GB2192733A

GB2192733A - Impact resistant and tempered optical elements

Info

Publication number: GB2192733A
Application number: GB08713905A
Authority: GB
Inventors: Randal W Tustison; Dennis Montanari; Thomas Varitimos; Bernard Dibenedetto
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1986-06-18
Filing date: 1987-06-15
Publication date: 1988-01-20
Anticipated expiration: 2007-06-15
Also published as: DE3720451C2; GB2225449A; JPS62299901A; JPH0578001B2; JPH04362901A; JPH0682164B2; FR2606165A1; GB8713905D0; FR2606165B1; GB2225449B; FR2606164B1; GB2192733B; FR2606164A1; DE3720451A1; FR2605417B1; GB8927705D0; FR2605417A1

Description

GB2192733A 1

SPECIFICATION

Impact resistant and tempered optical elements Background of the Invention 5

This invention relates genarally to optical elements and more particularly to impact protection and strengthening of optical elements.

As is known in the art, optical imaging systems generally include externally mounted optical elements which shield the remainder of the imaging system from an external environment. For example, with infrared (IR) airborne imaging systems, an IR transparent optical element such as a 10 window or dome is mounted on the airborne system to isolate the remainder of the IR imaging system from exposure to humid, corrosive and abrasive environments. Prolonged exposure to these environments generally degrade the optical and physical characteristics of the material of the optical element. Generally, the most severe environmental exposure encountered by these external elements appears to be high velocity water droplet impact which occurs when an 15 airborne system is flown through a rain field.

This problem of water droplet impact is more generally referred to in the art as rain erosion.

During flight through a rain field, water droplets impinge upon the surface of the external element producing subsurface fractures even at subsonic velocities. For very brittle materials these subsurface fractures are initiated at pre-existent microflaws lying near the surface of the optical 20 element. Rain erosion damage to such optical elements occurs prior to any significant removal of material. The mere propagation' of these pre-existent microflaws is sufficient to damage the optical element., In particular, these microflaws are propagated through- 1he optical element by the tensile component of the su rface stresswave created at the time of impact with the water droplet. Once formed, the continued propagation of the subsurface fractures through the optical 25 element will often produce large cracks in the optical element. In the region of the crack, scattering and refraction of incident IR energy occurs producing increased internal reflections and IR energy losses. With a significant number of such cracks, the transmissivity of the optical element is severely reduced. Furthermore, as cracks propagate through the optical element, catastrophic failure of the element may occur. When the optical element shatters or breaks, the 30 remaining optical elements of the IR imaging system are exposed to the external environment, resulting in potential catastrophic damage to the imaging system.

Typically,- materials which offer the best mechanical durability and optical performance for infrared imaging systems, particularly in the 8 um to 12 um infrared band, are limited to a relatively small number. Suitable materials include zinc sulfide, zinc selenide, germanium, gallium 35 arsenide, gallium phosphide, mercury cadmium telluride and cadmium telluride. Ternary sulfide materials such as calcium lanthanum sulfide are also currently being developed for IR applica tions, particularly in the 8-12 ym band. These -ternary sulfide materials may provide some improvement in durability but even these materials are susceptible to the environmental expo sures mentioned above. Generally, all of the aforementioned materials are relatively -brittle and 40 have a relatively low resistance to damage, particularly damage sustained during high velocity water droplet impact.

It is also known in the ari that optical energy incident upon a surface of an optical element will result iii-reflection of energy at such surface if the index of refraction of the material comprising the optical element is significantly different than the index of refraction of the medium 45 from which the energy originates. Generally, for airborne systems, the originating medium is air having an index of refraction of about one. Accordingly, it is standard practice in the optical industry to provide coatings of material of appropriate refractive index over the incident surface - of the optical element to reduce such reflection losses. At the deposited thicknessess, which are generally related to a fraction of an optical wavelength, these coatings are transparent in the IR 50 band. However, heretofore such optical coatings have served only to reduce reflection losses caused by a mismatch in refractive indices and have not served to increase the impact resis tance of the optical element.

It is known in thelart that a layer of hard carbon, that is, a carbon layer having quasi-diamond bonds and substantial optical transparency, when provided over germanium provides limited 55 protection to germanium optical elements from impact damage caused by rain erosion. Hard carbon cciatings on germanium are described in an article entitled- "Liguid Impact Erosion Mecha nisms In Transparenj Materials' ' ' by J.E. Fields et al, Final Report September 30, 1982 to March

31, 1983, Contract No. AFOSR-78-3705-D, Report, No. AFWAL-TR-83-4101. The hard carbon surfaces have not successfu I ly 'adhered to other IR materials such as zinc sulfide and zinc 60 selenide. Fu'rthermore, hard carbon coatings even on germanium as mentioned in the article are susceptible to debonding during high velocity water droplet impact. It was theorized there that the sheering force resulting from the radial outflow of water droplet impact causes debonding of- - the coating from the germanium layer. This phenomena of debonding is believed to significantly increase as the thickness of the hard carbon layer is increased. Therefore, thicker hard carbon 65 2 GB2192733A 2 coating layers which should have resulted in further impact protection for the optical element were not successful because of the aforementioned debonding problem. A further problem with hard carbon is that the index of refraction of hard carbon is about 2.45, substantially higher than the index refraction of many of the aforementioned optical materials such as zinc sulfide and zinc selenide. Accordingly, if an optical element is coated with a hard carbon coating, reflection 5_ losses at the incident surface of the optical element will be higher than if the optical element was not coated.

A third problem in the art concerns the fracture strength of these materials. Again, most materials which are suitable for IR transparent windows particularly in the 8 pm to 12 pm band R have low fracture strengths. This characteristic is particularly important in applications of these 10 elements where the element separates a high pressure region from a low pressure region, that is, in applications where the element is under some static or dynamic mechanical load. In an article entitled---ImpactDamage Threshold In Brittle Materials Impacted By Water Drops- by A.G. Evans et a], Journal of Applied Physics 51 (5), pps. 2473-2482 (May, 1980) at page 2481 it was theorized that martensite toughening (phase changes) at the surface of the brittle material 15 may be useful in. tempering such brittle materials. It was also theorized that surface compression stresses could be of benefit. However, the authors gave no specific description what they meant by -'-'surface compression;These brittle materials undergo surface compression when incident water drops impact the surface of the material.

