Effect of the addition of mullite–zirconia to the thermal shock behavior of zircon materials

Ceramics and refractories are subjected to local temperature and atmosphere gradients due to the successive application of heating cooling cycles, during service. These situations originate thermal stresses that cause certain degree of damage to the material defining the potential usage of the refractory. The favorable effect of the addition of a second phase for improving the thermal shock resistance (TSR) of dense ceramics have been proved previously. However the addition must be controlled because it can excessively affect the densification and microstructure. The objective of this work is to study the TSR of pure zircon ceramic materials and to quantify the influence of the addition of mullite–zirconia (MZ) grains in a range between 15 and 45 wt.%. Cool water quenching test was carried out on slip cast prismatic bars in order to study the TSR. For the thermal shock tests temperatures differences (ΔT) between 200 and 1200 °C were applied. The elastic dynamic modulus (E) was evaluated by the impulse excitation technique as a function of the temperature difference of the quenching test. The decrease in the E modulus was also correlated with the number of thermal cycles. The materials were characterized by density measurements and mechanical properties. The XRD and Rietveld method were employed for determining the crystalline phase composition. Finally, the microstructure of the materials was examined by SEM. Significant changes in the E modulus were observed for ΔT over 400 °C for all composites studied.