D. Li et al.Ceramics International 30(2004)213-217 215 700 600 z500 R400 0 LSB 300 b G=756P0289 200 G622P014 100 020406080100120 100 Displacement /um Indentation load/N ig. 3. Load-displacement curves of the two types of specimens with Fig. 5. Strength plotted against indentation load for the two materi- an indentation in the tensile surface:(a)MS (b) LSB als Curves for the two materials deviate slightly from slope of -1/3 loading, LSB also deformed in a linear elastic fashion with a low indentation load, the strength was not loid showed a progressive failure behavior. Upon initial Fig. 5 into two regions. In the left region, for specimens until the crack reached the same stress intensity as ms dependent, because it was microstructure-controlled However, rather than traveling right across the speci On the other hand, in the right region, where specimens men the crack was deflected at the first bn interface had a higher indentation load, strength was inversely that it reached. as shown in Fig. 4. Crack deflection related to the indentation load, because strength was along the weak interface allowed the load to continue controlled by external flaws. rising. Failure of the second SiC layer gave rise to the Fig. 5 shows that in the low-indentation-load region first load drop. This process was repeated until all the the strengths were higher for Ms than for LSB, in the SiC layers had cracked, resulting in a step-like load- high-indentation-load region the strengths for the two displacement response. The total area under the load- materials decreased linearly with indentation load, but displacement curve represented the work-of-fracture the slope being much steeper for MS than for LSB. This WOF. Evidently, woF of LSB was much higher than means that LSB have a higher retained strength than hat of ms MS for an equivalent indentation load, hence an improved damage tolerance in comparison with Ms. It 3.2. Indentation strengths and R-curves also suggested that LSB might have a higher fracture resistance than ms Indentation fracture strength, of, is plotted logarith R-curves of two materials were obtained from inden- mically against indentation load, P, in Fig. 5 for both tation-strength data. Linear regression was used to MS and LSB. For each of the two materials a knee in the obtain the best fit for the experimental data in Fig. 5. It corresponding curve(at P=P*)separated the plot in showed that the slopes of MS and LSB were-0. 289 and -o142, respectively. Griffith materials, for which the R-curve is flat, would follow the power law, or x P-k with k=1/ 3. The fact that k is lower than 1/3, suggests a rising R-curve behavior for both materials. If the Vick ers crack geometry is considered to be material-inde- pendent, the values y= 1. 24 [20]. x=0.071 can be calculated for the experimental value E/H=20 for MS we use the same values of y and x for the two materials Two families of Ka(c)curves can now be constructed from the indentation-strength data in Fig. 5, insertin 0A=Or at each value of indentation load P in Eq (1) The envelopes of tangency points for two materials are hown in Fig. 6. It can be seen from Fig. 6 that the envelope of tangency points for MS was approximately horizontal, indicative of a plateau R-curve be Fig. 4. Propagation of a major crack through the specimen of LSB. while, for LSB the envelope of tangency points yields a Note that crack deflection occurs along the SiC/BN interfaces. rising R-curve. This suggests that LSB possessed excellentshowed a progressive failure behavior. Upon initial loading, LSB also deformed in a linear elastic fashion until the crack reached the same stress intensity as MS. However, rather than traveling right across the speci￾men the crack was deflected at the first BN interface that it reached, as shown in Fig. 4. Crack deflection along the weak interface allowed the load to continue rising. Failure of the second SiC layer gave rise to the first load drop. This process was repeated until all the SiC layers had cracked, resulting in a step-like load– displacement response. The total area under the load– displacement curve represented the work-of-fracture (WOF). Evidently, WOF of LSB was much higher than that of MS 3.2. Indentation strengths and R-curves Indentation fracture strength,
f, is plotted logarith￾mically against indentation load, P, in Fig. 5 for both MS and LSB. For each of the two materials a knee in the corresponding curve (at P=P*) separated the plot in Fig. 5 into two regions. In the left region, for specimens with a low indentation load, the strength was not load￾dependent, because it was microstructure-controlled. On the other hand, in the right region, where specimens had a higher indentation load, strength was inversely related to the indentation load, because strength was controlled by external flaws. Fig. 5 shows that in the low-indentation-load region the strengths were higher for MS than for LSB, in the high-indentation-load region the strengths for the two materials decreased linearly with indentation load, but the slope being much steeper for MS than for LSB. This means that LSB have a higher retained strength than MS for an equivalent indentation load, hence an improved damage tolerance in comparison with MS. It also suggested that LSB might have a higher fracture resistance than MS. R-curves of two materials were obtained from inden￾tation-strength data. Linear regression was used to obtain the best fit for the experimental data in Fig. 5. It showed that the slopes of MS and LSB were 0.289 and 0.142, respectively. Griffith materials, for which the R-curve is flat, would follow the power law,
f / P k with k=1/3. The fact that k is lower than 1/3, suggests a rising R-curve behavior for both materials. If the Vick￾ers crack geometry is considered to be material-inde￾pendent, the values =1.24 [20], =0.071 can be calculated for the experimental value E/H=20 for MS. we use the same values of and  for the two materials. Two families of K0 A(c) curves can now be constructed from the indentation-strength data in Fig. 5, inserting
A=
f at each value of indentation load P in Eq. (1). The envelopes of tangency points for two materials are shown in Fig. 6. It can be seen from Fig. 6 that the envelope of tangency points for MS was approximately horizontal, indicative of a plateau R-curve behavior, while, for LSB the envelope of tangency points yields a rising R-curve. This suggests that LSB possessed excellent Fig. 3. Load–displacement curves of the two types of specimens with an indentation in the tensile surface: (a) MS, (b) LSB. Fig. 4. Propagation of a major crack through the specimen of LSB. Note that crack deflection occurs along the SiC/BN interfaces. Fig. 5. Strength plotted against indentation load for the two materi￾als. Curves for the two materials deviate slightly from slope of 1/3. D. Li et al. / Ceramics International 30 (2004) 213–217 215