Reduce Porosity in Ceramic Coatings

[rik]

Dear Forum Members..

We have Metco 9MC system which we use for spraying various pump sleeves, shafts, etc.. We spray ceramics like Alumina and Chromium Oxides, mostly -45 +15 mesh for ground finish applications.

Recently we observed that the porosity in the coatings have increased slighlty. We have not yet seen any considerable effect on the coating life or performance however we would like to keep the parameters optimized. We have not made any changes to the hardware and we are currently using GH Nozzle.

The coating porosity observed were reported during visual inspection and so far no metallographic inspection have been carried out. I was informed that the coating porsoity is dependednt on the following parameters:

For Chrome Oxide
1. Spray Distance: Currently 2.3 to 3.5" (while spraying with hand)
2. Powder Feed: 35 to 40 gm/min
3. Carrier Flow: 14 scfh
4. Voltage: 68 to 70 (Hydrogen adjusted accordingly)
5. Argon: 80

Could someone suggest how we could vary the above parameters to reduce porosity.

Please also comment if there are any other factors that could affect the porosity of ceramic coatings.

Rik

[Joris Kraak]

Dear Forum Members..

We have Metco 9MC system which we use for spraying various pump sleeves, shafts, etc.. We spray ceramics like Alumina and Chromium Oxides, mostly -45 +15 mesh for ground finish applications.

Recently we observed that the porosity in the coatings have increased slighlty. We have not yet seen any considerable effect on the coating life or performance however we would like to keep the parameters optimized. We have not made any changes to the hardware and we are currently using GH Nozzle.

The coating porosity observed were reported during visual inspection and so far no metallographic inspection have been carried out. I was informed that the coating porsoity is dependednt on the following parameters:

For Chrome Oxide
1. Spray Distance: Currently 2.3 to 3.5" (while spraying with hand)
2. Powder Feed: 35 to 40 gm/min
3. Carrier Flow: 14 scfh
4. Voltage: 68 to 70 (Hydrogen adjusted accordingly)
5. Argon: 80

Could someone suggest how we could vary the above parameters to reduce porosity.

Please also comment if there are any other factors that could affect the porosity of ceramic coatings.

Rik

---

Rik,

By what method have you measured and concluded that the porosity is increasing?
- roughness measurement
- metallographic analysis
- ??

If you are so strict on keeping parameters optimized my first suggestion would be to stop spraying by hand. You can use a simple X-Ytraverse late to keep spray distance the same. Furthermore you are referring to Chromium Oxide -45 +15, I suggest to dig a little deeper and check if powder properties are the same D10 D50 D90, Hall flow, Density, etc..

I hope this helps

[loriolo]

There are many others factors, to check,: 1-Gun geometry, 2-Carrier gas pressure and flow, 3- Grains distribution in the powder (it can be within the specification, but with grains distribution changed, for example increased percent of grains near 15 micron)

[rik]

Hello Joris & Luis,

Thank you very much for your response.
The components are sprayed with hand and also with a 1-axis slide. However in both cases the superficial porosity is visible. After grinding the surface we could easily see patches of porosity scatteres over the coated surface. We are currently using TAFA CRO-167 and some times we also use METCO-106.

We always keep the gun perpendicular to the cylindrical surface. Carrier gas pressure is 50 Psi and 14 Scfh flow. If i wish to tweak the carrier parameters to reduce the porosity in the coating, could you give me a few guide lines. It would be very helpful if you guys could give me a check list that i could use to inspect our spray system and try and locate where the problem lies.

Do you think reducing the spray distance to 2" help in achieving more denser coating?

All your suggestions are highly appreciated.

Cheers!
Rik

[Joris Kraak]

Rik,

It seems like the particles are not fully molten and break off during grinding. That could be that the plasma is not hot enough.

Below a checklist:

1. When is the 9MC system calibrated for the last time? We have to make sure the flows are actual flows.

2. Make sure you set the correct parameters as in Metco 106F Bulletin.

3. Start plasma system and check voltage (is this same as in bulletin?)

4. If voltage is too low check nozzle and electrode the check voltage again.

5. If voltage is still too low replace insulator

6. If voltage is still too low replace cables (when have you changed those last time)

7. Finally you can also crank up the Hydrogen (secondary gas) but not too much +5 on flowmeter

8. If all parameters are the same as in bullentin stop the plasma.

9. Measure feed rate approximately 40 gr/min

10. Check if the powder port is still ok and powder is injected into the middle of the plasma.

11. Do you use cooling air? If yes do not blow in the plasma but +/-90 degree on workpiece

12. Start plasma, Start powder.

13. Check powder injection good in plasma (use welding cap) You can crank carrier gas flow up so material is blow through the plasma the turn carrier gas back until you see the powder injected properly in the plasma. Sometime carrier gas setting totally not correspond with bulletin.

14. Make sure workpiece is not above 120 degrees celcius.

15. Make sure cooling air is not interfering with plasma too much, a little bit is no problem.

If you follow this ther is a great chance the problem is solved.

Of course there are many other things to check, but from my experience this are the most frequent parameters which cause problems.

I wont think reducing spray distance would help, particles have lower dwell time.

some questions
- How much Hydrogen do you use?
- Can you send me a photo of the coating?
- Did you change grinding procedure or lubricant to grind?

[loriolo]

In order to reduce porosity you must use the air jet unit with the cross jets setted at 6", and the spray distance keep withing 2 1/5", and spray rate withing 38 gramms/minute (5 lb/h), the spray distance is very critical.
Look at flame geometry while the powder is flowing, for particles of powder that come out of the strim as well as for low carrier flow or to high, this is very difficulty to set for oldest powder feeders, but in the knew twing is possible to set carrier pressure whit accuracy, becouse you do not hace powder rate changing the carrier gas pressure or flow.