D-H. Kuo, W.M. Kricen/ Materials Science and Engineering A210 (1996)123-1.34 after five cycles. In this way, interfacial delamination between LP and Lau was shown by sem at the notch tip(Fig. 12) To examine the interactions between the cracks and the microstructure, indentation cracks were introduced (Fig. 10). Radial cracks easily propagate across LP/A Fig 10(a))and LP/YAG( Fig. 10(b))interfaces. On the other hand, indentation cracks in the LP/LAI(Fig 10(c))system display a preferred LPE along the oundaries between Lp and LA. In 10(c), the broken line indicates the supposed direction of the 00400600800100012001400 Vickers radial crack, whereas the crack is actually Temperature(°C) deflected along the LP/LAn interface. An enlarged micrograph of the interfacial delamination in the in- ig. 11, Thermal expansion of LaAl, Os as measured by dilatome- dented LP/ LAu laminate, which in Fig. 10(c), is shown in Fig. 13. This is consistent with difficult to make. Densification by either hot pressing or the sem observation in Fig. 12 sintering at 1600C produced cracked specimens The morphological stability of the interface in fiber 3. 4. Pushout tests of fiber model systems reinforced composites is of concern in considering long term high temperature applications. Morphologic instability has been found between B-alumina-related 3.4.I. Fiber model systems materials and single-crystal Al2O3 fibers [11], which can The interfacial behavior of the Al,O, fiber/ LaPO4 fiber sliding to be difficult. Al2O3 matrix and YAG fiber/ LaPO. In the LP/A, LP/YAG and LP/LA, laminates, mor Al,O3 matrix model systems was studied by fiber phological instability is not evident. All the interfaces around the fiber coating for the two model systems are between the components of the laminates are stable and flat(Fig. 10) shown in Fig. 14. After sintering at 1550 some elongated magnetoplumbite B-alumina(LAu) crystals The CTE of LA, was not known before this study. are formed between the coating and the matrix. The After fabrication of single-phase LA, by hot pressing formation of LAn during sintering at 1550C may be at 1600C for 3 h, a density of 3.0 g cm-3was measured. The la, bar tested for cte had dimen- facilitated by the use of the high surface area Al2o powders for the matrix. This reaction is also observed sions of 17. 1 mm x 2. I mm x 1.7 mm. The thermal between the Al-O, fiber and the LP interphase, giving a expansion data for hot pressed LA u are shown in Fig 11. The ctes for la are8.8×10-6Cat200°C thin LAu layer. 10.0×106°C-lat600°andl0.8×10-6°-at 1000C, with an average value over this temperature 3. 4.2 Pushout test Examples of the pushout curves(load vs, crosshead displacement) for Al, O3 and YAG fiber model systems are shown in Fig. 15. On reaching the peak load (Pp), 3.3. Mechanical responses of laminated composites there is a subsequent load drop, which indicates that the bottom surface of the test fiber protrudes out of the Table 2 summarizes the four-point flexural strengths thin slice [22]. In other systems, after the fiber slide of single-phase and laminated materials, accompanied with its bottom surface protruded, the sliding resistance by the microstructural data. No available test data for decreases with increasing fiber displacement [23.How- the flexural strength of LP were obtained due to the ever, in the present case, both fiber systems in Fig. 15 difficulty in preparing specimens without cracking. The ide at about constant force, after passing the peak low strength of the LP/A laminate may be attributed to load the lower densification temperature of 1300C. Most The peak load(P), the easiest parameter to measure, with their tensile surfaces ground, indented or was used as the debonding load(Pa) in Eq. (1). Fig. 16 notched show brittle fracture under flexural tests. One gives the peak load(P)as a function of the embedded exception is the annealed and notched LP/LAu lami- fiber length(L). The experimental data plotted are the nate. The three notched LP /LA, bars show non- average peak loads, and the error bars represent one catastrophic fracture to some degree. One of the bars, standard deviation. By using an iterative regressive after the first load drop under a test, was subjected to curve fitting procedure, the interfacial shear strength td multiple unloading and re-loading. This bend bar broke was obtained. td values calculated by the linear andD.