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Microstructure and Fracture Toughness of Si 3 N 4 Ceramics: Combined Roles of Grain Morphology and Secondary Phase Chemistry
Author(s) -
Kleebe HansJoachim,
Pezzotti Giuseppe,
Ziegler Günter
Publication year - 1999
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1999.tb02009.x
Subject(s) - fracture toughness , materials science , microstructure , sintering , composite material , transgranular fracture , toughness , grain growth , grain boundary , grain size , ceramic , ostwald ripening , ultimate tensile strength , intergranular fracture , nanotechnology
Silicon nitride materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si 3 N 4 materials was evaluated. With a larger grain diameter of the Si 3 N 4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary‐phase chemistry. Si 3 N 4 grain growth was dominated by diffusion‐controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack‐propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si 3 N 4 matrix grains. Therefore, secondary‐phase chemistry plays a dominant role with respect to the mechanical behavior of liquid‐phase‐sintered Si 3 N 4 . Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary‐phase crystallization at triple pockets, and (iii) changing the Si 3 N 4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary‐phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.