Zirconium carbide doped with tantalum silicide: Microstructure, mechanical properties and high temperature oxidation

Materials Chemistry and Physics 143 (2013) 407e415

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Zirconium carbide doped with tantalum silicide: Microstructure, mechanical properties and high temperature oxidation Laura Silvestroni a, *, Diletta Sciti a, Marianne Balat-Pichelin b, Ludovic Charpentier b a b

ISTEC-CNR, Via Granarolo 64, 48018 Faenza, Italy PROMES-CNRS, 7 rue du four solaire, 66120 Font-Romeu-Odeillo, France

h i g h l i g h t s
Densification of ZrC at 1970 K with addition of TaSi2.
Thorough study of the microstructure by TEM and hypothesis on the densification mechanism.
Mechanical characterization: HV e 18 GPa, KIc e 3.6 MPa m1/2, s e 500 MPa.
Oxidation tests at 1800e2200 K for 20 min and discussion on the various oxidation mechanisms.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 January 2013 Received in revised form 12 September 2013 Accepted 18 September 2013

A zirconium carbide ceramic was hot pressed to full density thanks to the addition of TaSi2, which enabled the densification to occur at 1970 K and improved the mechanical properties as compared to monolithic ZrC. The microstructure was analysed by combined X-ray diffraction, scanning and transmission electron microscopy to investigate the effective role of the sintering additive. In addition, high temperature oxidation was performed using the reactor REHPTS (Réacteur Hautes Pression et Température Solaire) from 1800 to 2200 K for 20 min and this composite demonstrated to resist towards the highly oxidative conditions better than other carbides, thanks to the chemical modification of the oxide formed upon Ta addition. However from 2000 K, the specimen resulted very damaged. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Carbides Electron microscopy Microstructure Mechanical properties Oxidation

1. Introduction Zirconium carbide (ZrC) is an extremely hard and refractory ceramic, belonging to the class of Ultra-High Temperature Ceramics (UHTCs) in view of its high melting point exceeding 3800 K [1]. Like all the components of this family, ZrC possesses a combination of interesting properties, like for example high corrosion resistance in both acid and basic environments, a low thermal conductivity (20.5 W m1 K1) and high electrical conductivity thanks to the presence of metallic bonds. Moreover, the strong covalent ZreC bonds confer to ZrC high hardness (25 GPa), high Young’s modulus (440 GPa) and mechanical strength. Being a Zr-based compound, it possesses lower density compared to other transition metal carbides, like WC, TaC or HfC. Similar to other transition metal carbides, ZrC is often sub-stoichiometric and has a stability range of

* Corresponding author. E-mail address: [email protected] (L. Silvestroni). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.09.020

carbon to metal ratio from 0.65 to 0.98; when carbon content exceeds 0.98, the material contains free carbon too [2]. It is commercially used in tool bits for cutting and it is potentially suitable for re-entry vehicles, rockets or scramjet engines, where low density and high temperature load bearing capability are required. Other applications of ZrC ceramics can be found in nuclear sector, where it is used as refractory coating in reactors, thanks to its low neutron absorption cross-section and weak damage sensitivity under irradiation [3]. It has also been recognized that ZrC possesses favourable emissivity properties rendering it a promising material for use as absorber in concentrating solar power systems [4e6]. Besides these amazing chemico-physical properties, the two main drawbacks of ZrC consist in a difficult densification and a poor oxidation resistance, which characterizes all the transition metal carbides. Sintering is generally performed by pressure assisted techniques, like hot pressing (HP) [7] or spark plasma sintering (SPS) at very high temperatures [3,8,9], inducing however coarse microstructure with grain size in the order of 10e30 mm. Other

