INTEMA   05428
INSTITUTO DE INVESTIGACIONES EN CIENCIA Y TECNOLOGIA DE MATERIALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Hard problems with Soft Materials: Nanoindentation of Polymers
Autor/es:
L. SANCHEZ FELLAY; L. A. FASCE; P. M. FRONTINI
Reunión:
Encuentro; III Encuentro Nacional de Materia Blanda - III MAB; 2010
Resumen:
Depth-sensing
indentation (DSI, also called nanoindentation), in which the load and
displacement are continuously monitored during a contact test, has developed
into a standard technique for measurement of local mechanical responses of engineering
materials such as metals, glasses and ceramics. The two mechanical properties
measured most frequently are the elastic modulus, E, and hardness, H. In a
commonly used method, data obtained from a complete cycle of loading and
unloading are analyzed for properties deconvolution. The Oliver-Pharr approach
is the most accepted analysis technique [W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992)]
and it has been incorporated into a number of commercial systems largely due to
its robustness and simplicity. This technique relies on the isotropic time-independence
of indentation response in the experimental timeframe.
DSI
techniques are increasingly being used to probe the mechanical response of
polymers. In contrast to engineering materials, polymers behave in a
visco-elastic fashion, exhibit non-linear behavior at relatively small strains
and their responses to tension, compression and shear can be quite different.
Thus, a number of challenges exists when applying DSI methods to characterize
polymers.
This essay
summarizes the state of the art of nanoindentation of polymers based on the knowledge
available in literature, and our own experimental and finite element simulation
results.
Polymers exhibit
substantial time-dependent nanoindentation responses in an experimentally-relevant
timeframe: creep at fixed load, stress relaxation at fixed displacement, and
rate-dependent behavior. If the load history has a holding period, creep is
shown as a displacement of the tip even tough the load remains unchanged or
depending on the degree of creeping, the unloading curve may present a nose. Creep
effects lead to the overestimation of elastic modulus values due to high
contact stiffness and to the underestimation of hardness values due to large
viscous displacements. Popular experimental approaches use a long hold time at
peak load, aspiring to unload elastically after saturating the creep. Another
post-experiment approach and consists in the correction of the unloading
contact stiffness by accounting for the creep rate at maximum load [A.H.W. Ngan, H.T. Wang, B. Tang,
K.Y. Sze, Int. J. Solids Struct. 42 (2005)]. The disadvantage
of such approaches is that the viscoelastic response is discarded and not
measured, although time dependence is clearly a feature of the polymers mechanical
behavior. Directly accounting for the time dependence with phenomenological
modeling allows isolation of the elastic response while also directly measuring
an experimental time-constant. Soft polymers can also develop substantial
adhesive forces at the tip-sample interface, which largely affect elastic
modulus determination [S.
Grupta, F. Carrillo, C. Li, L. Pruitt, C. Puttlitz, Materials Letters 61 (2007)].
A false decreasing trend in E values is observed with increasing penetration
depth in the presence of adhesive forces. Adhesive forces can be measured and
included in E determination analysis by taking into account the JFK model.
When a
polymer deforms elasto-plastically beneath the indenter tip, some material is
displaced and push out to the side of the indenter forming a pile-up. The
actual contact area is higher than the one determined by the Oliver-Pharr
approach, and hence the elastic modulus and hardness data results to be
overestimated. Finite element simulations can be used to account for the
estimation of the actual contact area.
Several
examples of indentation tests on different polymers such as UHMWPE, PVC and
PMMA are shown and analyzed for illustrative purposes.
Prospective
works will be directed toward the understanding of the intrinsic deformation
behavior and the development of ad-hoc constitutive equations of polymeric
systems.