Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 111

ElectrophoreticDepositionofHydroxyapatite
Electrophoretic Deposition
Electrophoretic deposition for the fabrication of hydroxyapatite (HAp) coatings onto
metallic substrates was reported first in 1986 [96]. A survey of more recent studies (since
1997) is given in Table 3.6 [86,88,96-150]. Under an applied electric field, the HAp particles
exhibit a net positive charge and so they deposit on the cathode. Organic liquids have been
used consistently as the media when suspending and depositing HAp. Homogenous HAp
coatings can be formed easily on different metallic substrates, such as Ti, Ti6Al4V, and 316L
stainless steel.
The effects of coating and associated parameters, including medium, applied voltage,
current density, time, deposit weight and thickness, homogeneity, and crack formation
have been studied extensively [101,107]. These studies revealed that, as expected, the coat-
ing thickness increased with applied voltage and deposition time. However, an important
observation was that increasing the applied voltage also led to enhanced coating rough-
ness. Consequently, it was necessary to adjust the applied voltage and current so that the
coating morphology could be controlled appropriately [101]. At relatively low deposition
voltages, fine and densely packed HAp particles were obtained. In contrast, higher volt-
ages yielded porous coatings containing large HAp particles. Further, it has been reported
that, at high applied current densities, porous HAp scaffolds with interconnected pores
could be fabricated using electrophoretic deposition [107].
Many studies have indicated that the suspension stability plays an important role in the
electrophoretic deposition process [106,123,151]. Using fine, unagglommerated HAp par-
ticles favors the formation of a stable suspension and the fabrication of high-quality HAp
coatings. When submicron, well-dispersed, HAp particles were deposited under optimal
coating conditions, a high-quality, dense, HAp coating was achieved [106]. Figure 3.11
shows cross sections of the HAp coating before and after sintering. The as-deposited coat-
ing was homogeneous, with a thickness of ~30 µm; after sintering, the HAp layer became
translucent owing to significant densification and the coating thickness reduced to ~20 µm.
It also is known that the HAp synthesis method, particle size, and particle shape also play
important roles in determining the quality of green deposits [108,117]. Consequently, opti-
mization of the powder fabrication, suspension, and deposition parameters are standard
procedures for the fabrication of high-quality electrophoretically deposited coatings.
Ceramic-Metal Interface
One of the main shortcomings of electrophoretic deposition is the low adhesive bond-
ing strength between the deposit and the substrate. In general, a post-heating process is
required to densify the coating and improve the bonding but this process imposes con-
flicting requirements [96]. On the one hand, low densification temperatures result in poorly
bonded low-density coatings. On the other hand, high densification temperatures may
improve the bulk density and interfacial bonding but these come at the cost of potential
degradation of the HAp coating and deterioration of the metal substrate. In general, pure
bulk HAp decomposes in the temperature range 1250°C to 1450°C [152]. However, when
in contact with an underlying metal substrate, such as Ti, Ti6Al4V, or stainless steel, HAp
coatings may decompose at temperatures as low as 800°C to 900°C [101,153,154]. This results