Coatings of metastable ceramics deposited by solution-precursor plasma spray: I. Binary ZrO2–Al2O3 system

We show that the solution-precursor plasma spray (SPPS) method can be used to deposit ∼75% dense coatings of metastable ceramics in the ZrO2–10 mol% Al2O3 binary system. The microstructure of the resulting SPPS coatings contains a rich variety of features and phases, as determined using X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The SPPS coatings are predominantly nanostructured (grain size <100 nm), and made of tetragonal phase of ZrO2 (t-ZrO2) with Al3+ in solid solution. The Al3+ solute appears to stabilize partially the t-ZrO2. These coatings also contain some grains of monoclinic (m-ZrO2) and orthorhombic (o-ZrO2) phases, and Al-rich grain boundary amorphous phase. Crystalline Al2O3 phases are absent in the as-sprayed SPPS coatings. However, after a prolonged heat treatment (1400 °C, 30 h), α-Al2O3 grains precipitate out in the coating, and the t-ZrO2 grains coarsen and transform to m-ZrO2.