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Dienstag, 11 Januar 2022 09:00

Black Electrochemical Coatings for Aerospace and Allied Applications – Part 8 – Carbon Nanotube-Based Black Coatings

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Geschätzte Lesezeit: 8 - 16 Minuten

Coatings comprising carbon nanotubes (CNT) are super black, that is, characterized by uniformly low reflectance over a broad range of wavelengths from the visible to far infrared [1, 2]. This is attributable to the intrinsic properties as well as the morphology (density, thickness, disorder, and tube size) of graphitic material.

About two decades ago, diamond-like carbon (DLC) was the primary candidate for the black coating of the large optical baffles, as it has a low outgassing rate [3-5]. However, the reflectance of coating is greatly influenced by the film thickness and it is impractical to control the thickness on large and relatively complicated areas like optical baffles.

Among all the known materials, vertically aligned single- walled carbon nanotubes array (VANTA) behaves most similarly to a black body, a theoretical material that absorbs all incident light. It can absorb light almost perfectly across a very wide spectral range (0.2–200 μm). This behaviour is attributed to stem from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes [5, 6]. The carbon nanotubes can be grown on the substrates by various methods, viz, Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), thermal radiation, spray deposition, etc. Nevertheless, attaining sufficient adhesion requires embedding these CNTs in a structural matrix, which inherently decreases their light absorption. As these procedures are not in the scope of the present article, the discussion of CNT coatings is just briefly touched upon due to their inherent importance in realizing high absorptance black coatings.

The fabrication of NanoTube Black, a Vertically Aligned Carbon Nano Tube Array (VANTA) on aluminium substrates was reported for the first time by Theocharous et al. [7]. The coating on aluminium was realised using a process that employs top-down thermal radiation to assist growth, enabling deposition at temperatures below the substrate’s melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths range from 2.5 µm to 15 µm.

Spectrally selective carbon nanotube absorbers were prepared by electrophoretic deposition on aluminium by Chen et al. [8]. The homogeneous CNT coatings have a reasonable adhesion and exhibit fairly good selectivity for solar thermal collectors’ applications. A voltage threshold of 15 V was used in the CNT suspension system. The thickness of CNT coatings increases linearly with the deposition time. With thicker CNT coating, transition from low to high reflectance of solar absorbers shifts to longer wavelength which benefits the solar absorptance but the thermal emittance was also increased. Both higher peak temperature and longer dwell time at the peak temperature during the heat treatment improved the spectral selectivity of the CNT absorbers. The best solar selectivity was achieved with the CNT coating thickness of ~1.7 μm, where a solar absorptance of 0.88 and a thermal emittance of 0.28 was measured. As the spectral selectivity of the CNT absorbers was not good enough, further improvement by adding an antireflection layer on the CNT coating and/or by modifying the CNT layer composition was suggested.

CNT coatings were formed by spraying formulations consisting of a silicon binder and low cost multiwalled CNTs (MWCNT) on a pre-heated aluminium plate [9]. The diffuse reflectance of the coatings in the visible region (350–800 nm) was in the range of 2.6–5.11 %, depending on the MWCNT concentration in the coating. In the NIR region (850–2400 nm), the reflectance values were in the range of 4.0–6.5 %. Excellent adhesion of coating on the aluminium substrates was reported, when the CNT concentration was below 15 %. The coatings withstood the temperature cycling of -196 to 200 °C.

Surrey NanoSystems (SNS) reported two types of proprietary CNT-based coatings [10]. The Vertically Aligned Nanotube Array (VANTA) or Vantablack coating consists of a forest-like structure of vertically aligned, equally-spaced, high-aspect-ratio carbon nanotubes. The spacing of the tubes is such that virtually all of the light arriving at the surface enters the spaces between the tubes and is absorbed after multiple reflections between neighbouring tubes. Vantablack nanotubes are grown in a chemical vapor deposition (CVD) reactor. The coating can be grown on complex 3-D shapes, with optimization of the CVD plasma distribution for best results.

Another coating Vantablack-S was also reported which is compatible with aerosol application and has performance close to the CVD-grown Vantablack. Vantablack-S coating provides a significant cost reduction compared to the CVD-grown version; a lower process temperature allows application to a wider variety of materials. Further, it allows easy processing of much larger and more complex components. Both types of coatings show a broadband absorptance, Vantablack > 99 % and Vantablack-S ~ 0.96 %.

9 Conclusions

Black coatings have wide and varied technological importance. The ultra-high absorptance black coatings are extremely suitable in the design of terrestrial and space-borne highly sophisticated baffles/vanes of optical instruments to suppress the unwanted reflections or scattered light. This limits the deterioration of the geometric and/or radiometric image quality.

The solar selective black coatings exhibiting high solar absorptance and low infrared emittance have been used in solar collectors from a long time. The flat absorber black coatings with high solar absorptance and high IR emittance have immense technological importance in thermal control, optical instruments and sensors. A highly reliable passive thermal control system utilizes the known optical properties, viz., solar absorptance and thermal emittance of the surface. In this article, various electrochemical methods of black coatings on aerospace materials for various function applications are discussed. The Coating thickness and optical properties of some of the important black coatings are mentioned in Table 9.1.

Tab. 9.1: Coating thickness and optical properties of some of the important black coatings

Coating process

Coating thickness µm

Solar absorptance

IR emittance

Ref.

