The Effect of Internal Stresses and Interfacial Properties on Toughening of Brittle Matrices

Abstract

<p>The role of elastic, thermal and interfacial properties in the toughening of brittle particulate composites was studied. Three different systems were designed for these studies: 1) glass - partly oxidized Ni, where the thermal expansion coefficient of the glass is lower, equal to and higher than that of Ni; 2) glass - partly oxidized Al, in which both thermal expansion coefficients and elastic moduli are equal; and 3) the LiF-MgF₂ system, where the thermal expansion coefficient of LiF is 3.4 x that of MgF₂, and the elastic moduli of the two phases is similar. The first two systems were fabricated by mixing and hot-pressing the powders. In the LiF-MgF₂ system, second-phase particles were produced via a precipitation reaction.</p> <p>It was found that when the thermal expansion coefficient and elastic modulus of the second phase particles are higher than that of the glass matrix, a crack has a tendency to bypass the particles, propagating consistently in the matrix material. When the thermal and elastic stresses are eliminated and satisfactory bonding is achieved, a toughness increase up to 60 x that of the matrix results.</p> <p>In the LiF-MgF₂ composite, a 15 - 20 x toughness increase was achieved and is attributed to the precipitation of incoherent rod-shaped MgF₂ precipitates.</p>