Alumina nanocrystalline ceramic by centrifugal casting

Centrifugal casting is an established molding method to prepare ceramics with high strength and high reliability and it has been well demonstrated in Al2O3. However, it has not yet been applied to Al2O3 nanocrystalline ceramic with < 100 nm grain size, primarily due to the unavailability of high-quality alpha-Al2O3 nanoparticles. In addition, ultrafine nanoparticles may be difficult to cast from the solution unless high-speed ultracentrifuge (e. g., >60,000 rpm) is used. Here we addressed these two challenges by home-made dispersed alpha-Al2O3 nanoparticles with 10 nm average particle size and HCl-assisted casting under a normal centrifuge condition and report the first attempt to produce Al2O3 nanocrystalline ceramic by centrifuge casting and pressureless sintering. The sintering kinetics and microstructure were analyzed, which assists the design of optimal two-step sintering schedule. We showed that dense Al2O3 nanocrystalline ceramic with 65 nm average grain size and ultra-uniform microstructure (the standard deviation of the grain size distribution to the average grain size is 0.358) can be obtained by two-step sintering at 1175 degrees C without holding followed by holding at 1025 degrees C for 20 h. The ultrauniform microstructure may result from the denser and more uniform packing of particles in the green bodies produced by centrifugal casting. The two-step sintered Al2O3 nanocrystalline ceramic has a microhardness of 19.9 GPa. The microhardness indicates potential softening (inverse Hall-Petch relationship) of Al2O3 nanocrystalline ceramic at such a grain size.

Automated summary

Centrifugal casting has been successfully applied in the preparation of Al2O3 ceramics with high strength and high reliability, but it has not been used for Al2O3 nanocrystalline ceramics with < 100 nm grain size due to the lack of high-quality alpha-Al2O3 nanoparticles. In this study, we overcame these challenges by using home-made dispersed alpha-Al2O3 nanoparticles and HCl-assisted casting under normal centrifuge conditions. The results showed that an ultra-uniform microstructure with 65 nm average grain size can be obtained by two-step sintering. The microhardness of the sintered Al2O3 nanocrystalline ceramic was measured to be 19.9 GPa, indicating potential softening at this grain size due to the inverse Hall-Petch relationship.