rus
Issue #7-8/2022
V.N.Reshetov, I.V.Krasnogorov, V.V.Solovyov, E.V.Gladkikh, A.S.Useinov
EQUIPMENT FOR INSTRUMENTED NANOINDENTATION – PRINCIPLES OF OPERATION AND DESIGN FEATURES
EQUIPMENT FOR INSTRUMENTED NANOINDENTATION – PRINCIPLES OF OPERATION AND DESIGN FEATURES
INTRODUCTION
The instrumented indentation method has evolved since the middle of the twentieth century as a further evident development of the microindentation method. The key point of this new approach was the abandonment of visual control of the size of the imprint and the extraction of all the necessary information about hardness from the measured dependence of the indenter depth on the pressing force. At the same time, due to the possibility to control both the force and the depth of indentation during the loading and unloading stages, it became possible to measure not only hardness, but also the Young’s modulus of the material under test [1, 2]. Today, this method has become universally accepted and forms the basis of a number of international standards [3, 4].
In this method, there is no limitation inherent to optical microscopy on the minimum indentation size used to measure the hardness of the material. Since only depth and force data are used to determine mechanical properties, the factor limiting the indentation depth and contact force were the noise of the nanoindenter measuring system and the degree of sharpness of the diamond point used. Indenters for instrumented indentation are usually made in the form of a Berkovich type triangular pyramid which ensures self-similarity of the indenter in the maximum possible range of indentation sizes.
NANOINDENTER MACHINE DESIGN PRINCIPLES
Achievable indentation depths have now fallen below 10 nm and print sizes have become less than 100 nm, with a stable radius of curvature of the tip of the indenter pyramid less than 30 nm. The resolution of the measuring systems in terms of displacement and force is fractions of nm and μN, the maximum depths of immersion and indenting force are hundreds of μm and units of N. As a rule, a set of indenting modules is used to implement the specified dynamic range in terms of depths and forces.
When working in the nanoscale with normal forces of less than 50 mN and indentation depths up to 10 microns, electrostatic actuators combined structurally with a capacitive displacement sensor are most often used [5]. The most popular in the micron range is a scheme with an electromagnetic actuator and a differential capacitor as a displacement sensor [6]. There are variants of the indentation module using a piezoceramic actuator and a capacitive displacement measurement circuit [7].
The main arguments in favor of a particular indentation module design are the minimum temperature drift of the indentation depth measurement system and the maximum range of loads used. Purely capacitive and piezoceramic systems have the advantage in terms of temperature drift caused by the actuator. The electromagnetic actuator, by virtue of its operating principle, cannot fail to be warmed up by the contact force of the indentor. The thermal power released in the moving coil of the actuator is proportional to the square of the electric current, and the force developed by the actuator is proportional to the first degree of the current. Consequently, heating, and hence thermal expansion, is most pronounced when working with maximum indentation forces. The use of capacitive sensors in the form of a differential capacitor with a moving middle plate has de facto become the standard for tool indentation. Such sensors are not heat sources during their operation, create minimal force impact on the moving system, provide a low threshold level of the registered signal and sufficiently high linearity in the penetration depth.
Devices implementing the instrumented indentation test are often referred to as nanoindenters and are used with approximately the same precautions as atomic force and scanning tunneling microscopes. Nanoindenters are placed on vibration-isolating platforms and put inside thermally insulated boxes. As a rule, it is the level of seismic noise and temperature fluctuations in the room that are the main factors limiting the accuracy of measurements made and the minimum level of force during indentation.
To reduce the influence of vibration noise, the mass of the moving elements of the device related to the diamond indentor is to be minimized. Increasing the rigidity of the suspension of the moving elements, increasing the resonance frequency of the device and, as if reducing the influence of seismic noise, does not lead to improvement of metrological characteristics of the nanoindenter, since, against the background of increased rigidity of the indenter suspension system, the contact area stiffness, which characterizes the hardness and Young’s module of the tested material, becomes less noticeable, especially at small indentation depths. As a result, the measurement accuracy of the contact interaction force between the indenter and the material under test and the quality of the load-depth plunge curve decrease.