20 Summary of the Invention

In accordance with the present invention, an optical element which is resistant to high velocity water droplet impan includes a base layer of a first material having a first predetermined - modulus of elasticity and a coating of a second material having a second, higher modulus of 2-5 elasticity. The coating layer bonds to the material of the optical element and has a high degree 25 of resistance to debonding caused by sheer stresses encountered during high velocity water droplet impact. Preferably, the high modulus of elasticity coating comprises a material having an index of refraction less than the index of refraction of the material comprising the optical element. Preferably still, the material is substantially transparent to infrared radiation, and is substantially water insoluble. With this arrangement, the coating of the second higher elastic 30 modulus material protects the base comprised of the lower elasticity modulus material against impact damage, particularly such impact damage caused by high velocity droplet impact. Further more, the. coating material: havi'-ng a high resistance tp debonding from sheering forces will remain intact on the optical element during high velocity water droplet impact and thereby protect the optical element from environmental exposures such as rain erosion. 35 In accordance with a further-aspect of the present invention, the coating comprises a compo- site coating of a mixture, preferably homogeneous mixtures of first and second materials, each having a modulus- of elasticity at least twice as high as the modulus of elasticity of the material forming the base layer. The second material has a substantially higher modulus of elasticity than the first material, whereas the first material is insoluble and inert with water and the second 40 material is reactive with water. With this arrangement, the composite coating provides a layer over the optical element having a higher modulus of elasticity than the modulus of elasticity which would be provided by jLS st a layer of the first material. However, the composite coating - will also have a relatively low Water solubility and reactivity with water particularly if a layer of the first material is provided to- isolate the mixture from a source of water. 45 In accordance with further aspect. -of the present invention, the material of the base layer is selected from the group consisting of silicon, germanium, gallium arsenide, gallium phosphide, cadmium telluride, mercury -cadmium telluride, zinc sulfide and zinc selenide, and more preferably, - cadmium telluride, zinc sulfide, zind selenide or. a ternary sulfide such as calcium lanthanum suffide. The high modulus infrared transparent material comprising the first coating layer is 50 selected from the group corisIsting of yttrium 'oxide, scandium oxide, and a homogeneous composition of yttrium oxidejand magnesium Oxide, a composition of scandium oxide and magnesium oxide, and a composition of scandium oxide and yttrium oxide. With such arrange merits, an impact resistant anti-reflection coating for optical elements operative in the 8 urn to 12 urn wavelength band is provided making such elements more resistant to damage caused by 55 rain erosion or high veiocity'wbter droplet impact.

In. accordance with a still further -aspect of the present invention, an optically transparent element, comprising a first infrared transparent. material having a first modulus of elasticity, is protected from high velocity Water drop let impact by a composite coating comprising a first layer of a second, optically transparent material having a second, substantially higher 'modulus of 60 elasticity than that of the material of the optical element, and an index of refraction less than the index of refraction of the material of the optical element. The first coating layer material is substantially- resistant to debonding from the material of the optical element in response to sheer stresses resulting from radial dutf16w-of droplets during high velocify droplet impact. A second layer of the 'composite -coating -comprises a -third material having a third relatively high modulus 65 3 GB2192733A 3 of elasticity, said modulus of elasticity being higher than the modulus of elasticity of the first material comprising the optical element layer and, preferrably higher than the second material of the first coating layer. The third material comprising the second coating layer is substantially transparent to infrared radiation and has an index of refraction higher than the index of refraction of the second material of the first coating layer. The third material comprising the second 5 coating layer is also substantially resistant to debonding from the second material of the first layer of the composite coating, but may have a relatively poor resistance to debonding to the first material of the optical element. With such an arrangement, by interposing the first layer of material having substantial resistance to radial outflow induced debonding from the material of the optical element and further having substantial resistance to radial outflow induced debonding 10 of the third material of the second coating layer, a composite coating is provided which is substantially resistant to radial outflow induced debonding and further has a modulus of elasticity greater than that of the first material. The composite coating allows the effective physical thickness of the protective coating to increase, affording increased protection while still maintain- ing or possibly improving the optical properties of the combination of the composite coating 15 layer and the optical element.

In accordance with a further aspect of the present invention, the first material of the optical element is selected from the group consisting of silicon, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride, cadmium telluride, zinc sulfide and zinc selenide or a ternary sulfide. Preferably, the material is selected from the group consisting of cadmium tellu- 20 ride, zinc sulfide and zinc selenide. The second material of the first coating layer is selected from the group'consisting of yttrium'oxide, scandium oxide or mixtures of yttrium oxide or scandium oxide with magnesium oxide-. The third'material of the second coating layer is selected from the group consisting of cerium oxide, titanium oxide, zirconium oxide or hard carbon. With such an arrangment, by interposing the second material comprising the first coating layer which bonds 25 well to both the first material of the optical element and to the third material of second coating layer, in particular, the hard carbon layer, the adherence problems generally associated with hard carbon to most 8-12 um optical materials, are eliminated. Further, since materials such as hard carbon do not have the proper refractive index to antireflect materials such as zinc su i e, zinc selenide or cadmium telluride, the hard carbon layer can be used in combination with the lower 30 index of refraction first coating layer to provide an effectively lower index of refraction compo site layer.

In accordance with a still further aspect of the present invention, an optical element is protected by a composite layer comprising a plurality of layers of a, low index, high modulus of elasticity material followed by a high-index, high modulus of elasticity material stacked in an 35 iterative fashion to form a rhulti-layer, anti-reflective impact resistant coating. With Uch an arrangement,' this coating could be designed to provide broadband anti- reflection characteristics or other optical functions such as filtering with the advantage that the total physical thickness of the composite multi-layer coating could be large providing increased impact resistance. Prefera- bly, the low index, high modulus of elasticity material is selected from the group consisting of 40 yttrium oXide,'scandium oxide or mixtures of yttrium oxide or scandium oxide with magnesium - oxide and the high index, high modulus of elasticity material is selected from the group consist ing of cerium oxide, titanium oxide, zirconium oxide or hard carbon. The material comprising the optical element is selected from the group consisting of silicon, germanium, gallium, arsenide, gallium phosphide, cadmium telluride, mercury cadmium telluride, zinc sulfide and zinc selenide or 45 a ternary sulfide.

In accordance with a still further aspect of the present invention, an impact resistant, tempered optical element includes a base layer of an optical material having: an initial fracture strength.

Disposed'over the base layer of optical material is a coating comprising a compressed layer of material. The compressed layer of material has an overall thickness which is substantially smaller 50 in comparison to the thickness of the base of the optical material. With such an arrangement, the compressed layer of material will tend to mitigate the effects of the tensile stress wave ccmponent encountered cludrig droplet impact on surface microflaws, preventing their propaga tion through the surface of the optical element. The compressed regions, however,will tend to close these microflaws preventing their propagation in response to the tensile stress wave 55 component thereby reducing or compensating for the tensile stress wave component. By reduc-.

ing this tensile stress component, the damage resulting from water droplet impact on a surface of the optical element will-be mitigated and heince-the relatively brittle material is provided with a - tempered surface which is re,5i'stan t to damage caused by rain erosion. This tempered surface concomitantly increases the fracture strength of the -optical element. 60 In accordance with a further aspect of the present invention, an optical element, tempered to resist damage encountered during high velocity impact, includes a base comprising an- optical material having disposed on a surface thereof a compressed layer of said material. The com- - pressed layer of material comprises a plurality of furrows disposed in said layer, said furrows being separated by adjacent. regions of said compressed layer and having disposed under said 65 4 GB2192733A 4 furrows a portion of the compressed layer of, material.

