Bioceramic Coating on Titanium by Physical and Chemical Vapor Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 310

HAp

Dynamic
mixing layer

Ca + ion

Ion implantation
layer

Ti

Ti substrateTi

FIGURE 7.15
Schematic illustration of modified surface layer prepared by IBD.

range of degrees of freedom with regard to gas composition. (48) PLD can be used to prepare
apatite coatings containing OH groups by introducing water vapor into the PLD chamber
at a substrate temperature of 873 K by using an HAp target. (18,19,48) The Ca/P ratio and OH
content in calcium phosphate coatings can be controlled in PLD by changing the water
vapor pressure in a gas atmosphere. (19) Without water vapor, on the other hand, single-
phase HAp cannot be obtained; instead α-TCP and/or TTCP are formed. Water vapor plays
a significant role in the crystal phase of calcium phosphate in PLD. (20)

BioceramicCoatingbyCVD
CVD is a versatile process to prepare a wide range of materials in various forms and with
various microstructures, mainly because CVD has many deposition parameters such as
gas concentration, deposition temperature, geometric configuration of CVD chamber, and
so forth. (49) The morphology of deposits can be controlled by these parameters, in particular
deposition temperature and supersaturation of precursor gases as depicted in Figure 7.16.
Supersaturation can be defined as the ratio of an input source gas pressure to an equi-
librium gas pressure. A powdery deposit may form at low temperature and under high
supersaturation conditions, while platelike or epitaxial films form at high temperature and
low supersaturation conditions. In bioceramic coating, the substrate is commonly Ti or its
alloys, and the coating may be Ca-containing ceramics, mostly Ca-P-O films. Obviously,
metals and ceramics are different in nature, in particular thermal expansion, and they
may often be incompatible. The thermal stress due to thermal expansion mismatch at the
metal substrate/ceramic coating interface can be affected by the microstructure of the
coating, and it can be relaxed by a columnar microstructure having small gaps between
each column. There are many industrial applications of ceramic coatings on metals such
as yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) on Ni-base superalloy gas
turbine blade (50) and antiabrasive hard α-Al 2 O 3 coating on WC-Co cutting tools, (51) where
the columnar microstructure is essential to ensure the strong adherence of ceramic coat-
ings. A typical cross-sectional microstructure of YSZ TBC coating by CVD is presented
in Figure 7.17. (52) The columnar grains are well grown, and small gaps can be seen at the
columnar grain boundary.