JPH0682164B2 - Optical element with adjusted impact resistance and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

The present invention relates generally to optical elements, and more particularly to shock protection and reinforcement of optical elements.

As is known in the art, optical imaging systems generally include externally mounted optical elements that shield the rest of the imaging system from the external environment. For example, for an infrared (IR) aerial imaging system, the rest of the infrared imaging system was wet,
Infrared transmissive optics, such as windows or domes, are attached to aviation systems to block exposure to corrosive and abrasive environments. Prolonged exposure to such environments generally impairs the optical and physical properties of the material of the optical element. The most severe environmental exposures encountered by these external components are generally believed to be the high-velocity water droplet impacts that occur when an aviation system flies over a rainy area.

This water drop impact problem is more commonly referred to in the art as rain erosion. When flying in a rainy area, water droplets collide with the surface of the external element,
Subsurface destruction occurs even at subsonic speeds. For very brittle materials, these subsurface fractures begin with existing microflaw near the surface of the optical element. Rain erosion damage to these optical elements occurs before the material is significantly removed. The growth of these existing microdefects is sufficient to damage the optical element. In particular, these fine defects grow due to the tensile component of the surface stress wave generated when the liquid crystal is impacted by water droplets and penetrate the optical element. Once formed, subsurface fracture will often cause large cracks in the optical element by continual growth through the optical element. Scattering and refraction of incident infrared energy occurs in the region of the crack, increasing internal reflection and infrared energy loss. When a large number of such cracks are included, the transmittance of the optical element is significantly reduced. Furthermore, as cracks grow through the optical element, catastrophic failure of the element can occur. When the optics fracture or break, the remaining optics of the infrared imaging system are exposed to the external environment, causing potential catastrophic damage to the imaging system.

Materials which provide the best mechanical durability and optical performance to infrared imaging systems in general (especially 8 to 1).
(In the 2 μm infrared band) is relatively limited. Suitable materials include zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, and cadmium telluride. Ternary sulfide materials, such as lanthanum calcium sulfide, are also currently being developed for infrared radiation (especially in the 8-12 μm band). Although these ternary sulfide materials can improve durability somewhat, even these materials are susceptible to the above environmental exposures. Generally all of the above materials are relatively brittle,
Relatively low resistance to damage, especially to damage from high velocity water drop impact.

Also as known in the art,
If the refractive index of the material forming the optical element differs significantly from the refractive index of the energy-producing medium, the optical energy incident on the surface of the optical element will be reflected at this surface. Generally for aeronautical systems, the energy generating medium is air with a refractive index of about 1. Therefore, in the optical industry, in order to reduce this kind of reflection loss, it is common practice to provide a coating of a material having an appropriate refractive index on the incident surface of the optical element. At the deposited thickness (which generally relates to the portion of the wavelength of light), these coatings are transparent in the infrared band. However, these optical coatings have heretofore played a role of reducing reflection loss caused by a mismatch in refractive index, and have not been useful for enhancing the impact resistance of optical elements.

In the art, a hard carbon layer, ie a carbon layer with quasi-diamond bonding and substantial optical clarity, when applied on germanium, provides some protection for germanium optics from impact damage caused by rainfall erosion. It is known. Concerning the hard carbon on germanium, "Liquid Impact Erosion Mechanism in Permeable Materials" (Jay Fields et al.), Final Paper September 30, 1982-March 31, 1983, Contrast No. AFOSR-78-370
5-D, article no. AFWAL-TR-83-4101
It is described inside. Hard carbon surface is another infrared material,
For example, it does not necessarily adhere effectively to zinc sulfide and zinc selenide. Moreover, the hard carbon coating, even on germanium, is susceptible to delamination upon high speed water drop impact as described in the above paper. It is theorized that the shear forces caused by the radial outflow of water drop impacts cause the coating to separate from the germanium layer. It is believed that this debonding phenomenon increases significantly as the thickness of the hard carbon layer increases. Therefore, thicker hard carbon coatings should provide more protection to the optical element against impact, but this was unsuccessful due to the delamination problem described above. Another problem with hard carbon is that the refractive index of hard carbon is about 2.45,
It is substantially higher than the refractive index of many of the above optical materials, such as zinc sulfide and zinc selenide. Therefore, if the optical element is coated with a hard carbon coating, the reflection loss at the entrance surface of the optical element will be higher than if the optical element were not coated.

A third problem in the art relates to the fracture strength of these materials. Again, most materials suitable for infrared transparent windows (especially in the 8-12 μm band) have low breaking strength. This property is particularly important in applications where these devices separate the high pressure region from the low pressure region, ie where the device is under some static or dynamic mechanical loading. 2481, Journal of Applied Physics 51 (5) 2473-2482 (May 1980), A. G. Evans et al., A report entitled "Impact loss threshold for brittle materials impacted by water droplets". It has been suggested that martensite reinforcement (phase change) at the surface of brittle materials can be useful for tempering brittle materials of this type. There is also the theory that surface compressive stress may be beneficial. However, the authors of the above report do not give any specific explanation of what they mean by "surface compaction". These brittle materials undergo surface compression when an incident water drop impacts the surface of the material.

According to the invention, an optical element resistant to high-velocity water droplet impact comprises a base layer of a first material having a first predetermined modulus of elasticity and a second, higher layer. A coating of a second material having an elastic modulus of. This coating layer binds to the material of the optical element and is highly resistant to delamination caused by the shear stresses encountered during high velocity drop impact. Preferably, the high elastic modulus coating is made of a material having a refractive index lower than that of the material forming the optical element. Still preferably, this material is substantially transparent to infrared radiation and substantially water insoluble. According to this aspect, the higher elastic modulus second material protects the base layer of the lower elastic modulus material against impact damage, particularly impact damage caused by high velocity droplet impact. In addition, the coating material, which has a high resistance to delamination caused by shear forces, remains on the optical element upon high-speed water drop impact, thereby protecting the optical element from environmental exposure such as rainfall erosion.

