Effect of TiO2 content on the crystallization behavior and properties of CaO-Al2O3-SiO2 glass ceramic fillers for high temperature joining application

The effects of TiO2 content on the crystallization behavior, microstructure, microhardness, coefficient of thermal expansion (CTE) and softening temperature of CaO-Al2O3-SiO2 glass ceramic fillers were investigated. The results show that the crystallization activation energy (E) firstly decreases and then increases with the TiO2 addition from 10 wt% to 20 wt%. The microstructure and phase composition of glass ceramics importantly depend on the content of TiO2. Low TiO2 content results in formation of Ti-contained crystals (CaTiSiO5) due to phase separation. In contrast, high TiO2 concentrations lead to precipitation of small titanium oxide crystals that act as nuclei, facilitating the development of CaAl2Si2O8. Furthermore, the formation of crystalline phases is beneficial for improving the microhardness and refractoriness of glass ceramics. The CaO-Al2O3-SiO2 glass with 15 wt% TiO2 addition exhibits the highest crystalline volume fraction possibly due to the lowest activation energy of crystallization. As a result, its microhardness and softening temperature can reach as high as 898.2 HV and 1027 °C, respectively. However, its CTE slightly decreases from 7.44 × 10−6/°C to 6.35 × 10−6/°C after crystallization treatment because of the formation of CaTiSiO5.