《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_fibrous monoliths-15

+Model JECS-7698: No of Pages 8 ARTICLE IN PRESS Availableonlineatwww.sciencedirect.com Science Direct E噩≈RS ELSEVIER Journal of the European Ceramic Society xxx(2009)xXX-XXX www.elsevier.comlocate/jeurceramsoc Fabrication and properties of ZrB2-SiC-BN machinable ceramics Haitang wu,0. Weigang Zhang a State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering Chinese Academy of Sciences, Beijing 100190, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China Received 16 April 2009: received in revised form 5 September 2009: accepted 22 September 2009 Abstract ZrB2-Sic-Bn ceramics were fabricated by hot-pressing under argon at 1800C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB2-SiC-BN (40 vol%ZrB,-25 vol%SiC-3 )composite were 378 MPa and 4.1 MPam"2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB2-SiC (80 volgZrB2-20 vol%SiC). Noticeably, the hardness decreased tremen- dously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB2-SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB2-SiC-BN composites from 1100 to 1500C in air atmosphere showed that the weight gain of the 80 volgoZrB2-20 vol%SiC and 43.1 vol%ZrB2-269 vol%oSiC-30 vol%BN composites after oxidation at 1500C for 5h were 0.0714 and 0.0268 g/cm2, respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB2-SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300C and a compact film of zirconium silicate above 1300.C. The formed borosilicate and zirconium silicate on the surface of ZrB -SiC-BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB2-SiC-BN o 2009 Elsevier Ltd. All rights reserved. Keywords: Zirconium diboride; Boron nitride; Oxidation resistance; Mechanical properties; Machinability 1. Introduction sion components of structural ceramics are involved. The use of diamond tools and some special processing technologies such Ultra-high temperature ceramics (UHTCs)including refrac- as laser machining and ultrasonic machining are often ineffi tory diborides and carbides, such as ZrB2, HfB2, ZrC, Hfc cient and costly(machining cost usually accounts for 70-90% and TaC, are considered as the most promising materials for of the total cost)though those processes make some hard ceram- the application in critical thermal protection systems and other ics materials machinable. 6 Electrical discharge machining is components of future hypersonic aircraft or re-entry vehicles. another promising technology to machine ceramic components Compared to the single-phase monolithic UHTC, ZrB2-Sic of complex shape with high-dimensional accuracy and low sur composite is of particular interest because of its striking face roughness. Except for the inefficiency, electrical discharge property combination of high melting point, resistance to abla- machining requires a material resistance and can only machine tion/oxidation at high temperatures, high electrical and thermal components of small size. Comparatively, traditional mechan- conductivity and thermal-shock resistance, which makes it an ical machining is of both cost-effective and time-efficient attractive potential candidate for aerospace applications. I-S However, the extremely high strength and hardness of ZrB2-Sic Machining is an inevitable requirement for flexible use of due to the coexistence of strong covalent and metallic bond advanced ceramics, especially when the complex and preci- make mechanical machining very difficult or even impossi ble. This prevents the material from wide application. In recent years,attempts have been made to improve the machinability of ceramic materials by introducing in the matrix weak inter- Corresponding author at: State Key Laboratory of Multi-Phase Complex Sys- faces material, such as mica, h-BN, graphite, pores, rare-earth ems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190. PR China.Tel:+861062520135;fax:+86106252013 phosphates and Ti3 SiC2 analogous compounds, to facilitate E-mail address: wgzhang@home. pe ac cn(w. Zhang). crack deflection during machining . Among those materials, h 0955-2219/S-see front matter o 2009 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2009.09.022 Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.1016/ eurceramsoc2009.09.022

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 Available online at www.sciencedirect.com Journal of the European Ceramic Society xxx (2009) xxx–xxx Fabrication and properties of ZrB2–SiC–BN machinable ceramics Haitang Wu a,b, Weigang Zhang a,∗ a State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China Received 16 April 2009; received in revised form 5 September 2009; accepted 22 September 2009 Abstract ZrB2–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 ◦C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB2–SiC–BN (40 vol%ZrB2–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB2–SiC (80 vol%ZrB2–20 vol%SiC). Noticeably, the hardness decreased tremen￾dously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB2–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB2–SiC–BN composites from 1100 to 1500 ◦C in air atmosphere showed that the weight gain of the 80 vol%ZrB2–20 vol%SiC and 43.1 vol%ZrB2–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 ◦C for 5 h were 0.0714 and 0.0268 g/cm2, respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB2–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 ◦C and a compact film of zirconium silicate above 1300 ◦C. The formed borosilicate and zirconium silicate on the surface of ZrB2–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB2–SiC–BN. © 2009 Elsevier Ltd. All rights reserved. Keywords: Zirconium diboride; Boron nitride; Oxidation resistance; Mechanical properties; Machinability 1. Introduction Ultra-high temperature ceramics (UHTCs) including refrac￾tory diborides and carbides, such as ZrB2, HfB2, ZrC, HfC and TaC, are considered as the most promising materials for the application in critical thermal protection systems and other components of future hypersonic aircraft or re-entry vehicles. Compared to the single-phase monolithic UHTC, ZrB2–SiC composite is of particular interest because of its striking property combination of high melting point, resistance to abla￾tion/oxidation at high temperatures, high electrical and thermal conductivity and thermal-shock resistance, which makes it an attractive potential candidate for aerospace applications.1–5 Machining is an inevitable requirement for flexible use of advanced ceramics, especially when the complex and preci- ∗ Corresponding author at: State Key Laboratory of Multi-Phase Complex Sys￾tems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. Tel.: +86 10 62520135; fax: +86 10 62520135. E-mail address: wgzhang@home.ipe.ac.cn (W. Zhang). sion components of structural ceramics are involved. The use of diamond tools and some special processing technologies such as laser machining and ultrasonic machining are often ineffi- cient and costly (machining cost usually accounts for 70–90% of the total cost) though those processes make some hard ceram￾ics materials machinable.6 Electrical discharge machining is another promising technology to machine ceramic components of complex shape with high-dimensional accuracy and low sur￾face roughness. Except for the inefficiency, electrical discharge machining requires a material resistance and can only machine components of small size.7 Comparatively, traditional mechan￾ical machining is of both cost-effective and time-efficient. However, the extremely high strength and hardness of ZrB2–SiC due to the coexistence of strong covalent and metallic bond make mechanical machining very difficult or even impossi￾ble. This prevents the material from wide application. In recent years, attempts have been made to improve the machinability of ceramic materials by introducing in the matrix weak inter￾faces material, such as mica, h-BN, graphite, pores, rare-earth phosphates and Ti3SiC2 analogous compounds, to facilitate crack deflection during machining.8 Among those materials, h- 0955-2219/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2009.09.022

