Tantalum carbide products from chemically-activated combustion synthesis reactions

Manuscript Draft Manuscript Number:

Title: Tantalum Carbide Products from Chemically-Activated Combustion Synthesis Reactions

Article Type: Full length article

Keywords: Combustion synthesis; Chemical activation; Carbides; Spark Plasma Sintering; Microstructure

Corresponding Author: Dr. roberto orru, PhD

Corresponding Author's Institution: University of Cagliari First Author: Clara Musa, PhD

Order of Authors: Clara Musa, PhD; Roberta Licheri, PhD; Selena Montinaro, PhD; roberto orru, PhD; Giacomo Cao, PhD

Abstract: A rapid route based on the use of small amounts of

polytetrafluoroethylene to chemically activate and sustain the combustion synthesis reaction for the formation of TaC from its elements is

successfully exploited in this work. Other than a reaction booster, the polymer is found to play also a role as a carbon source, so that part of graphite can be replaced by Teflon to produce a single phase material. A relevant importance in the activation of the synthesis reaction is

provided by the intermediate phase TaF3, whose formation is clearly evidenced, along with that of Ta2C, by combustion front quenching

experiments. Additive free TaC products with relative density up to about 98% and grains size less than 5µm are finally obtained when combustion synthesized powders are processed for 20 min at 1800°C by Spark Plasma Sintering. A further increase in the sintering temperature to 2050°C and/or the dwell time to 30 min is found to negatively affect product densification. This outcome is mainly ascribed to the significant grains coarsening (above 20 µm) correspondingly observed as well as to other vapor-phase generating events, which could be more easily induced when powders are exposed to higher temperature conditions.

Suggested Reviewers: Olivia A. Graeve PhD

Professor, Department of Mechanical and Aerospace Engineering, University of California, San Diego, USA

ograeve@ucsd.edu

For her expertise in the field of Combustion Synthesis and Spark Plasma Sintering for the fabrication of various ceramics, including tantalum carbide

Claude Estournès

CIRIMAT UMR CNRS (Centre National de la Recherche Scientifique), Université Paul Sabatier, Tolouse, France

estournes@chimie.ups-tlse.fr

For his worldwide expertise in the field of SPS processes for the fabrication of various ceramics

Alexander S. Rogachev

Professor, Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences (ISMAN), Chernogolovka, Russian Federation rogachev@ism.ac.ru

For his well-recognized expertise in the field of Combustion Synthesis and Spark Plasma Sintering

Università degli Studi di Cagliari

Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali

Via Marengo 2, 09123 Cagliari - Italy

Tel.: +39-070675-5747-5055 Fax: +39-070675-5717-5067

Cagliari, 30-05-2017

Dear Editor,

please find attached our manuscript “Tantalum Carbide Products from

Chemically-Activated Combustion Synthesis Reactions” by C. Musa, R.

Licheri, S. Montinaro, R. Orrù, and G. Cao, submitted for publication in

Ceramics International.

With best regards.

Your sincerely,

Dr. Roberto Orrù

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Tantalum Carbide Products from Chemically-Activated Combustion Synthesis

Reactions

Clara Musa, Roberta Licheri, Selena Montinaro, Roberto Orrù*, Giacomo Cao

Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Unità di Ricerca del Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Università di Cagliari, Via Marengo 2, 09123 Cagliari, Italy

*Author to whom correspondence should be addressed:

R. Orrù (E-mail: roberto.orru@dimcm.unica.it; Ph.: +39-070-6755076; Fax: +39-070-6755057)

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Abstract

A rapid route based on the use of small amounts of polytetrafluoroethylene to chemically activate and sustain the combustion synthesis reaction for the formation of TaC from its elements is successfully exploited in this work. Other than a reaction booster, the polymer is found to play also a role as a carbon source, so that part of graphite can be replaced by Teflon to produce a single phase material. A relevant importance in the activation of the synthesis reaction is provided by the intermediate phase TaF3, whose formation is clearly evidenced, along with that of Ta2C, by combustion front quenching

experiments. Additive free TaC products with relative density up to about 98% and grains size less than 5m are finally obtained when combustion synthesized powders are processed for 20 min at 1800°C by Spark Plasma Sintering. A further increase in the sintering temperature to 2050°C and/or the dwell time to 30 min is found to negatively affect product densification. This outcome is mainly ascribed to the significant grains coarsening (above 20 m) correspondingly observed as well as to other vapor-phase generating events, which could be more easily induced when powders are exposed to higher

temperature conditions.

