Control of strength and toughness of ceramic/metal laminates using interface design
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The fracture strength and toughness of alumina can be increased by lamination with strategically placed nickel layers and by controlling the geometry of the interfaces. This paper describes the interface design of a ceramic/metal bonded system produced by changing the surface topography of the interface between the metal and ceramic layers in order to vary the strength of bonding. The tortuosity of the interface is described quantitatively using fractal geometry. Experiments and models of single ductile layer laminates show that the work of fracture of ductile layers which contribute to the increment of toughness of ceramic/metal laminates is dependent on the tortuosity of the interface. The more tortuous the interface, the stronger the laminate; the smoother the interface, the tougher the laminate. The results are used to design a ceramic/metal multilayer composite. The strength and toughness of the laminates can be controlled by the tortuosity of the interface and characterized using the fractal dimension.
I. INTRODUCTION Ceramics can be toughened and strengthened by bonding them with ductile lamina.1 The main mechanisms2 of toughening are ductile layer bridging of the advancing crack and plastic deformation of the metal layers during crack propagation. The work of fracture, which is mainly dependent on the plasticity of the ductile lamina, contributes to the increment of toughness of the ceramic. The residual compressive stress which is caused by a mismatch of thermal expansion in the ceramic/metal system contributes to the incremental strengthening of the ceramic. The amount of the increase is dependent on the specific ceramic/metal system and the bonding condition. It is important to study the plastic behavior of the ductile layers in these systems because, for most ductile materials, the plastic behavior varies with stress state. According to the von Mises criterion, plastic yielding in a multiaxially loaded sample is related to combined stresses by 3 :
ayf
4+
~crzf I 1/2
+
(1)
where cre is the equivalent stress which is determined by the tensile and shear stresses, ax, ay, az, Txy, Tyz, and TZX, acting on three orthogonal planes in a Cartesian coordinate system. When the applied stress reaches the yield stress, cr0 (ae = a0), yielding is predicted to occur. Therefore, the plasticity of the ductile layers which are bonded with brittle solids in the laminated composites is different from that of ductile materials 2362
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 9, Sep 1993
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which are in an unbonded state because the bonded layers are constrained by their neighbors (highly stiff brittle layers). This constrained state causes the ductile layers to be in a combined stress state rather than a uniaxial (unconstrained) state when a uniaxial tensile load is applied to the specimen parallel to the nickel layer direction. Several studies showed that the difference between the work of fracture and the strength of ductile materials depends on whether the ductile materials a
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