Strength evaluation of ceramic-metal joints

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Four-point bending tests were conducted for Si 3N 4S45C and Si 3N 4S45CSi 3N 4 joint smooth and precracked specimens. Analytical studies on joining residual stresses were also performed by the finite element method (FEM). The results obtained are as follows. 1. (1) There is a reason for the decrease in the fracture strength at the Si 3N 4S45C joint additional to the existence of the interface and the interlayer. 2. (2) Cracks are easily initiated from the inherent defect during mechanical cutting after joining. 3. (3) In the case when there is a crack and tensile residual stresses at the ceramic near the interface, the strength decreases drastically. The strength of the joint can be evaluated by the stress intensity factor using the plane strain fracture toughness of Si 3N 4. 4. (4) In the case when an indentation pre-crack is introduced at the ceramic near the interface, the pre-crack is kinked towards the interface. So, the indentation method is not suitable for introducing a pre-crack near the interface. 5. (5) The result of the two dimensional (2D) axisymmetric analysis on the joining residual stresses shows good agreement with the result of the three-dimensional (3D) analysis and the peak stress can be approximately estimated by superposing results of the two 2D plane stress analyses. 6. (6) The residual stresses redistribute after cutting of the Si 3N 4S45C joint and the peak tensile stress occurs on the new free surface at the ceramic near the interface. It is emphasized that the residual stress at a same point decreases after cutting, but the maximum value does not change. The effect is almost the same as that of changing the size and aspect ratio of the joint specimen. 7. (7) Residual stresses at the ceramic near the interface were measured non-destructively by the X-ray method for the Si 3N 4S45C joints before and after cuttings. The local residual stress distribution near the interface can be measured non-destructively by the PSPC/RSF system used in this study. The residual stresses redistribute because of the cutting. The distribution characteristics do not change, although the absolute value of the residual stresses at the same position decreases. The comparison between the results of the X-ray measurement and the results of the FEM analysis shows good agreement qualitatively. 8. (8) The singularity of residual stresses at the ceramic near the interface is characterized by elastic-plastic properties of the ceramic and insert materials. The exponent of the elastic-plastic singularity is larger than that of elastic singularity. 9. (9) The decrease in the fracture strength for the Si 3N 4S45C joints compared with those for the Si 3N 4Si 3N 4 joints is explained by considering the singular stress field of residual and applied stresses at the ceramic near the interface of the joint.