The demand for innovative solutions to enhance the safety of military personnel is continually on the rise. This includes the need to improve the performance of military vehicles and aircraft, in terms of both safety against laser attack and maximizing the information that can be presented to pilots without obstructing their view.
As well as safety, greater data is needed by pilots and soldiers, yet the systems gathering the data need to be compact, lightweight, and effective – and these requirements are driving the use of optical technologies in military and aerospace applications, including head-up displays and laser protection filters.
Coating Design and Precision Deposition Technology
All high-performance optical systems use coated components, which are improved by developments in optics and photonics, and deposition technology. Without the use of coatings, everyday objects such as cameras in mobile phones would not work as effectively as there would be too much reflection. This is eliminated with an efficient coating process. This is even more important in aerospace and defense applications.
Optical interference coatings are typically multilayer, nano-thickness structures applied to the surfaces of optical components to modify their optical performance. Using the principle of constructive and destructive interference, transmittance and reflection can be controlled within specific parameters. Absorption and non-optical properties such as electrical conductivity and surface hardness can also be influenced by the properties of the coating material.
Figure 1 shows the response of a high-performance filter designed to block a 532 nm (green) laser while transmitting other visible light.
Figure 2 shows the layer structure of this multilayer alternating refractive index design of around 500 layers with thicknesses varying from 5-70 nm.
To keep pace with progress in innovation, coating engineers use specialized design software in combination with advanced modelling tools to produce complex multi-element designs, which achieve the highest performance requirements for today’s systems.
Considerations When Designing Coatings
When designing a coating, it is essential that low stress is exerted on the substrate material. This allows for good coating adhesion and enables high optical flatness specifications required in modern applications.
Coatings that will be used in defense applications will be regularly exposed to large variations in weather conditions, which means that the coatings must be environmentally durable. And coatings need to be robust against high energy laser attacks.
Sensitive substrates, such as plastics, polymers, electronic sensors and structured devices should be carefully considered before coating, as coating an already sensitive material may decrease the product’s lifecycle by causing stress and making it susceptible to damage.
Performance versus cost is always a consideration. Will the price of coating a material reflect in its enhanced performance and durability? Do you really need to invest in an 800-layer coating when 400 layers would suffice?
And finally, the choice of coating materials needs to be carefully selected to operate at different wavelengths. This means that if an application needs to be suitable for night operation, the coating may need to be transparent in infrared.
Precision Material Deposition and Technology
The key coating material properties are: refractive index, absorption coefficient, stress coefficient, and durability. Design software is used to control the performance of the coating, allowing the material to be efficiently used. Figure 3 shows how the refractive index and absorption typically change by wavelength; this also depends on the nature of the deposition process.
There is a large range of coating materials available depending on the specific application. Materials used for visible wavelengths include silicon dioxide (SiO2), titanium dioxide (TiO2), aluminum oxide (Al2O3), tantalum oxide (Ta2O5), hafnium oxide (HfO2), and magnesium fluoride (MgF2). Materials used for infrared applications include zinc sulfide (ZnS), germanium (Ge), silicon (Si), silicon oxide (SiO), ytterbium fluoride (YbF3), and yttrium fluoride (YF3).
There are many processes used for the manufacture of optical thin films that are Physical Vapor Deposition (PVD) methods; this is the principle technique used to coat a surface. Under PVD, the coating is evaporated onto the surface using a powerful electron beam gun (EBG) under high vacuum, which reduces the risk of contaminating the growing film.
PVD includes several specific coating techniques, including Ion Assisted Deposition (IAD), Ion Beam Sputtering (IBS) and Plasma Enhanced Chemical Vapor Deposition (PECVD):
IAD adds a high-energy ion beam directed at the part being coated, which results in a higher density coating;
IBS directs a wide high-energy beam at the target (the coating material), which causes atoms to be sputtered off the surface. The atoms then deposit on to the parts to be coated resulting in a high-density super-smooth coating;
PECVD is a chemical vapor process, which deposits the film directly from the gas state on to the parts, while being influenced by a high energy plasma.
Plasma Ion Assist Deposition (PIAD) is another commonly used high volume production technique. Dense films are achieved by directing energetic ions from an intensive positively biased plasma at the growing films. This enables close control of the refractive index and absorption coefficient.