Current or power more influential? Spray rate?

[fhyuan]

Gordon:

1. For plasma spray parameters current and power(current * voltage ) , which one is more influential?

2. How to estimate deposition rate (micron/pass)? What is the definition of per pass?

thanks

[Gordon]

Hi Fhyuan

Quote:1. For plasma spray parameters current and power(current * voltage ) , which one is more influential?

As far as power output of the gun is concerned current and voltage are equally influential. The influence on the properties of the plasma are more complicated. Voltage across the plasma is determined by the plasma gas flow and the chemical make-up. With an argon or nitrogen primary gas only plasma, altering current is our main way of adjusting plasma power. When adding a secondary gas like hydrogen, we can drastically alter the voltage of the plasma and thus the power output of the plasma. So altering the chemical make-up and gas flows of the plasma can have more influence on plasma properties than just altering current or power output alone. See http://www.gordonengland.co.uk/sef/volta...t-257.html
http://www.gordonengland.co.uk/sef/colou...tml#pid817
http://www.gordonengland.co.uk/pft.htm

Quote:What is the definition of per pass?

The normal definition of a ?pass? in thermal spraying is one cycle or traverse of the gun across the coating area. For example one traverse of the gun along the length of a rotating part or one complete ladder scan of a non-rotating part. Confusion I suppose could arise from the fact that some overlapping can occur with this definition. For an extreme example the ?single pass method? where the total coating thickness is applied in one single traverse across a rotating part, where rotation speed is very high and traverse speed is very low. It could be said that this is not in fact a single pass, but a multitude of single passes (one per revolution) very slightly offset .

Quote:How to estimate deposition rate (micron/pass)?

More info here
http://www.gordonengland.co.uk/sef/surfa...-t-69.html
http://www.gordonengland.co.uk/sef/wc-co...t-226.html

Quote:Hi Fergus, Howied and all,

Fergus Wrote:Hi

Setting up speeds and feeds this way sounds logical, but where do we start . Is there a way of working this out or a formula to give us a starting point.

thanks

Fergus

I think most would start from a point from past experience, but you can theoretically calculate this using information supplied by powder/equipment supplier/manufacturer.

This only covers rotating cylindrical work pieces. For other types you will need to modify method.

1. Calculate coating area (3.14DL)
2. Use COVERAGE (sqft/hr/0.001" or sqm/hr/0.1mm) found hopefully in powder/equipment bulletin/manuals for your particular coating to calculate SPRAY TIME
3. Divide desired coating thickness by the thickness per pass (for carbides and ceramics I would aim for ~ 0.0002" or 5 micrometres per pass) to get number of passes required
4. Part length x No. of passes divided by the spray time will give you TRAVERSE SPEED
5. Traverse speed divided by traverse distance per revolution (this distance should be slightly smaller than your spray footprint) to get rotational frequency RPM.

The theoretical calculations do not always work exactly in practise, but should give a good starting point for fine tuning.

Example: 6" diameter x 12" long cylinder coated with Metco 73F-NS-2 (P73F-10 spec) to 0.10" thick.
1. 3.14 x 6 x 12 = 226 sqin or 1.57 sqft coating area
2. Coverage from Metco 73F-NS-2 tech bulletin says 135 sqft/hr for 0.001" coating thickness.
for 0.010" coating thickness thats 13,5 sqft/hr
for 1.57 sqft thats (1.57/13.5) gives us 0.116 hrs or 7 minutes spray time.
3. 0.010/0.0002 = 50 passes
4. 12" x 50/7 mins = 86"/min or 7.17 ft/min traverse speed
5. Lets say 0.16" traverse per rev 86/0.16 = 537 RPM
6. So that gives us a starting point of 537 RPM (850 SFPM) and a traverse rate of 86"/min using 50 passes and spray time around 7 minutes.

There is a point particularly with high spray rate/high deposit efficient materials on smaller diameters where the part revolution frequency RPM becomes difficult to manage. We can only compromise in these situations, by excepting higher deposits per pass or reducing spray rates. In many situations carbide and ceramic coatings are applied much thicker per pass than I personally would like, but have proved adequate for purpose (I just know they are not as good as they could be ).

Beware my maths may not be that good