Black Electrochemical Coatings for Aerospace and Allied Applications – Part 5 – Black Nickel and Cobalt Plating

Cobalt is used in manufacturing of super alloys to make parts of aircraft engines, gas turbine; high-speed steels; magnetic materials and many other products, including ceramics, cement, leather goods, paints, and rechargeable battery materials.

5.1 Black Nickel Plating

Black nickel coatings are typically deposited on steel, zinc, brass, bronze, zincated or zinc plated aluminium alloys to produce a non-reflective solar selective surface. Black nickel coating obtained by electrodeposition method has attracted the interest of researchers since it was introduced by Tabor [1] in the late 1950s. Since then, a large number of black nickel electroplating solutions based on salts of nickel, other metal (zinc, copper, or cobalt) and some sulphur compounds, e.g., thiocyanate have been reported [2–27].

A typical bath of black zinc-nickel alloy plating contains nickel chloride, zinc chloride, ammonium chloride, and sodium thiocyanate. Sodium thiocyanate is used as a complexing agent and in some of the baths boric acid is also added as a buffering agent. Chloride salts can be replaced with sulphates. A typical composition and operating conditions of chloride and sulphate baths are given in Table 5.1. The bath requires periodic correction / replenishment of zinc salt. Nickel or carbon anodes can be used.

Tab. 5.1: Typical composition and operating conditions of black zinc-nickel alloy plating baths. Values given in parenthesis are for reference [8]

Chloride Bath [3]		Sulphate Bath [3, 8]
Nickel chloride, NiCl2·6H2O	75 g/L	Nickel sulphate, NiSO4·6H2O	100 g/L (75 g/L)
Ammonium chloride, NH4Cl	30 g/L	Nickel ammonium sulphate, Ni(NH4)2(SO4)2.6H2O or Ammonium sulphate, (NH4)2SO4	15 g/L (45 g/L)
Zinc chloride, ZnCl2	30 g/L	Zinc sulphate	22 g/L (37.5 g/L)
Sodium thiocyanate, NaSCN	15 g/L	Sodium thiocyanate, NaSCN	15 g/L (15 g/L)
pH	3.5–5.5	pH	5.0-6.0 (5.6–5.9)
Temperature	25–35 °C	Temperature	26–32 °C (50–55 °C)
Current density	0.15 A/dm2	Current density	0.2 A/dm2) (0.15–0.30 A/dm2)

A solar absorptance value of ~ 0.95 and thermal emittances in the range of 0.10–0.25 can be achieved by Zn-Ni black alloy plating on different substrates. Surface morphology of black nickel found to influence the solar absorptance of the coating. The enhancement in solar absorptance occurs either by an array of fine particles causing intrinsic adsorption or by reducing the front surface reflections from an absorber surface [22].

Most of the earlier work on black nickel was concentrated in the development of selective solar collectors or solar absorbers exhibiting high absorbance in the range of solar spectrum and low emittance in the infrared range of radiation [15]. The thin black nickel plating may impart good solar selective characteristics, but its long-term reliability is poor. The coating is susceptible to discoloration when exposed to humid atmospheric conditions thereby resulting in the degradation of its thermo-optical properties, particularly at the elevated temperatures [7, 15, 23]. The applications of this type of coating are therefore limited to decorative and military purposes.

It has been observed that there is a strong relationship between the optical properties of black nickel coating and its thickness. The coating exhibits good solar absorber property only in a narrow range of thickness and with increase in thickness, the coating behaviour changes from a solar selective absorber (high absorbance and low emittance) to a flat absorber (high absorbance and high emittance) [18–20].

Sharma and Bhojaraj [14] investigated the black nickel plating on stainless steel (304 and 440C) for space applications. Following sequence of operations was adopted:

(a) Solvent degreasing in trichloroethylene
(b) Anodic electrolytic cleaning in 60–70 % sulphuric acid (V/V) using 10–11 V for 1–2 minutes, followed by water rinsing
(c) Activation in solution containing, nickel chloride NiCl₂·6H₂O: 250 g/L, hydrochloric acid (SG 1.16): 86 ml/L; room temperature; current density: 3 A/dm²; polarity and time: first the job is made anodic, after 2 minutes the polarity is reversed for 6 minutes. Water rinsing
(d) Black nickel plating in a bath consisting of nickel chloride NiCl₂·6H₂O: 75 g/L, ammonium chloride NH₄Cl: 30 g/L, sodium thiocyanate NaSCN: 15 g/L, and zinc chloride, ZnCl₂: 30 g/L; pH: 4.8–5.2; temperature: ambient; current density: 0.15 A/dm²; time: 2 hours. Water rinsing.

