High‐Temperature Characterization of Reaction‐Sintered Mullite‐Zirconia Composites

Abstract

Mullite‐zirconia composites are prepared by reaction sintering of zircon and alumina, either using reaction and sintering additives (titania (T‐MZ) or magnesia (M‐MZ)) or starting from highly reactive powder mixtures obtained by an ultra‐rapid‐quenching technique (URQ‐MZ). In the latter case, the presence of large amounts of an amorphous phase in the quenched powders enables sintering without the use of additives. The various composites differ mainly by their microstructure—i.e., the mullite grains aspect ratio and grain size—and by the quantity and nature of impurities or additives. Within this work, the modulus of rupture and critical stress intensity factor of each material are measured from room temperature to 1200°C. Bending creep is also investigated for various temperature and stress ranges. For the three materials, the fracture toughness (KIC) decreases with temperature, according to tetragonal zirconia stabilization, up to 600° to 700°C, above which a significant increase occurs. This toughening effect is attributed to plastic relaxation ahead of the crack tip associated with a glassy phase at the grain boundaries. The reinforcement is more efficient for T‐MZ because of the higher viscosity of the glassy phase. For the same material, strength does not follow the KIC curve because of subcritical crack growth. Creep data are explained on the basis of two distinct mechanisms, depending on stress and temperature range. At high stresses and/or low temperatures, creep is associated with microcrack formation, whereas, at low stresses and/or high temperatures, data are in agreement with a solution‐precipitation mechanism accommodating grain‐boundary sliding with interface reaction as the limiting stage. Despite its larger amount of glassy phase, the M‐MZ ceramic shows the better creep resistance because of its particular microstructure—consisting of cross‐linked elongated mullite grains—which impedes grain‐boundary sliding.