FR2864829A1 - Articles of ceramic matrix composite materials with improved lamellar strength for use as components for gas turbines subjected to elevated temperatures - Google Patents

Abstract

A composite article (10) with a ceramic matrix comprises: (A) multiple layers (12), each made up of a first reinforcing material (20) in a matrix of ceramic material (22), the reinforcing material being coated with an anti-adhesive agent that favours sliding between the reinforcing material and the ceramic matrix, with pairs of adjacent layers defining between them some interfaces (14); and (B) a second reinforcing material (16) physically connects the pairs of adjacent layers, with a stronger liaison existing between the second reinforcing material and the ceramic matrix than between the first reinforcing material and the ceramic matrix of the pair of adjacent layers. An independent claim is also included for the formation of this composite article.

Description

LAMINAR RESISTANCE CERAMIC MATRIX COMPOSITE

  The present invention relates generally to ceramic matrix composite (CMC) articles; more particularly, the present invention relates to CMC articles with out-of-plane resistance.

  In order to increase the efficiency of gas turbines, continuous attempts are made to achieve higher operating temperatures. Although great progress has been made in high temperature capabilities through the development of superalloys based on iron, nickel and cobalt, other possible materials have been investigated. Notable examples are ceramic matrix composite (CMC) materials, whose high temperature capabilities significantly reduce cooling air requirements. CMC materials, particularly continuous fiber ceramic (CCFC) composite materials are currently being considered for envelopes, combustion chamber liners, injectors and other gas turbine parts subjected to high temperatures. Silicon-based composites, such as silicon carbide (SiC), are particularly useful applications for high temperatures as a matrix and / or reinforcing material.

  Overall, the CMC materials comprise a fibrous or filamentary reinforcing material embedded in a ceramic matrix material. The reinforcing material serves as the carrier component of the CMC while the ceramic matrix protects the reinforcing material, maintains the orientation of the fibers and serves to dissipate the charges to the reinforcing material. CMCs frequently consist of multiple layers of "prepreg", which is typically a ribbon-like preform comprising the reinforcing material impregnated with a precursor of the matrix material. The preform has to undergo a treatment (including a firing) to transform the precursor into a desired ceramic.

  Frequently, CCFC material prepregs comprise a two-dimensionally arranged fiber assembly having a single single-axis aligned single-strand fiber wick impregnated with a precursor in the form of a slurry to create a laminate generally. two-dimensional. As a result of the impregnation, each laminate usually undergoes consolidation, densification and partial hardening (B-phase) of the matrix material. Multiple layers of the prepregs obtained are then superimposed in a mold and individualized to form a laminated preform, a process called "superposition". The prepregs are ordinarily arranged so that the adjacent prepreg strands are mutually transversely oriented (e.g. perpendicular to each other), giving greater mechanical strength in the laminar plane of the preform (corresponding to the main directions ( the load component) of the final CMC component) in comparison with out-of-plane directions, for example perpendicular to the laminar plane of the preform (i.e. the z-direction of the preform). Following the layering of layers, the laminated preform undergoes a new series of consolidation, densification and final curing (baking) processes to achieve the desired CMC element.

  It is known that CCFCs exhibit "degradation" due to the hardening action of the reinforcing fibers. The ceramic matrix may crack under the effect of a first stress (for example about 20,000 psi) while the short-term break strength of the CCFCs may be much larger (eg about 280 MPa (40,000 psi)). This effect results mainly from the action of the reinforcing material which fills the crack and redistributes the load. To promote this filling effect, the reinforcing materials are usually coated with a release agent, usually boron nitride (BN) with or without other compound, to allow limited and controlled slip between the fibers and the matrix. When single-axis alignment bits of a CCFC are densely grouped, the release coatings on the adjacent fibers are close to one another and can touch each other. Since the release liner itself is very weakly bonded to the fibers, the intralaminar (in which the fibers are in the layers) and the interlaminar (between adjacent layers) planes have relatively low tensile strengths (eg less than about 20 MPa (3000 psi). Because of their stratified structure, CCFC materials may exhibit relatively low tensile and shear strengths when subjected to off-plane loads, which necessarily limits use of CCFCs for many applications.

