The Use of Protective Coating on Carbon Nanofiber Composites to Guard against Space Generated Atomic Oxygen

Mayard Williams

University of Cincinnati

Abstract - The space environment is composed of complex systems that can destroy the overall reliability of materials used for space vehicles and structures. The synergistic effects from atomic oxygen (ATOX), UV radiation, thermal cycling is the primary source of these energies that creates unique challenges for designers. The study on protective coating used as a shield against primarily ATOX on the surface of a 3-phase Carbon Nanofiber (CNF) composite is undertaken in this research. SG120FD is a white silicone based off the shelf product that has significant properties in protecting against ATOX as well as other space generated energies. The capabilities of CNF along with a protective coating are presumed to enhance the CNF material while traversing thru Lower Earth Orbit (LEO) and deep space. The samples for this research were created using space rated epoxy to minimize outgassing while still maintaining the matrix’s overall integrity. Samples with and without SG120FD coating were then exposed to a simulated ATOX generator under vacuum equivalent to several months on orbit. The effects of the ATOX exposure were studied using Raman-Spectroscopy (R-S) and mass loss measurements. It was found that samples that had the protective coating did not have as much mass loss in comparison to the reference sample. The R-S analysis performed did not indicate any signal loss or peak to peak shifts that would indicate structural change.

Keywords: Carbon nanofibers, atomic oxygen, protective coating, space environment, composites.

1.0Introduction

The space vehicles traveling into space will not only be bombarded by cosmic particles and atomic oxygen (ATOX) but will also be exposed to radiation effects detrimental to sensitive equipment. To achieve the necessary protection for space vehicles and crewmembers it is imperative to design, understand, and implement proper shielding technologies. The only problem is that current technology has not caught up with our ambitions due primarily to the space radiation environment. Current technology has limited our space programs to near orbit exploration and development [1]. Understanding the Sun’s effect (which is the primary source for space radiation

n) on the space environment will lead to a thorough explanation on how cosmic rays and particle events are developed and where they come from. New technology and ideas are constantly being developed and researched, such as magnetic and electrostatic shielding, along with advances in some form of composite material [2]. The latter is where this research will attempt to develop the various possibilities for protection.

Nanocomposites or carbon nanofiber have emerged onto the scene as a technological revolution in more recent years. Studies have shown that they have the same or similar capabilities as aluminum including the all-important conductivity property. Carbon nanofiber’s (CNF) have a high Young’s modulus (which compares the strength and stiffness), very conductive, and can withstand a wide temperature range [3]. More important due to weight bogeys the CNF are much lower in density then aluminum, making it ever more attractive as an alternate.

CNF’s are shaped as a cylindrical structure measured by its length to diameter ratio. Their properties are so innovative that they are utilized in many applications across many industries. These CNF’s are manufactured thru a series of processes into the composites matrix to develop the nanocomposites. The chemical bonds are similar to graphite, which makes it much stronger than other bonds found in different elements. The CNF’s mechanical property is determined by measuring the amplitude of their thermal vibration, and has shown to have an average Young’s modulus of 1.8 TPa and a tensile of strength of 600Gpa. These characteristics make CNF hundreds of times stronger than steel and significantly lighter [4]. Carbon nanofiber specific properties are the capability to align themselves into ropes held together by the Van der Waals forces, which aids into the overall strength characteristics [5], [3]. CNF’s have demonstrated thru test to have a large surface area and subsequently a larger interface for stress within the composite they combine with. CNF’s are easier to adapt to various products and applications due the ability to disperse for long periods of time thru processing techniques.

With significant studies being done on nanocomposites and their appropriate role for applications on Earth, it is still unknown how these materials will hold up to the harsh space radiation environment. Research to date has already determined that regardless of the many benefits that nanocomposites afford, the harsh space environment will affect the material in some manner [6]. The major effect from radiation, particularly exposure to gamma, atomic oxygen, and electron radiation on polymeric or graphite/epoxy materials causes chemical radicals and thus material damages [7]. It is theauthor’s intentto study in depth what causes the onset of the CNF material degradation. To also determine the catalyst after irradiation or a simulated space environment that begins to compromises the material is of primary importance to this research. Forneset. al [7] has documented several mechanical measurements that showed little or no change in flexural properties of graphite or epoxy specimens. This research will make a case by using supporting literature on the mechanical properties such as tensile, modulus testing and conductivity measurements pre and post irradiation of similar material to note what will typically happen to these materials.Therefore, the focus will be on applications used to protect the composite matrix.

Raman Spectroscopy (R-S) is a widely developed technique of choice to support the detail properties of the carbon based materials. R-S is essential in the study of structural behavior, vibrational, and rotational in molecules. Raman can be very useful in studying solid, liquid and gaseous samples. Here in this study Ramen Spectroscopy is used to study the changes in CNF and understand how ATOX interacts with the composite structure.

