Ceramic plasma coating

by Mike Hammer

Posted to the 2strokes list by Mike:

I probably know and can answer most of the questions about plasma coating. Dale and I have probably heard the same questions 500 times or more each in the past 20 years, so I'll answer them to make a quick "ceramic plasma coating FAQ". DISCLAIMER: Your mileage may vary! This is the best of what I know reduced to principles and useful info. Your exact application of this information is completely and totally YOUR OWN RESPONSIBILITY! I am not responsible for your choices or what you do or anything that happens to your hotrod engine. That said, if you try it and find it works, at least tell folks I wasn't completely brain-dead. Also, if you can find it online read Dale's write-up of the development and specs of his class-winning RD350 longrod race engine. Much of what follows is in there in one form or another, though this probably has some more specific details.

That should be good enough. Here goes.

- When is plasma coating useful? IMO, ceramic/plasma coating is most useful in aircooled 2stroke motors, nitro-burning 4strokers and anytime you MUST get every last ounce of long-term performance out of an engine. The parameter I look for is very very high piston temps and long run times - the more of either or both of these, the greater the benefit from plasma coating. I have not seen enough of a performance increase to make it worth the expense and trouble in a properly watercooled 2stroke engine not used in world GP racing, such as privateer Honda RS125s or RS250s. RZ350's are borderline cases - if they're really pumped (70 hp and up) I think it may be worthwhile, maybe. Use in 2stroke GAS dragsters is debatable as the motors generally don't overheat in a single 1/4 mile run, though they will overheat if you do several runs very quickly back-to-back. We've all seen the pics of the Kawi triple dragger from the mid-70's that had five gallons of water sprayed on it between runs to cool it down... I have seen very positive results in aircooled RD350s and RD400s on roadrace tracks (heh heh), and old Kawi triples where the center cylinder is in a poor thermal position. So contrary to what some folks have said on the Internet (always beware your sources!), plasma coating is not something I recommend all the time, but it sure is helpful when you need it.

To give you another benchmark/useful mental reference, every single jet airplane engine I know about uses ceramic plasma coatings in several high-temp places, such as combustion chamber coatings, compression fins and turbine. Why? These parts run while exposed to very high temps for very long periods of time. The plasma coating reduces the heat soak into the part and thus allows it to maintain tolerance for a much longer time. Less heat in with same cooling equals lower part temperature. It's all physics, folks.

- What exactly is plasma coating and where does it go? Plasma coating is really an abbreviated name, like Kawasaki is short for Kawasaki Heavy Industries Limited, Inc. One possible more proper name is "ceramic coating applied in plasma form". The word "plasma" actually means "superheated material" or "really friggin' hot!!!" as in 3000 degrees and up. (For a few years I played with rare gas plasmas in the range of 10,000 degrees F.) As ceramic is a material that becomes solid at a very high temperature, it must be heated to very high temp, usually 3-5000 degrees F and sprayed on. For use in internal combustion engines there are four useful places for ceramic/plasma coatings - the piston crown (NOT the skirts or undercrown!!!), combustion chamber, inside of the exhaust port and outside of the header pipes.

Also, doing ceramic plasma coating is a batch process, so you should expect per-unit prices to decrease with large lots. "Back in the day" we had our pistons batched in with jet engine parts, which cut the per-unit price in half compared to just doing a pair of pistons. Of course, a smart coater will know this and will have figured out how to batch and price to keep their business alive, so don't be surprised to hear "we only triple coat one day per week, and it's $X per part". Get them talking about their business, what they do and who they do it for, and you can tell if they're a good smart operator. If so, KEEP THEM!!!

- What benefit(s) do plasma coating(s) offer? In engine use ceramics are an insulator, meaning they do not absorb heat as quickly as metals, in fact several orders of magnitude slower than metals such as aluminum or iron. To our problems, ceramics offer two simple answers that reduce to "ceramic/plasma coating helps reduce engine metal heating from combustion". First: Reduce the heating of engine parts. Uncontrolled heat is the enemy of internal combustion engines. We want ONLY the combustion gasses to be very hot, and we want everything else to run at "design temperature" or "not too hot". The ideal behind plasma coating is to surround combustion gasses with an insulated shell. In uncoated engines this shell is the aluminum or iron of the cylinder head, piston crown, cylinder and exhaust port. In an ideal plasma coated engine, all of these except the cylinder would be coated with one to three coats (depending on the part) of ceramic/plasma coating to contain the heat and keep it from heating the metal of the engine. This is good because when metal gets hot it expands. Aluminum expands a lot when it gets hot. Pistons are made from a variety of silicon-aluminum alloys. The more it expands the tighter clearances get. Motor parts are designed to run at a specific temperature which results in clearances becoming "as-designed". When you work to get more power out of an engine it runs hotter and these clearances shrink. If they get small enough (thinner than the film tolerance of the oil being used), the adjacent parts begin to rub against each other as metal-on-metal instead of metal. In 2stroke land we affectionately call this "seizing", which I don't think needs further explanation.