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pressureless sintering technologies enable the densification of ZrC at lower temperatures, but only upon addition of sintering additives such as MoSi2, ZrB2, HfC, carbon or SiC [10,11]. On the other side, poor oxidation resistance over 1070 K strongly limits the applications of ZrC and makes it necessary to work in protective environments. The final oxidation products of ZrC are ZrO2 and carbon oxides. ZrO2 forms a fine grained porous scale which allows gaseous diffusion of O2 through the pores to the ZrC surface, and therefore provides no oxidation protection. In addition, low-temperature formation of cubic modification of ZrO2, instead of thermodynamically stable monoclinic modification, may significantly contribute to the high oxidation rate as a result of about 5 orders of magnitude higher solid state oxygen diffusion in cubic zirconia compared to that in monoclinic [12]. One way to improve the oxidation resistance of ZrC is to make composites, adding for example ZrB2, SiC or silicides, able to hinder oxygen penetration through the B2O3-filled ZrO2 layer, or to form a protective silica scale [13]. In this work, TaSi2 was chosen as sintering additive for ZrC to enable densification at temperature lower than 2370 K and improve the oxidation resistance of the carbide matrix. The selection of TaSi2 was driven by its refractoriness, as it melts at 2370 K, and its proved efficacy as sintering additive for other transition metal carbides [14]. In addition, previous works on ZrB2 enabled to conclude that Ta-addition is beneficial for oxidation up to 1870 K owing to its capability to stuff oxygen vacancies in Zroxide by a higher valence cation, thus rendering the oxide layer more stable in the tetragonal crystal structure [15], or thanks to the tendency to increase the glass viscosity and induce immiscibility, which is translated in higher boiling point of the glass [16]. However tantalum becomes detrimental above 2150 K due to the melting of its oxide, Ta2O5 [17], or to the formation of a complex orthorhombic oxide, TaZr2.75O8, which has a needle-like morphology not favourable for adhesion to the unreacted bulk [15,18] and a lower melting point than pure tantalum- or pure zirconium-oxides [19]. In this work, the room temperature mechanical properties of the dense ZrCeTaSi2 were measured and compared to other ZrCcompounds, then the oxidation behaviour of the ceramic was studied in the temperature range from 1800 to 2200 K in order to evaluate the performances at the different temperatures and identify eventual operating limits.

The final density was measured by Archimedes’ method on a hydrostatic balance. The theoretical density was calculated by the rule of mixtures. Crystalline phases were identified by X-ray diffraction (Siemens D500, Germany) using the CuKa radiation, with 0.04 2q-step size and 1 s scan step time. The microstructure was analysed using scanning electron microscopy (SEM, Cambridge S360) and energy dispersive spectroscopy (EDS, INCA Energy 300, Oxford Instruments, UK) on fractured and polished surfaces. TEM samples were prepared by cutting 3 mm discs from the sintered pellets. These were mechanically ground down to about 20 mm and then further ion beam thinned until small perforation were observed by optical microscope. The detailed phase analysis was performed using a transmission electron microscopy (FEI Tecnai F20 ST), with an acceleration voltage of 200 kV, equipped with an EDAX EDS X-ray spectrometer PV9761 with Super ultra-thin window. Microstructural parameters, like amount of residual porosity or secondary phases, were determined through image analysis on the micrographs of polished surfaces by commercial software (Image Pro-plus 4.0, Media Cybernetics, Silver Springs, USA). 2.2. Mechanical properties Vickers microhardness (HV) was measured on the polished surface, with a load of 9.81 N, using a Zwick 3212 tester. Fracture toughness (KIc) was evaluated using chevron-notched beam (CNB) in flexure. The test bars, 25  2  2.5 mm (length  width  thickness, respectively), were notched with a 0.08 mm diamond saw; the chevron-notch tip depth and average side length were about 0.12 and 0.80 of the bar thickness, respectively. The flexural tests were performed on a semi-articulated silicon carbide four-point jig with a lower span of 20 mm and an upper span of 10 mm on a universal screw-type testing machine (Instron mod. 6025). The specimens were deformed with a crosshead speed of 0.05 mm min1. The slice model equation of Munz et al. was used to calculate KIc [20]. On the same machine and with the same flexural jig, the flexural strength (s), was measured on chamfered bars 25  2.5  2 mm (length  width  thickness, respectively), using a crosshead speed of 0.5 mm min1; five specimens were tested. 2.3. High temperature oxidation resistance

2. Experimental procedure 2.1. Material The ceramic was produced starting from the following commercial powders in amount 85 vol% ZrC and 15 vol% TaSi2: - cubic ZrC (H. C. Starck, Germany): Grade B, mean particle size: 3.5 mm; particle size range 0.8e6 mm; specific surface area 3 m2 g1; impurities (wt%): C 1.5, O 0.6, N 0.8, Fe 0.05, Hf 2.0. - hexagonal TaSi2 (Cerac Inc., Milwaukee, WI), purity 99.5%, 325 mesh, particle size range 5e10 mm. The powder mixture was ball milled for 24 h in absolute ethanol using silicon carbide milling media. Subsequently the powders were dried in a rotary evaporator, sieved through a 60-mesh screen and green shaped to 45 mm-pellet by linear pressing at 20 MPa. Hot-pressing was conducted in low vacuum (w100 Pa) using an induction-heated graphite die with a constant uniaxial pressure of 30 MPa, maximum sintering temperature of 1970 K, heating rate of 20 K min1 and free cooling.

