Process for preparing densified porous billets

Abstract: A densified porous billet is formed by depositing a matrix composition in the pores of a porous preform. The composition of the matrix is graded, varying with distance from the substrate surface. In one embodiment, pores 46 formed by the voids in a preform of three-dimensionally woven carbon fibres 42 are filled by chemical vapour deposition of a first layer 61 of carbon and subsequent layers 63, 65 of a carbon/silicon carbide composition and a final layer 67 of 100% silicon carbide to provide a step-wise gradation from carbon to silicon carbide.

Description: The invention is directed generally to densified porous preforms comprising porous preforms into which a matrix composition, as hereinafter defined, is deposited within the pores of the porous preform. The term preform shall apply to porous members such as multi-directional woven structures widely known as 2D,3D, 4D, --- nD structures. Preforms may also be constructed from fabrics, felts or cellular structures. Billet or Densified Preform shall mean a preform having its pores essentially filled with matrix material forming a composite structure. A graded matrix composition is defined as a composition in which the percentage of a matrix material across a section of the preform or within the pores of a preform is not uniform. The composition may vary discontinuously or continously. A graded carbon-ceramic composition is defined as a matrix composition in which the percentage of ceramic relative to carbon increases from zero on a substrate toward 100% remote from the substrate. The term, for example, may also apply to a matrix composition where the ratio of carbon to ceramic to carbon is zero in an interior region of a billet and increases toward 100% at the exterior surface of the billet. Fast or Rapid Densification is the process described and claimed in US-A-4472454 or a variation thereof. Substrate surfaces of a preform are the structural members such as fibers or cellular walls defining the pores of the preform. The process may be applied to any porous preform which can be heated sufficiently to effect chemical vapor deposition of a matrix material on a substrate surface. Generally, also the process lends itself to varying the composition of a matrix within the pores of a preform in a continuous deposition process. The process also lends itself particularly to forming graded matrices of selected materials for a variety of purposes e.g. oxidation protection, thermal shock and impact resistance etc. The invention has timely application to oxidation protected carbon preforms and particularly carbon preforms using ceramic compositions to improve the resistance of the carbon to oxidation. For this reason the invention will be described in the context of depositing a graded carbon-ceramic matrix within a carbon , specifically a 3D carbon preform. A broader selection of matrices will be covered later. Carbon-carbon composites (C/C) comprising carbon fibers in a carbon matrix exhibit many desirable properties for high temperature application. C/Cs are strong, tough, lightweight, highly refractory materials with unequalled strength retention and creep resistance to very high temperatures. C/C's combination of relatively low elastic modulus, extremely low coefficient of thermal expansing and high thermal conductivity results in a material that exhibits exceptional thermal shock resistance. Carbon-carbon composites require oxidation protection for many applications at high temperatures. it is generally recognized that dense ceramic coatings of silicon carbide (SiC) and silicon nitride (Si 3 N 4 ) as part of an oxidation protection strategy offer a potential for useful performance. (J.E. Sheehan, Carbon 27, 709 (1989)). The most serious problem in the use of such coatings is the thermal expansion mismatch between the composite and the ceramic coating which causes cracks in the coating and spallation. Use of C/C has been restricted , for the most part, to relatively short duration rocket propulsion, e.g. ITE's (integral throat entrances), or reentry applications, e.g. nose cones, due to its inherently poor oxidation resistance. Early reports laying out the basis for much of the subsequent work, were concerned with properties of basic interest for development of an oxidation resistant coating. These reports were interested with aspects such as diffusion of species through protective coatings, volatility of protective coatings, and chemical and mechanical compatibility of the coating materials with graphite. Also included were methods for applying coatings such as: 1) chemical vapor deposition(CVD), 2) Flame spraying, 3) Slurry dipping, painting or troweling, or 4) electrophoretic deposition. Recent work cited in US-A-4582751, US-A-4599256, US-A-4472476, US-A-4585675 