Coatings of metastable ceramics deposited by solution-precursor plasma spray: II. Ternary ZrO2-Y2O3-Al2O3 system

The solution-precursor plasma spray process has been used to deposit ∼75% dense, metastable coatings in the following ternary systems (mol%): (i) 10Al2O3-86.4ZrO2-3.6Y2O3 (10AlZrY) and (ii) 20Al2O3-76.4ZrO2-3.6Y2O3 (20AlZrY). The microstructures of these as-sprayed coatings consist of a rich variety of features and phases, as determined using X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The nanostructures (10-40 nm) in both coatings are primarily ZrO2 in tetragonal (t-ZrO2) form, with some cubic (c-ZrO2) phase present. Crystalline Al2O3 phases are absent in the nanostructured regions of these coatings, and the chemical compositions of these regions are close to the respective nominal compositions of the coatings. These results show clearly that Al3+, in addition to Y3+, is in solid solution with ZrO2, leading to the stabilization of t-ZrO2 and c-ZrO2. The sub-micrometer regions in these coatings, in addition to t-ZrO2, contain small amounts of crystalline Al2O3 phases. Both coatings contain small amounts of spherical grains of orthorhombic ZrO2 (o-ZrO2). A heat-treatment of 1400 deg. C for 30 h is insufficient to effect complete t → m transformation in ZrO2. Heat treatment at 1500 deg. C (30 h) results in the precipitation of α-Al2O3 and a new tetragonal form of Y2O3 in both coatings. This is accompanied by Al and Y depletion in and coarsening of the surrounding ZrO2 grains, and complete t → m transformation