Preferably the thickness of the compressed region of the optical material is 3 microns or less. The furrows are typically 10A to 10,000A deep and 0. 01 to 0:02 mm wide. With this arrangement, a tempered optical element is provided which is highly resistant to damage encoun- tered during- high velocity projectile impact. 5 In accordance with a -still -further aspect of the present invention, a method of tempering an optical element comprises. the steps of machining a plurality of furrows into the optical element, said furrows having a depth-generally in the range of 10A to 10,000A providing between adjacent furrows and under said furrows a compressed region of the optical material.

In accordance with a still further-aspect of the present invention, a method of tempering a 10' surface of an optical material includes the step of single-point machining of the optical element to provide a compressed layer 0.5 lim to 3.0 um thick into the surface of the optical element.

The compressed layer includes a plurality of furrows having a depth between 10 to 10,OOOA with adjacent furrows being spaced by a portion of the compressed layer of the material of the optical element. Preferablyl the furrows are introduced into the optical material by rotating the 15 optical element at a predetermined speed while a single-point diamond tool is brought into contact with the surface of the- rotating optical element, with said tool being fed across the surface of the Optical element at a predetermined rate until the aforementioned compressed layer is provided. With this arrangement, by single-point machining the surface of the optical element, a compressed layer of the material of the optical element is provided. This compressed layer will 20 strengthen the optical element and aid in preventing damage to the optical element resulting from highvelocity droplet imPa6t by mitigating or reducing the near-surface tensile forces resulting during high velocity water droplet' impbot.

In accordance with a still further aspect of the present invention, an impact resistant, tempered optical element includes a base layer of an optical -material having initial fracture strength and 25 having a firstpredetermined modulus of elasticity. Disposed over the base material layer is a compressed layer of optical material. The compressed layer of material has an overall thickness which is substantially small in comparison to the thickness of the base of the optical material generally in the range of 1-3 microns.. Disposed over the compressed layer is a coating layer of a second material having a second, higher modulus of elasticity than that of the material forming 30 the base layer of the optical element. The coating layer bonds to the compressed layer of optical material and has a high degree of resistance to debonding caused by sheer stresses encountered. during high velocity water droplet impact. With this arrangement, the combination of the ccmpressed layer and- the coating layer provide an optical element having enhanced impact resistance and strength characteristics. The outer coating layer provides a coating of a 35 material having a second, higher modulus of elasticity thereby protecting the underlying base layer comprised of the lower elasticity modulus material against impact damage such as encoun tered by high velocity droplet impact. Moreover, the compressed layer of material will tend to mitigate the effects of this tensile stress wave-comp. onent encountered during droplet impact.

Therefore, the combination ofthe two techniques will provide an- optical element having substan- 40 tially improved impact resistance and fracture strength.

Brief Description of the Drawings

The foregoing features of Ahis invention, as well as the invention itself, may be more fully understood from the following detai.led description of the drawings, in which: 45

FIG. 1 is an isometric view of- an optical- element, here a plate comprising a base layer and a protective layer in accordance with the present invention; FIG. 2 is an exploded crosssectional view--taken along line 2-2 of FIG. 1 showing the protective layer comprising a single layer coating in accordance with one aspect of the present invention: 50 FIG. 3 is an exploded cross-sectional view taken along line 3-3 of FIG. 1 showing the layer comprising a pair of coating layers in accordance- with an additional aspect of the present invention, - FIG. 4 is an exploded, cross-sectional view taken along line 4-4 of FIG. 1 showing t. he layer comprising a plurality of pairs of alternating high index of refraction and low index of refraction 55 coating layers in accordance- with a still further aspect of the present invention; FIGS. 5-A-5D are a series of graphs- (prior art) showing radial stress reduction as a function of normalized distance from the center-Of droplet impact, for coatings having different modu1ii, of elasticity.- 1 -.- -,-.

FIG. 6 is a photornicrograph- of a-h. uncoated ZhS surface exposed to a simulated rain field rate 60 of 1 inch per hour, velocity-of 450 mph, 90' impact angle and 2 mm rain drop size; FIG. 7 is a photornicrograph pf a coated surface in accordance with the present invention exposed-to the same simulated rain-fiel. d as described in FIG. 6; FIG. 8 is a plot of % -iransmissivity vs. wavelength for a 0.20 inch thick coated ZnS plate; FIG. 9 is a- cross-sectionaf view of a portion. of a dome; 65 GB2192733A 5 FIG. 10 is a cross-sectional enlarged view of a surface portion of the dome shown in FIG. 11 FIG. 11 is a diagrammatical view in cross section of a rain droplet incident upon a surface of a conventional optical element having a microstructure fault; FIG'. 12 is a diagrammatical view in cross section of a water droplet incident upon a com- pressed layer in accordance with a further aspect of the present invention; 5 FIG. 13 is an enlarged view of the water droplet impacting the compressed layer as shown in FIG. 12; FIGS. 14A, 14B are photomicrographs of a conventionally polished dome surface and a dome surface tempered in accordance with the present invention; FIG. 15 shows plots of KNOOP hardness numbers as a function of load for a conventional 10 polished ZnS surface and a ZnS tempered disk surface; FIG. 16 is a plot of typical hardness difference (KNOOP) as a function of penetration depth into the compressed surface of the ZnS tempered disk; FIGS. 17A, 17B are photomicrographs, respectively, of a surface of a ZnS lens which was conventionAy polished and 6 ZnS lens which was tempered in accordance with the present 15 invention each after exposure to a simulated rain field;

FIGS, 18A, 18B are photomicrographs, respectively, of a lapped ZnS lens which was conven tionally polished and a lapped ZnS lens which was tempered in accordance with the present invention showing distortion caused by the compressive layer; and FIG. 19 is a cross-sectional view, of a. portion of an optical element such as a plate or dome 20 having a compressed layer of optical material and a coating layer in accordance with a further aspect of' the present invention-.

DescriPtion -of the Preferred Embodiment

Referring now to FIG. 1, an optical element here a plate 10 is shown to include a base layer 25 12 comprising a material having predetermined optical properties. Although the optical element is herein described in particular as -being a plate, it is understood that other types of optical elements such as windows, domes, lenses, etc. having shapes other than planar may alterna tively be substituted for the aforementioned plate 10. Typically base layer 12 will have a thickness of-at least 0.05in. generally 0.1in. to about 0.5in. or thicker. The optical element further may have selective optical properties. For example, the optical element may be comprised of a material -which -is transparent to optical energy generally in the infrared, visible, and/or ultraviolet spectrums. The material may be a dielectric or a semiconductor material. In particular, for optical elements used in infrared imaging systems in the 8 um to 12 um wavelength range, examples of preferred materials include silicoq, germanium, gallium arsenide, gallium phosphide, 35 mercury cadmium telluride, cadmium telluride, zinc sulfide, zinc selenide, or one of the ternary sulfides. The selected material comprising layer 12 may be fabricated by any known technique such as powder compaction and densification or chemical vapor deposition. In particular for infrared applications, the materials selected for layer 12 are generally characterized as having a.

relatively -low modulus of elasticity typically in the range of 5 X 106 psi to 15 X 106 psi, a high 40 transmittance to infrared energy, typically in a range of 50% to 75% over at least a portion of the infrared wavelength band of 2.0 um to 30,um, and an index of refraction at 10 microns typically in the range of 2.2 to 4.0. The relevant mechanical and optical properties of some of these materials are shown in Table 1.

