Polycrystalline yttrium aluminium garnet (YAG) fibres produced from the steaming of an aqueous sol-gel precursor

Continuous ceramic fibres are constantly finding new applications as high temperature structural and engineering materials, and yttrium aluminium garnet (YAG) demonstrates the best all-around resistance to creep. A continuous, aligned, 5.5 μm diameter polycrystalline YAG fibre was manufactured from an aqueous sol-gel precursor which contained chlorine, and was compared to a similar nitrate containing YAG precursor fibre we have reported previously. The precursor resulted in denser gel fibres which demonstrated better sintering at equivalent temperatures. However, the fibres formed fully crystalline YAG between 800 and 900°C, a temperature 100°C higher than the fibres containing nitrate, and they were weakened by the presence of many hemispherical faults present in the fibre. It was shown that both of these features were due to the retention of chloride until the onset of formation of the crystalline YAG phase, and a series of steaming experiments were devised to remove the halide before this process could occur. Steaming at 500°C resulted in a poorly crystalline YAG fibre which also remained mechanically weak. It was found that steaming the precursor fibre from 200-500°C over 3 h, followed by firing to the required temperature in air, removed the chlorine and the problems it caused in the formation of the YAG phase with out any change in the sintering characteristics or grain size.