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Amorphous Carbon Film Coating

Amorphous Carbon Film Coating

Amorphous Carbon Film Coating

We're working with a piece of 17-4 PH stainless steel which is annealed.  We had applied one coat of metal containing hydrogenated amorphous carbon film to the steel.  The vendor then, without consenting us, coated the steel again; but this time with a hydrogenated amorphous carbon film (containing no metal).

My question is, by them doing this, will the two layers of film actually bond to each other?  If so, how apt will the bond be to chipping?

RE: Amorphous Carbon Film Coating

How were the films deposited?  If by PVD, you should be ok so long as they cleaned properly between layers.

Jim Treglio
Tribo Coating LLC

RE: Amorphous Carbon Film Coating

They haven't gotten back to us as to what process they used to deposit the film.  Is PVD the best process to deposit these types of films?  If not, what other processes could we use?


RE: Amorphous Carbon Film Coating

As they are hydrogenated, they could also have been deposited by plasma-assisted chemical vapor deposition, or PA-CVD.

Jim Treglio
Tribo Coating LLC

RE: Amorphous Carbon Film Coating

Oh, thank you so much!  In your opinion, which do you see as the most effective process?

Sorry to be asking so many questions, I'm just really new to this type of work and I'd like to understand as much as possible.

Thanks again!

RE: Amorphous Carbon Film Coating

The difference is in the deposition parameter range, not necessarily in the method.  Basically, all of these coatings rely on controlling the ratio of diamond to graphite bonds in the coating.  The higher the ratio, the harder and more brittle the coating; the lower the ratio, the lower the friction coefficient.  My understanding is that the hydrogen helps form the diamond bonds, so hydrogenated coatings tend to be harder than amorphous carbon coatings.  Then there is the metal component.  The metal type and quantity also affect performance, but the variations again are pretty extensive.

A guess a more pertinent question is why are you getting the part coated?  Is it for hardness or low friction?

Jim Treglio
Tribo Coating LLC

RE: Amorphous Carbon Film Coating

It's being coated for hardness.

RE: Amorphous Carbon Film Coating

My mistake on that last reply.  The steel is being coated for low friction.  I answered before I should've.

RE: Amorphous Carbon Film Coating

Actually, the quality of the bonding doesn't depend so much about the deposition method. It's the design of the interface an the parameters used that are important. Often, metallic interlayers are used without being mentioned by the manufacturer.
It should be no problem to deposit a-C:H on a-C:H:Me.
As for the chipping resistance: Try it out or ask the manufacturer about chipping tests close to your application conditions.

RE: Amorphous Carbon Film Coating

Ok, so here is the latest information I have to share with everyone on this.  We have been getting lots of the coated steel in for the past two months and they would either pass 100% or fail 100% when we received them back from coating.  

Right now, this is the process we perform on the parts before they are actually coated with the film.

Machined (no EDM at any time)
Solution annealed to 1900 degrees F
Heat treated to H900 (not vacuum furnaced)
Coated with TC40

Here is the latest information I have in regards to our vendors.  We're going to visit the coater in a few days and I'll have more information about their process at that time...

We went to visit our electropolisher the other day and they explained their whole process to us, which seemed to all be pretty much in control.  They tie up our parts with a copper wire.  Next, they are electrocleaned in a HCl and water bath to remove any oxidation from the heat treating.  This rinsing process is performed twice. Then they electropolish them for 30 seconds.  After those 30 seconds, they take the parts and rinse them in a warm alcohol bath (not hot, but warm).  After that, they dry the parts off with an air hose (compressed air).  We then receive the parts from them.

I have no experience with electropolishing, so if there is something that looks odd with their process, please let me know.  If no one thinks there's something wrong with what they're doing, I'll just assume that it's being done this way by everyone.

Also, does anyone here see something that could be causing the problem with flaking?  Some people have suggested that we vacuum furnace heat treat the parts before they're coated.  Is this a possible solution to the problem if the coater is indeed in control of their process?

Thanks a ton!

RE: Amorphous Carbon Film Coating

What is their electropolish (EP) solution?  The rinsing seems inadequate to remove the usual phosphate & sulfate residues left by most EP solutions.*  And, is the compressed air highly filtered (oil-free)???

