Synthesis and Characterization of Hydroxyapatite Nanocoatings by Sol–Gel Method for Clinical Applications - Biological and Biomedical Coatings: Processing and Characterization - page 68

extract intrinsic film properties (strength and fracture toughness) and interfacial adhesion
characteristics (Ignat 1996; Harry et al. 1998; Scafidi and Ignat 1998).
On tensile straining, the coating-substrate sample is examined to determine the instant
of first cracking in the coating, which corresponds to a strain ε c . Using Young's modulus of
the film ( E f ) the critical stress, σ c , for cracking may be calculated as follows:

σ c = ε c E f + σ r (2.9)

where σ r is the residual stress of the coating.
The fracture energy of the coating is obtained from the following equation (Hu and
Evans 1989):

2

h

σ σ

τ

c c

= g ( ) + (2.10) γ π α

f

E 3

f

where γ f is the fracture energy (J m −2 ), h is the thickness of the coating, and α is Dundar's
parameter α = ( E f − E s )/( E f + E s ), where E s is Young's modulus of the substrate. The g ( α ) is
obtained from Beuth and Klingbeil (1996) and τ = y / 3 , where σ y is the yield stress of the
substrate.
The toughness of the film is then given by

K = γ E

(2.11)

IC f f

Adhesion of the film to the substrate is determined by the measurement of the interfacial
fracture energy. During tensile straining, the instant of first debonding of the film corre-
sponds to a strain ε d . The interfacial fracture energy is calculated as follows:

= 1
2

2

E h (2.12)

γ ε

i f d

Nanoindentation

The methodology and instrumentation have been developed markedly since the early
1990s and it is now considered a routine and relatively effective means of obtaining accu-
rate measures of Young's modulus and hardness of coatings. It is the method of choice by
many practitioners in the biomaterials field for measuring the mechanical properties of
nanocoatings and implants due to the simplicity of operation. The main requirements for
obtaining the best possible results with such testing rely on adequate sample preparation,
calibration of equipment, and corrections for thermal drift, initial penetration, frame com-
pliance, and indenter tip shape (Oliver and Pharr 1992; Field and Swain 1993; Swain and
Mencik 1994; Field and Swain 1995; Mencik and Swain 1995; Gan et al. 1996; Mencik et al.
1997; Fischer-Cripps 2002).
In the well-known conventional microindentation testing, a load is applied through a
diamond tip of known geometry (typically Rockwell, Vickers, or Knoop) into the material
surface and then removed and the area of the residual impression is measured by optical
means to give the material hardness, an example of which is shown by Kealley et al. (2008)