A comparison of the microstructure and mechanical properties of two liquid phase sintered aluminas containing different molar ratios of calcia–silica sintering additives

The microstructures of two dense polycrystalline aluminas hot pressed with a total of 5 wt.% of liquid phase-forming sintering additives: calcium oxide (CaO) and silicon oxide (SiO2) but with differing CaO:SiO2 molar ratios, have been studied using analytical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray (EDX) spectrometry and electron energy loss spectrometry (EELS). These techniques allowed the direct imaging of secondary crystalline phases and amorphous films at grain boundaries and at triple pockets as well as both a qualitative and semi-quantitative determination of their composition and chemistry. Two samples CS5 and CSp, containing CaO:SiO2 molar ratios of 1:5 and 1:1.33 respectively, were studied in detail. SEM revealed big differences in median grain size between the CS5 sample, 0.56 μm, and the CSp sample, which was more than 2 times larger. TEM revealed that the CS5 sample exhibited a significant proportion of a secondary crystalline anorthite phase at triple pockets which resulted in a silica-rich glassy intergranular phase deficient in calcium possessing a composition lying in the mullite phase field of the ternary phase diagram. Conversely the CSp specimen possessed a heterogeneous glassy film which differed in composition between grain boundaries (where significant Ca was observed) and triple pockets (which were more silica-rich). The microstructure of CSp also showed evidence for dislocation arrays, strain and microcracks possibly due to the mismatch in thermal expansion coefficients between alumina grains, glassy and secondary and crystallised phases which, together with bigger grain size, may explain the reduced wear resistance relative to CS5.