Dust Testing

Dust testing splits into four separate problems with separate rigs: how much dust is on the surface and how it got there, how the dust wears material away, whether the dust gets past a seal into a bearing, and whether an active removal system works. Nine dust-caused hazards were identified from Apollo experience: obscured vision, clogged equipment, coated surfaces, loss of surface traction, inhaled dust, degraded radiators, fooled instruments, failed seals, and abrasion [1].

Applying the dust

An abrasion or coverage result is only as repeatable as the deposition. The Uniform Dust Deposition System at Glenn deposits simulant on test articles up to 60 cm in diameter and 15 cm high, in dry air below 1 percent relative humidity, with the operator isolated from aerosolized dust [2]. An imaging subsystem counts particles and measures size by machine learning, predicting surface deposition density from images alone at a coefficient of determination of 0.93, and coverage uniformity reaches a coefficient of variation below 0.11.

Single-run accuracy is the weak point: standard deviation of 18 to 24 mg, or 0.839 to 1.184 mg/cm2, over a 5 cm diameter area [2]. Building the deposit up over multiple runs tightens this to 0.2 to 0.6 mg, or 0.076 to 0.227 mg/cm2, over the same area [2].

Abrasion

Two-body scratch testing establishes the volumetric wear measurement from a single particle on a flat surface. Three-body testing is closer to the lunar case: a rotating rubber wheel loads a specimen while loose abrasive is fed into the contact, following ASTM B611 but with the water omitted. Mass loss is converted to wear volume through the specimen density [3].

Abrasives and specimens run in that campaign, and the result that matters for simulant selection:

Abrasive	Note
JSC-1A-F, nominal size distribution	Consistently more abrasive than the sieved version of the same simulant
JSC-1A-F sieved below 25 um	Less abrasive than nominal
NU-LHT-2M	Lunar highland simulant
Alumina, 50 um average, per ASTM G76	Dominated the abrasion; not representative of lunar dust
Silica, 50/70 mesh, per ASTM G65	Simulants resembled silica sand only on the softer specimens, aluminum and PMMA

Source: [3].

Specimens were PMMA, hardened 1045 steel, 6061-T6 aluminum and 1018 steel; the simulants abraded all of them [3].

The grain size result is repeated in fabric testing. Abrasion of Orthofabric, the outer layer of the Apollo suit, is more likely with larger grains than with the finest fraction, measured across a tumbler method, the Martindale method and an Accelerotor [4]. Ceramic coatings are tested against the same problem: air plasma sprayed alumina and 8 wt percent yttria-stabilized zirconia were compared by mass loss at intervals, surface roughness by profilometer, and microscopy, with alumina showing better wear resistance than 8YSZ, though 8YSZ remains attractive because it can be tailored. That campaign also established that the choice of wear medium has a distinct effect on the abrasion result, so the medium is a test variable rather than a background condition [5].

Textile screening for the VIPER mobility barrier used a Martindale abrasion test with a 150-grit garnet abradant at a 1 psi (6.89 kPa) load on a 1 inch (2.54 cm) plate for 10,500 cycles, with the abradant replaced every 10 percent of the test [6]. Garnet at 6.5 to 7.5 on the Mohs scale was chosen to match the upper range of JSC-1A. Evaluation covered air permeation, thickness and weight after wear cycles, flexibility and visual wear. The baseline air permeation of 22.5 cfm (38.23 m3/h) let all sizes of JSC-1A pass freely, which is what a candidate barrier has to beat [6].

Seals and bearings

Bearings are the mechanism elements most prone to failure from dust intrusion, which is why the sealing effort concentrates on them, and the seal materials themselves degrade: a spring-energized PTFE seal is damaged faster by larger particles, so it is placed last in a series behind coarser barriers [6]. Bushings carry a lower risk of failure from dust intrusion than bearings, and self-lubricating Vespel SP-3 bushings are used at pin joints for that reason, since a wet-lubricated joint attracts particulate.

The VIPER mobility system stacked four barrier types in series, in order of decreasing particle size admitted :

Barrier	Function and sizing
Global softgoods barrier (“Sock”)	Kevlar with integrated MLI over the whole mobility appendage; Nomex woven textile with a 0.5 inch (1.2 cm) graphite grid for surface resistivity; must survive thousands of motion cycles
Labyrinth	Coarse rejection of gravel-sized clasts; gaps sized by manufacturability at about 0.76 mm radial and 1.00 to 1.27 mm axial; three full teeth on the best-oriented application
Nomex felt, face or radial	Fine debris; face seals at 20 percent compression of seal thickness, from MER and MSL experience, against 5 to 10 percent suggested by Tri-ATHLETE
Spring-energized PTFE	Finest particles, placed last in the series because larger particles damage PTFE; rides on a surface held to Ra 0.8 um, lubricated with Braycote 600

Source: [6].

