ATOMIC OXYGEN-RESISTANT, LOW DRAG COATINGS AND MATERIALS

BACKGROUND OF THE INVENTION

Field of the Invention

[1] The present technology relates to coatings and materials that can minimize atomic oxygen (AO) degradation and reduce drag on an object, such as a spacecraft. The present technology also relates to a method of reducing drag and minimizing AO degradation of spacecraft intended to operate in harsh environments, such as stable Earth orbits at about 100 km-350 km, or orbits around other celestial bodies, such as Mars, that offer similar harsh conditions.

Description of Related Art

[2] Satellite orbital altitudes above Earth are typically categorized in three broad segments: low Earth orbit (LEO), medium Earth orbit (MEO) and geostationary Earth orbit (GEO). The general uses and characteristics of these orbits are shown in Table I and represent generally accepted usage of the terms LEO, MEO and GEO. Satellites can orbit at any altitude, and the gaps in altitude shown in Table 1 , such as between LEO and MEO, are also used, if less regularly. It is also common that satellites may orbit in eccentric, non-circular orbits, thereby passing through a range of altitudes in a given orbit.

[3] Atomic oxygen (AO) dominates the rarefied atmosphere at altitudes of 150 to 700 km (see Fig. 1 ). The concentration of AO varies with altitude and is dependent on several additional factors, including solar activity, season, local time, latitude, and variations in the Earth’s magnetic field. At altitudes lower than LEO, i.e. below about 400 km, atmospheric density increases exponentially. A typical O-atom number density at an altitude of 300 km is on the order of 10 ⁹ cm ^-3 , but it increases to more than 10 ¹ ° cm ^-3 as the altitude drops below 200 km. Orbiting spacecraft that fly at these altitudes travel at high velocities. The average relative velocity between such spacecraft and ambient oxygen atoms may be 7.4 - 7.8 km s ^-1 , depending on orbital altitude and inclination. The combination of impact velocity and O-atom density yields fluxes on the order of 10 ¹⁵ to 10 ¹⁶ O atoms cm ^-2 s ^-1 on ram surfaces of spacecraft. The gas-surface collisions are equivalent to oxygen atoms with a mean translational energy of ~4.5 eV striking a ram surface. The roughly 1000 K kinetic temperature of the atmosphere produces an energy spread (full width at half maximum) of 2 eV in the gas-surface collisions. Energetic collisions of ambient oxygen atoms [ground-state, O( ³ P)] with spacecraft surfaces lead to degradation of materials through oxidation and etching (erosion). [4] Structural and thermal-control materials consisting of organic polymers are particularly susceptible to oxygen-atom attack. High-energy collisions lead to the production of volatile products that carry mass away from the polymer surface and leave a roughened surface. Polymeric materials on spacecraft in LEO are also susceptible to high fluxes of vacuum ultraviolet (VUV) radiation, which may promote their degradation through multiple photochemical processes. Photons might act either alone or in combination with oxygen atoms to degrade polymers and thus limit their usefulness.

[5] Kapton (polyimide) and FEP Teflon are commonly used in thermal blankets on an oxidizing external surface of spacecraft. Both of these materials are subject to erosion in the space environment. Even though FEP Teflon is predicted to have a lower erosion depth, this material is still expected to erode -40 pm at 330 km, -200 pm at 240 km, and -1 .6 mm at 160 km during a three year mission. The estimates at lower altitudes are conservative, as collision-induced processes involving N2 are expected to play a significant role in assisting O atoms in the erosion of the material. It is clear that unprotected carbon-based materials (even fluorocarbon materials) will erode significantly during a three-year mission at relatively low Earth orbital altitudes. Given the harsh environment in LEO steps must be taken to protect polymers that are to be used on spacecraft that operate in this environment, or make such polymers more durable.

[6] Common approaches to make polymers more durable in LEO involve inorganic constituents, either as coatings, particle additives or fillers, or components of the polymer backbone. [7] Spacecraft coating materials for protecting polymers include metals, metal oxides, and semiconductor oxides. Potential metal coatings, such as silver and copper, have been tested in the LEO environment, but they oxidize readily and need to be protected. For some specialty applications, the noble metals, gold and platinum, should be durable in an oxidizing environment. The metal oxide coatings, AI2O3, TiO2, and SnO2, have shown remarkable resistance to atomic oxygen (AO) attack. Still, the most commonly used coating material is the semiconductor oxide, SiO2, which is almost as resistant to an AO attack as AI2O3 and, like AI2O3, does not alter the thermo-optical properties of the material in a deleterious way. However, such inorganic coatings are subject to cracking through handling and thermal cycling, and to damage such as pinholes, cracks, or scratches from micrometeoroid and debris impacts, leaving the underlying material exposed to AO attack.

