May 9, 2023
T-09: Window
When building products for space, one must optimize for conditions that are not typically considered for products on Earth. The crystal of a wristwatch provides an illustrative example. In this article, we explore key requirements and how they can be addressed through different paths in materials and technologies. Aside from excellent optical qualities, the aim for the Monolith window has been to strike a balance between shatter and scratch resistance - both critical attributes for products used in space.

Meeting the requirements of space

When venturing beyond the protective atmosphere and gravitational pull of our home planet, we find ourselves in an environment vastly different from that of Earth. In space, human life must be continuously safeguarded against extreme temperatures, vacuum, high levels of radiation, and the potential threat of space debris hurtling through space at supersonic speeds.

Even within the protective layers of a spacecraft, microgravity and high levels of radiation still pose significant risks. As a result, NASA and other space agencies impose strict requirements on equipment intended for use in space, to ensure their reliability and safety. For example, the crystal of a watch may present a much greater risk in a microgravity environment than on Earth.

Imagine shattering a regular crystal here on Earth. While unfortunate, this would usually not be much of a concern in terms of your health and safety. In contrast, a shattered crystal in space would result in a myriad of fragments floating around in your spacecraft and posing a serious health risk. Similarly, any compounds emitted from objects due to degassing can have a much more significant impact onboard a spacecraft than on Earth. Afterall, you can’t just open the window to get some fresh air.

NASA astronaut Tracy Caldwell Dyson gazes at the Earth from the cupola of the International Space Station. The windows are made of several layers of fused silica, which is a type of glass that is highly resistant to thermal shock and radiation damage. They are also designed to withstand micrometeoroids and other debris that could potentially damage the space station.

For this reason, NASA has strict requirements toward equipment used for Intra-Vehicular Activity (IVA) and even more so toward equipment designed for Extra-Vehicular Activity (EVA), which is a term used in the context of space exploration to describe any activity performed by astronauts outside of their spacecraft. This can include activities such as spacewalks, repairing and maintaining equipment, or conducting scientific experiments. 

For example, while here on Earth it is common to consider heavy atmospheric pressures (such as diving-related applications), in space we have to ensure that equipment operates well in the absence of pressure, ie. in vacuum. This is an even greater challenge due to extreme and rapid temperature changes that can lead to thermal expansion and contraction of materials. Due to the absence of an atmosphere, brightness and darkness are also much more intense. Objects with reflective surfaces can become a considerable distraction and limiting visibility.

Russian cosmonaut Alexander Skvortsov participates in the maintenance of the International Space Station during an extravehicular activity (EVA).

Striking the right balance

Choosing a material is always a quest for striking a balance between various trade-offs in order to match the requirements of a specific application. Watches often put an emphasis on scratch resistance, requiring hard materials such as sapphire and ceramic. While a high level of hardness makes scratches less likely, it also results in reduced flexibility and a greater chance of shattering; a crucial factor to consider in microgravity conditions.

During the development of Monolith, we explored and tested a variety of materials. This includes more traditional paths like sapphire and hesalite, as well as more exotic materials including aluminum oxynitride, ballistic glass, and various types of coatings. 

One of the first materials we looked into was diamond glass, which is created through a new deposition technology that enables diamonds to be lab grown as entire 2mm thick windows. It can also be used as a coating to deposit a thin protective film on top of an existing sapphire window. While this results in a window that is extremely scratch resistant (Mohs hardness of 10), the mechanical properties were actually not a good fit for our application because its extreme rigidity also causes diamond to be much more brittle, similar to a ceramic.

On the other end of the spectrum we have acrylic, also known as Plexiglas or Hesalite in the watch community as it was branded by Omega in the 1940s. This material is a great pathway for aerospace applications as it is highly impact resistant due to its flexibility, and even if pushed past its breaking point it simply cracks instead of shattering into millions of little pieces. This flexibility however comes with a tradeoff of being prone to scratches, having a Mohs hardness between 1 and 2. This tradeoff is also why it has become such a hot debate for as long as there have been watch forums; Acrylic or Sapphire?

In contrast to acrylic, traditional sapphire crystal is one of the hardest materials on Earth, with a hardness of 9 on the Mohs scale (second to diamond), which makes it highly scratch and corrosion resistant. Compared with many polymer-based materials, sapphire is also a lot more resilient to UV radiation and will not show yellowing or aging. Sapphire crystal is actually not a crystal at all, but a form of aluminum oxide that is transparent. Under extreme force or impact, however, sapphire can be chipped or cracked.

From this pathway we explored Bulletproof glass, combining the scratch resistance of glass with a ballistic TPU layer, similar to a car windshield. In the event the sapphire glass were to break, the TPU layer prevents the glass from "spalling" and shattering off into tiny fragments. The issue with this is that the TPU layer does not behave as well in the extreme temperature ranges of space, leading to potential delamination. The vacuum of space also causes the TPU to off-gas, releasing chemicals and changing its overall characteristics.

We also explored a material called transparent aluminum - no longer a dream of Star Trek science fiction. This material is also known as Aluminum oxynitride and is actually a transparent ceramic much like Sapphire. Unlike a monocrystalline sapphire window, Aluminum oxynitride is a polycrystalline structure allowing it to be made of powder which is then compressed and heated to extreme temperatures. The end result is a material which behaves much like Sapphire glass, however with slightly lower flexural strength and hardness. The primary advantage with this material is in how it is made. The compressed powder process allows the material to easily be shaped into more complex shapes than sapphire. In contrast to sapphire, this makes it especially suitable for large-scale components like windows for the ISS.

The Monolith window

Like for all the components of Monolith, the objective for the window has been to optimize it for the harsh conditions of space travel. The window illustrates that this is not a straight-forward task because different attributes need to be weighed against each other to match the requirements of a specific application. While a material such as hesalite provides the advantage of being highly shatter-resistant, this comes with a tradeoff of being prone to scratches and other forms of material fatigue. In contrast, traditional sapphire is more rigid, highly scratch resistant and offers very good optical properties that remain unchanged over long periods of time. 

To identify a middle way between these attributes, we have also been considering more exotic materials and “out-of-the-box” solutions. This means we are approaching product development with an open mind, ready to explore new paths. At the same time, we bear in mind that old and established paths have emerged for a reason, and we adopt innovations only if they provide a functional advantage.

For the Monolith window, we are currently developing a solution that involves optical grade C-plane sapphire crystals grown to the exact spec thickness to surpass all EVA/IVA impact testing laid out by NASA. The crystal will then be seated in a custom L ring gasket system to provide extra suspension shock absorption for heavy impact. This implementation will result in a very robust setup that is also extremely scratch and corrosion resistant. 

Until the launch of Monolith, we will continue to explore and iterate on our design and material choices which might result in new insights. We also look forward to any feedback, opinions and ideas from our community.

This behind the scenes image is part of an artistic series titled “Formation” captured by Michael Sorkin with a Leica M10 Monochrome and Summicron 35mm APO lens.
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