Effect of joining conditions on microstructure and flexural strength of long silicon nitride pipes fabricated by local heat-joining technique

The microstructure and flexural strength of long silicon nitride pipes joined by a local heat-joining technique at different mechanical pressures, temperatures, and holding times were examined. Silicon nitride pipes sintered with Y2O3 and Al2O3 additives were used as the parent material, and a powder slurry of a Si3N4–Y2O3–Al2O3–SiO2 mixture was brush-coated on the rough or uneven end faces of the pipes. The flexural strength gradually decreased with decreasing mechanical pressure from 5 to 1.4 MPa at 1600 °C for 1 h. A large number of small voids measuring less than 1 μm were present in the joint layers of the samples joined with mechanical pressures of 1.4 and 3 MPa at 1600 °C for 1 h, although there were very few voids formed at 5 MPa. When the joining temperature was increased to 1700 °C for 1 h, some voids were observed in both the joint layer and the parent silicon nitride, and the flexural strength decreased to 529 MPa. Furthermore, some voids were formed in the parent silicon nitride when the holding time was increased from 1 to 4 h at 1600 °C and 5 MPa, which indicated slight degradation of the strength from 677 to 644 MPa.