Nobuaki Kawai

Single microparticle launching method using two-stage light-gas gun for simulating hypervelocity impacts of micrometeoroids and space debris

A single microparticle launching method is described to simulate the hypervelocity impacts of micrometeoroids and microdebris on space structures at the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency. A microparticle placed in a sabot with slits is accelerated using a rifled two-stage light-gas gun. The centrifugal force provided by the rifling in the launch tube separates the sabot. The sabot-separation distance and the impact-point deviation are strongly affected by the combination of the sabot diameter and the bore diameter, and by the projectile diameter. Using this method, spherical projectiles of 1.0-0.1 mm diameter were launched at up to 7 km/s.

Reversible phase transition in laser-shocked 3Y-TZP ceramics observed via nanosecond time-resolved x-ray diffraction

The high-pressure phase stability of the metastable tetragonal zirconia is still under debate. The transition dynamics of shocked Y2O3 (3 mol. %) stabilized tetragonal zirconia ceramics under laser-shock compression has been directly studied using nanosecond time-resolved x-ray diffraction. The martensitic phase transformation to the monoclinic phase, which is the stable phase for pure zirconia at ambient pressure and room temperature, has been observed during compression at 5 GPa within 20 ns without any intermediates. This monoclinic phase reverts back to the tetragonal phase during pressure release. The results imply that the stabilization effect due to the addition of Y2O3 is to some extent negated by the shear stress under compression.

Complex structural dynamics of bismuth under laser-driven compression

With the aid of nanosecond time-resolved X-ray diffraction techniques, we have explored the complex structural dynamics of bismuth under laser-driven compression. The results demonstrate that shocked bismuth undergoes a series of structural transformations involving four solid structures: the Bi-I, Bi-II, Bi-III, and Bi-V phases. The transformation from the Bi-I phase to the Bi-V phase occurs within 4 ns under shock compression at ∼11 GPa, showing no transient phases with the available experimental conditions. Successive phase transitions (Bi-V → Bi-III → Bi-II → Bi-I) during the shock release within 30 ns have also been resolved, which were inaccessible using other dynamic techniques.

Two-Dimensional Observation of Grain Movements in Elongated and Aligned Grain Structure during High Temperature Deformation

It is understood that grains move by grain boundary sliding, and change their relationship to each other during superplastic deformation. Ideal two-dimensional observation of grain movements from the specimen surface is difficult even in the shear deformation because grains move three-dimensionally according to the stress distribution against the specimen surface. In this study, ODS steel with elongated grains aligned along one direction was deformed perpendicular to the aligned axis to achieve ideal two-dimensional grain movements. Surface height profiles with a laser microscope showed small amount of three-dimensional grain movements, while two-dimensional grain movements and rotations were appeared by observations before and after the deformation with SEM-EBSD.

Shock-induced disproportionation of mullite (3Al2O3⋅2SiO2)

Shock-recovery experiments have been performed on high-purity mullite polycrystals in the pressure range up to 72GPa. The recovered samples have been examined by an x-ray diffraction method, nuclear magnetic resonance spectroscopy, and transmission electron microscopy. The samples shocked above 65GPa contain an amorphous SiO2 phase and a γ‐Al2O3 phase, indicating that a rapid disproportionation reaction of mullite is induced by shock compression. The recovered amorphous SiO2 has a mean Si–O–Si bond angle roughly 7° narrower than that of the fused SiO2 glass, indicating the formation of the densified amorphous SiO2 phase. The γ‐Al2O3 phase is crystallized as very fine particles with grain sizes less than 10nm in the matrix of the desified amorphous SiO2 phase. The crystallization of γ‐Al2O3 is expected to occur during a pressure-release process owing to the crystal size effect concerning the phase stability of Al2O3.

Shock compression of cubic boron nitride

Hugoniot measurements have been performed on high-purity cubic boron nitride polycrystals in the pressure range up to 296GPa using a two-stage light-gas gun. Hugoniot parameters have been measured by a line reflection method and Fabry–Perot velocimetry. The Hugoniot elastic limit (HEL) is determined to be 44.3GPa, which is the second highest value after that of diamond. Above the HEL, the Hugoniot compression curve shows a considerable offset from its hydrodynamic compression curve, which is calculated from static-compression data. This result shows that cubic boron nitride preserves its shear strength in the plastic region. Hugoniot data indicate that the cubic phase of boron nitride is stable in the pressure range up to 296GPa.

Shock-induced intermediate-range structural change of SiO2 glass in the nonlinear elastic region

We study shock compressed fused quartz in the nonlinear elastic region using single-shot time-resolved x-ray scattering measurements. The first sharp diffraction peak (FSDP) of fused quartz shifts to the high Q region under shock compression. In contrast, the short-range order structure does not change around 3.5 GPa. The nanosecond FSDP shift provides clear evidence of intermediate-range order structural changes in the nonlinear elastic region. Because the intermediate-order structure is too short to produce the final structural state in the nonlinear elastic region, the FSDP shift is lower compared with hydrostatic experiments.

Instantaneous nano-order fragmentation in mullite ceramics triggered by a shock-induced phase transition

Mullite, a conventional refractory material, was observed to exhibit a peculiar nano-order fragmentation accompanying a phase transition induced by a shock wave. We propose a mechanism for this nanofragmentation, based on a comparative study of mullite-related materials. The microtextures of the mullite-related materials were affected by their initial crystal structure and chemical composition, indicating that oxygen vacancies in the crystal structure play an important role in the nanofragmentation. The results of the present study will help enable the deliberate control of the physical and mechanical properties of materials during high-velocity impact.

Observation of mass and velocity of projectile fragments produced by hypervelocity impact with lightweight ceramic targets

In order to characterize the dynamic fracture of Al projectiles caused by impact with lightweight ceramic targets, we perform hypervelocity impact experiments of lightweight ceramic targets using spherical Al projectiles accelerated by a mini two-stage light-gas gun, and we propose a new method for estimating fragment mass by quantitative image analysis. As materials for the targets, 1-mm thick mullite, silicon nitride, and alumina ceramics are chosen. Aluminum-alloy projectiles 2.0 mm in diameter are impacted onto the targets under normal impact conditions. The dynamic fracture of the targets and projectiles is observed using flash x-ray radiography. In comparison with silicon nitride and alumina targets, a mullite target breaks a projectile into smaller fragments, and the splay angle of the debris generated from a mullite target is larger than that of the other targets. These results suggest that mullite ceramic would be a promising structural member for a debris shield.

Nanofragmentation Controlled by a Shock-Induced Phase Transition in Mullite Related Ceramics and its Application

Mullite (3Al2O3•2SiO2) undergoes a phase transition at 30 GPa with forming aligned nanocrystalline fragments in an amorphous phase. The direction of the crystal axes of mullite nanocrystals with the grain sizes less than 10 nm is that preserved from the starting specimen. To clarify the mechanism of the nanofragmentation in mullite, compositional and structural effects are investigated by comparative studies using several mullite-related aluminosilicates. Consequently, we proposed that the oxygen vacancies in the crystal structure in mullite play an important role to formation of the nanofragmentation textures. Also, we performed impact experiments using mullite as a bumper material, simulating a Whipple bumper shield for spacecrafts. Damage of impact could be considerably less with mullite bumper shield than with aluminum alloy bumper shield, suggesting that mullite could be an candidate for a Whipple bumper materials in the next generation.