The coating obtained at these optimum conditions provide the solar absorptance of ~ 0.91 and thermal emittance of ~ 0.81. The space worthiness of coating was established by exposure to simulated environmental testing consisting of humidity, thermal cycling, thermovacuum performance tests.

Somasundaram et al. [15, 16] have reported the method of pulse electrodeposition of zinc-nickel alloy coatings on copper and stainless-steel substrates for thermal control applications of spacecraft. The black alloy coating was deposited from a modified Fishlock bath containing nickel sulphate: 75 g/L, nickel ammonium sulphate: 45 g/L, zinc sulphate: 28 g/L, ammonium thiocyanate: 30g/L and sodium lauryl sulphate: 0.25 g/L; pH: 3.0; temperature: 60 ± 2 °C; average current density: 0.6 A/dm²; pulse on-time: 0.1 milliseconds, pulse off-time: 0.9 milliseconds; plating time 40 minutes. An undercoat of 4–5 µm Watts nickel [nickel sulphate: 300 g/L, nickel chloride: 45 g/l, boric acid 45 g/l, sodium lauryl sulphate: 0.25 g/l; pH: 3.0; temperature: 60 ± 2 °C; average current density 0.6 A/dm²; pulse on-time 0.1 milliseconds; pulse off-time 0.9 milliseconds; plating time 40 minutes] was used to increase the corrosion resistance and adhesion of the black nickel coating. The 5–6 µm thick uniform jet-black zinc-nickel alloy coating exhibited an absorbance of 0.92 and an emittance of 0.83.

The coatings were characterized by cross-sectional SEM and EDX, XRD, XPS, Nanoindentation and Nano Vision analysis. EDX analysis shows the presence of zinc and nickel. XRD pattern indicates that the coating is mostly amorphous in nature. XPS spectra show the presence of Ni, Zn, S, O and ZnS. Nano Vision scan images of black nickel coating show an egg crate kind of morphology when viewed orthographically with a trough to crest distance of a few hundreds of nanometers. Surface topology of coating using NanoVision scan revealed the presence of lots of microcracks.

Mayanna et al. [17–19] reported the black Cu-Ni solar selective coatings on Cu and Mo substrates from an ethylenediaminetetraacetic acid (EDTA) or triethanolamine (TEA) complex baths for solar water heater collector panels. Deposition parameters were optimized using Hull cell experiments to achieve a high solar absorptance (0.94) and low thermal emittance (0.08–0.10) with a deposit thickness of 0.1–0.2 µm. The following bath formulations were finalized:

(a) Copper (II) nitrate: 2.5 g/L, nickel ammonium sulphate hexahydrate: 20 g/L, EDTA: 2.5 g/L and boric acid: 10 g/L
(b) Copper sulphate pentahydrate: 20 g/L, nickel sulphate: 20 g/L, ammonium persulfate: 10g/L, TEA: 20 g/L and boric acid: 10 g/L
(c) Copper sulphate: 10 g/L, nickel sulphate hexahydrate: 20 g/L, TEA: 20 g/L and boric acid: 10 g/L.

The baths were operated at pH: 5.0; temperature: 25–30 °C; current density: 0.5 (for a and b) and 1.0 A/dm² for c; time: 30 seconds. Stainless steel 316 was employed as anode. The coatings were characterized by EDX, XPS and XAES (X-Ray-Excited Auger Electron Spectroscopy).

