Effects of Silica on the Combustion Synthesis and Glass Formation of TiB sub(2)-Containing Calcium Aluminate Matrix Composites

Combustion synthesis has been used to study the combustion characteristics and microstructure in the TiB sub(2)-CaO-Al sub(2)O sub(3)-SiO sub(2) system. Both the combustion temperature and wave velocity decreased with an increase in the amount of silica. The reacted product consisted of solid crystalline TiB sub(2) phase and CaO-Al sub(2)O sub(3)-SiO sub(2) matrix. No significant reaction between the TiB sub(2) and the matrix was observed. It was found that the binary calcium aluminate matrix (Ca sub(12)Al sub(7)O sub(33) and CaAl sub(2)O sub(4) compounds) could form amorphous phase (glass) at a high cooling rate. Under a normal cooling rate, significant formation of glass in the matrix was found possible only by adding another glass-former, e.g., silica (SiO sub(2)). The addition of SiO sub(2) led to the formation of Gehelenite, to the reduction of the other crystal phases and to an increase in the glass phase in the matrix. A pure glass matrix was obtained for all compositions in the composition range from Ca sub(12)Al sub(7)O sub(33) to CaAl sub(2)O sub(4) compounds provided that an adequate amount of SiO sub(2) was added. The resulting glass matrix can be expressed by the formula Ca sub(2)(Si sub(x)Al sub(1-x)) sub(3)O, where x=0.32-0.58, depending on the composition. Effects of the TiB sub(2) phase on the combustion synthesis process and on the microstructure are also discussed. TiB sub(2) increased the reactivity and led to the formation of the molten matrix which was critical for glass formation. However, its effects on the vitrification of the matrix was considered to be minimum. The mechanism of glass formation in the matrix is discussed using the Zachariasen-Warren network theory.