TiC: Mechanical properties of the bulk materials

Accepted Manuscript SPS parameters influence on Ti3SiC2 formation from Si/TiC: Mechanical properties of the bulk materials Faten Turki, Houyem Abderrazak, Frédéric Schoenstein, Mohieddine Abdellaoui, Noureddine Jouini PII:

S0925-8388(17)30761-2

DOI:

10.1016/j.jallcom.2017.02.304

Reference:

JALCOM 41032

To appear in:

Journal of Alloys and Compounds

Received Date: 13 January 2017 Revised Date:

23 February 2017

Accepted Date: 28 February 2017

Please cite this article as: F. Turki, H. Abderrazak, F. Schoenstein, M. Abdellaoui, N. Jouini, SPS parameters influence on Ti3SiC2 formation from Si/TiC: Mechanical properties of the bulk materials, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.304. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT SPS Parameters Influence on Ti3SiC2 Formation from Si/TiC: Mechanical Properties of the Bulk Materials Faten Turkia*, Houyem Abderrazaka, Frédéric Schoensteinb, Mohieddine Abdellaouia, Noureddine Jouinib Laboratoire des Matériaux Utiles, Institut National de Recherche et d’Analyse Physico-chimique, Pôle

Technologique de Sidi Thabet 2020, Tunisie. b

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a

Université Paris 13, Sorbonne Paris Cité, Laboratoire de Sciences des Procédés et Matériaux, CNRS,

* Corresponding author: Faten TURKI

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E-mail address: [email protected]

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UPR 3407, 99 Avenue J.B. Clément, 93430 Villetaneuse, France.

Abstract

This paper reports on Ti3SiC2 formation, from 2Si/3TiC starting powders, using spark plasma sintering as a reactive process within the temperature range of

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1250–1400°C and for different holding times. The microstructure and mechanical properties of the bulk materials were studied as a function of temperature and holding time under a constant pressure of 60 MPa. The highest Ti3SiC2 conversion (87 wt%) was achieved for a temperature of 1400°C during 20 min. At these optimum

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conditions, the relative density was higher than 99%. The Young’s modulus, the Vickers hardness and the compressive yield strength of the corresponding sample

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were: 325 GPa, 4.9 GPa and 995 MPa, respectively.

Keywords:

Carbides; spark plasma sintering parameters, mechanical properties.

1

ACCEPTED MANUSCRIPT 1. Introduction Titanium silicon carbide Ti3SiC2 has been known as one of the notable ceramic materials elaborated for industrial applications. It has unique mechanical properties, which make it distinctive if compared with the conventional ceramics. Indeed, it has excellent machinability, important fracture toughness, moderately high

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strength, low density (4.52 g/cm3) and high elastic modulus (339 GPa) [1, 2]. In addition to these properties, Ti3SiC2 possess exceptional damage tolerance, high thermal shock resistance and stiff elasticity [1, 3]. In literature, for Ti3SiC2 synthesis, several mixtures were used as starting materials including Ti/Si/C [4-6], Ti/C/SiC [7-

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9], Ti/Si/TiC [10-13], Ti/SiC/TiC [14], Si/C/TiH2 [15], Si/C/TiC [16] and Si/TiC [1622]. Starting from TiC-Si mixtures to form Ti3SiC2 avoids the use of pure titanium for reasons of safety in the industrial scale. Fan et al. [16] succeeded in synthesize

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Ti3SiC2 from TiC/Si by addition of a small amount of Al or C. Hwang et al. [18, 19] reported a solid state reaction between TiC/Si powders without intermediate phases. However, Li et al. [20] proposed that the reaction between TiC/Si powders started at 1340 °C and postulated that the dominant intermediate phase would be Ti5Si3. SiC as well as TiC were obtained as secondary phases in the final products. Radhakrishnan et

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al. [17] pointed out that the intermediate TiSi2 was generated at first at a temperature of 1170 °C, as a result of the reaction of TiC and Si. Afterwards, TiSi2 was almost totally converted to Ti3SiC2 and SiC. This was confirmed by Kero et al. [22] who suggested that formation of relatively large quantities of Ti3SiC2 could occur in

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parallel with the evaporation of small amounts of Si(g). Nevertheless, regardless of the several starting powder mixtures cited above, pure Ti3SiC2 phase is often difficult

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to be obtained. The secondary phases such as titanium carbide, silicon carbide and/or titanium silicide were not necessarily detrimental to the material properties. Some studies [4, 7, 23, 24] have shown that the presence of these phases in a small amount can improve mechanical properties of Ti3SiC2. Several processes have been developed to synthesize Ti3SiC2 from TiC/Si such as isothermal treatment [25], pressure less heat treatment [18], cold isostatic pressing (CIP) [22] and hot pressing (HP) [17, 19]. It is interesting to note that the reactive spark plasma sintering have been used to elaborate Ti3SiC2 starting from 1Ti/1Si/2TiC [26] and 1Ti/1.3Si/2TiC [10] and never in the case of TiC/Si as raw materials. The work presented here is, to the best of our knowledge, the first synthesis of Ti3SiC2 by spark plasma sintering from TiC/Si. 2

ACCEPTED MANUSCRIPT The aim of this work is to show that SPS can be used as a reactive process to elaborate Ti3SiC2 with higher amount that obtained by previously cited methods. The influence of various SPS parameters, such as temperature and holding time, on the formation of Ti3SiC2 starting from TiC/Si powders mixture will be investigated. Moreover, the mechanical properties and relative densities of the obtained bulk

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materials will be studied as a function of the phase constituents.

2. Experimental Details 2.1. Synthesis and consolidation

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Commercially available TiC (<4 µm, 99%, Aldrich) and Si (<40 µm, 98%, Prolabo) were uniformly mixed in a GFL 3040 shaker mill for 48 h to synthesize

stoichiometry of reaction (1): 3TiC + 2Si → Ti3SiC2 + SiC

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Ti3SiC2. The stoichiometric molar ratio of Si-TiC was set at 2:3 corresponding to the (1)

Bulk samples were processed by Spark Plasma Sintering route (SPS) using a Syntex society model 515S apparatus. The powders were compacted into a graphite die of 8 mm in diameter. In this apparatus, the temperature is measured and

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controlled through an infrared camera (pyrometer CHINO IR-AH S2) focused on a small cavity inside of the graphite die. The sintering temperature was varied in the range of 1250 – 1400 °C. The sintering temperature was varied in the range of 1250 – 1400 °C and controlled through an infrared camera (pyrometer CHINO IR-AH

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S2). The heating rate was 60 °C/min and the holding time was within the range of 10 – 20 min. The applied pressure was constantly maintained at 60 MPa. After SPS, a

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disk of 8 mm in diameter and 4 mm in thickness was obtained. The sintering temperature, the axial pressure and the ram displacement were recorded as function of time to monitor the sintering process.

