M.Sc Thesis


M.Sc StudentDennis Esterlis
SubjectPreparation of Sol-gel Based Alumina Protective Coatings on
Optical Substrates
DepartmentDepartment of Chemical Engineering
Supervisors Full Professors Grader Gideon
Dr. Shter Gennady
Full Thesis textFull thesis text - English Version


Abstract

                An increasing number of military applications in electro-optical systems in recent years are associated with high velocity windows (missile domes) operating in the Visible - Far IR range. The requirements from the windows are to remain functional and durable under extreme environmental conditions (high velocity water drop and dust particles impact) while retaining optical transparency.

       Multispectral ZnS is an attractive window material having desired optical transmission in the Visible -Far IR range (0.5-11.5 microns). The main drawback of ZnS is an insufficient chemical and mechanical strength which leads to optical degradation of the window's properties when exposed to erosion caused by airborne particles and raindrops.

        The goal of this work was to develop a sol-gel derived protective coating on multispectral ZnS windows. Alumina was selected as a coating material due to its optical transparency in the desired range (0.4-5.0 microns), as well as its ability to function under harsh conditions. The main challenge with Alumina coatings is however to obtain the desired properties at sufficiently low processing temperatures, in order to avoid damaging the underlying ZnS. Chemical Solution Deposition (CSD) process offers a low-temperature, low cost route to ceramics, and therefore was the method of choice in the current research.  

       Deposition of alumina film on glass and quartz was the first step in development of protective coating on ZnS. Thermally stable and well adhesive alumina films have been developed on glass and quartz with good transmission over the whole range for both coated windows. The thermal behavior of ZnS under ambient and inert atmospheres was investigated to determine the optimal thermal treatment conditions for the coated windows. A new approach of 2-stage thermal treatment of coating was developed: Preheating in Air and Rapid Thermal Processing (RTP) in Ar. Alumina coatings were then applied using CSD method and results showed that it is possible to obtain stable alumina coatings up to 7 micron thickness on multispectral ZnS windows without oxidation of substrate and damage of its morphology. A TiO2 film, 50-200 nanometers in thickness, was used as an intermediate adhesion layer. The designed alumina films were thermally stable, uniform and crack-free with a good adhesion to substrate. The coated windows withstood a standard humidity test, followed by a pulling test, pointing that a chemical attack on the window was prevented. In addition, the good optical transmission at Visible, Near IR and Mid IR ranges was not affected by the humidity test.