NANOCOMPOSITES IN MULLITE-ZRO2 AND MULLITE-TIO2 SYSTEMS SYNTHESIZED THROUGH ALKOXIDE HYDROLYSIS GEL ROUTES - MICROSTRUCTURE AND FRACTOGRAPHY

The sol-gel process allows preparation of very homogeneous and reactive monolithic, optically clear gels. Low-temperature thermal treatments (700-1000-degrees-C) lead to amorphous optically clear samples ("glass"). Amorphous mullite compositions (0.4Al2O3-0.6SiO2 to 0.8Al2O3-0.2SiO2) retain large amounts of Ti and Zr elements. The crystallization has been studied by differential thermal analysis, dilatometry, X-ray and electron diffraction and Raman scattering. The nucleation begins above 1000-degrees-C with the departure of the last protonic species, the amorphous matrix being completely crystallized only above 1400-degrees-C. The addition of Zr and Ti elements leads to a homogeneous nucleation of phases with a composition close to ZrO2 and Al2Ti3O9 (EDX analysis) above the solubility limit. TEM and SEM analyses show that the precipitate size remains submicrometric over a wide temperature range (1000-1400-degrees-C) and consequently glass-like mechanical properties, as well as toughening effects, caused by the presence of nanoprecipitates, are observed.