Biomedical Engineering Reference
In-Depth Information
accompanying in-flight HA droplets and splats/coating formation during plasma spraying
of HA. The steam treatment during the plasma spraying is effective in reversing undesir-
able phases, ACP (the major effect), TCP, and so forth, to crystalline HA. The mechanism of
reversing HA decomposition involves mainly the entrapping of water molecules by indi-
vidual HA droplets upon their impingement. The results indicate that the phase changes
of HA during plasma spraying has already taken place during in-flight stage of the HA
particles. Furthermore, apart from reversing HA decomposition, the steam is also capable
of significantly increasing the adhesion strength of the coating with >100% (Li et al. 2006).
To provide steam is an easy and economical process during plasma spraying. The major
problem with plasma spraying of HA seems to be easily solved. It is also expected that
phase composition of the HA coatings can be altered through changing the pressure and
flow rate of the steam.
Vogel et al. (1996) also reported the crystallization of as-sprayed HA coatings at tem-
peratures around 500°C or above. It was perceived that the decomposition of apatite
during plasma spraying was reversible and could be described by a temperature- and
time-dependent equilibrium reaction. Furthermore, Tong et al. (1998) found a more stable
phase, nanocrystals of HA, after the heat treatment. However, in addition to the recrystal-
lization and element diffusion, one considerable phenomenon appears while the postheat
treatment is conducted; that is, new cracks appear as a result of thermal contraction (Gross
et al. 1998c; Kijk et al. 1996). HA is 3% denser than oxyapatite, thus the transformation to
HA will produce an additional 0.6% contraction (Gross et al. 1998c). The apparent con-
traction when crystallization occurs can correspond to ~1% in a direction parallel to the
lamellae within the coating. Additionally, the contraction is approximately proportional
to the amount of amorphous phase (Gross et al. 1998c). The appearance of cracking sig-
nifies that heat treatment of the amorphous phase is not a very favorable option when
large changes in crystallinity must be made. Furthermore, the reaction at the interface
between HA coating and titanium alloy substrate was observed (Gross et al. 1998c). Many
unfavorable phases, such as CaTiO
3
and CaTi
2
O
5
, formed owing to the element diffusion
and subsequent chemical reaction. Even though Ogiso et al. (1998a) found that the crystal-
lization of HA coating would occur in bony tissue after implantation, this crystallization
could not significantly decrease the solubility of the amorphous portion because the newly
crystallized HA is too fine to resist dissolution. Furthermore, apart from the solubility, the
recrystallization causes a stress accumulation within the HA coating because of volume
difference, resulting in a physical weakening of the HA coating, which is a different prob-
lem besides the solubility of the coating. In addition, Ogiso et al. also revealed that the
crystallization caused the adhesive bonding strength between HA coating and substrate
to be weakened because both portions were bound to each other through the amorphous
phase. Kijk et al. (1996) also found cracks in the HA coating annealed at a temperature
higher than 400°C, while 600°C was suggested to be probably the best annealing tempera-
ture to obtain a better coating.
Other methods to crystallize the coating with ACP content, such as laser treatment (Khor
and Cheang 1994b; Ranz et al. 1998) and water vapor treatment (Cao et al. 1996; Tong et al.
1998), were also reported. The latter was pursued under a low temperature, 125°C, with
good crystallization effect. However, the process can lead to a decrease in adhesive strength.
In other words, further studies still need to be done on the transformation of amorphous
phase, which is undesirable for the clinical application when compared to crystallized HA.
After all, the reduction of ACP content in as-sprayed HA coatings is primarily necessary
for the solution mentioned above. New deposition methods are required for the HA coat-
ing deposition with a low amorphous phase content to avoid the postspray treatment.
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