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Efficient Synthesis/Sintering Routes To Obtain Fully Dense Ultra-High-Temperature Ceramics (UHTCs) Roberta Licheri,† Roberto Orru` ,*,†,‡ Antonio Mario Locci,† and Giacomo Cao*,†,‡ Dipartimento di Ingegneria Chimica e Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA), Unita` di Ricerca del Consorzio InteruniVersitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), UniVersita` degli Studi di Cagliari, Piazza D’Armi, 09123 Cagliari, Italy, and PROMEA Scarl, c/o Dipartimento di Fisica, Cittadella UniVersitaria di Monserrato, S.S. 554 biVio per Sestu, 09042 Monserrato (CA), Italy
Two different synthesis/sintering routes are proposed in this work for the preparation of fully dense 2ZrB2SiC ultra-high-temperature ceramic (UHTC) composites. Both processes start from commercial powders of zirconium, boron carbide (B4C), and silicon, and both take advantage of the spark plasma sintering (SPS) apparatus. In the first synthesis route, reactive spark plasma sintering (RSPS), it is possible to gradually synthesize and fully consolidate the UHTC material in a single step. This result is achieved within a total time (tT) of 20 min and with a maximum temperature (TD) of 1900 °C. The second proposed synthesis method (i.e., SHS-SPS) consists in first synthesizing the composite powders, via self-propagating high-temperature synthesis (SHS), and subsequently consolidating them via SPS. In this case, the optimal operating conditions that are able to guarantee the obtainment of a fully dense material are TD ) 1600 °C and tT ) 30 min or TD ) 1800 °C and tT ) 20 min. Based on the results reported in this work, it can be stated that the two proposed methods represent particularly rapid and convenient preparation routes, in comparison to the techniques that are available in the literature for the preparation of analogous materials. 1. Introduction The so-called “ultra-high-temperature ceramics (UHTCs)” are ceramic-based materials that are characterized by extremely high melting points (i.e., >3000 °C), which make them suitable for use as thermal protection systems (TPS). In this study, particular interest is focused on metal refractory borides, such as zirconium diboride (ZrB2) and hafnium diboride (HfB2), which are considered to be potential candidate materials as TPS for space vehicles.1,2 Moreover, it is also well-established that the addition of silicon carbide (SiC) to these materials strongly improves their oxidation resistance at high temperature and improves their room-temperature strength.3-5 Regarding the relatively higher oxidation resistance, it was, in fact, demonstrated that pure diborides are oxidized remarkably at temperatures of >1200 °C, because of the formation of very volatile species (i.e., B2O3), while the presence of SiC in the range of 20-25 vol % promotes the formation of borosilicate glasses, which protect the material from oxidation up to temperatures of ∼1500 °C.3,4 Therefore, ZrB2-SiC- and HfB2-SiC-based composites have been investigated by several researchers, and various techniques for their preparation have been developed. Most of the methods proposed in the literature4-9 for obtaining ZrB2-SiC-based products in bulk form use the hot pressing (HP) technique, through which commercial ZrB2 and SiC powders are sintered. On the other hand, the reaction synthesis and densification in one step of the ZrB2-SiC composite was also accomplished * To whom correspondence should be addressed. For G. Cao: Tel.: +39-070-6755058. Fax: +39-070-6755057. E-mail address: cao@ visnu.dicm.unica.it. For R. Orru`: Tel.: +39-070-6755076. Fax: +39070-6755057. E-mail address:
[email protected]. † Dipartimento di Ingegneria Chimica e Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA), Unita` di Ricerca del Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Universita` degli Studi di Cagliari. ‡ PROMEA Scarl, c/o Dipartimento di Fisica, Cittadella Universitaria di Monserrato.
