Atmospheric plasma spraying coatings from alumina–titania feedstock comprising bimodal particle size distributions

In this work, Al2O3–13 wt% TiO2 submicron-nanostructured powders were deposited using atmospheric plasma spraying. The feedstocks were obtained by spray drying two starting suspensions of different solids content, prepared by adding nanosized TiO2 and submicron-sized Al2O3 powders to water. The spray-dried granules were heat-treated to reduce their porosity and the powders were fully characterised in both untreated and thermally treated state. Comparison with two commercial feedstocks was carried out. Characterisation allowed a temperature for the thermal treatment to be chosen on the basis of the sprayability of the feedstock and the preservation as much as possible of the submicron-sized structure of the unfired agglomerates. Optimisation of the deposition conditions enabled the reconstituted powders to be successfully deposited, yielding coatings that were well bonded to the substrate. The coating microstructure, characterised by SEM, was mostly formed by a matrix of fully molten particles where the presence of semi-molten feedstock agglomerates was also observed. Moreover, microhardness, toughness, adhesion and tribological behaviours were determined, and the impact of the granule characteristics on these properties was studied. It was found that changing the feedstock characteristics allows controlling the coating quality and properties. In general, good mechanical properties were obtained using a feedstock comprising a binary mixture of submicrometric Al2O3 and nanometric TiO2 particles in the spray-dried powder.This work has been supported by Spanish Ministry of Economy and Competitiveness (MAT2009-14144-C03 and MAT2012-38364-C03)