Hf B2 Research Articles

Oxidation ablation resistance of ZrB2-HfB2-SiC-TaSi2 coating prepared on C/C composite surface

In order to improve the oxidation and ablative properties of C/C composites, ZrB2-SiC-HfB2-TaSi2 coating was prepared on the surface of C/C composites by atmospheric plasma spraying. By means of microstructure analysis and phase structure analysis, the difference of the spray powder before and after IPS treatment and the microstructure and oxidation ablation resistance of the coating were studied. The results show that the IPS treated powder has better fluidity and densification. In the coating, part of ZrB2 and HfB2 exist in the form of unmelted particles, another part exists in the form of intermediate solid solution (Zr, Hf)B2, and TaSi2 exists in the particle gap. The linear ablation rate and mass ablation rate of the coating are −1.513 × 10−4 mm/s and 2.842 × 10−4 g/s, respectively, measured under the condition of oxygen acetylene heat flux of 1.8 MW/m2, 180 s were ablated. The coating has good ablative resistance, and TaSi2 can form dense Zr-Ta-O and Hf-Ta-O oxides with Zr and Hf elements. The Ta-Si-O glass phase can effectively seal the holes of the coating to prevent oxygen penetration. During the ablation process, the oxidation of (Zr,Hf)B2 also plays a certain role in the formation of densification zone on the coating surface.

Oxidation-affected zone in the sintered ZrB2–SiC–HfB2 composites

Understanding the behavior of ultra-high temperature ceramics (UHTCs) against oxidation is of particular importance in high-temperature applications. In this study, ZrB2–SiC–HfB2 UHTC composites were fabricated by spark plasma sintering (SPS) method at different temperatures, times, and pressures to investigate the effects of sintering process variables on their oxidation resistance. Before the oxidation tests, the as-sintered samples contained ZrB2 and SiC phases with (Zr,Hf)B2 solid solution. The samples were subjected to oxidative conditions at 1400 °C and their relative mass changes were measured as a function of oxidation time up to 20 hours. FESEM and EDS equipment were used for microstructural and elemental analyzes of cross-sections of different oxide layers. Due to the oxygen diffusion, ZrO2 and SiO2 phases appeared alongside (Zr, Hf)O2 in the surface layers. After identifying the several oxides and SiC-depleted layers in the oxidation-affected zone, a schematic model for the arrangement of such layers was proposed.

Solid solutioning in ZrB2 with HfB2: Effect on densification and oxidation resistance

Solid solution formation in ZrB2 with HfB2 addition along with SiC reinforcement is investigated. To impart enhanced oxidation protection, solid solution formed is synergistically reinforced with CNTs. With the formation of solid solution, an increase in densification, evaluated using different methods, is reported. Phase analysis shows the peaks of (Zr,Hf)B2 solid solution positioned between the corresponding peaks of ZrB2 and HfB2. Thus, lattice parameters (calculated by performing Rietveld refinement) were found to lie between the lattice parameters of monolithic ZrB2 (a = b = 3.169 Å, c = 3.530 Å) and HfB2 (a = b = 3.142 Å, c = 3.480 Å), confirming the formation of solid solution. The exothermic peak of ZrB2 oxidation shifted from 732 °C in ZrB2 + SiC composite to 745 °C in (HfB2 + ZrB2) + SiC with solid solutioning and further to 812 °C with CNT reinforcement in ZrB2 + (HfB2 + ZrB2) + SiC+CNT composite, elucidating an increase in oxidation resistance revealing the potential of these composites for aerospace applications.

Densification and flexural strength of ZrB2–30 vol% SiC with different amount of HfB2

Microstructure, flexural strength and densification behavior of spark plasma sintered ZrB2–30 vol% SiC composites doped with HfB2 were investigated. The composites contained 4, 8 and 12 vol% HfB2 were spark plasma sintered in 1650, 1725 and 1800 °C for 4, 9 and 14 min. Relative density and the ratio of open porosities were measured and used to investigate the densification behavior. Microstructural investigations as well as the flexural strength measurements were carried out using scanning electron microscopy and three-point bending instrument, respectively.Results indicated that a part of HfB2 may be solved in the matrix and form the solid solution of (Zr, Hf)B2, based on the sintering temperature, time and the applied pressure. Additionally, no meaningful correlation between the amount of HfB2 and the final grain size of the composites was found. The best outcome of the flexural strength was obtained as 470 MPa in sample containing 8 vol% HfB2, accompanied with increasing the shrinkage start temperature from 1328 to 1255 °C.

Effect of HfB2 on microstructure and mechanical properties of ZrB2–SiC-based composites

In this work, ZrB2–SiC-based composites with different additives were sintered in different temperature (1600, 1700, 1800 and 1900), time (4, 8, 12 and 16) and pressure (10, 20, 30 and 40MPa). The effect of HfB2 (0, 5, 10, 15vol.%) on ZrB2-based ceramics was investigated. Microstructure evaluation and phases were done by SEM and EDAX. Effect of SPS conditions (temperature, time and pressure) on microstructure was investigated. The results showed that Hf and Zr diffuse in each other and (Zr, Hf)B2 solid solution will be formed. In addition, it was concluded that by increasing temperature and time, solid solution formation increases. Pressure has little effect on diffusion. Also, MoSi2 and ZrB2 form (Zr, Mo)B2 solid solution. Finally it is cleared that HfB2 increases open porosity percent slightly. Finally, it was observed that HfB2 has negative effect on densification while improves flexural strength due to (Zr, Hf)B2 solid solution.

The processing and properties of (Zr, Hf)B2–SiC nanostructured composites

Abstract (Zr, Hf)B 2 –SiC nanostructured composites were fabricated by high energy ball milling and reactive spark plasma sintering (RSPS) of HfB 2 , ZrSi 2 , B 4 C and C. Highly dense composites with homogeneously intermixed ultra-fine (Zr, Hf)B 2 and SiC grains (100–300 nm) were obtained after RSPS at 1600 °C for 10 min. The densification was promoted by high energy ball milling and ZrSi 2 additive. The additives were almost completely transformed into ZrB 2 and SiC during densification. The improvement of flexural strength and fracture toughness (641 MPa and 5.36 MPa m 1/2 , respectively) was achieved. The relationships between the ultra-fine microstructure and mechanical properties were discussed.

Radiation defects in HfB2after neutron irradiation studied by perturbed angular correlations of181Ta

Abstract The perturbed angular correlation of the 133-482 keV γ-γ cascade in 181Ta was measured for thermal-neutron-irradiated Hf B2 as a function of the annealing temperature. Two distinct defect configurations D 1 and D 2 were established by the quadrupole frequencies: D 1 with ω q 1=146 MHz and D 2 with ω Q 2 = 166 MHz. The defect configurations D 1 and D 2 were identified as interstitial atoms at the nearest neighbour distance from the probe atom.