Examples of the role interfacial phenomena can play in materials processing are highlighted in the description of a number of recent research studies conducted at Imperial College. Interfacial phenomena play critical roles in increasing or retarding the rates of chemical reaction and in promoting or hindering wetting and dispersion of phases in each other.
The reduction of ilmentite to iron and titanium carbide or titanium oxycarbide has been studied with the ultimate aim of achieving separation of the titanium and its subsequent conversion to pigment grade titanium dioxide. The need to achieve good separation of iron from other reaction products is then a prime concern. The effects of reducing conditions on the wetting of titanium carbides and oxycarbides by iron alloys has therefore been studied. It seems that associative adsorption of titanium and carbon may be responsible for the observed effects of dissolved titanium and carbon on the wetting of TiC by liquid iron alloys.
As a result of this work a further project has been generated involving the identification of conditions for achieving good dispersions of refractory carbides including titanium carbide in iron alloys. The major motivation behind this work was the desire to develop a cheap casting based process for the production of iron based metal matrix composites capable of producing near net shape products. As a result of this work a novel rapid testing technique for the assessment of the wettability and compatibility of potential filler materials with liquid metal matrices has been developed. The technique employs levitation and quenching of liquid metal drops containing added filler materials to permit assessment of alloy composition, filler coatings and temperature on matrix/filler interactions. The levitation technique has been further utilised in a study of the conditions required for dispersion or non dispersion of second phase particles in liquid superalloys. In this case the cleanliness of the superalloys achieved during recycling procedures is determined by the ease with wich inclusions can be removed. Some observations with ternary oxides also indicate the importance of the associative adsorption phenomenon.
The importance of interfacial considerations has also been highlighted by our studies on the production of aluminium-titanium-boron grain refining master alloys from fluoride fluxes. Entrapment of the products can result from emulsification occurring during the reduction reactions. A detailed study of this phenomenon has been conducted using a modified sessile drop technique. The results obtained indicate the critical role that interfacial tension plays in determining the ease of metal-flux separations and in determining whether products are dispersed in the metal or slag.
The kinetics of metal-salt reactions were also found to be of importance. Fast transfer of Ti and B to the metal results in the build up to TiAl₃. TiB₂ or AlB₁₂ at the interface. These compounds when present at the interface can be wet by the flux and result in emulsion formation. Inhibition of emulsification can be achieved by the presence of surface active elements such as magnesium and calcium.
The role of interfacial phenomena in influencing the kinetics of the reduction of slags has been studied in an investigation of the kinetics of alkali metal oxide release from silicate melts. The kinetics of K₂O and Na₂O release during heating in graphite crucibles has been studied from binary alkali oxide-silicon dioxide melts and from a wide range of CaO-Al₂O₃-SiO₂ slags. The contribution of the wetting of the graphite by the slag towards influencing reaction kinetics represents a notable feature of the study.