[8] Particle additives to improve polymer durability include nanoclay, silica, and graphite. Adding particles of these materials is attractive because large-scale films of the composite materials can be produced simply and with relatively low cost. Some such polymer nanocomposites exhibited better AO resistance than control samples of pure polymer. For example, an epoxy/silica blend (with 5 wt % of 100 nm diameter silica particles) showed an erosion yield of only 0.18 X 10’ ²⁴ cm ³ atom ^-1 after exposure to an AO fluence of 1.97 X 10 ²¹ O atoms cm ^-2 . This erosion yield compares favorably to the commonly used Kapton H and HN polyimide films, whose erosion yields are 3.00 X 10’ ²⁴ cm ³ atom ^-1 and 2.80 x 10’ ²⁴ cm ³ atom ^-1 , respectively. On the other hand, polymer nanoclay composites do not have such high AO resistance, and erosion yields in an AO plasma of 0.78 X 10’ ²⁴ cm ³ atom ^-1 and 2.91 X 10 ^-24 cm ³ atom ^-1 for nylon/7.5 wt % nanoclay (Nanocor 1.30, with size of 8-10//m) and epoxy/6.0 wt % nanoclay (Nanocor 1.30, with size of 8-10 //m) composites, respectively, have been reported. As these inorganic additives do not dissolve in polymers, they usually aggregate to form inorganic protective layers as organic material erodes away. However, these protective layers are not continuous, so O atoms can penetrate between the gaps and continue the etching of the organic component. Another potential problem with particle additives is that they might lead to undesirable mechanical properties of the material.

[9] Copolymerization of organic polymers, such as polyimides, with an appropriate inorganic monomer has the advantage that the inorganic component is uniformly dispersed and can form an essentially continuous passivating oxide layer when the hybrid copolymer is subjected to AO bombardment. Because the polyimide (PI), Kapton, is often used on the external surfaces of spacecraft, the majority of the hybrid copolymers that have been studied involve PI as the main component.

[10] Polyimide copolymers that have shown good AO resistance are polyimide polymers that incorporate polyhedral oligomeric silsequioxanes (POSS). One type of polyimide-POSS copolymer (POSS-PI) incorporates the POSS monomer into the main polyimide polymer backbone, referred to as a main-chain (MC) POSS-PI polymer. Another type of polyimide-POSS copolymer incorporates the POSS monomer as a pendant group on the main polyimide polymer backbone, referred to as a side-chain (SC) POSS PI. One drawback of the POSS monomers that are functionalized for copolymerization with PI is that they are expensive to synthesize, and the copolymerization process is not conducive to low-cost and large-scale fabrication of

POSS PI materials or films. [11] In addition to being subjected to degradation, LEO spacecraft may be in a constant state of very slight atmospheric drag, requiring either regular boost to their altitude (e.g. fuel burns) or an end-of-useful-life caused by reentry and burn up similar to a meteor entering the Earth’s atmosphere. As an example, the International Space Station (ISS), orbiting at about 425 km, loses approximately 2-4 km/month of altitude and requires regular fuel burns to ensure it stays in proper orbit. But the atmospheric drag at this altitude is still very low and LEO spacecraft can remain in orbit for years without fuel burns.

[12] At altitudes below about 400 km to about 120 km, atmospheric density that causes atmospheric drag increases by an order of magnitude about every 50-75 km. Below an altitude of about 120 km, the atmospheric density increases by an order of magnitude about every 20 km. The key effect is that atmospheric density, and therefore drag, is about 5 orders of magnitude higher at an altitude of 100 km compared to the altitude of the ISS. Due to the atmospheric drag at these altitudes, thrust is provided on a continuous or near-continuous basis or the spacecraft’s orbit will decay in a matter of days, weeks or months, depending on altitude. Accordingly, very few spacecraft operate below about 400 km, and those that do are often in highly elliptical orbits, thus spending very little time at the lower altitudes. Altitudes in the range of about 100 km to about 350 km are herein termed “very low Earth orbit”.

[13] Spacecraft that can operate at lower altitudes of about 100 km-350 km offer advantages of reduced size, weight, and power consumption, which in theory allows for much lower cost spacecraft. However, atmospheric drag becomes a major consideration for orbits in this altitude range. Another consideration that is important regarding the magnitude of drag is the surface roughness that develops when a material is eroded by atomic oxygen in very low Earth orbit. When polymer and carbon surfaces become eroded, they typically develop very rough surfaces. This roughness has a profound effect on the scattering dynamics of gaseous species from the surface, which is an important consideration for drag.

