ty-Manwdsley et al Table ll. notched-Beam bend-Test results Total sample Bille eight(mm) delamination crack length delamination crack length 4.09 2.33 4.03 2.15 725 multilayer sample( sample 3lh cre disregarded. In the case of the diffractometry(XRD)was performed for phase identification.No inner and outer loading points w micrograph of the interface region that is shown in Fig. 2. The The delamination fracture energy Ti of the samples was calcu- thicknesses of the monazite layer in billets I and 2 are 210 and 160 lated using the equation um, respectively. The thickness of the monazite layers in billets 3 and 4 was 15 um, and the thickness of the alumina layers wa r-[2+(+) 125-150 um. The densities of the bars cut from these billets were measured using the Archimedes method, according to a standard where P is the load needed to cause debonding, L the distance from 4 were 4.04 and 4.10 g/cm, respectively, which is 99.0%-99.9% the outer load span to the inner load span(10 mm), v the Poissons of theoretical density, assuming that the samples contained 10 vol% monazite. The average measured density of bars from billet alumina. The variables h, and hz are the heights of the lower and 5 was 4.04 g/cm, which is 99.6% of theoretical density for an upper beams, respectively(the notch always being in the lower alumina sample that contains 4 vol% Y-TZP beam), and b is the width of the beam (2) Interfacial Fracture Energy Measurements (3) Measurement of Interfacial Frictional Sliding Resistance A representative plot of load versus specimen deflection(mea- Specimens for the measurement of interfacial frictional sliding sured using the lvdt deflectometer) for the notched-beam bend resistance(T, )were obtained by cutting a notch through the end of test is shown in Fig 3. The corresponding I, values, which are in the bar, parallel to the plane of the monazite/alumina interface and the range of 9.7-14.6 J/m2 at the shortest crack lengths(-3 mm) into a monazite layer. Wedges were inserted into the notches far are summarized in Table Il for billets 1-3. Compared to the earlier enough to initiate a delamination crack. Then, the wedges were results of Morgan and Marshall'on similar materials, these values removed from these samples, a razor blade was inserted into the are slightly greater notch, and a Mode I force was applied to split the specimen in two Figure 3 indicates that the applied load necessary to drive arts along the interface between the alumina and monazite. The delamination steadily increases, even when the crack is within the two pieces of the sample then were reassembled and placed into a inner loading span. The increasing load implies that the interfacial fully articulated, three-point bend fixture that was mounted on a fracture energy is dependent on crack length(R-curve behavior) screw-driven test machine. The testing machine was operated in R-curve behavior in the delamination fracture energy has been the same manner as described previously with the lvdt deflec- observed in earlier work and may be responsible for the signifi tometer directly beneath the loading point. The specimen was cant variability in the measured value of T in the alumina/ loaded until a nonlinearity was detected in the load-deflection response that corresponded to the point where slipping occurred along the cracked interface. Then, the sample was partially unloaded until slipping began in the opposite direction. A series of load-unload loops were generated and plotted individually to analyze sliding resistance. Using the model of Kovar and Thouless, and considering machine compliance, the interfacial frictional sliding resistance was calculated from the relation =3(△P)h1h2 where AP is the difference between the load at which slipping began and the minimum load during a load-unload cycle Alumina/Monazite Multilayer laminates Several bars made from multilayer bars were tested from billets 3, 4, and 5 by loading samples in four-point flexure. The tensile surface of each bar was polished and chamfered prior to testing. In ome cases, a notch 200 um wide and approximately half the eight of the bar was cut into the tensile surface of the specimen perpendicular to the long axis of the bar, using a diamond-edged wafering blade. In all cases, the testing machine was operated in 3 um displacement control at a crosshead speed of 0.5 mm/min II. Results Fig. 2. Backscattered SEM micrograph of a thermally etched (l Sample Characterization monazite interface, showing that no reaction phases have fom To ensure that no reactions occurred between the monazite and region is Al, Oa, and the bright inclusions in the Al O, matrix the alumina, 6, s, I5 the samples were inspected via SEM and X-ray particles.inner and outer loading points were disregarded. In the case of the multilayer sample (sample 3), the crack propagated along only one interface. The delamination fracture energy Gi of the samples was calcu￾lated using the equation Gi 5 F 3P2 L2 ~1 2 n2 ! 2E~h1 1 h2! 