1618 Journal of the American Ceramic Society-Karlsdottir and halloran Vol. 89. No 5 如=签 g,m,包m Fig. 6. (a) Oxidized surface of FMYB after 10 h at 1400.C in dry air. Large ytterbium silicates are apparent on the bn cell boundary phase;(b) magnification of the Bn boundary phase in(a) showing large ytterbium silicates; and(c)magnification of a Si3 N4 cell showing small ytterbium silicates on the surface Machined surface before oxidation used as sintering additives in the Si3 N4 phase the Sio, forms an SiaN O I\b2 For Sina with Y2O3 as a sintering additive,the oxide layer has been reported to consist of yttrium silicates (Y2Si2O,), which is the most stable compound in the Y-ShO ·Yb4S2O7N2 system21-23 Yb Si, O, skin has been reported to form on Si Na containing Yb2O3 as a sintering additive by a controlled oxida- tion process associated with the reaction between the Sio2 and Yb2O3. Yb2Si2O, has also been reported as one of the main Si3Na-SiC with Yb O3 as a sintering additive. nanocomposite oxidation products formed on the surface of Here, we have a 3D composite consisting of two different ce- ramic materials: Si3 Ng cells aligned in the uniaxial direction and Oxidized surface a bn cell boundary phase separating the Si3N4 cells. Si3N4 with Y2O3 and AlO3 as sintering additives have been shown oxidize at 1100%-1200C,.but bn starts oxidizing at lower temper atures; generally, the onset of measurable oxidation is about e|·Yb4s2OnN2 800C, but tends to decrease with higher oxygen impurity levels within the BN. At 800C the bn starts to oxidize into a liquid oxide(B,) by th tion reaction 2BN(s)+O2(g)=B2O3()+N2(g) The B,O3 liquid oxide is believed to start volatilizing at diffraction pattern of the surface of the FMY B before(a) temperatures above 1100.C The bn cell boundary phase in the FMYB sample consists of n grains surrounded by a glassy phase that migrates into the BI boundary phase during hot pressing. This can be seen from the oxidized FMYB sample; see Fig. 7. By comparing the two Fig 8, where the bright white spots represent the glassy phase diffraction patterns, it is evident that ther s considerable between the bn platelets, while the gray phase is the bn plate- formation of ytterbium silicates(Yb Si,O,)during the oxidation lets. After the oxidation and a decrease in the bn phase after the oxidation; see Fig. 7. an -100 um recess at the BN cell boundary while there was an It is well known that during oxidation of Si3 N4, a silica(SiO2) 4 um oxide layer on Si3N4 cells, and the oxidation zone was film is formed on the surface of the Si3 N4. If there are any oxides uniformly distributed around the surface; see Figs 9 and 10. 200 Fig 8. (a) Microstructure of the FMYB sample consisting of BN cell boundary phases and Si, N4 cells. (b) Magnification of the BN cell boundary, that consists of Bn grains surrounded by a glassy phase that migrated into the boundary phase during hot pressingthe oxidized FMYB sample; see Fig. 7. By comparing the two diffraction patterns, it is evident that there was considerable formation of ytterbium silicates (Yb2Si2O7) during the oxidation and a decrease in the BN phase after the oxidation; see Fig. 7. It is well known that during oxidation of Si3N4, a silica (SiO2) film is formed on the surface of the Si3N4. If there are any oxides used as sintering additives in the Si3N4 phase the SiO2 forms an oxide layer on the surface after reacting with the sintering ad￾ditives.14,16,21 For Si3N4 with Y2O3 as a sintering additive, the oxide layer has been reported to consist of yttrium silicates (Y2Si2O7), which is the most stable compound in the Y–Si–O system.21–23 Yb2Si2O7 skin has been reported to form on Si3N4 containing Yb2O3 as a sintering additive by a controlled oxida￾tion process associated with the reaction between the SiO2 and Yb2O3. 16 Yb2Si2O7 has also been reported as one of the main oxidation products formed on the surface of nanocomposite Si3N4–SiC with Yb2O3 as a sintering additive.17 Here, we have a 3D composite consisting of two different ce￾ramic materials: Si3N4 cells aligned in the uniaxial direction and a BN cell boundary phase separating the Si3N4 cells. Si3N4 with Y2O3 and Al2O3 as sintering additives have been shown oxidize at 11001–12001C,21,23 but BN starts oxidizing at lower temper￾atures; generally, the onset of measurable oxidation is about 8001C, but tends to decrease with higher oxygen impurity levels within the BN. At 8001C the BN starts to oxidize into a liquid oxide (B2O3) by the oxidation reaction24,25: 2BNðsÞ þ 3 2 O2ðgÞ ¼ B2O3ðlÞ þ N2ðgÞ The B2O3 liquid oxide is believed to start volatilizing at temperatures above 11001C.26 The BN cell boundary phase in the FMYB sample consists of BN grains surrounded by a glassy phase that migrates into the boundary phase during hot pressing. This can be seen from Fig. 8, where the bright white spots represent the glassy phase between the BN platelets, while the gray phase is the BN plate￾lets. After the oxidation testing on the FMYB sample, there was an B100 mm recess at the BN cell boundary while there was an B4 mm oxide layer on Si3N4 cells, and the oxidation zone was uniformly distributed around the surface; see Figs 9 and 10. 10 15 20 25 30 35 40 BN Intensity [Arb.unit] Intensity [Arb.unit] Oxidized surface 2θ 10 15 20 25 30 35 40 2θ Machined surface before oxidation Si3N4 BN Yb4Si2O7N2 Yb4Si2O7N2 Yb2Si2O7 Si3N4 (a) (b) Fig. 7. X-ray diffraction pattern of the surface of the FMYB before (a) and after (b) oxidation. Fig. 6. (a) Oxidized surface of FMYB after 10 h at 14001C in dry air. Large ytterbium silicates are apparent on the BN cell boundary phase; (b) magnification of the BN boundary phase in (a) showing large ytterbium silicates; and (c) magnification of a Si3N4 cell showing small ytterbium silicates on the surface. Fig. 8. (a) Microstructure of the FMYB sample consisting of BN cell boundary phases and Si3N4 cells. (b) Magnification of the BN cell boundary, that consists of BN grains surrounded by a glassy phase that migrated into the boundary phase during hot pressing. 1618 Journal of the American Ceramic Society—Karlsdottir and Halloran Vol. 89, No. 5