Mechanical Properties and Thermal Shock Resistance of SrAl₂Si₂O₈ Reinforced BN Ceramic Composites

Hexagonal boron nitride (h-BN) ceramics have become exceptional materials for heat-resistant components in hypersonic vehicles, attributed to their superior thermal stability and excellent dielectric properties.However, achieving densification during sintering poses challenges, and their mechanical properties are relatively poor.SrAl2Si2O8 (SAS), which has a low melting point and high strength, is introduced into h-BN ceramics and can play a role in facilitating sintering and reinforcing the strength and toughness.In this study, BN-SAS ceramic composites were fabricated via hot press sintering using h-BN, SrCO3, Al2O3, and SiO2 as raw materials, and the effects of sintering pressure on the microstructure, mechanical properties, and thermal properties of the composites were investigated.The thermal shock resistance of BN-SAS ceramic composites was evaluated.Results show that the phases of the BN-SAS ceramic composites prepared by hot pressure sintering are h-BN and h-SrAl2Si2O8.With the increase of sintering pressure, the composites' densities increased, and the mechanical properties showed a rising trend followed by a slight decline.At a sintering pressure of 20 MPa, the bending strength and fracture toughness of the composites were (138±4) MPa and (1.84±0.05)MPa•m 1/2 , respectively.Composites sintered at 10 MPa exhibited a low coefficient of thermal expansion (CTE), with an average CTE of 2.96×10 -6 K -1 from 200 ℃ to 1200 ℃.The composites prepared at 20 MPa displayed higher thermal conductivity ranging from 12.42~28.42W•m -1 •K -1 within the temperature range of RT~1000 ℃.Notably, BN-SAS composites exhibited remarkable thermal shock resistance, with residual bending strength peaking and subsequently declining sharply under a thermal shock temperature difference ranging from 600 to 1400 ℃ The maximum residual bending strength was recorded at a temperature difference of 800 ℃, with a residual strength retention rate of 101%.As the thermal shock temperature difference increased, the degree of oxidation on the ceramic surface and cracks due to thermal stress were also increased progressively.