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ZrB2-SiC composites' surface temperature response to dissociated oxygen at 1600 °C
Ceramics International 42 (2016) 14292–14297
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
ZrB2-SiC composites' surface temperature response to dissociated oxygen at 1600 °C Baihe Du a,b, Ning Li c,n, Bo Ke c, Pifeng Xing c, Xinxin Jin d, Ping Hu a, Xinghong Zhang a a
Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, PR China China Aerodynamic Research and Development Center, Mianyang 621000, PR China c Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, PR China d College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 May 2016 Received in revised form 5 June 2016 Accepted 7 June 2016 Available online 7 June 2016
The surface temperature response of ZrB2-30 vol% SiC composites exposed to dissociated oxygen was investigated using a side-arm oxidation facility. As the specimen was immersed in dissociated oxygen at 1600 °C, a sudden and spontaneous temperature jump accompanied by high degradation of composites was observed in the initial testing stage. For comparison, the surface temperature of pre-oxidized specimen was relatively low and maintained steady under the same testing conditions. The ZrB2-SiC composites' surface temperature response to dissociated oxygen could be linked to the competition between oxidation and catalytic reactions, and the temperature jump of the original specimen was mainly ascribed to the oxidation of composites with atomic oxygen. Pre-oxidation of ZrB2-SiC composites in static air could effectively inhibit the surface temperature jump phenomenon of composites under simulated service conditions. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. surfaces D. borides E. thermal applications Temperature response
1. Introduction Hypersonic vehicles as the next generation aircraft will encounter dissociated air in chemical nonequilibrium for a long time owing to unique flight trajectories, ultra-high speed and lifting body structure, which issues serious challenges to thermal protection materials [1–3]. High-temperature performance of materials is not merely dependent on their inherent properties, but related to surface temperature during service [4,5]. The surface temperature response to high-enthalpy flow has a significant influence on the reliability of thermal protection materials and structures associated with space vehicles. Zirconium diboride (ZrB2) based composites have been proven to possess a good combination of melting point [6], high-temperature retained strength [7], resistance to oxidation/ablation [8], as well as chemical inertness [9], and are being considered as promising candidates for the application in hypersonic flight. Systematic research on surface temperature response under simulated service conditions is necessary for ZrB2-based composites, which can provide a better understanding of the response relationship between environment and composites. High surface temperature is detrimental to material n
Corresponding author. E-mail address: [email protected] (N. Li).
http://dx.doi.org/10.1016/j.ceramint.2016.06.041 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
performance, such as mechanical properties, oxidation and thermal shock resistance, and then induce the degradations of materials. Temperature response is mainly responsible for the difference in surface temperature of various thermal protection materials under the same conditions. Ito et al. [10] have demonstrated the surface temperatures of SiC coating, high catalytic coating and ultra-high-temperature ceramic (UHTC) in subsonic high-enthalpy flow using plasma wind tunnel. The relatively high surface temperature of high catalytic coating and the change of UHTC from low catalytic region to high catalytic region with temperature could be attributed to surface catalytic effect and emissivity. Surface temperature jump phenomena were observed by Hald [11] for C/SiC, and Herdrich et al. [12] for SiC and C/C-SiC. In addition to the oxidation of materials, Hald pointed out that the catalytic reactions of dissociated atoms on surface also contributed to temperature jump, especially under active oxidation condition. Experimental results of Herdrich et al. showed a temperature jump of 400 K for SiC with the transition from passive to active oxidation, and similar phenomena have also been confirmed by Glass [13] and Sakraker et al. [14]. On the contrary, Panerai et al. [15] believed that the passive/active oxidation transition of SiC was not the reason for the sudden temperature increase, because the temperature jump occurred at conditions beyond active oxidation of SiC. The investigation into surface temperature response of ZrB2-based composites (generally with SiC additions) exposed to dissociated air lagged behind that of other Si containing thermal
B. Du et al. / Ceramics International 42 (2016) 14292–14297
protection materials. Hu et al. [16] investigated the effect of SiC content on temperature response of ZrB2-SiC composites. Compared with composites containing 15% and 30% SiC, ZrB2-10 vol% SiC composites exhibited the highest surface temperature. However, the surface temperature response of ZrB2-based composites has not been studied sufficiently, and more attention should be paid to the response mechanism of surface temperature to highenthalpy flow. In the present work, surface temperature response to dissociated oxygen of ZrB2-30 vol% SiC original specimen and the preoxidized specimen was investigated using a side-arm oxidation facility at 1600 °C. The effect of surface morphology on temperature response of ZrB2-SiC composites under simulated service conditions was determined in terms of the surface temperature history as well as microstructure of oxide scale after testing. It is believed that this work could provide a promising method to inhibit surface temperature jump, and lay the foundation for aerospace applications of ZrB2-based composites.
