Microstructure and room temperature mechanical properties of mullite fibers after heat-treatment at elevated temperatures

The composition and microstructure of Nitivy ALF 2880D fibers after heat-treatment at elevated temperatures are investigated by XRD, FT-IR, SEM and TEM analyses. Tensile properties of as-received and heat-treated fiber bundles have been studied. The results show that as-received fibers consist of γ-Al2O3, amorphous silica, and a little boron oxide. During heat-treatment process, boron oxide firstly melts and flows, resulting in large amount of liquid ravines, and then volatilizes, leaving several holes on fiber surface. Reaction between γ-Al2O3 and amorphous silica begins when heat-treated temperature is above 1100 °C, and completes at 1300 °C. As heat-treated temperature increases from 1100 °C to 1400 °C, grain growth of mullite starts and leads to the reduction of room temperature tensile strength of fibers. Tensile strength of fibers stays stable when heat-treated temperature is below 1200 °C, while the strength retention of fibers sharply decreased to 50% after heat-treatment at 1300 °C.