Investigation of the Surface and Boundary Layer ...

Investigation of the Surface and Boundary Layer Composition for Demising Aerospace Materials Adam S. Pagan (1) , Bartomeu Massuti-Ballester (1) , Georg Herdrich (1) , James A. Merrifield (2) , James C. Beck (3) , Volker Liedtke (4) , Benoit Bonvoisin (5) (1)

Institute of Space Systems, University of Stuttgart, Pfaffenwaldring 29, 70569 Stuttgart, Germany Email: [email protected] / [email protected] / [email protected] (2) Fluid Gravity Engineering Ltd, The Old Coach House, 1 West Street, Emsworth, Hants, United Kingdom, Email: [email protected] (3) Belstead Research Ltd, 387 Sandyhurst Lane, Ashford, Kent, United Kingdom, Email: [email protected] (4) Aerospace and Advanced Composites GmbH, Viktor-Kaplan-Straße 2 Objekt F, 2700 Wiener Neustadt, Austria, Email: [email protected] / [email protected] (5) ESA ESTEC, Keplerlaan 1, NL-2200 AG Noordwijk, The Netherlands, Email: [email protected]

ABSTRACT Five materials, encompassing the three metallic alloys type 316L stainless steel, grade 5 titanium and aluminium Al7075, silicon carbide, as well as a carbon fibrereinforced polymer (CFRP) variant, are subjected to different high-enthalpy air flow conditions relevant for uncontrolled atmospheric re-entries. Spectroscopic measurements in the boundary layer are used to identify primarily atomic species emanating from the material surface at different conditions and heating phases, and are accordingly correlated with the surface temperature history as well as with changes in the respective surface morphology. A demise-prolonging effect of surface oxidation, nitration and structural phase changes is observed and discussed. 1

INTRODUCTION

In what has been coined the Design for Demise (D4D) spacecraft design philosophy, disposable space vehicles operating in Low Earth Orbit (LEO) are to be constructed in such a way as to promote an early breakup and comprehensive aerothermal demise of its constituent components during re-entry upon End-of-Life (EoL) to minimise the risk for life and property on Earth [1]. Amongst structural design implications, this design philosophy requires the selection of materials to be performed under consideration of their respective tendency to succumb to harsh thermochemical environment experienced during re-entry Different types of exposed materials exhibit varying self-destructive responses towards such heat loads, one supposedly straightforward case being that of metals oxidising and melting. In the context of the ESA TRP Characterisation of Demisable Materials, a selection of eight representative aerospace material candidates were subjected to entryrelevant high-enthalpy air flow conditions of varying heat fluxes simulated in the Plasma Wind Tunnel (PWT)

facilities at the Institute of Space Systems (IRS) of the University of Stuttgart [2]. In the course of these tests, the thermal responses of the materials were monitored both at the front and at the back surface of the sample through pyrometry, in addition to a visual monitoring of the specimen’s state of demise. In addition, basic spectroscopic measurements within the boundary layer were conducted at different times during the respective test shot, with the intention of identifying occurring material-related atomic and diatomic species emanating from the exposed surface. The measurements were conducted around the visible spectrum (300 to 880 nm wavelength), and are evaluated in a primarily qualitative manner. Boundary layer spectra are normalised to one another to enable an assessment of relative intensity variations. In the following, a brief overview of the experimental setup and the selected materials is provided. Due to the extent of the activity, boundary layer spectra are shown only for selected test conditions for five materials. Atomic and diatomic species are identified through spectra taken at different times which are correlated with the corresponding front surface heating history and state of surface decomposition and discussed in that context. In addition, the relevance of thermochemical and physical surface and bulk material modifications to the process of demise is explored. 2

EXPERIMENTAL SETUP

The majority of the experiments discussed in the following were conducted in the IRS PWK4 facility as pictured in Fig. 1, using the arc heater or thermal plasma generator RB3 to generate a reproducible high-enthalpy air flow with thermochemical relevance to Earth entries for an extended period of up to several hours [3]. RB3 was designed to simulate high enthalpy air flows at pressure levels of up to and beyond 3 kPa and in effected heat flux ranges between 100 kW/m² and about 5

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measurement or material probe as required. Optical windows provide full optical access within the vacuum chamber.

