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Nickel silicon carbide coatings have a lot of adv...
US Chrome Corp. has nickel silicon carbide coated...
Machining New Metals - Nickel Silicon Carbide Coatings
Cylinder wear is something you want to minimize in an engine, so in some applications, the inside surface of the cylinders are coated with a hard, wear-resistant material such as nickel and silicon carbide. Such coatings go under various names that use a proprietary blend of nickel and silicon carbide to plate cylinders.
By Larry Carley
Cylinder wear is something you want to minimize in an engine, so in some applications, the inside surface of the cylinders are coated with a hard, wear-resistant material such as nickel and silicon carbide. Such coatings go under various names, such as “Nikasil” (Mahle Corp.), “Nicom” (U.S. Chrome Corp. of Wisconsin), “NSC” (Millennium Technologies) and others that all use a proprietary blend of nickel and silicon carbide to plate cylinders.
Nickel is the “glue” that holds and supports the particles of silicon carbide on the cylinder wall surface. Silicon carbide is a very hard abrasive material with a hardness second only to diamond. According to one company who uses the process, the coating has a hardness rating of 600 on the Vickers scale, and a sliding hardness of 58 to 60 Rockwell C. The hardness combined with a tendency to attract and hold oil (“oleophilic”) makes these coatings highly wear resistant. To keep it from wearing the piston rings, though, the size of the silicon carbide particles must be very small, typically around 3 to 4 microns in diameter. The particles make up only about 4 percent of the coating.
How is the coating applied? The nickel silicon carbide is electrodeposited on the surface of the cylinder by dipping the cylinder in a special plating tank. An electric current is then applied to the part so the coating will stick to it. The coating can be applied to most cast iron and aluminum alloys (but not all). It works great on cast iron cylinder liners and engine blocks, but it does not stick well to Compacted Graphics Iron (CGI). Up until a few years ago, most NASCAR times were coating their cylinders with some type of nickel silicon carbide for its lubricity and wear characteristics. But when they switched to engine blocks made of CGI, the natural lubricity of the graphite was all they needed for the rings to survive.
When a nickel silicon carbide coating is applied properly, a process which requires careful preparation of the cylinder surface by acid etching and repeated cleaning, the coating almost lasts forever. On a street-driven vehicle, it can last upwards of several hundred thousand miles because the coating wears very slowly compared to cast iron (anywhere from 3 to 10 times better depending on the application).
The coating process itself typically takes an hour or longer depending on the desired thickness of the coating. In the tank, the coating builds up at a rate of about .001˝ every 15 minutes. On most OEM and aftermarket applications, the applied coating is only about .0025˝ to .005˝ thick. It doesn’t take much to provide a hard, wear resistant surface.
The thickness of the coating can also be increased to as much as .025˝ to .040˝ if necessary to restore a worn out cylinder. This can be a real life-saver if you are trying to restore a vintage engine or a custom modified cylinder in a small engine or motorcycle that is worn to its maximum limit.
The same process can also be used to repair aluminum cylinder blocks that do not have iron cylinder liners. The cylinders can be bored to oversize, and plated to accept an oversize piston a process that is much easier than the factory reconditioning procedures that are recommended for some of these engines.
After a cylinder has been plated, it must be diamond honed to the desired finish and crosshatch. Many OEM nickel-silicone coated cylinders are typically finished to 30 Ra with a two-step diamond plateau. But many performance engine builders prefer a smoother finish ranging from 4 Ra up to 20 Ra, with additional valley depth (RVK) to hold more oil.
Break-in is similar to any engine. The cylinders can be prelubed with ordinary or synthetic motor oil, or assembly lube. One engine builder said its best to load the engine when it is first started up, otherwise you increase the risk of glazing the cylinders until the rings are fully seated.
As for ring compatibility, most of those we interviewed for this article said conventional chrome faced rings are not recommended. Most recommended low tension rings (cast iron, ductile iron or steel).
A Short History
It all started years ago when engineers were looking for ways to reduce engine weight. Aluminum was the most likely material to substitute for cast iron because of its weight advantage. But aluminum is a much softer metal. And you can’t run an uncoated aluminum piston against an uncoated aluminum cylinder at least not for every long.
Initially, iron cylinder liners were inserted into lightweight aluminum blocks to provide a reliable bore surface. But iron does not transfer heat as efficiently as aluminum, and iron adds weight. It also expands at a different rate than aluminum. So engineers came up with the idea of switching things around, and using iron-plated aluminum pistons in a bare aluminum bore. To increase the bore’s wear resistance, a high silicon alloy was used. The bores were then honed using a process that wore away the soft aluminum but left the hard silicon particles exposed.
The Chevy Vega engine was the first U.S. production application of this new technology and it proved to be a disaster. The engines burned oil from day one, and it only got worse as the miles added up. The fix, in most cases, was to bore out the block and install cast iron cylinder liners.
Back in the 1960s, a new type of engine called the “Wankel” rotary engine was going to revolutionize the automotive industry (at least that’s what everybody thought at the time). The Wankel used a triangular rotor that wobbled around inside a figure-eight shaped combustion chamber. The engine was capable of extremely high rpms and could produce a lot of horsepower, but sealing the rotor was an issue. The apex seals at the tips of the rotor tended to wear the surface of the engine housing very quickly.
