Aluminum titanate is a ceramic material with low thermal expansion owing to its large thermal expansion anisotropy. Low thermal expansion is important as it results in excellent resistance to thermal shocks. Aluminum titanate has been applied in refractories for molten aluminum industries, and as an element for diesel filters because of its high melting temperature and low thermal coefficients. However, this low thermal expansion is as a result of grain boundary cracks owing to the large thermal expansion anisotropy. Therefore, these ceramics have poor mechanical attributes.
Several studies have been proposed on the improvement of the mechanical integrity of ceramics. Introducing rectangular elongated grains into the granular grained structure has proved to be an efficient approach for self-reinforcement of ceramics. Elongated grains have been found to enhance fracture toughness via crack deflection and branching. Therefore, several additives have been identified to enhance aluminum titanate grain growth that lead to formation of rectangular elongated grains. In the present study, a self-reinforcement by elongated grains approach was applied to aluminum titanate ceramics.
While improving the mechanical properties of the ceramics, an increase in thermal coefficient is experienced. It is therefore important to enhance the mechanical strength while maintaining low thermal coefficient. Researchers led by Professor Yutaka Ohya at Gifu University in Japan studied the mechanical integrity of aluminum titanate ceramics and its thermal expansion when fabricated with and without doping. They investigated the relationship between the grain boundary cracking and the microstructure in order to understand its thermal expansion. Their work is now published in Journal of the European Ceramic Society.
Rectangular grains were generated implementing liquid-phase through doping with niobium pentoxide and lanthanum oxide. Lanthanum oxide content was fixed at 1% in order to obtain the self-reinforcement by elongated grains. The authors used raw powders of rutile and corundum and mixed them with co-doping additives, including, magnesium oxide-lanthanum oxide, magnesium oxide-niobium pentoxide and magnesium oxide-yttrium oxide. The powders were then mixed in an alumina ball mill and calcined.
The resulting powder was die-molded into rectangular bars. The authors then fired the ceramics at 1400, 1450 and 1500 °C for 2 h. They then determined the thermal expansion of the sintered ceramics and the occurrence of the grain boundary cracking. Four to six as-fired ceramics specimens were selected for mechanical testing and their results averaged.
The researchers observed that co-doping with lanthanum oxide and magnesium oxide in the mixture of rutile and corundum and firing at 1500° C led to ceramics with bimodal grained microstructure. Liquid-phase sintering and firing gave rise to elongated grains via Ostwald ripening. Self-reinforcement by the elongated grains led to sufficient resistance against crack propagation, low crack extension speed and preserved the very low thermal expansion coefficient. Increased grain length, grain pullout and bridging, crack branching and deflection gave rise to the high resistance against crack propagation.
However, co-doping of yttrium oxide and magnesium oxide led to moderate low thermal expansion coefficient and 34MPa bending strength. The ceramics’ brittle fracture was pegged at their granular microstructure. Magnesium-niobium oxides doped ceramic displayed poor mechanical integrity that was attributed to the formation of large rectangular grains owing to liquid-phase sintering.
Therefore, controlled bimodal grain microstructure by liquid phase sintering is essential for ceramic materials with very low thermal expansion and high crack extension resistance .
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Yutaka Ohya, Shu Yamamoto, Takayuki Ban, Makoto Tanaka, Satoshi Kitaoka. Thermal expansion and mechanical properties of self-reinforced aluminum titanate ceramics with elongated grains. Journal of the European Ceramic Society, volume 37 (2017), pages 1673–1680.Go To Journal of the European Ceramic Society