GB2291888A - Optical coating - Google Patents

Abstract

An optical component in combination with an optical coating adherent to said component comprises a coating having an outer stratum which is mechanically hard and an inner stratum disposed between the outer stratum and the component, the inner stratum being energy absorbent. The combination is arranged to have surface stresses which are compressive in order to cancel out the induced tensile stresses arising from repeated multiple projectile impact.

Description

OPTICAL COATING This invention relates to optical coatings for optical components.

It is known to provide optical components, whether of the refractive or reflective type with an optical coating for the purpose of enhancing the optical reflection or transmission characteristic of the component depending upon the nature of the component. Such coatings are known both for the visible and thermal infrared wavebands.

A particular problem arising from optical components which are transmissive in the infrared waveband (and which may also simultaneously be transmissive in the visible waveband) is that the material (or materials) forming the component is brittle and is liable to mechanical damage such as cracking and fracture when subjected to repeated projectile impact. Additionally the brittle nature of these materials is such that, even in the absence of severe mechanical damage, repeated impact by projectiles rapidly gives rise to severe optical degradation. Such optical components may be used, for example, to form a window on an aircraft or guided missile and, as such are subject to multiple impacts by airborne dust, insects on the wing, water droplets, hailstones or rain which, having regard to the speed of the aircraft or missile, collide with substantial relative momentum.

Various approaches have previously been proposed to minimise the mechanical and/or optical degradation of optical components caused by the impact of rain or water droplets, but none has mitigated this problem to any substantial extent whilst at the time being suitable for application to a wide range of optical materials, and also being of acceptably low optical loss by absorption, and so far as we are aware unacceptably large mechanical or optical degradation occurs in a time interval of the order of tens of seconds only.

It is an object of the present invention to obviate or mitigate the foregoing disadvantages by the provision of optical coatings for optical components, the coatings being adapted to enhance the durability of the coated component combination when subjected to repeated multiple projectile impact.

According to the present invention there is provided an optical component to which is adherent an optical coating, such coating comprising an outer stratum which is mechanically hard and an inner stratum disposed between said outer stratum and said component, said inner stratum being energy absorbent.

Conveniently each said stratum is in the form of a discrete layer having thickness of typically 10 to 100 microns. Alternatively, the inner and outer strata may form part of a continuously mechanically graded optical coating.

Conveniently, said inner stratum is bonded to either or both of the optical component and outer stratum by means of an optical matching stratum.

Conveniently also, at least said outer stratum incorporates compressive surface stresses in the absence of projectile impact.

The inner stratum may be mechanically elastic or non-elastic provided that it possesses optically useful properties following repeated multiple projectile impact.

By virtue of the present invention projectile impact damage to the coated optical component is substantially reduced and it is believed that this arises due to the presence of the energy absorbent stratum.

The optical component may be made of any one or more of a large number of materials which are suited to use in the particular wavebands of interest. For example, materials used in the band below 3 microns are silica and conventional glasses such as borosilicate, soda lime etc; materials mainly used in the 3-5 micron band are magnesium fluoride, magnesium oxide, aluminium oxide (sapphire), aluminium oxy-nitride (ALON), silicon nitride, magnesium aluminate, (spinel), calcium aluminate glasses, silicon and alkali earth halides such as calcium fluoride etc; materials mainly used in the 8-12 micron band are zinc sulphide, zinc selenide, germanium, gallium arsenide, chalcogenide glasses, alkali halides such as potassium chloride and binary and ternary sulphides.

The inner stratum may be formed from many polymeric materials which display elastic properties when in use such as polyethylene, polypropylene, polyisobutylene, polybutadiene or neoprene polymers, or copolymers such as ethylene/propylene or ethylene/normal butyl acrylate; or composites of the above polymers; or the above with crosslinking to optimise their properties; or the above with reinforcement; or it may be formed from plasma-polymerised hydrocarbon, chlorocarbon or fluorocarbon, films using saturated hydrocarbons such as, methane, ethane, butane and the like or using unsaturated hydrocarbons such as, ethylene and butene; and many other materials including many glasses and inorganic films some of which are nonelastic when in use.