According to another aspect of the invention, the coating comprises a composite coating of a mixture (preferably a homogeneous mixture) of the first material and the second material. Each of these materials has a modulus of elasticity at least twice that of the material forming the base layer. Second
The material has a substantially higher elastic modulus than the first material, and
The material is insoluble and inert in water and the second material is reactive with water. According to this aspect, the composite coating provides the optical element with a layer having a higher elastic modulus than that obtained by only one layer of the first material. However, the composite material will also have relatively low water solubility and reactivity with water, especially when the first material is applied to separate the mixture from the water source.

According to another aspect of the invention, the material of the base layer is silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride, cadmium telluride mercury, zinc sulfide and zinc selenide, more preferably telluride. Cadmium, zinc sulfide, zinc selenide or ternary sulfide,
For example, it is selected from the group consisting of lanthanum calcium sulfide. The infrared ray transmissive material having a high elastic modulus that constitutes the first coating layer includes yttrium oxide, scandium oxide, a homogeneous composition of yttrium oxide and magnesium oxide, a composition of scandium oxide and magnesium oxide, and a composition of scandium oxide and yttrium oxide. Selected from the group consisting of mixtures. This form provides an impact resistant anti-reflective coating for optical elements that can operate in the wavelength band of 8-12 μm, making such elements more resistant to damage caused by rain erosion or high speed water drop impact. Sex

According to yet another aspect of the present invention, the optically transparent element made of the infrared transparent first material having the first elastic modulus is substantially higher than the elastic modulus of the material of the optical element. Protected against high velocity water drop impact by a composite coating that includes a first layer having a two modulus of elasticity and an index of refraction less than that of the material of the optical element. The material of the first coating layer is substantially resistant to delamination from the material of the optical element in response to the shear stress caused by the radial outflow of the droplet when subjected to high velocity droplet impact. The second layer of the composite coating comprises a third material having a relatively high third elastic modulus. This elastic modulus is higher than the elastic modulus of the first material forming the optical element layer,
And preferably higher than that of the second material of the first coating layer. The third material forming the second coating layer is substantially transparent to infrared rays and has a refractive index higher than that of the second material of the first coating layer. The third material forming the second coating layer is also substantially resistant to the peeling of the first layer of the composite coating from the second material, but the peeling resistance of the optical element to the first material is relatively poor. May be. According to this form,
It has a substantial resistance to delamination of the material from the optical element induced by radial outflow, and a substantial resistance to the delamination of the third material of the second coating layer induced by radial outflow. The inclusion of the first layer of material provided results in a composite coating that is substantially resistant to delamination induced by radial outflow and that has a greater elastic modulus than that of the first material. To be This composite coating can increase the effective physical thickness of the protective coating and still maintain or possibly improve the optical properties of the composite coating layer and optical element combination.

According to another aspect of the present invention, the first material of the optical element is silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride mercury, cadmium telluride, zinc sulfide and zinc selenide, or tri-element. It is selected from the group consisting of former sulfides. Preferably this material is selected from the group consisting of cadmium telluride, zinc sulfide and zinc selenide. The second material of the first coating is selected from the group consisting of yttrium oxide, scandium oxide, or yttrium oxide or a mixture of scandium oxide and magnesium oxide. The third material of the second coating is selected from the group consisting of cerium oxide, titanium oxide, zirconium oxide or hard carbon. According to this type of embodiment, generally, the hard material is generally hard by interposing the second material forming the first coating layer that is sufficiently bonded to both the first material and the second coating layer (particularly, the hard carbon layer) of the optical element. Most of the 8 associated with carbon
The problem of adherence to ~ 12 μm optics is eliminated. In addition, materials such as hard carbon do not have a suitable index of refraction for antireflective materials such as zinc sulfide, zinc selenide or cadmium telluride, so the hard carbon layer is combined with the low index first coating layer. When used together, a composite material having an effectively lowered refractive index can be obtained.

According to still another aspect of the present invention, a plurality of materials having a low refractive index and a high elastic modulus, and then a material having a high refractive index and a high elastic modulus are alternately stacked to form a multilayer antireflection impact-resistant coating. The optical element is protected by the composite layer formed. According to this type of form, this coating is intended to provide broadband anti-reflection or other optical functions, such as a filter effect, with the advantage of increasing the total physical thickness of the composite multilayer coating and increasing impact resistance. Can be designed. Preferably, the low refractive index, high elastic modulus material is selected from the group consisting of yttrium oxide, scandium oxide, or a mixture of yttrium oxide or scandium oxide and magnesium oxide, and the high refractive index, high elastic modulus material is cerium oxide, oxide. It is selected from the group consisting of titanium, zirconium oxide and hard carbon. The material forming the optical element is selected from the group consisting of silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride, cadmium mercury telluride, zinc sulfide and zinc selenide or ternary sulfide.

In accordance with yet another aspect of the present invention, an impact resistant tuning optic comprises a base layer of optical material having an initial puncture strength comprising a layer of compressed material on a base layer of optical material. A coating is placed. This layer of compressed material has a substantially smaller total thickness as compared to the thickness of the base layer of optical material.
According to this type of morphology, the layer of compressive material mitigates the effects of the tensile stress wave components encountered during droplet impact on the microscopic defects of the surface, preventing them from growing over the entire surface of the optical element. Will show. This compression region will tend to close these microdefects in response to the tensile stress wave component and prevent them from growing, thereby reducing the tensile stress wave component and compensating for it. By reducing this tensile stress component, damage resulting from water droplet impact on the surface of the optical element is mitigated, thus providing a relatively brittle material with a conditioning surface that is resistant to damage caused by rainfall. This adjusting surface simultaneously increases the breaking strength of the optical element.

In accordance with another aspect of the invention, an optical element tuned to resist the damage encountered during high speed impact comprises:
It includes a base layer of optical material, on the surface of which is located a compressed layer of the material. The layer of compressive material includes a number of grooves disposed in the layer, the grooves being separated by the compressive layer in adjacent regions, under which some of the compressive layer material is disposed.