6 G132 192733A 6 TABLE 1

Properties of Materials for.Base Layer 12 Thermal Expansion Elasticity 5 Coefficient Modulus of " - - 70tyf Material- n @ 10.k m R% (10-6/ oC) X(106psi) CdTe 2.6 0.01 5.9 5.0 ZnSe 2.4 0.25 7.7 9.7 Zn$ 2.2 0.89 7.4 10.8 CaLa2S4 2.4 0. 25_ 14.7 13.8 15 Gal? 3.0 0037 5.3 20.6 GaAs 3.3 1.17 5.7 15.5 20 Ge 4.0- 4.10 6.1 15.0 11% is the reflection loss per surface resulting from a single quarter wavelength AR coating of 25 Y203 as will be described below, applied over the corresponding material. Disposed, over base 12 is an impact resistant anti-reflection coating layer 11. Suffice it here to say that coating layer 11 may have any one of the structures now to be discussed._ Referring now to FIG. 2, coating layer 1 -1 is shown to include a first protective layer 14 disposed over and preferably on the material comprising base 12. Protective layer 14 is corn- 30 prised of a material having a modulus of elasticity substantially higher than the modulus of the material of the base 12, a high-degree of infrared transparency in the deposited thickhess over the Selected wavelength band. Of the optical element, and preferably an index of refraction less - than the' index of refraction of the material comprising base layer 12. Furthermore, the material deposited has a high degree of adherence to the material of layer 12 and in particular is highly 35 resistant to debonding caused by Sheer stresses induced by radial outflow of high velocity droplet impactsuch as water droplet impact. Layer 14 may be deposited by any technique such as ion beam sputtering, diode sputtering or evaporation. Furthermore, layer 14 may alternatively be provided over the plate 12 by dipping plate 12 in a solution comprising an organic vehicle and the Ngh modulus of elasticity material. The plateafter being dipped in the solution of such 40 material is. removed from this -solution and placed in an oven whore the organic vehicle is driven off. Alternatively, the coating may be deposited by spray drying a mixture of a vehicle:and the coating material over the base layer' _12 heated.to a predetermined temperature. With these particular coating_ arrangements,, a relatively inexpensive technique is provided to for m a uniform layer of coating 14 on base 12. For the aforementioned base layer materials, suitable coating 45 materials include -yttrium oxide (Y,O,), magnesium oxide (M90) and scandium oxide (Sc,O,), as:h well as, homogeneous mixtures of these aforementioned materials. The relevant mechanical and optical properties for these materials care showri-in Table 2.

50 TABLE 2 _.,?roperties Of Materials.For Layer 14 Trans- Thermal (106 psi) missivity Expansion 55 range of Coefficient Modulus of H20 Material @ (1, 0 1.4, V.SA&m thick 10-6/' o C Elasticity Solubility.

Y20.3 1 - 63 0.21- 1 2.,U 7.2 25 insol.

Sc203 1.70 0.3-12 -8.5 26 insbl.

M90 1.70 0 1.21- 1 2A 12.0 48 Insol.

reacts with Hpl_ 65 7 GB2192733A 7 The principal factors to be considered in selecting materials for coating layer 14 are that the selected material must have optical properties suitable for the intended use of the optical element 10 at the thicknesses at which the material of coating layers 14 is deposited. Further, the coating layer material 14 must have a modulus of elasticity generally about at least twice the modulus of elasticity of the material of base layer 12. Further, when the intended use of the 5 optical element 10 will expose the coating layer 14 to water, the material of coating layer 14 must be insoluble and stable in water. In order to provide anti- reflection correction, the index of refraction of the material of coating layer 14 is preferably less than the index of refraction of the material of base 12. In general for anti-reflection correction between air having an index -of about 1.00 and the material of base layer 12, the index of refraction required of the coating (n14) is 10 approximately equal to the geometric mean of the indices of refraction of the material of base ' layer 12 and the surrounding medium (n14_V_n_j2). As is known in general for most Materials, the index of refraction varies as a function of wavelength dispersion. Accordingly, this anti-reflection correction also varies as a function -of wavelength.

Preferably, layer 14 is deposited over the base layer 12 to a physical thickness corresponding 15 to onq-quarter of an optical wavelength at the particular wavelength of interest for the optical element. In general, the optical thickness (t.) of such elements is defined as the product of the physical thickness (t) of the coating 14 and the refractive index (n) of the material of the coating 14 (t. = (t ' nc). The desired physical thickness for an optical thickness of A/4 is given by t (A/4)/ hc where A is the wavelength of particular interest for the optical element and nc is the 20 refractive index of the coating at the wavelength of interest. As will now be appreciated by those skilled in the art, the optical thickness (t.) may be a higher order thickness such as 3A/4 or 5A/4, and.the physical thickness t is then generally given by t = [(2N+ 1)A/4]/nc where N is an integer 0.1,2,3.... Thus, the physical thickness t of layer 14 may be increased, offering greater impact resistance protection for base 12 while still maintaining good anti-reflection and 25 optical transmissive properties. For a material such as Y203 having an index of refraction of nc= 1.63 at 10.6 micronsl the optimum thickness for a single A/4 layer at 10.6 microns would be about 1.63 microns.

Referring now to FIG ' 3, a plate 10 -is shown to include the base 12 and coating layer 11 disposed over at least a first surface thereof. Coating layer 11 is here a composite, antireflective 30 impact resistant layer 15. Layer 15 is shown to comprise the aforementioned coating layer 14 of the high elastic modulus material having an index of refraction less than the index of refraction of the material of the base 12, and having good adherence qualities to the material of the base layer 12. Disposed over this first, coating layer 14 is a second coating layer 16 comprising a second material having a substantially higher elastic modulus and having an index 35 of refraction higher than-those; of both the material of base 12 and first coating layer 14.

Suitable materials for the -aforementioned second coating layer 16 includes cerium oxide, titanium oxide, zirconium oxide and hard carbon. Of these examples, hard carbon is the preferred material since it has the highest modulus of elasticity. However, as mentioned previously, hard carbon does not adhere well to zinc selenide zinc sulfide etc. which are amongst the preferred materials 40 for base layer 12, particularly at optical wavelengths in the range of 8 um to 12 um. The relevant mechanical and optical. properties of these materials are shown in Table 3.

