Cylinder coating 2

[divergent]

I am in the process of integrating a small 2-stroke rc engine (5hp) into a UAV. The cylinder head has a Nikasil coating. The manufacturer has stated that I should expect to see approx. a 200hr life out of the coating but I'm suspicious as I have seen a significant amount of wear after only 20 hrs of run time. Granted, I am running the engine on a dyno where I am no doubt putting the engine through higher stresses then it will see in regular use.

Does anyone have any experience with this coating and what sort of practical life to expect?

Thanks.

 

[Metalguy]

It's a great coating. What oil are you using, at what ratio? Give pure castor a try.

 

[V12]

Great stuff. I have two engines that use Nikasil (or something very similar) a BMW 750iL has an all aluminum V12 with Nikasil coated cylinders. It's still running stong at 210K miles. I also have a Rotax FR125 2stroke kart engine. I've run it up over 19000 RPM on a regular basis on the track. No problems.
One interesting thing to note is with the Rotax engine the manufacturer will void the warranty if the engine is not broken in properly AND this HAS to be done on track. Not on an engine stand or dyno. The don't give reason, but you can speculate is has something to do with bedding in the pistons in the bore.

 

[divergent]

V12.
Thanks for the info.

You lead me into a follow-up question. Thou the manufacturer says that I should not need to do any sort of break in with the engine, I suspect otherwise. Is there a standard break-in methodology that I should be following before I do any serious testing? I'm quite new to the small two stroke world, been working with mostly rotorys and diesels to this point. Rotax engines are outstanding btw.

Metalguy.
I'm using a syntheic blend that the manufacturer recommended. Starting with ~60:1 and working down to 85:1 after a moderate amount of run time.

Thanks

 

[Metalguy]

Try running pure castor or at least 50% castor, and get the oil up to 40 to 1. See it this solves your wear problem, then slowly back off on the oil.

 

[Azmio]

The nikasil is a hard and durable coating. It works really well in many racing applications. I never knew that the coating will wear out in such a short time. However, there are many examples around the world where the nikasil got erodded by sulfur rich fuel. The combustion by product will settle in the sump and during warm up, the sulfur along with the water will vaporize forming sulfuric acid. Gee, we made some experiment on the cylinder coating, the acid eroded the coating badly

 

[bazeleur]

What I want to know exactly is whether Nicasil coating is applied directly to the (cast) cylinder wall or via a dry liner made of wrought aluminum. I've heard that cast Al surface doesn't make for a very reliable bond with the nickel-SiC matrix, but I've also heard otherwise... I was hoping you guys could clear up the confusion for me. THanks

 

[Azmio]

Bazeleur

Divergent mentioned about the 5hp 2 stroke engine. My guess is that the Nikasil is applied straight to the parent metal bore and not the liner.

One thing that I can share here is that Nikasil coating is very sensitive to the parent metal bore porosity. There is an example where the Nikasil coating is applied to high pressure die crankcase. High pressure die casting is known to have higher porosity if compared to other casting methods. We were quite shocked to know that the reject rate is as high as 50%.

Anyway, I do hope that Divergent's crankcase is not manufactured using high pressure die cast. If it is true, my recommendation is to ditch the Nikasil, instead, try Plasma Spray from Sulzer.

AO

 

[bazeleur]

Azmio,
Thank you for your input. I'm glad you brought up Sulzer's Plasma Spray coatings, as I've been doing quite a bit of research on Plasma Spray,HVOF, and Arc-Spray coatings lately, as frankly, I find the idea of paying >$300 per cylinder bore for Nicasil plating quite ludicrous. My question to you is this, what coatings would be most suitable for a high-performance cylinder bore? Thanks again

 

[divergent]

In our application the casting is a rather low end lost wax casting which is then bored to the appropriate size(.005" less then final.) The plating is then applied (US Chrome I believe) .007" thick and honed to the final size. Yes, porosity of the casting has been a major issue, but the required life of the head is on the order of 100hours so it hasn't presented a major problem for us. As long as the fuel is kept rather oil rich the plating seems to be lasting, we run 50:1 with a synthetic oil (Mobil 1 MX2T).

The engine is a rather cheap 2 stroke rc engine. Not the best engine to begin with but it does the job.

$300 a cylinder bore? That sounds rather high, if I remember our supplier is only paying $75 per head, and it's a small blind head.

Divergent.

 

[Tmoose]

Did someone ask what the ring material is?
And, for more description of the cylinder wear?

 

[Azmio]

Bazeleur

Here is some info in high performance engine applications that I can share with you:-

1) Cosworth F1 engine uses Sulzer's plasma spray.
2) Yamaha's sport bikes use Nikasil and also the electroplating version called YRPS (the composition is slightly different though).
3) Nikasil coating have been widely used by many racing engines

Considering the availability of such facility and supplier, my recommendation is to go with Nikasil. You do need to make sure that the cylinder bore has such a low porosity level. You also need to choose the right piston ring especially the surface treatment and the oil specs. Next is the need to coat the piston skirt with iron or molybdenum coating.

