Patent Publication Number: US-2015079297-A1

Title: Method of coating a fiber with pre-coating

Description:
The present invention lies in the field of fabricating parts out of metal matrix composite material. The invention relates to a method of depositing a coating of a first metal alloy on a fiber extending in a main direction D, the method comprising the following steps: 
     a) providing a first mass of a first metal alloy and heating the first mass to above its melting temperature so that this alloy is in the liquid state and occupies a space E; and 
     b) causing the fiber to move in translation from upstream to downstream through the liquid first mass along the direction in which the fiber extends at a first speed V1 such that the fiber becomes covered over at least a portion of its length by a coating of the first alloy, which coating presents a non-zero thickness over the entire periphery of the fiber in a plane perpendicular to the main direction D. 
     In certain applications, in particular in aviation for turbine engine parts, parts made of metal matrix composite material reinforced by fibers, e.g. ceramic fibers, present very considerable potential. 
     Such composites present performance in terms of stiffness and mechanical strength that is high, with the fiber reinforcement enabling weight to be saved compared with a part of equivalent performance but made of the same metal alloy without fiber reinforcement. 
     Such a composite is fabricated from a semi-finished product constituted by fiber reinforcement coated in a metal coating forming a sheath around the fiber. The alloy of the metal coating is the same as the alloy of the matrix in which the fibers sheathed in this way are to be embedded during the subsequent manufacturing step. 
     In order to coat the fiber in the metal alloy, it is possible for example to deposit the alloy by chemical vapor deposition in an electric field, by thermal evaporation, or by electrophoresis from a metal powder. 
     In the description below, the terms “upstream” and “downstream” are defined relative to the direction in which the fiber moves in translation. 
     Patent EP 0 931 846 describes a method of depositing alloy on a fiber by a liquid technique (referred to as “coating” the fiber). That device is described with reference to  FIG. 3 . 
     A mass  120  of the alloy is heated until it becomes liquid, and then a fiber  110  is moved in translation along its main direction (central axis of the fiber) through the liquid mass  120 . The fiber  110  extends between an upstream pulley  141  and a downstream pulley  142  that is situated on either side of the mass  120 , with the fiber being suitable for traveling relative to the pullies. In order to avoid leaving the fiber  110  in contact with the molten metal alloy  120  for too long with the risk of damaging it, the fiber  110  is initially held away from the alloy mass  120  while the mass  120  is being heated by using a pulley  148  that is situated on the portion of the fiber  110  that extends between the upstream pulley  141  and a downstream pulley  142 . The fiber  110  thus does not touch the alloy mass  120 . Once the mass  120  is liquid, the fiber  110  is caused to travel between the two pulleys from the upstream pulley  141  towards the downstream pulley  142 , and the fiber  110  is moved progressively towards the alloy mass  120  by moving the pulley  148  in translation until the fiber  110  comes into contact with the mass  120 , as shown in  FIG. 3  (the double-headed horizontal arrow shows the movement in translation of the pulley  148 , which pulley no longer touches the fiber  110  at the end of its movement). The portion of the fiber  110  that has passed through the liquid mass  120  then becomes covered by an alloy coating  125  of given thickness. 
     In that technology, the liquid mass  120  is kept levitated in a crucible  130  in which it is heated by a heater  135 , thereby presenting the advantage that the alloy mass  120  is not contaminated by the material constituting the crucible  130 . 
     That method nevertheless presents drawbacks. In order to obtain an alloy coating  125  on the fiber within a certain range of thicknesses (e.g. thicknesses of about 50 micrometers (μm)), it is necessary for the fiber  110  to pass through the liquid mass  120  of alloy at a high speed. Unfortunately, when the speed of the fiber  110  through the liquid mass  120  of alloy is too fast (more than several meters per second), the time of contact between the fiber  110  and the alloy is too short for the fiber to be completely wetted by the liquid alloy, thereby having the consequence of preventing the fiber  110  from penetrating into the alloy mass  120 , such that the fiber  110  remains at the periphery of the alloy mass  120 . Thus, by that method, at most approximately three-fourths of the periphery of the fiber  10  becomes coated (three-fourths in a cross-section plane perpendicular to the rectilinear fiber). 
