Experimental investigation of lunar dust impact wear for different grain sizes and impact angles

Structural samples removed from the Surveyor 3 probe and returned to Earth by the Apollo 12 astronauts have shown that close proximity lunar landing operations could damage previously landed lunar hardware. During the final landing approach the jet engines of the Apollo 12 lunar lander stirred up lunar soil. Parts of the dust flow impacted on the previously landed Surveyor 3 and “sandblasted” the surface of the probe. Similar scenarios might occur during future lunar missions as part of resupply or sample return scenarios. The presented experimental work focuses on the identification and quantitative characterization of possible damages to technical surfaces caused by lunar dust impacts for different impact angles and velocities. It can be assumed that, during lunar landing operations, the majority of the stirred-up dust particles will not impact perpendicular to the target surface. Therefore, impact experiments using particles of the lunar analogue material JSC-1A were conducted at different impact angles at impact velocities of approximately 200 m/s for grain sizes between 450 and 560 microns. In the current study 10 mg of the dust particles were accelerated with an electromagnetic eddy current accelerator. The accelerated lunar dust simulant particles were impacted on three different technical materials, Al6061-T6, silica glass, and Makrolon® (polycarbonate). Microscopic images of the impact area on the specimens and the mass change of the specimens resulting from the impacts were used to identify whether the major interaction between surface and particles is erosion or particle adhesion. The two ductile materials, aluminum and polycarbonate, are mainly affected by particle adhesion or the embedding of particles, while chipping is the major surface degradation effect for silica glass specimens. However material removal could only be measured for an impact angle of 15°. Macroscopic degradation phenomena were quantified by measuring the alteration of surface properties of the specimens. Change in surface properties were measured as the change of visible light transmission for both optical media, while the surface roughness (Ra) was measured to quantify the change of surface properties for the aluminum specimens. The results show a significant correlation between impact angle and the change of surface properties regardless of the investigated material. It was found, that the main degradation effects for surface roughness as well as for the transmission occur between impact angles normal to the surface and 15°, while the surface properties are less affected by dust impacts towards grazing impact angles. A comparison of these results with previously conducted experiments, which investigated the impact wear of JSC-1A particles, was done. The previous study impacted 10 mg of JSC-1A with grain sizes between 250 and 350 microns at different impact angles at impact velocities of approximately 360 m/s under the same experimental conditions. The comparison showed that the surface properties are more affected by the 360 m/s-impacts, although the single particle has less kinetic impact energy. The reason is that more particle impacts occur per unit area for the faster but smaller particles.