2.2. Characterization techniques X-ray diffraction analyses of the starting powders and the bulk SPS samples were performed on a Panalytical XPERT PRO MPD diffractometer using Cu Kα radiation. The existing phases were determined by the High Score Plus software based on the ICDD PDF2 data base. The cell parameters, the mean diffraction crystallite size and the weight content of each existing phase in the sintered samples were determined by the FullProf program [27] using the Rietveld method [28]. 3

ACCEPTED MANUSCRIPT Thermal Stability of the starting powder was performed by Differential thermal analysis (DTA) using a SETARAM-Setsys Evolution 16, thermal analysis instrument, with a heating rate of 10 K/min under Ar flow. The images of polished sections, fractures of the bulk samples and the damage induced by the Vickers indentation marks were recorded using Zeiss Supra 40 VP

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field emission gun scanning electron microscope (FEG-SEM). The microanalysis of each synthesized samples were carried out using the FEI Quanta 200 environmental scanning electron microscope coupled with EDX microanalyser.

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The relative densities of the consolidated samples were determined by the Archimedes’ method using O-xylene as measuring liquid (density of 0.88 at 25 °C). The microhardness measurements were conducted on the pellets’ plane using a

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Duramin 20 Vickers device under a test force of 4.91 N for 7 s. Each reported Vickers hardness value represents the average of ten measurements.

The compressive tests of parallelepiped specimens with dimensions of 2 × 2 × 3

3 mm were carried out using a Deben 5KN tensile compression stage with a compressive rate of 0.5 mm/min.

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The nanoindentation measurements were performed using an MTS nanoindenter XP equipped with a diamond tip of Berkovich type. The parameters fixed for the measures were: a depth imposed of 2000 nm, a deformation speed of

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0.05 s-1 and a frequency of 45 Hz.

3. Results and Discussion

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3.1. SPS Process

In the present work, SPS has been used not only as a consolidation process but

also as reactive one to elaborate Ti3SiC2 from Si and TiC mixture. In this process, two main factors play an important role: the applied pressure and the temperature rise by Joule effect. All the displacement profiles during the SPS process of the 2Si/3TiC powders mixture, recorded as a function of pressure and temperature were similar. Fig. 1 shows a typical punch displacement recorded during the SPS process of 2Si/3TiC powders mixture sintered at 1400 °C for 20 min under 60 MPa. The SPS process was conducted as follows: the pressure was increased to 60 MPa for 1 min and then kept constant during the compaction cycle. As to the temperature, it was maintained at a value close to 600 °C for 6 min and then increased to 1400 °C and 4

ACCEPTED MANUSCRIPT held constant for 20 min. A slight displacement of 0.08 mm was achieved during the first 13 min. This displacement is rather attributed to the particle re-arrangement of the initial powder. Then, between 13 and 16 min, the displacement increased swiftly to 0.75 mm. This last is probably due to the TiSi2 formation at about 1080 °C followed by a thermal particles softening for a sintering temperature of between

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1080 °C and 1230 °C. After 16 min, the displacement increased roughly to 1.9 mm. A eutectic mixture of Si-TiSi2 composition may be formed at 1330 °C, since the medium is rich in Si, thereby introducing a means for faster diffusion which increases the reactivity of the system. Ti3SiC2 is likely to form primarily in this melt.

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The bearing observed between 19 and 39 min is, in fact, attributed to the densification of the new formed phases as the temperature reached its maximum of 1400 °C. In the cooling stage from 1400 °C to 600 °C, the displacement increase (2.7 mm) is due to

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the thermal shrinkage, when the current is switched off and the temperature falls.

In order to investigate the effect of sintering temperature and holding time on the formation of Ti3SiC2, we plotted the variation of the piston displacement during sintering of mixtures versus time at 1250 °C, 1300 °C, 1350 °C and 1400 °C for 20

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minutes (Figure 2a) and at 1400 °C for 10, 15 and 20 minutes (Figure 2b). Figure 2a show that, at the stage "temperature rise", the piston displacement is more important for a higher temperature, which favors in advantage the Ti3SiC2 formation. However, from Figure 2b, the piston displacement varies only slightly by prolonging

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the holding time of 10 to 20 min. This leads us to conclude that Ti3SiC2 formation is

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rather attributed to the sintering temperature that the holding time.

3.2. DTA analysis

Differential thermal analysis is the technique which allowed us to determine

the temperature and type of all thermal changes (endothermic or exothermic). From the DTA thermogram of the starting powder mixtures of Si/TiC, two exothermic peaks can be distinguished at 1330 and 1415 °C (Fig. 3). These two exothermic peaks can be related to the formation of the TiSi2 and Ti3SiC2 phases. The results of XRD analysis at 1330 °C and at 1415 °C after DTA analysis show that the residual TiC is present in all samples with a significant amount. At 1330 °C, the formation of the new phases Ti3SiC2, SiC and TiSi2 is shown. Among these new phases, TiSi2 is the dominant phase at 1330 °C (which corresponds to a mass fraction of 31.2%) 5

ACCEPTED MANUSCRIPT and it is consumed as the temperature increases. Ti3SiC2 is present at 1330 °C in small amounts and it is the dominant phase at 1415 °C (which corresponds to a mass fraction of 69.8%). Some SiC is also present at two temperatures. We assume that there is a competition between the two following reactions: (1)

3TiC + 2Si → TiSi2 + 2TiC

(2)

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3TiC + 2Si → Ti3SiC2 + SiC

At 1330 °C, the two reactions take place but at 1415 °C, the reaction (1) was favored. This result is in agreement with The phases determined after DTA analysis prove the hypothesis proposed for SPS process. This was observed by Kero

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et al. [22]. They asserted that the TiSi2 and SiC phases were present at 1310 °C and the Ti3SiC2 phase started to appear in small amounts. However, Ti3SiC2 phase started dominant phase.

