Fracture Toughness of Ceramics Fired at Different Temperatures
AbstractThe fracture toughness test was performed at room temperature on sets of 5 ceramic samples made from material for high voltage insulators (kaolin 36 wt.%, Al2O3 30 wt.%, clay 12 wt.% and feldspar 22 wt.%) fired at temperatures 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1250, 1300, 1400, 1500°C at heating and cooling rate of 5°C/min. The precrack was made to each sample by indentation under the loads 10 N–200 N, the dwell time was 45 s and the loading rate was 10 N/s. Results of the fracture toughness tests were in accordance with changes of structure of the samples after the partial firings. Fracture toughness from 20°C to 500°C is almost constant and it varies between 0.1 MPa·m0.5and 0.2 MPa·m0.5. Dehydroxylation (420°C–600°C) does not influence the value of fracture toughness. At temperature interval where we assume sintering (700°C–1250°C) we observe exponential dependence of fracture toughness up to 1.5 MPa·m0.5. From comparison of the fracture toughness, Young’s modulus and flexural strength follows a correlation and proporcionality of these mechanical properties.
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