Degradation of oxide fibers by thermal overload and environmental effects

High strength (2-3 GPa) of polycrystalline oxide fibers used as reinforcements of ceramic bodies is achieved as a result of submicron grain size. The fine-grained microstructure, however, is not stable at high temperatures and hence grain coarsening must be considered. Coarsening of alumina in commercial 3 M Nextel fibers occurs above 1200 °C going along with significant strength reduction. Analysis of isothermal grain growth reveals relative little time dependency which is attributed to limited grain boundary mobility. Activation energy of alumina grain growth is 660 kJ/mol. Moderate grain boundary mobility of Nextel 650 and 720 fibers is attributed to second phase blocking while in case of Nextel 610 alumina fibers sub-nanometer silica grain boundary films are assumed to affect the grain boundary mobility. Pronounced grain growth occurs in Nextel 610 fibers as soon as stabilizing silica films dissipate either by outwards diffusion or by hydroxylation and subsequent volatilization in the presence of hot water vapour. Mullite grains are less prone against coarsening than alumina as evidenced in activation energy (900 kJ/mol) and very low grain boundary mobility at temperatures below 1600 °C.