Preferably the thickness of the compressed area of the optical material is 3
It is less than micron. Grooves generally have a depth of 10 to 10,000
The length is 0Å and the width is 0.01 to 0.02 mm. This form provides a conditioning, optical element that is highly resistant to damage encountered during high speed propulsion impact.

According to yet another aspect of the invention, a method of adjusting an optical element comprises machining a number of grooves into an optical element, the grooves generally having a depth of 10 to 10,000 Å. A compression zone of the optical material is provided between adjacent grooves and below the grooves.

According to still another aspect of the present invention, in the method for adjusting the surface of an optical material, the optical element is single-point machined to form a compression layer having a thickness of 0.5 to 3.0 μm on the surface of the optical element. Including steps. The compression layer has a depth of 10 to 10,000
Å multiple grooves, adjacent grooves separated by a portion of the compression layer of material of the optical element. Preferably the groove is
The single point diamond tool is introduced into the optical material by rotating the optical element at a predetermined speed while in contact with the surface of the rotating optical element.
The tool is applied to the surface of the optical element at a predetermined rate until the compression layer is applied. According to this method, a compression layer of optical element material is applied by single-point machining the surface of the optical material. This compression layer will reinforce the optical element and help prevent damage to the optical element caused by the high velocity drop impact by relaxing or reducing the near surface tensile stresses that occur during the high velocity drop impact.

In accordance with yet another aspect of the present invention, an impact resistant tunable optical element includes an optical element base layer having an initial puncture strength and a predetermined first modulus of elasticity. A compression layer of optical material is disposed on the base layer. The total thickness of the layer of compressive material is substantially smaller than the base layer of optical material, typically 1-3 microns. A coating layer of a second material having a second elastic modulus higher than that of the material forming the base layer of the optical element is disposed on this compression layer. This coating layer bonds to the compressed layer of optical material and is highly resistant to delamination caused by the shear stresses encountered during high velocity water drop impact. According to this aspect, the combination of the compression layer and the coating layer provides an optical element having high impact resistance and strength characteristics. The outer coating layer provides a coating of a material having a higher second modulus of elasticity, thereby protecting the lower substrate of lower modulus material from impact damage, such as under high velocity drop impact. In addition, the layer of compressive material will tend to mitigate the effects of this tensile stress wave component encountered during droplet impact. Therefore, the combination of these two techniques would provide an optical element with substantially improved impact resistance and puncture strength.

The foregoing features of the invention and the invention itself will be more fully understood from the following detailed description of the drawings.

FIG. 1 is an isometric projection of an optical element (here a plate) consisting of a base layer and a protective layer according to the invention.

FIG. 2 is an exploded cross-sectional view taken along line 2-2 of FIG. 1, showing a protective layer comprising a single layer coating according to one aspect of the present invention.

FIG. 3 is an exploded cross-sectional view taken along line 3-3 of FIG. 1, showing a layer of a pair of coating layers according to another aspect of the invention.

FIG. 4 is an exploded view of the assembly taken along line 4-4 of FIG. 1 and comprises layers of pairs of alternating high and low refractive index coating layers in accordance with yet another aspect of the present invention. Indicates.

FIGS. 5A-5D are a series of graphs showing radial stress drop as a function of normalized distance from the center of drop impact for coatings with different moduli (prior art).

FIG. 6 shows a change rate of 25.4 mm / hour (1 inch / hour), a speed of 724 km / hour (450 mph),
3 is a photomicrograph of uncoated ZnS surface exposed to an accelerated rainfall field with an impact angle of 90 ° and a raindrop size of 2 mm.

FIG. 7 is a photomicrograph of an antireflective (AR) coated surface according to the present invention exposed to the same accelerated rainfall field as shown in FIG.

FIG. 8 is a plot of transmission percentage versus wavelength for a 5.08 mm (0.20 inch) thick coated ZnS plate.

FIG. 9 is a sectional view of a part of the dome.

FIG. 10 is an enlarged cross-sectional view of the surface portion of the dome shown in FIG.

FIG. 11 is a schematic sectional view of raindrops falling on the surface of a general optical element having a microstructure defect.

FIG. 12 is a schematic cross-sectional view of water droplets falling on a compression layer according to another aspect of the present invention.

FIG. 13 is an enlarged view of water droplets that have impacted the compression layer shown in FIG.

14A and 14B are photomicrographs of the dome surface polished by a conventional method and the dome surface prepared by the method of the present invention (single point-diamond processing), respectively.

FIG. 15 shows the Knoop hardness number plotted as a function of load for ZnS and polished ZnS disk surfaces polished by conventional methods (ZnS microhardness).

FIG. 16 is a plot of typical hardness difference (Knoop) as a function of penetration depth into the compressed surface of a conditioned ZnS disk.

FIGS. 17A and 17B are a ZnS lens polished by a conventional method and a Zn prepared by the present invention.
3 is a micrograph of the surface of each S lens (DPM, diamond point processed) after being exposed to an accelerated rainfall field.

FIG. 18A and FIG. 18B respectively show a lapped ZnS lens (irregular) polished by a conventional method, and a diamond point processed according to the present invention.
3 is a photomicrograph of a wrapped ZnS lens (highly concave) showing the distortion caused by the compression layer.

FIG. 19 is a cross-sectional view of a portion of an optical element such as a plate or dome having an optical material compression layer and a coating layer according to another aspect of the present invention.

