Metallic Environmentally Resistant Coating Rapid Innovation Initiative (MERCRII)

Project Objective  

This project developed wear and radiation resistant coatings for moving mechanisms operating on the Lunar surface.

Project Description 

Lightweight alloys such as aluminum (Al) and titanium (Ti) are often specified for space systems to minimize mass while maintaining structural integrity. Such alloys however have poor tribological response (high friction and wear), especially in extreme space environments and in the presence of lunar regolith. This gap leads to reduced lifetimes and premature failures, ultimately limiting long term operations on the lunar surface.

To address this technology gap, this project proposed the development of advanced wear- and radiation-resistant coatings for lightweight parts to extend the lifetime and sustainability of lunar and Martian assets. Four novel coating materials were developed: hexagonal boron nitride-aluminum (hBN-Al), hexagonal boron nitride-titanium (hBN-Ti), nickel titanium (NiTi), and aluminum oxide (Al2O3). These were deposited using three different process technologies: cold-spray (CS), ambient plasma-spray (APS), and vacuum plasma spray (VPS). The coated substrate materials were Al6061 and Ti6Al4V (Ti64), both conventionally manufactured (CM) and additively manufactured (AM). Multiple down-selections were conducted throughout the project based on wear performance of the coatings in pin-on-disk tests run at ambient earth pressure. The coating and deposition process that presented the best wear performance of all was Ti-2Vol%hBN deposited via vacuum plasma spray. This best coating was applied to three mechanisms of action (rod and slot, ball and socket, and hinge joint), which were fabricated from Al6061 and Ti64, both by CM and AM techniques. The coatings were tested in two-body sliding (i.e., in the absence of regolith simulant) and three-body abrasive (i.e., in the presence of regolith simulant) wear (at earth ambient and at vacuum conditions), and in solid particle erosion (i.e., high speed regolith simulant particle impact) tests from -196°C to 150°C, after subjection to various extreme environmental conditions. In three-body abrasive wear tests, the abrasive third body used was one of three lunar regolith simulants: Zircon, Greenland Anorthosite, and JSC-1A. JSC-1A was used for most three-body abrasive wear tests and for all solid particle erosion tests, due to the abrasiveness, angularity and high lithic and glass fragments making this simulant recommended for wear testing. The environmental conditions that coatings were subjected to, prior to wear and erosion testing, include temperature cycles ranging from high (120°C) to cryogenic (-170°C), high vacuum (~10-7 Torr), and pre- and post-exposure to ionizing particle radiation (~6 Mrad). Further work was focused on different mechanisms for future missions and testing mechanisms for dust mitigation activities.

The proposed work highly engaged early careers from NASA MSFC, GRC and LaRC, working in collaboration with both academia and industry. Florida International University (Miami, FL) and Plasma Processes (Huntsville, Al) were the external partners on this technology development effort. The team was supported by a group of NASA mentors from different centers (MSFC, GRC and LaRC). They provided both technical and project management mentorship to ensure the success of the project and workforce development of the early career team. The coatings have been sent to space as part of two MISSE missions (MISSE-17 and MISSE-21) to evaluate real space exposure vs simulated space exposure. MISSE-17 contained coating applied on Metallic samples while MISSE-21 will be flying coatings applied on sofgoods.

This technology is in direct alignment with the Space Technology Mission Directorate (STMD) capabilities and technical focus areas. The Moon to Mars Focus of this project is to engineer more durable and dust tolerant systems. Ultralight, radiation-hardened, wear-resistant, and mechanically robust metal matrix composites (MMCs) are used to mitigate uncertainty and failures due to wear and enhance and enable key aspects of the Artemis missions [project was endorsed by Human Landing System (HLS) and In-Space Manufacturing (ISM) both projects housed at Marshal Space Flight Center].

