Thermal behavior of Mullite-Zirconia-Zircon composites. Influence of Zirconia phase transformation

Mullite-Zirconia-Zircon composites have proved to be suitable for high-temperature structural applications, with good mechanical and fracture properties and good thermal shock resistance. In this paper, the special dilatometric behavior of a series of Mullite-Zirconia-Zircon (3-40 vol.% ZrO2) composites is evaluated and compared with that of a pure Zircon material and explained in terms of the high Zirconia linear thermal expansion coefficient (alpha) and Zirconia martensitic transformation. Linear thermal expansion (alpha) up to 1273 K is studied and correlated with the phase composition of the composites; a linear correlation was found with the m-ZrO2 content evaluated with the Rietveld method. Zirconia (m-ZrO2) dispersed grains containing ceramics material showed a hysteresis in a reversible dilatometric curve (DC). The martensitic transformation temperatures could be evaluated and then compared with the endothermic and exothermic peaks temperatures obtained from the differential thermal analysis (DTA). Furthermore, the hysteresis area was correlated with m-ZrO2 content, where composites with less than 10 vol.% ZrO2 did not show this behavior, and from this content up to 40 vol.% of ZrO2 a linear increase of the hysteresis area was found.