2.1

– The samples of coating

In order to compare the different coatings, we carried out an optical test and an adhesion test on the samples.

The optical measurement consisted of the measurement of the total integrated scatter (TIS) on the SODERN TIS bench. The sample was illuminated by a laser beam (the bench is equipped with two He Ne lasers, 632.8nm and 543.5nm). The hemispherical scatter light was integrated by an integrating sphere. The signal was collected with a silicon cell. Figure 1 presents the photo of TIS bench.

Fig. 1:

Photo of the TIS bench

As the black coating must be used for the sharp edge of vanes and should not create particles, its adhesion is an important parameter. We performed the adhesion tests according to the ESA standard ESA-PSS-01-713 with the 3M853 pressure-sensitive tape. The configuration was a 4cm² loading face, an applied load of 5kg for 1 minute and a pulling apart tape and specimen at 0.2cm/min until separation. We thus classified in three groups the different coatings in function of their results : good adhesion (no particle visible on the scotchtape), adhesion acceptable (some particles visible on the scotchtape and adhesion acceptable) and adhesion non acceptable.

2.2

– The samples of vane edge

We compared the various technologies for machining edges with cross-sectional examinations (with the help of a microscope or a scanning electron microscope). To achieve that, we observed the edge and its morphology and measured the thickness.

2.3

– The coated vanes

After the results of the previous studies, we have selected the most promising technologies for machining edges and the most performance black coatings. We made complete coated vanes with the combination of various technologies and we characterised them, according to the test plan described on Figure 2.

Fig. 2:

Test plan for the coated vane characterisation

We performed optical tests to measure the scattering of the vanes. Then, we evaluated the behaviour of the vanes in the spatial environment. To achieve, we put them in a thermal environment for 64 cycles from +150°C to -50°C, and in thermal vacuum for one cycle from +80°C to –100°C. We observed the vanes and especially the edge and we performed new optical tests, before and after the environment cycling. We compared the scattering results and we take note of no change. These tests also allowed to control the adhesion of the coating on the sharp vane edges. And at the end, we cut all the vanes for a cross-sectional examination.

The optical test consisted of the measurement of the vane ESDF (Edge Scattering Distribution Function). This parameter characterises the vane by its scattering when it is illuminated and corresponds to the scattered irradiance divided by the incident irradiance. This test was carried out on the SODERN straylight bench. Figure 3 presents a schematic view of the ESDF set-up and Figure 4 shows a photo of the straylight test bench used for the ESDF measurements. For the ESDF test, we completely illuminated the vane edge. The incident beam, coming from a filament lamp (400 W), was collimated and reached the vane with a normal incidence. The detector, a CCD camera, was placed at a distance of 800 mm of the sample and moved around the sample. Different protective screens and light-traps were well-placed with the aim of reducing unwanted straylight.

Fig. 3:

Schematic view of ESDF measurements

Fig. 4:

Photo of the straylight test bench

3.1

– Black diffusing coatings

We have characterised the different black diffusing coatings in term of scattering, adhesion and thickness. The results of the most interesting coatings are presented in the following table :

We took the standard sulfuric anodic oxidation as a reference. We have tested different variants of sulfuric anodic oxidation which, on the one hand, allow better scattering performances and, on the other hand, increase the thickness of the coating. The adhesion performances are not altered. The black copper oxide film obtained by chemical deposition, has a weak scattering (between 0.8% and 2.1% in function of the substrate), and a weak thickness (between 5µm and 11µm). The film is less adherent than the sulfuric anodic oxidation. The adhesion is improved by the means of another deposition technology : the cathodic arc plasma deposition, which gives an high adherent and thin (5µm) film but with an increase of the scattering (TIS = 3.5%). The black nickel phosphorous layer has the same performances as the black copper oxide film deposited by arc plasma. The chemical black chromium oxide film has almost the same properties than the chemical black copper oxide film except for the scattering which is lightly less performance (TIS = 2 – 2.3%).

3.2

– Edge machining

In parallel of the coating study, we tested various technologies among the classical and controlled technologies of machining vane edges. The aim was to achieve the thinnest vane edge that could be coated by a scattering black coating with an acceptable level of adhesion.

After a visual selection, we cut the vanes for a cross-sectional examination. We measured the thickness of the edge with the help of microscope and electronic microscope. The results are reported in the following table :

The advanced grinding and the optical polishing create thinner edges than those obtained by standard grinding. These two technologies may be used for different substrates. The ultra-precise machining using a monocrystalline diamond, is not easy to implement, so we prefer the two former technologies.

Concerning the use of metal foil, the edge depends on the thickness of the foil and on the machining technology. We have tested different photochemical etching and photochemical electroforming techniques on nickel foil (50µm and 30µm thickness). All of them create edges with a thickness inferior to the thickness of the foil. The morphology of the edges vary according to the techniques.

3.3

– Coated vanes

Among the different technologies for edge machining and among the different coatings, we selected the most promising couples. Visual inspections and scattering measurements, before and after thermal cycling and vacuum, revealed no changes. All the coatings have a sufficient adhesion on the edge of the vanes.

Figures 5 and 6 presents the results of the most performant vanes, for illumination angles with a 0° and 30° incidence. The central peaks, observed in Figures 5 and 6, correspond to the incident beam passing through the vanes. We took as a reference the vane in aluminium alloy with an edge obtained by standard grinding and which is coated with the standard sulfuric anodic oxidation.

Fig. 5:

Scattering results for 0° incident illumination angle

Fig. 6:

Scattering results for 30° incident illumination angle

We note that the scattering performances of the standard vane could be improved by a significant factor, by using other technologies for edge machining and other coatings. Three vanes give the best scattering performances :

The first vane is slightly better for a 0° illumination angle. The followings, in terms of optical performance, are the vane in 6061 aluminium alloy with a polished edge and coated with a variant of the sulfuric anodic oxidation and the vane in stainless steel with an edge, manufactured by advanced grinding and coated with a black nickel phosphorous film.