Microstructure and strengthening mechanism of Si3N4/Invar joint brazed with TiNp-doped filler

Ag-Cu-Ti+TiNp composite filler was developed to braze Si3N4 ceramic to Invar alloy, and subsequently the effect of TiNp content on the microstructure and mechanical properties of the joints was investigated. The typical microstructure of the brazed joints is Si3N4/TiN+Ti2N+TiSi+Fe2Ti/Ag(s.s)+Cu(s.s)+TiNp/wavelike Fe2Ti+Ni3Ti/Ag-Cu eutectic/Invar alloy. With the increase of TiNp content, the reaction layers near the Si3N4 become thinner, and Ag-Cu eutectic appears less and finer between the ceramic and wavelike intermetallics. The maximum shear strength increases from 63 MPa to 229 MPa with the optimum TiNp content of 2 vol%, compared with the case without TiNp. The hardness and elastic modulus of the reaction phases were characterized by nanoindentation to reveal the strengthen of brazing seam. The following reasons are believed to strengthen the brazed joint: the thermal expansion coefficient (CTE) reduction of the composite filler, appropriate thickness of the Si3N4/braze reaction layer, the strengthen of the braze.