Coating processes – Heat decomposition of applied coating or base material
Patent
1996-05-24
1997-12-23
Group, Karl
Coating processes
Heat decomposition of applied coating or base material
501 964, 501 99, 501127, C04B 355835, B05D 302
Patent
active
057005176
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND RELATED ART
The present invention relates to a process for the densification of a porous structure by boron nitride e.g. making it possible to densify bidirectional multidirectional felts or fabrics, or porous ceramics. The densification consists of filling the gaps of the porous matrix so as to bring about an increase in the density thereof.
The invention also relates to a porous structure densified by boron nitride.
Boron nitride has the following particularly interesting properties: absence of a chemical reaction with carbon or carbon dioxide up to 1800.degree. C., up to 2400.degree. C. under nitrogen),
As a result of these specific properties, boron nitride is conventionally used in the construction of vacuum furnaces used at high temperatures, in the foundry field, in processes for the transformation of very pure metals or alloys in order to bring about a continuous casting of steels. It is also used in the coating of carbon fibres in order to protect them against oxidation or serving as an adaptation interface between the fibres and the ceramic composite matrixes.
Boron nitride is potentially usable for other very important applications in the aeronautical or space fields, e.g. in the manufacture of antenna windows, aircraft brakes, thermal shields or reentry vehicles (into the atmosphere) of a dielectric nature. Thus, boron nitride oxidizes much less rapidly than carbon-carbon matrixes and constitutes a very good electrical insulator. However, for said latter applications, boron nitride in ceramic form is not suitable, due to its excessive brittleness with respect to thermal or mechanical shocks. To obviate this problem, it is necessary to have materials in the form of composites incorporating a matrix reinforced by woven fibres, as is already the case for most non-metallic compounds of the carbon, carbide, boride, nitride or oxide type.
Several procedures have been studied in the prior art for producing composite materials having a boron nitride matrix.
Thus, a first method is known using the gaseous procedure or vapour phase chemical infiltration. This method has been industrially developed for carbon and silicon carbide and consists of chemically reacting by heating gaseous species within a porous network known as a preform. This method suffers from numerous disadvantages. It is difficult to obtain a high, homogeneous densification, because it is necessary to avoid the premature closure of the pores located at the periphery of the part to be densified. In addition, this method is extremely slow and production periods of several months are required for large parts.
Among the vapour deposition methods, the article by J. J. GEBHARDT, "Proceedings of the 4th CVD Conference", 1973, pp 460-472 discloses a process for densifying fibrous structures of silica and boron nitride by vapour thermal decomposition of a boron nitride precursor. This precursor is e.g. trichloroborazene (BClNH).sub.3. According to this document, the porous preform is placed within an enclosure into which the trichloroborazene is introduced in gaseous form, at a temperature of approximately 1100.degree. C. and under a pressure of 4.times.10.sup.2 Pa. The article also states that attempts to infiltrate boron nitride at higher pressures and temperatures rapidly leads to the sealing of the external pores of the preform and limits the penetration of the precursor and boron nitride into the porous structure.
The article by Hugues H. PIERSON, "Boron Nitride Composites by Chemical Vapor Deposition", J. Composite Materials, vol. 9, July 1975, pp 228-240, describes a composite material formed by chemical vapour deposition of boron nitride on a substrate based on boron nitride fibres. The reactive gases used are boron trifluoride (BF.sub.3) and ammonia (NH.sub.3). According to this article, a preform formed by a boron nitride felt is introduced into a CVD enclosure, in the centre of a graphite susceptor. Induction heating coils are placed in the enclosure walls, said enclosure being supplied by the two aforem
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Benazet Jean D.
David Patrick
Narcy Bruno
Commissariat a l''Energie Atomique
Group Karl
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