Piston coatings have become so common with OEM and aftermarket manufacturers that it seems as though they often go unrecognized. Often, consumers might understand the basic functions that piston coatings offer, but do not realize the full potential of the benefits. Engine builders, on the other hand, appreciate the benefits and can often specify which coatings they prefer to have applied to the pistons based on the application of the engine.
While many coatings are available, each coating has a unique function, so the individual benefits will only serve the purpose for which the engine is being used. This is not one of those situations where you just order your pistons with all the coatings, because it could prove to do more harm than good. Also, just like anything else, there is always the cost factor. Some coatings and processes can be very expensive, so you must determine if the benefits will out way the initial cost.
If my memory serves me correctly (and that’s a big if), piston coatings — along with internal engine coatings — were introduced around the mid-1990s. What I remember most were the do-it-yourself kits that were user applied. If you had an airbrush, and a conventional oven, you could successfully coat your engine parts. Back then, we were desperate for horsepower because the availability of aftermarket parts was slim compared to today’s market. So, I will have to admit that I was one of those guys that tried it. I will say that it was very time-consuming, which is probably why the cost of professionally coated parts is more expensive.
In order to get a better understanding of various piston coatings, we reached out to Mahle Motorsports to learn what coatings are offered and when each is beneficial. When coating a piston, there are target areas that need to be considered: the skirt, ring grooves, crown, and the entire piston. There are eight types of coatings that can be applied to the four target areas. Each one of the eight has a specific function and there are advantages and disadvantages associated with all of them. Hopefully, this information can clear up any misunderstandings that might cause some confusion about which coating is right for you.
Grafal Piston Coating
Grafal coating is proprietary to Mahle, and you will often see this coating on piston skirts. This is a dark coating that is actually a printed resin embedded with graphite. This will add approximately .001-inch to the diameter of the piston, so it must be considered when boring and honing the cylinders. Its purpose is to reduce sliding friction by adding a self-lubricating, protective layer.
This skirt coating acts as protection against cold start piston slap and over-fueling. It also aids with noise and friction reduction and even adds a layer of protects if there is a lack of proper lubrication.
Technically there are no disadvantages if the coating is applied properly. Mahle believes in this coating so much that it is applied to every piston it manufactures. This helps lower the cost substantially by incorporating the coating process into the main production process.
Note: People often think the Grafal is a break-in coating. This coating is meant to remain for the life of the piston. This coating will also save cylinder walls from abnormal conditions such as fuel wash, overheating, etc.
This is one we don’t often hear about because it has a small window of usage. Ferroprint is a dry-based lubricant applied to piston skirts. This is very similar to Grafal, but the resin structure is embedded with stainless steel. Its primary purpose is to provide a layer of protection for an aluminum piston to operate in an aluminum bore (i.e. small engines and motorsports).
Offers protection against scuffing from cold starts and lack of lubrication, and is necessary for this application.
Can be expensive to have applied. If the coating gets worn off, there will be unprotected surfaces and that may present problems. Note: Cylinders that have plating such as Nikasil will not use this type of coating. They will still use the Grafal. Again, this is for aluminum cylinder bore applications.
Piston Coatings Via Phosphating
If you have ever seen a piston that was dark gray and wondered why, the Phosphating process is what creates the dark-gray appearance on the entire surface of the piston. This is an aluminum-phosphate coating that is performed via an immersion process. Its primary usage is to provide break-in protection for the piston pin bores and ring grooves. This is a dry lubricant coating that is permanently bonded to the piston surface. The phosphate coating is practically immeasurable because the layer thickness is less than 4 microns.
The one and only function of the phosphate coating is to provide additional lubricity.
There are no disadvantages associated with the phosphate coating process.
Note: Often, there is a misconception that the phosphate piston coatings serve as some sort of thermal barrier for the piston crown. This is false. The dark-gray color serves no purpose on the crown and if the crown needs to be machined or the valve pockets recut, it will have no effect on the function of the coating since lubricity of the piston’s top surface is not an issue.
