3D crack reconstruction using acoustic emission

For better prediction of in-service component lifetime containing cracks, it is of utmost importance to have a precise knowledge of the crack shape, size and propagation rate. Research at Empa focused on developing new hardware and software to not only localize crack, but also to follow their propagation. The novelty is in the reconstruction of the complex geometry of the crack path and tracking its propagation in time, whereas existing methods focus only on the localization of the crack without any information about its geometry. The major achievement is a simple 3D crack reconstruction. The first results are very promising and a movie of a crack propagation can be viewed at  /web/s204/3dreconstruction_1

At present, further work are ongoing on the validation of the methods within a new project.

Selected publications

• Shevchik S.A., Meylan B., Violakis G., and Wasmer K., “3D Reconstruction of Cracks Propagation in Mechanical Workpieces Analyzing Non-Stationary Acoustic Mixtures”, Mechanical Systems and Signal Processing, Vol. 119, Issue March 2019, pp: 55-64, 2019,
  https://doi.org/10.1016/j.ymssp.2018.09.022

Multi-wire sawing of crystalline solar cells

Since 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.
Research at Empa focused on investigating the 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.
The two major achievements were (a) a fundamental understanding of MWSS process and its implication on surface quality and (b) the development of semi-empirical models for reducing the surface and sub-surface damages.

Selected publications

• Wasmer K., Bidiville A., Van der Meer M., and Ballif C., "Wire-Sawing Processes: Parametrical Study and Modelling", Solar Energy Materials and Solar Cells, Vol. 132, pp: 392-402, 2015,
  http://dx.doi.org/10.1016/j.solmat.2014.09.019

• Bidiville A., Neulist I., Wasmer K., and Ballif C., "Effect of Debris on the Silicon Wafering for Solar Cells", Solar Energy Materials and Solar Cells, Vol. 95, Issue 8, pp: 2490-2496, 2011,
  http://dx.doi.org/10.1016/j.solmat.2011.04.038

Grinding of single crystal sapphire

Sapphire (Al₂O₃) is the third hardest mineral material (after diamond and moissanite). Its combination of excellent chemical, electrical, mechanical, optical, thermal and durability properties makes sapphire to be the preferred material for the manufacturing of high performance systems and components, including the watch industry. However, its brittleness and hardness make the machining of sapphire very challenging and therefore it is carried out most often by grinding or laser machining.
Due to the high quality requirements of watch industry, the grinding process of hard material, in particular single crystal sapphire remains a veritable challenge.
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.
The two major achievements were (a) a fundamental understanding of crack propagation in sapphire and (b) the development of semi-empirical models for the grinding of sapphire.