Acoustic emission responses of plasma-sprayed alumina-3% titania deposits

Free standing alumina-3% titania samples were prepared by gas- and water-stabilized plasma (GSP and WSP) spraying. These free standing forms consisted of metastable γ and δ-alumina, as well as α-Al₂O₃. The microstructures of the materials were similar for both plasma spraying methods and typically exhibit distinct boundaries around splats, columnar grains, and inter- and infra-lamellar porosity. The modulus of rupture (MOR) and porosity were also similar for both GSP- and WSP-sprayed specimens. Four point bend tests with in situ acoustic emission (AE) were used to study the cracking behavior of these materials. Catastrophic failure was observed for GSP-sprayed samples while WSP-samples exhibited microcracking before failure. The amplitude and energy distributions were also investigated and it was thereby determined that the percentage of macrocracks for GSP-sprayed samples is much larger than for WSP-sprayed samples. The processing effects, which induced differences in microstructure, can account for the differences in AE responses.