Biomedical Engineering Reference
In-Depth Information
(a)
Plasma gas + current
Water-cooled anode
Cathode
Coating
Workpiece
Powder port
Insulator
(b)
Cooling
water
Oxy-hydroxygen
or Oxy-proplene
or Oxy-propane
Powder with
carrier gas
Water CAP body
Atomized
spray
Nozzle
Sprayed
materials
Water
CAP
Shock
diamond
Combustion
chamber
Substrate
FIGURE 4.3
(a) Sketch of the plasma spraying being used in the biomedical field for the production of HA coatings (http://
www.sulzermetco.com). (b) Schematic diagram of HVOF spray (http://www.nationalthermospray.com).
The plasma gas is injected into the gap between the two electrodes. As the gases pass
around the arc created between the electrodes, they are heated and partially ionized,
emerging from the anode nozzle at high velocity and high temperature. All materials with
melting point can be plasma-sprayed because of its high gas temperature.
The HVOF technique was developed in the early 1980s. The principle lies mainly in
the use of combustion of fuel gas such as hydrogen, propane, propylene, or acetylene, in
oxygen. Supersonic flame can be generated through the combustion of fuel gas in oxygen
under high pressure. As shown in Figure 4.3b, the characteristic of the supersonic flame
is the appearance of shock diamonds in the flame. Sprayed powders are axially fed into
the gun by means of inert carrier gas, then are accelerated and heated in the flame, which
is of a high velocity of more than 1000 m/s and a moderate temperature of up to 3400°C
depending on fuel gas and oxygen/fuel ratio. Impingement of the heated particles on sub-
strate or precoating could result in a satisfactory coating in terms of dense microstructure
and competitive mechanical properties. The unique heated efficiency of the particles can
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