The performance specifications given in the instrument descriptions in terms of minimum loads and indentation depths generally correspond to the conditions of minimal natural seismic noise, absence of industrial seismic interference and the use of a good vibration isolation system. The actual noise level on the force and displacement channels depends on the specific operating conditions of the instrument, and the manufacturer labels it as "lab dependent". The digital resolution of the force and displacement channel is usually an order of magnitude less than the declared "lab dependent" noise level.
DESIGN AND COMPOSITION FEATURES OF NANOSCAN-4D
Mechanical loading system
The detailed analysis of design and principles of work of the indenting module and electronics control will be carried out on an example of nanoindenter NanoScan-4D. This device is registered in the State Register of measuring instruments as a nanohardness tester under № 65496-16 and allows to perform the full range of techniques provided by the standards [8–10].
At development NanoScan-4D experience of work with foreign and domestic devices for instrumented indentation was taken into account and their advantages and disadvantages were studied. As a result of the analysis of the obtained results, a scheme with a paired electromagnetic actuator and a capacitive displacement sensor was chosen, Fig.1.
The use of an electromagnetic actuator in the form of a coil located in a cylindrical gap with an axial magnetic field made it possible to indent with loads over 2 N. This kind of actuators are actively used in acoustic systems and the technology of their production is well proven. They effectively convert current into force, are operable over a wide temperature range, are stable and have a linear response over the entire range of operating motions and forces.
A mechanical scheme was chosen that included two actuators that apply a joint force to the rod, on which a capacitive sensor and a diamond indenter are fixed. Using two actuators not only doubled the maximum force of indentation (with twice less thermal power), but also significantly simplified the control algorithms of the whole process of tool indentation. Two independent channels of force application simplify the procedure of indenting the surface, taking a stable load-depth curve, facilitate implementation of complicated measurement algorithms such as multiple indentation and dynamic measurements when loading is performed by an indentor oscillating with small amplitude. The device can be equipped with a spherical tip that allows reconstructing the stress-strain diagram in the partial-load loading mode [11]. Studying the properties of heterogeneous materials, such as steel samples that have been irradiated with heavy ions, can be carried out by applying dynamic instrumented indentation [12]. Examples of such measurements are presented in Fig.2.
When mapping the mechanical properties of the sample, one of the attenuators is used to compensate for large-scale surface topography roughness, while the second attenuator performs indentation with a minimum and stable control current, thus minimizing the thermal effect associated with the operation of the electromagnetic actuator.
The use of a carbon shaft also helps to reduce internal thermal drifts. This solution not only reduces thermal drift and the mass of the moving system, but also increases the bending stiffness of the shaft, which is important during sclerometry testing of samples. The transverse and bending stiffness is also increased by placing the membranes holding the shaft at a distance from the capacitive probe and the diamond indenter.
The symmetrical arrangement of the main elements of the indenting module design allows the module to be operated both horizontally and vertically. The presence of two independently controlled actuators makes it possible to programmatically correct the initial position of the indenter taking into account the direction and magnitude of gravity acting on the movable system of the indenting module. For the movable mass of the shaft with all the elements fixed to it equal to 40 g and the total stiffness of the two diaphragms of 10,000 N/m, the displacement of the middle pad of the capacitive sensor when the indentation module is turned upside down will be 80 µm. To compensate such displacement, one of the actuators must generate a force of 0.8 N, which is quite a lot and excludes the use of the same actuator to perform indentation in the load range of hundreds of μN. At a digit capacity of the used DAC of 18 bits and the maximum load of 2 N one bit will correspond to 4 μN, which is a little bit rough for work in the nanoscale range. The presence of the second actuator allows to use it in the mode of small loads, for example, up to 10 mN, and in this case one bit of DAC will correspond to 0,04 μN, which allows to conduct full measurements on any of the methods of instrumented indentation of the load range up to 10 mN.
