Biomedical Engineering Reference
In-Depth Information
TABLE 7.2. Comparison of Mechanical Properties of Ceramics and Bone and Other
Orthopedic Materials
Modulus of
elasticity/stiffness
(GPa)
Bending
strength
(MPa)
Hardness
(knoop hardness
number)
Ductility
(%)
Material
Cortical bone
10 - 16
140
0 - 2
Ceramics
Alumina
380
550
0
2100
Zirconia
170
350
0
1160
Hydroxyapatite
34.5
200
0
Metals
Implant alloy
(Co - Cr - Mo)
240
825
10
430
Stainless steel
annealed (316L)
190
485
40
235
Ti - 6Al - 4V (ELI)
110
900
12
325
Polymers
Polymethyl
methacrylate
(PMMA)
3
55
0 - 2
Polyethylene
(UHMW)
12
44
400
Adapted from [62] .
k
d
σσ
=+
0
y
(7.1)
Here
0
is a friction stress and
k
is a constant. Hardness and grain size also have
the above similar relationships. Many experimental results partly confi rmed the
above theoretical predictions. For example, as grain size is reduced to nanoscale
dimensions, the hardness of nanocrystalline metals (such as copper, palladium
and silver) typically increases and can be a factor of two to fi ve higher than
conventional metals [59,75-76]. Karch and Birringer reported that an increased
value of hardness and remarkably high values of fracture toughness (
σ
14 MPa · m
− 1/2
)
were observed for nanocrystalline titania [63]. Reducing the grain size of
tetragonal zirconia polycrystals (Y-TZP) from commercially used 0.3
∼
m to 10 nm
enhanced deformation rates by a factor of two and provided a material with
superplasticity properties [63]. Webster et al. also reported the modulus of
elasticity of 23 nm grain size alumina decreased by 70% compared to that of
177 nm grain size alumina [15]. However, since fl aws such as pores, dislocations,
and other defects of nanocrystalline ceramics frequently occur, sometimes
strength or hardness values of nanocrystalline ceramics deviate from Hall-Petch
predictions. This phenomena is called the inverse Hall-Petch effect, namely, a
decrease in strength or hardness as grain size is decreased below
μ
10 nm [54] .
Controlling manufacturing parameters may avoid the inverse Hall-Petch effect.
∼
Search WWH ::
Custom Search