by reactive hot pressing (RHP), starting from zirconium, SiB4, and graphite10 or zirconium, B4C, and silicon.11,12 Regardless of the two possible options (i.e., sintering or synthesis and densification in a single step), the main concern regarding the use of the traditional HP approach is related to its low heating rates, which leads to relatively long processing times (typically on the order of hours). To facilitate sintering phenomena, it has been suggested to add certain ceramic nitrides (e.g., Si3N4, ZrN) to the starting powder mixture, as a sintering aid.4,6,9 However, also in these cases, the total sintering time was still somewhat high (∼100 min). Analogous considerations can be made when the synthesis and consolidation of ZrB2/25 vol % SiC was performed in one step by RHP. In fact, the synthesis/densification time required to obtain 96.7%-97.7% dense product was ∼240 min (T ) 1900 °C) when starting from zirconium, silicon, and B4C.11,12 In addition, when the RHP of ZrB2/25 vol % SiC was accomplished, starting from zirconium, SiB4, and graphite,10 the holding time required to complete the chemical reaction was 1 h (T ) 1750 °C, P ) 5 MPa) and ceramic densification was attained after additional 30 min (T ) 2100 °C, P ) 20 MPa). However, note that the heating time, which was not specified in this work, still had to be added to the total holding time (90 min) previously considered, to calculate the total duration of the synthesis/consolidation process. A new sintering method, where the starting powders (either to be only consolidated or simultaneously reacted) are crossed by an electric current, was recently developed. This sintering technique typically makes use of the so-called “spark plasma sintering” (SPS) machine,13-18 although some other apparatuses based on this principle are available commerically. The mechanisms and phenomena on which SPS is based are not completely clarified yet. However, a pulse current passing through the processing powders is supposed to generate a plasma discharge within the voids among the particles.19 Typically,
10.1021/ie0701423 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/28/2007
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Table 1. Characteristics of the Starting Powders Used in the Present Investigation reactant
source
particle size [µm]
mean particle size, d50 [µm]a
purity [%]
zirconium (Zr) boron carbide (B4C) silicon (Si)
Alfa-Aesar Alfa-Aesar Aldrich
99.4 >99
a As determined in this work by means of particle size analysis (CILAS Model 1180, France).
processes conducted using SPS allow one to perform sintering at lower temperatures, with respect to traditional HP methods, and shorter times but are, nevertheless, sufficient to reach the complete densification of the material. A comparison of HP and SPS techniques was recently performed during sintering of the UHTC ZrB2/30 vol % ZrC/ 10 vol % SiC composite,20 and the possibility of obtaining the final material relatively faster via SPS was confirmed. In the present study, we report two recently patented21 processing routes for the fabrication of ZrB2-SiC (with a ZrB2/ SiC molar ratio of 2:1, corresponding approximately to ZrB225 vol % SiC) UHTC products in bulk form, starting from powders of zirconium, B4C, and silicon. Specifically, one of the two processes (hereafter called reactive spark plasma sintering (RSPS)) is based on the product synthesis and densification performed in a single step using the SPS apparatus. Alternatively, the second fabrication process (SHS-SPS) consists of, first, obtaining the UHTC product in powder form via self-propagating high-temperature synthesis (SHS) and, subsequently, consolidating it using the SPS apparatus. With regard to the SHS technique, it is a well-known combustion synthesis method based on the occurrence of strongly exothermic reactions that, once ignited, are able to propagate as a combustion wave through the entire reacting mixture, without requiring any other energy supply.22 Simple equipment, low-energy consumption, a high combustion temperature (up to 4000 K), and front propagation velocities up to ∼25 cm/s are the main characteristics of this technique, which often permits a rapid obtainment of final products (i.e., intermetallics, ceramics, composites, etc.), with purity and mechanical properties that are even better than those prepared using conventional methods. In the present work, the influence of holding temperature and processing time, relative to the SPS apparatus, on the RSPS or SHS-SPS fabrication routes is systematically investigated. The obtained optimal products are then characterized in terms of microstructure, hardness, fracture toughness, and oxidation resistance. The results are compared with those reported in the literature for relatively analogous ZrB2-SiC products prepared using other fabricating methods. 2. Experimental Materials and Methods Characteristics and sources of reactants used for the preparation of bulk ZrB2-SiC products are reported in Table 1. The starting mixture to be processed either by RSPS or SHS was prepared by mixing the reactants according to the following reaction:
2Zr + B4C + Si f 2ZrB2 + SiC
(1)
The powders were mixed in a SPEX 8000 shaker mill (SPEX CertiPrep, USA) for 30 min. The obtained mixture is used for either synthesis and simultaneous consolidation, according to the RSPS process, or for SHS of the UHTC powders to be subsequently densified via SPS.
