May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior Unaffected Interior The reason why larger Yb,, were found in the bn cell boundary than on the surface of the Si3N4 cells is that the 100 um recess BaTION boro-silicate glass in the boundary phase has a lower viscosit at bN cell ZONE than the glass in the grain-boundary phases, thus allowing more pid diffusion and thus larger Yb Si,O, grains emanated from the bn boundary phases than from the Si3N4 cells. Figure 1 l shows a schematic diagram of the oxidation mecha- 4 um oxide nism of the fmB layer on Si, cells IV. Conclusion The Si3 N4/BN FM with 8 wt% Yb2O3 showed a flexural strength of 340+91 MPa and graceful failure with a WoF of 2126+940 J/m". These properties are comparable with reported 0k50 100x Si3N4/BN FMs with 6 wt%Y2O3 and 2 wt% AL,O3. The ox- idation test of the FMs with Yb,O3 revealed a thin oxide scale Fig 9. FMYB sample after oxidation at 1400C for 10 h. The oxida- containing small Yb Si,O, on the Si3N4 cells but large Yb2Si,O tion zone was uniformly distributed around the surface on the Bn cell boundary. Also, microscopic analysis after the oxidation test showed an 100 um recess in the Bn cell bound- ry and an 4 um oxide scale on Si3 N4 cells, uniformly dis- tributed around the surface of the sample The Si3N4/BN FMs with La2O3 showed a flexural strength of 298+90 MPa and brittle failure in the majority of the samples Si The brittle behavior and low strength of the FMLA samples "Glassy" phase compared with the FMYB samples were believed to be due to disintegration of the sample, because of the hydration of La2O3 from the rare-earth apatite phase, LasSi3O12N. The surface of the FMLa sample after oxidation for 10 h at 1400.C in dry air showed severe oxidation In conclusion, the Si3N4/BN FM with Yb, O3 as a sinterin additive has proven to be similar in strength at room tempe ture to the commercially available Si3N4/BN FMs with 6 wt% Y,O3 and 2 wt% AlO,, and to have a promising oxidation be- havior. The rare-earth sintering aid, La2O3, has shown not to confer suitable mechanical properties or oxidation resistance to References b Si O 'S. Baskaran, S.D. Nunn, D. Popovic, and J. W. Halloran, "Fibrous Mono- Penetration in BN cell bound Ceramics,J. Am. Ceran. Soc., 80[10]2471-87(1997) R. w. Trice and J. w. Halloran, "Elevated-Temperature Mechanical Proper- ties of Silicon Nitride/ Boron Nitride Fibrous Monolithic Ceramics. Fig 10. Side view of a cross-section of an FMYB sample showing the Soc,832]31l-6(2000) Bn boundary phase after oxidation Trice and J. w. Halloran, " Enect of Sintering Aid Composition on the g of Si3 Na/BN Fibrous Monolithic Ceramics. J. Am. Ceram. Soc., 82 Thus, the BN cell boundaries started to oxidize before asts Monolithic ces haus Mim,H用时(m上m过 R. W. Trice and J. w. Halloran. "Influen the Interfacial Fracture Energy of Silicon Nitride/ Boron Nitride Fibrous into liquid oxide (b2O3). The B2O3 liquid oxide then started t platize at higher temperatures and yielded Yb2Si2O7 and left behind some residue of the amorphous glassy phase; see Fig 10 C. Goretta, F. Gutierrez-Mora N. Chen, J. L. Routbort, T A. Orlova, B I. M. Y. He, D. Singh, J. C. McNulty, and F. w. Zok, Thermal Expansion tional and Cross-Ply Fibrous Monoliths, Compos. Sci. Technol 1400°C.d ry a Clarke and G. Thomas. "Grain Boundary in a Hot-Pressed Mgo Fluxed J. Am. Ceram. Soc. 60 491-5(1977) ID. R. Clarke, " On the Equilibrium Thickness of Intergranular Glass Phases in Ceramic Materials. " J. Am. Ceran. Soc. 70[1] 15-22(1987). Y. Goto and G. Thomas. ""Microstructure of Silicon Nitride Co d with Rare-Earth o Growing SiO2 film semat-Nalsve Yb2Si207 glassy Is,and s M. Johnson. "Strength and Creep Be ase borosilicate havior of Rare-Earth Disilicate-Silicon Nitride Ceramics, J. Am. Ceram. Soc 8]2050-5(1992). Cinibulk, G. Thomas, and S. M. Johnson. "" Oxidation Behat of Rare-Earth Disilicate-Silicon Nitride Ceramics, J. Am. Ceran. Soc., 75[81 IH. Park. H.-E. Kim, and K. Niihara. ""Microstructural Evolution and Me Fig. 11. Schematic of the oxidation process for the exposed Si3N4 cells chanical Properties of SiN4 with Yb2O3 as Sintering Additive, "J. Am. Ceram and bn boundary phase Soc,803750-6(1997)Thus, the BN cell boundaries started to oxidize before the Si3N4 cells, where the BN grains in the cell boundary started to oxidize into liquid oxide (B2O3). The B2O3 liquid oxide then started to volatize at higher temperatures and yielded Yb2Si2O7 and left behind some residue of the amorphous glassy phase; see Fig. 10. The reason why larger Yb2Si2O7 were found in the BN cell boundary than on the surface of the Si3N4 cells is that the boro-silicate glass in the boundary phase has