Keywords: Combustion synthesis; Chemical activation; Carbides; Spark Plasma Sintering; Microstructure.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 1. Introduction

The growing interest in Ultra-High Temperature Ceramics (UHTCs) based on carbides and borides of early transition metals for high temperature applications is due to their unique combination of desirable properties as melting temperatures above 3000°C, good electrical and thermal

conductivities, high hardness, good thermal and chemical stability, etc. [1-2]. In this regard, carbide based UHTCs are mostly attractive for rocket propulsion components, which have to withstand to high thermal and mechanical loads [2]. Other promising applications include electrodes for low temperature fuel cells [3] and novel solar absorbers able to operate to higher temperature as compared to standard SiC systems [4-5].

Among the members of the UHTCs family, undoubtedly promising is tantalum carbide (TaC) which displays the highest melting temperature, exceeding 3800°C [2]. In spite of its technological interest, bulk materials made of TaC are difficult to obtain due to its primary strong covalent bonding and low self-diffusion coefficient. Therefore, extremely severe sintering conditions have to be applied to consolidate TaC powders without the introduction of secondary phases, as demonstrated by the several attempts conducted so far using different consolidation approaches [6-15]. For example, about 97.5% dense products were recently obtained by pressureless sintering only when sub-micrometer sized TaC powders were exposed for 30 min at 2300°C [7]. Such difficulty was also confirmed when a mechanical pressure is also simultaneously applied by conventional hot pressing (HP) methods

[6,10,15]. To support the latter statement, it should be noted that Zhang et al. [6] achieved their maximum relative density (96.2%) after 45 min at 2400°C with an applied pressure of 30 MPa. On the other hand, the attempts made with lower temperature levels (about 1900°C) produced sintered

materials only 92-93% dense [10]. Relative densities above 97.5% were recently obtained by Rezaei et

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or higher values under a mechanical pressure of 40 MPa. However, for the sake of comparison, it should be noted that the theoretical density of 14.3 g/cm3 was used by Rezaei et al. [15] to determine the relative density of their sintered samples, whereas values equal or superior than 14.5 g/cm3 are generally considered in the literature for tantalum monocarbide [4,6,10].

With respect to classical HP, the more efficient Spark Plasma Sintering (SPS), where the die/sample/punches ensemble is rapidly heated by an electric current flowing through it, is generally found to provide more favorable conditions for powders consolidation [16]. Despite that, the use of such technology did not necessarily ensure the complete densification of TaC powders, even when rather severe sintering conditions are applied [8-9,11-14]. Thus, SPS temperatures and holding times up to 2400°C and 20 min, respectively, were not sufficient to provide relative density above 96-97% [8]. Similarly, only 93% dense samples were produced by SPS after 5 min at 2100°C under a mechanical pressure of 40 MPa [11]. Temperature levels up to 2200°C were also required to consolidate

nanometric TaC powders [12]. Interestingly, Bakshi et al. [9] reported that an increase of the sintering temperature in the range 1800-2200°C produced only a marginal effect on powders densification, whereas a marked improvement was obtained only when the mechanical pressure was raised to 255 MPa.

The relative density and microstructure of TaC materials is not only affected by the

consolidation methods adopted but also depend on other factors, mainly the characteristics of starting powders, which are strictly related to their synthesis method. In this context, several processing routes were proposed to produce TaC powders, including carbothermal reduction of the Ta oxides [12], solvothermal processes [13], and self-propagating high-temperature synthesis (SHS) derived routes [17-20]. As far as the latter synthesis technique is concerned, it should be noted that, in spite of the high enthalpy of formation and adiabatic temperature, i.e. 144.097 kJ/mol [21] and about 2500 °C [22], respectively, the preparation by SHS of TaC from its elements is usually not possible, unless starting

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reactants are properly activated. This is because the two parameters mentioned above only provides the indication of the feasibility of the synthesis reaction from the thermodynamic point of view. However, as for the case of TaC, the self-propagating character of the synthesis reaction could be inhibited by kinetic limitations. To overcome such constraint, various attempts have been made in the literature [17-20]. Specifically, the occurrence of the synthesis reaction was made possible in presence of gas-phase transport agents such as iodine or carbon dioxide [17,19]. Alternatively, the self-propagating character was induced by the application of an external electric field by means the so-called FACS (Field Activated Combustion Synthesis) technique [18,20].

