Structural and functional reliability is vital for lunar surface operations using moving mechanical assemblies, such as life-support machinery, rovers, and scientific equipment. Lunar dust has been identified as the most important obstacle to this reliability, so the NASA Glenn Research Center, in collaboration with the Nippon Institute of Technology, studied the hardness of a lunar dust simulant (JSC-1) for future comparison with lunar materials. When contact surface material is removed by hard lunar dust particles, abrasive wear occurs. Particles may adhere to a second surface or may exist as loose particles between two contacting surfaces. The rate of abrasive wear is at least one to two orders of magnitude greater than that of other mechanisms, such as adhesive wear and fatigue. A material’s static hardness has been related to its abrasive wear resistance, and abrasive wear rate is inversely proportional to Vickers hardness for many annealed pure metals. Therefore, a hardness study of JSC-1 is important in determining how the hardness and elastic modulus of lunar dust will affect the wear behavior of materials proposed for lunar mechanisms.
Tests showed that JSC-1 is cohesive:its particles stick together under cold or hot pressing at compaction pressures of 4 to 6 GPa. Hot pressing at 1473 K produced a higher bulk density than cold pressing at 296 K did. The surfaces in contact with JSC-1 were flats polished with 1-μm diamond powder, and the root mean square roughness of the cold- and hot-pressed JSC-1 surfaces were 0.1 μm.
Examples of load-displacement data for compacted JSC-1. Top: Cold-pressed JSC-1. Bottom: Hot-pressed JSC-1.
A Berkovich indenter was used for nanoindentation measurements up to a predefined load of 1000 μN with the cold- and hot-pressed JSC-1, and load/unload curves were recorded for each indentation with a nanoindentation device mounted in an atomic force microscope (AFM). Reference measurements were conducted with fused silica, silicon, silicon carbide, amorphous silicon carbide films on silicon, and polycrystalline boron carbide. Both cold- and hot-pressed JSC-1 were characterized by low-vacuum scanning electron microscopy (SEM), three-dimensional measurements with SEM, and AFM for morphological and topographical analysis; x-ray diffraction for phase analysis; and x-ray fluorescence, energy dispersive x-ray spectroscopy with SEM, and electron probe microanalysis or x-ray microanalysis using wavelength dispersive x-ray spectrometers for chemical and elemental analysis.
Measurements similar to those in the graphs gave the nanoindentation hardness for cold- and hot-pressed JSC-1, fused silica, silicon, silicon carbide, amorphous silicon carbide films on silicon, and polycrystalline boron carbide. The photomicrographs show the resulting nanoindentations.
Scanning probe microscope images of the nanoindentations obtained by in situ imaging using the Berkovich indenter tip to scan the surface. Left: Cold-pressed JSC-1. Right: Hot-pressed JSC-1.
The measured hardness value of the cold-pressed JSC-1 was 2/3rd, 1/8th, and 1/9th, respectively, of that of silicon, silicon carbide, and polycrystalline boron carbide. The harder ceramics, such as boron carbide and silicon carbide, most likely possess greater abrasive resistance to JSC-1. In a separate investigation, the authors found that JSC-1 did readily abrade silicon. Thus, boron carbide and silicon carbide may be needed to provide suitable antiabrasion surfaces for long-life, lightweight components in surface mobility and power systems for lunar operations.
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Last updated: December 15, 2007
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