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Unlocking Wear Protection


Engine oil formulators continue pushing the envelope on lowering viscosity to achieve fuel efficiency. But ultra-low-viscosity lubes can compromise friction properties and wear protection, challenging manufacturers to find the right combination of base oil and additives.

Researchers are deciphering a possible solution by studying composite base fluids and their performance with additives.

Ultra-low-viscosity lubricants are coursing through the automotive segment. While SAE 0W-X grades held 11 percent of the North American passenger car motor oil market in 2017, additive manufacturer Infineum forecasts this share will triple to 33 percent by 2027 as SAE 0W-20-along with 5W-20-becomes the recommended grade for new vehicles. In addition, the much-delayed ILSAC GF-6 passenger car engine oil specification is expected to define the performance parameters for SAE 0W-16, the company pointed out in its 2017 Infineum Trends presentation.

Globally, ultra-low-viscosity grades made up 6 percent of the 7.2 million metric tons of PCMO demand in 2016, Parsippany, New Jersey-based Kline & Co. found. The market research firm projects that SAE 0W-X products will account for 23 percent of the PCMO market, slated to reach about 8.5 million metric tons, by 2026.

While worldwide PCMO demand will grow at a compound annual rate of 1.6 percent through 2026, ultra-low-viscosity engine oils will see growth of nearly 16 percent, albeit from a smaller base, said George Morvey, industry manager for Klines Energy Practice. A key factor is broader access to higher performance base stocks, including mineral API Group II and Group III as well as synthetic base oils.

What were seeing is, whether looking at a developed or developing country market, the demand for high performance, low-viscosity PCMO continues to accelerate, he said during a webinar about the global lubricants market. We are seeing continued expansion and availability of higher quality base stocks. That just makes it easier for formulators and suppliers anywhere to access quality base stocks and improve their product slate.

Picking the Low
Viscosity Lock

Original equipment manufacturers have started embracing ultra-low-viscosity motor oils while meeting increasingly strict technical demands, stemming from evolving engine designs and components, Morvey noted.

More and more OEMs, especially on the consumer passenger vehicle side, are moving to lower viscosity grades-0Ws-for factory and service fill, he said. Suppliers are certainly doing their part in terms of marketing and promotional efforts to get product into the market and into those engines.

Europe, North America and Asia-Pacific are the regions driving ultra-low-viscosity PCMO penetration. To meet demand, lubricants industry standards bodies have started defining the viscometrics of SAE 0W-X engine oils. In 2013-two years after Hondas petition to define ultra-low-viscosity grades-SAE Internationals Engine Oil Viscosity Classification Task Force added SAE 16 oils to its J300 oil viscosity standards, with SAE 12 and SAE 8 joining the system two years later.

The Japan Automobile Manufacturers Association, on the other hand, is hoping to go beyond defining properties to fully adopting an ultra-low-viscosity engine oil standard. Last year, it approved the introduction of a performance specification for SAE 0W-8, signaling that Japanese automakers want to take advantage of these oils fuel economy benefits.

Establishing a test method that measures fuel economy for lower viscosity oils such as 0W-8 is a very important task, Takumaru Sagawa, deputy general manager at Nissan Motor Co., commented in the Infineum Trends presentation. He added, We would not like to have a [volatility] requirement that is more severe for 0W-8 and 0W-12 than the [ILSAC] GF-6 requirements. The current proposal for GF-6 is to limit Noack volatility to 15 percent for all grades.

Although the switch to ultra-low-viscosity engine oils is clear, challenges remain in regards to their effectiveness in protecting internal combustion engine components. The ever more complex hardware systems that are being introduced present lubrication challenges in terms of maintaining wear protection and engine cleanliness while still contributing to fuel economy improvement, Infineum stressed.

Ensuring Protection

Lowering engine oil viscosity reduces fluid film thickness, which can put components at risk of higher friction and wear damage, including scuffing failure in extreme cases, echoed Cinta Lorenzo-Martin, principal materials scientist at Argonne National Laboratory in Argonne, Illinois.

Blending antiwear and friction modifier additives into the motor oil can usually address both issues, but these may not be sufficient when ultra-low-vis. synthetic base oils are used, she elaborated in a presentation given at the Society of Tribologists and Lubrication Engineers annual meeting in Atlanta.

We also run into the problem that, at high temperature, the base stock can give us some problems. We might have some temperature stability problems or evaporation rates [volatility] going up, she explained at the gathering in late May. So the question is, are the additives going to be enough to protect the surfaces?

Using a materials science approach, Lorenzo-Martin explained, a composite base oil that exhibits superior friction and wear protection properties to its two base fluid constituents could be developed to blend ultra-low-viscosity engine oils; this could represent a more cost-effective option as opposed to using the most expensive synthetic base stock alone. She proceeded to share the results of a study that analyzed the friction and wear performance of different combinations of polyol esters and polyalphaolefins.

