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Production of ultra-high temperature carbide (Ta,Zr)C by self-propagating high-temperature synthesis of mechanically activated mixtures
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 8885–8893 www.elsevier.com/locate/ceramint
Production of ultra-high temperature carbide (Ta,Zr)C by self-propagating high-temperature synthesis of mechanically activated mixtures E.I. Patseraa,n, E.A. Levashova, V.V. Kurbatkinaa, D.Yu. Kovalevb b
a National University of Science and Technology “MISIS”, SHS Research & Education Center MISIS-ISMAN, Leninsky Prospect, 4, Moscow 119049, Russia Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, ul. Academica Osipyana, 8, Chernogolovka, Moscow Region 142432, Russia
Received 10 February 2015; received in revised form 20 March 2015; accepted 23 March 2015 Available online 30 March 2015
Abstract The combustion temperatures and rates of mechanically activated (MA) Ta–Zr–C mixtures depending on the initial temperature T0 are determined. The self-heating phenomenon is observed in argon atmosphere at T0 4 380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. Zirconium oxide is formed in the combustion zone at the initial stage of chemical interaction; it is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone, while the binary tantalum–zirconium carbide (Ta,Zr)C is formed closer to the post-combustion zone. In order to maintain the layer by layer stationary combustion mode of SHS, the initial temperature T0 needs to be 298 K, while the duration of mechanical activation needs to be less than 5 min. After longer mechanical activation, the mixtures are prone to bulk combustion even at low initial temperatures. Single-phase (Ta,Zr)C carbide with the lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: SHS; Mechanically activated (MA); Mechanism; Phase formation; Tantalum–zirconium carbide
1. Introduction The advances in science and technology imply that novel functional materials and coatings with improved performance characteristics are designed and industrially implemented. Carbides TaC and ZrC are characterized by high melting point, hardness, resistance to corrosion and ablation (radiation-induced evaporation) [1–6]. Due to the latter property, they have been used to produce ultra-high-temperature composite materials for air and spacecraft industries. Furthermore, these carbides are used in manufacturing hard-alloy cutting tools [7–16], electrical engineering, nuclear industry, medicine, etc. Zirconium and tantalum carbides form a continuous series of solid solutions [9,16]. According to article [4,5] the melting points of binary (Ta,Zr)C carbides are higher than those of individual carbides. Zirconium and tantalum carbides form a n
Corresponding author. E-mail address: [email protected] (E.I. Patsera).
http://dx.doi.org/10.1016/j.ceramint.2015.03.146 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
continuous series of solid solutions [9,16]. According to the authors [4,5] the melting points of binary (Ta,Zr)C carbides are higher than those of individual compounds. In the Ta–Zr–C system, single-phase solid solution Ta1 xZrxC based on tantalum carbide is formed when ZrC content more than 50 mol% [9]. Meanwhile, the dissolution of tantalum carbide in the ZrC structure does not cause any noticeable shifts in phase peaks of solid solution, since a tantalum atom has a smaller radius compared to that of a zirconium atom. Broadening of the residual peaks of ZrC in a TaC–75%ZrC sample at high 2θ angles results from partial dissolution of tantalum carbide in zirconium carbide and formation of a series of Zr1 y TayC solid solutions [9]. Although there is a region of immiscible solid solutions in the quasi-binary TaC–ZrC system, the presence of the single-phase solid solution in the TaC–25%ZrC and TaC–50%ZrC compositions proves the stability of the resulting solid solution [9]. Along with the melting point, other physicochemical properties of complex carbides (such as specific conductivity [6],
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microhardness, coefficient of thermal expansion [5,6], vapor pressure [5], etc.) are also characterized by extreme dependence on carbide composition. The high melting temperature and corrosion stability of tantalum and zirconium carbides impose certain problems during production of materials and items made of them. The singlephase product with desired composition usually cannot be obtained by conventional powder metallurgy methods based on reduction reactions because of significant differences in chemical activity of metal oxides and diffusion coefficients of carbon in the carbide lattice. For example, zirconium carbide can be synthesized using one of the following methods [17,19]: either by direct saturation of zirconium with carbon or by reduction of zirconium oxide by carbon, when the initial components are taken in the powder form. The process runs through the formation of lower zirconium oxides followed by formation of zirconium carbide via reaction ZrOþ 2C=ZrCþ CO. This method is used for industrialscale synthesizing of technically pure zirconium carbide. The process is usually performed at 2000 1C. The reduction of Ta2O5 and subsequent formation of tantalum carbide is carried out at 1400 1600 1C in hydrogen atmosphere or under vacuum. Hence, coreduction of the initial components to obtain complex carbides usually fails. However, the authors of studies [17,19] proposed a hybrid method for synthesis and controlled hydrolysis of metal alkoxides and alkoxyacetylacetonates in the presence of a polymeric source of carbon (phenol–formaldehyde resin) and carbothermal reduction at low pressure and moderate temperatures (1300–1500 1C). They have successfully synthesized refractory nanocrystalline carbides with compositions Ta4HfC5 and Ta4ZrC5 [17]. However, since this process involves many stages and takes a rather long time, searching for the alternative methods to synthesize ultra-refractory carbides remains topical. Self-propagating high-temperature synthesis (SHS) [14,18, 20–22] can be used as an alternative method to produce tantalum–zirconium carbide with the highest melting point [4,5,9,16]. The ternary Ta–Zr–C and Ta–Ti–C systems are similar in terms of their combustion mechanism and kinetics. There are two dominant chemical reactions for both systems: one involves the diffusion mechanism through the melt, while the other one is mediated by gas-phase transfer of reagents. Thus, the temperature profiles of the combustion wave with two characteristic heat release maxima were detected in the Ta–Ti–C system within a broad range of tantalum concentrations (10–50%) [20,21], which was indicative of the splitting mode due to consecutive chemical reactions. However, the two maxima merge as Ta concentration and the initial temperature Т0 increase; the combustion now proceeds in the merging mode, when the consecutive reactions become parallel. The Ti–C and Zr–C systems are similar because the combustion runs through the diffusion mechanism after the formation of a metal–reagent melt, subsequent formation of the reaction surface by capillary impregnation of graphite or soot and carbon dissolution in the melt [23,24]. Solid-phase diffusion through the product layer is the ratelimiting stage during combustion of the Ta–C mixture; however, carbon transfer to the surface of tantalum particles
takes place via CO and CO2 recirculation through the Boudouard–Bell reaction [25]: interaction of a СО2 molecule with carbon yielding two CO moles; gas-phase transfer of 2CO to the surface of Ta particles; chemisorption of 2CO on the surface; two-stage interaction between tantalum and carbon yielding Ta2C semicarbide and subsequently tantalum carbide according to the scheme Та þ 2СО-ТаС þ СО2; desorption of a CO2 molecule from the surface of the newly formed tantalum carbide layer; transfer of CO2 to the surface of a carbon particle; interaction between СО2 and carbon yielding 2CO, etc. The adiabatic combustion temperature T ad for elemental c synthesis of composition 82.71% Taþ 10.43% Zrþ 6.86% С, which was calculated using THERMO software [26], is 2914 K. This value is higher than the melting point of zirconium (2125 K) but significantly lower than those of zirconium and tantalum carbides and much lower than the melting point of tantalum– zirconium carbide. Hence, the zirconium carbide phase in the combustion wave can be formed by crystallization from the oversaturated zirconium melt. Similar phenomena, observed early in Ta–Ti–C system [20,21], can be expected for combustion of ternary Ta–Zr–C mixtures: merging and/or splitting regime of combustion waves depending on mixture composition, degree of powder dispersion, mixture heterogeneity, and the initial temperature T0. Meanwhile, the difference compared to the Ti–Ta–C system is that a strong oxide film preventing zirconium from spontaneous ignition in air is formed on the surface of pyrophoric zirconium powder during delivery and storage. This fact may make it difficult to initiate the SHS reaction. One of the possible ways to eliminate the kinetic hindrance is to mix the Ta–Zr–C ternary system under conditions when oxide films on the surface of zirconium particles are mechanically destroyed, either completely or partially. This study focuses on the kinetics and mechanism of combustion, as well as on the stages of chemical and structural transformations in the Ta–Zr–C system during elemental synthesis with preliminary mechanical activation (MA SHS). 2. Experimental Zirconium (PTsrK-1 grade), tantalum (TaPM grade), and soot (P804T) powders were used as initial reagents. The composition of the reaction mixture was calculated based on the criterion of formation of a refractory binary Ta–Zr carbide [9,16,27] Before mixing, the initial powders were dried in a vacuum drying oven at 90 1C. The mixtures were mechanically activated in an AIR-0.015 ball mill with the operating parameters as follows: working drum volume, 250 cm3; centripetal acceleration along the drum axis, 250 m/s2; the ratio between the ball weight and weight of the raw mixture 20:1. MA was performed in an air atmosphere. The effect of the initial temperature (T0) of the reaction mixture on combustion temperature Tc and rate Uc was studied on a laboratory SHS reactor according to the procedure described in Ref. [22] using cylindrical briquettes (10 mm in diameter, 15 mm high, relative density of 60%). The
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Fig. 1. Microstructure of MA mixtures. Duration of mechanical activation: (a) – 5 min; (b) – 10 min; and (c) – 15 min. White particles – Ta; gray particles – Zr; and black particles – soot.
