IAM - Ceramic Materials and Technologies

Mechanisms of toughening in silicon nitrides: The roles of crack bridging and microstructure

  • Author:

    Fünfschilling, S; Fett, T; Hoffmann, MJ; Oberacker, R; Schwind, T; Wippler, J; Böhlke, T; Özcoban, H; Schneider, GA; Becher, PF; Kruzic, JJ

  • Source:

    ACTA MATERIALIA, 2011, Band 59, S. 3978–3989

Abstract

Crack-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.