MANUTECH-SISE > Axe Scientifique 4


The "Probing Dynamic Process" research axis focuses on dynamic processes and real-time control of surface phenomena upon the interaction with a processing tool, either light-driven or of mechanical nature. Two particular directions will be followed:
a) Dynamics of ultrafast laser-induced processes on materials
b) Dynamics of transient phenomena occurring at contact interfaces.
The thrust is to determine the characteristic time and space scales associated with transient processes appearing during material processing. In the course of the interaction between two surfaces or upon laser excitation, a confined medium with small space dimensions is formed (typically a nm to µm). Within this confined interface, fast dynamic phenomena occur on different characteristic timescales, from ps to ms, mirroring a range of non equilibrium, thermodynamic and thermomechanical processes. The knowledge of relaxation dynamics is essential for developing real time monitoring techniques and intelligent process control approaches. This research and development axis will also provide advanced tools and methods for observing, measuring, and controlling transient phenomena. It will equally promote further process understanding by the development of predictive theoretical or numerical multi-scale models for simulating interfacial phenomena. The emphasis is put on local phenomena in confined environments with the analysis of the collective dynamics during excitation or interactions at interfaces and the deriving consequences.

Research directions:
- Laser interaction, time-resolved diagnostic and control

The interaction regime and the associated process accuracy in ultrafast laser structuring of material is often associated with the degree of laser-induced nonequilibrium. This allows exploring novel structural phase transitions and metastable states capable of generating new classes of properties and material forms with updated functionalities.
The accurate knowledge of excitation channels and relaxation dynamics represents a great challenge in today material processing techniques providing at the same time a guideline for optimizing laser phenomena. Several tasks will be followed:
- Probing ultrafast laser phenomena on subwavelength scales: This task involves evaluating excitation transients, collective excitation dynamics, and primary hydrodynamic steps in laser nano- and micro-processing associated to a change of the topology on the nanoscale.
- Spatio-temporal pulse manipulation techniques and process control/optimization: The task proposes optimization methods based on the automated pulse design in space and time with a paramount importance in optimizing laser-matter interactions as well as in increasing their effectiveness.
- Predictive simulation tools of interface phenomena: The understanding of laser-induced processes is essential to the overall process control. This task involves the theoretical and numerical interrogation of electronic excitation and the subsequent thermodynamic material trajectories as a way of mastering laser processing events.

Conceptual design for probing and controlling laser-induced phenomena and structuring processes.

- Transient phenomena in mechanical contact:

Under tribological stress, transient (fast) phenomena are frequently observed but their influence on the evolution and degradation of a tribo-system is still largely unknown. However, the development of modeling and new acquisition techniques now makes possible the understanding of non-stationary regimes. These phenomena are related to the dynamics of the mechanical conditions but also the dynamic response of the interfacial material resulting from the contact interaction. Indeed, during a dry or lubricated contact, a confined medium is formed. It consists in an interfacial layer or a film lubricant whose tribological behavior and associated dynamics depend on its intrinsic properties (nature, mechanical properties) and the effects of confinement. The latter result in:
- A structuring effect near the walls or on the contrary a local disorder,
- A geometric confinement for which the lubricant film thickness is small compared to the size of the contact, which induces an amplification of experienced macroscopic mechanical stress (high gradient of stress or localized overstress, shear rate up to 106 s-1). The interface can then exhibit a complex nonlinear behavior (alignment of molecules, dynamic transition between solid- and liquid-like states) or instabilities (localized deformation, cavitation ...)
An illustration of these phenomena for lubricated contact is proposed below.


Dynamics of confined lubricated interfaces – Contact interferograms (IRIS tribometer, LTDS)

Another example, for dry conditions, is related to the process of friction stir welding. A textured pin rubs against a solid surface at high speed. This contact interaction generates local heating and stress gradients that induce important local microstructural changes localized at the interface. These structural changes depend on the local stress conditions but also on the materials in question and their surface topography. Moreover, they regulate the friction response of the interface thus controlling the quality of the process and its implementation. This process puts forward a series of fundamental questions related to the transient local conditions, their access and controllability: How can one precisely characterize these local phenomena, localized both in time and space, in this black box that is the interface. A major emerging scientific theme of this axis is the identification of local phenomena occurring in a confined area.