Wear, 115 (1987)
TRIBOLOGICAL BEHAVIOUR OF TWO- AND THREE-COMPONENT BONDED SOLID LUBRICANTS* WILFRIED
aTechnical Academy Esslingen, Ostfildern (F. R.G.) bDow Corning GmbH, Munich (F.R.G.) “Lanzhou Instiiute of Chemical Physics, The Chinese
Summary This report provides a comprehensive review of an investigation into the tribological behaviour of two- and three-component bonded solid lubricants performed in cooperation between the Technical Academy Esslingen, Dow Corning and Lanzhou Institute of Chemical Physics, China.
1. Introduction Bonded lubricating films can provide low friction at extreme temperatures, high pressures or under vacuum conditions. They can also prevent adhesion and wear of metal surfaces which are in frictional contact, simplifying the design of machine elements. Although these films have been widely applied in many areas, the limited wear life of bonded lubricating films and other factors which affect their performance restrict their application. In order to increase the wear life of bonded films and improve their performance, the factors influencing the tribological properties of films have been studied containing MoS *, graphite and Sb(Sb&) in one-, twoand three-component systems. This report provides a comprehensive review of an investigation programme performed in cooperation between the Technical Academy Esslingen, Dow Coming and Lanzhou Institute of Chemical Physics, China [l - 41. 2. Experimental
2.1. Test rig A ring-block frictional device is shown schematically *Paper Technology,
couple having line contact was used. The in Fig. 1. The test specimens consist of a
presented at the Nordic Symposium LuIeP, Sweden, June 15 18, 1986.
@ Elsevier Sequoia/Printed
in The Netherlands
TRANSfllSSION OF FRICTION FORCE
SPECIMEN BLOCK THERliOCOUPLE
MULTIPOINT DATA RECORDER FOR FRICTION FORCE l TEMPERATURE
rotating ring and a stationary block. There is pure sliding frict.iotl wrth iine contact between the cylindrical surface of the ring and the block. The ring is driven by a d.c. motor with infinitely variable speed. Thts rotational speed is digitally indicated. The load is applied hydraulically and can he controlled and indicated by a calibrated manometer. Load and speed can be selected according to the test program. The friction is measured using a dynamometer and recorded using an amplifier and multiple recorder. Using a limit value switch, the test machine as well as the measuring equipment can be automatically switched off when the frictional force reaches a critical value, The wear is evaluated by measuring the thickness of bonded films before start-up and at the end of a test or by measuring the width of the wear track of the block. A magnetic contact pin instrument with a digital display and a prmter measures the thickness of the bonded film on the ring and the transfer film on t.he block. The width of the wear track is measured with a stereo microscope. 2.2. Test lubricants As test lubricants, bonded solid film formulations have been produced containing MO&, graphite or Sb(Sb&) as single components or in combinations of two or even three components. These solid lubricants were dispersed
in polybutyltitanate, an organic air-curing binder. As solvents, chlorothene together with methylene chloride and butyl acetate have been used. For all test lubricants the same type and content of binder and the same solvent combination were chosen, only the solid lubricant combination has been changed. Tables 1 and 2 contain details of the test lubricant. TABLE
Complete formulations of single-component bonded film test lubricants
MSb-B CSb-B MC-D MO& Graphite Sb( SbS4)
3.45 3.45 4.60
45.00 59.95 57.65 45.00 63.40 49.60
10.70 13.00 13.00 8.40 13.00 13.00
10.00 10.00 10.00 10.00 10.00 10.00
9.00 9.00 9.00 9.00 9.00 9.00
MC-D MSb-B CSb-B MCSb-e
20.70 6.90 27.60
47 75 18.75
53 53 56.25
25 47 25
2.3. Preparation of test specimens Before applying the lubricant, the test specimens were degreased using a solvent and sand-blasted. For sandblasting, A&O3 particles with a size of 180 mesh were used. The bonded solid film was applied by spraying. The air-curing process lasted for about 0.5 h. After that the film thickness, always between 6 and 10 ,um, was measured. The lubricant was applied only to the cylindrical outer surface of the ring, whereas the surface of the block remained untreated. 2.4. Method of test and evaluation Under the selected test conditions, characterized by rotational speed (revolutions per minute) and normal load (newtons), test runs were performed until the end of the wear life of the bonded films was reached.