Polyol esters were selected because of their stability over a wide temperature range, low volatility and high flash point, in addition to inherent lubricity and wear protection properties, Lorenzo-Martin noted. They also provide seal swell, which expands elastomers to prevent oil leaking, added Ken Hope, global PAO technical services manager at Chevron Phillips Chemical Co. in The Woodlands, Texas.

Polyol esters are notable for their polar solvency, which assists additives to dissolve in non-polar base stocks such as PAO, Hope told LubesnGreases. The main disadvantage is their cost, depending on the chemistry, acknowledged Chris Donaghy, sales director for energy technologies at Croda. Generally, the cost of esters that would be used in engine oil applications could be double the cost of low-viscosity PAO.

PAO is increasingly being favored by blenders to formulate synthetic automotive lubes. They have moderate temperature stability and their non-polar molecules make them less reactive on metal surfaces, Lorenzo-Martin noted.

Commenting on the study done by Lorenzo-Martin, Hope stated that a composite of PAO and polyol esters can lead oil blenders to better balance viscosity and Noack volatility in ultra-low-viscosity lubes.

As you decrease the molecular weight of the fluid to get the low viscosity, there will also be much more volatile material, and those two points are opposed to one another, so youre looking for an optimal point. Combinations of PAOs and esters do provide an interesting opportunity in both aspects, he added.

Measuring Performance

Lorenzo-Martins study analyzed seven types of fluids: a 4 centiStoke PAO, a polyol ester, and five composite base fluids with varying ratios of PAO and ester (70-30, 60-40, 50-50, 40-60 and 30-70 percent). The fluids were evaluated for friction and wear performance using ball-on-flat-disk uni­directional sliding motion and reciprocating sliding motion.

The tests did not replicate operating conditions of internal combustion engines, as they didnt use actual components or run under real engine conditions, Lorenzo-Martin later told LubesnGreases. She explained that both bench tests served to pre-screen how fluids behaved under different contact conditions.

For PAO alone, the friction coefficient in the reciprocating sliding test was around 0.15, but when 30 percent ester was added, friction dropped more than 50 percent.

A similar result was recorded for the unidirectional sliding test, where friction coefficient for the pure PAO registered above 0.15, and a 45 percent friction reduction was observed when 30 percent ester was mixed in. The remaining composite fluid samples showed comparable results in reciprocating sliding and only slight variations in unidirectional sliding, Lorenzo-Martin highlighted.

Ball wear and flat wear protection were also measured for each of the composite fluids. With the different ratios of ester to PAO, the results for reciprocating and unidirectional sliding showed negative (adhesive) flat wear, meaning material was building up on the surface.

The results for ball wear, in contrast, showed a reduction in positive (abrasive) wear, meaning that the amount of material removed from the surface was significantly lowered when combining PAO with 30 and 40 percent ester, Lorenzo-Martin explained. However, with higher concentrations of ester, the reduction in wear was not as significant.

Its interesting that the reduction of wear seems to begin with relatively low additions of the ester, and as you go to higher and higher percentages, there isnt necessarily a continued benefit, Hope observed.

Additizing the Mix

The study also tested the friction and wear control properties of the composite fluids plus additives. Friction modifiers and antiwear additives brought down friction in all of the fluid samples in the reciprocating sliding motion test, but their effect was minimal in unidirectional sliding.

Very high concentrations of ester can be somewhat detrimental once they start competing for the surface of the metal, Hope noted. The ester moiety has an affinity to the metal surface and potentially, at real high concentrations, could even displace antiwear additives.

Additives had minimal effect on flat wear in unidirectional sliding, except for tests using pure PAO; ball wear was significantly reduced in the same test. In reciprocating sliding tests, additives changed the flat wear behavior of the composite base stocks and drastically reduced ball wear in the same test. Lorenzo-Martin pointed out that more tests need to be conducted to determine the reason for the additives behavior.

Lorenzo-Martin also analyzed the formation of surface films, or tribolayers. She found that the composite fluids alone formed a crystalline surface film, while fluids with additives formed an amorphous tribolayer.

[In the past] weve focused a lot on studying tribofilm behavior and how the structure of the films relates to performance, especially friction performance. We observe that an amorphous film will give you low friction, while a crystalline film will give you high friction, she explained.

Opening Up the Potential

Composite fluids certainly are not a new concept in lubricant formulations. Lorenzo-Martin believes that companies use mixtures of base fluids to blend engine oils, but only small amounts and not in the ratios that were measured for her tests.

What you find in commercial lubricants now is a certain percentage of other fluids that might improve additives dissolution in the system, she clarified. To my knowledge, its based more on how to make additives work with that base fluid, because some additives wont dissolve well in some base fluids.

Low viscosity has already been a game changer for engine oils and shows no signs of going away. I think it will be interesting to better understand what are the pluses and minuses of all these [base oil] mixtures, Lorenzo-Martin commented. Once we understand them better, we can look to optimization of new products.

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