PRIMERA INFANCIA Y DESARROLLO CREATIVO

When coatings are mentioned in the context of optical instrument design, one normally thinks first of thin films applied to optical substrates to change the spectral intensity distribution or polariza- tion state of transmitted or reflected radiation incident upon those components. Those types of coat- ings are not discussed here. Rather, we define a “coating” as a layer of some material or a finish added externally to the surfaces of optical and mechanical parts for the purposes of (a) protecting those parts from damage or degradation of performance due to adverse environmental exposure; (b) of performing a special function such as reducing or increasing absorptivity or emissivity of a com- ponent’s surface — usually for stray light suppression or thermal reasons; or (c) of providing a more suitable surface for diamond turning or polishing an optic such as a metal mirror to a smooth micro- roughness condition. For information regarding the design of the traditional thin film optical coat- ings, the interested reader is referred to other publications such as Macleod (1986), Rancourt (1987), and Willey (1996).

3.7.1 PROTECTIVECOATINGS

As mentioned in Chapter 2, corrosion is a common reaction between a material and its environment. It may exist in the form of fretting (due to small relative motions of parts that break down protec- tive oxides on surfaces), galvanic attack (due to electron flow between dissimilar materials — usu- ally in the presence of moisture), hydrogen embrittlement (due to diffusion of atomic hydrogen into a metal making it susceptible to brittle fracture), or stress-corrosion cracking (due to growth of a corrosion pit into a crack under tensile stress). To reduce the effects of corrosion, we apply various types of coatings to exposed surfaces of particularly susceptible materials. Common coatings are discussed here.

3.7.1.1 Paints

Most paints are a mixture of a film-forming binder (for continuity) and pigment (to provide color and opacity). Common types are oil- or water-based paints, varnish (a blend of resin and drying oil), enamel (using varnish or synthetic resin as the binder), or polymers such as acrylic or vinyl. For optical instruments, enamels and polymers are most commonly used, especially in production where fast drying is important. Powder coatings are solids applied to the surface and heated to form a continuous film. Ingredients such as aluminum flakes, zinc dust, glass beads, fungicides, sili- cones, or catalysts are sometimes added. Proper surface preparation (cleaning and sometimes roughening) and the use of multiple layers, including primers, are typically required for durability.

3.7.1.2 Platings and Anodic Coatings

Cadmium, chromium, and nickel platings are frequently added to other metals to protect their sur- faces from corrosion. Ferrous metals can be galvanized by dipping in molten zinc, by heating and tumbling in zinc dust (the Sherardizing process), by electrolytic deposition, or by flame spraying with atomized powdered metal (metalizing). Cadmium applied by electroplating is also used on iron parts, but its durability may be impaired by the presence of sulfur in the atmosphere. Carbon steels and the Invars are frequently protected by chrome plating. Usually, a nickel undercoat is applied before coating with chromium. The chromium layer coating is hard, and so serves as a protection against wear and abrasion.

Aluminum alloys are usually protected by a thin aluminum oxide layer formed by making the part serve as the anode in a bath of chromic, sulfuric, or oxalic acid electrolyte. The so-called

anodized surface so formed adds slightly to the dimensions of the part, so precision positioning and

alignment of joined parts must take this into account. The anodic coating is non-conducting and must not be applied to surfaces where electrical contact is required. Some stray light suppression is achieved at other than grazing incidence by a black anodized coating. Some other colors can also be achieved in the anodic coating for cosmetic effects.

To enhance the natural corrosion resistance of stainless steel parts, they may be passivated by making the part the cathode of a low-voltage circuit in a weak acidic electrolyte.

3.7.1.3 Proprietary Coatings

Certain proprietary coatings have become available for application to metals in optical instruments and other applications in recent years. Two such coatings are described here.

The first is a room temperature alkaline electrolytic process that imparts a hard coating to titanium. This coating reduces the tendency for a titanium surface to gall when rubbed on another

titanium surface and, in general, improves its wear resistance. In this Tiodize®_{process, produced by}

Tiodize Co., Inc., Huntington Beach, CA, the coating penetrates the surface and causes no dimen- sional buildup when a soft outer layer is removed by ultrasonic cleaning or burnishing. This process was used in the manufacture of a high-performance lens system involving interference-fit assembly of titanium lens cells in a titanium barrel as described in Section 5.3.

The second proprietary coating is the Magnadize® _{process by General Magnaplate Corp.,}

Huntington Beach, CA. It increases the corrosion resistance and hardens the surfaces of magnesium parts. After cleaning, the part surfaces are converted chemically to a thin layer of hydrated magne- sium oxide, a porous ceramic. This layer is then infused with a polymer or sealant to form the pro- tective coating. This coating is hard, displays low friction, and protects the otherwise vulnerable magnesium surface. Dimensional buildup on the surface is typically 0.0003 to 0.0020 in. (0.008 to 0.051 mm).

