Nanocrysrtalline Powders in the A12O3-ZrO2-CeO2-Y2O3 system for high-performance ceramics
X-ray diffraction, BET measurements, scanning electron and optical microscopy were used to analyze the properties variation of hydrothermal nanocrystalline powder with a composition 90 wt% Al<inf>2</inf>O<inf>3</inf>-10 wt%ZrO<inf>2</inf> (CeO<inf>2</inf>, Y<inf>2</inf>O<inf>3</inf>) after thermal treatment in the temperature range 400–1300 °C. The composition of the ZrO<inf>2</inf>-based solid solution was (mol %) 90ZrO<inf>2</inf>-8CeO<inf>2</inf>-2Y<inf>2</inf>O<inf>3</inf>. It was found that α-Al<inf>2</inf>O<inf>3</inf> is formed at 850 °C. Phase transformation F-ZrO<inf>2</inf>⇆T-ZrO<inf>2</inf> occurred at 1000 °C. Produced powder was sintered at 1300 °C. The present investigation demonstrates that heat treatment of composite nanocrystalline powders gives rise to complex processes. Understanding of such processes makes it possible to optimize conditions for fabrication of advanced ceramics with tailored properties.
Effect of heat treatment on the properties of nanocrystalline 80 wt % Al2O3-20 wt % ZrO2〈CeO2, Y2O3〉 powder
We have studied the evolution of nanocrystalline 80 wt % Al2O3-20 wt % ZrO2〈CeO2, Y2O3〉 powder prepared through hydroxide coprecipitation followed by hydrothermal decomposition of the hydroxides and firing at temperatures from 400 to 1300°C. α-Al2O3 has been shown to form at 850°C. The metastable phase F-ZrO2 persists up to this temperature. The variation in the morphology of the powder is topologically continuous. The processes induced by heat treatment of the nanocrystalline powder are interpreted in terms of the evolution of an open system.
Numerical Evaluation of Toughening by Crack‐Face Grain Interlocking in Self‐Reinforced Ceramics
Crack bridging associated with the pull-out process of interlocking grains in self-reinforced ceramic materials is studied through a micromechanical simulation. The pullout of a single inclined grain is modeled via the numerical solution of a general contact problem. The bridging-force versus crack-opening-distance curve indicates a nonlinear, springlike response for the pullout of interlocking grains. The sliding friction along the debonded interface, induced by highly localized contact stresses, dominates the total bridging force. The bridging force increases with grain inclination until eventual bridge failure. The pullout of misaligned grains mainly affects short-crack toughening, with a rising R-curve, whereas aligned grains contribute to long-crack toughening. The residual stresses of the thermal expansion anisotropy play a minor role in the pull-out process of grain interlocking and the resultant toughening. The proposed mechanism is operative in both single-phase and composite ceramics in which pullout of elongated grains/reinforcements occurs.