CETMIC   05378
Unidad Ejecutora - UE
capítulos de libros
Thermal analysis as a tool for studying zirconia containing materials
Thermal behavior of ceramics and glass-ceramic systems
Editorial Research Signpost
Año: 2011;
Introduction   In many industrial applications, components are required to work in extremely hard conditions, particularly at very high-temperatures for long times. For these applications, new ceramic matrix composites have been designed, with both micro and nano-sized reinforcing particles, located at inter or intra-granular positions, this can improve mechanical and thermomechanical performances as well as strongly reduce creep rate. Many attempts are in progress to develop biphasic [1-16]] and tri-phasic systems or even tetra-phasic systems [17-25]. Particularly much interest is still being carried out in dispersed zirconia containing ceramic composites with different contents of different types of ZrO2 grains. [1-27]. Zirconia ceramics have attracted great attention to industrial applications in oxygen pumps, sensors, fuel cells, and thermal barrier coatings due to their excellent electrical, thermal, and mechanical properties [26]. Pure Zirconia ceramic exhibits a phase transformation between monoclinic and tetragonal phases It can be seen that most of the reported m-t and t-m temperatures are in the temperature range 1400–1480 and 1250–1325 K, respectively, [27-33]. These transformations temperatures can be evaluated by different methods like differential thermal analysis, differential scanning calorimetry, high-temperature X-ray diffraction, neutron diffraction, dilatometry and Raman scattering. As well as theoretical calculations based in first principles [28, 29]. Over the last decades, considerable advances have been made to improve the fracture toughness of ceramic systems. One of the most important examples is the toughening contribution from stress – induced martensitic transformation of tetragonal grains. Lathabai et. al. [2] showed that significant toughening could be obtained by incorporating Zirconia particles (ZrO2) in a ceramic matrix. Different mechanisms are involved in the toughening of Mullite composite originated by Zirconia additions: stress-induced transformation, microcracking, crack bowing and crack deflection. In all cases, the operative toughening mechanism depends on such variables as matrix stiffness, Zirconia particle size, chemical composition, temperature and strength. Mullite Zirconia Zircon composites have proved to be suitable for high temperature structural applications [18] [24-25]. Undoubtedly, it is of great technological interest to know the dilatometric behavior of a family of materials for high temperature applications. It has been stated that the thermal expansion behavior of Zirconia ceramics from a given powder type can be “tailored” within limits by varying the chemical composition and processing variables [34]. This provides some physical property selection capability for engineering applications. One of the limiting behaviors of several Zirconia containing materials is the thermal shock resistance. This behavior is strongly related to the thermal expansion behavior of the materials [35-38]. Different thermal conditions cause dimensional changes and consequently the magnitudes of the thermal stress are also dissimilar. Hence it is of technological importance to improve the understanding of this behavior particularly in the Zirconia containing materials which present this special dilatometric behavior. Regarding to the dilatometric behavior dispersed Zirconia (ZrO2 = 40 wt. %) grains in a Mullite matrix showed a hysteresis in the reversible dilatometric curve of a Mullite Zirconia composite [3], and the martensitic transformation temperatures (Ms) could be evaluated. The thermal expansion hysteresis associated with the monoclinic-tetragonal transformation was also evident in the Mullite-yttria-stabilized ZrO2 composite from which the Ms temperature could be determined [41]. On the other hand, in a Mullite Zirconia composite (with 35% wt. ZrO2, and 1.5% Y203) a transformation hysteresis was not observed [42]. Perhaps because the monoclinic Zirconia content was not important since the tetragonal phase had been stabilized by the Yttrium oxide. While in the former material [41] the monoclinic was around 80 % of the total Zirconia. Below as an example of the possibilities of the thermal analysis techniques, particularly dilatometry in the understanding of zirconia containing composite materials we present the results of  a study of the dilatometric behavior of Mullite Zirconia Zircon composites. Firstly, the correlation between these behaviors with the processing variables especially with phase composition is presented. Secondly a special more deep study on the capability of this family of techniques in the Zirconia transformation consequences is also presented. A second objective is to find the critical ZrO2 content composition from which the hysteresis in the dilatometric curve appears for a Zirconia containing composite. The local thermal expansion mismatch between the constituent phases present in the composite is important, thermal expansion coefficients of these phases are shown in Table 1. The porosity, mean grain size and distribution together with the microcracks developed by the martensitic transformation during processing (cooling) and the local thermal expansion mismatch between grains will also influence the dilatometric behavior of the composites.