1. Field of the Invention
The present invention relates to a composite material having a reinforcing texture in refractory fibers and ceramic matrix with multiple interphases between the fibers of the texture and the matrix.
By refractory fibers is meant here fibers in carbon or in ceramic such as, for example the fibers constituted essentially of silicon carbide.
2. Prior Art
In the composite materials with refractory fibers and ceramic matrix, the fiber-matrix bond conditions the transfer of load and, as a result, the characteristics as well as the mechanical behavior of the materials.
One advantageous way of controlling the load transfer between fiber and matrix consists in interposing a fine layer of a material having a lamellar structure oriented in parallel to the axis of the fiber, as described in U.S. Pat. No. 4,752,503. The layer having the lamellar structure is in a material selected from rough laminar pyrolytic carbon (RL) PyC and boron nitride (BN). As shown diagrammatically in FIG. 1, the layer having a laminar structure constitutes an interphase LI defining two interfaces: one interface between fiber F and the lamellar interphase LI and one interface between said latter and the matrix M.
Without the lamellar interphase, a crack starting in the matrix M spreads directly through the fiber F, as shown in FIG. 2, this leading to a premature breaking of the fiber. The material has a fragile behavior.
When, on the contrary, there is a lamellar interphase, this prevents any cracks starting in the matrix from spreading directly through the fiber. Due to its resiliency under shear stress, the lamellar interphase permits a relaxing of the stresses exerted on the bottom of cracks. The material has a non-fragile behavior and improved mechanical properties, as shown by curve II in FIG. 3 which shows the relation between elongation and tensile strength. By way of comparison, curve I in FIG. 3 shows this relation in the case of a material without lamellar interphase.
The crack remains stopped in the lamellar interphase as long as the level of stress exerted on the bottom of the crack does not exceed the breaking strength of the weakest of the elements found in the immediate vicinity of the crack. Three elements have to be taken into account: the material constituting the lamellar interphase, the matrix-lamellar interphase interface and the fiber-lamellar interphase interface. Depending on which one of said elements has the lowest breaking strength, the progression of the crack will follow path a, b, or c, respectively, when the stress on the bottom of the crack increases, as illustrated in FIG. 4.
The most dangerous progression is that following path c, namely on the fiber-lamellar interphase interface. Indeed, the crack then can reach into the fiber and break it, if it meets with any surface defect of said fiber, which will cause a reduction of the mechanical properties of the composite material.