Figure 1. IRS experiment facility Plasma Wind Tunnel 4 (PWK4) MW/m². The test gas is heated in the discharge chamber through an electric arc and accelerated by a nozzle through thermal expansion PWK4 is connected to the inhouse central vacuum system enabling the creation and maintenance of a vacuum in operation at stable pressure levels as low as 10 Pa. A 4-axis numerically controlled moveable table supports and deploys a single

Based on earlier considerations, experimental conditions correlating with a range peak heat fluxes deemed typical for moderately-sized spacecraft components subjected to post-vehicle-fragmentation demise trajectories were established. It was accordingly determined that conditions with reference heat fluxes at 260 and 520 kW/m², respectively, would envelope most typical tumble-averaged peak values experienced by demising space debris. In addition, a condition providing a heat flux of 1400 kW/m² was set up to account for tumble-free stagnation point peak heating [4]. These reference conditions were calibrated using a water-cooled 50 mm diameter hemispherical calorimetric heat flux probe and a water-cooled Pitot probe of equal diameter. Due to the limited scope of this paper, only tests conducted at the moderate (520 kW/m²) and high (1400 kW/m²) heat flux conditions are discussed. The relevant conditions are presented in Tab. 1.

Table 1. Plasma generator (RB3) and test conditions Designation of condition Parameter RB3 Plasma generator conditions Current I Magnet coil current Imag Voltage U Electric power P Nitrogen mass flow mN2 Oxygen mass flow mO2 Total mass flow mtot Ambient pressure pamb Test conditions at reference position Reference position x (distance to generator nozzle) Reference heat flux qCuO (CuO, cold wall, calorimetric, hemispherical head) Reference stagnation pressure p0 (50 mm Pitot probe) Local mass-specific enthalpy h (Marvin and Pope, y = 0 mm) Mach number (Rayleigh-Pitot formula)

Unit

Moderate heat flux (MHF) Value

High heat flux (HHF) Value

[A] [A] [V] [kW] [g/s] [g/s] [g/s] [Pa]

460 ± 5 40 ± 1 88 ± 2 40.5 ± 1 5.00 ± 0.01 1.52 ± 0.01 6.52 ± 0.02 41 ± 5

605 ± 5 40 ± 1 88 ± 2 53.2 ± 1.1 5.00 ± 0.01 1.52 ± 0.01 6.52 ± 0.02 343 ± 5

[mm] [kW/m²] [Pa] [MJ/kg] [-]

