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

Abstract

Oxynitride glasses in the system M g - C a - S i - A l - O - N were prepared with the aid of a polymeric preceramic aluminum nitride precursor as nitrogen source and transformed into glass-ceramics at temperatures in the ränge of 950 to 1350°C To support the devitrification process, TiO₂ , Cr₂O₃, MgF₂ and ZrO₂ were added to the batches as potentially nucleating agents. Under the reducing meldng conditions applied. TiO₂ led to undesired reactions in the glass melt and was rejected as nucleadng 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 Ti02, Cr203 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.