12 10 For the B4C/BN microcomposites, the equations of R-curve 1-Ka=6.15-2.99exp(-△C246 behavior: 2-K=5.99-3.38eXp(-△C312) △C 3-K=6.83-349exp(-△C261) KR=599-3.38eXp For the B4C/BN nanocomposites, the equations of KR=6.83-3.49eXp (25) Fig 14 showed that the B4C monolith the B4C/BN microcom- B C monolith posites and the b4 C/BN nanocomposites all exhibited the rising R-curves behavior The B4C/BN nanocomposites exhibited the rel- ative higher rising R-curve behavior in comparison with the B4c B,C/BN nanocomposites monolith and the b4C/BN 600 8001000 3.5. The toughening mechanisms Crack length C(um) Fig. 14. The R-curves behavior of the B4C monolith, the B4C/BN microcom- The B4C monolith, the B4C/BN nanocomposites and the b4 C/BN osites and the BaC/BN nanocomposites expressed by the empirical equation: microcomposites exhibited the rising R-curve behavior. These were attributed to the different toughening mechanisms. The bacmono- lith exhibited the rising R-curve behavior. This was explained from microstructure. Fig. 1(a)showed that the weak phase YAG Y3Al5O12)existed in B4C monolith. During the fracture proces Kk=oo, m /2+XPCm-3/=Kx-(kK-Ko)expm*o when the microcracks reached to the weak phase YAG, the micro- cracks would be deflected and diverged along the weak phase, and prevented from further extending, the length of crack path was P was indentation load, or was fracture strength [26, 33]. The elongated and the work of fracture( wOF)increased. On the other cracks length Cm and Co were calculated by Eqs. (59)and (60)[26]: hand, the B4C monolith exhibited the compact and homogene microstructure, which resulted in the increase of crack propaga- (59) tion resistance. So the fracture toughness of B4 increased with the increase of crack length. The B4C/ BN nanocomposites and the Bac/BN microcomposites β P+1)/2 (60) exhibited the rising R-curves behavior: the toughening mecha- nisms were explained from microstructure Fig 15(a)and(b) show: the TEm crographs of the b4c Bn microcomposites and the so the value of a was calculated according to the above equations. B4C/BN nanocomposites. The weak interface between BAC matrix Fig 14 shows the R-curves behavior of the B4C monolith, the B4 C/B grains and h-BN particles, as well as the microcracks within h-BN microcomposites and the BA C/BN nanocomposites expressed by the particles existed in TEM micrographs. The weak interface between exponent equation B4C matrix grains and h-BN particles, and the microcracks within h-BN particles would improve the toughness of composites Dur- KR= Koo-(Koo-Ko)exp ing the fracture process, the microcracks would propagate along the weak interface between BC matrix grains and h-BN particles For the B4 monolith the equations of r-curve behavior: and propagate along the microcracks within h-BN particles. The cleavage behavior of the h-BN particles would disperse main cracks KR=6.15-299c/-4C (61) into many microcracks and prevent microcracks from extending, so the microcracks would be deflected and diverged and the defle (b) BA eak interfac 400nm 250nm Fig. 15. The TEM micrographs of the B4C BN microcomposites and the B4 C/ BN nanocomposites (a)The TEM micrograph of the Ba C/BN microcomposites; (b) the TEM212 T. Jiang et al. / Materials Science and Engineering A 494 (2008) 203–216 Fig. 14. The R-curves behavior of the B4C monolith, the B4C/BN microcom￾posites and the B4C/BN nanocomposites expressed by the empirical equation: KR = K∞ − (K∞ − K0)exp(− C/ ). KR = ϕfCm1/2 + PCm−3/2 = K∞ − (K∞ − K0) exp  −Cm − C0  P was indentation load, f was fracture strength [26,33]. The cracks length Cm and C0 were calculated by Eqs. (59) and (60) [26]: Cm = 3P ϕf 1/2 (59) Cm = C0  ˇ + 1 ˇ (ˇ+1)/2 (60) so the value of was calculated according to the above equations. Fig. 14 shows the R-curves behavior of the B4C monolith, the B4C/BN microcomposites and the B4C/BN nanocomposites expressed by the exponent equations: KR = K∞ − (K∞ − K0) exp − C For the B4C monolith, the equations of R-curve behavior: KR = 6.15 − 2.99 exp − C 246 (61) For the B4C/BN microcomposites, the equations of R-curve behavior: KR = 5.99 − 3.38 exp − C 312 (62) For the B4C/BN nanocomposites, the equations of R-curve behavior: KR = 6.83 − 3.49 exp − C 261 (63) Fig. 14 showed that the B4C monolith, the B4C/BN microcom￾posites and the B4C/BN nanocomposites all exhibited the rising R-curves behavior. The B4C/BN nanocomposites exhibited the rel￾ative higher rising R-curve behavior in comparison with the B4C monolith and the B4C/BN microcomposites. 3.5. The toughening mechanisms The B4C monolith, the B4C/BN nanocomposites and the B4C/BN microcomposites exhibited the rising R-curve behavior. These were attributed to the different toughening mechanisms. The B4C mono￾lith exhibited the rising R-curve behavior. This was explained from microstructure. Fig. 1(a) showed that the weak phase YAG (Y3Al5O12) existed in B4C monolith. During the fracture process, when the microcracks reached to the weak phase YAG, the micro￾cracks would be deflected and diverged along the weak phase, and prevented from further extending, the length of crack path was elongated and the work of fracture (WOF) increased. On the other hand, the B4C monolith exhibited the compact and homogenous microstructure, which resulted in the increase of crack propaga￾tion resistance. So the fracture toughness of B4C monolith increased with the increase of crack length. The B4C/BN nanocomposites and the B4C/BN microcomposites exhibited the rising R-curves behavior; the toughening mecha￾nisms were explained from microstructure. Fig. 15(a) and (b) shows the TEM micrographs of the B4C/BN microcomposites and the B4C/BN nanocomposites. The weak interface between B4C matrix grains and h-BN particles, as well as the microcracks within h-BN particles existed in TEM micrographs. The weak interface between B4C matrix grains and h-BN particles, and the microcracks within h-BN particles would improve the toughness of composites. Dur￾ing the fracture process, the microcracks would propagate along the weak interface between B4C matrix grains and h-BN particles, and propagate along the microcracks within h-BN particles. The cleavage behavior of the h-BN particles would disperse main cracks into many microcracks and prevent microcracks from extending, so the microcracks would be deflected and diverged, and the deflec￾Fig. 15. The TEM micrographs of the B4C/BN microcomposites and the B4C/BN nanocomposites. (a) The TEM micrograph of the B4C/BN microcomposites; (b) the TEM micrograph of the B4C/BN nanocomposites.