Biomedical Engineering Reference
In-Depth Information
started with the observation that HA in the pores of a metal implant with a porous coating
would significantly affect the rate and vitality of bone ingrowth onto pores [15,86].
The plasma spraying process was patented in 1960s, and the technical utilization of
plasma as a high-temperature source is realized in the plasma torch. The plasma gun con-
sists of a cone-shaped tungsten cathode and a cylindrical copper anode. The principle of
plasma spraying is to induce an arc by a high current density and a high electric potential
between the anodic copper nozzle and tungsten cathode. Gases flow through the annu-
lar space between these two electrodes, and an arc is initiated by a high-frequency dis-
charge. Noble gases of He and Ar are usually used as the primary plasma-generating gas.
Diatomic gases of H
2
and N
2
can be used as the secondary gas to increase the enthalpy of
plasma flame. Factors influencing the degree of particles melting during plasma spraying
include variables such as current density and gas mixture that control the temperature
of the plasma. The widely used plasma-generating gas is pure Ar (purity <99.95 wt.%).
Since the thermal conductivity and the heat conduction potential for diatomic gases such
as H
2
and N
2
are much higher than Ar [87], a mixed gas composition with Ar and H
2
/N
2
gives quite a hotter plasma torch than 100% Ar gas. When well-crystallized HA powders
are injected into the high-temperature plasma flame (normally in the range of 1 × 10
4
to
1.5 × 10
4
°C), small granules will be evaporated in the flame, and larger particles are melted
or partially melted quickly by the high-temperature plasma flame. Then these melted
droplets are accelerated to about 200 m/s before impacting the substrate [88,89]. The high-
impact velocity supplies high kinetic energy, which is expended in spreading the molten or
semimolten droplets and creating a lamellar microstructure. In addition, the high cooling
rate upon impact is estimated to be of the order of 10
6
to 10
8
K s
−1
[90]. Therefore, the large
contact area with the substrate and the rapid solidification result in producing amorphous
calcium phosphate (ACP) component within coatings, and it is more commonly found at
the coating/substrate interface.
Since the plasma spraying process involves high temperature and rapid solidification,
it will result in the dehydroxylation and decomposition of HA and the formation of an
amorphous structure within coatings. This decomposition sequence occurs in these steps
[91,92]:
Ca
10
(PO
4
)
6
(OH)
2
→ Ca
10
(PO
4
)
6
(OH)
2-2
x
O
x
V
x
+
x
H
2
O
(6.5)
Ca
10
(PO
4
)
6
(OH)
2-2
x
O
x
V
x
→ Ca
10
(PO
4
)
6
O
x
V
x
+ (1−
x
)H
2
O
(6.6)
Ca
10
(PO
4
)
6
O
x
V
x
→ 2α-Ca
3
(PO
4
)
2
+ Ca
4
P
2
O
9
(6.7)
α-Ca
3
(PO
4
)
2
→ 3CaO + P
2
O
5
(6.8)
Ca
4
P
2
O
9
→ 4CaO + P
2
O
5
(6.9)
The symbol “V” in the formulas of oxyhydroxyapatite (Ca
10
(PO
4
)
6
(OH)
2−2
x
O
x
V
x
, OHA) and
oxyapatite (Ca
10
(PO
4
)
6
O
x
V
x
, OA) refers to lattice vacancies in positions of the OH
−
groups
along the crystallographic
c
-axis in the structure of HA. Thus, the x-ray pattern of a
plasma-sprayed HA coating shows the presence of α-TCP, β-TCP, TP, and CaO phases in
addition to crystalline HA. The reduction in peak intensity and peak broadening of HA
peaks provide an evidence for the formation of ACP. The formation of these additional
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