The high transverse stiffness of the working shaft allows to realize an extension of the indentation force range up to 50 N. Schematic representation of the module for increasing the normal load on the indenter is shown in Fig.3. The use of such a module makes it possible to compare hardness data obtained by instrumented indentation and by micro-indentation, where hardness is determined by dividing the indentor pressing force by the area of the recovered indentation imprint measured with an optical microscope, in a timely manner using the same indenter.
In this load extension module, the role of the force actuator is performed by a linear actuator with a stepper motor, the force sensor is a strain gauge transducer, and the indentation depth sensor is a standard differential capacitor of nanoindenter with a movable middle pad.
Capacitive transducer schematics
Capacitive displacement sensors are actively used in a wide variety of measurement systems – capacitor microphones, seismic sensors, touch screens and security alarms. Traditionally, in precision measuring systems, they operate in differential mode, with the stationary terminals supplied by an anti-phase high-frequency voltage and synchronous detection of the signal taken from the middle moving terminal. Such a scheme allows measuring displacement of the moving shutter with a resolution of a hundredth of a nm in the registration bandwidth of hundreds of Hz. However, if the magnitude of displacement becomes commensurate with the working gap of the differential capacitor, the nonlinearity of the circuit increases dramatically. As a consequence, when using it in a nanoindenter displacement sensor, one has to make the working gaps of the differential capacitive sensor an order of magnitude greater than the working depths of indentation, in order to ensure nonlinearity at the level of 0.01%.
In NanoScan-4D devices along with such electronics scheme of a differential capacitor a modernized scheme is used, which allows providing the required level of linearity at a significantly smaller working gap of a differential capacitor, i.e. at a better threshold sensitivity for traditional inclusion. The linearity of the dependence of the output signal on the displacement of the moving pad of the differential capacitor is ensured by a modification of the supply circuit of the stationary pads, in which the incoming voltages on them are directly proportional to the working gap, Fig.4.
The voltage required for such compensation is formed by an analog multiplier, one of whose inputs receives the reference AC voltage, and the second one receives the integrated output signal of the synchronous detector. The applied circuit solution made it possible to use the capabilities of the differential capacitive sensor as effectively as possible, providing a low noise level when it is switched on traditionally and a high linearity when it is used in the negative excitation voltage feedback mode.
Methods of the dynamic range expansion
The control system of electromagnetic actuators used in nanoindentors also has a number of features. Since the coil of the actuator is made of copper wire, its resistance increases with increasing temperature, and as a consequence, when the ambient temperature changes or the coil is heated during operation, the coefficient linking the voltage applied to the coil and the force it generates will change. Therefore, the control circuit of such an actuator is built in the form of a current generator, Fig.4. In this case, the force channel calibration depends only on the magnetic field value in the working gap, and this value is practically independent of the external ambient temperature. Thus, it is guaranteed that the calibration of the force channel in the entire operating temperature range of the indentation module from –20 °С to +60 °С with an accuracy no worse than ±5%, and in the range from +15 °С to +30 °С no worse than ±1%. When working in a voltage generator mode, due to the change of resistance of the actuator winding (temperature coefficient of resistance of copper wire ~ 0,003 °C-1 ), the multiplier for DAC code conversion to force would fall by 30% at temperature change from –20 °С to +60 °С.
The full range of variation of normal force is divided into subranges with a maximum force of 3N, 0.3N and 30mN, by changing the parameters of the current generator that supplies the coils of the electromagnetic actuator. Displacements are measured in the same way with a breakdown into subranges of 300 microns, 100 microns, 30 microns and 10 microns. The two upper two ranges are implemented in the negative excitation voltage feedback mode, and the two most sensitive ones in the mode of an ordinary differential capacitor with a constant voltage on the fixed pads. Accordingly, the minimum value of digital displacement resolution is 0.05 nm, which is significantly less than the typical seismic response of a moving nanoindenter suspension system. The bandwidth processed by the software is from 0 Hz to 10 kHz. Digitization of data and generation of signals by the embedded microprocessor software is performed with frequencies up to 300 kHz and 18-bit digital resolution.
