Biomedical Engineering Reference
In-Depth Information
suggest that the net is nanocrystallined K 2 TiO 3 (Lee et al. 2009). Usually, the network struc-
ture obtained by chemical treatment needs a processing time of more than 10 h, whereas
the electrochemical treatment can be completed in no more than half an hour. In addition,
the pore size achieved by anodization is much smaller.
Fabrication of Compact Nanocrystalline Titania Coatings
Plasma Spraying
Titania coating with nanosized particles can be fabricated by plasma spraying. Both cold
spraying and plasma spraying can produce nanocrystalline coatings (Zhu and Ding 2000).
Usually, nanosized titania powders are utilized during the spraying process. The particles
size in the as-deposited coating depends mainly on the size of the powders.
Plasma spraying includes atmospheric plasma spraying (APS) and vacuum plasma
spraying (VPS). Figure 5.21 illustrates this process. An electrical arc is used to melt and
spray materials onto a surface. The high energy and density available in the plasma jet
make it one of the popular thermal spraying techniques. The density, temperature, and
velocity of the plasma plume are crucial to the formation of coatings. The temperature of
the plasma depends mainly on the degree of ionization which in turn is determined by
the type of plasma gas and parameters of the plasma. At present, gases such as Ar, He,
N 2 , and H 2 are commonly used in a plasma torch. To ensure environmental inertness, a
mixture of argon and hydrogen is commonly used. Formation of the plasma sprayed coat-
ing consists of three stages: (1) melting and accelerating of the particle from the supplier,
(2) transportation of the powder to the plasma torch, and (3) interaction between the mol-
ten materials and plasma beam and surrounding atmosphere. Currently, DC plasma arc
devices dominate the commercial market, whereas radio frequency (RF) and inductively
coupled plasma (ICP) have a few commercial applications (Liu et al. 2004).
Liu et al. have fabricated nano-TiO 2 coatings on Ti-6Al-4V substrate using atmospheric
plasma spraying. The deposited coating is about 100 μm in thickness. Figure 5.22 shows
the SEM and TEM pictures of the surface morphology and cross section (Liu et al. 2005).
High magnification views of the SEM picture show that the nanosized TiO 2 coating is com-
posed of particles less than 50 nm in size. TEM observation of the as-prepared coatings
also reveals that the surface mainly consists of grains less than 50 nm in size. The coating
Spray deposit
Anode
Plasma
Cathode
Electric arc
Powder and
carrier gas
Spray distance
Plasma
gas
Electrical (+)
connection
and water in
Electrical (-)
connection
and water out
Substrate
FIGURE 5.21
Schematic illustration of plasma-spraying technique. (From Liu et al., Mater. Sci. Eng. , R 47, 49-121, 2004. With
permission.)
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