Referring to FIG. 1, an optical element (here, plate) 10 is shown as including a base layer 12 of a material having predetermined optical properties. Although the optical element is specifically shown here to be a plate,
Other types of optical elements such as windows, domes, lenses, etc.
It is understood that a plate having a shape other than a plane can be used instead of the above plate. A typical base layer 12 is at least 1.3 mm (0.05 inches), typically 2.5-12.
It will have a thickness of 7 mm (0.1 to about 0.5 inch) or more. The optical element may also have selective optical properties. For example, optical elements are generally infrared, visible and / or
Alternatively, it may be made of a material transparent to light energy in the ultraviolet spectrum. This material may be a dielectric or a semiconductor. Particularly for optical elements used in infrared imaging systems in the 8-12 μm wavelength range, examples of preferred materials include silicon, germanium, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride, zinc sulfide, Includes zinc selenide, or one of the ternary sulfides. The selected material of which layer 12 is made can be made by any known method, such as powder compaction (powder).
compaction, densification) or chemical vapor deposition. Layer 12 especially for infrared
The materials selected for are generally 3.5 × 10 ⁵ -10.
5 × 10 ⁵ kg / cm ² (5 × 10 ⁶ to 15 × 10 ⁶ ps
i) relatively low elastic modulus, high infrared energy transmission (typically 50-75% over at least a portion of the infrared wavelength band of 2.0-30 μm), and refraction of typically 2.2-4.0 at 10 microns. It is generally characterized by having a rate. The relevant mechanical and optical properties of some of these materials are shown in Table 1.

[0041]

[Table 1] R% is the reflection loss (per surface) caused by the quarter wave anti-reflection (AR) monolayer coating of Y ₂ O ₃ described below on the corresponding material. An impact resistant antireflection coating layer 11 is disposed on the base layer 12. Suffice it to say here that layer 11 can be any of the structures which are to be discussed.

Referring now to FIG. 2, the coating layer 11 is above (preferably on) the material of which the base layer 12 is composed.
It is shown as including a first protective layer 14 disposed.
The protective layer 14 has a substantially higher elastic modulus than the elastic modulus of the base layer 12, a high degree of infrared transparency over the selected wavelength band of the optical element at the deposited thickness, and preferably a refractive index of the material forming the base layer 12. Also consists of a material with a low refractive index. In addition, the deposited material has a high degree of adhesion to the material of layer 12 and is particularly resistant to delamination caused by shear stress induced by the radial outflow of high velocity drop impact (eg, water drop impact). .
Layer 14 can be deposited by any method such as ion beam sputtering, diode sputtering or vapor deposition. Further, layer 14 may be applied to plate 12 by immersing layer 12 in a solution of organic vehicle and high modulus material. After immersing the plate in a solution of this type of material, it is removed from this solution and placed in an oven where the organic vehicle is run off. Alternatively, the coating can be applied by spray drying a mixture of vehicle and coating material onto the base layer 12 which has been heated to a predetermined temperature. According to each of these coating modes, a uniform coating layer 14 is formed on the base layer 12.
A relatively inexpensive method for forming the is provided. Suitable coating materials for the above-mentioned base layer materials include yttrium oxide (Y
₂ O ₃ ), magnesium oxide (MgO) and scandium oxide (Sc ₂ O ₃ ), and homogeneous mixtures of these materials. The relevant mechanical and optical properties for these materials are shown in Table 2.

[0043]

[Table 2] The main factor to consider when choosing the material used for the coating layer 14 is that the selected material must have suitable optical properties for the intended use of the optical element in the thickness at which the material of the coating layer 14 is deposited. That is not the case. Further, the material of coating layer 14 should generally have a modulus of elasticity of at least about twice that of the material of base layer 12. Further, when the intended use of the optical element 10 is to expose the coating layer 14 to water,
The material of coating layer 14 must be water insoluble and stable. The refractive index of the material of the coating layer 14 is preferably lower than the refractive index of the material of the base layer 12 in order to provide an anti-reflection modification. In general, for an anti-reflection modification between air and a material of base layer 12 having an index of refraction of about 1.00, the required index of refraction (n ₁₄ ) of the coating is the refractive index of the material of base layer 12 and the It is approximately equal to the geometric mean of the rates (n ₁₄ ≈√n ₁₂ ). As is generally known for most materials, the index of refraction changes as a function of chromatic dispersion. Therefore, this antireflection modification also changes as a function of wavelength.

Layer 14 is preferably deposited on substrate 12 to a physical thickness corresponding to a quarter wavelength at the particular wavelength of interest for the optical element. Generally, the optical thickness (t ₀ ) of such a device is the physical thickness (t) of the coating 14.
And the refractive index (n _c ) of the material of the coating 14 (t ₀ = t · n _c ). The desired physical thickness for the optical thickness λ / 4 is given by t = (λ / 4) / n _c , where λ is the specific wavelength targeted by the optical element and n _C is the target wavelength. Is the refractive index of the coating in. As will be appreciated by those skilled in the art, the optical thickness (t ₀ ) is then of a higher order thickness, eg 3λ / 4 or 5λ / 4, so the physical thickness t is t = (( 2N + 1) λ / 4)
/ N _c . Where N is an integer 0, 1, 2,
3, ... Thus, increasing the physical thickness t of layer 14 provides greater impact resistance protection to base layer 12, while still maintaining good antireflection and light transmission properties. For example, at 10.6 microns, the refractive index n _c
For the material Y ₂ O ₃ with = 1.63, the optimum thickness of the λ / 4 monolayer at 10.3 microns would be about 1.63 microns.

Referring now to FIG. 3, plate 10 is shown as including a base layer 12 and a coating layer 11 disposed on at least a first side thereof. In this case, the coating layer 11 is an antireflection impact-resistant composite layer 15. Layer 1
No. 5 is shown as including the coating layer 14 of a high modulus material having a refractive index less than that of the material of the base layer 12 and having good adhesion to the material of the base layer 12. On this first coating layer 14, a second coating layer made of a second material having a substantially higher elastic modulus and a higher refractive index than those of the materials of both the substrate 12 and the first coating layer 14. 16 are arranged. Suitable materials for this second coating layer 16 include cerium oxide, titanium oxide, zirconium oxide and hard carbon. Of these examples, hard carbon is preferred because it has the highest elastic modulus. However, as described above, hard carbon does not always adhere sufficiently to zinc selenide, zinc sulfide, etc. contained in the preferable material for the base layer 12 especially at a light wavelength of 8 to 12 μm. The relevant mechanical and optical properties for these materials are shown in Table 3.
Shown in.

