HVOF COATING CRACKING

[mqas]

I am looking for any suggestions regarding possible causes of cracking on a WC Co Cr coating that has been applied by HVOF.

The coating is ~ 0.006" thick and is applied to a 17/4 (H1150M condition)seal ring which is then pressure tested in an assembly. After pressure testing numerous radial cracks were observed throughout the complete circumference of the seal ring. These were confirmed using dye penetrant testing.

The seal ring design has been confirmed to be acceptable using FEA analysis and I am working on 2 possible root causes.

1/ Base material problem i.e. elastic limit has been exceeded

2/ Coating - wrong type

Any thoughts, ideas or previous experiences welcome

 

[Rich2001]

Cooling and solidification of the WC Co Cr coating is accompanied by contraction or shrinkage. As the particles strike they are rapidly cooled and solidified. Hence, generating Tensile stresses within the particle and compressive stresses within the surface of the substrate. As the coating is built up, so are the tensile stresses in the coating. As the coating thickness increases a thickness will be reached where the tensile stresses will exceed that of the bond strength or cohesive strength and coating failure will occur.

Have you tried a thinner coating?


Liquid penedtrant - Dye Checking - is of limited use, natural coating porosity fogs flaw indications, this could be why cracks were not detected prior to Pressure testing. The pressure induced an increase in stress that expanded the cracks.

 

[Carburize]

I would suspect that the stresses developed during the pressure test cracked the coating - A question is, do you need the extreme hardness of the WC Co Cr coating. There are many other more ductile coatings available such as those from Praxair like LW45 developed specifically to resist explosive decompression by having a tough matrix.If a thinner coating as suggested by Rich2001 does not overcome the cracking you may need to consider a coating formulation change.