3 b2GFS h1 h2 1 1D 3 21G (1) where P is the load needed to cause debonding, L the distance from the outer load span to the inner load span (10 mm), n the Poisson’s ratio of the bulk material (alumina), and E the Young’s modulus of alumina. The variables h1 and h2 are the heights of the lower and upper beams, respectively (the notch always being in the lower beam), and b is the width of the beam. (3) Measurement of Interfacial Frictional Sliding Resistance Specimens for the measurement of interfacial frictional sliding resistance (ts) were obtained by cutting a notch through the end of the bar, parallel to the plane of the monazite/alumina interface and into a monazite layer. Wedges were inserted into the notches far enough to initiate a delamination crack. Then, the wedges were removed from these samples, a razor blade was inserted into the notch, and a Mode I force was applied to split the specimen in two parts along the interface between the alumina and monazite. The two pieces of the sample then were reassembled and placed into a fully articulated, three-point bend fixture that was mounted on a screw-driven test machine. The testing machine was operated in the same manner as described previously with the LVDT deflec￾tometer directly beneath the loading point. The specimen was loaded until a nonlinearity was detected in the load–deflection response that corresponded to the point where slipping occurred along the cracked interface. Then, the sample was partially unloaded until slipping began in the opposite direction. A series of load–unload loops were generated and plotted individually to analyze sliding resistance. Using the model of Kovar and Thouless,14 and considering machine compliance, the interfacial frictional sliding resistance was calculated from the relation ts 5 3~DP!h1h2 b~h1 1 h2! 3 (2) where DP is the difference between the load at which slipping began and the minimum load during a load–unload cycle. (4) Fracture Behavior of Alumina/Monazite Multilayer Laminates Several bars made from multilayer bars were tested from billets 3, 4, and 5 by loading samples in four-point flexure. The tensile surface of each bar was polished and chamfered prior to testing. In some cases, a notch 200 mm wide and approximately half the height of the bar was cut into the tensile surface of the specimen perpendicular to the long axis of the bar, using a diamond-edged wafering blade. In all cases, the testing machine was operated in displacement control at a crosshead speed of 0.5 mm/min. III. Results (1) Sample Characterization To ensure that no reactions occurred between the monazite and the alumina,6,8,15 the samples were inspected via SEM and X-ray diffractometry (XRD) was performed for phase identification. No reaction phases were observed in the powder XRD spectra or the micrograph of the interface region that is shown in Fig. 2. The thicknesses of the monazite layer in billets 1 and 2 are 210 and 160 mm, respectively. The thickness of the monazite layers in billets 3 and 4 was 15 mm, and the thickness of the alumina layers was 125–150 mm. The densities of the bars cut from these billets were measured using the Archimedes method, according to a standard method.16 The average measured density of bars from billets 3 and 4 were 4.04 and 4.10 g/cm3 , respectively, which is 99.0%–99.9% of theoretical density, assuming that the samples contained 10 vol% monazite. The average measured density of bars from billet 5 was 4.04 g/cm3 , which is 99.6% of theoretical density for an alumina sample that contains 4 vol% Y-TZP. (2) Interfacial Fracture Energy Measurements A representative plot of load versus specimen deflection (mea￾sured using the LVDT deflectometer) for the notched-beam bend test is shown in Fig. 3. The corresponding Gi values, which are in the range of 9.7–14.6 J/m2 at the shortest crack lengths (;3 mm), are summarized in Table II for billets 1–3. Compared to the earlier results of Morgan and Marshall7 on similar materials, these values are slightly greater. Figure 3 indicates that the applied load necessary to drive delamination steadily increases, even when the crack is within the inner loading span. The increasing load implies that the interfacial fracture energy is dependent on crack length (R-curve behavior). R-curve behavior in the delamination fracture energy has been observed in earlier work7 and may be responsible for the signifi￾cant variability in the measured value of Gi in the alumina/ monazite system. Table II. Notched-Beam Bend-Test Results Sample Billet Total sample height (mm) Notch height (mm) Delamination fracture energy, Gi (J/m2 ) Measured at shortest delamination crack length Measured at longest delamination crack length 1 1 4.09 1.65 9.7 17.6 2 2 4.97 2.33 13.5 21.5 3 3 4.03 2.15 14.6 15.2 Fig. 2. Backscattered SEM micrograph of a thermally etched alumina/ monazite interface, showing that no reaction phases have formed; the bright phase in the upper region is LaPO4, the dark phase in the lower region is Al2O3, and the bright inclusions in the Al2O3 matrix are ZrO2 particles. 804 Journal of the American Ceramic Society—Mawdsley et al. Vol. 83, No. 4