STRUCTURAL CHARACTERIZATION OF ALUMINA METASTABLE PHASES IN PLASMA SPRAYED DEPOSITS

Plasma sprayed coatings were used as rapidly solidified pure alumina samples. The crystallographic structure of as-sprayed deposits has been previously shown to be a metastable one, the so-called τ modification. During heat treatments between 850°C and 1050°C, the τ to α transformation was studied through X-ray powder or single-crystal experiments, and transmission electron microscopy observations. This transformation occurs through a single intermediate phase, called δ*, with an orthorhombic unit cell (a=0.793 nm, b=0.795 nm,c=1.171 nm). δ* has a domain structure deriving from a quasi-periodic antiphase boundaries mechanism in the (100) planes of a defective spinel lattice. © 1990.