Investigation of cracking mechanisms of plasma sprayed alumina-13% titania by acoustic emission

Free standing alumina-13% titania samples were manufactured using high power water stabilized plasma spraying. Heat treatment was performed at 1450°C for 24 hours and then at 1100°C for another 24 hours. Four point bend tests were performed on the as-sprayed and heat-treated samples in both cross section and in-plane orientations with in situ acoustic emission monitoring to monitor the cracking during the tests. Catastrophic failure with less evidence of microcracking was observed for as-sprayed samples. Energy and amplitude distributions were examined to discriminated micro- and macro-cracks. It was found that the high energy (> 100) and high amplitude (say > 60 dB) responses can be characterized as macro-cracks. Physical models are proposed to interpret the AE responses under different test conditions so that the cracking mechanisms can be better understood.