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500 tion load, the fracture strength was related to the indentation load, 1-a,=465P6 because that the fracture strength was controlled by external flaws. 2-,=352P13 Fig 5 showed that the intersection points P"were different for the B4C monolith(P=2 N), the B4C/BN microcomposites(P=2N 3,=503P1 and the B4 C/BN nanocomposites (P*=3 N). This result indicated that the b4C/bn nanocomposites have the higher damage resistance than that of the BC monolith and the bac/BN microcompos- c巴 ites under the lower indentation load. As seen in Fig. 5, in the high-indentation-load region, the fracture strength of three mate- rials decreased linearly with the increase of the indentation load. 150 ■ BC monolith The B4C/BN nanocomposites maintained relative higher fracture strength in comparison with the B4C monolith and the B4C/BN microcomposites under the equivalent indentation load. The B4C 3▲ B,C/BN nanocomposites monolith retained higher fracture strength than that of the B4 C/BN microcomposites under the equivalent indentation load. These 1000 results indicated that the b4 C/ Bn nanocomposites have improved damage resistance in comparison with the B4C monolith and the bac/Bn microcomposites. It was suggested that the bc/Bn dentation load for the Ba nanocomposites would have the higher rising R-curve behavior than that of the B4 C monolith and the B4 C/BN microcomposites. R-curves behaviors of three materials were obtained from the indentation-strength data in Fig. 5. Linear regression was used to alculate the fit equation of each line, and the slope of each line content of h-BN. Fig. 4 showed that the fracture strength of the was calculated. The slope of the fit equation of the B4Cmonolith was BaC/BN nanocomposites was significantly improved in compari-.216, the slope of fit equation of the B4 C/BN nanocomposites was on with the B4C/BN microcomposites. The fracture strength of the -0.186 and the slope of fit equation of the B4C/BN microcomposites BAC monolith was 409 MPa. The fracture strength of the B4C/BN was -0.173. As seen in Fig. 5. in the high indentation load region, nanocomposites with the h-BN contents of 10 wt% and 20 wt% the relation of fracture strength of and indentation load P can be vere 424 MPa and 415 MPa, respectively. The fracture strength of expressed as following equations: for the B4c monolith the bacOn micr osites with the h-bn contents of 10wt% and 20 wt. were 360MPa and 327 MPa, respectively. The increase of=465P of fracture strength of the Bac/Bn nanocomposites was chiefly for the B4C/BN nanocomposites flaw size caused by the nano-sized h-BN particles homogenous of dispersions. Fig. 4 shows the effects of h-BN content on the fracture for the B4 C/BN microcomposites toughness of the B4C/BN nanocomposites and the B4C/BN micro- 0=352P-0 73 composites. The fracture toughness of the Bac/ BNmicrocomposites nd the B4C/Bn nanocomposites decreased gradually with the The slopes of fitequations of these three materials were all larger ncreasing content of h-BN. Fig 4 showed that the fracture tough 1/3; this result indicated that these three materials all have ness of the BAC/BN nanocomposites was significantly improve the rising R-curves behavior in comparison with the B4C/BN microcomposites. The fracture Griffith materials. for which the R-curve behavior was flat toughness of the B4C monolith was 5.369 MPam /2. The fracture would follow the power law [19-22] toughness of the b4 C/BN nanoc sites with the h-BN contents araP-k of 10 wt% and 20 wt% were 6.085 MPam1 2 and 5.34 MPam/2 respectively. The fracture toughness of the B4C/BN microcom- with k=1/ 3. The specimens have the rising toughness curves(r- posites with the h-BN contents of 10 wt% and 20wt were curves behavior), the relationship between or and Pwas expressed 4.57MPam'/ and 4.35 m /2, respectively. The increase of the as follows[19-22] fracture toughness of the B, C/BN nanocomposites was because that the nano-sized h-bN particles dispersed main cracks into many ng, which where P<1/3, or was fracture strength, P was indentation load. In this research, according to the results of the fit Eqs. (4-6). the facts that the calculated p values of three materials were all lower 3.4. The indentation strength and the r-curve behavior than 1/3, this result indicated that the B4C monolith, the B4 C/BN nanocomposites and the B4 C/BN microcomposites would have the Fig 5 shows the effects of the indentation load (P)on the frac- rising R-curves behavior If the Vickers cracks geometry was con- ure strength(or)of the B4Cmonolith, the B4C/BN microcomposites sidered to be materials independent, the values =1.24[19,20, 26] (20 wt% h-BN)and the B4C/BN nanocomposites(20 wt% h-BN) x values were calculated as the follows the two logarithmically axis systems. Fig. 5 showed that the intersection points P in the corresponding curves(at P=p") sep- X=s(H arated the plots into two regions for these three materials. In the left region, for specimens with the low indentation load, the frac- S=0.016[ 19, 20, 24]. x for the B4C monolith was 0.0737, X for ure strength was microstructure controlled. The fracture strength the B4C/BN nanocomposites was 0.0823, x for the B4 C/BN micro- retained high values under the low indentation load. On the other composites was 0.0923. The same values of and different values hand, in the right regions, where specimens had higher indenta- of x were used. According to Eq (1). the three families of206 T. Jiang et al. / Materials Science and Engineering A 494 (2008) 203–216 Fig. 5. The fracture strength plotted against the indentation load for the B4C monolith, the B4C/BN nanocomposites (20 wt.% h-BN) and the B4C/BN microcomposites (20 wt.