3.3. X-ray characterizations

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to form in abundance at the eutectic liquid temperature of 1330 °C and became the

Figure 3 gives the X-ray diffraction (XRD) patterns of 2Si/3TiC powder mixture and the bulk materials obtained after SPS for 20 minutes of holding time in

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the temperature range of 1250 - 1400 °C.

After spark plasma sintering of the starting mixture of powders of 2Si/3TiC, all the Si peaks disappeared in the XRD patterns, whereas, the TiC peaks is still present in the studied temperature range. Nevertheless, it has been noted that the intensity of the TiC

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peaks decreased obviously when the sintering temperature was increased. At 1250 °C, the formation of the new phases Ti3SiC2, SiC and TiSi2 is shown. The Ti3SiC2

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phase is the major phase for all temperatures and sintering times. Its peaks increase in intensity by increasing the temperature from 1250 to 1400 °C and the holding time from 10 to 20 min. However, a considerable decrease in the intensities of the SiC and TiSi2 phase peaks is noted when the temperature or the duration of sintering is increased. These are the minority phases in this mixture. The peak of carbon is related to the graphite foil used in order to prevent the powder from reacting with the surface of the graphite die and the graphite punches. It is interesting to note that while DTA analysis revealed that the formation of Ti3SiC2 occurred at 1415 °C, XRD of samples prepared by SPS indicated that this phase starts to be formed at significantly lower temperature (1250 °C). This is due to the nature of the SPS process. Indeed, in the presence of pressure (60 6

ACCEPTED MANUSCRIPT MPa) and electric current, localized necking occurs faster due to joule heating, this allows fast diffusion which favors chemical reactions at lower temperatures comparatively to DTA technique. Moreover, both TiSi2 and SiC phases appeared in overall samples and the intensities of their peaks decreased when the temperature increases. These are the minority phases in this mixture. However, Ti3SiC2 was

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almost the major formed phase particularly for a sintering temperature as high as 1400 °C.

To explain the Ti3SiC2 formation from Si/TiC powders, many authors have

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discussed the reaction sequence. According to Li et al. [20], as they detected the new phases of Ti5Si3 and SiC, they stated that these latter would be formed by reaction between Si and TiC. Then, Ti5Si3 would be consumed completely by TiC to form

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Ti3SiC2. However, Kero et al. [21, 22] haven't detected the presence of Ti5Si3 during the formation of Ti3SiC2 and suggested the formation of TiSi2 as an intermediate phase.

3.4. FEG-SEM characterizations

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Figure 5 illustrates the FEG-SEM micrographs in secondary electrons mode of polished surface of the bulk materials consolidated at 60 MPa during 20 min over a) 1250 °C, b) 1300 °C, c) 1350 °C and d) 1400 °C. The micrographs show dark particles embedded in gray regions. It can be noticed from this figure that the fraction

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of these dark and irregular shape particles decreases as the temperature increases. The

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dark particles belong to lighter phases whereas the clear regions depict heavy ones.

Figure 6 gives the energy dispersive X-ray analysis (EDX) of the bulk material

obtained by sintering at 1400 °C for 20 min. The corresponding EDX spectra of two different zones A and B are shown in Fig. 6b and 6c. According to its composition (Fig 6b), the gray zone (A) corresponds to the heavy phase Ti3SiC2. However, the darker zone (B) is indicative of a lighter material which matches with a mixture of SiC, TiSi2 and TiC phases (Fig 6c). These results are in a good agreement with the XRD results (Fig. 2).

3.5. Rietveld refinement results

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ACCEPTED MANUSCRIPT In order to determine the content of each obtained phase after sintering, all the XRD patterns of the obtained products have been refined by the Fullprof program, based on the Rietveld method. Figure 7 displays, as an example, the XRD pattern of the sample sintered at 1400 °C for 20 min after structural refinement. The lattice parameters, the lattice volume, the weight contents, the diffraction

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crystallite size and the refinement quality factors of the formed phases determined by the FullProf program are summarized in Table 1. From these results, it can be noticed that the refinement quality factors (χ2, RB and RF) are satisfactory.

Obtained by sintering at 1400 °C for 20 min, the Ti3SiC2 lattice parameters calculated

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based on Rietveld refinement are: a = b = 3.06787(5) Å and c = 17.67398(3) Å. These calculated values are close to those given in the literature by Barsoum (a= 3.0665 Å,

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c= 17.671 Å [29]). The lattice parameters decreased by increasing the sintering temperature. Regarding phase amounts, it is clear that the phase amounts of TiC and TiSi2 decrease as Ti3SiC2 and SiC amounts increase. Indeed, the weight content of TiC decreased significantly from 76%, used as starting content, down to 4% of the sample sintered at 1400 °C for 20 min. Similarly, the TiSi2 content decreased from 8% to 3%. Regardless of the temperature or sintering time, Ti3SiC2 represents the for 20 min.

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major phase with a weight content equal to ~ 87% of the sample sintered at 1400 °C

From these results, it can be concluded that the studied SPS parameters: holding time and sintering temperature have a strong effect on Ti3SiC2 formation. Indeed, at 1300

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°C and 1350 °C, we proposed that TiC reacted with Si to form Ti3SiC2 and SiC according to reaction (1). TiSi2 acted as an intermediate phase in the process of

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Ti3SiC2 formation from TiC/Si.

Thereby, TiC and TiSi2 phases were consumed giving rise to the formation of both Ti3SiC2 and SiC phases. There exist two chemical processes to obtain Ti3SiC2 starting to compete at 1250 °C: -

The first process consists on the reaction between TiC and Si as following

3TiC + 2Si → Ti3SiC2 + SiC -

(1)

The second process comprises two steps:

In the first step, the intermediate phase TiSi2 is formed following the reaction: 3TiC + 2Si → TiSi2 + 2TiC

(2) 8

ACCEPTED MANUSCRIPT In a second step, TiSi2 reacts with TiC leading to the final products as follows: TiSi2 + 2TiC → Ti3SiC2 + SiC

(3)

However, at higher temperature (1400 °C), which is close to Si melting temperature (1414 °C), the amount of TiC as well as SiC decreased when Ti3SiC2, the dominant phase, continued to form. The decrease of SiC amount can be explained by Si loss by

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evaporation during the sintering process. Based on the 1200 °C isotherm of the Ti-Si-C ternary phase diagram [30], the expected compositions of Si-TiC are located within the three-phase triangle bounded by SiC-TiSi2-Ti3SiC2. The phase contents are in good agreement with the expected

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ones. The remained TiC indicates an incomplete reaction between Si and TiC.