  In addition to CMC materials, laminated structure polymer matrix (CMP) composites can also have relatively low interlaminar strengths. CMPs differ significantly from CMCs in that they use a cured polymer resin as the matrix material. Methods are known for increasing the interlaminar properties of CMPs by providing reinforcing fibers or rods oriented in the z direction. If the reinforcing elements are stressed in the vicinity of their tensile strength, the force is transmitted without interlaminar cracking due to the high flexibility and plasticity of the resin matrix and the high stiffness of the reinforcement. . Simple mechanical models of CCFC materials with stratified constructions also reinforced with fibers or rods in the z-direction have predicted that if the reinforcing elements are stressed up to the point of tensile strength under tension, it is sufficient to small fraction by volume (about 5%) to double the interlaminar resistance of CCFCs. However, as a result of this stressing of the reinforcement, it has been observed that interlaminar cracks form and develop as the load is transmitted to the reinforcement through the rigid ceramic matrix. Especially in high temperature applications, cracks in the die are not acceptable as they may compromise the strength of the composite at ambient conditions.

  In view of the above considerations, it would be desirable to have a technique for improving the laminar strength, in particular the intralaminar strength, of CMCs, in particular CCFCs, and thus to increase the number of applications in which CMC can be used.

  The present invention provides an article of ceramic matrix composite material (CMC) such as a gas turbine part, as well as a method of manufacturing the article. In particular, the invention is directed to a CMC article which comprises multiple layers, each layer consisting of a first reinforcing material in a ceramic matrix, and the first reinforcing material is coated with a release agent which promotes the sliding between the first reinforcing material and the material of the ceramic matrix. The layers are superimposed so that the layers of each adjacent pair of layers each define an interface between them.

  According to the invention, the article also contains a second reinforcing material which physically connects the layers of the article to one another by bridging the interfaces between them. According to a preferred aspect of the invention, a stronger bond exists between the second reinforcing material and the ceramic matrix than between the first reinforcing material and the ceramic matrix, so that the mechanical strength, in particular the Intra-layer resistance of the article increases without inhibiting the ability of the first reinforcing material in the layers to permit limited and controlled sliding between itself and the ceramic matrix. This result can be achieved with a second reinforcing material free of release agent which would promote sliding between the second reinforcing material and the ceramic matrix of the layers.

  In one embodiment, the first reinforcing material consists essentially of single-axis alignment bits, and the ceramic matrix composite article is a continuous fiber ceramic composite article.

  In another embodiment, the second reinforcing material has discrete columnar elements inserted into the adjacent pair of layers so as to bridge the interface therebetween.

  In still another embodiment, the article further comprises preformed holes in the adjacent pair of layers and aligned from one edge to the other of the interface therebetween, the second reinforcing material being formed by a material comprising the ceramic matrix material such that the second reinforcing material is integral with the ceramic matrix material.

  The method of producing the CMC article generally comprises forming a preform composed of preform layers so that the layers of the adjacent pairs of layers of the preform define between them interfaces. According to a first embodiment of the method, the second reinforcing material is preformed as separate columnar elements, which are then inserted into two (or more) preform layers so as to bridge the interfaces therebetween. According to a second embodiment of the method, holes are previously formed in two (or more) layers of preform, then the second reinforcing material is formed in situ by filling the holes with a material comprising the material forming the ceramic matrix so that the different elements of the second reinforcing material form part of the material of the ceramic matrix.

  With the present invention, the laminar strength of a CMC article, and in particular of CCFC articles, can be improved, thereby improving the resistance of these articles to layer-like cracking, and therefore increasing the number of applications in which CMCs can be used.

  Other objects and advantages of the present invention will become more apparent from the detailed description hereinafter.

  The invention will be better understood on studying the detailed description of an embodiment taken by way of nonlimiting example and illustrated by the appended drawings in which: FIG. 1 is a cross-sectional representation of a laminated structure CCFC article and a separate interstratified reinforcing material formed according to a first embodiment of the present invention; and Figures 2 and 3 are cross-sectional representations of a laminated structure CCFC article and illustrate two steps by which interlaminar elements of a reinforcing material are formed in situ according to a second embodiment of the present invention. .

  The present invention is generally applicable to CMC parts and in particular to CCFC parts to operate in environments characterized by relatively high temperatures. Notable examples of such parts are combustion chamber parts, high pressure turbine blades, and other blades and parts of the hot gas turbine section, although the invention can be applied to other parts, including advanced steam turbines for power generation. Examples of CMC materials particularly concerned by the invention include those with reinforcing materials consisting of silicon carbide, silicon nitride and / or silicon in a ceramic matrix consisting of silicon carbide, silicon nitride and or silicon, for example a SiC / SiC CMC, although the invention also applies to other types of CMC materials.