In the present work, tantalum carbide powders are prepared for the first time by chemically activated combustion synthesis using polytetrafluoroethylene (PTFE), generally referred to as Teflon, as chemical promoter. It should be noted that this polymer was previously used in the literature to activate the SHS reaction in NbC [23], Al2O3-SiC [24], TiC-Ti [25], and B4C-TiB2 [26] systems. After

examining the effect produced by the introduction of Teflon to the stoichiometric mixture of tantalum and graphite, the role played by the polymer as a carburising agent in the synthesis reaction is then investigated. In this regard, the mechanism of TaC formation during the chemically activated process was studied taking advantage of the Combustion Front Quenching (CFQ) technique, which is based on the rapid extinction of the SHS front during its propagation [27]. Finally, the combustion synthesized powders are consolidated by SPS under different temperature and holding time conditions and the obtained bulk products are characterized from the microstructural point of view.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2. Experimental procedure

2.1 Synthesis of TaC powders

Tantalum, graphite and PTFE (-(C2F4)n-) powders, whose properties are reported in Table 1,

were used as raw materials. Specifically, when Teflon is considered only as a reaction promoter (booster), the starting mixture was prepared according to the following stoichiometry:

Ta + C TaC (1)

where, as a first attempt, the contribution of the polymer to the carburization of tantalum was not considered. In this study, the addition of Teflon to the elemental reactants was investigated in the range 0-10 wt%.

On the other hand, in a second stage, PTFE is assumed to participate as a carburizing agent to the TaC formation, in agreement to the following reaction:

Ta + (1-f)C(G) + f C(T) → TaC (2)

In the latter case, the required total carbon (C) is supposed to be given by the sum of graphite (C(G)) and the contribution (C(T)) obtained from the complete Teflon decomposition.

Mixing of starting powders was performed in a plastic jar with zirconia balls (2 mm diameter, 0.35g; Union Process, Akron, OH, USA) for 20 min by means of a SPEX 8000 shaker mill (SPEX CertiPrep, Metuchen, NJ, USA). SHS experiments were then conducted using cylindrical pellets (10 mm diameter and about 20-25 mm height) prepared by uniaxially pressing approximately 15 g of the obtained mixture. The SHS apparatus basically consists of a reaction chamber, a power supply (Belotti, Italy; output 0 – 100 V), to provide the energy required for reaction ignition, a video camera (Panasonic, mod. NV-DS 25), and a computer system equipped with a data acquisition board (Model PCI-MIO-16XE-50, National Instruments) supported by a software package (LabVIEW, National Instruments). Combustion temperature and front velocity during the synthesis process were determined by two or more thermocouples (W-Re, 127 m diameter, Omega Engineering Inc., Manchester, UK) embedded

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in the reacting samples. A two-color pyrometer (Ircon Mirage OR 15-990, Santa Cruz, CA, USA) focused on the pellet surface was also used.

The reaction chamber was first evacuated and then filled with Argon (about 1 atm overpressure). The combustion front was generated at the top of the pellet by using of a heated tungsten wire (Inland Europe, Varigney, France), which was immediately turned off as soon as the reaction was initiated. Then, for systems displaying a self-propagating character, the reaction front travels until reaches the opposite end of the sample. For the sake of reproducibility, each experiment was repeated at least twice. The resulting SHS product was converted in powder form by subsequent ball milling treatment. To this aim, about 4g of the obtained porous samples have been processed for 20 min using a stainless steel vial (VWR International PBI, Milan, Italy) with two steel balls (13 mm diameter, 4g), so that a ball-to-powder weight or charge ratio equal to two was obtained. Particle size of the resulting powders was determined using a laser light scattering analyzer (CILAS 1180, Orleans, France).

