KINETICS OF GAS-TO-LIQUID TRANSFER OF PARTICLES IN METAL MATRIX COMPOSITES

The distribution of reinforcing particles in metal matrix composites produced by casting techniques depends on the interaction between various solid and liquid phases during transfer of particles from gas to liquid, transfer of particles from liquid to solid and during particle-particle interaction in the liquid. Only the first issue will be addressed in this paper. In order to predict particle behavior during gas-to-liquid transfer thermodynamic and kinetic models have been proposed. The total free energy change involved in the transfer of a particle from gas to liquid must be negative for this transfer to occur spontaneously. A new model based on the force balance on the particle has been developed. The governing equation includes a surface energy component which can be calculated from sessile drop experiments assuming equilibrium surface thermodynamics. The critical acceleration a(cr) required for particle incorporation may then be evaluated. Nevertheless it must be emphasized that sessile drop data are for vacuum, not for atmospheric environment. In the latter case, oxide films forming on the melt will influence the transfer and alter the value of the required force. In order to try to include the influence of oxide films forming on the surface of the liquid metal in the calculation of the critical acceleration, an approach involving experimental work with a two-bucket centrifugal apparatus was used. From these experiments a critical angular speed for which incorporation occurs can be determined. Thus another critical acceleration for incorporation (omega-cr2R) can be calculated. If omega-cr2R is equal to the previous acceleration a(cr) the role of oxide films is negligible. If, on the contrary, there are differences they can be attributed to the presence of oxide films on the surface of the metal. Validation of the model has been carried out using data collected from sessile drop experiments and from experiments consisting of centrifuging spheroidal ceramic particles in aluminum melts. During the experiments alumina, zirconia, SiC-coated zirconia, and graphite-coated zirconia particles have been used. The basic criterion for selection of these materials was rho-p > rho-L. Calculation of the incorporation acceleration requires knowledge of wetting angles. Accordingly, sessile drop experiments were also performed to determine the wetting angles for some various metal-ceramic systems that were not available in the literature.