The experimental setup used in this work for SHS has been described in detail elsewhere.23 Approximately 8 g of the starting mixture was uniaxially pressed to form cylindrical pellets with a diameter of 16 mm, a height of 30 mm, and a green density of 50% of the theoretical value. The combustion front was generated at one sample end, using an electrically heated tungsten coil, which was immediately turned off immediately after the reaction was initiated. The reaction then self-propagates until it reaches the opposite end of the pellet. The temperature during the reaction evolution was measured using thermocouples (W-Re, 127 µm in diameter, Omega Engineering, Inc., USA). To convert the obtained SHS product to powder form, ∼4 g of it were milled (SPEX 8000 mixer mill) using a stainless steel vial with two steel balls (13 mm in diameter, 8 g in weight) for 20 min. Particle size distribution of the obtained powders was determined using a laser light scattering analyzer (CILAS Model 1180, France). An SPS 515 apparatus (Sumitomo Coal Mining Co., Ltd, Japan) was used in the temperature-controlled mode in both of the proposed processes. A detailed description of the SPS apparatus has been reported elsewhere, for the sake of brevity.24 Briefly, it combines a 50 kN uniaxial press with a DC pulsed current generator (10 V, 1500 A, 300 Hz) to simultaneously provide a pulsed electric current through the sample and the graphite die containing it, together with a mechanical load through the die plungers. Approximately 3 g of the starting powders, either already combustion-synthesized or to be reacted and simultaneously consolidated, were first cold-compacted inside the die (outside diameter ) 35 mm; inside diameter ) 15 mm; height ) 40 mm). To protect the die and facilitate sample release after synthesis, a 99.8% pure graphite foil (0.13 mm thick, Alfa Aesar, Karlsruhe, Germany) was inserted between the internal surfaces of the die and the top and the bottom surface of the sample and the graphite plungers (14.7 mm in diameter, 20 mm in height). Both the die and the plungers were composed of AT101 graphite and provided by Atal srl, Italy. In addition, for the purpose of minimizing heat losses by thermal radiation, the die was covered with a layer of graphite felt (Atal srl, Italy). The die was then placed inside the reaction chamber of the SPS apparatus and the system was evacuated down to 10 Pa. This step was followed by the application of mechanical pressure (20 MPa) through the plungers. The experiment is initiated with the imposition of a previously set temperature program for a given time. During SPS, the temperature, applied current, voltage, mechanical load, and vertical displacement of the lower electrode were recorded in real time. In particular, temperatures were measured using a C-type thermocouple (Omega Engineering, Inc., USA) that was inserted inside a small hole in one side of the graphite die. The measured displacement can be regarded as the degree of powdered compact densification. However, thermal expansion of the sample, as well as that of both electrodes, graphite blocks, spacers and plungers, are also responsible for the variation of this parameter. The contribution of the die/plungers ensemble is evaluated as reported elsewhere24 by performing “blank tests”, which consist of the application of same SPS conditions (temperature cycle, mechanical load, time, etc.) when a powder compact is not present. Thus, by subtracting the output from the “blank test” to the displacement measured during a typical SPS experiment, we obtain the sample shrinkage (δ), which will be considered in the following discussion. It is worth noting that, according to this procedure, thermal expansion of the processing powders is not considered. In any case, the real
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degree of consolidation was determined by measuring the density of the sample at the end of the process. For sake of reproducibility, each experiment was repeated at least twice. After the synthesis process, the sample was allowed to cool and then was removed from the die. The relative density of the product was determined by geometric and gravimetric measurements and also by the Archimedes method. Phase identification was performed using a Philips PW 1830 X-ray diffractometer using nickel-filtered Cu KR radiation (λ ) 1.5405 Å). The microstructures of ball-milled powders and SPS endproducts were examined by scanning electron microscopy (SEM) (Model S4000, Hitachi, Japan), and the local phase composition was determined using energy-dispersive X-ray spectroscopy (EDXS). Indentation methodology, using a Zwick 3212 hardness tester machine (Zwick & Co. GmbH, Germany), was used to determine the Vickers hardness of the obtained products. The applied load was equal to 1 kg, while the dwell time was 18 s. Based on the Vickers indentation and by assuming a Palmqvist cracks geometry, the fracture toughness (KIC) was also estimated through the following equation:25
KIC ) 0.0319
P al1/2
(2)
where P is the applied load, a the mean indentation half-diagonal length, and l is the crack length. The oxidation behavior of the ZrB2-SiC UHTC product was obtained by performing thermogravimetric analysis (TGA), using a TA SDA 2969 simultaneous DTA-TGA Instrument. Specifically, two types of experiments were conducted. One of the experiments was conducted under non-isothermal conditions. It consists of heating the specimen slowly (2 °C/min) from room temperature to 1450 °C in air while measuring the mass gain as a function of temperature. In the second test, after a rapid heating at a rate of 20 °C/min, TGA experiments were conducted isothermally at 1450 °C for ∼4 h, during which the temporal variation of the mass sample was monitored. 3. Results and Discussion
Figure 1. Temporal profiles of spark plasma sintering (SPS) outputs during the preparation of ZrB2-SiC using reactive spark plasma sintering (RSPS): (a) temperature and sample shrinkage, (b) current intensity and voltage (TD ) 1900 °C, tH ) 10 min, tT ) 30 min, P ) 20 MPa).