Project Results and Conclusions 

Main technological accomplishments achieved by the team through the project in key areas:

1. Background Study of Lunar Regolith Abrasive wear at extreme Temperatures (-196, RT, 150 °C)

During the stage of formulation of test parameters at the beginning of the project, three different Lunar simulants were analyzed and compared with the purpose of selecting the best fit for Tribological tests. The team tested the following simulants:

1) NU-LHT-2M (NASA/USGS Lunar Highlands Type 2nd generation, medium grain size). Greenland Anorthosite (GA) simulant, very fine and the smallest in particle size (8 μm) of all three tested simulants.

2) JSC-1A – mare simulant (reproduces fairly well the geochemistry of an Apollo 14 sample), large glass content (Size 36 μm).

3) Zircon (impact milled or crushed) (107 ± 27 µm) – this is one of the hardest minerals, it is found in lunar materials and can be found all over the Moon, and has a conchoidal (glass-like) fracture like the harder lunar minerals.

The most wear was seen with Zircon due to its hardness and the shape of its particles. The GA simulant caused the least wear under the same testing parameters.

The team selected JSC-1A for the project due to its suitability for abrasion tests, as it closely mimics the properties of lunar soil. This makes it ideal for evaluating materials and equipment intended for lunar environments. The composition and particle size of JSC-1A resemble the abrasive, angular nature of actual lunar dust, which is crucial for replicating the harsh wear conditions that equipment will face during lunar missions. Unlike spherical zircon or sub-micron-sized simulants, JSC-1A has a highly angular morphology, closely replicating the jagged, sharp edges of lunar soil particles formed due to the Moon’s lack of atmospheric weathering. This enhances its abrasive potential, making it a valuable material for simulating the degradation and wear caused by lunar dust.

Additionally, JSC-1A is a cost-effective and standardized material widely available for research. This makes it an essential tool for testing in preparation for missions like NASA’s Artemis program, where understanding dust mitigation and abrasion resistance is critical for the success of long-term lunar exploration.

This study examines the tribological behavior of Al6061 and Ti64 alloys under lunar regolith simulants at various temperatures.

JSC-1A particles (36 μm, sharp) cause more wear volume loss than GA particles (8 μm, flaky) due to their larger size and angular shape.

A novel “simulant embedding” mechanism was identified, where abrasive particles embedded in the alloy surface cause degradation.

Panoramic wear scar cross-section imaging with EDS mapping quantifies embedding fraction (Ef) and mean embedding depth (Ed).

Ef increases with temperature due to material softening, with smearing dominating in low-temperature conditions.

JSC-1A causes more embedding than GA at room and high temperatures by 12% and 15% on Al 6061 and 23% and 29% on Ti64, respectively.

Mean embedding depth increases linearly with temperature from low to high temperatures.

A non-dimensional embedding index (Ei) is proposed to assess embedding effects, incorporating simulant and material properties and test conditions.

There is a critical need for protective coatings to safeguard materials from the sharp, abrasive lunar regolith.

2. Coating Development Study 1- MERCRII- Multifunctional Protective Coatings for Protection Against Abrasive Regolith and Neutron Radiation

Plasma spray techniques (APS & VPS) developed multi-functional coatings using hBN-reinforced composite powders produced by cryomilling technique at 2% and 10% volume.

No significant differences or difficulties were observed when spraying coatings onto both AM and CM substrates. This benefits the project in several ways.

Versatility: Coatings can be applied to both additively and conventionally manufactured substrates.

Process Consistency: Ensures uniform coating performance across different substrate types.

Cost Efficiency: Reduces the need for separate coating processes, lowering production costs.

Time Efficiency: Simplifies the workflow, saving time in the manufacturing process.

Scalability: Supports the use of additive manufacturing without compromising coating quality.

Coating microhardness increased by ~3 times for 2 vol% hBN due to the formation of secondary phases like TiO, TiO2, and TiN.

Microhardness decreased with 10 vol% hBN due to agglomeration, weak interfacial bonding, and increased porosity.

