Effects of Y 2O 3 additives and powder purity on the densification and grain boundary composition of Al 2O 3/SiC nanocomposites

Abstract

Sub-micron sized SiC additions can be used to increase the wear resistance and change the fracture mode of Al 2O 3. However, these additions also restrict sintering. Al 2O 3 and Al 2O 3–5%SiC ‘nanocomposites’ were prepared from alumina powders of high purity and of commercial-purity, with or without the addition of Y 2O 3. The effects of these compositional variables on sintering rate, final density and grain boundary composition were investigated. A direct comparison with Al 2O 3–SiO 2 composites was also made, as it has been proposed that SiC partially oxidises during processing of Al 2O 3–SiC nanocomposites. The addition of 5 vol.% SiC to Al 2O 3 hindered densification, as did addition of 0.15 wt.% Y 2O 3 or 0.1 wt.% SiO 2. In contrast, the addition of 0.15 wt.% Y 2O 3 to Al 2O 3–5% SiC nanocomposites improved densification. The composition of Al 2O 3–Al 2O 3 grain boundaries in these materials was studied using STEM and EDX microanalysis. The addition of SiC and SiO 2 caused segregation of Si, and Y 2O 3 addition caused segregation of Y. The segregation of each element was equivalent to <10% of a monolayer at the grain boundary. However, if SiC and Y 2O 3 were simultaneously added the segregation increased to 40% of a monolayer. The enhanced segregation was attributed to increased oxidation of SiC in the presence of Y 2O 3 allowing formation of a SiO 2–Al 2O 3–Y 2O 3 eutectic phase or a segregated layer which may explain the improvement in sintering rate when Y 2O 3 was added to nanocomposites.