WO2015193176A1

WO2015193176A1 - Material testing device

Info

Publication number: WO2015193176A1
Application number: PCT/EP2015/063111
Authority: WO
Inventors: Jean-Marc Breguet; Gaylord GUILLONNEAU
Original assignee: Alemnis Gmbh; Empa, Swiss Federal Laboratories For Materials Science And Technology
Priority date: 2014-06-20
Filing date: 2015-06-12
Publication date: 2015-12-23
Also published as: EP3158312A1

Abstract

Material testing device (9) for performing indentation measurements, the material testing device (9) defining an axis (z) and comprising: - a support element (6) adapted to permit the material testing device (9) to be attached to a test rig (21); - a test tip (1) adapted to be brought into contact with a solid sample; - an actuation device (3) comprising an active material (20), the actuation device (3) being disposed between the support element (6) and the test tip (1), the actuation device (3) further being adapted to displace the test tip (1) in at least one direction comprising a component perpendicular to said axis (Z) by reaction of the active material (20) to an applied stimulus. According to the invention, the actuation device (3) is further adapted to act as a force sensor so as to be able to sense a force applied by the test tip (1) on the solid sample by means of measurement of a response of said active material (20) to application of said force.

Description

Material testing device

Technical Field

[0001] The present invention relates to the field of materials testing. In particular it relates to a material testing device for performing measurements such as indentation measurements for hardness and tribological testing of the surfaces of solid samples, and tensile, compression or bending testing for determining elastic and/or plastic properties of materials. The invention also relates to a corresponding method of performing a measurement utilizing the indentation device.

State of the art

[0002] Tensile, compression and bending testing of material is well-known.

Furthermore, indentation testing of material in general has also been known for at least a century, in the form of, amongst others, the Brinell, Vickers and Rockwell hardness tests. These tests are useful for determining hardness of bulk material, however have limited application for investigating thin coatings.

[0003] Nanoindentation, that is to say indentation testing on nanometre scale, allows not only interrogation of small volumes important for studying the mechanical properties of thin films and individual phases, but it can also yield valuable insights about rate-controlling deformation processes. The high resolution load-depth data obtained from nanoindentation testing is used to extract mechanical properties like hardness, modulus, and strain hardening parameters and also to study fundamental atomistic deformation processes such as dislocation source activation, shear instability initiation and phase transformations. The majority of nanoindentation measurements are performed at low, quasi-static strain rates, mainly because the existing instruments do not allow reliable high strain rate measurements.

[0004] An example of a device for such nanoindentation is disclosed in US201 1/265559. This document describes a 2-dimensional MEMS nanoindenter transducer comprising electrostatic capacitive actuators so as to actuate a nanoindenter tip. Initial positioning of the tip is performed via mechanical positioner and piezo positioner, and final positioning and movement of indenter tip is performed via the MEMS nanoindenter transducer. Force measurements are carried out by means of differential capacitive sensors. Such devices, when provided with appropriate tips, are also suitable for carrying out other material testing measurements such as the tensile, compressive, and bending tests referred to above.

[0005] A further example of an indenter in a tunneling electron microscope (TEM) device is given in Meas. Sci. Technol. 17 (2006) 1324-1329 doi: 10.1088/0957-0233/17/6/006, "A miniaturized TEM nanoindenter for studying material deformation in situ". This arrangement utilizes a separate force sensor with a separate spring element using a four bar mechanism based on flexible elastic hinges.

[0006] US 5,574,278 discloses another example of an AFM tip which comprises a segmented piezoelectric actuator and a separate ring-type sensor. Since this example uses separate actuators and sensors, it is again somewhat bulky.

[0007] WO 2012/122523 discloses a three-dimensional transducer with piezoelectric actuation and a capacitive sensor, which is complex.

[0008] Finally, EP 2 202 501 discloses a hollow piezoelectric device with segmented electrodes, which is intended to be used as a sensor for measuring forces and torques applied by an external actuator, but can also alternatively be used as an actuator. This document presents these two applications as either/or propositions, which thus implies use of two piezoelectric devices simultaneously to apply both piezoelectric actuation and sensing in one device.

