Outgassing problem on lamp reflector
On a parabolic reflector used with an HID lamp, discoloration is taking place on the coated surface. The base material of the reflector is aluminum 6061-T6 per ASTM B209 or ASTM B221. There is a 10-16 microinch roughness height called out on the parabolic surface and the coating is specified as follows:
MIRROR, ALUMINIZED COATING IN ACCORDANCE WITH
MIL-M-13508 WITH A MAGNESIUM FLUORIDE PROTECTIVE
FILM TO WITHSTAND TEMPERATURE OF 500 DEGREES FAHRENHEIT
Apparently, due to the high temperature environment created by the HID lamp, outgassing of the coating material is occurring, primarily the fluoride. I would greatly appreciate any information and advice regarding this matter as well as any suggestions of possible alternative processes.
Thank you in advance,Edward J. Hile, III
Goodrich Hella Aerospace - Oldmar, Florida
A spec is a spec, and if the customer wants MgF, then...
However, it seems to me a film of Al2O3, ZrO2 or even SiO2 might be adequate protection for an aluminum visible light reflector (HfO2 if you really need thermal protection); the caveat here is without more information on mechanical stress, thermal cycling, environment and other parameters the part may be subjected to, it is hard to give a definitive answer.
MgF2 is used in optics frequently as a polarizing material and because it is fluorescent under electric lights. There maybe reasons for the MgF2 spec other than protecting the reflective surface.Dale Woika
Surface Conversion Sciences - Bellefonte, Pennsylvania, USA
This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site