510 FABER EXAMPLES OF INTERFACE DESIGN In the last twenty years, a variety of concepts have been explored to provide the requisite low-fracture energy interface. Initial concepts concentrated on naturally lubricious materials, carbon and boron nitride. In addition to their easy cleavage planes, these materials also provide a compliant layer for easing asperity-asperity interaction between fiber and matrix. More recent work has been driven by the desire for coatings that can be used in oxidizing environ- ments and can maintain high-temperature stability in oxide systems. Those having most promise are the layered oxides, which demonstrate high-fracture anisotropy, and monazite systems, which are nonreactive with oxide fibers and matrices and have exceedingly low-interfacial fracture toughness. An alternate concept relies on the fact that the fracture energy of a material scales with den- sity, i.e. no energy is required to extend a crack across a void or pore. Hence low-toughness interfaces should be feasible in a porous system, or by total elim- ination ofthe interface through vaporization. Such concepts are explored below Carbon and boron Nitride layers The bulk of work on interfaces for ceramic-matrix composites has focused on carbon and boron nitride. The importance of carbon coatings was first recognized by Brennan Prewo(2)in SiC-reinforced lithium alumino-silicate (LAS)glass-ceramic matrices, which demonstrated exemplary strength and toughness. The carbon layer, in this case, is formed in situ, during processing in the presence of glass-ceramic nucleating agents, as shown in Figure 6a( Formation of the carbon layer in these materials has been described by Qi et al (47). They model the kinetics of carbon formation, which includes the nass transport of O2 and Co reaction products in the interface region. The Matrix Matrix NbC SiO interface Carbon interface Fiber 0.25m(B) Fiber 0.25μm Figure 6 Transmission electron micrograph of the interfacial region of a lithium-alumino-silicate matrix and Nicalon SiC fiber:(a)as-processed and(b)after 100 h at 850C in oxygen(fromP1: ARK/MBL/rkc P2: MBL/vks QC: MBL/agr T1: MBL May 16, 1997 13:47 Annual Reviews AR034-16 510 FABER EXAMPLES OF INTERFACE DESIGN In the last twenty years, a variety of concepts have been explored to provide the requisite low-fracture energy interface. Initial concepts concentrated on naturally lubricious materials, carbon and boron nitride. In addition to their easy cleavage planes, these materials also provide a compliant layer for easing asperity-asperity interaction between fiber and matrix. More recent work has been driven by the desire for coatings that can be used in oxidizing environ￾ments and can maintain high-temperature stability in oxide systems. Those having most promise are the layered oxides, which demonstrate high-fracture anisotropy, and monazite systems, which are nonreactive with oxide fibers and matrices and have exceedingly low-interfacial fracture toughness. An alternate concept relies on the fact that the fracture energy of a material scales with den￾sity, i.e. no energy is required to extend a crack across a void or pore. Hence, low-toughness interfaces should be feasible in a porous system, or by total elim￾ination of the interface through vaporization. Such concepts are explored below. Carbon and Boron Nitride Layers The bulk of work on interfaces for ceramic-matrix composites has focused on carbon and boron nitride. The importance of carbon coatings was first recognized by Brennan & Prewo (2) in SiC-reinforced lithium alumino-silicate (LAS) glass-ceramic matrices, which demonstrated exemplary strength and toughness. The carbon layer, in this case, is formed in situ, during processing in the presence of glass-ceramic nucleating agents, as shown in Figure 6a (46). Formation of the carbon layer in these materials has been described by Qi et al (47). They model the kinetics of carbon formation, which includes the mass transport of O2 and CO reaction products in the interface region. The Figure 6 Transmission electron micrograph of the interfacial region of a lithium-alumino-silicate matrix and NicalonTM SiC fiber: (a) as-processed and (b) after 100 h at 850◦C in oxygen (from 46)