     In order to improve the wetting of the fiber  110  at high speeds, one solution consists in depositing a compound that is wettable by the metal of the alloy on the fiber  110  by means of reactive chemical vapor deposition (RCVD). That method is described in patent FR 2 891 541. 
     It is then possible to cause the fiber  110  to pass through (the middle) of the alloy mass  120 , as shown in  FIG. 3 , and to obtain a deposit of alloy on the fiber  110 . 
     Nevertheless, that method presents drawbacks. Specifically, sporadic alloy-expulsion phenomena occur at the exit from the mass  120 , thereby leading to droplets of alloy becoming formed on the fiber  110  at more or less regular intervals. 
     This situation is shown in  FIG. 4 , which shows a fiber  110  in longitudinal section on exiting the alloy mass  120 , together with droplets  128 . 
     Such droplets  128  are undesirable, in particular because they prevent fibers  110  being distributed uniformly within the composite material once they are embedded in the matrix. Furthermore, they lead to the fiber breaking when they reach the downstream pulley  142 . 
     The present invention seeks to remedy those drawbacks. 
     The invention seeks to provide a method enabling the formation of these droplets to be prevented while continuing to ensure that the fiber passes through the alloy mass, even at high speeds. 
     This object is achieved by the fact that, prior to step a), the following steps are performed: 
     i) providing a second mass of a second metal alloy having a melting temperature T F2  that is strictly higher than the melting temperature T F1  of the first alloy; 
     j) heating the second mass to above its melting temperature so that the second alloy is in the liquid state and occupies a space E2, and then moving the fiber in translation from upstream to downstream through the second alloy, this translation taking place at a second speed V2 which is such that the condition under which the second alloy is taken up during this translation lies under visco-capillary conditions, such that the fiber becomes covered, over this portion of its length, by a coating of the second alloy, which coating presents a non-zero thickness over the entire periphery of the fiber; and 
     k) cooling the coating of the second alloy until it becomes solid. 
     By means of these provisions, since the fiber is coated in the second alloy, it is well wetted by the first alloy on passing through the first alloy, and the coating of first alloy on the fiber is of thickness that is uniform along the entire fiber, without droplets being formed. It is thus possible to coat a fiber with the first alloy even at high speeds (faster than 1 meter per second (m/s)), with a desired coating thickness, and with good adhesion of the coating, and good soundness for the fiber as coated in this way. 
     Advantageously, the second alloy does not form embrittling phases with the first alloy. 
     Thus, the second alloy and the first alloy present between them adhesion that is strong and tough, and the resulting composite tends to be stronger. 
    
    
     
       The invention can be well understood and its advantages appear better on reading the following detailed description of an implementation shown by way of non-limiting example. The description refers to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic view of a device using the method of the invention for covering a fiber by a liquid alloy; 
         FIG. 2  is a section on line II-II of  FIG. 1  showing a fiber coated in alloy by using the method of the invention; 
         FIG. 3 , described above, is a diagrammatic view of a device using the prior art method for covering a fiber by a liquid alloy; and 
         FIG. 4 , described above, is a longitudinal section of a fiber coated in an alloy by using the prior art method. 
     
    
    
     There follows a description of the method of the invention for coating a fiber  10 . 
     By way of example, the fibers  10  are made of ceramic. 
     In particular, the fibers  10  are made of silicon carbide (SiC) surrounding a core of tungsten or of carbon. 
     In general, each fiber  10  presents a pyrolitic carbon layer having a thickness of a few micrometers. This layer is advantageous since firstly it protects the SiC fiber chemically by acting as a diffusion barrier between the SiC fiber and the metal material external to the fiber, which material is often highly reactive, and secondly it protects the SiC fiber mechanically against the propagation of microdefects by limiting the effects of a nick and making it possible to avoid possible cracking, mainly as a result of the stratified configuration of the fine layer of pyrolitic carbon (see description below). 
     The term “coating” is used to mean depositing an alloy on a substrate as a result of moving the substrate (here a fiber) in contact with the alloy while the alloy is in liquid form, the alloy being solid at ambient temperature. The term “alloy” is used to include a pure metal, i.e. a metal that (ignoring trace elements) is constituted by a single element from the periodic table of the element (Mendeleev&#39;s table). 
     A certain quantity (a first mass) of a first alloy is provided, and this first mass  20  of this first alloy is heated until it is liquid (step a)). 
