MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 372

Beech (axial)
Beech (non-axial)
Poplar (axial)
Poplar (non-axial)
Mahogany (axial)
Mahogany (non-axial)
Red oak (axial)

1.80

1.60

1.40

Sapele (non-axial)

MSA model (axial)
MSA model (non-axial)

1.20

1.00

0.80

0.60

0.40

0.20

0.00

40 45 50 55 60 65 70

Porosity (%)

Figure 11.14. Fracture toughness as a function of porosity for cracks propagating perpen-
dicular (axial) and parallel (non-axial, transverse) to the axial direction of bioSiC derived from
several precursors (error bars equal one standard deviation and arrows refer to the axial direc-
tion). Values predicted by the MSA model for axial and transverse orientations are also shown
by the dashed lines.

50-70% higher than the values measured on porous samples due to the presence
of excess silicon, as expected [Presas, 2005].
The elastic moduli of porous bioSiC determined in compression in the axial
and transverse directions are plotted as a function of porosity in Figures 11.15a
and 11.15b [Kaul, 2006]. By grouping these data by the type of precursor (in this
case, diffuse-porous and ring-porous), it is apparent that the axial stiffness of dif-
fuse-porous SiC are more sensitive to porosity. Porous SiC is signifi cantly stiffer
in the axial direction than in transverse directions, in most cases by one order of
magnitude. A similar trend as the compression strength.
Such behavior is typical of honeycomb materials where the transverse (or in-
plane) strength and stiffness are controlled by cell-wall bending, whereas axial (or
out-of-plane) strength and stiffness depend on compression of the cell walls. The
elastic moduli reported here are about half that reported in the literature for
biomorphic Si/SiC at room temperature [Singh, 2002; Smirnov, 2003; Kardashev,
2005; Kardashev, 2006]. This is not surprising as the unreacted silicon stiffens the
composite.
Martínez et al. [Martínez, 2003] has compared the mechanical properties of
biomorphic SiC with other siliconized SiC. Figure 11.16 shows the strength data
for the reaction bonded SiC (RBSC), reaction formed SiC (RFCS), and BioSiC