2. Experimental procedure Commercially available ZrB2 powder (2 mm, purity 499.5%, Northwest Institute for non-ferrous metal research, China), and SiC powder (1 mm, purity 4 99.5%, Weifang Kaihua Micro-powder Co., Ltd., China) were used as raw materials. The powder mixtures of ZrB2 and SiC, weighed in proportion to the stoichiometric ratio of 70 vol% and 30 vol%, were ball-mixed for 10 h in a polyethylene bottle and then dried in a rotary evaporator. The as-received powder mixtures were hot-pressed at the temperature of 1950 °C for 1 h under a uniaxial load of 30 MPa in Ar atmosphere. Bulk density and theoretical density were calculated by the Archimedes method and the rule-of-mixture, respectively. Round-shaped specimens with Ø24 mm 3 mm were cut from the billet, and polished to a 1 mm surface finish. The pre-oxidation was carried out in static air using a furnace, and the specimens on zirconia crucible were oxidized at a constant temperature of 1500 °C for 0.5 h. Exposing ZrB2-SiC composites to dissociated oxygen was conducted using a side-arm oxidation facility assembled in our laboratory. This facility was described in detail previously [17]. Firstly, the specimen in a quartz tube was heated up to 1600 °C by using high-frequency induction device. The surface temperature was monitored by a two-color optical pyrometer (Raytek Ltd., Marathon MR1S, USA). The mass flow rate of pure oxygen which got through the experimental chamber was controlled at 50 ml/min, and the desired pressure managed by the pressure regulator and mass flow meter was 100 Pa. Then, opening the microwave generator (Nanjing Huiyan Ltd., MY1500S, China) with a frequency of 2.45 GHz and fixing the anode current at 350 mA, the specimen was exposed to dissociated oxygen
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subsequently. The surface microstructures of specimens were observed by scanning electron microscopy (SEM, FEI Sirion, Holland) equipped with energy dispersive spectroscopy (EDS, EDAX Inc., USA) for chemical analysis. Crystalline phases of the original and pre-oxidized specimens were analyzed by X-ray diffraction (XRD, Rigaku, Japan) that was operated at an incidence angle of 1.5° with the 2θ range from 10° to 70°. The chemical composition of the original specimen after testing was determined using X-ray photoemission (XPS, Thermo Fisher Scientific, USA) with monochromatic Al Kα radiation. The surface was cleaned by Ar þ ion sputtering for 1 min, and a charge correction was carried out with reference to the C 1s signal detected at 285.0 eV. The enthalpy change of oxidation reactions for both ZrB2 and SiC with molecular and atomic oxygen can be calculated using JANAF tables [18].
3. Results The bulk density of the sintered ZrB2-SiC composites was 5.18 g/cm3, which corresponded to a relative density of 99% using the rule mixtures based on the densities of 6.09 and 3.21 g/cm3 for ZrB2 and SiC, respectively [19]. Fig. 1a shows the surface microstructure of the polished specimen. Combined with EDS analysis (not shown here), the small dark SiC dispersed uniformly in the gray ZrB2, and a few small pits was observed in the polished surface because the grains were pulled out or desquamated during polishing. After pre-oxidation at 1500 °C in static air, the specimen surface was dark and completely covered with a tight and smooth borosilicate glass, as presented in Fig. 1b. The XRD patterns of original and pre-oxidized specimens are shown in Fig. 1c. ZrB2 as the main phase companied with a small amount of SiC was detected in the original specimen without any other phases. For the pre-oxidized specimen, the predominant component of the oxide scale was monoclinic zirconia, and a weak diffraction peak at around 30° corresponded to the tetragonal zirconia. Furthermore, a broad peak between 15° and 25° could be attributed to the presence of amorphous glass on surface, which was consistent with the specimen surface morphology [20]. The specimens were heated up to 1600 °C at 100 Pa. After opening the microwave generator, the original and pre-oxidized specimens showed a remarkable difference in the surface temperatures. The surface temperature histories of specimens exposed to dissociated oxygen are shown in Fig. 2. It was evident that the measured temperature fluctuates significantly as the plasma was generated. There are a large number of excited species in plasma. The de-excitation of these species in the form of radiation would affect the optical pyrometer. Additionally, arc instability, turbulence and unsteady fluid feeding also contribute to the drastic fluctuation of surface temperature [21,22]. However, this
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Fig. 1. Micrographs and XRD patterns of ZrB2-30 vol% SiC specimens: (a) original specimen, (b) pre-oxidized specimen, (c) XRD patterns of (a) and (b).