The letters DLC stand for “Diamond-Like Carbon”. It gets its name from the coating process in which an adhesion layer is applied and then followed by a layer of Hydrogenated Amorphous Carbon. The layer of carbon serves as a hard, slick surface to reduce friction. This carbon-based coating, combine the properties of diamond and graphite.
Offers superior friction reduction
The primary use for DLC is to be applied on hard, stable surfaces such as piston wrist pins. The soft structure of a piston, especially under extreme heat and load conditions, does not fit these requirements. It can be applied to the piston skirts, but the gains using DLC over the conventional Grafal coating are minimal. The tolerance for scuff protection is limited, and once the scuffing begins it can quickly progress to hard scuffing and catastrophic skirt failure. The DLC coating on the skirts is not very forgiving and the cost to benefit ratio is relatively weighted toward the cost.
Hardcoat Anodizing With PTFE Sealing
Hardcoat anodizing is a very specialized coating that offers increased resistance against abrasion and wear. The PTFE sealing offers increased lubrication. The coating process is performed through an immersion process that will coat the entire piston.
In extreme applications, hardcoat anodizing can offer increased resistance in abrasion and wear. The coating can also provide additional corrosion resistance for marine applications.
The increased resistance to abrasion and wear can lead to a long-term detriment to cylinder wall surfaces. Because of the processes that the piston must undergo for the coating, dimensional changes that will affect the piston have to be accounted for when designing the piston.
Hard Anodizing Ring Grooves
The primary function of this coating is to offer protection of the ring groove flanks against micro-welding. The term micro-welding is used to describe a situation when aluminum particles of the ring-groove bore transfer to the piston ring. This often causes the piston ring to stick in the ring groove, causing ring-sealing issues along with excessive blow-by and loss of power. While it has been proven that moving the top ring closer to the top of the piston helps make more power, when you do, especially under high cylinder pressures, micro-welding will occur.
Hard anodizing of the piston ring grooves does offer abrasion and wear resistance to micro-welding which will occur when the engine is used under extreme conditions. (i.e. supercharging, Nitrous Oxide, and turbocharging).
The surface of the ring groove flank becomes rough. Although this will offer protection against micro-welding, it can cause some difficulty with piston ring seating. Conditioning of the hard-anodized ring-groove flank does not occur during the break-in period. After a period of time, the anodized ring groove flank will become conditioned, but will still be rougher than an uncoated piston, therefore, possibly leading to a less-than-optimal piston ring seal.
Thermal Barrier Piston Coatings
A Thermal Barrier Coating is a spray-on coating usually applied to the top surface of the piston. Its function is to reduce heat transfer into the top of the piston. The benefit of Thermal Barrier Coating is highly dependent on the application and its use. Thermal Barrier Coatings are more effective if other components of the engine such as the combustion chambers, valves, and exhaust system are also coated.
Thermal barrier coatings will reduce heat transfer from the combustion chamber to the piston crown.
There is some additional cost added to the price of the piston for adding a Thermal Barrier Coating. While the cost may be somewhat insignificant, there will be additional expenses in coating the other combustion components in order to yield the full benefits of Thermal Barrier Coating. Once the piston is coated, there cannot be any machining or modifications done to the piston crown. If any modifications are needed, the piston will need to be recoated.
Mahle offers its Powerpak piston kits which come with the Grafal coated piston skirts and a phosphate coating. These two coatings are mostly utilized for street and racing applications and have been proven reliable for many years. They are applied during the production process, so it is very inexpensive and well worth the benefits when the cost equates to pennies on the dollar. The other coatings are beneficial but are structured mainly for more specialized applications where engines are under severe and extreme conditions.
The benefits of piston coatings have been debated for many years, but if properly utilized in the right situations, they can be a great way to extend the internal part’s life expectancy and even the performance it delivers. If you think a particular coating might be better for your application or you are not sure which would yield the best results for your engine, give the folks at Mahle Motorsports a call and get the best recommendation from professionals.