TABLE 3 45

Properties Of Materials For Layer 16 Themal Modulus _Expansion Of 50 n Coefficient Elasticity R20 Material-'' @(10,&m) 10-6"?C' i()6 psi Solubility Hard' 2.2-2.4 15b insol.

carbon 55 Ceo;.34 11.3 22 insol.

Ti02_ 2.4 8.2 44 insol.

ZrO2 _ -'..... 2. 1...'.. 7.2 20 insol. 60 Hard carbon films may be deposited by any suitable technique. For example, ion beam sputtering, as well as, chemical vapor deposition involving decomposition of hydrocarbon con- 65 8 GB 2 192 733A 8 taining vapors may be used. Although hard carbon layers have not generally adhered well to most IR materials, as mentioned previously, it is believed that such hard carbon layers will adhere to materials suitable for use as first coating layer 14. As mentioned above, suitable materials for layer 14 include, MgO, 9cO, and Y,O,. In general, hard carbon adheres very well to oxides of various types including the aforementioned ones. Thus, by interposing coating layer 14 5 between the very high modulus layer 16 and base 12, the impact resistant advantages of, the very high modulus layer 16 materials provide an anti-reflection, highly impact resistant composite layer 15 to protect the base 12 from high velocity water droplet impact.

Referring now to FIG. 4, an optical element here a plate is shown to include the base layer 12 and coating layer 1 1._ Coating layer 11 here is a broadband anti- reflection impact resistant 10 coating layer 17 having a plurality-of the aforementioned composite, anti- reflective impact resistant coating layers 15, as described in conjunction with FIG. 3. With this arrangement, a very thick anti- reflective coating 11 having excellent adhesion properties to base 12 and high impact - resistance is provided. Further, by appropriate -selection of thicknesses of the plurality of compo- site layers 15 and the individual coating layers 14 and 16 in accordance with multilayer coating 15 optical design principals,- a broadband anti-reflection coating may also be provided.

Referring now to FIGS. 5A-5D, a series of graphs from a Fields et al report entitled Liquid Impact Erosion Mechanisms In Transparent Materials AFWAL-TR-82-4022 shows radial stress reduction as a function of the gormalized distance from the center of the drop impact. Each graph has plots representative of the radial stress developed' on a coated surface having a 20 modulus of elasticity-higher'than t hat of the material of the optical elements in ccmparison with the radial stress developed on an uncoated surface. As shown in FIG. 5D, when the modulus of elasticity of the material of. the coating is approximately equal to 10 times the modulus of the material of the base layer, the tensile stresse& induced in the base layer are substantially equal to zero during water dropletimpact. 25 Referring now to FIGS. 6 and 7, photornicrographs of an uncoated surface (FIG. 8) and a coated surface (FIG. 9) each after exposure to a simulated rain field at the rate of 1 inch per hour, at a velocity of 450 miles per hour, With an incident angle of 90' and droplet diameter of 2mm is shown. As can be observed, the amount of damage exhibited by the uncoated surface of zinc sulfide is substantially higher than the damage exhibited by the yttrium oxide coated zinc 30 sulfide surface.

Referring to FIG. 8, a plot of a percent transmissivity vs. wavelength for a 0.2 inch thick coated plate of RAYTRAN (Trademark for Raytheon Co. Lexington, Ma.) type of zinc sulfide is shown. The coating was yttrium oxide a single quarter wavelength thick at 10.0 microns. The coating was approximately 2.45 microns thick.' The coating was selected to maximize the 35 transmittance of the surface at 10 microns, and was applied to both major surfaces of the plate.

Referring now to FIGS. 9-12, a portion of an optical elerrient, here a dome 110, is shown to include a layer 112 comprising. a material having predetermined. optical properties. Although the optical element is herein described in particular as being a dome, it is understood that other types of optical elements such as windows, plates, lenses, etc. ma, y alternatively be substituted 40 for the aforementioned dome 110. Typically, base layer 112 will have a thickness as base layer 12. The optical element further may have selected optical properties. For example, the optical element may be comprised of a material which: is transparent to optical energy generally in the infrared, visible, or ultraviolet spectrums. The material of the optical element may be a dielectric or a semiconductor material.' In particular, for optical elements used for infrared. imaging systems, 45 examples of preferred materials include silicon, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride, Cadmium telluride, zinc sulfide, zinc selenide or one of the ternary sulfides of the general form MN2S4 where,M is a monovalent ion, N is a ion selected from the lanthanide series and S is the sulfide ion S-2. The selected material comprising layer 112 may be fabricated by any known technique such as, powder compaction and densification or chemical vapor deposition. In general,_ rnaterials which are selected for layer 112 are characterized as having a elatively high transmittance -to a particular- energy, for example, infrared energy typically in excess of 50% at least over a portion -of th.e-infrared band, are generally very brittle and fairly - stiff materials having a modulus of elasticity in the range of 5 to 15 x 106psi, but are generally fairly weak materials having a fracture strength of typically Z,500 to 15, 000 psi. 55 The dome 110 further includes a'plastically deformed compressive layer 114 (FIG. 10) pro vided over surface 1 12a-. of the optical element 110. Preferably, the compressive layer 114 is a portion of the material of layer 11, 2. As shown more particularly in FIG. 12, the compressive layer 114 includes a plurality of furrows or grooves 113 with adjacent ones of said grooves or furrows 113 being spaced by compressed regions 113a of the material of layer 112 and having 60 disposed under said furrows a compressed layer 1 13b. The degree to which the layer 114 is compressed is a function of-the magnitude of the compressive forces which are provided during treatment of the dome 1 10;as will be described.

Referring now to FIGS. 11 and 12, the mechanism by which the compressive layer 114 65. strengthens and protects the dome 110 from. damage encountered during high velocity water 65 9 GB2192733A 9 impact or mechanical loading will now be described.

As shown in FIG. 11, a conventionally polished surface 11 2a has a water droplet 115 having a resultant velocity VO normal to a surface 1 12a impinging upon surface 1 12a of layer 112.

Present in surface 112a of layer 112 is a pre-existent microflaw 116 created during fabrication of the polished surface or otherwise associated with the morphology of base 112. During impact 5 of water droplet 115 with conventional surface 112a, a tensile component - of the resulting surface stress wave represented by arrows 118 is produced. In response to this tensile force 118 in the region of the microflaw 116, the microflaw propagates as crack 116'. If the tensile force is sufficiently high, the crack 116' can propagate completely through the base 112 of optical element 10. With a sufficient number of these cracks, the optical transparency of the 10 element can be severely reduced due to internal reflections and refraction in the region of the crack. More importantly, the optical element with sufficient cracks is susceptible to shattering or breaking thus exposing the remainder of the optical system (not shown) to catastrophic damage.

