Grit Blasting Parameters

[DanyGans]

Hi forum,
Since surface preparation is critical for adhesion mechanism between TS coating and ‎its substrate and most grit blasting operations are carried out with little parameters ‎controlling, I decide to ask the question about grit blasting recommended parameters.‎
What about parameters such as pressure,angle,nozzle,distance; I'm trying to make the surface-nozzle angle ‎perpendicular (as possible) but slight less 90deg. in order to reduce surface ‎contaminations. ‎
I use Al2O3 brown media blasted by NICOLIS160 machine with separation system.
B4C nozzle type with venturi shape.‎
Stand-off is approx. 6" and pressure 20-35psi.‎

I'm trying to understand if some requirements of recommendations exist for [u]nozzle ‎traverse speed and surface speed such in TS.‎[/u]

The second question is after how many BLASTING hours to change the media?

Thank you.
Dany.

[velocity]

I want to up this thread with extra questions:
Whats ideal Ra?
I saw many people are using vacuum blasting but isn' t angle of blasting important?

[Vadim Verlotski]

The thing with the sandblasting before the coating is very diverse and every thermal spray specialist has its own opinion for it. I will only give my opinion here without any claim to the scientifically proven truth:

Different methods of thermal spraying have different requirements and significantly different sensitivity to the surface condition prior to spraying.

The highest demands for the surface are the flame spraying with powder or wire, as well as arc spray (the coating with slow and big particles).These methods require maximum possible surface roughness (Ra > 20 microns, 1-2 mm abrasive grain) and are very sensitive to oil traces, moisture and other impurities on the blasted surface. In addition, the well-washed part before blasting must be heated to at least 80°C.

Although APS also belongs to "slow" spray methods, it is significantly less sensitive to surface contamination because hot plasma flame with a short spray distance cleans and activates the substrate surface. Plasma spraying "forgive" even the small traces of oil, which are quite critical for the flame spraying.

"High velocity" methods of thermal spraying (HVOF, HVAF, HV-APS, detonation spraying and cold spray) require significantly lower surface roughness than the "slow" methods. Very often the Ra > 2 microns (abrasive grains 100-150 microns) are sufficient. In some cases sandblasting can be completely deleted, although it already has the negative influence on adhesion.

And another couple of observations from my experience:

1. The finer corundum grains, the longer the blasting material can be used. With 1-2 mm grains, the grain breaks down after only 2-3 jet cycles; At 200 microns, the grain will last 50-100 times longer.

2. White corundum does not cost much more than a red or black, but has an important advantage: the contamination by the metal wear are better visible in white corundum. So you can change this white corundum in time (you change it when it gets "too gray").

3. For sandblasting, water and oil filters for compressed air are very important. Also, cleaning of oily and oxidized components must never be carried out in the sandblasting cabinet for spraying.

4. Angle of sandblasting, distance, surface speed and air pressure have a secondary influence on adhesion of the sprayed layer. It is only important that the whole surface is blasted evenly.

[darexpl]

I will add some from myself but it can be a deep theory than experience because I was limited to one type of coating and a few parts when I was a TS specialists (for a 5 years) and this is based on the specifications that needed to be met.

First at all with critical parts (for example for aviation engines) there is a requirement of adhesion for specific coating that is measurable in pressure on tensile test machine. Then you need to setup the process to meet this requirement. The second very important requirement is the separation between the substrate and coating. Evaluation of this is made for example in metallography . There are probably more requirements for the adhesion but mentioned above are known to me. Probably for those who are more experienced there are faster ways to find out the proper adhesion.

So if you known you requirement for the adhesion you should focus on the way how to achieve it. For what I know, the roughness of the prepared surface (Ra parameter) is critical. And if you would have chance to see a specification for TS process from a Aviation's Primes there is a specific range for each group of materials but in general "the high one" is on the safe side.

To be in the range of specific Ra you need to control the grit blasting process. Ra parameter can be fine tuned by two other parameters: distance and pressure. Of course you need to choose a correct type of media, grit size and nozzle ID.

There are a lot of good practices in grit blasting process:

1) surface-nozzle angle ‎should not be perpendicular when possible
2) movement of noozle shall be uniform and without spray pulsation
3) the exposure time shall be as minimal as possible to have full coverage of matte finish
4) do not touch blasting surface with bare hands
5) the blasted surface shall not be contaminated with oil and the air shall be very clean.
6) and more...

traverse speed and surface speed is not critical when blasting in manual suction cabinet.

You need to have a criteria for changing a media (for example when it contaminated and when it looses its properties).