[0009] Such arrangements, since they incorporate separate actuation and measuring components, are not compact, and are limited in the stiffness that can be achieved due to spring-based measurements in series with the actuators and the tip. Furthermore, at least partially due to the limited stiffness and the use of electromagnetic or electrostatic actuation (e.g. voice coil or MEMS), the frequencies that can be used in dynamic testing are limited to fairly low frequencies, and the forces that can be applied are likewise limited.

[0010] An object of the invention is thus to at least partially overcome the above- mentioned drawbacks of the prior art.

Disclosure of the invention

[001 1] This object is attained by a material test device for performing measurements of material properties. The material test device defines an axis and comprises a support element adapted to permit the indentation device to be attached to a test rig, a test tip adapted to be brought into contact with a solid sample, and an actuation device comprising an active material. Examples of such active material are shape memory alloys, piezoelectric or quartz material, magnetostrictive material, and electrostrictive material. The test tip may be a (micro- or nano-) indentation tip, a flat tip for applying a force without indenting the solid sample, or a gripping tip for gripping a sample for performing a tensile test, or any other convenient type of tip.

[0012] The actuation device is disposed between the support element and the test tip, and the actuation device is further adapted to be able to displace the test tip in at least one direction comprising one or more component(s) perpendicular to the axis of the material test device (i.e. at least partially laterally, or in other words in a direction other than parallel to this axis) by reaction of the active material to an applied stimulus such as an electrical voltage or current, a magnetic field, heat and so on.

[0013] According to the invention, the actuation device itself is further adapted to act as a force sensor so as to be able to sense a force applied by the test tip on the solid sample by means of measurement of a response of said active material to application of said force.

[0014] Since the actuation device, and in particular its active material component, also is used for sensing, no supplementary force sensing element is required.

In other words, the same volume, or portion, of active material can be used simultaneously for both functions. This reduces the complexity and the number of components required for the indentation device, as well as improving the rigidity and the force to weight ratio of the indentation device. Such an improved rigidity, together with the force that can be applied by an active material in comparison to e.g. a voice coil actuator, significantly increases the indentation force that can be applied to the solid sample.

[0015] Advantageously, the active material exhibits a response upon application of an electric voltage, such as is the case of a piezoelectric or electrostrictive material, and the actuation device comprises a plurality of electrodes, at least one of which is an inner electrode disposed towards the interior of the actuation device and at least one of which is an outer electrode disposed further towards the exterior of the actuation device than the inner electrode.

[0016] Such materials can respond very quickly, thus high frequencies of oscillation of the test tip can be attained, enlarging the possibilities for high strain rate dynamic indentation measurements.

[0017] Advantageously, a plurality of outer electrodes are provided, wherein the actuation device is arranged such that a voltage applied between an inner electrode and an outer electrode causes the test tip to displace in at least one predetermined direction, and wherein a voltage measured between an inner electrode and an outer electrode permits to measure a force applied by the test tip. This naturally includes application and measurement of various different voltages at the various different electrodes. These electrodes can be connected to external circuitry directly, or via connection pads. [0018] Alternatively, a plurality of inner electrodes may be provided, wherein an outer electrode surrounding the plurality of inner electrodes and disposed further towards the exterior of the actuation device than the inner electrodes is provided, and wherein the actuation device is arranged such that a voltage applied between an inner electrode and the outer electrode causes the test tip to displace in at least one predetermined direction, and wherein a voltage measured between an inner electrode and the outer electrode permits to measure a force applied by the test tip. Such an arrangement utilizing a single outer electrode provides excellent electromagnetic shielding of the active material and the inner electrodes from electromagnetic interference from outside, and shields nearby measurement equipment from electromagnetic interference emanating from the test tip itself.

[0019] Advantageously, the indentation device further comprises at least one strain gauge disposed on the actuation device, arranged so as to measure a deformation of the actuation device. Further force measurement is thus possible.

[0020] Advantageously, the active material is a piezoelectric material.

[0021] Furthermore, the object of the invention is attained by a method of performing an indentation measurement comprising the steps of:

- providing a solid sample to be tested;

- providing a test device as defined above;

- applying a force in at least one predetermined direction between the test tip and the solid sample, preferably by means of the actuation device, which force may be any combination of axial or lateral with respect to the axis of the indentation device and may be in traction or in compression;

- measuring a response of said active material to application of said force;

- determining the applied force based on a result of said measurement.