A usual rinsing process after is EP is: warm water rinse, flowing tapwater rinse, [unrack parts], 25 vol% nitric acid rinse, flowing rinse, flowing rinse, warm DI water rinse.

*A traditional electropolish solution is 63 wt% phosphoric acid + 15 wt% sulfuric acid + 22 wt% water (absolute wt. concentrations from titration & specific gravity).

RE: Amorphous Carbon Film Coating

I'm still waiting for a response from the electropolish vendor on their solution.

Here is the confusing thing.  We just had three lots of parts coated and the two that we've tested passed 100%.  The thing that's even more confusing is that if the lots pass or fail, it's always 100% pass or 100% fail.  There's never one part that's different from the rest.  

We've been looking to the surface finish on the area that's been flaking.  Our drawing calls for a 32 surface finish on the entire part, but the surface that has flaking issues is sometimes to spec and sometimes is not.

Other than the possible machining cause, does anyone see something that should be looked at more closely???

Thanks for your help!

RE: Amorphous Carbon Film Coating

We just inspected some of our parts and saw an uneven distribution of the coating on the lots that failed.  It was also observed that we have parts that have failed which have a better surface finish than some of the parts we have that passed.  I think after this observation, we can almost rule out the machining as an issue.

When we visited the vendor, they told us that our parts are not processed on their own.  They said the parts are processed when there are enough parts available for a full load, regardless of whether or not they’re our parts.  Therefore, our parts are mixed in with other parts, most likely larger than ours as the part we’re having an issue with is about 1”x4”x1/4.”

After observing the bad parts next to the good parts, we came up with a question.  

Could the load density be a leading factor in how well our parts are coated?  Also, could the size of the parts that are coated with ours be a reason that the coating doesn’t reach all areas of our parts?

Thanks again…

RE: Amorphous Carbon Film Coating

The load density will certainly have some effect on the coating processus.
Also, the other parts might not be as well cleaned as yours. This may affect your parts, if something is vaporized in the vacuum chamber and reaches the surface of your parts.

RE: Amorphous Carbon Film Coating

We're trying to make sure our process is in control before we send the parts to be coated.  In terms of electropolishing, is there a certain time that they should be dipped?  If there's an ASTM standard that they should be going by, could any of you share that with me?

Also, with the ultrasonic bath.  What should be used in that solution?  Right now we're using Softsoap from a drug store.  Is there something better to be using???


RE: Amorphous Carbon Film Coating

Get away from the soap as a lot of them don't rinse very will have an film that adheres to the part when it is removed from the bath.  You need to go to a cleaner designed for ultrasonic cleaning. You should be able to get with a local chemical (cleaning) supply house to get a cleaner or you can get one from a laboratory supply company.

If all the above fail here is a top of the line cleaner.


RE: Amorphous Carbon Film Coating

Good advice from unclesyd; handsoaps contain oils, lotions, etc.  Clothes washing detergent or baking soda would be better in a pinch.

ASTM B912, 'Standard Specification for Passivation of Stainless Steels Using Electropolishing,' doesn't specify polishing time.  http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B912.htm?L+mystore+ruht4924

Two important things about cleanliness for your parts:  
1) Prior to electropolishing, parts should pass a water-break test.  A simple one: wet part with DI water and verify that it forms a continuous smooth film w/o beading.

2)After the entire EP process (including handling with white linen gloves or similar) is completed, the surface can be tested for contaminants:  It is rinsed with an extractant (e.g., DI water-alcohol (spectroscopy grade) solution) which is caught in a pre-cleaned container and then analyzed for contaminants.  This is done by both the bio-pharmaceutical & semiconductor industries. Also described in ASTM F1374, 'Standard Test Method for Ionic/Organic Extractables of Internal Surfaces-IC/GC/FTIR for Gas Distribution System Components,' http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/F1374.htm?L+mystore+gevu32)656+1184337039

A very useful paper, 'STAINLESS STEEL TUBING INTHE BIOTECHNOLOGY INDUSTRY,' gives information on passivation, electropolishing, surface finish, testing & standards.  http://www.asepco.com/docs/PDF_files/Stainless_tubing.pdf

The SS parts will need plasma cleaning to activate the passsive surface within the coating chamber, so parts loading could be problematic from the process description given.

RE: Amorphous Carbon Film Coating

Thanks for the input on electropolishing guys!

