Surface prep issues before APS coating

[msiranni]

I am trying to grit blast a part to prepare it to be APS coated. When I am finished grit blasting, I check the Rz of the part - it is in spec. However, when I coat the part, the coating sometimes does not adhere. It appears that although the Rz is correct, the surface does not have the correct roughness for the coating to stick. I am using 80 grit aluminum oxide to grit blast, but it appears to break down very quick. Does anyone have any suggestions?

[MichiganMan]

What is the specified roughness? It is possible that you'll have to inform the customer that you need to get more roughness than specified.

It may also be an issue of surface contamination. Have you tested the surfaces for residual oils or greases? Small particles of aluminum oxide can sometimes lodge in the surface of softer metals. Have you ruled this out as a possible issue? You mention the aluminum oxide material seems to be breaking down quickly. Is it possible that residual dust from the oxide is resting on the surface and preventing proper adhesion?

When you say "the coating sometimes does not adhere." are you indicating that there are localized problems on a single piece, or that you are getting proper adhesion on some pieces but not others? It is a minor distinction, but a meaningful one.

If the roughness is in the typical range of what you'd expect for your process, then I would expect this to be a contamination issue of some sort.

[msiranni]

The specified roughness is less than 25 um. We have looked at the surface for residue, small particles of aluminum oxide, etc. We have noticed that when the coating does not adhere (and it is almost always in the same place - in a localized, flat area), that the surface resembles more like golf ball dimples than jagged edges - the Rz is the same in either case, but with the golf ball dimples, the coating doesn't adhere. I am curious if changing grit sizes will have an impact?

[derek]

The specified Rz is less than 25 um, what is your measurement? If your measurement is 20- 24 um will be good for your coating, if it's 10-15 um will be bad for your coating. In aircraft industry, people use 60 grit to do pre-coating preparation, in IGT industry people use 36 grit. Why don't you try the 60 grit to see what happens? Also make sure the part is clean before you blast the part. in another way, maybe you can change the spray pattern will help.

[MichiganMan]

The rounded dimples are no good. That jagged "anchor tooth" profile is always more coducive to good adhesion. It is unusual for aluminum oxide to round off rather than shatter into small pieces and remain jagged. You may have a lower purity oxide that would have a greater tendency to deform.

A few little tricks that may solve the problem would be:

1) If you've been reusing abrasive, then try some virgin material then view under magnification to see if you are getting better (more jagged) profile.

2) Depending on the geometry of the part this area may see an unusal amount of riccochet. If you think that this may be the case try blasting that area of the part last and see if that remedies the problem.

3) Coarser abrasive may help, but may also push you over the roughness limit. I'd also say that derek is accurate in saying that you need to be as close to the 25 um as you can.

[Met.Eng.]

80 is too fine for this.

[Justin]

Agree with Derek's opinion. You may try to use 60 grit. By the way, do you coat the part instantly after blasting it?