  Fig. 1 is a cross-sectional view of a laminated structure CCFC member 10 comprising multiple layers 12, each containing a reinforcing material 20 in a ceramic matrix 22. Each pair of adjacent layers 12 form an interlaminar interface 14. Reinforcing material 20 is preferably in the form of a set of fiber bundles, or wicks 20, as shown in FIG. 1. As a CCFC member 10, the wicks 20 are preferably oriented on a single axis in each layer 12, i.e. they are oriented side by side and parallel to each other, and that the orientation of the strands 20 in a given layer is transverse to the strands 20 of the immediately adjacent layers 12. The appropriate diameters for the strands 20 depend on the thickness of the layers 12 and the particular application. It is estimated that center-to-center distances of about 1 mm (about 0.040 inches) of the rovings are suitable for SiC wicks and serve to reinforce an SiC matrix 22 in layers 12 having a thickness of about 2.5. mm. (about 0.1 inches) as a result of their stratification and hardening.

  According to previous practice, the locks 20 are preferably coated with a release agent (not shown), for example boron nitride (BN) or carbon, which allows a limited and controlled sliding between the locks 20 and the ceramic matrix material 22 within each layer 12. When cracks appear in the workpiece 10, one or more wicks 20 filling the crack serve to redistribute the load between the adjacent wicks 20 and regions of the matrix material 22, which prevents or at least slows further crack propagation.

  In making the laminate piece 10, a desired number of laminate prepreg strips are superimposed to form a preform (not shown) of the workpiece. Prior to superimposition, each ribbon is ordinarily formed by impregnating a reinforcing architecture (formed by the wicks 20) with a precursor of desired die material 22, according to known techniques. For example, a prepreg ribbon can be formed in a single operation by applying the precursor during the winding of a filament on a spool. Various precursors are known for this purpose, and the preferred compositions will depend on the particular composition desired for the ceramic matrix 22 of the workpiece 10. Following the impregnation, the precursor is allowed to partially dry. After the superposition, the laminated tapes are generally individualized and hardened while being subjected to the application of a pressure and a high temperature, for example as in an autoclave. Preferably, the formed preform undergoes additional infiltration, for example with molten silicon supplied from the outside, so as to reduce the porosity and transform the ceramic precursor thereby forming the ceramic matrix 22 and resulting in to the desired CMC part 10. The appropriate processing techniques and parameters depend on the particular composition of the materials and will not be discussed here.

  According to a first embodiment of the present invention, FIG. 1 also represents a number of individual interlaminar elements of a reinforcing material 16 in holes 18 completely through the layers 12 of the part 10. The reinforcing material 16 may be in the form of wicks, large monolithic fibers or prefabricated chopsticks made of CMC or CCFC material. Suitable materials for the reinforcing material 16 include the same materials as are suitable for reinforcing (strands 20) laminate layers 12, for example silicon carbide, silicon nitride or silicon fibers, due to their thermal compatibility, although it is conceivable that other materials can be used provided that the chosen material is chemically appropriate for the operating environment of the part 10 and compatible with the matrix material of the laminated layers 12. The reinforcing material 16 is shown oriented substantially perpendicularly to the surfaces of the layers 12 and the interlaminar interfaces 14 present therebetween, that is to say oriented in the z direction of the part 10, although other orientations are conceivable.

  Unlike wicks 20 in layers 12, which are coated with a release agent to allow limited and controlled slip between the wicks 20 and the surrounding ceramic matrix 22, a strong bond is desirable between the material 16 and the matrix material 22 of the layers 12 in order to increase the out-of-plane mechanical resistance, in particular the intralaminar resistance, of the part 10. This is why the reinforcing material 16 is preferably free from any agent anti adhesive. More generally, it is desirable for the bond between the reinforcing material 16 and the ceramic matrix material 22 to be stronger than the bonding between the reinforcing strands 20 within the layers 12 and the ceramic matrix material 22. which therefore increases the laminar strength, and in particular the intralaminar strength, of the workpiece 10 without impairing the ability of the wicks 20 in the laminate layers 12 to prevent the propagation of a crack through the ceramic matrix material 22. In addition, it is desirable for the reinforcing material 16 to have a modulus of elasticity greater than the effective interlaminar modulus to facilitate fast charge transfer.