Consequently, the test runs were finished when the lubricant film was worn off and the metallic surface became visible or the friction coefficient exceeded a value of 0.1. This means the frictional force was measured continuously during the test. Also the film thickness was measured periodically during the test, in order to determine the wear rate of the bonded films. The following investigations have been carried out. (1) Wear depending on load with constant but different speeds and wear depending on speed with constant but different loads. (2) Load over speed for constant but different wear lives. (3) Film thickness over time; wear coefficient. (4) Friction coefficient over time. (5) Observations of film condition over time for evaluating the film failure mechanism. (6) Development of a mathematical function for the relationship between load, speed and wear life.
3. Wear behaviour
of solid lubricant
3.1. One-component systems The test lubricants on the basis of only one solid lubricant component contained 25 vol.% MO&, graphite or Sb(Sb&). Wear life data of those bonded films are shown in Fig. 2 for a moderate load and two speeds. These results show the greater lubricating properties of MO& under these
u s E
$ 2 4
190. SPEED 1% 100 1000 mln-'
140. 130. 120.
066 123 266
110. loo90. 80. 70. 60 50 LO 30 20 10 HOST
conditions compared with those of graphite. did not exhibit any lubricating properties.
3.2. Two-component systems 3.2.1. Optimum concentration The optimum concentration of two-component systems in terms of the longest wear life of the bonded films was determined for the combinations MoEG-graphite, MoSz-Sb(Sb&) and graphite-Sb(Sb&) using a load of 980 N and the speeds of 500 rev min-’ (1.23 m s-l) and 1000 rev min -’ (2.46 m s-r). Some of the results are shown in Fig. 3 (MO&--graphite system) and in Fig. 4 (MoS,-Sb(SbSJ system). y ;I
150 / 180
MoS*. Vol GRAPHITE. Vol. %
Fig. 3. Wear life of MoSz-graphite systems in bonded (500 rev min ‘); ii, 2.46 m s-l (1000 rev min ‘),
: 0. 1.23 m s ’
nos, , Vol. % SbfSbSJ. Vol. %
Fig. 4. Wear life of MoSz-Sb(SbS4) systems in bonded (500 rev min-‘); A, 2.46 m s-l (1000 rev min-I).
The curves of wear life us. concentration for the two lubrication additives of all systems exhibit a maximum. These maximum wear life data are higher than that of either single component, revealing that, obviously some synergistic effects occur. It should be mentioned that even Sb(SbS,), pas sessing no lubrication properties at all, can improve the lubrication hehaviour of MO& as well as graphite. 3.2.2. Comparison of one-component and optimum concentration twocomponent films The wear life data of all test lubricants listed in Table 1 have been measured at a load of 980 N and speeds of 500 and 1000 rev nun L for which the optimum concentration has been determined. A comparison of these results is shown in Fig. 2. In any case the tribological properties of bonded films containing two components are superior to those containing only one component. Not only is the excellent behaviour of the MO& and graphite-containing bonded film lubricant shown but also t,ha! of MoS, and Sb(SbS4). Therefore these two bonded film lubricants were investigated in more detail. 3.2.3. Tribological behaviour of two-component bonded films For characterizing the tribological behaviour of two-component bonded films the wear life data depending on load and speed have been measured, the wear life curves developed, the wear life dependence on t*ime measured and the wear coefficients calculated. The wear life of bonded films containing MoS,-graphite and MoS, Sb(Sb&) as a function of load at several speeds is shown in Figs. 5 and 6.
