The present invention relates to making composite material parts from fiber blanks made from one or more porous fibrous structures, in particular thermostructural composite material parts.
Thermostructural composite materials are remarkable for their good mechanical properties and for their ability to conserve these properties at high temperatures. They are used in particular for making structural parts in the fields of aviation and space. Typical examples of thermostructural materials are carbon/carbon (C/C) composite materials comprising carbon fiber reinforcement densified by a carbon matrix, and ceramic matrix composite (CMC) materials. CMC materials comprise fiber reinforcement made of refractory fibers (generally carbon or ceramic) and densified by a ceramic matrix or a combined carbon and ceramic matrix. An interphase layer, e.g. of pyrolytic carbon (PyC) or of boron nitride (BN) can be interposed between the reinforcing fibers and the ceramic matrix in order to improve the mechanical behavior of the material.
The manufacture of a CMC or C/C composite material part generally comprises preparing a fiber preform that is to constitute the reinforcement of the composite material, and then densifying the preform with a matrix of ceramic or carbon, possibly after forming an interphase layer on the fibers of the preform.
The preform is made from one- or two-directional fiber fabrics such as yarns, Lows, ribbons, woven cloth, unidirectional sheets, layers of felt. The preform is shaped, by steps of winding, weaving, braiding, knitting, or draping plies.
Densification may be performed by a liquid method, i.e. by impregnating the preform with a liquid composition containing a precursor of the ceramic or carbon material of the matrix. The precursor is typically a resin, which, after being cured, is subjected to heat treatment for ceramization or carbonization.
Densification can also be performed using a gas method, i.e. by chemical vapor infiltration using a reaction gas containing one or more precursors of the ceramic or carbon matrix. The gas diffuses within the pores of the fiber preform, and under particular conditions of temperature and pressure it forms a deposit of carbon or ceramic on the fibers, either by means of a component of the gas decomposing or else by means of a reaction taking place between a plurality of components.
The above processes of preparing CMC or C/C composite material parts are themselves well known.
The mechanical properties of a fiber-reinforced composite material part depend, in particular, on the ability of the fiber reinforcement to withstand various kinds of stress.
Thus, when the fiber reinforcement is constituted by a preform built up as a stack of two-dimensional plies, it can be necessary to provide good bonding between the plies. This ability of the reinforcement to withstand stresses in a direction extending transversely relative to the plies (or Z direction) can be obtained in well-known manner by needling together the superposed plies. Nevertheless, needling can be insufficient or difficult to perform. In particular, when using ceramic fibers, for example, needling can have a destructive effect on the fibers, thereby weakening the reinforcement in the plane of the plies.
Multilayer fiber structures are also known in which the bonds between layers are provided by weaving or braiding. Nevertheless, good mechanical strength in the Z direction requires a high concentration of bonds between layers, which leads to a fiber structure that is rigid, and relatively unsuitable for being shaped, even when shaping requires deformation of limited amplitude only.
This drawback is also to be found with fiber structures that are built up from plies that are bonded together by stitching. In addition, for ceramic fiber structures, it is difficult to use a ceramic thread for stitching plies together.
Furthermore, when the parts that are to be manufactured are complex in shape, it can be difficult or even impossible to make a one-piece preform having a shape close to that of the part that is to be manufactured. One known solution then consists in making the preform by assembling a plurality of fiber structures of simpler shape. Effective bonding between the fiber structures must then be achieved in order to ensure that the composite material part does not deteriorate in operation by loss of cohesion in the reinforcing fiber preform.
Document WO 97/06948 describes a method consisting in implanting rigid pins through a structure formed by superposed fiber plies that have been preimpregnated by a resin, or through a plurality of structures for assembling together, each being built up from resin-preimpregnated fiber plies. The pins are initially inserted in a block of compressible material such as an elastomer. The block of compressible material with the pins is brought against a surface of the structure made up of preimpregnated plies. Ultrasound energy is applied to the pins while simultaneously compressing the block in which they are inserted so that the pins are transferred into the structure built up from preimpregnated plies, thereby reinforcing such a structure or bonding it to an underlying structure. A resin matrix composite material part is then obtained by curing the resin.
Such a method is restricted to manufacturing composite materials having an organic matrix. Although document WO 97/06948 does indeed state that pins can be inserted after the resin has been cured, it will nevertheless readily be understood that the method can then only be applied to structures that are thin, unless the pins used are made of a material that is very rigid and strong, in particular metal pins and/or pins of relatively large diameter. Unfortunately, for thermostructural composite material parts that are to be exposed in operation to temperatures that are very high, the use of metal pins is undesirable, either because of the reduced strength of metal at such temperatures, or else because of differential expansion between the metal and the ceramic or carbon components of the composite material. In addition, the use of large-diameter pins can be undesirable because of the non-uniformity they impart to the structure of the composite material.
It is also stated in document WO 97/06948 that the pins can be inserted in dry fiber plies, i.e. plies that have not been preimpregnated. However, with a set of fiber plies or with a plurality of sets of plies that have been joined together, that cannot suffice to confer sufficient strength to enable them to be handled without being deformed. It is then necessary to use tooling in order to conserve the desired shape prior to densifying the fiber plies, which can be expensive and difficult to achieve, particularly when the composite material parts to be made are complex in shape.