112 Materials Chemistry and Physics, 36 (1993) 112-118 Effects of soxhlet extraction on the surface oxide layer of silicon nitride powders Vincent A. Hackley*, Pu Sen Wang and Subhas G. Malghan Ceramics Division, National Institute of Standards and Technology, Gaithersburg MD 20899 (USA) (Received November 16, 1992; accepted March 23, 1993) Abstract An aqueous sqxhlet extraction procedure was used to surface-clean five commercial silicon nitride powders. The solid-solution interface properties were characterized before and after extraction by electrokinetic sonic amplitude measurements. The isoelectric point (pH+) was found to increase significantly for some powders after treatment. The powder surface was analyzed using X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy before and after extraction. The surface oxygen content and oxide layer thickness decrease after treatment. A linear correlation was found between oxide thickness and pHicp, which yields a pristine pHicp of about 9.7 for the unoxidized S&N4particle. Introduction High-purity submicrometer silicon nitride powders are precursors in the manufacture of advanced structural ceramics for use in high-temperature load-bearing ap- plications such as turbine rotors and bearings. The bulk properties of this material (e.g., hardness, corrosion and oxidation resistance, thermal shock resistance) are ideal for such environments [l]. However, it is the microstructure of the sintered body that ultimately determines, or limits, its property performance. The microstructure is significantly influenced by the powder consolidation step in which the green body is formed. Colloidal processing of S&N, powders necessitates the dispersion of particles in a liquid medium (usually aqueous) which depends critically on the interfacial chemistry of the powder slip. Failure to control surface reactions may result in agglomeration, which causes poor slip rheology and produces microstructural het- erogeneity in the green body. This problem is com- pounded by complex interactions between the primary phase and the various additives (sintering aids, binders and dispersants) that are typically present during pro- cessing. Recently, the ubiquitous oxide coating that forms spontaneously on the surface of nitrides and carbides has received considerable attention [2-121. Oxidation at room temperature is thermodynamically favored, and *Author to whom correspondence should be addressed. the nitride surface can therefore be considered unstable in the presence of oxygen [2]: Si,N, + 30, ---+ 3Si0, + 2N, (1) Additionally, hydrolysis may occur in the presence of water vapor or in solution (eqn. (2)), which leads to the formation of oxide and the release of NH,: S&N, + 6H,O - 3Si0, + 4NH, (2) Silicon nitride powders often have a noticeable ammonia odor when first removed from a sealed container. Much of the variability encountered in the processing of different powders or between individual batches, as well as the wide range if isoelectric points (pH+) found for commercial S&N, powders, can probably be traced back to differences in the extent of surface oxidation. Understanding the formation, structure and chemical behavior of the surface oxide is critical to understanding Si,N, surface chemistry and to controlling slip rheology and stability. Oxide surface layers on S&N, powders, apparently amorphous and varying in thickness from 3 to 5 nm, have been imaged by electron microscopy [3]. X-ray photoelectron spectroscopy (XPS) studies indicate an oxide thickness of ~0.1 to 1 nm assuming a pure SiO, layer, and = 0.3 to 3 nm assuming a Si,ON, structure [4, lo]. Analysis of the XPS binding energy for Si 2p electrons indicates an intermediate surface state between SiO, and Si20N2, and shifts in the binding energy of the N 1s peak have been interpreted as being due to surface amines [6]. Additionally, infrared ab- 0254-0584/93/%6.00 0 1993 - Elsevier Sequoia. All rights reserved