+Model JECS-7698; No of Pages 8 ARTICLE IN PRESS H Wu, w. Zhang / Journal of the European Ceramic Society rxx(2009)ccr-ccx BN, which exhibits high thermal conductivity and high melting chemical compositions were evaluated by energy-dispersive point, is regarded as a suitable and effective interface material X-ray spectroscopy(EDS; Phoenix, EDAX, Mahwah, NJ) since the cleavage plane of h-Bn facilitates crack propaga- Flexural strength was tested in a three-point configuration tion and decreases the cutting resistance during machining. -- (3 mm x 4 mm x 36 mm chamfered bars), with a 30 mm span Besides these, the similar crystal structures of BN and ZrB2 and a crosshead speed of 0.5 mm/min. Fracture toughness ensure the good chemical compatibility between them. There- was evaluated by a single-edge notched beam test with fore, ZrB2-SiC-BN system may be a good candidate material 16 mm span and a crosshead speed of 0.05 mm/min for high temperature ceramics with excellent machinability and 2 mm x 4 mm x 22 mm test bars. Hardness was determined by mechanical properties Vickers indentation(Model HVS-5, Laizhou Huayin Experi In this paper, the fabrication of a machinable ZrB2-SiC-BN mental Instrument Limited Company, China)using a diamond composite with excellent mechanics properties and oxidation indenter with a load of 98N for 15s. resistance properties was reported. Influences of BN content on the hardness, fracture toughness, flexural strength and oxidation 2. 3. Oxidation tests resistance property of the composite were investigated. The oxi- dation resistance properties tested at from 1 100 to 1500C were Specimens were cleaned in an ultrasonic bath in acetone reported here. As the operative temperature for UHTC materi- before oxidation. The isothermal static oxidation tests were con- als is in excess of 2000"C, composition performing better than ducted in an electrical furnace at temperatures of 1100,1300 and others at 1500C may not necessarily apply to higher tempera- 1500 C in air with interruptions in the tests in order to measure ture ranges. The oxidation resistance of this composite and the weight(to an accuracy of 0.0001 g) change at fixed times.The optimization of the co ion at more than 2000C are also specific weight change was calculated according to the mass under investigation hange per surface area The oxidation resistance of specimen was also tested by a Net 2. Experimental procedure zsch STA449C thermogravimetric analyzer. The mass changes were followed at a rate of 5/min up to 1500C with an 2h 2.1. Samples preparation isothermal hold in a flowing air (50 ml/min). Four kinds ofZrB2-SiC-BN composites with various powder 3. Results and discussion compositions(vol%)were prepared(see Table 1) Commercially available ZrB2 powder(>99% purity, an aver- 3. 1. Mechanical properties and machinability age particle size of 3 um, Northwest Institute for non-ferrous metal research, China), SiC powder(>98.5% purity, an average Fig. I shows the XRd results of sintered ceramics contain- particle size of 1. 5 um, Weifang Kaihua Micro-powder Co ng various contents of BN. All samples contained the initial na)and h-BN powder(>99% purity, 4um, Chem Factory, phases of ZrB2, SiC and BN, except the sample ZSO in which Beijing, China)were used. Powders were mixed and ball-milled no BN existed. No new phase was formed during hot-pressing for 12 h in a polyethylene bottle charged with ethanol using ZrO2 and sintering. Therefore no chemical reactions occurred under balls. Solvent was then removed using a rotary evaporator. The the experimental condition, which benefits the formation of dried powder mixtures were sintered by hot-pressing in an argon weak interfaces between the boundaries of zrB,. si C and bn dies coated with pyrolytic By Pa pressure for 1 h in graphite grains atmosphere at 1800C and 23 2.2. Characterization ZrB D Sic Bulk density and theoretical density were measured and ZS3 assessed by the Archimedes method and the rule-of-mixture, CAAA respectively. Phase composition was identified by X-ray diffrac- tion(XRD; Rigaku, Dmax-rb) using Cu Ka radiation. The microstructure was characterized by field emission scanning 人人人 electron microscopy (SEM; $4700, Hitachi, Tokyo, Japan) and Table I Composition of the prepared composite samples. 口■■ 4:1:0 ZSI 4:1:2.14 26.9% 4:2.5:278 2 theta(degree) ZS3 40% 35% 4:25:3.5 Fig 1. XRD patterns of ZrB2 based composites. Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.10l6/ eurceramsoc2009.09.02