In 1967, Mahle solved the wear problem with a new nickel silicon carbide coating called “Nikasil.” Better yet, the same coating could also be applied to conventional engine cylinders, cylinder liners and sleeves to reduce wear and improve lubricity. Soon BMW, Porsche, Ferrari and Jaguar were using it on their engines. It also because a common coating for chain saw engines, marine outboard motor engines, snowmobile engines, ATV and motorcycle engines, and eventually even Formula One and NASCAR engines.
In the 1980s, BMW, Jaguar and Porsche got a bad rap when a lot of their Nikasil-coated engines failed. The problem wasn’t the coating but low quality fuel that contained high levels of sulfur. Over time, the sulfur would eat away the coating causing the cylinders to wear. As the story goes, the fuels that BMW, Jaguar and Porsche used to test their engines at the factory was high quality gasoline. So it came as a total surprise when these engines started to fail.
Today’s gasolines do not contain high enough levels of sulfur to cause any problems with these coatings, so that chapter is ancient history. Even so, nickel silicone carbide coatings can be damaged by nitrous oxide or racing fuels that contain nitro methane. The ingredients in these fuels can react with moisture to form nitric acid, which will strip the coating right off the surface. If the application is a performance engine used in drag racing, washing down the cylinder walls after a run with gasoline can minimize any adverse effects if the engine is nitrous boosted or runs on nitro methane. Otherwise, nickel silicon carbide coatings are not affected by pump gas, alcohol or dry fuels such as propane or compressed natural gas.
Charles Navarro of LN Engineering in Momence, IL works on a lot of older air-cooled Porsche engines. When he redoes a Porsche engine, he has the cylinders nickel silicon carbide coated by Millennium Technologies in Plymouth, WI.
“I’ve probably installed over 4,000 of these cylinders and never had a problem with the coating peeling or flaking off. The cylinders have a lifetime warranty on the coating. The cylinders are diamond plateau honed, and I use JEGs pistons with 12 lb. low tension rings. The low tension rings work well with the coated cylinders, and reduce friction and heat in the engine. The low tension rings and coated cylinders are good for a five horsepower gain, and the oil temperature runs cooler, too.”
Navarro says he typically charges around $63 per cylinder to bore and hone a Porsche cylinder, and $235 if the customer wants the cylinder coated with nickel silicon carbide. “The cost is not an issue with out customers because they are Porsche owners and are used to paying higher prices. They are also familiar with the benefits of the coating.
“We also install nickel silicon carbide coated aluminum sleeves to repair engine blocks. Newer liquid-cooled Porsche engines use a high silicon alloy called Lokasil that is brittle and tends to crack. We can fix these engines by boring out the cylinders and installing our coated aluminum cylinder sleeves. We’ve also done the same thing on LS Chevy V8 engines. The engines run cooler without the iron liners,” said Navarro.
Coating Company Comments
Chris Hackl at Millennium Technologies (www.mt-llc.com) said his company has done a lot of nickel silicon carbide coating and cylinder reconditioning on BMW, Mercedes, Ferrari and Porsche engines, go-cart engines, motorcycle engines, performance engines and vintage engines. They have also successfully used their coatings on diesel tractor pull engines to reduce wear up to 3 times over uncoated cylinders.
“Our coating process typically takes about 24 hours start to finish. That includes cleaning and preparation time, the time in the tank, and the time to finish hone the cylinders. We use diamond hones to plateau the cylinders. The finish will depend on the application, but usually ranges from 15 to 22 Ra microinches.”
Hackl said the coating process can be used to salvage vintage engines that would otherwise have to be sleeved or discarded. “We can build up the cylinders as much as .040˝ to save a block. On most applications, though, all you need is a .003˝ to .005˝ thick coating.”
US Chrome Corp. in Fon du Lac, WI (www.usnicom.com), has nickel silicon carbide-coated over one million cylinders since 1980. “We do cylinders for Mercury Marine as well as motorcycles, snowmobiles, ATVs and numerous racing teams, including Corvette LS engine blocks for LeMans racing,” said Scott Reath, plant manager.
“Since NASCAR switched to compacted graphite iron blocks, we don’t do those engines any more, but we still do a lot of cast iron sleeves and a lot of aluminum cylinder. Our Nicom will work with most aluminum alloys, including 356, 380, 390, 4032 and 6061, but it does not work with 7075 or any 2000 series aluminum alloys.”
Reath said cylinder preparation is the key to getting good adhesion and long lasting results. “We put a cylinder through a series of caustic and acid dips, with rinses in between to get down to a pure aluminum surface. The part then goes in the tank, and remains there as long as needed to build up the desired thickness. We normally apply a .0025˝ to .003˝ coating to the cylinder, but can go thicker as needed if we are building up a worn cylinder.”
Is It For You?
Nickel silicon carbide coatings have a lot of advantages, and can certainly add to the profitability of any job. For a daily driver, it’s obviously overkill. But in a highly stressed performance engine, or an engine that’s being built for durability, a wear-resistant oil retaining coating can significantly reduce cylinder wear. It’s also a good process for restoring worn cylinders, and is an alternative to sleeving. It also eliminates the need to use a cast iron liner in an aluminum block.
If an engine with a nickel silicon carbide coated cylinder is damaged as a result of a piston seizure or failure, the coating can be easily stripped off with nitric acid, or it can be honed way. The cylinder can then be reconditioned by reapplying the coating. Of course, this will require the services of an experienced coating company since it is not a do-it-yourself in-shop process.