Proper surface preparation is essential to remove contaminants and to ensure good adhesion of the coating with the base metal. One coating supplier said that 85 percent of the work that goes into coating a part is surface preparation. Surfaces can be prepped for coating by media or sand blasting. The material is then sprayed on, allowed to dry, then oven cured under carefully controlled conditions to achieve a long lasting bond. The oven temperature must also be limited when curing aluminum parts so the metal is not annealed (softened).
Dry Film Lubricants
Coated bearings are a good choice for any high revving engine application including drag racing, circle track or even sport compact street engines. Coated pistons are also a good choice for any engine that may be at risk of overheating or is subjected to unusually high combustion or operating temperatures.
Dry film lubricant coatings on piston skirts typically add about .001" to the piston diameter, so the question often comes up as to how this affects piston installation clearances. One leading piston manufacturer said it is not necessary to compensate for the coating when figuring piston-to-bore clearances. "Just pretend the coating isn't there," is their advice. Use the piston size on the box to calculate clearances, not the actual diameter of the coated piston.
A thinner coating is often used on bearings, typically .0002" to .0003? thick. The coating is thin enough that it should not affect normal bearing clearances so bearings can be assembled with the same clearances as if the coating wasn't there. Even so, you should still measure clearances to make sure adequate oil clearance exists.
Dry film coatings on bearings typically burnish over a period of time. But even if the coating appears to be wiped away, it typically penetrates the surface and is still able to provide lubricity.
One misconception about dry film lubricants is that they create more horsepower by reducing friction. That would only be true if there were no oil between the parts. But as long as there is an oil film between the parts, the coating does not change friction or the shear characteristics of the oil. Engine drag and parasitic horsepower loss can be reduced, though, when an oil-shedding coating is used on crankshaft counterweights. One coating supplier said coating the crank journals is good for 8 to 10 hp at 7,500 rpm. The oil also runs cooler and experiences less foaming.
Dry film lubricants do lower oil temperatures, anywhere from 15 to 30 degrees, which allows the use of thinner oils with a much greater safety margin. Lower viscosity oils create less drag, so there are some horsepower gains to be found by using thinner oil.
Dry film lubricants that contain a high amount of PTFE (Teflon) are typically oil-shedding coatings. Though Teflon is a slick, low-friction material, it repels oil rather than retains it. For this reason, some coating manufacturers say dry film lubricants that contain a high percentage of PTFE should not be used on bearings or piston skirts. Yet one major aftermarket supplier of engine bearings uses a coating that contains both PTFE and moly on their coated performance bearings. The moly in the coating helps hold oil, while the PTFE provides backup lubricity should oil pressure be lost.
Dry film lubricants can also be used to coat intake runners in manifolds and cylinder heads. According to one coating supplier, this helps reduce air/fuel separation in the intake runners while also insulating the mixture from engine heat. Depending on the formula of the dry film lubricant, some also act as an insulator or heat barrier.
Thermal Barriers
Thermal barrier coatings are typically about .001" to .003" thick, and help protect the pistons in supercharged or turbocharged drag racing engines, or ones that run on short bursts of nitrous oxide. Nitrous produces a tremendous amount of heat in a very short time, which can be very damaging to unprotected pistons. But a barrier coating also prevents heat from dissipating down into the pistons and rings, which may be counterproductive if the heat persists for a long period of time. That's why several piston manufacturers do not recommend a thermal barrier top coating on pistons for naturally aspirated engines (without nitrous) or ones that are built for endurance racing. On these kind of applications, you're probably better off without it.
When applied to the face of intake and exhaust valves, thermal barrier coatings can help the valves run cooler. Coating the face of both valves reflects heat back into the combustion chamber and helps the valves run cooler (especially exhaust valves). One coating manufacturer also recommends coating the backs and stems of exhaust valves with a dry film lubricant to improve lubricity and reduce valve guide wear. On titanium valves, such a coating can protect the valve from erosion. Dry film lubricant applied to the back of intake valves can also reduce the buildup of carbon deposits that disrupt airflow.
Inside the combustion chamber (especially in aluminum heads), a thermal barrier coating can prevent the head from absorbing so much heat. This also improves combustion efficiency, adds power and lowers coolant temperatures. The thermal barrier coating is usually applied to the combustion chambers and inside of the exhaust ports. It may also be sprayed on the intake manifold surface to keep heat away from the manifold.
Thermal barrier coatings can also be applied to the underside and flange surfaces of intake manifolds, and the carburetor or throttle body flange to keep heat away from the incoming air charge. On a normally aspirated engine, every 10 degree reduction in inlet air temperature can increase engine power almost one percent. On a turbocharged or supercharged engine, a 10 degree reduction in inlet temperature is good for up to two percent more horsepower.
One place where thermal barrier coatings work extremely well is on exhaust manifolds and headers. Coating the inside and outside of these parts keeps the heat in the exhaust which increases the velocity of the exhaust gas for reduced backpressure and reversion. The net result is better exhaust breathing and more horsepower, especially at higher rpm. The outside surface of the exhaust manifolds or headers also remains significantly cooler, which reduces underhood temperatures and the risk of serious burns should anyone come into contact with the hot surface of the pipes. One coating manufacturer said the temperature reduction is enough that coated exhaust manifolds or headers can be painted with high temperature paint and the paint won't burn off.
Exhaust coatings include bright near-chrome finishes that are more for looks and corrosion-protection than heat management (though they also provide a thermal barrier), and black and gray coatings that are primarily for heat management and performance (but also provide corrosion-protection). Some coating suppliers use a multi-step process and apply several different layers to manifolds and headers to get the best possible results. Though most exhaust coatings are wet sprayed-on products that are then oven cured, at least one is sprayed on as a molten ion stream similar to a plasma spray process. This type of coating forms a metallurgical bond with the base metal and can't delaminate. It also requires no oven curing and can be painted.
Chrome plating, by comparison, is a totally different process. The manifold or headers are immersed in an acid bath and chrome is electro-deposited on the surface of the metal. Chrome forms a thermal barrier on parts (which is good from a heat management standpoint for exhaust pipes but not oil pans or valve covers), and provides a bright, attractive finish. But corrosion-protection is limited, and it discolors and blues when it gets hot. Over time, chrome can also flake and peel off, something most ceramic-based coatings will not do (provided the surface is prepared properly and the coating is applied and cured properly).
Thermal Dispersants
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