Liquid Phase Sintering
Liquid phase sintering is a densification process in which a liquid phase increases the consolidation rate by facilitating particle rearrangement, enhancing transport kinetics, or both . The modern practice of liquid phase sintering evolved from the vitrification of traditional ceramic ware [45, 46]. During vitrification of clay-based ceramics, heating induces the formation of a high viscosity siliceous liquid phase . The liquid facilitates the dissolution of the remaining solid and the subsequent precipitation of primary mullite crystals with a needle-like morphology . The fraction of liquid depends upon the particular batch composition and the firing temperature, but can be well over 50 vol% for common triaxial whitewares . During vitrification, the
porous powder compact undergoes physical changes (shrinkage, pore removal) and chemical reactions (conversion of meta-kaolin to mullite, glass formation). The final vitrified body is generally free of pores and contains primary mullite that is formed during vitrification, secondary mullite that is formed by precipitation from the liquid during cooling, solidified glass, plus any inert fillers such as quartz that may have been added to the batch . The glassy phase serves as a bonding phase cementing the mullite crystals and fillers into a dense, strong ceramic . Unlike solid-state sintering, liquid phase sintering is an effective means for densification of large grain (10 pm or greater) materials.
The modern practice of liquid phase sintering uses additives to facilitate liquid phase formation . Effective liquid phase sintering minimizes liquid formation to avoid unintended deformation during densification . Liquid contents as low as 3-5 vol% are possible for well-designed liquid phase sintering operations . To promote densification, the liquid must form in appreciable quantities at the desired sintering temperature, it must wet the matrix, and it must be able to dissolve the matrix . As with vitrification, densification during liquid phase sintering occurs by particle rearrangement and solution precipitation, which are then followed by nondensifying grain coarsening through Ostwald ripening . Upon cooling, the liquid may form a glass or a crystalline phase. The solidified liquid can form a continuous film that surrounds the grains, an interpenetrating phase in the form of ligaments along grain boundaries, or an isolated phase that retreats to triple-grain junctions . Liquid penetration along the grain boundaries is a function of the ratio of solid-solid interfacial energy to solid-liquid interfacial energy, which is commonly expressed as the dihedral angle . To enhance performance at elevated temperatures, the amount of the residual second phase should be minimized if it is glassy upon cooling. Alternatively, some liquid phase sintering aids are designed to convert to crystalline phases that resist deformation. In either case, the resulting ceramic cannot generally operate above the temperature at which any glassy phase softens or the lower end of the melting range. In many instances, use temperatures are substantially below these limits. A representative liquid phase sintered microstructure, in this case for a mullite ceramic, has both a major phase and a solidified liquid (Fig. 4). The grain size in the liquid phase sintered ceramic is nearly an order of magnitude greater than in the solid – state sintered ceramic because of increased particle coarsening.
Liquid phase sintering processes can be designed for ceramic systems (and metallic ones for that matter) with the aid of phase diagrams . The first step in designing
Fig. 4 Microstructure of a liquid phase sintered mullite ceramic with a relative density > 99%. The ceramic had elongated grains with a grain size of > 5 pm and had a residual glassy phase surrounding the mullite grains
the liquid phase sintering process is to determine a range of compositions for the proposed additives that will promote liquid formation at the desired sintering temperature. This can be done using the appropriate binary, ternary, or higher order phase diagrams. Next, the composition of the liquid phase, after it becomes saturated with the matrix phase, can be predicted by constructing a join between the additive composition and matrix composition. Finally, the amount and composition of the phases that will be present after processing can be predicted by analyzing the cooling path for the matrix saturated liquid phase. One common example is the densification of a-Al2O3 with the aid of a CaO-SiO2 glass . Using the CaO-SiO2-Al2O3 ternary phase diagram , the first choice may be to select the CaO-SiO2 composition that results in the minimum melting temperature (64 wt% SiO2, 36 wt% CaO, which is the binary eutectic composition that melts at 1,426°C). However, analysis of the resulting liquid phase (19 wt% CaO, 34 wt% SiO2, 47 wt% Al2O3) indicates that CaO6Al2O3, 2CaOAl2O3SiO2, and CaOAl2O32SiO2 will form upon final solidification by peritectic reaction at 1,380°C. For example, the Al2O3-saturated liquid composition lies in the CaO6Al2O3- 2CaO Al2O3 SiO2-CaO Al2O3 2SiO2 compositional triangle. The resulting ceramic would contain 91.5 wt% a-Al2O3 for a composition containing a 4.0 wt% sintering aid addition. To increase the a-Al2O3 content of the final product, the initial additive composition can be shifted to 67 wt% SiO2 so that a liquid phase containing 19 wt% CaO, 36 wt% SiO2, 45 wt% Al2O3 forms when equilibrium is reached at 1,600°C. Upon cooling, Al2O3, CaO6Al2O3, and CaOAl2O32SiO2 will form by peritectic reaction at 1,495°C, increasing the resulting a-Al2O3 content of the final ceramic to 93.7 wt% for a composition containing a 4.0 wt% sintering aid addition. This change in composition also increases the temperature of first liquid formation by 115°C thereby allowing the ceramic to be used in higher temperature applications.