Shashikala et al. [20] obtained the solar selective black nickel-cobalt plating on pre cleaned aluminium alloy substrates with nickel undercoat. Process optimization was carried out by the hull cell experiments. The following sequence of operations was adopted:

(a) Solvent degreasing in trichloroethylene for 5–10 minutes
(b) Alkaline cleaning in a solution containing sodium carbonate: 20–25 g/L, sodium metasilicate: 8–12 g/L and tri sodium orthophosphate: 8–12 g/L, at 65 ± 5 °C for 3–4 minutes followed by water rinsing
(c) Chemical polishing in a solution containing orthophosphoric acid: 80 %, nitric acid: 3.5 % and copper: 0.01 % for 20–25 seconds at 90 °C, followed by hot water rinse
(d) De-smutting in an acid solution containing 10 ml/L sulfuric acid, 12 ml/L hydrofluoric acid and 25 ml/L nitric acid for 2–3 minutes at room temperature followed by water rinsing and air drying
(e) Zincating [29] for 45 seconds at room temperature
(f) Stripping of the first zinc layer by immersion in 50 % nitric acid for 1–2 minutes
(g) Re-zincating by repeating step 5 for 1 minute to provide uniform and compact fine grains zinc layer
(h) Nickel plating using Watts bath for obtaining a coating thickness of 5.0–7.5 µm
(i) Black nickel cobalt plating in a solution containing cobalt sulphate: 12.5 g/L, nickel sulphate: 7.5 g/L, ammonium acetate: 10.0–12.5 g/L and sodium thiocyanate: 10 g/L; pH: 5.5; temperature: 28–30 °C; current density: 4 A/dm² for 30–45 seconds.

At the above optimal conditions, a solar selective black Ni-Co coating with a solar absorptance of 0.948 and a thermal emittance of 0.17 was realized.

A process of black nickel solar absorber coatings on SS 316L sheet was described by Cantú et al. [24] for thermal solar cells applications. The stainless-steel sheet was sequentially washed with acetone, ethanol, concentrated solution of NaOH and finally rinsed with distilled water before use. A thin film of bright nickel coating was deposited using nickel sulphate 300 g/L, and nickel chloride 35 g/L; pH 6.1–6.5; current density: 20 A/dm²; temperature: 50 °C; time: 100 seconds. Black nickel coating was deposited from a bath containing nickel chloride: 75 g/L, sodium chloride: 30 g/L, temperature: 25–27 °C; current density: 0.14 –0.26 A/dm²; deposition time: 60–90 seconds. After the black nickel coating, a final sol-gel silica-based antireflection coating-tetra ethyl ortho silicate (TEOS), DYNASYL® was applied. The best solar absorptance and thermal emittance values obtained were 0.91 and 0.1, respectively.

Black nickel coatings were electrodeposited on to steel substrates from a modified Watts bath containing potassium nitrate by Ibrahim [25]. The best operating conditions to produce smooth and highly adherent black nickel were found to be NiSO₄·6H2O (0.63 M), NiCl₂·6H₂O (0.09 M), H₃BO₃ (0.3 M), and KNO₃ (0.2 M) at pH = 4.6; i = 0.5 A dm⁻²; temperature: 25 °C and time: 10 minutes. The modified Watts bath has a throwing power of 61 %, which is higher than that reported earlier. The potentiostatic current-time transients indicate instantaneous nucleation. XRD analysis shows that the black nickel deposit is pure metallic nickel with Ni(111) preferred orientation.

Mechanical properties and corrosion resistance of black nickel coatings are greatly enhanced when phosphorus is co-deposited with nickel. A process of flat absorber phosphorous black nickel alloy coating on SS 316 for space applications was developed by Maria Shalini et al. [26]. The test coupons were cleaned ultrasonically in trichloroethylene, followed by anodic electrolytic cleaning in a solution of sulphuric acid (SG 1.84): 600–700 ml/L at 10 V for 1 minute. The cleaned coupons were subjected to nickel strike using a solution of 225–275 g/L of nickel chloride and 84–88 ml/L of hydrochloric acid (35 %, V/V) at a current density of 3 A/dm², at 25 °C, followed by water rinsing. Thereafter, the test coupons were processed for conventional black Zn-Ni alloy plating [14] and phosphorous black nickel coating.

Phosphorous black coating was deposited in a bath con- taining 75 g/L nickel sulphate hexahydrate (NiSO₄.6H₂O), 30 g/L zinc sulphate (ZnSO₄), 30 g/L ammonium sulphate (NH₄)2SO₄, 37.5 g/L sodium thiocyanate (NaSCN), and 37.5 g/L sodium hypophosphite (NaH₂PO₂); operating at pH 4.8–5.2; current density 0.5 A/dm²; bath temperature 25–30 °C; for 15 minutes. Standard nickel was used as the anode.

