Method for manufacturing a part made of CMC

Abstract

A composite material part having a matrix made of ceramic, at least for the most part, is fabricated by a method including making a fiber preform from silicon carbide fibers containing less than 1 at % oxygen; depositing a boron nitride interphase on the fibers of the preform, deposition being performed by chemical vapor infiltration at a deposition rate of less than 0.3 m/h; performing heat treatment to stabilize the boron nitride of the interphase, after the interphase has been deposited, without prior exposure of the interphase to an oxidizing atmosphere and before depositing matrix layer on the interphase, the heat treatment being performed at a temperature higher than 1300 C. and not less than the maximum temperature to be encountered subsequently until the densification of the preform with the matrix has been completed; and thereafter, densifying the perform with a matrix that is made of ceramic, at least for the most part.

Claims

1. A method of fabricating a composite material part having a matrix, the majority of said matrix being made of ceramic, the method comprising: making a fiber preform from silicon carbide fibers containing less than 1 at % oxygen; depositing a boron nitride interphase on the fibers of the preform, deposition being performed by chemical vapor infiltration at a deposition rate of less than 0.3 m/h so as to obtain coated fibers; performing heat treatment of the thus obtained coated fibers, only after the end of the deposition of the boron nitride interphase, without prior exposure of the interphase to an oxidizing atmosphere and before depositing matrix layer on the interphase, the heat treatment allowing to stabilize the boron nitride of the interphase, the heat treatment being performed on the coated fibers which comprise the fibers and the boron nitride interphase and which are deprived of any matrix layer deposited on the boron nitride interphase, the heat treatment being performed at a temperature higher than 1300 C. and not less than a maximum temperature to be encountered subsequently until a densification of the preform with the matrix has been completed; and thereafter, densifying the preform with the matrix.

2. A method according to claim 1, wherein the boron nitride interphase is deposited at a deposition rate of less than 0.1 m/h.

3. A method according to claim 1, wherein the boron nitride interphase is deposited from a reaction gas mixture containing boron chloride BCl.sub.3, ammonia NH.sub.3, and gaseous hydrogen H.sub.2, at a temperature of less than 800 C. and at a pressure of less than 5 kPa.

4. A method according to claim 1, wherein the boron nitride interphase is deposited from a reaction gas mixture containing boron trifluoride BF.sub.3, ammonia NH.sub.3, and gaseous hydrogen H.sub.2, at a temperature lower than 1050 C., and at a pressure less than 20 kPa.

5. A method according to claim 1, wherein the duration of the heat treatment lies in the range 0.25 h to 4 h.

6. A method according to claim 1, wherein the duration of the heat treatment lies in the range 0.5 h to 2 h.

7. A method according to claim 1, wherein the boron nitride interphase is formed with the preform being held in shaping tooling, and after heat treatment, depositing at least one layer of ceramic on the interphase by chemical vapor infiltration in order to consolidate the preform held in the tooling, densification of the preform subsequently being continued after the consolidated preform has been extracted from the tooling.

8. A method according to claim 1, wherein the preform is densified at least in part by chemical vapor infiltration.

9. A method according to claim 1, wherein the preform is densified at least in part by impregnating the preform with at least one ceramic precursor polymer and by pyrolysis.

10. A method according to claim 1, wherein the preform is densified at least in part by introducing carbon and/or ceramic powder into the preform and by infiltrating silicon-based metal in the molten state.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other features and advantages of the invention appear on reading the following description made by way of non-limiting indication with reference to the accompanying drawing, in which:

(2) FIG. 1 is a flow chart specifying successive steps of an implementation of a method in accordance with the invention; and

(3) FIG. 2 is a flow chart specifying successive steps of another implementation of a method in accordance with the invention.

DETAILED DESCRIPTION OF IMPLEMENTATIONS

(4) FIG. 1 shows successive steps of an implementation of a method of the invention for fabricating a CMC part.

(5) In step 10, a coherent fiber structure is made from SiC fiber yarns as defined above. It is possible to use yarns supplied by the Japanese supplier Nippon Carbon under the reference Hi-Nicalon or, preferably, under the reference Hi-Nicalon Type-S having a higher elastic elongation limit. The term coherent structure is used herein to mean a structure that is possibly deformable while being capable of conserving its cohesion without assistance from support tooling or the equivalent. The fiber structure may be made by weaving, e.g. three-dimensional or multi-layer weaving. Other textile processes may be used, e.g. braiding or knitting.

(6) In step 11, the fiber structure is shaped and held in shape by means of shaping tooling, in order to obtain a fiber preform of a shape that is close to the shape of the part that is to be fabricated. Examples of shaping fiber preforms from a coherent fiber structure can be found in particular in US patent application No. 2011/0293828.

(7) In step 12, a BN interphase is formed on the SiC fibers by CVI, the preform being inserted together with its shaping tooling into a CVI oven. In well-known manner, it is possible to use tooling made of graphite that presents holes in order to facilitate the passage of the reaction gas during the CVI process.

(8) The parameters of the CVI process, and in particular the temperature and the pressure inside the oven, and also the composition of the BN precursor reaction gas, are selected so as to have a limited deposition rate in order to encourage the gas to diffuse to the core of the preform and avoid any significant thickness gradient in the BN deposit across the thickness of the preform. The deposition rate is selected to be less than 0.3 m/h, and preferably less than 0.1 m/h.

(9) For given parameters of the CVI process, the deposition rate can easily be determined by experiment, by measuring the thickness of a deposit formed on the surface of a testpiece, e.g. a monolithic block of SiC, as a function of the duration of the deposition stage. It can also be measured by taking the ratio of the thickness of the deposit as measured by examining a polished cross-section under an optical microscope or a scanning electron microscope, divided by the duration of the deposition stage.

