A. Driving
force for densification
Assuming as above that the liquid wets and spreads
over the solid surfaces, the solid / vapor interface of the particulate system
will be eliminated and pores will from in the liquid. The reduction of the
liquid / vapor interfacial area provides a driving force for shrinkage and
densification of the overall system. For a spherical pore of radius r in a
liquid, the pressure difference across the curved surface by the equation of
young and Laplace.
B. Enhancement
of densification
Compared to solid-state sintering, the presence of the liquid
phase leads to enhanced densification through (i) enhanced rearrangement of the
particulate solid and (ii) enhanced matter transport through the liquid.
Enhanced rearrangement
The friction between the particle is significantly reduced, so
they can rearrange more easily under the actions of the compressive stress
exerted by the liquid.
Enhanced Matter Transport
In solid –state sintering by, for example, grain boundary
diffusion, an important parameter that controls the rate if diffusion is the
product of the grain boundary diffusion coefficient Dgb and the grain boundary
thickness δgb.
In liquid-phase sintering, the corresponding parameter is the product of the
diffusion coefficient DL of the solute atoms in the liquid and the thickness of
the liquid bridge, δL. As noted earlier, δL is typically many times greater
than δgb, and diffusion through a liquid is much faster than in solid. The
liquid therefore provides a path for enhanced matter transport.
C. Source of the Liquid Phase
The consolidated powder
form for liquid-phase sintering is normally produced from a mixture of two
powders : a major component an additive phase. On heating, the additive
melts or reacts with a small part of the
major component to form a eutectic liquid. The production of the liquid phase
by melting of a additive is fairly common in metallic systems, e.g., Fe(Cu) and
W(Ni). In ceramic systems, the formation of a eutectic liquid is more common,
e.g., MgO(CaO-SiO2) and ZnO(Bi2O3). For systems that rely on the formation of a
eutectic liquid, phase diagrams play a key role in the selection of the
additive and in the choice of the firing conditions. The use of phase diagrams
in liquid-phase sintering will be discussed later in this chapter.
D. Amount of Liquid Phase
For the production of
most advanced ceramics by liquid-phase sintering, the amount of liquid phase produced at the firing
temperature is kept typically below =vol %, although in a few cases it csn
be as high as =10 vol %. The volume of liquid is
therefore insufficient to fill the interstices between the solid particles.
However, many traditional ceramics are fabricated by a process in which a much
higher volume of liquid is sufficient to fill the interstices between the solid
particles (after some rearrangement of the system), and no further
densification by other mechanisms is required for the production of the final
article. As we will recall from chapter 1, this type of sintering in which the
liquid volume is sufficient to fill up the interstices between the solid
particles is referred to as vitrification.
E. Persistent and Transient Liquid-Phase
Sintering
In most system, the
liquid persists throughout the firing process and its volume does not change
appreciably. This situation is sometimes referred to as persistent liquid-phase
sintering. On cooling, the liquid forms a glassy grain boundary phase, which,
as outlined earlier, may lead to a deterioration in high-temperature mechanical
properties. In a small number of systems, the liquid may be present over a
major portion of the firing process but then disappears by (i) incorporation
into the solid phase to produce a solid solution, e,g. , Si, N4(Al2O3-AIN);
(ii)crystallization of the liquid, e,g., Si3N4(Al2O3-Y2O3); or (iiii)
evaporation e,g., the system BaTiO3(LiF). The term transient liquid-phase
sintering is used to describe the sintering in which the liquid phase
disappears prior to the completion of firing. The interest in ceramic materials
for mechanical engineering applications at high temperatures has led to the investigation
of transient liquid-phase sintering in a few Si3N4 systems. However, the
process is generally difficult to control and requires much further work if it is to be practiced
successfully on a larger scale.
In this book, the term
liquid-phase sintering will refer most generally to the case of a persistent
liquid. A distinction between persistent and transient liquid-phase sintering
will be made only when it is convenient.
F. Cohesion of the Particulate Solid
During Firing
Despite the presence of a
viscous liquid between the particles during liquid-phase sintering, the
structure does not collapse unless the volume of liquid is very large. The
capillary stress produced by the liquid holds the solid particles together. The
creep (or shear) viscosity of the system is, however, much lower than that of a
similar system without the liquid phase.
G. Advantages and Disadvantages of
Liquid-Phase Sintering
The major advantages of liquid-phase
sintering, as discussed earlier, are (i) the enhanced densification leading to
the production of high density and (ii) the economic benefits arising from the
use of a lower firing temperature than that required for solid-state sintering
of the major component. However, the use of liquid-phase sintering is not
without its disadvantages. The liquid formed during firing normally remains as
a grains boundary phase on cooling (see
Fig. 1.15). this grain boundary phase
can cause a deterioration of the properties of the fabricated article. An
important example is the production of ceramics for structural applications at
high temperature. The grain boundary phase may soften prematurely, thereby
causing a reduction in the creep resistance of the solid. In many cases,
causing it is not easy to control the grain growth during liquid-phase
sintering. The liquid may also enhance the coarsening process so that the
achievement of a fine-grained microstructure may be difficult. Finally,
compared to solid-state sintering, the presence of an additional phase may make
the analysis and understanding of certain aspects of liquid phase sintering
more difficult.
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