Relation between Strength, Microstructure, and Grain‐Bridging Characteristics in In Situ Reinforced Silicon Nitride

Abstract

The grain size of in situ Si,N, is varied, and its effects on strength-flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging-zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress (p*) and length (Db) at the moment of the critical fracture. The results show that as grain size increases, p* decreases while D, and the critical bridging zone opening (u*) first increase and then decrease, resulting in a maximum for short-crack fracture toughness at an intermediate grain size. The initial increase of u* and D, with grain size is attributed to an increase in debonding length, while the decrease of p* is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes D, and u* to drop in the large-grain regime. Implications on microstructure design are discussed.

Microstructure and mechanical properties of silicon nitride ceramics prepared by pressureless sintering with MgO–Al2O3–SiO2 as sintering additive

“…Since a larger grain is associated with a higher residual stress in the surrounding glass, it is spontaneously believed that stress-induced microcracking predominantly occurs in exaggerated-grains-containing materials than in uniform grains material. [30][31] Therefore, the differences in the mechanical properties of Si 3 N 4 ceramics produced by different processing seem to arise partially from the changes in the homogenization of the grain size and their distribution throughout the bodies. Besides, the improvement of the sinterability of Si 3 N 4 with the improved homogeneity of the sintering additives distributed over the matrix and the resultant higher relative density are quite easily suggested partially responsible for the improvement of the mechanical properties.…”

Ceramics and ceramic coatings in orthopaedics

“…Si 3 N 4 's elongated grain structure can influence its Weibull modulus. Increases in size and aspect ratio of grains result in higher fracture toughness and strength due to R-curve behavior [281][282][283]. Without acicular growth, the mechanical properties of equiaxed Si 3 N 4 are similar to oxide ceramics [284,285].…”

The microstructure and boundary phases of in-situ reinforced silicon nitride

“…Under compressive loads, the acicular grains interlock, resulting in increased creep resistance by inhibiting grain-boundary sliding. Under tensile stresses, in addition to limiting grain-boundary sliding, the elongated grains improve the stress rupture properties by bridging the microcracks [3].…”