The Nature and Composition of Secondary Phases in α/β-SiAlON Ceramics Sintered with Yb2O3 Addition by Using TEM Techniques

Servet Turan1, Hilmi Yurdakul1

1Department of Materials Science and Engineering, Anadolu University, Eskisehir 26555, Turkey

SiAlON ceramics have attractive properties for high temperature and wear resistant applications, but are difficult to densify without densification aids. Although the densification aids form liquid phases at the sintering temperature and enable full densities to be obtained, they remain at triple junctions as amorphous or crystalline phases [1]. SiAlON pellets were prepared by using Yb2O3 sintering additives and then sintered by gas pressure sintering (GPS) at 1800oC for an hour under 22 bar nitrogen pressure. The aim of this study is to determine whether the composition of secondary phases is crystalline or amorphous and whether secondary phases are rich in oxygen or nitrogen since the high temperature properties of SiAlON ceramics are affected by the nature and composition of the secondary phases. In this research, electron transparent samples for analytical transmission electron microscopy (TEM) investigations were prepared by cutting, polishing, dimpling and finally ion beam thinning (Baltec RES 101). The prepared samples after coating (Baltec MED 020) with a thin carbon film were characterised to explain secondary phase nature and compositions by using 200 kV field emission transmission electron microscope (JEOL 2100F) attached with an energy filter (GATAN GIF TRIDIEM), parallel electron energy loss spectrometer (PEELS), a high angle annular dark field scanning transmission electron microscope (STEM-HAADF) detector and an energy dispersive x-ray (EDX) spectrometer (JEOL JED-2300T). The STEM-HAADF image in figure 1 (a) is showing the general microstructure of α/β-SiAlON ceramics. In this figure, black regions are corresponding to β-SiAlON grains which do not contain any sintering additives while grey and white regions were respectively α-SiAlON and secondary phases containing different amount of sintering additives confirming the EDX analysis (not given here). From the TEM image shown in figure 1 (b), it can be seen that the secondary phases are crystalline which is confirmed by XRD experiments (not given here). EFTEM-3 window elemental maps were obtained from the corresponding energy loss values of Al L2,3 (73 eV), Si L2,3 (99 eV), Yb N4,5 (185 eV), N K (401 eV) and O K (532 eV) (figure 2). The intensities of silicon and nitrogen elements in secondary phases were much less than the amount in α and β SiAlON grains whereas the intensities of aluminium, oxygen and ytterbium were very high in secondary phases. Therefore, it could be said that the composition of secondary phases was rich in oxygen, aluminium and ytterbium but does not contain silicon and nitrogen. However, silicon K (1839 eV) and nitrogen K (401eV) edges were observed in the PEELS spectra collected from secondary phases (figure 3 (a-b)). From the PEELS, EFTEM, STEM-EDX and XRD results, it can be concluded that the secondary phases formed after sintering in Yb2O3 added SiAlON are in crystalline nature and their chemical composition consist of oxygen, aluminium, ytterbium and a small amount of silicon and nitrogen. These crystalline secondary phases are solid solution of J phase (Jss=Yb4Si2-xAlxO7+xN2-x) formed towards the oxygen rich region on the line between Yb4Si2 Al7N2 and Yb4 Al2O9 phases at phase diagram in Yb-Si-Al-O-N system [2]. References [1] S. Turan et al., Materials Science Forum 383 (2002) 37. [2] Z.K. Huang et al., Ibid. 79 (1996) 2091.

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AMTC Letters Vol. 1 (2008)

© 2008 Japan Fine Ceramics Center

FIG 1. (a) STEM-HAADF, (b) TEM images showing α-SiAlON, β-SiAlON grains and crystalline secondary phases (SP)

FIG 2. The EFTEM-3 window elemental map results

FIG 3. The PEELS spectra of nitrogen K edge and silicon K edge collected from secondary phases in α/β-SiAlON ceramics sintered with Yb2O3 addition.

73 eV

N K Yb N4,5

0 eV

185 eV 401 eV O K

Al L2,3 Si L2,3 99 eV

Zero Loss

N K edge Si K edge a-) b-)

a-) b-)

SP

SP 200 nm

β α

10 nm

β

α SP

532 eV

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AMTC Letters Vol. 1 (2008)

© 2008 Japan Fine Ceramics Center