combustion temperature (Tc) was measured using a WRe5/ WRe20 W-Re thermocouple wire. The thermocouple was mounted through a hole ( 4 mm deep and 2 mm in diameter) drilled in the sample. The combustion rate (Uc) was measured by high-speed video recording using a Panasonic WV-BL600 camera at 15-fold magnification. The stages of phase transitions in the combustion wave were studied by dynamic X-ray diffraction analysis. An LKD-4 one-coordinate position-sensitive detector was used for single-frame recording of XRD patterns [28]. The diffraction pattern was recorded in the 2θ angle range of 18–801, since the initial reagents, hypothetical intermediates, and final products of the reaction have intense diffraction maxima in this range. The dynamics of structural transformations in the combustion wave were studied using the well-known method of stopped combustion front (SCF) by quenching in a copper wedge (the vertex angle of 51) [22] followed by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) of the characteristic SCF areas using a NORAN energy-dispersive X-ray spectrometer. This microscope was also used to analyze the microstructure of the synthesized samples. The phase composition of the combustion products was studied by X-ray diffraction analysis (XRD) using monochromatic CuKα radiation in the mode of step-by-step scanning in the 2θ range of 10–1101 with the scan increment of 0.11; the exposure time was 4 s per point. The spectra were processed using the JCPDS database. The technological parameters of force SHS-pressing (pressing delay time after the combustion was finished; time of exposure of hot synthesis products under pressure; and molding pressure) were optimized to produce dense ceramic samples.
3. Results and discussion Mechanical activation (MA) of the Та–Zr–C reaction mixture slightly reduces the size of tantalum particles due to the relatively high plasticity of tantalum. The particles are deformed during MA to get a flake shape; smaller soot and zirconium particles are embedded into the tantalum flakes, resulting in formation of agglomerated grains 200–300 mm in size (Fig. 1). As MA duration increases, the process of disintegration and agglomeration of powder particles (including agglomerates) is repeated manifold. As a result, their size is averaged and the internal layer structures forming the agglomerates become smaller. The average particle size at MA duration of 5 and 15 min is close both for agglomerates ( 50–150 mm) and for the agglomerate-forming layers (several micrometers wide and up to 50 mm long) (Fig. 1a and c). At MA duration of 10 min, agglomerates become larger and reach 200–300 mm in size; the size of individual layers formed by tantalum also increases. Meanwhile, the thickness of internal layers is significantly reduced at MA duration of 15 min. It should be mentioned that at MA duration of 3–15 min, reaction products were detected in this mixture neither by X-ray diffraction nor by EDS. Further studies were carried out for the reaction mixture exposed to MA for 5 min. The experimental studies of the combustion process demonstrated that the SHS reaction cannot be initiated in nonactivated mixtures with this composition prepared in a ball mill or in mixtures mechanically activated for less than 3 min. The interaction in the combustion mode starts in this system at MA duration of at least 3 min. The combustion temperature and rates of MA mixtures depending on the initial temperature T0 are shown in Table 1. The combustion temperature Tc of the mechanically activated mixtures turned out to be 600 K lower than the adiabatic (2914 K). Such a significant combustion temperature T ad c
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decrease in combustion temperature can be related both to heat losses for heating the surrounding medium and to the occurrence of the endothermic reaction of carbon gasification and CO formation. The effect of T0 on the combustion rate and temperature of the mixture exposed to mechanical activation for more than 3 min was unexpected. The combustion rate values remain virtually unchanged when the mixture is heated to temperatures higher than 373 K. However, even when the mixture is slightly heated above room temperature, Uc noticeably drops from 1.4 to 0.9 mm/s, while Tc increases. To answer the question what is the reason for such an unusual behavior, we studied the thermograms of combustion in detail. Fig. 2 shows the characteristic profiles of the combustion wave at different T0 and duration of MA of the mixture. A number of features were revealed by analyzing the temperature profiles of combustion of MA mixtures in argon atmosphere. Fig. 2 shows the thermograms for combustion initiated by a pulse from the electric spiral: at room temperature of 298 K in the first case (Fig. 2, curves 1 and 3) and after heating in a furnace to T0 ¼ 398 K (Fig. 2, curves 2 and 4). In both cases, recording of the temperature profile was turned on first, while heating was started later. The initiation of the combustion reaction at room temperature in both samples (after MA for 3 and 5 min) abruptly increased temperature as combustion started 10 s after the ignition. In the second case (T0 ¼ 398 K), the lateral surface of the samples was heated. Since the thermocouple is located deep in the center, the temperature readings in the center fall behind
Table 1 Effect of MA duration and initial temperature on combustion parameters. Duration of MA, (min)
T0, (K)
Theating, (K)
Uc, (mm/s)
Tc, (K)
3
298 373 398 578 718
– 753 898 941 998
1.4 0.86 0.9 0.95 0.97
2133 2358 2231 2176 2154
5
298 398
– 1050
2.2 Thermal explosion
2284 2298
2500
3
T, K
2250 2000
those on the sample surface. Hence, the samples were heated in the furnace at the minimal rate. For the sample taken from the mixture mechanically activated for 5 min (Fig. 2, curve 4), heating started at τ ¼ 30 s. The sample was heated to 678 K at τ ¼ 38 s and heating was stopped. The temperature continued to increase due to leveling of the temperature gradient between the sample center and surface; T ¼ 696 K at τ ¼ 42.5 s. However, the temperature did not decrease after leveling but continued to rise to T ¼ 711 K as a result of self-heating. After 3 s (τ ¼ 47.7 s) the temperature was 1071 K. The self-heating rate of the sample subsequently slowed down and the sample temperature was 1357 K at τ ¼ 57 s. Bulk auto-ignition (thermal explosion) took place at τ ¼ 58.8 s and the temperature reached 2298 K. Table 1 shows that the self-heating temperature (Theating) increases with increasing initial temperature T0. Recording of the temperature profile of combustion of a mixture sample exposed to MA for 3 min (Fig. 2, curve 2) showed that heating of the sample started at τ ¼ 18 s. At τ ¼ 23 s, when heating was stopped, the temperature was 680 K. However, identically to the previous case (Fig. 2, curve 4), self-heating of the sample from 701 K (τ ¼ 27 s) to 1000 K (τ ¼ 57 s) took place. The temperature remained almost unchanged at τ ¼ 57–67 s. At τ ¼ 67 (T ¼ 974 K), the mixture was ignited by local heating from the electric spiral. Since the combustion rate is low in this temperature range (see Table 1), the combustion wave reached the thermocouple junction only at τ ¼ 78 s; the combustion temperature of 2149 K was recorded at τ ¼ 83 s. The reasons for these features of thermal processes observed in thermograms can be as follows. After mechanical activation of the mixture in a ball mill, the reaction mixtures become active because oxide films on the surface of zirconium particles become mechanically disintegrated due to friction and deformation and concentration of adsorbed oxygen increases. The adsorbed oxygen interacts both with soot yielding CO2 or CO (at low temperatures via the direct exothermic reactions) or with Zr and Ta metals. Zirconium powder is pyrophoric; i.e., it can be ignited when contacting oxygen even at low temperatures. It seems that the amount of adsorbed oxygen in the described experiments was sufficient for zirconium oxidation even in argon atmosphere. 4 2
1
1750 1500
ignition
1250 1000 750 500 250
t, c 0
10
20
30
40
50
60
70
80
90
1- МА 3 min To = 298K
2- MA 3 min To = 398K
3- MA 5 min To = 298K
4- MA 5 min To = 398K
100
Fig. 2. Profiles of the combustion wave of mechanically activated Ta–Zr–C mixtures for MA duration of 3 and 5 min at initial temperatures of 298 and 398 K.