Grease damming at tight shaft clearances is used as a fifth, informal barrier, and on the suspension load cell bearing, where drag torque does not matter because motion is minimal, a deliberately thick grease application is the primary mitigation.

Radial seals are sized against parasitic torque: interference is swept while torque is measured at ambient and then at the actuator temperature extremes using an oven and chiller at ambient pressure, and drag torque is tracked at every seal interface against the actuator torque margin.

System-level dust exposure testing

The VIPER dust box held a mobility module inside a global seal with 1419 g of JSC-1A agitated by two fans every hour, with the drive, steering and suspension actuators sweeping continuously for 57 hours and 35 minutes, representing about 10 km of early mission traverse [6]. Current draw and internal temperatures stayed nominal; on teardown no measurable simulant was found inside the barrier or on internal hardware, and the mylar layers at high-flexion locations were undamaged [6].

Deposition on the article inside such a chamber is not uniform unless it is made so, which is the reason for a separate deposition standard [2].

Wheel-to-barrier interference was life tested separately at ambient for 40 km equivalent travel, about twice expected mission life, by cycling the mechanisms through full range of motion at the greatest inducible interference [6]. The germanium outer coating wore visibly, including at seams the wheel could not have touched; the Kevlar showed only minor stretching. The flight units delete the germanium coating in the interference regions as a result [6].

Active removal

The Electrodynamic Dust Shield is a substrate carrying interdigitated electrodes driven with out-of-phase high voltage pulses, producing a traveling electrodynamic wave that moves adherent dust off the surface by the dielectrophoretic force. It has been tested in vacuum chambers with lunar simulants and with Apollo regolith samples, and on reduced gravity flights that combine lunar gravity with vacuum. Twelve panels of glass, polyimide and prototype spacesuit fabric, some with a lotus leaf coating and some with thermal paint, flew on MISSE-11 in the wake position of the ISS for one year: two were energized and returned current and voltage data against a ground baseline, the remaining ten were passive and were to be operated in a vacuum chamber after return, and all twelve were imaged monthly to track change with time. Preflight ground testing on non-flight panels covered vibration and thermal vacuum, and early thermal runs failed by electrical breakdown across debonded surfaces before the final panels cleared dust nominally both before and after thermal testing [8].

The Apollo brushing precedent

A pre-mission study using soil returned from Apollo 12 concluded that a nylon bristle brush would remove lunar soil and dust effectively. Brushing was used on the Lunar Roving Vehicle radiators during Apollo 15, 16 and 17 and was almost wholly ineffective: the brush did not remove the finest particles and radiator thermal performance degraded despite it [7]. Re-testing the same technology under simulated lunar conditions found that two brushes, a Zephyr fiberglass fingerprint brush and an Escoda nylon fan brush, removed over 90 percent of the dust from AZ93 and aluminized FEP and recovered 80 percent or more of the original solar absorptance to emittance ratio after 20 strokes and 90 percent or more after 200 strokes. Absolute brush performance depends on which simulant is used, so the transferable result is the ranking, not the percentage.

What dust testing does not reproduce

Ambient pressure. The VIPER dust box, the Martindale campaign and the seal torque sweeps were all run at ambient pressure [6]. Dust cohesion and adhesion change character below a few torr, and adhesion to a specific surface pair changes at pressures far lower than that, so an ambient exposure test bounds mechanical infiltration but not the adhesive behavior [1].

Quantified exposure. The VIPER team states directly that it is very difficult to quantitatively define the expected dust exposure to a mechanism, and that its approach was to overdesign and run conservative tests rather than to specify an exposure [6]. Ingress gaps across the mechanism designs, labyrinths included, were never assessed against a specific debris size and were left large for manufacturability.

Seal comparison. No published basis exists for choosing between labyrinth, felt, spring-energized PTFE and grease damming on measured performance, nor for the design variables within each, such as felt compression and width [6]. Labyrinth seal effectiveness against gap size in isolation is explicitly named as future work.

Charge transfer. Terrestrial atmosphere dissipates surface charge, so tribocharging measured in air does not transfer. The Martian case is measured in a 10 mbar CO2 atmosphere with dust driven onto a cylindrical insulator by an impeller fan, using JSC Mars-1 alongside its separate mineral constituents [9].

Abrasive fidelity. The standard abrasives named in the wear standards are harder and more aggressive than lunar simulant: alumina dominated the three-body result and is not representative, while the simulants matched silica sand only on soft specimens [3]. A result quoted against ASTM G65 or G76 media is therefore not a lunar wear rate.

No combined-environment standard. There is no standardized, accessible method for evaluating materials and mechanisms in a facsimile of the harsh lunar environment, so abrasive regolith, vacuum, temperature and radiation are usually applied to an article separately rather than together, and that gap is itself a brake on dust-tolerant technology [10].