Conventional sintering route for the production of alumina-based nanocomposites: a microstructural characterization

Two α-Al2O3/YAG composite powders have been prepared by reverse-strike precipitation, starting from chlorides aqueous solutions, the former containing 50 vol% of the two phases (labelled as AY50) and the latter made of 90 vol% of alumina and 10 vol% of YAG (AY90). The as-prepared powders were characterised by DTA/TG simultaneous analysis as well as by XRD analysis performed after calcination at different temperatures. A systematic TEM analysis was performed on AY50 powders pre-treated at different temperatures, in order to investigate the crystallites size evolution as a function of the temperature. After that, samples were compacted by uniaxial pressing and sintered at 1600°C for 3h. SEM observations revealed a homogeneous microstracture made of micronic alpha-alumina and YAG grains. For limiting grain growth through the decreasing of the maximum sintering temperature, an innovative activation procedure by coupling suitable thermal and mechanical treatments of the powders was performed. After that, high densification (>95% of the theoretical density) was easily achieved by performing a free sintering in the temperature range between 1320° and 1420°C, with different soaking times at the maximum temperature. The resulting sintered bodies showed an effective retention of the nano-size of the primary particles. By SEM, highly-homogeneous nanostructures, with an average grains size of about 200 and 300 nm for AY50 and AY90, respectively, were observed.