[14] The combination of higher oxygen atom density and increased drag at altitudes below about 400 km presents a need for materials that can satisfy the requirements of oxygen atom resistance and reduced drag under very low Earth orbit (less than 400 km) conditions, or comparable harsh conditions surrounding other celestial bodies. Such materials should also be able to maintain atomic oxygen resistance and reduced drag for multiple months or even years, in order to enable practical missions where a spacecraft will continuously or regularly pass through altitudes of about 100 km- 350 km.

SUMMARY OF THE INVENTION

[15] The present technology provides atomic oxygen resistant, low drag materials that can be used in harsh environments, such as very low Earth orbit, or oxidizing orbits around other celestial bodies. The materials comprise an atomic oxygen resistant material having an atomically smooth surface, wherein (Eout/Ein) is between 0.5 and 1 .0, where Ein is the average incident kinetic energy and E _ou t is the average exit kinetic energy when a stream of atoms or molecules scatters inelastically from the surface, and 0 _ou t is greater than 0in/2, where Qin is the average incident angle and 0 _ou t is the average exit angle when a stream of atoms or molecules scatters inelastically from the surface, the angles being defined relative to a line normal to the surface and 0in and 0out are on opposite sides of the line normal to the surface. In one embodiment, the atomic oxygen resistant, low drag materials can be used on at least one surface of a spacecraft intended to orbit at latitudes of 100 km-350 km above the Earth.

[16] In another aspect, the present technology provides a method of increasing the atomic oxygen resistance and reducing drag on a spacecraft. The method comprises providing an atomic oxygen resistant material on at least a portion of the spacecraft such that the atomic oxygen resistant material has an atomically smooth surface, wherein (Eout/Ein) is between 0.5 and 1 .0, where Ein is the average incident kinetic energy and E _ou t is the average exit kinetic energy when a stream of atoms or molecules scatters inelastically from the surface, and 0 _ou t is greater than 0in/2, where 0in is the average incident angle and 0 _ou t is the average exit angle when a stream of atoms or molecules scatters inelastically from the surface, the angles being defined relative to a line normal to the surface, and 0in and 0 _ou t are on opposite sides of the line normal to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[17] Figure 1 illustrates graphical data of density of air versus altitude;

[18] Figure 2 is a cross-sectional view of one embodiment of the oxygen resistant material of the present technology, with an atomic oxygen resistant coating on a substrate that is also resistant to atomic oxygen;

[19] Figure 3 shows the Figure 2 embodiment with a passivating layer that forms on a surface that becomes exposed to an oxidizing space environment as a result of a defect in or damage to an atomic oxygen resistant coating; [20] Figure 4 illustrates the dynamics of the thermal and structural inelastic scattering regimes;

[21] Figure 5 shows diagrams of different gas-surface interactions;

[22] Figure 6 is a diagram illustrating a calculation of drag;

[23] Figure 7 shows diagrams of angular distributions of scattered O-atom flux.

DETAILED DESCRIPTION OF THE INVENTION

[24] The present technology provides materials that are atomic oxygen resistant and provide reduced drag on spacecraft intended to operate in harsh environments, such as the harsh conditions present at stable Earth orbits at about 100 km to about 350 km, or comparable harsh conditions present around other celestial bodies, such as Mars. The materials of the present technology comprise an atomic oxygen resistant material having an atomically smooth surface that provides for low drag (described further herein) even when exposed to an oxidizing environment.

[25] The term “atomic oxygen resistant” as used herein refers to materials that are not readily susceptible to oxidation and/or erosion from oxygen atoms.

[26] The terms “reduced drag” and “drag reduction” are used herein in comparison to a case of maximum drag, where all the incident atoms or molecules scatter diffusely from the surface, with the peak scattered flux occurring in the direction of a line normal to the surface.

[27] The term “low drag” as used herein refers to a surface that provides less than 50% of the drag in the diffuse scattering limit or maximum drag. Atomic Oxygen Resistant Materials