Kero et al. [31] have also synthesized Ti3SiC2 from TiC/Si by the combination of cold uniaxial pressing and cold isostatic pressing for compaction followed by heat

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treatment. Their obtained compound differed considerably from our overall phase composition. They found that the highest amount of Ti3SiC2 in the final product (58 wt%) was achieved by heat treating powder mixtures of initial composition 3TiC/2.2Si at 1350 °C for 1 h. Furthermore, it has been shown that Ti3SiC2 undergoes decomposition towards TiC and Si if it is maintained for longer time at this high

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temperature [32].

In comparison, the reactive SPS process seems to be more promising since it allows to reach a high amount of Ti3SiC2 (87 wt%) at 1400 °C in a significant lower time (1/5

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h), thus avoiding the decomposition of the as-obtained compound.

The diffraction crystallite size of phases was calculated based on the

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Rietveld method. The Ti3SiC2 diffraction crystallite size showed a weak variation over the temperature range corresponding to 1250 – 1350 °C regardless of the holding time used (10 – 20 min). However, it increased considerably when the temperature rose from 1350 °C to 1400 °C. These results indicated that the growth of the Ti3SiC2 crystallite size could be limited for sintering temperature below 1350 °C. Indeed, the crystallite size of Ti3SiC2 increased significantly from 56 nm to 79 nm when the sintering temperature increased from 1250 to 1400 °C.

3.6. Density measurement 9

ACCEPTED MANUSCRIPT Table 2 recapitulates the SPS process conditions and the corresponding densities of the obtained bulk materials, where the relative densities of the bulk materials were determined from the theoretical and the measured ones. The theoretical densities were calculated taking into account the overall composition of the bulk material deduced from XRD analysis in Table 1. From table 2, it can be noticed that

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the samples sintered at higher temperatures such as 1350 °C and 1400 °C showed higher densities. Then, it can be deduced that the density increased as a function of temperature. Thus, the relative density was raised from about 93% to about 99% when

min).

3.7. Mechanical properties of bulk materials

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the temperature was increased from 1250 °C to 1400 °C for the same holding time (20

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Figure 8 shows the variation of the Vickers microhardness of bulk materials, sintered at temperature ranging from 1250 °C to 1400 °C during 10, 15 and 20 min, recorded under a test force of 4.91 N for 7 s. It was found that the microhardness decreases by increasing the sintering time and temperature. A maximum Vickers microhardness value of about 950 ± 32 Hv (9.3 GPa) was obtained for the sample

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sintered at 1250 °C for 10 min. This value is close to that found by Radhakrishnan et al. [17] starting from a mixture of 3TiC/2Si sintered by vacuum hot pressing. Owing to the fact that Ti3SiC2 is rather a soft carbide with a Vickers microhardness of about 4 GPa [3], this highest value is related to the highest weight contents of titanium and

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silicon carbides as well as titanium disilicide. Indeed, TiC, SiC, and TiSi2 possess a high hardness values of about 28-35 GPa [33], 25 GPa [34] and 8-10 GPa [35],

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respectively. The lowest measured Vickers microhardness value close to 503 ± 20 Hv (4.9 GPa), corresponding to the material obtained after 20 min of sintering at 1400 °C, was attributed to the high Ti3SiC2 weight content in the corresponding sample. The dispersion of the microhardness values increased with the increase of the secondary phase weight contents. This is originated from the non-uniform microstructure as discussed above. The Vickers microhardness indentations of the bulk samples sintered for 20 min at 1250 °C and at 1400 °C are illustrated in Figure 9. As it can be shown from this figure, the indentation of the sample sintered at 1250 °C induced cracks which propagate along the ends of indentation diagonal to a large extent indicating a harder 10

ACCEPTED MANUSCRIPT bulk material (Fig. 9a). Whereas, for the surface of the sample sintered at 1400 °C (Fig. 9b), the damage areas were confined in the vicinity of indentation where the material was piled up, showing a quite soft material behavior.

The fracture toughness of the bulk sample sintered for 20 min at 1250 °C,

1/ 2

E  P  K IC = 0.016 H   C 3 / 2 

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shown on Figure 9a, was determined according to Anstis et al. equation [36]:

where KIC is the fracture toughness, E is the Young's modulus obtained by the the average crack length near the indent.

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nanoindentaion measurements, H is the hardness value, P is indentation load and C is The obtained value is about 5.62 MPa m1/2. It is intermediate between the fracture

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toughness of TiC and Ti3SiC2, which are, respectively, 5 [37] and 8.5 [38] MPa m1/2. Figure 10 shows the variation of the true stress as function of the true strain for samples sintered for 20 min at 1300, 1350 and 1400 °C. The compressive yield strength of the Ti3SiC2 based material, calculated from the plage of the stress-true

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strain curves, varies from about 990 MPa to about 995 MPa when the temperature increases from 1300 to 1400 °C. It could be seen that for all specimens, the stressstrain curve was nearly a linear curve indicating an elastic deformation behavior. It was remarkable to note that all Ti3SiC2 based materials obtained over the temperature

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range of 1300 – 1400 °C displayed an abrupt brittle fracture behavior and no macroplastic deformation can be observed. In fact, the polycrystalline Ti3SiC2 samples

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exhibit good resistance to damage during compressive tests. Zhang et al. [39] examined the compressive behavior of Ti3SiC2 with high purity (~99%) elaborated from Ti/Si/C and reported that the sample exhibits a yield strength of 935 MPa and that the deformation occurred by formation of classic shear bands. This value of compressive yield strength is close to those obtained for the present elaborated material (990 MPa).

