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Piston coatings have become so common among OEMs and aftermarket manufacturers that they often seem to be disapproved. Generally, consumers may understand the basic functions provided by piston coatings, but do not realize their full potential. On the other hand, engine manufacturers appreciate these benefits and can usually specify which coatings they prefer to apply to the piston based on the application of the engine.

Although there are a variety of coatings to choose from, each coating has its own unique functions, so their respective advantages are only suitable for the purpose of the engine. This is not one of the situations when you order a piston with all coatings, because it turns out that it does more harm than good. In addition, like anything else, there is always a cost factor. Certain coatings and processes can be very expensive, so you must determine whether the benefits will exceed the initial cost.

If I remember correctly (this is a big assumption), piston coatings and internal engine coatings were introduced in the mid-1990s. What I remember most is the DIY kit for user applications. If you have a spray gun and a conventional oven, you can successfully coat engine parts. At that time, we desperately needed horsepower because the supply of after-sales parts was small compared to today's market. So, I have to admit that I am one of those who have tried. I would say that this is very time consuming, which may be the reason why the cost of professionally coated parts is higher.

To better understand the various piston coatings, we contacted Mahle Motorsports to understand which coatings are available and when each is beneficial. When coating the piston, the target area needs to be considered: skirt, ring groove, top and the entire piston. There are eight types of coatings that can be applied to the four target areas. Each of the eight types has a specific function, and each has its own advantages and disadvantages. It is hoped that this information will eliminate any misunderstandings that may cause confusion as to which coating is suitable for you.

The Grafal coating is Mahle's patent, and you will often see this coating on the piston skirt. This is a dark coating, which is actually a printing resin embedded with graphite. This will increase the piston diameter by approximately 0.001 inches, so this must be taken into account when boring and honing the cylinder. Its purpose is to reduce sliding friction by adding a self-lubricating protective layer.

This skirt coating prevents the cold-start piston from flapping and excessive refueling. It also helps reduce noise and friction, and can even add a layer of protection if proper lubrication is lacking.

Technically, if the coating is applied properly, there are no disadvantages. Mahler believes in this coating so much that it is applied to every piston it makes. By incorporating the coating process into the main production process, this helps to significantly reduce costs.

Note: People usually think of Grafal as a running-in coating. The coating is designed to remain constant throughout the life of the piston. This coating can also protect the cylinder wall from abnormal conditions such as fuel flushing and overheating.

This is something we don't often hear because it has a very small window of use. Ferroprint is a dry base lubricant applied to piston skirts. This is very similar to Grafal, but the resin structure is embedded with stainless steel. Its main purpose is to provide a layer of protection for the aluminum piston so that it can run in the aluminum hole (ie, small engines and motorsports).

Provides protection against wear caused by cold start and lack of lubrication, which is necessary for this application.

Application can be expensive. If the coating wears out, an unprotected surface will appear, which can cause problems. Note: Cylinders with coatings such as Nikasil will not use this type of coating. They will still use Grafal. Again, this applies to aluminum cylinder bore applications.

The Ferroprinting process applies dry lubricant to the piston skirt. This coating is very similar to Grafal, but the resin structure is embedded in stainless steel.

If you have ever seen a dark gray piston and want to know why, the phosphating process is the cause of the dark gray appearance on the entire surface of the piston. This is an aluminum phosphate coating through a dipping process. Its main purpose is to provide running-in protection for piston pin holes and ring grooves. This is a dry lubricating coating that is permanently bonded to the surface of the piston. Phosphate coatings are actually not measurable because the layer thickness is less than 4 microns.

Phosphate coating is a dry lubricant that provides a thin film covering the entire piston. The coating covers the pin holes and ring grooves and is maintained until the engine reaches operating temperature.

The only function of the phosphate coating is to provide additional lubricity.

The phosphate coating process does not have any disadvantages.

