Ultrablack Coating Overcomes Fragility and Durability Bottlenecks

A broadband, ultrablack film from the University of Shanghai for Science and Technology and the Chinese Academy of Sciences could enhance the performance of telescopes and have other applications in space exploration and precision optics. In tests, the film achieved an average absorption as high as 99.4%, within a wavelength range of 400 to 1000 nm.

Optical devices that require ultrablack coating for stray light suppression often exhibit significant curvature and intricate shapes, which pose challenges for existing approaches to film preparation. In space exploration applications, payload is a critical factor, and black films that can be coated to lightweight materials are essential. The coating also needs to be robust enough to withstand the harsh environments found in space.

“Existing black coatings like vertically aligned carbon nanotubes or black silicon are limited by fragility,” researcher Yunzhen Cao said. “It is also difficult for many other coating methods to apply coatings inside a tube or on other complicated structures.”

The ultrablack coating can be applied to curved surfaces and magnesium alloys via atomic layer deposition to absorb more than 99% of light. Courtesy of Jin et al.

The researchers designed the ultrablack film by using alternating layers of titanium-aluminum-carbon (TiAIC) and silicon dioxide (SiO₂). They deposited the film using atomic layer deposition (ALD), a vacuum-based manufacturing technique that exposes a target object, placed in a vacuum, to different types of gas that adhere to the object’s surface in layers.

Using ALD, the researchers prepared an interlayer composed of aluminum oxide and titanium oxide (Al₂O₃ and TiO₂). The interlayer was deposited on the magnesium alloys prior to the deposition of the ultrablack film. With the presence of the Al₂O₃/TiO₂ interlayer, the ultrablack film on the surface of the alloy exhibited good adhesion and abrasion resistance.

By using ALD, the researchers were able to develop conformal coatings that provide broadband absorption for large-curvature surfaces. “One big advantage of the ALD method lies in its excellent step-coverage ability, which means we can obtain uniform film coverage on very complex surfaces, such as cylinders, pillars, and trenches,” Cao said.

The researchers applied the ultrablack coating to a large-curvature magnesium alloy surface. Magnesium alloys are the lightest structural metal and are widely used for aerospace applications. However, the strong electrochemical activity of magnesium leads to poor corrosion resistance. To prevent corrosion, the researchers created a barrier layer to go between the coating and the surface of the alloy.

Together, TiAIC and SiO₂ prevent nearly all light from reflecting off the coated surface of the optical device. “TiAlC acted as an absorbing layer, and SiO₂ was employed to create an anti-reflection structure,” Cao said. “As a result, nearly all of the incident light is trapped in the multilayer film, achieving efficient light absorption.”

In experiments, the ultrablack thin-film coating demonstrated an average absorption of 99.3% across a range of light wavelengths, from violet light at 400 nm to near infrared (NIR) at 1000 nm. After undergoing damp heat and thermal cycling tests, the absorptions of the ultrablack film-coated magnesium alloys remained at 99.1% and 99.0%, respectively, indicating that the film should hold up well under difficult environmental conditions. The researchers achieved uniform coverage on the complex structure and the coating’s performance remained stable.

“The film shows superb stability in adverse environments, and is tough enough to withstand friction, heat, damp conditions, and extreme temperature changes,” Cao said.

The researchers anticipate that the coating will be used to enhance space telescopes and optical hardware operating in the most extreme conditions, and they are working to further develop its performance.

“Now that the film can absorb over 99.3% of incoming visible light, we’re hoping to expand its light absorption range even further to include ultraviolet and infrared regions,” Cao said.

The research was published in the Journal of Vacuum Science & Technology A (www.doi.org/10.1116/6.0003305).