Tungsten carbide coating problem
05-19-2008, 01:11 PM, (This post was last modified: 05-19-2008, 01:13 PM by ycwbycwb.)
Tungsten carbide coating problem
Dear All,

Metco 73F-NS powder was plasma sprayed with 9MB. Two coating samples were checked with metallography. I had expect that coating 2 would be harder than coating 1 but the result wasn't that. I attach them here. Any advice would be appreciated. Thanks.

Coating 1

(link dead)

Coating 2

(link dead)
05-19-2008, 03:22 PM,
RE: Tungsten carbide coating problem
Hi ycwbycwb

My choice of coating would be No.2 even though apparent porosity appears higher, the content of primary WC is much higher. The first coating appears to have been sprayed with parameters leading to particle overheating, leading to oxidation/dissolving of WC forming a harder/brittle matrix. I would not be surprised to find that the first coating is the harder. Hardness values on WC/Co type coatings in itself is not a good indicator of wear properties.
05-19-2008, 03:39 PM,
RE: Tungsten carbide coating problem
Hi, Gordon,

Thanks for your comment.
Coating 1 has hardness about HV300 850 but coating 2 about 550. The hardness difference is bigger than I thought. Coating 2 seems to be more brittle than coating 1 during Vickers hardness test.
More preserved WC should result in acceptable hardness at least such as HV300 750. But what's wrong with this coating? (The hardness result was double checked.)

05-19-2008, 04:05 PM,
RE: Tungsten carbide coating problem
Hi ycwbycwb

The hardness does seem to be very low. I would expect above 750, more like 850-950 at top end. The second coating does look a little on the porous side. My experience is that using the Metco recommended parameters should give you better coating qualities than what you are getting. What parameters are you using?
05-20-2008, 01:13 AM, (This post was last modified: 05-20-2008, 01:13 AM by ycwbycwb.)
RE: Tungsten carbide coating problem
Hi, Gordon,

Coating 1 Coating 2
Ar: 140scfh 180scfh
H2: 7 6
Voltage: 65V 67V
Current: 450A 450A
Powder feed rate: 22g/min 22g/min
Powder gas pressure: 3.5bar 3.5bar
Powder gas flow: 9.5 9.5
Spray Distance: 60mm 60mm

05-20-2008, 02:53 AM,
RE: Tungsten carbide coating problem
Hi ycwbycwb

The only Metco parameters I have for 73F-NS-1 or 2 with Ar/H2 were standard may be 10 or more years ago for 7/9MB, so they may have been revised.

GE/731A Nozzle
#2 powder port
Ar 160 FMR @ 100 psi (6.9 bar)
H2 80% (low capacity flow tube) @ 50psi (3.4 bar)
400A @ 50 - 55V
50 FMR carrier gas flow
11lb/hr (5 kg/hr) powder feed rate
3" (75 mm) spray distance

Ar 160 FMR @100 psi should roughly be 180 SCFH
H2 flow I have no conversion info, but going by voltage your flows appear higher (assuming you are using same nozzle and hardware).

These parameters should give coatings ~3% porosity and cross-sectional hardness 750 - 950 HV/0.3. If you need better coatings, you will need to use Ar only or Ar/He parameters.

You can certainly see the difference in coating structure corresponding to higher primary flow (higher velocity, lower dwell time) in coating 2. Big Grin

Hope that helps
05-20-2008, 06:41 AM,
RE: Tungsten carbide coating problem
Hi, Gordon,

Powder bulletin for 73F-NS is temporarily stored at here

I understand why the user manual of 9MB recommends Ar for 184 scfh.
But it also recommends 66-68V voltage. This puzzles me.

I should follow the powder bulletin or the 9MB user manual?

05-20-2008, 01:10 PM,
RE: Tungsten carbide coating problem
Hi Ycwbycwb

Your powder bulletin is much the same as my old technical bulletin dated 1981 and the same parameters as the 9MB gun manual dated 1988 Happy0193

The newer plasma control panels now have different gas flow metering giving direct readout in SCFH which complicates the use of old parameters, particularly secondary hydrogen gas flows. Also gun voltage I believe is now measured at the gun, while all the parameters mentioned above were intended for the older consoles that measured voltage at the console and took into consideration the voltage drop along 15' cables. So you would expect the newer system to actually run on a lower indicated voltage reading (~ 2 - 3V lower) than an older system running exactly the same parameters. This makes the parameter discrepancies even more puzzling Rolleyes

I would try both parameter sets and compare. Setting the hydrogen flow in SCFH for the technical bulletin parameters will be a bit tricky as the old low capacity flow meter reading is not so easily converted as the Ar (Ar ~ 180 SCFH equiv to 160FMR @ 100psi or 180FMR @ 75psi) See https://www.gordonengland.co.uk/conversion/pgcalc.htm. Though I don't like to set plasma parameters up by voltage, preferring to always set by plasma gas flows and use voltage as an indicator that the system is running ok., in this case not knowing H2 SCFH I would set voltage between 50 and 55V or try comparing at either extreme.

It would be interesting to know what Sulzer Metco think of these discrepancies in parameters Big Grin

Good luck
05-20-2008, 01:42 PM,
RE: Tungsten carbide coating problem
Hi, Gordon,

Thanks a lot for guiding me out of these puzzles.


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