Hello! New Person Here! Question about Chromium Oxide.
04-29-2008, 10:59 PM,
Hello! New Person Here! Question about Chromium Oxide.

My name is Brian. I recently found this forum while doing some searching, and I found some useful information within. I recently graduated from school and I luckily found an entry level job with a Thermal Spray company. Prior to this, I had no formal training in Thermal Spray Technology and have essentially learned "on the field". I am not a Materials Science expert either. I went to school for Mechanical Engineering. We have a Detonation Gun here and I have been learning more and more about its theory and operation, and have become the main operator of this equipment. Does anyone here use any form of Detonation Technology?

My dilemma involves the application of Chromium Oxide powder. I have experimented with deposition, and unfortunately I cannot get any kind of decent coating. I can apply about, if I'm lucky, a 25-50 micron coating over a long period of time. The powder does get consumed and it just seems like the material cannot build up substantially. I know the melting point is relatively high, and I'm curious as to any input on the matter. Even if you're running with Plasma, HVOF, etc I would like to hear your opinion on it. Does it take that much energy vs a powder like Aluminum Oxide? How is the depositon efficiency? What kind of thickness can you achieve? Thanks in advance for any input!
05-02-2008, 06:06 PM,
RE: Hello! New Person Here! Question about Chromium Oxide.
Hi Brian

Sign0016 to the Surface Engineering Forum.

Chromium oxide generally is not known for good deposit efficiency Happy0193 well not when compared to most other powders.

Chromium oxide's high melting point, poor thermal conductivity and hard ceramic nature does not lend itself to very high velocity processing, certainly not in powder form anyway.

If you are managing to get a deposit, that is a promising sign. If you are getting problems with very slow build up or a situation where build up stalls, this may be due the fact that you are only melting/heat softening a small proportion of the powder, leading to poor deposit efficiency. Compounding this, the remaining non-melted hard powder then acts like a grit blasting media, which tries to remove the all ready deposited coating. When in equilibrium coating growth stalls. Another view is that the surface texture of the coating changes and becomes somewhat anti-stick and gradually reduces deposit efficiency.

To improve your deposit efficiency, more heat needs to be applied to the powder particles. This could be achieved by
  1. reducing powder particle size
  2. Changing oxygen/fuel ratio
  3. Changing gas flows/pressures
Problem with 2 and 3 is that to get more heat into the particles you need higher temperature or longer particle resident time in flame or both. Making changes to these parameters is not all ways straight forward and do not give necessarily the effect you first expect Happy0193 Effect of say changing oxygen/fuel ratio to get higher flame temperature may also increase velocity which may cancel or reduce any net change in particle heating Sad

Sorry, no real first hand experience with running detonation process, so this is about as far as can advise.

Union carbide/Praxair D-Gun as far as I'm aware did not offer chromium oxide coatings, only LC-4 which is a plasma coating. Most chromium oxide powder is applied via plasma spraying, also spraying using rod form via Rockide or HVOF wire process. Top Gun (TWI) HVOF claim chromium oxide coatings, I assume using acetylene fuel.

Good luck with your experimenting Big Grin
05-02-2008, 07:19 PM,
RE: Hello! New Person Here! Question about Chromium Oxide.

Thanks Gordon for the welcome and the reply. Much of what you mentioned was on par with what I thought too. I am able to get "some" deposit, which is hardly measurable. I estimate under a mil thickness, my micrometer isn't as specific as I'd like it to be. However I feel better knowing that Chromium Oxide is not a normally efficient powder Happy0193

I have experimented with varying oxy/fuel ratio as well as flowrates. Initially like you mentioned, the powder would hardly become molten at low flowrate and start to act as an abrasive. I have had this happen with some other cermets too. I can prevent this and I get "some" coating when I head towards maximum flowrate. After a certain rate, it gets difficult to control the deposit location so I am limited in that sense. As far as ratios, I normally run around 5:1 using propane. I have gone as low as 4:1 (about the lowest I can risk without stalling) and I may try the flip side, and head towards 7:1. The other option I think would be to try running MAPP gas instead of Propane.

Praxair had been using their Super D-Gun for coating tasks with Chromium Oxide. However I am unsure if this is still available as I was informed the process took a very long time, and I'm willing to believe they found a more efficient solution. I will keep you updated on my progress. Thanks again!
05-02-2008, 07:34 PM,
RE: Hello! New Person Here! Question about Chromium Oxide.
Hi Rhedalert

Thanks Big Grin

Forgot to mention fuel gas Ashamed0002 changing to higher temperature fuel gas should help MAPP, propylene etc. Can you run on acetylene?
05-02-2008, 08:37 PM,
RE: Hello! New Person Here! Question about Chromium Oxide.
I know of Detonation guns that can run with Acetylene but I do not believe I can with mine. I have a better chance with MAPP gas but even then, I'd be the first here to try it.

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