Moisture Protective Coatings for Alkali Halide IR Optics

• Main Authors: Yamagishi, Frederick G, Miller, Leroy J
• Format: Report
• Language: English
• Subjects: ALKAL HALIDE OPTICS; ALKALI METAL COMPOUNDS; Coatings, Colorants and Finishes; FORWARD LOOKING INFRARED SYSTEMS; HALIDES; INFRARED WINDOWS; MOISTURE; OPTICAL MATERIALS; OPTICS; PLASMA POLYMERIZATION; PLASMAS(PHYSICS); Plastics; POLYMERIC MATERIALS; POLYMERS; PROTECTIVE COATINGS; SURFACE CHEMISTRY; TRANSMITTANCE

The objective of this program is to develop plasma-polymerized films that are transparent, moisture-protective coatings to increase the lifetime and durability of press-forged KBr IR optical elements. Reaction conditions have been established that yield uniform films of plasma-polymerized ethane (PPE) on polished and etched single-crystal KBr flats. On polishing, polycrystalline KBr formed localized grains that could not be removed by chemical etching with HBr. Surface passivation could be achieved with a plasma of CF3Br or CF2C12. An air leak in the reactor caused extensive corrosion problems with CF3Br. Repairing the leak eliminated the corrosion problem. On a concurrent IR&D program, we are developing a new surface passivation method for NaCl using a non-IR-absorbing surfactant. PPE deposited on this treated surface afforded protection from damage by 98% relative humidity for 11 days. It is important to eliminate trapped free radicals in the film to reduce the chance of forming a more polar material caused by the reaction of the radicals with atmospheric oxygen. Treating the film briefly with a hydrogen plasma quenched the radicals, but the method was no more effective than the present method of storing under an ethylene atmosphere. The use of DEHA, a two hydrogen atom donor, is being studied in an IR&D program as a radical quencher and may be applied to this program if it proves successful.