To understand the mechanisms of micro-scale deformation in polycristalline Ti3SiC2, the cracking process and the fracture surface of the shear-fractured sample were examined. Figure 11 shows the FEG-SEM fractography of the bulk material’s

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ACCEPTED MANUSCRIPT fractured surface obtained by sintering for 20 min at 1400 °C. This figure shows an interesting phenomenon. In fact, unlike most brittle ceramic materials which are broken into small pieces upon failure, polycristalline Ti3SiC2 was not totally broken into small pieces but was shear fractured (Fig. 11b). The above phenomena suggested that Ti3SiC2 is a damage tolerant material, undergoing micro-scale deformation during

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compressive testing (Fig. 11a). The fracture mode is similar for all the bulk samples and is mostly intergranular (denoted by yellow arrows in the figure 11b). Cleavages with large flat and smooth surfaces can also be observed corresponding to the transgranular grain fracture (indicated by red arrows in the figure 11b). The presence

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of these two fracture modes confirms the existence of phases with different mode hardness. Moreover, figure 11b can obviously show the layered structure of Ti3SiC2.

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The nanoindentation measurements were carried out using a nanoindenter equipped with diamond tip of Berkovich type. These measures enable at a time to determine the Berkovich hardness (H) and the Young's modulus (E) of the sintered alloys using curves: load-penetration hardness-penetration and penetration-Young's modulus (Fig. 12).

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As shown by the load-penetration curves (Fig. 12a), applying the same load of about 740 mN, these curves follow the same look. The maximum depth of 1780 nm was obtained for the alloy sintered at 1400 °C for 20 min (Fig. 12a) because of its high mass content of Ti3SiC2 (~ 87%). The Berkovich hardness and the Young's modulus

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follow the same law of evolution that the Vickers microhardness. They decrease as the sintering temperature increases or when the holding time increases for a given

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sintering temperature. Thus, as an example, when the temperature increases from 1250 °C to 1400 °C, we note that the Berkovich hardness decreases from 9.14 ± 0.66 GPa to 6.44 ± 0.63 GPa and the Young's modulus decreases from 347 ± 16 GPa to 325 ± 14 GPa.

The alloy sintered at 1400 °C for 20 min, containing 87 wt% in Ti3SiC2, has Berkovich hardness and Young's modulus values similar to those reported by Kooi et al. [40] for pure ternary carbide Ti3SiC2.

4. Conclusion 12

ACCEPTED MANUSCRIPT In this work, Ti3SiC2 was elaborated for the first time through reactive spark plasma sintering starting from 2Si/3TiC in a temperature range of 1250-1400 °C for different holding times (10, 15 and 20 min). The effect of temperature and holding time on Ti3SiC2 formation was investigated in term of the phase constituents. It was found that Ti3SiC2 coexisted in all sintered samples with SiC and TiSi2 as secondary

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phases. TiC was present both as a reactant involved in the formation of Ti3SiC2 and as a secondary phase in the final sample. The crystallite size of Ti3SiC2 has increased from 56 nm to 79 nm when the sintering temperature increased from 1250 to 1400 °C. Vickers microhardness decreased with the increase of sintering time as well as

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temperature from 9.3 to 4.9 GPa. At 1400 °C, when the holding time was increased from 10 to 20 min, the weight content of Ti3SiC2 was improved up to 87 wt.% and a high dense compact material (99%) was obtained. Indeed, SPS can allow fast powder

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materials densification at lower temperatures comparatively to other sintering processes. Moreover, it can provide fully dense compacts with a very limited grain growth leading to the improvement of the physical and mechanical bulk material’s properties. At the optimum SPS conditions, the obtained properties of the synthesized material including Young’s modulus, Vickers hardness and compressive yield

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strength, 325 GPa, 4.9 GPa and 995 MPa, respectively, suggest its use for applications as moderate wear-resistance.

5. References

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[1] M.W. Barsoum, Physical Properties of the MAX Phases, in: R.W.C. K. H. Jürgen Buschow, Merton C. Flemings, Bernhard Ilschner, Edward J. Kramer, Subhash Mahajan, PatrickVeyssière (Ed.) Encyclopedia of Materials: Science and Technology (Second Edition), Elsevier, Oxford, 2006, pp. 1-11. [2] M.W. Barsoum, M. Radovic, Elastic and Mechanical Properties of the MAX Phases, Annu. Rev. Mater. Res., 41 (2011) 195-227. [3] M.W. Barsoum, D. Brodkin, T. El-Raghy, Layered machinable ceramics for high temperature applications, Scripta Mater., 36 (1997) 535-541. [4] H. Abderrazak, F. Turki, F. Schoenstein, M. Abdellaoui, N. Jouini, Effect of the mechanical alloying on the Ti3SiC2 formation by spark plasma sintering from Ti/Si/C powders, Int. J. Refract. Met. Hard Mater., 35 (2012) 163-169. [5] F. Meng, B. Liang, M. Wang, Investigation of formation mechanism of Ti3SiC2 by self-propagating high-temperature synthesis, Int. J. Refract. Met. Hard Mater., 41 (2013) 152-161. [6] X. Ji, Z. Yi, D. Zhang, K. Wu, F. Chang, C. Li, H. Tang, H. Song, Synthesis, characterization and tribological properties of High purity Ti3SiC2 nanolamellas, Ceram. Int., 40 (2014) 6219-6224.