Second: Reduce the heating of the incoming air-fuel mix (intake charge). Every engine builder eventually runs up against the problem of intake charge heating. The cooler the intake charge, the more dense it is. More dense means it carries more oxygen and gas molecules per cubic centimeter, or per cubic inch if you're working on American 2strokes. Intake charge is heated by compression in the intake path (airbox, carb, port and crankcase) - we can't do anything substantial about this short of running refrigerant lines in these areas. What we can affect is the heating of the intake charge in the crankcase, which is VERY significant. The hottest thing in the "box" surrounding the intake charge in the crankcase is the piston crown. We've all seen and diagnosed overheating by looking at the color of intake charge that is burned against the BOTTOM of the piston crown. Doesn't burning the intake charge before it gets to the combustion chamber sound like a bad idea?! Reducing piston crown temperature reduces the heating of the intake charge and thus maintain a higher intake charge density in the crankcase.

Here's a mental exercise you can do, I won't be held responsible for the results if you try this in real life and blow up your motor. Next time you get a chance, dyno your 2stroke motor when it first reaches proper running temperature. Now keep the engine on a varying high load for, say, 15 minutes to simulate conditions in a race. (Be sure to have fans running to cool the engine in a "normal life" manner! Don't blow up your motor.) Now dyno it again. Voila, the power has dropped. Why? THE ENGINE IS HOT! As long as it's not seizing that shouldn't matter - but it does because the intake charge is being heated and is thus less dense so your engine is getting less oxygen+gas, which means less power (assuming you have it tuned anywhere near decent). One guy who tried this told me his pumped RD400 engine lost 10 hp when it was super hot. Gee, wouldn't it be nice to have that 10 hp back at the end of a race when you most need it for the run to the flag... Another guy told me he only saw a 6 hp loss. Well, loss is loss I guess.

You also may recall Honda running all ceramic pistons in several 1980's GP class motorcycles for a short time, including a few sets in the oval bore NR500. While much progress was made, even mighty Honda realized that engine thermal cycles and the resulting metal expansion and contraction of cylinder walls, heads and studs are so severe that the very small-expanding ceramic pistons just couldn't do a proper job outside of a pure-race application. So they went back to aluminum alloy pistons, though unconfirmed rumor had it that the piston crowns were still plasma coated.

- Why triple coat instead of single coat? This is a very commonsense question, which is sometimes asked "if it's such a good insulator why do we need three coats?" The answer is metal expansion AND insulation. Obviously three coats will insulate better than one coat, we'll take that as given. The other coats are to buffer thermal expansion and shock loads in the ceramic itself.
Here is what each coat does:
- the first coat (against the piston) sticks to the piston and expands and contracts with thermal load the slowest of the three layers. Generally piston crowns are lightly etched before coating so the ceramic can adhere better.
- the second coat is a buffer coat between the first and third coats. It expands less slowly than the third coat, but more quickly than the first coat.
- the third coat (exposed to the combustion chamber) takes the brunt of the combustion thermal load and shock, and thus expands the most.

So three coats both accomplishes a higher insulation factor and a lower expansion RATE (not total physical expansion SIZE) to keep the plasma crown adhered to the piston crown. It's nice when it stays there.

If you only do one coat you will get some insulation benefit but not as much as three coats. You will also probably see the coating flaking off around the edges of the piston crown from two causes: number one being the high expansion of the piston metal in these areas, resulting in the piston metal literally pulling away from the ceramic; and number two the physical shock of the piston rocking in the bore and striking the cylinder. Some of the ringing you hear in an aircooled 2stroke engine when it is cold is the pistons slapping around in the bores, so to reduce this shock and the accompanying wear we all are used to a short warm-up period before we ride our precious children away at full throttle. At least I hope so. There is also some shock due to the explosion of the combustion charge itself, though this is not as much of a factor according to the Air Force.