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Hence, the exothermic oxidation of zirconium by oxygen accumulated during mechanical activation is a plausible reason for self-heating. Indeed, the chemical analysis data show that the total amount of oxygen increases almost threefold with increasing duration of MA: from 0.7 in the raw mixture in a conical mixer to 2.1% after MA for 5 min. As shown above (Table 1), preliminary heating of the mixture up to 373 K reduces Uc. This phenomenon can be interpreted by formation of an oxide film that can temporarily block the reaction surface of a Zr particle in the combustion wave. The oxidized zirconium particle interacts with carbon at a later stage by acting as an inert diluter (thermal ballast) in the combustion zone. The combustion rate drops as temperature is changed insignificantly. At concentrations of adsorbed oxygen up to 2.1% (after MA for 5 min), self-heating can spontaneously develop into bulk combustion, a process that can hardly be controlled. For this reason, it is undesirable to expose the system to long-term mechanical activation (for more than 5 min) and to increase the initial temperature (above room temperature) both because of fire and explosion hazard and in terms of providing quality of synthesis products. The duration of MA should be sufficient to decompose oxide films, expose the active zirconium surface, and initiate layer by layer stationary combustion. The duration of MA should be selected within the range of 3–5 min for specific conditions. X-ray phase analysis of final products and energydispersive spectroscopy of the stopped combustion front (SCF) were carried out to study the phase and structure formation processes. The microstructures with the characteristic areas are shown in Fig. 3. Fig. 3a shows the zone of heating of the initial mixture before the interaction was started. Individual tantalum and zirconium particles, as well as soot particles located between them, are clearly seen. Fig. 4b shows the microstructure in the combustion zone (i.e., the beginning of chemical interaction). A large tantalum particle with small carbide grains being formed along its edges is shown in the left portion of Fig. 3b; the composition of carbide grains is listed in Table 2. No carbide grain nuclei can be detected in the center of the particle yet. The gray areas (Fig. 3b, points 1, 2, and 4) are zirconium-rich. A large light tantalum particle can be seen in the bottom right corner (Fig. 3b, point 3). The reaction in the particle has not started yet, since no rounded carbide grains can be detected and the particle has an oblong fragmented shape that is typical of tantalum particles in the MA mixture. A more thorough examination of the combustion zone adjacent to the post-combustion zone allows one to distinguish two particle types with different morphologies: spherical (Fig. 3c) and flat ones (Fig. 3d). The share of spherical particles is low ( r 15%). The EDS data indicate that spherical particles are likely to be formed in areas where zirconium melt was spreading. Zirconium content is higher in all the points of particles under study (Table 3, points 1, 3–5) compared to the other adjacent particles (Table 3, point 2). A flat particle coated with extremely small carbide grains 100–200 nm in size can
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Fig. 3. Characteristic SCF areas on a sample taken from the mixture after MA for 5 min: (a) – heating zone; (b), (c) – combustion zone; (d) – postcombustion zone; and (e) – the final combustion product.
be seen in Fig. 4d. The grain composition is heterogeneous, but tantalum content is always higher than that of zirconium (Table 4). Diffusion processes resulting in leveling of component concentration (Table 5) and increasing the size of carbide grains take place in the post-combustion zone. The grain size increases to 5–10 mm (Fig. 3e). However, certain heterogeneity
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Fig. 4. Selected X-ray diffraction patterns of combustion of the mechanically activated Ta–Zr–C mixture, (a) – full time of analyses, (b) – 11 s, (c) – 19 s, and (d) – 33 s.
Table 2 Composition of the SCF sample in the combustion zone.
Table 3 Composition of the SCF sample in the combustion zone.