-H. Kuo, W.M. Kriven Materials Science and Engineering A210 (1996) 123-134 129 0 ;,< 1,4 ' '' I' ' ' I ' '' I ' ' ' I , ' ' I ' ' ' I 1.2 1 0.8 0.6 0.4 0.2 0 I 200 400 600 800 1000 1200 1400 Temperature (°C) Fig. 11. Thermal expansion of LaAl~Ot8 as measured by dilatome￾try. difficult to make. Densification by either hot pressing or sintering at 1600 °C produced cracked specimens. The morphological stability of the interface in fiber￾reinforced composites is of concern in considering long￾term high temperature applications. Morphological instability has been found between fl-alumina-related materials and single-crystal A1203 fibers [11], which can degrade the fiber and cause fiber sliding to be difficult. In the LP/A, LP/YAG and LP/LA~ laminates, mor￾phological instability is not evident. All the interfaces between the components of the laminates are stable and flat (Fig. 10). The CTE of LA~ was not known before this study. After fabrication of single-phase LA~ by hot pressing at 1600 °C for 3 h, a density of 3.0 g cm -3 was measured. The LAt~ bar tested for CTE had dimen￾sions of 17.1 mm x 2.1 mm × 1.7 mm. The thermal expansion data for hot pressed LAt~ are shown in Fig. 11. The CTEs for LA~ are 8.8 x 10 -6 °C I at 200 °C, 10.0 x 10 6 o C ~ at 600 °C and 10.8 x 10 6 °C ~ at 1000 °C, with an average value over this temperature range ofl0.0x 10 6oc 1. 3.3. Mechanical responses of laminated composites Table 2 summarizes the four-point flexural strengths of single-phase and laminated materials, accompanied by the microstructural data. No available test data for the flexural strength of LP were obtained due to the difficulty in preparing specimens without cracking. The low strength of the LP/A laminate may be attributed to the lower densification temperature of 1300 °C. Most laminates with their tensile surfaces ground, indented or notched show brittle fracture under flexural tests. One exception is the annealed and notched LP/LA1~ lami￾nate. The three notched LP/LA11 bars show non￾catastrophic fracture to some degree. One of the bars, after the first load drop under a test, was subjected to multiple unloading and re-loading. This bend bar broke after five cycles. In this way, interfacial delamination between LP and LA~ was shown by SEM at the notch tip (Fig. 12). To examine the interactions between the cracks and the microstructure, indentation cracks were introduced (Fig. 10). Radial cracks easily propagate across LP/A (Fig. 10(a)) and LP/YAG (Fig. 10(b)) interfaces. On the other hand, indentation cracks in the LP/LA~ (Fig. 10(c)) system display a preferred path along the boundaries between LP and LA~. In Fig. 10(c), the broken line indicates the supposed direction of the Vickers radial crack, whereas the crack is actually deflected along the LP/LA~ interface. An enlarged micrograph of the interfacial delamination in the in￾dented LP/LA~ laminate, which is marked by arrows in Fig. 10(c), is shown in Fig. 13. This is consistent with the SEM observation in Fig. 12. 3.4. Pushout tests of fiber model systems 3.4. I. Fiber model systems The interfacial behavior of the A1203 fiber/LaPO4 coating/Al203 matrix and YAG fiber/LaPO4 coating/ A1203 matrix model systems was studied by fiber pushout testing. Scanning electron micrographs taken around the fiber coating for the two model systems are shown in Fig. 14. After sintering at 1550 °C, some elongated magnetoplumbite/fl-alumina (LA~) crystals are formed between the coating and the matrix. The formation of LAI~ during sintering at 1550 °C may be facilitated by the use of the high surface area A1203 powders for the matrix, This reaction is also observed between the A1203 fiber and the LP interphase, giving a thin LA~ layer. 3.4.2. Pushout test Examples of the pushout curves (load vs. crosshead displacement) for A1203 and YAG fiber model systems are shown in Fig. 15. On reaching the peak load (Pp), there is a subsequent load drop, which indicates that the bottom surface of the test fiber protrudes out of the thin slice [22]. In other systems, after the fiber slides with its bottom surface protruded, the sliding resistance decreases with increasing fiber displacement [23]. How￾ever, in the present case, both fiber systems in Fig. 15 slide at about constant force, after passing the peak load. The peak load (Pp), the easiest parameter to measure, was used as the debonding load (Pd) in Eq. (1). Fig. 16 gives the peak load (Pp) as a function of the embedded fiber length (L). The experimental data plotted are the average peak loads, and the error bars represent one standard deviation. By using an iterative regressive curve fitting procedure, the interfacial shear strength ra was obtained, rd values calculated by the linear and