[28] A variety of materials that have atomic oxygen resistance can be used in the present technology. One example of an atomic oxygen resistant material is a polymeric material such as a copolymer of a polyimide monomer and polyhedral oligomeric silsesquioxane (POSS). POSS polymers have been copolymerized with the Kapton-like polyimide (PI), pyromellitic dianhydride oxydianiline (PMDA-ODA) to form a polymer having good atomic oxygen resistance. Alternative POSS copolymers that have been studied and shown to have good atomic oxygen resistance are a copolymer in which cyclopentyl POSS monomer is incorporated into the main polymer backbone, referred to as main-chain (MC) POSS PI, and a copolymer prepared by grafting a N-[(hepta- isobutylPOSS)propyl]-3,5-diaminobenzamide POSS monomer as a pendant group on the main polymer backbone, referred to as side-chain (SC) POSS PI. In a laboratory test environment with hyperthermal AO, and in exposures on the International Space Station as part of the Materials Interactions Space Station Experiments (MISSE-1 , MISSE-5, and MISSE-6) it was found that the erosion yield of these POSS PI materials depended primarily on the wt% of the silicon-oxygen cage and not on how the POSS monomer was bonded to the polymer backbone. With a hyperthermal AO fluence of 2.70 x 10 ²⁰ O atoms cm’ ² , the erosion yields of 7.0 wt% SisOi2 SC POSS PI and 7.0 wt% SisOn MC POSS PI were 0.15 and 0.13 x 10’ ²⁴ cm ³ atom’ ¹ , respectively. In a space-flight AO test with a fluence of 1 .80 x 10 ² ° O atoms cm’ ² on MISSE-5, the erosion yield of 7.0 wt% SisOn MC POSS PI was 0.14 x 10’ ²⁴ cm ³ atom ¹ . The erosion results from space-flight and hyperthermal AO beam tests at similar AO fluences were comparable, indicating that the hyperthermal AO beam faithfully replicates the AO effects observed in LEO experiments. In a space-flight AO test of 1 .97 x 10 ²¹ O atoms cm ² on MISSE-6, the erosion yields of 7.0 wt% SisOi2 SC and SisOn MC POSS Pls were 0.07 and 0.05 x 10’ ²⁴ cm ³ atom ^-1 , respectively. A much longer exposure on MISSE-1 to an AO fluence of 8 x 10 ²¹ O atoms cm ^-2 yielded a very low erosion yield of 0.026 x 10 ²⁴ cm ³ atom ^-1 for 3.5 wt% SisOn MC POSS PI.

[29] One advantage of the POSS PI copolymers is that the copolymers form an essentially continuous passivating SiO2 layer when the copolymer is subjected to AO bombardment. Higher AO fluence exposures of POSS Pls typically lead to lower erosion yields, as a result of the growing and increasingly impenetrable SiO2 layer on the surface. Although POSS PI copolymers have good AO resistance, the POSS monomers that are functionalized for copolymerization are typically expensive to synthesize, and the copolymerization process is not conducive to low-cost and large-scale fabrication of POSS PI films.

[30] A less costly alternative to POSS PI copolymers is a blend of a POSS monomer with a polyimide. Such blends have similar erosion rates and yields as the POSS-PI copolymers at similar weight percentages of the silicon-oxide cage, and likewise form an essentially continuous passivating SiO2 layer when subjected to AO bombardment. In some embodiments, the POSS-containing material can have a silicon oxide content of at least 3.5 wt%, where silicon oxide content refers to the total number of silicon and oxide atoms present in the material. It is expected that both the POSS PI copolymers and the blends of POSS and polyimide have acceptable thermal and mechanical properties at silicon-oxide cage loadings up to 7 wt%, and could provide an AO resistant substrate for use in the present technology. [31] Although POSS copolymers show good AO resistance, the POSS copolymer surfaces typically become rough as they are bombarded with atomic oxygen, and this surface roughness significantly affects the scattering dynamics of gaseous species from the surface. As the surface becomes rougher, the momentum transfer from gaseous species increases, resulting in increased drag. One aspect of the present technology is the recognition that, to minimize drag, the surfaces must be as smooth as possible on an atomic level, and remain as smooth as possible in an oxidizing environment. POSS-containing polymers, however, become rough as they react with atomic oxygen and form a passivating oxide layer. Such surfaces will therefore not minimize or reduce drag on a spacecraft operating in a very low Earth orbit.

[32] Other examples of AO resistant materials for use on spacecraft include metals, metal oxides, and semiconductor oxides. These materials have been employed as coatings or as grafts or as additives to protect polymeric substrates, such as Kapton, that are vulnerable to AO attack. Examples of such materials include SiO2, AI2O3, ZnO, TiO2, SnO2, ln20a, ZrO2 and other metal oxides, or atomic-oxygen resistant compounds, combinations of these oxides or O-atom resistant compounds, oxides or O-atom resistant compounds with dopants, paints that incorporate these oxides or oxide mixtures or doped oxides or O-atom-resistant compounds.

[33] Several processes have been used to deposit the metal, metal oxide and semiconductor oxides as protective coatings. Typical processes include vapor deposition, chemical vapor deposition, plasma-enhanced chemical vapor deposition, cathodic arc deposition, ion implantation, chemical grafting, or spraying or brushing. However, such processes can leave the coating surface rough, and the coatings themselves or coatings that form from reaction with atomic oxygen are subject to cracking through handling and thermal cycling, and to damage from micrometeoroid and debris impacts, leaving the underlying material exposed to AO attack.

