DETERMINATION OF THIN FILM'S MECHANICAL PROPERTIES

29
h
c
Pm
= hm
− ε
(3.8)
S
where Pm is the peak indentation load and ε is a constant which depends on the
geometry of the indenter. With these basic measurements, the projected contact area,
A, is derived by evaluating an indenter shape function at the contact depth, hc, that is
A=f(hc). Finally, substitute S in Equation (3.7) and the projected area A into
Equation (3.5) to obtain the Young’s modulus of the specimen. It is important to note
these equations were derived from pure elastic contact solution derived by Sneddon,
and how well they work for elastic/plastic indentation is not entirely clear. One
important way in which the elastic solution fails to properly describe elastic/plastic
behavior concerns pileup and sink-in of material around the indenter. In the pure
elastic contact solution, material always sinks in, while for elastic/plastic contact,
material may either sink in or pile up. Since this has important effects on the
indentation contact data, it is not surprising that the Oliver-Pharr method has been
found to work well for hard ceramics, in which sink-in predominates, but significant
errors can be encountered when the method is applied to soft metals that exhibit
extensive pileup. It is discussed the influences of pileup on the measurement of
Young’s modulus and pointed out that when pileup is large, the areas deduced from
analysis of the load displacement curves underestimates the true contact areas by as
much as 60% (A.Bolshakov and G.M. Pharr, 1998). This, in turn, leads to
overestimate the hardness and elastic modulus. The parameter,
h f
h max
which can be
measured experimentally and correlated with the material parameters E, ν, σ y and n
dσ
( ) which control indentation deformation, can be used as an indication of
dε
whether or not pileup is an important factor. Pileup is significant only when
h f
h max
>0.7 and the material does not appreciably work harden. When,
h f
h max