Strain measurement device and method for measuring mechanical strain

Abstract

A strain measurement apparatus (100) for measuring mechanical strain in a sample (1) comprises: a sample holder device (10) arranged for containing a sample (1) to be investigated; an indenter device (20) comprising an indenter tip (21) and an actuator stage (22) carrying the indenter tip (21), where the actuator stage (22) is arranged to apply a local mechanical load along a load axis z1 via the indenter tip (21) at an indentation zone (2) of a sample (1) contained by the sample holder device (10) when the sample holder device (10) is in a load application position; a confocal Raman microscopy device (30) having an imaging axis z2 and arranged for collecting at least one Raman spectrum in the indentation region (2) of the sample (1); and a computing device (40) arranged for calculating at least one strain parameter based on the at least one Raman spectrum, in which the sample holder device (10), the indenter device (20) and the confocal Raman microscopy device (30) are arranged such that the confocal Raman microscopy device (30) is able to collect the at least one Raman spectrum when the sample holder device (10) is in the load application position, and the indenter device (20) and the confocal Raman microscopy device (30) are arranged such that a load axis z1 of the actuator stage (22) and an imaging axis z2 of the confocal Raman microscopy device (30) coincide. Furthermore, a strain measurement method for measuring a mechanical strain in a sample (1) is described.

Description

Technical Field

[0001] The invention relates to a strain measurement device, which is configured for measuring mechanical strain in a sample, in particular in a laser transparent sample. Furthermore, the invention relates to a strain measurement method for measuring mechanical strain in a sample. The strain measurement is based on a combination of confocal Raman spectroscopy and indentation. Applications of the invention can be used, for example, in the field of materials science and materials research. Background Art

[0002] In this specification, the following prior art is cited to illustrate the technical background and related technologies of the present invention:

[0003] [1]AMKorsunsky.Chapter 8-Residual Stress "Measurement", in ATeaching Essay on Residual Stresses and Eigenstrains(eds Korsunsky AM).Butterworth-Heinemann(2017);

[0004] [2] W.Ecker et al. "Nanoscale evolution of stress concentrations and crackmorphology in multilayered CrN coating during indentation: Experiment and simulation", in "Materials&Design" 188, 108478 (2020);

[0005] [3] AJGLunt et al. "A review of micro-scale focused ion beam milling and digital image correlation analysis for residual stress evaluation and error estimation", published in "Surface and Coatings Technology" 283, 373-388, 2015;

[0006] [4] C. Gammer et al., “Measurement of local strain”, MRS Bulletin, 44(6), 459-464, 2019;

[0007] [5] J. Gim et al., “Nanoscale deformation mechanics reveal resilience innacre of Pinna nobilis shell”, Nat Commun, 10, 4822 (2019);

[0008] [6] A.Zeilinger et al. "In-situ Observation of Cross-SectionalMicrostructural Changes and Stress Distributions in Fracturing TiN Thin Filmduring Nanoindentation", in "Scientific Reports" 6, 22670 (2016);

[0009] [7] P. Manimunda et al., Chem. Commun, 2019, 55, 9200–9203;

[0010] [8] HC Loh et al., Commun Mater 1, 77 (2020);

[0011] [9]YBGerbig et al. "In-situ Raman spectroscopic measurements of thedeformation region in indented glasses", published in "JOURNAL OF NON-CRYSTALLINESOLIDS" vol.530, 2019, 119828;

[0012]

[10] WO 96 / 10737 A1;

[0013]

[11] RU 2 680853 C1;

[0014]

[12] WO 97 / 03346 A1;

[0015]

[13] JP 2017 146294 A; and

[0016]

[14] KR 101 783 541 B1.

[0017] Local stresses often define the performance or failure of solid materials. Therefore, studying the evolution of local stresses is an important factor in understanding the mechanical response of materials. Local stresses are determined by applied stresses, residual stresses and their interaction. While applied stresses can be measured and controlled, residual stresses are inherently difficult to detect, control or suppress. Over the past few decades, several techniques have been developed to evaluate internal strains or stresses.

[0018] Microscale focused ion beam milling and digital image correlation analysis is a destructive technique that provides direct measurement of residual strain on the sample surface. However, it is limited to evaluating residual strain and is not applicable to induced contact strain where external contact loads are present. In addition, mechanical milling will eventually destroy the area of ​​interest. In addition, digital image correlation cannot provide volumetric information and is limited to surface areas.

[0019] As non-destructive techniques, transmission electron diffraction and microscopy techniques allow the determination of local lattice strains with high spatial resolution down to a few nanometers (see, e.g., [4], [5]). However, transmission electron microscopy (TEM) requires rather thin samples, which is a disadvantage, since the sample has a given thickness and the resolution must be evaluated based on the volume that yields a single strain value. Strain relaxation effects may be induced during sample preparation and free edge effects may occur during in situ mechanical loading. Furthermore, the field of view in TEM methods is limited to a few micrometers and the technique does not support 3D characterization.

[0020] Microfocus X-ray diffraction is another known technique for real-time strain / stress measurement with micrometer-scale detection resolution [1]. However, this technique requires sophisticated measurement techniques, such as advanced synchrotron beamlines, and similar to electron diffraction techniques, X-ray diffraction has its own disadvantages in terms of size-restricted sample preparation, where the free surface of the sample can induce or release residual stress / strain on the sample [1], [2]. Similarly, the applied mechanical load may cause excessive deformation at the free edge of the sample. Therefore, the data collected from the illumination volume contains information from the deformation at the edge of the sample, which leads to unavoidable measurement errors. Finally, the transmission concept of X-ray diffraction measurement produces information from the illumination volume, rather than point-specific data. Therefore, this technique does not allow depth profiling of the volume along the illumination axis and produces a single strain value.