[0022] Such measurements are useful for extraction of hardness and modulus in addition to performing measurements of shear modulus, nanotribology, nanowear, fatigue testing, indentation measurements and so on.

[0023] Advantageously, the active material is responsive to electrical voltages (i.e. is electrostrictive or piezoelectric), and at least one inner electrode and at least one outer electrode are provided. The force is applied by applying at least one voltage between at least one inner electrode and at least one outer electrode. Furthermore, a response of said active material to the force applied to the solid sample is measured by measuring at least one voltage between at least one inner electrode and at least one outer electrode. The applied force can thus be determined from the measured voltage(s).

[0024] Advantageously, at least one electrode of the set comprising the at least one inner electrode and at least one outer electrode to which voltage is applied is not comprised within the set comprising the at least one inner electrode and the at least one outer electrode between which voltage is measured. In other words, at least one electrical connection to the various electrodes, or even all of the electrical connections, is/are different for application of voltage as for measurement of voltage. This simplifies the electronics and signal processing required

[0025] Alternatively, the set comprising the at least one inner electrode and at least one outer electrode to which voltage is applied is comprised within the set comprising the at least one inner electrode and the at least one outer electrode between which voltage is measured. That is to say that all electrical connections used for applying voltage are also used for measuring voltage.

[0026] Advantageously, the voltage comprises at least one component oscillating at a frequency of between 1 kHz and 50kHz, preferably between 10kHz and 40kHz. Additionally or alternatively, the voltage may comprise a component which comprises at least one harmonic frequency of at least 1 kHz, preferably at least 10kHz, further preferably at least 20kHz, even further preferably at least 50kHz. Ideally, this harmonic frequency can be situated between 50kHz and 200kHz, e.g. a step function rising in 5 microseconds or less.

[0027] Advantageously, the test tip is an indenter tip, preferably a micro- or nanoindenter tip, and the measurement is an indentation measurement, preferably a micro- or nanoindentation measurement. The terms "micro" and "nano" should be understood as relating to the micrometre and nanometre scales respectively. Brief description of the drawings

[0028] Further details of the invention will become apparent in the following description, in reference to the annexed drawings, in which:

- Fig. 1 is a perspective view of an material testing device according to the invention;

- Fig. 2 is a side-view and a lateral cross-sectional view of the material testing device of figure 1 ;

- Fig. 3 is a perspective view of the material testing device of figures 1 and 2 installed in a test rig;

- Fig. 4 illustrates a particular configuration of electrodes on the actuation device in end and perspective views;

- Fig. 5 illustrates a further particular configuration of electrodes on the actuation device in end and perspective views;

- Fig. 6 illustrates a particular polarization of the electrode configuration of figure 5 in end view; and

- Fig. 7 illustrates a yet further particular configuration of electrodes on the actuation device in end and perspective views.

Embodiments of the invention

[0029] Figures 1 and 2 illustrate a particular embodiment of a material testing device 9 according to the invention.

[0030] In this particular embodiment, the material testing device 9 defines an axis Z, which is the longitudinal major axis of the material testing device 9, and comprises a test tip 1 preferably situated on this axis Z and preferably extending parallel thereto. This test tip 1 may be of any known type, such as an indentation tip comprising a diamond or sapphire pyramid of any convenient form, and need not be described further. With such an indentation tip, the material testing device 9 constitutes an indentation device. For utilising the material testing device 9 for non-indentation applications such as compression or bending tests, test tip 1 may be flat or otherwise conveniently shaped so as not to indent the surface of the solid sample. For performing tensile tests, test tip 1 may comprise a gripping or clamping means for clamping a sample to be tested in tension. Test tip 1 may be fixed or removable. Test tip 1 is held in a tip support 2, which serves to interface the test tip 1 with an actuation device 3.

