rus
Issue #6/2018
E.Gladkikh, K.Kravchuk, I.Maslenikov, V.Reshetov, A.Useinov
Transparent diamond probe for nanoindentation
Transparent diamond probe for nanoindentation
The advantages of the nanoindentation, such as the speed of testing, the simplicity of sample preparation, the non-destructive principle of control, make it effective for studying a wide class of materials. Heterogeneous samples often require observation of their mechanical properties in local areas of the surface. In this regard, there is a need for precise positioning of the indenter using a transparent tip. An indenter made of transparent material, in addition to direct observation of the measurement area, allows radiation exposure of selected area of the sample. If there are no impurities and defects in the crystal from which the indenter is made, the radiation beam does not lose its intensity. The paper demonstrates the possibilities of observing the surface of a sample through an indenter, using liquid-crystal screens of electronic devices as an example. The image of individual pixels that passed through the indenter is resolved with great accuracy, and objects of tens of microns are distinguishable even without reaching the limit resolution of the microscope.
Теги: defects heterogeneous samples impurities indenter nanoindentation дефекты индентор наноидентирование неоднородные образцы примеси
At present, many specialists conducting studies of the mechanical properties of materials in the local surface area choose an instrumental indentation method [1]. The high speed of testing and control of properties without destroying a bulk sample are key advantages that determine the wide use of nanoindentation. However, in order to investigate the mechanisms of plastic deformation at the micro level, conventional indentation may not be enough. The load-unloading curve recorded during the measurement gives information about the hardness and elastic modulus of the sample, but does not allow tracing the formation of plastic deformation "banks" at the edges of the impression [2]. In this case, it is advisable to use various methods of controlling the surface topography, for example, optical microscopy, or atomic force or electron microscopy if the dimensions of an impression are so small that the resolution of light optics does not allow detecting it. In [3], using a scanning electron microscope, the deformation behavior of elastomers was studied directly during indentation using polydimethylsiloxane. However, indentation inside the electron microscope column is associated with certain difficulties.
It turns out that when it is important to control the shape of the impression of the indenter, one of the main advantages of indentation – the speed of testing is lost, since you should to place the sample under an optical microscope and to spend time finding the impression. The task is simplified if the nano-hardness tester is equipped with an additional module – an optical system (such as in NanoScan-4D manufactured by TISNCM [4]). However, even in this case, it will take time to move the sample stage and to adjust the focus of the microscope. In connection with the above, an urgent task is to observe the sample under study directly through the indenting tip.
The simplest solution was shown in [5, 6], but it is applicable only for transparent samples. Fused quartz and polymethyl methacrylate were studied on experimental installations in which a laser microscope and a CCD camera were placed under the sample (on the opposite side of the indenter). The use of a device of this design made it possible to observe a change in the thickness of the sample area in contact with the indenter. This, in turn, made it possible to study the deformation behavior of the sample and to determine the contact area in the process of application and removal of load.
There are several other ways to solve the problem. For example, you can use a transparent indenter and an optical microscope placed in its immediate vicinity. In this case, it is possible to investigate opaque samples, and the mirrors allows the use of an optical system located outside the indenter module [7].
An optical system coupled to a transparent spherical indenter immersed into a liquid was proposed for the study of biological objects [8]. The refractive index of the fluid in this case should be close to the refractive index of the indenter. For diamond (refractive index n = 2.4), there are no suitable immersion liquids [9], therefore indenters made of glass (n close to the refractive index of oil) or sapphire (n close to the refractive index for a number of substances listed on p. 311 in [9]) can be used in such an experimental design. Thus, significant limitations are imposed on the hardness of the objects under study.
In [10], by integrating the phase shift interferometer (Twyman – Green) into the measuring system, it was possible to determine with high accuracy the indentation-induced displacements of the surface layers of the sample. Due to this, based on the theory of elastic recovery and the finite element method, a method was developed for determining the Young’s modulus. During the test with a spherical indenter, the contact radii used to estimate the true strain curve of the material after reaching plastic deformation were also measured using the Tabor’s empirical relationship [11].