[0021] Raman spectroscopy is able to probe the molecular energy properties of Raman active materials. In addition to being able to characterize structural materials, the technique has also been used to measure mechanical strain / stress. This strain affects the frequency of molecular vibrations and thus the Raman spectrum through changes in peak position and width. Therefore, by measuring the position and / or width of the vibration bands of the strained sample and comparing it to the unstrained state (reference state), information about the magnitude of the mechanical strain / stress can be obtained. Compared to diffraction techniques, confocal Raman spectroscopy allows deep profiling within the penetration depth of the material. This penetration depth is a function of the laser wavelength and the sample absorption coefficient and can be in the millimeter range for laser transparent materials. Therefore, the sample thickness allows the strain / stress field to develop and propagate without free / cut edges.

[0022] Known approaches to study loaded / indented samples include post-indentation Raman spectroscopy and off-axis Raman spectroscopy under applied load. For post-indentation Raman spectroscopy, first, the sample is indented and the residual indentation is scanned after indentation. For example, an integrated indentation setup was proposed in [7], where a translation stage moves the sample from the indentation station to a separate measurement station (optical position) that is connected to the Raman spectrometer via a fiber optic cable. However, this approach does not allow real-time measurements. It does not allow the study of elastic responses (induced elastic strains) because they recover immediately after the sample is unloaded.

[0023] On the other hand, off-axis Raman spectroscopy under applied load (see, e.g., [9] to

[14] ) employs Raman spectroscopy while the sample is loaded. However, the off-axis coincidence of the incident laser with the applied load does not allow for coaxial scanning below the loading point (i.e. the sample volume cannot be measured), which means that the calculation of the strain tensor is more complicated. Furthermore, with available in situ Raman indentation equipment that includes off-axis characterization of the sample, part of the induced strain field will not be accessible under Raman microscopy because it lies in the shadow of the indenter. In addition, the tilt angle of the equipment further complicates the data analysis, since the intensity of the vibrational bands varies with the probe direction. Off-axis Raman spectroscopy has been performed on tensile / compressive samples (but not on indented samples) [8]. However, since distributed loading leads to distributed strains, this technique cannot study the strain development at the loaded region (since architectural materials such as bioceramics are not associated with the microstructural organization of the sample, which plays a key role in the development of concentrated stresses / strains).

[0024] In summary, the routine assessment of local stresses under applied loads has always been challenging as it requires a combination of two key aspects. First, the ability to measure the strain / stress field developed under the applied load and the interaction with possible residual strains / stresses in a “time-resolved” manner. Destructive techniques therefore cannot meet this requirement. Therefore, only non-destructive strain / stress measurement techniques (e.g. based on diffraction measurements) remain practical options. Second, the ability to probe the “local” strains / strains with sub-micrometer resolution. Therefore, synchrotron X-ray microdiffraction, transmission electron diffraction and microscopy, and confocal Raman microscopy [1], [2] remain the only feasible techniques for time-resolved measurements of local strains / stresses under contact loading. However, due to physical limitations, none of these techniques have been able to perform three-dimensional mapping of the induced contact strain / stress field in their known configurations so far.

[0025] Purpose of the Invention

[0026] The object of the present invention is to provide an improved strain measurement device and / or strain measurement method for measuring mechanical strain in a sample, which can avoid the limitations and disadvantages of conventional techniques. In particular, the strain measurement device and / or method can perform real-time three-dimensional mapping and avoid limitations caused by time and / or direction offsets. Summary of the invention

[0027] This object is achieved by a strain measuring device and / or a strain measuring method comprising the features of the independent claims. Advantageous embodiments and applications of the invention are defined in the dependent claims.

[0028] According to a first general aspect of the present invention, the above-mentioned object is achieved by a strain measurement device, which is configured to measure mechanical strain in a sample, and the strain measurement device comprises: a sample holder device, which is arranged to accommodate a sample to be studied; an indenter device, which comprises an indenter tip and an actuator stage carrying the indenter tip, wherein the actuator stage is arranged to apply a local mechanical load along a load axis at an indentation area of ​​the sample accommodated by the sample holder device via the indenter tip when the sample holder device is in a load application position; a confocal Raman microscopy device (preferably a confocal Raman microscope), which has an imaging axis and is arranged to collect at least one Raman spectrum in the indentation area of ​​the sample; and a computing device, which is arranged to calculate at least one strain parameter based on the at least one Raman spectrum.

[0029] According to the apparatus of the present invention, the sample holder device, the indenter device and the confocal Raman microscopy device are arranged so that when the sample holder device is in the load application position, the confocal Raman microscopy device is able to collect at least one Raman spectrum. In addition, according to the present invention, the indenter device and the confocal Raman microscopy device are arranged so that the load axis (indentation direction) of the actuator stage and the imaging axis of the confocal Raman microscopy device coincide.

