Article pubs.acs.org/IECR
Synthesis, Sintering, and Oxidative Behavior of HfB2−HfSi2 Ceramics Clara Musa, Roberta Licheri, Roberto Orrù,* and Giacomo Cao Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA), Unità di Ricerca del Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Università degli Studi di Cagliari, via Marengo n. 3, 09123 Cagliari, Italy ABSTRACT: The self-propagating high-temperature synthesis (SHS) of (1 − x)HfB2−xHfSi2 (x from 0 to 1) ceramics is successfully performed in this work starting from elemental reactants. The presence of HfSi2 in the composite system progressively reduces the exothermic character of the synthesis reaction. Bulk ceramics are then obtained after consolidation by spark plasma sintering (SPS) of the SHS crashed products. It is found that the HfB2−xHfSi2 composite powders synthesized in a single step by SHS require milder sintering conditions as compared to the mixtures consisting of the ceramic constituents obtained separately by the same route. In addition, 15 vol % is the minimum percentage of HfSi2 required to achieve the complete densification of the starting powders, under the SPS conditions investigated in the present work (I = 1350 A, P = 50 MPa, 30 min total time). Thermogravimetric analysis experiments performed in air flow up to 1450 °C clearly indicate that the introduction of HfSi2 plays a beneficial role for protecting the obtained material from oxidation.
1. INTRODUCTION
oxidation resistance of the resulting composite material, other than facilitating powder consolidation. Along these lines, one of the ceramics that might be used in combination with HfB2 to improve its resistance to oxidation is represented by hafnium disilicide (HfSi2). Owing to its interesting properties, the latter one is suitable for protection of refractory metals and alloys, electrodes, and electronic devices.9 Despite its importance, HfSi2 is, among the silicide phases, one of the less investigated systems. Indeed, only a few studies have addressed this material either in its monolithic9−11 or composite11−13 forms. For instance, this ceramic compound was prepared by mechanical alloying after 30 h ball milling of Hf and Si powders in stoichiometric ratio.10 More recently, HfSi2 was synthesized from elemental powders using classical furnace methods under flowing argon.11 The electrochemical synthesis of hafnium disilicide starting from a molten salt of NaCl−KCl−NaF−K2HfF6−K2SiF6 was also accomplished.9 As far as composite materials are concerned, a plate of HfB2− 7 vol % HfSi2 was prepared by HP to be subsequently coupled with HfSi2 and HfO2 layers in order to evaluate the compatibility of ceramic−ceramic phases in the Hf−Si−O system.12 The consolidation of HfB2−5 vol % HfSi2 powders by HP was also performed by Monteverde11 starting from commercial HfB2 powders and the disilicide phase, which was previously synthesized from elemental reactants as described above. More recently, HfC + 3Si mechanically activated powders were reactively sintered by high-frequency induction heated combustion synthesis to obtain the HfSi2−SiC composite.13 In the present work, the fabrication by SPS of dense HfB2− xHfSi2 ceramics is investigated for the first time using ceramic
It is well-known that HfB2 represents, along with ZrB2, the most important base material in the class of ultrahightemperature-ceramics (UHTCs) because of its interesting properties, such as high melting point (3380 °C), high thermal (104 W/mK) and electrical (9.1 × 106 S/m) conductivities, and thermal expansion coefficient (6.3 × 10−6 K−1).1 These and other attractive characteristics makes HfB2-based ceramics ideal candidates in various application fields where harsh environments have to be withstood, as for the fabrication of thermal protection systems of components in the aerospace industry (leading edges, nose tips).1 HfB2 has been also recognized as suitable material for cutting tools, refractory linings, microelectronics, and neutron absorbers.1,2 One of the main concerns related to the fabrication and utilization of this family of materials relates to the low intrinsic sinterability of HfB2 powders so that severe consolidation temperature and applied pressure conditions are needed to obtain nearly full dense products. It is well established that the spark plasma sintering (SPS) technology, where the powders to be consolidated and/or the mold containing them are rapidly crossed by an electric pulsed current, offers a powerful tool to overcome the drawback above, since relatively milder sintering conditions are needed in comparison with classical hot pressing (HP) methods.3 Specifically, several bulk HfB2-based ceramic materials, either monolithic or composite, requiring relatively lower sintering temperatures and few minutes holding times, have been fabricated so far by SPS.3−6 Nevertheless, it should be also noted that monolithic HfB2 is characterized by relatively modest resistance to oxidation at high temperatures.6 This fact clearly limits its possible application under such conditions. Several studies reported in the literature have shown that the introduction of suitable amounts of Si-containing phases, such as SiC,4,6−8 MoSi2,5 TaSi2,5 into HfB2 matrices, enhances the © 2014 American Chemical Society
