Hydroxyapatite and Other Biomedical Coatings by Electrophoretic Deposition - Biological and Biomedical Coatings: Processing and Characterization - page 118

0Ka, 71 PKa, 226

CaKa, 309 TiKa, 741

HAp Ti

FIGURE 3.12
Energy dispersive spectroscopy line scans across a HAp-Ti (left to right) interface (12 μm length; 1 μm divisions),
heat-treated at 950°C for 1 h. (Reproduced by courtesy of Journal of Materials Science Materials in Medicine .)

Previous studies revealed that many other oxides in contact with HAp also cause decom-
position [158,159]. These phases included Al 2 O 3 , SiO 2 , ZrO 2 , SiC, graphite, and 316L stain-
less steel, all of which reacted with HAp and reduced the decomposition temperature.

Metallic Substrates

Concerning titanium substrates, there are several problems that can be expected from heat
treatment at high temperatures, which are:

• Phase transformation. The α → β phase transformation in pure titanium occurs at
882°C [160]. Although this normally does not affect the mechanical properties, it
could disrupt the HAp microstructure. The use of alloying elements to stabilize
the α or β phase is well known [161]. Aluminum additions stabilize the α phase
by increasing the transformation temperature while vanadium additions stabi-
lize the β phase by decreasing it. As a result, the phase transition temperature of
Ti6Al4V is 1000°C.
• Grain growth. In general, grain coarsening in metals degrades the mechanical
properties. This has been demonstrated in titanium alloys [162].
• Oxidation. Both Ti and Ti6Al4V are very reactive and so they are susceptible to
oxygen dissolution and rapid oxidation reaction at high temperatures, which are
well known to cause embrittlement [154].

In light of these risks, it is desirable to minimize the heat treatment temperatures of HAp-
coated Ti and Ti6Al4V, which typically are in the range 600°C to 1050°C, depending on the
HAp powder, particle size, and substrate [154]. In general, for uncalcined and other fine
particles, densification can be done at relatively low temperatures; coarser calcined pow-
ders require higher temperatures.
In comparison, 316L stainless steel (16-18 wt.% Cr, 10-14 wt.% Ni, 2-3 wt.% Mo, 0.03
wt.% C, and 0.1 wt.% N) and Fecralloy (22 wt.% Cr and 4.8 wt.% Al) are less susceptible to
high-temperature oxidation and elevated temperature strength degradation than are Ti