[34] One alternative process that can form a continuous, angstrom-level- controlled, defect free coating is atomic layer deposition (ALD). ALD is a variation of chemical vapor deposition (CVD) that does not rely on line-of-sight to the substrate and does not employ the high substrate temperatures that CVD typically requires. The process is a gas-phase method based on two sequential self-limiting surface reactions. Each surface reaction allows only about 1 monolayer of deposition. For AI2O3, the binary reaction is:

(A) AIOH* + AI(CH ₃ )3 AIOAI(CH ₃ ) ₂ * + CH ₄ (1 )

(B) AICH3* + H2O AIOH* + CH ₄ (2) where the asterisks indicate the surface species. If each reaction is self-limiting and selfterminating, then the repetitive application of these reactions in an ABAB... sequence can produce atomic layer controlled AI2O3 deposition. AI2O3 ALD can be deposited at the low temperatures required for coating polymeric substrates.

[35] Other chemistries permit the deposition of a variety of different coatings, such as ZnO, TiO2, and SiO2, and multilayer coatings of different composition are possible. For the ALD of TiO2 and SiO2 films, the MCk + 2H2O — MO2 + 4HCI reaction for MO2 deposition (where M = Si or Ti) is divided into the following two half-reactions:

(A) MOH* + MCk MOMCh* + HCI (3)

(B) MCI* + H2O MOH* + HCI (4) [36] Conducting sequential reaction cycles enables the controlled growth of the metal oxide coating, thereby producing coatings of particular desired thicknesses. ALD coatings having a thickness of at least about 35 angstroms have been successful at preventing erosion of the underlying substrate due to atomic oxygen attack. Other coating thicknesses may be used as needed for a given design or application.

[37] Even though an ALD coating can be made very smooth and defect-free, the coating might eventually become compromised from handling, thermal cycling, or micrometeoroid impacts. While micrometeoroids cannot be avoided, ALD coatings can resist the formation of defects from thermal cycling and handling because they can be extremely thin and deform to some extent with the substrate. The pliability of the ALD coating can even be enhanced by depositing mixed inorganic/organic layers. Although ALD coatings would not be expected to have as many defects as conventional coatings, defects can still form from micrometeoroid and debris impacts, leaving the substrate vulnerable to attack and allowing deep etch pits to form near any defect. As etch pits in the substrate grow, they will undercut the coating and the unsupported coating will then fall off, exposing a larger area of substrate to atomic oxygen attack. Thus, etch pits will grow nonlinearly with atomic oxygen dose. Etch pits and other defects increase surface roughness, resulting in increased drag. Accordingly, while atomic oxygen resistance is an important property for the materials and coatings of the present technology invention, it alone may not be sufficient to provide and maintain an atomically smooth surface.

[38] The present technology is directed to the difficulty of providing materials and coatings that maintain atomic oxygen resistance and surface smoothness in a gaseous oxidizing environment. In one embodiment, this difficulty can be overcome by the combination of an atomic oxygen resistant substrate and an atomic oxygen resistant coating deposited on the substrate. An example of this embodiment is shown in Fig. 2, where an oxygen resistant substrate 10 has an oxygen resistant coating 12 deposited on the surface of the substrate 10.

[39] It is contemplated that any atomic oxygen resistant material could be used to form the atomic oxygen resistant substrate 10. Examples of atomic oxygen resistant materials for use as the substrate include any of the POSS-containing materials discussed above. Alternatively, metal oxides, semiconductor oxides, or oxide-forming elements or compounds that are atomic oxygen resistant, or combinations of such oxides, can be used as the substrate material. For example, a polished silicon wafer could be used as the substrate material.

[40] It is contemplated that the atomic oxygen resistant coating 12 can be any of the metal, metal oxide, or semiconductor oxide materials described above, or combinations thereof, provided they have an atomically smooth surface that provides for reduced or low drag even when exposed to an oxidizing environment.

[41] The coating 12 can be deposited on the AO resistant substrate 10 by any method that can form a continuous, angstrom-level controlled, defect free coating. Methods or techniques for depositing the coating may include ALD, low temperature or downstream CVD, or molecular beam epitaxy (MBE). Other deposition techniques known in the art may be used.

[42] In one embodiment, ALD is used to deposit the coating. The ALD coatings are conformal, so they are at least as smooth as the surface of the underlying substrate. In practice, the buildup of many atomic layers on the surface actually tends to smooth over imperfections in the substrate surface on the nanometer scale, making the resulting coating surface even smoother than that of the substrate. POSS polyimide surfaces can be prepared with an rms roughness of ~2.5 nm, so one would expect that the surface of an ALD coating on this substrate would be smoother than this. The value of using an atomic oxygen resistant substrate 10, such as a POSS-PI substrate or a polished silicon substrate, is that it will form a passivating SiO2 layer anywhere it comes into contact with O atoms. As shown in Fig. 3, if a defect 14 does occur in the coating 12, this passivating layer 16 will form upon contact with the atomic oxygen atoms. The passivating layer 16 may prevent any significant erosion of the atomic oxygen resistant substrate 12 around a coating defect 14 and maintain a fairly smooth surface. Although explained with respect to a particular embodiment, the present description of the ALD coating is exemplary, and not intended to limit modification or other embodiments that a person of ordinary skill would understand to be within the scope of the disclosed example.

