85Al₂O₃/SiC Nanocomposites: Preparation By Pressureless Sintering In Reduced Atmosphere And Surface Mechanical Properties Thereof

85Al2O3/SiC nanocomposites, in which different amount of nano-scale SiC particles were dispersed in 85Al2O3 matrix, were prepared by presureless sintering in reduced atmosphere formed by C powder and coarse SiC grits in a limited space. Densities and mechanical properties of the nanocomposites were measured. Fracture surfaces of the nanocomposites were characterized by SEM. The results revealed that SiC particles had great influences on the microstructure and mechanical properties of nanocomposites. Nanocomposites with low SiC content(≤10%) could be fully densified in reduced atmosphere without applying pressure. However, high SiC content(>10%) would greatly inhibit the sintering process and lead to low density. SEM observation of the fracture surface showed that there was an obvious change of facture mode from mainly intergranular fracture in 85Al2O3 to transgranular fracture in the nanocomposites by the addition of nanosized SiC particles. The polishing behavior and wet erosive wear resistance have been studied to evaluate the surface mechanical properties of 85Al2O3/SiC nanocomposites. For polishing behavior, the polished surface with various grit sizes were investigated by optical microscope, and the surface quality was quantified by the relative areas of smooth polished regions as apposed to darker rough regions. It was found that nanocomposites with 1~10% SiC had a better surface finish than 85Al2O3 at all stages of progressively polishing treatment. For wet erosive wear tests, the wear rates of nanocomposites with different amount of SiC nano-particles were measured and compared with 85Al2O3 ceramics. The results showed that 85Al2O3/1-10%SiC nanocomposites had lower wear rates than the pure 85Al2O3. Both results from polishing reponses and erosive wear resistance demonstrated much improved surface mechanical properties for nanocomposites over the 85Al2O3 ceramics. The material removal mechanisms of 85Al2O3 and 85Al2O3/SiC nanocomposites were dicussed based on the SEM observation of the ground and erosive wear surfaces. It was found that, during polishing and erosive wearing, the material removal processes are different for the two types of material. In 85Al2O3 ceramics, grain pullout and intergranular cracking are main removal processes, while local plastic deformation and fine scale removal might be main removal processes in nanocomposites.