+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/ eurceramsoc2009.09.02Please 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 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