The reactor used to perform high temperature oxidation is the REHPTS (Réacteur Hautes Pression et Température Solaire, High Pressure and Temperature Solar Reactor), implemented at the focus of the Odeillo 5 kW solar furnace [21]. A heliostat reflects the incident solar flux to a concentrator with faceted mirrors. A shutter enables to control the fraction of the concentrated solar flux delivered to the sample placed inside the reactor and therefore its surface temperature. In this set-up the disc (Ø ¼ 25 mm, h ¼ 2 mm) is placed 25 mm above the focus of the solar furnace, so that very high temperatures may be achieved on a homogeneous 10 mm diameter area and at very fast rate, up to 100 K s1. Two mirrors enable a monochromatic (5 mm) optical pyrometer (Ircon, Modline Plus) to measure the surface temperature of the sample through a fluorine window on a 6 mm diameter circle on the sample surface at a pyrometer-sample distance of 120 cm. To obtain the real temperature by monochromatic pyrometry, a normal spectral emissivity of 0.75 was used in these experiments, as the sample was immediately covered by an oxide layer mainly composed of zirconia. The pyrometer, together with all the parts present on the optical path, was calibrated on a blackbody.

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The accuracy of the temperature measurement is going from 1400  15 K to 2200  22 K. The oxidations were performed in air with an atmosphere continuously renewed. Due to the altitude of the laboratory, the total atmospheric pressure is around 87 kPa and the oxygen partial pressure pO2 is 17 kPa. The temperature of the samples was maintained at a constant plateau during 20 min and a video camera was used to follow in situ the oxidation process. A mass spectrometer (Pfeiffer Omnistar) enabled in situ gas phase analysis. CO is expected to be one of the main gaseous products during oxidation, but its molar weight is the same as N2 one (m/e ¼ 28), so it is impossible to separate the contribution of CO from the one of preponderant N2. We therefore mainly followed the signal corresponding to m/e ¼ 44, corresponding both to CO2 and gaseous SiO. The samples were weighted before and after oxidation in order to assess the mass variation and the surfaces and cross-sections were analysed after oxidation using XRD, SEM and EDS.

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being sintered at relatively low temperatures, 1970 K, the mean grain size was notably coarsened. TEM analysis allowed a deeper investigation of the matrix: Fig. 3a evidences the coreeshell morphology with denticulate interface; the EDS spectra in Fig. 3b show the chemical composition of the two regions. Particles incorporated in SiC large grains were identified as Zr-rich silicides, Zr2Si or Zr5Si3, as those displayed in Fig. 3c. At the triple points, further ZreSi phases with flat edges were recognized, as the examples reported in Fig. 3d,e. High resolution imaging revealed the presence of dislocations between ZrC-core and (Zr,Ta)C-shell (Fig. 4a) and generally clean grain boundaries between adjacent (Zr,Ta)C grains or in contact with Zrsilicides (Fig. 4b,c) were found. Occasionally, amorphous SiO2 was noticed at the grain boundaries between the carbide and the silicide phases (Fig. 4d).

