Oxidation Behavior of Spark Plasma Sintered HfB2-SiC-Graphite Composite at 1400 °C

Advanced Ceramics Progress: Vol. 8, No. 1, (Winter 2022) 9-17

Materials and Energy Research Center MERC Contents lists available at ACERP Advanced Ceramics Progress Journal Homepage:www.acerp.ir Original Research Article Oxidation Behavior of Spark Plasma Sintered HfB2-SiC-Graphite Composite at 1400 °C Mohammad Sakvand a, Maryam Shojaie-Bahaabad b, , Leila Nikzad c a MSc Student , Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Semnan, Iran b Assistant Professor, Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Semnan, Iran c Assistant Professor, Department of Ceramics, Materials, and Energy Research Center (MERC), Meshkindasht, Alborz, Iran  Corresponding Author Email: mshojaieb@shahroodut.ac.ir (M. Shojaie-Bahaabad) URL: https://www.acerp.ir/article_147536.html ARTICLE INFO ABSTRACT Article History: Received 04 March 2022 Received in revised form 20 March 2022 Accepted 05 April 2022 The current study aims to fabricate the HfB2-SiC-graphite composite through Spark Plasma Sintering (SPS) method at 1950 °C for 10 min. The oxidation behavior of the prepared composites was investigated at 1400 °C and different times of 4, 8, 12, and 16 h. In addition, the weight changes and thickness of the generated oxide layer were measured. The relative density, hardness, toughness, and strength of the composite made through the SPS method were calculated as 99.39 %, 10.16 GPa, 4.73 MPa.m1/2, and 464.12 MPa, respectively. The oxidation kinetic results of the composite exhibited linear-parabolic behavior. The chemical reaction during the oxidation process controlled the oxidation rate after 8 h. Followed by oxidation for more 12 h, the thickness of the oxide scale slowly increased, thus following a parabolic trend as a result of a decrease in the oxygen diffusion when HfOxCy and SiOxCy phases were formed. Therefore, it was concluded that the oxygen diffusion rate could control the oxidation process. Keywords: HfB2-SiC-Graphite Composite Spark Plasma Sintering (SPS) Ultra-High Temperature Ceramic (UHTC) Oxidation Behavior Kinetic https://doi.org/10.30501/acp.2022.332396.1083 1. INTRODUCTION Ultra-High Temperature Ceramics (UHTCs) consist of nitrides, carbides, and borides of transition metal [1-3]. UHTCs have recently drawn researchers’ attention owing to their ablation resistance and high oxidation at quite high temperatures while applied in Thermal Protection Systems (TPSs) such as nosecone, propulsion system, sharp leading edge, and rocket nozzle [3]. Among all types of UHTCs, HfB2 has received considerbale attention compared to other deborides due to its advantagous characteristics such as higher melting temperature (3380 °C), thermal conductivity (104 W/m.K), Young’s modulus (480 GPa), high resistance against oxidation, good hardness (28 GPa), and high chemical resistance [4]. Owing to the robust covalent bonds, low self-diffusion coefficient, presence of oxygen contaminants of non-oxide raw materials, and high temperature, mechanical pressure is often require d during a long period to achieve full density [3]. Numerous approaches such as hot pressing, pressureless sintering, reactive hot pressing, and plasma spark sintering, to name a few, are commonly used used for sintering HfB2 composites [5-7]. The spark plasma process requires low sintering temperature and shorter soaking time due to its higher heating rate than that of other methods [3]. Therefore, it is possible to make ceramics at high temperatures and speeds, i.e., a homogeneous fine grain structure with full density that requires less sintering. Upon adding the metallic additives (Fe, Ni, Co, W), carbides (SiC, HfC, WC, VC), nitrides (AlN, HfN, Si3N4), and desilicides (MoSi2, HfSi2, TiSi2, TaSi2) [3, 8-10], both sinterability and mechanical characteristics of HfB2 ceramics will be greatly improved. Some recent studies pointed out to the necessity of the presence of 10-30 vol. % SiC to achieve high oxidation resistance at high temperatures. Silicon carbide can improve the oxidation resistance of HfB2 ceramics by forming a B2O3-SiO2 glass layer above 1200 °C on the surface of the HfB2 matrix and inhibition oxygen diffusion into the bulk [11]. Followed by the oxidation of HfB2 at 800 °C, HfO2 (s) and B2O3 (l) will be formed. At 1200 °C, SiC is oxidized, and the liquid-formed SiO2 react with B2O3 (l) to produce borosilicate layer on the surface. This borosilicate layer fascilitates passive oxidation, which results