[43] In another embodiment, the atomic oxygen resistant low drag material could be a non-eroding material that also has an atomically smooth surface. One example of such a material is a metal oxide, such as sapphire. Due to its mechanical properties, sapphire could be used in the form of very thin sheets or layers for particular applications. In this embodiment, an additional atomic oxygen resistant coating would not be needed, since sapphire can be both atomic oxygen resistant and atomically smooth. Other materials that could be non-eroding and that also could have an atomically smooth surface include metals, metal oxides, semiconductor oxides, or combinations thereof. Gas-Surface Scattering Effect on Drag

[44] The drag on a spacecraft in very low Earth orbit depends mainly on the energy and momentum transfer between impinging gaseous atoms and molecules and the ram surfaces of the spacecraft. Thus, the inelastic scattering dynamics of atmospheric constituents (O, N2, and O2) on spacecraft surfaces at high relative velocities are necessary for the calculation of a drag coefficient.

[45] The nature of the inelastic scattering dynamics depends dramatically on the incident energy (see Fig. 4). At low incident energies, a metal surface appears extremely smooth to an incident atom, and the momentum of the atom parallel to the surface is conserved before and after the collision. Various cube models, such as the hard cube or soft cube model, have been proposed to describe the scattering in this so-called “thermal scattering regime”. In this regime, the final energy is the same as the incident energy for specular scattering, whereas the final energy is higher than the incident energy for angles smaller than the specular angle, and the final energy is lower than the incident energy for final angles that are higher than the specular angle. At high incident energies, the surface looks rough to the incident atom, and parallel momentum is not conserved during the collision. Sphere-sphere scattering models have been proposed to describe scattering in this so-called “structural scattering regime”. In this regime, the final energy tends to increase with final angle until very large final angles, where the final energy levels off or slowly decreases. In gas-surface scattering on ram surfaces in LEO, the structural scattering regime dominates.

[46] Two basic kinds of scattering are commonly observed for non-reactive collisions: direct (or impulsive) scattering and trapping desorption (also called more generally “thermal desorption”). Chemisorption or physisorption may also occur, and these processes lead to sticking of the incident species on the surface. These gassurface interactions are illustrated in Fig. 5. Impulsive scattering (IS) leads to atoms or molecules that exit the surface on a time scale too short for thermal equilibrium to be achieved, and the products exit the surface with high translational energies. Trapping (or thermal) desorption (TD) results in products that exit the surface with translational energies that are nominally given by the surface temperature, and these products have thermal translational energies which are typically low and given by a Maxwell-Boltzmann (MB) distribution at the surface temperature. When the incident energy is increased, there are two effects: (1 ) the fraction of impulsive scattering typically increases and (2) the impulsively scattered products get faster, while the thermally desorbed products keep the same velocities as given by the surface temperature. The two types of scattering have very different angular distributions. The impulsively scattered products tend to scatter with a lobular angular distribution roughly in the specular direction, whereas the thermally desorbed products have maximum flux in the direction parallel to a line normal to the surface and typically exhibit a cosine angular flux dependence with respect to this direction. When products desorb thermally, this situation is sometimes called “diffuse scattering”. When incident atoms or molecules undergo trapping desorption or chemi/physisorption, they lose essentially all their incident energy to the surface.

[47] On the other hand, atoms or molecules which scatter impulsively from the surface only deposit a fraction of their incident energy (perhaps 10% - 50%) on the surface. Thus, in order to minimize drag, it is important to find a set of conditions that maximizes impulsive scattering with minimum energy transfer to the surface. For the high incident energies of gas-surface interactions in environments such as very low Earth orbit, this set of conditions can be met with smooth surfaces and grazing incident angles. A diagram showing a calculation of drag is shown in Figure 6.

