 - 71. However, the interaction of additives, materials and loading conditions is so complicated that present knowledge about the basic mechanisms is not yet sufficient for current efforts to optimize the performance of higbly stressed lubricated systems to be ~oroughly carried out. A variety of lubricants with different additives were investigated to explore wear and friction behaviour, the wear mechanism and the mechanism of reaction layer formation under tribologieal conditions.
303
2. Test equipment Although the kinetics in the constant-velocity joint are rolling and superimposed sliding, the dominant loading situation results from the sliding component. To simulate the most critical situation, laboratory tests were performed with a pin-on-disc machine, only the sliding component being taken into account. The lubricant was circulated at a low flow rate and the wear of the pin was measured continuously by using radioactive pins and applying a radionuelide technique (Fig. 3).
-FN --
FN measurmg chamber
-
I
I
Fig. 3. Test arrangement for performing laboratory tests under boundary lubrication conditions with application of the radioisotope technique for wear measurements.
3. Materials, lubricants
and test parameters
The materials used are listed in Table 1. The materials 20 MnCr 5 EH (face hardened) and 100 Cr 6 H (hardened) are actually used for the original drive shaft, whereas the hardened chromium steel X 40 Cr 13 H was used for comparison as a more inert material with respect to chemical reactions.
TABLE 1 Materials investigated in pin-on-disc tests Material
Tempering conditions
Hardness (HV 10)
20 MnCr 5 EH I.00 Cr 6 H X40Cr13H
Face hardened Oil hardened Oil hardened
750 820 650
304 TABLE 2 Content of reactive elements in the lubricants Lubricant
and additive
package
Element
in the oil (wt.%)
concentrations
P
S
Paraffin oil (pure)
-
-
Base oil SAE 80 (plain)
-
0.5
-
-
-
Hypoid gear oil
0.16
2.4
0.034
-
-
Extreme pressure gear oil (ZnDTP; O~S~MO~(DTP)~; S-P compound)
0.23
2.7
0.01
0.13
-
Oil I (ZnDTP; PbDTP; OZS~MO~(DTP)~) Oil II
0.1
2.1
0.08
0.27
1.4
0.3
1.8
0.21
0.1
-
Zn
MO
Pb
-
-
(ZnDTP; S-P compound)
(ZnDTP; OzSzMoz(DTP)2; S-P compound)
Lubricants without additives and with different additive packages (dialkyldithiophosphates) were investigated (Table 2). Since the complex structure of the commercial lubricants was not known in detail, some lubricants were mixed in the laboratory, the additive being added to a plain SAE 80 base oil (Table 3).
TABLE 3 Content of reactive elements in the additive concentrates Additive Description and reactive elements
Symbol
Tricresylphosphate (P) S-P compound (S, P) Zn dialkyldithiophosphate (Zn, S, P) Pb dialkyldithiophosphate (Pb, S, P) MO compound in dithiophosphate (Mo, S, P)
Element concentrations concentmte (wt.%)
in the additive
P
s
Zn
MO
Pb
Al
8.2
-
-
-
-
A3
1.7
31.5
-
-
-
A5
1.2
1
0.5
-
-
-46
2
3.5
-
-
6
AI
2.5
7
-
5.5
-
305
Tests were carried out at 1 m s-i under different loading conditions (Fig. 4) as follows. (1) Approximate information can be obtained from a test where the load is increased continuously up to the failure or limiting load of the machine, which is at about 5000 N. (2) In a second step the load is first increased and than held constant at a certain level in some cases for up to 50 h to investigate the running-in behaviour and behaviour under steady state conditions.
1 m/s
/--U
-0
TI
B
8
/ time
increasing failure
load up to
or load limit
of machine
j/--f time
running time at constant load up to 50 h
time
stepwise
increasing
up to failure
load
or
limit of machine
(5000 N 2 400 N/mm’)
Fig. 4. Loading pattern for studying reaction layer formation under different conditions.
(3) The third loading pattern is a step-like loading with a hold time to investigate both the running-in processes and the failure load developing from steady state conditions.
4. Test results Two examples demonstrate the general test procedure and the information which can be obtained from pin-on-disc tests with two basically different commercial oils I and II. Oil I (Fig. 5) exhibits a high friction and temperature peak after each transition from one load to the other; however, each peak is only of short duration. At low and medium load, wear occurs only in that transition regime and declines when the coefficient of friction decreases to its previous level.
306
0
M
20
IO
LO
50
60
10
80
h
90
time
Fig. 5. Typical results of friction, 100 Cr 6 H lubricated with oil I.
wear
and
pin temperature
for
ball-bearing
steel
For oil II (Fig. 6) the transition conditions persisted over longer periods of time, leading to a higher wear rate, and finally the system became instable at a load of about 3000 N, whereas oil I still performed well even at a load of 5000 N.