[0030] According to a second general aspect of the present invention, the above-mentioned object is achieved by a strain measurement method for measuring mechanical strain in a sample, the method comprising the following steps: arranging a sample to be studied on a sample holder device; applying a local mechanical load to the sample using an indenter device, the indenter device comprising an indenter tip and an actuator stage carrying the indenter tip, wherein, when the sample holder device is in a load applying position, a mechanical load is applied via the indenter tip at an indentation area of ​​the sample along a load axis using the actuator stage; collecting at least one Raman spectrum in the indentation area of ​​the sample using a confocal Raman microscopy device having an imaging axis; and calculating at least one strain parameter based on the at least one Raman spectrum.

[0031] According to the method of the invention, at least one Raman spectrum is collected when the sample holder device is in the load application position. Furthermore, according to the invention, the indenter device and the confocal Raman microscopy device are arranged so that the load axis of the actuator stage and the imaging axis of the confocal Raman microscopy device coincide.

[0032] Preferably, the strain measurement method according to the second general aspect of the present invention or its embodiments is performed using the strain measurement device according to the first general aspect of the present invention or its embodiments.

[0033] The term "strain measurement" refers to the determination of strain, which is a measure of the deformation of a material under the action of a force, in particular an external force. The deformation leads to a change in the frequency of molecular vibrations in the material. In elastic deformation, the induced strain and stress are related. Therefore, the strain measurement device and method of the present invention can also be considered as a stress measurement device and method for measuring mechanical stress in a sample, wherein at least one stress parameter is obtained by correlation with the change in the peak width and / or position of the collected Raman spectrum.

[0034] The sample holder device is arranged in a load application position, and the sample holder device and the indenter device are arranged so that the tip of the indenter can contact and press against the sample on the sample holder device by displacement of the tip of the indenter. Preferably, the sample holder device and the indenter device have a fixed position relative to each other, so that the sample holder device is in a load application position during the entire strain measurement. The sample holder device preferably includes a support platform having a surface for placing the sample. The support platform has sufficient mechanical stability to resist any force caused by the indenter tip applying a local mechanical load on the sample. The support platform is preferably configured to directly expose an independent (thick) transparent sample through a hole (window) in the platform or to support a thin sample (<1mm) by using a transparent backplane, so that the detection light passes to collect at least one Raman spectrum. Particularly preferably, the sample holder, in particular the support platform of the sample holder, has a concentric window, which is inverted and mounted below the confocal Raman microscopy device.

[0035] The actuator stage of the indenter device is preferably arranged fixedly relative to the sample holder device and is configured to move the indenter tip relative to the sample holder device (in particular relative to a sample accommodated by the sample holder device) along a linear movement axis (preferably perpendicular to the surface of the support platform). Particularly preferably, the actuator stage comprises a piezoelectric actuator and a transducer, which has advantages in terms of the amount and time control of the applied load. Advantageously, the actuator stage can be configured for displacement of the indenter tip with a resolution step size as low as 5 nm. The indenter tip is made of a non-deformable material (e.g. a hard ceramic, such as diamond).

[0036] The mechanical load is applied to the sample by moving the indenter tip towards the sample holder device (in particular its support platform) so that the indenter tip contacts the sample and presses against the sample. By moving in the opposite direction, the load can be reduced to zero (releasing the sample). Due to the tip shape of the indenter tip, the direct application of the load is localized to the point-like contact section of the indenter tip with the sample. This contact section provides the indentation zone, and the load axis (axis of force applied to the sample) is the axis of movement of the indenter tip.

[0037] Confocal Raman microscopy devices typically include: a light source device (e.g., a laser source) for generating excitation light; an imaging optical device for directing the excitation light to the sample, focusing the excitation light at a selectable sample depth, and collecting Raman scattered light from the sample; and a spectrally resolved detector device (e.g., a spectrometer) for detecting the Raman scattered light. The imaging optical device can be configured to relay the excitation light in free space and / or in a light guide.

[0038] The imaging axis (detection direction) is provided by the direction in which the imaging optics illuminate the sample. The imaging axis coincides with the load axis, i.e. the two axes are preferably identical, or they are parallel at the load application location and at a distance such that the distance has a negligible effect on the measurement result to be obtained. The coincident axis can be provided by adjusting at least one of the imaging optics and the actuator stage. Preferably, the imaging axis is perpendicular to the surface of the support platform, in particular to the surface of a sample arranged on the support platform.

[0039] The calculation device for calculating at least one strain parameter comprises a computer circuit which is optionally contained in a control device of the strain measuring apparatus.

[0040] The arrangement of the sample holder device, the indenter device and the confocal Raman microscopy device of the present invention includes setting the sample holder device and / or the indenter device so that the sample holder device is in a load application position, that is, the advancing indenter tip can hit the sample. At the same time, the arrangement of the sample holder device, the indenter device and the confocal Raman microscopy device of the present invention includes setting the sample holder device and / or the confocal Raman microscopy device so that the imaging optical device can guide the excitation light to the sample on the support platform, in particular to the indentation area of ​​the sample, and collect the Raman scattered light from the sample, in particular the Raman scattered light from the indentation area of ​​the sample. Advantageously, when the sample holder device is in a load application position, in particular when the sample resting on the sample holder device can receive a local mechanical load (such as a static load or a dynamically changing load), at least one Raman spectrum can be collected.

