D. Liet al. Ceramics International 30(2004)213-217 86420 200N 864 0①cg3 86420864 2 100200300400500600700 100200300400500600700 Crack Crack size c/um Fig. 6. Toughness curve diagrams for(a) MS, (b) LSB. Families of solid curves are plots of KA(c)in Eq(1)using strength data in Fig. 5. Shaded lines are T(c) functions, plotted as locus of tangency points to Ka(c). damage tolerance and a rising R-curve behavior inin Fig. 8, further absorbed the fracture energy of mate- comparison with Ms rials, relaxed the stress at the crack tip and improved the fracture resistance and damage resistance 3.3. The main toughening mechanisms The above results showed that LSB possessed excel D27 lent damage tolerance and a rising R-curve behavior in comparison with Ms. This was thought be related to their different toughening mechanisms. Fig. 7 showed the crack propagation path of Ms, which introduced by Vickers indentation, crack deflection along weak grain interface is its main toughening mechanism. The main toughening mechanism of LSB is crack deflection along weak BN interface continually. As shown in ig. 4, when a crack propagates through LSB, crack deflection occurred along weak interface continually leading to a step-like crack propagation path which greatly extended effective crack length and absorbed more fracture energy, with an improvement of fracture resistance. Besides, crack branching and crack delani (a) nation which appeared at the weak interface, as shown D26 6um (b) Fig. 8. Two cracking mode of LSB for(a) crack branching, (b) crack Fig. 7. Crack propagation path of Msdamage tolerance and a rising R-curve behavior in comparison with MS. 3.3. The main toughening mechanisms The above results showed that LSB possessed excel￾lent damage tolerance and a rising R-curve behavior in comparison with MS. This was thought be related to their different toughening mechanisms. Fig. 7 showed the crack propagation path of MS, which introduced by Vickers indentation, crack deflection along weak grain interface is its main toughening mechanism. The main toughening mechanism of LSB is crack deflection along weak BN interface continually. As shown in Fig. 4, when a crack propagates through LSB, crack deflection occurred along weak interface continually, leading to a step-like crack propagation path which greatly extended effective crack length and absorbed more fracture energy, with an improvement of fracture resistance. Besides, crack branching and crack delami￾nation which appeared at the weak interface, as shown in Fig. 8, further absorbed the fracture energy of mate￾rials, relaxed the stress at the crack tip and improved the fracture resistance and damage resistance. Fig. 6. Toughness curve diagrams for (a) MS, (b) LSB. Families of solid curves are plots of K0 A(c) in Eq. (1) using strength data in Fig. 5. Shaded lines are T(c) functions, plotted as locus of tangency points to K0 A(c). Fig. 7. Crack propagation path of MS. Fig. 8. Two cracking mode of LSB for (a) crack branching, (b) crack delamination. 216 D. Li et al. / Ceramics International 30 (2004) 213–217