3.1. Synthesis and Simultaneous Densification by RSPS. 3.1.1. RSPS Process Dynamics. Temperature, sample shrinkage (denoted as δ, expressed as a percentage, relative to its final value), average electric current, and voltage temporal profiles measured during the synthesis process are shown in Figures 1a and 1b, when the dwell temperature (TD) was set equal to 1900 °C, while the heating time (tH) and the total sintering/ reaction time (tT) were 10 and 30 min, respectively. The applied mechanical pressure was 20 MPa. As shown in Figure 1a, only a slight decrease of the sample shrinkage δ is observed during the first 2.5 min. Afterward, δ first increases linearly to ∼14% (2.5-4.5 min) and then, during the subsequent 60 s, it further increases to ∼24%, at a relatively higher rate. Then, in the time interval of 5.5-7 min, δ not only stops increasing, but decreases slightly, to ∼20%-22% (see inset in Figure 1a). Subsequently, in the range of 7-10 min, it increases quite rapidly to ∼92%, until the temperature program is equal to its set-point value (TD). Finally, when the temperature is maintained constant to TD, δ changes modestly, thus reaching its final value (∼3 mm) at the end of the experiment (t ) tT ) 30 min). Regarding the electrical behavior of the system (cf. Figure 1b), it is observed that, immediately after the SPS experiment
was started, to obtain the selected thermal profile, the proportional-integral-differential (PID) controller current gradually augments the current with a consequent voltage increase. In particular, the current increases more rapidly during the sample displacement variation observed in the range of 7-10 min (cf. Figure 1a), because the sample becomes denser and, therefore, the corresponding electrical resistance decreases. Afterward, both the current and voltage rapidly decrease to the corresponding stationary values, i.e., ∼1200 A and ∼5 V, respectively. Qualitatively similar results were obtained by setting different TD and tT values. Note that, for safety reasons, both tH and the pressure (P) have been maintained constant in all experiments. In fact, preliminary experiments performed by setting tH too low (e5 min) and/or P too high (g30 MPa) were accompanied by breakage of the die/plungers during the RSPS process, which is most likely due to the occurrence of the exothermic reaction (reaction 1) under the combustion regime. 3.1.2. Evolution of the Synthesis/Densification Process. The formation of the UHTC product during the SPS process was studied as a function of the processing time (tT). Specifically, the influence of this parameter on the composition of the obtained RSPS products was investigated under the experimental conditions considered previously, i.e., the temperature-time
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Figure 3. Densities of end-products obtained via RSPS, as a function of the SPS holding time (tT) (TD ) 1900 °C, tH ) 10 min, P ) 20 MPa). The dotted line indicates the theoretical density of ZrB2-SiC (with a ZrB2/SiC molar ratio of 2).
