Adapting Metallic Coatings for Dynamic Applications

MARIE FREEBODY, CONTRIBUTING EDITOR

As with all products for which a variety of options is available to consumers, and for which application ultimately determines consumer selection, each type of metallic coating offers its own unique set of benefits and shortcomings. By carefully selecting the proper material(s) — whether aluminum, silver, nickel, or gold — as well as the necessary enhancements, manufacturers can expand the range of applications for mirrors from high-precision scientific instruments to commercial products and beyond.

Advancements in nanotechnology, plus chemical modifications and overcoat fabrication, are extending the utility of mirror coatings into specialized applications.

Fundamentally, coatings enable manufacturers to differentiate products based on performance metrics, such as reflectivity, wavelength selectivity, and environmental resistance. They also extend the lifespan of mirror components, reducing the need for frequent replacements and lowering the overall cost of ownership. As a result, the evolution of metallic coatings, broadly speaking, has revolutionized the design and application of mirrors, providing greater control of critical parameters and metrics.

Silver, aluminum, and gold optical coatings benefit from high reflectivity across a broad spectrum and durability, but they can be prone to tarnishing and absorption losses. Courtesy of Edmund Optics.

And as industries such as telecommunications, aerospace and defense, and medical imaging evolve, demand for highly specialized optical coatings is poised to continue to increase. Today, to engineer coatings to meet specific optical requirements is a decisive factor in determining the utility of products and the success of the manufacturers that bring them to market.

Reflectivity and metallic coatings

Reflectivity is a defining parameter for mirrors, and as it relates to the mirror coating, this quality depends significantly on the metal used for the coating. The coating process involves depositing a thin metal layer onto a substrate — typically glass or plastic — via methods such as physical vapor deposition or chemical vapor deposition. Beyond the distinct properties that different metals offer or achieve (sidebar), different enhancements can be made that influence the performance of the coating following the deposition stage.

“Metallic mirror coatings, such as silver, aluminum, and gold, offer high reflectivity across a broad spectrum, especially in the visible and near-infrared ranges, making them suitable for various optical applications,” said Tyler Christy, a thin-film manufacturing engineer at Edmund Optics.

“These coatings are durable and versatile, [and] applicable to different substrates. However, they can be prone to tarnishing, exhibit some absorption losses, and may suffer from surface roughness, which affects performance.” Metallic mirrors also have a tendency to absorb and are often susceptible to pinholing. These imperfections may stem from fabrication processes, material properties, substrate quality, or a combination of factors. Since these imperfections can leave small voids in the coating, they are apt to degrade the quality of the mirror and limit performance.

These diamond-turned aluminum parabolic mirrors are coated with a protective aluminum coating for a typical visible (400 to 700 nm) operating wavelength (top). A dielectric mirror for 405 to 415 nm on polymer. This molded parabolic mirror has 15 layers of material and reflectivity of >99.5% (bottom). Courtesy of AccuCoat Inc.

“Typically, the reflectance requirement and target wavelength will dictate the type of metallic mirror coating, as well as any changes or enhancements needed for optimal performance,” said Kelley Plats, business development manager at North American Coating Laboratories (NACL). “While most mirrors work well in various applications, they can be limited to wavelength.”

“For example, gold is an excellent reflector in the infrared spectrum but does not perform as well in the low-visible and ultraviolet wavelengths. Gold is also delicate and requires a protective layer to ensure it is not scratched,” Plats said.

A gold-coated material in a custom tooling for correct alignment and optimal coating uniformity. The coated part (with drilled holes) is a nonstandard shape. Courtesy of AccuCoat Inc.

“Aluminum is an amazing visible reflector and is relatively simple to use in most applications; however, if you need a high reflector, it will need to be enhanced using other dielectric materials. Silver has excellent reflective properties; however, it tarnishes easily.”

Advancements in metallic coatings

Customers’ needs for higher performance and resilience, as well as supply chain issues, especially in recent years, are the core drivers behind this current state of innovation in metallic coatings. Materials shortages have plagued procurement and lead times for coating developers, who have also contested with discontinuations of critical materials.

According to Christy, developers have had to explore alternate suppliers, consider different materials, and/or modify coating formulations to adapt to supply chain challenges. One such adaptation is the increased integration of nanotechnology, which has enabled the use of thinner, more efficient coatings that improve reflectivity while minimizing material usage. For example, nanoparticles of metals, such as aluminum and/or silver, are being engineered to increase the uniformity of coatings, enhancing reflectivity at specific wavelengths. Another advancement is a multilayered approach: Engineers or fabricators layer different metals to harness the strengths of each. For example, silver can be layered with aluminum to increase reflectivity in the UV range while maintaining the superior visible-light reflectivity of silver.