Because graphite fibers erode at a lower rate than epoxy matrix, they can partly protect the inner layers of matrix material from ATOX attack. Erosion rate predictions for short missions should be based on the characteristics of the matrix material. For long duration or missions encountering high ATOX doses, the fibers resistance becomes the dominant factor [8].

Atomic Oxygen is generated when the UV radiation is absorbed into the oxygen molecules. This will change their properties and can create negatively charged ions. The orbit where these effects are more prone is considered the upper atmosphere 100-650 kilometers. The ATOX densities at this level are a significant function of the activity generated from various solar events. The densities of ATOX are approximately 2 – 8 x109 atoms/cm3 with a flux of 1014 to 1015 atoms/cm2. The kinetic energy ofapproximately 5eV generated has the greatest effect on materials and can cause erosion of the surface, mass loss, and degradation in mechanical, thermal, and optical properties. Long term exposure can erode materials at 450 kilometer and up to 600 µm deep over 27 years [9]. While ATOX is considered the most hazardous the synergistic effects along with UV coupled with thermal changes provides the most detrimental challenges to materials.Synergistic action of atomic oxygen and thermal cycling degrades the matrix by chain scission, crosslinking and micro-crack damage, altering the composite's properties. The erosion induced by oxygen atoms provokes a decrease in flexural stiffness. This decrease may be drastically amplified for long duration exposure and could limit the service life of such materials in spacecraft applications.UV radiation generates energies great enough to separate the molecular polymer C-C and C-O bonds, which also alter material properties. High vacuum in the space environment less than 10-5Torr contributes to the overall affect on materials which results in loss of the materials dimensional stability and material contamination at the surface levels. Outgassing in poly based material is prevalent within the high vacuum condition and also contributes to the degradation of material properties. Therefore, designers must use low outgassing materials and epoxy in their design envelope to counter this effect. The overall synergistic effect in LEO and beyond can be damaging on composite materials. ATOX exposure by itself isvery harsh and along with UV radiation creates a very destructiveeffect on composites [9].

To attempt to protect space hardware from the natural space environment various methods have been researched and tested including the use of top coat paints. The use of specially qualified paints can provided a passive barrier against ATOX, UV, and thermal changes. The properties and characterization that classify these paints are the solar absorptance (αs), the effectiveness in absorbing radiant energyand IR emittance (ε) values, the ability to emit radiation energy. Essentially, to maximize heat removal and limit the heat absorption low αs/ε ratios should be utilized in the paint selection. The degradation of physical properties is exposed during the space environment life span for a material coated with paints. Since paints can be prone to damage caused by the space environment careful consideration to using paints that have excellent resistance to the space environment must be utilized. SG121FD is a protective coating that has exhibited superior thermal protection and excellent thermo-optical properties. In addition to the inherent characteristics it also exhibits excellentresistance to ATOX and UV radiation along with space rated electrons and protons. SG121FD has been recommended as suitable satellite radiator paint based on the study completed by Anvari, [10] which highlights these specific properties. The essential chemical makeup ofthis material is composed of a silicone binder and a modified zinc oxide pigment; both of these properties have been demonstrated to be durable under ATOX attack. Under SEM (scanning electron microscope) results show the SG121FD was the most efficient protective coating for ATOX in comparison to other commercially available products. The most significant detected effect of ATOX on the paint was a decrease in carbon atomic concentration from 24.4% at. to 11.9% at. after exposure to an ATOX fluence of 2.7 x 10 21 atoms/ cm2[11]. The study cited in this research promoted the use of this product as a thermal protector which minimized the effects in hot or cold conditions [10]. This research is using its critical properties to protect against the ATOX in the simulated space environment on a 3-phasecomposite nanofibers, which is different then what was done in any past research or its primary intended use.

2.0 Experimental Details

To better understand how ATOXaffects on compositesa 3-phase CNF matrix was developed with the following materials and designused in this research. The first ingredient to the 3-phase mix is EPO-TEK a low outgassing adhesive space grade epoxy. This epoxy is acceptable because the total mass loss (TML) is less than 1% and the collected volatile condensable mass (CVCM) is less than 0.1% of the initial sample mass as calculated below using initial (S1) and final (S2) data.

