Biomedical Engineering Reference
In-Depth Information
9.6 Plasma-Sprayed Coatings on
PEEK-Based Substrates
With differential scanning calorimetry (DSC),
thermal properties like melting points, phase tran-
sitions, crystallization temperatures, degree of
crystallinity, reaction kinetics, and decomposition
effects can be identified and measured. The DSC
principle is to compare the rate of heat flow between
the sample and an inert reference material that are
heated or cooled at the same rate. Changes in the
sample are associated with heat absorption or
release and cause a change in the differential heat
flow, which is then recorded as a peak. The area
under the peak is directly proportional to the
enthalpic change and its direction indicates whether
the thermal event is endothermic or exothermic. For
example, as a solid sample melts to a liquid, it will
require more heat flowing to the sample to increase
its temperature at the same rate as the reference.
This is due to the absorption of heat by the sample
as it undergoes the endothermic phase transition
from solid to liquid.
Figure 9.11 illustrates a DSC plot on a sample of
CFR-PEEK heated up to 380 Cat10 C/min: the
slope change around 170 C indicates the glass
transition, although the endothermic peak around
340 C is a consequence of melting.
Plasma spray coating of polymers and polymer
composites is not a commonly used process, mainly
because of the low thermal stability of polymers
compared to other metallic or ceramic substrates. For
polymer coating, the literature mostly describes the
use of flame spray or high-velocity oxy-fuel tech-
niques, because these processes involve lower
temperatures, thus minimizing thermal degradation.
These processes are, for example, used to resurface
polymer matrix composites in aerospace applications
with erosion-resistant coatings [79 e 81] .
PEEK is a high-temperature-resistant engineered
thermoplastic showing a relatively high glass transi-
tion temperature (about 140 C) and melting point
(about 340 C), also allowing short-time exposures at
temperature above 200 C. These features enable the
use of plasma spray processes for PEEK-based
components, provided that care is taken with accurate
regulation of those parameters that most affect the
cooling of the substrate: cooling system, position and
distance of the plasma torch from the substrate, and
relative movements during the coating process. An
Figure 9.11 DSC plot of PEEK Optima LT1CA30, heated from room temperature to 380 Cat10 C/min. The
y
-axis
indicates exothermal reactions.
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