146 F.F. Lange et al./ Materials Science and Engineering A195(1995)145-150 volume between the particles after pyrolysis, the matrix powder is packed from the slurry state by pressure phase contains a residual void phase even after multi- filtration, with interparticle forces controlled by surface ple infiltration-pyrolysis chemical methods through pH control, by polyelectro- We begin this review by describing the mechanical lytes etc. The highest packing density is achieved when properties of partially dense materials produced from the particles are repulsive. Whether long range or short powders to show that a porous matrix can be strong. range, the repulsive fo rce ac cts as a lubricant to allow Secondly, it will be shown that the packing density of particles to rearrange and pack to their highest density particles around fibers is highest when the particle-to- [26, 27. Also, above a critical pressure, their packing fiber diameter ratio is small. Thirdly, the kinetics and density is not affected by the applied pressure [26, 27 mechanics of the infiltration-pyrolysis process will be In contrast, the packing density achieved in bodies described. Lastly, the damage-tolerant mechanical produced from flocced slurries is much lower and pres- behavior of two composite systems will be demon- sure sensitive because strongly attractive interparticle forces produce a cohesive, connective network before they are packed[26, 27. In effect, the coefficient friction between particles is high when they are 2. Mechanical properties of partially dense strongly attractive and particle rearrangement during materials formed from powders packing is inhibited. Dry powders behave much like the flocced slurry. Repulsive particles are required to Porous materials can have high specific mechanical achieve a high packing density properties [9. Such solids, when formed from The particle-to-fiber diameter ratio R greatly affects powders, transmit force through structural units. These the packing density of particles around fibers [25]. One units have morphological characteristics which differ reason for the lower packing density is the"wall effect from those for cellular materials [10-24] and, unlike shown in Fig. 1(a). When particles are packed against a cellular materials, the mechanical properties are linear wall, such as a fiber surface, extra spaces exist that functions of porosity [24 when normalized by the would have been partially filled with particles if the corresponding values for the fully dense material. The surface did not exist. Zok et al. 28 have shown that the normalized Young's modulus E and the critical stress extra void volume introduced by the "wall effect intensity factor K are given by [24] increases with the ratio r. when r>0.1 a limited number of particles can fill the interstices between the E - and K (1) fibers as shown in Fig. 1(b). The geometrical restriction powder and inhibits the application of powder where po is the relative density of the initial powder methods in forming composite systems unless sub- compact. Similar relations are also obtained when micron particles of the desired powder can be increased density is achieved by cyclic precursor infil- obtained tration and pyrolysis [6]. Hence, both E and Kc can be optimized by starting with a powder compact having he highest possible particle packing density. More importantly, the strength of the partially dense body is controlled by the size of the crack-like flaws Fiber that pre-exist within the initial powder compact [24] Thus, partially dense powder compacts(and matrices in fiber-reinforced composites) could be strong, residual porosity, if the particles were packed Fiber disrupt particle packing Missing to a high relative density and the flaw size within the at surface Wall Effect Volume powder was minimized 3. Packing of particles and reinforcements Increasing particle fiber diame ter Ratio Particle morphology, interparticle forces and the Fig. 1. (a)The packing of particles at the fiber surface particle size distribution are the major factors control- a greater amount of porosity because of the"wall"eff ling the density to which mass can be consolidated the form of powders [25]. The effect of interparticle eter ratio is above 0.1, fewer particles can be packed within the forces on packing density is best illustrated when fiber interstices146 E E Lange et al. / Materials Science and Engineering A195 (1995) 145-150 volume between the particles after pyrolysis, the matrix phase contains a residual void phase even after multi￾ple infiltration-pyrolysis. We begin this review by describing the mechanical properties of partially dense materials produced from powders to show that a porous matrix can be strong. Secondly, it will be shown that the packing density of particles around fibers is highest when the particle-to￾fiber diameter ratio is small. Thirdly, the kinetics and mechanics of the infiltration-pyrolysis process will be described. Lastly, the damage-tolerant mechanical behavior of two composite systems will be demon￾strated. 2. Mechanical properties of partially dense materials formed from powders Porous materials can have high specific mechanical properties [9]. Such solids, when formed from powders, transmit force through structural units. These units have morphological characteristics which differ from those for cellular materials [10-24] and, unlike cellular materials, the mechanical properties are linear functions of porosity [24] when normalized by the corresponding values for the fully dense material. The normalized Young's modulus /~ and the critical stress intensity factor/(c are given by [24] E =P-Po and I?2c =p-p~° (1) 1-P0 1-P0 where P0 is the relative density of the initial powder compact. Similar relations are also obtained when increased density is achieved by cyclic precursor infil￾tration and pyrolysis [6]. Hence, both/~ and/(c can be optimized by starting with a powder compact having the highest possible particle packing density. More importantly, the strength of the partially dense body is controlled by the size of the crack-like flaws that pre-exist within the initial powder compact [24]. Thus, partially dense powder compacts (and matrices in fiber-reinforced composites) could be strong, despite residual porosity, if the particles were packed to a high relative density and the flaw size within the powder was minimized. 3. Packing of particles and reinforcements Particle morphology, interparticle forces and the particle size distribution are the major factors control￾ling the density to which mass can be consolidated in the form of powders [25]. The effect of interparticle forces on packing density is best illustrated when powder is packed from the slurry state by pressure filtration, with interparticle forces controlled by surface chemical methods through pH control, by polyelectro￾lytes etc. The highest packing density is achieved when the particles are repulsive. Whether long range or short range, the repulsive force acts as a lubricant to allow particles to rearrange and pack to their highest density [26,27]. Also, above a critical pressure, their packing density is not affected by the applied pressure [26,27]. In contrast, the packing density achieved in bodies produced from flocced slurries is much lower and pres￾sure sensitive because strongly attractive interparticle forces produce a cohesive, connective network before they are packed [26,27]. In effect, the coefficient of friction between particles is high when they are strongly attractive and particle rearrangement during packing is inhibited. Dry powders behave much like the flocced slurry. Repulsive particles are required to achieve a high packing density. The particle-to-fiber diameter ratio R greatly affects the packing density of particles around fibers [25]. One reason for the lower packing density is the "wall effect", shown in Fig. l(a). When particles are packed against a wall, such as a fiber surface, extra spaces exist that would have been partially filled with particles if the surface did not exist. Zok et al. [28] have shown that the extra void volume introduced by the "wall effect" increases with the ratio R. When R >0.1, a limited number of particles can fill the interstices between the fibers as shown in Fig. l(b). The geometrical restriction of particle packing limits the packing density of the powder and inhibits the application of powder methods in forming composite systems unless sub￾micron particles of the desired powder can be obtained. Fiber disrupts particle packing Missing at surface = Wall Effect Particle Volume Increasing particle/fiber diameter Ratio ) Fig. I. (a) The packing of particles at the fiber surface introduces a greater amount of porosity because of the "wall" effect where portions of particles (dark portions of particles) would have existed if fiber was missing. (b) When the particle-to-fiber diam￾eter ratio is above 0.1, fewer particles can be packed within the fiber interstices