Note: There is usually a misconception that the phosphate piston coating can act as some kind of thermal barrier for the piston crown. This is wrong. Dark gray has no effect on the top. If the top needs to be processed or re-cut the valve cavity, it will not affect the function of the coating, because the lubricity of the piston top surface is not a problem.

The letters DLC stand for "diamond-like carbon". Its name comes from the coating process, in which an adhesive layer is applied first and then a layer of hydrogenated amorphous carbon. The carbon layer is used as a hard and smooth surface to reduce friction. This carbon-based coating combines the properties of diamond and graphite.

The diamond-like carbon coating combines the properties of diamond and graphite to produce a very hard and smooth surface. This is an excellent choice for use on wrist stitches.

The main purpose of DLC is to apply to hard, stable surfaces, such as piston wrist pins. The soft structure of the piston, especially under extreme high temperature and load conditions, does not meet these requirements. It can be applied to piston skirts, but compared with traditional Grafal coating, the benefits of using DLC ​​are minimal. The resistance of wear protection is limited. Once wear begins, it quickly develops into hard wear and catastrophic skirt failure. The DLC coating on the skirt is not very forgiving, and cost-effectiveness is more important than cost.

Hard coat anodizing is a very specialized coating that can provide higher wear resistance and wear resistance. Teflon seals provide more lubrication. The coating process is carried out by a dipping process, which coats the entire piston.

In extreme applications, hard coat anodizing can improve wear resistance. The coating can also provide additional corrosion resistance for marine applications.

The increased wear resistance and wear resistance can cause long-term damage to the cylinder wall surface. Since the piston must be coated, the dimensional changes that affect the piston must be considered when designing the piston.

The main function of this coating is to protect the side of the ring groove from micro-welding. The term micro-welding is used to describe the transfer of aluminum particles from the ring groove to the piston ring. This usually causes the piston ring to get stuck in the ring groove, which can lead to ring sealing problems and excessive blow-by and power loss. Although it has been proven that moving the top ring closer to the top of the piston can help generate more power, when you do this, especially at high cylinder pressures, micro-welding can occur.

The hard anodizing of the piston ring groove does provide abrasion resistance and wear resistance for the micro welding that occurs when the engine is used under extreme conditions. (I.e. supercharging, nitrous oxide and turbocharging).

The surface of the side surface of the ring groove becomes rough. Although this will provide protection against micro-welding, it may cause difficulty in installing the piston ring. During the running-in period, no dressing of the hard anodized ring groove side will occur. After a period of time, the side surface of the anodized ring groove will deteriorate, but it will still be rougher than the uncoated piston. Therefore, the piston ring sealing effect may be poor.

Thermal barrier coating is a spray coating, usually applied to the top surface of the piston. Its function is to reduce the heat transfer to the top of the piston. The advantages of thermal barrier coatings depend to a large extent on the application and its use. If other parts of the engine (such as the combustion chamber, valves, and exhaust system) are also coated with thermal insulation, the thermal barrier coating will be more effective.

The thermal barrier coating will reduce the heat transfer from the combustion chamber to the piston crown.

The price of the piston with thermal barrier coating will add some additional costs. Although the cost may be a little trivial, in order to obtain the full benefits of thermal barrier coatings, additional costs are incurred when coating other combustion components. Once the piston is coated, no machining or modification of the piston crown is allowed. If any modification is required, the piston will need to be recoated.

MAHLE offers Powerpak piston kits with Grafal-coated piston skirts and phosphate coating. These two coatings are mainly used in street and racing applications and have been proven to be reliable for many years. They are applied in the production process, so when the cost is equivalent to a dollar penny, it is very cheap and very worthwhile. Other coatings are beneficial, but are mainly used for more professional applications, where the engine is under severe and extreme conditions.

The benefits of piston coatings have been debated for many years, but if used properly under the right conditions, they can be a good way to extend the life expectancy of internal parts and even the performance they provide. If you think a certain coating may be more suitable for your application, or you are not sure which coating will provide the best results for your engine, please call the staff at Mahle Motorsports and get the best advice from a professional.

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