The differences between nano- and coarse-grained materials in elastic, damping, strength, thermal, electric, magnetic and diffusive properties are caused not only by the small size of the grains in nanomaterials, but also by a special state of the surface or borders of grains in them . One of the areas of nanotechnology is the creation of samples from nanosized powders while maintaining a very small grain size. It is expected that these samples will have high ductility and density, which will affect their strength and hardness. It is known that the smaller is the grain size and the more developed is granular structure, the stronger and firmer are the samples. However, nanopowders used for obtaining such samples, contain strong hard-destructible agglomerates of nanoparticles , which requires use of non-standard methods of compaction, for example, hot pressing. The present paper analyses the samples created from nanopowder of nickel and silver. Nickel is the basis of most superalloys – heat-resistant materials. It is used in the plating (coating on the surface of another metal to protect it from corrosion), in the production of batteries, chemical and radiation technologies, medicine, coinage alloys, production of strings for musical instruments.
Silver is used in electrical engineering and electronics as coating or material of the contacts and conductor in high frequency circuits, in multilayer ceramic capacitors, solders, alloys for the manufacture of cathodes for electrochemical cells, jewelry, manufacturing of coins and medals, in microwave equipment for coating the inner surface of the waveguides, as a coating for mirrors with high reflectivity, as a catalyst in oxidation reactions, as a disinfectant substance (mainly for water disinfection) and in other areas. The aim of this project was the creation using SPS (Spark Plasma Sintering – hot pressing using sintering spark plasma) of tight and solid tablets with fine-grained (sub-micron) structure from nanodispersed powders of nickel and silver. DESCRIPTION OF EXPERIMENTAL CONDITIONS Nanopowders of nickel and silver created by the Russian company NskNano by the method of electrical explosion of wire were used in researches. Nickel powder had the following features: • average particle size (APS) of 60–70 nm; • specific surface area (SSA) of 10.0 m2/g; • purity of 95.3%; • color is black; • morphology (shape of particles) is spherical. Fig.1 shows electron microscopy of this nanopowder. Silver powder (a fully crystalline silver) had the following properties: • average particle size of 90–100 nm; • specific surface area (measured by BET) of 6.5 m2/g; • purity of 99.9%; • color is grey; • bulk density of 5.8 g/cm3. Previously, to demonstrate the differences in the properties of micro- and nanopowders experiments on the compressibility of nickel nanopowder used in this project and nickel micropowder PNE-1 made by the electrolytic method (the average particle size from 40 to 70 µm) were carried out. Fig.2 shows the dependence of the relative density of powder (relative to the theoretical density of nickel of 9.8 g/cm3) on the applied pressure in the range of 1–14 MPa (at the top are presented dependencies for PNE-1, at the bottom – for the nickel nanopowder). The figure shows that the compressibility of micro- and nanopowders is different. In the main experiments, the sintering of nickel and silver nanopowders was carried out on the Sinter Land Labox device in Lavrentyev Institute of Hydrodynamics of SB RAS by the method of SPS, when the electric current pulses pass through the pre-pressed powder (in these experiments, the current was up to 2 kA at a voltage of 3–4 V). The main difference between SPS and conventional pressing (at consecutive pressing and sintering) is the application of the pulse electric current directly to the sample, which contributes to the rapid heating of the powder and preservation in a large degree of microstructural parameters in the consolidated material. Pressing was carried out at a pressure of 40 MPa and maximum temperatures of 800 and 910 °C for nickel (melting point of 1 453 °C) and 600 °C for silver (melting point of 962 °C). The heating rate was 100 °C/min with holding at maximum temperature for 3 minutes. Microhardness of all samples was studied by means of hardness tester PMT-3. The yield strength in compression and a young's modulus were determined using Zwick / Roell Z005 (Germany) machines for strength testing of materials. Electron scanning microscope ZEISS EVO-50WDS-XVP-BU in Khristianovich Institute of Theoretical and Applied Mechanics of SB RAS was used to study of cleaved samples after deposition of gold layer. RESULTS AND DISCUSSION The diameter and thickness of obtained samples of nickel amounted to 10.3–10.4 mm and 1.8–1.9 mm, the density at Tmax = 800 °C was of 8.23 g/cm3, and at Tmax = 910 °C – of 8.3 g/cm3 (0.93 relative to table value). The diameter and thickness of the obtained sample of silver was of 10.5 and 1.4 mm and the density was of 10.2 g/cm3, i.e. very high (relative density of 0.97). The microhardness of the sample of silver was of 2.7 GPa, and the microhardness of samples of nickel Hv = 5.5 GPa (table values of hardness for compact silver and nickel are of 0.25 and 0.64 GPa, respectively). The yield strength in compression for silver sample σт = 150 MPa, and for nickel samples – approximately 400 MPa (table values of the yield strength for the compact silver and nickel are 50 and 120 MPa, respectively). The young's modulus for the of silver sample E = 100 GPa, and for nickel samples is approximately 300 GPa (table values for the compact silver and nickel are 80 and 210 GPa, respectively). Fig.3 shows electron microscopy of the cleaved nickel sample at Tmax = 800 °C. The grain size of the obtained sample was approximately 500–1 000 nm. Fig.4 shows electron microscopy of the cleaved silver sample at Tmax = 600 °C. The grain size of the obtained sample was about 1–2 µm. Fig.3 and Fig.4 show that fine-grained dense samples of nickel and silver were obtained using SPS. Thus, a solid, fine-grained (0.5–2 µm), dense (relative density from 0.93 to 0.97) samples with a hardness of 2.7 GPa (for silver) and 5.5 GPa (for nickel) were obtained from nano-powders of nickel and silver using spark plasma sintering. ■ The project is supported by SB RAS (project III.23.4.1).