Vol. 44 Supp.
SCIENCE IN CHINA (Series A)
August 2001
Investigation on tribology behavior of lubricants using the coefficient of friction test method CAI Jiyuan, GAN Quan & DAI Lixia Lanzhou Branch of PetroChina Lubricating Oil Research and Development Institute, Lanzhou 730060, China Received July 5, 2001
Abstract This test method is used to determine the property of lubricants by measure the parameters such as the coefficient of friction, wear value and seizure load on the Four Ball Wear Test Machine. Experiments were conducted using ASTM D5183-95 Standard Test Method (Standard Test Method For Determination Of The Coefficient of Friction of Lubricants Using the Four Ball Wear Test Machine) to measure the friction reducing ability, antiwear property and extreme-pressure property of different type of lubricants, the additives are also been studied at the same time. From the test result, this test method can distinguish not only the property of different type of lubricants rapidly, sensitively and effectively, but also can reflect the friction reducing ability, antiwear property and extreme-pressure property of various additive formula. Keywords: the coefficient of friction, lubricants, additive, synergistic mechanism.
Determination of lubricants friction coefficient is a kind of oiliness test in the friction and wear tests. However, determinations of lubricants anti-wear performance and load-carrying capacity of oil film belong to the category of wear test and extreme pressure (EP) test. It is very hard to determine the correlations among the performances of friction reduction, anti-wear and EP property of different formula by some old friction and wear test methods. At boundary lubricating condition on Four-Ball test machine, this method uses continue-loading to test friction coefficient, wear scar diameter and oil film cracking load under states of certain speed, certain temperature and different loads. Combined with conditions of oiliness, wear and extreme pressure, the test method provides a simple, comprehensive and reliable simulating test for oil formula researchers. In the past, lubricant friction coefficient was tested with Oiliness Testing Machine (Japan) and Falex Ring-Block Test Machine (America), which only determining lubricants friction coefficient at certain load and cycles. Usually, it adopted light-duty and moderate test conditions. As mechanical industry and automotive industry developing toward the direction of high-speed, high-temperature, high-pressure and over-loading, lubricating oils and additives are developing toward the direction of anti-high-temperature, anti-wear, EP and friction reduction. As a result, the differential ability of above test methods is very limited. It can only provide friction coefficient under certain load condition (light-duty). It also exists difficulty to determine the mechanism and interaction of lubricant additives, and the effects of wear reduction, anti-wear and EP during load variety from low to high. In 1995, ASTM issued “Lubricating Oil Friction Coefficient Test Method (ASTM D5183)” and this problem was solved. It is just the characteristics of the test
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method. Using China domestic Four-Ball friction specimens on MMW-1 Universal Test Machine, this research paper describes a process to establish friction coefficient simulating test method which equivalent ASTM D5183. Reference ASTM D4127, run long-cycle wear test for 1hr. and leave basic wear scars on the three steel balls, then increase load for the basic wear scars gradually. After a certain amount of test cycles, record the friction coefficients under each load stages, the maximum load is 980N. Measure average wear scar diameters on the three balls at the end of each test. The load, when oil film break and adhere, is regarded as the fail load. In this paper, the method was used to review the compound relationship among EP additive, anti-wear additive and friction improver, to review the effects of compound formulas for the performances of EP, anti-wear and friction reduction. Test results show, this method can be used to test friction and wear performances for vary kinds of lubricants because of its characteristics of simple operation, short test duration and better distinguish ability. 1
Test
1.1 Test equipment The test method was established on MMW-1 Vertical Style University Test Machine. The machine possess controllable shift gear systems (rotate speed can reach to 2000 rpm), spring loading system and computer controlling synchronized motor (load range is 101000 N), oil temperature adjustment system (range is 0200 Deg. C), friction force test system, test time control system and etc, multifunction. It can meet the requirement of ASTM D4172 for Four-Ball Wear Test completely. 1.2
Test specimens Test steel ball can meet the requirement of GB308 II (China national standard), GCr15,
D12.7 mm, HRC 6466. 1.3
Running oil It uses commercial white oil (Viscosity is 2428 m2/s@40) as running oil, which can
meet the standard of GB1791 and SH0006. 1.4
Specimens cleaning procedure Soak the four steel balls, clamp and oil cup in n-heptanes liquid for 1 min, use ultrasonic cleaner washing them for 10s, then repeat the above steps with acetone liquid, dry them with nitrogen finally. 1.5 Test method summary The test is suitable to determine friction coefficient for various lubricating oils. Index of average wear scar diameter, friction coefficient under each additional 98N loads, seizure load and final wear scar diameter is regarded as the test result.
