Materials under extreme environments

1. Materials Research in a microgravity environment

Microgravity alters many observable phenomena within the physical and life sciences. Systems and processes affected by microgravity include surface wetting and interfacial tension, multiphase flow and heat transfer, multiphase system dynamics, solidification. Extreme conditions in the ISS environment include exposure to extreme heat and cold cycling, ultra-vacuum, atomic oxygen, and high-energy radiation. Testing and qualification of materials exposed to these extreme conditions have provided data to enable the manufacturing of long-life, reliable components used on Earth as well as in the world’s most sophisticated satellite and spacecraft components.

Soldering is acknowledged as an important joining and repair technique that is likely to be utilized during extended space missions. To enable repairing electronics at the lowest component level for future exploration missions beyond low Earth orbit, it is essential to facilitate the production of soldering joints in microgravity conditions that have adequate mechanical properties. An experimental effort is proposed to characterize both the microstructure and resultant micro to the nanomechanical response of solder in terrestrial vs. microgravity environments, i.e., 1g vs. 10^-5g. Results from the In-Space Soldering Investigation (ISSI) experiments performed aboard the International Space Station (ISS) have shown that soldering in microgravity is expected to be considerably different than their ground-based counterparts due to Earth’s natural convective flow and buoyancy effects being minimized in microgravity during melting and solidification. The ISSI data has demonstrated that a lack of buoyancy forces in microgravity can internally trap the flux created during soldering at interfaces, such as repair joints, and produce the porosities. These internal porosities can be detrimental to the desired strength of the joint, as well as its thermal and electrical conductivity.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1. a) Photograph of a) a ground-based 40Pb-60Sn solder drop hanging from a silver-coated copper wire c) a microgravity solder (Football shape) where it was wrapped about the test wire, melted, and solidified. b) SEM image of the cross-section of ground-based solder d) cross-section of microgravity solder sample. Microgravity solder sample shows increased porosity.

 

The main aim of our work is to characterize the microstructure and nano-mechanical response of solders in terrestrial vs. microgravity environments using an array of nano-mechanical testing tools such as indentation and focused ion beam (FIB) fabricated micro-pillar compression and micro tensile testing, and nanoscale electrical contact resistance (nano-ECR) measurements.

A further aim of our work is to understand the substantial effect of aging on the solder microstructure and electrical and mechanical properties. Aging effects will be accounted for through testing of fresh ground-based solder prepared and aged in our lab, prepared using identical equipment and supplies as in the ISSI experiments.