EDX spectroscopy showed the inclusion of about 6 % phosphorous in the coating. The scanning electron microscopy studies revealed the amorphous nature of the coating. The electrochemical impedance spectroscopy (EIS) and linear polarization (LP) studies showed that, phosphorous addition confers better corrosion resistance in comparison to conventional black nickel coatings. The black nickel coating obtained from hypophosphite bath provides high solar absorptance and infrared emittance of the order of 0.93.

Black Ni-Co coatings have been electrodeposited by Karuppiah et al. [21] using the following experimental conditions: nickel sulphate hexahydrate: 10 g/L, cobalt sulphate heptahydrate: 10 g/L, ammonium acetate: 10 g/L; pH: 6.2, temperature: 35 °C, current density: 3.5 A/dm² for a duration of 30 seconds. The obtained film possesses a solar absorbance of 0.91 and a thermal emittance of 0.04. The coating consisted of particles of highly irregular shape, micro roughness and dendritic like structure. The high degree of solar absorption is attributed to optical interference and surface roughness which depends on the coating thickness.

Electrodeposition of black nickel-cobalt solar selective coatings on copper plates for solar thermal energy conversion was described by Karuppiah and John [27]. Hull Cell studies were employed to optimise the electrolyte composition and operating parameters. The optimised electrolyte contains nickel sulphate: 10 g/L, cobalt sulphate: 10 g/L and ammonium acetate: 10 g/L and operated at pH: 6.0–6.5; temperature: 30–35 °C; current density: 4 A/dm² for 30 seconds. An undercoating of 10 µm thick bright nickel with Watts nickel bath was provided prior to black nickel- cobalt coatings. The optimized process yielded a black nickel cobalt plating with the best solar selectivity of 9.5 (α = 0.95 and ε = 0.10).

SEM studies revealed the dendritic growth of the particles, the particles of highly irregular size and shape with micro hills and valleys. The high degree of solar absorption is attributed to optical interference, surface roughness. band gap absorption, graded index, metal-dielectric cermet coating or a combination of these effects.

A review on Electrodeposition of nanocrystalline nickel- cobalt binary alloy coatings containing history and summary of recent developments is presented by Ma et al. [28]. Nanostructure, morphology, physical and mechanical properties and corrosion resistance vs. bath composition and plating conditions, like pH, current density, temperature, etc. are summarised.

5.2 Black Cobalt Plating

Black cobalt oxides have been studied mainly for their optical and semiconducting properties which render them attractive for solar photothermal applications [30–34]. Common oxidation states of cobalt include +2 and +3, although compounds with oxidation states ranging from -3 to +5 are known. Cobalt (II) oxide (CoO) has rock salt structure. At temperatures of 600–700 °C, CoO oxidizes to cobalt (II,III) oxide, Co₃O₄ (CoO•Co₂O₃), which has a spinel structure [35]. Co₃O₄ exhibits spectrally selective surfaces with high values of solar absorbance (α) in the visible and the near infra-red (NIR) spectrum and low values of emittance (ε) in the IR spectrum which improves their thermal performance by reducing the radiative heat loss.

The works on the electrochemical black cobalt oxides can be divided into two groups: direct and indirect. In the former, a solution is prepared by dissolution of the chemical components that allow the direct formation of black cobalt on the substrate at the cathode during the electrolysis process itself. On the latter the formation of cobalt oxide can be accomplished in two steps, i.e., first the metallic cobalt is deposited on the substrate and then it is oxidized to black cobalt oxide through chemical or thermal oxidation in air at ~ 400 °C. In agreement with published results, these oxides show different optical properties depending on the type of oxide obtained, i.e., the properties are different when the obtained oxide is Co₃O₄, CoO, or CoO(OH). The stability studies with respect to optical absorptance and emittance measurements on the as grown and heat-treated cobalt oxide films spray deposited on aluminum and galvanized iron substrates were carried out by Choudhury and Sehgal [36]. The selective solar properties of oxide films grown in-situ on cobalt and cobalt alloys were investigated by Kwon and Douglass [37].

Kruidhof and van der Leij [38] electrochemically deposited cobalt on bright nickel-electroplated steel from a high efficiency cobalt sulphate bath containing, CoSO₄.7H₂O: 450 g/L, CoCl₂.6H₂O: 45 g/L and H₃BO₃: 40 g/L; operating at pH: 2–4; temperature: 55 °C; current density: 4–8 A/dm², film thickness: < 0.2 µm. The influence of addition of iron (III) sulphate in the bath was also investigated, because spinel Co-Fe oxide may have higher stability than a cobalt oxide. Additionally, a higher oxide such as Fe₂O₃ usually has a lower refractive index so as to yield a higher solar absorptance.

