The quantitative science of liquid phase sintering, a process to shape or form solid materials, began in the 1950s, but the practice dates from the 1400s when gold was used to bond platinum in Colombia and Ecuador. Today, it is a mainstay in a diversity of fields, such as metal cutting tools, armor piercing projectiles, automotive engine connecting rods, and self-lubricating bearings.
Future applications include use of liquid phase sintering to perform in-space fabrication and repair and using lunar regolith to fabricate structures on the Moon or using metal powder to fabricate replacement components during extraterrestrial exploration. The NASA Sample Cartridge Assembly-Gravitational Effects on Distortion in Sintering (MSL SCA-GEDS-German) investigation focuses on determining the underlying scientific principles to forecast density, size, shape, and properties for liquid phase sintered bodies over a broad range of compositions in Earth-gravity (1g) and microgravity (μg) conditions.
The final GEDS samples were processed on the International Space Station Low-Gradient Furnace on June 24, 2020. All experiments were completed without leaks or other failures. The first GEDS cartridge was returned to the Principal Investigator’s laboratory February 2020. The remaining GEDS samples are scheduled to return from the International Space Station via SpaceX-21 in January 2021 and SpaceX-22 in June 2021. Results will be shared via NASA open science databases upon completion of analysis and/or investigator publication.
There is considerable experience with liquid phase sintering on Earth, but the behavior under reduced gravity is not well understood. The NASA Sample Cartridge Assembly-Gravitational Effects on Distortion in Sintering (MSL SCA-GEDS-German) investigation in microgravity observes phase changes and product formation within solid mixtures undergoing spontaneous reaction. In the flight experiments GEDS sample cartridge assemblies were heated in the Low Gradient Furnace to 1200+ °C to induce liquid phase sintering densification of high-density tungsten alloys. The GEDS samples varied the liquid content and surface energy balance between solid-liquid and solid-solid by differing the liquid phase nickel-copper ratios. The returned samples will be analyzed by quantitative microscopy and current predictive numerical models. Densification in microgravity will be compared to ground controls. The pathway differences for microstructure-density-distortion on Earth and microgravity will be determined.
Sintering on Earth induces pore buoyancy and grain settling effects especially with high liquid contents. Gravity segregates the solid grain structure to increase grain bonding and reduced diffusion distances. The result is coarser grain size and denser skeletal structure that resists distortion. Gravity effects depend on component height, amount of liquid, grain size, density, porosity, and dihedral angle. This leads to grain size dependence along the dimension parallel to gravity. Initial results under microgravity conditions show that pore coalescence into large stable voids occur and cause component swelling. Also, several subtle factors, not typically of concern on Earth emerge to influence microgravity sintering.
Considerable opportunity exists for extraterrestrial repair and construction based on freeform fabrication from powders. Refractory materials are important for propulsion, radiation, and thermal systems. Future NASA efforts to extend human exploration back to the Moon and beyond require development of techniques and processes that permit fabrication and repair of critical components under reduced gravity conditions.
See the Space Station Research Explorer link below for more details.
Principal Investigator: Randall German, Ph.D., San Diego State University