in a parabolic mass gain, and decreases the oxidation rate of HfB2-SiC composites compared to pure HfB2 [12,13]. In this regard, many studies have been conducted on the optimization of the properties of HfB2-SiC ceramic composite compound and coatings in the past years [14-16]. However, low fracture toughness of HfB2-SiC ceramic composites is an obstacle to their wide applications, especially in harsh environments. It was observed that both sinterability and fracture toughness of HfB2-SiC composites were enhanced by adding geraphen or short carbon fibers [17,18]. To the best of the authors’ knowledge, the impact of SiC and graphite co-addition on the densification as well as the mechanical and oxidation behavior of HfB2 ceramics produced through the SPS method has not been examined yet. The previous study on the effect of the graphite addition on the mechanical properties of HfB2-30 vol. % SiC composites confirmed the good mechanical properties of the HfB2-20 vol. % SiC- 6 vol. % graphite composite. In this research, the spark plasma sintering method was employed to fabricate HfB2-30 vol. % SiC composite combined with 6 vol. % graphite. In addition, both densification and oxidation behavior as well as the mechanical characteristics of the mentioned composite at 1400 °C for 4, 8, 12, and 16 h were studied. 2. MATERIALS AND METHODS Commercial HfB2, SiC, and graphite flakes were used to synthesize the HfB2-20 vol. % SiC-graphite (6 vol. %) composite. Table 1 shows the properties of the starting powders. TABLE 1. Characteristics of starting materials Purity Dimension Supplier Materials 97 % D < 2 µm Beijing Cerametek Materials Co., China HfB2 99 % D < 10 µm Xuzhou Co., China SiC 99.9 % D < 50 nm L < 30 µm Qingdao Tiansheng Graphite Co.,China Graphite In order to mill HfB2 and SiC powder mixtures, first, high-energy planetary milling was applied using balls and a WC-Co cup at 300 rpm in ethanol medium with the ball-powder weight ratio of 10:1 for 3 h. The graphite nano-flakes were then ultrasonically stirred in 100 mL diluted ethanol for 1 h, and the mixture of HfB2 and SiC powder was added to the slurry and stirred for 30 min. Next, the slurry was dried on a hot plate equipped with magnetic stirring at 60 °C for 2 h. HfB2-30SiC-graphite composites were fabricated through the SPS method (Nanozint 10i, Khala Poushan Felez Co., Iran) at 1950 °C for 10 min under 40 MPa pressure in the vacuum of 0.05 mbar. The relative density of the composite and porosity values were calculated in the distilled water using Archimedes technique. The theoretical density was also measured based on the mixture law using the theoretical density of 11.2 g/cm3 for HfB2, 3.2 g/cm3 for SiC, and 2.26 g/cm3 for graphite. Phase analysis of composites was conducted using X-Ray Diffraction (XRD, Philips, Model: X’Pert MPD, Tube: Co, and λ: 1.78897 Å) pattern. Further, the surface and microstructure of the composite were examined using Field Emission Scanning Electron Microscope (FESEM, TESCAN, Model: MIRA) equipped with Energy Dispersive X-Ray spectroscopy (EDS) detector. The microstructure of the composite was investigated neglecting the thermal or chemical etching. The average grain size of the composite was measured by MIP Cloud software. The composite hardness was also measured using a Vickers hardness tester under 1 kg at the loading time of 10 s. In addition, the toughness of the composites was calculated through Equation (1) [14]: KIC = 0.073 (P/c1.5) (1) where KIC denotes the fracture toughness (MPa.m1/2), P the applied load (N), and c the average half-length of the crack (µm). The composite flexural strength was evaluated by a three-point flexural machine (Zwick Roell SP600, Germany) at the loading rate of 0.05 mm/min. The oxidation tests were carried out in an electric furnace at 1400 °C for 4, 8, 12, and 16 h. The composite oxidation resistance was evaluated considering the weight changes and thickness of the oxide layer after oxidation. 