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Molybdenum to the Rescue

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Who hasnt ogled one of Teslas all-electric cars and wondered how it would feel to drive past gasoline stations without glancing at the prices? Fuel economy-thats the dark cloud lurking in the back of your mind whenever you test-drive the latest SUV or pick-up truck, buy another tank of gas or develop a passenger car motor oil.

Automakers have been shaving tolerances on engine parts to improve fuel economy, and passenger car engine oil formulators are responding by pushing viscosity from SAE 10W-30 down to SAE 5W-X and 0W-X.

But formulators and engineers are aware that lower oil viscosity coincides with thinner lubricating films. A thinner film means a thinner cushion between asperities (microscopic bumps) on metal surfaces in bearings, gears, pistons and cylinders. Asperities collide when two surfaces undergo relative motion, causing friction and wear.

Can additive manufacturers provide friction modifiers for new generations of low-viscosity PCMOs that do not compromise fuel economy, wear protection or other performance aspects? Two speakers at the Society of Tribologists and Lubrication Engineers annual meeting in Atlanta discussed progress in the development of new molybdenum-based additives. Pinpointing the exact type and interactions of these chemistries holds promise for formulating better oils.

Efficient Use of Molybdenum

Brian M. Casey, senior research chemist at Norwalk, Connecticut-based Vanderbilt Chemicals, told his audience that molybdenum is a critical element present in many organometallic friction modifiers. Molybdenite ore can be processed to form molybdenum trioxide, which can then be used to synthesize molybdenum dialkylthiocarbamate (MoDTC) and molybdate ester friction modifiers. Each type of additive has specific friction control properties, as well as limitations.

Casey outlined two strategies for improving the performance of friction modifiers: molecular refinement to improve the chemistry of individual additives, and a fill-gaps approach to blend additives with complementary performance properties. For example, some additives are more effective at higher or lower temperatures, or in fresh versus aged oils.

His general approach was to use an MTM (Mini-Traction Machine, PCS Instruments) to evaluate additives in 0W-20 PCMO. The MTM used a steel ball on a steel disc to generate Stribeck-like curves of the coefficient of friction. He artificially aged oil samples by heating them at 165 degrees Celsius for 48 hours to simulate 100 hours of aging in the Sequence VID engine test, which is used to measure fuel economy performance of engine oils.

Casey provided examples from previous research on MoDTCs to demonstrate the two strategies for improving frictional performance in PCMOs. When MoDTC molecules adsorb on metal surfaces, they are stable initially, but additional friction can cause them to decompose and release molybdenum disulfide molecules. These MoS2 molecules can form a low-friction tribofilm on metal surfaces.

First, researchers at Vanderbilt used molecular refinement to modify two MoDTCs by changing the sulfur content and the hydrocarbon groups in fresh oil, and found friction data were comparable for the experimental MoDTCs and a commercial MoDTC. But in aged oil, friction coefficients for the modified MoDTCs were much lower than the commercial MoDTC. This improvement is significant because, traditionally, MoDTC performance is better in fresh PCMOs than in aged oils.

Second, researchers tried using organic friction modifiers that do not contain molybdenum in combination with MoDTCs (the fill-gaps approach). Examples of organic friction modifiers include monoglycerides of fatty acids, alkylamines, polyols, alkyletheramines and certain polymers. In one instance, an organic friction modifier increased the friction coefficient for an MoDTC in fresh oil by 0.02, but decreased the friction coefficient by 0.04 in aged oil.

Molybdenum Esters

Third, Casey evaluated experimental molybdenum esters, which contain molybdenum but not sulfur. The molybdenum ester molecules must scavenge sulfur (added to PCMOs from sources like ZDDP or sulfurized olefins) in order to form low-friction MoS2 films. Casey noted that molybdenum esters traditionally are more effective in aged oils than fresh oils. Also, the performance of organic friction modifiers typically declines in aged oils due to decomposition. To fill gaps and improve performance in fresh oils, Casey tested blends of molybdenum esters and organic friction modifiers. Through molecular refinement, Casey prepared novel organic friction modifiers that were chemically altered to minimize sites of oxidative and hydrolytic instability.

Caseys experimental design included seven chemistries: three organic friction modifiers alone (0.8 weight percent, two experimental, one commercial) and blends with two ratios of molybdenum esters and organic friction modifiers (180 parts per million of molybdenum with low and high levels of organic friction modifiers). MTM tests were performed at 40, 60, 80, 100, 120 and 140 C. To simplify analysis, Casey plotted the integral of the area under each Stribeck curve versus temperature.

In terms of friction, the best additive combinations in fresh and aged oils were the commercial organic friction modifiers (glycerol monooleate, no molybdenum ester) and molybdenum ester blended with a high proportion of an experimental organic friction modifier.

Casey also reported data for wear profiles-volumes of wear scars-from ASTM D5707 tests done at 80 C and 200 N load, using an SRV test machine. For the organic friction modifiers, average friction coefficients were comparable while wear was between 40 and 140 percent higher in aged versus fresh oils.

Casey suggested that thermal decomposition of organic friction modifiers via oxidation or hydrolysis might have had a negative effect. The experimental organic friction modifiers had lower wear scar volumes compared to glycerol monooleate. Conversely, for blends of organic friction modifiers with molybdenum ester, average friction coefficients and wear scar volumes were significantly lower in aged versus fresh oils.

