CC BY
21■SHS 2019 Moscow, Russia
NON-ISOTERMAL SYNTESIS OF NANOLAMINATES O. K. Lepakova", N. I. Afanasyev*", A. M. Shulpekov", and V. D. Kitler"
aTomsk Scientific Center SB RAS, Tomsk, 634055 Russia
*e-mail: af42@yandex.ru
DOI: 10.24411/9999-0014A-2019-10081
Creation of new materials and their production methods are one of the most important scientific and applied problems of physical materials science. At present, the obtaining of refractory high-strength materials possessing wear-resistance at elevated temperatures, high ductility at room temperature, and capable of operating under extreme conditions is an urgent problem.
MAX-materials are new materials and are of particular interest to develop refractory high-strength materials. These are ternary compounds that are described by the general formula Mn+1AXn, where M is transition metal; A is the element of IIIA or IVA periodic group, X is carbon or nitrogen (or both) and, possibly, boron. A distinctive feature of these materials is the structure of hexagonal crystal lattices, in which the layers of atoms of the M and A elements alternate in a certain sequence, and carbon atoms (or nitrogen) are located in octahedral pores between atoms of the M element. The structure of their crystal lattices provides the unique combination of metal and ceramics properties. To obtain materials based on MAX-phases, various methods are used [1-5]. The basic method for the obtaining of materials based on MAX-phases is sintering which requires high power and time consumption. Self-propagating high-temperature synthesis (SHS) can be considered to be an alternative to sintering.
In this work, the phase composition, microstructure, and some properties of SHS materials based on MAX-phases (Nb2AlC, Nb4AlC3) are studied. Powders of titanium (TPP8 grade, Avisma company, Berezniki), carbon (PM75 grade, < 0.033 ^m), niobium (TU 48-4-284-73, > 63), aluminum (ASD4 grade), and amorphous boron were used for the preparation of reaction mixtures. The phase composition of synthesized materials was determined using a DRON-2 diffractometer (CoKa radiation). Optical (Axiovert 200M, Karl Zeiss) and scanning electron (SEM515, Philips) microscopes were used to study the microstructure. X-ray microanalysis (CAMEBAX MICROBEAM) was performed to determine the concentration of components.
In the Nb-Al-C system, two compounds with a layered structure, Nb2AlC and Nb4AlC3, were detected [6, 7]. In these works, Nb2AlC was obtained by hot isostatic pressing and Nb4AlC3 was obtained by annealing Nb2AlC at a temperature of 1700°C. The combustion of a 2Nb + Al + C stoichiometric mixture proceeds in the spin mode. According to XRD analysis, the main phase in the synthesized product is Nb2AlC; NbC and niobium intermetallide are present. Figure 1 shows the structure of the break surface. Plate-shaped grains characteristic for MAX- phases are observed. Since the sample was not previously deformed, the layered structure of its grains was not detected. Since a mixture of 2Nb + Al + C is low-exothermic, a high-quality product based on Nb2AlC cannot be obtained by SHS without additional techniques.
The task of the study was to obtain a material with a nanolaminate structure in the four-component Nb-Al-C-N system (analogue to Ti2AlN0,5C0,5) in the combustion mode. The products of 2Nb + Al + 0.5C obtained in the combustion mode at a nitrogen pressure of 60, 30, 15, and 4 atm were synthesized and analyzed. According to XRD analysis, multiphase products consisting of AhNb3C, Nb4N3.9, and NbC are formed at a nitrogen pressures of 60, 30, and 15 atm (Fig. 2). And only at a nitrogen pressure of 4 atm, the XRD pattern of the synthesized product contains reflections of Nb2AlC. The material with the MAX-phase Nb2AlC was also obtained by the SHS method in a chemical furnace (Fig. 2d).
Fig. 1. Break structure in the SHS sample Nb2AlC.
261 degree)
Fig. 2. XRD patterns of SHS products (Nb-A-C system, Nb:Al:С = 2:1:1 molar ratio) synthesized under the nitrogen atmosphere at a pressure of (a) 60 atm; (b) 15 atm; (c) 4 atm; and (d) in a chemical furnace. 1 Nb2AlC; 2 NbC; 3 AhNb3C; 4 NbC0.5.
Figure 3 shows the microstructure of the SHS products (Nb-Al-C system) obtained at nitrogen pressure of 60 and 4 atm. In Fig. 3a, plate-shaped crystals belong to the ternary AhNb3C compound, the round phases belong to Nb4N3,9 and NbC. Figure 3b shows the microstructure of the sample synthesized at a pressure of 4 atm. Plate-shaped crystals typical for the samples with a nanolaminate structure are clearly observed.