3. RESULTS AND DISCUSSION Figure 1 shows the ram displacement and densification behavior of HfB2-30 SiC-graphite composite during the SPS process. Figure 1. Displacement-Temperature-Time (DTT) curves of the HfB2-30 SiC-graphite composite The compaction processes include three steps: (1) compaction resulting from the rearrangement of the powder particles, increase in the contact surface of the particles, formation of more sparks, and increase in the thermal efficiency due to an increase in both pressure and temperature [17,19,20]; (2) First a decrease and then an increase in the displacement at 1100 °C: the initial decrease was indicative of the expansion caused by the gases produced by the evaporation of impurities and surface oxide contaminants in the presence of graphite additive. The second increase was caused by an increase in the pressure on the powder particles up to a final pressure of 40 MPa. Bulk deformation is caused by high temperature, neck growth among the particles, complete contact of particles, and noticeable compaction in this area; (3) A very slow increase in the slope of the curve and smooth displacement-time curves which are indicative of the complete compaction of this composite [21,22]. Apparently, the contact surface of the HfB2 particles increased during the reaction of oxygen impurities (HfO2 and B2O3) with SiC and graphite. As a result of this chemical reaction, gaseous products such as SiO, CO, and BxOy were produced which, prior to the production of gaseous products, caused the formation of the liquid phase and increased the sinterability of the HfB2 powder [23]. Table 2 lists the physical and mechanical characteristics of the composite produce through SPS method. According to the results, the density of the obtained composite in this study was higher than the values reported by [24-27]. Since graphite can remove surface impurities on the SiC and HfB2 particles and promote the densification of the composite, its addition to the composite would increase density and decrease the porosity percentage. TABLE 2. Physical and mechanical properties of the HfB2-30SiC-graphite composite Ref Strength (MPa) KΙC (MPa m1/2) Hardness (GPa) Apparent porosity (%) Relative density (%) HfB2 average grain size (µm) Material composition [24] - 3.6 - - 98.9 - HfB2-20 vol. % SiC-10 vol. % TaSi2 [25] 4.7 20.4 - - - HfB2-10 vol.% SiC [26] 563 3.36 10.6 - 99.1 6.9 HfB2-20 vol. % SiC-10 wt. % WC [27] 585 3.72 21.07 - 98.8 2.57 HfB2-20 vol. % SiC-20 vol. % HfC [28] - 3 18 - 98.6 3.25 HfB2-15 vol. % SiC-15 vol. % MoSi2 [29] 545 3.59 19 - 99.2 2.07 HfB2-20 vol. % SiC-8 vol. % HfC Peresent work 464.12 4.73 10.16 0.61 99.39 HfB2: 3.268 SiC: 2.155 Graphite: 0.125 HfB2-30 vol. % SiC-6 vol. % Graphite The hardness of HfB2-30 SiC-graphite composite was lower than that of HfB2-SiC composite (18-20.4 GPa) in other researches [25,27-29]. Although this composite is characterized by a high density due to the inherent softness of graphite, its hardness is reduced. The toughness and bending strength of HfB2-20 SiC-graphite composite was higher and similar to those values reported in the literature, i.e., 4.65 MPa.m1/2 and 465 MPa, respectively [24], mainly due to the laminar structure of the graphite and its role in siterability improvement of the composite. The effect of carbon fibers on the toughness of HfB2 composites was also reported in [17]. Figure 2 shows the XRD pattern of the composite after the SPS process according to which, both HfB2 and SiC phases can be observed in the samples, and there are no unwanted phases in the composites. Figure 2. XRD pattern of the HfB2-30 SiC-graphite composite sintered at 1950 °C for 10 min Figure 3 shows the SEM images with elemental analysis (EDS) of the composite surface after the SPS process. According to EDS analysis, the dark and light areas represent the SiC and HfB2 phases, respectively. Figure 3. SEM images analysis (EDS) of the composite surface after the SPS process at 1950 °C for 10 min Figure 4 depicts the SEM images of the fracture cross-section of the composite after the SPS process. The fracture surface of the sintered composite was a combination of the intergranular fractures. Particles pull-out and sharp edges observed in the images are possibly related to the intergranular fracture that occurred in grain boundaries (thin arrows). Certain areas, particularly in the compared surfaces with a typical grain growth compared to others, represent wide and smooth surfaces, indicating the intragranular fracture (thick arrows). An oxide layer on the surface of non-oxide particles (such as HfO2, SiO2, and B2O3) causes the generation of borosilicate glass phases. In their research work on the HfB2-SiC composite sintered through hot pressing technique, Monteverde et al. stated that the glass phase in the SEM images was visible in the forms of coating, uniform and brittle fracture surface or a low thickness layer [30]. Such a glass phase was clearly visible in the SEM images of the cross-section of the composite prepared in the present research work (dashed