Biomedical Engineering Reference
In-Depth Information
HT-HACs compared with those high-temperature heat-treated HACs displayed in Figure
6.24. Since both Young's modulus and the bonding strength of the plasma-sprayed HACs
are significantly improved after the hydrothermal treatment (Figure 6.23), it is worth not-
ing that the improvement of mechanical properties for HT-HACs can be pinpointed due
to the self-healing effect of hydrothermal crystallization. The evidences can not only be
observed from surface morphologies as shown in Figure 6.16, but also be demonstrated
from the fracture specimens of HT-HAC test pieces after three-point bending tests. Figure
6.27b represents a significant reduction of pores and cracks within the HT-HAC coat-
ings, and the inter-lamellar boundaries are obviously healed with the nanocrystalline as
indicated by arrows in Figure 6.16f. It can be seen that the microstructural homogeneity
and the contact between splats boundaries are significantly improved after hydrothermal
treatment. Thus, besides the new-growth HA crystallites observed on the coating surface,
the hydrothermal crystallization with a self-healing effect occurred throughout the whole
HA coating layers under the abundant saturated steam pressure environment. According
to the above-mentioned results, it can be recognized that the overall bonding strength
of plasma-sprayed HACs is effectively improved by the low-temperature hydrothermal
treatment with increasing both the cohesive strength and the adhesive interfacial strength
through the self-healing effect and the HAC/Ti substrate interfacial chemical bonding
(Ti-OH), respectively.
Erosive Wear Behaviors
Plasma-sprayed ceramic coatings have been widely employed to provide an improved wear
resistance to various industrial parts [50,223-225]. The erosion of ceramic coatings can be
influenced by both coating properties and impacting particle conditions including its size
and morphology, velocity, incident angle, and materials properties in a similar way to
that of bulk materials. Considering the relationship between microstructural features and
properties of thermal sprayed coatings, it can be recognized that the properties of thermal
spraying coating will be significantly influenced by the lamellar structure inherent in the
thermal spraying processes [226-229]. The difference of microstructural features of the
coatings from the bulk materials makes their erosive wear behaviors in a different way.
With the applications of plasma-sprayed HA-coated implants for the artificial joints
replacement, the abrasive wear and particle erosion behaviors of coatings are of particu-
lar interest. Clinical studies have indicated that the release of particles and subsequent
inclusions in the surrounding bone will result in coating degradation and wear problems
[49,230]. Some reports have focused on the wear behaviors that resulted from particle
release of HACs by the fretting and abrasive wear tests [116,225,231-233]. Different from
the fretting and abrasive wear tests, the particle erosion method [234-236] can also reflect
the erosive wear behaviors of a material from an oblique incident angle to a normal inci-
dent angle through changing various erosion impact angles (θ is varied from 0° to 90°). The
previously mentioned presentations have indicated that biological responses, microstruc-
ture homogeneity, Young's modulus, and bonding strength can be effectively improved
by performing appropriate crystallization treatments [120-122]. The wear behaviors,
which resulted from the particle release with chemical dissolution of HACs, will also be
improved with HA crystallization. Since the particle erosion test can reflect various wear
behaviors of ductile fracture and brittle fracture from changing the erosion impact angles,
it will adequately simulate the wears of acetabular components in the human body.
The particle erosion test is generally conducted at room temperature using a sandblast-
conducted at room temperature using a sandblast-
ing type erosion machine. The setup for the experiment is illustrated schematically in
is generally conducted at room temperature using a sandblast-
generally conducted at room temperature using a sandblast-
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