Characterization of the Si3N4/42CrMo joints vacuum brazed with Pd modified filler alloy for high temperature application

• The Si3N4 ceramic and 42CrMo steel was successfully joined by using Pd modified filler alloy.
• The maximal joint strength of 352.3 MPa was obtained while Pd modified Ag–Cu–Ti filler alloy was used.
• The bend joint strength could maintain 216.56 MPa while the testing temperature was 500 °C.
• The bonding mechanism was clarified in the research by TEM analysis.
• The mechanism of oxidation resistance for the joint was unveiled.

The Si3N4 ceramic were brazed to 42CrMo steel by using Ag54.67–Cu35.96–Ti4.37–Pd5 (at.%) filler alloy. Effect of brazing temperature and dwelling time on the microstructure and mechanical properties of the joints were investigated. The results show that a double reaction layer which is composed of TiN and Ti5Si3 was formed at the Si3N4 ceramic/filler alloy interface. A continuous reaction layer consisting of TiC was also detected adjoining to 42CrMo steel. The central part of the joint was made up of Ag–Cu eutectic and CuPdTi reaction phases. With increasing brazing temperature, the thickness of brazing layer was gradually decreasing. Besides, partial stripy Cu[Ag] solid solution was transformed into blocky Cu[Ag, Pd] solid solution. The interfacial reaction layer was not affected with prolonging the dwelling time. However, more CuPdTi reaction phases were precipitated. The maximal bending strength of 352.3 MPa was obtained when the joint was brazed at 920 °C for 30 min. The bending strength of the joint could still maintain 216.56 MPa while the testing temperature was 500 °C. Pd modified filler alloy could significantly improve the joint high temperature properties since Pd element altered the phase composition and reduced the probability of oxidation at high temperature.