N
N
1 2 3 4
Content, (at%) Та
Zr
С
42.6 42.9 53.3 38.1
14.5 6.2 0.1 19.0
42.9 50.9 46.4 42.9
of phase composition is retained. X-ray phase analysis of a portion of the post-combustion zone showed that three phases are present: stoichiometric tantalum monocarbide TaC with the lattice parameter of 0.4455 nm ( 55%);
1 2 3 4 5
Content, (at%) Та
Zr
С
25.1 46.7 25.1 12.4 29.4
32.0 7.7 31.9 42.7 35.9
42.9 45.6 43.0 44.9 34.7
binary tantalum–zirconium carbide (Ta,Zr)C with the lattice parameter of 0.4489 nm ( 33%), which corresponds to 14.0% of Zr in the carbide; and a small amount (1–2%) of tantalum semicarbide Ta2C. The incompleteness of diffusion
E.I. Patsera et al. / Ceramics International 41 (2015) 8885–8893 Table 4 Composition of the SCF sample in the post-combustion zone. N
1 2 3
Content, (at%) Та
Zr
С
35.1 56.8 43.5
5.3 8.5 16.7
60.6 34.7 39.8
Table 5 Composition of the SCF sample in the final product zone. N
1 2
Content, (at%) Та
Zr
С
38.4 40.1
13.3 9.2
48.3 50.7
processes in the samples is associated with the high cooling rate of quenched samples. In order to prove the stages of chemical reactions in the combustion wave for a mixture exposed to MA for 5 min, we determined the phase composition by dynamic X-ray diffraction analysis [28]. The results are presented as a selected series of XRD patterns recorded during the combustion in a cell located directly in the plane of the goniometer. Fig. 4 shows the diffraction pattern of evolution of the crystalline structure of the initial reagents and synthesis products. Fig. 4a (τ ¼ 0 s) corresponds to the XRD pattern of the initial mixture. Reflections from the (110) and (211) planes of Ta can be seen. Zirconium is represented by two reflection peaks from the (100) and (002) planes; its intense (101) line is superimposed with the (110) line of Ta. The (201) line of Zr and the (211) line of Ta are also superimposed. No reflections from carbon are observed, since amorphous soot was used in this study. The phase composition of the mixture corresponds to Ta þ Zr. Immediately after the combustion front passes through τ ¼ 11 s (Fig. 4b), (100) Zr and (002) Zr peaks disappear from the XRD pattern and the ZrO2 (111) peak starts to appear (its intensity increases with time). The ZrО2 peaks are seen well starting with τ ¼ 11 s. The intensity of Ta (110) and (211) peaks decreases simultaneously, which suggests that the interaction between tantalum and carbon was started or that the intensity was reduced because the content of unbound zirconium was decreased due to its oxidation. The (101) Zr and (110) Та reflections are superimposed because of line broadening caused by MA and smaller size of powder particles. Furthermore, reflections of the corresponding planes of all the carbides formed are also found in this 2θ range: (100) and (002) Та2С; (111) and (200) ТаС; (111) and (200) ZrС. The initial size of carbides is less
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than 100 nm, which also favors line broadening. Ta2C formation can be observed from (110) reflections in the 2θ CuKα range of 59–60о; ZrC formation, from (220) reflections at 2θ CuKα ¼ 55.45о. The intensity of (200) Ta reflection is low and was not observed in the XRD pattern of the initial mixture under these conditions of recording. At τ ¼ 11 s, the peaks corresponding to (220) ZrС and (110) Та2С reflections are seen in the XRD pattern at 2θ CuKα angles of 55.451 and 59–601. However, the question regarding Ta2C formation is ambiguous, since the (110) Та2С and (220) ТаС lines may be superimposed. It can be seen in Fig. 4 (τ ¼ 11 and 19 s) that line intensity is low; the line width is 51. The intensity of these peaks increases with time. The Ta (110) and (211) peaks decrease with time: they initially shift toward smaller 2θ angles and starting with τ ¼ 19 s, toward greater angles. The shift toward smaller angles is obviously caused by zirconium melting, formation of zirconium carbide, and its dissolution in tantalum. The shift in tantalum lines toward greater angles may suggest that tantalum carbide is formed and zirconium is dissolved in it. The dynamic X-ray phase analysis data (Fig. 4) proved that interaction in this system starts with zirconium oxidation at τ ¼ 8 s and is followed by formation of zirconium and tantalum carbides starting with τ ¼ 11 s. After τ ¼ 19 s, the XRD pattern is characterized by narrowing and splitting of X-ray lines. However, no fundamental changes in the XRD pattern were observed, suggesting that the phase formation process is finished. Line intensity is enhanced (Fig. 4, τ ¼ 33 s) due to growth of crystals of the newly formed phases. The X-ray phase analysis data demonstrate that as opposed to the quenched samples, the products synthesized by forced SHS compaction in a sand mold (Fig. 5a) are the single-phase (Ta,Zr) C solution with the lattice parameter of tantalum–zirconium carbide of 0.4479 nm, which corresponds to 10–11% of zirconium in complex carbide. The microstructure of the compacted product is shown in Fig. 5b. 4. Conclusions 1. The macrokinetic features of combustion of mixtures in the Ta–Zr–C system were studied. The combustion temperatures and rates for MA mixtures depending on the initial temperature T0 were determined. The self-heating effect was observed in argon atmosphere at Т0 4380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. 2. To conduct SHS in the layer by layer stationary combustion mode, the initial temperature Т0 needs to be 298 K, while the duration of MA needs to be less than 5 min. After a longer MA, the mixtures become prone to bulk combustion even at low initial temperatures. 3. The dynamics of structural transformations in the combustion wave were studied. Zirconium oxide is partially formed in the combustion zone at the initial stage of chemical interaction; remainder is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone
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Fig. 5. X-ray diffraction pattern (a) and microstructure (b) of single-phase solid solution (Ta,Zr)C synthesized by forced SHS compaction from the mixture mechanically activated for 5 min.
and binary tantalum–zirconium carbide (Ta,Zr)C emerges closer to the post-combustion zone. 4. Single-phase carbide (Ta, Zr)C with lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold.
Acknowledgments This work was carried out with partial financial support from the Ministry of Education and Science of the Russian Federation in the framework of state assignment No. 11.233.2014/K in the part of mechanism of combustion and structure formation and in the framework of Increase Competitiveness Program of NUST «MISiS» (No. К2-2014-012) in the part of dense product obtaining using force SHS-pressing technology. The authors are
grateful to senior researcher N.A. Kochetov for his assistance in conducting combustion experiments.
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E.I. Patsera et al. / Ceramics International 41 (2015) 8885–8893 [5] G.V. Samsonov, I.M. Vinitskii, Handbook of Refractory Compounds, Metallurgiya, Moscow, 1976. [6] I.G. Barantseva, V.N. Paderno, Yu.B. Paderno, Some physical properties of alloys of the systems ZrC–NbC and TaC–HfC, Powder Metall. Met. Ceram. 6 (2) (1967) 139–141. [7] X. Zhang, N. Liu, Effects of ZrC on microstructure, mechanical properties and thermal shock resistance of TiC–ZrC–Co–Ni cermets, Mater. Sci. Eng. 561 (2013) 270–276. [8] A.G. Merzhanov, A.S. Rogachev, A.S. Mukas'yan, V.M. Khusid, I.P. Borovinskaya, B.B. Khina, The role of gas phase mass transfer during combustion in the tantalum–carbon system, Inzhenerno-Fiz. Zhurnal 59 (3) (1990) 1–15. [9] S.A. Ghaffari, M.A. Faghihi-Sani, F. Golestani-Fard, M. Nojabayy, Diffusion and solid solution formation between the binary carbides of TaC, HfC and ZrC, Int. J. Refract. Met. Hard Mater. 41 (2013) 180–184. [10] X. Zhang, N. Liu, C. Rong, J. Zhou, Microstructure and mechanical properties of TiC–TiN–Zr–WC–Ni–Co cermets, Ceram. Int. 35 (3) (2009) 1187–1193. [11] Peng Zhou, Yingbiao Penga, Yong Du, Shequan Wang, Guanghua Wen, W.e.n. Xieb, Keke Chang, A thermodynamic description of the C–Ta–Zr system, Int. J. Refract. Met. Hard Mater. 41 (2013) 408–415. [12] U. Rolander, G. Weinl, M. Zwinkels, Effect of Ta on structure and mechanical properties of (Ti, Ta,W)(C, N)–Co cermets, Int. J. Refract. Met. Hard Mater. 19 (4–6) (2001) 325. [13] G. Oestberg, K. Buss, M. Christensen, S. Norgren, H.-O. Andren, D. Mari, et al., Effect of TaC on plastic deformation of WC–Co and Ti (C, N)–W–Co, Int. J. Refract. Met. Hard Mater. 24 (2006) 145–154. [14] Takeshi Tsuchida, Satoshi Yamamoto, Mechanical activation assisted self-propagating high-temperature synthesis of ZrC and ZrB2 in air from Zr/B/C powder mixtures, J. Eur. Ceram. Soc. 24 (1) (2004) 45–51. [15] I.S. Grigor'ev, E.Z. Meilikhov (Eds.) FiPhysical Values, Energoatomizdat, Moscow, 1991, pp. 303. [16] Kh. Khollek, Binary and Tertiary Carbide and Nitride Systems of Transition Metals. Handbook, Metallurgiya, Moscow, 1988.