flash). In the cross-section images of HfB2-SiC composites, graphite layers in different amounts can be observed in the cross-section, indicating that the graphite was not converted into new phases. Therefore, it can be concluded that the sintering and compressibility in the HfB2-30 SiC-graphite systems was non-reactive. Figure 4. SEM of the fracture cross-section of the composite after the SPS process at 1950 °C for 10 min Figure 5 presents the SEM image of the composite surface after oxidation at 1400 °C and different times according to which, the composite surface is coated with a glass layer. Oxidation of SiC particles according to reaction (2) at the temperatures above 1100 °C would form a glass layer of SiO2 on the surface of the composite [31-35]. 2 SiC (s) +3 O2 (g) = 2 SiO2 (l) +2 CO (g) (2) The glass layer is evenly distributed on the HfB2-30 SiC-garaphite composite surface after oxidation for 12 and 16 h. As observed in the EDS analysis, the glass layer is evenly composed of Si and O. In addition, the white crystals of different sizes and shapes were found on the surface of composites after oxidation. According to the EDS analysis, the spherical crystals with high amounts of Hf and O and angular crystals with equal amounts of Hf and Si were HfO2 and HfSiO4 crystals, respectively. Figure 5. SEM image of the surface of the HfB2-30 SiC-graphite composite after oxidation at 1400 °C for different Times, (a) 4 h, (b) 8 h, (c)12 h, and (d) 16 h The formation of crystalline phases in the present study can be elaborated base on thermodynamic calculations (Figure 6) [31]. To be specific, followed by the formation of SiO2 (reaction (2)) and HfO2 (after oxidation at 800-1700 °C according to reaction (3)) [31], HfO2 was first dissolved in the borosilicate melt, thus forming SiO2-B2O3 (HSB) liquid in the glass layer. As the oxidation process continued, the HSB liquid would flow from the top of the glass layer. In the case of B2O3 evaporation, HfO2 particles were precipated from the HSB liquid. In addition, HfO2 reacted with SiO2 at temperatures above 1200 °C, according to reaction (4), thus forming HfSiO4 particles [35]. Figure 6. Gibbs free rnergy versus temperature for possible reactions in this study 2 HfB2 (s) + 5 O2 (g) = 2 HfO2 (s) +2 B2O3 (l) (3) SiO2 (l) + HfO2 (s) = HfSiO4 (s) (4) SiC (s) + O2 (g) = SiO (g) + CO (g) (5) B2O3 (l) = B2O3)g) (6) 3 C (s) + O2 (g) = 2 CO (g) (7) Two types of bubbles were observed on the surface of this oxide layer some of which grew to the surface, yet the others could not find their way to the surface. Researchers believe that this layer cannot flow near these bubbles and cover the open bubbles owing to the high viscosity of the glass layer. These bubbles can be formed by the accumulation of gaseous products resulting from active oxidation of SiC at the high temperature of 1400 °C (reaction (5)) and B2O3 evaporation at the temprature of above 1100 °C (reaction (6)) and oxidation of graphite at 500 °C (reaction (6)) [2, 34]. Figure 7 illustrates the cross-section of the composite after the oxidation test at different times. According to the EDS analysis (Figure 8), the first layer was rich in Si while the second one had moderate amount of Si and Hf. Figure 7. cross-section of the HfB2-30 SiC-graphite composite sample after oxidation test for different times Figure 8. Line EDS analysis of the HfB2-30 SiC-graphite composite sample after oxidation test for (a) 8 and (b) 16 h Figure 9 shows the thickness of each layer formed on the surface of the HfB2-30 SiC-graphite composite followed by conducting the oxidation test. The obtained results showed that followed by increasing the time of graphite oxidation during the oxidation process at 1400 °C and forming new channels for oxygen to better penetrate into the composite, the oxidation rate and thickness of SiO2 rich layer would increase. Given that the formation of the thicker glass layer as a result of a decrease in the oxygen penetration into the composite after oxidation for 12 h, the thickness of SiO2 rich layer would slowly increase.As observed in the EDS analysis (Figure 10), HfO2, HfOxCy, and SiOxCy phases were formed followed by oxidation for 12 h. Zapata et al. reported the formation of MeOxCy (Si, Zr,Hf) after oxidation of MeB2-SiC (Zr,Hf) composites at 1500 °C for 3 h [36,37]. Figure 9. Thickness of the oxide layer formed on the surface after conducting the oxidation test at different times MeOxCy phases were proposed as the novel protective coatings for