Dependence of the Stabilization of α-Alumina on the Spray Process

Abstract

A phase change from a-alumina (corundum) in the feedstock powder to predominantly other alumina phases, such as c-alumina in the coating normally takes place, as a result of the spray process. It is expected that the prevention of this phase transformation will significantly improve the mechanical, electrical, and other properties of thermally sprayed alumina coatings. The results regarding the possibility of stabilization of a-alumina through addition of chromia published in the literature are ambiguous. In this work, stabilization using different spray processes (water-stabilized plasma (WSP), gas-stabilized plasma (APS), and high-velocity oxy-fuel spray (HVOF)) was studied. Mechanical mixtures of alumina and chromia were used, as were prealloyed powders consisting of solid solutions. The investigations focused on mechanical mixtures with both APS and WSP and on prealloyed powders with WSP. The coatings were studied by x-ray diffraction, including Rietveld analysis, and analysis of the lattice parameters. Microstructures were investigated by optical microscopy using metallographic cross-sections. It was shown that in the case of the mechanically mixed powders, the stabilization predominantly depends on the applied spray process. The stabilization of the a phase by use of the WSP process starting from mechanical mixtures was confirmed. It appears that stabilization exhibits a complex dependence on the spray process, the process parameters (in particular the thermal history), the nature of the powder (mechanically mixed or prealloyed), and the chromia content.

Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement

“…It has also been reported that suppression of metastable phase formation, i.e., retention of the α phase is likely to improve the mechanical and other properties of the coating [5,7,8]. Hence, it is important to know the phase fractions of the alumina coatings form a technological point of view.…”

Basic principles of reaching triboactive coatings by mixing of nanosized feedstock powders in the suspension plasma spraying process

“…In the literature (e. g. [23]) it was shown that a mixing of oxide-ceramics depending upon the spraying conditions can be achieved. With conventional APS this is however not possible (e. g. [24]), presumably due to the small surface to volume ratio of the feedstock. Therefore the SPS process may be suitable, partic- ularly to mix a larger number of oxide ceramic systems in the plasma and to examine the resulting phases.…”

D.C. Plasma Spraying

“…Thus plasma-sprayed alumina coatings cannot be used over 800 C. However spray conditions have been sought, which would keep the α phase. Stahr et al [146] have studied the stabilization of α alumina by spraying, with APS, WSP, and HVOF, mechanical mixtures of alumina and chromia, as well as pre-alloyed powders consisting of solid solution α-Al 2 O 3 with additions of Cr 2 O 3 . Stabilization of the α phase using the WSP process starting from mechanical mixtures was confirmed.…”