3.2. Densification behaviour With the aim to identify the densification mechanisms activated by TaSi2, the main outcomes regarding the microstructural evolution of ZrCeTaSi2 can be summarized as follows:

3. Results and discussion 3.1. As sintered microstructure The addition of TaSi2 to ZrC matrix enabled the achievement of full density, 7.11 g cm3, at 1970 K, which is a notably lower temperature as compared to conventional ZrC-based ceramics for which 2170e2370 K are required, see Table 1. X-ray diffraction did not evidence the presence of either ZrC or TaSi2. On the contrary a new phase with reduced cell parameter was identified as a (Zr,Ta)C solid solution (Fig. 1). The substitution of Zr atoms by Ta from TaSi2 provoked a shrinkage of the lattice from 4.692  A for ZrC to 4.646  A, which corresponds to a carbide with formula (Zr0.8Ta0.2)C, according to Vegard’s rule. The final microstructure of this composite was very complex, as the SEM images display in Fig. 2. Fig. 2a shows a homogeneous and dense microstructure with around 5 vol% of black particles of SiC. In Fig. 2b a magnification of the polished surface also shows dark grey phases with irregular faceted shape containing ZreTae SieC in different amounts, probably deriving from the original TaSi2, which is instead not clearly detected. The inset of Fig. 2b evidences the morphology of the matrix grain. As a general guideline, bright contrast indicates higher Ta content, whilst darker contrast indicates higher Zr percentage. The core with jagged edges is ZrC, moving outwards the contrast becomes brighter and EDS revealed the presence of Ta, giving rise to an approximate formula (Zr0.85Ta0.15)C. Inside the grain, 500 nm particles of ZreTaeSieC phases are visible, but the stoichiometry is difficult to define. The mean grain size of the newly formed carbide is around 5.2 mm with grains achieving up to 8 mm. Despite

- ZrC original grains were surrounded by Ta-containing solid solutions, which grew epitaxially on the matrix grain (Fig. 3a). The misfit between ZrC and the (Zr,Ta)C solid solution was accommodated by corrugated grain boundaries and dislocations (Fig. 4a). These features suggest great solubility of Ta in ZrC; - after sintering, no TaSi2 was observed anymore, but ZreSi phases with low dihedral angles took its place, revealing a mutual solubility of Zr in TaeSi-based phases. The new formed silicides were located mainly at the triple points (Fig. 3e). - High resolution TEM showed evidence of generally clean grain boundaries between carbide matrix and silicide (Fig. 4a). Previous studies of the interaction between carbides and TaSi2 revealed that during sintering, in presence of CO, TaSi2 tends to dissociate and decompose into Ta, gaseous SiO, liquid Si and eventually to form new TaC phase [14], according to reaction (1). The formation of liquid silicon at relatively low temperature is compatible with the enhanced sintering activity of this composite which started to shrink below 1770 K, close to the melting temperature of Si. TaSi2 þ CO(g) / TaC þ SiO(g) þ Si(l)

(1)

Free Si was not clearly observed in the final microstructure; however in the highly reducing environment of the hot pressing chamber with graphite dies and rams, it is very probable that liquid silicon was reduced to SiC, as testified by Fig. 2:

Table 1 Sintering parameters, microstructural features and mechanical properties of some ZrC-based ceramics. Legend: HP-hot pressing, SPS- spark plasma sintering, PLSepressureless sintering, m.g.s. e mean grain size, HV e hardness, KIceCNB fracture toughness, s e 4-pt flexural strength. Label

ZCT ZCM ZCM ZCM ZCZg ZCS a b

Sintering

Sint. add.

Rel. r

m.g.s

HV

K, min, atm, MPa

vol%

%

mm

GPa

HP 1970, 6, vac, 30 HP 2170, 12, vac, 30 SPS 1970, 3, vac, 100 PLS 2220, 60, Ar, HP 2170, 60, vac, 30 PLS 2370, 120, Ar, -

15 TaSi2 15 MoSi2 9 MoSi2 20 MoSi2 8 wt Zr þ 2.3 wt graphite 20 SiC

99.9 96.8 99.0 96.8 98.3 96.7

5.2 3.9 3.5 6.0 w10 3.1

17.9 19.2 20.0 12.7 16.2 11.8

Direct crack measurements. 3-pt bending.

s

KIc 1/2

(MPa m      

0.7 0.4 0.5 1.0 0.9 0.8

3.6 3.4 3.3 3.5 4.7 e

    

0.4 0.5 0.4a 0.2 0.4a

)

Ref

MPa 503  474  591  272  e 474b

55 41 48b 12

This work [23] [8] [11] [7] [25]

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Fig. 1. X-ray diffraction pattern of the as sintered ZrCeTaSi2 composite showing peaks shift from the pure ZrC phase.