Special Issue: Massimo Morbidelli Festschrift Received: Revised: Accepted: Published: 9101
October 1, 2013 January 7, 2014 January 7, 2014 January 7, 2014 dx.doi.org/10.1021/ie4032692 | Ind. Eng. Chem. Res. 2014, 53, 9101−9108
Industrial & Engineering Chemistry Research
Article
mixture to be processed by SPS was obtained by combining them in appropriate proportions to produce the desired system composition, hereafter indicated as (HfB2)SHS−x(HfSi2)SHS. A Spark Plasma Sintering apparatus (SPS 515 Sumitomo Coal Mining Co. Ltd., Japan) under vacuum (20 Pa) conditions was utilized to consolidate SHS powders. This machine combines a DC pulsed current generator (10 V, 1500 A, 300 Hz), to provide an electric current through the processing powders (6 g) and/or the graphite cylinders containing them, with an uniaxial press (max 50 kN) for the simultaneous application of a mechanical load through the punches. SPS experiments were generally carried out using the graphite cylinder configuration, hereafter indicated as A and characterized by external diameter (De) and inside diameter equal to 35 mm and 15 mm, respectively. Alternatively, a different die configuration, hereafter indicated as B, which has smaller De value (30 mm) and the same inner diameter, was also adopted. Both the dies and related plungers were composed of AT101 graphite (Atal s.r.l., Italy). A prescribed electric current cycle was applied to the SPS system during sintering experiments. This procedure was chosen, instead of the temperature control mode, for the sake of comparison with recent studies reported in the literature on the fabrication of monolithic HfB2.6 Specifically, the maximum value of the mean electric current (I = 1350 A) was achieved in 10 min (tH) and maintained constant for additional 20 min (tD). The mechanical pressure (P = 50 MPa) was held constant during the entire SPS run. The most relevant SPS parameters, temperature, current, voltage between the machine electrodes, mechanical load, and vertical sample displacement (δ), were recorded in real time. The temporal evolution of the δ parameter is important, as it provides an indication of compact densification, although the thermal expansion of the electrodes/ spacers/die/plungers/sample ensemble also contributes to the measured value.19 Temperature−time profiles were obtained by means of a two-color pyrometer (Ircon Mirage OR 15-990, USA) focused on the lateral surface of the graphite die. To limit heat losses by thermal radiation, thus reducing thermal gradients in radial direction, a graphite felt layer (Atal s.r.l., Italy) was placed around the die, while a graphite foil liner (Alfa Aesar, 0.13 mm thick, 99.8% purity) was used to facilitate product release after sintering. The final density of ceramics was determined on polished dense samples through geometric/gravimetric measurements and using the Archimedes method. In this regard, the theoretical densities of the monolithic and composite products were calculated by considering the density values of HfB2, and HfSi2 as 11.18 and 7.97 g/cm3, respectively. A Philips PW 1830 X-ray diffractometer using Cu Kα radiation (λ = 1.5405 Å) and Ni filter was used for phase identification. Particle size distribution was determined by means of a laser light scattering analyzer (CILAS 1180, France). The microstructure of sintered materials was examined by scanning electron microscopy (SEM) using a Zeiss EVO LS15 microscope equipped with energy dispersive X-rays spectroscopy (EDS), Oxford X-MAX Probe. The oxidation resistance of bulk ceramics was evaluated by performing thermogravimetric analysis (TGA) measurements in air flow (100 mL/min) under isothermal conditions (1450 °C) using a NETZSCH STA 409PC Simultaneous DTA-TGA instrument.
powders prepared by self-propagating high-temperature synthesis (SHS).14 In this regard, it should be noted that the combination of SHS and SPS routes was successfully adopted in previous studies for the acquisition of dense MB2−SiC and MB2−MC−SiC (M = Zr, Hf, or Ta) products.4,15−17 In this work, the composite mixtures are obtained by blending HfB2 and HfSi2 powders synthesized separately by SHS from elemental reactants. Alternatively, the SHS route is also exploited to obtain the composite product in a single step. Possible benefits in term of resistance to oxidation deriving from the introduction of HfSi2 on hafnium diboride matrix are evaluated by comparing the behavior of the obtained nearly dense composite materials when exposed to high temperatures in air flow during thermogravimetric analysis. It should be noted that, to the best of our knowledge, no studies have been addressed so far in the literature on such a characterization relatively to the HfB2−HfSi2 system.
2. EXPERIMENTAL MATERIALS AND METHODS SHS experiments were carried out starting from mixtures prepared according to the following stoichiometries: Hf + 2.2B → HfB2
(1)
Hf + 2Si → HfSi 2
(2)
(1 + α)Hf + 2.2B + 2α Si → HfB2 + α HfSi 2
(3)
Commercially available hafnium (Alfa-Aesar, 99.6% purity), silicon (Aldrich,