MATERIALS AND METHODS
It may be noted the following key requirements for the indenter, through which the sample surface is supposed to be observed. First, the indenter must be optically transparent. Indenters are made of artificially grown diamonds, which are crystals of high purity (compared to natural). The quality of the material of the indenter, as a rule, is checked using the X-ray diffraction. In addition, the use of high-purity indenters opens up the possibility of "delivering" radiation to a specific area of the sample without losing power – you can modify the surface of the object under study with a laser beam directly through the indentation tip, and also combine the study of local mechanical properties with optical methods, for example, with Raman spectroscopy .
Secondly, an important factor for indentation and simultaneous observation of the test object is the shape of the indenter. With a significant deviation of the macrogeometry of the indenter, the optical path of the rays changes significantly. The shape control can be performed using a 3D optical profilometer/confocal microscope (manufactured by Sensofar, Spain, Fig.1).
Thirdly, the quality of the obtained image of the sample is influenced by the surface roughness of the indenter itself, which can be monitored using an atomic force microscope.
The indenter for the experiment was made in the form of a cylinder, both ends of which were sharpened in the shape of the Berkovich pyramid. The indenter was pressurized into a brass holder with a round hole on the side surface perpendicular to the axis of rotation of the cylinder (Fig.2). In order to direct the optical rays from the object under study through the hole, a mirror was placed inside the holder. This approach does not require the use of immersion liquids.
DISCUSSION OF RESULTS
In this paper, liquid-crystal displays of electronic devices were used to demonstrate the possibility of observing the surface of the sample through the indenter and the quality of the resulting image. Fig.3 shows the obtained images of LCD screens with resolutions of 167 ppi (Fig.3a), 233 ppi (Fig.3b) and 440 ppi (Fig.3c), each pixel of which consisting of three color components (red, blue and green), has dimensions of about 150 μm, 110 μm and 60 μm, respectively. For each pixel size, the required magnification of the optical microscope was selected.
As can be seen in Fig.3, the image obtained through the indenter and appearing in the field of view of the lens is resolvable with great accuracy (objects of tens of microns are distinguishable even without reaching the limit resolution of the microscope). Due to such observation of the sample surface, the study of its local areas selected immediately prior to testing can be carried out.
CONCLUSION
In the case of studying the characteristics of heterogeneous and composite materials (with a phase size of micron scale), it is necessary to control the choice of the area of study. Analytical devices, combined with the optical system of registration of the sample image, make it possible to carry out operational control of properties. Thus, the use of a transparent tip for nanoindentation significantly reduces the time spent searching for the desired area.
The optically transparent indenter with the correct geometry opens up the possibility of a local impact on the study area using radiation that is not dissipated due to the low concentration of defects in the artificially grown crystal. ■
The study was carried out with the financial support of the Russian Foundation for Basic Research in the framework of research project No. 18-08-00558.
It turns out that when it is important to control the shape of the impression of the indenter, one of the main advantages of indentation – the speed of testing is lost, since you should to place the sample under an optical microscope and to spend time finding the impression. The task is simplified if the nano-hardness tester is equipped with an additional module – an optical system (such as in NanoScan-4D manufactured by TISNCM [4]). However, even in this case, it will take time to move the sample stage and to adjust the focus of the microscope. In connection with the above, an urgent task is to observe the sample under study directly through the indenting tip.
The simplest solution was shown in [5, 6], but it is applicable only for transparent samples. Fused quartz and polymethyl methacrylate were studied on experimental installations in which a laser microscope and a CCD camera were placed under the sample (on the opposite side of the indenter). The use of a device of this design made it possible to observe a change in the thickness of the sample area in contact with the indenter. This, in turn, made it possible to study the deformation behavior of the sample and to determine the contact area in the process of application and removal of load.
