May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior dissipation. During an SEM/XEDS procedure, lanthanum- rich Side surface crystals were observed spread over the side surfaces of the FMLA specimens; see Fig. 5. XEDS analyses of these crystals nowed peaks of N, O, Si, and La. XRD analyses were also performed, where B-Si3 N4, BN, and small rare-earth apatite Lassi3o12N phases were identified. The rare-earth apatite phase. Las Si3O12N, was identified in grain-boundary phases in an in situ reinforced silicon nitride with La,o Tensile initiation he lanthanum-rich crystals growing from the side surfaces was 如mmH believed to be La(OH)3 crystals that were formed in air during hydration of La2O3 from the rare-earth apatite phase, Las. Fig.2. Scanning electron micrograph of a side view of an FMYB D3012N. This could not be confirmed with XRD analysis be- cause of the small amount of the crystals on the samples. Meadowcroft found that a strontium-doped lanthanum chro- mite prepared to comprise LacrO3+x mol% Sro disintegrated at room temperature, owing to the formation of La(oh)3, whicl was established by XRD analysis. The strontium was believed to have displaced the lanthanum in order to dope the lanthanum chromite, and then the free LayO, hydrated with time, where the 00 associated expansion caused the ceramic to disintegrate. The brittle behavior and low strength of the FMLa samples com- pared with the FMYB samples also gave us reason to believe that the ceramic disintegrated because of hydration of LayO3 available from the rare-earth apatite phase, LasSi3O12N (2) Characterization of the Oxidation Surface Layer The surface of the FMLa sample after oxidation for 10 h at 1400.C in dry air showed severe oxidation; SEM analysis of the 0.000080.170.250.330.420.500.58 Crosshead Displacement [mm] ered with burst bubbles and cracks. Because of the disintegra tion of the FMLA samples, we ot discuss the oxidation Fig 3. Flexural response of Si3N4/BN FMs with lanthanum oxide the FMla here in this paper but will focus on the oxidation the fmrb The oxidation behavior of SiNa ceramics with additive tems has been reported to be strongly dependent on the chemical composition of the oxidation surface layer formed by the oxidation products of the Si3 N4 and the sintering additives. 20 shows an Sem picture of th Side surface dized surface of FMYB after 10 h at 1400C in dry air where an oxide scale has formed. With XEDS and X-ray diffractiomet he oxidation product was identified to be mainly ytterbium sil icates (Yb2Si207). Large ytterbium silicates, 20 um, were found on the bn cell boundary but small ytterbium silicates were found on the Si3 N4 cells of 0.8 um. This can be seen in Figs. 6(b)and (c), which shows a higher magnification of the Shear initiation ytterbium silicates on the bn boundary phase and the sigN 加出知 cells in Fig. 6(a); the ytterbium silicates are the white flattened crystals in Figs. 6(aHc). XRD was performed before and after Fig 4. Scanning electron micrograph of a side view of an FMLA flex- the oxidation testing for FMYB. The XRD analysis of the un- iral bar showing a shear-initiated fracture between the outer and inner oxidized FMyB sample revealed B-Si3N4, BN, and Yb Si2O,N peaks, and B-si3N4, BN, Yb4si2O7N2, and Yb, peaks of SinA B SiaNA SE 102 OR Fig. 5. (a) Lanthanum-rich crystals formed on the side surface of FMLA samples. (b) Magnification of the box in(a) of the crystals formeddissipation. During an SEM/XEDS procedure, lanthanum-rich crystals were observed spread over the side surfaces of the FMLA specimens; see Fig. 5. XEDS analyses of these crystals showed peaks of N, O, Si, and La. XRD analyses were also performed, where b-Si3N4, BN, and small rare-earth apatite La5Si3O12N phases were identified. The rare-earth apatite phase, La5Si3O12N, was identified in grain–boundary phases in an in situ reinforced silicon nitride with La2O3. 12 The composition of the lanthanum-rich crystals growing from the side surfaces was believed to be La(OH)3 crystals that were formed in air during hydration of La2O3 from the rare-earth apatite phase, La5- Si3O12N. This could not be confirmed with XRD analysis be￾cause of the small amount of the crystals on the samples. Meadowcroft19 found that a strontium-doped lanthanum chro￾mite prepared to comprise LaCrO31x mol% SrO disintegrated at room temperature, owing to the formation of La(OH)3, which was established by XRD analysis. The strontium was believed to have displaced the lanthanum in order to dope the lanthanum chromite, and then the free La2O3 hydrated with time, where the associated expansion caused the ceramic to disintegrate. The brittle behavior and low strength of the FMLA samples com￾pared with the FMYB samples also gave us reason to believe that the ceramic disintegrated because of hydration of La2O3, available from the rare-earth apatite phase, La5Si3O12N. (2) Characterization of the Oxidation Surface Layer The surface of the FMLA sample after oxidation for 10 h at 14001C in dry air showed severe oxidation; SEM analysis of the surface of the sample showed that the oxidation layer was cov￾ered with burst bubbles and cracks. Because of the disintegra￾tion of the FMLA samples, we will not discuss the oxidation of the FMLA here in this paper but will focus on the oxidation of the FMYB. The oxidation behavior of Si3N4 ceramics with additive sys￾tems has been reported to be strongly dependent on the chemical composition of the oxidation surface layer formed by the oxidation products of the Si3N4 and the sintering additives.20 Figure 6(a) shows an SEM picture of the top view of the oxi￾dized surface of FMYB after 10 h at 14001C in dry air where an oxide scale has formed. With XEDS and X-ray diffractiometry, the oxidation product was identified to be mainly ytterbium sil￾icates (Yb2Si2O7). Large ytterbium silicates, B20 mm, were found on the BN cell boundary but small ytterbium silicates were found on the Si3N4 cells of B0.8 mm. This can be seen in Figs. 6(b) and (c), which shows a higher magnification of the ytterbium silicates on the BN boundary phase and the Si3N4 cells in Fig. 6(a); the ytterbium silicates are the white flattened crystals in Figs. 6(a)–(c). XRD was performed before and after the oxidation testing for FMYB. The XRD analysis of the un￾oxidized FMYB sample revealed b-Si3N4, BN, and Yb4Si2O7N2 peaks, and b-Si3N4, BN, Yb4Si2O7N2, and Yb2Si2O7 peaks of 0 50 100 150 200 250 0.00 0.08 0.17 0.25 0.33 0.42 0.50 0.58 Stress [MPa] Crosshead Displacement [mm] Fig. 3. Flexural response of Si3N4/BN FMs with lanthanum oxide (FMLA). Fig. 4. Scanning electron micrograph of a side view of an FMLA flex￾ural bar showing a shear-initiated fracture, between the outer and inner loading pins. Fig. 2. Scanning electron micrograph of a side view of an FMYB flexural bar showing a tensile initiated fracture. Fig. 5. (a) Lanthanum-rich crystals formed on the side surface of FMLA samples. (b) Magnification of the box in (a) of the crystals formed. May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior 1617