  With CMC materials, it may be preferable to form, rather than machining, a through hole because machining operations may damage the wicks 20. According to their structure, the elements constituted by the reinforcing material 16 of the Fig. 1 can be introduced by pushing them into the preform of the piece in the green state. Alternatively, the holes 18 may be formed using a tipped mandrel or other suitable tool for moving the wicks 20 in the planes of the layers 12. Then, in the absence of a release agent, the reinforcing material 16 may be closely bonded to the material of the ceramic matrix 22 of each layer 12 during subsequent infiltration and cooking steps. After cooking, it is possible to remove any part of the material 16 protruding from the part 10.

  Although shown as having a planar shape, the workpiece 10 could be formed by winding a precursor impregnated filament (eg wick) onto a cylindrical mandrel. After partial drying and curing, the reinforcing material 16 could be inserted into the annular shaped preform made, for example with a radial orientation relative to the preform. Therefore, this use of the reinforcing material 16 is also covered by the present invention.

  Figures 2 and 3 show a second embodiment of the invention in which holes 118 are formed by removing material (by drilling, punching, etc.) from a green part preform 130 (Figure 2), which, by cooking, forms a piece 110 in CCFC (Figure 3). As in the case of the embodiment according to FIG. 1, the workpiece 10 and its preform 130 have a laminated structure comprising multiple layers 112 and laminated ribbons 132, respective adjacent pairs of layers 112 and ribbons 132 defining interlaminar interfaces 114. Each ribbon 132 of the preform is shown as containing a reinforcing material 120 in a ceramic precursor material 134 which, by firing, forms the ceramic matrix material 122 of the workpiece 110. As in the case of the embodiment according to FIG. 1, the reinforcing material 120 is shown as single-axis wicks 120, coated with a release agent (not shown) to allow limited and controlled slip with the matrix material 122.

  The tapes 132 of the preform 130 are superposed and processed as described above with reference to FIG. 1, but without the introduction of the distinct reinforcing material 16. On the other hand, the holes 118 are preferably formed in the individual ribbons 132 of the preform, after which the ribbons 132 are superimposed and consolidated to form the preform 130. preform 130 is then infiltrated with silicon or other suitable material so that the holes 118 of the preform 130 are filled with the infiltrating agent and the precursor material 134 from the surrounding regions of the ribbons 132 of the preform. To increase the mechanical strength, an additional precursor material 134 may be directly introduced into the holes 118, after which the preform 130 is put into phase B, consolidated and fired. During firing, the precursor material 134 and the infiltrated agent in the holes 118 form an interlaminar reinforcing material 116 which is substantially integral with the ceramic matrix material 122 of the workpiece 10, and thereby mechanically connects one to the other. other layers 112 of the workpiece 110 to increase the laminar strength and in particular the tensile intralaminar strength of the workpiece 10. To obtain a 100% increase in intralaminar / interlaminar tensile strength, it is estimated that the holes 118 must constitute at least approximately 10% of the area of the interlaminar interface 114.

  In a variant of the embodiment according to Figures 2 and 3, the holes 118 are formed after lamination and consolidation of the preform 130. The holes 118 may be formed through the thickness of the preform 130 (or up to some intermediate depth) by drilling, for example by mechanical or laser drilling, or by a fugitive material forming technique. A precursor material 134 is then directly deposited in the holes 118, after which consolidation, infiltration and firing are performed to form in situ the interlaminar reinforcing material 116, which again substantially forms a body with the ceramic matrix material 122. .

  The invention can be extended to CMCs whose reinforcing material comprises a three-dimensional woven or braided architecture. In such an embodiment, the reinforcing material (e.g., fibers, rods) oriented in the main (load bearing) directions of the architecture is coated with a release agent, while reinforcement introduced into the reinforcement architecture so as to be perpendicular to the principal directions (i.e. in the z direction) is free of release agent to provide intralaminar and interlaminar reinforcement.

