Bioceramic Coating on Titanium by Physical and Chemical Vapor Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 314

of Ca-P-O coating is greater than that of CaTiO 3 coating. A functionally graded coating
changing from a CaTiO 3 to a Ca-P-O layer prepared by the present authors is a good can-
didate for biomedical coating as depicted in Figure 7.21. (57) The columnar CaTiO 3 layer was
prepared first and then the Ca-P-O layer was stacked, forming bilayers.
A few reports on the preparation of Ca-P-O coatings by CVD have been published.
Ca-P-O coating was prepared on an Al 2 O 3 substrate at 1123 K by using Ca(dpm) 2 and P 2 O 5 .
(58) As-deposited coatings were β-Ca 2 P 2 O 7 containing AlPO 4 , which was transformed to
β-Ca 3 (PO 4 ) 2 by heat treatment at 1273 to 1623 K. The Ca/P molar ratio increased from 1.33
to 1.66 by the heat treatment.
A fluorine-containing carbonated HAp coating was conducted on Ti-6Al-4V substrate
by using Ca(hfpd) 2 (hfpd: hexafluro-pentadione) and tributylphosphate at 873 K. (59) The
as-deposited coating had a cauliflower-like nodule microstructure with a Ca/P molar ratio
of 1.3 close to OCP containing impurity C-H. This coating was heat-treated at 1073 K, and
then the Ca/P molar ratio changed from 1.3 to 1.71 close to that of HAp, whereas the crystal
structure was not investigated. No biomedical properties of this coating were reported.
The present authors have prepared Ca-P-O coatings on Ti substrate by using Ca(dpm) 2
and (C 6 H 5 O) 3 PO precursors by changing the deposition temperature from 873 to 1073 K
and the Ca/P molar ratio from 0.1 to 2.5. (22,26,27) Single phase HAp and α-TCP were first
prepared in as-deposited forms at 950 to 1000 K. The crystal phase of Ca-P-O coatings
changed widely with deposition temperature and Ca/P molar ratio in source gases as
illustrated in Figure 7.22. Mixed phases of CaO and CaCO 3 formed below 900 K, whereas
HAp and α-TCP formed over 900 K. Single-phase HAp and α-TCP coatings were prepared
at 973 to 1073 K and at a Ca/P molar ratio below 1.0.
CVD is in general advantageous for controlling the preferred orientations by controlling
deposition conditions, and (002) preferred orientation, which is often observed in bone,
can be achieved in HAp coating on Ti substrate. The typical surface morphology of (002)-
oriented HAp along with that of (510)-oriented α-TCP coating by thermal CVD is presented
in Figure 7.23. Both have a grainlike surface microstructure and columnar cross section.
The deposition rates of these Ca-P-O coatings are 10 to 20 µm/h, which are 10 to 100 times
higher than that of common sputtering.

(a)

(b)

5 µm 5 µm

FIGURE 7.21
Microstructure of functionally graded CaTiO 3 /Ca-P-O coating. (a) Surface, (b) cross section.