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Ceramic foam filters (CFFs) have been used by steel foundries for several years to remove non-metallic inclusions from the metal castings. In order to improve their filtration efficiency, promising approaches which involve “active” and “reactive” coatings applied on the filters have recently been proposed. In this work, a new coating system containing multi-walled carbon nanotubes (MWCNTs) is presented. A special water-based dispersing technique for carbon nanotubes was developed. Xanthan gum was used both as a dispersing agent and as a rheology modifier for the spraying slurries. The synthetic coal tar pitch Carbores® P was added as a binder for the carbon nanotubes. The coatings were cold-applied on 10 pores per inch (ppi) carbon-bonded alumina filters previously manufactured via the Schwartzwalder process. After drying, the samples were heat treated at 800 °C. The performance of the coated filters was investigated with the aid of a special steel casting simulator in fully-controlled atmosphere.

The carbon bonded magnesia (MgO C) coatings deposited on the carbon bonded alumina (Al 2 O 3 C) substrate are intended to be used as reactive filters for filtration of steel melts containing high oxygen levels and inclusions. The aim of this study was to describe the chemical reactions running at 1600 °C at the interface between the MgO–C coating and the liquid 42CrMo4 steel, at the interface between the MgO C coating and the Al 2 O 3 C substrate as well as at the interface between Al 2 O 3 inclusions and the liquid steel. The MgO C/Al 2 O 3 C interface was prepared by slip casting. The steel/MgO C interface was formed between the quickly molten metal and the MgO C/Al 2 O 3 C stack, the steel/Al 2 O 3 interface between the quickly molten metal and the Al 2 O 3 particles added to the steel melt. In order to imitate the temperature profile during a casting process and concurrently to avoid the metal convection, the spark plasma sintering was used for the steel melting. The chemical reactions at the interfaces were concluded from the phase formation after different reaction times. The results of the electron microscopy, X-ray spectroscopy and electron backscatter diffraction indicate the reduction of MgO and Al 2 O 3 by carbon contained in the binder, the formation of CO gas, the evaporation of metallic Mg and Al and the dissolution of the vaporised metals in the steel. The above experimental methods revealed formation of MgAl 2 O 4 whiskers and dense MgAl 2 O 4 layers at the MgO C/Al 2 O 3 C and at the steel/MgO C interfaces, respectively. The exogenous Al 2 O 3 inclusions inserted into the steel melt were also converted to MgAl 2 O 4 , which proves that Mg from MgO C was dissolved within the steel melt.