+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/ eurceramsoc2009.09.02Please 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.