Research conducted during the development of the NanoScan-4D showed that the requirements to the linearity of the suspension system of the working shaft imply the development of flat springs of a membrane type with a special pattern of elastic elements, providing deviation from linearity in dependence on the force displacement not more than 0.01% over the whole range of working movements, which is hundreds of microns.
The suspension system and its time and temperature stability are key parameters affecting the quality of the load-depth relationship measured. In this regard, it turned out to be extremely important to ensure the same coefficient of thermal expansion of the flat spring material and the indentation module support structure. This is especially strong when working with a thermal stage and making measurements at increased and decreased temperatures, when the device is located in a climatic chamber [13]. The difference in the thermal expansion coefficient of the flat spring and the supporting structure leads to an additional tension or compression of the flat spring and as a consequence not only the resonance frequency, but also the zero position of the displacement sensor is an order of magnitude greater than would be expected with a simple thermal expansion of the portion of the stem near the hot region of the thermal stage [9].
CONCLUSIONS
The principles of modern nanoindenter design considered in this work demonstrate the high performance of the instrumented indentation method as well as its informativeness for localized studies. These devices, providing the possibility to study mechanical properties with a spatial resolution better than 100 nm and about 10 nm in depth, allow to work with heterogeneous materials and thin functional coatings, mapping the value of hardness.
This class of instruments is de facto becoming the accepted standard for mechanical testing, gradually replacing micro-hardness testers not only from the field of scientific research, but also actively penetrating into the field of industrial diagnostics. Nevertheless, the equipment for instrumented nanoindentation is a complicated hardware-software complexes, carrying out precision measurements of small signals on the sensitivity limit of primary transducers, included in their design. For correct operation of such measuring devices, for obtaining reliable results about investigated objects, as well as for providing metrological basis for such measurements, all paths through which the primary measured signal passes, should be optimized and coordinated by their resolution, linearity and noise.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The instrumented indentation method has evolved since the middle of the twentieth century as a further evident development of the microindentation method. The key point of this new approach was the abandonment of visual control of the size of the imprint and the extraction of all the necessary information about hardness from the measured dependence of the indenter depth on the pressing force. At the same time, due to the possibility to control both the force and the depth of indentation during the loading and unloading stages, it became possible to measure not only hardness, but also the Young’s modulus of the material under test [1, 2]. Today, this method has become universally accepted and forms the basis of a number of international standards [3, 4].
In this method, there is no limitation inherent to optical microscopy on the minimum indentation size used to measure the hardness of the material. Since only depth and force data are used to determine mechanical properties, the factor limiting the indentation depth and contact force were the noise of the nanoindenter measuring system and the degree of sharpness of the diamond point used. Indenters for instrumented indentation are usually made in the form of a Berkovich type triangular pyramid which ensures self-similarity of the indenter in the maximum possible range of indentation sizes.
NANOINDENTER MACHINE DESIGN PRINCIPLES
Achievable indentation depths have now fallen below 10 nm and print sizes have become less than 100 nm, with a stable radius of curvature of the tip of the indenter pyramid less than 30 nm. The resolution of the measuring systems in terms of displacement and force is fractions of nm and μN, the maximum depths of immersion and indenting force are hundreds of μm and units of N. As a rule, a set of indenting modules is used to implement the specified dynamic range in terms of depths and forces.
When working in the nanoscale with normal forces of less than 50 mN and indentation depths up to 10 microns, electrostatic actuators combined structurally with a capacitive displacement sensor are most often used [5]. The most popular in the micron range is a scheme with an electromagnetic actuator and a differential capacitor as a displacement sensor [6]. There are variants of the indentation module using a piezoceramic actuator and a capacitive displacement measurement circuit [7].
