NASA Sample Cartridge Assembly (MSL SCA-GEDS-German) - 04.19.17

Overview | Description | Applications | Operations | Results | Publications | Imagery

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Science Objectives for Everyone
Liquid phase sintering is an important means to fabricate net-shape composite materials for applications over a range of industries. The science of liquid phase sintering is about 50 years old, but practice dates from the 1400s when gold was used to bond platinum in Columbia 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 as a means to perform in-space fabrication and repair, and for example using lunar regolith to fabricate structures on the moon or using metal powder to fabricate replacement components during extraterrestrial exploration. The MSL SCA-GEDS-German (NASA Sample Cartridge Assembly) experiment 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.
Science Results for Everyone
Information Pending

The following content was provided by Randall German, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details

OpNom:

Principal Investigator(s)
Randall German, Ph.D., San Diego State University, San Diego, CA, United States

Co-Investigator(s)/Collaborator(s)
Eugene Olevsky, Ph.D., San Diego State University, San Diego , CA, United States

Developer(s)
Information Pending

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
NASA Research Office - Space Life and Physical Sciences (NASA Research-SLPS)

Research Benefits
Space Exploration, Earth Benefits, Scientific Discovery

ISS Expedition Duration
September 2015 - March 2016; April 2017 - September 2017; -

Expeditions Assigned
45/46,51/52,55/56

Previous Missions
Information Pending

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Experiment Description

Research Overview

  • Densification and distortion during sintering are the focus of the MSL SCA-GEDS-German investigation. During sintering at low temperatures, powder compacts gain strength through interparticle bonding, usually induced by solid-state surface diffusion, followed by further strengthening from densification at high temperatures.
  • Thermal softening of the solid which occurs at high temperatures further reduces the strength. Sintering densification requires the compact to become thermally softened to a point where creep strain rates often reach levels near 10-2 s-1 (1% per second) when the liquid forms. Dr. German’s measurements show the in situ viscosity during rapid sintering densification often falls below 1 GPa-s. Hence sintering densification is accelerated when the liquid phase forms.
  • Therefore, the in situ strength provides a means to separate compact densification (as required for better properties) from distortion (as required for net-shaping). This research examines the interplay that allows densification without distortion.
  • Most surprising is the beneficial role of gravity, where the gravity stress due to gravitational force acting on the powder structure induces skeletal formation to reduce distortion in contrast to sintering in the absence of gravity where no such force exists. Microgravity liquid phase sintering results in components exhibiting less densification and more distortion.
  • Microgravity data and observations are supplemented by extensive Earth-based experimentation. Based on the observations, we believe strength and viscosity evolution during sintering are the keys to understanding how components densify during liquid phase sintering. Further, the contrast and comparison of gravity effects with parallel microgravity experiments provides insight on how powder systems densify in general. Such insight on the role of gravity suggests routes for minimized distortion.

Description

The MSL SCA-GEDS-German investigation in microgravity observes phase changes and product formation within solid mixtures undergoing spontaneous reaction. Basically, during heating a powder compact gains strength through low-temperature interparticle bonding, usually induced by solid-state surface diffusion, followed by further strength contributions from high-temperature sintering densification. In cases where a liquid phase forms, densification is accelerated because of solid transport in the liquid, capillary forces, and liquid lubrication leading to grain sliding along contacts. As long as there are solid bonds or open pores in the sintering body, then there is sufficient rigidity to avoid distortion. However, rapid sintering densification leads to a loss of structural rigidity. Substantial weakness occurs when the pores are closed by rapid densification. This is because the solid skeleton is dissolved by newly formed liquid – a condition that is evident with high liquid contents or when pores rapidly coalesce. When this semisolid system also rapidly densifies, then the capillary forces associated with open pores are also lost, leaving saturated pores and no secondary source of strength.
 
In a sense, liquid phase sintering is analogous to building sand castles at the beach. The extremes exist as - 1) loose, dry sand with no strength and poor ability to hold shape 2) sand with all of the pores saturated with liquid, and although there is more strength there is still no ability to hold shape, and 3) intermediate (three phase conditions) consisting of solid grains, wetting liquid, and pores, where the liquid pulls the grains into contact and gives the greatest strength. Most surprising is the gravitational role in suppressing distortion; the gravitational stress acting on the solid grain structure induces grain-grain contact to reduce distortion at full densification on Earth. In contrast, sintering in microgravity results in significantly more distortion.
 
Further, since there is no buoyancy force on the pore space the compacts distort freely as the pore agglomerate. Also, there is extra liquid since pore filling does not consume excess liquid. Most surprising is the evolution toward hollow spheres, where all of the pores have coalesced into one single central pore and the whole body has become spheroidal. Such behavior is contrary to the earlier suggestions that sintering in space would lead to greater precision. Instead, we now see that microgravity sintering leads to lower performance, an inability to eliminate pores, and more distortion.
 
This becomes evident in trying to explain the causes of significant distortion when sintering in microgravity. Gravity induces grain settling, with top-bottom grain size, solid volume fraction, and contiguity gradients. Grain settling does not occur in microgravity, rather surface to core gradients arise. Pore elimination (densification) is difficult in microgravity. Pore coarsening and coalescence produce large pores in microgravity resulting in samples which are more porous and more distorted when compared to Earth-based sintering.

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Applications

Space Applications
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.

Earth Applications
Significant knowledge gains have come from the microgravity research, changing how Earth-based liquid phase sintering is performed, resulting in higher precision sintered products. One of the most important findings from the combined 1g and μg studies is that the rules isolated by mankind over many years of Earth-based processing do not carry over into the microgravity environment. Liquid-phase-sintering studies in microgravity are expected to provide design parameters for the low-cost fabrication of parts, for example, engine components in automobiles.

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Operations

Operational Requirements and Protocols
On Earth, densification and distortion are sequential events, and various strategies have emerged to improve distortion control in liquid phase sintering.  The first focus is on densification which occurs prior to distortion. Only when open pores are eliminated and no solid skeleton has formed can gross distortion be observed. If a solid skeleton has formed prior to pore closure the powder compact resists distortion.  The absence of gravity-induced solid grain contacts reduces the skeletal strength and the compact forms a thick shell of dense material.  One of the surprising observations in microgravity is the migration of pores to the compact center with a dense outer shell. GEDS experiment samples are contained in Sample Cartridge Assemblies (SCAs) and are fully processed the Materials Science Research Rack (MSRR) Materials Science Laboratory (MSL) Low Gradient Furnace (LGF). After return to Earth, the SCAs are disassembled and the experiment samples are destructively analyzed by the Investigators. The Crew members insert the SCAs one at a time into the MSRR MSL LGF for processing. Following power on, the temperature/power profile is adjusted to achieve the isothermal conditions required to perform the experiment. Temperature sensor data as well as other data are downlinked to Earth during the experiment run to enable the investigators to tweak parameters as needed. All data will be downlinked and provided to the investigators for detailed and in-depth assessment by the investigators.

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Decadal Survey Recommendations

CategoryReference
Applied Physical Science in Space AP3
Applied Physical Science in Space AP5
Applied Physical Science in Space AP11

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Results/More Information

Information Pending

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Related Websites

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Imagery

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Example of liquid phase sintered tungsten alloy.  Image courtesy of San Diego State University.

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Schematic of the International Space Station furnace liquid phase sintering.  Image courtesy of San Diego State University.

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The sintering furnace prior to launch on the International Space Station.  Image courtesy of San Diego State University. 

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