There is an increasing demand, both from industry and research community, to characterize small-scale and site-specific mechanical properties of materials.
The motivation behind such demand is two folds:
- the need to reliably predict the performance of miniature mechanical systems (MEMS, UV-LIGA watch parts, coated components like machining tools etc.) and microelectromechanical systems (MEMS/NEMS/MOEMS)
- to develop detailed and accurate hierarchical models of the mechanical behavior of complex, multicomponent structural materials.
This requires the development of new micro-scale instruments and testing methods to study the deformation behavior of small mechanical parts and to investigate size effects in mechanical properties.
Those properties are affected by the machining process (eg. Deep Reactive Ion Etching, Electro Discharge Machining, Laser machining, High Speed Milling, Blanking, Stamping, ¡K) that have an important impact on the surfaces properties (roughness, micro-cracks, chemical modification, ..) and by the manufacturing process (electrodeposition, laser sintering, ¡K.) that create non-uniform microstructures, interfaces and residual stress in the bulk material.
The Scientific Instrumentation group aims at developing the methods and innovative instruments necessary to extract material properties under complex load cases (fatigue, cycling, multi-axial loading, low and high temperature, ¡K) at the micro- and nano- scales. Those developments are in most of the cases conducted through collaborative projects with instruments manufacturers.
Mechanical properties of tailored nanostructured Ni and NiW alloys produced by electrodeposition
The project addresses electrodeposition and micromechanical characterization of nanostructured Ni and Ni alloys miniature components.
The ultimate goal is to improve their mechanical properties such as strength, Young's modulus and creep resistance. These properties cannot be improved and optimized simultaneously by changing a single microstructural parameter like grain size.
In addition, the mechanical characterization techniques need to be adapted for small scale testing of the developed electrodeposits. A commercially miniaturized tensile testing device is modified to accommodate specimens with gauge dimensions of 250ƒÝm * 250ƒÝm. Consistent and reliable mechanical properties are obtained from commercially used and in-house electrodeposited Ni samples. Similar mechanical behavior was also measured from in-situ SEM micro-compression. Further a miniaturized uniaxial creep test will be developed. In order to gain insights into the rate controlling deformation mechanisms, it is proposed to adapt variable temperature micropillar compression testing to enable strain rate jump and load relaxation tests to extract deformation related activation parameters.
Ex-situ elevated temperature nanoindentation in vacuum - CSM Instruments
An elevated temperature nanoindenter capable of operating in vacuum up to 10-7 mBar is being developed in collaboration with CSM Instruments. The aim is to attain testing temperatures up to 800°C. This instrument is based on active surface referencing technology that utilizes two independent vertical axes: one for indentation and the other for the referencing the sample surface. Due to symmetric architecture, differential displacement measurement and use of low CTE materials in the load frame, thermal stabilization times of the indenter tip with the heated sample has been drastically reduced. Average thermal drift rates of only 10nm/min are obtained on fused silica at 400°C without tip heating. Currently work is progressing on integration of non-contact tip heating so as to minimize the temperature gradient between the indenter tip and the sample surface and eliminate thermal drift completely. It is anticipated this development will enable us to perform not only short term measurements like indentation but also long term measurements like indentation creep testing accurately.
Dynamic Nanoindentation Testing
Majority of nanoindentation studies deal with application of constant, linear or proportional loading profiles. Therefore, each indentation cycle provides only one set of mechanical properties (hardness, modulus). At least 5-l 0 indents must be performed on a homogeneous sample in order to obtain some reliable statistics in experimental data. Due to this, nanoindentation is limited to homogeneous samples, comparatively thicker films and extensive experimentation in order to avoid substrate and boundary effects and scatter in data. These limitations can be overcome by superimposing a sinusoidal signal on the normal force during indentation which is referred to as dynamic testing. From the local unloading data obtained from each sinus segment, it is possible to extract the modulus and hardness values. To illustrate, with a 50Hz sinus frequency and 5 seconds loading time, about 50x5= 250 sets of hardness and modulus values can be extracted from a single curve. An example is shown in the left figure showing the fused silica hardness as a function of the depth, obtained with only one indentation test at 1Hz. The apparatus is actually developed for indentations at higher frequencies.
This project aims at developing the necessary devices (sensors, actuators …), testing procedures and signal processing to push further the performances of existing instruments. The Alemnis SEM Indenter will be used as instrument platform.
High Temperature Deformation of Bulk Metallic Glasses (BMGs)
The elevated temperature mechanical response of a Zr-based bulk metallic glass (BMG) was examined using in situ indentation and micro-compression in the SEM. This allowed direct observation of shear band formation and propagation as a function of strain rate and temperature as well as measurement of uniaxial stress-strain relationships. The flow stress has been found to remain constant with temperature at ~2 GPa below the glass transition temperature, Tg. The magnitude of the stress drops/serrations in the indentation load-displacement curves, and compression stress-strain curves increases with temperature. Above Tg, plastic flow was observed to be homogeneous without any shear band formation, and the flow stress was observed to decrease significantly. (Materials Science and Engineering, 2011, Scripta Materialia, 2012)
Microcompression testing in air
Many applications require testing in ambient conditions. For example, cement and polymeric samples cannot be used in SEM due to dehydration and electron beam damage respectively. This necessitates nanomechanical testing under optical microscope in air. The Alemnis indenter was integrated under a commercially available microscope for precise positioning of the indents for site-specific requirements. Microcompression testing of high-speed melt-spun fiber samples (~100 microns diameter) was carried out to determine their elastic modulus. Another recent application was integration of the indentation set-up on to a synchrotron beamline to study the strain fields in indentation of a Zr-based bulk metallic glass in transmission geometry. The addition of a small microscope onto the indenter enables site-specific indents and microcompression testing to be performed in a remote controlled environment eg. synchrotron beamline.