Carbon-bonded or sintered ceramic foam filters are widely used for the filtration of metal melts. The filtration efficiency depends on a variety of factors such as the material and the morphology of the filter surfaces. In this study, the surface modification of carbon-bonded alumina foams with ceramic coatings by electrophoretic deposition (EPD) was investigated. The gas bubble formation by electrolysis during the EPD from aqueous suspensions was utilized to generate channel-like pores in the coatings.
Owing to its sufficiently high electrical conductivity, carbon-bonded alumina foam can act as 'deposition electrode'. Two different alumina powders and, in some preliminary experiments, yttria-stabilized zirconia were used for the coatings. Different suspension compositions and EPD parameters were tested. The coatings were sintered in non-oxidizing atmosphere.
A non-destructive three-dimensional characterization of the coated samples was possible by means of X-ray computed tomography (CT). Both the homogeneity and the pore structure of the coatings were evaluated on the basis of 3D and 2D CT images. It was shown that coatings with pore channels, which were oriented perpendicularly to the strut surface, could be successfully produced on Al2O3-C foams. Furthermore, the CT images visualise how the depth of the EPD in a given foam volume and the porous structure of the coatings depend on the suspension composition and the EPD parameters.