Acoustic Emission Studies of Alumina‐13% Titania Free‐Standing Forms during Four‐Point Bend Tests

Free‐standing alumina‐13% titania samples were manufactured with a water‐stabilized plasma spray gun to a thickness of ∼5 mm. A thin layer of aluminum was arc sprayed prior to depositing the thick coating and etched away using hydrochloric acid. The so‐obtained free‐standing plate was cut into four‐point‐bend‐test specimens with dimensions of ∼5 mm × 5 mm × 50 mm. The as‐sprayed material consisted of a supersaturated solid solution, where titania was frozen inside the alumina matrix. Heat treatment was performed at 1450°C for 24 h and then at 1100°C for another 24 h. After heat treatment, titania precipitates were observed. The major phases of the as‐sprayed and heat‐treated samples were γ‐ and α‐alumina, respectively. The porosity was ∼10% for as‐sprayed samples; this value was reduced to ∼3% after heat treatment. Four‐point bend tests were performed on the as‐sprayed and heat‐treated specimens in cross‐section and in‐plane directions. An acoustic emission technique 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 direction, catastrophic failure with less evidence of microcracking occurred. For heat‐treated specimens, microcracks were usually observed when tests were performed in either of the orientations. Energy and amplitude distributions for each testing condition were examined. These distributions changed after heat treatment; however, no significant differences were distinguished when tests were performed in the cross‐section or in‐plane directions.