Black chromate on Mg

8–11

0.91

0.90

[11]

Galvanic black anodising on Mg

12–15

0.95

0.93

[12]

Black molybdate on Al

5 ± 1

0.92

0.70

[13]

Black molybdate on Mg

12–15

0.90

0.90

[14]

Black permanganate on Mg

6

0.90

0.80

[15]

Copper blackening

 

0.90– 0.95

0.30–0.40

[16]

Anodizing-inorganic black colouring on Al

25 ± 5

0.97

0.90

[17]

Integral black anodizing (AA1100 /2024/6061)

30–32

0.80/0.82 /0.90

0.82/0.82/0.85

[18]

Hard anodizing

80–85

0.90

0.88

[19]

Black chrome on Al

5

0.93

0.11

[20]

Black trivalent chromium on Cu

2

0.96

 

[21]

Black Zn-Ni

5–6

0.92

0.83

[22]

Black Cu-Ni

0.1–0.2

0.94

0.10

[23]

Black Ni-Co

Ni undercoat 5.0–7.5

0.94

0.17

[24]

Black Ni-P

Ni-P 30 ± 2 Oxidation ~0.7

0.99

0.76

[25]

Black PEO on Al

50–70

0.89

0.72

[26]

Black PEO on Mg

 

0.96

0.95

[27]

Black PEO on Ti

1.3

0.93

0.88

[28]

CNT-based coatings Vantablack Vantablack-S

 

0.99 0.96

 

[10]

Black MAC 2-ML-H75

63.5–216.0

0.97

0.92

[29, 30]

Black Polyurethane, Aeroglaze® Z306

50.8–76.2

0.96

0.91

[29, 30]

Black Polyurethane, Aeroglaze® Z307

50.8–76.2

0.97

0.88

[29, 30]

The organic polymeric materials like paints, dye-stuffs, tapes, adhesives, are easy to implement on different surfaces/ materials and also provide excellent optical properties. But these materials have poor thermal stability and are known molecular contaminants. The outgassed species from these materials can deposit onto other exposed surfaces and can alter their properties and consequently their performance will degrade. The cleavage can occur as a result of UV irradiation, chemical oxidation or combination of both. The organic dye stuff is found to degrades at elevated temperatures (> 65 °C). Application of these materials on sensitive instruments, like optics, laser systems, detectors, thermal control surfaces is therefore restricted.

The % Total Mass Loss (TML) and % Collected Volatile Condensable Material (CVCM) values measured for some of the electrochemical black coatings, which are stable inorganic materials were found as low as 0.05–0.06 and 0.01–0.02 % [17, 25], as traces of absorbed water and gases were the only species to outgas. While even for specific low outgassing polymeric black paints, mass loss % was reported as high as 2.3 % [29, 30], confirming their non suitability for critical systems.

In general, the electrolytic processes provide far superior corrosion and abrasion resistance than the chemical conversion processes. Chemical conversion coatings, viz, black chromate, chrome-manganese, molybdate / permanganate coatings, galvanic anodizing, black oxide coatings provide a mild corrosion resistance and moderate abrasion resistance. The absorptance of most of these coatings is also just modest except the galvanic black anodizing on magnesium alloys that provides high solar absorptance value.

Among the electroplating processes (black- Cr, Ni, Co, Zn-Ni, Ni-Co, Cu-Ni, Zn-Ni-P, etc.), black chrome has emerged as a strong candidate because of its good solar selective properties and long-term durability. Black chrome plating provides a good wear resistance, low friction, excellent corrosion resistance, high hardness, and long-time durability. In spite of hazardous nature, the hexavalent chromium electrolytes are still widely used to produce thin black chrome coatings due to the versatile properties of coating. Nickel under coat significantly improves the corrosion resistance, enhances the thermal resistance and helps in lowering the thermal emissivity of black chrome coatings. Black nickel and black cobalt coatings also possess good solar selective properties. But their corrosion resistance and thermal stability are poor to moderate, particularly at high humidity and elevated temperatures. Black Zn-Ni-P coating imparts improved corrosion resistance and good flat absorber characteristics. The solar selective black chrome with nickel undercoat is therefore the best choice for the solar collector appliances.

Anodizing is a versatile process on aluminium alloys. Colouring of anodic film with organic dye stuffs is the simplest and most widely accepted process for decorative finishes. However, the colour fastness due to the bond cleavage within the organic molecule restricts their application in critical areas. The integral colour anodizing produces stable coating, but the absorptance of coating is not very high and there is difficulty in obtaining an acceptable colour match on different alloy composition and temper. Among the two inorganic colouring techniques: adsorption and electro-deposition, the latter delivers a stable black coating, however, the process is very sensitive to bath composition and operating conditions. The inorganic colouring absorption process is simple and provides high absorptance black coating with good stability in adverse space conditions. The process stands for far superior coatings on aluminium components for both stray light mitigation and thermal control applications. Of late, there has been increasing interest in the plasma electrolytic oxidation coatings due to their improved surface mechanical properties. The plasma electrolytic oxidation with metavanadate and tungstate additives results in black coatings with excellent mechanical properties and corrosion resistance, but the optical properties or solar selectivity of the coatings reported so far are not good enough to make them a suitable candidate for optical assemblies or solar collector systems.

Black electroless nickel coating formed by acid etching of a low phosphorus electroless nickel imparts a rich black finish with high absorptance, and exquisite corrosion resistance. Although the darkest material available today are carbon nanotubes, but the CNT forest is quite fragile and its reflectance is highly sensitive to the film thickness. Nevertheless, embedding the CNTs in a structural matrix intrinsically decrease the light absorption. Based on the rich optical properties, ease of implementation, surface mechanical characteristics, and environment stability, the black Ni-P coating is preferable for the safe implementation on the dimensionally critical large size baffles.

Acknowledgement

The author thankfully acknowledges the support of his former colleagues, A. Rajendra, R. Uma Rani, N. Sridhara, Anju M. Pillai, T. Harikrishna and Arjun Dey, of U. R. Rao Satellite centre, Indian Space Research Organization, Bangalore, India.

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  • Ausgabe: 1
  • Jahr: 2022
  • Autoren: Dr. Anand Kumar Sharma

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