Tribological performance of organic molybdenum in the presence of organic friction modifier

The tribological performance of organic molybdenum in the present of organic friction modifier was investigated in this study. Three types of organic friction modifiers were selected, which are Glycerol monooleate, Pentaerythritol and N,N-Dimethylhexadecylamine. The organic molybdenum are MoDTC, MoDDP and molybdenum amide. Friction coefficient and wear were studied in block-on-ring test rig with steel test specimens. Experimental results indicate the Pentaerythritol shows synergistic effect with MoDTC in wide range temperature, while increased the friction coefficient of molybdenum amide in high temperature. N,N-Dimethylhexadecylamine shows synergistic effect with molybdenum amide, while hindered the friction reduction performance of MoDTC in low temperature. The presence of Glycerol monooleate reduced friction coefficient of MoDTC in low temperature, while increased the friction coefficient of molybdenum amide in most situations. All the tested organic friction modifiers improved the friction reduction performance of MoDDP. Most of the tested organic friction modifiers reduced the wear of organic molybdenum. The PT shows the best anti-wear performance with MoDTC. The tribo-chemical products in test specimens lubricated with different lubricant formulas indicate that the presences of Pentaerythritol promotes the production of MoS2 in MoDTC. N,N-Dimethylhexadecylamine promotes the production of MoS2 in molybdenum amide. The side products of MoO1.6S1.6 and Cr/MoS2 of MoDDP in high temperature lead to high friction coefficient.


Introduction
Organic molybdenum has long been applied as an effective additive for reducing friction [1][2][3][4], which shows significant friction reduction performance in steel, iron and other surface [5,6]. Therefore, it draws extensive attention for the study on friction reduction mechanism.
Typical organic molybdenum are Molybdenum dithiocarbamates (MoDTC), molybdenum dialkyl dithiophosphate (MoDTP), molybdenum dialkyldithiophosphate (MoDDP) and molybdenum amide. In general, the tribo-chemical decomposition of organic molybdenum and the generation of molybdenum disulphide (MoS 2 ) are the key to reducing friction coefficient. By means of Raman spectroscopy, Khaemba  decomposition in steel/steel contacts [7]. According to the analytical results, the products of MoDTC decomposition include MoS 2 , FeMoO 4 and sulfur-rich molybdenum compounds, as well as MoSx (x>2). FeMoO 4 is derived from the side reaction between iron oxides and molybdenum compounds at low temperatures and low MoDTC concentrations. As for the tribo-chemical product, surface roughness will have an impact on the tribo-chemical production of MoDTC. A rough surface contributes to the increased generation of MoS 2 from MoDTC, while a smooth surface leads to the mixture of MoS 2 , MoSx (x>2) and FeMoO 4 , which indicates the partial decomposition of MoDTC [8].
Lubricants are the mixture of base oil and different functional additives. The organic molybdenum will be impacted by the presence of other additives. Both synergies and antagonisms can be found between functionalized additive and organic molybdenum [6].
Syngernistic tribological effect could be produced when MoDTC is mixed with zinc dialkyldithiophosphate (ZDDP). The research results indicate that the lubricant formulations and tribo-chemical reaction films could make a difference to the deformation of subsurface layers, which plays an important role in the anti-wear performance [9]. Syngernistic tribological effect could also be found when mixing sulfur-and phosphorus-free molybdenum amide (MA) with ZDDP [10]. As indicated by the results of four-ball machine, the 2%wt MA and 1% 1.25% ZDDP are optimal for friction reduction and anti-wear performance. The similar synergistic results were also found by Cai T and Zhang J [11,12]. The mixture of 0.5% MA and 0.5% ZDDP reduced the friction coefficient to almost 35% compared with pure MA and ZDDP, with amorphous MoS 3 and MoS 2 as the intermediate product of MA and ZDDP mixture. In severe friction condition, the main product is MoS 2 [13]. In addition, fatty triamine was also identified as effective in enhancing the performance in friction reduction [14]. The MoS 2 long sheets were found on friction surface, which aligned along the sliding direction [15]. Yan et al compared the tribological behaviors of sulfur and phosphorus free organic molybdenum, ZDDP and MoDTC. When the concentration of sulfur-and phosphorus-free organic molybdenum reached 2% wt, the friction coefficient and wear scar were minimized. The wear scar of four-ball test with organic molybdenum was the lowest compared with ZDDP and MoDTC [16]. Compared with MoDTC, the friction coefficient was reduced by 25% in the presence of highly sulfurised moly-trimer additive and MA additive [17]. The production of MoS 2 /MoS 2−x O x shows solid lubricant character on steel surfaces. The MoS 2 /MoS 2 −x O x sheets are mainly embedded in an oxygen-rich amorphous matrix while the tribo-chemical product of MoDTC-ZDDP mixture is polyphosphate/phosphate glass matrix.
Apart from syngersitic effect, some additives may have adverse effect on the performance of organic molybdenum in reducing friction, especially organic friction modifiers, which have a polar functional group to form adsorption boundary film on friction surfaces and reduce friction coefficient. Typically, organic friction modifiers include ester, amide, fatty acid and long chain fatty amine. The polar group-containing organic friction modifiers can improve the solubility of polar additives in non-polar base oil [20]. It is demonstrated that the increasing ester concentrations will increase the friction coefficient of MoDTC in steel/iron contacts [21]. Though the friction coefficient of lubricant containing ester and MoDTP can lead to low friction coefficient at the beginning of friction test, the low friction phenomenon is not long-lasting in the presence of ester [22]. Therefore, in order to optimize the composition in lubricant, it is necessary to figure out the influence of organic friction modifier to organic molybdenum compound.
In this paper, an investigation was conducted about the tribological performance of organic molybdenum in the presence of organic friction modifier. The sulfur-and phosphorus-free molybdenum amide (MA), the sulfur-containing Molybdenum dithiocarbamates (MoDTC), and the sulfur-phosphorus containing molybdenum dialkyldithiophosphate (MoDDP) were selected as the organic molybdenum. The Glycerol monooleate (GM), Pentaerythritol (PT) and N,N-Dimethylhexadecylamine (AM) were selected as organic friction modifiers. Sliding friction was performed on block-on-ring test ring. The friction coefficient and wear of test specimens were recorded. The interaction mechanism among organic friction modifiers and organic molybdenum were discussed.

