J Mater Sci(2006)41:7425-7436 7431 transparent, typically less than 100 nm thick, and the standard eBsd pattern, shown in Fig. 6b, was first area of the sample thin enough for electron transmis- taken from a polished sapphire substrate for compari sion is only of the order of the typical grain size of TA, son. Figure 7 shows the EBSD patterns taken from the so that TEM unsuitable for comparing grain orienta- TA grains and the EBSD results demonstrate that the tions in these materials. In the present work EBSD was local orientation of each grain relative to the surface used to examine the regional grain orientation rela- normal deviates only slightly from the 0001 zone axis tions(microtexture)in the c-axis textured a-alumina These results were plotted in the orientation of the pellets. Figure 6a shows a forward-scatter secondary plane of the sample as discrete and inverse pole figures electron(FSE)image taken from an area examined in shown in Fig 8. The above EBSD measurement results a tilted in-plane surface of the TA with 9.1% initial confirm the X-ray diffraction macrotexture results and platelet content. A dozen alumina grains from this area show the strong out-of-plane c-axis texture. In addi- (marked by letters) were examined by EBSD and a tion, no preferred in-plane orientation was detected from the ebsd measurement similar results can also be obtained from ebsd characterization of the cross. section of the same TA sample(not shown). Formation of rbaota. atta and rbm/ta laminates Based on the above texture analysis, we decided to 9.1 vol% alumina platelets in the TA interlayers in all D three types of laminates. These TA tapes were first used,together with texture-free, fine-grain, RBAO alumina layers, to fabricate the rBaoTA laminate, in which the two'different'layers contain the same phase alumina, but in different morphologies. The thermal mismatch between the textured and non-textured la ers in this laminate is small and is associated with the 10% difference in thermal expansion coefficient of a-alumina parallel and perpendicular to the c axis. The textured layers in this laminate structure are expected to reinforce the alumina by crack deflection along the grain boundaries formed with the basal planes of the 山m6 textured grains, which leads to the formation of low 百21 energy basal plane free surfaces In order to adapt traditional rBAo processing for the texture-free layers in this texture-reinforced, sin gle-phase alumina laminate, a fine(usually submicron) 231022113 o precursor powder mixture of aluminum and alumina was prepared by attrition milling. The particle size 565416 distribution after attrition milling is shown in Fig. 9a, 2423 110u while Fig 9b and c compare the as-received aluminum powder (plasma-sprayed, 45-90 um) with the submi cron particle size and irregular grains in the attrition 45-13 5413 milled RBAO powder mixture. The heating cycle for RBAO samples requires a low heating rate below 1100oC to ensure sufficient oxidation of the metal 2201 particles Phase evolution in a series of RBAO samples was evaluated by X-ray diffraction. After heating at taken from an area examined in a tilted in-plane surface of the 1 C/min to 1100 Fig 6(a) Forward-scatter secondary electron (FSE) image the rAO samp fully textured alumina(TA)with 9. 1% initial platelet content;(b oxidized to a-alumina, but a large fraction of the met indexed standard electron back-scatter diffraction(EBSD) oxidized below the melting point (660C). Figure 10a pattern from polished a basal sapphire substrate shows the X-ray diffraction patterns from a RBAO 2 Springertransparent, typically less than 100 nm thick, and the area of the sample thin enough for electron transmis￾sion is only of the order of the typical grain size of TA, so that TEM unsuitable for comparing grain orienta￾tions in these materials. In the present work EBSD was used to examine the regional grain orientation rela￾tions (microtexture) in the c-axis textured a-alumina pellets. Figure 6a shows a forward-scatter secondary electron (FSE) image taken from an area examined in a tilted in-plane surface of the TA with 9.1% initial platelet content. A dozen alumina grains from this area (marked by letters) were examined by EBSD and a standard EBSD pattern, shown in Fig. 6b, was first taken from a polished sapphire substrate for compari￾son. Figure 7 shows the EBSD patterns taken from the TA grains and the EBSD results demonstrate that the local orientation of each grain relative to the surface normal deviates only slightly from the 0001 zone axis. These results were plotted in the orientation of the plane of the sample as discrete and inverse pole figures shown in Fig. 8. The above EBSD measurement results confirm the X-ray diffraction macrotexture results and show the strong out-of-plane c-axis texture. In addi￾tion, no preferred in-plane orientation was detected from the EBSD measurement. Similar results can also be obtained from EBSD characterization of the cross￾section of the same TA sample (not shown). Formation of RBAO/TA, ATZ/TA, and RBM/TA laminates Based on the above texture analysis, we decided to use 9.1 vol% alumina platelets in the TA interlayers in all three types of laminates. These TA tapes were first used, together with texture-free, fine-grain, RBAO alumina layers, to fabricate the RBAO/TA laminate, in which the two ‘different’ layers contain the same phase, a-alumina, but in different morphologies. The thermal mismatch between the textured and non-textured lay￾ers in this laminate is small, and is associated with the 10% difference in thermal expansion coefficient of a-alumina parallel and perpendicular to the c axis. The textured layers in this laminate structure are expected to reinforce the alumina by crack deflection along the grain boundaries formed with the basal planes of the textured grains, which leads to the formation of low energy basal plane free surfaces. In order to adapt traditional RBAO processing for the texture-free layers in this texture-reinforced, sin￾gle-phase alumina laminate, a fine (usually submicron) precursor powder mixture of aluminum and alumina was prepared by attrition milling. The particle size distribution after attrition milling is shown in Fig. 9a, while Fig. 9b and c compare the as-received aluminum powder (plasma-sprayed, 45–90 lm) with the submi￾cron particle size and irregular grains in the attrition milled RBAO powder mixture. The heating cycle for RBAO samples requires a low heating rate below 1100 C to ensure sufficient oxidation of the metal particles. Phase evolution in a series of RBAO samples was evaluated by X-ray diffraction. After heating at 1 C/min to 1100 C, the RBAO sample was fully oxidized to a-alumina, but a large fraction of the metal oxidized below the melting point (660 C). Figure 10a shows the X-ray diffraction patterns from a RBAO Fig. 6 (a) Forward-scatter secondary electron (FSE) image taken from an area examined in a tilted in-plane surface of the textured alumina (TA) with 9.1% initial platelet content; (b) indexed standard electron back-scatter diffraction (EBSD) pattern from polished a basal sapphire substrate J Mater Sci (2006) 41:7425–7436 7431 123