smooth as-sprayed carbide coatings

[durkie]

Hi folks....I'm after some information about how smooth (< 14microinch Ra, < .35micron Ra, > 320 grit) carbide coatings are produced as-sprayed, with no grinding. I'm trying to duplicate a WC/Co coating that I have that I am almost 100% certain was not ground. However, almost all thermal spray shops I talk to say that it's near impossible without a secondary grinding operation. What's going on? Would this require very specialized equipment or powder?

[Gordon]

Hi durkie

You will struggle to get much below about 100 microinch (2.5 micrometre) Ra for as-sprayed surface texture with HVOF carbide coatings and that's even with using the finer grade powders on very fine grit blasted or non-grit blasted substrates. 14 microinch Ra is practically not far off a ground finish.

[durkie]

i see. the coatings i can produce now are rated in terms of grit, and the smoothest one is nominally 320 grit, which I understood to be close to 14 microinch Ra. however i've only been able to come to that number from conversion tables i've found on the internet (and even those say Ra and grit don't exactly correspond). and if what you're saying is true about 14 microinch Ra being close to a ground finish, then I have to be off in the conversion there, because this surface is still quite rough.

[Gordon]

Hi durkie

The 320 grit reference is really an indication to the size of the grit in an abrasive product and the conversion to surface texture Ra is probably based on the finish obtained on steel using that product (not necessarily same as carbide coating).

I can only guess that someone has compared say the texture of a 320 grit silicon carbide grinding paper with that of a as-sprayed coating in a sight and feel sensory fashion. I would say that they are probably not too far apart , but texture of abrasive grit (or as-sprayed coating) is far different to the actual finish obtained on steel using that abrasive. Hope that makes sense

[durkie]

Yep, I think I got it. The confusing part was that the Ra of the grit -> Ra conversion doesn't actually refer to the abrasive itself (even though grit does), and that a rough surface can produce a finish smoother than itself (makes sense though).

Basically, my 320 grit coating will produce a 14 Ra steel surface finish, but the actual Ra of the coating proper is much higher.

Thanks!