Alumina-base plasma-sprayed materials - Part II: Phase transformations in aluminas

Aluminum oxide is widely used for plasma spraying. Alumina deposits consist of a number of metastable crystallographic modifications, which at elevated temperatures, transform to the stable ? phase. It was shown that additions of various oxides changed the phase composition and shift phase transformation temperatures. This paper addresses the variation of phase compositions and temperatures of the phase changes for plasma-sprayed alumina deposits manufactured with alumina-base materials containing O2O3 and TiO2. This study combines the results obtained from energy dispersive analyzer of X-rays (EDAX) and scanning electron microscopy (SEM) chemical analysis, differential thermal analysis (DTA), and X-ray powder diffraction (XRD) quantitative phase analysis of as-sprayed and annealed samples of alumina deposits and shows how the two additives change the phase composition and the ?-phase formation temperature. This transformation temperature varied by nearly 200 °C. The metastable alumina sequences were also influenced by the chemical composition; for example, the content of 6 alumina varied between 0 and 55 wt %.