Fabrication and plasma arc thermal shock resistance of HfB2-based ultra high temperature ceramics

 Due to their excellent properties such as high melting point, high electrical conductivity, high oxidation resistance and good chemical stability, transition metal diboride (ZrB2, HfB2) based ceramics have good potential for various aviation applications, such as hypersonic flight, atmospheric reentry or rocket propulsion. Such severe usage environment requires a good oxidation resistance and thermal shock resistance (TSR) of ultra high temperature ceramics (UHTC).On-ground arc-jet testing has recently shown as a more effective way on investigating the thermal shock behavior of UHTCs than conventional water quenching method , because it can provide a very good ground-based simulation of the hypersonic flight conditions, including rapid temperature rising, high temperature (>2 000 °C), high heat flow and relatively high gas pressure.

Preparation of composites 

Commercial available HfB2 powder, SiC powder  and AlN powder  were used. The compositions of materials were: 1) HfB2-20% (volume fraction) SiC (H20S); 2) HfB2-20%SiC-15%AlN (H20S15A). The starting powder mixtures were milled for 24 h, and hot pressed in Ar gas atmosphere. The sintering temperature was 2 000 °C and 1 800 °C for composites H20S and H20S15A, respectively. The applied pressure was 30 MPa and the soaking time was 30 min for both the materials.

Composites characterization 

Specimens were firstly polished and ultrasoniccleaned by acetone and pure ethanol. The final densities of composites were measured by Archimedes water-immersion method, and the theoretical density was estimated by the rule of mixture. Flexural strength was measured with three-point bending tests at a crosshead speed of 0.5 mm/min. The sample size is 36 mm  3 mm  4 mm with the span of 30 mm. Fracture toughness was evaluated using single-edge notched bend (SENB) beams (2 mm  4 mm  20 mm, notch depth and radius of 2 mm and 0.2 mm, respectively) with a span of 16 mm and a crosshead speed of 0.05 mm/min. The hardness of composites was measured by a Vickers indenter with 50 N as applied load for 10 s on polished sections. The thermal shock tests were conducted in a self-building alternating plasma arc heater. The experimental conditions for thermal shock are listed in Table 1. The dimensions of specimens are 15 mm in diameter and 20 mm in height. The microstructure of materials was studied by scanning electron microscope (SEM, Hitachi S-4700) equipped with an energy dispersive X-ray spectroscope. Crystal-phase identification of samples was determined by X-ray diffractometry (Rigaku, Dmax-rb, Cu Kα=1.541 8 Å).

Conclusion

1) HfB2-SiC-AlN composite has a better thermal shock resistance than HfB2-SiC composite. Microstructure analysis of cross section of samples shows a different surface morphology.

2) The improvement of thermal shock resistance of HfB2-SiC-AlN composite is mainly due to the formation of HfO2-Al2O3 glassy phase, which fills in the voids and cracks of oxidized layer. This endows it a moderate strength and prevents it from being broken.

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