Gold, shown here protected on polycarbonate, is an excellent reflector in the infrared spectrum. The material does not perform as well in the low-visible and UV wavelengths. Courtesy of NACL.

Additionally, researchers are exploring organic-inorganic hybrid coatings that combine the flexibility of organic materials with the robustness of inorganic compounds. This combination can yield coatings that offer advantages both for their reflectivity and durability.

In terms of fabrication, Christy believes that the future now centers increasingly around advancements in precision deposition technologies, such as atomic layer deposition and advanced sputtering, which are currently focused on reactive processes. These techniques offer finer control over coating thicknesses and compositions, resulting in highly customized coatings for specific wavelengths and applications. Innovations such as low-emissivity windows (either polymer or, more commonly, glass) and the use of thin metallic coatings in biomedical surface activation have illuminated the expanding applications of these coatings, but the fundamental manufacturing processes have largely remained consistent.

“The ongoing emphasis on precision will continue to enhance the performance, reflectivity, and durability of metallic mirror coatings, supporting further innovations across various fields,” Christy said.

Also, Christy said, manufacturers use simulation software to optimize coating designs before fabrication. The careful selection of substrate and adequate preparation also hold a direct line to improving adhesion and surface quality.

Complex shapes and space applications

The range of market-available solutions that optics companies (for example, Edmund and Altechna) offer is evident in more than just a variance of materials. The various shapes and forms of consumer optics that these and other companies offer include prisms, parabolic off-axis mirrors, cylindrical and spherical mirrors, and computer numerical control (CNC)-machined custom-designed parts.

Use cases for complex-shaped optics are evolving steadily, and this flexibility in shape is helping to supplant conventional monolithic glass bank primary mirrors in telescope imaging systems. Historically, these components imposed limitations on both the maximum size and coating options — with primary mirrors constructed of lightweight hexagonal segments. Since each segment is relatively easy to manufacture and can be coated using standard-size equipment, size limitations are removed and many more coating options become available. Ground-based examples include the Hobby-Eberly (11 m) and Keck (10 m) telescopes. Perhaps the most notable system is the recently launched James Webb Space Telescope. It features 18 gold-coated hexagonal mirrors arranged in a honeycomb pattern, effectively creating a singular and massively powerful primary mirror.

Within industry, one of the niche areas of focus currently at coatings developer and manufacturer AccuCoat Inc. is extending capability to uniformly coat polygons in aluminum, gold, and silver. Polygons are used in and for UPC (universal product code) barcode readers. In this application, a rapidly spinning polygon located in front of a red laser ensures that a barcode can be read from any direction.

The challenge in coating a polygon is to coat all the outside edges uniformly.

While continually rotating the polygon on its own axis, and inside of a chamber, AccuCoat overcame the challenge of balancing the speed of rotation in relation to deposition rate to maintain uniformity across each face of the part.

For AccuCoat, this achievement is tied directly to a common customer demand.

“Many times, customers want <1% change in uniformity of reflectivity from surface to surface,” said Patrick Iulianello, AccuCoat’s vice president of operations and cofounder.

“During deposition, generally the face that is orientated toward the deposition at the bottom of the chamber is the face that gets coated, so when you continually rotate a spinner/polygon, you have to control speed, orientation, and deposition rate to achieve the proper total deposition for reflectivity.”

Dielectric overcoats

Dielectric overcoats, which consist of nonmetallic materials, such as magnesium fluoride and silicon dioxide, serve as protective and performance-enhancing layers for metallic mirrors. These coatings are applied over the metal to prevent tarnishing and oxidation. Both are common issues with metallic coating materials, including silver and aluminum.

More than just a protective layer, dielectric overcoats also increase reflectivity by minimizing surface losses. “Dielectric materials give metals a boost in performance: They allow a coating designer or engineer the ability to specifically target reflectance at a specific wavelength,” said NACL’s Plats. “They can also increase the overall durability of the coating on the lens, which extends the life of the lens and enhances the performance of the complete system.”

Dielectric coatings are particularly useful in applications with high laser-induced damage threshold exposure. Evaporated Coatings Inc. (ECI), for example, manufactures highly reflective low-loss dielectric laser optical coatings with up to 99.9% reflection that can be optimized for use from 248 to 2500 nm for laser line wavelengths or multiband applications.

Also, overcoat dielectric layers of alternating high/low refractive index can enhance the reflectivity of less expensive metals while simultaneously enhancing durability. ECI’s coatings can be deposited onto various types of optical materials, including glass substrates, fiber optic devices, and crystals and semiconductor materials.