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The test is separated into two procedures, procedure I is white oil running test, procedure II is friction coefficient test under gradually increased load conditions (table 1). Table 1 Test Conditions Temperaturep Speed Duration
Wear-in 75±2k(167±4Ň) 600 r/min 60 min
Test
Load
392N(40 kgf) per 60 min
600 r/min 10 min 98.1N(10 kgf) per 10 min increment to a load that indicates incipient seizure (sudden increase in friction force value oversteady state) on the friction trace
1.6 Test oils 5 standard commercial lubricating oils, 7 laboratory oils with different kinds of wear reduction additives, anti-wear additives and EP additives were selected as reference oils to review differential ability of the test method and interaction mechanism of the additives (tables 13). Table 2 Quality Level of Testing Oil No 1 2 3 4
Oil Mobil 80w/90 vehicle gear oil Mobil 632 industry gear oil N100 50CD
viscosity degree 80w/90 N320 N100 50
quality level GL-5 USS224 HM CD
producer Mobil CD Mobil CD LANLIAN LANLIAN
Table 3 Additive Formula for Testing Oil (I) No 1 2 3 4 5
Oil TO-1 TO-2 TO-3 TO-4 TO-5
Base Oil SAE90 SAE90 SAE90 SAE90 SAE90
T 406 / 0.2 0.2 0.2 0.2
T 304 / / 0.5 0.5 0.5
18EPI / / / 0.5 0.5
T321 / / / / 2.0
Table 4 Additive Formula for Testing Oil (II) No 1 2 3
Oil TO-7 TO-8 TO-9
Base Oil 200SN 200SN 200SN
LAN403B 0.4 0.4 0.4
NSPN / 0.1 0.1
MoTP / / 0.2
2 Results and discussion Fig. 1 is the characteristic of friction coefficient varying as the load increasing on 5 commercial lubricating oils. Fig. 2 describes wear increasing at the end of the 5 commercial lubricating oils test, it equals to the differentia of wear scar diameter after both the end of test and running test. The smaller the value is, the better the test result is. Fig 1 is the performance of diesel engine oil which detergent and dispersant as the major additives in its formula. Bigger friction coefficient appears in the initial stages (µ = 0.1), then friction coefficient continue increase when load aggravating, the biggest friction coefficient happened at the load 392N, then the curve become flat and
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Friction coefficient, µ
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! "#
Load, 1
Fig. 1. The load-friction coefficient curve of 4 oils. 1. 50CD diesel engine oil, 2. N100 anti-wear hydraulic fluid, 3. mobil-632, 4. Mobil HD 80w/90.
until adhere happened at the load 980N. From fig. 2, anti-wear ability is poor (wear increment is 0.33 mm). So, Mechanism of diesel engine oil effect belongs to a tiny physical and chemical absorber. Curve 2 is a friction coefficient of anti-wear hydraulic oil which phosphite ester and ADTP as the major additives in its for
mula, the friction coefficient variety range is
0.10.115, wear increment is 0.07 mm, but
Fig. 2. Anti-wear property comparison of 4 test oils. 1. 50CD diesel engine oil, 2. N100 anti-wear hydraulic fluid, 3. mobil-632, 4. Mobil HD 80w/90.
friction reduction is not obvious. Curve 3 is a friction characteristic of ferric phosphate film formed when industrial gear oil with S-P type EP anti-wear additives during chemical reaction, load range is 98N~980N, friction coefficient is smaller (µ = 0.075) at the initial stage,
friction coefficient increases a little when load aggravating, after that, friction coefficient reaches 0.95. Friction coefficient tends to decreases while load increasing, wear increment is 0.03 mm, it shows a better result of anti-wear and friction reduction and the test state is in boundary lubrication. Curve 4 is a new kind of S-P type GL-5 automotive gear oil. The frictional characteristic of chemical adsorptive film and strong solid lubricating film formed on friction surface can get the lowest friction coefficient during the whole test processes and can reduce wear to the lowest level, it reflects additive compound effects on S-P additive well assorted and consonance operation. Research for reaction mechanism of friction improver, anti-wear and EP additives is very crucial in lubricant additives researches, and their disciplinarian is especially important. Fig. 3 and Fig. 4 state the mechanism of friction improver, anti-wear and EP additives reduce lubricant friction coefficient, increase load-carrying capacity and reduce wear. Curve 1 is the friction performance of SAE90 base stock (TO-1) ,friction coefficient is bigger at lower loadadhere appears in the load of 785N and wear increment reaches 0.28mm. Curve 2 is the friction effect of TO-1 oil with 0.2% benzotriazole 18 amine salt (oily additive) in it. It has a certain function to improve
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Friction coefficient µ
seizure
test, load %
%
%
%
%
Fig. 3. Load-friction coefficient diagram of the synergistic effect among the friction improver, anti-wear additive and EP additive.
