Transparent diamond probe for nanoindentation
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 .
An optical system coupled to a transparent spherical indenter immersed into a liquid was proposed for the study of biological objects . 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 , 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 ) can be used in such an experimental design. Thus, significant limitations are imposed on the hardness of the objects under study.
In , 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 .
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.
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.