FRACTURE TOUGHNESS OF KAOLIN FIRED AT DIFFERENT TEMPERATURES

Abstract eng:
The fracture toughness (FT) test was performed at room temperature on ceramic samples made from kaolin at temperatures of 400–1250 °C at a heating and cooling rate of 5 °C/min. The precrack was made by indentation under the loads 10–200 N, the dwell time was 45 s and the loading rate was 10 N/s. Results of the FT tests were in accordance with the changes in structure of the samples after the partial firings. The FT from 20 °C to 500 °C is almost constant and it varies between 0.1 MPa·m0.5 and 0.2 MPa·m0.5. Dehydroxylation (420–600 °C) and solid state sintering (700–950 °C) does not influence the value of FT. At temperature interval where we assume liquid state sintering (1000–1250 °C) we observe an exponential increase of FT up to 0.6 MPa·m0.5. From comparison of FT, Young modulus and flexural strength, a correlation and proportionality of these mechanical properties follows.

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