DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

When the powders are densified in simple configurations by hot-pressing within a
die, the criticality of the identical sintering rate under similar conditions is somewhat
reduced. The die may resist the deformation and potential failure of the powder compact;
and the applied high pressure may induce macroscopic material flow to compensate for
the shrinkage differences while maintaining the shape of the component. This technique
was found advantageous over hot isostatic pressing for one-dimensional FGM disk
fabrication, 44 however not much better than hot isostatic pressing for more complex FGM
geometries.
Liquid phase sintering is an attractive process for FGM preform densification,
with major advantages including: 1) low sintering temperature, 2) much faster
densification rate than in solid-phase sintering, 3) high volume fraction of a refractory
phase may be accommodated in some systems, and 4) stresses during sintering are low,
38, 44
since sintering rate differentials can be compensated by macroscopic compact flow.
However, the presence of a liquid phase permits motion of liquid and solid phases
relative to one another, which could under circumstances reduce or even eliminate
38, 44
gradients initially placed in the structure during powder packing.
Infiltration method involves first producing a preform of a refractory phase with
graded porosity, and then infiltrating it with a melted phase. The refractory phase must be
insoluble in the liquid, be sufficiently percolating to stand on its own weight before
infiltration, and be strong enough to resist compression by the melt during pressure-
assisted infiltration. Depending on the processing route used, infiltration processing can
both be constructive or transport-based. Compared to liquid phase sintering, the
infiltration method has the advantage of no homogenization of the refractory phase by
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