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Fig. 2. Surface temperature histories of ZrB2-30 vol% SiC specimens exposed to dissociated oxygen.
fluctuation of temperature has little effect on both evaluation of surface temperature and characterization of temperature response. When the original specimen was immersed in atomic oxygen, a sudden and spontaneous temperature increase up to 1810 °C was observed in the time range from 3 to 80 s, and then the temperature stabilized at 1890 °C for 100 s. Subsequently, the surface temperature gradually decreased and finally reached a steady state at 1610 °C. It should be noted that, when the microwave generator was opened again, the temperature jump did not occur and the surface temperature was maintained steady throughout the test. At the same time, the surface temperature history of the pre-oxidized specimen was fundamentally different from that of the original specimen. The pre-oxidized specimen placed in dissociated oxygen reached a plateau of 1605 °C during the test. Fig. 3 shows the surface SEM images of the ZrB2-SiC specimens after testing in dissociated oxygen. The difference in surface microstructure between original and pre-oxidized specimens resulted from their different surface temperature histories. It was observed in Fig. 3a that the surface of the original specimen after being exposed to dissociated oxygen was very rough, and mainly composed of zirconia confirmed by EDS (not shown here). There were many irregular clusters and pores on the surface, which could be attributed to the evaporation of gaseous products as the surface temperature rises rapidly. The original specimen during testing underwent high degradation. For the pre-oxidized specimen tested under the same conditions, the surface was quite flat,
and no other condensed phase except for the fine zirconia grains was identified due to the evaporation of borosilicate glass at very low pressure (Fig. 3b). The surface composition of the original specimen after testing in dissociated oxygen was further identified by XPS. Fig. 4a shows the survey XPS spectra of the products on surface of the oxide scale. The peaks of Zr 4p, Zr 4s, Zr 3d, Zr 3p, Zr 3s, O 1s and C 1s were found in the spectra. A large number of Zr and O peaks indicated that the outer layer of the oxide scale was mainly composed of zirconia. The curve fitting Zr 3d spectra was illustrated in Fig. 4b, measured binding energies of Zr 3d5/2 and Zr 3d3/2 were 182.7 and 185.0 eV, respectively, which were assigned to Zr–O bond. Similarly, the O 1s signal at the binding energy of 530.8 eV was also attributed to the presence of ZrO2 on the surface (Fig. 4c). Based on C 1s as presented in Fig. 4d, the peak located at 285.0 eV was low and confirmed as C–C bond from a small quantity of adventitious carbon. It should be noted that no characteristic peak of Si and B could be identified in the survey XPS spectra. It was suggested that specimen tested at low pressure did not produce borosilicate glass due to the active oxidation of SiC.
4. Discussion The surface component and microstructure have a significant influence on the surface temperature, as indicated in Fig. 2, which is responsible for the high-temperature performance of ZrB2-based composites during service. Nonetheless, the mechanism of surface temperature response for ZrB2-SiC composites to high-enthalpy flow is still unclear. It has been widely recognized that the temperature response of composites in atomic oxygen can be associated with the surface catalytic effect [10,23,24]. When ZrB2-based composites are exposed to high temperature, low pressure and atomic oxygen environment, the atomic species, which partially diffuse to the surface of composites, will recombine into molecules accompanied by the release of dissociation energy to surface. In the present experiment, a surface temperature of pre-oxidized specimen increased by 5 °C, which could be related to the catalytic reaction of atomic oxygen on surface. Indeed, the catalytic coefficient of zirconia is larger than that of silica [25,26]. However, such a large increase in surface temperature of the original specimen could not be reasonably explained by the catalytic coefficient disparity between original and pre-oxidized specimens, meanwhile, the sudden temperature increase did not occur as the microwave generator was opened
100μm Fig. 3. Micrographs of ZrB2-30 vol% SiC specimens after testing: (a) original specimen; (b) pre-oxidized specimen.
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Fig. 4. XPS spectra of the ZrB2-SiC original specimen after testing: (a) the survey spectra, (b) Zr 3d, (c) O 1s and (d) C 1s.
again. Consequently, the catalytic reaction of atomic oxygen might contribute to the massive increase in surface temperature, but it is not the main reason. Similarly, heat radiation of composites, also dependent on the surface component and microstructure, could influence the surface temperature response. The emissivity of zirconia is relatively low compared with borosilicate glass, but the emissivity for the oxidation products of ZrB2-SiC composites varies slightly over a wide temperature range [16,27]. Furthermore, the microstructural evolution of oxide scale for the original specimen is not instantaneous. Heat radiation associated with composites surface is not the main factor responsible for the sudden temperature increase after testing in dissociated oxygen. On the contrary, it is possible that the change in surface component and microstructure is attributed to the rapid increase in surface temperature, rather than acts as its trigger. Li et al. [28] noted that heat conduction contributes to the temperature histories of ZrB2-SiC specimens exposed to high-enthalpy flow. During testing at elevated temperatures, the surface of ZrB2-SiC specimen will be covered with a porous zirconia layer, whose heat conductivity is much smaller than that of ZrB2-SiC composites. Therefore, heat transport becomes less efficient or slower in composites as the thickness of oxide scale increases, and surface temperature rises rapidly due to the surface overheating. However, in the present experiment, the specimen was heated from the interior using high-frequency induction device, rather than heated by heat conduction of high-enthalpy flow. It is reasonable to consider that heat conduction has negligible effect on the temperature difference between original and pre-oxidized
specimens. Based on the above-mentioned discussion, the main possible reason for surface temperature jump of original specimen exposed to dissociated oxygen is the oxidation of composites accompanied by releasing a large amount of heat. According to thermodynamic calculations as shown in Fig. 5, oxidation of ZrB2 and SiC in different oxidizing atmospheres is exothermic reactions in the temperature range of 1000–2000 °C. It should be noted that the
Fig. 5. Variations of reaction enthalpies as a function of temperature for reactions of ZrB2 and SiC in different oxidizing atmospheres.