Wear performance improved due to the synergy between solid lubrication and hard secondary phases.

COF friction increased by 40% for APS Ti/2 vol% hBN coatings compared to the Ti substrate.

APS and VPS Ti/2 vol% hBN coatings improved wear performance by 90% in tests with JSC-1A lunar simulant, while 10 vol% hBN coatings showed reduced performance due to defects.

Wear scar analysis revealed JSC-1A particle embedding, potentially impacting structural component performance.

Radiation shielding improved by 27% for VPS Ti/2 vol% hBN against Al substrates and by 7% against Ti substrates.

VPS Ti/2 vol% hBN coatings had better mass absorption coefficients than APS coatings due to higher hBN retention and fewer oxide phases.

The best-performing coatings were VPS Ti/2 vol% hBN and APS Ti/10 vol% hBN, ideal for aerospace wear and radiation shielding applications.

An optimal balance between retained hBN and secondary phases is crucial for enhanced wear and radiation properties.

The Al2O3 coating was evaluated under the same test parameters as the novel hBN coatings to compare off-the-shelf products with MERCRII coatings. The Al2O3 coating failed to withstand the thermal cycle, resulting in detachment from both Al6061 and Ti64 substrates. Multiple attempts to make this coating survive the thermal cycle conditions were implemented applying different interlayer materials. However, the coating did not pass this test for any tests.

3. Coating Development Study 2- MERCRII- Multifunctional Protective Coatings for Protection Against Abrasive Regolith and Neutron Radiation in Simulated Thermal Radiation Cycles

The study examines the impact of lunar-like conditions, including radiation exposure and thermal cycling, on titanium-hBN composite coatings in MSFC TVAC chamber

Radiation-induced hardening leads to brittle fracture, increased porosity, and crack formation due to thermal stresses.

XRD and TEM analyses reveal high dislocation densities, black dot defects, and dislocation clusters caused by combined environmental stresses.

Both APS and VPS Ti/2 vol% hBN coatings show substantial wear resistance improvements, with a 90-95% reduction in wear rate compared to Ti substrates.

VPS coatings slightly outperform APS, while Ti/10 vol% hBN coatings exhibit reduced wear performance due to agglomeration and porosity.

Distinct wear track features like particle embedding and flattening were observed, especially in Ti/2 vol% hBN coatings exposed to JSC-1A lunar regolith.

Synergistic coatings provide improved neutron shielding, with a 28% increase in mass absorption coefficient compared to uncoated Al6061 and Ti64.

VPS Ti/2 vol% and VPS Ti/10 vol% hBN coatings are the best choices for aerospace applications requiring wear resistance and radiation shielding under extreme conditions.

hBN reinforcement in titanium via plasma spray techniques offers significant potential for future lunar structures and human landing systems.

4. Coating Development Study 3- MERCRII- Erosion Behavior of Ti-hBN Multifunctional Coatings in a Custom-Made Planetary Test Rig at Extreme Lunar Temperatures

Lunar Erosion Performance: Ti/2 vol.% hBN and NiTi coatings were tested using a custom planetary erosion test rig to simulate extreme lunar conditions.

Superior VPS Performance: VPS Ti/2 vol.% hBN coatings showed better erosion resistance compared to APS, offering an optimal balance between hardness and ductility.

Mass Loss Reduction: VPS coatings reduced mass loss by 31% at room temperature (RT) and 5% at high temperatures (HT), compared to Ti substrates.

Impact Velocities: APS coatings experienced significant mass loss at high velocities (250 mph), while VPS coatings resisted failure at lower velocities (50 mph).

Temperature Effects: LT conditions caused brittleness, leading to higher mass loss, while HT conditions resulted in plastic flow and particle embedding.

NiTi-Hf Coatings: NiTi-Hf coatings exhibited superior erosion resistance compared to Ti64 substrates; this is attributed to their super elasticity and shape memory effects.