[0046]

[Table 3] The hard carbon film can be deposited by any suitable method. For example, an ion beam sputtering method and a chemical vapor deposition method involving decomposition of a hydrocarbon-containing vapor can be adopted. Although the hard carbon layer does not necessarily adhere well to most infrared materials as described above, it is believed that this kind of hard carbon layer adheres to materials suitable for use in the first coating layer 14. Suitable materials for layer 14 as described above include MgO, Sc ₂ O ₃ and Y ₂ O ₃ . Hard carbon generally adheres very well to various types of oxides, including these. Therefore, by interposing the coating layer 14 between the layer 16 having an extremely high elastic modulus and the base layer 12, the advantage of the impact resistance of the material having an extremely high elastic modulus 16 is a composite layer having an antireflection property and a high impact resistance. 15
And protect the base layer 12 from high velocity water drop impact.

Referring now to FIG. 4, the optical element is shown here as including a base layer 12 and a coating layer 11. Here, the coating layer 11 is a broadband antireflection impact-resistant coating layer 17 including a plurality of the antireflection impact-resistant composite coating layers 15 described above with reference to FIG. According to this form, a very thick antireflection coating 11 having excellent adhesion to the base layer 12 and a high impact resistance can be obtained. Furthermore, a plurality of composite layers 15 and individual coating layers 14 and 1
It is also possible to obtain a broadband antireflection coating by appropriately selecting the thickness of 6 according to the optical design principle of the multilayer coating.

Referring now to FIGS. 5A-5D, "Mechanisms of Droplet Impact Erosion in Transparent Materials" by Field et al.
A series of graphs taken from the paper entitled (AFWAL-TR-82-4022) show radial stress drop as a function of normalized distance from the drop impact center. Each graph plots a typical radial stress on a coated surface having a higher elastic modulus than that of the material of the optical element compared to the radial stress on an uncoated surface. As shown in FIG. 5D, if the elastic modulus of the material of the coating is 10 times that of the material of the base layer, the tensile stress induced in the base layer upon a water drop impact is substantially equal to zero.

Referring now to FIGS. 6 and 7, rate of change 25.4 mm / hour (1 inch / hour), velocity 724 km / hour (450 miles / hour), incident angle 90 °, respectively. And micrographs of the uncoated surface (FIG. 8) and the coated surface (FIG. 9) after exposure to an accelerated rainfall field with a droplet diameter of 2 mm are shown. As can be seen, the amount of damage exhibited by the uncoated zinc sulfide surface is substantially higher than that exhibited by the yttrium oxide coated zinc sulfide surface.

Referring to FIG. 8, the thickness is 5.1 mm (0.
Percent transmission versus wavelength plots are shown for zinc sulfide coated plates of the type 2 inch RAYTRAN (trademark of Raytheon Company, Lexington, Mass.). The coating was quarter wave thick yttrium oxide at 10.0 microns. The coating was about 2.45 microns thick. The coating was chosen to maximize surface transmission at 10 microns and was applied to both major sides of the plate.

Referring now to FIGS. 9-12, a portion of the optical element (in this case dome 110) is shown as including a layer 112 of a material having predetermined optical properties. Although the optical element is shown here as a dome, it is understood that other types of optical elements, such as windows, plates, lenses, etc., may be substituted for this dome 110. Generally, the base layer 112 will have a similar thickness as the base layer 12. The optical element may further have selected optical properties. For example, the optical element may be composed of a material that is generally transparent to light energy in the infrared, visible or ultraviolet spectrum. The material of the optical element may be a dielectric or a semiconductor. Particularly for optical elements used in infrared imaging systems, examples of preferred materials include silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride mercury, cadmium telluride, zinc sulfide, zinc selenide, or the general formula MN ₂ S
₄ (wherein M is a monovalent ion, N is an ion selected from the lanthanide series, and S is a sulfide ion S ²⁻
It is one of the three ternary sulfides. Layer 1
The selected materials making up 12 are any of the known methods,
For example, powder compaction, densificati
on) or chemical vapor deposition. In general, the material chosen for layer 112 is characterized by having a relatively high transmission for certain energies, eg, 50% or more of the infrared energy, generally over at least a portion of the infrared band, generally 3.5-10. .5 ×
It is an extremely brittle and fairly rigid material with an elastic modulus of 10 ⁵ kg / cm ² (5 to 15 × 10 ⁶ psi), but it is generally 387 to 1055 kg / cm ² (5,500 to 1).
It is generally a fairly soft material with a breaking strength of 5,000 psi).

The dome 110 further includes a plastic deformation compression layer 114 (see FIG. 1) provided on the surface 112a of the optical element 110.
0) is included. Preferably, the compression layer 114 (thickness t _c ) is part of the material of layer 112. As shown in more detail in FIG. 10, the compression layer 114 includes a number of furrows, gr
oove) 113, adjacent ones of these grooves 113 are separated by a compressed region 113a of the material of layer 112,
A compression layer 113b is arranged below these grooves. The degree of compression of layer 114 is a function of the amount of compressive force applied during processing of dome 110, as described below.

Referring now to FIGS. 11 and 12, the mechanism by which the compression layer 114 reinforces the dome and protects it from damage encountered during high speed water impact or mechanical loading is described.

