Advanced Materials Processing  
Dynamical processes
Optical materials
Nanocomposites fabrication and coatings
Electromagnetic processing of functional materials

Dynamical processes

Machining of brittle materials at industrially interesting rates with high efficiency, as well as micro- and nano-structuring of hard materials, composites, and polymers are the foci of this research group.

From a scientific point of view, the main goal is to gain a fundamental understanding of wanted or unwanted (wear) re-moval processes such as: laser machining, crack initiation and propagation dependence on materials properties such as microstructure, impurity densities etc.
From a company point of view, the main goal is to enhance industrial process via a deep understanding of the process.

To achieve both goals, each process is investigated by combining specific, various but complementary in-situ methods. It includes high-speed camera, acoustic emission, fibre Bragg gratings and interferometry systems. The results are ana-lysed using high level signal and image processing.

The combination of specific knowledge of the group members with the different technics allows us to answer question to a wide range of industrial processes where there is an interaction between any kind of material and any kind of tools such as grinding, laser, electrical discharge, etc.

In-situ tribological wear measurement at high temperature (800°C) and high relative humidity (80%)

Today’s battle for increasing performance and efficiency in engines leads to con-stantly increasing operation temperatures. As a result the materials used in these systems need to be adapted to be able to perform at higher temperatures. In order to improve the material’s resistance at high temperature, thermal spray coatings can be applied (e.g. atmospheric plasma spraying (APS), flame spraying (FS), High Velocity Oxygen/Flame  spray(HVOF) and Wire arc spraying (WAS)). To perform well, these coatings’ wear resistances in a harsh environment must be ex-tremely high.

In order to test the properties of such coatings at high temperature (T) as well as high relative humidity (RH), a new unique multifunctional tribometre to measure wear rates and friction coefficients simultaneously at high T and high RH is built by industrial partners. This tribometre will enable in-situ measurements thanks to an incorporated optical device capable of characterizing wear during experiments.

Based on these new tribometre, the wear properties of the coatings can be tested under severe conditions of wear and oxidation and observed in these environ-ments without stopping the experiment and cooling the sample. Based on these information obtained in situations similar to real engines the coatings’ performanc-es can be validated or improved.

Tribological optimization of lubricated sliding systems by Laser Surface Texturing

The relative sliding contact between crucial components of common industrial machines that work under the combined action of high loads and low sliding speeds, involves harsh lubrication conditions. As a consequence, wear and eventually seizure may take place and hence dramatically reduce the machine lifetime. Among the different methods developed to improve the tribological performance of such challenging sliding systems, surface texturing is one of the most employed. In this method surface depressions are produced to act as micro-hydrodynamic bearings, micro-reservoirs for lubricant and micro-traps for wear debris. Several surface texturing techniques are available, however laser texturing seems to offer the most promising concept, since it is extremely flexible in terms of materials, shapes and sizes of the created structures, fast enough to be industrially implemented and clean to the environment. The scientific goal of this industrial project is to understand the influence of the geometrical and microstructural texturing parameters on the tribological behavior, and not simply to do a trial and error optimization of the textured surface as is usually done. This understanding will not only help solving the problem of our industrial partner but also generate general knowledge that can be extended to several other tribo-systems. For this purpose, Empa is using state-of-the-art laser texturing to develop a tribologically-optimized surface on journal bearing cast iron/steel components. The tribological testing of the textured surfaces is being supported by fluid dynamics simulations, thus providing a better understanding of the physical mechanisms of the lubrication process.

Contact : Kilian Wasmer

Multi-wire sawing of crystalline solar cells
A. Bidiville, K. Wasmer et al. Sol.EnergyMater.Sol.C 132(1)(2015)392-402,DOI:10.1016/ j.solmat.2014.09.019

In recent years, photovoltaic industry (PV) has been under commensurable pres-sure at every stage of the manufacturing chain to reduce costs. This has led to thinner wafer slicing and so new challenges arose. In particular, the handling and cleaning of thin wafers (< 200 ìm) as well as the overall downstream solar cells manufacturing processes to avoid breakage. Breakage is a consequence of the brittleness of silicon once a microscopic crack is present. Without taking into ac-count the probable defects generated during the ingot casting process, these mi-croscopic cracks originating at the silicon surface mainly occur during the shaping of the silicon (Si) bricks and during the wafering/slicing process. The key for break-through lowering of raw material cost, hence a major part of solar cell costs, is the reduction of kerf loss and the ability for slicing thinner wafers. Research at Empa focused on the state of the art slicing technology by means of multi-wire slurry saw (MWSS) in order to understand the fundamental of dynamical processing during the sawing mechanisms to minimize the influence of sub-surface defects on the overall wafer quality. Finally, investigations on dynamical processes of new technologies such as diamond-plated wire-sawing have been carried out. The commercial goal is to develop a process technology that allows mass production of thin crystalline silicon solar cells, including the cutting of ultra-thin silicon wafers (<150um) via multi-wire sawing..

Contact : Kilian Wasmer

Grinding of single crystal sapphire
S. Graça, K. Wasmer et al. Acta Mat. 67(6)(2014) 67–80, DOI: 10.1016/ j.actamat.2013.12.004

Due to the high quality requirements of watch industry, the grinding process of hard material, in particular single crystal sapphire remains a veritable challenge. This is the case, even if grinding is the most commonly used machining process for the fabrication of structural components made of hard materials such as ce-ramics, including sapphire. In the last several decades, the high cost associated with machining of ceramics components has spurred a considerable research ef-fort aimed at developing efficient grinding processes. Abundant literature exists about the effect on grinding of the abrasive (type, size and concentration), the material properties of the specimen, the wheel characteristics (e.g. vitrified bond diamond wheels) and specimen and wheel speeds, in order to control surface quality.

Hence, this project aims at increasing the productivity of watch glass manufactur-ing a by combining a fundamental study of the sapphire with a systematic technological investigation of the current grinding practices to establish processing maps relating both the process parameters and process outcome. Research at Empa focused on industrial grinding technology in order to understand the fundamental grinding process of sapphire via deformation (dislocations) and failure mechanisms (cracks formation, crack propagation, chipping) to suppress or minimize to a maximum the sub-surface defects on the overall watch glass.

To solve this industrial process, bridging high level scientific research and industrial application is a must.

Contact : Kilian Wasmer

Affiliation

Laboratory for Advanced Materials Processing
Empa - Materials Science & Technology
Feuerwerkstrasse 39
CH-3602 Thun

Tel.: +41 58 765 1133
Fax.: +41 33 228 44 90
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