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 2 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx BN, which exhibits high thermal conductivity and high melting point, is regarded as a suitable and effective interface material since the cleavage plane of h-BN facilitates crack propaga￾tion and decreases the cutting resistance during machining.9–12 Besides these, the similar crystal structures of BN and ZrB2 ensure the good chemical compatibility between them. There￾fore, ZrB2–SiC–BN system may be a good candidate material for high temperature ceramics with excellent machinability and mechanical properties. In this paper, the fabrication of a machinable ZrB2–SiC–BN composite with excellent mechanics properties and oxidation resistance properties was reported. Influences of BN content on the hardness, fracture toughness, flexural strength and oxidation resistance property of the composite were investigated. The oxi￾dation resistance properties tested at from 1100 to 1500 ◦C were reported here. As the operative temperature for UHTC materi￾als is in excess of 2000 ◦C, composition performing better than others at 1500 ◦C may not necessarily apply to higher tempera￾ture ranges. The oxidation resistance of this composite and the optimization of the composition at more than 2000 ◦C are also under investigation. 2. Experimental procedure 2.1. Samples preparation Four kinds of ZrB2–SiC–BN composites with various powder compositions (vol%) were prepared (see Table 1). Commercially available ZrB2 powder (>99% purity, an aver￾age particle size of 3 m, Northwest Institute for non-ferrous metal research, China), SiC powder (>98.5% purity, an average particle size of 1.5m, Weifang Kaihua Micro-powder Co., Ltd., China) and h-BN powder (>99% purity, 4 m, Chem Factory, Beijing, China) were used. Powders were mixed and ball-milled for 12 h in a polyethylene bottle charged with ethanol using ZrO2 balls. Solvent was then removed using a rotary evaporator. The dried powder mixtures were sintered by hot-pressing in an argon atmosphere at 1800 ◦C and 23 MPa pressure for 1 h in graphite dies coated with pyrolytic BN. 2.2. Characterization Bulk density and theoretical density were measured and assessed by the Archimedes method and the rule-of-mixture, respectively. Phase composition was identified by X-ray diffrac￾tion (XRD; Rigaku, Dmax-rb) using Cu K radiation. The microstructure was characterized by field emission scanning electron microscopy (SEM; S4700, Hitachi, Tokyo, Japan) and Table 1 Composition of the prepared composite samples. Sample ZrB2 SiC BN ZrB2:SiC:BN ZS0 80% 20% 0% 4:1:0 ZS1 56% 14% 30% 4:1:2.14 ZS2 43.1% 26.9% 30% 4:2.5:2.78 ZS3 40% 25% 35% 4:2.5:3.5 chemical compositions were evaluated by energy-dispersive X-ray spectroscopy (EDS; Phoenix, EDAX, Mahwah, NJ). Flexural strength was tested in a three-point configuration (3 mm × 4 mm × 36 mm chamfered bars), with a 30 mm span and a crosshead speed of 0.5 mm/min. Fracture toughness was evaluated by a single-edge notched beam test with a 16 mm span and a crosshead speed of 0.05 mm/min using 2 mm × 4 mm × 22 mm test bars. Hardness was determined by Vickers indentation (Model HVS-5, Laizhou Huayin Experi￾mental Instrument Limited Company, China) using a diamond indenter with a load of 98 N for 15 s. 2.3. Oxidation tests Specimens were cleaned in an ultrasonic bath in acetone before oxidation. The isothermal static oxidation tests were con￾ducted in an electrical furnace at temperatures of 1100, 1300 and 1500 ◦C in air with interruptions in the tests in order to measure weight (to an accuracy of 0.0001 g) change at fixed times. The specific weight change was calculated according to the mass change per surface area. The oxidation resistance of specimen was also tested by a Net￾zsch STA449C thermogravimetric analyzer. The mass changes were followed at a rate of 5◦/min up to 1500 ◦C with an 2 h isothermal hold in a flowing air (50 ml/min). 3. Results and discussion 3.1. Mechanical properties and machinability Fig. 1 shows the XRD results of sintered ceramics contain￾ing various contents of BN. All samples contained the initial phases of ZrB2, SiC and BN, except the sample ZS0 in which no BN existed. No new phase was formed during hot-pressing and sintering. Therefore, no chemical reactions occurred under the experimental condition, which benefits the formation of weak interfaces between the boundaries of ZrB2, SiC and BN grains. Fig. 1. XRD patterns of ZrB2 based composites.

+Model JECS-7698: No of Pages 8 ARTICLE IN PRESS H. Wu, W. Zhang /Journal of the European Ceramic Sociery xxx(2009)rxx-ccX Table 2 Density and mechanical properties of ZrB2-SiC and ZrB2-SiC-BN composites. Sample Composition(vol%o) Apparent density Relative density Flexural strength Fracture toughness Vickers hardne (g/cm) (%) (MPa) MPam/ (GPa) 5.129 B2+14%SC+30%BN4.102 3.5 ZrB2+26.9%SiC+30%BN3.775 317 ZS340%ZrB2+25%SiC+35%BN3.732 378 Results of density and mechanical properties are listed in interface at the ZrB2-Sic-Bn grain boundaries is the main rea Table 2. An increase in the flexural strength of ZrB2-SiC com- son for the improvement of the machinability, which can enhance posites doped BN was found compared to that of ZrB2-Sic the crack deflection and avoid the catastrophic failure of the composite without BN. This mainly results from the fact that material during drilling the h-BN crystals were homogeneously dispersed around the Fig. 4 shows the fracture surface of specimens for a test of matrix grains of ZrB2 and Sic during sintering(as shown in fracture toughness. It can be seen that abnormal grain growth Fig. 2), which limits the grain growth and improves their flexural occurs in the ZSO specimen with a main fracture model of trans- granular fracture. For the ZS1, ZS2 and ZS3 specimens, fractures It is assumed that the soft h-BN particles with layered- propagate parallel to the layer crystals because bn grains pos- structures could relax stress and absorb energy at the crack sess a layered crystal structure and are readily delaminated due tip through microcracking or crack-particle interactions, then to its low cleavage energy. Crack deflections, branching and tious to improve fracture toughness. 13-15 However, compat i- blunting during machining of layered crystal BN are benef ZrB2-SiC, the fracture toughnesses of all ZrB2-SiC-BN com- local cutting area, which lead to fracture modes dominated by posites decreased in the study. the intergranular fracture. This phenomenon confirms the for- On the other hand, Table 2 shows that the hardness of the mation of weak ZrB2-SiC-BN interfaces by the addition of BN ZrB2-SiC-Bn composite decreased greatly with 30 vol%Bn and is the main reason for the improved machinability of this additive compared to pure ZrB2-SiC. Hardness is an impor- composite. tant indicator for ceramic machinability. Generally, a lower hole made by cemented carbide drills on the ZS2 specimen. It 3.2. Oxidation resistance can be seen that the ZrB2-Sic-BN composite is successfully 3.2. 1. Thermal gravimetric analysis(TGA) machined. However, due to high hardness, the ZSO specimen Fig 5 shows the mass changes of the four specimens. It is ithout bN additive cannot be machined using such drills. As shown that there is a similar tendency as the temperature below stated above, the layered structure of BN resulting in a weak Fig.2. Cross-sectional SEM micrograph from polished section of ZS2 compos- Fig 3. Demonstration of the prepared machinable ZrB2-SiC-BN ceramic com- posite using cemented carbide drill. Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.1016/ eurceramsoc2009.09.022