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reaction enthalpies for both ZrB2 and SiC with oxygen atoms are much lower than those with oxygen molecules, indicating that oxidation of ZrB2-SiC composites in atomic oxygen will release more energy under the same conditions. Additionally, the releasing energy per unit time is also dependent on the oxidation rate. The increase in oxidation rate owing to the high activity of oxygen atoms has been confirmed by some literatures [17,29], which is beneficial to the surface temperature jump. As ZrB2-SiC composites are exposed to dissociated oxygen at high temperature, both oxidation reactions and catalytic recombination reactions exist and compete with each other. It can be expected that a large amount of oxidant is consumed due to rapid oxidation in the initial stage of testing process, which inhibits the catalytic reactions of dissociated atoms on surface of specimen. In addition, based on Eqs. (1)–(5), the energy released from oxidation reaction with atomic oxygen is actually equal to the sum energy of the catalytic reaction of oxygen atoms and oxidation reaction with molecular oxygen. Therefore, the temperature jump of original specimen in the initial testing stage is mainly ascribed to the oxidation of composites. As the thickness of oxide scale rises, the surface temperature of composites as well as the oxidation rate decreases gradually. Because the oxidation rate is smaller than that of original specimen in the initial testing stage, the surface temperature does not jump but increases by 10 °C after opening the microwave generator again, which could be related to the combination of oxidation and catalytic reactions. For the pre-oxidation specimen, the amorphous borosilicate glass exposing on surface, as a protective barrier to limit the diffusion of oxidant into the inner part of matrix and improve the resistance to oxidation, is the essential reason for the relatively low surface temperature of specimen during testing. Consequently, before using the composites in the high temperature, low pressure and atomic oxygen environment, making ZrB2-SiC composites pre-oxidized in static air can be considered as a promising method for inhibiting the temperature jump phenomenon of composites during service.
2O (g) + SiC = SiO (g) + CO (g) + 667.3 kJ/mol
(1)
O2 (g) + SiC = SiO (g) + CO (g) + 157.3 kJ/mol
(2)
2O (g) + 2/5ZrB2 =2/5ZrO2 +2/5B2 O3 (g) + 1146.1 kJ/mol
(3)
O2 (g) + 2/5ZrB2 =2/5ZrO2 +2/5B2 O3 (g) + 636.0 kJ/mol
(4)
2O (g) = O2 (g) + 510.1 kJ/mol
(5)
5. Conclusions The ZrB2-30 vol% SiC original specimen and the specimen preoxidized in static air were exposed to dissociated oxygen using a side-arm oxidation facility at 1600 °C, in order to investigate the effect of surface morphology on the temperature response of ZrB2-SiC composites under simulated service conditions. As the original specimen was immersed in dissociated oxygen, a sudden and spontaneous temperature jump up to 1890 °C, accompanied by high degradation of composites, was observed in the initial testing stage. Then the surface temperature gradually decreased to 1610 °C, and a sudden temperature increase did not occur after opening the microwave generator again. On the contrary, the surface temperature of pre-oxidized specimen rose to 1605 °C and was maintained steady under the same testing conditions. The ZrB2-SiC composites' surface temperature response to dissociated oxygen can be related to the competition between
oxidation and catalytic reactions. The released energy from the catalytic recombination reaction of oxygen atoms contributes to the increase of surface temperature, however, the temperature jump of the original specimen is mainly ascribed to the oxidation of composites with atomic oxygen. Violent oxidation of composites associated with low reaction enthalpy and high oxidation rate results in the extraordinary temperature response, which is detrimental to the high-temperature performance of composites. It is advisable to make ZrB2-SiC composites pre-oxidized in static air for inhibiting the temperature jump phenomenon before using the composites in the high temperature, low pressure and atomic oxygen environment.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Project Nos. 51272056, 11121061, 51372047 and 91216301), the National Fund for Distinguished Young Scholars (No. 51525201) and the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRETⅢ.201506).
