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

Biomedical Engineering Reference

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º —HA

* —α-TCP

+—β-TCP

(c)

º º

* *

º º º º

(b)

(a)

º º º º

20 30

50 [ º20 ] 60

2 θ (deg)

FIGURE 4.34

XRD patterns of heat-treated HA coatings under (b) 500°C and (c) 750°C annealing with the comparison to (a)

the as-sprayed coating. (From Li et al., Biomaterials , 23, 2015-2112, 2002a. With permission.)

at more than 600°C for 3 h in the presence of water vapor. It was already detected by DSC

test that the crystallization temperature of the ACP is about 720°C (Li et al. 2002a).

Postspray heat treatment would further promote the interfaces bonding between the

coating and substrates, and is deemed to be a necessary step to transform the amorphous

calcium phosphate into HA structure (Yang et al. 1997). As well, the cracks generated by

heat treatment should be considered (Cheang et al. 1996a). As discussed earlier, besides the

improved phases in the HA coatings, enhanced coating mechanical properties were also

reported (Li et al. 2002a; Yang 2007). For example, for HVOF-sprayed HA coatings (Li et

al. 2002a), the adhesive strength is significantly improved by the heat treatment at 750°C,

increasing from 26 ± 2MPa for the as-sprayed coating to 34 ± 3 MPa. The shear strength is

also increased from 11.2 ± 0.9 to 14.1 ± 0.8 MPa.

Generally, according to the mechanism of HA decomposition (Aoki 1994), loss of water

is the key reason accounting for the undesirable phase changes of HA during the plasma

spraying. It was found that HA gradually releases its OH - ions and transforms into oxy-

apatite (OHAP) in the temperature range of 1000°C to 1360°C. Above 1360°C, OHAP would

decompose into TTCP and α-TCP (Liao et al. 1999). It was proposed that the dehydration of

HA gives a solid HA-OHAP solution in which chains of OH − might be replaced by chains

of OH − , O 2− , and vacancies (MacPherson et al. 1995). The leave of OH − groups in terms of

water at high temperatures attained by the HA particles leads to lattice changes associated

with structure changes (Aoki 1994). Consequently, the plasma-sprayed HA coatings are

composed of several CPs with different amounts depending on the extent of the dehydra-

tion of HA. Therefore, the possible approaches used for reversing CPs to crystalline HA

must involve providing extra water, and hence OH − . Some researchers have reported their

findings that postspray steam treatment on the plasma-sprayed HA coatings from their

surface showed evidence of significant reversing effect (Shirkhanzadeh 1994; Yamashita

et al. 1994; Cao et al. 1996; Tong et al. 1998; Li et al. 2006). It was believed that oxyapatite

remains stable under dry conditions and would readily transform to HA in the presence of

moisture (Gross et al. 1998c; Yamashita et al. 1994). It was reported that water vapor treat-

ment pursued under a low temperature, 125°C, was effective in achieving good crystalliza-

tion (Shirkhanzadeh et al. 1994; Tong et al. 1998).

Biological and Biomedical Coatings: Processing and Characterization

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