[0041] Compared to conventional techniques (e.g. [7]), the technique of the present invention aligns the indentation direction (load axis) with the Raman microscope (coaxial imaging axis) and co-locates the load point and the contact area, making it possible to determine the amount (single value) and optional 1D distribution, 2D distribution or 3D distribution of the contact strain / stress induced under elastic-inelastic deformation. By Raman spectroscopy, changes in molecular vibration bands in the indentation area of ​​the sample are measured with improved accuracy. In particular, the present invention allows real-time loading and measurement of induced contact strain / stress fields with sub-micron spatial resolution in Raman active materials and preferably laser transparent and / or sufficiently thin materials. Advantageously, the use of the arrangement of the indenter device and the confocal Raman microscopy device of the present invention with coinciding load axis and imaging axis eliminates the limitations of off-axis techniques (e.g. [8]). In addition, using the arrangement of the confocal Raman microscopy device of the present invention, at least one Raman spectrum can be collected during and / or after the application of the load without moving the sample, so that strain measurement can be accurately performed in the indentation area. In particular, the recovery rate of the sample can be measured during load reduction.

[0042] According to a preferred embodiment of the present invention, the sample holder device, the pressure head device and the confocal Raman microscopy device are arranged so that the confocal Raman microscopy device can collect at least one Raman spectrum while applying a local mechanical load. In terms of the method, at least one Raman spectrum is preferably collected while applying the local mechanical load (i.e. during the load application). Advantageously, at least one strain parameter is estimated after the load is applied without being affected by the relaxation process. Alternatively or additionally, at least one Raman spectrum can be collected before and / or after applying the local mechanical load.

[0043] According to a particularly preferred embodiment of the present invention, the indenter device (in particular at least the indenter tip of the indenter device) and the confocal Raman microscopy device (in particular at least a part of the imaging optics of the confocal Raman microscopy device) are arranged on opposite sides of the sample holder device, in particular on opposite sides of the support platform of the sample holder device, so that a local mechanical load can be applied at a load point on a first side of the sample and at least one Raman spectrum can be collected in the indentation area (in particular from an opposite second side of the sample at the load point). For the method, preferably a local mechanical load is applied at a load point on a first side of the sample and at least one Raman spectrum is collected in the indentation area (in particular from an opposite second side of the sample at the load point). Advantageously, sufficient space is obtained for the indenter and the confocal Raman microscopy device (or its components) with a relative arrangement.

[0044] Applying a load and collecting Raman scattered light from the opposite side preferably includes providing a vertical orientation of the load axis and the imaging axis, applying the load from above in the direction of gravity and collecting at least one Raman spectrum from below the support platform. Alternatively, applying the load from below parallel to the direction of gravity and collecting at least one Raman spectrum from above the support platform can be provided.

[0045] According to a further preferred embodiment of the invention, the sample holder device is provided with a translation stage, which is arranged for adjusting the xy position of the sample holder device in a plane perpendicular to the imaging axis of the confocal Raman microscopy device. For the method, the xy position of the sample holder device in a plane perpendicular to the imaging axis is preferably adjusted. Advantageously, the translation stage facilitates adjustment of the sample relative to the load axis and the imaging axis, thereby allowing two-dimensional (in a plane parallel to the surface of the support platform) or three-dimensional scanning measurements by operating the translation stage.

[0046] Another particular advantage of the invention is if the confocal Raman microscopy device is configured for time-resolved collection of multiple Raman spectra, thereby allowing for studying e.g. relaxation properties of the sample. Alternatively or additionally, mapping of the sample in a range including the indentation area can be provided. Advantageously, this mapping feature provides additional reference information of the sample in areas with less strain or even no strain. For mapping the sample, an optical scanner device can be provided, which is arranged for successively focusing the excitation light to different positions within and / or outside the indentation area.

[0047] According to a further advantageous embodiment of the invention, the indenter device comprises a load sensor connected to the actuator stage and arranged for measuring the mechanical load applied to the sample. For the method, the load sensor connected to the actuator stage can be used to measure the mechanical load applied to the sample. Advantageously, the load sensor allows determining a correlation of at least one collected Raman spectrum and / or at least one strain parameter with the amount of load applied to the sample. Furthermore, the load sensor can be used for load control and / or can apply the load according to a predetermined time function, thereby allowing calculating at least one strain parameter according to a given test protocol.

[0048] Advantageously, various features of the indenter tip can be used to improve strain measurement. Preferably, the indenter tip has a distal contact section exposed for contacting the sample and has a size of less than 1 μm. Alternatively, larger sizes are also feasible, such as 1 μm to 1 mm or to 3 mm or even to 5 mm. Due to the small size of the contact section, the application of high external pressure and position-resolved measurement can be improved in an advantageous manner. Alternatively or additionally, the indenter tip can be replaceable. Therefore, the measurement setting can be conveniently adapted to a specific sample. Alternatively or additionally, the indenter tip can be a diamond tip. Diamond has a high hardness, so that it allows measuring almost all sample materials of interest. Other indenter tip materials can include, for example, zirconium oxide, sapphire, ruby ​​or tungsten carbide. Alternatively or additionally, the indenter tip can be a conical spherical tip. The conical spherical shape is conducive to uniform load application and the development of strain in the sample. Alternatively, the indenter tip can have a tip geometry including a cubic corner or a Bosch geometry to further locate stress concentrations or deform the sample part in an inelastic manner. Other tip geometries may include, for example, Vickers geometry or flat punch geometry.