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Short communication
ZrB2-SiC composites' surface temperature response to dissociated oxygen at 1600 °C Baihe Du a,b, Ning Li c,n, Bo Ke c, Pifeng Xing c, Xinxin Jin d, Ping Hu a, Xinghong Zhang a a
Science and Technology on Advanced Composites in Special Environments Laboratory, Harbin Institute of Technology, Harbin 150001, PR China China Aerodynamic Research and Development Center, Mianyang 621000, PR China c Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, PR China d College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, PR China b
art ic l e i nf o
a b s t r a c t
Article history: Received 27 May 2016 Received in revised form 5 June 2016 Accepted 7 June 2016 Available online 7 June 2016
The surface temperature response of ZrB2-30 vol% SiC composites exposed to dissociated oxygen was investigated using a side-arm oxidation facility. As the specimen was immersed in dissociated oxygen at 1600 °C, a sudden and spontaneous temperature jump accompanied by high degradation of composites was observed in the initial testing stage. For comparison, the surface temperature of pre-oxidized specimen was relatively low and maintained steady under the same testing conditions. The ZrB2-SiC composites' surface temperature response to dissociated oxygen could be linked to the competition between oxidation and catalytic reactions, and the temperature jump of the original specimen was mainly ascribed to the oxidation of composites with atomic oxygen. Pre-oxidation of ZrB2-SiC composites in static air could effectively inhibit the surface temperature jump phenomenon of composites under simulated service conditions. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. surfaces D. borides E. thermal applications Temperature response
1. Introduction Hypersonic vehicles as the next generation aircraft will encounter dissociated air in chemical nonequilibrium for a long time owing to unique flight trajectories, ultra-high speed and lifting body structure, which issues serious challenges to thermal protection materials [1–3]. High-temperature performance of materials is not merely dependent on their inherent properties, but related to surface temperature during service [4,5]. The surface temperature response to high-enthalpy flow has a significant influence on the reliability of thermal protection materials and structures associated with space vehicles. Zirconium diboride (ZrB2) based composites have been proven to possess a good combination of melting point [6], high-temperature retained strength [7], resistance to oxidation/ablation [8], as well as chemical inertness [9], and are being considered as promising candidates for the application in hypersonic flight. Systematic research on surface temperature response under simulated service conditions is necessary for ZrB2-based composites, which can provide a better understanding of the response relationship between environment and composites. High surface temperature is detrimental to material n
Corresponding author. E-mail address: [email protected] (N. Li).
http://dx.doi.org/10.1016/j.ceramint.2016.06.041 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
performance, such as mechanical properties, oxidation and thermal shock resistance, and then induce the degradations of materials. Temperature response is mainly responsible for the difference in surface temperature of various thermal protection materials under the same conditions. Ito et al. [10] have demonstrated the surface temperatures of SiC coating, high catalytic coating and ultra-high-temperature ceramic (UHTC) in subsonic high-enthalpy flow using plasma wind tunnel. The relatively high surface temperature of high catalytic coating and the change of UHTC from low catalytic region to high catalytic region with temperature could be attributed to surface catalytic effect and emissivity. Surface temperature jump phenomena were observed by Hald [11] for C/SiC, and Herdrich et al. [12] for SiC and C/C-SiC. In addition to the oxidation of materials, Hald pointed out that the catalytic reactions of dissociated atoms on surface also contributed to temperature jump, especially under active oxidation condition. Experimental results of Herdrich et al. showed a temperature jump of 400 K for SiC with the transition from passive to active oxidation, and similar phenomena have also been confirmed by Glass [13] and Sakraker et al. [14]. On the contrary, Panerai et al. [15] believed that the passive/active oxidation transition of SiC was not the reason for the sudden temperature increase, because the temperature jump occurred at conditions beyond active oxidation of SiC. The investigation into surface temperature response of ZrB2-based composites (generally with SiC additions) exposed to dissociated air lagged behind that of other Si containing thermal
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protection materials. Hu et al. [16] investigated the effect of SiC content on temperature response of ZrB2-SiC composites. Compared with composites containing 15% and 30% SiC, ZrB2-10 vol% SiC composites exhibited the highest surface temperature. However, the surface temperature response of ZrB2-based composites has not been studied sufficiently, and more attention should be paid to the response mechanism of surface temperature to highenthalpy flow. In the present work, surface temperature response to dissociated oxygen of ZrB2-30 vol% SiC original specimen and the preoxidized specimen was investigated using a side-arm oxidation facility at 1600 °C. The effect of surface morphology on temperature response of ZrB2-SiC composites under simulated service conditions was determined in terms of the surface temperature history as well as microstructure of oxide scale after testing. It is believed that this work could provide a promising method to inhibit surface temperature jump, and lay the foundation for aerospace applications of ZrB2-based composites.
