Classes of materials used in medicine - Biomedical Engineering Desk Reference - page 270

Vapor-deposited carbons are also used in heart-valve
applications. Typically, the coatings are thin ( < 1 m m) and
may be applied to a variety of materials in order to confer
the biochemical characteristics of turbostratic carbon.
Some examples are vapor-deposited carbon coatings on
heart-valve sewing cuffs and metallic orifice components
(Haubold et al. , 1981).

Mechanical properties

Mechanical properties of pure pyrolytic carbon, silicon-
alloyed pyrolytic carbons and glassy carbon are given in
Table 3.2.11-1 . Pyrolytic carbon flexural strength is high
enough to provide the necessary structural stability for
a variety of implant applications and the density is low
enough to allow for components to move easily under the
applied forces of circulating blood. With respect to or-
thopedic applications, Young's modulus is in the range
reported for bone (Reilly and Burstein, 1974; Reilly et al. ,
1974), which allows for compliance matching. Relative
to metals and polymers, the pyrolytic carbon strain-to-
failure is low; it is a nearly ideal linear elastic material and
requires consideration of brittle material principles in
component design. Strength levels vary with the effec-
tive stressed volume or stressed area as predicted by
classical Weibull statistics (De Salvo, 1970). The flexural
strengths cited in Table 3.2.11-1 are for specimens tested
in four-point bending, third-point loading (More et al. ,
1993) with effective stressed volumes of 1.93 mm 3 . The

Fig. 3.2.11-4 Electron micrograph of pure pyrolytic carbon
microstructure showing near-spherical polycrystalline growth
features formed during deposition (Kaae and Wall, 1996).

carbons so that overall the material exhibits isotropic
behavior.
Glassy carbon, also known as vitreous carbon or
polymeric carbon, is another turbostratic carbon form
that has been proposed for use in long-term implants.
However, its strength is low and the wear resistance is
poor. Glassy carbons are quasi crystalline in structure and
are named ''glassy'' because the fracture surfaces closely
resemble those of glass (Haubold et al. , 1981).

Table 3.2.11-1 Mechanical properties of biomedical carbons

Property Pure PyC Typical

Si-alloyed
PyC

Typical
glassy
carbon

Flexural strength
(MPa)

493.7 12 407.7 14.1 175

Young's modulus
(GPa)

29.4 0.4 30.5 0.65 21

Strain-to-failure (%) 1.58 0.03 1.28 0.03

Fracture toughness
(MPa O m)

1.68 0.05 1.17 0.17 0.5-0.7

Hardness
(DPH, 500 g load)

235.9 3.3 287 10 150

Density (g/cm 3 )

1.93 0.01 2.12 0.01 <1.54

CTE (10-6 cm/cm C) 6.5 6.1

Fig. 3.2.11-5 Electron micrograph of silicon-alloyed pyrolytic
carbon microstructure showing near-spherical polycrystalline
growth features formed during deposition (Kaae and Wall, 1996).
Small silicon carbide particles are shown in concentric rings in the
growth features.

Silicon content (%) 0 6.58 0.32 0

Wear resistance Excellent Excellent Poor