- What other options are there? Polishing is the only other similar option available. Polishing reduces the surface area of metal exposed to combusion heat, so the part literally absorbs less heat. There is a slight reflective effect but this obviously goes away as carbon slowly accumulates on running parts. If you are not using plasma coatings all combustion chambers and exhaust ports should be polished for this effect, and if not plasma coated piston crowns can be carefully polished to get some benefit. You have to be a bit careful when polishing piston crowns as polishing any alloy containing silicon probably won't result in a perfect polish, and you don't want to remove very much metal from a piston crown. So stop polishing a bit sooner rather than trying to get that chrome wheel shine! Proper water cooling consisting of correct water flow rates, proper coolant exposure to hot spots in the engine, proper radiator sizing and proper air flow through the radiator, and will overshadow the benefits of plasma coating as the piston temps are held down quite a bit compared to aircooled engines. But even these engines will often benefit from simple exhaust port coating and expansion chamber coating to keep exhaust heat from soaking back into the cylinder.

- What do I need to be careful of? The first concern is obviously to get someone who knows what they're doing. We found the jet engine rebuilders were very very knowledgeable, probably because of all the required training just to get a process and operator certified to rebuild commercial jet engines. Many of the older sprint car builders have enough experience that they also know what they are doing very well. I won't recommend or diss any one company because a lot of your success will depend on who you get and how well you can express to them what you are trying to accomplish. There are many ceramic coatings available and you or I probably can't become well enough educated in all the factors to know what we want. Like top suspension, sometimes you have to find and rely on the pros!

The second concern is engine assembly. Assume you just got back your new plasma coated pistons. Chuck 'em in and test them, right? WRONG! Assemble the engine and check the squish clearance first! You just added to the height of the piston crown, so you will probably have to machine the squish band back to reset the proper clearance for your engine. Proper squish clearance depends on rod stretch, piston expansion and head expansion, so this can be a different number depending on your engine. For Yamaha aircooled RD350s and RD400s the optimum clearance is 0.035 inches with a tolerance of -0.000 inches and +0.005 inches. Of course, remachining the squish changes the combusion chamber volume, so you will want to check that and adjust as appropriate. Note that plasma coating does NOT change the desired spec for piston-to-cylinder clearance.

The third concern is engine warm-up prior to full throttle operation. Assume you have only coated the piston crown, and that you have remachined your heads to the proper squish clearance and combustion chamber volume. You will find, no surprise, that your engine takes a lot longer to warm up! Of course it does, the piston is no longer absorbing combustion chamber heat and passing it through the skirts to the cylinder at the same rate. This is, after all, what we were after! On the RDs it takes about five minutes to warm up the engine at the usual 2500-4000-5500 rpm "blip pattern", which we're all pretty familiar with. Then you will need to take it a bit easy for the first 1-1.5 minutes on track to allow the piston and cylinder to come up to speed. If you have ridden a water-cooled 2stroke you will recognize this as exactly the same pattern. If not, talk to an RS250 or TZ250 pilot to find out what they do from cold engine startup to full temp use, and you'll get the idea. Again, this can vary a bit and I won't pay for your motor if you screw this up, so be a bit careful OK?!!!

- Where to get it done. The two biggest applications I have seen outside of heavy industry are jet engine rebuilders and sprint car racing. Another I have recently become aware of is snowmobile and ATV drag racing. Sprint cars are built to rather limiting engine rules so they tend to need all the thermal help they can get. There are a wide variety of sprint car engine rebuilders who plasma coat the inside of exhaust ports and combustion chambers. Fewer do piston crowns. If they don't know about triple coating find someone else who does. The snowmobilers are very fond of coating the exhaust ports and expansion chambers to help with overall engine cooling and maintaining exhaust gas temps as are the Banshee drag racers, and as a result these two areas are very well known. I would rather wrap the exhaust header than ceramic coat it if I had to do it on a budget, as exhaust wraps are fairly cheap and probably insulate better. But if you have cooling problems definately get the inside of the exhaust ports coated after you finish the porting job. Do a Google search for "coat exhaust port yamaha banshee" and you'll get a list of companies as a starting place. If and when you find someone who knows triple coating and will do it for pistons, email the list so the rest of us know too!

- My recommendations? The biggest gain for aircoolers is in getting the piston crown triple coated. This is less effective for well-cooled watercoolers. Generally, the blacker the residue under a used piston crown, the more the engine will benefit from coating the top of the piston crown. The next biggest gain is getting the exhaust port coated, which also helps general cooling for watercoolers by reducing how much heat gets into the cylinder. Coating the combustion chamber is very tricky because it expands and contracts so much -- I've seen several "combustion chamber caps" come completely dislodged, with predictable results. I can't recommend it, polish the combustion chamber instead. Finally, wrap the header of the expansion chamber to both keep exhaust heat from blowing back on the motor, and keep up the temp of the exhaust gasses in this critical region.

Well, that's about it, I hope this covers most of the questions.


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