WC-Co16 coating problem enquiry

[howied]

Hi all,

Anyone here having any idea about RR?s coating application?

Anyway, I am spraying with METCO 73F-NS-2 to MSRR9507/1, which is WC-16Co powder.

I found that there is micro cracking among coating; I suspect that it is because deposit efficiency is 0.00075? per pass. May be too higher.

1, anyone can tell me the proper surface speed and traverse speed for this coating?
2, how to reduce micro cracking if you are facing this problem?
3, how to adjust to reduce deposit efficiency?

[Gordon]

Hi Howied

I take it that the amount of microcracking or shrinkage cracks in your coatings are deemed as unacceptable by RR standards. Microcracking is acceptable up to a certain level.

What (I assume plasma spray) process/parameter/coating type?

Deposit efficiency from a thermal spray point of view is really the %wt of your powder that ends up in your coating. I assume you mean deposit rate/thickness per pass/cycle of the spray gun.

Quote:1, anyone can tell me the proper surface speed and traverse speed for this coating?

My way of looking at surface speed and traverse rates on rotating parts:

Determine spray foot print width (probably ~4mm). Set traverse rate in conjunction with surface speed (via rpm, diameter dependant) to give desired deposit thickness per pass while traversing a spray foot print width minus say 20% per revolution.(minimum coating overlap, while maintaining even coating deposition with no barber pole effect). Note the traverse rate determines the deposit thickness per pass, while the rotational speed/surface speed influences the traverse distance travelled per revolution.

I think a deposit thickness per pass for this type of coating is best at ~ 0.0002"

Localised over heating of the coating due to insufficient cooling or too heavy deposit per pass or over heating powder particles during spraying (incorrect plasma parameters) causing oxidation/decomposition of WC primary particles and embrittlement of Co matrix are probable reasons for increased microcracking or macrocracking.

Quote:2, how to reduce micro cracking if you are facing this problem?

Make sure you are using recommended parameters, try to keep coating/substrate temperatures as constant as possible throughout spraying (low deposit thickness per pass will help here). Make sure your well within the recommended coating thickness limitation (~0.015" for this type of coating).

Quote:3, how to adjust to reduce deposit efficiency?

I think I covered that above if I understand correctly.

Hope that helps.

[fhyuan]

I also think it is right the thickness per pass of the spray gun is the deposit rate.

The deposit rate should be mostly dependent on traverse rate and the surface speed is set to resolve the barber pole mark and to achieve the improved microstructure of the coatings.

Gordon Wrote:you mean deposit rate/thickness per pass/cycle of the spray gun.

My way of looking at surface speed and traverse rates on rotating arts:

I think a deposit thickness per pass for this type of coating is best at ~ 0.0002"

I think I covered that above if I understand correctly.

Hope that helps.

[Gordon]

Hi fhyuan

to the Surface Engineering Forum.

Thanks for your comments, it made me realise i had not written part of my previous post very well:

Quote:My way of looking at surface speed and traverse rates on rotating parts:

1. Determine spray foot print width (probably ~4mm)
2. Set surface speed (via rpm, diameter dependant) to achieve desired thickness per pass.
3. Set traverse rate to give spray foot print width minus say 20% per revolution.(minimum coating overlap, while maintaining even coating deposition with no barber pole effect).

I will edit the post so it reads:

Quote:My way of looking at surface speed and traverse rates on rotating parts:

Determine spray foot print width (probably ~4mm). Set traverse rate in conjunction with surface speed (via rpm, diameter dependant) to give desired deposit thickness per pass while traversing a spray foot print width minus say 20% per revolution.(minimum coating overlap, while maintaining even coating deposition with no barber pole effect). Note the traverse rate determines the deposit thickness per pass, while the rotational speed/surface speed influences the traverse distance travelled per revolution.

(Admin note: This post does not appear in threaded mode ??)

[Fergus]

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

[fhyuan]

Fergus, I think Gordon has given the above answer for your question. First test to have the foot print width, then calculate the relationship between the traver rate and the surface speed (RPM).

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

[Gordon]

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

[howied]

Sorry,

the cracking i found should be Macro-cracking,as described by Gordon.
As the coating has lifted from the substrate adjacent to the
crack.Cracking reach to substrate in most situation.

Now i applied coating thickness to 0.009".I found there will be much less or none crackings should i spray to almost .004".

I preheat test piece to 100 Celsius degree and three aux. cooling jets pointing to test piece at 40Psi.Should i not preheat?
should i take away aux cooling jets?

How should i adjust to get rid of cracking?

[Gordon]

Hi Howied

I would only reduce preheat if you are using aluminium or magnesium based substrates and with these use indirect preheating (heat from back side and not the coating surface).

I would only reduce auxillary cooling if your coating/substrate temperature has a tendency to fall during spraying. Try to achieve a constant temperature throughout spraying.

The advice in this thread is equally applicable to macro-cracking. If you are having these problems with coatings at 0.009" thickness, then it sounds like something within your process is not right. Is your surface preparation and grit blasting as good as it could be? Are your plasma spray parameters as recommended? etc.