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SURFACE TREATMENTS for Space Applications

Surface treatments play an important, multi-functional role in satellite components. They are often critical in terms of optical properties, corrosion resistance, wear resistance, ability to withstand humidity, heat cycling and vibrations and also to reduce or eliminate degassing and particle emissions. Steiger Galvanotechnique SA has participated in the manufacture of many components for numerous telecommunication satellites and missions; as for instance: ISO, XMM, MERIS, STENTOR, METOP, EOS, ROSETTA, MXT CAMERA OF SVOM, EUCLID, CHEOPS.

Surface treatments for Space applications include:

  • Alcaline anodisation of Titanium
  • Chemical conversion coatings on aluminium alloys
  • Black anodisation of aluminium alloys
  • Electroless nickel on aluminium
  • Gold plating
  • Multicoatings
  • 3D Additive Finishing – Innnosurf® advanced Coatings

Alcaline anodisation of titanium

Weighing less than steel, but stronger than most stainless steels, its superior strength-to weight ratio (particularly at high temperatures) and its resistance to several forms of corrosion have made titanium and its alloys significantly more attractive to the aero-space and aircraft designer.

A problem arising with Titanium is due to its severe galling tendency. An efficient way to reduce to galling problems is to apply an alcaline anodisation according to the AMS 2488c. By this process, a coating of a few μm of TiO² is formed by anodic conversion on the titanium surface. Steiger Galvanotechnique SA offers this coating under the trade name “Biodize®”.

More details on the characteristics of this coating are given in our Brochure for Medical Techniques. Today the largest application of this coating is for the surface treatment of medical implants.

Conversion coatings

The range of chromate conversion coatings are described in detail in various literature and standards (1–3). The coatings based on hexavalent chromium salts, for safety and environmental reasons, are increasingly substituted by chromite conversion coatings based on trivalent chromium salts.

The chromate conversion coatings are formed by a chemical reaction between the aluminium alloy surface and the hexavalent chromium in the solution bath. Cr +VI is reduced to Cr +III while the aluminium is oxidized to Al +III. During the course of the reaction the pH at the metal-liquid interface rises and causes the chromium and aluminium compounds to precipitate on the surface in the form of a gel containing chromates, chromium hydroxides, as well as aluminium oxides, oxyhydrates and bifluorides depending on the type of chromate conversion coating applied. The gel, once dry, achieves mechanical stability. The coating should not be heated above 80°C to prevent excessive dehydration of the layer. If overheated, the coating will disintegrate.

Different types of chromate conversion coatings are available under different trade names, for instance: Alodine 1000 and Alo-dine 1200s. The chromate conversion coating is chosen for two reasons, namely: corrosion protection and low electrical resistance. An increasing thickness of the coating improves the corrosion resistance but simultaneously also increases the electrical contact resistance. The thickness of the coating is influenced by the concentration of chromate in the solution, by the pH, and the immer-sion time. The Alodine 1000 coating is almost colorless, but the Alodine 1200s coatings are brass-colored; tending toward brown-ish for higher thicknesses.

The chromite conversion coatings based on trivalent chromium salts are available under different trade names; for instance: Surtec 650. Its appearance is a faint, but visible, iridescent blue. This conversion coating, as applied by Steiger, has been validated by prime users, namely Airbus Defence & Space and Thales Alenia Space. The validation criteria are the same as for chromate conversion coatings, including visual test, wipe test, peel test, layer weight, corrosion test, damp heat, ageing, thermal cycling and contact resistivity.

The physical properties of 3 types of conversion coatings are presented in the Table hereunder.

  Alodine 1000 Alodine 1200s Surtec 650
Appearance Almost colorless Almost colorless Almost colorless
Maximum utilisation temperature 80°C 80°C 80°C
Layer weight < 0.1 g/m² 0.2 – 0.4 g/m² 0.2 – 0.4 g/m²
Corrosion resistance; number of hours in salt spray test (NF X 41-002) >72 h >168 h >168 h
Contact resistance for a surface of 6.45 cm²; (MIL-C-81706) 0.3 – 1 mΩ 1 – 5 mΩ 1 – 5 mΩ
Applicable Norm MIL-DTL-5541F Type I MIL-DTL-5541F Type I MIL-DTL-5541F Type II

The thermo-optical properties of the conversion coatings are characterized by a low absorption value α, a low emission ε and high reflectivity R.

Black anodisation of aluminium according to ECSS-Q-ST-70-03C

The black anodisation coating is an aluminium oxide layer with a porous structure containing a UV resistant mineral pigmentation. Black anodisation coatings show good scratch resistance compared to black paints.

