Advanced Materials Processing  
Processing dynamics and optical materials
Nanocomposites fabrication and coatings
Electromagnetic processing of functional materials

Processing dynamics and optical materials
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. The main goal is to gain a fundamental understanding of wanted or unwanted (wear) removal processes such as: laser machining, crack initiation and propagation dependence on materials properties such as microstructure, impurity densities etc.. We are comparing different machining methods such as grinding, laser cutting, laser drilling, laser ablating.
The cost efficient production – shaping and machining of optically active materials is one of the specific challenges for IT applications (i.e. on chip, or chip to chip optical interconnects with amplification, modulation and routing functionality). For these applications, the needed micro- to nano- structuring range is covered by applying laser or electron beam induced processing for either new materials – device prototyping or for the important step towards showing feasibility of industrial fabrication or master mould production and repair.
Finally, for multilayered systems such as coatings or other types of assembly a fundamental challenge has to be faced, interfacial adhesion. We are trying to gain more insight by studying simple model systems and transferring this knowledge to industrially relevant systems.
Tribological optimization of lubricated sliding systems by Laser Surface Texturing
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Laser surface texturing of a cast iron sleeve in a half-journal bearing

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.

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 constantly 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  spraying (HVOF) and Wire arc spraying (WAS)). To perform well, these coatings’ wear resistances in a harsh environment must be extremely 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 tribometer to measure wear rates and friction coefficients simultaneously at high T and high RH is built by CSM Instruments. This tribometer will enable in-situ measurements thanks to an incorporated optical device capable of characterizing the wear track during the experiments.
Based on these new tribometer, the wear properties of the coatings can be tested simultaneously under severe conditions of wear and oxidation and observed in these environments without interrupting the experiment and superimposed post oxidation during cooling of the sample. Based on these in-situ observations obtained with conditions similar to real engine loads, the coatings’ performances can be validated or improved.
Dynamical processes Multi-wire sawing of crystalline solar cells
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Bidiville A., PhD thesis, University of Neuchatel, 2010
In recent years, photovoltaic industry (PV) has been under commensurable pressure 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 account the probable defects generated during the ingot casting process, these microscopic 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 breakthrough 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
Saphire watch glass and its failure surface

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 ceramics, including sapphire. In the last several decades, the high cost associated with machining of ceramics components has spurred a considerable research effort 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 manufacturing 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

Laser processing
Eximer laser system XXL (1.5x2m)

Mask projection imaging of excimer laser pulses for direct ablation has been developed already 2 decades ago. Still today, a very large scale test facility has not been available for research and development purposes until recently. The KrF Excimer lasers emitting at 248 nm with pulses of up to 600 mJ per pulse and repetition rates of up to 400 Hz structure 2 x 1.5 m2 polymer sheets down to 80 micrometer depth of the structures. Strongly differning from the focussing and pixel scanning laser machining systems the mask projection ablation results in unique efficiency. Air bearings, interferometric positioning, short wavelenght and stable lasing are the combined requisites of the laser center @ empa, Thun.

Alternative laser processing with longer pulse duration for laser soldering are presently developed in our labs. For these applications we apply a long pulse, fibre coupled Nd:YAG laser, providing pulses with high power (up to tens of J) and variable length (0.1-20 ms). In addtition this Nd:YAG diode pumped system offers large variability in temporal laser intensity shaping. The latter is ideal for applications including relatively slow dynamic processes such as surface diffusion or even bulk diffusion. Optimizing laser processes can always be carried out by two fundamentally different ways, either adapting the laser parameters, or adapring the materials properties. In our laboatory we focus on tailoring of materials for specific laser applications such as laser debinding, laser sintering, laser soldering, laser welding.     
Contact : Patrik Hoffmann & Kilian Wasmer

Optical materials
Cross section SEM image of HV-CVD LiNbO3 film on c-cut saphire ; TEM of film substrate interface

Optical materials is a mature, but still an actively developing field today. Miniaturisation and integration of the optical devices impose new challenges – shaping the right material into the right form is not always an easy task. This issue we address in our lab, where we develop methods of fabrication and processing of optical materials for integrated optics.
High vacuum chemical vapour deposition technique has been developed during many years in our group for deposition of thin films of oxide materials. Our HVCVD reactor is designed for 4” wafers, healable up to 700°C. Three independent precursor delivery lines are available on the reactor. Precursor behaviour in high vacuum may significantly differ from the observed in standard CVD conditions. Our expertise and research interest include the important part of precursor testing and selection.
Both crystalline, functional (e.g. electro-optic) materials, doping for active laser properties and amorphous films for passive waveguiding functionality have been deposited our the past years. List of materials, which we have been or are working with, includes, but is not limited to LiNbO3, BaTiO3, TiO2, Al2O3, Nb2O3, HfO2, …
Characterization of the thin films’ structural and functional properties is also an important task in the investigations and provides feedback for the fabrication process.
Currently, we are working on the integration of the crystalline materials such as LiNbO3 and BaTiO3 on technological substrates to enable up scaling. Processing methods of thin films on the wafer scale, such as structuring, selective deposition with irradiation by laser or electron beam, lift-off techniques, are critical aspects of integration and device fabrication and also receive high attention in our research.
Contact: Patrik Hoffmann & Yury Kuzminykh

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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