F. FLOREA, N. N. ANTONESCU

Friction Coefficient and Wear Variation for Grease With Different Additives

RESEARCH The paper present the studies made with some lubricating compositions based on lithium soap grease and a number of different additives. The friction, wear and EP properties are evaluated at variable loads and durations using a four-ball machine. The friction coefficient are evaluated at long durations (30…120 minutes) because show more exactly the comportment of lubricant compositions. The investigations regard the performance of different additive combinations and some synergetic effects.

Keywords: friction; wear; synergetic effect

1. INTRODUCTION The tests were made with the grease sample including the following additive types: The wear and friction coefficient values are affected by a number of parameters depending upon the * AO – antioxidant (aromatic sulphur lubricant (type, viscosity, the content and the type of combination) additives, etc.), the material of the friction couplings * SC – sulphur compound (≈ 33% wt. sulphur) (their nature and composition, roughness, thermo- * FM – friction modifier (oxidized vegetable oil) chemical treatments, etc.) and upon the physical and * ZnDTP – zinc ditiophosphate. mechanical conditions (speed, temperature, loading, The results obtained with different additive type of contact, etc.). Usually, the friction coefficient concentrations and combinations and efficacy of this is measured at reduced values for load and duration are presented in table1. and, in this conditions, the determined friction coefficient value is more for information, as during 2.1. Friction coefficient variation such a reduced duration, the wear of the metallic surfaces does not begin, especially at relatively low For this parameter, most often, like in the case of loading. At increased durations (30…120 minutes) tests made on four-ball machine, the testing duration of tests, the friction coefficient variations show more is approximately 60 sec. The determined friction exactly the comportment of the lubricant, especially coefficient value, in these conditions is more for additivated. information, as during such reduced length of time for testing, the wear does not begin, especially at 2. RESULTS AND DISSCUTIONS relatively low loading. At the same loading values, but at increased durations (30…120 min), the A four-ball machine was utilized to investigate the friction coefficient variation show more exactly the friction and wear comportment for all samples comportment of the lubricant, especially additivated. tested. In these conditions, friction coefficient is influenced, Different type and additives concentration was in the same time, by the shear stress of the chemical dispersed in a grease type lithium soap with compounds developed at the metal surfaces by the following characteristics: additives action, and exist possibility from decreasing the value of the friction coefficient in the * Penetration at 250C, mm/10 …… 275 o testing period. In the figures 1…7, are presented the * Dropping Point, C ……………. 190 variation of friction coefficient at long duration for * Rolling test (4 h), mm/10 ……… 305 lithium grease (NLGI 2) free and with different * Oil separation (30 h, 1000C), % .. 1.9 additives content. The efficacy for reduction of the friction coefficient is presented in table1. For this F. FLOREA, was utilized the average values of the friction N. N. ANTONESCU, coefficient obtained for 120 minutes of testing Mechanical and Electrical Engineering Faculty, duration. For samples prepared with more additives, Petroleum-Gas University, Ploiesti, Romania the total concentration is 3% wt.

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Table 1.

Additive content, % wt. Efficacy, % Sample AO FM ZnDTP SC Friction coeff. Wear Weld 300 N 600 N 300 N 600 N Load A - - - - 100 100 100 100 100 A 1 0.5 - - - 104 114.4 113.3 138.1 126.7 A 2 1.5 - - - 118.2 108.8 108 129.8 140 A 3 - 2 - - 103.3 104.1 106.7 102.3 100 A 4 - 5 - - 114.4 120.1 110.7 106.6 106.7 A 5 - 10 - - 118.6 121 121 107.5 113.3 A 6 - - 1 - 87.4 110 133.8 138.1 126.7 A 7 - - 2 - 103.3 114 142.7 147 140 A 8 - - 3 - 116.5 116.7 153.3 159.5 153.3 A 9 - - - 0.3 96 108.1 108.7 135.7 126.7 A 10 - - - 1.0 115.4 118.7 133.3 158 146.7 A 11 - - - 1.5 109.5 108 123 153.4 153.3 A 12 - - - 2.0 102.6 105.2 122 152.7 160 A 13 - - - 3.0 90 101.8 115 147.6 166.7 A 14 - - 1.5 0.5 121.4 97.5 148 147.3 133.3 A 15 - - 1.5 1.5 109.8 110.4 138.3 159.3 173.3 A 16 - - 1.0 2.0 119.3 109.8 122 154 173.3 A 17 0.5 - 2.5 - 107.2 113.2 147.7 163.5 153.3 A 18 1.0 - 2.0 - 109.7 113.8 144 162.9 146.7 A 19 1.5 - 1.5 - 106.5 113.7 140.3 161.7 140 A 20 0.5 - 0.5 2.0 122 120.3 110 140.6 186.7 A 21 0.5 - 1.0 1.5 113.9 112 130 159.4 180

