Water vapor resistance of plasma‐polymerized coatings on potassium bromide windows

Plasma‐polyme rized (PP) coatings of chlorotrifluoroethylene (CTFE), tetraluoroethylene (TFE), and ethylene were deposited in a capacitively‐coupled 13.56‐MHz radio‐frequency glow discharge on potassium bromide (KBr) infrared windows for moisture protection. The PPTFE coatings deposited slightly downstream of the internal plasma reactor electrodes provided the best moisture protection for the KBr windows. The PPTFE‐coated windows tolerated an upper limit relative humidity of about 80 percent at 297°K without visible damage occurring to either window or coating within 24 h (86.4 ks). It appeared from environmental tests that some of the coatings failed to protect the KBr windows from moisture because of defects in the coatings (pinholes, cracks); others failed because of their high water permeability. Elemental analysis of the bulk and photoelectron spectroscopy (ESCA) of the surface of PPTFE coatings deposited downstream of the electrodes revealed that these samples contained less oxygen (from the atmosphere) than coatings deposited between the electrodes. The lower oxygen content of the coatings deposited downstream may have accounted for the improved moisture resistance of these coatings. The PPTFE coatings were posttreated by heating and by cross‐linking with activated species of inert gases (CASING). Neither post‐treatment technique was effective in improving the moisture resistance of the PPTFE coatings.