Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 64

where R i is the radius of curvature before coating, R f after coating, E s and v s the modulus
and Poisson's ratio of the substrate, t s the substrate thickness, and t f the film thickness.
Assuming that the scan length L of the substrate is much greater than the final bow B of
the substrate, the radius R can be calculated using Equation 2.6.

2

L
B

R = (2.6)

8

Compressive stress in the film leads to a convex deflection, whereas tensile stress results
in concave deflection. Measurements can be made in situ during deposition or thermal
treatment with optical means by interferometry or deflection method (cantilever beam)
using a low-power laser. Similarly, elastic strain in films can be measured by x-ray dif-
fraction from the change in crystal lattice d -spacing, from which the stress can be deter-
mined given knowledge of Young's modulus and strain-free lattice spacing (which may be
unknown in many cases) (Tsui et al. 1998).
The most commonly used methods for characterizing the performance of micro- and
nanocoatings on substrates can generally be divided into the measurement of coating prop-
erties and adhesion strength. There are a number of excellent methods that can be used in
thin film mechanical properties evaluation, and some of the commonly used methods are
nanoindentation, tensile testing, scratch testing, adhesion and wear testing, pin-on-disk
testing, pull-out test, and bending and bulge testing. Some of these are shown in Figure
2.12a, b, c, and d.

(a) (b)

P max

Acoustic sensor

Force
sensor

d P
d h

Holder

Diamond tip

Sample

Displacement, h

(c) (d)

P /2 P /2

substrate

crack

h

coating

FIGURE 2.12
Some of the thin film testing methods: (a) typical scratch testing device (based on the Quad Group Inc. Romulus
III universal test equipment), (b) schematic of a nanoindentation load vs. displacement load and unloading
curve from an indentation experiment, (c) schematic of the four-point bend delamination method, and (d)
Orthopod® pin on disk wear tester.