Functional Coatings on Carbon‐Bonded Ceramic Foam Filters for Steel Melt Filtration

Ceramic foam filters (CFFs) have been used by steel foundries for several years as an effective physical refining solution after the ladle treatment. Recently, new approaches involving “active” and “reactive” coatings applied on the filters have been proposed, in order to improve their efficiency during operation. In this work, two different coating methods are presented. The first exploits the reaction of carbon‐bonded alumina–magnesia filters when immersed in a steel melt at about 1650 °C. In this case, the coating is formed directly during operation: due to the carbothermal reduction of magnesia and dissolution of alumina, in situ spinel formation is observed. This also results in a volume expansion that counteracts the shrinkage of the filter material related to sintering and pyrolysis phenomena. Next, multi‐walled carbon nanotubes (MWCNTs) are applied on carbon‐bonded alumina filters by means of spray coating to increase the reactivity of the surface. The performance of the coated filters is assessed in comparison to uncoated ones, using a special steel casting simulator under controlled atmosphere. The microscope investigations after 10 and 30 s of immersion indicate a better performance of the MWCNTs‐coated filters.