What are the biggest factors

[erick212]

What are the biggest factors that effect oxide content in a 75/25 Chromium Carbide,Nickel,Chromium sprayed by HVOF. This would be with a DJ2600 with H2 and O2.

Thank you,


Erick

[Gordon]

Hi Erick,

My list really applies to most oxidation sensitive materials and also some other thermal spray processes as well.

• Powder/particle size - smaller the particle, bigger the surface area, greater the oxidation potential.
• Particle velocity - higher the velocity, lower the dwell time, lower the oxidation potential.
• Temperature - higher the temperature, higher the oxidation potential.
• Chemistry of spray stream - oxygen/fuel ratio
• Spray distance - longer distance, longer dwell time, higher oxidation potential.
  

Substrate/coating temperature (under normally conditions) usually has no significant impact on oxidation compared to above factors.

Manipulation of process parameters to control above factors are complex For example; changing H2/O2 ratio to give reducing flame (less oxidising) may result in changing flame temperature and velocity, so that the overall effect may in fact be increasing oxidation. Less obvious parameters, like air flow/pressure, feed rate and carrier gas flow may also have their influences.

[erick212]

Gordon Wrote:Hi Erick,

My list really applies to most oxidation sensitive materials and also some other thermal spray processes as well.

• Powder/particle size - smaller the particle, bigger the surface area, greater the oxidation potential.
• Particle velocity - higher the velocity, lower the dwell time, lower the oxidation potential.
• Temperature - higher the temperature, higher the oxidation potential.
• Chemistry of spray stream - oxygen/fuel ratio
• Spray distance - longer distance, longer dwell time, higher oxidation potential.
  

Substrate/coating temperature (under normally conditions) usually has no significant impact on oxidation compared to above factors.

Manipulation of process parameters to control above factors are complex For example; changing H2/O2 ratio to give reducing flame (less oxidising) may result in changing flame temperature and velocity, so that the overall effect may in fact be increasing oxidation. Less obvious parameters, like air flow/pressure, feed rate and carrier gas flow may also have their influences.

I have been manipulating the parameter but keeping the same gas ratios. It seem the system will not allow me to go over a 40 flow on the O2 and this is limiting me on being able to get higher velocities and keep the same ratio. I have tried high pressure cylinders and liquid dewers banked together and still cannot get over a 40 O2 flow @ 200psi. With this limitation I have just tried various feed rates with the highest flows(keeping ratios) that I can get. My next step is to see if there is a flow control setting that is causing the problem. Thank you for your input Gordon.


Thank you,

Erick

[Gordon]

Hi Erick,

I take it that your metallographic evaluation is showing too much oxide. I would also consider trying a different powder brand or powder with a tighter particle size distribution. Powder with excessive fines certainly does not help and even batch to batch differences can be significant. You can only go so far with gun parameter changes while keeping all aspects of coating quality in check and that usually is not a lot . Powder changes on the other hand can make a significant difference.

I will be interested how you get on. Good luck

[erick212]

I am trying to make this coating with a plasma cut powder, it is what the customer has requested. I do agree with you on the powder size and lot, I have seen a big difference from lot to lot...... mostly from one powder source though. I have quite a few coupons in he lab right now and waiting on result. I will let you know if there are any major changes.


thanks

Erick

[erick212]

Well the results are back. Nothing to write home about but, I think we made the oxide requirement. It was a mix of cranking up the velocity and feed rate adjustments. Also if anyone does not know you can replace air with nitrogen and run it through the gun without any adverse effects.

Erick

[Gordon]

Hi Erick

Thanks for feedback, it sounds like you are moving in the right direction. I did try using nitrogen in place of air many years ago in an attempt to generate some form of shroud, but seem to remember the effect was somewhat less than I was expecting. Certainly, doing the same type of thing with arc spray makes a big difference.

[erick212]

Yes, I did use the nitrogen in place of the air. The sample came out with almost the same oxide content..