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[17] V.G. Sevast'yanov, E.P. Simonenko, N.A. Ignatov, Y.u.S. Ezhov, N.P. Simonenko, N.T. Kuznetsov, Low-temperature synthesis of nanodispersed titanium, zirconium, and hafnium carbides, Zhurnal Neorg. Khimii 56 (5) (2011) 707–719. [18] J.M. Córdoba, M.A. Avilés, M.J. Sayagués, M.D. Alcalá, F.J. Gotor, Synthesis of complex carbonitride powders TiyMT1 yCxN1 x (MT: Zr, V, Ta, Hf) via a mechanically induced self-sustaining reaction, J. Alloy. Compd. 482 (2009) 349–355. [19] N.A. Ignatov, Synthesis of highly dispersed and nanocrystalline binary and mixed carbides of tantalum and group IVB metals under mild conditions, Cand. Sci. (Chem.) (Dissertation), Moscow, 2011. [20] E.A. Levashov, A.S. Rogachev, Y.u.K. Epishko, N.A. Kochetov, Poroshkovaya metallurgiya i funktsional'nye pokrytiya, Izv. vuzov. 1 (2007) 14. [21] E.A. Levashov, V.V. Kurbatkina, A.S. Rogachev, N.A. Kochetov, E. I. Patsera, N.V. Sachkova, Characteristic properties of combustion and structure formation in the Ti–Ta–C system, Russ. J. Non-Ferr. Met. 49 (5) (2008) 404–413. [22] E.A. Levashov, A.S. Rogachev, V.V. Kurbatkina, et al., Prospective Materials and Technologies of Self-Propagating High-Temperature Synthesis, MISiS, Moscow, 2011. [23] A.S. Rogachev, A.S. Mukas'yan, A.G. Merzhanov, Dokl. Akad. Nauk. 297 (6) (1987) 1425. [24] P. Mossino, Ceram. Int. 30 (3) (2004) 311. [25] S.L. Kharatyan, Self-Propagating High-temperature Synthesis: Theory and Practice, in: A.E. Sychev (Ed.), Territoriya, Chernogolovka, 2001, p. 157. [26] A.A. Shiryaev, Thermodynamic of SHS: modern approach, Int. J. SHS 4 (1995) 351–362. [27] X.-G. Wang, Effect of solid solution formation on densification of hotpressed ZrC ceramics with MC (M ¼V, Nb, and Ta) additions, J. Eur. Ceram. Soc. 32 (8) (2012) 1795–1802. [28] V.I. Ponomarev, I.O. Khomenko, A.G. Merzhanov, The laboratory method of dynamic X-ray diffraction, Kristallografiya 40 (1) (1995) 14–17.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 8885–8893 www.elsevier.com/locate/ceramint
Production of ultra-high temperature carbide (Ta,Zr)C by self-propagating high-temperature synthesis of mechanically activated mixtures E.I. Patseraa,n, E.A. Levashova, V.V. Kurbatkinaa, D.Yu. Kovalevb b
a National University of Science and Technology “MISIS”, SHS Research & Education Center MISIS-ISMAN, Leninsky Prospect, 4, Moscow 119049, Russia Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, ul. Academica Osipyana, 8, Chernogolovka, Moscow Region 142432, Russia
Received 10 February 2015; received in revised form 20 March 2015; accepted 23 March 2015 Available online 30 March 2015
Abstract The combustion temperatures and rates of mechanically activated (MA) Ta–Zr–C mixtures depending on the initial temperature T0 are determined. The self-heating phenomenon is observed in argon atmosphere at T0 4 380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. Zirconium oxide is formed in the combustion zone at the initial stage of chemical interaction; it is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone, while the binary tantalum–zirconium carbide (Ta,Zr)C is formed closer to the post-combustion zone. In order to maintain the layer by layer stationary combustion mode of SHS, the initial temperature T0 needs to be 298 K, while the duration of mechanical activation needs to be less than 5 min. After longer mechanical activation, the mixtures are prone to bulk combustion even at low initial temperatures. Single-phase (Ta,Zr)C carbide with the lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: SHS; Mechanically activated (MA); Mechanism; Phase formation; Tantalum–zirconium carbide
1. Introduction The advances in science and technology imply that novel functional materials and coatings with improved performance characteristics are designed and industrially implemented. Carbides TaC and ZrC are characterized by high melting point, hardness, resistance to corrosion and ablation (radiation-induced evaporation) [1–6]. Due to the latter property, they have been used to produce ultra-high-temperature composite materials for air and spacecraft industries. Furthermore, these carbides are used in manufacturing hard-alloy cutting tools [7–16], electrical engineering, nuclear industry, medicine, etc. Zirconium and tantalum carbides form a continuous series of solid solutions [9,16]. According to article [4,5] the melting points of binary (Ta,Zr)C carbides are higher than those of individual carbides. Zirconium and tantalum carbides form a n
Corresponding author. E-mail address: [email protected] (E.I. Patsera).
http://dx.doi.org/10.1016/j.ceramint.2015.03.146 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
continuous series of solid solutions [9,16]. According to the authors [4,5] the melting points of binary (Ta,Zr)C carbides are higher than those of individual compounds. In the Ta–Zr–C system, single-phase solid solution Ta1 xZrxC based on tantalum carbide is formed when ZrC content more than 50 mol% [9]. Meanwhile, the dissolution of tantalum carbide in the ZrC structure does not cause any noticeable shifts in phase peaks of solid solution, since a tantalum atom has a smaller radius compared to that of a zirconium atom. Broadening of the residual peaks of ZrC in a TaC–75%ZrC sample at high 2θ angles results from partial dissolution of tantalum carbide in zirconium carbide and formation of a series of Zr1 y TayC solid solutions [9]. Although there is a region of immiscible solid solutions in the quasi-binary TaC–ZrC system, the presence of the single-phase solid solution in the TaC–25%ZrC and TaC–50%ZrC compositions proves the stability of the resulting solid solution [9]. Along with the melting point, other physicochemical properties of complex carbides (such as specific conductivity [6],
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microhardness, coefficient of thermal expansion [5,6], vapor pressure [5], etc.) are also characterized by extreme dependence on carbide composition. The high melting temperature and corrosion stability of tantalum and zirconium carbides impose certain problems during production of materials and items made of them. The singlephase product with desired composition usually cannot be obtained by conventional powder metallurgy methods based on reduction reactions because of significant differences in chemical activity of metal oxides and diffusion coefficients of carbon in the carbide lattice. For example, zirconium carbide can be synthesized using one of the following methods [17,19]: either by direct saturation of zirconium with carbon or by reduction of zirconium oxide by carbon, when the initial components are taken in the powder form. The process runs through the formation of lower zirconium oxides followed by formation of zirconium carbide via reaction ZrOþ 2C=ZrCþ CO. This method is used for industrialscale synthesizing of technically pure zirconium carbide. The process is usually performed at 2000 1C. The reduction of Ta2O5 and subsequent formation of tantalum carbide is carried out at 1400 1600 1C in hydrogen atmosphere or under