The standard anodisation of aluminium has a thickness of approximately 20 μm. ²⁄³ of the layer thickness grows inside the substrate material and ¹⁄³ grows over the initial dimension of the part. The pigmentation consists of cobalt sulphides, which are directly obtained chemically inside the pores of the aluminium oxide. The pigmentation is followed by a sealing step in hot water by hydratation and the formation of hydroxides that occupy more volume than aluminium oxide and “tighten-up” porous structure.

Typical aluminium alloys for space applications are:

  • AlMgSi0.5-1 6000 Series
  • AlMg3 5754: A-G3M and 2024: A-U4G
  • AlCu2MgNi 2618A: A-U2GN
  • AlZn6MgCu1.5 7025

The suitability of the different alloys to black anodisation depends not only on the compatibility of the alloy composition but also on the surface finish resulting from the machining of the parts.

For parts made of 7075 aluminium alloy, the anodisation may lose its mechanical stability, especially when the alloy is machined with a ball-nosed cutter, producing a surface having a ‘saw-tooth’ profile. It has been observed that fine particles can detach during hard cleaning. This is due to the fact that the sharp edges of the ‘saw-tooth’ profiles, the oxide layer of the black anodisa-tion has a disturbed structure and consequently loses its mechanical stability. Flat, smooth surfaces show much better coating stability. It should be emphasized that 7000-Series and 2000-Series alloys remain under ESA alert and should be avoided for space applications.

The oxide structure of 6061-alloys is much more compact than the 7075-alloys and therefore the black anodisation layer can be better applied on rough surfaces. 6061-alloys used by Aerospatiale for Meris parts were sand-blasted before the application of the black anodizing treatment. These parts could be ultrasonically cleaned in hot water but were not resistant enough to withstand a mechanical cleaning by rubbing with a cloth moistened with acetone.

Coating properties

The functional properties of ESA PSS-01-703 black anodisation*) have been tested at the ESA (1, 2).
The results are summarized hereunder.
*) ESA PSS-01-703 is the previous specification number to ECSS-Q-70-03

1) Outgassing kinetic (4)
The test performed according to ESA specification 232-1985 gives long term outgassing at 25°C as well as at 0, 50, 75, 100 and 125°C for a period of approximately 11 years in a space Environment.

Fig. 1
Outgassing of black anodisation coatings at 25°C (long-term predictions at 25°C): Colinal 3100 and ESA PSS-01-703.
Colinal is a black anodisation containing a tin oxide pigment

2) Chemical spray test (5)
This is a cleaning test consisting of spraying the samples with iso-propyl alcohol for 1 minute at room temperature.
No visible degradation was observed as a result of the test and the samples passed the test.

3) Humidity test (5)
The samples were subjected to 7 days at 50°C and a relative humidity of 95%. No visible degradation was observed as a result of the test and the samples passed the test.

4) Thermal cycling test (5)
The samples were thermally cycled 100 times between +100°C and –100°C in a vacuum of 1.10-5 torr according to ESA  specification PSS-01-704. The dwell time at maximum and minimum temperatures was 15 minutes and the average heating and cooling rate was 10°C ± 2°C per minute between the two temperature extremes. The samples passed the test.

5) Thermal shock test (5)
The samples were transferred 10 times from an oven at +50°C to liquid nitrogen. The dwell time at 50°C was 10 minutes and 5 minutes in liquid nitrogen. The transport time from oven to liquid nitrogen and vice versa was approximately 5 seconds.  The samples passed the test.

6) Adhesion test (5)
The adhesion has been tested by the peel and pull-off strength of coatings and finishes with pressure-sensitive tapes, according to ESA PSS-01-713. The black anodisation had not degraded as a result of the evaluation test and was thus considered to be satisfactory.

7) Thermo-optical properties (5, 6)
The measurement of the thermo-optical properties was performed according to ESA specification PSS-01-709.

  • The solar absorptance αs was measured compared to a reference sample of known absolute absorptance using an Elan 510 solar reflectometer.
  • The αs values for 24 samples ranged between 0.930 and 0.940. These values were not altered by the chemical spray, humidity testing, thermal cycling, and thermal shock.
  • The normal emittance (εn) of the samples was measured with a Gier Dunkle DB 100 emissometer.
  • The εn values for 24 samples ranged between 0.853 and 0.857 and were not altered by the chemical spray, humidity testing, thermal cycling, and thermal shock.
  • The reflectance R of the black anodisation has been measured with a CARY 5 spectrophotometer using an integrating sphere for the total reflectance (Rt) and a VW reflectance accessory for the specular reflectance.

