The surface treatment technology company ENBIO based at Belfield Campus, Nova UCD received a contract from the ESA (European Space Agency) in May 2014 to provide a patented surface treatment (CoBlast) to their Solar Orbiter Mission’s main heat shield. The contract was awarded after ENBIO’s surface treatment solution was subjected to extensive tests at ESTEC (European Space Research and Technology Centre), the technical headquarters of ESA in the Netherlands. The approved ENBIO solution makes use of new as well as old technologies. It incorporates a pigment that has been used in cave paintings 30,000 years ago into the company’s proprietary CoBlast process.
The M-class mission Solar Orbiter, currently being developed by the ESA in collaboration with the NASA (National Aeronautic and Space Administration), is scheduled to be launched in January 2017. The aim of the seven-year mission is to find answers to questions about the sun and the heliosphere, the electromagnetic bubble that covers the solar system. Further, the spacecraft will travel as close as 42 million kilometres to the sun during the mission, approximately one-fourth of the distance between the sun and the Earth, making the man-made object to go so close for the first time. The Solar Orbiter is designed to take photos of the sun at close quarters and at a higher angle, than has ever been done before.
Therefore, the sensitive optical instruments of the spacecraft need to be protected against the radiation from the sun just as it is required for human eyes. It is the complex heatshield that is provided at the spacecraft’s front that ensures protection. When the Solar Orbiter goes around the sun, the heatshield will always be facing the sun. As a result, the heatshield gets exposed to heat which is 20 times more than the heat experienced on earth, approximately 520 degrees Celsius. The heatshield consists of multiple layers of insulation and titanium foils. This is to ensure that the electronic gadgets and instruments provided on-board operate under temperatures that are more manageable.
Additionally, the surface of the heatshield facing the sun is to be provided with a coating which maintains stable as well as favourable thermo-electric properties throughout the duration of the mission in order to control the absorption and radiation of sun’s heat. As such, the coated surface should be capable of converting solar radiation into infrared rays of lower energy which will be radiated back into space so as to control the heatshield’s surface temperature.
The thermal stability of the coating is important because it helps to eliminate outgassing, a phenomenon in which volatile material is released at decreased pressure or higher temperature. Volatile material released during outgassing may condense once again and settle on the lenses of instruments. This can cause on-board instruments to show erroneous readings. Additional, the electrical conductivity of the coated surface should be good. This prevents building up of static charge on the coated surface of the heatshield. Excessive build-up of electrical charge results in arcing, causing damages to the instruments.
Another requirement to be met by any coating solution is its suitability for use on titanium foils that are 50 micrometer thick (half as thick as human hair). Prior to accepting the technology, the coating as well as the process was subjected to rigorous quality tests as part of which a full-size structural thermal model of the heatshield was constructed.
Finally, the tests carried out on the model and other samples are as follows:
#1: Accelerated radiation tests at 500 degree Celsius and under hard vacuum at ESA’s Synergistic Temperature-Accelerated Radiation Facility
#2: Fast thermal cycling (between 100 degree Celsius and 700 degree Celsius) under vacuum
#3: Outgassing in the XTES facility of ESA
#4: Measurement of electrical conductivity.
Prior to and after each test the thermo-optical properties were determined to see if there was any coating degradation.
ENBIO’s CoBlast Technology
ENBIO’s coating technology CoBlast, originally developed for titanium implants on artificial bone, is made use of to replace the oxide layer on metals and provide a customised and thin mechanochemically bonded layer. It is a clean process as it neither requires thermal input or wet chemistry. Further, CoBlast executes the abrasive blasting and deposition processes in one step. This leads to exceptional bonding between the substrate and coating.
Weight is always critical in space missions and, at just 3-5 microns thick, the coating also adds very little weight – even when applied over large components. In honour of the mission, ENBIO decided to name the new coating SolarBlack. In all space missions, weight is a critical factor. The weight added by the 3 to 5 microns coating is extremely low. ENBIO has named this coating as SolarBlack in honour of the mission.
On completion of the tests at the laboratory level, the full-scale STM developed by the Italian company Thales Alenia Space was tested. As many as 30 titanium foils were coated using the CoBlast technology and stitched together to form a heatshield of size 3.1 × 2.4 metres. During testing, extreme cold conditions prevailing in space was simulated by cooling the back wall of a vacuum chamber having a diameter of 10 metres and height of 15 metres to –170 degrees Celsius using liquid nitrogen. On the other hand, light from as many as 19 xenon lamps of 25 kW each was focused using mirrors in order to heat the front side of the heatshield. This was done to ensure that the STM model accurately represented the actual working condition in space. The tests that lasted for about two weeks were completed by June 2014.
Following the successful testing of the heatshield, ESA requested ENBIO to increase its capacity so as to provide coating for more complex as well as larger parts such as the High and Medium Gain Antenna of the Solar Orbiter. ENBIO has expanded its facility with the help of Enterprise Ireland and the ESA.
Currently, a coating of ceramic material for other parts of the Solar Orbiter – including the main antenna, solar arrays and instrument booms – that are subjected to sun’s radiation is also being tested. The coating, referred to as SolarWhite, is provided in order to reflect the solar radiation instead of absorbing it. This is being developed jointly by Dr Kenneth Stanton, a senior lecturer, and Kevin Doherty, a PhD researcher.
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