vacuum. Hence, coreduction of the initial components to obtain complex carbides usually fails. However, the authors of studies [17,19] proposed a hybrid method for synthesis and controlled hydrolysis of metal alkoxides and alkoxyacetylacetonates in the presence of a polymeric source of carbon (phenol–formaldehyde resin) and carbothermal reduction at low pressure and moderate temperatures (1300–1500 1C). They have successfully synthesized refractory nanocrystalline carbides with compositions Ta4HfC5 and Ta4ZrC5 [17]. However, since this process involves many stages and takes a rather long time, searching for the alternative methods to synthesize ultra-refractory carbides remains topical. Self-propagating high-temperature synthesis (SHS) [14,18, 20–22] can be used as an alternative method to produce tantalum–zirconium carbide with the highest melting point [4,5,9,16]. The ternary Ta–Zr–C and Ta–Ti–C systems are similar in terms of their combustion mechanism and kinetics. There are two dominant chemical reactions for both systems: one involves the diffusion mechanism through the melt, while the other one is mediated by gas-phase transfer of reagents. Thus, the temperature profiles of the combustion wave with two characteristic heat release maxima were detected in the Ta–Ti–C system within a broad range of tantalum concentrations (10–50%) [20,21], which was indicative of the splitting mode due to consecutive chemical reactions. However, the two maxima merge as Ta concentration and the initial temperature Т0 increase; the combustion now proceeds in the merging mode, when the consecutive reactions become parallel. The Ti–C and Zr–C systems are similar because the combustion runs through the diffusion mechanism after the formation of a metal–reagent melt, subsequent formation of the reaction surface by capillary impregnation of graphite or soot and carbon dissolution in the melt [23,24]. Solid-phase diffusion through the product layer is the ratelimiting stage during combustion of the Ta–C mixture; however, carbon transfer to the surface of tantalum particles
takes place via CO and CO2 recirculation through the Boudouard–Bell reaction [25]: interaction of a СО2 molecule with carbon yielding two CO moles; gas-phase transfer of 2CO to the surface of Ta particles; chemisorption of 2CO on the surface; two-stage interaction between tantalum and carbon yielding Ta2C semicarbide and subsequently tantalum carbide according to the scheme Та þ 2СО-ТаС þ СО2; desorption of a CO2 molecule from the surface of the newly formed tantalum carbide layer; transfer of CO2 to the surface of a carbon particle; interaction between СО2 and carbon yielding 2CO, etc. The adiabatic combustion temperature T ad for elemental c synthesis of composition 82.71% Taþ 10.43% Zrþ 6.86% С, which was calculated using THERMO software [26], is 2914 K. This value is higher than the melting point of zirconium (2125 K) but significantly lower than those of zirconium and tantalum carbides and much lower than the melting point of tantalum– zirconium carbide. Hence, the zirconium carbide phase in the combustion wave can be formed by crystallization from the oversaturated zirconium melt. Similar phenomena, observed early in Ta–Ti–C system [20,21], can be expected for combustion of ternary Ta–Zr–C mixtures: merging and/or splitting regime of combustion waves depending on mixture composition, degree of powder dispersion, mixture heterogeneity, and the initial temperature T0. Meanwhile, the difference compared to the Ti–Ta–C system is that a strong oxide film preventing zirconium from spontaneous ignition in air is formed on the surface of pyrophoric zirconium powder during delivery and storage. This fact may make it difficult to initiate the SHS reaction. One of the possible ways to eliminate the kinetic hindrance is to mix the Ta–Zr–C ternary system under conditions when oxide films on the surface of zirconium particles are mechanically destroyed, either completely or partially. This study focuses on the kinetics and mechanism of combustion, as well as on the stages of chemical and structural transformations in the Ta–Zr–C system during elemental synthesis with preliminary mechanical activation (MA SHS). 2. Experimental Zirconium (PTsrK-1 grade), tantalum (TaPM grade), and soot (P804T) powders were used as initial reagents. The composition of the reaction mixture was calculated based on the criterion of formation of a refractory binary Ta–Zr carbide [9,16,27] Before mixing, the initial powders were dried in a vacuum drying oven at 90 1C. The mixtures were mechanically activated in an AIR-0.015 ball mill with the operating parameters as follows: working drum volume, 250 cm3; centripetal acceleration along the drum axis, 250 m/s2; the ratio between the ball weight and weight of the raw mixture 20:1. MA was performed in an air atmosphere. The effect of the initial temperature (T0) of the reaction mixture on combustion temperature Tc and rate Uc was studied on a laboratory SHS reactor according to the procedure described in Ref. [22] using cylindrical briquettes (10 mm in diameter, 15 mm high, relative density of 60%). The
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Fig. 1. Microstructure of MA mixtures. Duration of mechanical activation: (a) – 5 min; (b) – 10 min; and (c) – 15 min. White particles – Ta; gray particles – Zr; and black particles – soot.
combustion temperature (Tc) was measured using a WRe5/ WRe20 W-Re thermocouple wire. The thermocouple was mounted through a hole ( 4 mm deep and 2 mm in diameter) drilled in the sample. The combustion rate (Uc) was measured by high-speed video recording using a Panasonic WV-BL600 camera at 15-fold magnification. The stages of phase transitions in the combustion wave were studied by dynamic X-ray diffraction analysis. An LKD-4 one-coordinate position-sensitive detector was used for single-frame recording of XRD patterns [28]. The diffraction pattern was recorded in the 2θ angle range of 18–801, since the initial reagents, hypothetical intermediates, and final products of the reaction have intense diffraction maxima in this range. The dynamics of structural transformations in the combustion wave were studied using the well-known method of stopped combustion front (SCF) by quenching in a copper wedge (the vertex angle of 51) [22] followed by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) of the characteristic SCF areas using a NORAN energy-dispersive X-ray spectrometer. This microscope was also used to analyze the microstructure of the synthesized samples. The phase composition of the combustion products was studied by X-ray diffraction analysis (XRD) using monochromatic CuKα radiation in the mode of step-by-step scanning in the 2θ range of 10–1101 with the scan increment of 0.11; the exposure time was 4 s per point. The spectra were processed using the JCPDS database. The technological parameters of force SHS-pressing (pressing delay time after the combustion was finished; time of exposure of hot synthesis products under pressure; and molding pressure) were optimized to produce dense ceramic samples.
