Main parameters used for the powder flame spray process:
Why you want to decrease it and on which material you are working ? Please share your exact requirement in details.
Hi Lalit,
Thanks for your reply!
The powder material that I am using now is NiCrlAl-Bentonite.
The coating specification that I am using for this powder indicates that the tensile bond strength shall be 300-800 psi, but currently my coating’s tensile bond strength is at about 1100 psi.
The results of the tensile bond strength testing shows that it fails at the coating, indicating that the tensile bond strength of the coating itself is strong.
What's the micro-hardness of the coating ? Surface roughness of substrate ? And of machined surface ? Tell me the percentage porosity? Check the level of diffusion/metallurgical bonding at coating-substrate interface. Maintain the substrate temperature near to 250°C. Increase the traverse speed of gun. Optimize your spraying distance and spray above stoichiometric ratio i.e. excess oxygen.
The coating specification has only testing requirements for erosivity number and tensile bond strength. The surface roughness of substrate is about 100 μin Ra, which is well within specification range (80-150 μin Ra). I am unable to determine the coating's percentage porosity.
Is it that right now I can only 1) increase the traverse speed of gun, 2) optimize spraying distance, and 3) increase oxygen flow rate?
If the microhardness is very hard there will be no wear/abrasion. If porosity is high if will affect the cohesive bond strength(intersplats) i.e. lower bond strength inside the coating. Please check the microstructure of cross-section by SEM technique to ensure the type of bonding with the substrate, whether it is metallurgical or mechanical. You need mechanical bonding with substrate so diffusion or haz should not exceed 0.5 micron. You also need to control the substrate temperature. Yes you can vary these factors and can apply Design of experiments to optimize the process parameters.
The microstructure of the NiCrAl/Bentonite coating itself is porous. The bonding of the coating with the substrate is mechanical.
Currently the microstructure shows that there are more NiCrAl alloy particles than bentonite particles (clay-like particles), possibly indicating that the temperature of my flame is too high. My current oxygen-acetylene ratio is about 1.33. With regards to reduce the flame temperature, is adjusting the oxygen flow rate or the acetylene flow rate a better idea?
See, if you are spraying at stoichiometric ratio the combustion products will travel with high speed and maximum temperature. If you increase the oxygen level above stoichiometry it wll cause cooling of flame and increase the combustion chamber pressure and then powder particles will travel with high velocity. The result will be lower degradation of particles but reduced porosity. If you decrease the oxygen flow rate below stoichiometry then it will cause incomplete combustion flame is again cool, less degradation or metallurgcal changes but high porosity. You can change either oxygen or acetylene but I think you should start with stoichiometry and then gradual decrease the O/F ratio.
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