     This heating is performed by placing a quantity of this first alloy in a container, e.g. a crucible  30 , and heating it by means of a heater  35  until the temperature throughout the first alloy is higher than its melting temperature T F1 . In known manner, the liquid first mass  20  of this first alloy is kept levitated in the crucible  30 , thus presenting the advantage that the first mass  20  does not touch the crucible  30  and is therefore not contaminated by the material from which the crucible  30  is made. 
     By way of example, the heater  35  is an inductor arranged around the crucible  30 , the inductor also keeping the first mass  30  of this first alloy in levitation. 
     Once liquid, this first mass  20  occupies a space E1, i.e. the first mass  20  completely fills this space E1, but does not extend beyond it. 
     If the first alloy is not a pure metal, then the melting temperature T F  is the liquidus temperature for the particular composition of the alloy. 
     By way of example, the first metal alloy is a titanium alloy. 
     For example, this first alloy may be Ti-6242 having the following composition by weight: 
       6%Al+2%Sn+4%Zn+2%Mo 
     the balance being Ti. 
     A fiber  10  is placed in such a manner as to extend between an upstream pulley  41  and a downstream pulley  42 , between which it is suitable for traveling from the upstream pulley  41  towards the downstream pulley  42  in a direction given by arrow F in  FIG. 1 . 
     The fiber  10  thus moves in translation along the main direction D in which it extends, in such a manner that between a first instant t1 and a subsequent instant t2 an arbitrary first section S1 of the fiber  10  (other than its downstream end) moves so as to occupy at the subsequent instant t2 the position that was occupied at the first instant t1 by a second section S2 of the fiber  10  situated downstream from the first section S1. 
     Between two pulleys, the fiber  10  is tensioned and therefore extends along a main direction D that is the same for each cross-section of the fiber  10 . For other portions of the fiber  10 , the fiber  10  need not necessarily be rectilinear and its main direction D may vary along the fiber  10 , e.g. the fiber  10  (and its main direction) follows a circular arc around a pulley. The upstream pulley  41  is situated upstream from the mass  20  and the downstream pulley  42  is situated downstream from the first mass  20 . 
     The upstream pulley  41  and the downstream pulley  42  form part of a drive mechanism  40  for driving the fiber  10 , the fiber  10  being driven for example by a motor (not shown) included in the drive mechanism  40 . 
     The upstream pulley  41  and the downstream pulley  42  are positioned in such a manner that when the fiber extends in rectilinear manner from one pulley to the other (i.e. when it extends along a straight line interconnecting these two pulleys), the fiber  10  passes through (the middle) of the first mass  20  of the first alloy, and thus through the space E1 (step b)). 
     The drive mechanism  40  may include a guide mechanism other than pulleys for guiding the fiber  10 , providing the fiber  10  passes through the first mass  20  as described above. 
     Advantageously, the main direction D of the fiber  10  is constant (the fiber  10  is rectilinear) between a point upstream from the space E1 and a point downstream from the space E1. The fiber thus tends to conserve a rectilinear shape once it has been coated. 
     In order to coat the first alloy on a portion of the length of the fiber  10  (e.g. the majority thereof), this portion is caused to pass through the first mass  20  and the space E1 as described above. A coating  25  of first alloy is then deposited on the fiber  10 . 
     The fiber  10  passes through the first mass 20 of alloy at a first speed of translation V1. In the method of the invention, this first speed V1 is high, e.g. faster than 2 m/s. 
     In the invention, before coating the fiber  10  as described above, the fiber  10  is subjected to pre-coating (steps i), j), and k)). 
     This pre-coating is performed in a manner similar to the above-described coating, but nevertheless with differences. 
     Firstly, the pre-coating takes place through a second liquid mass  220  of a second alloy that is different from the first alloy of the first mass  20 . The second alloy thus differs in composition from the first alloy, i.e. it is not made up of the same chemical elements, or it is made up of the same chemical elements but in different proportions. 
     Furthermore, this pre-coating takes place at a speed of translation V2 (second speed V2) that is such that the condition under which the second alloy is taken up during this translation lies under visco-capillary conditions of taking-up of the alloy by the fiber  10 . Such visco-capillary conditions correspond to the situation in which the thickness of the alloy that is taken up by a fiber (i.e. that becomes deposited on and that remains on the fiber—this being then called the taking-up of the alloy) is proportional to the two-thirds power of the speed V (i.e. proportional to V 2/3 ). The thickness of the alloy that is taken up is small, being of the order of a few micrometers (μm). 