70
M
WI
50
60
70
W
h
90
time
Fig. 6. Typical results of friction, 100 Cr 6 H lubricated with oil II.
wear
and
pin temperature
for
ball-bearing
steel
307
Test runs with continuously increasing load using different oils and materials (Fig. 7) indicate that two different mechanisms are acting. 20 MnCr 5 EH and ball-bearing steel 100 Cr 6 H do not come to failure with any of the three lubricants, but reach the limit load of the machine. Therefore no differentiation between these oils was achieved. When the pin diameter was reduced from 4 to 3 mm to increase the maximum pressure level the different quality of the lubricants became obvious, but still no failure occurred with oil I. The tests with hardened chromium steel X 40 Cr 13 H confirmed that drastic differences exist between oil I and the two other lubricants. Oil I leads to a high load-carrying capacity whereas extreme pressure gear oil and commercial oil II were not effective on X 40 Cr 13 H steel.
B 2
l!L time
EP extreme pressure I
oil1
II
0ilU
gear
oil
1 1000
0 EP
I
II
EP
I
II
EP
I
II
Lubricant Fig. 7. Comparison of lubrication efficiency of different oils in combination with different materials under boundary lubrication conditions with a pin-on-disc machine.
The same tendency was obtained when the load was increased step like in such a way that steady state conditions were reached during each load step (Fig. 8). The hypoid gear oil behaves similarly to the high pressure gear oil and the commercial oil II. These three lubricants are very effective on low alloy carbon steels but fail in combination with the chromium steel. Commercial oil I, however, has the same efficiency on all three materials and does not fail within the loading capacity of the machine.
308 500
1
pin/disc, v=lm/c
I
-
I
--
oil1 oil
II
time
Sr 3"I
I
--hypoidgearoil
o, 250-
-4
-extreme pressure gearoil
5
I
2
-.,i_ ZOMoCr5EH L---
5
XbOCrtJH
0
~ 0
I
I
I
I 3000 load
IO00 al00
N
5000 0
1000
2000
3000 load
N
5000
Fig. 8. Wear behaviour of different materials as a function of load under boundary conditions with different lubricants.
The explanation of this phenomenon may be found in the reactivity of the additive in connection with the material. Tests with ball-bearing steel using SAE 80 base oil with different additives (Fig. 9) led to a more detailed understanding of the mechanisms that occurred. Obviously the lead dithiophosphate is able to form a reaction layer without participation of the bulk
pin/disc1OOCr6H, base oii SAE 60, v=lm/s 150,
I
I
50
1000
2000 load
N
3000
Fig. 9. Effect of oil additives on wear behaviour of hardened ball-bearing steel under boundary lubrication conditions (pin-on-disc machine; 100 Cr 6 H; base oil SAE 80; u = 1 m s-l).
309
metal and therefore the optimum wear behaviour is independent of the reactivity of the material. As soon as a certain peak temperature is reached, a lead compound is being precipitated on the sliding surface, filling up the grooves and causing smoothing of the surface so that on the one hand hydrodynamic and elastohydrodynamic lubrication is supported and on the other hand adhesion processes are avoided through the separating reaction layer (Fig. 10). These reaction products adhere extremely well to the bulk material.
T: 9
i
time
(a)
(cl
sliding direction
@I
(d)
Fig. 10. Reaction layer formation on a hardened ball-bearing steel pin formed under boundary lubrication conditions with oil I (pin-on-disc machine; 100 Cr 6 H; u = 1 m s-l; 3000 N): (a) pin; (b) region A in (a); (c) region B in (b); (d) region C in (c).
Microanalyses of the surface using energy-dispersive X-ray (EDX) analysis indicate that the grooves are substantially filled with a lead component. Phosphorus as well as sulphur play only a minor role (Fig. 11). When the molybdenum sulphur dithiophosphate was used, no significant reaction layer was found on 100 Cr 6 H using EDX analysis. However, from the surface appearance and wear behaviour it is assumed that the smooth
310
time
(b)
Fig. 11. Reaction layer formation on hardened ball-bearing steel lubricated with oil I (tribological conditions equivalent to those shown in Fig. 10) and corresponding lead and phosphorus distribution: (a) pin; (b) enlarged view of part of the pin (a); (c) lead distribution for the region shown in (b); (d) phosphorus distribution for the region shown in (b).
surface results from a thin reaction layer which does not cause microseizure (Fig. 12). As a more sensitive analysis method, Auger spectroscopy was applied to this surface. In the outermost layer the presence of reaction products could be demonstrated (Fig. 13). The amount of reaction products decreases with increasing sputter time so that the thickness of the layer could roughly be estimated to be about 10 nm. From a variety of test runs with different oils and additive mixtures pin surfaces were investigated using EDX or microprobe analysis. From these investigations two groups of additives could be differentiated in general (Tables 4 and 5).
