These materials offer combinations of high hardness, low melting temperature, low density, and formability like a plastic.
Spacecraft shielding is defined as the outer layer of a satellite or spacecraft that protects it against micrometeorite and orbital debris (MMOD), radiation damage, and re-entry temperatures. There are several problems with the design and implementation of shields, particularly in the area of MMOD shielding. Spacecraft and satellites need to have the lowest possible mass due to the enormous cost per pound of putting them into orbit or deep space. However, low Earth orbit (LEO) is currently littered (and increasingly so) with orbital debris, primarily remnants of rocket upper stages, satellites, and pieces of spacecraft that have broken away or have collided with other objects. The major threat is that this debris is traveling at 8 to 18 km/s, and any piece larger than a few centimeters has the kinetic energy to potentially become a “spacecraft killer.” Large debris is tracked with radar, but the smaller debris (below a centimeter or so) is too small to track and must be mitigated by shields in the event of a collision. The International Space Station, for example, employs over 500 different shield designs into its outer skin, which are designed to protect a variety of vital components.
Designing new materials for MMOD shielding is a compromise among ballistic performance, areal density, volume, and geometry. Shields are often comprised of multiple materials stacked in layers or as foams and function by vaporizing, diffusing, and catching a projectile without compromising the inner wall of the spacecraft. Materials that have been used in shields include aluminum panels and foams, titanium, carbon fiber, Kevlar, and Nextel cloth, among others. Aluminum is widely used both as panels and as cellular structures due to its low areal density and easy fabrication; however, aluminum is a soft metal with a low ballistic limit compared with other metals (like titanium and steel, for example). Empirical ballistic limit equations have been developed for many potential shield materials, which has lead to the observation that their hardness is one of the best indicators of shielding effectiveness. A new class of metal alloys, which seems to have an optimal combination of properties for MMOD shielding, are bulk metallic glasses (BMGs, also called amorphous metals) and their composite derivatives. BMGs are metal alloys that have been designed with chemical composition and high cooling rates such that they freeze in an amorphous (non-crystalline) state. This gives them unique mechanical properties, including ultra-high strength and hardness, low stiffness, density similar to titanium, and formability like plastics. BMG matrix composites, which are reinforced with soft, crystalline phases that grow as dendrites, are alternative materials that exhibit the same beneficial mechanical properties as the monolithic BMGs, but with the added benefits of ultra-high fracture toughness and ductility, making them suitable for highperformance structural applications. Particularly, BMGs have high hardness (6 times harder than Al alloys), relatively low density (twice as dense as Al alloys), and low melting temperatures (the same as Al alloys), which makes them effective at vaporizing incoming debris while assuring that the part of the shield that is impacted also melts or vaporizes, preventing solid debris from hitting the spacecraft wall.
Over the last two years, JPL, in collaboration with Johnson Space Center and the University of Southern California, have been performing the first hypervelocity impact tests on BMGs and their composites, with promising results. Thin, corrugated panels of BMG composites, which have been formed into multi-layered cellular structures through capacitive joining, have shown to be extremely effective at mitigating impacts from Al projectiles (see the figure). In recent work, metallic glass sheets were substituted for Kevlar in shields designed for the International Space Station and tested one-to-one against conventional shields at impacts up to 7 km/s. The new shields outperformed the heritage ones in the preliminary testing. Since BMGs and their composites can be formed into large sheets and panels with unmatched mechanical properties, they are potentially optimal materials for integration into future spacecraft and satellites.