Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 51

large volume of material that is lost, and therefore the corresponding shrinkage that takes
place (Hench and West 1990). This can be avoided through the use of drying control chemi-
cal additives (DCCA), careful control of drying conditions, and schedules and increasing
the degree of cross-linking. However, the drying of sol-gel derived nanofilms is not such
a problem compared with that of monoliths, as they are constrained in the plane of the
substrate and all shrinkage takes place normal to the substrate (Pettit et al. 1988). Coatings
less than 400 to 500 nm thick can be dried at controlled rates, to minimize the potential for
cracking. However, when thickness of the films exceed approximately 500 nm, cracking
becomes difficult to avoid (Sakka et al. 1984; Strawbridge and James 1986; Dislich 1988; Gan
and Ben-Nissan 1997; Aksakal and Hanyaoglu 2008). The critical thickness for cracking or
debonding to occur in coatings is defined (Hu and Evans 1989) as:

= 2

E

γ

h

(2.1)

2

σ

where h is the film thickness, E is the plain strain (Young's modulus) of the film, γ is the
fracture energy to create new surface, and σ is the stress in the film.
Firing or sintering is the final stage in the production of ceramic materials via the sol-
gel process. By heating dried gels to elevated temperatures, usually approximately 600°C,
any remaining organic materials are combusted (Figure 2.7). Due to the small grain sizes
obtained in this process, sintering temperatures are between 400°C and 800°C. Some spe-
cific chemistry allows lower temperatures such as 90°C. By controlling the atmosphere
during the firing process, gel can be converted to oxides (Yoldas 1984; Anast 1996), nitrides
(Jimenez and Langlet 1994; Kraus et al. 1996; Gao et al. 1997; Cassidy et al. 1997), carbides
(Raman et al. 1995; Hasegawa 1997), and mixtures thereof (Gabriel and Riedel 1997).

The Sol-Gel Process: Coating Techniques

There are two major coating techniques: dip and spin coating.

Dip Coating

Dip coating is primarily used for the fabrication of coatings on items such as flat glass
substrates. However, dip coating can also be used for coating complex shapes such as
rods, pipes, tubes, and fibers. The dip coating process has the advantage of being capable
of producing multilayered coatings with a high degree of thickness uniformity up to 1000
nm thick. In addition multilayered coatings can be manufactured with specific optical
characteristics and are used commercially (Dislich 1988).
The film thickness of a single layer is determined by the withdrawal speed for a given
solution (Figure 2.8). On the other hand, the thickness can also be modified by altering the
physical properties of the solution, for example, viscosity, as well as the number of layers
deposited (Turner 1991; Lee et al. 1993; Paterson et al. 1998). The film thickness can be esti-
mated from the following (Scriven 1988):

1 2 /

U
g

η
ρ

h = c (2.2)