DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

For a composite with two constituents, there are various scenarios to consider: 1)
at a sufficiently low volume fraction of one phase in the other, densification of the
composite is primarily controlled by the single percolating matrix phase; and the second
isolated phase, however, still influences local densification kinetics; 2) when the
inclusion volume fraction becomes sufficiently high for significant percolation to occur,
discrepancies in densification rate are much more drastic between compositional graded
layers: above the specific volume fraction, the secondary inclusion phase may carry load;
for densification of the composite, both percolating phases must densify. If it does not
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also densify, it resists contraction of the composite powder mixture.
In the case of single phase densification with a non-percolating and non-
densifying refractory phase, the reduction of densification rate for the powder matrix 57-60
can be rationalized and modeled: when the matrix and inclusion particles have
commensurate radii (R = Rp,matrix/Rp,inclusion ≈ 1), the inclusions alter both the initial
powder packing density and the evolution of inter-particle neck geometries. Therefore
greater deformation of the matrix particles is required before reaching a given compact
density. 60, 61 When the radius of the inclusion particles is significantly larger than that of
the matrix particles, i.e. R « 1, the modeling should instead focus on local stresses in the
densifying matrix, resembling a continuum having a stress-dependent density evolution
function. 62-64 In the non-percolating regime, although the retarding effect of isolated
inclusion phase on the densification of local matrix phase could be significant, the
difference in global densification rates between compositions with varying inclusion
volume fractions is generally insignificant.
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