THERMOMECHANICAL PROPERTIES AND CREEP-BEHAVIOR OF ZIRCONIA BASED ADVANCED MATERIALS PREPARED BY ULTRA-RAPID QUENCHING

Mullite-zirconia and forsterite-zirconia composites were prepared from raw materials synthesised by the ultra rapid quenching (URQ) techniqu e. Compared to more conventional preparation routes, the URQ-technique leads to powders with improved homogeneity, which can be sintered up to densities close to the theoretical ones without the use of sinterin g aids. After firing, the dense bodies were characterised by their the rmomechanical properties as well as by their creep behaviour. The URQ- MZ composite displays higher strength and toughness values than the UR Q-FZ material owing to a reinforcement effect associated with zirconia phase transformation. Such a reinforcement is however rapidly lost wh en increasing temperature and can therefore account for the sharp decr ease of the thermomechanical properties under 600 degrees C. In the ca se of URQ-FZ, because phase transformation does not arise when loading , the room temperature characteristics are obviously maintained up to 600 degrees C. At 800 degrees C and above, the mechanical properties e volve in a similar way for both materials and are governed by the resi dual glassy phase at the grain boundaries. Taking into account the pre sence of this intergranular glassy phase, it is proposed to explain th e superplastic deformations and the high strain rates reached during c reep tests on the basis of a solution-diffusion through liquid phase-p recipitation mechanism. Amongst the two steps involved in this sequent ial mechanism, diffusion rather than matter exchange at the crystal/li quid interface appears as the limiting step controlling creep at low s tresses.