Abstract
Plasma-sprayed ceramic coatings are formed by a stream of molten particles which impact, spread and rapidly solidify onto a substrate to form a lamellar microstructure. The lamellae often consist of metastable phases and are typically 1 – 2 μm thick with columnar grains 0.1 – 0.4 μm in cross-section extending through their thickness. The grain size is determined by the rapid solidification rate which cannot be controlled. Zirconia coatings, partially or fully stabilized with Y 2O 3 or CeO 2, were prepared in which the latter contained varying proportions of transformable tetragonal phase. The double cantilever beam technique was used to determine whether these coatings had improved toughness over conventional ZrO 2-based coatings containing no transformable tetragonal phase. The results show that tetragonal coatings have significantly higher fracture toughness than fully stabilized (cubic) ZrO 2 coatings. Cohesive fracture toughness is greater than adhesive toughness. The highest toughness was achieved with ZrO 2-CeO 2 coatings which contained approximately 20% transformable tetragonal phase. The results are interpreted in terms of the influence of grain size and composition on the tetragonal-to-monoclinic phase transformation.
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