Chemically, the key to fabricating an effective mirror coating lies in understanding the interaction between the metal and the surrounding environment. For example, adding a layer of magnesium fluoride on top of aluminum prevents oxidation and improves the reflection of UV light. The thickness and composition of the coating are carefully controlled to mini- mize absorption and scattering, thereby preserving reflectivity. Similarly, chromium or silicon dioxide overcoats are commonly applied to protect silver mirrors, preventing tarnishing while also enhancing durability.

“In applications where high reflectivity in the infrared is crucial, a metal like gold may be chosen for its superior IR reflectivity,” Christy said. “For applications needing high UV reflectivity, aluminum would be the metal of choice and additional layers may be added to enhance performance. The coating’s material, thickness, and protective measures are adjusted based on the application’s operating environment, wavelength range, and performance requirements.”

Chemical tweaks in high-precision applications may also involve doping the metallic coatings with small amounts of other elements to modify the refractive index or improve the adhesion of the metal to the substrate. For example, adding titanium or chromium to aluminum can improve the mirror’s adherence to the substrate, making it more durable under mechanical stress.

Dielectric coatings can also be fine-tuned to reflect specific wavelengths while allowing others to pass through for optical applications, where controlling the reflection and transmission of light is critical. This property is especially useful in beamsplitters, antireflective coatings, and laser mirrors, in which precise control over reflectivity is required.

Still, dielectric coatings are unsuitable in certain situations. Vilnius, Lithuania-based EKSMA Optics emphasizes that although dielectric metallic coatings provide greater reflection across the operating bandwidth, they alter the polarization state of an incident beam. This quality makes them inappropriate for polarization-sensitive applications. The company offers variously sized round, rectangular, and spherical mirrors coated with gold, silver, or aluminum.

Pricing pressures

Even as consumers frequently opt for advanced multilayer dielectric coatings for high-performance applications, the effectiveness of metallic coatings alone often suffices in applications that require broad-spectrum reflectivity and durability. “Metallic mirrors are a [cost-effective] alternative to dielectric mirrors with high reflectivity,” said Audrius Jakštas, sales technical director at EKSMA Optics. “For example, when the customer needs a mirror for a broadband wavelength range and there are no requirements for high reflectivity (>98% to 99%), then a metallic mirror is a good alternative.”

Reflectivity and operational wavelength typically guide materials selection for coatings. But the rising costs of precious metals are increasingly influencing the direction of the industry. For example, gold coatings can be up to 10× that of aluminum and certain other materials. Though materials with lower levels of purity may be usable substitutes, a less pure material may hinder durability, reflectivity, or other factors. Such a degradation may not be acceptable for a customer or its application.

“We typically use ‘5 nines’ (99.999%) pure gold, silver, and aluminum to give the best-performing product to our customers,” AccuCoat’s Iulianello said. “At this purity, you are procuring the material at [the] market price of the day, and today that is around $2400/oz (28.35 g) and typically, we can use about 2 to 4 g per coating run.”

Of course, these values are evolving, as are the factors that are apt to influence these values. Supply chain considerations, plus increased demand for specialized coatings amid the development of materials and methods, are ensuring a forthcoming dynamic wave of progress in mirror optics and coatings.

Metallic Coating Materials

Aluminum: Aluminum is known for its high reflectivity across a broad spectrum of wavelengths and is particularly useful in UV, visible, and infrared applications. Its reflectivity ranges between 85% and 95% in the visible spectrum and can reach 90% in the UV. Due to its versatility and cost-effectiveness, aluminum is often used in telescopes, laser optics, and solar panels. Aluminum’s susceptibility to oxidation requires it to be used with protective coatings to preserve its reflective properties.

Silver: Silver boasts the highest reflectivity in the visible range, exceeding 97%, which makes it ideal for use in high-performance mirrors, such as those used in imaging, solar energy collection, and other applications. Silver tarnishes quickly when exposed to sulfur or oxygen, limiting its application in harsh environments unless adequately protected by overcoats. Recent advancements in protective coatings and dielectric layers have helped mitigate silver’s tendency to degrade, allowing for extended use in challenging environments.

Nickel: Nickel is primarily valued for its durability and resistance to corrosion: It finds application in demanding environments in which mechanical stress, heat, and oxidation are concerns, such as in military optics and aerospace. Nickel’s lower reflectivity compared with aluminum and silver limits its use in applications in which brightness and optical precision are paramount.

Gold: Gold excels in the infrared spectrum, with reflectivity reaching up to 98%. It is widely valued for infrared telescopes, thermal imaging systems, and applications in which heat must be efficiently reflected. Gold also has excellent resistance to oxidation and corrosion, which enhances its longevity. The material’s high cost often restricts its use to specialized and high-budget applications.

Segmented diamond-turned aluminum mirrors coated with a protective silver coating, with reflectivity >97% over a wide wavelength range. Courtesy of AccuCoat Inc.