2. Experimental procedure Commercially available ZrB2 powder (2 mm, purity 499.5%, Northwest Institute for non-ferrous metal research, China), and SiC powder (1 mm, purity 4 99.5%, Weifang Kaihua Micro-powder Co., Ltd., China) were used as raw materials. The powder mixtures of ZrB2 and SiC, weighed in proportion to the stoichiometric ratio of 70 vol% and 30 vol%, were ball-mixed for 10 h in a polyethylene bottle and then dried in a rotary evaporator. The as-received powder mixtures were hot-pressed at the temperature of 1950 °C for 1 h under a uniaxial load of 30 MPa in Ar atmosphere. Bulk density and theoretical density were calculated by the Archimedes method and the rule-of-mixture, respectively. Round-shaped specimens with Ø24 mm 3 mm were cut from the billet, and polished to a 1 mm surface finish. The pre-oxidation was carried out in static air using a furnace, and the specimens on zirconia crucible were oxidized at a constant temperature of 1500 °C for 0.5 h. Exposing ZrB2-SiC composites to dissociated oxygen was conducted using a side-arm oxidation facility assembled in our laboratory. This facility was described in detail previously [17]. Firstly, the specimen in a quartz tube was heated up to 1600 °C by using high-frequency induction device. The surface temperature was monitored by a two-color optical pyrometer (Raytek Ltd., Marathon MR1S, USA). The mass flow rate of pure oxygen which got through the experimental chamber was controlled at 50 ml/min, and the desired pressure managed by the pressure regulator and mass flow meter was 100 Pa. Then, opening the microwave generator (Nanjing Huiyan Ltd., MY1500S, China) with a frequency of 2.45 GHz and fixing the anode current at 350 mA, the specimen was exposed to dissociated oxygen
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subsequently. The surface microstructures of specimens were observed by scanning electron microscopy (SEM, FEI Sirion, Holland) equipped with energy dispersive spectroscopy (EDS, EDAX Inc., USA) for chemical analysis. Crystalline phases of the original and pre-oxidized specimens were analyzed by X-ray diffraction (XRD, Rigaku, Japan) that was operated at an incidence angle of 1.5° with the 2θ range from 10° to 70°. The chemical composition of the original specimen after testing was determined using X-ray photoemission (XPS, Thermo Fisher Scientific, USA) with monochromatic Al Kα radiation. The surface was cleaned by Ar þ ion sputtering for 1 min, and a charge correction was carried out with reference to the C 1s signal detected at 285.0 eV. The enthalpy change of oxidation reactions for both ZrB2 and SiC with molecular and atomic oxygen can be calculated using JANAF tables [18].
3. Results The bulk density of the sintered ZrB2-SiC composites was 5.18 g/cm3, which corresponded to a relative density of 99% using the rule mixtures based on the densities of 6.09 and 3.21 g/cm3 for ZrB2 and SiC, respectively [19]. Fig. 1a shows the surface microstructure of the polished specimen. Combined with EDS analysis (not shown here), the small dark SiC dispersed uniformly in the gray ZrB2, and a few small pits was observed in the polished surface because the grains were pulled out or desquamated during polishing. After pre-oxidation at 1500 °C in static air, the specimen surface was dark and completely covered with a tight and smooth borosilicate glass, as presented in Fig. 1b. The XRD patterns of original and pre-oxidized specimens are shown in Fig. 1c. ZrB2 as the main phase companied with a small amount of SiC was detected in the original specimen without any other phases. For the pre-oxidized specimen, the predominant component of the oxide scale was monoclinic zirconia, and a weak diffraction peak at around 30° corresponded to the tetragonal zirconia. Furthermore, a broad peak between 15° and 25° could be attributed to the presence of amorphous glass on surface, which was consistent with the specimen surface morphology [20]. The specimens were heated up to 1600 °C at 100 Pa. After opening the microwave generator, the original and pre-oxidized specimens showed a remarkable difference in the surface temperatures. The surface temperature histories of specimens exposed to dissociated oxygen are shown in Fig. 2. It was evident that the measured temperature fluctuates significantly as the plasma was generated. There are a large number of excited species in plasma. The de-excitation of these species in the form of radiation would affect the optical pyrometer. Additionally, arc instability, turbulence and unsteady fluid feeding also contribute to the drastic fluctuation of surface temperature [21,22]. However, this
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Fig. 1. Micrographs and XRD patterns of ZrB2-30 vol% SiC specimens: (a) original specimen, (b) pre-oxidized specimen, (c) XRD patterns of (a) and (b).