The mechanically hard stratum may be formed from aluminium oxide, spinel, zirconium dioxide, zirconium nitride, zirconium titanate, hafnium oxide, yttriumaluminium garnet, yttrium oxide, niobium oxide, mixed oxides of zirconium, hafnium, niobium, lanthanum, boron nitride, boron carbide, silicon carbide and silicon nitride, all of which materials are suited to the spectral band below 5 microns, and in respect of the 8-12 micron region diamond-like amorphous carbon, such as ARG 4 marketed by Barr & Stroud Limited, germanium carbide, and certain rare earth sulphides such as cerium sulphide.

The bonding stratum may be any one of the above mentioned materials appropriate to the spectral region of interest being either a mechanically hard material or a mechanically elastic material, but by way of example where the mechanically hard strata is diamond-like carbon, such as ARG 4, the preferred bonding layer is silicon.

The optical coating may be deposited on the optical component using the plasma CVD process which is known per se, individual strata or a graded composition of the coating being achieved by varying the plasma gas composition and/or flow rate, and/or substrate temperature and/or the electromagnetic deposition conditions as the coating is grown. For example, a plasma gas in the form of butane produces a mechanically elastic hydrocarbon polymer at low rf power input and high gas pressure whereas with increased rf power input and decreased gas pressure (and/or possibly the introduction of other gases such as silane or germane), a hard film of diamond-like amorphous carbon (or silicon carbide or germanium carbide) is derived.

It will be appreciated that the coating in addition to minimising damage preferably confers good antireflection properties to the optical component and consequently the optical thickness of each stratum, together with the refractive index thereof, requires to be selected in relation to the refractive index of the optical component. For example, if the optical component is made of zinc sulphide (with refractive index 2.3) and the coating has a mechanically elastic stratum of polythene (refractive index 1.5) and a mechanically hard stratum of diamond-like carbon (refractive index 2.0) and if each stratum is one quarter wavelength in thickness (at 10 microns), the reflectance is increased and this may be unwanted.Introduction of a bonding stratum between the mechanically elastic stratum and the zinc sulphide and having a refractive index of 3.0 gives rise to reduced reflectance when the bonding stratum is one quarter wavelength thick. Non one-quarter-wave-strata thicknesses may be used in accordance with the present invention and, for example, arranging the optical thickness of each of the mechanically hard and mechanically elastic strata of the preceding example at 0.35 one quarter wavelengths reduces the average reflectivity to about 3.5% in the 8-12 micron waveband.

Where the optical component is made of germanium (refractive index 4.0) and the coating consists of a mechanically elastic stratum of glassy chalcogenide (refractive index about 2.9) and a mechanically hard stratum of diamond-like carbon in the form of a modified ARG 4 film (refractive index about 1.8), low reflectivity is achieved with only two strata.

By way of a further example, where the optical component is zinc sulphide the coating may take the form of an optically matching stratum followed by a mechanically elastic stratum followed by a bonding stratum followed by the mechanically hard stratum, the order and magnitude of refractive indices being 2.3/3/1.5/3/2 and the optical thickness of the coating in terms of one quarter wavelengths at 10.4 microns being 0.15/2.0/0.52/1.15 the combination of component and coating has an average reflectivity of 1.6% in the 8.2-11.6 micron band, maximum reflectivity being about 3%. In this case the mechanically elastic stratum may be a polymeric layer, such as, polythene and the mechanically hard stratum may be diamond-like carbon, such as ARG 4.