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx 3 Table 2 Density and mechanical properties of ZrB2–SiC and ZrB2–SiC–BN composites. Sample Composition (vol%) Apparent density (g/cm3) Relative density (%) Flexural strength (MPa) Fracture toughness (MPa m1/2) Vickers hardness (GPa) ZS0 80%ZrB2 + 20% SiC 5.129 93.0 281 4.8 15.9 ZS1 56%ZrB2 + 14%SiC + 30%BN 4.102 90.3 301 3.5 5.9 ZS2 43.1%ZrB2 + 26.9%SiC + 30%BN 3.775 90.6 317 3.7 5.6 ZS3 40%ZrB2 + 25%SiC + 35%BN 3.735 92.6 378 4.1 52 Results of density and mechanical properties are listed in Table 2. An increase in the flexural strength of ZrB2–SiC com￾posites doped BN was found compared to that of ZrB2–SiC composite without BN. This mainly results from the fact that the h-BN crystals were homogeneously dispersed around the matrix grains of ZrB2 and SiC during sintering (as shown in Fig. 2), which limits the grain growth and improves their flexural strengths. It is assumed that the soft h-BN particles with layered￾structures could relax stress and absorb energy at the crack tip through microcracking or crack-particle interactions, then prevent the main crack from extending which should be propi￾tious to improve fracture toughness.13–15 However, compared to ZrB2–SiC, the fracture toughnesses of all ZrB2–SiC–BN com￾posites decreased in the study. On the other hand, Table 2 shows that the hardness of the ZrB2–SiC–BN composite decreased greatly with 30 vol%BN additive compared to pure ZrB2–SiC. Hardness is an impor￾tant indicator for ceramic machinability. Generally, a lower hardness leads to an improved machinability. Fig. 3 shows a hole made by cemented carbide drills on the ZS2 specimen. It can be seen that the ZrB2–SiC–BN composite is successfully machined. However, due to high hardness, the ZS0 specimen without BN additive cannot be machined using such drills. As stated above, the layered structure of BN resulting in a weak Fig. 2. Cross-sectional SEM micrograph from polished section of ZS2 compos￾ite. interface at the ZrB2–SiC–BN grain boundaries is the main rea￾son for the improvement of the machinability, which can enhance the crack deflection and avoid the catastrophic failure of the material during drilling. Fig. 4 shows the fracture surface of specimens for a test of fracture toughness. It can be seen that abnormal grain growth occurs in the ZS0 specimen with a main fracture model of trans￾granular fracture. For the ZS1, ZS2 and ZS3 specimens, fractures propagate parallel to the layer crystals because BN grains pos￾sess a layered crystal structure and are readily delaminated due to its low cleavage energy. Crack deflections, branching and blunting during machining of layered crystal BN are beneficial to prevent macroscopic fractures from propagation beyond the local cutting area, which lead to fracture modes dominated by the intergranular fracture. This phenomenon confirms the for￾mation of weak ZrB2–SiC–BN interfaces by the addition of BN and is the main reason for the improved machinability of this composite. 3.2. Oxidation resistance 3.2.1. Thermal gravimetric analysis (TGA) Fig. 5 shows the mass changes of the four specimens. It is shown that there is a similar tendency as the temperature below Fig. 3. Demonstration of the prepared machinable ZrB2–SiC–BN ceramic com￾posite using cemented carbide drill

+Model JECS-7698; No of Pages 8 ARTICLE IN PRESS H. Wu, w. Zhang/Journal of the European Ceramic Society rcx(2009)ccx-cox Fig 4. SEM micrographs of the fracture cross-sections of samples ZSO(A), ZSI (B), ZS2(C)and ZS3(D) 1100C and no significant weight gain was observed for the four isothermal oxidation stage at 1500C, the weight gain rate slows pecimens. while the weight gain rates of all the four samples down compared to those during heating up or anisothermal stage increase abruptly as the temperature is up to 1500C, which from 1 100 to 1500oC, which results from the formation of oxide indicates an accelerated oxidation. Moreover, the weight gain films on the surfaces. The formed oxide film actually acts as a rate of Zso is the fastest among the four samples. During the barrier for further diffusion of oxygen into the fresh interface of ZrB2-SiC-BN. It is interesting that the addition of bn restrains the oxidation during not only heating up stage but also the high 0.018 perature isothermal oxidation stage(1500C for 120 min) 1400 especially for ZsI and Zs2 with medium contents of BN 0.012 3.2.2. Oxidation resistance of the composites in static air environment e0.009 The isothermal oxidation resistances for all the specimens were studied at 1100, 1300 and 1500C. The specific weight changes versus oxidation time are given in Fig. 6a-c. In general, 0.003 the weight gain rates of all samples increase with the temperature and the specific weight change with time basically follows a 0.000 parabolic oxidation law. The latter implies that the oxidation kinetics is controlled by transport of oxidant through the growing oxide filr At 1100C, the sample ZS3 containing 35 vol%oBN pres Fig. 5. TGA oxidation of ZrB, based composites in air up to 1500C. the highest rate of specific weight gain. However, mini Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.10l6/ eurceramsoc2009.09.02

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 4 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx Fig. 4. SEM micrographs of the fracture cross-sections of samples ZS0 (A), ZS1 (B), ZS2(C) and ZS3 (D). 1100 ◦C and no significant weight gain was observed for the four specimens. While the weight gain rates of all the four samples increase abruptly as the temperature is up to 1500 ◦C, which indicates an accelerated oxidation. Moreover, the weight gain rate of ZS0 is the fastest among the four samples. During the Fig. 5. TGA oxidation of ZrB2 based composites in air up to 1500 ◦C. isothermal oxidation stage at 1500 ◦C, the weight gain rate slows down compared to those during heating up or anisothermal stage from 1100 to 1500 ◦C, which results from the formation of oxide films on the surfaces. The formed oxide film actually acts as a barrier for further diffusion of oxygen into the fresh interface of ZrB2–SiC–BN. It is interesting that the addition of BN restrains the oxidation during not only heating up stage but also the high temperature isothermal oxidation stage (1500 ◦C for 120 min), especially for ZS1 and ZS2 with medium contents of BN. 3.2.2. Oxidation resistance of the composites in static air environment The isothermal oxidation resistances for all the specimens were studied at 1100, 1300 and 1500 ◦C. The specific weight changes versus oxidation time are given in Fig. 6a–c. In general, the weight gain rates of all samples increase with the temperature and the specific weight change with time basically follows a parabolic oxidation law. The latter implies that the oxidation kinetics is controlled by transport of oxidant through the growing oxide film. At 1100 ◦C, the sample ZS3 containing 35 vol%BN presents the highest rate of specific weight gain. However, minimum

+Model JECS-7698: No of Pages 8 ARTICLE IN PRESS H. Wu, W. Zhang /Journal of the European Ceramic Sociery xxx(2009)rxx-ccX HZSO zs!1100℃ 1300℃ 0008 1012141618 Time(hours) Time(hours) 1500℃ ---ZS2 06-△ 000 Fig. 6. Weight change of composites at(A)1100". ( B)1300C and(C)1500C specific weight gain after 64 h, 0.0020 g/cm2, was obtained all processes of oxidation may proceed through the following for ZS2(43. 1%ZrB2+26.9%SiC +30%BN), which is slightly steps: lower than that of Zso(80%ZrB2 20%SiC), 0.0023 g/cm.At 1300 and 1500oC, the highest rate of specific weight gain is ZSO, (a) At the temperature from 600"C to 1100C. The generated and ZS2 presents the lowest oxidation rate. The weight gain for liquid B2O3 would heal the cracks in the oxide scale, leading tSO and ZS2 after oxidation at 1500C for 5h were 0.0714 and to the partially or completely sealing of the cracks depending 0. g/cm, respectively. Therefore, it is concluded that the on the formation rate of B2O3 from the reaction(1)and (3) specimen ZS2 exhibits the best oxidation resistance from 1100 nd the viscosity of B203 liquid. 6 to1500°C For the modified composites, the expected ■ ZrSic describing the oxidation process are as follows o Zro2 2ZrB2(s)+502(g)=2Zo2(s)+2B2O3( C(s)+ 302(g)= 2SiO2(s)+ 2Co(g) 4BN(s)+302(g)=2B2O3()+2N2(g) (3) wi ini. B2O3()=B2O3(g) (4) SiO2(s)+xB,O3( )= B2O3.xSio () B2O3-xSio2()=B2O3(g)+xSio2(s) The reaction(1)(3)lead to weight gains, and the reactions (4)and(6)lead to weight loss. The weight change of sample 2 theta(degree) is accumulative results of reactions(1)(4)and(6). The over- Fig. 7. XRD patterns of ZSo(A)and ZS2(B)oxidized at 1500. Ih Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.1016/ eurceramsoc2009.09.022

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx 5 Fig. 6. Weight change of composites at (A) 1100 ◦C, (B) 1300 ◦C and (C) 1500 ◦C. specific weight gain after 64 h, 0.0020 g/cm2, was obtained for ZS2 (43.1%ZrB2 + 26.9%SiC + 30%BN), which is slightly lower than that of ZS0 (80%ZrB2 + 20%SiC), 0.0023 g/cm2. At 1300 and 1500 ◦C, the highest rate of specific weight gain is ZS0, and ZS2 presents the lowest oxidation rate. The weight gain for ZS0 and ZS2 after oxidation at 1500 ◦C for 5 h were 0.0714 and 0.0268 g/cm2, respectively. Therefore, it is concluded that the specimen ZS2 exhibits the best oxidation resistance from 1100 to 1500 ◦C. For the modified composites, the expected main reactions describing the oxidation process are as follows: 2ZrB2(s) + 5O2(g) = 2ZrO2(s) + 2B2O3(l) (1) 2SiC(s) + 3O2(g) = 2SiO2(s) + 2CO(g) (2) 4BN(s) + 3O2(g) = 2B2O3(l) + 2N2(g) (3) B2O3(l) = B2O3(g) (4) SiO2(s) + xB2O3(l) = B2O3·xSiO2(l) (5) B2O3·xSiO2(l) = B2O3(g) + xSiO2(s) (6) The reaction (1)–(3) lead to weight gains, and the reactions (4) and (6) lead to weight loss. The weight change of sample is accumulative results of reactions (1)–(4) and (6). The over￾all processes of oxidation may proceed through the following steps: (a) At the temperature from 600 ◦C to 1100 ◦C. The generated liquid B2O3 would heal the cracks in the oxide scale, leading to the partially or completely sealing of the cracks depending on the formation rate of B2O3 from the reaction (1) and (3) and the viscosity of B2O3 liquid.16 Fig. 7. XRD patterns of ZS0 (A) and ZS2 (B) oxidized at 1500 ◦C for 1 h

