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.