Abstract: The present invention relates to: a novel process for the preparation of mesophasic polyborazylene. Said process is of particular value in that it affords a quality product from an appropriate polyborazylene, rapidly (virtually instantaneously), reproducibly and with a good yield. Furthermore, said process is easy to carry out. Said process comprises the preparation of polyborazylene by the polycondensation of borazine in a closed reactor and the addition, to said polyborazylene obtained by polycondensation, of a solvent selected from aromatic solvents, borazine solvents and mixtures thereof, mesophasic polyborazylene which is novel in that it is in the presence of a particular solvent and/or by virtue of its quality, the use of said polyborazylene of the invention, and/or prepared according to the invention, as a boron nitride precursor.

Abstract: The matrix of the composite material is made in sequenced manner by chemical vapor infiltration within fiber reinforcement. Each elementary sequence of the matrix comprises a relatively flexible layer and a relatively rigid ceramic layer, and the sequences are of thicknesses that increase going away from the reinforcing fibers, with the thickness at least of the first-deposited sequence being selected to be sufficiently small for the fibers to be coated in essentially individual manner. The flexible layers are made of an anisotropic material having sufficient capacity for elastic deformation in shear and transversely, and although a mainly ceramic character is conserved for the matrix, the thickness of the flexible layers is nevertheless selected to be sufficiently large to be capable of absorbing differential expansion of the components of the composite material while it is being made without inducing the existence of an initial network of cracks.

Abstract: The present invention relates to synthetic phyllosilicates having a lamellar structure of the type T--O--T devoid of OH.sup.- and F.sup.- ions. Said phyllosilicates are very resistant to oxidation and possess a high thermal stability. The invention further relates to a process for the preparation of said phyllosilicates.

Abstract: A layer (16) forming interphase in a material having a lamellar structure is situated between the fibers of the texture and the matrix, and at least one layer (14) forming control interphase is situated between the surface of the fibers (10) and the lamellar interphase (16) in such a way as to define, for those cracks which have spread through the matrix (18) and the lamellar interphase (16), a preferential propagation zone away from direct contact with the actual fibers (10).

Abstract: A method of manufacturing a composite material formed of a fibrous reinforcement and a silica based ceramic or vitro-ceramic matrix. The method includes impregnating the fibrous reinforcement by means of a sol-gel process wherein premature crystallization of the matrix obtained from the sol is hindered by addition to the sol of a crystallization retarding agent, and hot pressing the resulting material. The composite material formed has a residual porosity without the addition of nucleating agents of about less than 10%.

Abstract: A porous structure made of refractory fibers or densified by carrying out a sequential deposition of a matrix, the deposition comprising at least two chemical vapor infiltration phases of an essentially ceramic material separated by a chemical vapor infiltration of a less rigid material chosen among pyrolytic carbon and boron nitride.

Abstract: The material is formed by a fibrous reinforcement constituted by a texture of carbon or silicon carbide fibers and a matrix deposited by chemical vapor infiltration through the porosity of the fibrous fabric and containing free carbon co-deposited with the zirconia, the weight proportion of free carbon co-deposited with the zirconia in the matrix being preferably between 0.1 and 10%. In comparison with pure or partly stabilized fritted zirconias, the composite zirconia according to the invention exhibits an improved tenacity, preservation of the mechanical strength and dimensional stability under high temperatures, as well as an improved density, without the introduction of the fibrous reinforcement reducing too excessively the properties of heat insulation and resistance to oxidation.

Abstract: The present invention concerns processes for making composite materials.The process is characterized essentially by the fact that it consists in making a fibrous preform 1, in pre-treating this fibrous preform by a fluorine containing flux 3 and in wetting the said pre-treated preform by a light alloy.Utilization for the making of composite pieces with a fibrous armature such as carbon wet by a light aluminum-based alloy.

Abstract: A refractory fibrous preform in ceramic material is placed inside an infiltration chamber where it is heated within a reacting gaseous mixture introduced into the chamber and containing at least one gaseous hydrolyzing or oxidizing agent and a volatile hydrolyzable or oxidizable compound of the one element or of each element of which the oxide is a constituent of the matrix of the structure to be produced, in order to obtain by chemical reaction an adhesive deposit of said oxide or oxide constituting said matrix and having a melting point higher than 1750.degree. C.