Lubricants
The organic molybdenum friction modifiers used for friction study includes molybdenum dithiocarbamate (MoDTC), molybdenum dialkyldithiophosphate (MoDDP) and molybdenum amide (MA), which were provided by Minglanchem. Co. Ltd. The molecular structures of organic molybdenums given by the suppliers are shown in Fig 1. The organic friction modifiers include Glycerol monooleate (purity 97.7%), Pentaerythritol (purity 95%) and N,N-Dimethylhexadecylamine (purity 98%). The organic friction modifiers, which contain no sulfur and phosphorus, were supplied by Shanghai Aladdin Bio-Chem Technology Co. Ltd. Group I mineral base oil was selected for its wide application in machinery lubrication. Because Group I base oil was distilled from crude oil, impurities as sulfur often exist in the base oil. The sulfur concentration of base oil in this study was 0.11%wt, which was analysed by OSA 4 metallab on-site oil analyzer.

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1 wt% organic molybdenum and 1 wt% organic friction modifier were added into the base oil, which is an effective concentration [23]. The base oil and additives were weighed by analytical balance, then mixed thoroughly with Magnetic Stirrer. Viscosity of test oil samples were measured with capillary viscometer. For the description to be simplified in the following sections, the abbreviations of base oil and additives are listed in Table 1. The test oil formulations are shown in Table 2.

Test specimens
The ring and block of test specimens were of GCr15 steel, which is typically used for fabricating bearings and friction specimens in tribological test. The ring and block are shown in Fig 2. The test specimens have a hardness of HV 751 for the ring and block. The surface roughness Ra of ring and block are 0.1 μm and 0.3 μm. The ring is 45mm in inner diameter, 50mm in outer diameter and 25mm in height. The block is a cylinder which is 10 mm in height and diameter.

Experimental apparatus
Block-on-ring test rig was fabricated to verify the lubricating performance. Block-on-ring is a typical line contact, which represents the contact state of cylinder liner and piston ring in diesel and gasoline engine. Therefore, it has been widely used in lubrication performance test. The schematic of test rig is illustrated in Fig 3. Photos of test rig is shown in Fig 4. The ring was fitted on the rolling shaft driven by the servo motor, while the block was fixed on the  loading device. A normal load was applied on the top of block. Sliding friction appeared between ring and block when shaft rolling. The loading device was fixed on a guide rail, which can move freely along the friction force direction. When the friction force pushing the loading device, the friction force was detected by the pressure transducer fixed between the loading device and damper screen. Lubricant bath was located beneath the ring. With the rolling of ring, the lubricant contained in the lubricant bath was entrained onto the friction surface.
Heater and temperature sensor were embedded in the lubricant bath, temperature of lubricant bath was controlled by the temperature controller. The oil entrained in the contact surface is hard to be detected, therefore, the temperature of lubricant bath represents the lubricant temperature in this paper. The temperature can be controlled from 40˚C to 150˚C, which is

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typical temperature for engine bearing in cold starting and long term running. The speed at which the shaft spun can be controlled from 0 r/min to 3000 r/min. The load was applied with weights, which are varied from 10 to 100N. The pressure transducer Longlv-LLLBM, which measures the friction force, is produced by Shanghai Longlv Electronic Technology Co., Ltd. The measuring range is ±100N, with accuracy of ±0.1N. Coupled with the charge amplifier, the friction force signal was read by data Acquisition Card YAV-USB 8AD Plus, which is produced by Wuhan YAV Electronic Technology Co., Ltd. Friction data was measured by every second, totally 21600 data points.
The friction coefficient was obtained with Eq 1 Where μ is friction coefficient, F is friction force (N), N is applied load (N)

Experimental procedure
Prior to experiment, the test specimens were cleaned with petroleum ether and alcohol to remove any contaminants left on the surface. Then, the ring and block were fixed onto the test rig. 20 ml of lubricant was added into the lubricant bath. The test conditions were set according to Table 3. The additive concentration is low, the viscosity of formulated lubricants varies little compared with base oil. Therefore, the base oil viscosity in different test temperature can be the representative of test lubricants, which was shown in Fig 5.

