Boundary face between fiber and matrix
What, then, is the ideal design of the fiber/matrix boundary face? The most significant characteristic is that, the fibers remain uncut even if the matrix (i.e., ceramic-fixing fibers) is destroyed. Reference for such design is represented by the ratio of destruction-resistant indexes of matrix and fiber. We use a value called energy-release rate, which is used in fracture dynamics, as a destruction-resistant index. The energy-release rate denotes the energy spent to make cracks per unit of area. Material with a large energy-release rate indicates that a great amount of energy is required to destroy it.
Then, assuming that the crack starts in the matrix and extends to the fibers, measures to prevent fibers from breaking are necessary to retain damage tolerance. To this end, we need to set up an appropriate ratio between the energy-release rate necessary for cracks to extend to fibers, and the energy-release rate necessary for cracks to extend along fibers. The ratio of these two energy-release rates can be obtained by numerical analysis and it is determined from the elasticities of fiber and matrix. Although the value varies according to variation of combined materials, one reference is its lowest value. The lowest value is given when two semi-infinite plates adhere to each other and the elasticity of fiber and matrix is the same. And when the energy-release rate to extend along fibers is less than about one quarter of the energy-release rate necessary to break fibers, the direction of the crack extension changes without cutting fibers. Based on the above premise, we should set up, as one reference, the energy-release rate of the fiber/matrix boundary face to below one quarter of the matrixEenergy-release rate.
When joining ceramics to ceramics directly, the strength of the boundary becomes the same as that of ceramics and, therefore, we cannot satisfy the condition that it must be one quarter of the energy-release rate of ceramics. To meet the condition, we changed the design to create a weak boundary layer on the fiber/matrix boundary face in case of fiber-reinforced ceramics. What kind of material should we select to build a weak boundary face? As stated above, ceramic does not have a large energy-release rate. The boundary face must also have a small energy-release rate of less than one quarter, and its material must have high resistance to heat like ceramics. So we adopted a thermal-resistant material, which has weak direction to crystal structure such as carbon and boron nitride, and aligned the weak direction along the fibers. Thus, we were able to realize a boundary layer with ideal conditions.
The composite material called carbon-fiber-reinforced carbon (C/C) is made of carbon crystal (graphite crystal), where the matrixEcarbon or carbon fiber possesses such a boundary property in its own right. Therefore, unlike SiC composite materials, we need not care about the boundary layer providing damage tolerance. Nevertheless, the importance of control of the fiber/matrix boundary face is still valid because the mechanical properties of the C/C material vary depending on the adhesiveness of the boundary face. Boundary control is important for both fiber-reinforced ceramics and carbon, and therefore R&D on this matter continues.
Current applications and further advancement
Although there are various heat-resistant composite materials with damage tolerance, actual applications are unfortunately rare. This is because of the problem with strength deterioration caused by reaction with oxygen at high temperatures. Although C/C materials are used relatively frequently in the aerospace field, actual applications are few, namely: nozzles (Fig. 3) for solid rockets such as M-V, S-520 and SRB-A; nose cones and leading edges of the Space Shuttle; and aircraft brake disks. These applications only take advantage of carbons high heat-resistance, which cannot be replicated by other materials. With fiber-reinforced ceramics (e.g., C/SiC) or C/C using carbon fibers, the fibers themselves deteriorate due to the generation of CO or CO2 gases in reaction with high-temperature oxygen. Apart from special applications where the material consumes itself, antioxidative treatment by surface coating on ceramics is required for reuse, as with the Space Shuttle. A reliable coating technique still remains to be developed.