Biomedical Engineering Reference
In-Depth Information
where
λ
is the average steady-state crack spacing and
σ
f
is the tensile strength of the film
(i.e., the coating).
σ
f
can be determined experimentally by measuring the maximum elastic
strain,
ε
f
, of the coating (at which the initial formation of cracks is detected):
σ
f
=
ε
f
E
f
where
E
f
is the Young's modulus of the coating.
The interfacial shear strength evaluated with the shear lag strain method are reported
to be at least an order of magnitude greater than the pull-out shear strengths reported
for plasma-sprayed and high-velocity oxy-fuel sprayed HA (Li, Khor, and Cheang 2002;
Brossa et al. 1994; Gan, Wang, and Pilliar 2005). On the other hand, the reported interfacial
shear strength between the coating surface and surrounding tissues
in vivo
was about
16.65 MPa after a 24-implantation (Yang et al. 1997). Therefore, from the standpoint of
interfacial shear strength, sol-gel derived HA coatings appear quite promising for long-
term load-bearing implants. However, it should be noted that the determination of interfa-
cial shear strength using the shear lag analysis requires a limited film thickness, hence the
low transverse residual stress within the coating.
Scratch Test
The scratch test is generally accepted as one of the simple and popular methods in assess-
ing the adhesion properties of coating-substrate interface (Arias et al. 2003; Zhang et al.
2006). Basically, it is carried out by drawing a diamond tip over the coating surface to pro-
duce a scratch. The applied normal load is increased linearly until a critical load is reached
at which the adhesion failure is induced at the coating-substrate interface. Thus, the criti-
cal load can be taken as a semiquantitative measurement of the coating-substrate adhe-
sion strength, and the failure mode can provide further qualitative information upon the
coating-substrate interface (Zhang et al. 2006). Figure 1.8 shows a typical scanning scratch
100
Tip radius: 15 μm
Scratch speed: 2 μm/s
Scanning amplitude: 50 μm
a
b
80
2
2
60
Peeling point
1
1
40
20
Higher critical load
0
0
100
200
300
400
500
600
Load (mN)
FIGURE 1.8
Coefficient of friction in terms of relative output voltage as a function of normal load in the scratch test of sol-
gel derived coating: (a) HA coating, (b) fluoridated HA coating. At point 1, the indenter starts to plough into the
coating, resulting in a steeper increase in coefficient of friction; at point 2, the indenter completely peels off the
coating and scratches onto the substrate causing an abrupt increase in friction. (From Zhang et al.,
Surface and
Coatings Technology
, 200, 22-23 Spec. Iss., 6350-6354, 2006. With permission.)