[0031] Actuation device 3 comprises an active material 20, that is to say a material that fundamentally changes shape on a microscopic level, or expands or contracts in response to a stimulus such as electrical voltage/current, heat, or so on. Examples of such material are shape memory alloys, which respond to heat (which may be applied externally or by passing a current through the material itself leading to resistive heating), magnetostrictive and/or electrostrictive material which respond to magnetic and/or electric fields, and piezoelectric or quartz material. Note that structures which are merely subject to a force when subject to a stimulus, such as wires or coils that are subject to magnetic forces when a magnetic field generated by a current passing through them interacts with a further magnetic field, are not active materials, since no fundamental change to the material occurs, and any changes of shape are merely due to the force applied by the magnetic interactions. The same applies equally for electrostatic devices such as capacitors. In the present case, actuation device 3 is a tubular piezoelectric actuator, and thus the active device in the present embodiment is a piezoelectric material. Other configurations of actuator are equally conceivable rather than a piezo tube, such as a piezo beam arrangement.

[0032] Actuation device 3 further comprises a plurality of connection pads 4p, 5p, which are electrically connected to a plurality of electrodes (not visible on figures 1 -3; see figures 4, 5 and 7 for various electrode configurations), which in the present example are four outer electrodes 4, here situated underneath the outer connection pads 4p which are themselves situated on the outside of the actuation device, and an inner electrode 5, situated further towards the interior of the actuation device 3 than the outer electrodes 4 and connected with inner connection pad 5p. Outer electrodes 4 are typically covered by the material of the actuation device 3 itself, or by a separate sheath, however they may also be exposed on the outside of the actuation device 3, which may also result in outer connection pads 4p being redundant and hence not necessary. Connection pads 4p, 5p are adapted to permit connecting the electrodes 4, 5 to appropriate circuitry, so as to be able to apply one or more voltages between electrodes, and also to measure the voltage(s) generated in the piezoelectric material. Other configurations and numbers of inner and outer electrodes are naturally possible, and some examples of such configurations will be discussed below. Furthermore, actuation device 3 and/or holder 6 may comprise one or more strain gauges bonded to its outside, or integrated into its structure for permitting further force measurements.

[0033] Actuation device 3 is attached to holder 6, which constitutes a mechanical interface with a test rig. Holder 6 comprises a cylindrical body with a reduced diameter section 6a upon which actuation device 3 is sheathed, in abutment with a shoulder joining the cylindrical body with the reduced diameter section 6a. Holder 6 further comprises a circumferential V-groove 6b, for clamping the material testing device 9 in a test rig 21 (see figure 3), and a radial through-hole 6c which forms a T-junction with an axial hole 6d, providing access for an electrical connection to inner electrode 5. Holder 6 may likewise also comprise one or more strain gauges bonded to its outside, or integrated into its structure. Naturally, the exact form of the holder 6 can be changed according to requirements.

[0034] Figure 3 illustrates a test rig 21 provided with a material testing device 9. Test rig 21 comprises a frame 7, upon which is mounted a tool head 8, into which the material testing device 9 is fixed securely and removably, and which may comprise conventional measurement apparatus. Tool head 8 is movable axially with respect to the axis of the material testing device 9. Facing the material testing device 9 is a load cell 10, to which the solid sample is attached such that the test tip 1 can be brought into contact therewith. Load cell 10 is itself fixed to an X-Y table 1 1 which is movable perpendicularly to the axis of the indenter head 8 so as to position the solid sample. Alternatively, X-Y table 1 1 may also move along the Z direction, i.e. additionally parallel to the axis of the material testing device 9. Other configurations of test rig 21 are naturally possible, and the force applied by the force applied to the sample may be applied exclusively by an element of the test rig 21 such as an X-Y-Z table, or the tool head 8, i.e. the material testing device 9 acts exclusively as sensor.

[0035] Due to the configuration illustrated in figures 1 -3, by applying a voltage between inner electrode 5 and one or more outer electrodes 4, the active material 20 reacts so as to displace the test tip 1 and thus to apply a force to the solid sample in any direction according to the Cartesian axes Z (parallel to the axis of the material testing device), X and Y (perpendicular to the axis of the material testing device and perpendicular to one another). This direction preferably includes at least one component in the plane of axes X and Y, i.e. not exclusively an axial displacement along axis Z. In the case of an indentation measurement, this direction thus can include a component not in the direction of the indentation. Conversely, by measuring voltage between inner electrode 5 and one or more outer electrodes 4, the active material 20 reacts to the force applied to the tip so as to generate a voltage between the inner electrode 5 and each outer electrode 4. From measurement of these voltages, the force applied to and/or by the tip according to Cartesian axes X, Y and Z can be determined.