Test duration influence because the wear spot increase and, at the same load, the contact pressures decrease. The * After 30 minutes from testing, the values of lubricant have in these conditions a great friction coefficient practically remain constants, influence. for the majority of the samples. Therefore, for correct characterization of the lubricants, he The load influence must be not less than 30 minutes. * The results presented in table 1 and figure 1, * Between the friction coefficient values after 1 show the decrease of the friction coefficient in minute and 30 minutes are great differences: presence of antioxidant (AO) additive at reduced 42% and 55% for grease with 1% wt. and 3% load (300 N) and relatively high (600 N). A wt. ZnDTP, respectively. reduction with 4…18.2% for friction coefficient are obtained at 300 N and with 14.4…8.8% at * After the first minutes, the wear begin, but 600 N, for 0.5…1.5% wt. AO content. during the test, the variation of friction Therefore, the optimum content of this additive coefficient is small. This aspect is possible type is 0.5…1% wt.

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* The friction coefficient decrease, especially at * For SC + ZnDTP combinations, the best results low load (table 1 and fig.2) if content of friction are obtained for samples with 2% wt. SC and 1% modifier (FM) increase. But, for 10% wt. FM wt. ZnDTP (table 1 and fig.5). content, obtain the decrease for dropping point * The comportment of samples with AO and (5…60C) and penetration (10…16 mm/10). ZnDTP additives are presented in table 1 and Therefore, the maximum content of this additive figure 6. For these samples, at higher loads, the is 5% wt. friction coefficient values decrease and the best * The friction coefficient decrease if ZnDTP results is obtained for combination: 1% wt. AO content increase but, for 1% wt. at 300 N, the + 2% wt. ZnDTP. value of this parameter exceeds the value * In case of AO + ZnDTP + SC combinations, the obtained for grease additive (fig.3, table 1). A best result is obtained for sample with 0.5% wt. possible explication consists in the fact that at AO + 0.5% wt. ZnDTP + 2% wt. SC (fig.7). low load, the temperature generated by friction is low and the additive remains inactive. * The synergetic effects appear for following combinations: * For sulphur content, the very low values for friction coefficient is obtained for 1% wt. SC (≈ • at 300 N: 0.5% wt. SC + 1.5% wt. ZnDTP; 2% 0.33% wt. sulphur). At low load, for low (0.3% wt. SC + 1% wt. ZnDTP; 0.5% wt. AO + 0.5% wt.) and high (3% wt.) content for SC, the wt. ZnDTP + 2% wt. SC; 0.5% wt. AO + 1% wt. friction coefficient values are high. ZnDTP + 1.5% wt. SC; At reduced load, because the temperature generated • at 600 N: 1% wt. ZnDTP + 2% wt. SC; 1% wt. by friction is low, the inactivity of sulphur is AO + 2% wt. ZnDTP; 1.5% wt. AO + 1.5% wt. possible. For great sulphur content is possible to ZnDTP; 0.5% wt. AO + 0.5% wt. ZnDTP + 2% appear the corrosive wear (fig.4). wt. SC.

