Protective Coating for ZnS Windows & Domes

Zinc sulfide (ZnS) is an attractive candidate for missile windows and domes because of its broadband optical transparency. However, ZnS is a relatively soft and weak material (compared to Sapphire, ALON and Spinel) and is therefore subject to erosion damage when exposed to the rigors of environmental loading, during captive carry as well as high speed missile flight. Surmet proposes a two-layer coating system consisting of an outer durable, abrasion resistant, oxide coating (~0.5-1.5 ??m), deposited over a thick (3-10 ??m) strengthening/hardening layer (e.g., silicon for LWIR or SiN, SiC, SiON, SiCON for extended MWIR applications). The hard outer layer provides anti-reflection (AR) function, abrasion resistance and oxidation protection while thicker layer increases the strength/hardness of the underlying substrate. The oxides of interest include ZrO2, HfO2 and Y2O3. The effort will begin with the reactive sputter deposition of ZrO2 and HfO2 coatings. For Y2O3, Surmet will adapt its plasma assisted-chemical vapor deposition (PA-CVD). Surmet has already established PA-CVD process for thicker (3-10 microns) strengthening/hardening layer. A series of coatings with different thickness ratios of the outer oxide and inner strengthening/hardening layers will be characterized for optical properties, hardness, strength, abrasion and rain erosion resistance, thermal stability and oxidation protection.