Government regulations may be subject to changes in the political climate, Becker said, but OEMs have been re-engineering powertrains to increase miles per gallon for new vehicles.

Making smaller engines and operating them at faster speeds produces greater mechanical stresses and higher temperatures under the hood. Under these conditions, steel and iron surfaces do not tolerate rubbing well. As their surfaces degrade, gears and cam shafts become noisy, piston rings wear and allow carbon dioxide and other gases to leak from engine cylinders, and drivers lose engine performance and fuel economy, he explained.

So OEMs investigated wear-resistant coatings to protect iron and steel powertrain components, Becker continued. In 2014, GM started commercial production of small automobiles with coated piston rings, and Nissan is selling cars with coated valve lifters. In both cases, diamond-like carbon coatings are used to protect critical parts from wear.

Arup Gangopadhyay, technical leader of lubrication science & mechanical friction in base engine design at Ford Motor Co., agreed. Specialized coating materials are very much in common use today in volume-produced engines. A number of different types of new coatings are under development or in use to protect specific powertrain components.

DLC coatings can reduce friction for piston rings and cam followers and improve their durability. They are also used on piston pins, probably for improving wear resistance. Gangopadhyay explained, Its now becoming clear that tribofilms can form on DLCs. Tribofilms form when surfaces undergoing friction react chemically with additives in the lubricant.

Predicting the performance of DLCs is not a simple matter because DLCs are available in many different compositions, and lubricants come in many different formulations, Gangopadhyay pointed out. In laboratory tests, we observed significant friction reduction with DLC-coated cam followers depending on temperature, lubrication and operating speed. Friction reduction and wear protection depended on the specific chemistry and preparation of the DLC coating and the chemistry of the oil-based lubricant.

Not Just Carbon

DLC coatings are a relatively expensive option for OEMs. To apply the coatings, batches of piston rings and other parts are placed in an industrial-scale vacuum chamber. Carbon atoms are deposited to gradually build up microscopically thin layers using physical vapor deposition (PVD) techniques. Similar methods are used to manufacture semiconductor devices, aluminum coatings on plastic films and even synthetic diamonds.

In natural and synthetic diamonds, each carbon atom bonds to four other carbon atoms. The result is a clear, colorless crystal that sparkles in jewelry and resists wear as a grinding medium or coating for engine parts.

Carbon atoms can also form opaque, gray graphite, which is used as a solid film lubricant for metal forming, wire drawing and other applications, and as an extreme pressure additive. In graphite, each carbon atom bonds to three others.

The root cause of the differences between diamond and graphite is the distribution of electrons around nuclei of carbon atoms, a property called sp3 or sp2 hybridization, respectively.

Many DLC coatings are made for commercial and research purposes. Each type of DLC contains a specific mixture of sp3 and sp2 carbon atoms along with hydrogen, silicon, tungsten, titanium, chromium or other dopants to customize performance.

But DLC coatings are not the only game in town, according to Dean Clarke, chemical engineer with Infineums lubes development division. Clarke presented a paper comparing DLCs with a ceramic coating at the Leeds-Lyon Symposium on Tribology held in Lyon, France, last fall. According to Clarke, a ceramic PVD coating outperformed two DLCs in laboratory tests where sections of piston rings and liners underwent reciprocating (back-and-forth) motion. Clarke reported a coefficient of friction below 0.04 for PVDC-coated specimens lubricated with fully formulated SAE 0W-20 oils that contained high levels of molybdenum friction modifiers.

On the other hand, he noted that friction and wear were higher for DLC-coated specimens lubricated with these oils, but less with other formulations. He emphasized the importance of formulating engine oils to be compatible with specific coatings.

According to Clarke, the surface analysis proved that much less tribofilm formed on both the DLC-coated ring and the paired liner compared to PVD chromium nitride-carbide coated rings. This is only an advantage for DLC in the absence of friction modifiers. Once such additives are included, incorporating them into the surface tribofilm is critical, and the PVD CrNC/C coating is more effective at receiving friction modifiers.

Ali Erdemir and Osman Eryilmaz of Argonne National Laboratory in Illinois developed a family of new coatings that interact with polyalphaolefin base stocks to generate DLCs. They deposited mixtures of nitrides of molybdenum or vanadium with copper or nickel catalysts on AISI 52100 steel. These catalytic coatings interacted with base stocks to generate DLC tribofilms under rubbing conditions. As these DLCs underwent wear, they self-healed by cracking base stock molecules to make fragments that re-formed the DLC coating.

Ball-on-disc tests…reveal that these tribofilms nearly eliminate wear and provide lower friction than tribofilms formed with zinc dialkyldithiophosphate, Erdemir reported in Nature journal in August 2016.