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[7] H. Abderrazak, F. Turki, F. Schoenstein, M. Abdellaoui, N. Jouini, Influence of mechanical alloying on Ti3SiC2 formation via spark plasma sintering technique from Ti/SiC/C powders, Ceram. Int., 39 (2013) 5365-5372. [8] Y. Zou, Z. Sun, H. Hashimoto, L. Cheng, Reaction mechanism in Ti–SiC–C powder mixture during pulse discharge sintering, Ceram. Int., 36 (2010) 1027-1031. [9] J. Qin, D. He, Phase stability of Ti3SiC2 at high pressure and high temperature, Ceram. Int., 39 (2013) 9361-9367. [10] M.A. El Saeed, F.A. Deorsola, R.M. Rashad, Influence of SPS parameters on the density and mechanical properties of sintered Ti3SiC2 powders, Int. J. Refract. Met. Hard Mater., 41 (2013) 48-53. [11] Y. Liu, F. Luo, W. Zhou, D. Zhu, Dielectric and microwave absorption properties of Ti3SiC2 powders, J. Alloys Compd., 576 (2013) 43-47. [12] D. Chen, X. Tian, H. Wang, Z. Huang, Rapid synthesis of TiC/Ti3SiC2 composites by laser melting, Int. J. Refract. Met. Hard Mater., 47 (2014) 102-107. [13] Z. Li, F. Luo, C. He, Z. Yang, P. Li, Y. Hao, Improving the microwave dielectric properties of Ti3SiC2 powders by Al doping, J. Alloys Compd., 618 (2015) 508-511. [14] Z.F. Zhang, Z.M. Sun, H. Hashimoto, T. Abe, A new synthesis reaction of Ti3SiC2 through pulse discharge sintering Ti/SiC/TiC powder, Scripta Mater., 45 (2001) 1461-1467. [15] X. Liu, Y. Jiang, H. Zhang, L. Yu, J. Kang, Y. He, Porous Ti3SiC2 fabricated by mixed elemental powders reactive synthesis, J. Eur. Ceram. Soc., 35 (2015) 13491353. [16] X. Fan, X. Yin, L. Wang, P. Greil, N. Travitzky, Synthesis of Ti3SiC2-based materials by reactive melt infiltration, Int. J. Refract. Met. Hard Mater., 45 (2014) 1-7. [17] R. Radhakrishnan, C.H. Henager Jr, J.L. Brimhall, S.B. Bhaduri, Synthesis of Ti3SiC2/SiC and TiSi2/SiC composites using displacement reactions in the Ti-Si-C system, Scripta Mater., 34 (1996) 1809-1814. [18] S.S. Hwang, S.W. Park, T.W. Kim, Synthesis of the Ti3SiC2 by solid state reaction below melting temperature of Si, J. Alloys Compd., 392 (2005) 285-290. [19] S. Hwang, S.C. Lee, J. Han, D. Lee, S.-W. Park, Machinability of Ti3SiC2 with layered structure synthesized by hot pressing mixture of TiCx and Si powder, J. Eur. Ceram. Soc., 32 (2012) 3493-3500. [20] S.-B. Li, J.-X. Xie, L.-T. Zhang, L.-F. Cheng, In situ synthesis of Ti3SiC2/SiC composite by displacement reaction of Si and TiC, Mater. Sci. Eng., A, 381 (2004) 51-56. [21] I. Kero, M.L. Antti, M. Odén, Preparation and firing of a TiC/Si powder mixture, in, IOP Conference Series-Materials Science and Engineering Bristol, UK : Institute of Physics Publishing (IOPP), 2009. [22] I. Kero, R. Tegman, M.-L. Antti, Phase reactions associated with the formation of Ti3SiC2 from TiC/Si powders, Ceram. Int., 37 (2011) 2615-2619. [23] B. Liang, M. Wang, X. Li, Y. Mu, Fabrication and characterization of Ti3SiC2SiC nanocomposite by in situ reaction synthesis of TiC/Si/Al powders, Bull Mater Sci, 34 (2011) 1309-1311. [24] S.-B. Li, J.-X. Xie, L.-T. Zhang, L.-F. Cheng, Mechanical properties and oxidation resistance of Ti3SiC2/SiC composite synthesized by in situ displacement reaction of Si and TiC, Mater. Lett., 57 (2003) 3048-3056. [25] J.M. Córdoba, M.J. Sayagués, M.D. Alcalá, F.J. Gotor, Synthesis of Ti3SiC2 Powders: Reaction Mechanism, J. Am. Ceram. Soc., 90 (2007) 825-830. [26] N.F. Gao, J.T. Li, D. Zhang, Y. Miyamoto, Rapid synthesis of dense Ti3SiC2 by spark plasma sintering, J. Eur. Ceram. Soc., 22 (2002) 2365-2370. 14

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[27] J. Rodríguez-Carvajal, An Introduction to the Program FullProf 2000, 2001. [28] H. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr., 2 (1969) 65-71. [29] M.W. Barsoum, The MN+1AXN phases: A new class of solids: Thermodynamically stable nanolaminates, Prog. Solid State Chem., 28 (2000) 201281. [30] W.J.J. Wakelkamp, F.J.J. van Loo, R. Metselaar, Phase relations in the Ti-Si-C system, J. Eur. Ceram. Soc., 8 (1991) 135-139. [31] I. Kero, R. Tegman, M.-L. Antti, Effect of the amounts of silicon on the in situ synthesis of Ti3SiC2 based composites made from TiC/Si powder mixtures, Ceram. Int., 36 (2010) 375-379. [32] C. Racault, F. Langlais, R. Naslain, Solid-state synthesis and characterization of the ternary phase Ti3SiC2, J. Mater. Sci., 29 (1994) 3384-3392. [33] H.O. Pierson, Handbook of Refractory Carbides & Nitrides: Properties, Characteristics, Processing and Apps, Elsevier Science, 1996. [34] I. Yonenaga, Thermo-mechanical stability of wide-bandgap semiconductors: high temperature hardness of SiC, AlN, GaN, ZnO and ZnSe, Physica B: Condensed Matter, 308–310 (2001) 1150-1152. [35] J.K. Sonber, T.S.R.C. Murthy, C. Subramanian, S. Kumar, R.K. Fotedar, A.K. Suri, Investigations on synthesis of HfB2 and development of a new composite with TiSi2, Int. J. Refract. Met. Hard Mater., 28 (2010) 201-210. [36] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements, J. Am. Ceram. Soc., 64 (1981) 533-538. [37] A. Teber, F. Schoenstein, F. Têtard, M. Abdellaoui, N. Jouini, The effect of Ti substitution by Zr on the microstructure and mechanical properties of the cermet Ti1xZrxC sintered by SPS, Int. J. Refract. Met. Hard Mater., 31 (2012) 132-137. [38] Y. Zhou, Z. Sun, Micro-scale plastic deformation of polycrystalline Ti3SiC2 under room-temperature compression, J. Eur. Ceram. Soc., 21 (2001) 1007-1011. [39] Z.F. Zhang, Z.M. Sun, Shear fracture behavior of Ti3SiC2 induced by compression at temperatures below 1000 °C, Mater. Sci. Eng., A, 408 (2005) 64-71. [40] B.J. Kooi, R.J. Poppen, N.J.M. Carvalho, J.T.M. De Hosson, M.W. Barsoum, Ti3SiC2: A damage tolerant ceramic studied with nano-indentations and transmission electron microscopy, Acta Mater., 51 (2003) 2859-2872.