(a) (b)
Fig. 3. Microstructures of the products (Nb-Al-C system) synthesized under the nitrogen atmosphere: (a) 60 atm, (b) 4 atm.
As already mentioned, Nb4AlC3 MAX-phase was initially obtained by the high-temperature (1700°C) heat treatment of the Nb2AlC MAX-phase [7]. Later, single-phase Nb4AlC3 samples were synthesized by hot pressing and spark plasma sintering (SPS) [8, 9]. The microstructure and electrical, thermal and mechanical properties of synthesized Nb4AlC3 MAX-phases were studied in these works. Excellent mechanical properties of Nb4AlC3 at high temperatures were noted. The bending strength of Nb4AlC3 (346 MPa) remains without worsening in the range from room temperature to 1400°C, the Young's modulus of Nb4AlC3 can be maintained up to 1580°C that is much higher than that of Nb2AlC (1400°C), Ta4AlC3 (1200°C), and Ta2AlC (1200°C), which indicates prospective using the MAX-phase (Nb4AlC3) at high temperatures.
Considering the above, the studies were conducted to find the optimal conditions for obtaining the Nb4AlC3 MAX-phase by the SHS method. As preliminary studies showed, the combustion of the 4Nb + 1.2Al + 2.7C mixture developed in a spin mode. To increase the exothermicity of the mixture, a mixture of Ti + 2B powders was added to the initial mixture in an amount of 1, 5, and 10 wt % as an additional source of heat. Figure 4 shows the X-ray diffraction patterns of the SHS products based on Nb4AlC3 and with the addition of Ti + 2B
_SI IS 2019_Moscow, Russia
powders. XRD analysis shows that the addition of 1wt % Ti + 2B to the initial mixture during SHS forms a product consisting of Nb4AlC3, NbC, and a small amount of the intermetallic phase NbAl3. The addition of a larger amount of Ti + 2B leads to a significant decrease in the MAX-phase of Nb4AlC3 and the presence of niobium monoboride in the synthesized product. With the addition of 10 wt % Ti + 2B, the product consists of NbC, NbAb, and NbB.
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4- | 3
T—1—I—'—I—'—I—1—I—1—I—1—I—1—T
20 30 40 50 60 70 80 90 100
20, degree
Fig. 4. XRD patterns of the SHS products (Nb-Al-C system, Nb:Al:C = 4:1.2:2.7 molar ratio) and with the addition of exothermic Ti + 2B powders, (a) Nb:Al:C = 4:1.2:2.7; (b) 1 wt % of Ti + 2B; (c) 5 wt % of Ti + 2B; (d) 10 wt % of Ti + 2B. 1 Nb4AlC3; 2 Nb2AlC; 3 NbC; 4 NbAb; 5 NbB.
Currently, research is underway to develop and manufacture film heating elements. The electrical conductivity of polymer composite materials varies widely. This allows the use of the composition as a conductive material in electrical engineering. The requirements for electrically conductive composite materials can be met by selecting and optimizing the characteristics of the powder fillers used. It was previously shown that titanium carbosilicide (Ti3SiC2) with high values of thermal and electrical conductivity can be used as a conductive material when creating film heaters. Studies have been conducted of the electrically conductive properties of the compounds, in which crushed SH-synthesis products based on Nb2AlC and Nb4AlC3 were used as electrically conductive fillers. Polymethylphenylsiloxane (PFMS) and its mixture with EDP epoxy resin were used as a binder in polymer compounds (Fig. 5).
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0 50 100 150 200 250 300 350 400
t,°c:
Fig. 5. The dependence of the electrical resistance of the coating on the heat treatment temperature with fillers: №23-based on Nb4AlC3, №24-based on Nb2AlC. 1 PFMS, 2 PFMS + EDP.
A comparative analysis of the electrically conductive properties of coatings based on Nb2AlC and Nb4AlC3 with coatings based on carbide and carbosilicide titanium showed that the electrical resistance of the latter is many times less.
Composite materials based on the MAX-phases, such as, Nb2AlC, Nb4AlC3 were obtained by the method of self-propagating high-temperature synthesis. The SHS modes that ensure the maximum amount of, Nb2AlC and Nb4AlC3phases in the material were found. Composite polymer coatings based on Nb2AlC and Nb4AlC3 can be used as film electric heating elements.
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