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D. MANESSIS *, H. DU *, I. L. SINGER **,AND R. LARKER *** * Department

of Materials Science and Engineering, Stevens Institute of Technology, Hoboken,

NJ 07030

"Chemistry NavalMaterials, ResearchLulea Laboratory, Washington, D.C 20375 "***DivisionofDivision, Engineering University of Technology, Lulea

S-97187, Sweden

ABSTRACT Silicon oxynitride (Si2N 2O) ceramics were oxidized in 1 atm dry oxygen at 11 00IC and 1300 0 C. The oxidized samples were studied using x-ray photoelectron spectroscopy and crosssectional transmission electron microscopy in conjunction with energy dispersive x-ray analysis. TEM characterization revealed the chemical abruptness of the Si0 2 and Si2N2 0 interface. Further investigation indicated the inclusions of residual Si0 2 in Si2N20, which contributed to the broad XPS elemental distribution in the oxide-substrate interface region.

INTRODUCTION Considerable technological interests exist in silicon oxynitride (Si 2N2O) ceramics because of the prospects it offers for high-temperature structural applications [1-4]. However, Si 2N 2O is thermodynamically unstable in oxidizing environments. Oxidation and associated degradation processes can significantly limit the performance and long-term reliability of Si 2N 20 components in many projected applications. Further interest in the intrinsic oxidation characteristics of Si2N20 stems from the formation of a silicon oxynitride interlayer during oxidation of Si3N 4, which has been shown to be responsible for the excellent oxidation resistance of Si3N 4 [5-7]. The role of this interlayer in the oxidation behavior of Si 3N 4 is still being debated. Documented studies of oxidation of pure Si2N20 ceramics have been focused primarily on the kinetic aspects of the oxidation process; and far less extensively on the characterization of the oxidation products [3,4,8]. Studies of oxidation of powdered Si 2N2O at 1100 0-1300 0 C attributed the oxidation weight gain to two successive processes, one controlled by interface reaction and the other by diffusion. It was also concluded that molecular oxygen diffusion in silica was the rate-limiting mechanism in the parabolic region of oxidation [4]. Another study of the oxidation kinetics of pure hot-isostaticallypressed (HIPed) Si2N20 using the thermogravimetric method at 1300 0 -1600'C indicated that a parabolic law was followed below 1350 0 C where the oxide scale was silica and above 1500'C where the oxide was cristobalite. At 1350'-1500'C where the oxide layer was partially crystallized, a non-parabolic weight gain was observed [8]. There is a dearth of information on the chemical and structural characteristics of the oxide layer formed on Si2N 20 as well as the chemical nature of the oxide-Si 2N 20 interface. This paper describes our investigation of the structural, chemical and interface nature of the oxide layers formed on hot-isostatically-pressed Si 2N 2O without sintering additives using various analytical techniques.

393 Mat. Res. Soc. Symp. Proc. Vol. 410 0 1996 Materials Research Society

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