The crystalline phases were identified using an X-ray diffractometer (Philips PW 1830, Almelo, The Netherlands) equipped with a Ni filtered Cu K radiation ( = 1.5405Å).

2.2 Mechanistic study

The kinetic mechanism of reaction (1) was performed taking advantage of the CFQ technique which makes use of a cylindrical copper block (60 mm high with a diameter base of 50 mm) with a conical-shaped cut (50 mm high with a base diameter of 16 mm) where the starting mixture to be reacted was placed and pressed. The synthesis reaction was initiated at the base of the cone by means of a heated coil and the combustion front was extinguished due to the combination of intense heat transfer to the copper block and the progressive shrinkage of the cross section. As a consequence, the intermediate and end reaction products can be simultaneously frozen and the evolution of the SHS reaction may be followed by analyzing the composition of the various zones relatively to the location

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of the combustion front. Therefore, useful information on the chemical transformation of reactants to products during SHS can be deduced using the CFQ technique [27].

2.3 Powder densification

Powders obtained by SHS as described in section 2.1 were consolidated using an SPS equipment (515 model, Fuji Electronic Industrial Co. Ltd., Kanagawa, Japan) under vacuum (20 Pa) conditions. Details on the SPS apparatus are reported elsewhere [28-29]. Sintering experiments were carried out using approximately 7.5 g of powders preliminarily cold-compacted inside the die (30 mm outside diameter; 15 mm inside diameter; 30 mm height) to produce 14.7 mm diameter specimens and approximately 3 mm thickness. Sample release after sintering was facilitated by lining the internal die surface with graphite foils (0.13 mm thick, Alfa Aesar, Karlsruhe, Germany). Temperature, current, intensity, voltage and sample displacement, were recorded in real time during the process. In particular, temperature was measured using a C-type thermocouple (W-Re, 250 μm diameter, Fuji Electronic Industrial Co. Ltd., Kanagawa, Japan) inserted in a small hole drilled on the lateral surface of the die. SPS runs consisted of the imposition of a prescribed thermal cycle, where the temperature was first increased from the room value to the maximum level (TD) in 10 min. Then, the TD value was kept

constant for a prescribed duration (tD). The effect of TD and tD on the density of the sintered product

was investigated in the range 1600-2050°C and 5-30 min, respectively; the mechanical pressure of 60MPa was applied from the beginning of each SPS experiment. As for the case of SHS experiments, each SPS run was reproduced at least twice.

Relative densities of the sintered specimens were determined by the Archimedes’ method, using distilled water as immersion medium and by considering the theoretical value of TaC equal to 14.65 g/cm3 [30].

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The morphology of the SHSed powders, as well as the microstructure and the local phase compositions of the sintered samples, were examined by scanning electron microscopy (SEM) (mod. S4000, Hitachi, Tokyo, Japan, and mod. EVO LS15, ZEISS NTS Gmbh, Oberkochen, Germany).

3. Results and discussion

3.1. Powder synthesis

3.1.1 PTFE as a reaction promoter

In agreement with previous results reported in the literature, the stoichiometric mixture consisting of Ta and graphite only did not display a self-sustaining character upon local ignition. The effect produced by the introduction of Teflon as a booster of reaction (1) was then examined. It was found that the addition of polymer amounts equal or lower than 1 % wt. was not sufficient to make the synthesis reaction self-propagating. On the other hand, when its content was raised to 3 wt.%, the generated combustion wave was able to travel completely across the pellet with an average front velocity of 3.1± 0.2 mm/s. Correspondingly, a maximum combustion temperature (Tc) of 1950 ± 50°C

was recorded. Consequently, 3 wt.% can be regarded as the threshold amount of this polymer required for inducing the SHS character in reaction (1). Above this limit, the Tc and vf values were found to

increase as the Teflon percentage was progressively augmented. In particular, the latter parameters increased to 2130±130°C and 3.6 mm/s, respectively, for the system containing 10 wt.% of Teflon. A marked elongation of the pellet, as compared to its original height, is observed after the SHS reaction, particular when considering higher polymer percentages. This outcome could be readily ascribed to the amount of gases developed as a consequence of Teflon decomposition. The XRD patterns of the products obtained using different amounts of Teflon are compared in Figure 1 along with that one of starting reactants.

