Influence of nucleating agents on the crystallization of Mg-Ca-Si-Al-O-N oxynitride glasses

Oxynitride glasses in the system Mg-Ca-Si-Al-O-N were prepared viith the aid of a polymeric preceramic aluminum nitride precursor as nitrogen source and transformed into glass-ceramics at temperatures in the range of 950 to 1350 degrees C. To support the devitrification process, TiO2, Cr2O3, MgF2 and ZrO2 were added to the batches as potentially nucleating agents. Under the reducing melting conditions applied. TiO2 led to undesired reactions in the glass melt and was rejected as nucleating agent. Cr2O3 could only be dissolved in the glasses to a maximum content of 1 wt% and did not lead to obvious effects concerning nucleation and crystal growth. The addition of fluorine results in the formation of a fine-grained microstructure but by analogy to Cr2O3, did not lead to phase separation. Adding ZrO2. provoked phase separation in the oxynitride glasses, which was even promoted by the presence of nitrogen. These glass-ceramics possessed an extremely fine-grained microstructure containing nonstabilized tetragonal ZrO2, which results in an additional nitrogen content independent Improvement of the mechanical properties. By comparison to TiO2, Cr2O3 and MgF2, zirconia has a twofold effect: it is not only an efficient nucleating agent in the case of oxynitride glasses but also an efficient toughening agent for the resulting glass-ceramics.