3. Results and discussion Mechanical activation (MA) of the Та–Zr–C reaction mixture slightly reduces the size of tantalum particles due to the relatively high plasticity of tantalum. The particles are deformed during MA to get a flake shape; smaller soot and zirconium particles are embedded into the tantalum flakes, resulting in formation of agglomerated grains 200–300 mm in size (Fig. 1). As MA duration increases, the process of disintegration and agglomeration of powder particles (including agglomerates) is repeated manifold. As a result, their size is averaged and the internal layer structures forming the agglomerates become smaller. The average particle size at MA duration of 5 and 15 min is close both for agglomerates ( 50–150 mm) and for the agglomerate-forming layers (several micrometers wide and up to 50 mm long) (Fig. 1a and c). At MA duration of 10 min, agglomerates become larger and reach 200–300 mm in size; the size of individual layers formed by tantalum also increases. Meanwhile, the thickness of internal layers is significantly reduced at MA duration of 15 min. It should be mentioned that at MA duration of 3–15 min, reaction products were detected in this mixture neither by X-ray diffraction nor by EDS. Further studies were carried out for the reaction mixture exposed to MA for 5 min. The experimental studies of the combustion process demonstrated that the SHS reaction cannot be initiated in nonactivated mixtures with this composition prepared in a ball mill or in mixtures mechanically activated for less than 3 min. The interaction in the combustion mode starts in this system at MA duration of at least 3 min. The combustion temperature and rates of MA mixtures depending on the initial temperature T0 are shown in Table 1. The combustion temperature Tc of the mechanically activated mixtures turned out to be 600 K lower than the adiabatic (2914 K). Such a significant combustion temperature T ad c
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decrease in combustion temperature can be related both to heat losses for heating the surrounding medium and to the occurrence of the endothermic reaction of carbon gasification and CO formation. The effect of T0 on the combustion rate and temperature of the mixture exposed to mechanical activation for more than 3 min was unexpected. The combustion rate values remain virtually unchanged when the mixture is heated to temperatures higher than 373 K. However, even when the mixture is slightly heated above room temperature, Uc noticeably drops from 1.4 to 0.9 mm/s, while Tc increases. To answer the question what is the reason for such an unusual behavior, we studied the thermograms of combustion in detail. Fig. 2 shows the characteristic profiles of the combustion wave at different T0 and duration of MA of the mixture. A number of features were revealed by analyzing the temperature profiles of combustion of MA mixtures in argon atmosphere. Fig. 2 shows the thermograms for combustion initiated by a pulse from the electric spiral: at room temperature of 298 K in the first case (Fig. 2, curves 1 and 3) and after heating in a furnace to T0 ¼ 398 K (Fig. 2, curves 2 and 4). In both cases, recording of the temperature profile was turned on first, while heating was started later. The initiation of the combustion reaction at room temperature in both samples (after MA for 3 and 5 min) abruptly increased temperature as combustion started 10 s after the ignition. In the second case (T0 ¼ 398 K), the lateral surface of the samples was heated. Since the thermocouple is located deep in the center, the temperature readings in the center fall behind
Table 1 Effect of MA duration and initial temperature on combustion parameters. Duration of MA, (min)
T0, (K)
Theating, (K)
Uc, (mm/s)
Tc, (K)
3
298 373 398 578 718
– 753 898 941 998
1.4 0.86 0.9 0.95 0.97
2133 2358 2231 2176 2154
5
298 398
– 1050
2.2 Thermal explosion
2284 2298
2500
3
T, K
2250 2000
those on the sample surface. Hence, the samples were heated in the furnace at the minimal rate. For the sample taken from the mixture mechanically activated for 5 min (Fig. 2, curve 4), heating started at τ ¼ 30 s. The sample was heated to 678 K at τ ¼ 38 s and heating was stopped. The temperature continued to increase due to leveling of the temperature gradient between the sample center and surface; T ¼ 696 K at τ ¼ 42.5 s. However, the temperature did not decrease after leveling but continued to rise to T ¼ 711 K as a result of self-heating. After 3 s (τ ¼ 47.7 s) the temperature was 1071 K. The self-heating rate of the sample subsequently slowed down and the sample temperature was 1357 K at τ ¼ 57 s. Bulk auto-ignition (thermal explosion) took place at τ ¼ 58.8 s and the temperature reached 2298 K. Table 1 shows that the self-heating temperature (Theating) increases with increasing initial temperature T0. Recording of the temperature profile of combustion of a mixture sample exposed to MA for 3 min (Fig. 2, curve 2) showed that heating of the sample started at τ ¼ 18 s. At τ ¼ 23 s, when heating was stopped, the temperature was 680 K. However, identically to the previous case (Fig. 2, curve 4), self-heating of the sample from 701 K (τ ¼ 27 s) to 1000 K (τ ¼ 57 s) took place. The temperature remained almost unchanged at τ ¼ 57–67 s. At τ ¼ 67 (T ¼ 974 K), the mixture was ignited by local heating from the electric spiral. Since the combustion rate is low in this temperature range (see Table 1), the combustion wave reached the thermocouple junction only at τ ¼ 78 s; the combustion temperature of 2149 K was recorded at τ ¼ 83 s. The reasons for these features of thermal processes observed in thermograms can be as follows. After mechanical activation of the mixture in a ball mill, the reaction mixtures become active because oxide films on the surface of zirconium particles become mechanically disintegrated due to friction and deformation and concentration of adsorbed oxygen increases. The adsorbed oxygen interacts both with soot yielding CO2 or CO (at low temperatures via the direct exothermic reactions) or with Zr and Ta metals. Zirconium powder is pyrophoric; i.e., it can be ignited when contacting oxygen even at low temperatures. It seems that the amount of adsorbed oxygen in the described experiments was sufficient for zirconium oxidation even in argon atmosphere. 4 2
1
1750 1500
ignition
1250 1000 750 500 250
t, c 0
10
20
30
40
50
60
70
80
90
1- МА 3 min To = 298K
2- MA 3 min To = 398K
3- MA 5 min To = 298K
4- MA 5 min To = 398K
100
Fig. 2. Profiles of the combustion wave of mechanically activated Ta–Zr–C mixtures for MA duration of 3 and 5 min at initial temperatures of 298 and 398 K.
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Hence, the exothermic oxidation of zirconium by oxygen accumulated during mechanical activation is a plausible reason for self-heating. Indeed, the chemical analysis data show that the total amount of oxygen increases almost threefold with increasing duration of MA: from 0.7 in the raw mixture in a conical mixer to 2.1% after MA for 5 min. As shown above (Table 1), preliminary heating of the mixture up to 373 K reduces Uc. This phenomenon can be interpreted by formation of an oxide film that can temporarily block the reaction surface of a Zr particle in the combustion wave. The oxidized zirconium particle interacts with carbon at a later stage by acting as an inert diluter (thermal ballast) in the combustion zone. The combustion rate drops as temperature is changed insignificantly. At concentrations of adsorbed oxygen up to 2.1% (after MA for 5 min), self-heating can spontaneously develop into bulk combustion, a process that can hardly be controlled. For this reason, it is undesirable to expose the system to long-term mechanical activation (for more than 5 min) and to increase the initial temperature (above room temperature) both because of fire and explosion hazard and in terms of providing quality of synthesis products. The duration of MA should be sufficient to decompose oxide films, expose the active zirconium surface, and initiate layer by layer stationary combustion. The duration of MA should be selected within the range of 3–5 min for specific conditions. X-ray phase analysis of final products and energydispersive spectroscopy of the stopped combustion front (SCF) were carried out to study the phase and structure formation processes. The microstructures with the characteristic areas are shown in Fig. 3. Fig. 3a shows the zone of heating of the initial mixture before the interaction was started. Individual tantalum and zirconium particles, as well as soot particles located between them, are clearly seen. Fig. 4b shows the microstructure in the combustion zone (i.e., the beginning of chemical interaction). A large tantalum particle with small carbide grains being formed along its edges is shown in the left portion of Fig. 3b; the composition of carbide grains is listed in Table 2. No carbide grain nuclei can be detected in the center of the particle yet. The gray areas (Fig. 3b, points 1, 2, and 4) are zirconium-rich. A large light tantalum particle can be seen in the bottom right corner (Fig. 3b, point 3). The reaction in the particle has not started yet, since no rounded carbide grains can be detected and the particle has an oblong fragmented shape that is typical of tantalum particles in the MA mixture. A more thorough examination of the combustion zone adjacent to the post-combustion zone allows one to distinguish two particle types with different morphologies: spherical (Fig. 3c) and flat ones (Fig. 3d). The share of spherical particles is low ( r 15%). The EDS data indicate that spherical particles are likely to be formed in areas where zirconium melt was spreading. Zirconium content is higher in all the points of particles under study (Table 3, points 1, 3–5) compared to the other adjacent particles (Table 3, point 2). A flat particle coated with extremely small carbide grains 100–200 nm in size can
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Fig. 3. Characteristic SCF areas on a sample taken from the mixture after MA for 5 min: (a) – heating zone; (b), (c) – combustion zone; (d) – postcombustion zone; and (e) – the final combustion product.
be seen in Fig. 4d. The grain composition is heterogeneous, but tantalum content is always higher than that of zirconium (Table 4). Diffusion processes resulting in leveling of component concentration (Table 5) and increasing the size of carbide grains take place in the post-combustion zone. The grain size increases to 5–10 mm (Fig. 3e). However, certain heterogeneity
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Fig. 4. Selected X-ray diffraction patterns of combustion of the mechanically activated Ta–Zr–C mixture, (a) – full time of analyses, (b) – 11 s, (c) – 19 s, and (d) – 33 s.
