| This report represents the results of an experimental study of the effects of Na2O and CaO on the microstructure, flexural strength, fracture toughness and thermal shock resistance of aluminosilicate refractory ceramics. The addition of 4--6 wt% of Na2O is shown to alter the microstructure and also improve the thermal shock resistance, which is characterized by the number of cold shock cycles to failure; while the addition of CaO is shown to strengthen the bridging ligaments. The photomicrographs obtained using the scanning electron microscope, SEM, and the elemental maps obtained using the energy dispersive X-ray spectroscopy, EDS, show that the addition of Na 2O decreases the formation of mullite as the level of Na2O is increased. Also, the formation of tiny needles/laths is seen as the level of Na2O is raised. The volume of the needle-like crystals is much more pronounced in the batch of materials doped with 5wt.%Na2O and 6wt.%Na2O.; The doping with Na2O changes the viscosity-temperature characteristics and the glass transition temperatures in ways that enhance the crack-tip shielding due to viscoelastic bridging, which is modeled using fracture mechanics concepts. Following an initial microstructural characterization, basic mechanical properties determination, and thermal shock experimentation, a fracture mechanics framework is utilized to determine the crack driving force which is pertinent to the characterization of toughness of the refractory ceramics material. It is shown that the batch of the material doped with 4wt.%Na2O + 0.5wt.%CaO provides the optimum crack-tip shielding, improved flexural strength, and enhanced crack growth resistance. The improved thermal shock resistance is attributed to the shielding effects of viscoelastic crack bridging by glassy phase between mullite platelets. Optimization of the material processing and the assessment of the level of the dopant (CaO) are considered for future work. |
