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?