1990 L. Zou et al. /Journal of the European Ceramic Society 23(2003)1987-1996 water,glycerin, and paraffin, milled, and incorporated thinner BN interfacial sheet. The samples then were into a 20 wt polyvinyl alcohol solution; this mixture stacked, placed in a graphite die, and sintered, by hot was milled again and then degassed, under vacuum, at pressing, at 1820C for 1.5 h in an atmosphere of N2, 1.013x 10 Pa pressure. The homogeneous slurry was under a pressure of 22 MPa. Initially, the heating rate used for tape casting, and green sheets 40-60 um thick was slow, to allow the binder in the interlayer tape to obtained pyrolyze and burn out below 500C. Strict control of gle-interface samples were prepared by sandwich- the heating rate was not necessary, because the inter- products of the Si3 N4 matrix around a facial layer was so thin. After sintering, the thickness of the interfacial layer was 15 to 30 um 2.3. Experimental method 3×4×50 machined. Because the phase angle of loading, v, defined as the angle having a tangent equal to the ratio of the shearing to the opening stress-intensity factors, 3 was influenced by sample dimensions, that value varied with the thickness ratio, h/h? Test errors resulting from fuctuation of the sample dimensions were reduced and the effect of phase angle y on interfacial toughness was fixed in the present study by making the thickness of the upper and lower beams as identical as possible. The KIMM 28kU"5nx3,009 sample was notched along the source of the crack to a certain depth, where it was near the interphase. The Fig. 2. SEM micrograph of the BN interphase crack loading system as illustrated in Fig. I was adopted to Displacement u(mm) Displacement, u(mm) 40 20 0 0.000.010.02003004005006007 Displacement, u(mm) Fig. 3. The three-point bending test and interfacial toughness calculation results for the bn+ Si3N4 interphase system. (a) BN;(b)bn+ 15vol %Si3 N4;(c)BN+ 25vol %Si3 N4: (d)BN 50vol %Si3N4water,glycerin,and paraffin,milled,and incorporated into a 20 wt.% polyvinyl alcohol solution; this mixture was milled again and then degassed,under vacuum,at 1.013
105 Pa pressure. The homogeneous slurry was used for tape casting,and green sheets 40–60 mm thick was obtained. Single-interface samples were prepared by sandwich￾ing two green products of the Si3N4 matrix around a thinner BN interfacial sheet. The samples then were stacked,placed in a graphite die,and sintered,by hot pressing,at 1820 C for 1.5 h in an atmosphere of N2, under a pressure of 22 MPa. Initially,the heating rate was slow,to allow the binder in the interlayer tape to pyrolyze and burn out below 500 C. Strict control of the heating rate was not necessary,because the inter￾facial layer was so thin. After sintering,the thickness of the interfacial layer was 15 to 30 mm. 2.3. Experimental method Test samples measuring 3
4
50 mm3 were machined. Because the phase angle of loading, , defined as the angle having a tangent equal to the ratio of the shearing to the opening stress-intensity factors,13 was influenced by sample dimensions,that value varied with the thickness ratio, h1/h2 .Test errors resulting from fluctuation of the sample dimensions were reduced and the effect of phase angle on interfacial toughness was fixed in the present study by making the thickness of the upper and lower beams as identical as possible. The sample was notched along the source of the crack to a certain depth,where it was near the interphase. The Fig. 2. SEM micrograph of the BN interphase crack. loading system as illustrated in Fig. 1 was adopted to Fig. 3. The three-point bending test and interfacial toughness calculation results for the BN + Si3N4 interphase system. (a) BN; (b) BN + 15vol.%Si3N4; (c) BN + 25vol.%Si3N4;(d) BN + 50vol.%Si3N4. 1990 L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996