[0049] The tip can be replaced to install one of different tip geometries and / or materials. The tip size and contact radius can also vary. In practice, the choice of tip depends on the measurement conditions, such as sample size, sample hardness or sample structure. For example, the radius of a conical spherical tip can be from 300nm to several millimeters, while the tip size of a cube corner tip can be about 70nm to 100nm, and the tip size of a Bosch tip can be about 120nm.

[0050] The features disclosed in the strain measurement device and its embodiments also represent preferred features of the strain measurement method of the present invention and its embodiments. The above aspects and preferred features of the present invention, in particular regarding the configuration of the device and the size and composition of the various components associated with the device, also apply to the method. The above preferred embodiments, variants and features of the present invention can be combined with each other as required. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Further advantages and details of the invention will be described below with reference to the accompanying drawings, which schematically show:

[0052] Figure 1 The features of a strain measurement device according to a preferred embodiment of the present invention are shown;

[0053] Figure 2 and Figure 3 Shows Figure 1 Further features of the strain measuring device;

[0054] Figure 4 shows a scanning configuration using a strain measurement method according to a preferred embodiment of the present invention;

[0055] Figure 5 Experimental results of in situ Raman indentation studies in point measurement mode under incremental load are shown;

[0056] Figure 6 shows experimental results of in situ Raman indentation studies in 3D mapping mode; and

[0057] Figure 7 Experimental results of in situ Raman indentation in 2D mode under incremental load at the same location are shown. DETAILED DESCRIPTION

[0058] Embodiments of the present invention will be described below with specific reference to the arrangement of the sample holder device, the indenter device and the confocal Raman microscopy device of the in-situ strain measurement apparatus and the operation of the above-mentioned devices. The present invention is preferably implemented using a confocal Raman microscope (such as a confocal Raman microscope known per se for strain measurement). Therefore, with respect to the prior art, the details of the confocal Raman microscope, its available control schemes and available schemes for Raman signal acquisition will not be described.

[0059] Calculating at least one strain parameter is preferably performed based on at least one measured load (as known per se from conventional Raman-based strain measurements) and at least one collected Raman spectrum. In particular, it comprises signal evaluation of at least one Raman spectrum, correlating features of at least one Raman spectrum (such as band peak positions and / or bandwidths and / or variations thereof) with deformation quantities (e.g., indenter tip displacement), and calculating at least one stress or strain parameter analytically or numerically from the load and deformation quantities. Calculating at least one stress or strain parameter may comprise calculating a single stress or strain parameter (such as a local strain) as a single value, or calculating multiple stress or strain parameters as a mapping and / or time function.

[0060] Typically, the sample comprises a solid Raman-active material, such as a ceramic or a plastic. With reference to a plate-like sample in an exemplary manner, the sample is preferably transparent in the wavelength range of the confocal Raman microscope, in particular at the wavelength of the detection light of its light source and the wavelength of the Raman scattered light. The wavelength range of the confocal Raman microscope extends, for example, from about 400 nm to 1.5 μm. The thickness of the transparent sample ranges, for example, from 10 μm (supported by a laser transparent backplane such as quartz) to 5 mm (freestanding). It should be noted that optical transparency is not a necessary feature of the sample. In the absence of optical transparency, strain measurements may be limited to thin samples (e.g., tooth enamel) with a thickness range of, for example, 10 μm to 100 μm.

[0061] Figure 1 A side view of an embodiment of a strain measurement device 100 for measuring mechanical strain in a sample 1 is schematically shown. The strain measurement device 100 comprises a sample holder device 10, an indenter device 20 having an indenter tip 21 and an actuator stage 22, a confocal Raman microscope 30 and a computing device 40, which is included in a control device 50 (e.g., a control computer). The load axis z1 of the indenter device 20 (in particular the indenter tip 21 of the indenter device 20) and the imaging axis z2 of the confocal Raman microscope 30 extend together along a vertical z-axis. The sample holder device 10 is suitable for arranging the sample in a plane perpendicular to the load axis and the imaging axis, in particular in a horizontal xy plane. Figure 2 Additionally shown are apparatus for adjusting the position of the sample holder device 10 and the indenter device 20 relative to the confocal Raman microscopy device 30 and / or for scanning a sample to measure a sample image (see Figure 4 )'s preferred translation stage 60. Figure 3 Details of the sample holder device 10 are shown in the top view of FIG.

[0062] The sample holder device 10 comprises a support platform 11 extending in a horizontal xy plane for accommodating the sample 1. The support platform 11 is mounted on a support plate 13 via four support columns 12 (see Figure 1 and Figure 3 ). Preferably, the support platform 11 includes an adjustable sample holder frame 14 having a central laser detection window 15 for passing detection light and collecting at least one Raman spectrum. The sample 1 can be fixed on the surface of the sample holder frame 14. The position of the sample holder frame 14 in the xy plane can be adjusted via a sample adjustment knob 16. With the sample 1 arranged in the sample holder frame 14, the sample holder device 10 is in a load application position, wherein the clamped sample 1 will be exposed to the tip of the indenter (from the lower surface) and the light / laser (from the upper side). The sample adjustment knob 16 is configured to position the sample 1 relative to the load axis z1 and the imaging axis z2. The translation stage 60 can provide a scanning movement of the sample in the xy plane. Therefore, by operating the translation stage 60 in the xy plane, the contact area / region in the sample 1 can be scanned through the light / laser detection window 15, and by operating the translation stage 60 in the z direction and / or by moving the focus of the confocal Raman microscopy device 30 along the z axis, the measurement depth in the sample 1 can be scanned.