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Fig. 1: Variation of sintering temperature, sample shrinkage and applied pressure during the SPS process of Si/TiC powder mixture heated at 1400 °C for 20 min. Left

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y-axis corresponds to the sintering temperature and right y-axis corresponds to displacement and pressure.

Fig.2. Variation of the piston displacement during sintering of mixtures versus time a) at 1250 °C, 1300 °C, 1350 °C and 1400 °C for 20 min and b) at 1400 °C for 10, 15

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Fig. 3: DTA thermogram curve of the starting powder mixture of Si and TiC. Fig. 4: XRD patterns of 2Si/3TiC powder and samples obtained by sintering for 20

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Fig. 5: FEG-SEM micrographs of the polished surface of the bulk material obtained after sintering for 20 min at a) 1250 °C, b) 1300 °C, c) 1350 °C and d) 1400 °C. Fig. 6: (a) Micrograph of the bulk material obtained after sintering at 1400 °C for 20 min and the EDX analyses of (b) the A zone and (c) the B zone.

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Fig. 7: Example of Rietveld fit, using Fullprof program of XRD pattern of Si/TiC sample obtained after SPS sintering at 1400 °C for 20 min. The points are the observed intensities and the black pattern is the calculated one. The small vertical bars display Bragg positions of hkl reflections. The difference between the experimental

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and calculated intensities is given at the bottom of the plot. Fig. 8: Variation of the average Vickers micro-Hardness of sintered samples as function of holding time at different sintering temperatures.

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Fig. 9: FEG-SEM micrographs of the Vickers indentations marks under a charge of 5 N for the bulk material obtained after sintering for 20 min at (a) 1250 °C and (b) 1400 °C.

Fig. 10: True stress vs. true strain curves obtained from compression tests of the samples sintered under 60 MPa for 20 min at 1300, 1350 and 1400 °C. Fig. 11: FEG-SEM micrographies of (a) the parallelepiped specimen used for the compressive test of the bulk material obtained by sintering for 20 min at 1400 °C and (b) the corresponding fracture surface : presence of two fracture modes denoted by yellow arrows (intergranular) and red arrows (transgranular).

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Fig.12: Variation of a) load, b) Berkovich hardness and c) Young’s modulus as a function of the displacement into surface for samples sintered at different temperatures for 20 min.

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Table 1:Rietveld refinements results of the bulk materials sintered at 60 MPa by SPS starting from mixture of Si-TiC.

1250°C-20min

SiC (P 63 m c)

Latticeparameters(Å)

a = 3.06935(5) c = 17.67614(4)

a = 4.32378(9)

a = 3.08241(9) c = 15.10287(6)

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144.211 65.29 (0.24) 55.8(2) 5.24 4.39

80.833 17.74 (0.11) 42.6(3) 1.94 1.27

124.268 8.14 (0.09) 40.6(5) 6.64 5.75

Latticeparameters (Å)

a = 3.06931(5) c = 17.67607(6)

a = 4.32297(3)

a = 3.08239(5) c = 15.10283(4)

Lattice volume (Å3) Weight contents (%) (nm)

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144.207 67.33 (0.27) 56.7(4) 4.76 3.55

80.788 15.52 (0.12) 54.6(4) 2.54 1.92

124.266 8.55 (0.10) 41.8(3) 6.98 5.61

Latticeparameters (Å)

a = 3.06894(5) c = 17.67567(8)

a = 4.32291(5)

a = 3.08194(5) c = 15.09941(7)

Lattice volume (Å3) Weight contents (%) (nm)

144.169 67.79 (0.28) 58.6(5) 4.28 3.64

80.785 15.28 (0.11) 59.6(6) 1.74 1.32

124.201 8.65 (0.09) 43.4(4) 6.43 5.36

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Lattice volume (Å3) Weight contents (%) (nm)

TiSi2 (Fddd)

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TiC (Fm-3m)

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Rietveld refinements results

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SPS conditions

a = 8.26475(7) b = 4.80547(6) c = 8.55421(9) 339.739 8.60 (0.06) 52.7(3) 4.58 3.82 a = 8.26398(4) b = 4.80523(5) c = 8.55409(8) 339.686 8.29 (0.06) 53.8(1) 4.12 3.08 a = 8.26335(4) b = 4.80516(7) c = 8.55364(6) 339.637 8.19 (0.07) 57.2(6) 4.18 3.66

C (P 63/m m c)

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a = 2.47147(3) c = 6.70431(4) 35.464 0.23 (0.01) 33.1(4) 8.11 9.55

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a = 2.46874(6) c = 6.70422(4) 35.385 0.31 (0.01) 34.6(1) 4.67 10.11

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a = 2.46712(9) c = 6.70224(3) 35.328 0.09 (0.01) 37.6(3) 9.38 7.01

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SiC (P 63 m c)

Latticeparameters (Å)

a = 3.06875(5) c = 17.67485(9)

a = 4.32283(3)

a = 3.08187(4) c = 15.09856(8)

Lattice volume (Å3) Weight contents (%) (nm)

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144.144 75.59 (0.29) 56.8(4) 5.59 4.21

80.780 11.46 (0.11) 44.7(3) 2.44 1.97

124.189 7.68 (0.10) 42.9(5) 7.68 7.54

Latticeparameters (Å)

a = 3.06856(5) c = 17.67466(3)

Lattice volume (Å3) Weight contents (%) (nm)

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144.125 76.28 (0.31) 56.7(6) 5.51 3.96

Latticeparameters (Å)

a = 3.06844(5) c = 17.67432(3)

Lattice volume (Å3) Weight contents (%) (nm)

144.111 76.83 (0.33) 58.8(3) 5.88 4.42

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TiC (Fm-3m)

a = 4.32198(6)

a = 3.08174(6) c = 15.09791(8)

80.732 9.92 (0.11) 58.7(2) 2.35 2.15

124.173 8.52 (0.10) 43.7(7) 6.31 5.09

a = 4.32179(9)

a = 3.08152(6) c = 15.09766(5)

