The fiber-matrix interfacial domain is a critical part of composites because load transfers from the matrix

to the fiber and vice versa occur through the interface. Most authors support the concept of weak interfaces to increase fracture toughness. They assign toughening to crack-bridging and fiber pullout. Weak interfaces are detrimental to the composite strength. A high strength requires that the matrix carry a part of the load. This is obtained with strong interfaces, which implies that the deflection cracks at interfaces are short and/or that significant sliding friction takes place. These latter requirements, to be met for strong composites, are therefore incompatible with the former ones for tough composites, if toughening is based solely upon crack-bridging and fiber pullout.

Fiber-matrix interfaces exert a profound influ­ence on the mechanical behavior and lifetime of composites. Efforts have been directed toward opti­mization of interface properties. Fiber-matrix inter­faces in CVI SiC/SiC composites consist of a thin coating layer (<1-pm thick) of one or several materi­als deposited on the fiber (interphase). CVI SiC/SiC composites with rather strong interfaces have been obtained using fibers that have been treated in order to increase the fiber/coating bond.35,42 The concept of strong interfaces has been established on CVI SiC/SiC composites with PyC and multilayered (PyC/SiC)n fiber coatings. Less interesting results have been achieved with BN interphases.43 Table 6 gives the various values of the interfacial shear stres­ses measured using various methods on CVI SiC/SiC composites with PyC-based fiber coatings: the inter­facial shear stresses range between 10 and 20 MPa for the weak interfaces, whereas they are larger than 100-300 MPa for the strong interfaces.43-47

In the presence of a weak bond between the fibers and the matrix or coating, single, long interface cracks are created during matrix cracking (adhesive failure type, Figure 10). The associated interface shear stresses are low, and load transfers through the interface crack are poor. The matrix is subjected to low stresses and the volume of matrix that may experience further cracking is reduced by the pres­ence of the long interface cracks. The matrix crack density is small. The crack spacing at saturation as well as the pull-out length tends to be long (>100 pm). Toughening results essentially from slid­ing friction along the cracked interfaces. However, as a result of matrix unloading due to long interface cracks, the fibers carry most of the load, which reduces the composite strength. The corresponding tensile stress-strain curve exhibits a narrow curved domain limited by a stress at matrix saturation which is distinctive of the ultimate strength (Figure 1).

Source: Rebillat, F.; Lamon, J.; Guette, A. Acta Mater. 2000, 48, 4609-4618; Lamon, J.; Rebillat, F.; Evans, A. G. J. Am. Ceram. Soc. 1995, 78, 401-405; Lissart, N.; Lamon, J. Acta Mater. 1997, 45, 1025; Rebillat, F.; Lamon, J.; Naslain, R.; Lara-Curzio, E.; Ferber, M. K.; Besmann, T. J. Am. Ceram. Soc. 1998, 81, 965; Rebillat, F.; Lamon, J.; Naslain, R.; Lara-Curzio, E.; Ferber, M. K.; Besmann, T. J. Am. Ceram. Soc. 1998, 81, 2315-2326.

In the presence of stronger fiber/coating bonds, the matrix cracks are deflected within the coating (cohesive failure type, Figure 10) into short and branched multiple cracks. Short interphase cracks as well as improved load transfers allow further cracking of the matrix via a scale effect, leading to a higher density of matrix cracks (which are slightly opened). Sliding friction within the coating as well as multiple cracking of the matrix increases energy absorption, leading to toughening. Short interphase cracks and improved load transfers reduce the load carried by the fibers, leading to strengthening. The associated tensile stress-strain curve exhibits a wide, curved domain and the stress at matrix cracking saturation is close to the composite strength (Figure 1).

The interphase is ineffective when fiber surface is too rough, although deflection of matrix cracks occurs. Because of strong fiber-matrix interactions in the interface cracks, premature fracture of com­posite occurs under small strains close to the strain at proportional limit. This phenomenon is observed on CVI SiC/SiC reinforced with Tyranno-SA3 fibers.38