In a further example with the optical component being made of zinc sulphide and the optical coating being in the form of a mechanically elastic stratum followed by a bonding stratum followed by a mechanically hard stratum having respective refractive indices 1.5/3/2 and respective optical thicknesses in terms of one quarter wavelengths at 10.4 microns of 4.39/0.39/1.19, the average reflectivity is 3.8% in the 8.4-11.4 micron band.

In each of the foregoing examples it is preferred that the outer stratum of the coating, which is the mechanically hard stratum, is deposited by the CVD process with inbuilt compressive stresses because it has been found that as regards projectile impact the impact-induced stresses within the contact area are themselves compressive whereas in an annular region bordering the contact area the induced stresses are tensile and by virtue of the inbuilt compressive stresses of the coating there is a tendency to cancel out the induced tensile stress which in turn reduces damage both optically and mechanically to the coating/substrate combination.

A stress analysis of zinc sulphide substrates in both coated and uncoated form is illustrated in Fig. 1 showing the stress magnitudes at 4 microns below the surface of the zinc sulphide in terms relative to the centre of the projectile contact area. In Fig. 1 r represents distance from the centre of the contact area which has a radius of a (a typically is about 150 pm). It will be seen that where the zinc sulphide substrate is uncoated (curve A) the level of tensile stress is relatively great. In the presence of a coating which is 2 microns in thickness and consisting solely of a mechanically hard coating (curve B) the tensile stress level is marginally reduced. In the presence of a coating comprising a 10 micron thick mechanically elastic strata and a 2 microns thick mechanically hard strata (curve C) the tensile stress level is very substantially reduced. Such stress can be shown to be further reduced, approaching zero, when an increased thickness of elastic layer is used. Elimination of resultant tensile stress by virtue of the present invention is of notable significance in that an analysis of damaged optical components indicates that the greatest severity of the mechanical and/or optical damage occurs in those areas which are subjected to tensile stressing.

By way of example numerical data concerning mechanical elasticity and hardness of optical materials is recited in chapter 7 of THE INFRARED HANDBOOK (Wolfe & Zissis).

Claims (7)

1. An optical component in combination with an optical coating adherent to said component, said coating comprising an outer stratum which is mechanically hard and an inner stratum disposed between said outer stratum and said component, said inner stratum being energy absorbent.

2. A combination as claimed in claim 1, wherein each said stratum is in the form of a discrete layer.

3. A combination as claimed in claim 1, wherein the inner and outer strata form part of a continuously mechanically graded optical coating.

4. A combination as claimed in any preceding claim, wherein said inner stratum is bonded by means of an optical matching stratum to at least one of said optical component and said outer stratum.

5. A combination as claimed in any preceding claim, wherein said outer stratum incorporates compressive surface stresses in the absence of projectile impact.

6. A combination as claimed in any preceding claim, wherein said inner stratum is made of a mechanically elastic material possessing optical useful properties following repeated multiple projectile impact.

7. An optical component in combination with an optical coating adherent to said component and substantially as hereinbefore described.

7. An optical component in combination with an optical coating adherent to said component and substantially as hereinbefore described.

Amendments to the claims have been filed as follows 1. An optical component in combination with a thin film optical coating adherent to said component, said coating being suitable for radiation transmission in the infrared wavelength region and comprising an outer stratum which is mechanically hard and an inner stratum disposed between said outer stratum and said component, said inner stratum being absorbent of energy resultins from mechanical impact.

2. A combination as claimed in claim 1, wherein each said stratum is in the form of a discrete layer.

3. A combination as claimed in claim 1, wherein the inner and outer strata form part of a continuously mechanically graded optical coating.

4. A combination as claimed in any preceding claim, wherein said inner stratum is bonded by means of an optical matching stratum to at least one of said optical component and said outer stratum.

5. A combination as claimed in any preceding claim, wherein said outer stratum incorporates compressive surface stresses in the absence of projectile impact.

6. A combination as claimed in any preceding claim, wherein said inner stratum is made of a mechanically elastic material possessing optical useful properties following repeated multiple projectile impact.