Biomedical Engineering Reference
In-Depth Information
Additionally, the operation temperature is low, and process variables can be easily
monitored and controlled. However, the resorbability of the coating is a big con-
sideration since the coating is very thin.
4.
Physical vapor deposition (PVD).
Radio frequency sputtering methods have been
reported (Yamashita et al. 1994; van Dijk et al. 1998). The main advantage of this
method is that a thin coating (~3 μm) with a “clean” interface and dense microstruc-
ture as well as good adhesion to substrate can be obtained. However, the potential
decomposition of complex target molecules may occur, thus the materials arriving
at the substrate may be composed of individual atoms, simple molecules, or ions,
rather than stoichiometric HA molecules. The resultant coating may not possess
the chemical and structural integrity of initial HA target. Furthermore, the PVD
technique cannot produce a thick coating, which is very important for bony tissue
fixation with the total implant.
5.
Plasma spraying
(Reis et al. 1996; Gross et al. 1998a; Wang et al. 1993). Plasma spraying
is by far the most widely used and the main industrial process to deposit thick HA
coatings. The attraction lies in the easy operation and high efficiency to produce HA
coatings, as well as relatively low substrate temperature, available technology and
equipment, acceptable production costs, and its ability to apply tailored coatings
rapidly and reproducibly on complex shapes. However, it is more or less limited by
low cohesion and adhesion strength, as well as significant phase transformation of
HA that is induced by the extremely high temperature of the plasma torch.
6.
Suspension plasma spraying
(Kozerski et al. 2010). There have been successful efforts
in recent years in applying suspension plasma spraying in depositing HA coat-
ings. The major advancement of this technology is its capability of easily produc-
ing nanostructured HA coatings. While the challenges for this approach are the
potential overheating of the fine HA suspension particles during the spraying,
which might trigger unfavorable HA phase transformation.
7.
HVOF spraying
(Cheang et al. 1995; Lugscheider et al. 1996; Sturgeon et al. 1995;
Lewis et al. 1998). So far, only preliminary experimental results have been col-
lected on HVOF-sprayed HA coatings. Due to its attractive performances (i.e., high
flame velocity and moderate flame temperature), HVOF has received increased
attention since its first inception in the early 1980s. Studies showed that the HVOF
flame temperature was relatively low, less than 3400°C, which is believed to be an
advantage concerning the phase transformation of HA material at elevated tem-
peratures. This process was believed to be one potentially interesting method for
HA coating fabrication.
8.
Blast coating
(Ishikawa et al. 1997). The advantage of this method is that the process-
ing can be pursued at room temperature, but adhesion is poor. The low operating
temperature leads to little phase transformation, but the mechanical properties of
the obtained coatings are rather poor.
9.
Ion beam dynamic mixing (IBDM) method
(Yoshinari et al. 1994; Luo et al. 1999). This
method is a combination of ion implantation and physical vapor deposition. The
adhesive bonding strength of HA on Ti alloy substrate was reported to be up to
65 MPa (Yoshinari et al. 1994). Thus, the IBDM method was suggested to be use-
ful for creating adherent films. However, the layer is too thin, which is less than 2
μm. Moreover, phase transformation from crystalline HA to amorphous calcium
phosphate has been detected.
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