As shown in FIGS. 12 and 13 in accordance with the present invention, as compressed layer 114 is provided over the dome 110, the pre-existing microflaws 116 provided in the base 112 15 are healed by compression of the material in region 114. During fabrication of the compressed layer 114, opposing compressive forces 114 are provided which cause the microflaws to shrink in size resulting in smaller microflaws 117. Furthermore, the material surrounding the microflaw 117 is still under compression as represented by arrows 120 pushing in on the material around the microflaw 117. During water droplet impact, as mentioned above, a tensile stress compo- 20 nent 118' is provided at the site of the microflaw 117. Accordingly, since the microflaw 117 is smaller, the velocity at which the water droplet may impact the surface layer without causing damage has increased since the velocity threshold is increased by reducing the size of the microflaw,' Furthermore the material remains in compression after the furrows 113 are provided and the reduced tensile force component 118' created at microflaw 117 will not propagate 25 through the compressive layer 113 and into the base layer 112 unless the tensile force compo nent 118' can overcome the compressive force representing the degree to which the layer 114 is under compression. Accordingly, the compressive layer 114 provides two mechanisms to increase the velocity threshold at which impact damage will occur: it generally reduces the extent of the microflaws present in the material thereby creating for a given water droplet impact 30 velocity a smaller resulting tensile component; and provides a compressive force to dampen or reduce the propagation of the tensile force produced through the compressed layer 13'.

The preferred method for providing the compressive layer 114 is to machine a surface portion, of the optical element with a single diamond point. Generally, the surface may be machined in two steps. The first step, a "rough cut," has machining parameters selected to remove substan 35 tial amounts of material 119 as shown in FIG. 10, this may be on the order of 1 to 5 mils or.

more of material. The second cut, a "finishing cut," may be of one or several passes or cuts where a small amount of material, typically 0.1 to 0.5 mile is removed to provide a substantially flat, yet furrowed surface.

Typical surface characteristics for the tempered compressive layer 114 shown in FIG. 1.0 are 40 as follows:

The furrows 113 typically may have a width wf typically of 0.0 1 to 0.02 mm. The. sidewall Z portions 113a generally have a height hf of 10-A to 10,000 A.

Typical machining parameters that are useful to machine a compressive layer 114 in 1 inch zinc sulfide discs are as follows: 45 GB2192733A 10 Useful Single-Pt Machining Parameters for Creating Compressive Surface Layers -5 Rough Cut 5 Depth of Cut = 0.003 inch Rotation.Speed = 750 rpm Feed Rate = 0.5-inches/min. 10 Tool Radius = 0.125 inches Finish Cut Depth of Cut 0.0002 inches 15 - Rotation Speed 550 rpm - -Feed Rate 0.250 inches/min.

Tool Radius --0.125 inches 20 Sixty samples of zinc sulfide discs were fabricated in accordance with the present techniques.

Mechanical parameters measured on these discs were knoop microhardness (Kg/MM2) and frac ture strength, Fracture strength was generally at least 19,500 psi which is greater than the fracture strength (15,500) of conventional polished samples. Accordingly, this technique provides 25 about a 25% increase in f racturer -strength.

As shown in FIGS. 14A, 14B the conventionally polished surface is substantially smooth, uniform and featureless, whereasl the surface prepared in accordance with the present invention has substantially regularly spaced apart-furrows or-grooves.

As shown-in FIGS. 15 and 16, typical microhardness plots of knoop hardness numbers vs. 30 load for conventionally polished zinc sulfide discs and zinc sulfide discs having the compressed layer 114 are shown. The knoop hardness for discs having the compressed layer 114 are generally between 50 and 100 numbers higher than the knoop hardness numbers for conven tionally polished discs for loads of 30 grams or less. Furthermore, as shown in FIG. 16, the hardness -difference expressed -as difference in knoop hardness numbers as a function of pene- 35 tration depth into the compressed layer 114- indicates that a significant increase in hardness is provided for penetration depths of 2 microns or less. By extrapolation of this data, it is inferred that the, hardness effect is confined within a -3 micron surface portion of the machined samples.

A comparison of FIGS. 17A and 17B, conventionally polished vs. diamond point machined zinc sulfide plates following 5 minutes of exposure at 450 miles per hour to a 1 inch per hour 40 simulated rain field, 2 mm drop diameter, shows that the conventionally polished sample sus tained significantly more damage consisting primarily of subsurface fractures; whereas, the dam age sustained by the plate having the compressed layer provided by single diamond point machining a zinc sulfide sample (FIG. 17B) is substantially reduced.

Comparison of FIGS. 18A and 18B indicates that layer 13 is a compressed layer of optical 45 material. Two zinc sulfide lens blanks from the same lot of zinc sulfide material were polished.

One lens had its surface polished optically flat using conventional polishing techniques, the other lens had its surface machined flat in accordance with the present invention. Following fabrication, each sample was mounted face down on a lapping pad and thinned down from approximately 0.2 inches to 0.01 inches in t hickness and to a quarter wavelength flatness in the visible 50 spectrum across the lapped' surface. When these samples were released from the lapping pads, some distortion in each sample took place. This distortion is shown in the interference topo graphs of FIGS. 18A and 18B. As shown in FIG. 18A, the distortion in the conventionally polished sample was minimil resulting in a final irregular surface. However, as shown in FIG.

18B, for the sample having -th& compressed layer 114 in accordance with the present invention, 55 the distortion was so severe that it could not be measured on the interferometer. Furthermore, once the specimen in FIG.1813 was released from the lapping pad, the sample distorted into a highly concave sUrfabe. This distortion is related to the inherent stresses contained in the layers of the lapped sample. Accordingly, it is clear that substantially no compressive stress is pro- vided in the conventionally polished blank (FIG" '18A); whereas, the sample represented in FIG. 60 18B was provided with a highly compressed layer 114. The radius of the concave surface was optically measured and this radius was used to estimate the amount of compressive stress present at the surface of the machined sample-(FIG. 18B). Compressive stress (S) as related to the radius (R) of the curvature of the surface was as follows:

1 GB2192733A 11 S E'd2 6(1-)tR 5 where E is Young Modulus given as 10.8 x 106 psi; d is the sample thickness estimated to be -0.009 inches; R is the radius of curvature measured as 129.2 x 10-2M; t is the thickness of the compressed layer, estimated as 1 X 10-6 m and V is the Possion's ratio estimated as 0.28.