[0063] The indenter device 20 includes an indenter tip 21, an actuator stage 22, and a load sensor 23. The indenter tip 21 has a conical spherical rigid tip section 21A made of, for example, diamond (see Figure 4 ) and a tip support body segment 21B. This arrangement allows the replacement of different indenter tips having different sizes and geometries, which are suitable for forming and applying different stress fields. The actuator stage 22 is a piezoelectric actuator, which is supported by a support plate 13 and / or a translation stage 60, and is capable of applying a displacement with a minimum step size of, for example, 5 nm and a maximum applied load of, for example, 20 N. The load sensor 23 is arranged between the actuator stage 22 and the tip support body segment 21B for measuring the load applied to the sample 1. Specifically, the load sensor 23 can have a reading resolution of, for example, 0.01 N and a maximum load of, for example, 20 N, and the load sensor 23 is connected to the handle of the piezoelectric actuator. The load sensor 23 is connected to the control device 50 for analyzing the load sensor output, and optionally for providing loop control of the actuator stage 22 based on the load sensor output.

[0064] The confocal Raman microscope 30 preferably includes a microscope body 34 having a vibration-damping microscope base plate 35, for example, made of granite ( Figure 2), and the confocal Raman microscope 30 is provided with a light source device 31, an imaging optical device 32 and a detector device 33. The light source device 31 is, for example, a laser (λ=488nm, 532nm, 633nm or 785nm). The detector device 33 is connected to a computing device 40. The confocal Raman microscope 30 is, for example, a confocal Raman microscope (e.g., WITec Alpha 300R of the German manufacturer WiTec). Preferably, the confocal Raman microscope 30 is provided with a conventional confocal microscope imaging section, thereby allowing microscopic imaging of the sample for visual monitoring of the sample.

[0065] The translation stage 60 is fixedly arranged on the microscope base plate 35 to support the sample holder device 10, adjust the position of the sample holder device 10 on the xy plane, and adjust the load axis z1 of the indenter tip 21. In addition, the translation stage 60 is configured to adjust the position of the sample holder device 10 along the z direction, that is, to adjust the focal position of the confocal Raman microscope 30 along the z direction in the sample 1. The adjustment can be performed by a motorized actuator and / or a manually driven micrometer screw gauge 61.

[0066] The indenter device 20 is inverted and mounted below the confocal Raman microscope 30, and the sample 1 is located between the indenter device 20 and the confocal Raman microscope 30 (see Figure 1 ) for applying a given load (applied by the displacement of the indenter tip 21) to the sample 1. The load causes a change in the vibration band of the Raman signal collected by the confocal Raman microscope 30 (in particular in at least one Raman spectrum of the sample). The change will be filtered and mapped to evaluate the induced elastic strain / stress of the sample 1 by the computing device 40. Since the indenter device 20 and the confocal Raman microscope 30 are arranged with coinciding load axes z1 and imaging axes z2, the confocal Raman microscopy device 30 is able to collect at least one Raman spectrum without interference when the sample holder device 10 is in the load application position and even during the application of the load.

[0067] As an alternative to the illustrated embodiment, the arrangement of the indenter device 20 and the confocal Raman microscope 30 can be reversed, wherein the confocal Raman microscopy device 30 is mounted upside down below the indenter device 20 and the sample 1 is located between the indenter device 20 and the confocal Raman microscope 30, or the coinciding load axis and imaging axis can be tilted relative to the vertical z-axis.

[0068] For strain measurement of the present invention, the strain measurement device 100 is preferably operated as follows. Prior to strain measurement, the xy position of the indenter tip 21 can be adjusted using the micrometer screw gauge 61 of the translation stage 60 (see Figure 2). After the sample 1 is mounted on the support platform 11 of the sample holder device 10, the region of interest (ROI) can be selected using the sample adjustment knob 16 while the sample 1 is monitored using the microscope imaging section of the confocal Raman microscopy device 30. Subsequently, the position of the sample can be fixed using the sample holder clamping screws 17 (see Figure 3 As a preliminary reference measurement, the vibration bands of the molecular structure of the sample under no-load conditions can be acquired using a confocal Raman microscope 30 before indentation. The characteristics of the vibration bands can be used as a reference for subsequent strain measurements.

[0069] The strain measurement includes the step of applying a local mechanical load to the sample 1 using the indenter tip 21 of the indenter device 20. The indenter tip 21 is displaced until it contacts the sample 1 and forms an indentation area 2 (see Figure 4 ). With further displacement of the indenter tip 21, a load is applied to the sample 1, causing elastic and / or inelastic deformation at the indentation area 2 (ie, in the direct contact area and adjacent areas of the sample 1).

[0070] Furthermore, the strain measurement comprises the step of collecting at least one Raman spectrum in the indentation region 2 using the confocal Raman microscopy device 30. While the load is applied, the confocal Raman microscopy device 30 is used to collect at least one Raman spectrum. Calculating at least one strain parameter from the at least one Raman spectrum comprises filtering the collected at least one Raman spectrum of the detection region (e.g., for selected peak positions and / or bandwidths), and correlating the collected at least one Raman spectrum with the indentation data (load, displacement and / or contact geometry) using the computing device 40. The correlation between the features of the at least one Raman spectrum and the induced strain / stress results in at least one strain parameter to be obtained.

