MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 377

Figure 11.17). This approach explains the large decrease in strength of Cerastar
RX. At these silicon contents, the contact sites are small enough to be fractured
by the applied stresses, leading a deformation controlled by the fl ow of silicon.
The reaction formed silicon carbide (RFSC) microstructure can be modeled
by a stacking of spherical pores (silicon regions), as it is generated from the silicon
infi ltration of a carbon preform with a sponge-like microstructure. Its strength is
close to the curve expected for this ideal microstructure (curve three of Figure
11.17) although the experimental slope is a bit higher. The strength of BioSiC
fabricated from mango wood (58% of SiC) is included together with the RFSC
data, because it shows very little anisotropy due to its short longitudinal cells
[Mart í nez, 2000 ].
The pores/channels in BioSiC have cylindrical/tubular shape, so this mate-
rial must be modeled by a stacking of cylindrical/tubular pores. This implies that
the behavior will be anisotropic and the strength in the axial and radial direction
has to be different. It must be noted that the compressive strength of RBSC,
RFSC, and BioSiC in the axial and radial direction, follow the same order in
strength level as predicted by a stacking of spherical particles, spherical pores,
cylindrical pores in the longitudinal and perpendicular direction, respectively
(Figure 11.17 ).
In the case of axial compression of longitudinal tubular pores, the type of the
stacking and shape of the tubular pores does not have infl uence on the theoretical
strength (the strength is simply equal to the volume fraction). The experimental
results are lower than the ones predicted by the model (curve four of Figure
11.17) and the slope of the experimental curve is slightly higher. There are several
microstructural factors that can account for the differences with the idealized
model:
1. The cells in biomorphic SiC are not perfectly aligned, which could pro-
mote some bending of the cell walls. This effect must be more important
for the lower densities and could be partially responsible for the higher
slope of the experimental curve.
2. The cell shape differs from a perfect cylinder. The walls present “ defects ”
related with the nature of the precursors, as pores. Additionally, the sepa-
ration between contiguous cells may not be fl at.
3. There is a certain amount of radial channels, so the theoretical strength
should be taken as an average of the strength of a solid with longitudinal
tubular pores, and a small proportion of tubular pores oriented perpen-
dicularly to the applied stress. This fact, may explain the differences in
strength of the model and experimental values. In this regard, it is remark-
able that microstructures like the one of BioSiC fabricated from Cistus
ladanifer (rockrose), without radial channels, show a strength very close
to the theoretical one (Figure 11.17, 75% SiC).
When the compression is done in the radial direction, the models predict a
strong dependence with the type of stacking of the tubular pores and a slight