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Dienstag, 16 November 2021 10:59

Black Electrochemical Coatings for Aerospace and Allied Applications – Part 6 – Black Electroless Nickel-Phosphorus

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Geschätzte Lesezeit: 15 - 30 Minuten

Electroless Ni-P coating with subsequent oxidization is a well-established technique for ultra-high absorptance black finish [1–4], although the darkest man-made surfaces are made of carbon nanotubes (CNTs) [5]. The robust surface of the black Ni-P coating is preferable for minimising the risk of accidentally damaging the dimensionally critical baffles of large size. On the contrary the CNT forest is quite fragile. The black electroless nickel has outstanding mechanical properties and stability against sunlight exposure in humid as well as extreme space conditions.

6.1 Electroless Ni-P plating

The unusual combination of properties of electroless nickel-phosphorous coating such as corrosion and wear resistance, hardness, lubricity, bondability and uniformity of deposit regardless of geometry of the substrate makes it ideal for a widespread engineering applications. Electroless plating, also referred as autocatalytic or chemical plating, is a plating process in which metal deposition on the substrate proceeds autocatalytically based on the reduction of metallic ions from an aqueous metal salt-based solution, without passage of external current. The reducing agent presents in the solution act as electron donors to metal ions. The metal ions get reduced to metal after receiving the electron and deposit on the substrate. The substrate surface acts as a catalyst, which accelerates the electroless chemical reaction allowing oxidation of the reducing agent and providing self-catalytic continuous deposition of metal. The deposit is not pure nickel, but contains co-deposited phosphorous or boron, depending on the reducing agent employed hypophosphite or borohydride [6–15].

Components of the electroless bath include an aqueous solution of metal ions, reducing agent(s), complexing agent(s), and bath stabilizer(s) operating in a specific metal ion concentration, temperature, and pH range [7]. The electroless bath provides a deposit that follows all contours of the substrate exactly, without building up at the edges and corners. A sharp edge receives the same thickness of deposit as does a blind hole. The workpiece being plated must be catalytic in nature. A properly prepared workpiece provides a catalysed surface and, once introduced into the electroless solution, a uniform deposition begins. Minute amounts of the electroless metal (e.g., nickel) itself will catalyse the reaction to continue the deposition, provided that the metal ion and reducing agent are replenished. If air or evolved gas are trapped in a blind hole or downward facing cavity, the electroless deposition will be prevented in these areas.

The electroless deposition process differs from immersion plating or galvanic displacement, where the base material is dissolved and displaced by a metallic ion in the solution having a lower oxidation potential than the displaced metal ion [8–10]. In such processes the reducing agents are not required to reduce the metal ions to metal, as the base material itself behaves as a reducing agent. Immersion deposits are typically very thin as deposition is not sustainable. Once the surface of the substrate is fully covered with novel metal (lower oxidation potential), the deposition stops, as no more substrate can be dissolved in the solution and displaced. The immersion coatings generally do not adhere well to the substrate, these processes therefore have not gained wide acceptance. On the other hand, the electroless processes bestow adherent uniform deposits. The electroless processes have acquired a wide importance for engineering applications due to the coatings’ excellent mechanical properties and corrosion resistance.

Although, the spontaneous reduction of nickel cations by hypophosphite anions was first observed by Wurtz in 1844 [16–18], but the credit of practical application of electroless Ni-P deposition goes to Brenner and Riddell [19–22] when they accidentally discovered the nickel deposition on the inner walls of tubes in 1946 [23]. Since then the electroless deposition processes have undergone numerous modifications to cater the needs of various industrial applications [15]. Electroless nickel-boron plating was developed a few years later, in 1955 [24], just after the discovery of the borohydride ion. Ni-B coatings have higher hardness and wear and abrasion resistance than Ni-P coatings, but the corrosion resistance of the latter is far superior [25, 26]. Furthermore, the reducing agent (sodium borohydride or dimethylamine borane) used for Ni-B are much costlier than the hypophosphite of Ni-P coatings, consequently, the Ni-B are used much less frequently. Here, our discussion will be limited to Ni-P coatings which are used as a precursor for black Ni-P coatings.

