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 t emperature structural and engineering materials, and yttrium aluminium garn et (YAG) demonstrates the best all-round resistance to creep. A continuous, aligned, 5.5 mu m 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 previous ly. The precursor resulted in denser gel fibres which demonstrated better s intering at equivalent temperatures. However, the fibres formed fully cryst alline YAG between 800 and 900 degrees C, a temperature 100 degrees C highe r than the fibres containing nitrate, and they were weakened by the presenc e 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 experimen ts were devised to remove the halide before this process could occur. Steam ing at 500 degrees C resulted in a poorly crystalline YAG fibre which also remained mechanically weak. It was found that steaming the precursor fibre from 200-500 degrees C over 3 h, followed by firing to the required tempera ture in air, removed the chlorine and the problems it caused in the formati on of the YAG phase with out any change in the sintering characteristics or grain size. (C) 1999 Elsevier Science B.V. All rights reserved.