Designing for ultrahigh-temperature applications: The mechanical and thermal properties of HfB₂, HfC _x , HfN _x and αHf(N)

Abstract

The thermal conductivity, thermal expansion, Young's Modulus, flexural strength, and brittle-plastic deformation transition temperature were determined for HfB₂, HfC_0.98, HfC_0.67, and HfN_0.92 ceramics. The mechanical behavior of αHf(N) solid solutions was also studied. The thermal conductivity of modified HfB₂ exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2.5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC_0.98 to 1100°C for HfC_0.67 ceramics. The transition temperature of HfB₂ was 1100°C. Pure HfB₂ was found to have a strength of 340 MPa in 4 point bending, that was constant from room temperature to 1600°C, while a HfB₂ + 10% HfC _x had a higher room temperature bend strength of 440 MPa, but that dropped to 200 MPa at 1600°C. The data generated by this effort was inputted into finite element models to predict material response in internally heated nozzle tests. The theoretical model required accurate material properties, realistic thermal boundary conditions, transient heat transfer analysis, and a good understanding of the displacement constraints. The results of the modeling suggest that HfB₂ should survive the high thermal stresses generated during the nozzle test primarily because of its superior thermal conductivity. The comparison the theoretical failure calculations to the observed response in actual test conditions show quite good agreement implying that the behavior of the design is well understood.