Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 108

Increasing
voltage
Time
FIGURE 3.10
Effect of voltage on rate of deposition.
since the resistances are the same, then the current associated with the higher voltage must
be higher, and so the rate of deposition also must be higher.
Electrical Conductivity of Substrate
Most electrophoresis work is done using conducting substrates, including metals and
graphite. It is obvious that a highly conducting substrate is desirable in order to reduce
the resistance of the circuit. However, there have been a number of studies of deposi-
tion of ZrO 2 on porous insulating substrates backed by an electrically conducting surface.
These reports are summarized in Table 3.5 [90-95]. In these studies, it is considered that
the porosity is essential as it provides a pathway to complete the circuit to the conducting
back surface.
However, doubt has been cast on this requirement by recent work by the Australian
authors of the present work [35]. This work revealed that tin oxide (SnO 2 ) could be depos-
ited on single-crystal sapphire (Al 2 O 3 ) backed by graphite tape. The deposits were very
thin but they were coherent, flat, and of even thickness.
Surface Area of Substrate
When the current is reported, it usually is done in terms of the current density,
which is the current divided by the surface area of the substrate. It is clear that this will
have a significant impact on the rate of deposition because a higher current density depos-
its more quickly. That is, for the same voltage setting, a small substrate will be coated more
rapidly than a large one owing to the greater current.
Electrode Separation
The intensity of the electric field (V/cm) is a direct function of the electrode separation.
Since the voltage affects the rate of deposition, as shown in Figure 3.10, then a higher field
strength resulting from a smaller electrode separation increases the deposition rate.