Electroless nickel-phosphorus coatings are generally favoured over electroplated nickel deposits because of their enhanced corrosion-resistance properties and the coating uniformity achievable with complex objects. Each system has its advantages and disadvantages, but the properties of electroless nickel and its ability to incorporate composites have enabled the process to be exploited widely. The advantages of the electroless coatings [27] include:

(a) Good throwing power
(b) Uniform coating thickness on complex objects
(c) No levelling required
(d) Coatings are harder (480 HV) than electrodeposited metal (200 HV) because phosphorus is incorporated in the structure
(e) Deposits can be further hardened to about 1,050 HV by heat treatment at about 400 °C for 1 hour
(f) Good wear resistance
(g) Low ductility (1–3 % elongation)
(h) Low porosity, which leads to good corrosion resistance
(i) Excellent solderability and brazability characteristics, and
(j) Low labour cost.

The disadvantages associated with electroless system are:

(a) Solutions are expensive,
(b) Deposition rates are slow,
(c) Welding characteristics are poor,
(d) Careful analytical control of the bath is required, and
(e) Electroless baths have limited life due to the formation of reaction by-products.

The coating properties can be tailored by optimizing the alloy composition, the phosphorus content between 1 % and 15 % by weight. According to ASTM 733B-04 standard, electroless Ni-P films can be classified as low phosphorus (up to 5 wt.% P), medium phosphorus (5–9 wt.% P) and high phosphorus (10–14 wt.% P) layer, based on their phosphorus content. The incorporation of phosphorus greatly affects physical and chemical properties of the coating, such as density, hardness, corrosion resistance, etc. The structure of electroless nickel is responsible for some of its unique properties. It differs greatly from the crystalline structure of electrodeposited nickel and it can normally be described as having an amorphous structure or one consisting of ultra-fine crystallites [28]. The amorphous nature of the deposits becomes more dominant with increasing phosphorus content. The deposits with about 12 % and above phosphorus, are considered as truly amorphous. However, the coatings below 12 % phosphorous are metastable solid solutions within which high phosphorous species such as Ni3P or Ni5P2 exist extensively, or as inclusion in low P content alloys [29]. The absence of a well defined crystal structure eliminates the possibility of intergranular corrosion that can be a problem with crystalline deposits, such as electrolytic nickel. Electroless nickel, therefore, provides a more effective barrier coating in protecting a substrate from corrosive attack.

The phosphorus content of the deposit depends mainly on the pH control of the bath during plating, but it is also affected by its formulation (complexing and buffering agents), the plating bath temperature, and the molar ratio of nickel to hypophosphite [30]. As the hydrogen phosphite concentration increases during the life of the bath, the phosphorus content of the deposit gradually increases.

The density of pure nickel is 8.9 g/cm3. The density of electroless nickel is not constant and decreases appreciably with increasing phosphorus content. For instance, a deposit containing 3 % phosphorus has a density of 8.52 g/cm3 while at 11 %, the density is only 7.75 g/cm3 [31]. Other differences in deposit properties are frequent because of variation in heat treatment temperature and time, after plating. As plated, electroless nickel is in a metastable state consisting of a supersaturated solid solution of phosphorus in nickel, because the equilibrium solid solubility of phosphorus in nickel is essentially zero; however, the second phase cannot form because the time interval between the deposition of successive layers is too short for the necessary diffusion. The structure of the deposit changes from microcrystalline to amorphous with increasing alloy content. The second phase, which is Ni3P, can form on annealing and results in precipitation hardening [32]

Heat treatment of nickel-phosphorus deposits can cause significant changes in properties and structure [33–40]. The hardness of ‘as-deposited’ is greater for low phosphorus coatings, but the hardness of all types of Ni-P coatings increases dramatically after heat treatment. When electroless nickel deposits are heated above 220 °C, they begin to crystallize, and nickel phosphide precipitates from supersaturated nickel phosphide solid solutions. XRD examination shows substantial crystallinity and segregation of the deposit into small crystallites of two distinct phases, nickel metal and nickel phosphide (Ni3P). The primary effects of crystallization and nickel phosphide precipitation are hardening and shrinkage of the coating alloy (leads to microcracking particularly in high and medium phosphorus deposits). This results in the improvement of the hardness, adhesiveness, creep and wear resistance of the Ni-P coatings while the formability, ductility and corrosion resistance are reduced.

The maximum hardness in electroless nickel deposits can be attained by 1-hour heating at about 400 °C or in 10 hours at 260 °C. The ability of deposits to maintain their hardness under elevated temperature service conditions increases with increasing phosphorus content but decreases rapidly above 385 °C [28, 41].

Electroless nickel is most frequently used in wear applications in the precipitation-hardened condition, because of improved hardness and natural lubricity. The corrosion resistance deteriorates upon heat treatment above 250 °C because of cracking; therefore, under corrosive wear applications, it may be prudent to forgo the higher hardness. Electroless nickel is also applied to improve solderability and brazeability of surfaces and is used with moulds and dies to improve lubricity and part release.