on oxidation resistant C/C's have utilized innovative methods of coating or inhibiting oxidation of C/C's. The new methods are similar to the previous methods. However the coating or inhibitors were produced using unique combinations of materials and processes. In some cases the materials of concern arose out of the reduction to practice of previously proposed materials. The application of silicon- or silicate ceramics- based coatings provide only limited oxidation resistance as cracks develop in the coating or the coatings peel off. Oxygen and atmospheric moisture passes through the cracks to the underlying material with deleterious or disastrous effect. Silicon nitride and silicon carbide ceramic deposits made by chemical vapor infiltration and other methods require hours to make and have undesirable byproducts and are generally made in a vacuum. The aforementioned procedures exhibit thermal shock problems due to the inherent differences particularly in the coefficient of thermal expansion between the carbon and the ceramic coating. A raison d'etre of ceramic-ceramic composites is to provide the toughness and damage tolerance unavailable in monolithic ceramics. As currently understood, the key to achieving these properties is the proper interaction of the preform and the matrix at their interface. If bonding across this interface is strong, toughness is not achieved because cracks propagate readily through the interfaces resulting in catastrophic failure. If the bonding is too weak optimum mechanical properties may not be achieved and toughness may be diminished by too facile fiber pull out. The process described in the application could also be used to tailor the inter-facial properties through graded chemical composition with the object of optimizing the fiber matrix bond. Past approaches have included fiber coating prior to preform fabrication or chemical vapor infiltration of woven preforms. These prior approaches suffer from mismatch of dissimilar material properties and very long process times. It is an object of the invention to provide a process for making a densified preform which avoids the limitations and disadvantages of prior art densified billets. It is another object of the invention to provide a process for making a densified preform or billet containing a vapor deposited matrix. It is another object of the invention to provide a process for densifying a preform with graded matrix composition. It is yet another object of the invention to provide a process for making a continuously varying or a discontinuous graded matrix composition. It is also an object of the invention to provide a process for making an oxidation resistant carbon billet containing a graded carbon-ceramic matrix. It is also an object of the invention to provide a process for optimizing the interfacial properties between the fibers and the matrix. Aspects of the invention are set forth in the accompanying independent claims 1, 4, 13 and 20. In accordance with the invention there is provided a process for making a densified preform comprising a preform having an interior porous region bounded by exterior surfaces. The pores of the preform contain a graded vapor- deposited matrix composition. Also in accordance with the invention there is provided a process for making a densified carbon preform comprising a porous carbon preform having on interior porous region bounded by exterior surfaces. The interior porous region contains a graded composition. The graded composition may be a carbon-silicon carbide, a carbon-silicon nitride, carbon-ceramic, or ceramic-ceramic composition. In another embodiment of the invention there is provided a process for making an oxidation inhibited carbon billet having a porous carbon preform with exterior surfaces and a porous interior structure. The pores contain graded a carbon ceramic -composition. The exterior surfaces of the carbon preform are coated with ceramic. The preferred ceramics are silicon carbide and silicon nitride, or mixtures thereof. Another embodiment of the invention comprises processes for densifying a porous preform by depositing a continuously varying or discontinuous graded composition within the interior region of the preform or within the pores. The novel features that are considered characteristic of the invention both as to its organization and method of operation, together with additional objects and advantages thereof , will be better understood from the following description of a specific embodiment , when read in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic representation of an induction heated reactor for making densified preforms in accordance with the invention. Fig. 2 is across section of the billet formed from the preform of Fig. 1 taken along line 2-2. Fig. 3 is a graphic