The measurements have been performed in the wavelength range of 350 to 1150 nm on two types of black anodized samples: one was sand-blasted and the other not. The sand-blasted sample had significantly lower reflectance (Rt) values. The specular reflectance (Rs) is estimated at Rs < 0.1% for the sand-blasted sample and at Rs ~= 0.2% for the un-sanded sample.

The specular reflectance (Rs) has been measured for the black anodized samples in the IR range of 2.5 μm to 15 μm. Below a wavelength of 10 μm, Rs is smaller than 0.01 and in the wavelength range of 10 to 15 μm Rs increases up to 0.05.

In a further study (7) made at ESA under contract for ONERA / CERT in Toulouse, the influence of a long-term exposure to UV and particulate radiation on the thermo-optical properties of different coatings including black anodisation has been investigated. The experimental conditions simulated those of a low polar orbit environment with enhanced UV exposures (i.e. accelerated aging). UV irradiation was produced by a sun simulator based on a Bi-Lambda 4, using a short Xenon arc with interference filters eliminating wavelengths above 400 nm. The total UV irradiation was 18’206 EHS (Equivalent Solar Hours) which corre-sponds to 6.53 space years.

  • Resistance to irradiation

The black anodised samples also have been exposed to particles for 6.53 years equivalent dose. The particles involved were 200 keV electrons, and 50 keV and 300 keV protons. The temperature of the sample holder was maintained constant at 25°C. The performance of the coating is characterised by the variation of the ratio αs/εn throughout the exposure duration.
High stability of the αs/εn is required.

The relevant results of this study are presented hereunder:

initial absorptance in air α = 0.910
initial absorptance in vacuum α = 0.913
final absorptance in vacuum α = 0.912
final absorptance in air α = 0.907

initial emittance in air ε = 0.854
final emittance in air ε = 0.855
initial αs/εn value in air α/εn = 1.09
final αs/εn value in air α/εn = 1.06

The variation of αs/εn values decreases by less than 3% after an equivalent of 6.53 space years in a low polar orbit environment.

  • The thermo-optical properties of the black anodisation coating, measured at ESA (8) are presented in the table below:
  Initial (as received) After thermo-cycling After humidity test After thermal-shock
Aluminium 6082 α solar 0.961 0.955 0.952 0.956
Aluminium 6082 ε n-IR 0.90 0.90 0.90 0.90
Aluminium 6082 α solar 0.956 0.951 0.950 0.951
Aluminium 6082 ε n-IR 0.91 0.91 0.90 0.90
Aluminium 6082 α solar 0.951 0.948 0.951 0.951
Aluminium 6082 ε n-IR 0.01 0.90 0.90 0.90

Electroless nickel

Electroless nickel is applied on functional parts, as, for instance, on cap cone brackets of solar panels in order to provide the parts with corrosion and wear resistance. Electroless nickel satisfies the requirements of MIL-C-26074E. The usual thickness is 20 to 30 μm with an excellent distribution over the part.

Electroless nickel is obtained by reduction of nickel ions with phosphorous com-pounds. Therefore atomic phosphorous is incorporated in the nickel layer. Its concen-tration varies between 3 and 13 weight-% depending on the bath formulation. Most applications use a phosphorous content between 8 and 10%. Its hardness is about 600 HV in “as plated” conditions and can be increased up to 1000 HV through appropriated thermal treatment. The hardness increase is due to the precipitation of Ni3P compounds in the nickel matrix.

According to the MIL-C-26074E, a thermal treatment at 120°C during 1 h is required after plating on aluminium alloys. This thermal treatment does not change the struc-ture of the NiP alloy and has no influence on the hardness of the coating but provides evidence of good adhesion. Adhesion failures would cause blisters, detectable by visual inspection.

The main properties of the nickel-phosphorous coating are its wear and corrosion protection. It is also well suited as a diffusion barrier layer. With a phosphorous content above 10.5 weight-%, the deposit is amorphous and non-magnetic if not thermally treated.

Gold plating

The most frequently required function of the gold plating is IR-reflectivity.

Gold plating on aluminium alloys

Bright gold plating on aluminium alloys (6061 and 7075) has been performed for Contraves Space and CSEM for the XMM-RGS project. The requirement was to achieve near mirror quality. The adhesion was tested with a tape lift test and thermal shock testing in liquid nitrogen.

The gold plating on aluminium involves several steps. They include: degreasing, deoxidizing of aluminium, applying an adhesion layer, an undercoat of copper, a diffusion barrier of nickel and a final coating of gold. The total thickness is 15 to 20 μm with about 3 μm of gold. The main challenge is to guarantee good adhesion of the coating while keeping a high brightness.

