Removal of grit after grit-blasting?

[pulaunias]

Hi Dear members,

I came across an issue and will appreciate if any of you can comment on that.

After we grit blasted a metal specimen using alumina, a small number of grits were found to be embeded at the surface (as when we clean this specimen in a sonication bath, we saw them coming off). I guess this should eventually decrease the coating-substrate bonding strength.

So my question is, do you typically eliminate these embedded grits in your operation as an attempt to increase the bonding strength? I imagine that this can be practical for small parts, but might be difficult for larger ones.

Moeover, if we really do this, say by sonication in some solvent (water? ethanol? air-blasting?), are we runing the risk of introducing new "contaminants"?

thanks.

[LEN WOOD]

Interface contamination by blasting media is an issue and the degree of which surmounts to coating failure and (hopefully prior to this) Laboratory/QA rejection. The reason is that bonding is most definitely compromised. The phenomenon varies according to substrate material, finishing and geometries. Obviously a flat test piece is not always indicative of the actual intended target substrate or component.

While I have seen a variety of methods employed to remove some of the embedded particles, care should be taken. I know of a few applications where post-blast media removal is essential to achieving the optimal bond. However, IDEALLY the only operation between media blast preparation and TS process should be clean air (or suitable gas) blast.

You should look for ways to minimise media entrapment. Send me an email (len@metal-monster.com) and I can help you with this.

[Gordon]

Hi Pulaunias

Generally agree with Len's comments.

Quote:After we grit blasted a metal specimen using alumina, a small number of grits were found to be embeded at the surface (as when we clean this specimen in a sonication bath, we saw them coming off). I guess this should eventually decrease the coating-substrate bonding strength.

Retention of some contaminants from grit blasting is a fact of life. Even post treatments will only remove the weakly attached stuff. "a small number of grits were found to be embedded at the surface" could in fact be interpreted as a good situation (emphasis on small number) To give some idea of quantity, many aerospace coating specs call for limits at around 25% contamination as measured on interface cross-section. Increasing amounts of contaminants, particularly those that are poorly bonded will obviously have negative effects on coating-substrate bonding strength, so efforts to minimise this is important.

Quote:So my question is, do you typically eliminate these embedded grits in your operation as an attempt to increase the bonding strength? I imagine that this can be practical for small parts, but might be difficult for larger ones.

I would rephrase to minimise, you will not total eliminate. As len said "IDEALLY the only operation between media blast preparation and TS process should be clean air (or suitable gas) blast." I have come across people using ultra-sonic cleaning and brushing post treatments, but this is usually in extreme cases and then I'm not totally sure that it had any significant effect on bond strength anyway.

Quote:Moeover, if we really do this, say by sonication in some solvent (water? ethanol? air-blasting?), are we runing the risk of introducing new "contaminants"?

Grit blasting not only roughens the surface, but also "activates" the surface. Hence, the reason why we recommend coating as soon after blasting as possible. The surface becomes more reactive, so time and environment are important factors in retaining this ideal condition for bonding coatings. So, to answer your question, yes.

Before thinking of post treatments, make efforts to tune your grit blasting to give the very best conditions possible.

[pulaunias]

Thank you Len. I will contact you.

Gordon, thanks a lot too.

[MarcelvW]

Dear All,
As mentioned earlier, gritblasting is first of all used for "activating" (meaning taking off the natural oxide layer of a metal) and secondly for a certain roughness (more surface area). Blast angle and airpressure has to most influence on embedded particles. So, too high air-pressures (depending of the hardness of the surface) and angles between 80 and 100 degrees should be avoided.
After gritblasting a part has to be thermal sprayed within a certain time to avoid the natural oxide layer "growing" back again (Co/Ni alloys within 4 hours, Fe alloys within 2 hours, Ti-alloys within 1 hour, Mg/Al alloys within half an hour).
Indeed a certain contamination by gritparticles is almost unavoidable, but does not need to be a problem as long as you meet the tensile bond strength values.

[Gordon]

Hi MarcelvW

to the Surface Engineering Forum.

Thank you for your input.

[derek]

I did some DOE on pink, white and brown grit. The reault indicated that the pink is the best one, but the most expensive. The brown is the worst but the cheapest.

[k09]

it is indeed very difficult to clean large areas with entrapped blasting medium and most often, cleaning it with only compressed air is not possible.

Last time we had this problem during coating of large rolls. We were able to clean it with light brushing with a SS-bristle brush with simultaneous application of compressed air. We were able to get the grits out faster with this. In this case the removal of grits was important because the coating itself was just 80 microns thick.

Regards

K09

[loriolo]

It is not recommendable to apply any strange chemical or meccanical product on the grit blasted surface, because this can decrease the bond strength, because all chemical products can oxide the surface, and brishing cat alter the grit blasted surface preparation, so only blowing with clean and dry air, or in some special cases with nitrogen. If there is too much embedded grains, that do not came out blowing with air, change the blasting angle or the grit blasting material, I had 25 years of experience setting aero industry coatings.
Regards

[Kopi]

I have only two fast comment for this topic:

- the grit quality must be controlled
- try to use shallow angle ( roughly 30°- 40° )
- you must not use short blasting distance ( 150 - 200 mm is the best )


I think if you follow the upper thing you will not have any problem with the surface.

Sincerly

[jaydev choudhary]

Before selecting particular grit for any substrate all the physical properties of the substrate material and applying grit must be known carefully, if substrate material is too soft then must use balls in form of grit eg. glass beads so that it would not embed in substrate material.

usually tensile strength of substrate must be more than used grit for avoiding this problem.

(04-17-2010 03:24 PM)pulaunias Wrote:  Hi Dear members,

I came across an issue and will appreciate if any of you can comment on that.

After we grit blasted a metal specimen using alumina, a small number of grits were found to be embeded at the surface (as when we clean this specimen in a sonication bath, we saw them coming off). I guess this should eventually decrease the coating-substrate bonding strength.

So my question is, do you typically eliminate these embedded grits in your operation as an attempt to increase the bonding strength? I imagine that this can be practical for small parts, but might be difficult for larger ones.

Moeover, if we really do this, say by sonication in some solvent (water? ethanol? air-blasting?), are we runing the risk of introducing new "contaminants"?

thanks.

[Gordon]

Quote:if substrate material is too soft then must use balls in form of grit eg. glass beads so that it would not embed in substrate material.

While bead/shot blasting may help with reducing interface contamination, it will not provide a suitable texture for good bonding.