FR2649393A1 - BORON NITRIDE COATING, PROCESS FOR PRODUCING SUCH A COATING ON FIBERS AND COMPOSITE MATERIAL COMPRISING FIBERS SO COATED - Google Patents

Abstract

The present invention relates to a boron nitride coating, said coating having a thickness of at least 0.40 microns, surrounding fibers to be coated within a matrix. This coating is characterized in that it is sufficient to inhibit both the physical bond and the chemical interaction between the matrix and the fibers, the tensile strength being increased by deflection and blunting of the cracks, causing protrusion. fibers, when stress is applied to the matrix / fiber system.

Description

The present invention relates to the coating of fibers and especially the

  fiber coating by means of a thick layer of boron nitride and the fibers

resulting coatings.

  The reinforcement, by means of fibers, of glass, glass-ceramic, ceramic, metal, thermosetting and thermoplastic materials, can increase their resistance,

  thus expanding the range of their possible applications.

  However, high bonding strengths between fibers and matrix, especially in the case of ceramic composite materials, can cause a brittle fracture which results in substantially planar failure surfaces having substantially no protruding fibers and causing decrease in breaking strength. Projecting fibers increase the distance traversed by the crack initiation cracks, increasing their surface

  specific and the strength of the composite.

  Applying a coating, for example carbon, to the fibers, prior to matrix formation, can reduce the bond strength between fibers and matrix and the chemical interaction, improving the breaking strength. Carbon coatings (see, e.g., U.S. Patent Nos. 4,425,407 and 4,731,298 cited as reference) which are readily oxidized and are good electrical conductors, have seen their limited use in

certain conditions.

  Boron nitride, having high electrical resistance, excellent thermal shock resistance and non-combustibility, is similarly used as a fiber coating (see US Patent No. 4,642,271 cited for reference ). Because of its hexagonal planar structure similar to graphite, boron nitride is similar to carbon without any of its limitations, and it is an interesting substitute. Boron nitride coatings can be applied using DCV (Chemical Vapor Deposition) in temperature ranges from about 850 ° C to about 2200 ° C. However, stoichiometric boron nitride coatings have only been obtained on at deposition temperatures above 17000C, temperatures which usually cause significant fiber degradation When the fibers are heated to such temperatures, cooled, and tested at room temperature, they do not retain their original strength. more boron nitride coatings, such as those made in the prior art, are thin coatings, generally less than 0.35 micron, and thick coatings are only obtained at temperatures above 1400 ° C (patent US 4,481,257 cited for reference). As a result, research is continuing to improve

  fiber coatings and methods of application.

  The present invention relates to the coating of fibers with boron nitride in order to increase the tensile strength and to decrease the chemical reactions between the fibers and the matrix. The fibers are placed inside a DCV reactor and are heated. The reactive gases, containing both boron and nitrogen, are introduced into the reactor by means of a carrier gas. The gases react to produce boron nitride, deposition occurs, and a thick (ie, greater than 0.40 micron) coating of boron nitride is formed around the fibers; this thickness being necessary to obtain the desired resistance and to reduce the chemical reaction between the fibers and the matrix. One embodiment of the present invention will be described hereinafter by way of nonlimiting example, with reference to the appended drawing, in which: FIG. 1 represents a graph of the variation of the bending strength, as a function of temperature, coated and uncoated fibers inside a

composite material.

  FIGS. 2A, 2B and 2C show a comparison of the fracture surfaces for different thicknesses of the

boron nitride coating.

  FIG. 3 represents a DCV reactor assembly

  used in the application of boron nitride coating.

  Brittle fracture is a significant problem in ceramic composite materials made of uncoated fibers, or very finely coated, embedded in a ceramic matrix. The fragility of composite materials causes cracks, resulting from cracks due to stresses, which propagate in a straight line through the composite material. However, if the fibers inside the matrix are sufficiently coated with boron nitride, the boron nitride coating increases the breaking strength, by deflection and blunting of the cracks, preventing breakage of brittleness. The application of a stress to the composite material comprising boron nitride-coated fibers causes the fibers to protrude away from a smooth fracture surface. However, all boron nitride coatings do not result in a protrusion of the fibers. Indeed, if the thickness of the boron nitride coating is around or below 0.30 micrometer, a smooth fracture surface results, similar to that of composite materials containing

uncoated fibers.

  Figure 1 shows the variation in flexural strength of fiber composite materials known as "NEXTEL", coated and uncoated, embedded in a glass matrix. The composite material comprising boron nitride-coated fibers according to the invention (curve 1) has a significantly greater flexural strength than that of the composite material comprising fibers.

uncoated (curve 5).

