Influence of simultaneous Cr2O3 and TiO2 additions on the microstructure and properties of APS alumina coatings

Alumina is an important material for thermal spray coating solutions. For improving the coating properties, additions of TiO2 or Cr2O3 are common, but their simultaneous addition has not been studied so far. One alumina-rich ternary feedstock powder composition (81 wt% Al2O3-12.5 wt% Cr2O3-6.5 wt% TiO2) was prepared in three different ways: 1) blending of commercial fused and crushed plain oxide powders; 2) blending of two commercial binary jointly fused and crushed feedstock powders (Al2O3-13 wt% TiO2 and Al2O3-25 wt% Cr2O3); 3) preparing an experimental agglomerated and sintered powder. Plain oxide powders and the two binary powders were used for comparison. The feedstock powders were studied by a standard procedure, including particle size distribution by laser diffraction, the morphology and microstructure by SEM and EDS, and phase composition by XRD. Coatings were prepared by APS with an Ar-H-2 plasma gas mixture, using a single spray parameter set optimized for the plain Al2O3 powder. The influence of the feedstock powder properties on coating deposition was evaluated by the average thickness per layer and coating roughness. The coating microstructures were studied by optical microscopy and SEM, including the investigation of local compositions by EDS. The phase compositions and lattice parameters of the alumina phases in the powders and coatings were investigated by XRD. The coatings were characterized for their microhardness HVO.3, and tribological (unidirectional dry sliding wear resistance), corrosion (exposure testing against H2SO4) and electrical (resistivity, dielectric breakdown field strength and impedance) properties. Chromia additions, existing as a (Al,Cr)(2)O-3 solid solution in the feedstock powder, were found to have a positive effect on the stabilization of the corundum phase for APS coatings, while titania additions were found to have no influence on the corundum phase stability. The corrosion resistance was found to be positively influenced by the joint addition of chromia and titania compared to plain alumina.