+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/ eurceramsoc2009.09.022Please 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