Study of the structural materials properties by instrumental indentation using a portable nanohardness meter
A modification of the "NanoScan-4 D" nanohardness meter, which allows of measuring the mechanical properties of articles by the instrumental indentation according to GOST R 8.748-2011 under conditions close to industrial fabrication, has been developed. The main advantage of the described device, unlike most modern portable hardness testers, is the ability to work with a wide class of materials (from metals to solid polymers) since the study of the mechanical properties of products does not require preliminary information on the elastic modulus of the material being tested. Presented are the experimental data obtained on standard samples of the enterprise: polycarbonate and aluminum, as well as on various metal articles used as parts of machines and mechanisms of the oil and gas industry. The measured values of hardness coincide with the values obtained on a laboratory nanohardness meter taking into account the inherent errors of this type of equipment.
Control of mechanical properties is a necessary procedure in manufacturing and operation of various parts and mechanisms which makes it possible to make conclusions on their characteristics and an expected lifetime of the product. Testing of materials is a special field of material science, where measurements of the products in use are the most important and actual task. Several types of hardness meters intended for use in the field have been developed in parallel with the development of the mechanical properties testing methods.
The most popular portable hardness meters function on the basis of two principles: analysis of the probe tip withdrawal and measurements of the contact acoustic impedance of the material . The results obtained in use of these devices feel the impacts of mass and rigidity of measured objects, substrate properties on which the article is located and a multiple factors difficult to control. Ultrasonic or impedance hardness meters use indirect methods based on calculation of strength according to the relationships binding various mechanical and physical properties with strength value determined by direct methods. In other words, interpretation of the tested material hardness data obtained by an impedance hardness meter depends on the elastic modulus (Young). So, such type of equipment should be operated provided we know the data on the elastic modulus value because it is used at the primary data processing determined at the contact of the indenter and the material . According to the instrumental indentation method [3,4], the presented portable nanohardness meter detects the mechanical properties under controlled loading applied to the indenter when it contacts the sample and indenter displacement in the course of measurements. Automatic calculation of hardness and the elastic modulus of the material under the fixed load or penetration depth is performed in accordance with the obtained data.
Fig.1a presents the general view of the device. The main elements placed inside the case are: a long rod (stem) with the indenter fixed at its end, a force producing element (electric wiring located in the magnetic field of a constant magnet and plates of a capacity detector for registration of displacement, and elastic membranes intended to provide the plan-parallel motion of the indenter. The indenter of a portable hardness meter is a pyramidal trihedral diamond tip of the Berkovich type.
The instrument set includes two nozzles differing in purpose: for measuring bulk samples and for measuring thin samples, for example, knives, as shown in Fig.1b. Both types of nozzles at the point of contact with the sample have 3 spherical supports that protect the measuring system of the nanometer hardness meter from external vibration and temperature influences.
When using nanohardness meters, it is necessary to regularly (at least once per thousand measurements) carry out calibration of the tip shape by comparing it with the appropriate nanoindentation standard (multiple indentation of fused silica). This procedure is necessary not only for the correct measurement of hardness, but also for controlling the degree of wear of the tip and for making a decision on its replacement. An example of such curve is shown in Fig.2.
The traditional construction of the nanohardness meter provides for a possibility to smoothly feed the indenting head to the test sample. In this portable modification there are no mechanized movements, except an electromagnetic actuator. Therefore, when replacing the indenter and setting up the device, it is necessary to calibrate the relative vertical location of the spherical support pairs with respect of the indenter tip. Movement of the support head along the rod is carried out by its rotation; fastening in the working position is carried out using a safety nut. The range of the working position of the tip: from 20 to 80 microns from the test surface.
The distance is controlled by the operator visually, or by inserting a calibrated probe into the gap between the indenter and the surface, with the power supply of the nanometer hardness meter turned on. For flat surfaces, the procedure for calibrating the vertical position is carried out simultaneously with the change of the working head and, in real measurements, is not further performed. When working with uneven concave and convex surfaces, this procedure is carried out at the place of measurement.
When carrying out measurements on free surfaces, it is necessary to maintain the constant force that presses the device to the article during the entire measurement cycle (the procedure time depends on the maximum load and takes about one minute on average) of at least 30 N and not more than 60 N, as well as the angle between the force application line and the perpendicular to the surface of the sample less than 10 degrees. In case of magnetic articles, the nanohardness meter is kept on the surface with the aid of magnetic inserts. On completing the indentation and before the next test the operator should move the device at a distance of at least 1 mm along the surface of the article. Calculation of hardness and modulus of elasticity occurs automatically.
When working with less than 10 mm thick articles you can use a special clamp that provides tight contact of the test product with spherical supports and the clamping surface of the clamp.
The level of surface roughness, as with any hardness measurements, should be lower than the value specified in GOST for the selected forces and indentation depths. By default, Ra should be no more than 5% of the indentation depth.
Before testing, the tip shape was calibrated by a series of measurements on a test block (fused silica) with an increasing load. The obtained function of the tip shape was used in future to calculate hardness.
Verification of the results obtained using a portable hardness meter was carried out by comparing them with the data of the device included in the state register of measuring instruments, "NanoScan-4D" nanohardness meter (FSBI TISNCM, Russia). The "NanoScan-4D" nanohardness meter occupies an exceptional position among the devices manufactured in Russia, since it allows measurements of the physical and mechanical properties of materials on a submicron and nanometer scale of linear dimensions .
To demonstrate the capabilities of a portable hardness meter, samples of different materials were studied: metal (nuts and pins), polycarbonate (a standard sample of the enterprise), D16T aluminum and a tourist knife blade. The loading force during the experiments was 1 N (the maximum load range of a portable nanohardness meter is 10 N). A larger load was not required, since the surface of the samples was prepared before testing – in order to reduce roughness it was polished. Before testing by the instrumental indentation method it is necessary to make sure that the surface roughness does not exceed 1/20th of the indenter recess .
Table 1 shows the data on the modulus of elasticity and hardness obtained on the samples. As can be seen from Table 1, the series of measurements carried out on the samples yielded hardness values that coincided, within the measurement error, with the values obtained by the laboratory nanohardness "NanoScan-4D" meter. As for the modulus of elasticity, underestimated values were obtained on a portable device as compared with the data obtained by the "NanoScan-4D" device, however, the difference in values does not exceed 15%. This deviation can be explained by the variability of the effective stiffness of the nanohardness meter – test article system and by the impossibility of taking into account this variability when processing the obtained primary data.
Fig.3 shows the loading-unloading curves obtained on the studied nut sample, as well as on fused silica (the load value in the quartz test was 500 mN).
For many tasks of modern materials science, instrumental indentation is a powerful research tool. The tests of the compact version of the "NanoScan-4D" device demonstrated a possibility of using this method for the on-line diagnostics of the mechanical characteristics of components and articles without removing them from the working process. A key advantage of this method is the ability to determine hardness without a priori information about the elastic modulus of the material. The design of the portable hardness meter is suitable for measuring both massive and thin samples thanks to two different nozzles. Equipping this device with a wireless communication and global positioning module in future will make it possible to link the measured values to the geographical coordinates of the object and create a cloud-based system for storing and processing data on mechanical properties, which makes the use of this device extremely promising as an additional tool for monitoring the state of engineering structures, bridges, pipelines, railways and land, sea and air transport vehicles. ■