a lower viscosity than the glass in the grain–boundary phases, thus allowing more rapid diffusion and thus larger Yb2Si2O7 grains emanated from the BN boundary phases than from the Si3N4 cells. Figure 11 shows a schematic diagram of the oxidation mecha￾nism of the FMYB. IV. Conclusion The Si3N4/BN FM with 8 wt% Yb2O3 showed a flexural strength of 340791 MPa and graceful failure with a WOF of 21267940 J/m2 . These properties are comparable with reported Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3. The ox￾idation test of the FMs with Yb2O3 revealed a thin oxide scale containing small Yb2Si2O7 on the Si3N4 cells but large Yb2Si2O7 on the BN cell boundary. Also, microscopic analysis after the oxidation test showed an B100 mm recess in the BN cell bound￾ary and an B4 mm oxide scale on Si3N4 cells, uniformly dis￾tributed around the surface of the sample. The Si3N4/BN FMs with La2O3 showed a flexural strength of 298790 MPa and brittle failure in the majority of the samples. The brittle behavior and low strength of the FMLA samples compared with the FMYB samples were believed to be due to disintegration of the sample, because of the hydration of La2O3 from the rare-earth apatite phase, La5Si3O12N. The surface of the FMLA sample after oxidation for 10 h at 14001C in dry air showed severe oxidation. In conclusion, the Si3N4/BN FM with Yb2O3 as a sintering additive has proven to be similar in strength at room tempera￾ture to the commercially available Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3, and to have a promising oxidation be￾havior. The rare-earth sintering aid, La2O3, has shown not to confer suitable mechanical properties or oxidation resistance to Si3N4/BN FMs. References 1 S. Baskaran, S. D. Nunn, D. Popovic, and J. W. Halloran, ‘‘Fibrous Mono￾lithic Ceramics: I, Fabrication, Microstructure, and Indentation Behavior,’’ J. Am. Ceram. Soc., 76 [9] 2209–16 (1993). 2 D. Kovar, B. H. King, R. W. Trice, and J. W. Halloran, ‘‘Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 80 [10] 2471–87 (1997). 3 R. W. Trice and J. W. Halloran, ‘‘Elevated-Temperature Mechanical Proper￾ties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 83 [2] 311–6 (2000). 4 R. W. Trice and J. W. Halloran, ‘‘Effect of Sintering Aid Composition on the Processing of Si3N4/BN Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 82 [11] 2943–7 (1999). 5 R. W. Trice and J. W. Halloran, ‘‘Influences of Microstructure and Temper￾ature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 82 [9] 2502–8 (1999). 6 Y.-H. Koh, H.-W. Kim, H.-E. Kim, and J. W. Halloran, ‘‘Thermal Shock Resistance of Fibrous Monolithic Si3N4/BN Ceramics,’’ J. Eur. Ceram. Soc., 24, 2339–47 (2004). 7 K. C. Goretta, F. Gutierrez-Mora, N. Chen, J. L. Routbort, T. A. Orlova, B. I. Smirnov, and A. R. de Arellano-Lope´z, ‘‘Solid-Particle Erosion and Strength Degradation of Si3N4/BN Fibrous Monoliths,’’ Wear, 256, 233–42 (2004). 8 M. Y. He, D. Singh, J. C. McNulty, and F. W. Zok, ‘‘Thermal Expansion of Unidirectional and Cross-Ply Fibrous Monoliths,’’ Compos. Sci. Technol., 62, 967–76 (2002). 9 D. R. Clarke and G. Thomas, ‘‘Grain Boundary in a Hot-Pressed MgO Fluxed Silicon Nitride,’’ J. Am. Ceram. Soc., 60 [11–12] 491–5 (1977). 10D. R. Clarke, ‘‘On the Equilibrium Thickness of Intergranular Glass Phases in Ceramic Materials,’’ J. Am. Ceram. Soc., 70 [1] 15–22 (1987). 11Y. Goto and G. Thomas, ‘‘Microstructure of Silicon Nitride Ceramics Sinte￾red with Rare-Earth Oxides,’’ Acta Metall. Mater., 43 [3] 923–30 (1995). 12M. Liu and S. Nemat-Nasser, ‘‘The Microstructure and Boundary Phases of In-Situ Reinforced Silicon Nitride,’’ Mater. Sci. Eng., A254, 242–52 (1998). 13M. K. Cinibulk, G. Thomas, and S. M. Johnson, ‘‘Strength and Creep Be￾havior of Rare-Earth Disilicate–Silicon Nitride Ceramics,’’ J. Am. Ceram. Soc., 75 [8] 2050–5 (1992). 14M. K. Cinibulk, G. Thomas, and S. M. Johnson, ‘‘Oxidation Behavior of Rare-Earth Disilicate–Silicon Nitride Ceramics,’’ J. Am. Ceram. Soc., 75 [8] 2044–9 (1992). 15H. Park, H.-E. Kim, and K. Niihara, ‘‘Microstructural Evolution and Me￾chanical Properties of Si3N4 with Yb2O3 as Sintering Additive,’’ J. Am. Ceram. Soc., 80 [3] 750–6 (1997). Fig. 9. FMYB sample after oxidation at 14001C for 10 h. The oxida￾tion zone was uniformly distributed around the surface. Fig. 10. Side view of a cross-section of an FMYB sample showing the BN boundary phase after oxidation. 1400°C, dry air B2O3 volatilizing Si3N4 Si3N4 Growing SiO2 film Yb2Si2O7 + glassy phase + borosilicate Intact BN Fig. 11. Schematic of the oxidation process for the exposed Si3N4 cells and BN boundary phase. May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior 1619