100 = TML% (1)

The commercial product identifier is301-2 and has a TML of 0.89% and a CVCM of 0.01% when cured at 800 C for 3 hours. It should be noted that physical and mechanical degradation is observed in certain materials when 3 to 5 percent of the mass is lost. Therefore, it’s important to consider this criterion in the epoxy profile selection. The carbon fiber fabricused was developed at ACP Composites and offers high strength to weight ratio, thermal and electric conductivity properties that can be tailored to various applications. The fabric properties were .009 inch thick 3K Carbon Standard Modulus plain weave at 5.78 oz/yd2 which makes up the 2nd ingredient in the 3 part CNF. The last addition to the matrix isthe carbon nanofibers (XT-LHT-AM) developed by Pyrograf Products, Inc. The density and fiber diameters are 1.55 to 1.70 g/cm3 and 50-200 nanometers respectively, and were chosen because of mechanical and electrical property enhancement and the ease to process and disperse. The additional protective coating SG121FD White Silicone Coating was manufactured by MAP France, which details were described in the previous section. The 3-phase composite was manufactured and processed at the University of Cincinnati laboratories. The CNF was applied as a percentage of the total weight of the epoxy mix using a drill to fully disperse the CNF into the epoxy. The epoxy/CNF mixture was placed in a vacuum chamber for up to 3 minutes or when visible bubbles were removed. The next step was to apply four layers of the composite fabric into a 2x2x.25 inch quad Teflon mold and apply the vacuumed down epoxy mixture between layers. Once set in the mold the unit went into the oven and cured for 4 hours at 800 C as noted in Table 1.

Table 1. 3-Phase CNF Manufacturing and development process

Post curing samples were removed and weighed then sectioned further into 1x1inch pieces to go into their respective testing block as shown in Table 2. The white silicone paint was then added to the samples with a broad brush leaving a 100 µm layer of coating behind on the sample.

Table 2. Testing Matrix used for CNF samples

To first establish a baseline the samples that were created were studied under R-S where a spectra was obtained for each sample with and without the protective coating. The samples were then sealed and shipped for ATOX and UV analysis at Cal Polytechnic State University [12]. The ATOX simulation consisted of a vacuum chamber, sample containment system capable of holding 4 - 2x2 inch samples, including the Kapton HN sample. The atomic oxygen simulation was delivered using RF power to create capacitively coupled plasma operating at 13.56 MHz.The UV radiation simulation comprised of a Hamamatsu L10706 vacuum ultraviolet (VUV) light source. After the ATOX simulation was performed the samples were shipped back and analyzed under R-S system for pre and post comparison.The simulation duration was equal to the orbital exposure of weeks to months which equates to an ATOX flux of 1.7 +/- 0.07x1016 atoms/cm2 and the equivalent sun power of 120-200 nm UV light or 108 ESH (equivalent sun hours). The base pressure of the simulation system was near 175 mTorr. When combined as an overall system with the testing time of 24 hours there is a calculated impact of 1.70 x 1021 atoms/cm2of ATOX on the sample. All samples were weighed before and after exposure to the 24 hour ATOX test and mass measurements were taken with a precisely calibrated analytical balance.

Raman spectroscopy machine using a 785 nm line diode was used to capture the spectra of the samples. The spectra was acquired over 250-2500 cm-1 with an operating power level of 100mW. The Raman system consists of four major components. 1. The excitation source (Laser). 2. Sample illumination system and light collection optics. 3. Wavelength selector. 4. Detector. The sample is illuminated with the laser beam in the UV or near infrared (NIR) range. The scattered light is then collected with the lens and is sent through an interference filter or spectrophotometer to get a Raman spectrum of the choice samples. During analysis the prime focus will be mainly on the D-band which is a function of the defects in the CNF structure. TheR-S will be able to highlight any defects that may be attributed to the exposed samples. The baseline sample was used to ground the findings and determine if and how the samples changed. Our study will also focus on the G band which will determine if any defects where caused by the ATOX effects and how it may be related to the carbon properties. The D-band which represents the Lorentzian curves is to a function of the vibrational modes generated by the atoms and also highlights defects and the changes present in the material.

3.0 Results and Discussion

Kapton HN was used as a reference sample along with 7 other CNF samples with no differences associated to processing techniques. To capture the total mass loss at average initial and average final was calculated from five measurements. The characterization of potential of a material exposed to ATOX can also be described as erosion yield. Erosion yield is the ratio of volume or mass of material loss per each incident oxygen atom [13]. Due the ATOX attack on the material erosion yield data from experimental studies is used to calculate the overall effective mass loss Table 3.

Table 3.Target materials and corresponding ATOX reactivity [14].

Table 4 represents the average mass loss due to the effective ATOX fluence on the sample. The Kapton HN reference sampleand A4a CNF sample underwent the same exposure time of 24 hours to the ATOX. The Kapton sample after ATOX measured a 3.0% total mass loss versus the coated composites. The SG120FD white paint on the CNF composite with an erosion yield of 1.7x10-24 cm3/atom and after an equivalent ATOX fluence of 1.68 x1021atoms/cm2shows the effectiveness of the paint product with reference to TML. While there was mass loss on the A4a samples and all samples under this study, the effect of the coating on the CNF demonstrates that the mass loss is significantly small enough where there was no mechanical and physical degradation of the CNF.

Table 4. Mass loss and fluence data for 3 phase CNF coated composites under study, samples of note are A4a, A2a, and Kapton HN reference.