Surface roughness after grit blasting

[Leanne]

Hi there all SEF members! Although I am new to this forum, I have been viewing some of the threads posted.

In need of some assistance please!

We are critically looking at our grit blasting procedure and the results obtained from several micros we have evaluated.

I would like to know - is there a specified surface roughness after grit blasting for thermal spraying applications?

Would appreciate your help with this one!

Thanks

[kschewe]

Its all different for different processes. Most oems use glass beads for hvof. The surface roughness would be very little. For other processes like plasma oems use grit mesh 16-60. I personally believe surface roughness has very little effect on bond strength.

[shantanu]

(06-20-2012 07:31 AM)Leanne Wrote:  Hi there all SEF members! Although I am new to this forum, I have been viewing some of the threads posted.

In need of some assistance please!

We are critically looking at our grit blasting procedure and the results obtained from several micros we have evaluated.

I would like to know - is there a specified surface roughness after grit blasting for thermal spraying applications?

Would appreciate your help with this one!

Thanks

in my opinion, the preferred abrasive for almost thermal spray applications would be Al2O3. Different mesh sizes of this grit under varied air pressures is used or varied profiles. Typically a 80 grade size aluminum oxide using 6 kg air pressure should give a surface Ra value of 12-15 microns

For spray coatings like gas or powder flame and ARC spray, I think a very well prepared surface profile is very essential as the particles being sprayed do not have the inherent velocity as in the case of HVOF and hence have a lower impact strength on the substrate.

Overall, however, surface preparation is the key to a successful thermal spray coating as far as I am concerned.

Best of luck

Hope it helps

Shantanu

Hope this helps

[sreenuvundela]

Freinds

I fully agree with the comments of Shantanu.

Regards

SREENIVAS

[MichiganMan]

Leanne,

Sometimes there is a specified surface roughness for a particular process, but as the others have stated sometimes there is not, and what you should concern yourself with is rather specific to the application. Media selection also tends to have an impact on results as well, particularly if the media you use has a tendency to embed in the substrate or leaves residual dust on the surface.

Generally speaking lower velocity sprays will benefit more (at least as far as adhesion is concerned) from greater surface profile, but angular profile is generally better than not across the board. If you are not experiencing the desired adhesion results, then posting application specifics may help the users here to make a suggestion for you.

[loriolo]

I agree with Shantanu, every process need a very accurate surface preparation method, and sand blast operation as well as other preparation method before sandblast, like surface cleaning and so on, can determine the failure of the coating if not corretly done.

[Leanne]

Thank you all for your input.

This seems like a relatively difficult question to ask....and it seems like most of the detail originates mostly from experience rather than specifications....?

If the surface roughness is application "sensitive", what would the surface roughness then be for an HVOF / Plasma / Arc sprayed coating?

We currently have a surface roughness range of between 4 to 6 Ra....? It is difficult to determine if we comply or not...

Your thoughts please !!!

[Gordon]

Hi Leanne

Quote:Hi there all SEF members! Smile Although I am new to this forum, I have been viewing some of the threads posted.

to the Surface Engineering Forum.

Quote:We are critically looking at our grit blasting procedure and the results obtained from several micros we have evaluated.

That is good, as has been said before surface preparation is the key to a successful thermal spray coating.

Quote:I would like to know - is there a specified surface roughness after grit blasting for thermal spraying applications?

If it has not been specified by the customer or by any specific specification your required to meet, then no, there is not a generic numeric roughness figure.

In practice roughness may vary from no grit-blasting to extremely rough depending on what is wanted from the specific coating and substrate. I know people like and are comforted having measurable numerical figures to work to and asses quality, but roughness alone is a very poor indicator of how well a coating will bond. Compare two surfaces blasted to the same Ra value; one shot blasted (smooth rounded profile) and one grit blasted (sharp angular profile), as different as chalk and cheese when it comes to bonding a thermal spray coating. Look at grit blasting as a means of activating the surface: increasing surface area, providing hooks for mechanical keying, cleaning and making surface more chemically/physically reactive. Increasing roughness does not necessarily increase exposed surface area nor bond strength.

Quote:This seems like a relatively difficult question to ask....and it seems like most of the detail originates mostly from experience rather than specifications....?

Yes, but I think answering is even more difficult At the end of the day specifications are only any good if they are derived from good experience.

Quote:If the surface roughness is application "sensitive", what would the surface roughness then be for an HVOF / Plasma / Arc sprayed coating?

Surface roughness I think is also "sensitive" to coating properties and function, and substrate properties. I'm running short on time so I will leave this open for more discussion.

Quote:We currently have a surface roughness range of between 4 to 6 Ra....? It is difficult to determine if we comply or not...

If the quality of blasting is good (sharp angular profile, even coverage, clean from contaminants with minimum wait time before spraying etc) it may well be suitable. If you don't have a specific specification to comply with, it does not matter, what's important is whether the coating is fit for purpose.

[Leanne]

Hi Gordon and SEF members!!

Thank you so much for the valuable input! This has helped me a lot!

Kind regards