Solution for S provides S = 7100 kg/CM2 or 1 X 105 psi. Accordingly, the strengthening and hardened effects demonstrated in the previous data are clearly the result of the formation of a 10 sufface compressive surface layer on the zinc sulfide blanks during the operations described above. Therefore, by proper choice of machining parameters such as the tool speed, tool type, depth of cut, the feed rate, tool angle, etc. the magnitude of the compressive layer provided on the zinc sulfide surface may be selected and, hence, the degree of strengthening/hardening of the zinc sulfide may also be controlled. 15 Referring now to FIG. 19, a portion of an optical element, here a plate 130 is shown to include the layer 12 comprising a material having predetermined optical properties as described above. Disposed over layer 12 is a compressed layer of material 114 as described in conjunction with FIGS. 9-18. Disposed over compressed layer 114 is a coating layer 11 comprising one of the aforementioned single layer or multilayer coatings described in conjunction with FIGS. 1-8. 20 With this particular arrangement, it is believed that the enhanced hardening and rain erosion resistant properties of both techniques described previously will aid in providing an optical element having substantially improved rain erosion resistance and fracture strength. Having described preferred embodiments of this invention, it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is felt, 25 therefore, that this invention should not be limited to the disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.

Claims

1. An optical element compCising: 30 a base comprising a first material having a predetermined optical transmissivity over a predet ermined range of the optical wavelengths and a predetermined resistance to damage caused by high velocity droplet impact; means disposed on such base for increasing the resistance of the base to damage caused by droplet impact ccmprising a second material having an index of refraction less than 2.0. 35 -2. The optical element as recited in claim 1 Where in said means comprises a compressed region of said second material.

3. The optical element as recited in claim 2 wherein said compressed region of second material is a compressed region of said first material.

4. The optical element as recited in claim 3 wherein the compressed layer includes a plurality 40 of furrows disposed in a surface portion of said layer having disposed under said furrows a compressed layer of said first Material and having disposed between adjacent furrows com pressed regions of the first material.

5. The optical element as recited in claim 4 wherein the first material comprises a material selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, 45 gallium phosphide, mercury cadmium telluride, cadmium telluride and a ternary sulfide havingr the, general chemical formula MN2S4 where M is a cation selected from the Group 1A elements, N is a cation selected from the lanthanide rare earth series elements, and S is the sulfide anion S2-.

6. The optical element as recited in claim 4 wherein the furrows have a depth in the range of 10 A to 10-,000 A and have a'width generally in the range of 0.01 to 0.02 mm. 50 7. The optical element as rebited in claim 1 wherein said first material has a first predeter mined modulus of elasticity and wherein said second material has a second, higher modulus of elasticity than the modulus of elasticity of the first material-to provide the increased impact resistance to the base.

8. The optical element as recited in claim 7 wherein the modulus of elasticity of the second - 55 material is at least twice the modulus of elasticity of the first material.

9. The optical element as recited in claim 8 wherein the second material has a physical thickness t given by t = (2N + 1)A/4/n. where N is an integer equal to 0, 1,2,3,..., A is a particular Wavelength of interest and n. is a refractive index of the second material at the wavelength 60 10. The optical element as recited in claim 9 wherein the first material comprising the base layer is selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, ballium phosphide, Mercury cadmium telluride, and cadmium telluride and a ternary sulfide having the general chemical formula MN2S1 where M is a monovalent cation selected from the Group' 1A elements, N is a cation selected from the lanthanide rare earth series-elements 65 I 12 GB2192733A 12 and S is the suffide anion S2-; and wherein the second material is selected from the group consisting of yttrium oxide, scandium oxide, magnesium oxide or mixtures of yttrium oxide, scandium oxide and magnesium oxide.

11. The optical element as recited in claim 1 wherein the first material has a first predeter mined modulus of elasticity and wherein the means comprises a first layer of the second 5 material, said second material having a second, higher modulus of elasticity than the modulus of elasticity of the first material and a second layer of a third material having a modulus of elasticity higher than the modulus of elasticity of the first material and the second material, and having an index of refraction greater than 2.0. - 11. The optical element as recited in claim 11 wherein the thickness of the composite layer 10 has a physical thickness t given by t = (2N + I)A/4/n,, where N is an integer 0, 1,2,3,...,), is a particular wavelength of interest and n,, is the index of refraction of the composite layer.

13. The optical element as recited in claim 11 wherein:

the base comprises a material selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride and cadmium teliuride 15 and a ternary sulfide having the general chemical formula MN., S, where M is a cation selected from the Group 1A elements, N is a cation selected from the lanthanide rare earth series elements and S is the sulfide anion S2- wherein the second material is selected from the group consisting of yttrium oxide, scandium 26 oxide, magnesium oxide'.and. a mixture of yttrium oxide, scandium oxide and magnesium oxide; 20 and j wherein the third material isselected from the_group consisting of hard carbon, cerium oxide, titanium oxide and- zirconium oxide. 1 14 ' -The optical element as recited in claim 1 wherein the first material of the base has a first predetermined modulus of elasticity- and wherein- the means further comprises: 25 a compressed region of said second. material and a layer of a third material having a second, higher modulus of elasticity than the material of the first layer.

15. The optical element as recited in claim 1 wherein the compressed region of second material is a compressed segion of the: first material and is disposed in the base and wherein the layer of the third material is disposed in the compressed region of first material. 30 16. The optical element as -recited in claim 15 wherein the modulus of elasticity of the third material is at least twice that of the: material of the base.

17. The optical element as recited in claim 16 wherein the first material is selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride and cadmium telluride and a ternary sulfide having the general chemical formula IVIN,S, where M. isa monovalent ion selected from the Group 1A elements, N is a divalent ion selected from the lanthanide rare earth series elements and S is the sulfide anion S2-; and - wherein the third materiaCis selected from the group consisting of yttrium oxide, sdandium oxide, magnesium -oxide or mixtures of yttrium oxide, scandium oxide and magnesium oxide. 40 18. The optical element as reci ' ted in claim 17 wherein the third material has a physical thickness-t given by t _ (21\1+1)A/4/n. where.N is an integer equal to 0, 1,2,3,..., A is a particular wavelength of interest and n. is a refractive index of the third material at the wave length 19. The optical-element as-recited in claim 14 wherein said means further comprises a layer 45 comprising a fourth material- -disposed over the third material said fourth material having a modulus- of -elasticity higher than the modulus of elasticity of either the first or third material and an index of refraction greater than _2.0.

20. The optical element as recited in: claim 19 wherein the thickness of the composite layer is 56 given by t = (2N + 1) A/4/n, where N is an integer 0,1,2,3, A is a particular wavelength and 50 n. is the index of refraction of the composite material at the wavelength 2V. The optical element as recited in claim 20 wherein the first material is selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride 6nd cadmium telluride and a ternary sulfide having the general chemical formula NINS, when M is a group 1A element N is a lanthanide rare earth element and 55 S is the sulfide anion S2- said third material is selected from the group consisting of yttrium oxide, scandium oxide, magnesium oxide or mixtures of yttrium oxide, scandium oxide and magnesium 'oxide, and said fouhh material is selected from the-group consisting of hard carbon cerium oxide-, -titanium oxide and zirconium oxide.

22. An optical element comprising. 60_ a base comprising a first material having a predetermined optical transmissivity over a predet ermined range of optical wavelengths and a predetermined modulus of elasticity; a layer of a second, different material having a modulus of elasticity at least twice the modulus of the material of the first layer and having an index of refraction less than 2.0.

