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18XV International Symposium on Self-Propagating High-Temperature Synthesis
FORMATION OF STRUCTURE OF SHS- AND MA-POWDERS Ti3SiC2 BY SPS
S. A. Oglezneva*", T. L. TalakoA, A. A. Smetkin", A. I. Letskofi, and M. N. Kachenuk"
aPerm National Research Polytechnic University, Perm, 614990 Russia bO.V. Roman Powder Metallurgy Institute, Minsk, 220005 Belarus *e-mail: osa@pm.pstu.ac.ru
DOI: 10.24411/9999-0014A-2019-10112
The development of effective methods for producing composites based on MAX-phases is an urgent task in materials science. Many researchers have shown that with the help of reaction sintering, SHS method it is possible to obtain composites with different MAX-phase content [1-4]. The aim of the present work was a comparative analysis of consolidation composition of Ti-SiC-C upon SPS of the mechanically activated (MA) powder mixtures and the powders obtained by SHS.
For consolidation, (1) mechanically activated mixture of powders of titanium, silicon carbide, and carbon in a molar ratio of 3Ti:1.25SiC:0.75C, which was obtained by processing in a planetary mill and (2) the reaction mixture of titanium, silicon carbide and carbon in a molar ratio of 5Ti:2SiC:1C, which was crushed after after SHS sinters, containing 75% of Ti3SiC2, 10 % TiC, and 15% TixSiy were used. The average particle size of MA and SHS powders was 20 and 1.5 |im, respectively. Consolidation of powders was carried out by SPS using Dr. Sinter SPS-1050b plant (SPS Syntex Inc.) at temperatures of 1100, 1200, 1300 and 1400°C, holding time for 5 min, and under a pressure of 30 MPa. XRD analysis of consolidated powders is shown in Table 1.
Table 1. Phase composition of samples after SPS of MA and SHS-powders.
Method of
processing Tsps, oC T13S1C2 TiC TixSiy Ti SiC
mixtures
MA 1100 28 52 -- 10 10
SHS 33 54 13 -- --
MA 1200 45 55 -- -- --
SHS 6 66 28 -- --
MA 1300 82 18 -- -- --
SHS 8 68 24 -- --
MA 1400 80 20 -- -- --
SHS 12 84 4 -- --
From the presented data it follows that the greatest number of Ti3SiC2 phase (82%) was obtained during consolidation at T = 1300oC of MA mixture. This is due to the fact that the process of formation of Ti3SiC2 occurs at such a temperature T = 1300°C due to the interaction of intermediate phases TixSiy with TiC formed at lower temperatures. At temperatures of 1100 and 1200°C diffusion processes are limited by temperature and time parameters, so the composites are preferred phases of titanium carbides and silicides. At a higher temperature T = 1400°C the content of Ti3SiC2 is slightly reduced due to the decomposition of titanium silicon carbide on titanium carbide and silicon, partially evaporating in a vacuum.
With the consolidation of the milled powders obtained by SHS method, the degradation Ti3SiC2-phase to carbide and the silicides of titanium, due to its dispersion in the initial state and probably the presence of impurities. Already at T = 1100oC Ti3SiC2 decomposition occurs,
■SHS 2019 Moscow, Russia
its share decreases from 75 to 33%. Further temperature rise even more dramatically reduces the content of Ti3SiC2 to 12% at 1400oC.
Figure 1 shows the dependence of the shrinkage of MA and SHS powders on the sintering temperature (up to 1400°C). The nature of their compaction differs significantly from each other. MA powder is compacted almost monotonously throughout the temperature range. The formation of titanium silicon carbide starts at temperatures above 1100°C and is accompanied by a slowing of the shrinkage due to the fact that the resulting Ti3SiC2 has a large volume.
---- --- - - -
/ r
/
_ l / ^ 1
t t
/
_ ____
MA — SHS - -
600 700 800 900 1000 1100 1200 1300 1400
T, °c
Fig. 1. Shrinkage dependence on the sintering temperature of MA and SHS powders.
SHS powders due to their high dispersion are actively consolidated, starting with T = 900°C up to the holding temperature T = 1200°C. Intensive shrinkage of briquettes from SHS powders can be connected, including, with the change in the phase composition during the decomposition of Ti3SiC2. Above the temperature of 1200oC, there is an inhibition of the process of shrinkage, associated with the two simultaneously occurring processes -decomposition of Ti3SiC2 leading to a decrease in the volume of the sample and synthesis of Ti3SiC2 from silicides and carbides of titanium leading to an increase in sample volume. The quantity of Ti3SiC2 with an increase in the temperature from 1200 to 1400oC increases from 6 to 12%, Table. The density of Ti3SiC2/TiC composites based on MA powders consolidated at 1400°C is 4.5 g/cm3, and TiC/Ti3SiC2 composites based on SHS powders is 4.8 g/cm3.
Thus, SPS of MA powders at temperatures of 1300 and 1400°C makes it possible to form a Ti3SiC2/TiC composite material due to reaction sintering, while the Ti3SiC2 phase is thermally stable. In SPS of highly dispersed SHS powders occurring reversible processes of decomposition-synthesis Ti3SiC2-phase.
The work is executed at financial support of Russian Foundation for Basic Research and Belarusian Republican Foundation for Fundamental Research, grants no. 18-58-00031 Bel_a and T18P-101.
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