Growth of Al2O3/ZrO2(Y2O3) eutectic rods by the laser floating zone technique: effect of the rotation

The laser floating zone technique has been applied to the growth of Al2O3/ZrO2(Y2O3) rods in the eutectic composition to reveal the effect of forced convection induced by rotation on the rod microstructure. A systematic experimental study of this effect has been carried out combining different source rod and/or eutectic rod rotation (0-200 rpm) and travelling speeds (10-1500 mm/h) in an axial thermal gradient close to 6 × 105 C/m. The results indicate that the rotation is useful to achieve a more homogeneous temperature distribution, especially in thick rods but it has a limited effect in the change of the solidification front shape. The forced convection in the floating zone caused by rotation slightly flattens the solidification interface enhancing the homogeneity of the phase distribution across the sample. However, it introduces several new microstructural features like banding and phase coarsening that can deteriorate the mechanical behaviour of the rods. On the other hand, rods above 1.6 mm in diameter cannot be grown without cracks, even with fast eutectic rod rotation. Rotation does not change the pulling rate threshold (50 mm/h) at which the transition from coupled to dendritic and cellular growth morphology takes place.