+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/ eurceramsoc2009.09.022Please 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 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