The electroless nickel is deposited by an autocatalytic chemical reaction between the job surface and plating solution. The principal redox reaction in the electroless nickel solution is the reduction of nickel ion to nickel metal and oxidation of hypophosphite ion [H2PO2] to hydrogen phosphite ion [HPO3]2– and phosphorus. Because the reaction can only take place at a catalytic surface, dehydrogenation of reactant is therefore proposed as a first step of the reaction mechanism [42–44].

[H2PO2] + H2O Catalytic surface → H+ + [HPO3]2– + 2H

The hydrogen atoms formed react with nickel ions, and a charge transfer occurs resulting in the deposition of nickel metal.

Ni2+ + 2H → Ni°(s) + 2H+

The hydrogen atom again reacts with hypophosphite ion and a decomposition reaction takes place leading to the co-deposition of phosphorus with nickel metal.

[H2PO2] + H → H2O + OH + P (s)

The hypophosphite ion further reacts with water molecules resulting in the formation of hydrogen phosphite ions along with hydrogen ions and hydrogen gas. Hydrogen phosphite anion or phosphite usually refers to [HPO3]2− but includes [H2PO3] ⇌ ([HPO2(OH)]). These anions are the conjugate bases of phosphorus acid.

[H2PO2] + H2O → [HPO3]2– + H+ + H2 (g)

The Overall reaction can thus be represented as:

2Ni2+ + 8[H2PO2] + 2H2O → 2Ni°(s) + 2P(s) + 6[HPO3]2– + 8H+ + 3H2(g)

In electroless nickel plating, once the nickel deposition is initiated, each deposited metal layer acts as a catalytic base, enabling further growth for thicker and more coherent coatings [45] compared to its electrochemical counterpart. As with any chemical process, the efficiency of the electroless nickel plating is determined by the experimental conditions employed. The plating rate and metal coating purity are highly dependent on the pH and temperature of the nickel bath and the ratio of the concentrations of the metal ion and reducing agent [10, 46–49].

6.2 Blackening of Electroless Ni-P Coating

Blackening of electroless nickel alloy coating bestows a high absorptance, conductive and rich black surface finish with enhanced wear and corrosion resistance [50–62]. The black finish is formed by acid etching of a low phosphorus electroless nickel coating. During acid etching, the nickel atoms in the nodular Ni-P surface are preferentially removed and some nickel atoms are oxidised. The etching has undergone progressive enrichments, though the results of earlier processes were encouraging but finishes required improvements for implementation on critical baffle components [50]. It has been now established that compared to medium or high phosphorus, a low phosphorus Ni-P alloys imparts an electrically conductive deeper black finish with good wear resistance.

Phosphorous contents affect the etching behaviour and properties of nickel black coating. Higher phosphorous Ni-P alloys are not suitable for blackening. As the phosphorous contents in the electroless coating increases, it becomes more resistant to acid etching and consequently a higher concentration of oxidizing acids is required for blackening. This may adversely affect the uniformity of the coating and its solar absorptance may drop. When the phosphorous contents in the electroless nickel exceeds 14 %, it becomes extremely difficult to etch the coating, even with high concentration of oxidizing acids. Lower the initial phosphorous content in the electroless nickel, the greater the extent of the etching and lower the reflectance of the resulting surface. However, the coating below 6 % phosphorous show poor corrosion resistance without any appreciable improvement in absorptance value. A base layer of medium or high phosphorus electroless nickel can be used, where an increased corrosion resistance is required. Low phosphorous electroless Ni-P baths operating at a medium acidic pH of 4.5 to 6.0 are recommended for obtaining ultra-high absorptance black coating. Bath constituents and operating conditions of some typical baths are presented in Table 6.1 [58–60, 62].

Tab. 6.1: Typical bath formulation and operating conditions for low phosphorous electroless Ni

Bath constituents / Operating conditions

Bath-1

Bath-2

Nickel sulphate hexahydrate, g/L

30

26.3

Sodium hypophosphite, g/L

10

26.4

Tri sodium citrate dihydrate, g/L

12.5

Sodium acetate, g/L

5

12.3

Thiourea, mg/L

1

0-2

D, L-Malic acid, g/L

4.0

Citrate acid, g/L

3.8

Succinic acid, g/L

5.9

Lauryl sodium sulphate, mg/L

8.0

pH

4.5–5.0

5.8

Temperature, °C

88–90

85±2

Time, minutes

75–90

120

Plating rate, µm/ hour

25±5

John et al. [51] reported the process of deposition of electroless nickel at room temperature and then blackened it with dilute HNO3 solution. A solar absorptance (α) value of 0.94 and an IR emittance (α) value of 0.15 was accomplished. The coatings were tested for thermal stability and compared with standard black chrome panels. At a solar radiation level of 800 W/m2 an equilibrium temperature of 155 °C was recorded compared with 160 °C and 96 °C for black chrome panels and black paint, respectively under identical conditions.

