News | July 27, 2023

Zygo Announces Uniquely Durable & Repeatable Salt Fog Silver Coating

(Middlefield, CT, USA 26th July 2023) Due to unprecedented customer demand, Zygo Corporation — a business unit of Ametek Inc. — has developed a salt fog silver coating that can be applied to silver reflector optics used either in harsh marine salt spray environments or for optics exposed to extreme temperatures (from -62OC to +800C).

Because of the fact that it possesses the highest broadband reflectivity, the lowest emissivity, and the lowest polarization splitting of all metals, silver is used in the most demanding visible and infrared mirror applications such as space-based sensors, and large telescopes. It out-performs gold and aluminum which have lower levels of reflectivity. However, silver is much more prone to atmospheric corrosion, and is easily tarnished and corroded in air —and even more so in harsh environments — which negatively effects its reflectivity and overall optical performance.

Salt fog silver is a silver mirror coating that offers high reflectivity from visible through to infrared wavelengths. A protective layer of salt fog silver is added to standard metallic silver coatings ensuring that optics are better able to resist corrosion and mechanically induced damage. 

Standard protective measures are not suitable to protect a silver reflector in a marine environment. Most existing industry solutions are sub-optimal and either exhibit rapid degradation in the field, the formation of pin hole blemishes in the coating, or generally inconsistent performance. For optics typically used in maritime defense applications and in astronomy and top-end research applications, such issues are not acceptable, and a salt fog silver solution is required.

Jennifer Chen, Sr. Product Manager at Zygo says, “Very few coating houses offer a salt fog silver solution, without which numerous applications will fail in the field or suffer from dramatically reduced service life. Zygo began working on salt fog silver coatings back in 2016, driven by the requirements of one particular customer project, but over the last few years has seen a significant increase in interest from customers needing salt fog silver protection to help optical component survival in harsh and extreme environments, or when exposed to particularly low or high temperatures. The salt fog silver coating capability now places Zygo in a strong position as one of the only optics companies in the world that can offer this level of protection to the optics it manufacturers for defense, military, astronomy, and research applications.”

The salt fog silver coating capability is the result of much work undertaken by Zygo’s world-class coating design engineers and technicians who work closely with the company’s optical manufacturing teams. With Zygo also being a pre-eminent supplier of optical metrology equipment, customers can take advantage of a “one stop shopping” advantage, which simplifies the supply chain, reduces program risks, and reduces overall project timelines. Such a vertically integrated supplier is crucial when it is critical that your optical substrate meets operational requirements after a coating is applied.

Chen continues, “There are a very few companies that provide coatings that protect silver reflector optics in harsh environments, and Zygo’s salt fog silver coating exhibits superior spectral performance and spectral uniformity than any alternatives. Our salt fog silver coating is especially durable, passing the adhesion cellophane tape test, and showing no degradation when exposed to 95-100% humidity at 120oF over 24 and 240 hours. It also passed the moderate abrasion test (50 strokes with a minimum force of one pound) and the lifetime operational durability test (-55oC to +55oC at a rate of 4oC per minute for chamber air temperature between extremes, holding at extreme chamber air temperature for one hour for 20 cycles).”

Some customers request extended testing to be able to make an accurate evaluation of the likely service life of a silver mirror coating. This is frequently done via salt spray or salt fog testing, which is a way of measuring the corrosive impact of harsh marine environments on optics. It is standard that components used in harsh marine environments are evaluated to tight military specifications in controlled conditions.

“Chen concludes, “Our salt fog silver underwent an extended 5-day salt fog test, a lifetime storage durability test (-62oC to +80oC at a rate no greater than 5oC per minute between extremes, holding at extreme temperature for 8 hours over 4 cycles), as well as a solubility and cleanability test, which requires immersion in Isopropyl alcohol or methyl alcohol, Acetone, and Ethyl alcohol for a minimum or 10 minutes each. ZYGO’s salt fog silver coating passed all these performance requirements, which when coupled with its high repeatability makes it uniquely a uniquely reliable protection for silver reflector optics. In addition, while ZYGO’s salt fog silver coating has proved highly effective for silver reflector optics, the company continues to research its adherence to other substrate materials.”

To ensure quality for its customers, Zygo maintains complete control over every facet of optics manufacturing from grinding and polishing, to coating and assembly. At each stage of production, skilled opticians meticulously test and verify the components using sophisticated measuring instruments, most of which are designed and manufactured by Zygo. As such the company holds a unique position as a precision optics manufacturer and welcomes the opportunity to discuss its vertically integrated approach to optics development, and how the use of its salt fog silver coating may be beneficial in specific customer applications.

Source: Zygo Corporation