hvof & bonding & amorphous coating

[hustzc]

Dear all;

i HVOF sprayed amorphous coatings on mild steel surface, and then tested the bonding strength of the coating/sub system according to ASTM C633, found the BS is only in the range of 30-40 MPa. (the coating thickness is ranged from 300-600μm, and it seems good bond with the substrate, and the porosity is below 1%)

1) why the BS is so low ?

2) when i exam the cross-section of the fractured sample, i found a layer amorphous coatings with thickness of around 1-10μm on the sub. surface, why ??

[djewell]

I had some discussions with Liquid Metal about amorphous coatings several years ago. The powder must be kept relatively cool during spraying or it will crystallize on solidification. Since your particles were molten when they hit your substrate, they have a "weak" bond. Colder (solid) particles would embed themselves in the surface and have a stronger bond.

[Simon Kim]

I had also applicated Armacor M to medium carbon steel 17 years ago.
The HVOF sprayed layer was not amorphous, just microcrystalline structure.
The amorphous structure reached by arc spray, because arc has more bigger temperature gradient.
And I guess the bonding strength is related with physical bonding at the interlayer.
So thermal fused bond, surface area as blasting condition, thermal expansion etc. are all related in my opinion.
If the bonding strength is bigger than shear stress during working, it should be safe.
My tip is coating layer thickness, thinner is stronger than thicker case under the shear stress condition.

I wish your success.

Simon

[hustzc]

Thanks for reply.

First of all, i did not understand why molten particles corresponding to "weak" bond ? lots of solid particle always means relatively large porosity due to poor splat structure in my opt.

Secondly,

Do you have any suggestions to keep the powders are solid or even improve the bonding strength ?
Anyway, the HVOF WC-Co coating owns a bonding strength of as high as 70MPa. 30-40 MPa is really low as compared.

Best Regards

I had some discussions with Liquid Metal about amorphous coatings several years ago. The powder must be kept relatively cool during spraying or it will crystallize on solidification. Since your particles were molten when they hit your substrate, they have a "weak" bond. Colder (solid) particles would embed themselves in the surface and have a stronger bond.

---

Thanks, Kim,

In my case, the coating is actually almost amorphous by XRD and TEM using HVOF spraying. Maybe the bonding strength of 30-40 MPa is more than enough in many cases, however, i need to improve it to more than 50 MPa, or even higher according to the demands.

So, any method or suggestions to give me?

Best Regards

I had also applicated Armacor M to medium carbon steel 17 years ago.
The HVOF sprayed layer was not amorphous, just microcrystalline structure.
The amorphous structure reached by arc spray, because arc has more bigger temperature gradient.
And I guess the bonding strength is related with physical bonding at the interlayer.
So thermal fused bond, surface area as blasting condition, thermal expansion etc. are all related in my opinion.
If the bonding strength is bigger than shear stress during working, it should be safe.
My tip is coating layer thickness, thinner is stronger than thicker case under the shear stress condition.

I wish your success.

Simon

[sreenuvundela]

Dear Freinds

Very nice explanation of Mr. Kim and i totally agree with your comments.

Higher coating thickness, Poor surface preparation, liquid stage of Powder particles (During rapid solidification, lot of internal stresses takes place and these stresses acts against bond strength) are causes for poor bond strength.

Regards

SREENIVAS

[musheer]

Hi All,

we are planning to buy a HVOF system, preferably JP 5000 of Praxair. before coming to a decision i want to do the cost analysis.
1. area of deposition per hr
2. operating life of JP 5000 gun spares ( mean to say, how often do we need to replace parts )
3. surface preperation cost & time consumed for all these processes.

where can i get a list of gun spares with their operating life, i want to estimate the operating cost of just a JP 5000 gun.

Thanks,
Musheer