Gionata Schneider
This thesis investigates the interaction between mechanically induced flow of a liquid metal into a porous solid, and kinetic effects at the triple line. The approach is experimental and focuses on metal-ceramic systems for which the literature gives values of wetting and local kinetic parameters (Simga, SimgaLV, and interfacial phenomena near the triple line) at high temperature, gleaned by means of the sessile drop experiment. We investigate the infiltration with Al of Al2O3 preforms at 1000°C, 1050°C and 1100°C, and the infiltration with Cu-x (x=Si, Ti or Cr) of porous CGr between 1050°C and 1200°C. Those three alloys are known to form carbides better wetted by the liquid than is graphite.
If measurements are taken at various values of applied metal overpressure, in the presence of thermally activated processes at the triple line, time intervenes in capillarity and the front does not stabilise. We observe that the metal continues to invade slowly the porous medium, gradually ¿creeping¿ into its pores under the action of local phenomena that take place at the triple line. We characterise such dynamic infiltration by measuring rates of steady isobaric infiltration, measuring also the variation of infiltration rates at Pth and Pth+0.05MPa. Saturation rates are then computed by means of a linear regression over 100s of the measured saturation for each pressure step. Data are interpreted using an analogy with high-temperature plasticity. This leads to assume that the applied pressure contributes a proportional reduction in the activation energy of the process that governs the rate of preform infiltration, the constant of proportionality defining an activation volume that characterises the thermally activated process.
Cu-46at.pct.Si infiltrating carbon is a mildly reactive system that forms SiC at the interface. Isobaric saturation rates of infiltration and their dependence on variations in the applied pressure were conducted between 1050°C and 1200°C. It is found that an activation volume on the order of ~200nm3 makes the data compatible with the Arrhenius law, with an estimated activation energy value of ~400kJ/mol, which is realistic for a process limited by the rate of SiC growth along the interface.
Aluminium infiltrating three different Al2O3 preforms shows similar behaviour as does the mildly reactive Cu-Si/C system if particles in the preform are highly angular, regardless of the presence or absence of Na impurities. Isobaric saturation rates measured between 1000°C and 1100°C give a similar activation volume as the Cu-Si/C system, on the order of ~200nm3, and an activation energy ~300kJ/mol, suggesting that the kinetics are likely limited by solid phase diffusion. It is also shown that sodium impurities in alumina serve to facilitate its infiltration by aluminium, likely because sodium alters the oxide skin that initially covers the melt.
Finally, in the infiltration of Cu-1at.pct.Cr or Cu-10at.pct.Ti into CGr, one finds that interfacial reaction does not aid pressure infiltration. In the former, it is because initial reaction between melt and preform depletes the alloy in reactive Cr, leading to similar infiltration as with pure Cu. In the latter, initial carbide formation is so extensive that it blocks ingress of the metal into the preform. For alloying elements forming a better wetted interface to aid pressure infiltration, reaction must thus be sufficiently slow compared to the rate of metal flow.
EPFL2019
