ACOUSTIC-EMISSION STUDIES OF ALUMINA-13-PERCENT TITANIA FREESTANDING FORMS DURING 4-POINT BEND TESTS

Free-standing alumina-13% titania samples were manufactured with a wat er-stabilized plasma spray gun to a thickness of similar to 5 mm. A th in layer of aluminum was are sprayed prior to depositing the thick coa ting and etched away using hydrochloric acid. The so-obtained free-sta nding plate was cut into four-point-bend-test specimens with dimension s of similar to 5 mm x 5 mm x 50 mm. The as-sprayed material consisted of a supersaturated solid solution, where titania was frozen inside t he alumina matrix, Heat treatment was performed at 1450 degrees C for 24 h and then at 1100 degrees C for another 24 h. After heat treatment , titania precipitates were observed. The major phases of the as-spray ed and heat-treated samples were gamma- and alpha-alumina, respectivel y. The porosity was similar to 10% for as-sprayed samples; this value was reduced to similar to 3% after heat treatment. Four-point bend tes ts were performed on the as-sprayed and heat-treated specimens in cros s-section and in-plane directions. An acoustic emission techniques was used to examine the cracking during the tests in situ. Microcracking prior to failure was observed for as-sprayed samples that were tested in the cross-section direction. However, when tested in the in-plane d irection, catastrophic failure with less evidence of microcracking occ urred. For heat-treated specimens, microcracks were usually observed w hen tests were performed in either of the orientations. Energy and amp litude distributions for each testing condition were examined. These d istributions changed after heat treatment; however, no significant dif ferences were distinguished when tests were performed in the cross-sec tion or in-plane directions.