As shown in FIG. 11, the surface 112a polished by a conventional method is applied to the surface 112a at a final speed V _0.
On the other hand, in the direction of the normal, there is a water drop 115 that is colliding with the surface 112a of the layer 112. Surface 112a of layer 112 has pre-existing microdefects 116 during processing of the polished surface as well as with the morphology of base layer 112. A tensile component (represented by arrow 118) of the surface stress wave generated when the water droplet 115 collides with the surface 112a according to the conventional method is given.
In response to this pulling force 118 in the area of the microdefects 116, the microdefects grow as cracks 116 '. If the tensile force is high enough, the crack 116 ′ may propagate through the base layer 112 of the optical element 10. If the number of these cracks is sufficient, the optical transparency of this device can be significantly reduced due to internal reflection and refraction in the area of the cracks. More importantly, an optical element containing sufficient cracks is prone to breakage or fracture, resulting in catastrophic damage to the rest of the optical system (not shown).

As shown in FIGS. 12 and 13, since the compression layer 114 is applied on the dome 110 in accordance with the present invention, the existing microdefects 116 in the base layer 112 are caused by the compression of the material in the region 114. Heal. When the compression layer 114 is processed, an opposing compression force 114 is applied, which reduces the size of the fine defects, and thus the smaller fine defects 1
It becomes 17. Further, the material surrounding the fine defects 117 is
It is still under compression, as indicated by the arrow 120 pushing the material in region 113b. As described above, the tensile stress component 118 is applied to the fine defect portion upon impact with water droplets. Since the microdefects are smaller, the rate at which a water drop impacts the surface layer without causing damage is increased. This is because the speed threshold increases as the size of the fine defects decreases. Further, the material is still in a compressed state after the groove 113 is formed, and the reduced tensile stress component 118 ′ generated at the fine defect 117 site is the tensile stress component 118 ′.
Will not grow until it penetrates through the compression layer 114 and into the base layer 112 unless the compression force exceeds a compression force that represents the degree of compression of the layer 114. Thus, the compression layer 114 provides two mechanisms that enhance the velocity threshold at which impact damage occurs. That is, it generally reduces the degree of micro-defects present in the material, thereby reducing the resulting tensile component for a constant drop impact velocity; and slowing or reducing the growth of tensile forces created in the compression layer 114. It gives a compressive force.

The preferred method for applying the compression layer 114 is to machine the surface portion of the optical element with single point diamond. Generally the surface can be processed in two steps. The first step, "roughing", has the machining parameters selected to remove a substantial amount of material 119 as shown in FIG. This would be a material of the order of 25-127 microns (1-5 mils) or more. The second cutting step, "finishing", may be one or several passes or cutting steps, with a small amount of material, generally about 2.5 to 12.7 microns (0.1 to 0. 5 mils) is removed resulting in a substantially flat but grooved surface.

The general surface characteristics of the adjustment compression layer 114 shown in FIG. 10 are as follows.

The groove 113 generally has a width Wf (typically 0.01
˜0.02 mm). The side wall portion 113a generally has a height hf of 10 to 10,000Å.

The general processing parameters used to machine the compression layer 114 into a 1 inch zinc sulfide disk are as follows:

Single Point Machining Parameters Useful for Creating Compressed Surface LayersRoughing:  Depth of cutting = 0.076 mm (0.003 inch) Rotational speed = 750 rpm Supply speed = 12.7 mm / min (0.5 inch / min) Tool radius = 3.175 mm (0.125 inch)Finish shaving  Cutting depth = 0.0051 mm (0.0002 inch) Rotational speed = 550 rpm Feed rate = 6.35 mm / min (0.250 inch / min) Tool radius = 3.175 mm (0.125 inch) 60 by this method One zinc sulfide disc sample was machined
It was Mechanical parameters measured on these discs
Knoop micro hardness (kg / mm²) And breaking strength
there were. Fracture strength is generally at least 1371 kg / c
m²(19,500 psi), which is
Breaking strength of polished sample 1090 kg / cm²(1
Greater than 5,500 psi). Therefore this method is
Increases breaking strength by about 25%.

As shown in FIGS. 14A and 14B, the surface polished by a conventional method is substantially smooth and uniform and has no characteristic. In contrast, the surface prepared according to the present invention comprises substantially regularly spaced ridges or grooves.

15 and 16 show typical Knoop hardness number versus typical microhardness plots for conventional zinc sulfide disks and zinc sulfide disks containing compression layer 114. The Knoop hardness number for a disc containing the compression layer 114 is generally 50-100 higher than the Knoop hardness for a conventionally ground disc for loads of 30 g or less. Further, as shown in FIG.
The hardness difference, expressed as the depth of penetration into it, indicates that the hardness increases significantly for penetration depths of 2 microns and below. By extrapolating this data, it is estimated that the hardness effect is limited to within 3 microns of the surface area of the machined sample.

17A and 17B, ie 724
25.4m at km / h (450 miles / h)
Rainfall area (2 m) accelerated to m / hour (1 inch / hour)
of the conventional method and the diamond point processed zinc sulfide plate after being exposed to a droplet diameter of m) for 5 minutes, the samples polished by the conventional method were significantly more damaged mainly by surface destruction. Indicates that. In contrast, the plate containing the compression layer applied by single-point diamond machining a zinc sulfide sample suffered substantially less damage (FIG. 17B).

A comparison of FIGS. 18A and 18B shows that layer 114 is a compressed layer of optical material. Two zinc sulfide lens blanks obtained from the same lot of zinc sulfide material were polished. One lens had its surface polished optically flat by conventional polishing methods, and the other lens had its surface machined flat according to the present invention. After processing, each sample was mounted on the lapping pad with the surface facing down, and the entire lapped surface had a thickness of about 5.1 mm (0.2 mm).
From 0.01 inch to 0.25 mm (0.01 inch) and to quarter-wave flatness in the visible spectrum. When these samples were removed from the lapping pad, some distortion occurred in each sample. This distortion is shown in the interference topography of Figures 18A and 18B. As shown in FIG. 18A, the distortion of the sample polished by a conventional method was small, which shows that an irregular final surface was obtained. However, as shown in FIG. 18B, the sample including the compression layer 114 according to the present invention could not be measured by the interferometer because the distortion was remarkable. Furthermore, when the test piece of FIG. 3 was removed from the lapping pad, the sample was distorted and became a significantly concave surface. This distortion is related to the inherent stresses contained in the layers of the lapped sample. Therefore, it is clear that the blank polished by the conventional method had substantially no compressive stress (FIG. 18A);
The sample shown in 8B comprised a highly compressed layer 114. The radius of the concave surface was measured optically and the amount of compressive stress present on the machined surface was estimated using this radius (Fig. 1
8B). The relationship between the radius of curvature (R) of the surface and the compressive stress (S) is as follows.