     Advantageously, the coating speed V1 is strictly faster than the pre-coating speed V2, i.e. the pre-coating speed V2 is strictly slower than the coating speed V1. It is thus possible to deposit a coating of first alloy of desired thickness on the fiber  10 , e.g. 
     thickness of the order of 50 μm, and to do so without droplets forming along the fiber  10 . 
     For example, the speed V2 is equal to 1 m/s or slower. 
     In certain configurations, it is desired for the volume fraction of fibers  10  in the final composite material (i.e. after the fibers  10  have become embedded in the metal matrix) to be as high as possible, in order to obtain superior mechanical performance. For this purpose, the total thickness of the coating deposited on the fiber  10  during the pre-coating and during the coating should be as small as possible. To obtain a thickness of first alloy  25  (as deposited during coating) that is as slow as possible, the first speed V1 should be as small as possible. The speed V1 is then under certain circumstances slower than the second speed V2, and lies within visco-capillary conditions. 
       FIG. 1  is a diagram showing the fiber  10  being subjected to this method of being pre-coated with the second alloy, the second mass  220  of the second alloy being situated in a crucible  230  heated by a heater  235  to a temperature higher than its melting temperature T F2 . 
     The fiber  10  is tensioned between a third pulley  243  situated upstream from the second mass  220  and a fourth pulley  244  situated downstream from the second mass  220 . The fiber is moved in translation from the upstream pulley  243  to the downstream pulley  244  and it passes through the second mass  220  of the second alloy, which mass occupies a space E2. The fiber extends along a main direction D2. 
     While the second mass  220  of alloy is being heated, the portion of the fiber  10  between the third pulley  243  and the fourth pulley  244  is held away from the mass  20  of alloy by an intermediate pulley (not shown), after which it is moved towards the second mass  220  of alloy (in a method similar to that for the pulley  148  described with reference to  FIG. 3 ). 
     Given that the second speed V2 lies in visco-capillary conditions, the fiber  10  is well wetted by the second alloy, and the fiber penetrates fully into the second mass  220 . On leaving the second mass  220 , the fiber  10  presents a coating  225  of second alloy of thickness that is substantially constant over its entire circumference and over the entire length of the portion that is to be coated. This thickness is small relative to the diameter of the fiber  10 , i.e. less than one-tenth of this diameter. 
     Once the entire portion of fiber  10  that is it desirable to coat has become coated in the second alloy, the coating is allowed to cool so that it becomes solid (step k)). 
     In order to accelerate this cooling, it is advantageous to use a cooler that cools the second alloy on this portion of fiber  10 . 
     The cooler is thus situated on the path of the fiber  10  downstream from the space E2 (and upstream from the subsequent coating device, and possibly from the fourth pulley  244 , such that the second alloy is solid when it comes into contact with the fourth pulley  244 ). 
     By way of example, the cooler is a sheath through which the fiber  10  passes, and it delivers a stream of gas or air (e.g. at ambient temperature) filling the inside of the sheath and in which the fiber  10  is immersed so as to be cooled. 
     Thereafter the fiber  10 , as already coated in this coating  225  of the second alloy, is coated in the first alloy. For this purpose, the fiber  10  is caused to pass through the first mass  20  of the alloy at a speed V1, using the method described above. 
     On exiting the first mass  20 , the coating  225  of second alloy on the fiber  10  presents a coating  25  of a substantially constant thickness of the first alloy over its entire circumference and along the entire length of the portion that is to be coated. 
     Given that the downstream pulley  42  is touched by the fiber  10  carrying the coating  25 , it is necessary for the coating  25  to be solid when it comes into contact with the downstream pulley  42 . 
     After coating, in order to cool the coating  25  sufficiently for it to be solid when it comes into contact with the downstream pulley  42 , a cooler  60  is used, which cooler is then situated downstream from the space E1 and upstream from the pulley  42 . By way of example, the cooler is similar to the above-described cooler downstream from the pre-coating operation. 
     The second alloy presents a melting temperature T F2  that is higher than the melting temperature T F1  of the first alloy. 
