Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si3N4 ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si3N4 matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated beta-sialon grains and a few ball-like beta-Si3N4 grains exist, and (III) diffusion zone in Si3N4 matrix containing a thin dark layer and a similar to 25 mu m thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225 MPa, when the joints were prepared at 1600 degrees C for 30 min under a pressure of 1.5 MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000 degrees C and 1200 degrees C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10 h at 1200 degrees C, the residual strength of the joints was still 143 MPa, attributed to the existence of YbAG phase.