1010 Journal of the American Ceramic Sociery-Kovar et al. Vol 81. No. 4 a Deflectometer Displacement, (mm) Fig. 12. Several hysteresis loops, shown over different loading ranges, for the material containing 50 vol% Si3 N4 in the interface, the width of the loops increases as the load increases 10um o-0% u=0.76±0.06 203 028±0.04 024士 0.17±003 0,2 leviation of the coefficient o increasing interfacial resistance with delamination crack length (R-curve behavior), and (ii) the presence of defects along the interface that draw the delamination crack out of the inter It has been suggested that delamination cracks can kink out Fig. 11. SEM micrographs, each at the same magnification, of the of an interface under certain conditions when the interfacial interfacial fracture surfaces for specimens containing(a) 10 and( b)50 fracture resistance increases as the crack extension increases. 19 vol% Si,N4 in the interphase, taken prior to the sliding experiments Such R-curve behavior has been observed during the growth of he large platelike grains are BN, whereas the finer grains are Si3N4) other all-ceramic layered systems. 21 22 R-curve behavior in ce- ramics is usually the result of frictional sliding that occi sorption are observed only in materials that have the lowest the crack wake during extension. 23 In the Sia N,/BN system interfacial fracture resistance(30-50 J/m2) examined in this study, however, the measured sliding resis These observations suggest that the energy-absorption capa tance was extremely low(see Fig. 13)and R-curve behavior bility of a material is not determined merely by whether or not was not observed during the growth of delamination cracks crack deflection occurs. Rather, the extent of energy absorption Thus, R-curve behavior does not contribute to crack kinking in is primarily influenced by the crack path after the initial crack this material system deflection occurs. Specifically, the energy-absorption capabil Another explanation that has been previously proposed to ity is greatly reduced when the delamination cracks kink out of xplain crack kinking behavior involves flaws in the SiaN the interphase after traveling only a short distance. Thus, de- termination of the nature of the transition between delamina- tion cracking and crack kinking is essential to the development ity. There are at least two possible explanations for the crack that is c出m价men of layered ceramIcs that have high energy-absorption capabil kinking behavior observed in these layered ceramics: (i)ansorption are observed only in materials that have the lowest interfacial fracture resistance (30–50 J/m2 ). These observations suggest that the energy-absorption capa￾bility of a material is not determined merely by whether or not crack deflection occurs. Rather, the extent of energy absorption is primarily influenced by the crack path after the initial crack deflection occurs. Specifically, the energy-absorption capabil￾ity is greatly reduced when the delamination cracks kink out of the interphase after traveling only a short distance. Thus, de￾termination of the nature of the transition between delamina￾tion cracking and crack kinking is essential to the development of layered ceramics that have high energy-absorption capabil￾ity. There are at least two possible explanations for the crack kinking behavior observed in these layered ceramics: (i) an increasing interfacial resistance with delamination crack length (R-curve behavior), and (ii) the presence of defects along the interface that draw the delamination crack out of the inter￾phase. It has been suggested that delamination cracks can kink out of an interface under certain conditions when the interfacial fracture resistance increases as the crack extension increases.19 Such R-curve behavior has been observed during the growth of delamination cracks in polymer/ceramic systems20 as well as other all-ceramic layered systems.21,22 R-curve behavior in ce￾ramics is usually the result of frictional sliding that occurs in the crack wake during extension.23 In the Si3N4/BN system examined in this study, however, the measured sliding resis￾tance was extremely low (see Fig. 13) and R-curve behavior was not observed during the growth of delamination cracks.‡‡ Thus, R-curve behavior does not contribute to crack kinking in this material system. Another explanation that has been previously proposed to explain crack kinking behavior involves flaws in the Si3N4 ‡‡If the interfacial fracture resistance increases as the crack extension increases, the load necessary to propagate a delamination crack will also increase. The plateau load that is observed during delamination cracking in these materials (see Fig. 9) indicates that R-curve behavior does not occur. Fig. 11. SEM micrographs, each at the same magnification, of the interfacial fracture surfaces for specimens containing (a) 10 and (b) 50 vol% Si3N4 in the interphase, taken prior to the sliding experiments (the large platelike grains are BN, whereas the finer grains are Si3N4). Fig. 12. Several hysteresis loops, shown over different loading ranges, for the material containing 50 vol% Si3N4 in the interface; the width of the loops increases as the load increases. Fig. 13. Frictional sliding resistance (ts), plotted versus the normal pressure on the interphase for specimens with varying Si3N4 content of the interphase; the mean and standard deviation of the coefficient of friction (m) are also shown. 1010 Journal of the American Ceramic Society—Kovar et al. Vol. 81, No. 4