     Surprisingly, tests undertaken by the inventors have shown that when the melting temperature T F2  of the second alloy is lower than the melting temperature T F1  of the first alloy, the coating  25  of the first alloy and the fiber  10  run the risk of being embrittled. Furthermore, the coating  25  of first alloy does not wet the surface of the coating  225  of second alloy in uniform manner, i.e. certain portions of the coating  225  of second alloy are not covered by the first alloy. 
     This is due to the fact that while passing through the mass of the first alloy (coating), the second alloy is heated to above its melting temperature T F2  and the second alloy (as deposited during pre-coating) remelts and becomes dissolved in the first alloy, and the second alloy shrinks by the capillary effect, thereby laying bare the surface of the fiber  10 . Furthermore, since the second alloy is in liquid form, it reacts with the first alloy, which is likewise in liquid form, so as to form chemical compounds that, during subsequent cooling of the first and second alloys, serve to embrittle the coating  25  of the first alloy and to embrittle the fiber  10 . 
     For example, for a fiber made of silicon carbide (SiC) that is to be embedded in a matrix of Ti-6242 titanium alloy (first alloy) having a melting temperature T F1  of 1670° C., and with a second alloy of zirconium-vanadium Zr—V with a melting temperature T F2  of 1500° C., it is observed after pre-coating (step k)) that carbides (Zr, Ti—C) form during coating (step b)) around the fiber  10  and at the old β grain boundaries of the titanium first alloy. 
     These carbides embrittle the coating  25  of first alloy. Furthermore, remelting the coating  225  of second alloy tends to embrittle the fiber  10 . 
     Surprisingly, tests undertaken by the inventors have shown that when the melting temperature T F2  of the second alloy is equal to the melting temperature T F1  of the first alloy, i.e. when the second alloy and the first alloy are identical, the fiber  10  is not completely covered by alloy. 
     This is due to the fact that the layer of alloy deposited on the fiber  10  during pre-coating is thin (because of the low speed at which the fiber passes through the alloy) and tends to fracture during subsequent cooling. In addition, this layer can become dissolved during the subsequent coating operation. Consequently, the coating that is performed subsequently is not effective, with regions of the fiber  10  that are laid bare being poorly wetted. 
     For example, for a fiber made of silicide carbide (SiC) that is to be embedded in a matrix made of Ti-6242 titanium alloy, after the pre-coating, a brittle layer of TiC is formed on the surface of the fiber  10 . During subsequent cooling, this layer breaks because of its small thickness. Decohesion thus occurs between the fiber  10  and the coating  225  of the second alloy, leaving regions of the fiber  10  that are bare. These bare regions of the fiber  10  are poorly wetted during the coating operation, and consequently the fiber  10  is not covered by alloy in some locations. 
     In contrast, when pre-coating is performed with a second alloy having a melting temperature T F2  that is strictly higher than the melting temperature T F1  of the first alloy used during the subsequent coating operation, then a coating  25  of first alloy is obtained that is of uniform thickness over the entire surface of the coating  225  of second alloy that covers the fiber  10 , i.e. over the entire periphery of the fiber  10  in a plane perpendicular to the main direction D. 
     This situation is shown in  FIG. 2  which is a cross-section of the fiber  10  (i.e. a section in a plane perpendicular to the direction in which this portion of the fiber  10  extends (main direction D)) after it has been coated. 
     During pre-coating, the second alloy wets the fiber  10  well since the second speed V2 is slow. 
     During subsequent coating through the first alloy, the first alloy wets the second alloy coating  225  well and a first alloy coating  25  of uniform thickness becomes formed over the entire surface of the second alloy coating  225 , which coating adheres thereto. Because the melting temperature T F2  of the alloy is higher than the melting temperature T F1  of the first alloy, the coating  225  of the second alloy remains solid throughout coating, thereby protecting the fiber  10 . When present, the pyrolytic carbon layer at the surface of the fiber  10  is not damaged during this coating operation. 
     Thus, the wetting of the fiber  10  by the first alloy is improved relative to a prior art method without pre-coating, thereby enabling the fiber  10  to penetrate fully into the first mass  20  of alloy even at speeds that are fast (several meters per second), thereby covering the fiber over its entire surface without forming droplets. 
     Advantageously, the second alloy (of the pre-coating) does not form embrittling phases with the first alloy (of the coating). 