80.722 9.13 (0.10) 64.2(7) 2.99 2.36

124.153 9.19 (0.09) 46.8(8) 7.64 6.81

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Rietveld refinements results

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SPS conditions

TiSi2 (Fddd)

a = 8.26322(5) b = 4.80397(5) c = 8.55334(8) 339.536 5.12 (0.06) 53.6(8) 7.91 5.01 a = 8.26273(3) b = 4.80349(9) c = 8.55301(5) 339.468 5.01 (0.06) 57.9(2) 6.82 4.61 a = 8.26241(4) b = 4.80267(8) c = 8.55295(9) 339.395 4.85 (0.07) 58.6(4) 7.27 5.96

C (P 63/m m c)

χ2

a = 2.46682(6) c = 6.70222(2) 35.319 0.15 (0.01) 33.2(6) 3.22 3.48

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a = 2.46651(6) c = 6.70168(4) 35.308 0.27 (0.01) 35.8(6) 3.76 4.79

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Latticeparameters (Å) Lattice volume (Å3) Weight contents (%) (nm)

SiC (P 63 m c)

a = 3.06829(5) c = 17.67425(6)

a = 4.32176(2)

a = 3.08149(3) c = 15.09763(3)

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144.096 78.34 (0.29) 57.6(3) 5.51 3.92

80.720 7.25 (0.11) 53.6(8) 3.03 2.29

124.150 9.16 (0.09) 46.7(3) 7.95 7.83

Latticeparameters (Å)

a = 3.06826(5) c = 17.67422(2)

a = 4.32169(6)

a = 3.08143(7) c = 15.09757(6)

Lattice volume (Å3) Weight contents (%) (nm)

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144.093 79.96 (0.32) 59.8(5) 5.76 4.12

80.716 6.18 (0.10) 63.7(5) 3.01 1.93

124.145 9.18 (0.11) 47.7(5) 8.89 8.64

Latticeparameters (Å)

a = 3.06822(5) c = 17.67418(2)

a = 4.32163(2)

a = 3.08139(4) c = 15.09751(6)

Lattice volume (Å3) Weight contents (%) (nm)

144.089 81.84 (0.52) 63.2(2) 5.43 4.16

80.713 5.92 (0.12) 70.9(6) 3.17 2.50

124.141 8.92 (0.11) 52.8(4) 8.68 8.91

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TiSi2 (Fddd) a = 8.26234(4) b = 4.80258(1) c = 8.55268(7) 339.375 4.95 (0.06) 55.8(7) 7.09 5.41 a = 8.26228(7) b = 4.80249(8) c = 8.55263(3) 339.364 4.43 (0.06) 58.9(2) 7.95 7.02 a = 8.26219(3) b = 4.80241(4) c = 8.55257(7) 339.352 3.01 (0.06) 59.8(4) 8.34 7.25

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TiC (Fm-3m)

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Ti3SiC2 (P 63/m m c)

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Rietveld refinements results

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SPS conditions

C (P 63/m m c)

χ2

a = 2.46592(3) c = 6.70153(5) 35.290 0.30 (0.01) 34.6(4) 3.51 8.78

6.89

a = 2.46588(6) c = 6.70148(7) 35.288 0.25 (0.01) 36.8(6) 5.03 4.96

7.11

a = 2.46584(7) c = 6.70151(2) 35.287 0.31 (0.01) 41.6(4) 3.15 7.76

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Latticeparameters (Å) Lattice volume (Å3) Weight contents (%) (nm)

SiC (P 63 m c)

a = 3.06796(5) c = 17.67411(3)

a = 4.31992(7)

a = 3.08135(2) c = 15.09723(1)

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144.064 82.67 (0.41) 71.6(8) 5.26 3.28

80.617 5.52 (0.11) 59.7(8) 4.46 2.78

124.136 7.53 (0.10) 54.9(3) 6.62 5.72

Latticeparameters (Å)

a = 3.06792(5) c = 17.67402(2)

a = 4.31988(4)

a = 3.08132(4) c = 15.09719(3)

Lattice volume (Å3) Weight contents (%) (nm)

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144.059 83.57 (0.55) 73.5(6) 4.86 3.06

80.615 5.08 (0.10) 67.6(7) 5.79 3.34

124.133 7.47 (0.12) 57.5(4) 7.63 6.45

Lattice parameters (Å)

a = 3.06787(5) c = 17.67398(3)

a = 4.31984(4)

a = 3.08130(5) c = 15.09708(8)

Lattice volume (Å3) Weight contents (%) (nm)

144.054 86.54 (0.43) 78.7(6) 5.34 4.15

80.613 3.95 (0.11) 77.8(5) 5.38 3.32

124.131 6.52 (0.11) 60.9(5) 5.08 5.92

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TiSi2 (Fddd) a = 8.26205(6) b = 4.80212(5) c = 8.55224(8) 339.313 4.28 (0.08) 56.8(6) 4.43 3.82 a = 8.26201(3) b = 4.80204(6) c = 8.55218(6) 339.303 3.88 (0.08) 59.7(4) 5.61 4.82 a = 8.26197(4) b = 4.80201(2) c = 8.55202(5) 339.293 2.99 (0.09) 62.8(7) 5.86 4.34

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TiC (Fm-3m)

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Ti3SiC2 (P 63/m m c)

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SPS conditions

C (P 63/m m c)

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ρth (g.cm-3) 4.41 4.40 4.39 4.41 4.39 4.37 4.38 4.37 4.39 4.39 4.39 4.41

drel (%) 92.74 93.41 93.62 93.42 93.62 94.73 94.59 94.97 94.99 95.90 98.41 99.32

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ρmes (g.cm-3) 4.09 4.11 4.11 4.12 4.11 4.14 4.14 4.15 4.17 4.21 4.32 4.38

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t (min) 10 15 20 10 15 20 10 15 20 10 15 20

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Si/TiC was used for Ti3SiC2 synthesis by using reactive spark plasma sintering. Optimum sintering conditions for Ti3SiC2 formation were 1400 °C, 20 min and 60 MPa.

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The purity of Ti3SiC2 was as high as 87 wt.%.

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The fully dense compact had an interesting mechanical behaviors.

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