311
time
(cl Fig. plex 20% gion
(d) 12. Reaction layer formation on hardened ball-bearing steel lubricated with a commolybdenum dithiophosphate in base oil SAE 80 (pin-on-disc machine; 100 Cr 6 H; O~S~MO~(DTP)~; u = 1 m s-l): (a) pin; (b) enlarged view of part of the pin; (c) reA in (b); (d) region B in (b).
(1) Lubricants with additives mainly based on sulphur and phosphorus generate a reaction layer with a high content of iron. This is the common mode of action and is widely described in the literature. This type of reaction can only occur on materials with appropriate reaction kinetics as these are the low alloy steels. The removal of the reaction layer due to the tribological stressing implements the loss of material and results in an equilibrium between reaction layer formation and removal. (2) Lubricants with metal dithiophosphates (e.g. lead and zinc) create a reaction layer that contains almost no iron, i.e. effectiveness of these additives is not dependent on the reactivity of the material and the wear process of the reaction layer does not necessarily implement the loss of bulk material.
17.
&utter t,A
*in
6
Fig. 13. Element distribution (Auger spectroscopy) across the thickness (corresponding sputter time) of a reaction layer formed on hardened ball-bearing steel lubricated with molybdenum dithiophosphate in base oil SAE 80. TABLE 4 Composition
of reaction product@ on 100 Cr 6 H at 3000 N
Lubricant
Element concentrations
Hypoid gear oil Extreme pressure gear oil Oil I Oil II
(wt.%)
Fe
P
s
Zn
MO
Pb
56 50 2 61
19 6 1 4
3 12 15 32
22 1 1 1
31 1 2
80 -
aDetermined by EDX analysis. TABLE
5
Composition
of reaction productsa on 100 Cr 6 H at 2000 N with base oil SAE 80
Additive
5% S-P compound fP> S) 100% Zn dialkyldithio-
in the reaction layer (wt.%)
Element concentrations
I?;
P
58
38
9
Zn
MO
Pb
4
_
-
-
21
1
69
-
-
3
5
23
-
-
69
18
5
13
-
-
64
18
5
13
-
S
phosphate (P, S, Zn) 20% Pb dialkyldithiophosphate (P, S, Pb) 20% MO compound in dithiophosphate (P, S, Pb) 20% Pb dialkyldithiophosphate aDetermined by EDX analysis.
64
313
The amount of iron in the reaction layer probably results from a chemical reaction of the additives on organic sulphur and phosphorus compounds since the oil contained a combination of both types of additives.
5. Conclusions With respect to highly stressed machine parts which operate in the regime of boundary lub~cation under high hertzian pressure, sliding tests with a pin-on-disc machine were carried out to explore the tribological behaviour with the focus being on reaction layer formation. To summarize the results it can be stated that two groups of additives, on the one hand organic sulphur and phosphorus compounds and on the other hand organometallic dithiophosphates, can be differentiated according to their mode of action. With additives based on sulphur and phosphorus, iron of the bulk material does participate in reaction layer formation which results in a low but controlled loss of material from the partners. The reactivity of the bulk material influences the effectiveness of the lubricant. With additives based on organometalhc dithiophosphates, with metals such as lead and zinc, the bulk material does not necessarily participate in the reaction layer formation and therefore the tribological process runs almost in a state of zero wear. For several applications this is not always desirable. In opposition to the commercial additives based on sulphur and phosphorus, organometallic dithiophosphates are suitable as lubricants also for relatively inert materials, e.g. high chromium alloyed steels. To render the running-in processes possible and to keep the wear under steady state conditions at a low level, the development of lubricants must probably be aimed at a combination of both types of additives.
References 1 J. Fohl, Untersuchung von Triboprozessen in der Grenzfllche von Metallpaarungen bei Mischreibung, insbesondere im Hinblick auf Werkstoffiibertragung, Reaktionsschtichtbildung und Verschleiss, Dissertation, Universitiit Stuttgart, 1976. 2 M. A. Khosrawi, Untersuchung iiber die Reaktion~h~chtbildung bei hoehbelastbaren Schmieriilen, ~i~e~tation, Univ~si~t Suttgart, 1983. 3 E. S. Forbes, Antiwear and extreme pressure additives for lubricants, Tnbology, (1970) 255 - 265. 4 K. Wagner, Untersuchung zur Wirkung von Schmierstoffadditiven auf Sehichtbildung, Reibung und Verschleiss, Schmierungstechnik, 11 (1980) 330 - 336. 5 A. G. Papay, Gear oils today and tomorrow, ASLE Trans., 30 (1974) 446 - 454. 6 J. Hickmann and H. Giilsing, Druckbelastbarkeit und Verschleissschutz von Schmierstoffen mit neuartigen Additiven, MineraZGZtechnik, 20 (3) (1975) 1 - 28. 7 T. Sakamoto, H. Uetz, J. FShl and M. A. Khosrawi, The reaction layer formed on steel by additives based on sulphur and phosphorus compounds under conditions of boundary lubrication, Weor, 77 (1982) 139 - 157.