  FIGS. 2A, 2B, and 2C show a comparison of fracture surfaces for a boron nitride coating with a thickness of 0.08 micrometer (FIG. 2A) and 0.16 micrometer (FIG. 2B) (as claimed in FIG. U.S. Patent No. 4,642,271). Both surfaces show relatively smooth fracture surfaces. As for FIG. 2C, it shows a sensitive projection of the fibers provided with a boron nitride coating with a thickness of 1.14 microns. Different fibers; such as silicon nitride, mullite and alumina fibers can be used in this process, "Nextel 480" fibers produced by SUMITOMO Company 3M, Co., MN and A1203 being preferred. Matrices such as glass, glass-ceramic, ceramic matrices (including CVI silicon carbide and silicon nitride), thermoplastic materials, thermosetting materials and metal can be used to enclose the fibers.

previously mentioned.

  In addition, different reactive gases containing beer and nitrogen may also be used; BC13 (boron trichloride) and NH3 (ammonia) have demonstrated their capacity. These reactive gases are introduced into the reactor

by a vector gas.

  Hydrogen (H2), often used as a carrier gas, has demonstrated its inability for this process. Hydrogen causes degradation and weakening of the fibers. An inert gas, such as argon (Ar), usually degrades the fibers less than the usual H2 gas. As a result, an inert gas, argon in particular, has been used as a gas

vector for this invention.

  The following table shows the results of coated "Nextel 480" fibers at the specified temperature

  for 3 minutes with H2 or Ar as carrier gas.

at____________

  Vector gas: Temp.OC: RT tensile strength (MPa)

H2: 1050: 1068

H2: 1080: 841

  H2: 1100: too weak to be tested Ar: 1050: 2604 Ar: 1060: 1729 Ar: 1100: 889 Ar: 1150: 675 Ar: 1200: 579 The present invention will be clarified by reference

to the examples below.

Example 1

  5. The following procedure was used (FIG. 3) to form a boron nitride coating on "Nextel 480" fibers. 1. A graphite mandrel 10, 5.8 cm in diameter (0.3 mm diameter wall thickness) and 10.2 cm long, which has "Nextel" fibers 11 disposed therein, is loaded into a DCV reactor 12 7.6 cm in diameter and 40.6 cm in length (see FIG.

Figure 3).

  2. The reactor is pressurized to 27 Pa, by a vacuum generator 13 and heated to

10500C.

  3. The reactant and carrier gases are allowed to flow into the reactor 12, passing through three-way valves 14, flow meters 15 and needle valves 16: BC13 at 67 cc / min, NH3 at 67 cc / min and Ar at approximately 208 cm3 / min, for 3 minutes, resulting in a coating of approximately 1.3 boron nitride

micrometer thickness.

Example 2

  The parameters given in Example 1 are followed, and by changing the fiber "Nextel 480" for a fiber "Sumitomo", it is possible to obtain a coating of approximately

0.80 micrometer thick.

  The thickness of the coating, given in Examples 1 and 2, can be increased by modifying the concentration of BC13 and NE3 (reduction of the amount of carrier gas Ar)

  or by increasing the reaction time.

Claims (7)

  1.- coating with boron nitride, said coating having a thickness of at least 0.40 micrometer, surrounding fibers intended to be coated inside a matrix, characterized in that this coating is sufficient to inhibit both the physical bonding and the chemical interaction between the matrix and the fibers, the breaking strength being increased by deflection and blunting of the cracks, which causes a protrusion of the fibers, when a   stress is applied to the matrix / fiber system.   2. A process for producing a boron nitride coating on fibers intended to be coated inside a ceramic matrix, using a carrier gas, this coating being produced by reactive gases, characterized in that it comprises the steps of using the reactive gases, said boron-containing gases and nitrogen, to use an inert gas as carrier gas, and to apply the coating at a temperature of between about 1000 C and 13000C , the temperature being determined by a balance between the desired purity of the coating in   boron nitride and the possible degradation of the fiber.   3. A process according to claim 2 characterized in   argon is used as the carrier gas.   4. A process according to claim 2 characterized in that boron trichloride (BC13) and   ammonia (NH3) as reagent gases.   5. A fiber selected from a group comprising mullite-based fibers, silicon nitride and alumina, said fiber comprising, on its surface, a coating of boron nitride at least 0.40 micrometer thick. approximately, said coated fiber being embedded in a matrix selected from the group consisting of glass, glass-ceramic, ceramic materials (including CVI silicon carbide, and silicon nitride), thermosetting materials, thermoplastic materials, and metallic materials.   6. Composite material comprising fibers coated with boron nitride, said fibers being arranged in a network, and a matrix, said composite   showing, during a break, a protrusion of fibers.   7. Composite material according to claim 6, characterized in that the thickness of the nitride coating of   * boron is at least 0.40 micrometer.