Alternatives to grit blasting?

[durkie]

Hello folks...I am having a HVOF WC or ceramic (probably some mixture of aluminum oxide/chromium oxide/titanium oxide) applied to a knurled aluminum surface. The aluminum is knurled to act as a grip surface, and the coating is applied to reduce wear of that surface (believe me, I've definitely tried using the coating by itself on smooth aluminum as the grip surface, just can not get it to work). Knurl peaks are on the order of .016" , and the coating is around .003" - .005".

The problem I'm having is that the grit blasting process destroys my knurl peaks. The company that I'm working with for my product claims that they are using a low pressure and only doing as much blasting as necessary.

What I want to know is: are there any alternatives here? Could I blast the surface prior to knurling? Could they blast at an angle instead of perpendicular to the piece in order not to directly blast my knurl peaks? Could they forgo blasting completely, since the surface already has an aggressive texture to it? Would spraying an intermediate Ni (maybe NiAl) layer and then the WC layer help anything? Could I chemically etch the aluminum oxide off the surface instead?

Basically, any way at all I can get around this?

Thanks

[Gordon]

Hi Durkie

to the Surface Engineering Forum.

The aluminum is knurled to act as a grip surface, and the coating is applied to reduce wear of that surface (believe me, I've definitely tried using the coating by itself on smooth aluminum as the grip surface, just can not get it to work). Knurl peaks are on the order of .016" , and the coating is around .003" - .005".

HVOF coatings will generally produce a relatively fine textured surface, which in many other applications is desirable. Have you tried using coarser particle size powder or powder product that lends itself to producing a rougher coating? Alternatively, spraying processes like arc or plasma would give much more latitude in achieving desired surface texture.

The problem I'm having is that the grit blasting process destroys my knurl peaks. The company that I'm working with for my product claims that they are using a low pressure and only doing as much blasting as necessary.

What I want to know is: are there any alternatives here? Could I blast the surface prior to knurling? Could they blast at an angle instead of perpendicular to the piece in order not to directly blast my knurl peaks? Could they forgo blasting completely, since the surface already has an aggressive texture to it? Would spraying an intermediate Ni (maybe NiAl) layer and then the WC layer help anything? Could I chemically etch the aluminum oxide off the surface instead?

Basically, any way at all I can get around this? Smile

Try blasting with finer grained blasting abrasive to produce a fine etched surface, but without destroying your knurled texture.

With thin HVOF WC based coatings you may find there is sufficient bond strength with clean soft aluminium without blasting,, so this is certainly worth exploring.

[durkie]

With thin HVOF WC based coatings you may find there is sufficient bond strength with clean soft aluminium without blasting,, so this is certainly worth exploring.

Thank you thank you...glad to be here. I'm wondering what exactly you would mean by clean - the aluminum is anodized prior to knurling, and there's not a whole lot I can do about that right now. Should I assume that the anodized layer is going to make things worse?

[Gordon]

Hi Durkie

Should I assume that the anodized layer is going to make things worse?

Yes. I think it would. I would experiment with your various options to see what works best in practice. Consider grit blasting before knurling.

I feel sure that your thermal sprayers could come up with a coating system that would work even without knurling.

What is your coating gripping against? A rough arc sprayed high chromium steel coating or similar may be a better choice.

[Johnson E]

A common grip coating applied at my shop has been delivered by plasma. We mix our own blend of macro crystalline Wc (two thirds by volume) to one third Ni/Al (again, by volume). Obviously, the exothermic constituent is the "binder", not only to the substrate, but to the carbide particles as well. Should work on aluminum (although getting a good bond to aluminum is notoriously difficult). I'll report back on the carbide particle size, as I'm not at work right now. I would most certainly eliminate the anodizing. Like Gordon suggested, a viable alternative might be an arc sprayed coating....say...95mxc or Ranotung. I'd be inclined to bond first with 95/5 though.
Hope this is of some help.

[Johnson E]

Particle size of Wc in my last post is -40/+140 mesh. I'd like to add that fused/crushed SiC in the same particle size has proven to be an economical alternative to Wc.

[djewell]

We regularly spray onto aluminum substrates without grit blasting using our AC-HVAF gun. The particles are not melted in the flame, and when they hit the surface of the part, they blast away whatever is there which is not strongly enough bonded to resist the impact. In this way, there is a continuous grit blasting effect on the surface and the coating. Ideally, you should remove the anodizing before starting to spray your WC so you can bond directly to the metal.

[durkie]

I feel sure that your thermal sprayers could come up with a coating system that would work even without knurling.

What is your coating gripping against? A rough arc sprayed high chromium steel coating or similar may be a better choice.

I'd love to try such a thing, but I think the shop I'm working with can only do WC, Ni, and some ceramics. If anyone knows of a shop that could do such a thing in the Atlanta, USA region, I'd like to hear about it.

But anyways, the surface is gripping against a polyurethane pad. Pretty much every grit level of the WC coatings has had poor performance, from 60 grit to 320. The only real difference has been in how much it shreds the urethane pad when it slips.

I can probably get them to mix in some Ni powder with the WC when spraying, but since they haven't really had any ideas about how to improve the grip of the coating, I've taken it upon myself to try to improve the grip of the substrate.

Any ideas?

[Gordon]

Hi Durkie

I would be incline to find someone who can arc spray rough textured coatings. I'm hoping some of our USA members could make suggestions on who to try.

Though HVOF/HVAF are pretty much the "state of the art" thermal spray processes, they don't really lend themselves to producing grip coatings. If you want to continue with WC based coatings, I would follow Johnson E suggestion using plasma.

Failing the above, continue experimenting with substrate surface profile, which may be good anyway in addition to the above coatings.