[0071] By collecting and evaluating individual spectra, a 0-dimensional map of the sample is obtained, resulting in a single strain value (see Figure 4 A). Preferably, when scanning the indentation area 2 by operating the translation stage 60, a plurality of Raman spectra are collected to obtain the vibration bands of the molecular structure of the sample 1 in and / or around the indentation area 2 with local resolution. Thus, a 1D map, a 2D map or a 3D map of the sample can be obtained, such as Figure 4 B to Figure 4 D schematically shows line scan, area scan and volume scan. Figure 4 The line scan of B preferably provides a deep profile of the strain in the sample. Alternatively, other directions of the line scan can be set. Additionally, as Figure 4 A to Figure 4 The mapping shown in D can be performed with time resolution, for example by repeatedly collecting and evaluating Raman spectra.

[0072] In summary, in order to perform time-resolved three-dimensional mapping of local strain / stress using a confocal Raman microscope 30, two key elements are combined. First, a pointed micro- or nano-indenter (indenter tip 21) with a controllable displacement or load forms a local contact stress field. This task cannot be accomplished by a conventional micro-tension or bending tester because the applied stress field is not local and / or cannot be fully accessed by the laser used to scan the stress / strain zone. Secondly, the confocal Raman microscope 30 and the computing device 40 provide at least one strain parameter, preferably with spatial resolution, for mapping the indentation area. Combining the Raman microscopy device 30 with a translation stage 60 advantageously provides sub-micron scanning resolution in the x-, y-, and z-directions.

[0073] Optionally, the detection spatial resolution can be increased by selecting an objective lens of the imaging optics 32 with a larger numerical aperture (NA) and a laser with a shorter wavelength (λ). Thus, for example, using a 532 nm laser of the light source device 31 and an objective lens with NA=0.9, the spatial resolution (0.61*λ / NA) can be reduced to 360 nm. Furthermore, in order to detect changes in the vibration bands of the sample 1 exposed to external forces, the detection device 33 of the confocal Raman microscope 30 should be provided with a spectrometer, preferably with high resolution (<1 cm) in the Raman shift. -1 This resolution can be adjusted, for example, by choosing a high grating number (>1800 g / mm) and a large focal length (>300 mm), which is achievable in available confocal Raman microscopes.

[0074] For example, by using the piezoelectric actuator of the actuator stage 22 to apply and control the displacement of the indenter tip 21, through a diamond tip with a known geometry, and using the load sensor 23 to record the resulting load, the Hertzian contact stress σ and the indentation strain ε applied at the contact point can be obtained from the following equations:

[0075]

[0076]

[0077]

[0078] Where P is the load, a is the contact radius, R is the radius of the indenter tip 21, and h is the applied contact depth (displacement into the sample 1). By collecting the Raman spectrum from the loaded sample 1 and comparing it to the spectrum of the unloaded state, any changes in the vibration band can be realized and can be used to correlate the induced peak displacement and the applied contact stress. Specifically, the contact stress can be calculated using the above equation based on the measured load, known radius, and contact depth derived from the induced peak displacement.

[0079] The application of the invention is not limited to determining the Hertzian contact stress as a strain parameter. Alternatively, any correlation between the applied force or pressure and the changes in the scanned vibrational band of the Raman-active material can be calculated, in particular in real time and / or in a time-resolved manner.

[0080] Through actual testing, the inventors have used the strain measurement device 100 and method of the present invention to study the strain / stress and Raman peak shift caused by a series of point measurements, area scans and 3D mapping (stacked area scans to obtain volume data) on a geological fluorapatite sample (thickness: 1 mm) under different contact loads. For example, Figure 5 The test results of in-situ Raman indentation in point measurement mode under incremental loads (0 to 170 g) are shown. Figure 5 A, Raman peaks extracted for the ν1 phosphorus vibration band of the phosphate group of fluorapatite at the contact point in the indentation area showing the changes in peak position and width. Figure 5 B shows the measured load associated with the applied displacement of the indenter tip 21. The linear behavior of the extracted load-displacement curve indicates the elastic response of the sample. Figure 5 As shown in C, the correlation between the Raman peak change (induced peak shift) and the applied displacement (contact depth) can be obtained. This correlation is used to calculate the strain parameter (Hertzian contact stress)

[0081] Using further tests, such as Figure 6 As shown, in situ Raman indentation in mapping (3D) mode at a constant load of 1 N or 1.45 N allows the characterization of stress distribution around the indentation area and / or at different depths in a fluorapatite sample. Figure 6 A and Figure 6 B, shows the spatial distribution of indentation stress and indentation strain in the sample under the action of the indenter tip with a radius of 125 μm, while Figure 6 C shows a cross-sectional view of the sample in plane i.

[0082] Furthermore, using the in situ capability of the method and implementing load-sweep cycles, the evolution of mechanical strain in calcite samples was revealed ( Figure 7 ). In this study, a series of images at the same location in the xy plane (2D scan) under defined applied contact loads P = 84 mN, P = 147 mN, and P = 0 (scan after removal of the 147 mN load) represent the elastic (1), elastic-inelastic (2), and residual strain (3) states, respectively.

[0083] The features of the invention disclosed in the above description, drawings and claims are of great significance for the realization of the invention in its various embodiments, whether alone or in combination or sub-combination. The invention is not limited to the preferred embodiments described above. On the contrary, there are many variations and derivative forms, which also use the concept of the invention and therefore fall within the scope of protection. In addition, the invention also claims protection for the subject matter and features of the dependent claims, regardless of the features and claims to which they refer.

