Acktar Fractal Black™ as a Dual-Function Thermal-Optical Control Coating and Structural Primer

Acktar Advanced Coating’s Fractal Black™ is an inorganic vacuum-deposited thermal-optical control coating with well-established absorptivity and emissivity from ultraviolet to thermal infrared ranges. This coating has been used in the aerospace industry for stray light control and passive thermal management for its low reflectance in the visible and infra-red spectra, large working temperature range, and low outgassing. Fractal Black has proven to outperform traditional thermal-optical spray coatings for complex geometries in terms of conformance to small radii of curvature and substrate adhesion for a wide variety of substrates ranging from metals to glasses. As a result, Fractal Black has been used as a thermal-optical coating in many space flight applications, such as James Webb and BepiColombo. This work explores the dual function of Fractal Black as a structural primer for bonding as well as a thermal-optical coating on the linear variable filter (LVF) mount baffles on Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx)’s spectrometer.SPHEREx’s LVF is fabricated from ASTM B386, titanium-zirconium-molybdenum (TZM). The LVF has strict stray light and geometric conformance requirements that are challenging to meet with traditional spray coatings. Additionally, the LVF requires structural bonds of its sapphire-based filters to the TZM frame, further challenging downstream assembly processes. Acktar Fractal Black was identified as a potential coating for the unique substrate and required sharp radii edges due to its strong substrate adhesion, geometric conformance, and dual-function potential to control stray light and serve as a structural primer.Fractal Black was subjected to typical aerospace coating qualifications including: thermal-optical property verification, cleanability, tolerance to mechanical damage, thermal shock resistance, and adhesion. Thermo-optical properties were compared across the as-received condition, solvent-rinsed, solvent-wiped, and plasma-cleaned conditions. All measurements resulted in high absorptivity of approximately 0.98 and high emissivity of approximately 0.90 with negligible changes between cleaning methods. Substrate adhesion and thermal shock resistance was verified by ASTM D3359 tape adhesion testing under pre- and post-thermal shock conditions, from 110°C to liquid nitrogen temperatures. No Fractal Black was removed during adhesion testing, indicating perfect conformance to stringent coating requirements.Fractal Black was compared to BR-127 NC/ESD for conformance to complex geometries. Optical and scanning electron microscope imaging revealed full coverage of the edges and undercut regions on coupons replicating complex geometry of SPHEREx baffles. Additionally, microscopy confirmed higher uniformity and smaller achievable radii using Fractal Black relative to standard black coatings.Fractal Black was also evaluated as a structural primer on ten TZM lap shear coupon sets bonded with EC-2216. Al6061-T6 coupons primed with BR-127, a standard aerospace structural primer, and bonded with EC-2216 were tested as a control. Results indicated that Fractal Black-primed coupons performed nominally, with an average of 3.6 ksi shear strength, identical to what was achieved with BR-127-primed coupons. Additionally, all coupons failed cohesively in the EC-2216, indicating high process repeatability. Nominal performance of Acktar Fractal Black as a dual-function thermal-optical coating and structural primer demonstrates its potential to streamline fabrication and assembly processes throughout the industry.