(10) When using a reaction gas mixture of the BCl.sub.3NH.sub.3H.sub.2 type, the temperature is preferably selected to be less than 800 C., e.g. to lie in the range 650 C. to 800 C., and the pressure is selected to be less than 5 kPa, e.g. lying in the range 0.2 kPa to 5 kPa. When using a gas mixture of the BF.sub.3NH.sub.3H.sub.2 type, the temperature is selected to be less than 1050 C., e.g. lying in the range 900 C. to 1050 C., and the pressure is selected to be less than 20 kPa, e.g. lying in the range 2 kPa to 20 kPa.

(11) The thickness of the interphase lies on average in the range 0.1 micrometers (m) to 1 m, e.g. in the range 0.1 m to 0.3 m, it being observed that this thickness may vary in particular as a function of location relative to the holes in the shaping tooling. The deposit may be formed by continuous CVI, i.e. with the reaction gas flowing continuously through the oven, or by pulsed CVI, by performing successive cycles, each comprising introducing reaction gas into the oven, maintaining it therein for a predetermined duration, and then discharging the residual gas from the oven.

(12) In step 13, heat treatment is performed to stabilize the interphase BN under an inert atmosphere, e.g. under argon, without prior exposure of the BN interphase to an oxidizing environment and prior to forming a layer of matrix on the BN interphase. The heat treatment is advantageously performed in the CVI oven immediately after the end of depositing the BN interphase. The purpose of the heat treatment is to stabilize the BN chemically by causing volatile species derived from the reaction gas and present in the BN deposit to degas and by eliminating the presence of active sites where oxygen could be come grafted if the interphase were to be exposed to an oxidizing environment while the part made of CMC is in use.

(13) The temperature of the heat treatment is selected to be higher than 1300 C., e.g. lying in the range 1300 C. to 1450 C. This temperature is also selected to be not less than the maximum temperature subsequently encountered up to the end of fabricating the CMC part, in particular the maximum temperature to be encountered while densifying it with the matrix. As a result, any subsequent degassing of residual volatile species from the BN deposit is avoided, as might otherwise result from being exposed to a temperature higher than the heat treatment temperature, where such volatile species could then become trapped in the CMC, in particular at the interphase, or could pollute the matrix while it is being formed, thereby affecting the properties of the CMC.

(14) The duration of the heat treatment, i.e. the time during which the specified temperature is maintained, preferably lies in the range 0.25 h to 4 h, and more preferably in the range 0.5 h to 2 h.

(15) In step 14, after heat treatment, with the preform still held in the shaping tooling in the CVI oven, a layer of ceramic matrix is formed on the BN interphase by CVI in order to consolidate the preform, i.e. in order to bond the fibers of the preform together sufficiently to enable the preform to conserve its shape without assistance from the shaping tooling. This matrix layer may be made of SiC, for example. It should be observed that the formation of a matrix layer for consolidating the preform might be unnecessary if the BN interphase suffices for consolidation purposes.

(16) After consolidation, the consolidated preform may be withdrawn from the shaping tooling (step 15) in order to be densified by a matrix that is at least essentially made of ceramic. The densification may be performed in two steps 16 and 18 that are separated by a step 17 of machining the part to its desired final shape. The following known densification processes may be used: forming a matrix by CVI as a single layer or as a plurality of superposed layers; forming a matrix by a liquid technique known as polymer infiltration and pyrolysis (PIP) with a plurality of cycles, each comprising impregnation with a liquid composition containing at least one ceramic precursor, followed by pyrolysis; and forming a ceramic matrix by impregnating with a slip containing one or more carbon or ceramic powders, e.g. SiC or Si.sub.3N.sub.4, known as a slurry casting (SC), followed by drying and infiltration with fused silicon or with a molten alloy containing a majority of silicon, known as melt infiltration (MI).

(17) The use of CVI to form ceramic matrix layers of SiC, B.sub.4C, or SiBC is described in particular in U.S. Pat. Nos. 5,246,756 and 5,965,266.

(18) The ceramic precursor for PIP densification may be an organo-silicon compound such as a polysilazne, polysiloxane, polycarbosilane, or silicone resin.

(19) A densification process by an MI technique is described in particular in U.S. Pat. Nos. 4,889,686, 4,994,904, and 5,015,540. Under such circumstances, when selecting the temperature for heat treatment, it is necessary in particular to take account of the fact that the MI process with infiltration of molten silicon takes place at a temperature that is generally not less than 1420 C., which temperature may nevertheless be a little lower when using a silicon-based alloy.

(20) The two densification steps may be performed using the same process or by using different processes.

(21) Finally, in step 19, the outer surface of the part or a portion of the outer surface, may be coated in a layer of ceramic paint or of an environmental barrier coating (EBC) having a thermal protection function and/or a function of providing protection against corrosion in an oxidizing and/or wet environment. Reference may be made in particular to the following patent applications: WO 2010/063946, WO 2010/072978, US 2009/0169873, and US 2010/003504.

(22) The method of FIG. 2 differs from that of FIG. 1 merely in that step 12 of forming the BN interphase by CVI is performed using a reaction gas mixture of the BF.sub.3NH.sub.3H.sub.2 type, while the other steps 10, 11, and 13 to 19 are similar to those of the method of FIG. 1. The CVI process is performed at a temperature that is preferably less than 1050 C., e.g. lying in the range 900 C. to 1050 C., and the pressure is preferably selected to be less than 20 kPa, e.g. lying in the range 2 kPa to 20 kPa.