Commercial Applications

Gangopadhyay explained that OEMs perform lab tests to check that engine oils are compatible with a DLC coating before they commercialize it. In North America, all OEMs use ILSAC GF-5 engine oils with similar additives. But in Japan, OEMs tend to use different friction modifiers and higher treat levels than in North America.

Molybdenum dithiocarbamate additives are very effective for reducing boundary friction between ferrous surfaces and are used commonly in lubricant formulations in Japan. They improved fuel economy in tests run using Japanese drive cycles. But they can produce negative interactions and increase wear when used to lubricate a DLC-coated surface rubbing against a ferrous surface. We observed an increase in friction and wear of DLC-coated surfaces in engine tests. It is unclear which lubricant additives may be responsible for this behavior, he continued.

It is very important to understand positive and negative effects of lubricant formulations and DLCs because the whole purpose of spending money for DLCs is to reduce friction and wear, Gangopadhyay concluded.

Becker noted, Theres a whole lot that is not known about how DLCs interact with lubes. We do know that DLCs do not behave like ferrous surfaces in contact with engine oils. Engine oils contain a lot of reactive additives, and some DLCs contain hydrogen and metal atoms that are also reactive. More research is needed to investigate possible reactions between DLCs and additives.

Some work has been done to explore this topic, including a study conducted by Maria Isabel De Barros Bouchet of the University of Lyon, France, described in the 2017 issue of Scientific Reports. Bouchet tested bearing steel coated with DLCs and lubricated with oleic acid, a fatty oil present in many seed oils. She observed macroscopic super-low friction (coefficient of friction below 0.01) when oleic acid promoted tribochemical reactions that modified the tops of colliding asperities inside wear scars.

Becker is not aware of any durability or performance issues related to commercial use of DLCs on drivetrain components and incompatibility with engine oils. However, there need to be quantitative test results to validate and justify more commercial applications of these costly materials.

Becker and Gangopadhyay expect that OEMs will continue to customize coatings for protecting various components operating at different temperatures, pressures and lubrication regimes within a single powertrain. Testing will become necessary for checking compatibility of multiple coatings used on different surfaces, and their eventual wear and degradation byproducts.

Becker expressed concern about possible long-term implications if individual OEMs develop proprietary engine oil formulations to optimize performance of their unique coatings, something that Nissan is already doing. Proprietary OEM engine oils could disrupt current practices for standardizing engine oils, defining powertrain warranties and servicing consumer markets.

Testing in Vehicles

Current qualification procedures for API and ILSAC engine oil standards specify the use of older model engines that probably dont contain DLC-treated surfaces. Those engines eventually will become unavailable, and there will be no choice but to run tests in newer engines with coated parts.

Peter M. Lee, staff engineer and chief tribologist at Southwest Research Institute in San Antonio, Texas, is part of a group working on low-friction coatings for piston rings to improve fuel economy. Speaking at the Society of Tribologists and Lubrication Engineers annual meeting in May, Lee discussed TiSiCN coatings formed by depositing atoms of titanium, silicon, carbon and nitrogen on piston rings using plasma-enhanced magnetron sputtering.

SwRI successfully developed low-friction, wear-resistant TiSiCN nanocomposite coatings for diesel piston rings. These coatings consist of microscopic titanium-carbon-nitrogen nanocrystals embedded in silicon-carbon-nitrogen amorphous solid, similar to chocolate chips in cookies.

The lab used a series of methods, from bench instruments to actual vehicles, to test these piston coatings. First, sections were cut from piston rings and cylinders, lubricated with SAE 10W-30 heavy-duty diesel engine oil, and tested using a Plint TE-77 tribometer. Lee reported a 10 percent reduction of the coefficient of friction for coated versus uncoated steel.

Second, the research team used a gasoline engine fitted with sensors to determine that coatings on piston rings reduced the coefficient of friction by 39 percent compared to uncoated steel. Tests with a diesel engine showed that coatings significantly reduced wear-induced weight loss of rings and cylinder liners.

OEMs prefer fuel economy tests performed in actual vehicles, even though they are less repeatable than friction and wear tests. Lees group developed an automated drive control system to remove some of the variability from vehicle tests where a human driver performs a standard sequence of speed intervals in a controlled setting.

SwRI tuned their drive control system to mimic the accelerator pedal position for a human driver to minimize variations. Initial tests with the automated system and a 2017 passenger car with a 2.4-liter inline four-cylinder engine showed a measureable increase in fuel economy resulting from SwRIs piston ring coating.

Mary Moon, Ph.D., is a professional chemist, science writer and technical editor of the NLGI Spokesman. She has hands-on R&D and management experience formulating, testing and manufacturing lubricating oils and greases. She is skilled in industrial applications of tribology, electrochemistry, and spectroscopy. Contact her at mmmoon@ix.netcom.com or (267) 567-7234.