Figure 2. X-ray diffraction (XRD) patterns of RSPS products obtained for different values of the time interval during which the pulsed electric current is applied (TD ) 1900 °C, tH ) 10 min, P ) 20 MPa): (a) reactants, (b) tT ) 3 min, (c) tT ) 4 min, (d) tT ) 4.5 min, (e) tT ) 5 min, (f) tT ) 5.5 min, (g) tT ) 6 min, (h) tT ) 8 min, and (i) tT ) 10 min.
program as reported in Figure 1a and P ) 20 MPa. The experiments were performed by interrupting the application of the current at different times that correspond to the most significant changes in the displacement time profile, i.e., in the range of 3-10 min (cf. Figure 1a). The final samples obtained under these conditions were analyzed via X-ray diffraction (XRD). The resulting patterns are reported in Figure 2, along with that which corresponds to the starting powder mixture (cf. Figure 2a). These results clearly indicate that the synthesis reaction of ZrB2 and SiC starting from the zirconium, B4C, and silicon proceeds gradually, as indicated by the compositional changes of the RSPS product observed as the reaction time increases. Specifically, under the adopted experimental conditions, the first evidence of product formation (ZrB2) is found within 4 min (cf. Figure 2c), while all starting reactants are almost completely converted within 6 min (cf. Figure 2g) and a product containing only the desired carbide and boride phases is obtained within 8 min (Figure 2h). In addition, the maximum transformation rate is observed in the temporal range of 4.5-5.5 min (cf. Figures 2d-f). Because the synthesis reaction is almost completed within 99.5% of the theoretical density, when TD was augmented from 1400 °C to 1900 °C. 3.1.4. Product Characterization. Based on the previously reported results, it can be concluded that the optimal operating conditions that are able to guarantee the obtainment, via RSPS, of a fully converted and dense ZrB2-SiC product are TD ) 1900 °C, tH ) 10 min, tT ) 20 min, P ) 20 MPa. Consequently, the product synthesized under such conditions was further characterized in terms of SEM investigation, as well as mechanical and oxidation resistance properties. Two back-scattered SEM micrographs at different magnification of the RSPS composite synthesized under the optimal conditions previously reported is shown in Figures 5a and 5b. Two different regions are readily observed, specifically, a dark one, which corresponds to the ZrB2 phase, and a light one (SiC); these regions are found well-distributed throughout the entire sample.
Figure 5. Scanning electron microscopy (SEM) back-scattered micrographs of the RSPS UHTC product at different magnifications: (a) 200× and (b) 2000× (conditions: TD ) 1900 °C, tH ) 10 min, tT ) 20 min, P ) 20 MPa).
The Vickers hardness and fracture toughness were determined following the procedure described in the experimental section. The Vickers hardness was determined to be 15.9 ( 0.4 GPa, whereas the fracture toughness was determined to be 4.1 ( 0.6 MPa m1/2. These values are very similar tosand, in some cases, higher thansthe best results reported in the literature for ZrB2/SiC dense composites that have the same stoichiometry and have been obtained by alternative methods.6,7,11,27 The oxidation resistance of the UHTC material produced via RSPS has been measured using TGA by monitoring the mass change of the sample subjected to an oxidizing environment (air) at high temperature. Specifically, two types of measurements have been performed, and the obtained results are reported in Figures 6a and 6b. During the first test (cf. Figure 6a), the specimen was heated at a relatively low heating rate (2 °C/ min) from room temperature up to 1450 °C. Conversely, the second test (cf. Figure 6b) was conducted isothermally at 1450 °C, for ∼4 h, after a rapid heating stage (applying a rate of 20 °C/min). The ZrB2-SiC composite produced via RSPS shows a low and thermally stable oxidation rate up to 1450 °C, because of the well-known formation of a protective layer of borosilicate glass on the surface of the oxidized material.4,10 Specifically, the onset of the oxidation reaction, which corresponds to the point where the sample starts to gain weight, was recorded to occur at ∼770 °C. Afterward, the weight first increases up to a maximum level at T ) 1033 °C, then decreases, reaching a minimum value at 1156 °C, and, finally, increases again until the final temperature is achieved. These results are similar to those found by other authors when characterizing a UHTC
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Figure 7. Temperature profile recorded during self-propagating hightemperature synthesis (SHS) of the ZrB2-SiC composite, according to reaction 1.
