Mechanical behaviour of zirconia–mullite directionally solidified eutectics

• Directionally solidified ZrO2-mullite eutectic grown by laser floating zone.
• Ultra-fine eutectic morphology.
• A remarkable hardness value of 21.2 GPa for the fastest as-grown fibres.

Laser floating zone technique (LFZ) is used to grow directionally solidified eutectic (DSE) zirconia–mullite composite fibres (30:70 in wt.%). A notable increase in hardness is observed from 11.3 to 21.2 GPa as the pulling rate increases from 10 to 500 mm/h, due to the ultra-fine eutectics developed at very high growth rates. The indentation fracture toughness reaches a maximum value of 3.5 MPa m1/2 for the fibre pulled at 100 mm/h, almost three times the value of 1.2 MPa m1/2 determined for LFZ single-crystal mullite. The eutectic dendrites that develop along the growth direction are immersed in a glassy phase whose brittleness is counteracted by the beneficial ultra-fine morphology, giving a bending strength maximum of 534 MPa. Yet, the soft nature of the glassy matrix prevails at the high temperature testing (1400 °C), causing a decrease to about one-half of the RT value in the fibres with less glassy phase content.