Microstructure development of Al2O3-13wt.%TiO2 plasma sprayed coatings derived from nanocrystalline powders

The development of constituent phases and microstructure in plasma sprayed Al2O3-13wt.%TiO2 coatings and reconstituted nanocrystalline feed powder was investigated as a function of processing conditions. The microstructure of the coatings was found to consist of two distinct regions; one of the regions was completely melted and quenched as splats, and the other was incompletely melted with a particulate microstructure retained from the starting agglomerates. The melted region predominantly consisted of nanometer-sized gamma-Al2O3 with dissolved Ti4+, whereas the partially melted region was primarily submicrometer-sized alpha-Al2O3 with small amounts of gamma-Al2O3 with dissolved Ti4+. The ratio of the splat microstructure to the particulate microstructure and thus the ratio of the gamma-Al2O3 to alpha-Al2O3 can be controlled by a plasma spray parameter, defined as the critical plasma spray parameter (CPSP). This bimodal distribution of microstructure and grain size is expected to have favorable impact on mechanical properties of nanostructured coatings, as has been observed before. (C) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.