Flame Sprayed Tungsten Carbide coating

[Yehah]

I have an application in which we used a flame sprayed tungsten carbide coating on a rod under a reciprocating o-ring seal. The coating was polished to a 16 rms (or better) finish. Now, there is a leak past the o-ring. The fluid is a water based hydraulic fluid. However, it also leaks using nitrogen. All our testing seems to indicate that the leaking is occuring through a porosity in the coating and not past the o-ring seal surface. Has anyone ever heard of sprayed and polished tungsten carbide exibiting this kind of porosity?

 

[TVP]

The following is from ASM HANDBOOK Volume 5 Surface Engineering:

Thermal spray coatings consist of many layers of thin, overlapping, essentially lamellar particles, frequently called splats. Generally, the higher particle-velocity coating processes produce the densest and beter bonded coatings, both cohesively (splat-to-splat) and adhesively )coating-to-substrate). Metallographically estimated porosities for detonation gun coatings and some HVOF (High Velocity Oxy Fuel) coatings are less than 2%, whereas most plasma sprayed coating porosities are in the range of 5 to 15%. The porosities of flame sprayed coatings may exceed 15 %.

So, it is very well known that thermal spray coatings can exhibit substantial porosity. You should consult with your vendor to determine the following:

a) how much porosity is present?
b) how much porosity is tolerable?
c) can they produce the coating to that requirement?

 

[boo1]

I have used HVOF tungsten carbide Metco powders on hundreds of shaft overhauls with zero defects. Recommend changing to the HVOF process (see post above) to decrease the porosity. Proper preparation of the shaft is as important to sealing performance as the selection of the seal itself. Conditions that affect the seal are the shaft configuration, tolerance, hardness, materials, speed and eccentricity.


Recommended shaft finish is 10-20 micro-inches Ra (arithmetic average), plunge ground , with a machine lead angle of zero ±3 minutes and a burr-free chamfer or radius. The chamfer acts as a ramp for the sealing lip to ride up on. Without it, it is virtually impossible to install the seal without damaging the lip.

I hope you find this usefull! Cheers