The main arguments in favor of a particular indentation module design are the minimum temperature drift of the indentation depth measurement system and the maximum range of loads used. Purely capacitive and piezoceramic systems have the advantage in terms of temperature drift caused by the actuator. The electromagnetic actuator, by virtue of its operating principle, cannot fail to be warmed up by the contact force of the indentor. The thermal power released in the moving coil of the actuator is proportional to the square of the electric current, and the force developed by the actuator is proportional to the first degree of the current. Consequently, heating, and hence thermal expansion, is most pronounced when working with maximum indentation forces. The use of capacitive sensors in the form of a differential capacitor with a moving middle plate has de facto become the standard for tool indentation. Such sensors are not heat sources during their operation, create minimal force impact on the moving system, provide a low threshold level of the registered signal and sufficiently high linearity in the penetration depth.
Devices implementing the instrumented indentation test are often referred to as nanoindenters and are used with approximately the same precautions as atomic force and scanning tunneling microscopes. Nanoindenters are placed on vibration-isolating platforms and put inside thermally insulated boxes. As a rule, it is the level of seismic noise and temperature fluctuations in the room that are the main factors limiting the accuracy of measurements made and the minimum level of force during indentation.
To reduce the influence of vibration noise, the mass of the moving elements of the device related to the diamond indentor is to be minimized. Increasing the rigidity of the suspension of the moving elements, increasing the resonance frequency of the device and, as if reducing the influence of seismic noise, does not lead to improvement of metrological characteristics of the nanoindenter, since, against the background of increased rigidity of the indenter suspension system, the contact area stiffness, which characterizes the hardness and Young’s module of the tested material, becomes less noticeable, especially at small indentation depths. As a result, the measurement accuracy of the contact interaction force between the indenter and the material under test and the quality of the load-depth plunge curve decrease.
The performance specifications given in the instrument descriptions in terms of minimum loads and indentation depths generally correspond to the conditions of minimal natural seismic noise, absence of industrial seismic interference and the use of a good vibration isolation system. The actual noise level on the force and displacement channels depends on the specific operating conditions of the instrument, and the manufacturer labels it as "lab dependent". The digital resolution of the force and displacement channel is usually an order of magnitude less than the declared "lab dependent" noise level.
DESIGN AND COMPOSITION FEATURES OF NANOSCAN-4D
Mechanical loading system
The detailed analysis of design and principles of work of the indenting module and electronics control will be carried out on an example of nanoindenter NanoScan-4D. This device is registered in the State Register of measuring instruments as a nanohardness tester under № 65496-16 and allows to perform the full range of techniques provided by the standards [8–10].
At development NanoScan-4D experience of work with foreign and domestic devices for instrumented indentation was taken into account and their advantages and disadvantages were studied. As a result of the analysis of the obtained results, a scheme with a paired electromagnetic actuator and a capacitive displacement sensor was chosen, Fig.1.
The use of an electromagnetic actuator in the form of a coil located in a cylindrical gap with an axial magnetic field made it possible to indent with loads over 2 N. This kind of actuators are actively used in acoustic systems and the technology of their production is well proven. They effectively convert current into force, are operable over a wide temperature range, are stable and have a linear response over the entire range of operating motions and forces.
A mechanical scheme was chosen that included two actuators that apply a joint force to the rod, on which a capacitive sensor and a diamond indenter are fixed. Using two actuators not only doubled the maximum force of indentation (with twice less thermal power), but also significantly simplified the control algorithms of the whole process of tool indentation. Two independent channels of force application simplify the procedure of indenting the surface, taking a stable load-depth curve, facilitate implementation of complicated measurement algorithms such as multiple indentation and dynamic measurements when loading is performed by an indentor oscillating with small amplitude. The device can be equipped with a spherical tip that allows reconstructing the stress-strain diagram in the partial-load loading mode [11]. Studying the properties of heterogeneous materials, such as steel samples that have been irradiated with heavy ions, can be carried out by applying dynamic instrumented indentation [12]. Examples of such measurements are presented in Fig.2.