[0036] By applying voltage between the inner electrode 5 and some of the outer electrodes 4 while measuring voltage between inner electrode 5 and others of the outer electrodes 4, the tip can be simultaneously displaced so as to apply an axial and/or lateral force to a solid sample, while simultaneously measuring the forces applied, using the same portion of active material for both applications.

[0037] Indeed, it is also possible to simultaneously apply and measure force by means of the same pair of electrodes, as illustrated in the following papers:

Dosch J etc. al, "A Self-Sensing Piezoelectric Actuator for Collocated Control", Journal of Intelligent Material Systems and Structures, 1992 3: 166; and

"Duo-bimorph actuator made of PMN-PT [01 1] : 3D modelling, development and characterization", Ivan, loan Alexandru ; Ciubotariu, Adrian ; Clevy, Cedric ; Lutz, Philippe ; Chaillet, Nicolas, IEEE/ASME International Conference on Advanced Intelligent Mechatronics, ΑΙΜΊ 3., 2013.

[0038] Such simultaneous actuation and sensing naturally requires appropriate electronics and processing, for instance to measure the applied voltage and the free charge on the piezoelectric, if following the principle disclosed in

Dosch et al, cited above.

[0039] Figures 4-7 disclose several configurations of inner electrodes 5 and outer electrodes 4 in the material testing device 9.

[0040] The configuration of figure 4 is similar to that of figures 1 -3, and comprises a single, tubular inner electrode 5, serving as ground electrode, and four substantially segmental tubular outer electrodes 4 extending over approximately 90° of circumference. The space between outer electrodes 4 and inner electrode 5 comprises a piezoelectric material, or other active material. Applying a voltage between inner electrode 5 and one or more of the outer electrodes 4 induces the reverse piezoelectric effect in the material and thus causes the piezoelectric material to displace the test tip 1 and thereby apply a force to the surface of the solid sample with which it is in contact. Measuring a voltage between inner electrode 5 and one or more of the outer electrodes 4, which may be the same and/or different to those to which voltage is applied, permits to measure the force applied by the tip by means of the voltage generated by the direct piezoelectric effect.

[0041] By using only 3 outer electrodes 4, each extending over an angle of substantially 120°, the construction can be simplified along with the associated electronics, and thus the cost of the whole system can be reduced.

[0042] Alternatively, by arranging the outer electrodes 4 as six or eight, or even more segments, every second outer electrode 4 may be used for actuation (i.e. application of voltage), and the intervening outer electrodes 4 may be used for sensing (i.e. measurement of voltage).

[0043] Figure 5 shows yet another electrode configuration. Inner electrode 5 is unchanged from the arrangement of figure 4, however outer electrodes 4 are divided longitudinally into a first section 4a and a second section 4b, each of four segments. One of first section 4a and second section 4b is then used for actuation, the other of the sections 4a, 4b being used for sensing. Alternatively, some electrodes in each section 4a, 4b may be used for sensing and others used for actuation. As illustrated, the electrodes of first section 4a are directly adjacent to the corresponding electrodes of second section 4b, however they may also be disposed rotated with respect to one another by any desired angle, such as 45°. Again, although 4 outer electrodes are illustrated in each section 4a, 4b of outer electrodes 4, any convenient number may be used, and different numbers of electrodes may be used in each section 4a, 4b. Essentially, the electrode configurations can be patterned to optimise sensitivity and/or displacement of the test tip 1 , and any number of inner electrodes and any number of outer electrodes, shaped as desired, are possible.

[0044] Figure 6 shows the electrode configuration of figure 4 polarised in an ad-hoc manner, with the upper two (in respect of the view of figure 6) outer electrodes 4 polarised positively, and the lower two outer electrodes 5 polarised negatively. This permits an extremely simple configuration of the associated electronics, requiring only two amplifiers. However, this comes at the cost of only permitting lateral actuation and sensing.