representation of the graded carbon-ceramic composition within the billet. Fig. 4 is a representation of a 3D porous woven preform containing a graded ceramic matrix composition within its pores. Fig. 5 is a schematic representation of the fiber -ceramic distribution about several of the fibers of the Fig. 4 billet. The preferred embodiment described in detail herein addresses the problem of providing thermal expansion compatibility between a carbon preform and an oxidation inhibited densified preform. Both thermal expansion compatibility and oxidation inhibition may be accomplished through grading the matrix composition to include a ceramic component. The process of constructing a discontinuous graded matrix within the preform 30 will be described in relation to Figs. 1 -3. . Referring to Fig. 1 there is shown a schematic representation of a reactor 10 useful for making the densified porous preform through a process in accordance with the invention. The reactor 10 contains a vessel 12 which includes a exhaust port 14, an outlet port 16, and an inlet port 18. Connected to the inlet port 18 is a conduit 20 which communicates through a mixer valve M, two reagent inlets 22 and 24. A preferred reagent in the process for making a carbon-ceramic graded matrix is a liquid hydrocarbon such a cyclohexane from which carbon can be vapor deposited. Positioned within the reaction 12 is a densified preform 30 which is to be densified in accordance with the invention. In this instance, the preform 30 contains a heat susceptor 32. The susceptor 32. in this illustration is used to concentrate heat in the center of the preform 30 to initiate radial heat transfer through the preform 30. In many cases the susceptor will not be required. This is particularly true where the density of the porous preform is relatively high and where induction heating can be effective without a susceptor. The fast or rapid densification/CVD process wherein a carbon preform is immersed in cyclohexane and heated to at least the decomposition temperature of the cyclohexane is described in US-A-4472454. In that process carbon is deposited in the walls of the pores and within the pores of the preform producing a carbon-carbon billet. In the embodiment where the purpose is to deposit a graded matrix within the interior of the preform, carbon will be deposited adjacent to the inside surface of the cylindrical preform 30. The densification will proceed generally outwardly radially. As the densification proceeds outwardly toward the exterior surface of the preform 30 the proportion of carbon deposited will be decreased while the percentage of ceramic deposited is increased. The graded portion of the deposition is obtained by controlling the liquid stoichiometry. Initially the deposit is pure carbon or very carbon rich and then proceeds toward stoichiometric ceramic remote from the substate surface. The actual temperature distribution varies as a function of both the deposit time and the spatial coordinates due to the progression of the deposit front and the heat of vaporization of the mixture respectively. To densify the preform 30 in Fig. 1 a hydrocarbon such as cyclohexane 38 is introduced via inlet 24, mixer M and inlet 18 until the cyclohexane covers and impregnates the preform 30. Excess reagent leaves the vessel 12 through outlet 16. An induction heater 36 is provided to heat, initially the susceptor 32 until the region adjacent to the inside surface 39 of preform 30 is heated above the decomposition temperature of the cyclohexane. The preferred decomposition range for depositing and coating is 1000°C to 1500°C nominally 1200°C to 1300°C. Carbon is deposited on the substrate surfaces defining the pores adjacent to the inside surface 39. The hydrogen released is exhausted through the exhaust 14. See layer 41 in Fig.2 and the graph in Fig. 3. Referring to Figs. 2-3 the foregoing process steps create a layer 41 comprising 100% carbon and zero% ceramic. To construct a discontinuous graded carbon-silicon carbide matrix within the interior of the preform as illustrated in Fig.2 a silicon carbide precursor such as a chlorsilane,e.g.methyltrichlorsilane, is supplied to inlet 22(Fig.1) until a desired mixture of carbon and silicon carbide precursors is achieved. The time-temperature is adjusted within the preferred range to cause the deposit front to progress outwardly until layer 43 is formed. Subsequently the liquid stoichiometry is adjusted again so that the layer 45 comprising 75% silicon carbide and 25% carbon is formed. Finally, the liquid stoichiometry is adjusted to 100% chlorosilane and 100% silicon carbide is deposited adjacent to the exterior surface of the preform 30 and on the exterior surfaces 37 of the preform 30. See curve 51 in Fig.3 for a graphic representation of the just described