Hi Brian
to the Surface Engineering Forum.
Chromium oxide generally is not known for good deposit efficiency
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
- reducing powder particle size
- Changing oxygen/fuel ratio
- 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
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
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