Mechanical and thermal properties of light weight boron‐mullite Al5BO9.

Al5BO9 is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al5BO9 have not been carried out. Herein, we report the mechanical and thermal properties of bulk Al5BO9 prepared by spark plasma sintering of solid‐state reaction synthesized Al5BO9 powders. The bulk (B), shear (G), and Young's (E) moduli are 148 GPa, 85 GPa, and 214 GPa, respectively, which are close to the theoretical values. The Pugh's ratio G/B is 0.574, indicating its intrinsic damage tolerance, which is also revealed by Hertzian contact test. The Vickers hardness (Hv) is 10.8 GPa, being lower than mullite. The flexural strength, compressive strength, and fracture toughness are, respectively, 277 ± 35 MPa, 814 ± 75 MPa, and 2.4 ± 0.3 MPa·m1/2, which are close to those of mullite. Al5BO9 has anisotropic coefficient of thermal expansion (CTE) in three crystallographic directions, ie αa = (4.40 ± 0.21) × 10−6 K−1, αb = (7.11 ± 0.18) × 10−6 K−1, αc = (6.70 ± 0.29) × 10−6 K−1 from Debye temperature to 1473 K, which are underpinned by its structural feature, ie lower αa is resulted from the edge‐shared AlO6 octahedron chains along the [100] direction. The average CTE is (6.05 ± 0.06) × 10−6 K−1. The thermal conductivity declines with temperature as κ = 1336.39/T + 1.97, consisting with predicted trend from Slack's model. The low thermal conductivity and low density guarantee Al5BO9 a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles. [ABSTRACT FROM AUTHOR]