Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 52

Immersion Startup Deposition

and drainage

Evaporation Drainage

Hydrolysis and structural changes

FIGURE 2.8
Schematic diagram of the five stages of dip coating process.

where c is a constant and is about 0.8 for Newtonian liquids, η is the sol viscosity, U is the
withdrawal speed, ρ is the density, and g is the acceleration due to gravity.
The dip coating process is normally a batch process but can be adapted to coat long flex-
ible sheets and wires (Scriven 1988). The overall process can be broken up into five stages:

1. Immersion

2. Startup

3. Deposition and drainage

4. Evaporation

5. Drainage

The first two stages are always sequential; the third and fourth may take place concomi-
tantly throughout the entire process, unless the necessary precautions are taken (Figure
2.8).
When substrates are dip-coated, the thickness of the film will be different at the edges.
This might generate cracks during firing. Faster pullout rates generate thicker coatings due
to less drainage and evaporation during pullout. By proper control of pullout rate we can
produce about 70- to 100-nm thick crack-free coatings if all other factors are adequate.

Spin Coating

Spin coating is a process suited to flat shapes such as disks and plates. Similar to dip coat-
ing, spin coating can be separated into four stages and evaporation can take place through-
out the entire coating process.
The first stage of spin coating involves delivering excess liquid to the substrate while
it is stationary or spinning. During the second stage, rotation causes the liquid to move
radially outward due to centrifugal forces. This may occur while the substrate is being
accelerated to maximum speed. In the third stage, excess liquid flows to the perimeter and
is detached in the form of droplets. If the substrate has a central hole, drainage takes place
in the same manner as in the dip coating process. The film thins down to a fairly uniform
thickness, apart for the edges, which is effected due to the surface tension changes. As
the film thins, the flow decreases as drag forces increase. The thinner films are affected
more by evaporation, which raises the solution viscosity by concentrating nonvolatiles.
Evaporation becomes the dominant process once spinoff has ceased. Film thickness
will reach uniformity provided that its viscosity is insensitive to shear rate (Newtonian