2Si(l) þ CO(g) / SiC þ SiO(g)

(2)

Those findings are in good accordance with the microstructural features, as pure TaSi2 is not found anymore and TaC is instead present in the solid solution with ZrC according to Eq. (3):

The well-defined boundary between core and shell and the morphology of the interface between them with trapped particles also put forward a re-precipitation from liquid phase over a diffusion process. 3.3. Mechanical properties

TaC þ ZrC / (Zr,Ta)C

(3)

Indeed, it is well known that the solubility between carbides of Group IV and mono-carbides of Group V is complete and they are expected to form solid solutions [1]. Microstructural features and thermodynamics suggest that densification occurred through transient liquid phase. This hypothesis is further strengthened by the irregular shape displayed by the residual silicide phases, as well as the relevant coarsening of the carbide grains. We can reasonably suppose that liquid phase containing TaeSieCeO, where Zr was soluble in, formed during densification and crystallized at the triple points in form of ZreSi phases upon cooling leaving clean grain boundaries. As solid solutions formed, Ta from TaSi2 substituted Zr atoms in ZrC lattice. This may occur either by cations diffusion or by solution-reprecipitation. Given the low self-diffusion coefficient of this class of materials, it is presumed that lattice diffusion can occur only at very high temperature. Indeed, solution re-precipitation seems to be the dominant mechanism, in light of the sintering behaviour characterized by a relatively low shrinkage temperature.

Table 1 summarizes the main sintering parameters, microstructural features and mechanical properties of the ZrCeTaSi2 ceramic, compared to other ZrC-based materials taken from literature. The hardness of the ZrCeTaSi2 composite was about 18 GPa, in the range of the data reported in the literature for similar materials. The discrepancy between monolithic ZrC, 25 GPa, and this composite is due to the presence of softer silicide phases which have hardness below 16 GPa [22] and the quite coarse microstructure. The fracture toughness resulted in the range or higher than the other ZrC-composites, indicating that this property is mainly dictated by ZrC matrix toughness [7,8,11,23,24]. ZrCeTaSi2 composite displayed strength around 500 MPa, higher than similar composites containing MoSi2 processed by hot pressing and pressureless sintering [7,8,11,23,24]. The highest value shown in Table 1 for the SPS-composite refers to measurements carried out on 3-point bending and very small specimens [8]. So it is concluded that this ceramic displayed very good mechanical properties also after sintering at such low temperature (1970 K).

Fig. 2. SEM images of the polished surface of ZrCeTaSi2 composite showing (a) the formation of SiC dark particles and (b) the morphology of the matrix.

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Fig. 3. TEM images of the as sintered ZrCeTaSi2 composite showing (a) the morphology of the matrix grain with the corresponding EDS spectra in (b), (c) SiC grains incorporating ZreSi phases, (d) and (e) ZreSi phases at the triple point.

3.4. Microstructure of oxidized specimens Pictures of the samples after testing at 1800, 2000 and 2200 K for 20 min, are shown in Fig. 5. The front faces are covered with a grey layer becoming progressively white indicating that the expected oxidation occurred. The oxide layer that formed at 1800 K, Fig. 5a, looks quite smooth and well adherent to the carbide, indicating that at this stage the sample well survived the test. At

2000 K, Fig. 5b, the sample broke upon cooling and some bubbles can be seen in the more heated zone. At 2200 K, Fig. 5c, bubbling phenomena are more evident, the surface is whiter and a wavy surface is also noticeable, indicating a change in the oxidation resistance at this temperature. Fig. 6 shows the evolution of CO2 and SiO as a function of the oxidation temperature, determined using mass spectrometry. We can observe that at 1800 K the material evolves very low amount of

Fig. 4. HR-TEM images of the as sintered ZrCeTaSi2 composite showing (a) the interface between ZrC-core and (Zr,Ta)C shell with dislocations indicated by arrows, (b), (c) examples of clean grain boundaries between (Zr,Ta)C e (Zr,Ta)C e ZrSi2, or partially wetted grain boundaries in (d).

