Evaluation of Weak Interface Effect on the Residual Stresses in Layered SiC/TiC Composites by the Finite Element Method
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Jining Qin State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 1954 Hua Shan Road, Shanghai 200030, People’s Republic of China (Received 14 May 2001; accepted 18 February 2002)
A symmetrically layered SiC/TiC ceramic with a gradual structure was designed by the finite element method (FEM). After sintering, proper thermal residual stress was introduced into the ceramic due to the coefficients of thermal expansion mismatch between the different layers. After different SiC + C interlayers were inserted into the layers to weaken the interface, the effect of the composition of the SiC + C interlayers between the layers on the residual stress was evaluated. It was found that the weak SiC + C interlayer had little relaxation effect on the residual stress distribution. These ceramics were then fabricated by aqueous tape casting, stacking, and hot-press sintering. An x-ray stress analyzer was used to test the surface stress conditions of the sintered materials. The tested surface stress of the layered SiC/TiC ceramic without interlayer was very close to the FEM calculation. However, there were differences between the tested and calculated results of the layered SiC/TiC ceramics with interlayers; the reason for this was analyzed.
I. INTRODUCTION
In the past decade, layered ceramics have been studied extensively because they can show dramatically improved strength, fracture toughness, and damage and fatigue resistance compared with monolithic ceramics.1–6 Layered ceramics can be divided into two categories. One is surface-compression-strengthened layered ceramics with strong interface bonding, such as the ZrO2/ Al2O3 system.7,8 The other is laminated ceramics with weak interface bonding, such as SiC/C and Si3N4/BN systems.9–11 The former is strengthened or toughened by the introduction of surface compressive stress due to coefficient of thermal expansion (CTE) mismatch between different layers. The latter is toughened by crack propagation along the weak interface between layers. If both the above strengthening and toughening mechanisms could synergetically work together, such a ceramic body could possibly show even better properties. However, few studies have been done along this approach. It is evident that when a pure bending load is applied to a structure with a rectangular cross section, the stress condition varies linearly from maximum tension on the surface to zero stress on the central plane.12 Thus, we 1118
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J. Mater. Res., Vol. 17, No. 5, May 2002 Downloaded: 17 Mar 2015
believe that thermal residual stress in a symmetrically layered ceramic should be designed to vary gradually from surface compressive stress to inner tensile stress. Therefore, such a structure should have gradual compositions from the surface to the center. In such a case, the finite element method (FEM) is proper for designing the structure because FEM is a powerful tool for calculating residual stress distribution in composites.13,14 Furthermore, FEM is also suitable for evaluating the e
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