When mapping the mechanical properties of the sample, one of the attenuators is used to compensate for large-scale surface topography roughness, while the second attenuator performs indentation with a minimum and stable control current, thus minimizing the thermal effect associated with the operation of the electromagnetic actuator.
The use of a carbon shaft also helps to reduce internal thermal drifts. This solution not only reduces thermal drift and the mass of the moving system, but also increases the bending stiffness of the shaft, which is important during sclerometry testing of samples. The transverse and bending stiffness is also increased by placing the membranes holding the shaft at a distance from the capacitive probe and the diamond indenter.
The symmetrical arrangement of the main elements of the indenting module design allows the module to be operated both horizontally and vertically. The presence of two independently controlled actuators makes it possible to programmatically correct the initial position of the indenter taking into account the direction and magnitude of gravity acting on the movable system of the indenting module. For the movable mass of the shaft with all the elements fixed to it equal to 40 g and the total stiffness of the two diaphragms of 10,000 N/m, the displacement of the middle pad of the capacitive sensor when the indentation module is turned upside down will be 80 µm. To compensate such displacement, one of the actuators must generate a force of 0.8 N, which is quite a lot and excludes the use of the same actuator to perform indentation in the load range of hundreds of μN. At a digit capacity of the used DAC of 18 bits and the maximum load of 2 N one bit will correspond to 4 μN, which is a little bit rough for work in the nanoscale range. The presence of the second actuator allows to use it in the mode of small loads, for example, up to 10 mN, and in this case one bit of DAC will correspond to 0,04 μN, which allows to conduct full measurements on any of the methods of instrumented indentation of the load range up to 10 mN.
The high transverse stiffness of the working shaft allows to realize an extension of the indentation force range up to 50 N. Schematic representation of the module for increasing the normal load on the indenter is shown in Fig.3. The use of such a module makes it possible to compare hardness data obtained by instrumented indentation and by micro-indentation, where hardness is determined by dividing the indentor pressing force by the area of the recovered indentation imprint measured with an optical microscope, in a timely manner using the same indenter.
In this load extension module, the role of the force actuator is performed by a linear actuator with a stepper motor, the force sensor is a strain gauge transducer, and the indentation depth sensor is a standard differential capacitor of nanoindenter with a movable middle pad.
Capacitive transducer schematics
Capacitive displacement sensors are actively used in a wide variety of measurement systems – capacitor microphones, seismic sensors, touch screens and security alarms. Traditionally, in precision measuring systems, they operate in differential mode, with the stationary terminals supplied by an anti-phase high-frequency voltage and synchronous detection of the signal taken from the middle moving terminal. Such a scheme allows measuring displacement of the moving shutter with a resolution of a hundredth of a nm in the registration bandwidth of hundreds of Hz. However, if the magnitude of displacement becomes commensurate with the working gap of the differential capacitor, the nonlinearity of the circuit increases dramatically. As a consequence, when using it in a nanoindenter displacement sensor, one has to make the working gaps of the differential capacitive sensor an order of magnitude greater than the working depths of indentation, in order to ensure nonlinearity at the level of 0.01%.
In NanoScan-4D devices along with such electronics scheme of a differential capacitor a modernized scheme is used, which allows providing the required level of linearity at a significantly smaller working gap of a differential capacitor, i.e. at a better threshold sensitivity for traditional inclusion. The linearity of the dependence of the output signal on the displacement of the moving pad of the differential capacitor is ensured by a modification of the supply circuit of the stationary pads, in which the incoming voltages on them are directly proportional to the working gap, Fig.4.
