Hi Alexangel
Could write a book trying to answer these questions
briefly though:
Quote:1. What would be the main causes towards porosity over max condition?
Basically this down to the condition of the particles on impacting the substrate. Too cool or too slow and particles don't splat well and thus don't fill or flow into all the spaces and then causing another effect called shadowing. Imagine particle flight as a beam of light, poorly deformed particles will cast shadows. These shadowed areas become difficult to fill (remember thermal spray is a line of sight process) leaving voids. The angle at which the particles strike the substrate is also important. The shallower the angle the greater the shadowing effect. This image shows the effect: click to enlarge
If this is your problem you should be able to recognize it by the lay of the splats, they start to align themselves roughly perpendicular to the line of spray
.
Some powders contain hollow spherical particles which naturally tends to increase porosity and helps with deposit efficiency. This should not really be a factor here unless the powder has been changed.
Shrinkage of particles on cooling naturally contribute towards porosity and cracking. Again this should not be significant factor here.
Quote:2. What is the parameter that contribute to this? Theoretically Increasing of 1st gas or decreasing of 2nd Gas, Increasing of current? and particle size and increasing of spray distance will contribute to it.. but what is the mechanism of porosity formed during the spraying?
I think I answered that above.
- Heating of particles and velocity
- Secondary material entering coating
- Spray distance
- powder characteristics, feedrate and injection
- Spray angle
Quote:3. If all my parameter is established and constant, what can a defective equipment contribute to this ?
Well this could almost be anything. Just make sure equipment is serviced and calibrated. Regularly check for leaks particularly powder feeder. Check hardware condition like nozzle/electrode, powder port (also alignment) and hoses. Check powder condition.
Quote:4. What could a sprayer defective practices that contribute to this ?
This unfortunately is probably your highest risk area. Apart from not setting correct parameters say like spray distance or using the wrong nozzle etc. etc... an unobservant operator can miss things like pulsating powder feeding, pressure drops in gas supplies, fouled powder ports, gun moving from mounted position etc... Really any change from the ideal set-up is capable of effecting coating quality overall not just porosity.
Finding the cause to your problem will probably be very difficult. Look at your microstructures again to see if they hold any other clues, like splat angle as in the case of iincorrect spray angle as discuss above. Are unmelted particles higher or lower than usual? Is porosity evenly distributed through out coating? Is porosity large or small? etc..
Finally I must mention metallographic preparation. I doubt this is a problem in your case, but I have come across it so many times. Company sprays TBC just like yours, gets outside metlab to evaluate coating metallography. Metlab reports 30-50% porosity (should be 5-15%). Company revises spraying parameters. Metlab reports no improvement. I regrind and polish same mounted specimens and report less than 5% porosity (too dense). Take care!