The deposited cobalt was then oxidized in air for two hours at 400 °C. Of all bath parameters, the pH-value had the greatest influence on the optical properties of the oxide. The best results were obtained at a pH of 2.3. The ratio of solar absorptance and IR emittance (α/ε) at 100 °C was observed as follows: 0.76/0.05 (pH = 4); 0.95/0.11 (pH = 2.3); 0.90/0.07 [(pH = 2.3 with addition of 6–8 g/L Fe₂(SO₄)₃.7H₂O)]. The high absorptance found at pH = 2.3 was mainly due to the needle-like structure of top surface, that changes to rod-like with the addition of iron (III) sulphate. Heating tests at different temperatures in air and vacuum showed that of the two-high solar absorptance coatings obtained at pH 2.3, without and with addition of iron (III) sulphate to the bath found to be stable up to 300 °C.

The optical properties of the different black cobalt films, plated cobalt sulphide, plated cobalt oxide‐hydroxide, and cobalt oxide prepared by thermal oxidation of electroplated cobalt metal were examined using a range of surface analytical techniques [39, 40]. The black cobalt sulphide had severe thermal degradation, cobalt oxide‐hydroxide behaved as a good selective absorber up to about 400 °C, while the thermally oxidized cobalt black started degrading only at around 500 °C. A limited amount of optical degradation upon heating the oxide black cobalt in air is due to oxidation of hydroxide, but the major degradation is associated with the substrate oxidation rather than the film oxidation. The black cobalt oxide coatings exhibit high solar absorptance due to porous outer layer grading into non-dense oxides of cobalt (CoO or Co₃O₄).

McDonald [41] has deposited black cobalt oxide on a thin interlayer of silver or gold (diffusion barrier coatings) on oxidized stainless steel. Samples of SS304 and Incoloy 800 were first oxidized at 950 °C for 15 minutes in air to form oxide barrier layers. A noble metal reflector layer was deposited on the oxidized surface by vacuum evaporation or thermal decomposition of organometallic compounds at 650 °C in air. It was then coated with black cobalt oxide absorber layer either by thermal decomposition of a metal compound or by electrodeposition. Electrodeposition can be carried out from a strong oxidizing solution containing CoSO₄: 100 g/L and H₂O₂ (30 %): 10 ml/L with a current density 8 A/dm², followed by a dip in a solution of ammonium persulfate: 100 g/L and sodium hydroxide: 50 g/L for 1 minute and drying at 200 °C. The Co₃O₄/Ag or Au/oxidized stainless steel film had absorptance > 0.90 and emittance of ~ 0.20, even after 1000 hours of exposure at 650 °C in air.

The chemical composition and crystal structure of electrodeposited black cobalt on steel and annealed at 400 °C was examined by Abdel Hamid et al. [42]. The crystal structure analysis showed that the bulk composition of the films was mainly cobalt oxide. The topmost surface layers of the films are made of a multivalence state of cobalt oxide with an oxidation state of ≥ +2. The surface morphology of coating was changed from dendritic structure to lamellar at higher current density.

Barrera et al. [43–45] reported electrodeposition of black cobalt onto a stainless-steel substrate from cobalt sulphate and boric acid aqueous solution in presence of small quantity of nitrate as an oxidant that allows the deposition of a black spinel-type cobalt oxides, Co₃O₄. The electrolyte formulation was similar to that proposed by John et al. [46]. The following process was optimized by using the standard 267 ml Hull cell experiments: cobalt sulphate: 275 g/L, cobalt chloride: 35 g/L, boric acid: 30 g/L, cobalt nitrate: 2.5 g/L; pH: 3.5; temperature: 20 °C; current density: 10 A/dm2. Approximately, 2.3 µm thickness of coating was deposited in 45 seconds.

