Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 142

10. Pulsed laser deposition (Cottel et al. 1992). This approach is potentially very interest-
ing because of conservation of the stoichiometry of HA.
11. Others. Preparation of HA films by surface-induced mineralization (Campbell et
al. 1996), sol-gel technique (Piveteau et al. 1999; Hwang et al. 1999; Weng et al.
1998), and others, has also been reported.
Generally, the mechanical properties and phase composition, as well as operation feasi-
bility, are the main factors used to evaluate processing techniques. HA coatings that are
deposited by these techniques possess differences in chemistry and crystallinity, which
can affect the biological response to HA and ultimately the coating performance. Among
all the surface coating techniques, thermal spraying has shown wide application in depos-
iting HA coatings due to its identified characteristics: low substrate temperature, large
coating thickness that can be up to several centimeters, good coating mechanical perfor-
mance, low cost, and so forth.
PrincipleofThermalSpray
Thermal spray is a generic term for a group of coating processes used to apply metallic or
nonmetallic coatings (Davis 2004). These processes are grouped into four major categories:
flame spray, electric arc spray, cold gas spray, and plasma arc spray. The coating material
(in powder, wire, or rod form) is heated by the energy sources and then projected at a
high velocity onto a component surface as molten or semimolten or even slightly heated
(cold spray) particle that flattens, undergo rapid solidification or solid deformation, and
form a deposit through successive impingement. Figure 4.2 shows a schematic diagram
of particles impinging onto a substrate in the process. The coatings are generated through
continuous overlapping of splats. Intensive cooling at spraying is of primary importance
because the temperature gradients within the coating and the substrate, together with the
quenching stresses within the individual lamellae (group of splats sprayed per layer), will
control the residual stresses of the coatings as well as the formation amorphous phases
(Sampath et al. 1996). Thus, a thermal sprayed coating consists of layers of splats (solidified
droplets; Pawlowski 2008, Figure 4.2b), which have a lamellar cross section. In fact, virtu-
ally all materials can be sprayed. However, special care has to be made when handling the
materials that decompose before melting as hydroxyapatite or sublimate as graphite. The
effort concerns the powder preparation and choice of spray parameters.
The strength of as-sprayed coatings depends substantially on the features such as
porosity and interlamellar strength, originated from the building-up process of solidified
droplets, or splats (Pawlowski 1995). These features result from the splat process on the
substrate, and thus the properties of coatings are correlated closely to the temperature and
velocity of the particles upon impact.
Thermal spraying covers a range of spraying processes that can be employed depending
on the materials and desired coating performance. Table 4.1 compares the main thermal
spraying techniques being used in the biomedical field for the production of bioactive
coatings (Espanol Pons 2003). For comparison purposes, the values on the particles veloc-
ity have been given for an alumina powder with an average medium size of 30 μm. It
must be noted that thermal spray is an active research area and there has always been