Fabrication, mechanical properties and thermal shock resistance of a dense SiC NWs/α-Si3N4 composite coating for protecting porous Si3N4 ceramics

To prevent porous Si3N4 ceramics from absorbing moisture in practical application, a dense SiC NWs/alpha-Si3N4 composite coating was fabricated using the two-step technology involving slurry and liquid infiltration method. The results revealed that the SiC NWs not only can promote densification sintering of the coating but also can reduce thickness of the coating effectively due to the block effect of the SiC NWs. Furthermore, the SiC NWs in the coating had a good interfacial bonding with alpha-Si3N4 matrix, meanwhile the toughening effects of the SiC NWs involving crack deflection, crack branching, SiC NWs bridging and pullout, also presented during mechanical damage. The strengthening and toughening mechanisms of the SiC NWs not only enhanced mechanical properties of the coating but also raised significantly critical crack size (L), thereby which made the coating have a good ability for thermal shock resistance. After thermal shock at Delta T = 1000 degrees C for 15 times, water absorption of the coated porous Si3N4 ceramics increased only to 7.21% from 4.54%, and its residual strength ratio was up to 83.4%. More importantly, the coating exhibited an ability for crack healing during thermal shock testing. All those above results indicated that as-prepared SiC NWs/alpha-Si3N4 composite coating can meet the applied requirements in long-term extreme conditions.