MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 370

processed from beech, taking into account the differences in density. The fl exure
strength of the high-density biomorphic composites processed from eucalyptus
and beech was similar to that of the materials obtained from beech and mahog-
any, respectively, even though their density was signifi cantly higher. The fracture
surfaces were always very brittle, and little information on the fracture micro-
mechanisms could be extracted from their analyses, although the most obvious
reason for the better properties of the latter materials was the absence of residual
C. Thus, the presence of residual C did not seem to infl uence signifi cantly the
compressive strength of biomorphic Si/SiC composites but it was clearly negative
in bending. The bending strength of brittle materials—as opposed to compressive
strength—is controlled by the presence of defects where cracks are nucleated.
Regions containing amorphous residual C enhance the total volume fraction of
these regions in the biomorphic composites and reduce the overall bending
strength while its infl uence on the compressive strength was very limited.
The compressive strengths of the porous bioSiC (with Si removed) [Kaul,
2006] are considerable lower (decrease up to 60%) than those of fully dense
bioSiC/Si (Figure 11.13) [Singh, 2002; Presas, 2005]. This is not surprising as the
residual Si serves to reinforce the bioSiC.
The values measured here agree with compressive strength values for bioSiC/
Si measured above the softening temperature of silicon [Varela, 2002; Martínez,
2003]. In addition to the infl uence of orientation on mechanical properties, pre-
cursor type, independent of porosity, appears to play a signifi cant role. For a given
porosity, SiC samples fabricated from diffuse-porous precursors have better
mechanical properties than those fabricated from ring-porous precursors. In com-
pression, failure is the result of accumulated damage. The lower compressive
strength of ring-porous SiC cannot be explained only by the larger pores in these
materials. The difference in strength between ring-porous and diffuse-porous SiC
is hypothesized to be infl uenced by the amount of residual carbon.
It can be seen that for a given porosity ring-porous hardwoods tend to have a
higher carbon content than that produced from diffuse-porous hardwoods in the
fi nal biomorphic SiC. In the diffuse-porous samples, residual carbon is generally
10 vol.% of the solid phase. However, this value ranges from fi ve percent to 50%
for ring-porous samples, and the amount of residual carbon tends to increase with
decreasing porosity in the precursor. These measurements can be a consequence
of SiC formation mechanism in the two types of porosity. To fully convert the
carbon scaffold to SiC, the molten silicon must infi ltrate all pores. In diffuse-
porous preforms, the uniform distribution of sap channels allows for easier infi l-
tration. In ring-porous samples, with a non-uniform distribution, there are areas
of lower density where sap channels are present that are easily infi ltrated as well
as areas of higher density where only fi ber cells are present that are not as easily
infi ltrated. The majority of the pores in ring-porous precursors are smaller fi ber
cells, so incomplete reaction is likely. Also from Table 11.2, it is evident that there
is greater variability in the amount of residual carbon for the ring-porous than for
the diffuse-porous SiC, which indicates that the residual carbon is not homo-
genously distributed in the material.