There are several other ways to solve the problem. For example, you can use a transparent indenter and an optical microscope placed in its immediate vicinity. In this case, it is possible to investigate opaque samples, and the mirrors allows the use of an optical system located outside the indenter module [7].
An optical system coupled to a transparent spherical indenter immersed into a liquid was proposed for the study of biological objects [8]. The refractive index of the fluid in this case should be close to the refractive index of the indenter. For diamond (refractive index n = 2.4), there are no suitable immersion liquids [9], therefore indenters made of glass (n close to the refractive index of oil) or sapphire (n close to the refractive index for a number of substances listed on p. 311 in [9]) can be used in such an experimental design. Thus, significant limitations are imposed on the hardness of the objects under study.
In [10], by integrating the phase shift interferometer (Twyman – Green) into the measuring system, it was possible to determine with high accuracy the indentation-induced displacements of the surface layers of the sample. Due to this, based on the theory of elastic recovery and the finite element method, a method was developed for determining the Young’s modulus. During the test with a spherical indenter, the contact radii used to estimate the true strain curve of the material after reaching plastic deformation were also measured using the Tabor’s empirical relationship [11].
MATERIALS AND METHODS
It may be noted the following key requirements for the indenter, through which the sample surface is supposed to be observed. First, the indenter must be optically transparent. Indenters are made of artificially grown diamonds, which are crystals of high purity (compared to natural). The quality of the material of the indenter, as a rule, is checked using the X-ray diffraction. In addition, the use of high-purity indenters opens up the possibility of "delivering" radiation to a specific area of the sample without losing power – you can modify the surface of the object under study with a laser beam directly through the indentation tip, and also combine the study of local mechanical properties with optical methods, for example, with Raman spectroscopy .
Secondly, an important factor for indentation and simultaneous observation of the test object is the shape of the indenter. With a significant deviation of the macrogeometry of the indenter, the optical path of the rays changes significantly. The shape control can be performed using a 3D optical profilometer/confocal microscope (manufactured by Sensofar, Spain, Fig.1).
Thirdly, the quality of the obtained image of the sample is influenced by the surface roughness of the indenter itself, which can be monitored using an atomic force microscope.
The indenter for the experiment was made in the form of a cylinder, both ends of which were sharpened in the shape of the Berkovich pyramid. The indenter was pressurized into a brass holder with a round hole on the side surface perpendicular to the axis of rotation of the cylinder (Fig.2). In order to direct the optical rays from the object under study through the hole, a mirror was placed inside the holder. This approach does not require the use of immersion liquids.
DISCUSSION OF RESULTS
In this paper, liquid-crystal displays of electronic devices were used to demonstrate the possibility of observing the surface of the sample through the indenter and the quality of the resulting image. Fig.3 shows the obtained images of LCD screens with resolutions of 167 ppi (Fig.3a), 233 ppi (Fig.3b) and 440 ppi (Fig.3c), each pixel of which consisting of three color components (red, blue and green), has dimensions of about 150 μm, 110 μm and 60 μm, respectively. For each pixel size, the required magnification of the optical microscope was selected.
As can be seen in Fig.3, the image obtained through the indenter and appearing in the field of view of the lens is resolvable with great accuracy (objects of tens of microns are distinguishable even without reaching the limit resolution of the microscope). Due to such observation of the sample surface, the study of its local areas selected immediately prior to testing can be carried out.
CONCLUSION
In the case of studying the characteristics of heterogeneous and composite materials (with a phase size of micron scale), it is necessary to control the choice of the area of study. Analytical devices, combined with the optical system of registration of the sample image, make it possible to carry out operational control of properties. Thus, the use of a transparent tip for nanoindentation significantly reduces the time spent searching for the desired area.
The optically transparent indenter with the correct geometry opens up the possibility of a local impact on the study area using radiation that is not dissipated due to the low concentration of defects in the artificially grown crystal. ■
The study was carried out with the financial support of the Russian Foundation for Basic Research in the framework of research project No. 18-08-00558.
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