Structure and chemistry of grain boundaries in SiO₂-doped TZP

The addition of glass phase can control the grain boundary structure and hence the mechanical properties of tetragonal zirconia polycrystals (TZP). To reveal the effect of the glass dopant on the high-temperature deformation behavior of TZP, SiO2-doped TZP, (SiO2–Al2O3)-doped TZP, (SiO2–MgO)-doped TZP and undoped TZP were prepared and their grain boundary structure, chemical composition and chemical bonding state were investigated by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) using a field-emission-type transmission electron microscope (FE-TEM). It was found that no amorphous film was formed along the grain boundaries in any of the specimens examined, but amorphous pockets formed at multiple grain boundary junctions in three kinds of glass-doped specimens. In the glass-doped specimens, the segregation of yttrium, silicon and the added metal ions (Al3+ or Mg2+) was observed over a width of several nm across the grain boundaries. The addition of pure SiO2 much enhanced the ductility in TZP, although further addition of a small amount of Al2O3 or MgO to SiO2 phase resulted in a marked reduction in the tensile ductility of SiO2-doped TZP. EELS measurements and molecular orbital (MO) calculations using a cluster model revealed that the ductility of TZP was related to the bond overlap population (BOP) at the grain boundaries, which was influenced by the kinds of segregated dopants. That is, the presence of Si4+ increases the BOP, strengthening the grain boundary bonding strength and thus preventing cavity formation, but Al3+ and Mg2+ decrease the BOP, enhancing the grain boundary cavitation and thus reducing the ductility. Furthermore, the dynamic behavior of SiO2 in TZP was observed using a TEM in situ heating technique, and the results supported the fact that that Si segregates along the grain boundaries.