MECHANICAL AND THERMAL SHOCK BEHAVIOUR OF Al2O3-SiO2-ZrO2 REFRACTORIES

N. RENDTORFF; A. SCIAN; E. AGLIETTI

Bahia, Brasil

Congreso; UNITECR 09; 2009

Abstract Refractories of the Al2O3-SiO2-ZrO2 system are widely used in the glass industry in fore hearth, distributor, feeders, and as expendable materials as plungers, spouts, tubes, orifice rings, etc. These materials are commonly subjected to thermal stresses during installation. So they are recognized as having good thermal shock properties Once installed, the service life is then determined mainly by its corrosion characteristics.. In this work three kind of refractories were studied to observe and correlate mechanical properties with thermal shock behavior. The materials and their principal crystalline phases are: AM (alumina-mullite 35), Am (alumina-mullite 10), and Az (alumina-zircon) All the samples had similar open porosity and pore size distribution (mercury intrusion). The mechanical characterization comprises: fracture intensity factor (KIC), fracture initiation energy (ãNBT) and work of fracture (ãwof) using notched bars. The dynamic elastic modulus E of the composites was measured by the excitation technique. The AM material showed the highest E , MOR (óf) and K1c values. For the thermal shock experiments (TS), the water quenching method was used. Thermal cycles with quenching temperature differentials, ÄT of 400, 600, 800, and 1000 oC were applied. The TS behavior was evaluated by measuring the decrease in E1/E0 ratio where E0 and E1 are the dynamic elastic modulus before and after one quenching, respectively. Also the strength (modulus of rupture, MOR) of the materials before and after the TS test was measured. The elastic modulus remains relatively high (near 80%) up to a ÄT of 500 oC for the three samples. Strength decreases to 80% of the initial value at a ÄT near 400 oC. Materials showed an experimental ÄTc of 400-500 oC and up this temperature no clear differences in the TS behavior is observed. At a ÄT higher than 600 oC,  AM showed a highest reduction of E and MOR than Am and Az. Considering the retained MOR and E with ÄT Am and Az have a similar behavior. However clear differences with AM are observed at  ÄT > 600 oC. The usage of these materials in glass service indicates that the AM material have the low TS resistance. A correlation between the decrease of MOR and E is observed in all the samples. The first approaches to determine the thermal stresses and to introduce the damage resistance parameters of ceramic material are a thermo-elastic theory, which is focused in the initiation of the fracture (R and R’)  A second approach focuses on crack propagation for conditions of thermal shock more severe than those for crack initiation ( R’’’). A unified theory of the thermal shock resistance considering the initiation and crack propagation was proposed. For high strength refractories with short initial cracks (R’’’’) and for lower strength refractories with larger initial cracks (Rst). Both models consider ã wof and E in themselves. The parameters considering crack initiation (R) are very similar between them and quite different from those observed experimentally (ÄTc). This fact can be explained if we considerer that microstructure of refractories have initially defects and microcraks. The R’’’ parameters are the same for all materials.The energy balance approach to define the propagation of inherent, pre-existing cracks in a refractory and for our materials the Rst parameter reflected the TS damage.  The best TS resistance of Az and Am is due to the micro cracks size and its distribution in the microstructure of the materials. In AM refractory  the high content and great grain size of mullite produce the appearance of more great cracks than in the other materials.