Another Daytona 500 watched from a hotel bar, thanks to the long standing tradition of the American Association for the Advancement of Science to schedule their meeting opposite the Daytona 500. I would have loved to have seen the race in person, but between TrackPass and television coverage, I’ll be skipping out of sessions long enough to keep enough track of what’s happening.
Daytona and Talladega are such fast tracks that NASCAR has to limit speeds. Restrictor plates reduce the amount of air that can flow into the engine. The chemical reactions that are combustion are annoyingly picky. For example, two octane molecules require exactly 25 oxygen molecules to undergo combustion. The oxygen molecules come from the air, so how much air gets into the engine determines how much gasoline can be let into the engine and that determines how much power the engine produces. The restrictor plates lower engine horsepower at Daytona and ‘Dega from 850 hp to about 460 hp. I remain amused by the fact that the pace car (a spiffy Corvette Z06) actually has a more powerful engine than the cars it will lead around the track under caution.
The inherent tug-of-war at Daytona is between the force the engine exerts to make the car move forward and the opposing forces of friction and air resistance. Having even a few extra horsepower can make a big difference in how you race, which is why we’re hearing complaints from the Chevy, Dodge and Ford camps about recent chassis dyno results showing Toyota has anywhere from a 15-30 peak hp advantage (at the rear wheels). More about what those numbers actually mean in a later post.
We heard a lot about engines this week for another reason, which is that a number of Chevy and Toyota teams either had engines expire or opted to change engines just before the Gatorade Dual 150s. While there is still some speculation about whether the Toyotas and the Chevys are suffering from the same problem, there has been a lot of talk about lifter and cam coatings, so I asked a few questions of my engine consultant, Dr. Andy Randolph of Bill Davis Racing, while I was at Daytona earlier this week.
A sociological note: Engine builders (and engine tuners) are interesting NASCAR archetypes. I found that vehicle dynamicists (for example) often will answer a question, and then ask me not to use, for example, specific spring rates they mentioned. The engine culture is different. You can tell you’re onto something interesting when they a) get sly little grins on their faces and clam up or b) yell loudly at you to stop looking under the hood of their car. Luckily for me, Andy is in the former category. Between the information from Andy and a search of Google patents, I think I can shed a little light on the problems teams have been having this week with engines.
Engines are full of parts that move against each other at high temperature and/or high pressure. The ideal material for such parts would be lightweight, strong and hard. This ideal material, unfortunately, doesn’t exist. This is one of those compromise issues, just like the trade-off between brittleness and strength in chassis steel.
The camshaft has egg-shaped lobes on which the lifters ride. The lifters lift the pushrods and the pushrods activate the rocker arms that open the valves. The cams and the lifters are in constant moving contact. Restrictor plate engines run at about 8000 rpms at Daytona pretty much continuously during green flag racing. Any time two pieces rub against each other, friction between the parts produces heat and wear. Wear removes atoms from one part due to rubbing by the other part. Wear is a great thing if you’re trying to sand a piece of wood; however, wear is a major problem for engines.
Cams and lifters usually are made of tool steel, a fairly hard material that doesn’t expand much when it gets hot. But tool steel isn’t durable enough for a NASCAR engine. If you run two pieces of tool steel against each other at the temperatures and pressure found in a high-performance engine, you’ll end up with either much smaller pieces, or the two pieces will cold weld and you’ll have a single stationary piece of steel where the two moving pieces used to be.
There are materials harder than tool steel (like tungsten carbide and diamond); however, making parts from these materials presents its own challenges. First, ‘exotic’ materials like these tend to also be expensive. Second, what kind of tool do you use to cut something that’s really hard? (It’s like the ancient quest for the alkahest (the universal solvent). What would you store it in if you found it?)
Tungsten carbide tooling often is made by combining very small pieces of tungsten carbide powder with cobalt powder (which serves as a binder). The mixture is pressed into the desired shape at very high temperature and pressure. This works well for tungsten carbide, but tungsten carbide isn’t as hard as diamond and, unfortunately, the heating/pressing process doesn’t work very well with diamond. So how might you be able to take advantage of the low cost and pretty respectable properties of tool steel, while giving your parts the extra hardness a material like diamond would provide?
The answer is to coat the part made from the base metal with a thin, but effective, layer of the material with the desired properties. Vermeil, for example, is sterling silver coated with a thin layer of gold. The outside of a piece of jewelry, for example, looks like gold, but its cost reflects the silver interior. Gold-colored drill bits are usually tungsten carbide coated with titanium nitride (TiN), another wear-resistant material.
Diamond, which is a form of carbon, is very hard. Graphite, which is used as a lubricant, is also made of carbon atoms. Why is one very hard and the other very soft? The answer lies in their crystal structure. In graphite, the carbon atoms are arranged into sheets of hexagons, with a carbon atom at the vertex of each hexagon. I’ve shown one such sheet in the diagram below.
If you lifted that sheet, you’d find similar sheets below it. The carbon atoms make four bonds. Each carbon atom in graphite (like the one I’ve colored red) has three bonds within the plane (indicated by the three atoms with red outlines) and one bond between its plane and the plane above or below it (which isn’t shown in my figure). The weak interplanar bonds are pretty easy to break. A graphite pencil writes because the shearing action of the pencil scrapes off layers of carbon atoms onto the paper.
Diamond, on the other hand, has carbon atoms connected by four covalent bonds of equal strength, each pointing in different directions, as shown in the diagram below. This type of bonding makes diamond incredibly hard, even though it is made of the same carbon atoms as graphite. (The diamond bonds are sp3 bonds, which are covalent sigma bonds. The graphite bonds are sp2 bonds, which aren’t nearly as strong.) Natural diamond is mostly found in the cubic crystal structure I drew above, but there also is a much less common hexagonal crystal structure called lonsdaleite. Both have sp3 bonding.
