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JTST HIGHLIGHTS
torch sprayed an Inconel coating on the foil’s surface. A burner
test rig was built to produce hot combustion gases that flowed
over the exposed face of the heat exchanger. Cool air flowed
through the foam heat exchanger at rates of up to 200 SLPM.
Surface temperature and air inlet/exit temperature were mea-
sured. Heat transfer to air flow through the foam was signifi-
cantly higher for the thermal sprayed heat exchangers than
for the brazed heat exchangers. On average, thermal sprayed
heat exchangers show 36% higher heat transfer than conven-
tionally brazed foam heat exchangers. At low flow rates, the
convective resistance is large (~4 × 10−2
m2 K/W), and the effect of thermal con-
tact resistance is negligible. At higher flow
rates, the convective resistance decreases
(~2 × 10−3 m2 K/W), and the lower contact
resistance of the thermally sprayed heat
exchanger provides better performance
than the brazed heat exchangers.
PLASMA-SPRAYED
HYDROXYLAPATITE-BASED
COATINGS: CHEMICAL,
MECHANICAL, MICROSTRUC-
TURAL, AND BIOMEDICAL
PROPERTIES
Robert B. Heimann
The salient properties and func-
tions of hydroxylapatite (HA)-based plas-
ma-sprayed coatings, including the effect
on biomedical efficacy of coating thick-
ness, phase composition and distribution,
amorphicity and crystallinity, porosity
and surface roughness, cohesion and
adhesion, micro- and nano-structured
surface morphology, and residual coating
stresses are discussed. In addition, details
of the thermal alteration that hydroxylap-
atite particles undergo in the extremely
hot plasma jet that leads to dehydroxyl-
ated phases such as oxyhydroxylapatite
(OHA) and oxyapatite (OA) are discussed.
Thermal decomposition products such
as tri-(TCP) and tetracalcium phosphates
(TTCP), and quenched phases such as
amorphous calcium phosphate (ACP), are
also discussed. The article further explains
the role of amorphous calcium phos-
phate during the in vitro interaction of as-
deposited coatings with simulated body
fluid resembling the composition of extra-
cellular fluid (ECF) as well as the in vivo
responses of coatings to the extracellu-
lar fluid and the host tissue, respectively.
Fig. 3. — Test rig.