Four-ball wear tests (ASTM D4172) were carried out at 75 C. Unlike the SRV test, wear scar diameters increased with oil aging in every case. The coefficient of friction was higher with aging except for blends of molybdenum ester with high levels of two of the organic friction modifiers. There was no significant difference between fresh and aged oils.

Casey noted that molybdenum ester blends with higher levels of organic friction modifier outperformed blends with lower levels of the organic compounds throughout this study.

The blend of molybdenum ester with a high level of one particular organic friction modifier gave the best overall performance (small scar diameter and low friction coefficient) for both fresh and aged PCMOs in MTM, SRV and four-ball wear tests.

This study demonstrated the potential importance of evaluating additives in aged as well as fresh model lubricants and using both friction and wear data from multiple bench tests for a holistic overview of performance.

Branched MoDTCs

According to Kenji Yamamoto of Japan-based Adeka Corp., the combination of molybdenum and sulfur atoms in MoDTCs is primarily responsible for their friction-reducing properties. But little is known about possible effects of branched hydrocarbon chains on MoDTC performance.

Yamamoto tested five experimental MoDTC additives, each with four branched hydrocarbon chains of eight or 13 carbon atoms. These additives contained different combinations of C8 and C13 chains, i.e., 0, 25, 50, 75 and 100 percent C8 (no C8/four C13; one C8/three C13; two C8/two C13; three C8/one C13; four C8/no C13).

Yamamoto added each MoDTC to a commercially available SAE 0W-16 Japanese PCMO with low levels of molybdenum. He evaluated these five test fluids using tribometers in the laboratory.

All five MoDTCs gave comparable performance and decreased the coefficient of friction, relative to the as-supplied PCMO, by 55 percent in pure sliding and 45 percent in rolling-sliding tests.

Engine Tests

Next, Yamamoto compared five experimental MoDTC formulations in tests using 1.4/2.0L engines identical to those used in commercially available cars. All five experimental MoDTCs in SAE 0W-16 oils reduced friction inside the engines.

Unlike the bench tests, he observed different levels of friction modifier performance for the five experimental MoDTC formulations in this engine test. As the proportion of the smaller C8 chains increased from 0 to 100 percent, friction decreased and friction reduction relative to SAE 0W-16 PCMO increased at three engine speeds (700, 1,200 and 2,000 rpm)-typical for cruising and fuel economy tests with actual vehicles.

Yamamoto also tested MoDTCs in SAE 0W-20 PCMO in the engine of a medium-sized car. The test was performed under controlled conditions using a chassis dynamometer, which resembles a treadmill for cars, while the torque and power outputs delivered to the rear wheels were measured.

A mixture of MoDTCs in SAE 0W-20 oil slightly increased engine torque and power output relative to as-supplied SAE 0W-20 oil between 3,500 and 7,000 rpm, Yamamoto reported.

Solubility

Additives must be soluble in order to dissolve in a lubricant. But in PCMOs, Yamamoto pointed out, somewhat lower solubility could favor adsorption of MoDTC on metal, where it can form a tribofilm and reduce friction. This trade-off between MoDTC solubility and friction reduction explains results from his engine tests.

Yamamoto showed that MoDTC solubility in SAE 0W-16 PCMO decreased as the proportion of C8 branched chains increased over C13 chains. The larger C13 hydrocarbon groups improved the compatibility of the MoDTC with the oil, while the shorter C8 groups favored adsorption and tribofilm formation, he noted.

In his bench tests, the friction may have been so severe that all five MoDTCs decomposed and formed similar low-friction films, regardless of their solubility in the SAE 0W-16 PCMO.

Fuel Economy Tests

MoDTC chemistry can affect friction control performance, but what about fuel economy?

Yamamoto reminded his audience that PCMOs affect fuel economy primarily through two opposing effects. Oil gets thinner as its temperature increases from 40 to 100 C. Thinner oil provides less fluid resistance to moving parts, which is good for fuel economy, but provides less protection against rubbing of metal surfaces, which is detrimental to fuel economy and wear protection.

Yamamoto conducted fuel economy tests for SAE 0W-16 engine oils under both the New European Driving Cycle and its successor, the Worldwide Harmonized Light Vehicle Test Procedure. In both tests, a commercially available vehicle is operated on a dynamometer through a sequence of conditions that simulate driving. Fuel efficiency improvement is calculated relative to performance using reference oils. He used an SAE 0W-20 reference oil.

Yamamotos fuel efficiency improvement results for MoDTCs differed significantly in NEDC and WLTP tests. For example, in the NEDC, an MoDTC gave 0.23 percent improvement in an SAE 0W-16 PCMO, which amounted to around 10 percent of that oils total fuel efficiency advantage over the reference oil.

In the WLTP, the same MoDTC gave 0.62 percent improvement in the 0W-16 PCMO, which represented 65 percent of that oils overall fuel efficiency advantage relative to the reference oil.

Yamamoto learned that the effect of MoDTC was greatest when the PCMO was hot and vehicle and engine speeds were high or dynamic. These relatively severe operating conditions are more prevalent during the WLTP than the NEDC.

While less than one percent fuel efficiency index from MoDTC might appear to be negligible, it translates to a significant improvement of this commercial SAE 0W-16 PCMO-a valuable contribution to fuel economy.

Mary Moon, Ph.D., is a professional scientist, technical writer and editor. She has worked as a chemist with hands-on R&D and project management experience formulating, testing and manufacturing lubricating oils and greases and other specialty chemicals. Contact her at mmmoon@ix.netcom.com or (267) 567-7234.

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