[0045] Figure 7 illustrates a further configuration of inner electrodes 5 and outer electrodes 4. In this example, outer electrode 4 is a single, tubular electrode, whereas the inner electrodes 5 are divided in the same manner as for the outer electrodes 4 in figures 4-6, and to which all comments in respect of figures 4-6 apply equally mutatis mutandis, i.e. all comments in respect of outer electrodes 4 in figures 4-6 apply to inner electrodes 5 in figure 7, and likewise all comments in respect of inner electrode 5 in figures 4-6 apply to outer electrode 4 of figure 7.

[0046] In this configuration, since outer electrode 5 completely surrounds the piezoelectric material 20 and the inner electrodes 4, it can be used as a ground electrode and thus provide an electromagnetic shielding function, shielding both the active material and inner electrodes from outside electromagnetic interference, and shielding external devices from electromagnetic interference emanating from the inner electrodes.

[0047] The material testing device 9 according to the invention may be used as follows.

[0048] The material testing device 9 is mounted in a suitable support, such as the indentation head 8 of the test rig illustrated in figure 3, or any other suitable test rig. A solid sample is suitably prepared as is known to the skilled person, and is mounted in the test rig facing the test tip 1. The test tip 1 is then brought into contact with the surface of the solid sample, and the sample is indented by actuation of the active material so as to move the test tip 1 axially and/or laterally with respect to the axis of the material testing device 9 by applying appropriate voltages to the various electrodes 4, 5. Additionally, conventional actuators situated in the test rig may contribute to applying force. The force applied is then measured by measuring the response of the active material, and displacement measurements may be carried out by sensors on the test rig and/or by microscope (e.g. electron, atomic force etc.) measurement of the resulting indentation on the surface of the solid sample.

[0049] The forces applied may be any combination of constant forces and time- varying forces, in directions depending on the configuration of the inner electrode(s) 5 and outer electrode(s) 4 and the voltages applied thereto. Due to its use of an actuator using an active material, particularly a piezoelectric material, the material testing device of the invention is capable of applying forces varying at a frequency of up to several tens of kilohertz, such as up to 50kHz axially and 15kHz laterally, which is not possible with conventional electromagnetic actuators such as voice coils. Furthermore, the range of forces which can be applied by the test tip is significantly greater than that which can be applied using voice coils for actuation, and can extend from e.g. 10 micro-Newtons up to 10N axially and 3N laterally. Also, extremely fast step-changes in displacement can be achieved with the present device, such as a step change in less than 5με for small displacements. In other terms, such a step change may be defined as one incorporating at least one harmonic of at least 1 kHz frequency, preferably at least 10kHz, at least 20kHz, at least 50kHz, and optionally from any of these frequencies up to 200kHz when the step change is decomposed into its constituent frequencies e.g. by fast Fourier transform.

[0050] As such, the material testing device 9 according to the invention enables high strain rate, dynamic mode and fatigue testing (or any combination thereof) to be performed at the micrometre to nanometre scale. Such high strain rate measurements are of particular interest in the nanocrystalline and ultra-finegrained material community as these material show enhanced strain rate sensitivity, meaning that the flow stress or hardness changes with the applied loading/strain rate. This type of measurement is also interesting for comparison to ballistic impact testing which is of interest for developing armour material and protective coatings. Dynamic mode testing, in which a small oscillation amplitude is superimposed on a load signal to enable modulus and hardness measurements as a function of depth, is necessary for measuring the mechanical properties of thin films and multilayers, and it can be performed with the material testing device 9 according to the invention. The material testing device 9 according to the invention can furthermore enable, among others, extraction of hardness and modulus as a function of indentation depth in addition to performing measurements of shear modulus, nanotribology and nanowear, of material at small length scales.

[0051] The material testing device 9 according to the invention can work both in load- or displacement- controlled loop without any special effort or special controller, and can thus be retrofitted to existing test rigs. Furthermore, since both actuation and sensing are combined in the same element, it is extremely compact, with a minimum of components, and furthermore combines both axial and lateral actuation/sensing without separate components being required for each. This aspect also increases the robustness of the material testing device 9 compared to those of the prior art. Furthermore, the material testing device is vacuum compatible. Although the invention has been described with reference to particular embodiments, variations thereto are naturally possible while remaining within the scope of the invention as defined in the claims.

