The sintering behaviour, mechanical properties and creep resistance of aligned polycrystalline yttrium aluminium garnet (YAG) fibres, produced from an aqueous sol-gel precursor

Continuous ceramic fibres are finding applications as reinforcements in cer amic matrix composites, and yttrium aluminium garnet (YAG) is a particularl y attractive candidate material on account of its creep resistance at high temperatures. A continuous, aligned, 5.5 mu m diameter polycrystalline YAG fibre was manufactured from an aqueous sol-gel precursor which contained ch lorine, and compared to a similar nitrate containing YAG precursor fibre we have reported previously. The precursor sol was found to be stable at a hi gher concentration than the nitrate containing sol, and this resulted in de nser gel fibres which demonstrated better sintering at equivalent temperatu res, giving a 98.5% sintered YAG fibre at 1550 degrees C with a grain size of only, 1 mu m. However, on firing in air, the fibres formed fully crystal line YAG between 800 and 900 degrees C, a temperature 100 degrees C higher than the fibres containing nitrate, and they were weakened by the presence of many hemispherical faults. It was shown that both of these features were due to the retention of chlorine until the onset of formation of the cryst alline YAG phase, and a series of steaming experiments were devised to remo ve the halide before this process could occur. It was found that steaming t he precursor fibre from 200 to 500 degrees C over 3 h, followed by firing t o the required temperature in air, removed the chlorine and the problems it caused in the formation of the YAG phase without any change in the sinteri ng characteristics or grain size. The steamed fibres were of a strength and quality comparable to fibres drawn from organometallic precursors. Empiric al friability measurements showed the strength was maintained after firing to 1550 degrees C, although there was a deterioration irt apparent strain t o break of the aligned blanket product above 1200 degrees C. Conversely, th e creep resistance, measured using the BSR test, improved with increase in temperature. The fibres fired to 1550 degrees C were fully, relaxed at temp eratures 100-150 degrees C below that of coarser, larger YAG fibres previou sly reported with a 3 mu m grain size and 120 mu m diameter. However, when allowance was made for grain size, the difference in creep rates,vas within the range obtained by extrapolating previous data using lattice diffusion and grain boundary, effect models. Fibres fired to 1400 degrees C were much finer grained but only slightly inferior to the 1550 degrees C fibre in te rms of creep. The alumina sol used in this work contained a significant lev el of sodium, and this suggests that the creep rates ape effected by grain boundary impurities, especially sodium. A sodium free sol has been procured and fur ther work is recommended to clarify the effect of impurities and i mprove fibre properties. (C) 1999 Elsevier Science Limited. All rights rese rved.