Fig. 5. Wear life of MoS--graphite bonded films us. load for several speeds 1000; m, 1250; .,1500;~, 1750; n, 2000. per minute): X, 500; 0, 75O;c;,
Fig. 6. Wear life of MO& -Sb(SbSq) b onded films us. load for several speeds per minute): X, 500:‘1, 750:~. 1000;:7, 1250;*, 15OO;V, 1750;e. 2000
The load effect on wear life at higher loads is more pronounced than at lower loads. As expected, higher sliding speeds result in lower wear life. Obviously, the wear rate of the bonded films increases with speed. Using the results of wear life data as a function of speed as well as load, so-called speed-load curves for different wear life data can be developed. They can be regarded as “fingerprints”, characterizing the lubricating properties of the specific bonded lubricating films. Figure 7 shows these speed-load curves for the two-component system MoSz-graphite, whereas from Fig. 8, representing a reference lubricant based on the same solid lubricant composition, very similar lubrication behaviour may be deduced. As an example of the film thickness variation over time, Fig. 9 shows the results obtained with the MO&-graphite formulation. Operational conditions were selected under which the bonded films exhibit stable friction coefficients and rather long wear lives. These bonded films reveal the following details regarding the wear process. (1) At the beginning of sliding the wear rate of bonded films is high but after running-in it decreases remarkably. (2) For all these bonded films a transfer film is formed on the block, the thickness of which reaches a maximum and then decreases. (3) With higher loads at the same speed the transfer films are formed remarkably quicker than at lower loads.
“x ” d
\ 1250. 1000.
‘; .G E
V \ 0
7. Wear life curves of MO&-graphite 50 000; 0,100 000; a, 150 000; 0,200
Fig. 8. Wear life (cycles) curves 150000;n, 200000;a, 300000;~,
Load. N bonded films (code 000; V, 300 000.
a reference 400000.
life in cycles:
0, 100 000;
SLIDING DURATION, KILO CYCLE
9. Film thickness as a function r, block). (a) Load 1470 rev min ’
of sliding duration for MoS~~graphite N, speed 500 rev min ’ (b) Load 1370
SLIDING DURATION, KICO CYCLE
bonded N_ speed
Wear coefficient Graphite Load, Speed,
245 N 500 rev min
Load, Speed, Speed,
980 N 500 rev rnin-~’ 1000 rev min
Load, 1470 N Speed, 500 rev min Speed, 1000 rev min
” mm’ MoS2
N ~’ mm graphite
’ ’ ’
(4) With higher speeds at the same load the transfer films are formed more slowly than at lower speeds. (5) MoSz exhibits better lubricating behaviour than graphite. The lowest wear rate was obtained with the optimum MO&-graphite combination. These results are confirmed by the wear coefficients calculated, using the slopes of the film thickness time curves after running-in. The good
behaviour of MO& in combination shown (Table 3).
3.3. Three-component system 3.3.1. Behaviour of optimum concentration system Owing to the beneficial effect of Sb(SbS,) on the lubrication properties of MO& (and graphite) a new bonded film was developed containing all three components. A third component was added to a two-component bonded lubricant with good properties (graphite to a MoS,-Sb(Sb&) combination, Sb(SbS4) to several MoS,-graphite combinations). The formulation MCSb-e (Table 2) contains the optimum concentration of the three components; therefore its tribological properties have been evaluated in more detail. Figure 10 shows the wear life depending on load and speed of this formulation. 1
z x u 0
z Is 650.?z .-I L
Fig. 10. Wear life depending on speed and load of MoSz-graphite-Sb(SbS4) A, 612 N; n, 980 N;A, 1470 N; q,196O N. (Code 3-e.)
Y, 50000;:-~. 100000;‘,
curves 150 000:
filmb ~vodc 3.~
’ . 200 000; :, 300 000; 0, 400 000; *. 500 000
Fig. 12. The dependence of the coating graphite and Sb(SbS4) on speed and load.
