Biomedical Engineering Reference
In-Depth Information
et al. 2004). It is anticipated that this new patented process and material can be applied to
bone graft applications where high strength requirements and longevity are pertinent.
MechanicalPropertiesofThinFilms
Understanding of the long-term mechanical reliability of biomedical thin films is pertinent
in clinical applications. Hence, a number of methods capable of quantitatively measuring
the mechanical properties are required. Since the 1980s there have been constant improve-
ments and developments in equipment capable of extracting the structure and properties
of thin films and also the adhesion of the coating to an underlying substrate (Ben-Nissan
and Pezzotti 2002).
A general review of thin film mechanical properties of coatings is given by Nix (2006).
According to his work on both thin and thick film coatings, the main factors that can influ-
ence mechanical reliability are many; however, the most important ones are the interfacial
properties, residual stresses for thick coatings, substrate, and thin film thickness and its
geometry.
The evaluation of stress in a nanocoating due to deposition technique and heat treat-
ments applied plays an important role in regards to mechanical stability. The potential
for cracking and spalling of the coating due to inbuilt stresses (whether they are tensile or
compressive) or from external mechanical loading plays a key part in the successful use
of biomaterial implants. The stress in a film is a combination of the following components
and is described in Equation 2.4: intrinsic (deposition, mode of growth, structure), ther-
mal expansion mismatch between film and substrate (specifically in thick films) due to
changes in temperature and externally applied stresses (Lepienski et al. 2004):
σ f = σ (intrinsic) + σ (thermal) + σ (external)
(2.4)
Stress measurement by substrate curvature is a simple method requiring either (1) a
stylus profilometer capable of scans 10 mm in length or more, or (2) using a phase shifting
interferometer. The advantages of using an optical method such as interferometry are high
accuracy of about 6 nm, substrates with curvature of up to 3 µm are acceptable, and it is a
rapid and straightforward technique to use.
In stylus profiometry, the substrate needs to be sufficiently thin for adequate measure-
ment of the curvature. The method requires careful and precise placement of the sample,
so that a cross scan in the x and y directions can be made on the surface (preferably both
sides) before and after coating.
The stress in the coating (intrinsic + thermal) can be evaluated using the Stoney formula
without prior knowledge of the coating properties. The information that is required is
coating thickness, substrate thickness, and its Young's modulus. Stoney's equation (Stoney
1909) shows the relationship between the change in the radius of a coated and uncoated
substrate and the corresponding stress in the layer (Equation 2.5):
2
1
6
1
1
E
t
t
=
s
s
f
(2.5)
σ
R
R
(
1
)
ν
f
i
s
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