grades matrix structure. Alternatively the stoichiometry could have been varied continuously to provide a continuous variation shown in curve 53 of Fig 3. Referring to Fig. 1 there is depicted an irregular aura like line surrounding the preform 30. This is a stable vapor film which is created by the decomposition of the liquid and precursor vaporization processes. The film acts to cool the preform in front of the deposit front so that no deposition takes place in front of the deposit front. The formation of the film is also characteristic of the use of liquid to immerse the preform. Because a liquid is used and because liquids characteristically form vapor films,it possible to create a large temperature gradient across the preform. It is possible to effect rapid densification and it is possible to control the position of the deposit front. Figs. 4-5 are used to illustrate another embodiment where a graded carbon-ceramic composition is deposited within the pores of a preform. Figure 4 depicts a 3D woven preform 40. The fibers may be carbon, silicon carbide. or another material which is capable of being heated, preferably by induction. In the case of a carbon 3D preform a susceptor is not needed and is therefore not illustrated. It must be emphasized that the invention is not limited to carbon-ceramic which has been illustrated in detail because of the timely demand for oxidation inhibited carbon. Other possible configurations will be set forth below. Typically, a 3D block structure such as illustrated in Fig. 4 is formed by weaving three mutually orthogonal courses of fibers, such as fibers 42. The symbol 44 is intended to identify "virtual" exterior surfaces of the preform 40. That is, these would be the surfaces if the preform were a solid block. The pores of the preform are identified by the symbol 46. Clearly the pores 46 are the voids between adjacent fibers. The surfaces of the fibers defining these voids and also the substrates on which graded matrices will be deposited. The object here is to heat the preform uniformly so that deposition will take place on all of the substrate surfaces defining the pores of the preform at substantially the same time. The sequence of steps needed to construct a graded matrix composition within the pores are essentially the same as previously described. The stoichiometry of the deposit is controlled by stoichiometry of the liquids. The actual deposition temperature is a function of both the deposit time and the location of the the deposit front as well as the heat of vaporization, as before. Initially, for example, the preform is immersed in an organic solvent such as cyclohexane - or for that matter benzene, hexane etc. The fibers from which the preform is constructed are heated uniformly to the decomposition temperature of the cyclohexane. As a result a carbon layer 61 deposited on each fiber. See Fig 5. The liquid stoichiometry is changed to increase the chlorosilane to increase the percentage of silicon carbide deposited. Consequently, layer 63 is formed. Layer 65 with still more silicon carbide follows. Finally, the liquid contains only the silicon carbide precursor and 100% silicon carbide is achieved in layer.67. Layer 67 is shown to extend beyond the exterior surface 44 of the carbon preform. Clearly if the stoichiometry of the liquids is varied in a continuous fashion a continuously varying gradation such as shown in curve 53 in Fig. 3 would result. Deposit compositions could be varied by changing the precursor material. For example deposition of titanium, zirconium , or hafnium carbide may be accomplished from dispersions of titanium, zirconium, or hafnium tetrachloride and cyclohexanes. Other systems could include organo metallics such as titanocene, zincocene, hafnocene in Toluene. Other possibilities would be boron carbide from boron trichloride and an organic compound. Ceramic nitrides could be deposited by reaction of one of the chlorides with hydrazine. An important ceramic is Silicon Nitride which may be formed by bubbling ammonia (NH 3 ) through such liquid precursors as silicon chloride. Silicon nitride is stable at high temperatures and has a smaller coefficient of expansion than silicon carbide. TiN would conceivable result from bubbling ammonia through titanium tetrachloride liquid. Clearly, it is possible by means of the inventive process to form matrices from mixtures of ceramics as well as carbon-ceramic compositions. The mode of heating the carbon preform will vary with the configuration and the material. For example the preform 30 in Figure 1 could have been heated by passing current through the carbon. Induction heating may also include r-f and microwave heating where applicable. The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the scope of the invention as defined by the claims.