Compared with the data obtained with one- and two-component bonded films the excellent behaviour of this optimum concentration threecomponent bonded film is obvious. The variation in film thickness with the sliding duration was also determined for this bonded film. This test was run at 980 N load and a speed of 500 rev mini. Using the slope of this film thickness vs. sliding duration curve after running-in, a wear coefficient of 0.23 X lo-i0 mm3 N-’ mm-’ was determined. Compared with the data of the two-component films, the excellent behaviour of this three-component bonded film can be recognized. It not only lengthens the wear life but also decreases the wear rate. The wear life curves of this optimum formulation are shown in Fig. 11. The wear life results of the three-component system containing optimum concentrations of MO&, graphite and Sb(SbS4) as a function of load and speed are shown in Fig. 12 three dimensionally. 3.3.2. Comparison of the three-component system component systems A comparison of the wear life data over speed for the component and two-component systems together with the cant at a load of 980 N is shown in Fig. 13. The wear life
z -;, u
optimum threereference lubricurves of these
Speed, Cycles min-’ Fig. 13. Comparison of wear life of different bonded films vs. speed for 980 N load: q, MO&-graphite Sb(SbS4) (code 3-e); q, MO& graphite (code MC-D); n, MO& -Sb(SbS4) (code MSb-B); 0, graphite-Sb(SbS4) (code CSb-B); El, commercially available film (MoS2graphite).
12 500 750 000
” , .-c
; : v z-
Ez (I) 250 1 0
14. Wear life (cycles) curves of solid lubricants containing bondeu films. Reference bonded films: 0, 100000; “, 300 000. MoSz graphite bonded films (code %lC-Dj: e. 100 000; V, 300 000. MoS-graphite Sb( SbS4) bonded films: 0, 100 000; vl, 300 000.
lubricants are plotted in Fig. 14. The excellent behaviour of tht lubricant containing MO&, graphite and Sb(Sb&) when compared with all other bonded lubricant films can be clearly recognized. The outstanding lubricant effectiveness is remarkable, especially at the lower speed.
During the test runs for the evaluation of the wear life of the bonded solid films the friction coefficient over time was also recorded. This type of friction coefficient curve as well as the stable coefficient of friction can be used to characterize the friction behaviour of bonded solid lubricating films. 4.1. Type of friction coefficient curves In investigating the frictional behaviour of bonded solid !ubricating films, three types of friction coefficient curves over time were observed. Type 1 of the friction coefficient curves over time obtained with the bonded lubricant film containing an optimum concentration of MO& and Sb(SbS4) is shown in Fig. 15(a). This friction behaviour is typical for
0.05 0.06 0.03 0.04
0.12 0,i 1 0,lO 5 0.07 ‘Z g o,of3-~yj B F 0.05 .z ;=” 0.04 . 6 0” 0.03 -
0.09 0.08 5 .g ‘-
B r; al 2
50 Time 40[min]
Fig. 15. (a) Coefficient of friction us. time (type 1): load, 980 N; speed, 500 rev min-’ (1.23 m s-‘). (b) Type 2: load, 980 N; speed, 1000 rev min-’ (2.46 m s-‘). (c) Type 3: load, 980 N; speed, 1750 rev min-’ (4.3 m so.‘).
pressure-velocity values of less than 1200 N m s-’ and characterized by stable friction. Type 2 of the friction coefficient curves over time obtained with the same bonded lubricant film is shown in Fig. 15(b). The frictional behaviour revealed in this figure could be typical for pressure-velocity values of higher than 1200 N m s-l (2400 N m s-l). This frictional behaviour is the result of conditions in which no clear running-in period with decreasing friction is observed, followed by a stable coefficient of friction. The friction coefficient reaches a peak before again decreasing. Bonded films exhibiting this kind of friction behaviour have a rather short wear life.
