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Item type:Publication, Access status: without files , Mechanisms of toughening in silicon nitrides : the roles of crack bridging and microstructure(Elsevier, 2011-06-01); ; ; Schwind, ThomasCrack-bridging mechanisms can provide substantial increases in toughness coupled with strength in ceramics. Herein, we describe the various bridging mechanisms, their toughening contributions and how they are affected by microstructure in silicon nitride ceramics, which are a classic example where both high strength and toughness are achieved. Crack growth resistance curves (R-curves) for seven different silicon nitrides doped with various metal oxides and with different microstructures were measured, and bridging stress distributions were calculated for each. Based on an analysis of those results combined with results of the relative interfacial toughness of two of the ceramics, a new mechanistic theory for the evolution of the R-curve is proposed. The initial steep rise in toughness is attributed to the formation of elastic bridges that experience no debonding. This mechanism has not previously been recognized in the literature and the high strength of these materials (up to 1 GPa) is here attributed primarily to that mechanism. As those bridges begin to fracture and the mechanism becomes saturated, a change in the R-curve slope is observed and the more traditional mechanisms of partially debonded elastic and fully debonded frictional bridges dominate the continuing rise in toughness. Furthermore, the saturation of each mechanism is associated with a change in slope of the R-curve. The results of this study provide a fundamental insight into how to optimize silicon nitride microstructures for high strength and toughness.Publicationtype: Journal Article10 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Crack-tip toughness from vickers crack-tip opening displacements for materials with strongly rising R-curves(Wiley-Blackwell, 2011); ; ; Accurate measurements of the intrinsic crack-tip toughness, KI0, are essential to understanding the strength and toughness of bridging ceramics. Furthermore, even the most carefully made R-curve measurements cannot accurately assess this initial toughness point. In this manuscript, both rigorous and approximate methods are proposed for determining KI0 from crack-tip opening displacement (COD) measurements on Vickers indent cracks for materials with steeply rising R-curves. Such approaches are attractive because of the relative ease of producing indentation cracks and analyzing the CODs; further, the latter method is of particular interest because it saves considerable computational effort. Both the advantages and limitations of the proposed methods are discussed. Finally, based on applying the above methods to three Si3N4 ceramics, it was concluded that there is a common crack-tip toughness of KI0≈2.2±0.3 MPa·√m. © 2011 The American Ceramic Society.Publicationtype: Journal Article5