Fracture Toughness of Ceramics Fired at Different Temperatures

The fracture toughness test was performed at room temperature on sets of 5 ceramic samples made from material for high voltage insulators (kaolin 36wt.%, Al(2)O(3) 30 wt. %, clay 12wt.% and feldspar 22 wt.%) fired at temperatures 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1250, 1300, 1400, 1500 degree C at heating and cooling rate of 5 degree C/min. The precrack was made to each sample by indentation under the loads 10N-200 N, the dwell time was 45 s and the loading rate was 10N/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 degree C to 500 degree C is almost constant and it varies between 0.1 MPa times m(0.5) and 0.2 MPa times m(0.5). Dehydroxylation (420 degree C-600 degree C) does not influence the value of fracture toughness. At temperature interval where we assume sintering (700 degree C-1250 degree C) we observe exponential dependence of fracture toughness up to 1.5 MPa times m(0.5). From comparison of the fracture toughness, Young's modulus and flexural strength follows a correlation and proporcionality of these mechanical properties.