HVOF WCCo on Chilled Cast Iron(55HRC)

[tobjung]

Dear forum,

Recently we coated a calender roll which is made of chilled cast iron (Hardness of roll is 55HRC) with WC17Co using HVOF. 
The coating looked good but, when it is put into operation, it has random spot peel off (looks like localized bonding issue). Picture

Pretreatment: The roll was blasted by silicon carbide because it was very hard (Alumina oxide cannot blast it). 
We blasted it to 9 Ra but we are not really confident about the blasted texture profile on the substrate surface. 
After that, we sprayed 75B as a bond coat layer before spraying HVOF.

Now, the blasted texture profile of silicon carbide is our main suspect

Anybody has some suggestions ?

[Vadim Verlotski]

Dear forum,

Recently we coated a calender roll which is made of chilled cast iron (Hardness of roll is 55HRC) with WC17Co using HVOF. 
The coating looked good but, when it is put into operation, it has random spot peel off (looks like localized bonding issue). Picture

Pretreatment: The roll was blasted by silicon carbide because it was very hard (Alumina oxide cannot blast it). 
We blasted it to 9 Ra but we are not really confident about the blasted texture profile on the substrate surface. 
After that, we sprayed 75B as a bond coat layer before spraying HVOF.

Now, the blasted texture profile of silicon carbide is our main suspect

Anybody has some suggestions ?

If you had a problem with improper sandblasting of a part, then the coating would not peel off pointwise, but in large pieces. Such small (point) defects are usually associated not with poor surface preparation of the part, but with problems that arose during the spraying process itself. For example, a droplet of oil or water in the cooling air can cause localized degradation of adhesion. In addition, small coating defects occur due to the "spitting" of the burner during the spraying process. Such "spits" are visible as bright sparks in a flame.