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The XRD analysis indicates that the SHS product obtained when considering 3 wt.% consists of cubic TaC (card n. 77-0205) only with no residual reactants or secondary phases. On the other hand, above this limit, particularly for Teflon amounts equal or higher than 10 wt. %, free graphite was also detected by XRD. Furthermore, a progressive shift in the position of TaC peaks towards lower theta values is observed as the booster percentage was increased (see inset of Figure 1). The latter feature clearly provides an indication of the fact that the C/Ta ratio in the carbide phase increases as the polymer percentage was augmented. Both the presence of residual graphite in the final product and the enhancement of the carbon fraction in the formed tantalum carbide as the Teflon content is increased, could be readily associated to the fact that the polymer not only plays a role as a booster for the synthesis reaction, but also directly contributes to the carburization of Ta.

3.1.2 PTFE as a carburizing agent

Due to its carburizing action described in section 3.1.1, Teflon was then considered, according to reaction (2), as a carbon source in the combustion synthesis process. In this regard, it is assumed that all carbon present in the polymer (C(T)) participates, along with that provided by graphite (C(G)), to the

carburization of tantalum to form TaC. Specifically, the value of 0.1154, corresponding to the minimum weight amount (3 wt.%) of Teflon required to make the synthesis reaction self-sustaining, was adopted for the f parameter appearing in reaction (2), so that the following specific reaction stoichiometry was examined:

Ta + 0.8846 C(G) + 0.1154 C(T) → TaC (3)

The related powder mixture still exhibited a SHS character. In particular, the generated reaction front travelled spontaneously through the specimen with average velocity and combustion temperature values very similar to those ones measured when 3 wt.% of PTFE was added to the stoichiometric 1:1 mixture of Ta and graphite reactants, as reported in section 3.1.1. The end product showed the typical

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gold brown color and entirely consisted of tantalum carbide, as revealed by the related XRD pattern reported in Figure 2(a). The powders resulting after the ball milling treatment were characterized in terms of particle size and morphology. In this regard, laser light scattering analysis provided the following data, i.e. 0.34 ± 0.05, 1.06 ± 0.08, 5.54 ± 0.42, and 2.18 ± 0.12m, for d10, d50, d90, and

average diameter, respectively. These measurements are perfectly consistent with SEM observations, as confirmed by the micrograph reported in Figure 2(b), which indicates that powders are constituted of agglomerates of few micrometer or sub-micrometer sized particles.

3.2. Mechanistic study

The formation mechanism of the TaC phase by the Teflon activated combustion synthesis was investigated in this work using the CFQ technique, where the synthesis reaction was allowed to take place inside a copper block. In particular, experiments were carried out by considering the system where an amount of -(C2F4)n- equal to 3 wt.% was added to the elemental reactants. The obtained

results evidenced that, after ignition, the SHS reaction propagated through the mixture until it was extinguished, as shown in Figure 3(a), few millimeters from the apex of the cone, as a consequence of the intense conductive heat losses occurring from the sample to the copper block (cf. full arrows in

Figure 3(a)). In this regard, previous studies evidenced that cooling rates up to 103 K/s can be achieved

using such technique [27 and references therein]. A layer to layer XRD analysis was carried out at various axial locations of the quenched region enclosed within the dashed box drawn in Figure 3(a) and the obtained results are reported in Figure 3(b). The analysis corresponding to the sample slice nearer to the apex of the cone (cf. Figure 3(b), pattern (1)) indicates that only reactants are present. On the other hand, the subsequent analyzed layer, corresponding to XRD pattern (2), evidenced a significant content of the TaC phase along with a noticeable amount of initial reactants, but the presence of hexagonal Ta2C (card n. 73-1321) and TaF3 as by-products is also clearly detected. The

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same signals are also found when considering the XRD pattern (3), although in this case TaC is undoubtedly the predominant phase, whereas peaks intensities of residual reactants are drastically reduced and consequently those ones corresponding to Ta2C and TaF3. Finally, only signals of the TaC

product were found in the last slice of the pellet examined (cf. Figure 3(b), pattern (4)), as an evidence of the synthesis completion.

