中波红外减反射硬质薄膜研究进展(特邀)

Research Advances in Medium Wave Infrared Antireflection Hard Coating(Invited)

中波红外即3~5μm波段红外线,在大气中具有良好的透过率。中波红外探测器被广泛应用于红外制导、红外成像、气体检测分析、空间遥感等领域。为提高系统性能和增强探测器对各种恶劣工作环境的抵抗力,在光学窗口和薄膜表面常镀制高硬度薄膜进行保护。本文从硬质薄膜入手,阐述了中波红外减反射硬质薄膜的设计、材料选择及应用,根据不同材料体系总结了金属氧化物、氮化物、类金刚石及碳化物薄膜等常见中波红外波段硬质薄膜的光学、力学性能等,并概述了晶界强化、模量差理论、纳米复合材料等目前主要的薄膜硬度强化方法。

Medium Wave Infrared(MWIR)light(@3~5μm)has high transmittance in the atmosphere.MWIR detectors have been widely used in the fields of infrared guidance,infrared imaging,gas detection,and space remote sensing,especially in military application prospects.High-temperature infrared radiation is generated by high-speed aircraft and missiles due to air friction and jet exhaust,which is often detected by infrared detectors.Compared with short-wave infrared,radar,and laser detectors,MWIR detectors possess higher sensitivity.In addition,the absorption lines of massive gas molecules concentrate in the midwave infrared band,thus it is often used for gas detection and industrial analysis.In practical applications,detector windows and lens of MWIR systems will be deposited with Anti-Reflection(AR)coating to reduce the energy loss during propagation and improve the imaging quality by eliminating stray light.However,coating materials are typically soft.Anti-reflection coating materials commonly used in midwave infrared band e.g.Ge,ZnS,MgF_(2),have excellent optical properties,but poor mechanical properties.To improve the optical performance of the systems and enhance the resistance to various harsh working environments,the optical windows and the surfaces of the films are usually coated with high hardness film for protection.This article describes the design,material selection and application of the MWIR antireflection hard coating.A summary of common hard film materials including the metal oxides,nitrides,and carbides are presented with the optical and mechanical properties.And the grain size strengthening,Koehler theory,nanocomposites methods are outlined to improve the film hardness.The first section illustrates the design theory of anti-reflection film,solutions to the high stress issue of hard protective film,and the design strategy of hard anti-reflection film.The second part introduces representative hard film materials in the mid-wave infrared band.The relationship between substrate temperature,gas flow rate,deposition rate,film stack design and mechanical properties of the films during the preparation process is analyzed.Among these hard coating materials,metal oxide films such as Al_(2)O_(3),Y_(2)O_(3),and HfO_(2) own excellent comprehensive properties and can be used as optical windows or protective films in some industrial applications.Differently,elements such as carbon and nitrogen have the advantages of small atomic bond length and thus strong chemical bond.For example,Diamond-Like Carbon(DLC)and BN films posses the advantages of extremely high hardness and relatively high light transmittance in the midwave infrared band,which have great potential in MWIR military applications.The third part of this paper summarizes the effective methods to improve the hardness of the films including grain boundary strengthening,Koehler theory,and doping other elements to form nanocomposite materials.Koehler theory is only applicable to multi-layer films and requires appropriate matching between film layers,which limits its application in most cases.Element doping and grain boundary strengthening are mature,which are able to improve the hardness of thin films effectively.The last part of the article describes some applications of MWIR hard AR coatings in military and industrial fields.Common AR coatings are considered difficult to apply in high temperature working environments.In recent research,the new mid-wave infrared hard AR coating materials with excellent high temperature stability and optical properties,such as B_(4)C and BN,are expected to greatly improve the stability of AR film system in high temperature working environment.They are considered to promote the application of mid-infrared detectors in industrial high temperature monitoring and infrared lasers,and even protect infrared windows in high-speed fighter jets and missiles.It is pointed out that exploring materials with better performance,optimizing the film deposition process,and combining various hardness enhancement technologies will still be the focus of future research on mid-wave infrared hard AR coatings.