Enhancement of thermal shock resistance in β-Si3N4 coating with in situ synthesized β-Si3N4 nanowires/nanobelts on porous Si3N4 ceramics

A dense beta-Si3N4 coating toughened by beta-Si3N4 nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si3N4 ceramics in this work. A porous beta-Si3N4 nanowires/nanobelts layer was synthesized in situ on porous Si3N4 ceramics by chemical vapor deposition, and then Y-Si-Al-O-N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion beta-Si3N4 nanowires/nanobelts, fine beta-Si3N4 particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 +/- 0.05 MPa m(1/2), and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at Delta T = 1200 degrees C, the coated porous Si3N4 ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment.