Evaluation of Si3N4 joints: bond strength and microstructure

Joining of pressurelessly sintered silicon nitride ceramics was carried out using adhesive slurries in the system Y-Si-Al-O-N in a nitriding atmospher e. The effects of bonding parameters, such as joining temperature (1450-165 0 degrees C), applied pressure (0-5 MPa) and holding time (10-60 min), on t he bond strength of joint were evaluated. A typical microstructure of the j oint bonded with the optimum adhesive was investigated. The three point ben d testing of joined samples with 3 x 4 x 36 mm(3) in dimension was employed to study the bond strength of joints. The results show that an optimum joi ning process was achieved by holding at 1600 degrees C for 30 min under an external pressure of 5 MPa and the maximum bond strength was 550 MPa, compa red to 700 MPa of unbonded Si3N4 ceramic, using the adhesive having the Si3 N4/Y2O3 + SiO2 + Al2O3) ratio of 0.39. The good bond strength is attributed to the similarity in microstructure and chemical composition between joint zone and ceramic substrate. The fracture modes were classified into two ty pes according to the values of bond strength. (C) 1999 Kluwer Academic Publ ishers.