Type 3 of the friction coefficient curves over time also obtamed with the same bonded lubricant film is shown in Fig, 15(c). This friction behaviour is typical for rather high pressure--velocity values of more t,han 4200 N m s-l. The pressure-velocity load value is too high to permit normal operation, This condition is characterized by a high starting coefficient of friction of about 0.1. Even after the running-in process, the coefficient, of friction is higher (0.06 - 0.07) than the starting coefficient of friction for types 1 and 2. The coefficient of friction remains at an unstable high value and fluctuates remarkably. The wear life of the bonded film untft~r t~hcse conditions is very short. 4.2. Stable friction coefficient The so-called stable friction coefficient established after thrl running-in process may also be used to characterize the friction behaviour of bonded solid lubricating films. Table 4 contains the values of the stable c,oefficient of friction for graphite, MO& and the two-component systems investigated containing optimum concentrations. Graphite exhibits no stable friction region at all; the friction coefficient fluctuates between 0.12 and 0.18. The addition of Sb(Sb&i) to this formulation results in a stable friction region at low pressure velocity component exhibit, values. Formulations containing MoS, and another lower friction coefficients than MoS2 alone. The MO&--graphite system showed the lowest friction coefficients and stable friction regions :it. all the pressure-velocity values which were investigated. The stable friction coefficients for the bonded film containing optimum concentrations of MO&, graphite and Sb(Sb&) under different speed and load conditions are listed in Table 5. We recognize here that the friction coefficient is stable and low for pressure-velocity values below 7200 N m se-‘. In comparison with the data contained in Table 4, the highest pressure-velocity values with a stable friction region can obviously bt> attained with this lubricant.
Stable coefficient Operational
Speed (rev min- ’ )
Pressure-velocity value (N m s- ‘)
245 980 1470
500 500 500
300 1200 1800
values for a two-component Coefficient Graphite
0 f friction CSb-B
- 0.04 0.03
0.04 - O.UB
0 01 0.02 0.03 0.05
Stable coefficient of friction graphite and Sb( SbS4)) Speed (rev min-‘) 500
for an optimum
Coefficient (m C’)
750 0.04 1500 0.04 2250 0.02 3100 0.03
- 0.06 - 0.05 - 0.03 - 0.04
1200 0.04 2400 0.03 3600 0.03 4800 0.03
1800 0.03 3600 0.02 4000 0.03 7200 -
2400 0.03 4800 0.02 - 0.03 7200 0.02 - 0.03 9600 -
- 0.05 - 0.05 - 0.04 - 0.04
using a mathematical
5.1. Equations The results of this investigation have shown that the wear life of all bonded films decreases with increasing load and/or speed. Accordingly, the wear life tends to zero for infinite loads and speeds. The wear behaviour therefore might follow a mathematical function in the form of F, X Llu = F2 X Lzu = F, X L, U = constant
where F is the load (newtons), L is the wear life (minutes), (revolutions per minute) and U is an exponent. It follows that wear life over load F1 X LluF = F2 X L,‘?F = constant
(1) G is the speed
wear life over speed G, X LluG = G2 X LzuG = constant The logarithm
of eqns. (2) and (3) result in
lg F + U, X lg L = lg CF
lg G + u, X lg L = lg Co
Given y=lgL x = lg F or lg G eqns. (2) and (3) can be transformed y=BXX+B,
where B and B,, are constants.
Using measured load wear life and speed wear life data, the corresponding U and C values can be calculated. Using reasonable Lr and C values, which resulted in a minimum relative error between the experimental data
Fig. 16. Lifetime at constant speeds
us. load for formulations (revolutions per minute):
containing MoSa and graphite (code 0, 500; :I, 1000; 0, 1500; +, 2000
E a /
Fig. 17. Lifetime at constant loads:
500 1000 Life Time,
us. speed for formulations 0. 245 N; :i, 612 N;;‘,, 980
containing N; +. 1225
MoSa and graphite N; n, 1470 N
Fig. 18. constant
Lifetime us. load speeds (revolutions
for a formulation containing MO&, graphite and Sb(SbS4) per minute): 0, 500; A, 1000; 0, 1500; X, 2000.
