Biomedical Engineering Reference
In-Depth Information
For example, nanostructured metals are most commonly shaped
by machining of bulk metal into their i nal form. h e studies that have
addressed machining of nanostructured metals generally show that nano-
structured metals possess superior machinability. Documented advantages
include reduced tool wear and superior surface i nish [61-63]. Cutting
forces for ultrai ne grained copper and its conventional metal counterpart
have been shown to be undif erentiated [62]. Lapovok
et al.
[61] noted that
the thermal conductivity of nanostructured metals decreases with decreas-
ing grain size, thereby reducing the length of machining chips during
turning and enhancing machinability. However, it should be noted that the
chips formed by machining can be signii cantly stronger and harder than
derived from machining of conventional metals. h us drilling or machin-
ing of internal cavities can be more dii cult due to the need to remove the
extra hard chips. In addition, the higher yield strength of nanostructured
metals results in higher elastic stresses during boring or machining of inte-
rior cavities. h is can accelerate tool wear under these circumstances.
h ere are distinct advantages to shaping and forming of bulk nanostruc-
tured metals. h eir ultrai ne grain size enables them to deform by grain
boundary sliding, and therefore they can be formed superplastically at
lower temperatures and higher strain rates than conventional i ne grain
metals [4, 26, 64-69]. h e availability of grain boundary sliding as a defor-
mation mechanism aids formability even during conventional intermedi-
ate temperature and high rate forging. One consequence of this advantage
is that multi-step forging operations, for example, to produce high strength
hip implant structures, can be accomplished in fewer steps. Similarly, in
closed die forging it is easier to achieve complete die i ll at lower tempera-
tures and with lower forces.
Metals used in medical devices are commonly subject to surface modi-
i cations or coating processes during manufacturing. A signii cant body
of knowledge has emerged on the viability of coating bulk nanostructured
metals [70-76]. For example, hybrid oxide coatings or hydroxyapatite
adhere readily to nanostructured titanium, providing enhanced biocom-
patibility and osseointegration [73, 77, 78]. While nanostructured surfaces
have been shown to have intrinsically superior biocompatibility [28, 29,
36, 37, 79-82], the additional enhancement of their biological properties
through surface treatments are notable [27, 83-91].
1.1.3.2
Superior Physical and Mechanical Properties
Perhaps the most distinctive characteristic of nanostructured metals
is their superior mechanical properties compared to their conventional
