Influence of the elementary mixing scale on HVOF-sprayed coatings derived from nanostructured aluminosilicate/mullite feedstock

Aluminosilicate/mullite coatings were prepared from nanosized sources of aluminium and silicon by means of a high velocity oxy-fuel thermal spray process. X-ray diffraction, analytical electron microscopy, differential thermal analysis and Fourier transform infrared spectroscopy were used to characterize the phase composition, elementary distribution, and microstructure in the feedstock and in the coatings. It was revealed that the chemical environment of the alumina component, as well as the inherent elementary distribution in the nanoparticulate aluminosilicate feedstock, showed the greatest influence on the microstructure and the crystallization of nanosized ceramic phases in the sprayed coatings. A uniform distribution of alumina and silica in a short-range ordered monophasic-like nanostructure was the most appropriate physico-chemical conditions of the feedstock to achieve nanosized crystallites of mullite in a monolithic-like aluminosilicate coating. In turn, when using diphasic feedstock, clearly heterogeneous but nanostructured coatings with a good abrasion resistance were obtained. The agglomerated and heat-treated powders were considered as the most suitable type of feedstock for providing nanostructured aluminosilicate coatings.