Table 2 Composition of the SCF sample in the combustion zone.
Table 3 Composition of the SCF sample in the combustion zone.
N
N
1 2 3 4
Content, (at%) Та
Zr
С
42.6 42.9 53.3 38.1
14.5 6.2 0.1 19.0
42.9 50.9 46.4 42.9
of phase composition is retained. X-ray phase analysis of a portion of the post-combustion zone showed that three phases are present: stoichiometric tantalum monocarbide TaC with the lattice parameter of 0.4455 nm ( 55%);
1 2 3 4 5
Content, (at%) Та
Zr
С
25.1 46.7 25.1 12.4 29.4
32.0 7.7 31.9 42.7 35.9
42.9 45.6 43.0 44.9 34.7
binary tantalum–zirconium carbide (Ta,Zr)C with the lattice parameter of 0.4489 nm ( 33%), which corresponds to 14.0% of Zr in the carbide; and a small amount (1–2%) of tantalum semicarbide Ta2C. The incompleteness of diffusion
E.I. Patsera et al. / Ceramics International 41 (2015) 8885–8893 Table 4 Composition of the SCF sample in the post-combustion zone. N
1 2 3
Content, (at%) Та
Zr
С
35.1 56.8 43.5
5.3 8.5 16.7
60.6 34.7 39.8
Table 5 Composition of the SCF sample in the final product zone. N
1 2
Content, (at%) Та
Zr
С
38.4 40.1
13.3 9.2
48.3 50.7
processes in the samples is associated with the high cooling rate of quenched samples. In order to prove the stages of chemical reactions in the combustion wave for a mixture exposed to MA for 5 min, we determined the phase composition by dynamic X-ray diffraction analysis [28]. The results are presented as a selected series of XRD patterns recorded during the combustion in a cell located directly in the plane of the goniometer. Fig. 4 shows the diffraction pattern of evolution of the crystalline structure of the initial reagents and synthesis products. Fig. 4a (τ ¼ 0 s) corresponds to the XRD pattern of the initial mixture. Reflections from the (110) and (211) planes of Ta can be seen. Zirconium is represented by two reflection peaks from the (100) and (002) planes; its intense (101) line is superimposed with the (110) line of Ta. The (201) line of Zr and the (211) line of Ta are also superimposed. No reflections from carbon are observed, since amorphous soot was used in this study. The phase composition of the mixture corresponds to Ta þ Zr. Immediately after the combustion front passes through τ ¼ 11 s (Fig. 4b), (100) Zr and (002) Zr peaks disappear from the XRD pattern and the ZrO2 (111) peak starts to appear (its intensity increases with time). The ZrО2 peaks are seen well starting with τ ¼ 11 s. The intensity of Ta (110) and (211) peaks decreases simultaneously, which suggests that the interaction between tantalum and carbon was started or that the intensity was reduced because the content of unbound zirconium was decreased due to its oxidation. The (101) Zr and (110) Та reflections are superimposed because of line broadening caused by MA and smaller size of powder particles. Furthermore, reflections of the corresponding planes of all the carbides formed are also found in this 2θ range: (100) and (002) Та2С; (111) and (200) ТаС; (111) and (200) ZrС. The initial size of carbides is less
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than 100 nm, which also favors line broadening. Ta2C formation can be observed from (110) reflections in the 2θ CuKα range of 59–60о; ZrC formation, from (220) reflections at 2θ CuKα ¼ 55.45о. The intensity of (200) Ta reflection is low and was not observed in the XRD pattern of the initial mixture under these conditions of recording. At τ ¼ 11 s, the peaks corresponding to (220) ZrС and (110) Та2С reflections are seen in the XRD pattern at 2θ CuKα angles of 55.451 and 59–601. However, the question regarding Ta2C formation is ambiguous, since the (110) Та2С and (220) ТаС lines may be superimposed. It can be seen in Fig. 4 (τ ¼ 11 and 19 s) that line intensity is low; the line width is 51. The intensity of these peaks increases with time. The Ta (110) and (211) peaks decrease with time: they initially shift toward smaller 2θ angles and starting with τ ¼ 19 s, toward greater angles. The shift toward smaller angles is obviously caused by zirconium melting, formation of zirconium carbide, and its dissolution in tantalum. The shift in tantalum lines toward greater angles may suggest that tantalum carbide is formed and zirconium is dissolved in it. The dynamic X-ray phase analysis data (Fig. 4) proved that interaction in this system starts with zirconium oxidation at τ ¼ 8 s and is followed by formation of zirconium and tantalum carbides starting with τ ¼ 11 s. After τ ¼ 19 s, the XRD pattern is characterized by narrowing and splitting of X-ray lines. However, no fundamental changes in the XRD pattern were observed, suggesting that the phase formation process is finished. Line intensity is enhanced (Fig. 4, τ ¼ 33 s) due to growth of crystals of the newly formed phases. The X-ray phase analysis data demonstrate that as opposed to the quenched samples, the products synthesized by forced SHS compaction in a sand mold (Fig. 5a) are the single-phase (Ta,Zr) C solution with the lattice parameter of tantalum–zirconium carbide of 0.4479 nm, which corresponds to 10–11% of zirconium in complex carbide. The microstructure of the compacted product is shown in Fig. 5b. 4. Conclusions 1. The macrokinetic features of combustion of mixtures in the Ta–Zr–C system were studied. The combustion temperatures and rates for MA mixtures depending on the initial temperature T0 were determined. The self-heating effect was observed in argon atmosphere at Т0 4380 K due to oxidation of the surface of zirconium particles by adsorbed oxygen. 2. To conduct SHS in the layer by layer stationary combustion mode, the initial temperature Т0 needs to be 298 K, while the duration of MA needs to be less than 5 min. After a longer MA, the mixtures become prone to bulk combustion even at low initial temperatures. 3. The dynamics of structural transformations in the combustion wave were studied. Zirconium oxide is partially formed in the combustion zone at the initial stage of chemical interaction; remainder is subsequently transformed into zirconium carbide. In addition, tantalum carbide is formed in the combustion zone
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Fig. 5. X-ray diffraction pattern (a) and microstructure (b) of single-phase solid solution (Ta,Zr)C synthesized by forced SHS compaction from the mixture mechanically activated for 5 min.
and binary tantalum–zirconium carbide (Ta,Zr)C emerges closer to the post-combustion zone. 4. Single-phase carbide (Ta, Zr)C with lattice parameter of 0.4479 nm was synthesized by forced SHS compaction in a sand mold.
Acknowledgments This work was carried out with partial financial support from the Ministry of Education and Science of the Russian Federation in the framework of state assignment No. 11.233.2014/K in the part of mechanism of combustion and structure formation and in the framework of Increase Competitiveness Program of NUST «MISiS» (No. К2-2014-012) in the part of dense product obtaining using force SHS-pressing technology. The authors are
grateful to senior researcher N.A. Kochetov for his assistance in conducting combustion experiments.
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