The surface morphology plays a crucial role in obtaining the high absorptance coating. The black colour of Ni-P coating arises from its unique surface morphology combined with the formation of nickel oxides (NiO, Ni2O3) and some nickel phosphate [52–54]. This black layer is less than 1 micron thick. The surface morphology of black nickel depends on the phosphorous contents. The coating with low phosphorous produces a very pronounced ‘crater’ morphology and with high phosphorous a ‘stalagmite-like’ morphology. Ni-P alloys with a lower phosphorous content are etched more completely, with large crater formation as large portions of the alloy are dissolved [52–56]. As the phosphorus content in the coating increases, it is more difficult for large swaths of Ni-P to be etched and hence narrow sharper features are observed.

The high absorptance of the coating is associated with unique surface morphology consisting of a dense array of microscopic, conical pores perpendicular to the surface. This structure produced by selective etching of Ni–P acts as light traps and is capable of absorbing 99.5 % light in the solar region (300-2300 nm). The pore diameter, pore depth and pore spacing range from a fraction of micrometre to a few micrometres or about a fraction to several wavelengths of light. Consequently, the pores trap any incident light in a wide spectral range (Fig.6.1) [56–58].

Fig. 6.1: SEM of electroless nickel-phosphorous coating after etching [56]      Fig. 6.1: SEM of electroless nickel-phosphorous coating after etching [56]

Sharma et al. [56–58] have investigated the blackening process of electroless nickel on AA6061, Invar and Ti6Al4V for applications on the space telescopes. Studies were conducted on Ni-P deposits with P content 5–7 % with a coating thickness of 30±2 μm [56]. The influence of various process parameters on the physico-optical properties of the coating was investigated. Effect of concentration of blackening solutions, pH, immersion time, temperature of etching solution and heat treatment was investigated to optimise the blackening process.

The following three etching solutions were used for blackening of electroless nickel coating on aluminium alloys and Invar [56, 57]. The optimal concentration, operating parameters and the optical properties of consequential blackened finish are as given below:

Solution-1: Nitric acid: 9 M; 40 °C; time: 40 seconds; α: 0.995; ε: 0.76
Solution-2: Sulphuric: 6 M + nitric acid: 4 M; 60 °C; time: 20 seconds; α: 0.971; ε: 0.43
Solution-3: Nitric acid: 1.1 M + sulphuric acid: 0.3 M + potassium permanganate: 0.1 M; 40 °C; time: 50 seconds; α: 0.878; ε: 0.38

With an increase in etching time and / or increase in temperature of the etching solution, the nickel content in the coating decreases whereas the phosphorus and oxygen content increases. This suggests that only nickel atoms in the nodular Ni-P surface are preferentially removed and some of them converted to oxides such as NiO and Ni2O3 [1, 56–58]. This caused the phosphorus and oxygen contents to increase at the black Ni-P surface. The nodular structure becomes slightly larger with increase in temperature of the etching solution. As solution-1 (nitric acid: 9 M) was found to provide the highest solar absorptance value, it was selected for further studies.

Cross sectional views of the electroless nickel coating as obtained and after etching showed very uniform deposition. The coating after etching shows a peak to trough height of 6–7 μm, with a top layer of Ni–P black oxide being ~0.7 μm. However, the thickness of this top layer in excess of 1.0 μm is often powdery and nonadherent [56].

The pH of the electroless nickel plating solution should be regulated to obtain the most favourable results. The phosphorous contents of the electroless Ni-P deposits depend upon the pH value of the solution. At high pH, phosphorous content decreases, but the solution may decompose and at low pH value phosphorous contents are higher and the resultant coating has high stress [1]. At lower pH of 4.0, solar absorptance of black coating was lower (0.98) and at pH 5.5 and above the electroless nickel plating solution starts decomposing. The optimum results were obtained at a solution pH value of 4.7.

Black Ni-P coatings show slightly higher corrosion potential and passivation potential when compared to unetched Ni-P coatings. Regardless of whether a heat treatment is applied, they show a higher corrosion current because the black layer is very thin and has a special crack structure. Nonetheless, heat treatment is recommended to harden the black layer and stabilize the deposit colour. Heat treatment also marginally improves the corrosion resistance of the black Ni-P coating [59].