[48] The calculation of drag in Figure 6 is described in comparison to maximum drag that occurs when all the incident atoms or molecules scatter diffusely from the surface. This diffuse scattering represents full momentum and energy accommodation on the surface, i.e., the incident kinetic energy of the atoms or molecules is lost to the surface. As shown in Figure 6, when the angle of incidence is large relative to the surface normal (grazing incident angle) and the exit angle is also large, the atoms or molecules scatter impulsively and momentum accommodation is reduced. Impulsive scattering can be increased and diffuse scattering decreased with a surface that is sufficiently smooth on an atomic level. A sufficiently smooth surface is one in which Eout/Ein is between 0.5 and 1 .0, where Ein represents the average incident kinetic energy and E _ou t represents the average exit kinetic energy when a stream of a atoms or molecules scatters inelastically from the surface, and 0 _ou t is greater than 0in/2, where Qin is the average incident angle and 0out is the average exit angle when a stream of atoms or molecules scatters inelastically, the angles being defined relative to the a line normal to the surface (surface normal), as shown in Figure 6. Such a surface is atomically smooth and provides a substantial reduction of the diffuse scattering limit or maximum drag. In one example calculation, shown in Figure 6, where Eout/Ein is 0.8, 0out is 60° and a is 20°, the atomically smooth surface can provide a drag reduction of approximately 57.5% compared to a surface that provides maximum drag, where all the incident atoms or molecules scatter diffusely, with the peak scattered flux occurring in the surface normal direction. In some embodiments, an atomically smooth surface can provide a drag reduction of at least about 10%, for example a reduction of about 10% to about 60%, alternatively about 15% to about 60%, alternatively about 20% to about 60%, alternatively about 25% to about 60%. In some embodiments of the low drag coatings and materials of the present technology, the atomically smooth surface can provide a drag reduction of about 50% to about 60% compared to a surface that provides maximum drag. Stated differently, in some embodiments, the atomically smooth surface can provide approximately less than 50% of the drag in the diffuse scattering limit or maximum drag.

[49] A molecular beam apparatus can be used for the study of gas-surface interactions. Such an apparatus, and the techniques for using it in the study of gassurface interactions is disclosed in Murray, V. J.; et al., “Gas-Surface Scattering Dynamics Applied to Concentration of Gases for Mass Spectrometry in Tenuous Atmospheres”, J. Phys. Chem. C 2017, 121, 7903-7922, which is hereby incorporated by reference.

[50] For studies of gas-surface interaction dynamics, a velocity-selected beam is directed at a (temperature-controlled) target surface at a selected angle of incidence. The products that scatter from the surface (either inelastically or reactively) are collected as a function of their flight time and scattering angle with the use of the rotatable mass spectrometer detector. The angularly resolved time-of-flight distributions for mass- selected products are used to derive angularly resolved velocity distributions, 1(9, v), or translation energy distributions, 1(9, ET), where I is proportional to product flux. Knowing the translational energies and angles of the scattered products, which are identified by mass spectrometry, allows several inferences to be made about the interaction mechanisms at the surface. For example, a preference for specular scattering with relatively large final translational energies indicates direct impulsive scattering. On the other hand, if the products exit the surface with a MB translational energy distribution in a cosine angular distribution about the surface normal, then one would conclude that the interaction with the surface occurs in thermal equilibrium (TD).

[51] The beam-surface scattering technique has been used recently to study the scattering dynamics of hyperthermal O atoms on three representative surfaces: Gold (Au), silicon dioxide (SiC ), and highly oriented pyrolytic graphite (HOPG). Figure 7 shows angular distributions of scattered O-atom flux, and Table 2 contains the experimental conditions for each sample.

Table 2. Experimental conditions for each sample

[52] With the experimental conditions for HOPG in the table above and an angle of incidence of 70 degrees, O atoms scatter with only 5% - 10% energy loss in the super- specular direction. These scattering dynamics would be ideal to minimize drag; however, this material is not suitable for use in the harsh conditions of very low Earth orbit, because HOPG reacts with atomic oxygen and will erode and form a rough surface after extended exposure to oxygen atoms. The roughened surface will promote multiple-bounce scattering on the surface and will drive incident atoms into thermal equilibrium, resulting in nearly complete energy and momentum accommodation of the incident atoms or molecules.

[53] Gold is a chemically inert, noncatalytic surface that might be useful as a coating for some applications. Smooth thin films of gold can be produced on a variety of different substrates and geometries via a variety of different deposition techniques. In addition, gold is the only transition metal that does not have a stable oxide; AU2O3 is metastable and decomposes quickly at temperatures above 423 K. Although the broad angular distribution of O atoms that scattered from the gold surface (Fig. 7a) suggests that there is a large fraction of diffuse scattering and therefore significant energy accommodation, the translational energy distributions of the O atoms indicate that energy transfers in the range of 10% to 40% are the norm. Thus, the broad angular distribution is not the result of energy accommodation but rather the highly corrugated potential energy surface for the O-Au interaction. Similar results have been observed for O2 scattering on gold. This result suggests that gold might be a suitable AO-resistant material that could be used in space to minimize gas-surface energy transfer and therefore drag. However, the optical properties of gold would not be suitable for thermal control.