Diamond-like carbon (DLC) is an amorphous material, which means that it doesn’t have a regular crystalline structure. Amorphous and crystalline can be compared using an analogy of the seating of graduates and guests at commencement. The graduates are ushered in by the marshals and every seat is filled. The seating is highly ordered. The families, on the other hand, sit where they want and fill seats in a random way. There are regions where all seats are filled; however, there are also empty seats. The long-range order of the seated graduates is not there. DLC is the same way: There are some locally ordered regions, but there is no long-range order.
There are a number of different forms of DLC, but all share the property of being amorphous and having significant amounts of the sp3 bonding that gives diamond its hardness. There is only one type of DLC that is all sp3 bonding, which is a tetrahedral hydrogen-free amorphous carbon. Most other forms have either a mix of graphitic (sp2) and diamond (sp3) bonding, while others include hydrogen and/or other metals. These variations aren’t as strong as ‘pure’ DLC’, but they can have other advantages that can compensate for slight decreases in hardness. DLC coatings are also used on hard disk read heads to protect them in case of head crashes.
Wear is characterized by the wear factor, which is measured by scraping a piece of the material and seeing how much comes off. DLC has a wear factor about 300 times less than steel and about 10 times less than titanium nitride, which means that the DLC film will last much longer than a titanium-nitride-coated or an uncoated part. (Steel has a wear factor of about 15,000×10-9 mm3/Nm, titanium nitride 500×10-9 mm3/Nm and DLC about 50×10-9 mm3/Nm.)
In addition to being hard, DLC coatings also significantly reduce friction between the moving parts. The coefficient of friction characterizes the interaction between two materials. The larger the coefficient of friction, the harder it is to slide the two past each other. Steel on steel has a coefficient of friction of 0.7. Titanium nitride on steel has a coefficient of friction of 0.3 and DLC has a coefficient of friction of 0.2 with steel. Nanocomposite coatings (called DLN for diamond-like nanocomposite) can have coefficients of friction with steel as low as 0.1. There is a lot of interest in these coatings from the perspective of energy savings, since friction losses in the engine account for 15-20% of the energy expenditures in commercial cars.
DLC films are grown using processes such as PVD (Physical Vapor Deposition) or PACVD (Plasma-Assisted Chemical Vapor Deposition). These techniques avoid forming the longer sp2 bond by sending carbon atoms really quickly at the part to be coated. The speed of the deposition process forces the atoms to bond in ways they wouldn’t bond if given time to do otherwise. Growing good DLC films is part science and part art. The type of DLC used on most engine parts for motorsports is a form that has a fair amount of hydrogen, which means that there is a combination of sp2 and sp3 bonding.
It’s not as easy as grabbing a camshaft and some lifters off the shelf, sticking them in a high-vacuum chamber and tossing carbon atoms at them. At the high temperatures encountered in an engine, carbon atoms from the DLC coating can migrate into the steel and form iron carbides like Fe3C (aka cementite). Cementite is extremely brittle and that compromises the durability of the coating, so you can’t (in general) coat steels directly with DLC. One solution is to coat the tool steel with an intermediate material that serves as a diffusion barrier, meaning that it prevents carbon atoms from getting into the tool steel. Amorphous silicon oxide (Si:O) is an ideal material for this role because it provides good high-temperature stability.
Another problem with coating is the internal stress of the coating. If you look at a DLC coating at very high magnification, you find that the surface of a DLC film resembles cobblestones, with each stone having a diameter of a few micrometers. This randomness introduces large internal stress, making the film behave like a piece of paper that’s been stored rolled up. If you try to make the paper lie flat, it doesn’t want to: It wants to curl up. The internal stress makes the coating harder, but it also makes the coating want to pop off the surface it is supposed to be covering.
The coatings used on camshafts and lifters are thus multilayered coatings. On top of the amorphous Si:O diffusion barrier, for example, you might use titanium nitride to help with adhesion, or just to have something hard underneath in case there was a breach, as has been done for piston skirt coating applications. A DLC coating follows as the top coat, to provide hardness and decrease the friction.
It doesn’t take much of a coating to improve the part’s hardness. Coatings usually are on the order of a few to tens of microns (A micron is a millionth of a meter). For comparison, a typical human hair is about 70 microns in diameter. The parts have to be polished extremely well — if the surface roughness is greater than the coating thickness, some of the part won’t have any coating on it. The thickness of the coating applied is determined by how much is likely to be worn through during the lifetime of the part and how well you can get the DLC coating to stick.
The reports I’ve heard say that some of the coatings on the cams and/or lifters were flaking off. Remember that the whole point of these coatings is that they are hard. Small pieces of them are still very hard and small pieces of very hard material falling into the engine spells disaster. It’s a potentially catastrophic enough effect that a number of teams changed engines rather than take a chance. It looks now as though the problem was a single or a limited number of batches of parts, so it’s not something systemic that is likely to be a recurring problem. The coating process is so complex and dependent on precise processing conditions that if even one thing in the complex process isn’t exactly correct, the coatings won’t adhere. I’m sure there are materials engineers working overtime with lifters from the same batch trying to identify where the problem was.
Thanks to Dr. Randolph for pointing me in the right direction, and congrats to him and the No 22 team on running in the top 10 toward the end of the Daytona 500. At least, that was until the No. 22 was punted by the No. 29. Kevin Harvick thus the “Darwin’s Doghouse” list this week.