23. The optical element as recited in claim 22 wherein the base comprises a material selected. 65 13 GB2192733A 13 from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium - phosphide, mercury cadmium telluride, cadmium telluride and a ternary sulfide having the general chemical formula MN2S4 where M is a monovalent cation from the Group 1A elements, N is a cation selected from the lanthanide rare earth series elements and S is the sulfide anion S2-; and wherein the second material is selected from the group consisting of yttrium oxide, scandium 5 oxide, magnesium oxide or homogeneous mixtures of yttrium oxide, scandium oxide and magne sium oxide.

24. The optical element as recited in claim 23 wherein the layer of second material has a physical thickness t given by t = Q21\1+1) A/4)/ric where N is an integer 0,1,2,3,..., A is a- particular wavelength of interest and ric is the refractive index of the second -material at the 10 wavelength A.

25. An optical element comprising:

a base comprising a material having a predetermined optical transmissivity over a predeter mine d range of wavelengths, said material having a first predetermined modulus of elasticity and a first predetermined index of refraction; 15 a composite coating disposed over at least a portion of said base comprising:

(i) a first layer comprising a first high modulus of elasticity material having a modulus of elasticity of at least twice the -modulus of elasticity of said base material and having an index of refraction less than the index of refraction of the material of said base; (ii) a second layer comprising a s econd high modulus of elasticity material having a modulus of 20 elasticity of at least twice the modulus of elasticity of said base material and having an index of refraction greater than 2.0.

26. The combination as recited in claim 25 wherein: the base comprises a material- selected' from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, Mercury cadmium'telluride and cadmium telluride and a ternary sulfide having the 25 general chemical formula MX2 S, where M is a monovalent ion selected from the Group 1A elements, X is a divalent ion selected from the lanthanide rare earth series elements and S is the sulfide anion S2-; wherein the first high modulus material is selected from the group consisting of yttrium oxide, - scandium oxide and magnesium oxide and mixtures of yttrium oxide, scandium oxide and 3 0 magnesium oxide; and wherein, the second high modulus material is selected from the group consisting of hard carbon, cerium oxide, titanium oxide and zirconium oxide.

27. The combination as recited in claim 26 wherein the first layer has a physical thickness t given by t = ((2N + 1)A14)1n, where N is an integer 0,1,2,3,,A is a particular wavelength of 35 interest and nc, is the index of refraction of the'material of the first layer; and wherein the physical thickness of the second layer is given by t = ((2N + 1)A/4/ric2 where ncl is the index of refraction of the material of the second layer.

28. A method of protecting a region of an optical element comprised of a first material having a first modulus of elasticity from impact damage encountered in a high velocity droplet environ- 40 ment comprises the step- of:

providing a layer of a second material over the region to be protected having a modulus of elasticity at least twice the modulus of elasticity of said first material and which is resistant to debonding from the first material in response to sheer stress caused by radial outflow during droplet impact. 45 291 Th e method as recited in claim 28 wherein the material comprising the optical element is selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide, mercury cadmium telluride and cadmium telluride; and wherein the second material is selected from the group consisting of yttrium oxide, scandium oxide, magnesium oxide or homogeneous mixtures of yttrium oxide, scandium oxide and magne- 50 sium oxide.

30. The method as recited in claim 28 further comprising the ste p of:

providing a layer of a third material over the layer of second material, said third material having a modulus of elasticity at least twice the modulus of elasticity of said first material and an index of refractiop greater than the index of refraction of the second material. 55 31. The method a's recited in. claim 30 wherein the third material is selected from the group consisting of hard carbon, cerium oxide, titanium oxide and zirconium oxide.

32. A method of bonding a layer of hard carbon to a base. comprising a material -selected from the group conisting of a Group 11-V or Group 111-V material comprising the step of:

providing a first layer comprising a material selected from the group consisting of yttrium 60 oxide, scandium oxide and magnesium oxide between the base and the hard carbon layer.

83. Thd method of claim U wherein the,Group 11-VI material is selected from the group consisting of zinc sulfide, zinc selenide, mercury cadmium telluride, cadmium telluride and the Group 111-V material is selected from the group consisting of gallium arsenide and gallium phosphide. 65 14 GB2192733A 14 34. The method as recited in claim.33 wherein the base of the first layer and hard carbon layer provide an optically transparent element.

35. The method as recited in claim 34 wherein the first layer has a physical thickness (t) related to t = (2N+ 1),Z/4/nc where A is a wavelength of interest, ric is the refractive index of the material of the first layer at A and N is an integer 0, 1,2,3,. 5 36...An: optical -element comprising.

a base comprising a material having a predetermined optical transmissivity, with a surface portion of said optical, material being in compression, said compressed surface portion having a plurality of furrows disposed in said surface- portion having underlying said furrows and disposed between adjacent, ones of said furrows compressed regions of the material of said base. 10 37. The optical element as recited in claim 36 wherein said base comprises a material selected from the group consisting of zinc sulfide, zinc selenide, germanium, gallium arsenide, gallium phosphide,, mercury cadmium telluride, cadmium telluride, and a ternary sulfide having the general chemical formula. IVINS, -where M is a monovalent. cation selected from the group 1 A elements, N is cation selected from the lanthanide rare earth series elements and S is the sulfide 15 anion S,-.

38. The optical element of claim 37 wherein said compressed region of material has a thickness -of--about three microns. - 39. A method of providing-a tempered, strengthened surface to a material comprising the step 2Q of-converting a surface Portion of said material into a compressed surface portion of said 20 material to Provide said layelF having a higher f racture strength than the fracture strength of the material.

40.- A method of strengthening the outer surface of an optical element includes the step of:

single- point turning the surface of the optical'element to introduce onto such outer surface a 254 compressed- 'surface 18yer portion of said optical element. 25 - 41. The method as recited. in claim 40 wherein said turning step is accomplished with a single diamond point tool.

42. The method as recited in claim 41 wherein the turning step comprises the steps of first turning the optical element to remove a surface layer portion having a thickness of at least 1 mil from said outer surface; and 30 then turning said optical' element to remove a surface portion of at least 0.01 mils thick from - said outer surface to provide on said outer surface of said optical element furrows in said surface portion, said furrows spaced by regions of the compressed material.

43. The method of claim 42 wherein said optical element is selected from the group consist ing of -a dome, a lens, a plate and a window. 35 44. The method as recited in. claim 43 wherein said material of the optical element is selected from the group consisting. of zinc sulfide, zinc selenide, mercury cadmium telluride, cadmium telluride, germanium, gallium- arsenide and gallium phosphide.

45. The method as recited in claim 44 wher ein said material is selected from the group 4b consisting of zinc selenide and' zinc sulfide. 40 Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC111 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 31). Printed by Burgess & Son (Abingdon) Ltd. Con. 1/87.