The space worthiness of the black Ni-P coatings was evaluated by the simulated environmental tests, viz, humidity, thermal stability, thermal cycling and thermovacuum tests and measurement of optical properties. The optical properties (solar absorptance and infrared emittance) of the coatings were measured before and after each environmental test to ascertain their stability. The % Total Mass Loss (TML) and % Collected Volatile Condensable Ma- terial (CVCM) values were measured as low as 0.05 and 0.01 %.

The humidity test was carried out to examine the resistance of the coating to the corrosive pre-launch atmosphere. The test was conducted in a thermostatically controlled humidity chamber for 48 hours. A relative humidity of 95 %±0.5 % was maintained in the chamber at 50±1 °C. To ascertain the stability of coating at elevated temperatures, a thermal stability test was conducted in an oven at 200±2 °C for 48 hours, followed by quenching in ambient air.

The thermal cycling test is designed to evaluate the effect of cycling temperature on the deposit that is likely to be encountered throughout the lifespan of a spacecraft. The test was conducted in thermostatically controlled hot and cold chambers. A total of 100 cycles was applied. A cycle consists of placing the samples into a chamber operating at −45 °C for 5 minutes, bringing them to an ambient temperature with a dwell of 15 minutes and shifting them to a hot chamber at 80 °C for 5 minutes.

To further examine the effect of cycling temperature in the simulated space environment, the electroless nickel black-coated specimens were subjected to a thermo-vacuum hot and cold soak test. The test consists of lowering the temperature of −45 °C for 2 hours and then raising the temperature to 80 °C for another 2 hours. A total of 10 cycles of hot and cold soak were applied. A vacuum level of 10–5 Torr was maintained inside the chamber during the entire period of testing. The black electroless Ni-P coatings have passed these tests without any degradation in their optical properties.

In another study, etching of electroless plating nickel on medium carbon steel sheets was accomplished by Cui et al. [60] by immersion in a solution containing 5.5 M sulfuric acid and 4.1 M sodium nitrate at 50 °C for 10 seconds. After rinsing and drying, the samples were again etched in the same solution for 5 seconds. AFM analysis showed that after etching the nodular structure of Ni-P film changed to small convex closure-like structure and the film’s surface became smoother. In addition, the etching led to an increase of phosphorus content in the film due to a preferential dissolution of nickel atoms. The electrochemical impedance spectroscopy studies showed that after black treatment the corrosion resistance of low-phosphorus deposits improved, but of high-phosphorus it significantly reduced. Visible reflectance measurements on the films showed that the optimum pre-etch phosphorus content range to prepare low reflectance (~0.45) black nickel is from 3 to 7 wt %.

Black Ni-P (92.16:7.84) and Ni-W-P (93.02:5.79:1.19) solar absorber coatings on aluminium substrate were characterized by Khalifa et al. [61]. The surface morphology of the as deposited Ni-P and Ni-W-P showed a dense coating having spherical nodular structure with very smooth and high coalescence. The diameters of nodules were 0.019 μm and 1.388 μm, respectively. After etching the blackened Ni-P and Ni-W-P coatings became further denser with spherical nodular particles of 1.4 μm and 2.857 μm diameter. The particles were closed packed to each other to provide improved corrosion resistance. The absorption percentage range in wavelength 250–550 nm, was 99.45 %. Corrosion resistance in 3.5 % NaCl solution was reported in the following order: black Ni-W-P > Ni-W-P > black Ni-P > Ni-P > substrate.

Azli et al. [62] deposited nickel-phosphorous (Ni-P) coating onto a carbon steel and focused the study on the effect of plating bath pH on the light absorption and corrosion properties of the black surface. The authors have obtained electroless nickel plating at various plating bath pH (4.5, 5.0, and 6.0) for 3 hours at a constant bath temperature and composition. Thereafter, the Ni-P coating surfaces were blackened by etching with 9 mol/L nitric acid solution and the surface morphology, chemical composition, optical and other surface properties of coating were examined. The SEM-EDX analysis revealed that black Ni-P coating formed at pH 4.5 had the highest porosity, which is 47.1 % of the surface area with highest phosphorus content. The coatings so formed were also reported to have the highest light absorption in Visible-Near IR region (92 %), due to the high surface roughness.

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  • Ausgabe: 11
  • Jahr: 2021
  • Autoren: Dr. Anand Kumar Sharma

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