Influence of the zirconia transformation on the thermal behavior of zircon-zirconia composites

During a heating-cooling cycle, zirconia (ZrO2) undergoes a martensitic transformation from monoclinic to tetragonal structure phases, which presents special hysteresis loop in the dilatometry curve at temperatures between 800 and 1100 degrees C. Monoclinic zirconia (m-ZrO2) particles reinforced ceramic matrix composites not always present this behavior. In order to elucidate this fact a series of zircon-zirconia (ZrSiO4-ZrO2) ceramic composites have been obtained by slip casting and characterized. The final properties were also correlated with the zirconia content (0-30 vol.%). The influence of the martensitic transformation (m-t) in well-dispersed zirconia grains ceramic composite on the thermal behavior was analyzed. Thermal behavior evaluation was carried out; the correlation between the thermal expansion coefficients with the zirconia content showed a deviation from the mixing rule applied. A hysteresis loop was observed in the reversible dilatometric curve of composites with enough zirconia grains (>= 10 vol.%). Over this threshold the zirconia content is correlated with the loop area. The transformation temperatures were evaluated and correlated with the zirconia addition. When detected the m-t temperature transformation is slightly influenced by the zirconia content (due to the previously evaluated decrease in the material stiffness) and similar to the temperature reported in literature. The reverse (cooling) transformation temperature is strongly decreased by the ceramic matrix. The DTA results are consistent with the dilatometric analysis, but this technique showed more reliable results. Particularly the endothermic m-t transformation temperature showed to be easily detected even when the only m-ZrO2 present was the product of the slight thermal dissociation of the zircon during the processing of the pure zircon material.