Claims

1. A strain measurement device (100) configured to measure mechanical strain in a sample (1), the strain measurement device comprising - a sample holder device (10) arranged to receive a sample (1) to be investigated, - an indenter device (20) comprising an indenter tip (21) and an actuator stage (22) carrying said indenter tip (21), wherein The actuator stage (22) is arranged to apply a local mechanical load along a load axis (z1) via the indenter tip (21) at an indentation area (2) of a sample (1) accommodated by the sample holder device (10) when the sample holder device (10) is in a load application position, - a confocal Raman microscopy device (30) having an imaging axis (z2) and arranged to collect at least one Raman spectrum in an indentation area (2) of said sample (1), and - a calculation device (40) arranged for calculating at least one strain parameter based on the at least one Raman spectrum, Features - the sample holder device (10), the indenter device (20) and the confocal Raman microscopy device (30) are arranged such that when the sample holder device (10) is in a load application position, the confocal Raman microscopy device (30) is capable of collecting the at least one Raman spectrum, and The indenter device (20) and the confocal Raman microscopy device (30) are arranged such that a load axis (z1) of the actuator stage (22) and an imaging axis (z2) of the confocal Raman microscopy device (30) coincide.

2. The strain measurement device according to claim 1, wherein The sample holder device (10), the indenter device (20) and the confocal Raman microscopy device (30) are arranged such that the confocal Raman microscopy device (30) is capable of collecting the at least one Raman spectrum while applying the local mechanical load.

3. The strain measuring device according to claim 1, wherein - The indentation device (20) and the confocal Raman microscopy device (30) are arranged on opposite sides of the sample holder device (10) so that the local mechanical load can be applied at a load point on a first side of the sample (1) and at least one Raman spectrum can be collected in the indentation area (2), in particular from an opposite second side of the sample (1) at the load point.

4. The strain measuring device according to claim 1, wherein - the sample holder device (10) is provided with a translation stage (60) arranged for adjusting the xy position of the sample holder device (10) in a plane perpendicular to the imaging axis (z2) of the confocal Raman microscopy device (30).

5. The strain measuring device according to claim 1, wherein - the confocal Raman microscopy device (30) is configured for at least one of: time-resolved collection of a plurality of Raman spectra and mapping of the sample (1) in a range including the indentation region (2).

6. The strain measuring device according to claim 1, comprising at least one of the following features: - the indenter tip (21) has a distal contact section exposed for contacting the sample (1), - the indenter tip (21) is replaceable, - the indenter tip (21) is a diamond tip, a sapphire tip or a ruby ​​tip, - the indenter tip (21) is a conical spherical tip, and - The indenter tip (21) has a tip geometry comprising a cube corner or a Bohr geometry or a Vickers geometry or a flat punch geometry.

7. The strain measuring device according to claim 1, comprising at least one of the following features: - the actuator stage (22) comprises a piezoelectric actuator and a transducer, - the indenter device (20) comprises a load sensor (23) connected to the actuator stage (22) and arranged to measure a mechanical load applied to the sample (1), and - The sample holder device (10) has a concentric window which is mounted upside down below the confocal Raman microscopy device (30).

8. A strain measurement method for measuring mechanical strain in a sample (1), comprising the following steps - arranging the sample (1) to be investigated on the sample holder device (10), - applying a local mechanical load to the sample (1) using an indenter device (20), the indenter device comprising an indenter tip (21) and an actuator stage (22) carrying the indenter tip (21), wherein: When the sample holder device (10) is in the load application position, a mechanical load is applied along a load axis (z1) at the indentation area (2) of the sample (1) via the indenter tip (21) using the actuator stage (22), - collecting at least one Raman spectrum in the indentation region (2) of said sample (1) using a confocal Raman microscopy device (30) having an imaging axis (z2), and - calculating at least one strain parameter based on said at least one Raman spectrum, Features - collecting said at least one Raman spectrum when said sample holder device (10) is in a load application position, and The indenter device (20) and the confocal Raman microscopy device (30) are arranged such that a load axis (z1) of the actuator stage (22) and an imaging axis (z2) of the confocal Raman microscopy device (30) coincide.

9. The strain measurement method according to claim 8, - collecting said at least one Raman spectrum simultaneously with the step of applying said local mechanical load.

10. The strain measurement method according to any one of claims 8 to 9, - collecting said at least one Raman spectrum before and / or after the step of applying said local mechanical load.

11. The strain measurement method according to any one of claims 8 to 10, - applying the local mechanical load at a load point on a first side of the sample (1), and collecting the at least one Raman spectrum at the indentation area (2), in particular collecting the at least one Raman spectrum from an opposite second side of the sample (1) at the load point.

12. The strain measurement method according to any one of claims 8 to 11, comprising: - adjusting the xy position of the sample holder device (10) in a plane perpendicular to the imaging axis of the confocal Raman microscopy device (30).

13. The strain measurement method according to any one of claims 8 to 12, comprising at least one of the following: - Time-resolved collection of multiple Raman spectra, and - Mapping the sample (1) in a region including the indentation area (2), preferably three-dimensional mapping.

14. The strain measurement method according to any one of claims 8 to 13, further comprising - Measuring the mechanical load applied to the sample (1) using a load cell (23) connected to the actuator stage (22).

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