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Project C5: Mechanics of surface recovery and reconstruction after deformation

Principal Investigators: R. Mülhaupt (Freiburg) / H. Pelletier (Strasbourg)
Collaborators: C. Gauthier (Strasbourg)

PhD Student: Joseph Lejeune


Current state of the research

Materials able to retain their function and/or structural integrity after being damaged are of great scientific interest. In the past decade, there have been several attempts to prepare self-repairing materials or concepts that would lead to them [1-3]. Reported self-healing approaches mainly deal with retaining the integrity of the material by either filling the cracks of the damaged materials by an encapsulated liquid reactive agent or by regenerating surface functional groups. In this context, hybrid organic-inorganic materials, polyhedral oligomeric silsesquioxanes or graphene-filled elastomers, have attracted considerable interest over the past decade as “self-healing” high-temperature nanocomposites and coatings. On the other hand, the recovery of materials can already result from careful optimization of their viscoelastic properties. Thus, linking viscoelastic materials properties with investigations of surface recovery is expected to provide relevant information for the design of intelligent materials for self-lubricating surfaces and self-healing materials. The proposed project here wants to establish such a link.


Contributions of the proposers

The Strasbourg group has significant experience on creep and recovering phenomena that govern the relaxation of an indentation or scratch experiments on a polymer [4]. At a micrometer scale (for contact sizes more than few tenth of microns of radius), the viscoelastic recovery of residual imprints as a function of the contact conditions (loading rate, holding time, tip radius, temperature) can be followed in-situ through transparent samples by observing the contact area and the groove left on the surface. Finite-element numerical simulations allow to evaluate the influence of plasticity and in a more general way the rheological parameters of the tested material. This opens up possibilities to study relaxation processes leading to recovery and/or self-healing of surfaces. The Freiburg group has a long-standing and profound experience in synthesis [5] of mixed macromolecular systems. In particular, the group investigated systems with surface active components which can (re-)organize at surfaces in an original way to increase the self-healing rate. This opens up possibilities to study relaxation processes leading to recovery and/or reconstruction of surfaces (containing hard and soft polymers, polymers filled with nanoparticles, semi-crystalline polymers, …), which are, for instance, applicable for dry self-lubrication.


Research project

The lifetime of polymeric surface is often reduced because of poor surface resistance, such as wear and mar resistances. Thus, we will develop and test polymer systems which have the intrinsic capability to regenerate or rejuvenate their surface properties. The contribution of the Freiburg group will be to functionalize (e.g. via appropriate coatings) the surface in order to decrease the friction coefficient, to increase the molecular mobility and to increase the recovery of imprints. In addition, systems filled with nanoparticles or nano-reservoirs will simultaneously be able to increase strain hardening and to provide self-healing features.

As a simplification of the complex process, indentation and scratching tests will be performed by the Strasbourg group using diamond indenters which are commonly employed as tribological testings. Classical methods used to determine the mechanical properties of the surfaces of materials from load-displacement curves obtained in micro- or nano-indentation tests assume that the materials behaves in an elastic-plastic manner during the loading phase and in an elastic manner during the unloading phase. At room temperature, amorphous polymers exhibit time and temperature dependences [6,7]. When a constant normal load is applied to the surface, the creep, which occurs during the loading phase, ncreases the indentation depth and consequently the true contact area. After withdrawal of the indenter, we can follow the recovery and  self healing of the material. The figure below describes the experimental devices used for indentation and scratching. Micro-indentation tests may be conducted with spherical tips under different conditions of temperature (-20 °C  to 100 °C) and holding time (10 s to 105 s). The applied normal load will be chosen so as to perform tests at different ratios a/R (a is the contact radius, and R the tip radius), typically ranging from 0.1 to 0.2. These limits are chosen to prevent full yielding of the polymer around and under the contact area.


Recently, the Strasbourg group has developed a scratch test allowing to record the residual groove profile for rather short periods of lifetime (ca. 0.4 s) and to observe the evolution of the contact geometry and the morphology of the groove, as a function of variable scratch conditions, for example as a function of the ratio a/R, the sliding velocity Vtip or the true friction coefficient µ.  

Such time resolved experiments will thus allow us to establish clear relations between the degree of surface deformation (or damage) and the rate of recovery, both in topography but also in surface chemistry and thus frictional properties. The behavior of various systems (filled elastomers, systems with surface active and structural groups) prepared by the Freiburg group will be tested and compared. Processes of intrinsic self-repair of polymer surfaces after damage will be investigated.


[1]S. R. White, et al., Nature 409 (2001) 794.
[2]D. G. Shchukin, H. Möhwald, Small 3 (2007) 926.
[3]C. M. Brick, E.R. Chan, S. C. Glotzer, J. C. Marchal, D. C. Martin, R. M. Laine, Adv. Mater. 19 (2007) 82.
[4]C. Gauthier, A.-L. Durier, C. Fond, R. Schirrer, Tribology International, 39 (2006) 88.
[5]C. Bolln, A. Tsuchida, H. Frey, R. Mülhaupt, Chem. Mater. 9 (1997) 1475.
[6]B. Darlix, P. Montmittonet, B. Monasse, Polymer Testing, 6 (1986) 189.
[7]L. Cheng, X. Xia, L. E Scriven, W. W.Gerberich, Mechanics of Materials, 37 (2005) 213.


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