Note: Descriptions are shown in the official language in which they were submitted.
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The invention concerns the forming on a substrate
of a coating which strongly absorbs electromagnetic
radiation over a broad band of wavelengths, from visible
light up to and including the far infra-red (from
wavelength in the order of one micron up to wavelengths
of at least 100 to 200 ~m, preferably up to around
500 ~m).
A coating of this kind is in particular required
to cover the baffles of space telescopes, i.e. generally
cylindrical parts through which incident radiation passes
and the purpose of which is to absorb as much as possible
of the spurious radiation which compromises the quality
of observation (for example the inherent radiation of the
Earth and/or the reflection from the latter of sunlight,
or any other albedo, etc). It should be borne in mind
that, generally speaking, the quality of observation
improves as the wavelength extends farther into the
infra-red, provided of course that sources of infra-red
radiation near the place of observation can be
eliminated.
A coating of this kind can have other applications,
in the field of scientific (:in particular optical)
instrumentation among others, or in the simulation on the
Earth of the conditions applying in deep space.
At present there are only two types of coating
capable of meeting the a~sorption requirements in this
wide a spectral band (beyond 80 to 100 ~m).
One is a rough paint, HERBERTS 1002E (there is an
electrically conductive variant with the reference
1356H), which in principle is applied to a smooth
substrate. This paint is in practise very difficult to
use (including in cases where the substrate is flat or
practically so) as the roughness needed to trap the
radiation is created as layers are built up in accordance
with extremely strict rules which are not always easy to
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follow: in particular, the evaporation of the solvenks
must be strictly controlled between successive layers
(not too much, nor too little), and depends on many
parameters including the conditions (aging, etc) of the
paint; because of the large number of parameters, the
best way to use this paint is under "visual" human
control; the results are somewhat random.
A coating called "MARTIN BLACK" developed by the US
company MARTIN MARIETTA DENVER AEROSPACE is described in
US patent 4 111 762. This coating is black, resembles
velvek and offers excellent performance up to around
80 ~m. It has numerous interesting characteristics,
especially for space applications, such as low
outgassing, excellent resistance to thermal cycling and
lS low parkicle contamination. It has the disadvantage of
being obtained by anodization, however, so that it can be
applied only to aluminum alloy substrate. It is also
difficult to prepare and, most importantly, extremely
fragile: the slightest contact damages it irreperably,
which considerably complicates handling.
This coating has been improved as described in US
patent 4 589 972 and is still under development in this
new version under the name " INFR~ BLACK". This coating
has absorption capacity to around 180 to 200 ~m, but has
the same drawbacks as "MARTIN BLACK", namely its extreme
fragileness and its restriction to aluminum alloy
substrates.
Materials such as stainless steel, invar and
titanium conventionally used in space applications,
especially in optical instruments (where there is o~ten
khe requirement to minimize thermal expansion) cannot be
coated with "MARTIN BLACK" or " INF~A BLACK" and coating
them with the aforementioned HERBERTS paint yields highly
random results.
The invention is directed to alleviating khe
aforementioned disadvantages by proposing a coating
providing absorption which varies little with wavelength
over a wide band of wavelengths from the visible up to a
wavelength beyond 100 ~m, typically beyond 200 ~m and even
up to 500 ~m, suitable for a great variety of substrates
(metals, and also ceramics, even plastics materials, etc)
by a method that is easy to use and reliable (it is
easily automated) and at moderate cost.
This coating will be advantageously adapted to
withstand thermal cycling, low temperatures (infra-red
observations are often conducted near 0 KELVIN), and to
give rise to only limited outgassing.
To this end the invention proposes a body adapted
to absorb visible light and infra-red radiation
comprising a substrate covered with a coatingr
characterized in that this coating comprises a porous
layer formed of pigments adapted to absorb infra-red
radiation in a thermoplastic binder.
