Viscous flow behaviour of fluorozirconate glasses

The isothermal viscous flow properties of two fluorozirconate glass compositions have been studied by parallel plate rheometry at temperatures approaching the fibre-drawing viscosity region. An unusual flow behaviour has been found, in which sharp end points occur where deformation ceases under load. This behaviour has been characterised in terms of a threshold yield stress with hysteresis effects. The materials are prone to crystallisation, but no obvious direct link has been found between such effects and the flow behaviour, which may thus be intrinsic.