Thermal Spray Grit Blast

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Thermal spray engineers everywhere know of the myriads of variables that can affect the outcome of the process from the stand point of quality. This is because the thermal spray coating process itself is built up of various sub-processes, each of which has its own set of variables and therefore problems. It IS the job of thermal spray coatings process engineers to pay specific attention to EACH of these sub processes and institute proper controls on each of them in order to ensure final product quality.

It is my opinion that none of these sub processes can be taken for granted and none of them is either more important or less important than the other. I know, as must you, that in many thermal spray coatings houses, the plasma spray operator is generally considered the king ( or the queen to be politically right ) in comparison to the parts cleaning operator and this is not correct from a processing stand-point. Everyone is equally important and I do not mean this from a socialism stand-point but rather from a technical point of view.

Grit blasting prior to thermal spray is one such sub-process that is very important in itself. The variables dealing with proper grit blasting cannot be over emphasized. You do want the proper grit blast coverage and the appropriate level of grit blasting, but not over grit blasting by way of too high a grit blasting pressure or too aggressive a grit size. I know of thermal spray coatings companies that require their operators to have a grit blast release coupon at the beginning of every job that gets tested for proper surface roughness measured by way of a profilometer and that is the right way to ensure that the surface has been properly prepared.

Grit blasting is a key link in the overall thermal spray process because at the very heart of proper adhesion is a properly prepared surface. Grit blasting prior to thermal spray is considered a *significant process* by many OEM aerospace companies and rightfully so, because thermal spray is inherently a mechanically bonded phenomenon other than in cases where a diffusion heat treatment is followed up.

Personally, the variables in grit blasting are so many and the process itself can become problematic at times that I felt that it deserved a separate blog all by itself and you are welcome to visit my blog on abrasive grit blasting entitled http://abrasivegritblasting.blogspot.com wherein I plan to discuss not only grit blasting as a surface preparation process prior to follow on processes but also as a means of coating removal. ( Of course, some of us will claim never to have had a need to strip and recoat, right? wink, wink ) Anyway, that is another site that you may want to visit in your spare time.

Thermal spray companies that perform shot peening have another headache to deal with; I guess that is why they pay them the big bucks :) But shot peening becomes another process with variables that need to be controlled and monitored.

A case in point to be remembered if you are planning to institute grit blast cabinet release by way of coupon control, ( if you have not already done so far ) is to ensure that the coupon base material is the SAME as the part base material, otherwise the results wont mean much.

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10 comments:

Scooter said...

Are you a Thermal Engineer? Your article was interesting to read though I don't know anything about Thermal Engineering.

Raj K said...

I am a coatings engineer with quite a bit of experience in the field of thermal spray coatings. Thank you for stopping by.

Renergy said...

The article is very informative ion the sense that it emphasises
the importance of grit blasting
which is normally not given much
significance.Since we do metallising it will really help
us to control our process
parameters and help us to get
better coating results.
Thanks for information

Ken said...

I agree that grit blasting is very important. Not only for surface roughness but mostly for cleaning the surface. If you look at a wire arc process, some substrates don't even need to be blasted just cleaned and the bond strength is almost the same.

Renergy said...

Sir
We will liketo kinow if twin wire
spray coatings can be applied to
oil expeller worms for increasing the life of the part.The oil expeller worms wear out very fast due to abrasion and also work under
high load.Can twin wire spray
coating be successful to improve life and please suggest the measures to be taken to get best
bonding results.Can we use nichrome
80/20 wire as bond coat after grit
blasting with brown aluminium oxide
grit.Kindly suggest sice you are an
expert in this field and your
valuable advise can be of immense
help to us.
Thanks with regards
for Renergy
Gurmit Singh

Renergy said...

We tried metal spray of oil expeller worms for improving life.
We did not get very good results and we now feel the importance of
grit blasting.The coatings in
some cases peeled off.We feel
the bonding falt could have been
due to imperfect grit blasting
Media for grit blasting used was
aluminium oxide 25 mesh brwon
colour type.

Renergy said...

Dear Ken,
We have found out the importance of
grit blasting in twin wire spray as
improper grit blasting has resulted in poor adhesion and coating failure.We feel even right
and proper control of grit blasting is needed.I would request
Raj to kindly suggest the capacity of the air compressor for grit blasting and pressure required.
Thanks

Gurmit Singh

Raj K said...

I would recommend aluminum oxide grit #36 blended with #40 in a 50-50 ratio at an air pressure of 80 psi with a 5 to 6 inch stand off at a gun angle of approximately 45 degrees. Hope this will help. Also, ensure that the surface has been properly degreased prior to the grit blasting operation. Additionally, only clean gloves to be used after grit blast prior to coat. Thank you.

Renergy said...

Thanks Raj
We shall strictly follow your advise on grit blasting and try
to achieve as good results as
possible.This will help us in spray
coating adhesion and improve quality of coatings.

Thanks once again for most valued
advise.

With regards
for Renergy
Gurmit Singh

Monaliza Legarto said...

Plasma Spray , the deposits remain adherent to the substrate as coatings; free-standing parts can also be produced by removing the substrate.