  Although presented above from the standpoint of treating a prepreg, the invention can also be extended to fiber reinforced ceramic composites made by other methods, including chemical vapor infiltration (CVI) and slurry casting. For example, a preform consisting of superimposed fiber fabrics can be coated with a release agent and then silicon carbide using an IVC technique to achieve the equivalent of a second prepreg coating. . The holes may be formed in appropriate directions through the preform after application of the release agent or silicon carbide, optionally followed by the insertion of a reinforcing material (e.g., in FIG. 1). The preform can then be infiltrated under vacuum using a suitable release agent (eg SiC / C) similar to that used to form the prepreg ribbons, after which drying and infiltration by a molten agent are realized. In a hybrid process that combines aspects of the prepreg and suspension casting techniques, locks are coated as in the formation of a prepreg and then woven to form a fabric. The fabric can then be impregnated with a matrix material, in a similar manner to the suspension casting, after which infiltration by a melt agent is achieved. Reinforcements can be inserted after forming the part. In all cases, the holes and reinforcements could be inserted after infiltration by a molten agent, which requires another melting operation.

Claims (10)

  A ceramic matrix composite article (10, 110), characterized in that it comprises: multiple layers (12, 112), each layer (12, 112) comprising a first reinforcing material (20, 120) in a material of the ceramic matrix (22, 122), the first reinforcing material (20, 120) being coated with a release agent which promotes sliding between the first reinforcing material (20, 120) and the matrix material ceramic (22, 122), adjacent pairs of layers (12, 112) defining between them interfaces (14, 114); and a second reinforcing material (16, 116) physically connecting to each other at least one of the adjacent pairs of the layers (12, 112), a stronger bond existing between the second reinforcing material (16, 116) and the ceramic matrix material (22, 122) of the adjacent pair of layers (12, 112) between the first reinforcing material (20, 120) and the ceramic matrix material (22, 122) of the pair adjacent layer (12, 112).   Ceramic matrix composite article (10, 110) according to claim 1, characterized in that the first reinforcing material (20, 120) essentially consists of single-axis alignment locks, and the composite article ( 10, 110) is a continuous fiber ceramic composite article.   Ceramic matrix composite article (10, 110) according to claim 1, characterized in that the second reinforcing material (16, 116) is free of release agent which promotes sliding between the second reinforcing material (16,116) and the ceramic matrix material (22,122) of the adjacent pair of layers (12,112).   A ceramic matrix composite article (10, 110) according to claim 1, characterized in that the second reinforcing material (16) comprises discrete columnar elements inserted into the adjacent pair of layers (12) so as to bridge the gap. interface (14) therebetween.   A ceramic matrix composite article (10, 110) according to claim 1, characterized in that it further comprises preformed holes (18) in the adjacent pair of layers (112) and aligned from one edge to the other. other of the interface (14, 114) therebetween, the second reinforcing material (116) being made of a material comprising the ceramic matrix material (122) so that the second reinforcing material (116) is integral with the ceramic matrix material (122).   A method of forming a ceramic matrix composite article (10, 110), the method characterized by comprising the steps of: forming a preform comprising multiple layers (132) of preform between layers adjacent pairs of preform layers (132), each preform layer (132) having a first reinforcing material (20, 120) coated with a release agent and surrounded by a ceramic matrix precursor material (134) the preform comprising a second reinforcing material (16, 116) which physically connects the layers of adjacent pairs of the preform layers (12, 112) to each other; and baking the preform layers (132) to convert the ceramic matrix precursor material (134) into a ceramic matrix material (22, 122) and thereby result in a multi-layered ceramic matrix composite (12, 112), the slip-preventing release agent between the first reinforcing material (20, 120) and the ceramic matrix material (22, 122), so that a stronger bond exists between the second material reinforcement (16, 116) and the ceramic matrix material (22, 122) only between the first reinforcing material (20, 120) and the ceramic matrix material (22, 122).   7. The method of claim 6, characterized in that the forming step comprises forming the second reinforcing material (16, 116) so that it is free of a release agent which would promote slippage between the second reinforcing material (16,116) and the ceramic matrix material (22,122) of the layers (12,112).   Method according to claim 6, characterized in that it further comprises the steps of forming the second reinforcing material (16) as separate columnar elements (16), and then inserting the separate columnar elements (16). 16) in the preform layers (132) to fill the interfaces (14) therebetween.   The method of claim 6, characterized in that it further comprises the steps of forming holes (118) in the preform layers (112), and then forming the second reinforcing material (116) in situ in filling the holes (118) with a material comprising the ceramic matrix precursor material (134) so that the second reinforcing material (16) is integral with the ceramic matrix material (122) after the baking step.   10. Method according to claim 6, characterized in that the preform is made by a prepreg process, a suspension casting process or a combination thereof.