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Fig. 2. Surface temperature histories of ZrB2-30 vol% SiC specimens exposed to dissociated oxygen.
fluctuation of temperature has little effect on both evaluation of surface temperature and characterization of temperature response. When the original specimen was immersed in atomic oxygen, a sudden and spontaneous temperature increase up to 1810 °C was observed in the time range from 3 to 80 s, and then the temperature stabilized at 1890 °C for 100 s. Subsequently, the surface temperature gradually decreased and finally reached a steady state at 1610 °C. It should be noted that, when the microwave generator was opened again, the temperature jump did not occur and the surface temperature was maintained steady throughout the test. At the same time, the surface temperature history of the pre-oxidized specimen was fundamentally different from that of the original specimen. The pre-oxidized specimen placed in dissociated oxygen reached a plateau of 1605 °C during the test. Fig. 3 shows the surface SEM images of the ZrB2-SiC specimens after testing in dissociated oxygen. The difference in surface microstructure between original and pre-oxidized specimens resulted from their different surface temperature histories. It was observed in Fig. 3a that the surface of the original specimen after being exposed to dissociated oxygen was very rough, and mainly composed of zirconia confirmed by EDS (not shown here). There were many irregular clusters and pores on the surface, which could be attributed to the evaporation of gaseous products as the surface temperature rises rapidly. The original specimen during testing underwent high degradation. For the pre-oxidized specimen tested under the same conditions, the surface was quite flat,
and no other condensed phase except for the fine zirconia grains was identified due to the evaporation of borosilicate glass at very low pressure (Fig. 3b). The surface composition of the original specimen after testing in dissociated oxygen was further identified by XPS. Fig. 4a shows the survey XPS spectra of the products on surface of the oxide scale. The peaks of Zr 4p, Zr 4s, Zr 3d, Zr 3p, Zr 3s, O 1s and C 1s were found in the spectra. A large number of Zr and O peaks indicated that the outer layer of the oxide scale was mainly composed of zirconia. The curve fitting Zr 3d spectra was illustrated in Fig. 4b, measured binding energies of Zr 3d5/2 and Zr 3d3/2 were 182.7 and 185.0 eV, respectively, which were assigned to Zr–O bond. Similarly, the O 1s signal at the binding energy of 530.8 eV was also attributed to the presence of ZrO2 on the surface (Fig. 4c). Based on C 1s as presented in Fig. 4d, the peak located at 285.0 eV was low and confirmed as C–C bond from a small quantity of adventitious carbon. It should be noted that no characteristic peak of Si and B could be identified in the survey XPS spectra. It was suggested that specimen tested at low pressure did not produce borosilicate glass due to the active oxidation of SiC.
4. Discussion The surface component and microstructure have a significant influence on the surface temperature, as indicated in Fig. 2, which is responsible for the high-temperature performance of ZrB2-based composites during service. Nonetheless, the mechanism of surface temperature response for ZrB2-SiC composites to high-enthalpy flow is still unclear. It has been widely recognized that the temperature response of composites in atomic oxygen can be associated with the surface catalytic effect [10,23,24]. When ZrB2-based composites are exposed to high temperature, low pressure and atomic oxygen environment, the atomic species, which partially diffuse to the surface of composites, will recombine into molecules accompanied by the release of dissociation energy to surface. In the present experiment, a surface temperature of pre-oxidized specimen increased by 5 °C, which could be related to the catalytic reaction of atomic oxygen on surface. Indeed, the catalytic coefficient of zirconia is larger than that of silica [25,26]. However, such a large increase in surface temperature of the original specimen could not be reasonably explained by the catalytic coefficient disparity between original and pre-oxidized specimens, meanwhile, the sudden temperature increase did not occur as the microwave generator was opened
100μm Fig. 3. Micrographs of ZrB2-30 vol% SiC specimens after testing: (a) original specimen; (b) pre-oxidized specimen.
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Fig. 4. XPS spectra of the ZrB2-SiC original specimen after testing: (a) the survey spectra, (b) Zr 3d, (c) O 1s and (d) C 1s.
again. Consequently, the catalytic reaction of atomic oxygen might contribute to the massive increase in surface temperature, but it is not the main reason. Similarly, heat radiation of composites, also dependent on the surface component and microstructure, could influence the surface temperature response. The emissivity of zirconia is relatively low compared with borosilicate glass, but the emissivity for the oxidation products of ZrB2-SiC composites varies slightly over a wide temperature range [16,27]. Furthermore, the microstructural evolution of oxide scale for the original specimen is not instantaneous. Heat radiation associated with composites surface is not the main factor responsible for the sudden temperature increase after testing in dissociated oxygen. On the contrary, it is possible that the change in surface component and microstructure is attributed to the rapid increase in surface temperature, rather than acts as its trigger. Li et al. [28] noted that heat conduction contributes to the temperature histories of ZrB2-SiC specimens exposed to high-enthalpy flow. During testing at elevated temperatures, the surface of ZrB2-SiC specimen will be covered with a porous zirconia layer, whose heat conductivity is much smaller than that of ZrB2-SiC composites. Therefore, heat transport becomes less efficient or slower in composites as the thickness of oxide scale increases, and surface temperature rises rapidly due to the surface overheating. However, in the present experiment, the specimen was heated from the interior using high-frequency induction device, rather than heated by heat conduction of high-enthalpy flow. It is reasonable to consider that heat conduction has negligible effect on the temperature difference between original and pre-oxidized
specimens. Based on the above-mentioned discussion, the main possible reason for surface temperature jump of original specimen exposed to dissociated oxygen is the oxidation of composites accompanied by releasing a large amount of heat. According to thermodynamic calculations as shown in Fig. 5, oxidation of ZrB2 and SiC in different oxidizing atmospheres is exothermic reactions in the temperature range of 1000–2000 °C. It should be noted that the
Fig. 5. Variations of reaction enthalpies as a function of temperature for reactions of ZrB2 and SiC in different oxidizing atmospheres.
