1.1 Introduction 3 thermal conductivity makes them very attractive for electronic packaging (e.g., heat sinks).Besides good thermal properties,their low density makes them par- ticularly desirable for aerospace electronics and orbiting space structures;orbiters are thermally cycled by moving through the Earth's shadow. Compared to the metal itself,a carbon fiber metal-matrix composite is char- acterized by a higher strength-to-density ratio (i.e.,specific strength),a higher modulus-to-density ratio (i.e.,specific modulus),better fatigue resistance,bet- ter high-temperature mechanical properties(a higher strength and a lower creep rate),a lower CTE,and better wear resistance. Compared to carbon fiber polymer-matrix composites,a carbon fiber metal- matrix composite is characterized by higher temperature resistance,higher fire resistance,higher transverse strength and modulus,a lack of moisture absorp- tion,a higher thermal conductivity,a lower electrical resistivity,better radiation resistance,and absence of outgassing. On the other hand,a metal-matrix composite has the following disadvantages compared to the metal itself and the corresponding polymer-matrix composite: higher fabrication cost and limited service experience. Fibers used for load-bearing metal-matrix composites are mostly in the form of continuous fibers,but short fibers are also used.The matrices used include aluminum,magnesium,copper,nickel,tin alloys,silver-copper,and lead alloys. Aluminum is by far the most widely used matrix metal because of its low density, low melting temperature(which makes composite fabrication and joining relatively convenient),low cost,and good machinability.Magnesium is comparably low in melting temperature,but its density is even lower than aluminum.Applications include structures(aluminum,magnesium),electronic heat sinks and substrates (aluminum,copper),soldering and bearings(tin alloys),brazing (silver-copper), and high-temperature applications(nickel). Although cement is a ceramic material,ceramic-matrix composites usually refer to those with silicon carbide,silicon nitride,alumina,mullite,glasses and other ceramic matrices that are not cement. Ceramic-matrix fiber composites are gaining increasing attention because the good oxidation resistance of the ceramic matrix(compared to a carbon matrix) makes the composites attractive for high-temperature applications (e.g.,aerospace and engine components).The fibers serve mainly to increase the toughness and strength(tensile and flexural)of the composite due to their tendency to be partially pulled out during the deformation.This pullout absorbs energy,thereby tough- ening the composite.Although the fiber pullout is advantageous,the bonding between the fibers and the matrix must still be sufficiently strong for the fibers to strengthen the composite effectively.Therefore,control over the bonding between the fibers and the matrix is important for the development of these composites. When the reinforcement is provided by carbon fibers,the reinforcement has a second function,which is to increase the thermal conductivity of the composite, as the ceramic is mostly thermally insulating whereas carbon fibers are thermally conductive.In electronic,aerospace,and engine components,the enhanced ther- mal conductivity is attractive for heat dissipation.1.1 Introduction 3 thermal conductivity makes them very attractive for electronic packaging (e.g., heat sinks). Besides good thermal properties, their low density makes them par￾ticularly desirable for aerospace electronics and orbiting space structures; orbiters are thermally cycled by moving through the Earth’s shadow. Compared to the metal itself, a carbon fiber metal-matrix composite is char￾acterized by a higher strength-to-density ratio (i.e., specific strength), a higher modulus-to-density ratio (i.e., specific modulus), better fatigue resistance, bet￾ter high-temperature mechanical properties (a higher strength and a lower creep rate), a lower CTE, and better wear resistance. Compared to carbon fiber polymer-matrix composites, a carbon fiber metal￾matrix composite is characterized by higher temperature resistance, higher fire resistance, higher transverse strength and modulus, a lack of moisture absorp￾tion, a higher thermal conductivity, a lower electrical resistivity, better radiation resistance, and absence of outgassing. On the other hand, a metal-matrix composite has the following disadvantages compared to the metal itself and the corresponding polymer-matrix composite: higher fabrication cost and limited service experience. Fibers used for load-bearing metal-matrix composites are mostly in the form of continuous fibers, but short fibers are also used. The matrices used include aluminum, magnesium, copper, nickel, tin alloys, silver-copper, and lead alloys. Aluminum is by far the most widely used matrix metal because of its low density, lowmeltingtemperature(whichmakescompositefabricationandjoiningrelatively convenient), low cost, and good machinability. Magnesium is comparably low in melting temperature, but its density is even lower than aluminum. Applications include structures (aluminum, magnesium), electronic heat sinks and substrates (aluminum, copper), soldering and bearings (tin alloys), brazing (silver-copper), and high-temperature applications (nickel). Although cement is a ceramic material, ceramic-matrix composites usually refer to those with silicon carbide, silicon nitride, alumina, mullite, glasses and other ceramic matrices that are not cement. Ceramic-matrix fiber composites are gaining increasing attention because the good oxidation resistance of the ceramic matrix (compared to a carbon matrix) makes the composites attractive for high-temperature applications (e.g., aerospace and engine components). The fibers serve mainly to increase the toughness and strength (tensile and flexural) of the composite due to their tendency to be partially pulled out during the deformation. This pullout absorbs energy, thereby tough￾ening the composite. Although the fiber pullout is advantageous, the bonding between the fibers and the matrix must still be sufficiently strong for the fibers to strengthen the composite effectively. Therefore, control over the bonding between the fibers and the matrix is important for the development of these composites. When the reinforcement is provided by carbon fibers, the reinforcement has a second function, which is to increase the thermal conductivity of the composite, as the ceramic is mostly thermally insulating whereas carbon fibers are thermally conductive. In electronic, aerospace, and engine components, the enhanced ther￾mal conductivity is attractive for heat dissipation