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Fig. 5. Photographic images of the ZrCeTaSi2 discs after oxidation tests at (a) 1800, (b) 2000 and (c) 2200 K.

gaseous species, which notably increases after 10 min of oxidation at 2000 K. This trade off can be due to the migration of silicide phases to the surface, which form a protective glassy scale until the dissociation of ZreSi silicides prevails, resulting in further development of SiO. At 2200 K the gases escaping is notably higher, owing to the complete dissociation of the silica-glass and the lack of a protective oxide scale, or to CO2 escape through fissures. As far as concerns mass variation, as expected, the weight gain increased with increasing temperature going from 1.20 mg cm2 min1 at 1800 K, to 2.37 mg cm2 min1 at 2000 K and 3.85 mg cm2 min1 at 2200 K. X-ray diffraction patterns collected on the surface of the three specimens after oxidation tests are shown in Fig. 7. The main crystalline phases at all temperatures is a mixed oxide with composition TaZr2.75O8, which has an orthorhombic structure and preferred orientation along the [020] planes; the peaks become sharper increasing the temperature, indicating improved crystallization. Monoclinic ZrO2 is also present and the peaks signal increases with temperature to about 25%1 at 2200 K, indicating the higher stability of pure oxide over the mixed one. Cubic and tetragonal ZrO2 could be present too, but the superimposition of the main peaks with the mixed oxide hinders a conclusive analysis. Other authors [25,26] and similar tests on ZrC-based composites showed indeed that the presence of carbon, coming from the oxidation of the carbide, stabilizes c-ZrO2 at low temperatures. Fig. 8 presents SEM images of the surfaces of the central region of the discs at 1800, 2000 and 2200 K. The addition of TaSi2 to a ZrC matrix generated a variety of elaborated morphologies varying the oxidation temperature; as a rule of thumb, dark regions correspond to silica-based glass, bright phases to oxides. At 1800 K the surface of the composite presents a rough cracked aspect with TaZr2.75O8 and ZrO2 being the main phases where discrete pockets of silica-based glass are found, Fig. 8a. The surface at 2000 K is mainly composed of petal-like grains of TaZr2.75O8 which form volcanos and tend to microcracking, Fig. 8b. At 2200 K melting and recrystallization of the oxide occurred and 20 mm large grains formed leaving residual silica, containing Zr traces, at the grain boundaries, Fig. 8c. These grains are in turn composed of polyhedral structures of ZrO2 and TaZr2.75O8 showing the growth planes decorated by a dendritic irregularly shaped phase identified as ZrO2 and ZrSiO4 containing small traces of Ta, inset in Fig. 8c.

1 Since the scattering coefficient for the orthorombic TaZr2.75O8 phase has not been published in the literature and is not available in the ICSD database, we considered this phase as a solid solution between 0.5 mol orthorombic Ta2O5 (#54e 514) and 3 mol tetragonal ZrO2 (#42.1164) and estimated a scattering coefficient of 4.7.

Fig. 9 presents the cross-section of the sample oxidized at 1800 K, where a 140 mm thick layer underwent oxidation with formation of the mixed Zr, Ta oxide with granular shape covered by 3 mm thin and discontinuous silica layer (Fig. 9b). Moving further inward, down to around 400 mm from the surface, a complex mixture of (Zr,Ta)Si2, (Zr,Ta)-oxy-carbide and SiC phases were found, Fig. 9c. The complex (Zr,Ta)eCeO phase is the result of partial oxidation of the starting matrix grain, made also of (Zr,Ta)C, as outlined above. The interfaces between the oxide, oxy-carbide and the bulk are crack free and continuous. The formation of an oxy-carbide standing between the pure oxide external layer and the carbide core, had been already reported for ZrC and HfCcompounds [25,26]. After oxidation at 2000 K, the specimen resulted in a layered structure with the outermost scale composed of 5 mm long TaZr2.75O8 grains standing out on a continuous silica-based scale (Fig. 10b), which topped about 300 mm of coarse ZrO2 and TaZr2.75O8 grains, partially filled with silica and where vigorous bubbling clearly occurred Fig. 10a. This thick external layer was partially detached from the underneath surface, which is not shown, since it had the same aspect as the section oxidized at 1800 K (Fig. 9). The cross-section of the ceramic oxidized at 2200 K is shown in Fig. 11, where all the thickness of the disc resulted partially oxidized. The outermost thick scale is composed by a compact ZrO2 layer where 20e30 mm large porosities can be found (Fig. 11a). In this region, magnified in Fig. 11b, SiO2 droplets (dark) containing Zr traces are surrounded by a brighter phase, identified as a solid

Fig. 6. Measured concentration of SiO and CO2 (m/e ¼ 44) produced at (a) 1800, (b) 2000 and (c) 2200 K during the oxidation of ZCT.

