In the present paper, wetting experiments of silver, copper and indium based fillers on the silicon nitride ceramic have been investigated. The effects of active metal elements on its mechanism on silicon nitride ceramic and 40Cr steel also have been studied. The optimum filler metal and its concentration of active metal element have been proposed. The mechanical properties of brazing joints were improved by using intermediate metal layers. In order that the intermediate metal acts as a buffer metal which will yield and absorb the heat stresses, arisen from the great diffefrence of thermal expansion of 40Cr steel and Si_3N_4 the intermediate metal should have low elastic modulus and low yield strength. The brazing mechanism of joining silicon nitride ceramic to 40Cr steel was offered based on the analyses of SEM, EDX, EPMA etc. Active Ti in fillers will diffuse into the interface and react with silicon nitride to form TiN, Ti_5Si_3 and TiSi_2 to form a strong joint. The concentration of active elements in fillers and technological parameter of brazing would directly affect the strength of Si_3N_4/40Cr steel joints.
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