Measurement of Residual Thermal Stress and Its Distribution on Silicon Nitride Ceramics Joined to Metals with Scanning Acoustic Microscopy

Residual thermal stress on the ceramics joined to metals was determined with a reflection-type acoustic microscope. This report is concerned with an apparatus for measuring surface acoustic wave (SAW) propagation velocity and an analytical method for determining residual thermal stress from the change in SAW velocity as well as the results obtained for the silicon nitride ceramics joined to stainless steel with Ni/W/Ni interlayers.The maximum stress in the vicinity of the ceramic/metal interface, which was tension in nature, was estimated to be 400, 200, and 600 MPa for joints with 0.5 mm Ni (A), 1.25 mm Ni (B), and 2.0 mm Ni (C) interlayers. The stress decreased gradually with the distance from the ceramic/metal interface for the joint C, whereas for the joint A the stress profile was complex, showing a small compressive stress at a distance of 0.7 mm and a maximum tensile stress between 1 and 2 mm. For the joint B, the tensile stress was relatively low all over the ceramics. A four-point bending test at room temperature showed that joints A and C had a relatively low fracture strength, while the joint B had the highest strength of 230 MPa, because of low residual thermal stress.It is proposed that the scanning acoustic microscope shows considerable promise for the non-destructive evaluation of the stress and stress distribution at the ceramic surface of ceramic/metal joints.