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Fig. 7. X-ray diffraction patterns of the ZrCeTaSi2 composite after oxidation at (a) 1800, (b) 2000 and (c) 2200 K.

solution with possible formula (Zr0.8Ta0.2)O2. Right underneath this 550 mm layer, the mixed TaZr2.75O8 oxide is present in form of a thick dense scale including elongated porosity and a Ta-rich white oxide (Fig. 11c, upper part). Moving further inward, fine grained ZrO2 with Ta traces and SiO2 discrete phases (Fig. 11d) stand above the already mentioned (Zr,Ta)-oxy-carbide and SiC phases (Fig. 11c lower part and Fig. 11e). The core of the disc is composed by a mixed (Zr,Ta)-oxy-carbide containing progressively lower oxygen amounts. These articulate morphologies are the result of complex oxidation mechanisms, including melting, phase separation and reprecipitation, occurring at 2200 K.

be the migration of silica glass to the surface, as observable in Fig. 10b, which after a certain period loses its shielding action and is no longer protective to gases escape, as indicated by the large voids in the cross-section of Fig. 10a, and by the bubbling and fracture of the sample in Fig. 5b. From this temperature on, vigorous gas escape was reported to occur, as demonstrated also by the turbulent microstructure in Fig. 11a. It is almost established that the oxidation of carbides of groups IVeVI transition metals occurs through formation of an oxy-carbide of the metal plus carbon, which is subsequently oxidized to CO and CO2, ending with formation of the metal oxide [25,26], according to reactions of the type: MeC þ 2O2 / MeCxOy þ C1x þ O4y / MeO2 þ CO2

(4)

3.5. Oxidation behaviour Pictures and SEM images clearly reveal this composite to undergo various oxidation mechanisms depending on the different temperature range. Fig. 6 evidences that the evolution of CO2 and SiO gaseous species has a slow and parabolic trend at 1800 K for the whole duration of the test, but at 2000 K an abrupt increase occurs after 10 min of oxidation. This variation was ascribed to the melting and decomposition of Zr-silicides, occurring at around 1900 K [27], which resulted in further SiO release. A second explanation could

This general reaction can be extended to the particular case of ZrCeTaSi2, where a solid solution grain is oxidized to the corresponding mixed oxy-carbide and then to the mixed oxide (5): (Zr,Ta)C þ 2O2 / (Zr,Ta)CxOy þ C1x þ O4y / TaZr2.75O8 þ CO(5) The formation of an intermediate oxy-carbide or cubic ZrO2 phase next to the cubic ZrC phase ensures that the scale adheres more firmly to the specimen substrate, owing to the same crystal structure, thereby improving its protective qualities. The partial

Fig. 8. SEM images of the surface of ZrCeTaSi2 after 20 min oxidation in air at (a) 1800, (b) 2000 and (c) 2200 K. In the insets a magnification of the microstructure.

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Fig. 9. SEM images of the polished cross-section of ZrCeTaSi2 after oxidation at 1800 K. (b), (c) Magnification of the areas as indicated in (a).

Fig. 10. SEM images of the fractured cross-section of ZrCeTaSi2 after oxidation at 2000 K. Only the outermost scale is shown.

sintering of the scale in the presence of carbon also impedes the diffusion of the reaction components and hence lowers the rate of oxidation [27]. The generated oxide layer is partially protective, as testified by the parabolic rate of gases evolution which is believed to control the oxide growth up to 1800 K. A different behaviour is observed upon oxidation at higher temperatures. Whilst for other carbides, like HfC the evolution of gases becomes less important owing to the sintering of the oxide [16,17], in the present case the gases evolution notably increases, probably owing to the melting of the mixed oxide. These considerations imply that the sintering and melting of the surface oxides determines a change in the oxidation behaviour. In particular, some authors recognized that the melting point of the TaZr2.75O8 phase could be significantly lower than that of pure zirconia with heavy consequences on the high temperature stability [28]. This assumption seems to find a confirmation in the test performed at 2200 K, where evident melting, dissociation, evaporation and re-precipitation phenomena occurred. The addition of significant amounts of silicides could partially alter the oxidation behaviour, so the oxidation reactions involving the Zr-silicides and the other secondary phases, like SiC, should be considered as well. Since the tests were performed from 1800 K on,

we can say that the surface of the specimens is mainly subjected to active oxidation regime that involves formation of gaseous products according to: ZrSi2 þ 2O2 / ZrO2 þ 2SiO