+Model JECS-7698; No of Pages 8 ARTICLE IN PRESS H. Wu, w. Zhang/Journal of the European Ceramic Society rcx(2009)ccx-cox 1# ZrSio 10. Oum 1000u KEnt 3# 1# 2 0.6 3.00 1002003.00 Energy -keV Energy -kev Fig 8. Cross-sectional micrographs of Zs2 after oxidation at 1500C for 2h (b) During the temperature from 1100C to 1300C. Large it contains an appropriate amount of BN (30 vol%)com- mount of volatile B2O3 would form. And very important pared with the sample Zso, so more B203 was generated. the formation of sio derived from the oxidation of sic and more easily spread in the material surface for oxide film. becomes significant, reaction between SiO and B2O3 leads Moreover, the sample ZS2 contains a higher proportion of to a stable borosilicate glass. The borosilicates would act silicon carbide than Zso and ZSI. therefore more silicon Is a protective layer to reduce oxidation rate more effec oxide forms in ZS2 than that does in zso and ZsI. The ively than B2O3 due to lower volatility and smaller oxygen higher SiO2 content in borosilicate glass, the higher the vis- ffusivity.7-19The sample ZS3 gets the highest rate of spe cosity and melting point of borosilicate glass are, which can cific weight gain, which is attributed to the largest content more effectively cover and protect the surface of samples of BN (35 vol%). The formation of B2O3 from the oxidation Therefore. the oxidation resistance of Zs2 with a suitable of bn shows the obvious weight gain. For the sample ZS2, amount of bn additive is better than other samples Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.10l6/ eurceramsoc2009.09.02

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 6 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx Fig. 8. Cross-sectional micrographs of ZS2 after oxidation at 1500 ◦C for 2 h. (b) During the temperature from 1100 ◦C to 1300 ◦C. Large amount of volatile B2O3 would form. And very important, the formation of SiO2 derived from the oxidation of SiC becomes significant, reaction between SiO2 and B2O3 leads to a stable borosilicate glass. The borosilicates would act as a protective layer to reduce oxidation rate more effec￾tively than B2O3 due to lower volatility and smaller oxygen diffusivity.17–19 The sample ZS3 gets the highest rate of spe￾cific weight gain, which is attributed to the largest content of BN (35 vol%). The formation of B2O3 from the oxidation of BN shows the obvious weight gain. For the sample ZS2, it contains an appropriate amount of BN (30 vol%) com￾pared with the sample ZS0, so more B2O3 was generated, and more easily spread in the material surface for oxide film. Moreover, the sample ZS2 contains a higher proportion of silicon carbide than ZS0 and ZS1, therefore more silicon oxide forms in ZS2 than that does in ZS0 and ZS1. The higher SiO2 content in borosilicate glass, the higher the vis￾cosity and melting point of borosilicate glass are, which can more effectively cover and protect the surface of samples. Therefore, the oxidation resistance of ZS2 with a suitable amount of BN additive is better than other samples

+Model JECS-7698: No of Pages 8 ARTICLE IN PRESS H. Wu, W. Zhang /Journal of the European Ceramic Sociery xxx(2009)rxx-ccX I Matrix Fig. 9. Cross-sectional micrographs of Zso after oxidation at 1500C for 2h (c)With increasing temperature, 1300-1500oC. The viscosity cosity, it can efficaciously cover the sample surface and seal the of borosilicate glass decreases with increasing temperature, cracks, which effectively limits the inward transport of oxygen which benefits a healing of cracks and the diffusion velocity and correspondingly enhances the resistance to oxida of oxygen. When temperature is up to 1500C, the ini- tially formed silica-enriched glass will be gradually lost in ZrO2+SiO2= ZrsiO4 response to the reaction( 6)due to the substantive volatilize- Figs. 8 and 9 show SeM results for the oxidized ZSo and ZS2 tion of B2O3. While Sioz is significantly less volatile than after oxidation at 1500oC for 2h, respectively. It is noticeable B2O3 at these temperatures(the vapor pressure of B2O3 is that the oxide scales of both zso and zS2 consist of two dis- O times higher than that of SiO2 at 1500 C), 0 the remain- tinct layers and the oxide layer of the specimen ZSo(214 um) ing silicon oxide may react with zirconium oxide to generate is thicker than that of ZS2(169 um), which also indicates that a new anti-oxidation coating, zirconium silicate, which the oxidation resistance of zs2 is better than that of zso. The was confirmed by phase analysis in the study for the first outer layers of both them are identified as ZrSiO4, according to the combination of the EDS and XRD. In the inner layer, it is observed that white zirconia particles as a skeleton dis- Fig. 7 shows the XRD patterns of the surface coatings of tribute in grayer zirconium silicate. The ZrO2 does not enhanc ZS2 and Zso oxidized at 1500C for I h. Monoclinic ZrO2 the oxidation protection, but may provide mechanical integrity and tetragonal ZrSiO4 were observed. Since the B2O3 and and strength like a framework for the liquid glass. At the same borosilicate are amorphous, some undetected B2O3 probably time, cracks were found in oxide layer during the quenching pro- may remain dissolved in the Sio2. The presence of ZrSiO4, pre- cess, which attributes to the coefficients of thermal expansion sumably derived from the chemical reaction between ZrO2 and mismatch between the oxide layer and matrix SiO2(reaction(7), stabilizes SiO2 and inhibits the volatilization EDS shows that zirconium, oxygen and silicon are present as of silica. Besides these, ZrSiO4 has high melting point and vis- the primary elements in the oxidized layer. Although quantitative Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.1016/ eurceramsoc2009.09.022