Wear increment, nm
72
72
72
72
72
Fig. 4. Anti-wear ability of the synergistic effect among the friction improver, anti-wear additive and EP additive.
friction performance at low load, and formation of physical and chemical film is the major tasks. Curve 3 shows the improvement of TO-2 oil friction and wear performance when 0.5% acidic phosphite ester (anti-wear additive) is added. Compare with benzotriazole 18 amine salt, acid phosphite ester has stronger ability of absorb and more active of chemical reaction. It not only reduces friction coefficient, but also reduces the wear rate to the lowest point. It proves an excellent performance of wear reduction and effect of anti-wear. Curve 4 is the wear reduction effect of TO-3 oil with 0.5% long-chain acid phosphite ester (friction improver) in it. Because of strong absorb ability of long-chain, it can improve friction performance of metallic surface and get the lowest friction coefficient during the load test. However, as it occupies the metal surface, the per-
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formance effect of acidic phosphite ester is obstructed completely and its anti-wear performance is weakened (wear increment is 0.14) too. Curve 5 describes effect of TO-4 oil with sulfurized isobutene in it. Sulfurized isobutene is the typical EP anti-wear agent, when it contacts with metal frictional surface at high-temperature, chemical friction reaction is happened, high-melting-point solid film of sulfurized ferrite is created which has extremely strong effect of anti-wear and anti-adhere. Because it is added in TO-5 oil, EP anti-wear performance of the oil has been enhanced, and wear increment has been decreased into the lowest (0.01 mm), but it counteracts friction reduction effect on partial long-chain acid phosphite ester. MoDTP has an excellent friction reduction effect in friction improvers (fig. 5 and fig. 6). Curve 1(TO-7), Fig. 5, is the characteristics of load vs. friction coefficient when 0.4% EDMO (oleinic acid glycol ester) is added in 200SN base stock, and its friction coefficient tends to decrease as load increasing. The friction coefficient varies at the range of 0.093~0.41 during the load However, another 0.2% MoDTP is added in the oil TO-9, friction coefficient is reduced greatly (µ = 0.07ü0.09), wear increment is decreased about 68%, and proves a excellent effect of friction
Friction coefficient, µ
reduction and wear resistant when MoDTP is combined with NSPN. Mechanism of MoDTP is different from other friction improvers, improves surface roughness is the major character. Tiny
test load
Fig. 5. The friction improvement of MoDTP.
Wear increment, mm
72
72 Fig. 6. The anti-wear effect of MoDTP.
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test oil
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platform is previously formed on the friction surface under function of MoDTP, then decades nano layer of boundary lubricating film is formed on the platform, the boundary lubricating film continues formed and continues worn out in order to create a dynamic balance and stable friction effect, the anti-wear performance is improved also. Although MoDTP has a better performance in friction reduction, it erodes metal surface severely. As a result, its application range is very limited. Through above research, lubricating state of lubricants in Four-Ball friction coefficient test mainly depends on additive absorption at metal surface and boundary film formed after chemical friction reaction. If friction coefficient is between 0.05ü0.1, it is in boundary lubricating state. As additive types and performances are differences in lubricating oils, boundary-lubricating effects to be improved are difference at different load conditions too. Organic phosphides in boundary lubricating frication reaction create phosphate ferric film, active sulfide can produce sulfide ferric film with new metal surface exposure in the air at high-temperature. All of the formed boundary films can reduce friction coefficient, decrease wear rate, increase load-carrying capacity and improve boundary lubricating effect, especially when friction improver, anti-wear and EP additive are well compounded, and get an ideal wear reduction and EP anti-wear performances. 3
