HIGH-TEMPERATURE FRACTURE-TOUGHNESS OF DUPLEX MICROSTRUCTURES

The temperature dependence of the fracture toughness of ceramics exhib iting duplex microstructures was studied relative to their single-phas e constituents using two test methods: bend testing of chevron-notched beams, and the indentation-crack-length technique. The two materials systems studied were Al2O3:c-ZrO2(Y) and Al2O3:Y3Al5O12 (YAG), and the testing temperature ranged from room temperature to 1200 degrees C, T he study showed that in both systems the duplex materials showed highe r toughness values than their single-phase constituents above 800 degr ees C, This result was attributed to the contribution of low-energy in terphase boundaries to the overall composite toughness, Indentation cr ack length measurements gave toughness values and trends comparable to those determined by the chevron-notched beam method, By comparing the results of the two test methods it was possible to demonstrate that t he indentation calibration constant (xi) shows no significant temperat ure or material dependence, For the zirconiacontaining materials, home r:er, indentation at elevated temperatures is accompanied by significa nt localized plasticity, which suppressed the radial cracking, Under s uch conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the fracture toughness.