Effect of sulfuric acid solution on fracture strength of silicon nitride ceramics/nickel joints

The nickel metal joined to silicon nitride ceramics using silver-copper-tit anium brazing and titanium filler was placed in a sulfuric acid solution at 323 K for up to 214h. Joining strengths were measured by the 4-point bendi ng test. Microstructures of the reaction layer and porous layer were invest igated with a scanning electron microscopy and an electron probe microanaly zer, Fracture strength of the normal joint was 245 MPa before soaking and d ecreased to 170 MPa after 16h and to 100 MPa after 144h in 1N H2SO4. Three regions were observed on the fracture surface: the region I at the reaction layer/brazing layer interface, the region II at the bonding interface neig hborhood in porous layers (in silicon nitride) and the region III at the si licon nitride/reaction layer interface, In the porous layer of the silicon nitride was formed yttrium-oxide and aluminium-oxide elute by the soaking t he sintering aid in the grain boundary of silicon nitride. The growth of th e porous layers occurred on the joint surface of silicon nitride ceramics w ith different roughness due to the ground surface roughness of the test pie ce. The wedge-shaped porous layers were formed a residual stress on the int erfacial side of the silicon nitride ceramics joints.