Lytle (1995) described special coatings that can be applied to plastic optics to make them reflec- tive. These are usually vacuum-deposited metals such as aluminum or chromium plated onto the opti- cal surface. To make the surface more durable, a dielectric overcoat may be applied over the metal. This is done by vacuum deposition over aluminum films or by spraying or dipping an organic mate- rial over chromium. Lytle (1995) also described techniques for applying antireflection, antiabrasion, and antistatic coatings to refracting surfaces of polymer optics such as optical instrument lenses and eyeglasses. These coatings are vacuum deposited or applied by dipping, spraying, or spinning. 3.7.2 OPTICAL BLACKCOATINGS

The interior surfaces of many optical devices are blackened to reduce stray light reflections within the system. Absorbing finishes are also needed on baffles and on some structural members such as spiders and vanes as well as on the sensitive surfaces of radiometric detectors and solar collectors and temperature control surfaces of space borne systems. A broad summary of the many types of

paints, coatings, and surface finishes available for such purposes, typical measurements, and a large number of references to other pertinent publications may be found in Pompea and Breault (1995). For simplicity, here we define all these surface treatments as optical black coatings.

An optical black coating has a combination of characteristics that determine its function. These are (1) inclusion of a light-absorbing material such as a black dye, carbon black particles, or silicon carbide particles; (2) multiple reflections within cavities, craters, or cracks; (3) scattering of the radiation from surface irregularities; and (4) interference of radiation in a multilayer structure. Each of these characteristics is somewhat wavelength dependent, so the optical black coating must be tai- lored to the specifics of the application. Absorption of energy at one wavelength may also cause reradiation at another wavelength, thereby disturbing the thermal balance of the system.

One widely used type of optical black coating is Martin Black, a dyed anodized coating with pro- tuberances and cavities that works well on aluminum over a broad spectral range (see Figure 3.20). The textures created in some other optical black coatings are similar to that of Martin Black, while others resemble arrays of overlapping elongated particles or arrays of vertical tubular structures. All have characteristic features with lateral dimensions measuring a few micrometers in size.

Important considerations in the choice of type of optical black coating include sensitivity to temperature extremes and changes, to vibration and shock, to solar and nuclear radiation, to abra- sion, and to moisture. For space applications, the potential exists for micrometeorite damage, out- gassing, problems related to gravity release, weight loss due to exposure to atomic oxygen, and aging. A variation of Martin Black (called Enhanced Martin Black) has been created to enhance its durability when exposed to atomic oxygen in low Earth orbit. Most optical black coatings are frag- ile and hard to clean when they pick up dust or other debris. Some coatings are especially sensitive to damage during application or from subsequent handling. These coatings sometimes fail from flexure of the substrate, so structural stiffness is important. Pike and Mehrotra (1999) described a

rugged proprietary coating originally developed for thermographic applications in the 8 to 14 µm

spectral band that has been found to be useful as a stray visible light suppressant in optical instru- ments.

A common measurement of performance of an optical black coating is its bidirectional reflectance distribution function (BRDF), which is defined as the ratio of reflected radiance to inci- dent irradiance. This function typically varies with angle of incidence and wavelength of the radia- tion. The various sources of attenuation mentioned above tend to diffuse incident radiation at any given localized area on the coated surface so that the usual specular reflection from the surface is converted into a three-dimensional lobe of finite lateral extent ranging from fractions of a degree to tens of degrees. The surface therefore becomes more Lambertian as it reduces radiance.

FIGURE 3.20 Scanning electron micrgraph of Martin Black, a dyed anodized aluminum surface for UV, vis-

ible, and IR light attenuation applications. (Photo courtesy of Stephen M. Pompea, Pompea and Associates, Tucson, AZ.)

Pompea and Breault (1995) correctly pointed out that selection of the appropriate optical black coating for any application is a systems issue and should be considered early in the design process. All aspects, including location in the system, substrate materials, spectral requirements, availability of information needed for design, performance requirements, effects elsewhere in the system, manufac- turability, environmental degradation, maintenance, cost, and schedule should be thoroughly examined. 3.7.3 COATINGS TOIMPROVESURFACE SMOOTHNESS

3.7.3.1 Nickel

The optical surfaces of metal mirrors are sometimes plated with electrolytic nickel (EN) or, more frequently, with electroless nickel (ELN). The latter plating is Ni with 8 to 11% phosphorus. Both platings produce a smoother surface than can be produced on the bare substrate. This reduces the radiation scattering characteristics of the surface. Typical requirements for mirror surface rough- ness for infrared applications are about 40 Å rms. This can only be achieved by plating, followed by diamond turning and polishing of the nickel surface. These platings are discussed further in Section 13.7.

Because the CTE of nickel, 13.5⫻ 10⫺6°C⫺1, is significantly different from that of typical

metals used for mirrors, such as aluminum with a CTE of 20.3⫻ 10⫺6°C⫺1or beryllium with a

CTE of 11.5⫻ 10⫺6°C⫺1, bimetallic effects can occur as the temperature changes. Vukobratovich

et al. (1997) and Moon et al (2001) analyzed these effects for a 180 mm-diameter 6061-T651 alu- minum alloy mirror of different configurations to be used at 65 K. As discussed further in Section 13.7, a previous contention that plating both sides of the mirror with the same type and thickness nickel coating would eliminate the bimetallic effect was not found to be universally true. Nor was it found that increasing mirror thickness would prevent mirror distortion. Finite element analysis is needed to accurately define the magnitude of the problem for any given design.

3.7.3.2 Alumiplate®

This proprietary coating is 99.9⫹% amorphous aluminum plated onto aluminum parts, such as mir-

ror substrates, to form a layer that can be diamond turned or polished to extremely fine micror- oughness. It is produced by AlumiPlate, Inc., Minneapolis, MN. The layer is about 0.005 in. (0.125 mm) thick. Because there is minimal bimetallic effect, the coating works well at cryogenic temper- atures (see Vukobratovich et al., 1998).