It should be noted that, albeit our analysis of the quenched sample was confined to the region delimited by the dashed box reported in Figure 3(a), a thin layer of unreacted powders was also found in correspondence of the lateral interface between the reacting mixture and the copper block. This outcome indicates that, as a consequence of the heat losses, the extinction of the synthesis reaction occurred also in the radial direction. Consistently, the combustion front was not flat once quenched. The formation of metal fluorides as intermediate phases during combustion synthesis processes activated by Teflon is in agreement with previous results reported in the literature [23,25]. Indeed, NbF3 and TiF3 were formed when such polymer was used to induce the SHS reaction in NbC [23] or

TiC-Ti [25] systems, respectively. Nersisyan et al. [23] also proposed a reaction mechanism to justify the direct role played by NbF3 in the synthesis process.

Along the same line, the occurrence of the following list of reactions is postulated in this work, where the two intermediate Ta2C and TaF3 phases detected in the quenched sample are involved:

(4) (5) (6) (7) (8) (9)

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The activation of the synthesis reaction has to be preceded by Teflon decomposition, according to reaction 4 [23]. The resulting fluorine could then quickly combine with Ta to form TaF3 (reaction

(5)), whose presence was evidenced in the quenched region (cf. Figure 3). Subsequently, the highly reactive tantalum trifluoride interacts with C, either initially present or produced by reaction (4), to form the subcarbide Ta2C phase (reaction (6)), also detected in the sample by the CFQ test, or directly

TaC (reaction (7)). In this regard, interesting results are reported by Larson et al. [31] who investigated the mechanism of TaC and Ta2C formation from Ta and C using synchrotron radiation in

time-resolved X-ray diffraction. It was observed that the synthesis of the TaC from a stoichiometric 1:1 mixture of elemental powders is preceded by the formation of Ta2C. Furthermore, the obtained data

indicated that the rate of formation of Ta2C was double with respect to that one of TaC. Accordingly, in

the present study Ta2C is first formed and subsequently converted to the end product phase following

reaction (8). Finally, it is also possible that tantalum carbide can be formed with reaction (9), particularly when considering the C reactant formed in-situ from PTFE decomposition (reaction (4)), which is expected to be more reactive as compared to graphite powders initially present in the mixture.

Nevertheless, the most relevant and novel finding evidenced by CFQ tests conducted in the present study is the formation of TaF3 as intermediate phase that, as for the case of most fluorides of

transition elements, is characterized by high reactivity which enables the combustion synthesis of TaC. This feature clearly provides the motivation for the activation mechanism of the process induced by Teflon.

3.4 Spark Plasma Sintering of TaC powders

The final important target of the present work is represented by the obtainment by SPS of dense products from the combustion synthesized TaC powders. The optimal sintering conditions were identified after systematically studying the effect of the dwell temperature and holding time on product

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density. As shown in Figure 4a, it is observed that about 95% dense samples can be produced when SHSed powders are processed for 20 min at 1600°C by SPS. Powders consolidation is progressively improved as the temperature was increased to 1800°C, so that relative density values above 98% are correspondingly achieved. Nonetheless, data reported in Figure 4a indicate that a further increase in the TD value up to 2050°C produces negative effects in terms of product densification.

This finding seems to be in contrast with Bakshi et al. [9] results who did not observe worth mentioning effects on the densification of TaC powders when the SPS temperature was raised from 1800 to 2200°C under an applied pressure of 60 MPa and 5 min holding time. Indeed, both sintered samples displayed a very low density (86%). Such discrepancy, could be very likely ascribed to the different characteristics of the powders undergoing SPS in the two investigations. Apparently, the combustion synthesized TaC product obtained in the present work possesses a higher sensitivity to temperature with respect to the sub-micrometer sized powders processed by Bakshi et al. [9].