Fric. Coeff. 0,16 0,14 0,12 A -a A-b 0,1 A1-a 0,08 A1-b 0,06 A2-a 0,04 A2-b 0,02 0

1 Duration, min. 20 40 60 80 100 120

Figure 1. Friction coefficient variation with AO content: a) 300 N; b) 600 N

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Fric. 0,14 0,12 A -a 0,1 A-b 0,08 A3-a 0,06 A3-b 0,04 A4-a 0,02 A4-b 0 A5-a A5-b 1 0 20 40 60 80 2 100 1 Duration,

Figure 2. Friction coefficient variation with FM content: a) 300 N; b) 600 N

Fric. Coeff. 0,14 A -a 0,12 A-b 0,1 0,08 A6-a 0,06 A6-b 0,04 A7-a 0,02 A7-b 0 A8-a 1 0 0 20 40 60 80 A8-b Duration,10 12 min.

Figure 3. Friction coefficient variation with ZnDTP content: a) 300 N; b) 600 N

Fric. Coeff. 0.14 A -a A-b 0.12 A9-a 0.1 A9-b 0.08 A10-a 0.06 A10-b A11-a 0.04 A11-b 0.02 A12-a 0 A12-b A13-a 1 0 0 0 0 2 4 6 80 00 1 12 A13-b Duration,

Figure 4. Friction coefficient variation with SC content: a) 300 N; b) 600 N

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Fric. Coeff. 0.14 A -a 0.12 A-b 0.1 0.08 A14-a 0.06 A14-b 0.04 A15-a 0.02 A15-b 0 A16-a

1 0 0 0 0 0 0 2 4 6 8 0 2 A16-b Duration,1 1 min.

Figure 5. Friction coefficient variation with SC + ZnDTP combination: a) 300 N; b) 600 N

Fric. Coeff. 0.14 A -a 0.12 0.1 A-b 0.08 A17-a 0.06 A17-b 0.04 A18-a 0.02 A18-b 0 A19-a

1 0 A19-b 20 40 6 80 00 1Duration,120 min.

Figure 6. Friction coefficient variation with AO + ZnDTP combination: a) 300 N; b) 600 N

Fric. Coeff. 0.14 0.12 A -a 0.1 A-b 0.08 A20-a 0.06 A20-b 0.04 A21-a 0.02 A21-b 0

1 0 Duration, min. 20 40 60 80 00 2 1 1

Figure 7. Friction coefficient variation with AO + ZnDTP + SC combination: a) 300 N; b) 600 N

2.2 Antiwear effects The result are presented in table 1 (efficacy) and figures 8…14 (wear spot diameter). The studies have been carried out on the four-ball machine at 300 N and 600 N for 120 minutes * For the majority of samples containing one maximum duration. single additive type, the wear spot diameter

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(“d”) value decrease when additive concen- decreases. The best result is obtained for the tration increase (fig.8…11), excepting the sample with 0.5% wt. AO + 2.5% wt. ZnDTP. samples with contents over 0.5% wt. AO and 1% * The combinations containing AO, ZnDTP and wt. SC. For great content of sulphur corrosive SC decreased wear (fig.14, table 1). For 0.5% (chemical) wear appear. wt. AO + 1% wt. ZnDTP + 1.5% wt. SC content * The samples with ZnDTP and SC, compor-tment result the minimum value for wear. differs with loading. For 600 N, maximum The synergetic effects are obtained for the samples efficacy is obtained for 1.5% wt. SC + 1.5% wt. with following combinations: ZnDTP while at 300 N, minimum wear are obtained for 0.5% wt. SC + 1.5% wt. ZnDTP • at 300 N: 1.5% wt. ZnDTP + 0.5% wt. SC; 1.5% (fig.12 and table 1). The choice bet-ween these, wt. ZnDTP + 1.5% wt. SC; 0.5% wt. AO + 2.5% another parameter is necessary; for example, wt. ZnDTP; 1% wt. AO + 2% wt. ZnDTP; 1.5% weld load. wt. AO + 1.5% wt. ZnDTP; 0.5% wt. AO + 1% wt. ZnDTP + 1.5% wt. SC. * For greases with AO + ZnDTP combinations obtain a great reduction of wear (fig.13, table 1). • at 600 N: all the samples excepting the The better efficacy is obtained, in this case, for combination with 0.5% wt. AO + 0.5% wt. 600 N because AO additive contain sulphur. In ZnDTP + 2% wt. SC. the same time, with the increase of sulphur content (> 0.5% wt. AO), the antiwear efficacy

d, mm 3 2,5 A-a A-b 2 A1-a 1,5 A1-b 1 A2-a 0,5 A2-b 0 30 45 60 75 90 105 120 Duration,

