Viscoelastic toughening of refractory ceramics
Authors:   , Akpan Edem T. , Gogotsi Yury 
Published: July 2021

This dissertation 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. 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 Na₂O 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 apparent porosity of the batch of material doped with 4wt.%Na₂0 + 0.5wt.%CaO was 20%, and that of the as-received material was 30%, while that of the batch of material doped with 6wt.% of Na₂O was 23%. 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. Actually, the number of cycles to failure was seen to marginally decrease at the temperature range 1200°C to 1350°C for the material doped with 4wt.%Na₂O + 0.5wt.%CaO, compared to the temperatures, where the number of cycles to failure rapidly declined. It is, therefore, believed that the slight decrease in the number of cycles to failure in the temperature range from 1200°C to 1350°C is attributed to the fact that shielding due to the bridging forces is active at these temperatures. Similar toughening characteristics were observed for the materials with different compositions, except that improvement was the greatest for the material doped with 4wt.%Na₂O + 0.5wt.%CaO. A similar improvement in the fracture toughness for the batch of material doped with 4wt.%Na₂O + 0.5wt.%CaO was observed. The fracture toughness for this material was 1.6±0.02 MPa [square root of]m, compared to 1.17±0.02 MPa [square root of]m for the as-received material and 1.1±0.02 MPa [square root of]m for the material doped with 6wt.%Na₂O. A similar trend was observed in the flexural strength measurement. The specimen doped with 4wt.%Na₂O + 0.5wt.%CaO has a relatively higher flexural strength at room temperature compared to the as-received and those doped with 6wt.%Na₂O. Finally, in order to further improve the thermal shock resistance of the refractory ceramics, it is recommended that a hot isostatic pressing (HIP) technique be employed so that the need of additives during processing can be eliminated and the apparent porosity can significantly be reduced or eliminated. Isostatic pressing can also reduce the sintering temperature and holding time which therefore translates into reduced material processing cost. This processing technique is only feasible for the production of items such as electronic components, and hence is not recommended for use in processing of refractory bricks due to the high cost of HIP. It is further recommended that different chemistry that can change the temperature-viscosity characteristics of the viscous phase need to be pursued. A self-consistent solution should be adopted to account for the crack opening profile without resorting to the assumption of a crack opening profile which may not give an accurate account of actual crack path phenomenon.