+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/ eurceramsoc2009.09.022Please 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 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