Erosion Mechanisms: Key mechanisms identified include particle embedding, erosion pitting, mass loss, and particle rebounding, with superior energy absorption by VPS coatings.

Modified Erosion Index: A new erosion index and models were proposed, incorporating fracture toughness, exposure time, and temperature.

Implications for Lunar Applications: The findings highlight the potential of Ti-BN coatings for lunar infrastructure, offering excellent erosion, abrasion, and radiation resistance.

5. Coating Development Study 4 - MERCRII- MISSE-17: Coatings exposed to Radiation and atmospheric Oxygen in ISS

Increased Porosity and Cracks: Higher porosity and crack density were observed in APS and VPS coatings post-exposure, especially at initial radiation impact areas.

Elemental Distribution: Even boron nitride distribution, with increased oxygen due to atmospheric exposure.

Oxidation: VPS coatings show a 25% rise in oxide phases, higher than APS (6%) after exposure.

Microhardness: APS coatings exhibit a 12% increase, while VPS coatings show a 56% increase in microhardness due to dislocation clusters from GCR and SPR.

Unexposed Side: The hardness of the unexposed side remains similar to that of the control, indicating hBN's role in radiation absorption.

Better and more standard polishing method would facilitate the wear measurements quantitatively and qualitatively on the coatings. Very rough coatings are difficult to characterize.

6. Coating Development Study 5 – MERCRII best coating applied to prototype and tested in a relevant environment: VPS 2vol%hBN was applied to three different relevant mechanisms prototypes and tested at vacuum with regolith after being exposed to thermal cycle.

Addressing the challenge of coefficient of thermal expansion mismatch was paramount when applying coatings to full-scale test mechanisms. We meticulously controlled substrate temperatures to minimize thermal expansion and prevent localized substrate melting. Additionally, we incorporated a bond coat material to alleviate thermal expansion discrepancies, thereby reducing the risk of coating cracking.

Wear of the mechanisms is more difficult to measure. The parts needed to be polished to some extent in order to run. Wear patterns on mechanisms are larger (i.e., less concentrated/more diffuse) than those found for the tribometer tests with pucks. The contact pressures for the mechanism tests were much lower than those used in the tribometer tests with the pucks. The contacts with the mechanisms were conformal (i.e., the surfaces in contact have the same, or almost the same, radius of curvature), which is typical for the design of most contacting mechanisms if it can be achieved. The tribometer tests with the pucks were nonconformal – a small radius ball was slid against a nominally flat puck. This is typical of standard tribological tests, as it increases the contact pressure and leads to more, and more concentrated, wear; this makes the wear generally easier to measure.

7. Coating Development Study 6 – MERCRII coating variations applied to Ortho-fabrics to provide wear and radiation resistance to softgoods.

As a side study the winner MERCRII coating and some variations form this one were applied on top of Orthofabric samples to evaluate the wear, radiation resistance and the flexibility of these when applied on softgoods. These results are still being evaluated by the MERCRII team, but some preliminary lessons learned have been noted. Pin-on disk, erosion and Tumbler tests were some of the tests conducted to evaluate the coatings. This study was in collaboration between FIU, Plasma Process LLC, and three NASA centers (MSFC, LaRC, and JSC). These samples will fly as part of the MISSE-21 mission; further evaluation will be executed after receiving samples back from space.

Enhancements in the fixturing of Ortho fabrics significantly improved coating quality. While small test samples were initially requested, it was discovered that coating larger areas distributed heat more effectively, resulting in more secure fixturing and a reduction in charring of both the fabric and the coating. It was noted that scaling up or down for test and evaluation purposes may not yield representative or optimal results.

Higher hBN content (20%) enhances adhesion strength with FEP matrix

Develop advanced wear- and radiation-resistant coatings for lightweight parts for use in lunar and Martian architectures, and to design unique wear characterization techniques comparing different lunar simulants.

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