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reaction enthalpies for both ZrB2 and SiC with oxygen atoms are much lower than those with oxygen molecules, indicating that oxidation of ZrB2-SiC composites in atomic oxygen will release more energy under the same conditions. Additionally, the releasing energy per unit time is also dependent on the oxidation rate. The increase in oxidation rate owing to the high activity of oxygen atoms has been confirmed by some literatures [17,29], which is beneficial to the surface temperature jump. As ZrB2-SiC composites are exposed to dissociated oxygen at high temperature, both oxidation reactions and catalytic recombination reactions exist and compete with each other. It can be expected that a large amount of oxidant is consumed due to rapid oxidation in the initial stage of testing process, which inhibits the catalytic reactions of dissociated atoms on surface of specimen. In addition, based on Eqs. (1)–(5), the energy released from oxidation reaction with atomic oxygen is actually equal to the sum energy of the catalytic reaction of oxygen atoms and oxidation reaction with molecular oxygen. Therefore, the temperature jump of original specimen in the initial testing stage is mainly ascribed to the oxidation of composites. As the thickness of oxide scale rises, the surface temperature of composites as well as the oxidation rate decreases gradually. Because the oxidation rate is smaller than that of original specimen in the initial testing stage, the surface temperature does not jump but increases by 10 °C after opening the microwave generator again, which could be related to the combination of oxidation and catalytic reactions. For the pre-oxidation specimen, the amorphous borosilicate glass exposing on surface, as a protective barrier to limit the diffusion of oxidant into the inner part of matrix and improve the resistance to oxidation, is the essential reason for the relatively low surface temperature of specimen during testing. Consequently, before using the composites in the high temperature, low pressure and atomic oxygen environment, making ZrB2-SiC composites pre-oxidized in static air can be considered as a promising method for inhibiting the temperature jump phenomenon of composites during service.
2O (g) + SiC = SiO (g) + CO (g) + 667.3 kJ/mol
(1)
O2 (g) + SiC = SiO (g) + CO (g) + 157.3 kJ/mol
(2)
2O (g) + 2/5ZrB2 =2/5ZrO2 +2/5B2 O3 (g) + 1146.1 kJ/mol
(3)
O2 (g) + 2/5ZrB2 =2/5ZrO2 +2/5B2 O3 (g) + 636.0 kJ/mol
(4)
2O (g) = O2 (g) + 510.1 kJ/mol
(5)
5. Conclusions The ZrB2-30 vol% SiC original specimen and the specimen preoxidized in static air were exposed to dissociated oxygen using a side-arm oxidation facility at 1600 °C, in order to investigate the effect of surface morphology on the temperature response of ZrB2-SiC composites under simulated service conditions. As the original specimen was immersed in dissociated oxygen, a sudden and spontaneous temperature jump up to 1890 °C, accompanied by high degradation of composites, was observed in the initial testing stage. Then the surface temperature gradually decreased to 1610 °C, and a sudden temperature increase did not occur after opening the microwave generator again. On the contrary, the surface temperature of pre-oxidized specimen rose to 1605 °C and was maintained steady under the same testing conditions. The ZrB2-SiC composites' surface temperature response to dissociated oxygen can be related to the competition between
oxidation and catalytic reactions. The released energy from the catalytic recombination reaction of oxygen atoms contributes to the increase of surface temperature, however, the temperature jump of the original specimen is mainly ascribed to the oxidation of composites with atomic oxygen. Violent oxidation of composites associated with low reaction enthalpy and high oxidation rate results in the extraordinary temperature response, which is detrimental to the high-temperature performance of composites. It is advisable to make ZrB2-SiC composites pre-oxidized in static air for inhibiting the temperature jump phenomenon before using the composites in the high temperature, low pressure and atomic oxygen environment.
Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Project Nos. 51272056, 11121061, 51372047 and 91216301), the National Fund for Distinguished Young Scholars (No. 51525201) and the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRETⅢ.201506).
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