Multispectral glasses and coatings for defence applications

Abstract

Electro-optical infrared (EO/IR) systems are integral to providing situational awareness and target detection in defence and various other applications. These systems employ both passive and active detection across a wide spectrum of wavelengths, ranging from ultraviolet (<0.4 µm) to long-wave infrared (>8 µm), thereby offering multiple detection methodologies even in conditions of low visibility. Presently, EO/IR systems utilise different optics that are specialised for transmitting specific wavelength bands. The introduction of a multispectral material capable of transmitting the entire wavelength range could potentially enable the use of a single optic, thus reducing the overall weight and size of the system. This enhancement could broaden the scope of EO/IR system applications, such as unmanned aerial vehicles typically constrained by weight limitations. Gallium lanthanum oxy-sulfide (GLS(O)) glasses emerge as a promising candidate for these applications, with 1 mm thick samples produced for this project transmitting from 0.53-9.6 µm. Oxide can be added to the base GLS(O) glass, but the base glass itself contains an amount of oxide (signified by the ‘(O)’ in the GLS(O) formula), this underlying oxide content was not, in general, quantified for this work. Other advantages include GLS(O) having a higher Vickers hardness (4.6 GPa) than ZnS (2.25 GPa), a contemporary material, while also being glass mouldable. Within this project, halide and nitride-based additives (CsCl, CsI, Si3N4, AlN) were incorporated into GLS(O) compositions to modify their optical and mechanical properties. Noteworthy findings include the addition of Si3N4 to 65Ga2S3:35La2S3:(O) glass, yielding a composition of 65Ga2S3:34.7La2S3:0.3Si3N4:(O), also containing oxide brought in by [O] contamination of Ga2S3 and La2S3 due to adventitious oxygen ingress during synthesis. This resulted in a blue shift from 530 nm to 500 nm for a 1 mm thick sample, accompanied by a minor blue shift of the multiphonon edge from 9.6 μm to 9.52 μm, and a slight increase in hardness from 4.67 to 4.75 GPa. Protective coatings were also deposited on GLS(O) via radio-frequency magnetron sputtering, including SixOyNz, AlxOyNz, and BxOyNz. In a notable example, a 1.8 μm-thick Al42O3N55 coating demonstrated a high hardness of 20.7 GPa, while maintaining transmittance (> 50 %) across the 0.375-10.6 μm range. This coating could be suitable for safeguarding GLS(O) on an EO/IR system against adverse defence and aerospace environments, including erosion from solid and liquid particles.

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Identifiers

ORCID for Ellis Archer: orcid.org/0000-0002-4389-9269

ORCID for Ioannis Zeimpekis: orcid.org/0000-0002-7455-1599

ORCID for Pier-John Sazio: orcid.org/0000-0002-6506-9266

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