Compared with conventional instrumented indentation (e.g., nanoindentation), the microscale compression test of a geometrically defined pillar has the obvious advantage of a relatively uniform stress-strain field. The main issue with instrumented indentation is that the stress-strain field underneath the indenter is highly complex and depends on the specific indenter tip geometry. In addition, the extraction of uniaxial stress-strain constitutive relations from indentation is very difficult and requires complicated iterative methods.
Micropillars can be manufactured by a range of techniques, including Focused Ion Beam (FIB) milling, replication techniques, or LIGA. They provide a small volume of material with a very defined geometry (commonly cylindrical or square). Various experimental variables may affect the stress-strain characteristics of the material being tested, namely the aspect ratio (the ratio of height and diameter of the pillar), the size of the substrate under the pillar, the taper angle (micropillars are often tapered due to the FIB milling process), the stiffness of the substrate and the misalignment between the pillar axis and the compression direction.
Examples of Si micropillar arrays (left) made by lithography and a single micropillar (right) just before contact with a diamond flat punch indenter
Compression testing is usually performed using a flat punch indenter which has a slightly larger diameter than the micropillar itself. Alemnis can provide flat punch indenters in a wide range of materials (e.g., diamond, tungsten carbide, sapphire, ruby, silicon carbide, boron nitride, stainless steel, etc) and a wide range of diameters, from 2 to 400 µm with a typical cone angle of 60°. Custom indenter geometries (e.g., pyramidal, spherical, knife, wedge, etc.) are also possible, on request. Special geometry indenters may be used for specific applications where simple compression is replaced by cutting or fracture.
Example of a micropillar compression test on a micropillar built from a multilayer stack of different coatings
There may be various size effect phenomena in pillar compression experiments, one of the most common being that the flow stress of the material increases as the size of the pillar is decreased. This has been documented in a number of cases in single phase materials at small scales. The micropillar compression test can therefore be very useful to study size effects, in terms of deformation behavior, dislocation movement or strengthening mechanisms. In a typical test, the initial compression will cause linear elastic behavior up to the elastic limit (or yield point), after which plastic deformation will become dominant. In the plastic regime, various types of deformation mode may be observed, e.g., serrated plastic flow which is manifested by sudden load drops and reloading periods where dislocation “slip” is occurring.
The Alemnis Standard Assembly (ASA) is designed for micropillar compression experiments which is why it operates with true displacement control, thus allowing any sudden load excursions to be monitored by a load cell in real time.
The unique strain rate options possible with the ASA mean that the strain rate can be varied even within an experiment, often referred to as a strain rate jump test. This can be used to investigate the strain rate sensitivity of a material over many orders of magnitude. For example, with a maximum actuation speed of ̴8 mm/s and a micropillar with gage length of ̴4 µm, the effective strain rate can cover the range 0.0001/s up to a maximum of 2000/s, which equates to 8 orders of magnitude of strain rate!
The ASA can also be used to conduct creep tests on micropillar specimens (by holding the applied load constant and measuring the creep in the displacement signal) or stress relaxation experiments (by holding the micropillar under a fixed deformation and monitoring the variation of applied load). Other variants of the compression test include fracture studies, where the micropillar may be intentionally fractured by indenting with a sharp indenter (e.g., cube corner), or split using a wedge-shaped indenter over a defined area. Micropillars can also be produced with a notch to force the fracture at a specific location. Fatigue tests can be made on micropillars by cycling the load or displacement in a continuous or increasing manner to investigate when a specific volume of material fails by fatigue fracture, e.g., in Micro Electro Mechanical Systems (MEMS).
Example of a series of strain rate jump tests performed on nanocrystalline Ni micropillars at various temperatures (from Ref. 3)
Although many micropillar compression tests are performed in-situ inside an SEM, it is also possible to mount the ASA ex-situ and use an optical microscope to align the flat punch indenter on the micropillar. Such an ex-situ approach can also be used to perform unique experiments which could not be performed inside the SEM, e.g., non-conductive or fragile materials, biomaterial micropillars submerged in liquid for simulating true “in-service” conditions, electrical contact resistance experiments, electro-chemical evaluations, etc.
Experimental setup for ex-situ micropillar compression of a polymeric melt-spun fibre (from Ref. 2)
Load-depth plot for a micropillar compression test on a sapphire pillar containing a 20 nm Nb layer (Courtesy of Imperial College, London, UK)
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