Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 175

sizes lead to low sintering rates, thus the development of pores can be controlled (Eckert et
al. 1995). However, commercial titania powder is very fine, around 1 μm. Therefore, certain
methods such as thermal and mechanical processing are required to produce the sintered
porous TiO 2 materials. The mechanical requirements of the prostheses, however, severely
restrict the use of low-strength porous ceramics to nonload-bearing applications. The envi-
ronmental sensitivity of the ceramics and the loss of strength of porous ceramics with
aging are negative aspects (Hench et al. 1982). In vitro investigation of nanoalumina and
nanotitania powders has shown the presence of a critical grain size for osteoblast adhesion
for the powder (Webster et al. 1999). The study provides evidence of the ability of nano-
phase alumina and titania to simulate material characteristics of physiological bone that
enhance protein interactions and subsequent osteoblast adhesion. Therefore, the compos-
ite coating made from titania and HA has attracted attention owing to its ability of com-
bining advantages of the both materials (Weng et al. 1994; Vu et al. 1997). It was believed
that titania could work as a skeleton in the composite system. Additionally, according to
the formula utilized for coating fracture toughness determination (Beshish et al. 1993), the
high Young's modulus of titania material could be useful for the improvement of the frac-
ture toughness of HA, which is very critical for the reliability of the implant for long-term
use. Influence of an addition of titanium dioxide on thermal properties of sintered HA has
attracted significant attention (Weng et al. 1994; Vu et al. 1997) because titania ceramics are
potentially useful as porous cell carrier material whose properties, such as good perme-
ability and high biocompatability, serve to enhance cell vitality. The efficacy of different
titanium dioxide materials on cell growth and distribution has been studied (Blum et al.
1996). Study showed that the as-sprayed pure HA coating is mainly composed of crystal-
line HA and α-TCP (Li et al. 2002b). The crystalline HA, anatase TiO 2 , α-TCP, amorphous
calcium phosphate, as well as some rutile TiO 2 , CaTiO 3 , and CaO are detected simultane-
ously in both the composite coatings (Figure 4.28) (Li et al. 2003). The chemical reaction
between HA and titania during the thermal spraying was revealed by a high-temperature
DSC analysis to be ~1400°C (Li et al. 2002b). And indeed a reaction layer between the HA
and titania was distinguished by TEM (Figure 4.29) (Li et al. 2003).
Despite the advantages introduced by TiO 2 addition, a certain negative effect was
revealed in the HA/titania composites (Weng et al. 1994; Vu et al. 1997). The main prob-
lem is that the existence of TiO 2 can result in the decomposition of HA at a relatively low
temperature, such as the considerable decrease in the decomposition temperature from
1300°C to 1400°C for pure HA to 750°C to 1150°C for HA containing oxide additives (Vu et
al. 1997). Figure 4.30 shows the influence of the addition of titania powder on phase com-
position of sintered HA under high sintering temperatures. It shows that the higher the
temperature, the more easily the decomposition of HA and chemical reaction between the
two components can occur. Although the negative effect of the ceramic additives must be
considered, the influence can be neglected if the processing temperature is far lower than
the decomposition temperature. HVOF could achieve this due to its relatively low flame
temperature. Fortunately, the addition of titania did not trigger obvious deterioration of
biocompatibility of the coatings (Li 2002), while the mechanical properties were signifi-
cantly enhanced (Li et al. 2002b).
Based on the observations, an impact formation model has been proposed (biomat-
impact), which correlates the parameters of the in-flight titania particles with the impact for-
mation on preflattened HA splat by elasmic/plasmic deformation (Li et al. 2003). According
to the research on HA/titania composite coating (Li et al. 2002b; Li et al. 2003), the addition
of TiO 2 is found to improve Young's modulus, fracture toughness, and shear strength of
HVOF-sprayed HA-based coatings. This consequence is attributed to the weak chemical