Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-10-22
2001-02-13
Lorin, Francis J. (Department: 1775)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C264S068000
Reexamination Certificate
active
06187123
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method for producing a ceramic sandwich type honeycomb-shaped structure, especially for large lightened mirrors, as well as the ceramic structure obtained by said method.
2. Description of Background Information
A large number of telescopes have been sent into space and carried by satellites for extremely varied applications. These telescopes have a solid glass mirror so as to withstand the stresses of space, basically mechanical and thermal stresses, while providing the mirror portion with the best possible optical quality so as to obtain extremely high performance.
Glass is well-known for:
its dimensional stability due to its extremely low expansion coefficient,
its capacity to be cast into a given shape,
its polishing capacity,
its mechanical qualities.
Nevertheless, glass does have one drawback which is particularly important for space applications, namely its weight.
Any additional weight requires an additional thrust to be placed into orbit, which leads to significantly increased costs.
Moreover, once in orbit, the weight remains prejudicial as it is necessary to have a power motorization proportional to the weight and it is necessary to combat the focus of inertia, a force which limits accuracy.
In addition, the fragility of glass, especially during mounting but more generally during the various machining phases, is also a serious drawback.
The space industry and connected industries which use the same techniques, especially the aeronautics industry, are seeking a structure which possesses good mechanical characteristics, especially high rigidity, as well as an extremely low coefficient of expansion and being as light as possible.
Moreover, for certain applications, it is essential that the material resist oxidation at extremely high temperatures of about 1600° C. without its mechanical qualities being significantly altered at these temperatures.
Silicon carbide, SiC, is one of several high performance materials having properties compatible with these strict and highly specific specifications since its density is low, d=3.25, its longitudinal elasticity module is high, E=600 Gpa, its coefficient of thermal expansion is low &agr;=4.10
−6
° C.
−1
, and its coefficient of thermal conduction is sufficiently high for the applications envisaged, &lgr;=200 W/m.° K.
Moreover, SiC is insensitive to radiations emitted in a synchrotron, a cyclotron or in high power lasers by virtue of its low atomic number, although it can be used in these applications as a deviation mirror.
Other applications may be possible with this SiC-based ceramic. The method includes applications involving not only extremely lightened mirrors with high dimensional stability, but also thermal screens for aircraft, aircraft fire protective screens, thermic exchangers or catalyst supports.
It is also known that in cryogenic optical systems, the thermic mass effect is reduced by the use of SiC with the result of facilitating cooling of the mirror and maintaining the right temperature.
A large number of techniques have been developed to produce SiC structures, but a large number of difficulties limit the possibilities for lightening the products manufactured. The size and shape are also subjected to production stresses.
WO 88/07688 concerns a lightened mirror made from SiC and a method for producing this mirror.
This particular mirror includes an SiC foam preform made from polyurethane foam cut to the sought-after profile and from which all the heteroatoms are eliminated, except carbon. SiC is then deposited on this preform by any suitable method so as to obtain a monolithic porous block of SiC which is densified by depositing a fine coat of SiC on the surface so as to provide the surface with good adhesion capacity.
The block obtained is machined so as to provide it with surface evenness or the desired curve. The machined faces receive a thin SiC coating by means of chemical vapor deposition. This coating reinforces the mechanical resistance of the block and which may be polished so as to provide the surface with optical qualities.
This block nevertheless exhibits inadequate resistance for certain applications in which the mechanical stresses are high and the dimensions are reduced, with the result that the strength, especially mechanical resistance, of a SiC block with a foam structure is too weak. It is also possible that the penetration of SiC into a thick foam block poses problems.
SUMMARY OF THE INVENTION
The aim of the invention is to provide a method for producing a SiC structure, especially for lightened mirrors including a honeycomb-shaped core, also made of SiC, as well as the structure obtained by this method.
The structure obtained has the shape and dimensions sought-after and possesses extremely high mechanical rigidity, even at high temperatures, and is extremely resistant to oxidation, is lighter and compatible with the applications envisaged, especially airospace applications ones, it can be machined and its faces can receive a coating for being polished to provide optical qualities, and it may also comprise localised inserts so as to receive mounted parts, especially mechanical fixing parts.
To this effect, the invention concerns a method for producing a ceramic SiC structure, especially a lightened mirror, which is characterised by the succession of the following steps:
cutting along the desired profile of a preform of a honeycomb-shaped structure whose cell walls have an organic matrix with a fibrous reinforcement while taking into account any possible shrinkages,
pyrolysis of this honeycomb structure preform so as to retain solely the carbon whilst keeping the initial profile,
silicidation of the carbon of the preform so as to obtain a microporous SiC core,
infiltrating type reinforcement by SiC chemical vapor deposition of the cell walls of the honeycomb structure,
formulation of a felt type sheet, assembling the sheet on a one face of the core, thus covering the cells and densification of this sheet so as to obtain SiC skins on the surface of the core so as to form a SiC structure.
According to the invention, the embodiment of the sheet, the assembling with covering of the alveoles and densification are carried out by the sequence of the following steps:
placing a tufted felt with short ex-raxon carbon fibers on each of the faces of the core,
infiltrating type stiffening and linking of said felt to the cell walls of the cells by means of a SiC chemical vapor deposition,
binding by a SiC-based cells so as to reduce porosity on the surface, and
infiltrating type chemical vapor deposition of a thin coating of SiC with a slow kinetics phase for densifying the surface coating, and a thicker coating of about hundreds of u during a second phase with faster kinetics.
As part of making the skins, each face of the core is plated with a sufficient pressure on the felt sheet so that the cell walls of the cells pierce the felt to a certain depth, the imprinted sheet being partially housed in the cells by covering them.
According to one variant, the felt is directly embodied in the cells.
According to one variant of making of the ceramic structure, the invention involves the following steps:
making of a SiC felt sheet of whiskers coated with an organic binder with a low voluminal percentage of fibers of about between 5 and 20%, a small quantity of binder between 3 and 15% in weight and a small thickness of about one millimeter,
cutting of portions of this felt sheet of whiskers to the profile of the faces of the core with a reinforced honeycomb structure and placing of these cut portions on these faces with a subsequent piercing to imprint the sheet with the core which constitutes a sandwich,
baking of the sandwich produced so as to polymerize the organic binder and fix this sandwich in the desired shape,
pyrolysis of the sandwich so as to eliminate the organic binder,
chemical vapor deposition of SiC with slow kinetics so as to form the skins on the core and a predensification
Boursereau Frédéric
Chenier Jean-Pascal
Rousseau Gerard
Aerospatiale - Societe Nationale Industrielle
Lorin Francis J.
Young & Thompson
LandOfFree
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