E D. Rodeghiero et al/ Materials Science and Engineering 4244(1998)11-21 EHT=10.00 KV 8 mm 30um Photo No =18 Detector= Q Fig. 4. Backscattered electron SEM micrograph of the microstructure of a 50/50 vol. Fe/a-Al20 site: (Fe= light contrast, Al,,= dark surfaces were prepared by flattening the pellet faces 2.3. Ceramic-ceramic composite synthesis with a 20 um metal bonded diamond wheel(Struers), coarse polishing with Sic paper, and then fine polishing The preparation of the Sic(particulate)-reinforced with 6 um diamond paste impregnated Texmet polish lumina composites consisted of the following steps ing cloth(Buehler). Finally, an aqueous ultrasonic First a 0.15m absolute ethanol solution of aluminum to remove pellet surface contamination ier polishing isopropoxide was again prepared and heated to boil- cleaning bath was utilized immediately ing. Next, an appropriate amount of either SiC whiskers(≈ I um diameter by≈l5 um long) or Sic 2. 2. Doped metal-ceramic composite synthesi platelets(0.5-5 um thick by 5-70 um diameter)(John In certain instances. some of the metal-ceramic com- son Matthey) was added to the ceramic precursor solu- osites were doped with additional phases or com tion while stirring. After 5 min, just enough water to pounds. Two of the more extensively investigated gel the precursor solution was added. The mixture was xamples were Ni/a-Al2O3 doped with ZrO2 and Ni/a left to stir continuously at 70C until the gel was Al,O, doped with Cr2O3. In the case of the Zro viscous enough to prevent SiC settling. The gel was then transferred to crystallization dishes and dried for pared by simply adding small amounts of the zirconium 24 h at 100C. Finally, the dried gels were delicately alkoxide to the initial aluminum isopropoxide solution. hand ground and sieved in the same fashion as the Reduction and hot-pressing were then carried out as metal-ceramic precursors. To fully transform the ce- described above. To produce the Cr,O, doped material, ramic gel to A2 O, the dried powders were calcined in a chromium formate salt (added to the nickel formate a quartz tube furnace at 900 C in air for 1 h. This was dihydrate solution) was used as the dopant source. then followed by uniaxial hot-pressing at 1750.C and Again, reduction and hot-pressing were carried out 35 MPa for 3 h. The 0.5 in. inside diameter dies used normally. In this case, however, even though the in this case consisted of high strength graphite(Poco chromium source was incorporated along with the N raphite, grade ZxF-5Q) and were enclosed in a the form of a salt, the employed hydrogen reduction chamber backfilled with argon for the duration of the temperature of 1000 C was not high enough to reduce high temperature exposure. Handling and post-process- the chromium to its metallic state and hence, an alu- ing of the sintered Sic/a-Al2O3 was performed in a mina-rich Al,O3/Cr,O similar fashion as described for the metal-ceramic duced as the ceramic phase composites.14 E.D. Rodeghiero et al. / Materials Science and Engineering A244 (1998) 11–21 Fig. 4. Backscattered electron SEM micrograph of the microstructure of a 50/50 vol.% Fe/a-Al2O3 composite; (Fe=light contrast, a-Al2O3=dark contrast). surfaces were prepared by flattening the pellet faces with a 20 mm metal bonded diamond wheel (Struers), coarse polishing with SiC paper, and then fine polishing with 6 mm diamond paste impregnated Texmet® polish￾ing cloth (Buehler). Finally, an aqueous ultrasonic cleaning bath was utilized immediately after polishing to remove pellet surface contamination. 2.2. Doped metal–ceramic composite synthesis In certain instances, some of the metal–ceramic com￾posites were doped with additional phases or com￾pounds. Two of the more extensively investigated examples were Ni/a-Al2O3 doped with ZrO2 and Ni/a￾Al2O3 doped with Cr2O3. In the case of the ZrO2 modified Ni/a-Al2O3, the doped composites were pre￾pared by simply adding small amounts of the zirconium alkoxide to the initial aluminum isopropoxide solution. Reduction and hot-pressing were then carried out as described above. To produce the Cr2O3 doped material, a chromium formate salt (added to the nickel formate dihydrate solution) was used as the dopant source. Again, reduction and hot-pressing were carried out normally. In this case, however, even though the chromium source was incorporated along with the Ni in the form of a salt, the employed hydrogen reduction temperature of 1000°C was not high enough to reduce the chromium to its metallic state, and hence, an alu￾mina-rich Al2O3/Cr2O3 solid solution (ruby) was pro￾duced as the ceramic phase. 2.3. Ceramic–ceramic composite synthesis The preparation of the SiC(particulate)-reinforced alumina composites consisted of the following steps. First, a 0.15 M absolute ethanol solution of aluminum isopropoxide was again prepared and heated to boil￾ing. Next, an appropriate amount of either SiC whiskers (:1 mm diameter by :15 mm long) or SiC platelets (0.5–5 mm thick by 5–70 mm diameter) (John￾son Matthey) was added to the ceramic precursor solu￾tion while stirring. After 5 min, just enough water to gel the precursor solution was added. The mixture was left to stir continuously at 70°C until the gel was viscous enough to prevent SiC settling. The gel was then transferred to crystallization dishes and dried for 24 h at 100°C. Finally, the dried gels were delicately hand ground and sieved in the same fashion as the metal–ceramic precursors. To fully transform the ce￾ramic gel to Al2O3, the dried powders were calcined in a quartz tube furnace at 900°C in air for 1 h. This was then followed by uniaxial hot-pressing at 1750°C and 35 MPa for 3 h. The 0.5 in. inside diameter dies used in this case consisted of high strength graphite (Poco Graphite, grade ZXF-5Q) and were enclosed in a chamber backfilled with argon for the duration of the high temperature exposure. Handling and post-process￾ing of the sintered SiC/a-Al2O3 was performed in a similar fashion as described for the metal–ceramic composites