top of page
This project aims to reduce the amount of Regolith exposure EVA suits must endure by removing any possible sources of magnetism and therefore reducing interactions between the suits and Regolith. Stainless steel is ferromagnetic, which makes it a point of attraction for Regolith. The joints in EVA suits are an assembly of steel raceways and ball bearings to facilitate freedom of movement. Manufacturing flaws and friction can magnetize the stainless steel, making it difficult to remove Regolith and decontaminate EVA suits. The proposed bearing replaces the metal with bearings made of the nonmagnetic ceramic Silicon Nitride (Si3N4), thus removing the risk of magnetized joints.
Design & Analysis
Led material and performance analyses on the proposed design to eliminate lunar dust build-up. Contributed to the design, research, and testing process.
Timeframe
1 year
Software
SolidWorks
This design requires a material capable of handling significant friction wear and extreme thermal shock while preserving the functionality and durability of the joints in EVA suits. The proposed material is powdered silicon nitride that will be obtained by compressing Si3N4 or via selective laser sintering additive manufacturing to create, sinter, and heat-treat the Z2 external bearing assembly as needed. Si3N4 is classically heat-pressed into shape via an 8-14 ton ‘press’ and then post-baked in a vacuum oven at approximately 1000-1200°C to sinter the core and ensure a crystallized matrix rather than a ‘powder’ core. Post-production smoothing is via the creation of a diamond paste or a hardened ceramic paste to get each piece. The team proposes the use of Si3N4 nano-powder.
Stainless steel and Si3N4 have similar reactions to externally applied electric fields. Si3N4 has a very low dielectric constant change in between 8 and 10, which means it matches the ratio of electric permittivity relative to vacuum. Hence, Si3N4 is a great insulator with 8 or 10 times greater electrical permissibility compared to a vacuum. Therefore, any externally applied electric field would be not effective on Si3N4 bearings. Stainless steel is a metal with low electrical conductivity and does not permit electricity due to being a conductor to keep the electric field lines on the steel surface. Its electrical resistivity is between 1.5e-4 and 4.5e-6 ohm-cm, whereas Si3N4 electrical resistivity is around 7.2e-4 ohm-cm. Si3N4 is a diamagnetic material that repels the magnetic field lines exposed from outside, such as magnetized lunar dust, and opposes any magnetic field line that tries to go through Si3N4. Hence, no such magnetized particles can cause damage to Si3N4 bearings. On the other hand, 300 series stainless steel is paramagnetic, which can be weakly attracted by a strong magnetic field applied from the outside. Thus Si3N4 is a safer material for bearings in a Regolith environment.
Kinetic friction in a pure silicon nitride system is higher than kinetic friction in a purely stainless steel system in most of the motion cases. At the same velocity, 0.1 m/s, the stainless steel friction coefficient is 0.31, and silicon nitride kinetic friction varies between 0.60 - 0.39 depending on the force acted and from room temperature to degrees up to 1000°C. This helps the bearing ball and inner and outer race resistance against any deviation and displacement when assembled. Silicon nitride friction factor increases as the temperature increases, which means at higher temperatures, it is more resistant to motion. On the other hand, in a vacuum, stainless steel demonstrates a higher friction coefficient, 2.9, while silicon nitride has around the same as its air environment case, between 0.60 - 0.39.
bottom of page