The possibility of obtaining dense TaC products at relatively shorter processing times was also explored in the present work. In particular, the effect of tD parameter on the density of the SPSed

product can be deduced in Figure 4b for the case of TD =1800°C. It is clearly seen that relatively less

dense samples are produced when the dwell time was reduced at 5 or 10 min. Furthermore, a negative effect on powder consolidation is also produced when the sintering time was prolonged to 30 min. Therefore, based on the systematic study results, it is possible to conclude that the optimal TD and tD

values for reaching the highest relative density from combustion synthesized TaC powders are 1800°C and 20 min, respectively. It should be noted that such conditions are among the milder ones adopted so far for obtaining comparable densification levels for the TaC system. Thus, in agreement with previous studies [30-31], the aptitude of the SHS technique to produce powders with higher sintering ability with respect to otherwise synthesized materials is confirmed.

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The effect determined by a temperature increase on product microstructure can be deduced by Figures 5a-5b, where three micrographs relative to samples sintered at 1800, 1900 and 2050°C, respectively, are shown. The achievement of a quite high densification degree in the material produced at 1800°C (cf. Figure 5a) is further proved, albeit a residual amount of closed porosity with pore size in the submicron range is present. The sintered product is characterized by a rather uniform microstructure consisting of fine grains, generally less than 5m sized, with equi-axed morphology. Thus, it can be stated that, under such conditions, grain growth was retained to a reasonable level during SPS. In contrast, Figures 5b-5c evidenced that a marked increase in grains size occurred if the sample is exposed to progressively higher thermal levels. In particular, when the TD value was raised to

2050°C, the end product consisted of grains up to 20 m or larger in size. Furthermore, in agreement with data plotted in Figure 4, total porosity was found correspondingly higher, with respect to that one present in samples sintered under milder conditions, with pores size typically larger than 1m (Figures 5c).

The origin of the presence of trapped porosity in the microstructure of TaC products obtained by SPS was recently discussed in detail by Kelly and Graeve [13]. Specifically, based on their

experimental evidences obtained when nanosized TaC powders (73 nm) were processed for 10 min up to 2200°C under the action of 50 MPa pressure, three mechanisms were postulated to justify pore formation in the resulting sintered material. All pore-forming mechanisms are thought to be caused by the generation of a vapor phase during the sintering process. In particular, the first type porosity was associated to the reduction of Ta oxide impurities, whose presence could be significant, i.e. up to 7.9 wt.% in Kelly and Graeve [13] study, when starting from nanopowders. On the other hand, oxygen content is generally markedly lower (less than 1 wt.%) when considering micron-sized powders. Beneficial effects in removing oxygen impurities, and consequently promoting powders consolidation, were obtained when proper amounts of C were added to the carbide material to be sintered.

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Nonetheless, the introduction of excessive quantities of this reactant induces the second type mechanism of pore-forming through the production of carbon oxides or, in the absence of oxygen, carbon sublimation. Finally, the third cause assumed to be possibly responsible for pore formation is represented by the incongruent sublimation of TaC. In this regard, the authors specified that the conditions to make the vapour formation possible might be fulfilled in correspondence of hot spots locally established during the SPS process. In any case, Kelly and Graeve [13] suggested that the critical sintering temperature of about 1900°C should not be overcome to avoid an excessive vapor production and, consequently, pore generation. This feature is consistent with our outcomes reported in Figures 4 and 5, showing that pore amounts trapped in product microstructure progressively increase for temperatures equal or exceeding 1900°C.

Regarding the specific mechanism to which the presence of pores in the SPSed samples produced in the present work could be ascribed, it should be recalled that starting powders are in the micron-meter range so that the amount of oxygen impurities is expected to be very low, as mentioned above. Nonetheless, traces of oxides possibly present in metal reactants might be not completely removed during the SHS process, so that first type mechanism is not excluded. This hold also true regarding the presence of residual minor amounts of unreacted C after the synthesis process, which might generate porosity according to the second proposed mechanism. Furthermore, the possible sublimation

phenomena involving C and TaC are more likely expected to occur when operating at sintering temperatures higher than the optimal one of 1800°C.

