Effect of extreme service temperatures on microstructure and strength of additively manufactured silicon nitride

Additive manufacturing technology provides a novel approach for the production of complex-structured silicon nitride ceramics. In this study, the microstructural and strength evolution of additively manufactured silicon nitride after continuous thermal exposure for 24 hours in an oxygen-containing atmosphere at 1200-1500 ℃ are investigated. The morphology, phase compositions and element distribution are characterized by SEM, XRD, EBSD and EPMA. The results show that with increasing exposure temperature, α→β phase transformation occurs, and the volume fraction of β-Si3N4 increases from 63.02% to 74.15%. Meanwhile, the grain size of silicon nitride grows from 1.33 μm at 1200 ℃ to 1.97 μm at 1500 ℃. The flexural strength exhibits a rise-then-fall trend with increasing temperature, reaching a peak value of 722.67 MPa at 1200 ℃ and dropping to a minimum of 242.67 MPa at 1500 ℃, which represents a reduction of approximately 66.00% compared to the unexposed condition. Grain coarsening, as well as the formation of pores and microcracks during thermal exposure, are the primary causes of strength degradation. In addition, high-temperature oxidation reactions lead to the formation of mechanically weak SiO2 phases and introduce dimensional inaccuracies, further compromising the mechanical performance of the additively manufactured silicon nitride. As a result, flexural strength continues to decrease with increasing exposure temperature. This study reveals the microstructural and mechanical evolution mechanisms of additively manufactured silicon nitride ceramics under extreme high-temperature service conditions, providing a theoretical foundation for improving their service reliability and process optimization.