Fig. 19. Coating lifetime us. speed for a formulation containing MO&, Sb(SbS4) at constant loads: 0, 612 N;.h, 980 N; 0, 1470 N; X, 1960 N.
and the mathematical function, load-time curves for several speeds and speed-time curves for several loads were plotted on paper with a double logarithmic scale. As an example in Figs. 16 and 17 the life time of the MO&-graphite system has been plotted. It may be recognized that for these formulations the life time can be expressed in a form of the mathematical function developed above. Figures 18 and 19 exhibit the results of this approach for the three-component system. Most of the values represent the relative error range of 5%. Only a few experimental results obtained at high loads exceed this range.
6. Conclusions A test rig and a test procedure have been developed using a ring-block system as test specimen in order to evaluate the wear behaviour of bonded solid lubricating films by laboratory methods. As test lubricants, bonded films have been used containing MO&, graphite and Sb(Sb&) as two- and three-component systems. The results can be summarized as follows. (1) It was found that using optimum concentrations of MoSz-graphite and MoS,-Sb(Sb&) greater wear lives and lower film wear rates were exhibited compared with the single components. (2) Even Sb(SbS& which has no intrinsic lubricating properties, can improve the lubrication behaviour of MO& and graphite. (3) A new bonded film was developed containing an optimum concentration of MO&, graphite and Sb(Sb&). The wear life and wear rate data for this three-component film proved to be superior to the one- and twocomponent bonded films. (4) With all lubricants, the wear life depended on speed for several constant loads and the wear life depended on load for several constant speeds. (5) Using the life-load and the life-speed curves, so-called “wear life curves” have been developed, representing speed over load providing constant wear life data.
(6) The wear life curves are characteristic of the lubricating properties of bonded film solid lubricants and can be used to compare different lubricants. (7) The limit of the wear life of bonded solid lubricant films can be characterized by the wearing away of the film and the formation of blisters, (8) The wear endurance of such films decreases with increasing speed and load and is more sensitive to speed than load. (9) Pressure-velocity data, as criteria for extreme pressure behaviour. increase with increasing load but decrease with increasing speed. (10) Independent of the original film thickness, the wear life hmit is reached at a film thickness of about 2 - 4 pm. (11) An important phase of the gradual destruction of the bonded lubricant film is the formation of blisters. (12) Three types of friction coefficient curves over time were observed. Low pressure-velocity values Friction characterized by a clear running-m process with decreasing friction coefficients and stable friction. Medium pressure-velocity values No clear running-in process characterized by decreasing friction coefficients. Rather short wear life of the bonded film. High pressure-velocity values Characterized by high friction coefficients which remain unstable. Very short wear life of the bonded film. (13) A so-called stable friction coefficient, established following the running-in process, may be used to characterize the frictional behaviour of bonded solid lubricating films. (14) The varying frictional behaviour at different operational condition levels can be observed by investigating the wear tracks on the bonded solid lubricating films. (15) A mathematical function has been developed which c”an be used to describe and characterize the wear behaviour, for example wear life depending on load and speed, of bonded solid lubricants in the region of stable friction. References 1 W. J. Bartz, R. Holinski and 3. Xu, A study on the behaviour of bonded lubricating films containing molybdenum disulfide (MO&-graphite and Sb(Sb&)), hoc. 3rd Int. Conf. on Solid Lubrication, Denver, CO, August 7 - 10, 1984, in ASLE Spec. Publ. 14, 1984 (American Society of Lubrication Engineers, Chicago, IL) 2 W. J. Bartz and J. Xu, Describing wear life of bonded solid lubricants by laboratory methods, Lubr. Erzg., 41 (1985) 607 613. 3 W. J. Bartz, R. Holinski and J. Xu, Wear life and frictional behaviour of bonded solid lubricants, ASLE--ASME Tribology Conf., Atlanta, GA, October 8 ! 0, 198Ei. 4 W. J. Bartz and J. Xu, Wear behaviour and failure mechanism of bonded solid lubricants, ASLE Ann. Meet., Toronto, May 12 - 15, 1986, American Society of Lubrication Engineers, Chicago, IL, 1986, Reprint 86-AM-6F-1