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx 7 Fig. 9. Cross-sectional micrographs of ZS0 after oxidation at 1500 ◦C for 2 h. (c) With increasing temperature, 1300–1500 ◦C. The viscosity of borosilicate glass decreases with increasing temperature, which benefits a healing of cracks and the diffusion velocity of oxygen. When temperature is up to 1500 ◦C, the ini￾tially formed silica-enriched glass will be gradually lost in response to the reaction (6) due to the substantive volatiliza￾tion of B2O3. While SiO2 is significantly less volatile than B2O3 at these temperatures (the vapor pressure of B2O3 is 105 times higher than that of SiO2 at 1500 ◦C),20 the remain￾ing silicon oxide may react with zirconium oxide to generate a new anti-oxidation coating, zirconium silicate, which was confirmed by phase analysis in the study for the first time. Fig. 7 shows the XRD patterns of the surface coatings of ZS2 and ZS0 oxidized at 1500 ◦C for 1 h. Monoclinic ZrO2 and tetragonal ZrSiO4 were observed. Since the B2O3 and borosilicate are amorphous, some undetected B2O3 probably may remain dissolved in the SiO2. The presence of ZrSiO4, pre￾sumably derived from the chemical reaction between ZrO2 and SiO2 (reaction (7)), stabilizes SiO2 and inhibits the volatilization of silica. Besides these, ZrSiO4 has high melting point and vis￾cosity, it can efficaciously cover the sample surface and seal the cracks, which effectively limits the inward transport of oxygen and correspondingly enhances the resistance to oxidation. ZrO2 + SiO2 = ZrSiO4 (7) Figs. 8 and 9 show SEM results for the oxidized ZS0 and ZS2 after oxidation at 1500 ◦C for 2 h, respectively. It is noticeable that the oxide scales of both ZS0 and ZS2 consist of two dis￾tinct layers and the oxide layer of the specimen ZS0 (214 m) is thicker than that of ZS2 (169 m), which also indicates that the oxidation resistance of ZS2 is better than that of ZS0. The outer layers of both them are identified as ZrSiO4, according to the combination of the EDS and XRD. In the inner layer, it is observed that white zirconia particles as a skeleton dis￾tribute in grayer zirconium silicate. The ZrO2 does not enhance the oxidation protection, but may provide mechanical integrity and strength like a framework for the liquid glass. At the same time, cracks were found in oxide layer during the quenching pro￾cess, which attributes to the coefficients of thermal expansion mismatch between the oxide layer and matrix. EDS shows that zirconium, oxygen and silicon are present as the primary elements in the oxidized layer. Although quantitative

+Model JECS-7698; No of Pages 8 ARTICLE IN PRESS H Wu, w. Zhang / Journal of the European Ceramic Society rxx(2009)ccr-ccx analysis for B is not possible by EDS since B is beyond the limit 2. Levine, S.R., Opila, E.J. Halbig, M. C,Kiser, J.D., Singh, M and Salem, f the detection capability, a small amount of B element may J. A, Evaluation of ultra-high temperature ceramics for aeropropulsion u still exist as borosilicate, though a previous SIMS investigation J. Eur Ceran.Soc,2002,22,2757-2767 showed the amount being minimaL. 20 3. Zhou, X.J., Zhang. G.J., Li, Y. G Kan, Y. M. and Wang, P. L, Hot pressed ZrB2-SiC-C ultra high temperature ceramics with polycarbosilane It is worth noting that the outer layer of Zs2 appears compact as a precursor. Mater. Lett, 2007, 61, 960-963 and adherent, while that of Zso is discontinuous and incompact. 4. Monteverde, F, Bellosi, A and Scatteia, L, Processing and properties of urthermore, the thickness of ZS2 is remarkably lower than that ultra-high ure ceramics for space applications. Mater. Sci. Eng. A of Zso. This is due to the fact that the formation of this layer 2008,485,415-421 is mostly dependent on the SiC content. The silicon carbide 5. Yang, F. Y, Zhang, X. H, Han, J. C. and Du, S. Y, Characteriza- content of Zs2 is higher than that of Zso. a high Sic content tion of hot-pressed short carbon fiber reinforced ZrB2-SiC temperature ceramic composites. J. Alloys Compd., 2009, 472, 395- is beneficial for the formation of ZrSiO4, which can act as effective barrier against the inward diffusion of oxygen. 6. Zhang, w.Y. and Gao, H, Microstructures, properties and fabrication of machinable ceramics. J. Synth. Cryst. 2005. 34. 170-173 4. Conclusions 7. Wang, R. G, Pan, w,Jiang, M.N., Chen, J, Luo, Y M. and Sun, R F, Devel- pment in machinable ceramics and machining technology of engineering ceramics. Bull. Chin Ceram Soc. 2001.3 27-35 (1)ZrB2-SiC-BN ceramics were successfully prepared by 8. Xu, HH. K and Jahanmir, SS, Scratching and grinding of a machinable hot-pressing under t1800°cand23 MPa pres glass-ceramic with weak interfaces and rising T-curve J. Am. Ceram. Soc. sure. With the addition of bN. the flexural strength of 1995,78,497-500. the ZrB2-SiC composite was improved, and the fracture 9. Zhang, G J, Yang, J., Ando, M and Ohji, T, Nonoxide-boron nitride toughness decreased slightly, but the hardness decreased composites: in situ synthesis, microstructure and properties. J. Eur Ceram. enormously, and the machinability of this composite was Soc.,2002,22,2251-2254 10. Cho, M. w. Kim, D. W and Cho, w.s., Analysis of micro-machining mproved noticeably. characteristics of Si3 N4-hBN composites. J. Eur. Ceram. Soc., 2007, 27 (2)Below 1300C, the addition of BN significantly improved 1259-1265 the oxidation resistance of ZrB2-SiC ceramics due to the 11. Li, Y L, Zhang, J.X., Qiao, G.J. and Jin, Z.H., Fabrication and properties formation of ample borosilicates. At 1300.C and above, zirconium silicate deriving from the reaction between sil- 12. Kusunose. T. Sekino, T. Choa. Y. H and Nihara. K. Fabrication and ica and zirconium oxide played as a major anti-oxidation microstructure of silicon nitride/boron nitride nanocomposites. J.Am. coating, which could inhibit the diffusion of oxygen and Ceram.Soc.2002,85,2678-2688 protect the material underneath from further oxidation after 13. Wang, R.G. Pan,WChen, J.Jiang, M.N. and Fang, M H, Fabrication and evaporating of borosilicates (3)The composite with components of 43. 1%ZrB2, 26.9%SiC 14. Li Y. L. Qiao, G.J. and Jin, Z H. Machinable Al2O3/BN ceramics with strong mechanical properties. Mater. Res. Bull. 1500C. A total weight gain as low as 0.0268 g/cm after 1401-1409 oxidation at 1500oC for 5h was observed. The addition 15. Wang, X D. Qiao, G.J. and Jin, Z H, Fabrication of machinable silicon of BN in the appropriate amount is implied as an effec carbide-boron nitride ceramic nanocomposites. J. Am. Ceram Soc., 2004 87,565-570 tive method to simultaneously improve the flexural strength, 16. Naslain, R. Design, preparation and properties of non-oxide CMcs for machinability and the oxidation resistance of ZrB2-SiC hes and nuclear reactors: an overview. Compos. Sci. ceramIcs Technol.,2004,64,155-170. 17. Jacobson, N S and Morscher, G. N, High-temperature oxidation of boron Acknowledgements nitride: ii. Boron nitride layers in composites. J. Am. Ceram. Soc., 1999, 82, 1473-1482. 18. Baskaran S and Halloran, J. w, Fibrous monolithic ceramics: Ill. mechan- from the Chinese Academy of Sciences ical properties and oxidation behavior of the silicon carbide/boron nitride under the Program for GF Basic Research(No. Al320070102) stem.J.Am.Cerm.Soc.,1994,77,1249-1255 is gratefully appreciated 19. Guo, Q.G Song, J.R., Liu, L and Zhang, B J, Relationship between oxi- dation resistance and structure of B4 C-SiC/C composites with self-healing References properties. Carbon, 1999, 37, 33-40 20. Rezaie. A, Fahrenholtz, w. G. and Hilmas, G. E, Oxidation of zirconium diboride-silicon carbide at 1500C at a low partial pressure of oxygen. J. 1. Zimmermann, J. w,Hilmas, G. E, Fahrenholtz, w. G, Monteverde, F and Am. Ceran.Soc,2006,89,3240-3245 Bellosi, A Fabrication and properties of reactively hot pressed ZrB2-SiC ramics.J. Eur Ceran. Soc. 2007. 27. 2729-2736 Please cite this article in press as: Wu, H, Zhang, w, Fabrication and properties of ZrB2-SiC-BN machinable ceramics, J. Eur Ceram. Soc. (2009),doi:10.10l6/ eurceramsoc2009.09.02