Claims: A process for making a densified billet containing a graded composition, comprising: providing a porous preform (30,40) of a first material, immersing the preform (30, 40) in a mixture of respective pre-cursor liquids of two materials to be deposited in the pores of the preforms, and heating the preform to vaporize the liquid near the preform and to deposit a composition of the materials within the pores (46) of the preform (30,40) from the pre-cursor liquids the relative amounts of the pre-cursor liquids being varied during the deposition to cause grading of the deposited composition. A process as defined in claim 1, characterised in that the relative amounts of the pre-cursor liquids are varied continuously during the deposition. A process as defined in claim 1, characterised in that the relative amounts of the pre-cursor liquids are varied discontinuously during the deposition. A process for densifying a preform, comprising: providing a porous preform (30,40) of a first material; immersing said preform in a liquid reagent which will decompose on contact with the heated preform to deposit a second material in said pores (46); heating the preform (30,40) to decompose the liquid reagent and deposit the second material in the pores (46); and altering the composition of the liquid reagent to a composition which will decompose to deposit a third material, thereby depositing a graded composition of said second and third materials in said pores. A process as defined in claim 1, characterised in that the composition of the liquid reagent is altered continuously thereby to deposit a continuously graded composition. A process as defined in claim 1 characterised in that the composition of the liquid reagent is altered discontinously thereby to deposit a discontinuously graded composition. A process as defined in claim 1, 2 or 3 wherein the first material is carbon. A process as defined in claim 5, 6 or 7, characterised in that the second material is carbon and the third material is a ceramic. A process as defined in claim 5, 6 or 7, characterised in that the second material is carbon and the third material is SiC. A process as defined in claim 5, 6 or 7, characterised in that the second material is carbon and the third material is an oxidation inhibitor. A process as defined in claim 5, 6 or 7, characterised in that the reagent in which the preform is immersed initially is a carbon precursor and the third material is a ceramic. A process as defined in claim 5, 6 or 7, characterised in that the reagent in which the preform is immersed initially is cyclohexane and the third material is a ceramic. A process for densifying a preform, comprising: providing a porous preform (30,40) of a first material; immersing said preform in a liquid first reagent comprising a precursor for a second material, the first reagent decomposing on contact with the heated preform (30,40) to deposit the second material in said pores (46); heating the preform (30,40) to the temperature at which the pre-cursor decomposes; altering the composition of the liquid reagent by adding a liquid second reagent comprising a precursor for a third material, the second reagent decomposing on being heated to deposit the third material in the pores (46) and; increasing the proportion of said second reagent to said first reagent to deposit a graded composition of said second and third materials in the pores. A process as defined in claim 13, characterised in that the relative proportion of the first and second reagents is varied continuously. A process as defined in claim 13, characterised in that the proportion of the first and second reagent is varied discontinuously. A process as defined in claim 13, 14 or 15, characterised in that the second reagent is a SiC precursor. A process as defined in claim 13, 14 or 15, characterised in that the second reagent is a silicon nitride precursor. A process as defined in claim 13, 14 or 15, characterised in that the second reagent is a ceramic precursor. A process as defined in claim 13, characterised in that the first and second materials are carbon and the third material is an oxidation inhibitor. A continuous deposition process for the densification of a porous preform structure comprising a porous substrate having voids extending inwardly from the surfaces thereof which render said body porous, comprising the steps of: (a) providing a reaction vessel for coating-precursor liquids which are heat-decomposable at elevated temperatures to vapor-deposit coatings on the surfaces of said substrate body to fill said voids and densify said preform strcuture; (b) filling said reaction vessel with a coating-precursor liquid which is heat-decomposable to a vapor and deposits a ceramic or organometallic layer on the surfaces of said porous substrate body; (c) inserting a said porous substrate body within said reaction vessel to submerge said body in said liquid; (d) heating the porous substrate body submerged within the coating precursor liquid to an elevated temperature sufficient to decompose the precursor liquid and cause the vapor deposition of a ceramic or organometallic coating on said surfaces, and (e) continuing said heating while the preform is submerged in said liquid to cause the vapor deposit to fill said voids and form a densified preform structure.

Inventors: CARROLL THOMAS J, CONNORS DONALD F JR, SUPLINSKAS RAYMOND J, THURSTON GARRETT S, CARROLL, THOMAS J., CONNORS, DONALD F., JR., SUPLINSKAS, RAYMOND J., THURSTON, GARRETT S.
Applicants: AVCO CORP, AVCO CORPORATION
Application year: 1992
Main classification: C04B35
IPC codes: C04B41, D03D25, C04B35, C23C18, C23C16