Besides, another relevant aspect to be considered is the possible negative effect on powder densification produced by the significant grains coarsening taking place in parallel. This option was only marginally considered by Kelly and Graeve [13], who observed that a rapid grain growth was not accompanied in their investigation by pore generation. This is in contrast with our findings, as clearly evidenced in Figure 5. Thus, as far as the present investigation is concerned, the marked coarsening in

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the microstructure occurring when the TD value is raised above 1800°C likely contributes, along with

the other mechanisms previously described, to make product densification progressively worse. Also in this case, the different characteristics of the starting powders undergoing SPS in the two investigations could explain the observed discrepancies.

4. Concluding remarks

In the present work, polytetrafluoroethylene was utilized to promote and support the combustion synthesis reaction for the formation of tantalum carbide from its elements. In this regard, the role played by the polymer as carburizing agent, other than reaction booster, was evidenced. The threshold amount of Teflon identified to activate and self-sustain the synthesis process was 3 wt.%. The resulting powders consisted of tantalum carbide with no residual reactants or secondary products.

Interesting insights on the mechanism of TaC formation were inferred by combustion front quenching experiments, which allowed us to identify TaF3 and Ta2C as intermediate phases of the

PTFE activated synthesis reaction. Based on the obtained results, the related kinetic mechanism was postulated.

The combustion synthesized TaC powders exhibited a good sintering ability and can be consolidated by SPS under relatively mild conditions without the support of additional phases. Indeed, monolithic products with relative density exceeding 98% were achieved after 20 min at 1800°C, under the action of a mechanical pressure of 60 MPa. On the other hand, powders exposed to higher temperature levels or for prolonged time periods lead to bulk product characterized by increased porosity along with a coarser microstructure. The latter feature is most likely responsible for the density decrease observed for sintering temperatures above 1800°C. Furthermore, as proposed by Kelly and Graeve [13], other different mechanisms involving the local generation of a vapor phase during the SPS process are discussed in this work, to explain the origin of porosity found in the sintered products.

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Acknowledgements

This activity was carried out in the framework of the FIRB2012-SUPERSOLAR (Programma “Futuro in Ricerca”, prot. RBFR12TIT1) project funded by the Italian Ministry of Education, University and Research.

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Table 1. Properties of the reactant powders used for the chemically activated combustion synthesis of TaC.

Reactant Vendor Particle Size Purity

Tantalum Alfa Aesar cod. 00337 < 44 μm 99.6 %

Graphite Aldrich cod. 282863 > 20 μm 99.99 %

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Captions for figures.

Figure 1. XRD patterns of (a) the starting reactants and SHS products obtained according to reaction (1) for different amounts of Teflon as a reaction promoter: (b) 3 wt. %; (c) 10 wt. %.

Figure 2. XRD patterns of starting reactants and SHS product (a) obtained according to reaction (2) and SEM micrograph of TaC powders (b) obtained after 20 min ball milling.

Figure 3. Quenched sample obtained after CFQ test (a) and XRD patterns of different layers (b) obtained in the region where the reaction front was extinguished (dashed box on the left).

Figure 4. Effect of (a) the sintering temperature (tD=20 min) and (b) the holding time (TD=1800 °C) on

the density of SPSed products obtained from chemically activated combustion synthesized TaC powders.

Figure 5. SEM micrographs (fractured surfaces) of TaC products obtained by SPS at different sintering temperatures (tD=20 min): (a) 1800°C, (b) 1900°C, and (c) 2050°C.

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Figure 1. XRD patterns of (a) the starting reactants and SHS products obtained according to reaction (1) for different amounts of Teflon as a reaction promoter: (b) 3 wt. %; (c) 10 wt. %.

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Figure 2. XRD patterns of starting reactants and SHS product (a) obtained according to reaction (2) and SEM micrograph of TaC powders (b) obtained after 20 min ball milling.

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Figure 3. Quenched sample obtained after CFQ test (a) and XRD patterns of different layers (b) obtained in the region where the reaction front was extinguished (dashed box on the left).

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Figure 4. Effect of (a) the sintering temperature (tD=20 min) and (b) the holding time (TD=1800 °C) on

the density of SPSed products obtained from chemically activated combustion synthesized TaC powders.

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Figure 5. SEM micrographs (fractured surfaces) of TaC products obtained by SPS at different sintering temperatures (tD=20 min): (a) 1800°C, (b) 1900°C, and (c) 2050°C.