Peraluman samples (5101.24) have been fine-ground to an N4 surface finish and then coated with electrodeposited copper and nickel underlayers and a top layer of gold. The thermo-optical properties of these samples i.e. the total reflectance, the normal emittance and the solar absorptance have been measured at the ESTEC. The values obtained for Steiger Au were close to the ones obtained with electrodeposited gold on polished nickel (reference mirror) or with vacuum deposited gold on polished glass surface (Table below). It has been demonstrated that the polishing of aluminium substrates is not necessary for obtaining a high mirror quality provided the surface finish is high enough (typically N4).

Thermo-optical properties of gold coated samples

No Sample Normal emittance εn Solar absorptance
1 Reference Mirror of ESTEC (electrodeposited Au on polished nickel surface) 0.021 0.230
2 Au on glas (ESTEC) Vacuum deposited Au on polished glass surface 0.020 0.198
3 Steiger Au-Co; 2–3 μm on Peraluman sample 0.036 0.244
4 Steiger Au-Ag; 2–3 μm on Peraluman sample 0.029 0.235

The picture shows components of the MXT camera for the SVOM mission, the aim of which is the observation and precise study of sudden gamma emissions that can be traced back to intense bursts of high-energy photons during the explosion of massive stars or the fusion of two neutron stars. Gold-plated and black anodized parts are clearly visible in the picture.

Gold plating on titanium

Housing components in titanium were gold-plated for the “Rosina” experiment within the “Rosetta” satellite. 
The “Rosina” equipment was fabricated under the direction  of the University of Bern and the gold plating work was performed by Steiger SA.

The gold plating was applied in order to meet the following requirements:

  • No magnetic material
  • Good corrosion protection
  • High reflectivity of electrons
  • High superficial electrical conductivity
  • High ratio of α/εn values

Multicoatings

Most applications require two coatings on the same part, as, for example, black anodizing and Alodine 1200s or Surtec 650. The purpose of the multicoating is to provide a part with high emissivity and electrical continuity between adjoining parts. This requires sophisticated masking techniques in order to guarantee sharp edges of the selected areas. In some applica-tions even triple coatings have been applied, for instance, in the baffles of the blocking shell done under contract with Apco Technologies for the XMM project. Black anodisation ESA PSS-01-703 was combined with thick gold plating (100 μm) and a chromate conversion coating.

The picture on the right shows a triple coating of black anodi-sation, Alodine 1200s and electroless nickel for a good electri-cal connection. The application of a third treatment increases the level of sophistication very much because of electrochemi-cal restrictions and masking challenges.

3D Additive Finishing – Innnosurf® advanced coatings

The manufacture of parts by the 3D laser additive printing method is rapidly gaining industrial importance. It makes it possible to produce parts of very complex geometry which cannot be obtained in using conventional machining tech-niques. Space applications use these technologies for the manufacture of lightweight aluminium parts on which a func-tional surface must still be provided. Galvanic coatings are well-suited for processing 3D laser printed aluminium having a high silicon content. The desired surface functionalities are optical properties, resistance to corrosion and wear, surface condition and, in particular, the appearance, which must have a reduced roughness compared to the part coming out of 3D laser printing. The surface treatment must enhance and com-plement the advantages of 3D manufacturing, i.e. adapting to complex geometries on both external and internal faces. With this in mind, Steiger-Innosurf® has developed purely chemical coatings with excellent penetration into cavities due to the absence of an electric field during deposition. Typically, when a conductive coating is requested, the Nickel-Copper-Silver coating system is applied. If the coating should be non-conductive, anodic oxidations are appropriate. They can be of the conventional type or achieved with glow discharge techniques.

Microsection of 3D additive coating of 3D laser printed aluminium. The coating system comprises a NiP12 underlayer – Cu layer and a top coat of Ag

Bibliography

1) H. Ketterl; Aluminium 34 (7), p 398–405 (1958)
2)  MIL C-5541
3)  DIN 50939
4) J. Dumas, ESA Memorandum ref. QM E 205/92K/594/J Du, 25/11/92
5) M. Froggatt, ESA Memorandum ref. QM/94/MF, 16/12/94
6) J.-L. Bézy, ESA Memorandum ref. NTO/JLB/4141, 15/6/94
7) ONERA/CERT, Complexe Aerospatial, BP 4025, F-3155 Toulouse, Document N° 4397/00/RF
8) ESA Report: 00/005 D-TOS/QMC