What causes luminescence in sandblasted coated materials?

[Ash]

I have a small vacumm chamber configured for the sputter deposition of silicon in an oxygen/nitrogen/argon plasma. The resultant thin film compounds are used as transparent dielectric layers for optical filters and comprise SiN(x), SiO(x)or SiO(x)N(y).

When I clean the plant by sandblasting the coated areas, the coated material glows with visible light. This only happens when the coated deposit is blasted by the sand and stops when the coated material is balsted away to reveal the aluminium or stainless steel underlying structure (tooling, chamber wall etc).

What causes this?

Some finely divided metals burn in air. You get similar effects when blast cleaning titanium coated chamber furniture, but the effect there is due to elemental metal burning as it's blasted away into a finely divided powder. This effect is more like a real luminescence. Pressures of having to make a living prevent me from doing anything other than be puzzled at the moment. Anyone any ideas?

 

[Tyger]

Sand is of course Silica

which if in the form of quartz crystals is a piezoelectric, so it might involve the piezoelectric effect.

You might try other insulating materials, expecially on a conducting substrate. Also Zinc Sulfide.

 

[Jonathan]

I have heard that dust devils or sand storms can create strong electrostatic fields through the piezoelectricity of sand (no one really understands it though), and maybe this can create some kind of coronal discharge, but this is beyond me. How bright is the light?

 

[LURCH]

Probably just the kenetic energy of the impact. When I worked in a Fyberglass afctory for a while, I was once given the particularly nasty job of cleaning off an old iron water tank. The tank was coated with Gelcoat, an epoxy resin used in the manufacture of fyberglass. I was instructed to hold the sandblaster at such a distance from the tank that the impact of the abrasive sand caused the material to glow. When I reached bare metal, the glow stopped.

It was my thinking at the time that the impact must cause heat (from almost any distance). Enough impact energy, and that EM radiation moves up into the visible frequencies. But the bare metal radiates the heat much more effeciently, and does not build up enough energy to start glowing.

Suggestion for experimentation:
If you're truly curious, you might try altering the impact energy slightly, either by a change in the blaster's air pressure or it's distance from target, and see if you get red or orange light from lower energies and blue light from higher energies.