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The mechanical properties of Si3N, based ceramic bodies are largely controlled by the phases formed on the grain boundaries during sintering. For this reason determination of the state and the chemical composition of the surface of the starting Si,N, powder is of prime theoretical and practical importance, especially in the cases when advanced ultrafine powders are used. In this work an ultrafine Si,N, powder, obtained by high temperature plasma nitridation of silicon, has been characterized by XPS. Oxygen, carbon and also a small amount of potassium were detected as surface impurities. The Si 2p and Si (KLL) spectral lines could be decomposed into two components corresponding to silicon in Si,N, and SiO, phases. From angle-resolved and Ar-ion depth-profiling experiments a layer model, consisting of a Si3N, core covered by a relatively thick SiO, layer and a carbon contaminant overlayer, could be elucidated.

Structure of Inorganic Phosphorus-Nitrogen Tetrahedral Compounds-(39 refs.). -(VITOLA, A.; RONIS, J.; AVOTINS, V.; MILLERS, T.; Latv.

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The hot press sintering of ultrafine Al2O3-TiN and Al2O3-TiN composites was studied. Initial materials were prepared by the hydrolysis of ultrafine AIN-TiN composite powder with the followed preheating at 450 in vacuum and by mixing Al2O3 and TiN pow ders, all made by the RF plasma chemical synthesis. The hot pressed samples were dense with no observa ble open porosity. Even when sintered at a tempera ture as low as 1500, the composites exhibited good strength, hardness and fracture toughness. The highest bending strength values were 48130MPa for the hydrolyzed and 47260MPa for the mixed compo sites. Both composites differ by phases content and changes in lattice parameters of Al2O3 and TiN. This is due to the different water content in initial aluminas used, Al2O3 and Al2O3, which resulted in different oxidation states of Ti for the mixed and hydrolyzed composites. The enlargement of the Al2O3 and TiN unit cells caused by the dissolution of Ti2O3 and TiO, results in the lattice softening which together with the other characteristics, is an important factor governing the mechanical properties of sintered composites.