Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Molybdenum dialkyl dithiophosphate (MoDDP) is generally considered as a friction-reducing and antiwear additive, but it also can act as extreme pressure additive. Its chemical structure is as follows:
Extreme pressure performance of molybdenum dithiophosphate is mainly reflected in its ability to significantly improve last non-seizure load by reducing the four-ball wear scar diameter and steady state friction coefficient of the oil treated with this additive. A four-ball friction and wear tester was used in the evaluation of friction, wear and extreme pressure properties of a 650 SN base oil treated with this additive. Standard ASTM D4172 four-ball wear test method conditions were used, except test load was reduced from 40 kgf to 30 kgf for the base oil, only. All the other tests were run at 40 kgf load, 1200rpm, 60
minutes and 75℃. The base oil without additives could sieze at 40 kgf. The ASTM D2783 four-ball EP test method was used to measure the extreme pressure properties . Standard test conditions of 1760 rpm, 10 seconds and room temperature were observed. The molybdenum dialkyl dithiophosphate additive evaluated is a typical product of MoDDP (molybdenum content of 8.1%,phosphorus content of 6.1%, sulfur content of 12.3%) [10]. Test results are shown in Table 3.
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In general, though extreme pressure additives in grease can greatly improve the weld point, this often leads to lower antiwear performance of the grease, resulting in an increase in four-ball wear scar. As we know, MoDDP additive not only can provide excellent anti-friction and antiwear properties, but also can significantly improve the last nonseizure load. While on the other hand, DMTD derivatives can improve the EP performance in either 4-ball weld point or Timken OK load or both. Although for any lubricant oil or grease, four-ball weld point, the last nonseizure load (LNSL), four-ball wear scar diameter and friction coefficient are often mutually restrained since all these additives are competing for the same metal surface area for adsorption. In this paper, a combination of DMTD derivatives as EP additive and MoDDP as antiwear additive and friction reducer in various base greases were studied with several common grease performance evaluation test methods. It is expected that the combination of DMTD extreme pressure additive and molybdenum dialkyl dithiophosphate antiwear agent can provide the best balance of extreme pressure, antiwear and anti-friction properties in treated greases.
2. Experimental / Materials and Methods
2.1 Base Greases and Additives Base greases used in this study include a lithium complex grease, a polyurea grease and a calcium sulfonate complex grease. All three base greases were obtained from real production batches courtesy of several grease manufacturers. DMTD dimer [10] used in this study has a sulfur content of 62.0-67.0% and a nitrogen content of 17.4-19.4%. DMTD dimer/polyalkylene glycol complex [10] has a sulfur content of 20-28%, and a nitrogen content of 6.0 to 10.0%. Molybdenum dialkyl dithiophosphate (MoDDP) has molybdenum, phosphorus and sulfur contents of approximately 8.1%, 6.1% and 12.3% respectively. All three additives are available commercially [10].
2.2 4-Ball Friction and Wear Test Standard ASTM D2266 four-ball friction and wear test method was used to evaluate additive performance in lubricating grease. Experimental conditions were: 40 kgf load, 1200 rpm
speed, 60 minutes, 75℃. Real-time recording of coefficient of friction during the test is also available with the specific four-ball test machine used, in addition to measurement of wear scar diameters.
Table 3. Four-ball friction and wear tests on 650 SN base oil with MoDDP.
As can be seen from Table 3, LNSL value of oil treated with MoDDP will increase from 36 kgf to 95-100 kgf, showing good extreme pressure performance but the weld point is only improved marginally. For four-ball wear test, the wear scar diameter under load of 30 kgf for base oil is 0.802 mm, by adding 1.0% and 2.0% MoDDP , the wear scar diameter under 40 kgf load is reduced to only 0.478 mm and 0.376 mm, respectively. It can also be seen from Table 3, the coefficient of friction (C of F) for oil with MoDDP will be reduced by 20%, from 0.083 to 0.077.
34 LUBE MAGAZINE NO.126 APRIL 2015
2.3 Four-Ball EP test Standard ASTM D2596 four-ball EP test method was used to evaluate additive EP performance in greases. Experimental conditions were: 1770 rpm speed, time duration 10 seconds. Both last nonseizure load (LNSL) and weld point were measured.
3. Results and Discussion
3.1 DMTD derivatives and molybdenum dialkyl dithiophosphate synergistic combination in lithium complex grease MoDDP, DMTD dimer and DMTD/PAG complex were added to the lithium complex base grease at the indicated treat levels. Four-ball wear and extreme pressure tests were performed on the treated grease, see results in Table 4.
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