S = Ed ² / (6 (1-V) tR) In the formula, E is Young's modulus and is 7.6 × 10 ⁵ kg / cm ^2.
(10.8 × 10 ⁶ psi); d is the sample thickness, estimated to be 0.229 mm (0.009 inches); R is the radius of curvature, 129.2 × 10 ^-2 m T is the thickness of the compression layer, estimated at 1 × 10 ^-6 m; V is the position's ratio,
It is estimated to be 0.28. By solving for S, S =
7100 kg / cm ² (1 × 10 ⁵ psi) is obtained.
The reinforcing and hardening action demonstrated in the above data is therefore clearly the result of the formation of a surface compression layer on the zinc sulphide blank during the operation. Therefore, by appropriately selecting the machining parameters such as tool speed, tool type, cutting depth, feed rate, tool angle, etc., the scale of the compression layer applied to the zinc sulfide surface can be selected. Therefore, it is also possible to control the degree of reinforcement / hardening of the zinc sulphide.

Referring now to FIG. 19, a portion of the optical element (here, plate 130) is shown as including a layer 12 of material having the aforementioned predetermined optical properties. Disposed on layer 12 is a layer of compressive material 114 as described in connection with FIGS. 9-18. Overlying the compression layer 114 is a coating layer 11 consisting of one of the single layer or multilayer coatings described above in connection with FIGS. According to this aspect, it is believed that the enhanced stiffening and rainfall erosion resistance of both methods will help provide an optical element with substantially improved rainfall erosion resistance and fracture strength. To be

Although the preferred embodiments of the present invention have been described above, it will be apparent to those skilled in the art that other embodiments incorporating the concept can be adopted. Therefore, it is believed that the present invention should not be limited to the presented forms, but only by the spirit and scope of the appended claims.

[Brief description of drawings]

1 is an isometric projection of an optical element (here a plate) consisting of a base layer and a protective layer according to the invention.

2 is an exploded cross-sectional view taken along line 2-2 of FIG. 1 showing a protective layer comprising a single layer coating according to one aspect of the present invention.

3 is an exploded cross-sectional view taken along line 3-3 of FIG. 1 showing a layer of a pair of coating layers according to another aspect of the invention.

FIG. 4 is an exploded assembly elevation view taken along line 4-4 of FIG. 1 showing layers of alternating pairs of high and bottom refractive index coating layers in accordance with yet another aspect of the present invention. Show.

5A-5D are a series of graphs showing radial stress drop as a function of normalized distance from the center of drop impact for coatings with different moduli (prior art).

FIG. 6 shows the metallographic structure of an uncoated ZnS surface exposed to an accelerated rainfall field with a rate of change of 25.4 mm / hour (1 inch / hour), a speed of 724 km / hour (450 mph), an impact angle of 90 ° and a raindrop size of 2 mm. It is a microscope picture shown.

7 is a photomicrograph showing the metallographic structure of a coated surface according to the present invention exposed to the same accelerated rainfall field as shown in FIG.

FIG. 8 is a plot of transmission percentage vs. wavelength for a 5.08 mm (0.20 inch) thick coated ZnS plate.

FIG. 9 is a sectional view of a part of the dome.

10 is an enlarged cross-sectional view of the surface portion of the dome shown in FIG.

FIG. 11 is a schematic cross-sectional view of raindrops falling on the surface of a general optical element having a microstructure defect.

FIG. 12 is a schematic cross-sectional view of water droplets falling on a compression layer according to another aspect of the present invention.

FIG. 13 is an enlarged view of water droplets that impact the compression layer shown in FIG.

14A and 14B are photomicrographs showing the metal structures of the dome surface polished by a conventional method and the dome surface prepared by the method of the present invention (single point-diamond processing), respectively.

FIG. 15 shows the Knoop hardness number plotted as a function of load for ZnS and polished ZnS disk surfaces polished by conventional methods (ZnS microhardness).

FIG. 16 is a plot of typical hardness difference (Knoop) as a function of penetration depth into the compressed surface of a conditioned ZnS disk.

17A and 17B are respectively a ZnS lens polished by a conventional method and a Zn prepared by the present invention.
It is a microscope picture which shows the metallographic structure of the surface of S lens after each being exposed to the accelerated rainfall field.

18A and 18B are respectively Lapping ZnS lenses polished by conventional methods (irregular), and Lapping Z showing the distortion caused by the compression layer prepared according to the present invention (diamond platinum processing).
It is a micrograph (metallic structure) of an nS lens (highly concave).

FIG. 19 is a cross-sectional view of a portion of an optical element such as a plate or dome with an optical material compression layer and a coating layer according to another aspect of the present invention.

