Biomedical Engineering Reference
In-Depth Information
microcracks resulting from thermal contraction during the rapid cooling stage after
plasma spraying. The cross-sectional feature of the plasma-sprayed HACs is shown in
Figure 6.6b. According to the quantitative calculation by an image analyzer OPTIMAS 6.0
[63,64,101], the spraying defects content (in volume %), including microcracks and pores
within the entire cross-sectional area of HACs, is about 4% for plasma-sprayed HACs in
the case of Figure 6.6b. Although the plasma-sprayed coatings possess inevitable spray-
ing defects, including pores and thermal-induced microcracks, they can be subjected to
high densification due to the unapparent lamellar structure. Since the structural density of
plasma-sprayed coatings is significantly affected by the variation of spraying parameters
such as plasma gas flow rate (l min
−1
), plasma power (kW), powder feed rate, surface speed,
standoff distance, and so forth, the coating microstructure displayed in Figure 6.6b can be
recognized as a dense HA coating (following the definition of the maximum porosity less
than 5% by volume [8,9]) obtained from an appropriate spraying parameter. Besides the
microstructural defects, a noteworthy feature with a mixture of dark gray and light gray
regions is observed within plasma-sprayed HACs, as shown in Figure 6.6b. This distin-
guishable color contrast is resulted from the difference between amorphous and crystal-
line area of the coatings [102]. Other phases may be present in small quantities, but they
cannot be distinguished from HA. During plasma spraying, the unmolten and partially
molten particles are transferred to coatings with a morphology representative of the start-
ing powders. Therefore, these dark gray regions marked by circles can be thought of as
the crystalline region from the residual partially molten particles within plasma-sprayed
HACs. This phenomenon is important in the performance of the HA coatings because
the dissolution of amorphous regions could lead to failure for implants after a period of
implantation. Therefore, it is possible to increase the crystallinity and in some cases the
bonding strength of plasma-sprayed HACs by performing postheat treatments.
Crystallization of Plasma-Sprayed HACs during Heat Treatments
Referring to the reports about HA, many researchers have investigated the material and
medical properties of plasma-sprayed HACs in the past 10 years. Amorphous calcium
phosphate (ACP) is thermodynamically metastable and impurity calcium phosphate
(including TCP, TP, and CaO, called the impurity phases) is undesirable for its dissolution
problems in human body fluids. Therefore, previous studies pointed out that controlling
spraying parameters [112] or performing appropriate thermal treatments (in vacuum,
in an atmosphere with moisture or steam pressure and the spark plasma sintering
(SPS) technique) are available methods that significantly promote the HA crystallization,
improve the crystallinity and dissolution behaviors of coatings, and enhance the surface
activity to the growth of apatite layers [16,59,65,66,99,113-115]. Additionally, reports have
focused on the changes in phase composition, microstructural homogenization, and
reduction in residual porosity of HACs [55-59,63,101], as well as the mechanism of crystal-
of HACs [55-59,63,101], as well as the mechanism of crystal-
55-59,63,101], as well as the mechanism of crystal-
-59,63,101], as well as the mechanism of crystal-
59,63,101], as well as the mechanism of crystal-
], as well as the mechanism of crystal-
lization of the coating layers [116].
Figure 6.7 illustrates the XRD patterns of plasma-sprayed HA-coated Ti-6Al-4V speci-
mens with postvacuum and atmospheric heat treatments at 400°C, 500°C, 600°C, 700°C,
and 800°C. The three strongest HA main peaks tend to become sharper with increasing
heating temperatures, revealing that the plasma-sprayed HACs possess different degrees
of crystallization after postheat treatment. For the atmospheric heat-treated HACs, the
partial water vapor pressure of atmospheric moisture can help to recover and promote
the reconstitution of TCP, TP, and ACP into crystalline HA. However, the vacuum heat-
treated specimens possess more TCP and TP than the atmospheric heat-treated specimens.
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