Cracking of WC-Co-Cr coatings

[Graham Bulman]

I have a problem with cracks appearing in WC-Co-Cr coatings applied by HVOF. What are the process parameters that should be adjusted to alleviate this problem?

[Gordon]

Hi Graham

Welcome to the Surface Engineering Forum.

Making assumptions that equipment and powder manufacturer/supplier recommended parameters are being followed. Coating cracks are likely to be related to poor management of temperature control, gun/part speeds and feeds or coating thickness limitation. If the cracks are appearing after finishing, then grinding may also be at fault. If you are not using recommended parameters/powders, then there could be other problems, which we would need more info on to comment.

Ideally, part temperature should remain constant from start to finish of spraying (easier said than done). Carbide coatings should be applied at a build up rate of around 0.0002" (5 micrometres) per pass by adjusting speeds and feeds. Make sure you are well within the recommended thickness limitation for your coating.

Hope this helps.

[Vladimir]

Hi Graham,

to my experience with HVOF WC-Co-Cr coatings, cracking comes when
1) the base material has the own stress f.ex. casting or forget part without annealing treatment for reduction of inside stress
2) you spray the inside cylindric surface
3) the border of coating is sudden

the other problem is when the coating hasn?t the adhesion and fall off. This comes when
1) base material is to hard
2) on the surface is rest of old coating
3) the thickness is to hight
4) during the process comes to overheating

Vladimir