Flame Sprayed Tungsten Carbide coating
Flame Sprayed Tungsten Carbide coating
(OP)
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?





RE: Flame Sprayed Tungsten Carbide coating
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?
RE: Flame Sprayed Tungsten Carbide coating
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.
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