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

Abstract The solution-precursor plasma spray process has been used to deposit ∼75% dense, metastable coatings in the following ternary systems (mol%): (i) 10Al 2 O 3 –86.4ZrO 2 –3.6Y 2 O 3 (10AlZrY) and (ii) 20Al 2 O 3 –76.4ZrO 2 –3.6Y 2 O 3 (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 ZrO 2 in tetragonal ( t -ZrO 2 ) form, with some cubic ( c -ZrO 2 ) phase present. Crystalline Al 2 O 3 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 Al 3+ , in addition to Y 3+ , is in solid solution with ZrO 2 , leading to the stabilization of t -ZrO 2 and c -ZrO 2 . The sub-micrometer regions in these coatings, in addition to t -ZrO 2 , contain small amounts of crystalline Al 2 O 3 phases. Both coatings contain small amounts of spherical grains of orthorhombic ZrO 2 ( o -ZrO 2 ). A heat-treatment of 1400 °C for 30 h is insufficient to effect complete t → m transformation in ZrO 2 . Heat treatment at 1500 °C (30 h) results in the precipitation of α-Al 2 O 3 and a new tetragonal form of Y 2 O 3 in both coatings. This is accompanied by Al and Y depletion in and coarsening of the surrounding ZrO 2 grains, and complete t → m transformation.