(6)

SiC þ O2 / SiO þ CO

(7)

Gases formation introduces porosity which allows diffusion via pores. Gas formed below the oxide layer can also lift and disrupt the oxide layer, like in the present case from 2000 K on. In principle, oxidation of silicides should produce glassy silica that diffuses through the surface and form a stable and continuous silica layer. However, previous studies on the oxidation of similar composites have shown that even at temperatures lower than 1500 K, no continuous oxidation layer was observed on the surface, although silica partially filled cavities in the oxide cross-section [11]. This can be due to several reasons: there is not enough silica to fill all the volume expansion associated with the formation of the porous oxide layer and large CO escape resulting from oxidation of the carbide can further accelerate the dissociation of silica to gaseous SiO. Although no continuous silica layer was found on the

Fig. 11. SEM images of the polished cross-section of ZrCeTaSi2 after oxidation at 2200 K. (b)e(e) Magnification of the areas as indicated in (a) or (c).

L. Silvestroni et al. / Materials Chemistry and Physics 143 (2013) 407e415

surface, the presence of partially filled porosity in the cross-section can hinder the fast diffusion of gaseous species towards the unreacted bulk up to 2000 K (Fig. 10). 4. Conclusions A zirconium carbide ceramic was hot pressed at 1970 K with addition of tantalum silicide which enabled the complete densification. The matrix was composed by an inner ZrC-core surrounded by (Zr,Ta)C solid solution forming grains of about 5 mm. These were bordered by ZreSi phases, formed upon cation exchange between ZrC and TaSi2, and SiC particles, formed after carbo-reduction of the silica-based species. This ceramic displayed good mechanical properties with hardness of 18 GPa, fracture toughness of 3.6 MPa m1/2 and flexural strength of 500 MPa. Oxidation tests from 1800 to 2200 K for 20 min duration evidenced the formation of ZrO2 and of the mixed oxide TaZr2.75O8 with platelet shape. The ZrC-based composite resisted well up to 1800 K and until the first 10 min at 2000 K. After this controlled oxidation regime, concurrent phenomena of silicide decomposition, vigorous gas evolution and oxide melting took place, resulting in final oxide breaking and spalling. Acknowledgements We greatly acknowledge the financial support of the US Air Force Research Laboratory to partial of this activity through grant N. FA8655-12-1-3004, with Dr. Ali Sayir as contract monitor. D. Dalle Fabbriche is acknowledged for hot pressing, G. Celotti for X-ray diffraction and C. Melandri for mechanical testing. References [1] H.O. Pierson, Handbook of Refractory Carbides and Nitrides, William Andrew Publishing/Noyes, 2001, pp. 55e99. [2] Floyd B. Baker, Edmund K. Storms, Holley Jr., E. Charles, Enthalpy of formation of zirconium carbide, J. Chem. Eng. Data 14 (1969) 244e246. [3] H.F. Jackson, D.D. Jayaseelan, W.E. Lee, M.J. Reece, F. Inam, D. Manara, C. Perinetti Casoni, F. De Bruycker, K. Boboridis, Int. J. Appl. Ceram. Technol. 7 (2010) 316e326. [4] B. Pierrat, M. Balat-Pichelin, L. Silvestroni, D. Sciti, High temperature oxidation of ZrCe20%MoSi2 in air for future solar receivers, Sol. Energy Mater. Sol. Cells 95 (2011) 2228e2237. [5] E. Sani, L. Mercatelli, P. Sansoni, L. Silvestroni, D. Sciti, Spectrally selective ultra-high temperature ceramic absorbers for high-temperature solar plants, J. Renewable Sustainable Energy 4 (2012) 033104. [6] E. Sani, L. Mercatelli, F. Francini, J.-L. Sans, D. Sciti, Ultra-refractory ceramics for high temperature solar absorbers, Scr. Mater. 65 (2011) 775e778.

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