A scanning electron microscopic study to observe the changes in the growth morphology of the α phased Alumina–13 wt.% titania coatings during plasma spraying

Abstract

Plasma spraying of Alumina–13 wt.% titania coatings shows preferential stability of α phase, in the as sprayed conditions, to an extent of 15–40%. As reported in the literature, the quantity of this α phase can be considered to be closely related to the processing parameters of the air plasma spray process, such as arc current, plasma gas flow rate etc. Normally, a lower value of arc current, results in larger quantity of preferentially stabilized α phase. In this paper, we have attempted to measure the orientation image mapping (OIM) of these α phased Alumina–13 wt.% titania coatings with Electron Back Scattered Diffraction. From the OIM, the pole figures for the α phase were established and from appropriate pole figures, such as (0001), (0006), (01ī1) and (10ī1) the growth morphology of the α phase could be determined. For larger values of arc current, at which the growth rate is faster and due to the imposed rapid solidification conditions, the hexagonal basal plane of (0001)-H type of the α phase tends to align with the (001) type FCC faces of the metastable γ phase. This alignment can be considered to be similar to a directionally solidified chill cast texture. On the other hand, at slower growth rate and at lower arc current value not only a larger quantity of α phase forms, but for accommodation of these larger number of α grains in the thinner section of splats there appears to be a need for a change in the growth morphology. (100)-R type rhombohedral faces, with shorter axial dimensions were found to align in preference to the directional solidified (0001)-H type basal planar arrangement. This type of change in growth morphology can possibly accommodate more α grains for the same thickness of the splat as compared to a basal plane (0001)-H type arrangement.