Claims

Material testing device (9) for performing material measurements, said material testing device defining an axis (Z) and comprising:

- a support element (6) adapted to permit the material testing device (9) to be attached to a test rig (21 );

- a test tip (1) adapted to be brought into contact with a solid sample;

- an actuation device (3) comprising an active material (20), the actuation device (3) being disposed between the support element (6) and the test tip (1 ), the actuation device (3) further being adapted to displace the test tip (1 ) in at least one direction comprising a component perpendicular to said axis (Z) by reaction of the active material (20) to an applied stimulus;

characterised in that the actuation device (3) is further adapted to act as a force sensor adapted to sense a force applied by the test tip (1 ) on the solid sample by means of measurement of a response of said active material (20) to application of said force.

Material testing device (9) according to claim 1 , wherein the active material (20) exhibits a response upon application of an voltage, and wherein said actuation device (3) comprises a plurality of electrodes (4, 5), said plurality of electrodes (4, 5) comprising at least one inner electrode (5) disposed towards the interior of the actuation device (3) and at least one outer electrode (4) disposed further towards the exterior of the actuation device (3) than the at least one inner electrode (5).

Material testing device (9) according to the preceding claim, wherein a plurality of outer electrodes (4) are provided, wherein the actuation device (3) is arranged such that a voltage applied between at least one inner electrode (5) and at least one outer electrode (4) causes the test tip (1 ) to displace in at least one predetermined direction, and wherein a voltage measured between at least one inner electrode (5) and at least one outer electrode (4) permits to measure a force applied by the test tip (1 ).

4. Material testing device (9) according to claim 2, wherein a plurality of inner electrodes (5) are provided, and wherein an outer electrode (4) surrounding the plurality of inner electrodes (5) and disposed further towards the exterior of the actuation device (3) than the inner electrodes (5) is provided, wherein the actuation device (3) is arranged such that a voltage applied between at least one inner electrode (5) and at least one outer electrode (4) causes the test tip (1 ) to displace in at least one predetermined direction, and wherein a voltage measured between at least one inner electrode (5) and at least one outer electrode (4) permits to measure a force applied by the test tip (1 ).

5. Material testing device (9) according to any preceding claim, further comprising at least one strain gauge disposed on the actuation device (3) and is arranged so as to measure a deformation of the actuation device (3).

6. Material testing device (9) according to one of claims 1 -5, wherein said active material (20) is a piezoelectric material.

7. Method of performing a measurement comprising the steps of:

- providing a solid sample to be tested;

- providing a material testing device (9) according to any preceding claim;

- applying a force in at least one predetermined direction between the test tip (1 ) and the solid sample;

- measuring a response of said active material (20) to application of said force; - determining the applied force based on a result of said measurement.

8. Method according to the preceding claim, wherein said force is applied by means of the actuation device (3). 9. Method according to one of claims 7 and 8, wherein the test device (9) is according to one of claims 2 to 4, and wherein said force is applied by applying a voltage between at least one inner electrode (5) and at least one outer electrode (4).

10. Method according to one of claims 7-9, wherein the material testing device (9) is according to one of claims 2 to 4, and wherein said measuring a response of said active material (20) comprises measuring a voltage between at least one inner electrode (5) and at least one outer electrode (4).

1 1. Method according to claims 9 and 10, wherein at least one electrode of the set comprising the at least one inner electrode (5) and at least one outer electrode

(4) to which voltage is applied is not comprised within the set comprising the at least one inner electrode (5) and the at least one outer electrode (4) between which voltage is measured. 12. Method according to claims 9 and 10, wherein the set comprising the at least one inner electrode (5) and at least one outer electrode (4) to which voltage is applied is comprised within the set comprising the at least one inner electrode

(5) and the at least one outer electrode (4) between which voltage is measured.

13. Method according to any of claims 8-12, wherein said voltage comprises at least one component oscillating at a frequency of between 1 kHz and 50kHz, preferably between 10kHz and 40kHz. 14. Method according to one of claims 8-12 and 1 1 , wherein said voltage comprises at least one component describing an electrical signal of any shape comprising at least one harmonic of at least 1 kHz, preferably at least 10kHz, further preferably at least 20kHz, even further preferably at least 50kHz.

15. Method according to any of claims 7-14, wherein said test tip (1 ) is an indenter tip, preferably a nanoindenter tip, and wherein said measurement is an indentation measurement, preferably a nanoindentation measurement.