According to preferred fea~ures of the invention,
some of which may be combined:
- the porous layer also comprises mineral additives,
- the porous layer also contains carbon black,
- the binder is an equal parts mixture of polyamide and
polyoxymethylene,
- the pigments comprise a mixture of "VAT BLUE 4" and
"VAT BROWN 3" pigments,
- said porous layer is deposited onto a rough surface,
- said porous layer covers a rough sub-layer deposited
onto the substrate,
- said sub-layer has a roughness of at least 100 ~m Ra,
- the substrate is metal and the sub-layer is nickel
aluminide,
- said coating has a roughness RT of at least 200 ~m.
The invention further consists in a me-thod of
manufacturing a body adapted to absorb visible light and
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infra-red radiation. in which method a coating is deposited onto a
substrate, characterized in that it comprises a stage during which a
porous layer of powder comprising a mixture of pigmen-ts adapted to
absorb infra-red radiation and a thermoplastic binder is deposited by
thermal sputtering.
It will be understood that because of its porous character
the surface layer is fundamentally different from a layer of paint.
According to further preferred features of the invention,
some of which may be combined with each other ~
- said thermoplastic sputtering is carried out usinq an oxypropane
flame qun,
- said layer is deposited in a number of passes,
- the layer also contains mineral additives,
- the porous layer also contains carbon black,
- the binder is a mixture of equal parts of polyamide and
polyoxymethylene,
- the pigments are "VAT ~LUE 4" and "~AT ~ROWN 3" pigments,
- the powder has a particle size of less than 100 ~m,
- the porous layer is deposited onto a rough surface.
- this rough surface is formed by depositing by thermal sputtering a
rough sub-layer on the substra~e,
- this sputtering is achieved using an oxyacetylene flame-gun,
- the substrate bein~ metallic, the sub-layer is nickel aluminide.
It will be understood that the method in accordance with the
invention applies to a qreat variety of subsrates, metals or
otherwise, and is easy to use, even on non-plane substrates, provided
that the ~eometry of the body concerned does not prevent access to a
thermal sputlering gun.
Obiects, characteristiGs and advantages of the invention
will emerge from the following description given by way of
non-limiting example with reference to the appended drawings in
which :
- fiqures lA through lD show four successive phases in the
preparation of an absorbent body in accordance with the invention,
shown in cross-section,
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- figure 2 is a diagram showing the equipment
employed, and
- figure 3 is a graph showing, for each of a number
of absorbent bodies in accordance with the invention and
from the prior art, the percentage of radiation reflecte~
as a function of the wavelength of the incident
radiation.
Figures lA through lD shown in cross-section the
preparation of an absorbent body 10 in accordance with
the invention starting from an initially smooth substrate
l made from metal, for example (in practise stainless
steel).
In a first phase the surface of the metal substrate
1 is prepared by fine sand blasting to obtain the surface
state of figure lB (this typically represents a roughness
in the order of 4 to 5 ~m Ra).
In a second phase, which is recommended to achieve
satis~actory absorption at wavelengths beyond 100 ~m, a
rough sub-layer 2 is applied by thermal sputtering of a
material which adheres strongl~r to the substrate in
question (this is preferably a sub-layer of nickel
aluminide which adheres strongly (by way of an exothermal
reaction) to most metals used in space applications
(stainless steel, aluminum alloy, invar - preferably
coated with chemical nickel -)).
This material is deposited with an oxy-acetylene
flame gun with the parameters deliberately set to obtain
a high degree of roughness.
To give a numerical example, a 4.75 mm diameter rod
of nickel aluminide is employed, the thermal sputtering
parameters being:
- oxygen pressure : 4.8 bars,
- acetylene pressure: 1.2 bars,
- SF air nozzle; pressure 4 bars.
Figure 2 is a schematic diagram showing the flame
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gun 11 of any appropriate known type with its powder feed
12 and oxygen/acetylene feed 13. A gun of this kind
genera~es a considerable amount of heat and the substrate
is therefore advantageously cooled on the side opposite
the surface to be coated. To reduce the generation of
heat the flame gun can advantageously be replaced with an
electrical arc gun.