Figure 8. Wear spot variation for samples with AO: a) 300 N; b) 600 N

d, mm 3

2,5 A3-a 2 A3-b A4-a 1,5 A4-b 1 A5-a 0,5 A5-b

0 30 45 60 75 90 105 120 Duration, min.

Figure 9. Wear spot variation for samples with FM: a) 300 N; b) 600 N

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d, mm 2 A6-a 1,5 A6-b A7-a 1 A7-b 0,5 A8-a A8-b 0 30 45 60 75 90 105 120 Duration, min.

Figure 10. Wear spot variation for samples with ZnDTP: a) 300 N; b) 600 N

d, mm 1,8 A9-a 1,6 A9-b 1,4 A10-a 1,2 1 A10-b 0,8 A11-a 0,6 A11-b 0,4 A12-a 0,2 A12-b 0 A13-a 0 0 0 0 20 4 6 80 2 A13-b Duration,10 1 min.

Figure 11. Wear spot variation for samples with SC: a) 300 N; b) 600 N

d, mm 1,6 1,4 A14-a 1,2 A14-b 1 A15-a 0,8 A15-b 0,6 0,4 A16-a 0,2 A16-b 0 30 45 60 75 90 105 120 Duration,

Figure 12. Wear spot variation for samples with ZnDTP + SC: a) 300 N; b) 600 N

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d, mm 1.2 A17-a 1 A17-b 0.8 A18-a 0.6 A18-b 0.4 A19-a 0.2 A19-b 0

5 0 Duration, min. 30 45 60 75 90 0 2 1 1

Figure 13. Wear spot variation for samples with AO + ZnDTP: a) 300 N; b) 600 N

d, mm 3 2.5 A-a A-b 2 A20-a 1.5 A20-b 1 A21-a 0.5 A21-b 0 30 45 60 75 90 105 120 Duration, min.

Figure 14. Wear spot variation for samples with AO + ZnDTP + SC: a) 300 N; b) 600 N

2.3. EP characteristics REFERENCES For this, on utilized as parameter the weld load. The [1.] Cholakov, G.S., Stanulov, K.G., Cheriisky, results obtained are presented in table 1 (efficacy) I.A., functional Properties of EP Additive and figure 15 (values). Package Containing Zn-Dialkyl-Ditiopho- sphate, Sulfuruzed EP Additive and Metal Based on these results can be draw: Deactivator, 7th International Colloquium, * When the additive concentrations increase, the Technische Akademie Esslingen, 16-18 Ian., weld load increase. 1990, vol.1, 3.8-1…3.8-11 * For the majority of EP greases, 2400 N is the [2.] Florea, F., Antonescu, N.N., The Effect of value for weld load and thus, the samples Duration and Contact Pressure on the Friction containing following additives are accepted: > Coefficient of Some Lubricating Greases, 11th 2% wt. SC; 1.5% wt. ZnDTP + 1.5% wt. SC; 1% International Colloquium, Technische Akade- wt. ZnDTP + 2% wt. SC; 0.5% wt. AO + 0.5% mie Esslingen, 13-15 Ian., 1998, vol.3, 2257- wt. ZnDTP + 2% wt. SC and 0.5% wt. AO + 1% 2259 wt. ZnDTP + 1.5% wt. SC. [3.] Nabil, Al-H., Antonescu, N.N., Florea, F., In this case, synergetic effects are put in evidence Synergetic Effects at High Additive Concen- for all additive combinations. trations, VAREHD 7, Conference of Lubrica- tion and Elastohydrodinamic Traction, Sucea- va University, 2-4 June, 1994, 77-9

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