Residual Stress Measurement of Near Interface Region in Silicon Nitride and Stainless Steel Brazing Joint
Nobuyuki FUJII, Kinichi KANNO, Shigeru KOZAKI
High residual stresses are known to exist in the ceramic part near the ceramic-metal joint. However, in carrying out their measurement, we sometimes find it very difficult to get a tendency in stress distribution. One of the reasons for the difficulty seems likely to be the presence of residual stresses produced when ceramic material is sintered. These initial stresses are considered to affect the residual stresses caused by brazing.
In this study we measured both residual stresses before and after brazing and then examined if the measured stress values are Proper as such. For this purpose, we compared the result of computer simulation, that of a 4-point bending test, and the measured stress values with one another.
As a result, we found out the following. (1) Over 180 MPa of residual stress was measured in the surface layer of the as-sintered silicon nitride. The area of the stress concentration was as small as below 1.0 mm in diameter. (2) The stress distribution obtained by subtracting the initial stresses before brazing from the residual stresses measured after brazing agreed with the simulation result qualitatively. (3) The tensile residual stress measured with a micro X-ray beam was about 270 MPa at maximum at a point 0.2 mm away from the interface. The strength of the silicon nitride decreased by 335 MPa due to brazing. (4) The cracking position roughly agreed with the point where the maximum principal stress was measured. The direction of cracking also agreed with the normal to the direction of the maximum principal stress.