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Tribological performance of organic molybdenum with organic friction modifier After experiments, the test specimens were cleaned with petroleum ether and alcohol. The wear scar and surface morphology of test specimens were measured / observed with ZEISS Axio Observer, Chemical elements of friction surfaces were detected by JEOL JSM-7610F Energy dispersive spectrometer (EDS). Tribo-chemical products were detected by Thermo Fisher Scientific K-Alpha 1063 X-ray photoelectron spectroscope (XPS).

Results and discussion
Friction and wear Fig 6 shows the friction coefficient variation of lubricants containing MoDTC and organic friction modifiers during 6 hours test, during which the temperature is 40˚C. In the initiation, friction coefficient rises sharply to a high level, which is close to 0.16, and then decreases to a relative low value. All the friction coefficients keep stable except for BO-MC, which rises gradually from 0.075 to 0.085 after 280 min test. After 6h test, all the test oil samples show stable value.
To summarize the friction results and get a clear conclusion, the average friction coefficients of lubricant samples at stable period were illustrated in Fig 7. Fig 7A shows the influence of organic friction modifiers to MoDTC (MC). In general, with the rise of the test temperature, the friction of BO and BO-MC-GM increases gradually, while the friction of BO-MC-PT, BO-MC-AM and BO-MC drop significantly. Throughout the test, base oil without any additive showed the highest friction coefficient, which rose from 0.094 to 0.116. The BO-MC-GM demonstrated an excellent friction reduction performance at 40˚C, before a slight increase from 40˚C~130˚C. At 150˚C, friction coefficient of BO-MC-GM increases from 0.079 to 0.093, which is close to the maximum friction coefficient of BO and BO-MC-AM at 40˚C. The friction coefficient of BO-MC declined sharply from 0.084 to 0.051 when temperature rise from 40˚C to 70˚C, which may be caused by the sparking temperature for the tribo-chemical reaction of organic-molybdenum composition. Then, it increased slightly to 0.062, which may result from the decreasing base oil viscosity with temperature rise.

Discussion
The analysis of EDS and XPS was conducted on wear surface. Fig 11 shows the EDS peaks of the friction surfaces after tests. The friction surface lubricated with MC ( Fig 11A) and MA ( Fig  11B) mainly contains Fe, Chrome, Molybdenum and Sulfur. The surface lubricated with MP ( Fig 11C) contains Phosphorus as well.

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Tribological performance of organic molybdenum with organic friction modifier The tribo-chemical products were analyzed by XPS photopeaks, and corresponding data were verified with XPS data base [24].
The photopeaks of the D orbital in the third electronic shell (Mo3d) were generally used to analyze the chemical composition of Mo containing substance. Fig 12 illustrates the XPS photopeaks of Mo3d lubricated with MoDTC (MC) containing lubricants. Obvious change can be found on the XPS photopeak of Mo3d. In Fig 12A, the wear product of BO-MC mainly composed of MoO 3 and MoS 2 . In Fig 12B, the MoS 2 peak in BO-MC-PT is higher than that of BO-MC, which means more MoS 2 was produced on friction surface in the present of PT. In Fig 12C, the wear product of BO-MC-AM also shows clear MoS 2 peak, while in Fig 12D, there is nearly no MoS 2 peak shown for BO-MC-GT. It has been proved that the preferential conversion of organic molybdate to from MoS 2 can improve the tribological performance [25]. Therefore, the absence of MoS 2 is the main reason that impact the friction reduction perfor-

Conclusion
In this paper, an investigation was conducted into the impact of organic friction modifier on the tribological performance of organic molybdenum friction modifier using a block-on-ring

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Tribological performance of organic molybdenum with organic friction modifier

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test rig, based on which the friction and wear of different additive mixture were recorded and analyzed. The findings of this study are summarized as follows: 1. With the increase of test temperature, the present of GM in MoDTC increased the friction coefficient, while the friction of MoDTC with AM and PT reduced dramatically. The MC-PT is suitable to reduce friction in a wide range of temperature, while BO-MC-AM only shows the lowest friction coefficient at 150˚C. BO-MA-AM mixture shows obvious synergistic friction reduction performance. All the tested organic friction modifiers reduced the friction coefficient of MoDDP. All the tested organic friction modifiers reduced the wear of organic molybdenum except for the BO-MA-PT formulation. The PT shows the best anti-wear performance with MoDTC.