Consideration may also be given to using a
mechanical process to roughen the substrate, provided
that its thickness enables it to withstand such treâtment
without overall deformation (more than 1 to 2 mm).
Chemical attack may also be considered (in the case of
plastics material substrates, for example).
When the sub-layer 2 has been formed (see figure
lC) a porous absorbent layer 3 is deposited in the form
of a powder made up of a fine ground mixture of pigments
which are highly absorbent in the infra-red band (in
practise known organic pigments) and a thermoplastic
binder. Other additives of any appropriate known type
may be added to meet other complementary objectives
(for example electrical conductivity using carbon black,or
mineral additives).
Note that it is standard practise to use such
pigments in a heat-cured binder (paints) but not in a
thermoplastic binder.
A thermoplastic binder is easier to use because,
unlike heat-cured binders, it changes very little with
time. On the other hand, its chemical nature (presence
of polar groups on the polymer) must be carefully chosen
to achieve strong adhesion to the metal. This adhesion
will be favo~ed by the use of a rough substrate (the sub-
layer 2).
As the pigments are usually organic, in theory it
is necessary to keep their temperature below a relatively
low threshold, typically in the order of 20~ C and the
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20~4642
associated binder's softening temperature must not be
significantly higher than this threshold.
In this example the powder comprises 60~ of a
mixture of equal parts of polyamide (PA-ll) and
polyoxymethylene and 40% of a mixture of equal parts of
~V~T BLUE 4 and VAT BROWN 3 pigments. The combination
is ground to obtain a particle size below 100 ~m.
The porous absorbent layer 3 is deposited by
thermal sputtering, under less severe thermal conditions
than those for the sub-layer 2, which conditions are
compatible with the nature of the pigments.
An oxypropane flame gun is used, for example. This
is preferably of a special type (SP2) for plastics
material powders, featuring lateral injection. The
lS sputtering parameters are, for example:
- oxygen pressure: 3 bars,
- propane pressure: 3 bars,
- air pressure: 1.1 bars,
- powder distribution: pressure 3 bars.
This layer is advantageously ~ormed in a number of
passes (for example, four pass~s with a thickness of
50 ~Im~ each pass taking a few Eractions of a second);
this, combined possibly with cooling of the back of the
substrate, makes it possible to keep the temperature of
2S the powder to around 100C.
For ~he combination (sub-layer, if any and layer) a
valley dep~h (roughness coefficient RT) is chosen in
practise which is near (or even greater than) the nominal
observation wavelength (200 ~m in the numerical example
given here) with a Ra-roughness coefficient slightly
lower than this (from 100 to lS0 ~m).
Tests have shown that a 2+3 coating of this kind
offers good resistance to thermal cycling (30 cycles
between 4K and 300K, for example).
Figure 3 provides a comparison of various ma-terials
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from the point of view of specular reflection at T = 7K
as a function of the wavelength between 20 and 450 -
500 ~m.
Curve I (in full line) represents a body coated
with a 400 ~m layer of HERBERTS 1002E paint. Note that
the reflection coefficient R remains below 20% up to
around 400 ~m.
The dotted line curve II represents a sample in
accordance with the invention comprising a layer
deposited directly onto the smooth substrate. The
coefficient R remains below 20% up to around 180 ~m.
The dashed line curve III represents a sample as in
figure lD, and therefore comprising a porous layer on a
rough sub~layer (with the substrate possibly being bare
from place to place, at the bottom of the valleys). Note
that the coefficient R remains below 20~ up to
wavelengths of more than 350 ~Im~ being equivalent to that
of layer I (the difference is less than the accuracy of
the measurement).
It will therefore be understood that the invention
has surprisingly enabled a level of performance
appro~imating that of HERBERTS 1002E paint to be achieved
with an entirely rudimentary and therefore inexpensive
method of implementation.
It goes without saying that the foregoing
description has been given by way of non-limiting example
only and that numerous variations may be put forward by
those skilled in the art with departing from the scope of
the invention.