[54] SiO2 has been used as an AO-resistant coating in LEO, and the transparency of this coating makes it suitable for protection of thermal control materials.

The angular distributions of hyperthermal O atoms scattering from SiO2 (Fig. 7b) suggest predominantly impulsive scattering with little energy accommodation. The energy distributions of scattered O (and O2) products indicate that energy transfers are in a similar range as those from gold. An energy accommodation coefficient of 0.5 would thus be a conservative estimate from a design point of view. The SiO2 surface in the experiments was smoother than those of HOPG and gold (Table 2), but with surfaces on this roughness scale, the corrugation of the potential energy surface for the gas-surface interaction often plays a more significant role in determining the angular distribution of scattered products, especially when energy transfers are very low. The surface corrugation is expected to be more pronounced for the insulating SiO2 surface than for gold, because the electrons are tightly bound in covalent bonds. As a result of the highly corrugated interaction potential between the incident species and the surface, the scattering dynamics on perfectly smooth Au and SiO2 will lead to broader angular distributions of scattered products than would be observed on HOPG, which has a minimally corrugated interaction potential. The result of the nature of the interaction of hyperthermal O atoms with a fairly smooth SiO2 surface is a lobular angular distribution that is broader than that from HOPG but narrower than that from gold. The resistance of SiO2 to atomic oxygen attack and the fairly low energy accommodation of hyperthermal O and O2 on this surface (as long as it is smooth on the nanometer scale) make SiO2 a suitable spacecraft coating material for minimizing drag in very low Earth orbit environments.

[55] AI2O3 is another material that is AO-resistant and would likely have similar gas-surface scattering dynamics as SiO2. In fact, AI2O3 is even more resistant to AO attack than SiO2. The diffusion length for 5-eV O atoms on SiO2 is about 5 nm, whereas it is only 3 nm for AI2O3. There is no data available for hyperthermal O-atom scattering on AI2O3, but experience would suggest that the scattering dynamics should be similar, for a given surface roughness, on SiO2 and AI2O3. Moreover, both SiO2 and AI2O3 can be deposited on a variety of substrates by ALD, producing smooth coatings that are defect-free. The energy accommodation coefficient on a smooth AI2O3 surface is expected to be low (<0.5), and the combination of the AI2O3 coating with a polymer substrate that can form a passivating SiO2 layer when under AO attack is expected to provide an extremely durable material even in very low Earth orbit.

Applications for AO Resistant Low Drag Materials and Coatings

[56] The AO resistant, low drag materials and coatings described herein can be applied to or form one or more surfaces of a spacecraft intended to orbit in a gaseous environment. In one embodiment, the material can be used as the top layer of multilayer insulation on the external surface of a spacecraft. In this embodiment, the material comprises a layer of an AO resistant material, such as a POSS PI copolymer or blend, and an AO resistant coating such as SiO2 or AI2O3, deposited on the outer-facing surface of the polymer layer. A metal coating, such as an aluminum or silver coating, can be deposited on the opposite inner-facing side of the polymer material. This same material, without the metal coating, could also be used as a durable structural material for solar panels, if a polymer support is needed.

[57] Other uses contemplated for the materials and coatings of the present technology include, but are not limited to, thread for sewing or joining layers of multilayer insulation together, insulating for cables, as an insulating layer beneath solar cells, as a structural material and/or coating for light baffles, including support posts; coatings for sensitive components and electrical connections, and as an AO deflecting nose of a spacecraft.

[58] In another embodiment, an AO resistant low drag material or coating could be used on the surface of an inlet for an air-breathing ion engine. A further description of an air-breathing ion engine is provided in co-pending U.S. Patent Application Ser. No. 15/439,533 filed on February 22, 2017, entitled “A Satellite System”. For an inlet to an air-breathing ion engine that would be appropriate for flight in very low Earth orbit (e.g., less than 400 km), the incoming O, N2, and O2 must be able to be concentrated efficiently into the engine, and the inlet surfaces must not be roughened by O atom impact. For efficient concentration, the inlet surface must promote impulsive scattering, and the angular distributions of the scattered atoms and molecules must be narrow and peaked near, or preferably beyond, the specular angle. The specular scattering characteristics of SiO2 described above suggest that a material comprising an AO resistant substrate and a SiO2 or AI2O3 coating deposited on the substrate could be used on or as the surface of an inlet for an air-breathing ion engine.

[59] The presently technology is now described in such full, clear, concise, and exact terms as to enable any person skilled in the art to which it pertains, to practice the same. It is to be understood that the foregoing describes preferred embodiments of the invention and that modifications may be made therein without departing from the spirit or scope of the invention as set forth in the appended claims.