Joining Technology And Mechanism Of Porous Si₃N₄ Ceramic And Dense Si₃N₄ Ceramic Joints

Porous Si3N4 is a structural and functional ceramic material with a large number of connected or closed holes. By means of its low dielectric, anti-oxidation, thermal shock performance, it has been widely used as aerospace wave-transparent material. Porous Si3N4 was joined to densified Si3N4 using RE2O3(RE = Yb, Y or Nd)-Al2O3-Si O2-Si3N4 mixed powders as solder by in-situ forming β-sialon/glass interlayer. The influence of type of rare-earth in the solder and process parameters on the microstructure and properties of joints was studied. The forming process of phases in the seam and the joining mechanism were analysed and discussed. The high-temperature damage behavior of joints were evaluated by studying mechanical properties, anti-oxidation and thermal shock properties at high-temperature.Results indiated that the four-point bending strength of joints could reach the peak value of 88 MPa and 158 MPa, respectively, when using Yb(and Nd)-β/glass composite solder at 1600?C for 30 min under 0.8MPa joining pressur.. When using Y-β/glass composite solder, the maximum bending strength of 103 MPa could be obtained at 1650℃ for 30 min. Increasing holding time or joining temperature, can promote the production β-Sialon phase in the joint, but it also causeed infiltration of much liquid glass into the porous Si3N4 ceramics. However, decreasing the joining temperature, holding time or joining pressure resulted in the densification of seam decreasing, and thus the strength of joints decreased, although the infiltration of liquid glass into porous Si3N4 ceramics.When Joing was executed at lower temperature of 1500℃ for 15 min under the pressure of 0.8MPa followed by a pressless heat-treatment at 1550℃ for 30 min, the infiltration of glass phase into the porous Si3N4 ceramics side can be released. Compared with one-step joining, the strength of joints decreased. When the heat-treatment temperature was increased to 1600℃, the strength of the joint can be improved, but the glass infiltration was aggravated again.The high-temperature properties of joints prepared at 1600℃ for 30 min under the pressure of 0.8MPa were investigated. The high-temperature flexural strength of the joint using Yb(Y or Nd)-β/glass composite solder is 22 MPa, 52 MPa, 81 MPa, respectively, at 1000?C. The remained strength of joints. No obvious oxidation layer was observed on the surfaces of joints after 12 h oxidation at 1200?C, but some cracks appeared on the oxidized surfaces. Compared with joints joined using Y and Yb-β/glass solder, the joints using Nd-β/glass solder presented the maximum residual intensity of 14 MPa after 12 h oxidation, together with 13 MPa after 8 times thermal shock at ?T=1170℃.