[Explanation of symbols]

 10, 110: Optical element 11, 111: Antireflection coating layer 12, 112: Base layer 14: First coating layer 16: Second coating layer 113: Groove 114: Compressed layer 115: Water droplet 116, 117: Fine defect 118, 118 ′: Tensile stress component 120: Compressive force

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Thomas Valitimos East Cross Street, Noord, Mass., USA 28 (72) Inventor Bernard Di Benadetto Concorde Road, Wayland, Mass. 292 (56) ) References Japanese Patent Publication No. 5-78001 (JP, B2)

Claims (16)

[Claims] 1. Zinc sulfide, zinc selenide, gallium arsenide, gallium phosphide, cadmium telluride mercury, cadmium telluride, and a general chemical formula: MN ₂ S ₄ (wherein M
Is a cation selected from Group 1A elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S ^2- ), and is 10 microns selected from the group consisting of ternary sulfides. A base layer composed of a first material having a refractive index of about 2.2 to 3.3 at a wavelength of, a predetermined light transmittance in the wavelength range of 8 to 12 microns, and a predetermined elastic modulus; At least twice the elastic modulus of the material of the layer, a refractive index of less than 2.0 at a wavelength of 10 microns, and ((2N +
1) The physical thickness given by λ / 4) / n _c , where λ is the wavelength in the range 8 microns to 12 microns, N is an integer, and n _c is the second material at wavelength λ. An optical element comprising a second different material disposed on the base layer and transparent to the optical wavelength range having a refractive index of .about. 2. The optical element according to claim 1, wherein the second material is selected from the group consisting of yttrium oxide, scandium oxide or magnesium oxide, or a homogeneous mixture of yttrium oxide, scandium oxide and magnesium oxide. 3. The optical element according to claim 2, wherein the layer of the second material is yttrium oxide. 4. A material selected from the group consisting of zinc sulfide and zinc selenide, having a predetermined transmittance for optical energy having a wavelength in the range of about 2 to 12 microns, and having a thickness of about 0. A base layer having a predetermined thickness in the range of 0.5 to 0.5 inches; and a material selected from the group consisting of scandium oxide, yttrium oxide and magnesium oxide at about 1 / wavelength selected within the above wavelength range.
An optical element having an anti-reflection, impact-resistant optical coating consisting of a coating having an optical thickness equal to 4. 5. The device of claim 4 wherein said wavelength is in the range 8-12 microns. 6. A device according to claim 5, wherein the coating having a thickness of ¼ of the wavelength is yttrium oxide. 7. Zinc sulfide, zinc selenide, gallium arsenide, gallium phosphide, cadmium mercury telluride, cadmium telluride, and a general chemical formula: MN ₂ S ₄ (wherein
M is a cation selected from Group 1A elements, N is a cation selected from lanthanide series rare earth elements, and S
Is a sulfide anion S ²⁻ ) having a refractive index of about 2.2 to 4 at a wavelength of 10 microns, a predetermined light transmittance at a wavelength in a predetermined range, and A base layer composed of a material having a predetermined first elastic modulus; and (i) a refractive index which is at least twice the elastic modulus of the material of the base layer and is lower than the refractive index of the material of the base layer. A first layer composed of a first high modulus material having: and (ii) a second layer having a modulus of elasticity of at least twice the modulus of elasticity of the base layer material and a refractive index of 2.0 or more. An optical element comprising: a second layer composed of a high modulus material of; a composite coating layer disposed on at least a portion of the base layer; 8. The first high modulus material is selected from the group consisting of yttrium oxide, scandium oxide and magnesium oxide, or a mixture of yttrium oxide, scandium oxide and magnesium oxide;
The optical element according to claim 7, wherein the second highly elastic material is selected from the group consisting of hard carbon, cerium oxide, titanium oxide and zirconium oxide. 9. The layer is t = ((2N + 1) λ / 4) / n
_c1 (where N is an integer 0, 1, 2, 3 ...
Is the particular wavelength of interest and n _c1 is the physical thickness t given by n _c1 is the refractive index of the material of the first layer) and the physical thickness of the second layer is t = ((2N + 1) λ /
_9. The optical element of claim 8 given by 4) / n _c2, where n _c2 is the refractive index of the material of the second layer. 10. Zinc sulfide, zinc selenide, gallium arsenide, gallium phosphide, cadmium telluride mercury, cadmium telluride, and the general chemical formula: MN ₂ S ₄ (wherein M
Is a cation selected from Group 1A elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S ^2- ), and is selected from the group consisting of ternary sulfides. Elastic modulus and about 8-12
A method of protecting an area of an optical element composed of a first material having a predetermined light transmission in the micron wavelength range from impact damage encountered in a high velocity droplet environment, the method comprising: Has a modulus of elasticity at least twice that of the first material, and has a predetermined light transmittance in the wavelength region,
And the step of applying a second material that is resistant to delamination from the first material in response to shear stress caused by radial outflow upon droplet impact. 11. A material constituting an optical element contains a material selected from the group consisting of zinc sulfide, zinc selenide, gallium arsenide, gallium phosphide, cadmium mercury telluride, and cadmium telluride. The material of claim 1 is selected from the group consisting of yttrium oxide, scandium oxide, or a homogeneous mixture of yttrium oxide, scandium oxide and magnesium oxide.
The method described in 0. 12. A third material having a modulus of elasticity at least twice that of the first material and having a refractive index greater than that of the second material on the layer of the second material. The method of claim 10, further comprising the step of providing. 13. The method of claim 12 wherein the third material is selected from the group consisting of hard carbon, cerium oxide, titanium oxide and zirconium oxide. 14. Zinc sulfide, zinc selenide, comprising the step of providing between the base layer and the hard carbon layer a first layer made of a material selected from the group consisting of yttrium oxide, scandium oxide and magnesium oxide. Gallium arsenide, gallium phosphide, cadmium telluride mercury, cadmium telluride, and the general chemical formula: MN ₂ S ₄ (where M is 1 A
A cation selected from a group of elements, N is a cation selected from lanthanide series rare earth elements, and S is a sulfide anion S ^2- ) and is composed of a material selected from the group consisting of ternary sulfides. A method of bonding a hard carbon layer to a base layer. 15. The first layer is yttrium oxide,
The material of the base layer is selected from the group consisting of zinc selenide and zinc sulfide, and the base layer, the first layer of yttrium, and the hard carbon layer have a high rate of effect at least in the wavelength range of 8 to 12 microns. The method of claim 14, wherein the optical element is provided. 16. The first layer is t = ((2N + 1) λ /
4) / n _c (where λ is the specific wavelength of interest, n _c is the refractive index of the material of the first layer at wavelength λ, and N is an integer 0, 1, 2, 3, ... 16. The method of claim 15, having a physical thickness given by: