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29HIGH-TEMPERAURE SYNTHESIS OF CAST CERAMIC MATERIALS BASED ON Cr2AlC MAX PHASE IN LAYERED AND MIXED SYSTEMS
V. A. Gorshkov*", P. A. Miloserdov", V. I. Yukhvid", N. Yu. Khomenko", and N. V. Sachkova"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10049
Recently in Russia and abroad there has been a great scientific and practical interest in ceramic materials based on MAX phases of carbide and nitride compounds. MAX phases have a layered structure in which layers of transition metal carbides alternate with aluminum. They attract considerable attention because of the unusual combination of properties that make them promising to work in extreme conditions and have great potential for use in the aerospace, automotive and industrial fields, because they have a unique combination of features like metals and ceramics with excellent mechanical, chemical, thermal, and electrical properties [1, 2].
Of the methods for producing ceramic materials based on MAX phase Cr2AlC, the most well-known methods of hot pressing, sintering spark plasma (SPS Spark Plasma Sintering) and in the thermal explosion mode are in the literature. Using these methods, small samples are obtained, and the processes are carried out at high pressure and on complex equipment [3, 4]. These methods are inefficient and energy intensive. The most promising way to obtain such materials is the self-propagating high-temperature synthesis (SHS) [5]. One of the directions of this method is metallothermic SHS using initial mixtures consisting of metal oxides, a reducing metal (aluminum), and a non-metal (carbon, boron, silicon). The burning temperatures of such mixtures exceed, as a rule, the melting points of the final products obtained in the combustion wave in a liquid-phase (cast) state. Cast materials with various contents of the MAX phase Cr2AlC [6, 7] were obtained by metallothermic SHS. In these studies, the authors used initial mixtures containing chromic anhydride (CrO3), which is hygroscopic and thermally unstable, to obtain the desired products, which limits the practical implementation of the method.
In this work, we studied the patterns of high-temperature synthesis of cast ceramic materials based on Cr2AlC MAX phase in layered and mixed systems containing thermally stable and non-hygroscopic reagents. The synthesis process (Fig. 1) was carried out on the basis of chemically (I) and thermally (II) coupled reactions:
(I) Cr2O3xAlxC + (CaO2xAl) ^ Cr2AlC + AhOs-CaO
(II) (Cr2O3xAlxC)/(CaO2 xAl) ^ Cr2AlC + AhOs-CaO
(I) (II)
Fig. 1. Schemes of the organization of the synthesis process. (I) based on chemically coupled reactions; (II) based on thermally coupled reactions.
iSHS 2019
Moscow, Russia
The finished mixture was placed in a transparent quartz cups with a diameter of 15-25 mm and a height of 50-60 mm. Syntheses were carried out in a SHS reactor with a volume of 3 l under an initial excess pressure of argon of 5 MPa. In the experiments, the burning rate, the yield of the metal phase into the ingot, and the mass loss of the mixture during combustion were determined. The phase composition of the combustion products was determined by X-ray phase analysis and electron microscopy. X-ray phase analysis was performed using a DRON-3M diffractometer. Quantitative analysis was performed according to the Rietveld method. The study of the microstructure and elemental composition of the samples was carried out using an ULTRA plus Zeiss ultra-high-resolution field-emission scanning electron microscope equipped with an INCA 350 Oxford Instruments microanalysis system.
In the experiments carried out according to scheme (I), the ratios between the masses M2/(M + M2) of the weakly exothermic Cr2O3xAlxC (Mi) and highly exothermic CaO2xAl (M2) mixtures were varied.
It was found that with an increase in the ratio M2KM1 + M2) from 0.1 to 0.4, the burning rate monotonically increases from 1.3 to 3.4 cm/s, the yield of the target product in the ingot relative to the mass of the charge increases from 27 to 38% and reaches a maximum (38%) at a ratio of
0.3. and then decreases to 31%. In this case, the mass loss of the mixture is 0.6-4.5%.
In the experiments carried out according to scheme (II), the ratios between the layers (D -d)/d of weakly exothermic Cr2O3xAlxC (1) and highly exothermic CaO2xAl (2) mixtures were varied. It was found that with an increase in the ratio (D - d)/d from 1/3 to 3/2, the yield of the target product in the ingot increases and reaches a maximum (35%) at a ratio of 1/1, and then decreases. The mass loss of the mixture is 0.8-5.6%.
It should be noted that the combustion of the weakly exothermic Cr2O3xAlxC mixture in both cases failed.
The analysis of the obtained ingots showed that when organizing the process according to scheme (I), they are composite materials based on MAX phase Cr2AlC (from 53 to 67%), carbides (Cr3C2, Cr7C3) and aluminides (Al8Cr5) of chromium, and during synthesis on based on thermally coupled reactions (II), the target products are composite materials based on MAX phase Cr2AlC (from 50 to 65%), ca rbides (&3C2, &7C3) and aluminides (Al8Cr5)
chromium. The highest content of the MAX phase Cr2AlC is in the samples obtained in experiments conducted according to scheme (I), with the ratio M2AM1 + M2) = 0.3, and in the samples obtained in experiments conducted according to scheme (II), the ratio between the layers (D - d)/d = 1/1.
The diffraction lines of MAX Cr2AlC phase are narrow, which indicates a high degree of perfection of its crystal structure. At the same time, there is a nanolaminate structure with a layer thickness of several nanometers.
The research was supported by the Russian Foundation for Basic Research (project no. 19-08-00053).
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