If your engine has significant porosity level that you can see with your naked eye. Try Plasma spray because several papers claim that the sprayed particles can fill the hole created by the casting porosity. However, Suzuki tried their own version of plasma spray and reported some quality problem. Some element of risk but it is better than applying Nikasil to bore with visible casting porosity.

AO

 

[kmpillai]

Dear Azmio,
Using Plasma sparay also whether they are using Nickel-Silicon Carbide coating or some other wear resistance coating. I found widely elctro palting technique is used for Nickle-Silicon carbide coating. Other than electro plating any other technique is there for Nickle-Silicon carbide coating for cylinders.

Thanks
KMPillai

 

[Tmoose]

Have we heard what are your rings faces with?

 

[bazeleur]

From the literature I have seen, the best ring coating for Nicasil bores in Titanium Nitride coated rings, followed by Chromed rings, which are slightly inferior, and plain cast iron, which is waaay behind.

Azmio,
I did a bit of research yesterday and according to a Ward's Autoworld article, Sulzer's plasma coatings were superior to Nicasil in every single way; friction, wear, and corrosion resistance. They's cheaper to boot... What do you make of this? As an aside, what are the most suitable (plasma spray) coatings to use? From what I understood it was something along the lines of an iron/molybdenum coating. More info would be most appreciated, thanks!

 

[Azmio]

Bazeleur

Yup, you're right, plasma spray sprays combination of iron and molybdenum. Volkswagen was the one that came up with the combination. I am not sure whether we need to pay royalty to VW for the use of the powder.

As for Plasma spray vs Nikasil. One major advantage is the fact that plasma spray does not use any Nickel as what contains in the Nikasil. The Ni is sensitive to corrosion from the acidic vapor during warm up. Once the Ni is gone, the silicone and carbide particles would be gone as well.

At this moment, I have no data from the engine motoring for friction loss between Nikasil and Plasma spray. I have to ask around whether the R&D group has conducted such test because I believe they did.

As for which one is cheaper, we first have to look whether it is available nearby or not. In Japan, Sulzer has it's rep. In Europe, the facilities are increasing and you wont have problem to find one. However, I'm not sure whether plasma spray facility can be found in US. Even if the spraying facility is available, you need to find a honing shop that is capable to hone the sprayed surface. From what I heard from Gehring's engineers in Germany, the honing requirement is different from cast iron or Nikasil surface.

I know that Nikasil coating has been widely used and they are many facilities to do so around the globe. On the other hand, environmental group is fighting to get rid of this Nikasil facility. AS for the honing of the Nikasil, I believe such machining know-how has been mastered by so many machine shops around the world.

 

[bazeleur]

Another question for you guys,
What is the proper way to bore/hone Alusil blocks, the ones made out of 390 aluminum, where the surface aluminum is supposedly "etched away revealing the hard silicon crystals to provide the necessary wear resistance"?

 

[bazeleur]

To answer my own question, I just heard that rubbing the bore surface with a leather swab dipped in acid/etching solution is supposed to melt away the aluminum matrix on the surface thereby exposing the silicon crystals... However, I was hoping for a little more detailed information on the subject matter. Ideas anyone?

 

[Azmio]

Bazeleur,

You should be able to get some information on the honing information from Kolbenschmidt ATAG website. Gehring in Germany is also a specialist in the honing process. I am not sure if Gehring has a website that describes the honing process for various bore surface.

This is your lucky day because I happened to have the know how in the design of hypereutectic Alusil block. Anyway, the etching process has been ditched and replaced with 3 step (if my memory is correct) mechanical honing. Kolbenschmidt no longer perform the etching process. The block that we have also uses mechanical honing and not etching.

Anyhow, you can only do the etching or mechanical honing for aluminum block with high silicone content and not regular secondary aluminum. The silicone content should be at least 17% whereas the regular aluminum alloy is in the range of 7-9%. The bore surface is also specially cooled down during casting or else the grain properties wont be suitable.

You should check on the websites and I can help you out on the design side.

 

[Tmoose]

I don't know if the alloys are the same, but Sunnen, the honing people, have a web site.

http://www.sunnen.com/product/pdf/60-67.pdf

They say this

"Tooling for Resizing High-
Silicon Aluminum Alloy
Blocks
Porsche, Mercedes Benz, Honda,
GM Vega 2300
High silicon aluminum die-cast blocks
do not require a steel or cast iron liner
in the cylinder bore. Pure particles of
silicon, about .001" (0,03 mm) in diameter,
are dispersed in the aluminum
alloy. To achieve a proper surface finish,
the cylinders should be prepared
so piston and rings contact only silicon.
Silicon is very hard, so there will be
very little wear in the cylinder bore.
The aluminum’s function is merely to
“hold” the silicon particles which
actually form the wall.
Excessively scored or worn cylinders
can be reconditioned to accept oversize
pistons. A special “Conditioning”
operation is needed to remove aluminum
from and between the silicon
particles so the wall will be a silicon
wall."