Repeatability investigation of lubricating oil friction coefficient test
Equivalent ASTM D5183-95, on MMW-1 Universal Test Machine, this paper describes a test method for evaluate lubricating oil friction coefficient. Test repeatability is investigated with 13 kinds of different reference oils, and each oil is repeated for three time (tables 5 and 6). Test results show an excellent repeatability of the test method, the maximum deviation of friction coefficient is 0.016 and wear increment is 0.06 mm. Table 5 The standard bias of friction coefficient load(N) Oil 1# 2# 3# 4# 5# 6# 7# 8# 9#
result x S x S x S x S x S x S x S x S x S
98
196
294
392
490
588
0.0789 0.0199 0.0758 0.00574 0.0850 0.0115 0.0932 0.00890 0.0888 0.00603 0.110 0.0162 0.0819 0.0104 0.0842 0.00910 0.0818 0.0192
0.0823 0.0120 0.0811 0.00475 0.0899 0.00392 0.114 0.00850 0.0958 0.00537 0.117 0.0128 0.100 0.00862 0.0922 0.00506 0.0873 0.0167
0.0804 0.0137 0.0848 0.00642 0.0958 0.00410 0.120 0.00709 0.111 0.0144 0.124 0.00985 0.110 0.0118 0.0940 0.00463 0.101 0.0190
0.0758 0.0115 0.0935 0.00473 0.0952 0.00035 0.123 0.0116 0.124 0.00245 0.119 0.0114 0.115 0.0125 0.0939 0.00455 0.102 0.0217
0.0776 0.0139 0.0954 0.00404 0.0948 0.00146 0.123 0.0122 0.123 0.00200 0.110 0.0133 0.117 0.0144 0.0927 0.00524 0.105 0.0248
0.0762 0.0129 0.0957 0.00457 0.0914 0.00273 0.123 0.0108 0.124 0.00273 0.106 0.0137 0.113 0.0147 0.0918 0.00473 0.0993 0.0247
687 0.0722 0.0129 0.0941 0.00631 0.0897 0.00035 0.118 0.0112. 0.128 0.00586 0.102 0.0120 0.109 0.0107 0.0895 0.00404 0.118 0.000 (To be
785 0.0711 0.0122 0.0911 0.00440 0.0886 0.00147 0.117 0.00953 0.126 0.00529 0.100 0.0163 0.103 0.00894 0.0856 0.00495 0.0929 0.0244 continued
883
981
0.0693 0.0688 0.0139 0.0137 0.0922 0.0875 0.00315 0.00228 0.0859 0.0837 0.00225 0.00220 0.115 0.110 0.00899 0.00929 0.124 / 0.00874 / 0.100 / 0.0194 / 0.973 0.00925 0.00608 0.00471 / / / / 0.0956 0.110 0.0320 0.00990 on the next page)
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load(N) Oil 10# 11# 12# 13#
98
196
294
392
490
588
687
785
883
981
0.0822 0.0115 0.0979 0.00824 0.0948 0.00260 0.0932 0.0148
0.0830 0.00981 0.105 0.00551 0.111 0.00346 0.0958 0.0136
0.0843 0.00895 0.111 0.00845 0.128 0.00700 0.0944 0.0115
0.0905 0.00547 0.118 0.00794 0.136 0.00757 0.0899 0.0167
0.0901 0.00583 0.117 0.0118 0.137 0.00929 0.0889 0.0123
0.0873 0.00581 0.117 0.0121 0.126 0.00833 0.0897 0.00719
0.0848 0.00581 0.112 0.00755 0.118 0.00586 0.0924 0.00601
0.0838 0.00679 / / 0.107 0.00808 0.0954 0.00836
/ / / / 0.100 0.00472 0.0964 0.00301
/ / / / 0.0952 0.00251 0.0896 0.00869
result x S x S x S x S
Table 6 The parallel test result and standard bias of the increment of the diameters of the scars ( m m ) oil
1st test
2nd test
3rd test
X
S
1#
0.02
0.02
0.03
0.023
0.0058
2#
0.05
0.03
0.06
0.047
0.015
3#
0.03
0.02
0.01
0.020
0.010
4#
0.06
0.08
0.05
0.063
0.015
5#
0.38
0.26
0.33
0.32
0.060
6#
0.30
0.25
0.28
0.28
0.025
7#
0.09
0.10
0.07
0.087
0.015
8#
0.01
0.03
0.01
0.017
0.012
9#
0.12
0.16
0.12
0.13
0.023
10#
0.07
0.09
0.05
0.070
0.020
11#
0.32
0.27
0.30
0.30
0.025
12#
0.24
0.24
0.25
0.24
0.0058
13#
0.10
0.07
0.08
0.083
0.015
4 Conclusions 1. This test method can be an efficient way for screening the friction reducing ability, antiwear property and extreme-pressure property of different type of lubricants, it can distinguish the comprehensive property of lubricants with good repeatability. 2. If friction coefficient is between 0.05ü0.1, it is in boundary lubricating state. And it can reduce friction coefficient, decrease wear rate, increase load-carrying capacity and improve boundary lubricating effect, especially when friction improver, anti-wear and EP additive are well compounded, and get an ideal wear reduction and EP anti-wear performances. 3. The establishment of this test method is the unique one in domestic, and it will push the research of super lubricants go forward. References 1. 2. 3. 4. 5.
ASTM D5183-95 ASTM D2670-81 ASTM D2714-81 ASTM D4172 Wen Shizhu, Principles of Tribology, Beijing: Tsinghua University Press, 1990.