UHTCs in cases where the oxygen diffusion coefficients were reduced, and oxidation resistance of composites was improved for a long exposure time. In this study, followed by the formation of the HfOxCy and SiOxCy phases and their oxidation, oxygen diffusion into the composite would decrease after 12 h. As a result, the thickness of the SiO2 layer slowly increased. Therefore, it can be expected that the composite will follow a stabilized trend at long exposure times. Figure 10. cross-section of the HfB2-30 SiC-graphite composite sample after the oxidation test for 16 h Figure 11 illustrates the oxidation kinetics of HfB2–30 SiC–graphite composites oxidized at 1400 °C. The oxidation mechanism is determined by the changes in the weight gain per unit surface area (∆W/S) as an oxidation time function. Figure 11. Oxidation kinetics of HfB2-30 SiC-graphite composites oxidized at 1400 °C The oxidation kinetic variables, x and k, are calculated using Equation (8): (∆W/S)x = kt (8) where x is the oxidation exponent, and k the constant of the oxidation rate. According to the literature, while the linear trend (x=1) is indicative of the reaction rate-controlled kinetics, the parabolic one (x=2) is indicative of the diffusion rate-controlled kinetics. The value of x in the current study was calculated as 1.563 which was indicative of a linear-parabolic behavior. While the linear oxidation kinetics were observed for 8 h, the parabolic oxidation kinetics were observed for up to 16 h. The thickness of the oxide scale rapidly increased 8 h past the oxidation, thus making the trend linear. According to Figure 5, HfO2 and HfSiO4 were the main phases on the top oxide layer at the oxidation time of less than 12 h, suggesting that under this condition, the oxidation was controlled by the reaction rate. After 12 h of oxidation, the thickness of the oxide scale gradually increased, following a parabolic trend mainly due to the impact of the produced HfOxCy and SiOxCy phases that reduced the oxygen transport. With the development of these phases, the rate of oxidation began to increase steadily. As a result, the diffusion rate determined the oxidation rate. There was a barrier for oxygen diffusion in this stage. The oxidation rate was liner at first; however, after the formation of HfOxCy and SiOxCy phases, oxygen was used for oxidation of these phases. Consequently, the oxygen diffiusion rate in the bulk decreased, hence improvement in the oxidation resistance of the composite and gradual increase in the oxidation rate. 4. CONCLUSION In the current research, HfB2-30 vol. % SiC-6 vol. % graphite composite was fabricated through SPS method. The obtained composite was charaterized by high relative density (99.39 %) and good mechanical properties (the toughness and strength values were obtained as 4.73 MPa.m1/2 and 464.12 MPa, respectively). The oxidation behavior of the composite was studied at 1400 °C and different times. The thickness of the oxide layer formed on the surface of the HfB2-30 SiC-graphite composite increased from 3.2 µm to 16.1 µm upon increasing the oxidation time. The composite exhibited a linear-parabolic behavior during the the oxidation test, and the oxidation exponent was obtained as 1.563. The results from the elemental analysis revealed that HfO2 and HfSiO4 were the main oxide specieses on the top oxide layer at the oxidation time below 12 h, suggesting that under this condition, the oxidation was controlled by the reaction rate. After oxidation for more 12 h, the thickness of the oxide scale slowly increased, thus making the trend parabolic mainly due to the impact of the decreasing oxygen diffusion on the formed HfOxCy and SiOxCy solid solutions. Of note, the oxidation was controlled by the diffusion rate. For this reason, it expected that the composite would follow a stabilized trend at long exposure times. ACKNOWLEDGEMENTS The authors would like to acknowledge Sharood University of Technology and Materials and Energy Research Center for their supports throughout this study. REFERENCES 1. 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Please cite this article as: Sakvand M., Shojaie-Bahaabad M., Nikzad. L., “Oxidation Behavior of Spark Plasma Sintered HfB₂-SiC-Graphite Composite at 1400 °C”, Advanced Ceramics Progress, Vol. 8, No. 1, (2022), 9-17. https://doi.org/ 10.30501/acp.2022.332396.1083 2423-7485/© 2022 The Author(s). Published by MERC. This is an open access article under the CC BY license (https://creativecommons.org/licenses/by/4.0/).