Novel design of a Si3N4/BaO-Al2O3-SiO2 coating with a heterogeneous- layer structure on porous Si3N4 ceramic

During the preparation of coatings on porous Si3N4 ceramics, stress cracks often occur due to the sintering mismatch between the coating and substrate. To solve this problem, in this work, a Si3N4/BaO-Al2O3-SiO2 (BAS) coating with a heterogeneous-layer structure was designed and prepared, and the problem of sintering mismatch was effectively solved by suppressing the horizontal shrinkage of the coating. First, to verify the feasibility of the design, thin slices were prepared by alternately stacking the Si3N4 and BAS layers. The results showed that the thin slices prepared at 1550 degrees C had a dense and uniform morphology. During sintering, the liquid phase formed in the BAS layers infiltrated into the adjacent Si3N4 layers that can not only promote the densification but suppress the horizontal shrinkage of the Si3N4 layers. The largest vertical shrinkage rate of thin slices was 47.30% while the horizontal shrinkage rates were less than 2%. Based on these results, the coating with a heterogeneous-layer structure was prepared by alternately spraying Si3N4 and BAS slurries on porous Si3N4. The strength of interfacial bonding between the coating and substrate was 16.60 MPa, and the water absorption and Vickers hardness were 4.07% and 3.02 GPa, respectively. Conversely, the coating with a homogeneous structure prepared by the same spraying parameters displayed numerous macroscopic cracks. This study provides a novel design for the structure of the coating and promotes the application of porous Si3N4 ceramics in extreme environments.