The voltage required for such compensation is formed by an analog multiplier, one of whose inputs receives the reference AC voltage, and the second one receives the integrated output signal of the synchronous detector. The applied circuit solution made it possible to use the capabilities of the differential capacitive sensor as effectively as possible, providing a low noise level when it is switched on traditionally and a high linearity when it is used in the negative excitation voltage feedback mode.
Methods of the dynamic range expansion
The control system of electromagnetic actuators used in nanoindentors also has a number of features. Since the coil of the actuator is made of copper wire, its resistance increases with increasing temperature, and as a consequence, when the ambient temperature changes or the coil is heated during operation, the coefficient linking the voltage applied to the coil and the force it generates will change. Therefore, the control circuit of such an actuator is built in the form of a current generator, Fig.4. In this case, the force channel calibration depends only on the magnetic field value in the working gap, and this value is practically independent of the external ambient temperature. Thus, it is guaranteed that the calibration of the force channel in the entire operating temperature range of the indentation module from –20 °С to +60 °С with an accuracy no worse than ±5%, and in the range from +15 °С to +30 °С no worse than ±1%. When working in a voltage generator mode, due to the change of resistance of the actuator winding (temperature coefficient of resistance of copper wire ~ 0,003 °C-1 ), the multiplier for DAC code conversion to force would fall by 30% at temperature change from –20 °С to +60 °С.
The full range of variation of normal force is divided into subranges with a maximum force of 3N, 0.3N and 30mN, by changing the parameters of the current generator that supplies the coils of the electromagnetic actuator. Displacements are measured in the same way with a breakdown into subranges of 300 microns, 100 microns, 30 microns and 10 microns. The two upper two ranges are implemented in the negative excitation voltage feedback mode, and the two most sensitive ones in the mode of an ordinary differential capacitor with a constant voltage on the fixed pads. Accordingly, the minimum value of digital displacement resolution is 0.05 nm, which is significantly less than the typical seismic response of a moving nanoindenter suspension system. The bandwidth processed by the software is from 0 Hz to 10 kHz. Digitization of data and generation of signals by the embedded microprocessor software is performed with frequencies up to 300 kHz and 18-bit digital resolution.
Research conducted during the development of the NanoScan-4D showed that the requirements to the linearity of the suspension system of the working shaft imply the development of flat springs of a membrane type with a special pattern of elastic elements, providing deviation from linearity in dependence on the force displacement not more than 0.01% over the whole range of working movements, which is hundreds of microns.
The suspension system and its time and temperature stability are key parameters affecting the quality of the load-depth relationship measured. In this regard, it turned out to be extremely important to ensure the same coefficient of thermal expansion of the flat spring material and the indentation module support structure. This is especially strong when working with a thermal stage and making measurements at increased and decreased temperatures, when the device is located in a climatic chamber [13]. The difference in the thermal expansion coefficient of the flat spring and the supporting structure leads to an additional tension or compression of the flat spring and as a consequence not only the resonance frequency, but also the zero position of the displacement sensor is an order of magnitude greater than would be expected with a simple thermal expansion of the portion of the stem near the hot region of the thermal stage [9].
CONCLUSIONS
The principles of modern nanoindenter design considered in this work demonstrate the high performance of the instrumented indentation method as well as its informativeness for localized studies. These devices, providing the possibility to study mechanical properties with a spatial resolution better than 100 nm and about 10 nm in depth, allow to work with heterogeneous materials and thin functional coatings, mapping the value of hardness.
This class of instruments is de facto becoming the accepted standard for mechanical testing, gradually replacing micro-hardness testers not only from the field of scientific research, but also actively penetrating into the field of industrial diagnostics. Nevertheless, the equipment for instrumented nanoindentation is a complicated hardware-software complexes, carrying out precision measurements of small signals on the sensitivity limit of primary transducers, included in their design. For correct operation of such measuring devices, for obtaining reliable results about investigated objects, as well as for providing metrological basis for such measurements, all paths through which the primary measured signal passes, should be optimized and coordinated by their resolution, linearity and noise.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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