crack in WC/Co

[Panf]

Hi,Forum. Hi, Gordon.

I want continue WC/Co thread.
We make this coating on plungers. We have HVOF GTV-K2 equipment and WC/Co 88/12 powder (-53+15mkm). And we don't have problems with coating thickness 0,5mm or less. But coating crack when thickness 1,5mm.
Before spraying surface was preparation by cuting tool (turning machine), gritblasting and preheating to 100 C. Thickness per pass was 5...10 mkm.
Temperature during spraing was 110...140 C.

Question: in principle can I make WC/Co coating with thickness 1,5mm and more?

Thanks and sorry for my English

[Gordon]

Hi Panf

In my experience most HVOF WC/12Co (tungsten carbide/12% cobalt) coatings are reliable up to about 0.5 mm thick, but are somewhat unpredictable above this thickness. Your equipment, powder and parameters are also critical factors. HVOF WC/17Co coatings I have seen in excess of 2.5 mm thickness.

If your coating thickness is mainly for dimensional reasons rather than providing a large wear limit, consider undercoating a thinner WC/12Co coating by using a low stress coating like NiAl (nickel aluminium) to provide the bulk thickness.

[Panf]

Hi Gordon
Why my equipment and parameters is critical factors?
and whose the powder NiAl you recommended for HVOF spraying?

[Gordon]

Hi Panf

Brand X and brand Y of a WC/12Co may not give you the same quality of coating or thickness limitation using your particular equipment and parameters. WC/12Co can vary in particle size distribution and also in form (cast and crush, sintered and crushed, agglomerated and sintered). Using say a Sulzer Metco Diamalloy 2004 powder (primarily designed for use with the Diamond Jet HVOF equipment) with say TAFA JP5000 HVOF will probably give different coating qualities than if sprayed using the Diamond Jet. Generally powders are tailored to suit a particular HVOF set-up and parameter set. If you do not have the facilities and expertise to do your own coating development/optimisation, then I would suggest that you initially use the powder and parameters recommended by your equipment supplier.

A nickel 5% aluminium alloy powder suited to your equipment. I know some use arc spray coatings of NiAl and 13%Cr steel for heavy back-up coatings.

[Panf]

Dear Gordon
Thanks for you reply

Can I ask few qestions once more?
1. Some times we make heavy back-up coatings by wire flame spray. But in this work we can't do it. WFS coatings don't have sufficient adhesion.
2. NiAl alloy must be clad or gasatomized and his must be sprayed by HVOF or APS?
3. Maybe use NiCr coatings for precoat?

Thank you

[Gordon]

Hi Panf

I would go for a good quality gas atomised nickel ~5% aluminium alloy powder for HVOF spraying. NiCr based alloy coatings will tend to be more stressed and have lower bond strength than NiAl. NiCr could be a good choice where a good corrosion resistant base coat is needed and the total coating combination thickness is less than about 1.5 mm.

[hvofhamid]

hi every body, sorry for my bad english .i made last week a test on hvof thikness carbide .i success to spray more than 2 mm on 76 diameter without pre-heat ,and i haven t got any problem.next week I ll confirm the test on 100 diameter.

[hvofhamid]

HI EVERYONE HERE
the test was a sucess. now i can tell you how you can coat more than you are used to do
first you determine the critical point of temperature;i mean when your WC/Co crack.once you do it ,you never depass it .you coat for exemple 6 pass then you let the substrat temperature coming down in order to stop dilatation and recupear the mechanical carasterics,after you do the same for the other passes till you get the thikness you desire.have nice coat

[fhyuan]

The cracks you mentioned are macrocracks, right?