Hardness and Indentation Fracture Toughness of Slip Cast Alumina and Alumina-Zirconia Ceramics

Alumina (Al₂O₃) and zirconia (ZrO₂) have good overall properties and thus are widely used oxide technical ceramics. The biggest drawback of Al₂O₃ is its low fracture toughness. In contrast, ZrO₂ is relatively tough, but is also much more expensive. In order to improve the alumina toughness, composite ceramics are being developed. Slip casting technology has economic advantages over the conventional hot isostatic pressure technology, but problems may arise when preparing stable highly-concentrated suspensions (slip) for filling the mold. The purpose of this study is to prepare aqueous suspensions using 70 wt. % α-Al₂O₃, with 0, 1, 5 and 10 wt. % of added t-ZrO₂. Suspensions were electrosterically stabilized using the ammonium salt of polymethylacrylic acid, an alkali-free anionic polyelectrolyte dispersant. Also, magnesium oxide in form of magnesium aluminate spinel (MgAl₂O₄) was used to inhibit the abnormal alumina grain growth during the sintering process. Minimum viscosities were used as stability estimators, where an increase in ZrO₂ content required adding more dispersant. After sintering, the Vickers indentation test was used to determine the hardness and the indentation fracture toughness from the measurement of the crack length. Also, the brittleness index (B_i, μm^−1/2) was calculated from values of Vickers hardness and the Vickers indentation fracture toughness. It was found that with increasing ZrO₂ content the fracture toughness increased, while the hardness as well as the brittleness index decreased. Zirconia loading reduces the crystallite sizes of alumina, as confirmed by the X-ray diffraction analysis. SEM/EDS analysis showed that ZrO₂ grains are distributed in the Al₂O₃ matrix, forming some agglomerates of ZrO₂ and some pores, with ZrO₂ having a smaller grain size than Al₂O₃.