K. K. Chawla 4. INTERFACIAL BONDING IN METAL MATRIX COMPOSITES We provide a summary of salient features of the interfacial region in some of the ost important metal matrix composites 4.1. Crystallographic nature In crystallographic terms, ceramic/metal interfaces in composites are, generally, inco- herent and high-energy interfaces. Accordingly, they can act as very efficient vacancy sinks, provide rapid diffusion paths, segregation sites, sites of heterogeneous precipita- tion, as well as sites for precipitate free zones. Among the possible exceptions to this are the eutectic composites [3] and the newer XDm type particulate composites [4] 4.2. Mechanical bonding Some bonding must exist between the ceramic reinforcement and the metal matrix for load transfer from matrix to fiber to occur. two main categories of bonding are mechanical and chemical. Mechanical keying effect between two surfaces can lead to bonding. Hill et al. [5] confirmed this experimentally for tungsten filaments in an aluminum matrix while Chawla and Metzger [6] observed mechanical gripping effects at AlO3/Al interfaces. The results of Chawla and Metzger [6] are shown in Fig. 1 in the form of the linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness. The main message of this figure is that the crack density continues to increase to larger strain values in the case of a rough interface( deeply etched pits) vis-a-vis smooth or not very rough interface, i. e. the rougher the interface, the stronger the mechanical We can make an estimate of the radial stress at the fiber/ matrix interface due to roughness induced gripping [7] Emer E(1+m)+Em(1-) where E is Youngs modulus, v is Poisson's ratio, A is the amplitude of roughness, r is the radius of the fiber, and the subscripts m and f indicate matrix and fiber respectively. For a given composite, the compressive radial stress increases with the roughness amplitude and decreases with the fiber radius. Al uch an MMC, i.e. non-reacting components with a purely mechanical bond at the interface, is the filamentary superconducting composite consisting of niobium-titanium alloy filaments in a copper matrix 4.3. Chemical bon Ceramic/metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. One needs to have knowledge of the chemical reaction products and, if possible, their properties. Molten290 4. INTERFACIAL BONDING IN METAL MATRIX COMPOSITES We provide a summary of salient features of the interfacial region in some of the most important metal matrix composites. 4.1. Crystallographic nature In crystallographic terms, ceramic/metal interfaces in composites are, generally, inco￾herent and high-energy interfaces. Accordingly, they can act as very efficient vacancy sinks, provide rapid diffusion paths, segregation sites, sites of heterogeneous precipita￾tion, as well as sites for precipitate free zones. Among the possible exceptions to this are the eutectic composites [3] and the newer XDTM type particulate composites [4]. 4.2. Mechanical bonding Some bonding must exist between the ceramic reinforcement and the metal matrix for load transfer from matrix to fiber to occur. Two main categories of bonding are mechanical and chemical. Mechanical keying effect between two surfaces can lead to bonding. Hill et al. [5] confirmed this experimentally for tungsten filaments in an aluminum matrix while Chawla and Metzger [6] observed mechanical gripping effects at A1203 /A1 interfaces. The results of Chawla and Metzger [6] are shown in Fig. 1 in the form of the linear density of cracks in alumina as a function of strain in an alumina/aluminum composite for different degrees of interface roughness. The main message of this figure is that the crack density continues to increase to larger strain values in the case of a rough interface (deeply etched pits) vis-a-vis smooth or not very rough interface, i.e. the rougher the interface, the stronger the mechanical bonding. We can make an estimate of the radial stress at the fiber/matrix interface due to roughness induced gripping [7] where E is Young's modulus, v is Poisson's ratio, A is the amplitude of roughness, r is the radius of the fiber, and the subscripts m and f indicate matrix and fiber, respectively. For a given composite, the compressive radial stress increases with the roughness amplitude and decreases with the fiber radius. An important example of such an MMC, i.e. non-reacting components with a purely mechanical bond at the interface, is the filamentary superconducting composite consisting of niobium-titanium alloy filaments in a copper matrix. 4.3. Chemical bonding Ceramic/metal interfaces are generally formed at high temperatures. Diffusion and chemical reaction kinetics are faster at elevated temperatures. One needs to have knowledge of the chemical reaction products and, if possible, their properties. Molten