The freshly prepared black cobalt showed a solar absorptance value of 0.93 and thermal emittance of 0.26 on stainless-steel while on the nickeled stainless-steel these values were 0.93 and 0.18, respectively. The thermal stability of coatings was good up to 300 °C, but even a short heating above temperatures of 400 °C lead to big modifications in the values of the optical properties that worsen their selectivity and damage their characteristics of solar radiation absorptivity. The structure of black cobalt is amorphous granular, after thermal treatment, surface recrystallisation leads to changes in shape of grain. This influences the optical properties; the surface absorptivity decreases while the emissivity is increased. This is primarily due to an oxidation and diffusion processes during the thermal treatment, where CoO transforms to Co₃O₄ [40].

In another work [44] the authors have reported the cobalt electrodeposition from an electrolyte containing cobalt (II) salt: 1.17 M, sulphuric acid: 0.98 M and boric acid: 0.2 M, with and without potassium nitrate: 0.1 M. The electrodeposited cobalt from the electrolytic without potassium nitrate was white‐gray, whereas with potassium nitrate was black‐coloured. Although both deposits were composed of metallic cobalt, the white cobalt deposit was a smooth, 2D film while the black deposit consisted of many dispersed, nano‐sized clusters of 150 to 250 nm in diameter. Formation of black cobalt involves the simultaneous processes of 3D nucleus formation and growth, limited by mass transfer, and the reduction of nitrates in the medium onto the surfaces of these nuclei. The phenomenon of cobalt‐nitrate interaction strongly depends on the nitrate concentration. These authors have also prepared the cobalt oxide and cobalt oxide-silver selective surfaces in a galvanostatic mode and tried to establish the relation with the roughness factor of the selective surfaces with deposition time and the solar absorptance of the deposits [45].

Kadirgan and Sohmen [47] developed a black cobalt selective absorber coating on copper sheet for solar collectors’ applications. Copper was chemically cleaned and nickel plated for a thickness of 10 µm using a Watts nickel bath containing nickel sulphate: 300 g/L, nickel chloride: 25 g/L, boric acid: 35 g/L, saccharin: 0.5 g/L; pH 4.0; temperature 50 °C; current density 4 A/dm². Thereafter, the cobalt was deposited from a solution of CoSO₄.7H₂O: 319 g/L, CoCl₂.6H₂O: 45 g/L, boric acid: 24.8 g/L and an oxidant (HNO₃: 0.075 M); temperature: 20–22 °C; current density: 6A/dm²; time: 18 seconds. Under heat treatment in air, the conversion of CoO is followed by the formation of grey black Co₃O₄. A solar absorptance of 0.92 and thermal emittance of 0.04 was reported. Thermal stability of the coating was evaluated at various time intervals after thermal cycling at 300 °C. A drop in α from 0.92 to 0.885 was observed after 40 seconds exposure. The possibility of deposition of black cobalt coatings from fluoride-containing electrolytes based on cobalt sulphate was studied by Ivanova and his co-workers [48].

Electrodeposition of black cobalt thin films on bright nickel-plated brass and copper substrates were reported by Toghdori et al. [49]. Brass and copper plates were thoroughly degreased and then subjected to a 1-minute acid etch in 5 % sulphuric acid. Bright nickel plating was carried out in watts bath: nickel sulphate 250 g/L, nickel chloride 50 g/L, boric acid 50 g/L; 0.5 A/dm², 70 °C. Finally, black cobalt was deposited with electrolyte containing cobalt sulphate: 400 g/L, cobalt chloride: 50 g/L, cobalt nitrate: 4 g/L, boric acid: 40 g/L; pH: 4; current density: 3 A/dm²; temperature: 30 °C. The anode was cobalt metal with 99.9 % purity. It was then annealed in the air at 400 °C for 20 minutes. The maximum solar absorptance was 98 %-99.55 % at wavelength range of 400–1200 nm.

A process of solar selective black cobalt-cadmium coatings was optimised using the Hull cell technique by John and Santhi [50]. The electrolysis process consisted of cobalt sulphate: 10 g/L, cadmium sulphate: 10 g/L, ammonium thiocyanate: 10 g/L, cetyl trimethyl ammonium bromide (CTAB): 0.5 g/L, operating at 30 °C, pH of 5.0, current density of 2 A/dm² for 20 seconds. Optimised coating exhibited optical properties of α = 0.94 and ε = 0.14. Heat treatment of the coating at 300 °C for 64 hours resulted in an increase in absorptance (0.96) and a decrease in emittance (0.08) thereby showing its suitability for use in high temperature solar thermal systems. SEM studies reveal a structural modification due to recrystallization and an increased void formation upon heat treatment.