April 1998 Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics 25%SN4 50 50% SL N N 0 .2 0.0 Deflectometer Displacement(mm) Deflectometer Displacement(mm) Fig. 9. Load-LVDT-deflectometer-displacement plot for notched specimens containing(a)10, (b)25,(c)50, and(d)80 vol% Si,Na in the interphase. Propagation of the initial delamination crack occurs at an almost-constant load in Figs. 9(aHc) increases. This observation indicates that there is some cou- served for the material that contained 50 vol% Si3Na in the lombic contribution to the frictional sliding resistance Ts. Based interphase, whereas the materials that contained 0, 10, and 25 on a simple coulombic model in which the sliding resistance is vol% SiaN4 in the interphase had narrower lo given b sliding resistance, Ts, is plotted as a function of th (3) pplied pressure on the interface for specimens that differing values of Si3N4 in the interphase in Fig. 13. The solid lines represent the best fit, based on a least-squares linear re- sure, p the normal pressure, and u a coulombic coefficient of gression. Only the material that contained 50 vol% Si3N4 in the friction,it was found that the friction coefficients for the ma- interphase exhibited significantly higher sliding resistance; the terials generally increase as the SigN, content in the interphase sliding resistance in this material is 2-3 times that of the other s. Values for u varied from 0. 17 to 0.76 as the Si3N The fact that BN, a well-known solid lubricant, is present on At a given load range, the widest hysteresis loops were ob- the sliding surfaces may explain why the value of Ts is very low in these materials. Because bn platelets are significantly larger than the Sis N4 grains, the roughness of the fracture surfaces decreases as the Siy Na content in the interphase increases(see r for Bulk si, na Figs. Il(a)and(b). Contrary to previous observations made on mposites however. the sliding resis- tance decreased as the roughness of the sliding surfaces in- creased. The presence of lubricious BN on the interface seems to have a much-larger role than the interfacial roughness in the 80 determination of the sliding resistance in these materials Comparison of the delamination cracking morphologies for the Si3 N,/Bn layered ceramics shown in Figs. 4(aHd)with the interfacial fracture resistance shows that, in materials with 0 very high interfacial fracture resistance values(80 J/m),no 100 crack deflection is observed and very little energy is absorbed Si, N, in Interphase(%) Specimens with moderate interfacial fracture resistance values (50-80 J/m2)exhibit crack deflection; however, the delamina- tent in the interphase c plotted N:用2时increases. This observation indicates that there is some cou￾lombic contribution to the frictional sliding resistance ts. Based on a simple coulombic model in which the sliding resistance is given by ts 4 to + mp (3) where to is the intrinsic sliding resistance at zero normal pres￾sure, p the normal pressure, and m a coulombic coefficient of friction, it was found that the friction coefficients for the ma￾terials generally increase as the Si3N4 content in the interphase increases. Values for m varied from 0.17 to 0.76 as the Si3N4 content in the interphase was increased. At a given load range, the widest hysteresis loops were ob￾served for the material that contained 50 vol% Si3N4 in the interphase, whereas the materials that contained 0, 10, and 25 vol% Si3N4 in the interphase had narrower loops. Frictional sliding resistance, ts, is plotted as a function of the normal applied pressure on the interface for specimens that contained differing values of Si3N4 in the interphase in Fig. 13. The solid lines represent the best fit, based on a least-squares linear re￾gression. Only the material that contained 50 vol% Si3N4 in the interphase exhibited significantly higher sliding resistance; the sliding resistance in this material is 2–3 times that of the other materials. The fact that BN, a well-known solid lubricant, is present on the sliding surfaces may explain why the value of ts is very low in these materials. Because BN platelets are significantly larger than the Si3N4 grains, the roughness of the fracture surfaces decreases as the Si3N4 content in the interphase increases (see Figs. 11(a) and (b)). Contrary to previous observations made on fiber-reinforced composites,17,18 however, the sliding resis￾tance decreased as the roughness of the sliding surfaces in￾creased. The presence of lubricious BN on the interface seems to have a much-larger role than the interfacial roughness in the determination of the sliding resistance in these materials. IV. Discussion Comparison of the delamination cracking morphologies for the Si3N4/BN layered ceramics shown in Figs. 4(a)–(d) with the interfacial fracture resistance shows that, in materials with very high interfacial fracture resistance values (>80 J/m2 ), no crack deflection is observed and very little energy is absorbed. Specimens with moderate interfacial fracture resistance values (50–80 J/m2 ) exhibit crack deflection; however, the delamina￾tion cracks are short because the delamination cracks kink out of the interphase. These specimens also do not absorb much energy. Extensive delamination cracking and high energy ab￾Fig. 10. Interfacial fracture resistance (Gi ), plotted versus Si3N4 con￾tent in the interphase; the fracture resistance for bulk Si3N4 is also plotted. Fig. 9. Load–LVDT-deflectometer-displacement plot for notched specimens containing (a) 10, (b) 25, (c) 50, and (d) 80 vol% Si3N4 in the interphase. Propagation of the initial delamination crack occurs at an almost-constant load in Figs. 9(a)–(c). April 1998 Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics 1009