Silicon Carbide Reinforced Alumina and Mullite Ceramics

Doctoral thesis, 2006

The aim of this work was to develop homogeneous ceramic nanocomposite materials with good thermal and structural stability as well as good mechanical properties. The development of microstructure was related to the material properties. This thesis reports on the electron microscopy characterization of alumina (Al2O3) and mullite (3Al2O3·2SiO2) reinforced with 5 vol % SiC nanoparticles. Al2O3 / 5 vol% SiC nanocomposite materials were pressureless sintered to near full density at 1780 °C. Smaller additions of MgO promoted densification at lower temperatures. The addition of both MgO and SiO2 resulted in a slightly reduced density and a larger Al2O3 grain size as compared to doping with MgO only. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The hardness varied in the range 17.0-18.5 GPa and the indentation fracture toughness varied between 2.3 and 2.6 MPam1/2 when the material was sintered at 1780 °C. The crack front propagated transgranularly without interacting with intragranular SiC. Crack bridging was observed but not crack deflection. Al2O3 / 5 vol% SiC composites were also fabricated by the in-situ synthesis of nano-sized SiC particles. It was found that SiC and mullite formed during sintering. SiC nanoparticles with a size ranging from a few ten nanometers up to ~300 nm were located predominantly to the interior of both mullite and alumina matrix grains. The relationship between the fine-scale micro- and nanostructure and the creep deformation process in single-phase mullite and mullite reinforced with 5 vol% nano-sized SiC particles was investigated. Creep deformation of polycrystalline mullite was dominated by diffusional processes and unaccomodated grain boundary sliding facilitated by softening of the intergranular glass. Diffusional creep in the nanocomposite was inhibited by the slower diffusional transport in the SiC particles compared to self diffusion of the matrix, leading to an enhanced creep resistance by grain boundary pinning. Grain boundary sliding due to softening of the intergranular glass reduced the beneficial effect of the nanoparticles at higher stresses.