DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

Powder densification processes generally include four techniques: 1) conventional
solid state powder consolidation, 2) liquid phase sintering, 3) infiltration, and 4) reactive
powder processes. As the solid state powder consolidation and liquid phase sintering are
major processing methods in the current study, they are described in more details than the
rest FGM fabrication techniques.
Conventional solid state powder processing techniques fabricate ceramic-metal
FGM by first making a powder preform containing materials with desired gradient. The
preform is then consolidated by conventional techniques, such as cold pressing,
pressureless sintering, and hot isostatic pressing. The initial distribution of materials is
considered unchanged after the consolidation process, provided that the mutual diffusion
of constituent phases is negligible during the processing time frame.
Discrepancy in densification rate between compositionally graded layers is un-
desirable, as they cause warping and possibly cracking in the FGM during densification.
Therefore, these effects must be considered. Particularly, these effects require that a
relatively uniform initial powder density within the green compact has to be established;
if powder blends are packed initially to different volume fractions for compositionally
graded layers, subsequent full densification will cause uneven shrinkage in the
component, resulting in warpage or cracking in the dense FGM.
Prediction of the densification kinetics of an FGM structure requires understand-
ing of the densification rate of each composition in the structure; various theories have
been proposed to address the question, however their suitability is depending on the
volume and size ratios of the phases present. 44
65