     Thus, the interface between the coating of the second alloy and the coating of the first alloy does not present any phases (i.e. metallurgical phases or compounds) that embrittle this interface, and there is no risk of the interface becoming a zone that generates breaks or decohesion between these coatings. 
     For example, the second alloy (of the pre-coating) contains at least one chemical element that is present in the first alloy (of the coating). 
     Thus, the matrix of the composite (which matrix is made of the first alloy) is not modified chemically in harmful manner in the vicinity of the fiber  10   
     Alternatively, the chemical element has a beta-generating effect on titanium, i.e. it presents a body centered cubic structure like that of niobium. Alternatively, this chemical element has an alpha-generating effect. 
     For example, with a fiber made of silicon carbide (SiC) that is embedded in a matrix of Ti-6242 titanium alloy (first alloy) having a melting temperature T F1  of 1670° C., the second alloy is selected to be a titanium niobium alloy (Ti—Nb) comprising 51% by weight of Ti and 49% by weight of Nb, and having a melting temperature T F2  of 1870° C. 
     During pre-coating, the second alloy wets the fiber  10  well because the second speed V2 is slow (equal to about 1 m/s or slower), and a coating  225  of this second alloy is formed over the entire surface of the fiber  10  with a constant thickness of 4 μm, and this coating adheres to the fiber  10 . This coating  225  is made up of grains of beta phase titanium with the carbides TiC and NbC at the boundaries between grains. 
     During the subsequent coating, the Ti—Nb alloy is well wetted by the titanium alloy. Little niobium diffuses into the titanium, thereby avoiding the appearance of a supercooling phenomenon such as a eutectic phenomenon (a portion of the alloy going to the liquid state). 
     Advantageously, the niobium content in the second alloy is greater than 3% in order to obtain some beta phase in the titanium of the second alloy (below which content the titanium is entirely in alpha phase), and less than 50% in order to avoid overheating the fiber  10  during pre-coating (since the melting temperature T F2  increases with the percentage of niobium). 
     Alloys other than Nb—Ti that are suitable for pre-coating SiC fibers when the first alloy is a titanium alloy are alloys of titanium and one (or more) additional element(s) present in the first alloy. The melting temperature of the additional element should be higher than the melting temperature T F1  of the first alloy. Advantageously, the additional element does not form a eutectic with titanium, and on the contrary it forms a total solid solution (a single solid phase below the solidus temperature in the phase diagram), or else it generates a peritectic reaction. 
     Such additional elements are as follows: zirconium (Zr), chromium (Cr), vanadium (Va), hafnium (Hf), molybdenum (Mo), tantalum (T), rhenium (Re), and tungsten (W). 
     Thus, and advantageously, when the first alloy is Ti-6242 titanium alloy, the second alloy (or pre-coating) includes at least one of the elements in the group constituted by Nb, Zr, Cr, V, Hf, Mo, Ta, Re, W. 
     The second alloy may thus be an alloy of titanium with a plurality of elements from this group, such as Ti—Nb—Zr, Ti—Nb—V, Ti—Ta—Zr. 
     In a variant, after the fiber  10  has been pre-coating with the second alloy and before the fiber  10  is coated with the first alloy, the fiber  10  (with its coating of second alloy) is subjected to a second pre-coating operation with a third alloy having a melting temperature T F3  that is strictly lower than the melting temperature T F2  of the second alloy and strictly higher than the melting temperature T F1  of the first alloy. 
     Thus, after step k), and before step a), the following steps are performed: 
     l) providing a third mass of a third metal alloy having a melting temperature T F3  that is strictly lower than the melting temperature T F2  of the second alloy and that is strictly higher than the melting temperature T F1  of the first alloy; 
     m) heating the third mass to above its melting temperature so that the third alloy is in the liquid state and occupies a space E3, and then moving the fiber from upstream to downstream in translation through the third alloy, this translation taking place at a third speed V3 that is faster than the second speed V2, which is slower than the first speed V1, and that is such that the condition under which the third alloy is taken up during this third translation lies under visco-capillary conditions, such that the fiber becomes covered over a portion of its length (already coated in the second alloy), by a coating of the third alloy presenting a thickness that is not zero and occupying its entire periphery; and 
     n) cooling the coating of the third alloy until it becomes solid. 
     The method of the invention is applicable to any combination of fibers, in particular ceramic fibers, and of metal alloy constituting the matrix in which the fibers are embedded.