Biomedical Engineering Reference
In-Depth Information
2
2
(a)
(b)
1
1
0
0
-1
-1
-2
-2
-3
-3
0.0
0.4
0.8
1.2
1.6
2.0
2.4
0.0
0.4
0.8
1.2
1.6
2.0
2.4
X
distance (mm)
X
distance (mm)
FIGURE 1.10
Typical profilometry results of measured curvatures for sol-gel derived fluoridated HA coating: (a) before
removing the coating and (b) after removing the coating. The reduction of curvature caused by the removal
of fluoridated HA coatings indicates a tensile residual stress existing in the coatings. (From Zhang et al.,
Engineering Fracture Mechanics
, 74, 12, 1884-1893, 2007. With permission.)
curvature radius after removing the coating indicates a tensile residual stress existed in
the coating.
A great number of possible parameters can cause residual stresses, depending on the
deposition technique. Structure-related residual stress results from the following three
aspects: thermal stress, growth-induced stress, and structural mismatch-induced (includ-
ing lattice distortion) stress (Zhang et al. 2004). In a sol-gel deposition process, the thermal
stress comes from the drying and firing process coupled with the difference in the coef-
ficient of thermal expansion (CTE) between the coating and the substrate, the growth-
induced stress comes from the coating shrinkage driven by capillary stresses during the
drying and firing process, and the structural mismatch-induced stress comes from the
difference in crystal structure between the coating and the substrate as well as the lattice
distortion within the coating as a result of defects or incorporation of foreign molecules or
molecule groups. Mainly, thermal stress,
σ
Rt
, always contributes more to the total residual
stress in the coating, and it can be calculated by (Watanabe et al. 2002):
∆ ∆
TE
1
α
f
=
σ
Rt
−
ν
S
where
Δα
is the difference of CTE between the interested coating and the metallic sub-
strate and
Δ
T
is the temperature variation during drying or firing. For example, CTE of
HA is 15 × 10
−6
/K, whereas that of Ti6Al4V substrate is about 8.9 × 10
−6
/K (Zhang et al.
2006). Thereafter, the residual stress caused by thermal mismatch between HA coating
and Ti6Al4V substrate should be tensile.
The presence of residual stress in the coating serves as an important influencing factor
in determining the durability of the coated implants. Previous work has demonstrated
that the existence of residual stress in HA coatings can alter the concentration of superna-
tant species in solution: tensile residual stresses enhancing dissolution and compressive