Figure 6. Specific weight change of the ZrB2/SiC ceramic composite obtained by RSPS (TD ) 1900 °C, tH ) 10 min, tT ) 20 min, P ) 20 MPa) during thermogravimetric analysis (TGA) oxidation in air as a function of (a) temperature (non-isothermal run with a heating rate equal to 2 °C/ min) and (b) time (isothermal run at 1450 °C).
product that has an analogous composition and has been obtained via alternative processing methods.4 Different specific studies have been reported in the literature to interpret such behavior.4,28 Specifically, when ZrB2 is in contact with O2 at high temperature, they react to form ZrO2 and B2O3. The latter one is in a liquid state (its melting temperature is ∼450 °C) and is able to fill porous ZrO2, thus acting as an oxygen diffusion barrier. However, as the temperature increases, the evaporation of B2O3 becomes more significant. Thus, although up to 1000-1050 °C, the formation of B2O3 in the material surface is predominant and the sample weight gain increases, the evaporation of B2O3 prevails on its formation when the temperature is relatively higher. Correspondingly, the weight of the oxidizing material starts to decrease. However, at a temperature of ∼1200 °C, SiC oxidation leads to silica formation, which combines with B2O3 to form a silica-rich borosilicate glass layer. The latter phase consequently reduces the B2O3 evaporation, in addition to providing improvement of the oxidation resistance of the UHTC material. 3.2. SHS and Subsequent Densification by SPS. 3.2.1. Synthesis of ZrB2-SiC by SHS. According to the high enthalpy of reaction 1, i.e., - ∆H°r ) 647.266 kJ/mol,29 the synthesis of the resulting 2ZrB2-SiC composite displayed self-
Figure 8. Comparison of XRD patterns of (a) the starting reactants and (b) the UHTC product obtained via SHS, according to reaction 1.
propagating behavior. A typical experimental temperature profile recorded during combustion synthesis by a thermocouple embedded in the sample is reported in Figure 7. The temperature suddenly increases when the reaction front approaches the position where the thermocouple is placed, thus reaching its maximum value, which is equal to ∼2200 °C. The front velocity was calculated from temperature profiles and was equal to 11 ( 1 mm/s. The XRD pattern of the obtained UHTC SHS product, along with that of the starting mixture, are reported in Figure 8. Phase analysis of the XRD patterns showed that the reaction proceeds to completion, with the formation of the desired boride and carbide products. Following the procedure described in the experimental section, before proceeding to the powder consolidation stage by SPS, the obtained SHS product was milled for 20 min and the size distribution of the resulting powders, and an SEM backscattered micrograph, are shown in Figures 9a and 9b, respectively. Figure 9a shows that a particle size of 3 min are needed (cf. Figure 2)) and occurs under relatively slow heating conditions (cf. Figure 1a). Conversely, during SHS, the transformation is very rapid (on the order of seconds) and it is accompanied by drastic heating and cooling steps (cf. Figure 8). These aspects most likely affect the microstructure formation of the obtained product, which is rather fine, as shown in Figure 9b. This fact can also explain the relatively lower sintering temperature required for the SHS-SPS process to obtain a fully dense UHTC material, in comparison to that obtained via the RSPS method. This finding is also consistent with other experimental results recently observed when comparing the sinterability of SHS and commercial powders in the ZrB2-ZrCSiC system, under the same SPS conditions.30 In fact, it was observed that, although the commercial mixture consisted of relatively finer particles, in comparison to SHS powders, the latter ones were characterized by finer grain sizes of each individual phase, thus requiring a lower sintering temperature to obtain a fully dense product. Regarding mechanical properties, it was found that the Vickers hardness, using a load of 1 kg, was ∼16.7 ( 0.4 GPa, whereas the fracture toughness was 5.0 ( 0.3 MPa m1/2. These values are slightly better than those obtained when considering products prepared via RSPS. The mass change of the SPS UHTC material, when oxidized in air at high temperature, is reported in Figures 14a and 14b, where the corresponding dependence on temperature (non-
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Figure 14. Specific weight change during TGA oxidation in air of the sintered ZrB2-SiC sample (TD ) 1800 °C, tH ) 10 min, tT ) 20 min, P ) 20 MPa) obtained starting from powders synthesized using SHS, as a function of (a) temperature (non-isothermal run with a heating rate of 2 °C/min) and (b) time (isothermal run at 1450 °C).