Since we’ve had a chamber dedicated to our parts, we’ve gotten 90% passing parts.  This is good, but we still need to find out why we have some parts that are failing.

We got some of our stripped parts back (ones that had the coating stripped with 50% benzyl peroxide).  However, some of them had a little bit of the coating left over on the part.  Would it be alright to go ahead with coating these parts with the leftover coating on the parts, or should we continue to remove all of the coating?

RE: Amorphous Carbon Film Coating

We have had many issues with DLC(diamond-like coating) from many different vendors.  The coating varies from batch to batch and from part to part.  The one big factor we found was the same you did which is what other parts were in the batch with our parts.  All of the vendors do a plasma etch step before they coat.  We found this process had varying depths on different regions of the parts and affected the adhesion greatly.  Also there was the assumption that the etch process is also a sputtering process in which material from all of the surfaces in the chamber end up on your part to some degree.

One other thing to note is most of the vendors have their own cleaning process which normally gave us better adhesion results.  

Other variables that affect adhesion is surface finish (better finish can even give worse adhesion when it becomes too good), thermal history in the chamber,  chamber gas pressure and fixturing.  

RE: Amorphous Carbon Film Coating

Also forgot to mention, the thicker the coating the worse the adhesion.  5um is about the limit where the internal stress of the coating start to cause adhesion failures.  

Also areas that have relatively thin coating can indicate regions that didn't get plasma etched enough and also be a region of adhesion failure.

RE: Amorphous Carbon Film Coating

The stress in the coating is compressive, so another factor is the curvature of the part.  The coating will adhere better to convex surfaces than concave surfaces if the stress is significant.

Jim Treglio

RE: Amorphous Carbon Film Coating

Thanks for your input guys!

I had another question about the electropolishing.  At times, we’ve received parts of the same lot that have come back with different finishes on the 17-4.  Some come back with a matte finish, while others come back with a shiny mirror-like finish.  Is there any reason that you can see this happening?  Could the location in the tank affect the finish of certain parts?  All parts are fixtured in the same location, so that shouldn’t be a factor…

RE: Amorphous Carbon Film Coating

If the parts from the same lot are identical in alloy chemistry, heat treatment, machining or other mechanical treatment, and pre-cleaning, then the matte finish is poor electropolishing (etching) due to improper current density. A matte or snowy finish can also be from gas bubbles trapped on the surface.
But, more likely, the current density issue.  Anode (part) to cathode spacing should be nearly uniform, both anode and cathode must have enough current carrying capacity (cross-section relative to surface area), plus the rectifier must be able to supply enough power to do the entire load simultaneously. If underpowered, do fewer parts at a time.

Please describe the relative geometries of shiny and matte parts vs. cathodes.

RE: Amorphous Carbon Film Coating

The parts are all the same part and they are shaped like a "V" where the arms flex and crush a piece of titanium between the ends of the arm tips.  The base of the V shape is a rectangle about a half inch long.  The entire part is about 4" long, 1/8" thick, and the outermost tips of the top of the V shape are about 1" wide.

The parts are typically hung 3 pieces to one copper wire.  Each one of the parts is about 2-3" from the closest part.  All of the parts are always the same part when they are sent to be electropolished, so the part geometry is always constant.  

Does this give you any more information that you didn't already know?

RE: Amorphous Carbon Film Coating

Do you know the wire size & whether the parts are positioned with the flat surfaces vertical (to avoid trapping gas bubbles)?  Need 5-10 amps per part (assuming 8 sq. inches area).
IMHO, it's better to rack the parts (e.g., with copper, brass, or titanium spring clips on a copper or titanium rod) than hang from a wire. If loose or undersize wire, amperage to the part maybe below the critical current density (which varies depending on electrolyte, e.g., minimum 50 Amp/sq. ft. for 63 wt% H3PO4-15 wt% H2SO4-22 wt% H2O).

It's also essential that parts be clean (no oil!) when electropolishing. If the racker has dirty hands or gloves... See any oil sheen on the electrolyte surface?

Surfactant in the EP solution helps if parts are less than perfectly clean. Also, helps gas bubbles exit surface of part.

RE: Amorphous Carbon Film Coating

After seeing the wire, I can recall it being about 1/8" diameter.  As far as the orientation of the flat surfaces of the part, I don't recall them having them set in any particular way.

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