% h-BN). The curves for the three materials deviated slightly from the slope of −1/3. The indentation loads in the figure were from 0.1 N to 294 N. content of h-BN. Fig. 4 showed that the fracture strength of the B4C/BN nanocomposites was significantly improved in comparison with the B4C/BN microcomposites. The fracture strength of the B4C monolith was 409 MPa. The fracture strength of the B4C/BN nanocomposites with the h-BN contents of 10 wt.% and 20 wt.% were 424 MPa and 415 MPa, respectively. The fracture strength of the B4C/BN microcomposites with the h-BN contents of 10 wt.% and 20 wt.% were 360 MPa and 327 MPa, respectively. The increase of fracture strength of the B4C/BN nanocomposites was chiefly attributed to the refined B4C matrix grains and the reduction of flaw size caused by the nano-sized h-BN particles homogenous dispersions. Fig. 4 shows the effects of h-BN content on the fracture toughness of the B4C/BN nanocomposites and the B4C/BN microcomposites. The fracture toughness of the B4C/BN microcomposites and the B4C/BN nanocomposites decreased gradually with the increasing content of h-BN. Fig. 4 showed that the fracture toughness of the B4C/BN nanocomposites was significantly improved in comparison with the B4C/BN microcomposites. The fracture toughness of the B4C monolith was 5.369 MPa m1/2. The fracture toughness of the B4C/BN nanocomposites with the h-BN contents of 10 wt.% and 20 wt.% were 6.085 MPa m1/2 and 5.34 MPa m1/2, respectively. The fracture toughness of the B4C/BN microcomposites with the h-BN contents of 10 wt.% and 20 wt.% were 4.57 MPa m1/2 and 4.35 MPa m1/2, respectively. The increase of the fracture toughness of the B4C/BN nanocomposites was because that the nano-sized h-BN particles dispersed main cracks into many microcracks and prevented main cracks from extending, which improved fracture toughness. 3.4. The indentation strength and the R-curve behavior Fig. 5 shows the effects of the indentation load (P) on the fracture strength (f) of the B4C monolith, the B4C/BN microcomposites (20 wt.% h-BN) and the B4C/BN nanocomposites (20 wt.% h-BN) in the two logarithmically axis systems. Fig. 5 showed that the intersection points P* in the corresponding curves (at P = P*) separated the plots into two regions for these three materials. In the left region, for specimens with the low indentation load, the fracture strength was microstructure controlled. The fracture strength retained high values under the low indentation load. On the other hand, in the right regions, where specimens had higher indentation load, the fracture strength was related to the indentation load, because that the fracture strength was controlled by external flaws. Fig. 5 showed that the intersection points P* were different for the B4C monolith (P* = 2 N), the B4C/BN microcomposites (P* = 2 N) and the B4C/BN nanocomposites (P* = 3 N). This result indicated that the B4C/BN nanocomposites have the higher damage resistance than that of the B4C monolith and the B4C/BN microcomposites under the lower indentation load. As seen in Fig. 5, in the high-indentation-load region, the fracture strength of three materials decreased linearly with the increase of the indentation load. The B4C/BN nanocomposites maintained relative higher fracture strength in comparison with the B4C monolith and the B4C/BN microcomposites under the equivalent indentation load. The B4C monolith retained higher fracture strength than that of the B4C/BN microcomposites under the equivalent indentation load. These results indicated that the B4C/BN nanocomposites have improved damage resistance in comparison with the B4C monolith and the B4C/BN microcomposites. It was suggested that the B4C/BN nanocomposites would have the higher rising R-curve behavior than that of the B4C monolith and the B4C/BN microcomposites. R-curves behaviors of three materials were obtained from the indentation-strength data in Fig. 5. Linear regression was used to calculate the fit equation of each line, and the slope of each line was calculated. The slope of the fit equation of the B4Cmonolith was −0.216, the slope of fit equation of the B4C/BN nanocomposites was −0.186 and the slope of fit equation of the B4C/BN microcomposites was −0.173. As seen in Fig. 5, in the high indentation load region, the relation of fracture strength f and indentation load P can be expressed as following equations: for the B4C monolith: f = 465P−0.216 (4) for the B4C/BN nanocomposites: f = 503P−0.186 (5) for the B4C/BN microcomposites: f = 352P−0.173 (6) The slopes of fit equations of these threematerials were all larger than −1/3; this result indicated that these three materials all have the rising R-curves behavior. Griffith materials, for which the R-curve behavior was flat, would follow the power law [19–22]: f ∝ P−k (7) with k = 1/3. The specimens have the rising toughness curves (Rcurves behavior), the relationship between f and P was expressed as follows [19–22]: f = ˛P−ˇ (8) where ˇ < 1/3, f was fracture strength, P was indentation load. In this research, according to the results of the fit Eqs. (4–6), the facts that the calculated ˇ values of three materials were all lower than 1/3, this result indicated that the B4C monolith, the B4C/BN nanocomposites and the B4C/BN microcomposites would have the rising R-curves behavior. If the Vickers cracks geometry was considered to be materials independent, the values ϕ = 1.24 [19,20,26], values were calculated as the follows: = E H 1/2 = 0.016 [19,20,24]. for the B4C monolith was 0.0737, for the B4C/BN nanocomposites was 0.0823, for the B4C/BN microcomposites was 0.0923. The same values of ϕ and different values of were used. According to Eq. (1), the three families of K A(c)