isothermal test) and time (isothermal run), respectively, are plotted. Also, in this case, the oxidation resistance results are determined to be comparable to those observed when considering the RSPS samples (cf. Figures 6a and 6b). Thus, analogous considerations can be made. Generally, it can be concluded that the ZrB2-SiC composite first synthesized by SHS and subsequently sintered by SPS under the optimal operation conditions previously reported shows a relatively low and thermally stable oxidation rate, up to 1450 °C. 4. Summary and Concluding Remarks With the final goal of obtaining a fully dense ZrB2-SiC composite (with a ZrB2/SiC molar ratio of 2), two different processing methodssi.e., the reactive spark plasma sintering (RSPS) and self-propagating high-temperature synthesis, followed by spark plasma sintering (SHS-SPS)shave been proposed in this work. Both processes start from commercial powders of zirconium, boron carbide (B4C), and silicon, and make use of the spark plasma sintering (SPS) apparatus. Using the RSPS method, it was possible to synthesize and consolidate the desired UHTC material in a single step. An investigation of the evolution of the synthesis process revealed
that, under the experimental conditions considered (maximum temperature of TD ) 1900 °C, heating time of tH ) 10 min, and pressure of P ) 20 MPa), the initial reactants were transformed gradually to ZrB2 and SiC, thus obtaining their complete conversion within a total SPS time (tT) of 8 min. However, to produce highly dense samples, the electric current needed to be applied for time intervals longer than those required to obtain the complete conversion. Finally, a pure and completely dense product (with a relative density of >99.5%) was obtained at tT ) 20 min. The other proposed method (SHS-SPS) consists of two steps. Specifically, taking advantage of the highly exothermic reaction (reaction 1), the starting reactants were first reacted, using SHS, to form the ZrB2-SiC composite. Thus, the obtained powders were subsequently consolidated via SPS. It was determined that, in this case, the optimal operating conditions that are able to guarantee the obtainment of a fully dense material (for a relative density of >99.5% of the theoretical value) are TD ) 1600 °C and tT ) 30 min or TD ) 1800 °C, tT ) 20 min, while the applied pressure is P ) 20 MPa. Regarding the characteristics of the obtained products investigated in the present work (i.e., hardness, fracture toughness and oxidation resistance), the obtained results are similar and, in some cases, better, in comparison to similar ultra-hightemperature ceramic (UHTC) products synthesized using competitive methods. In addition, both the RSPS and SHS-SPS processes are characterized by shorter processing times and lower sintering temperatures, when compared to the other fabrication methods proposed in the literature. For instance, as indicated in the Introduction, the total time required to obtain, in one step using reactive hot pressing (RHP), a dense ZrB2-SiC was ∼240 min (T ) 1900 °C), when starting from zirconium, B4C, and silicon,11,12 or >90 min (T ) 17502100 °C), when the initial powders were zirconium, SiB4, and carbon.10 Analogous considerations can be made when considering pure sintering processes performed by hot pressing (HP) and starting from commercially available ZrB2 and SiC powders. As an example, it was reported5,7 that, for conditions of T ) 1900 °C and P ) 32 MPa, the total time required to obtain a near full dense ZrB2/20-30 vol % SiC was 450 min. Therefore, it can be concluded that the two proposed reactive methods to obtain completely dense ZrB2/25 vol % SiC from zirconium, B4C, and silicon, represent particularly rapid and convenient preparation routes, in comparison to the techniques available in the literature. Moreover, when focusing on the consolidation aspect, the results of this general consideration are even more appropriate for the SHS-SPS method, where the sintering temperature are relatively reduced. Such demonstrated advantages of the proposed processing routes can be explained because they are both based on a combination of in situ methods for the synthesis of the ZrB2SiC composite, either performed directly during the densification process or preliminarily using SHS, with the use of a very efficient sintering method, such as SPS. Acknowledgment We gratefully acknowledge our colleague, Dr. A. Cincotti (Dipartimento di Ingegneria Chimica e Materiali, Universita` degli Studi di Cagliari, Italy), for useful discussions and valuable support. We also gratefully acknowledge the Regione Autonoma della Sardegna (Italy) for financial support through the project POR Sardegna 2000-2006 (Misura 3.13).
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ReceiVed for reView May 30, 2007 ReVised manuscript receiVed September 3, 2007 Accepted October 1, 2007 IE0701423