Please cite this article in press as: Wu, H., Zhang, W, Fabrication and properties of ZrB2–SiC–BN machinable ceramics, J. Eur. Ceram. Soc. (2009), doi:10.1016/j.jeurceramsoc.2009.09.022 ARTICLE IN PRESS +Model JECS-7698; No. of Pages 8 8 H. Wu, W. Zhang / Journal of the European Ceramic Society xxx (2009) xxx–xxx analysis for B is not possible by EDS since B is beyond the limit of the detection capability, a small amount of B element may still exist as borosilicate, though a previous SIMS investigation showed the amount being minimal.20 It is worth noting that the outer layer of ZS2 appears compact and adherent, while that of ZS0 is discontinuous and incompact. Furthermore, the thickness of ZS2 is remarkably lower than that of ZS0. This is due to the fact that the formation of this layer is mostly dependent on the SiC content. The silicon carbide content of ZS2 is higher than that of ZS0. A high SiC content is beneficial for the formation of ZrSiO4, which can act as an effective barrier against the inward diffusion of oxygen. 4. Conclusions (1) ZrB2–SiC–BN ceramics were successfully prepared by hot-pressing under argon at 1800 ◦C and 23 MPa pres￾sure. With the addition of BN, the flexural strength of the ZrB2–SiC composite was improved, and the fracture toughness decreased slightly, but the hardness decreased enormously, and the machinability of this composite was improved noticeably. (2) Below 1300 ◦C, the addition of BN significantly improved the oxidation resistance of ZrB2–SiC ceramics due to the formation of ample borosilicates. At 1300 ◦C and above, zirconium silicate deriving from the reaction between sil￾ica and zirconium oxide played as a major anti-oxidation coating, which could inhibit the diffusion of oxygen and protect the material underneath from further oxidation after evaporating of borosilicates. (3) The composite with components of 43.1%ZrB2, 26.9%SiC and 30%BN showed excellent oxidation resistance up to 1500 ◦C. A total weight gain as low as 0.0268 g/cm2 after oxidation at 1500 ◦C for 5 h was observed. The addition of BN in the appropriate amount is implied as an effec￾tive method to simultaneously improve the flexural strength, machinability and the oxidation resistance of ZrB2–SiC ceramics. Acknowledgements Financial support from the Chinese Academy of Sciences under the Program for GF Basic Research (No. A1320070102) is gratefully appreciated. References 1. Zimmermann, J. W., Hilmas, G. E., Fahrenholtz, W. G., Monteverde, F. and Bellosi, A., Fabrication and properties of reactively hot pressed ZrB2–SiC ceramics. J. Eur. Ceram. Soc., 2007, 27, 2729–2736. 2. Levine, S. R., Opila, E. J., Halbig, M. C., Kiser, J. D., Singh, M. and Salem, J. A., Evaluation of ultra-high temperature ceramics for aeropropulsion use. J. Eur. Ceram. Soc., 2002, 22, 2757–2767. 3. Zhou, X. 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