205 ± 0.01 519 ± 100 415 ± 10 13.7 ± 1.6 2.92

350 ± 0.01 1400 ± 140 1900 ± 20 17.3 ± 1.8 2.10

Figure 2. Schematic of IRS 50 mm material probe head

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In the initial phase of this activity, samples of five different materials, including a high-temperature ceramic (sintered silicon carbide SSiC), an organic laminate composite (Carbon Fibre-Reinforced Polymer composed of a TenCate EX-1515 cyanate ester resin and Torayca M55J carbon fibers) and three metallic alloys (type 316L stainless steel, grade 5 titanium Ti-6Al-4V, and aluminium alloy 7075-T651) were subjected to these environments until either a (quasi) steady state or the onset of demise was achieved. Sample geometries constituted flat coins resembling conical segments with a thickness between 3 mm and 4 mm. As shown in Fig. 2, these sample coins were embedded in a well-insulated and radiatively-cooled sample holder setup. The sample is pressed against the conical opening of a silicon carbide cap via three spring-loaded zirconium oxide rods with minimal contact edges. An intermediate silicon nitride ring with a knife-edge contact effectively minimises radial heat fluxes, which are further reduced by a comparatively low temperature gradient between the sample and the SiC cap, as neither are in any significant thermal contact with the water-cooled foot of the material probe due to an effective interior insulation. Heat fluxes from the sample through the insulated interior of the probe, which is further removed through a small gap to the sample, are thus reduced. Through a silicon carbide tube and an optical fibre connected to a collimator, pyrometric measurements were conducted of the back surface temperature history using the IRS Mini-PYREX pyrometer, assuming a nearblack-body environment within the cavity enclosed by the sample, its gap to the insulation foam and the SSiC tube. Front surface pyrometry was conducted using a linear pyrometer LP3 80/20 and a LumaSense MCS640 thermographic imaging camera, operating at wavelengths around 958.1 and 960 nm, respectively [2]. These temperature measurements were corrected in postprocessing using according device-specific spectral emissivities obtained in the Emissivity Measurement Facility EMF at IRS both for virgin material samples and such specimens that had previously been subjected to plasma wind tunnel testing [5]. Where such temperature-specific emissivities were not available, a constant value was assumed based on previous experience with similar materials. Spectroscopic measurements in PWK4 were conducted using an Ocean Optics S2000 spectrometer within a range of approximately 300 to 880 nm. Through use of a collimator, the measurement volume was restricted to a diameter of approximately 3 mm in direct proximity to the sample’s front surface centre as indicated in Fig. 3. An Ulbricht integration sphere with a known intensity distribution function as well as a well-characterised HgAr-lamp were used for the calibration of the intensity and wavelengths, respectively [2]. Test conditions were initially established while the

Figure 3. Spectroscopy measurement volume as indicated through a projected laser dot material probe was outside of the plasma plume. Once the conditions were set up, a free stream spectrum was recorded and the probe was moved into the centreline of the RB3 plasma generator, at which point the test commenced. Spectroscopic measurements were then taken manually at roughly regular intervals until the conclusion of the experiment through a shutdown of RB3. Exposure times were iteratively varied as required. 3

SPECIES IDENTIFICATION

It is attempted to identify both molecular and atomic emission bands in a selection of spectra recorded at different heating phases of a sample subjected to plasma wind tunnel testing. Intensities are given in arbitrary units and thus yield no absolute values, however they are normalised with regards to one another to enable a comparison of relative line intensities at different times. Free stream spectra, typically considerably lower in their overall intensity, are normalised independently. Molecular bands are identified through comparisons with spectral analyses of air or nitrogen plasmas in literature [6, 7, 8] and, in the case of the two non-metallic Table 2. Overview of atomic and diatomic candidate species considered in each case, originating from the air plasma, the material or boundary-layer interactions Primary candidates Secondary Diatomic (atomic) candidates (atomic) candidates Originating from the air plasma N, O O+, (Cu) N2, N2+ Sintered silicon carbide (Atom%) Si (55.9), C (44.1) Type 316L stainless steel (Atom%) Fe (66.4), Cr (18.8), Si (1.2), Al (0.6), C C2, CN, CO Ni (9.5), Mn (2.2), (traces), Mo (1.3) S (traces) Grade 5 titanium Ti-6Al-4V (Atom%) Ti (84.3), Al (13.0), V Fe (traces) TiO (2.6) Aluminium alloy Al7075-T651 (Atom%) Al (93.1), Mg (3.5), Cr (0.18 – 0.28), Fe, Zn (2.5), Cu (0.9) Mn, Ti, Si (traces) Carbon Fibre-Reinforced Polymer EX1515/M55J C, H, N, O Na C2, CN, CH, NH, CO

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Atomic lines are identified by isolating sufficiently prominent line features in the respective intensity- and wavelength-calibrated spectrum. A shortlist of abundant and trace candidate species is compiled according to the composition of the test material and the air plasma (see also Tab. 2). Any lines identified in spectra extracted both from the free plasma jet and the boundary layer, or in only the latter, during PWT testing are associated exclusively with air-related species. Conversely, lines found only in the boundary layer spectrum are initially assumed as material-related species, unless no according match can be found. Line identities are attributed using the National Institute of Standards and Technology’s (NIST) Atomic Spectra Database [12] as a reference. 4

spectrum. Where possible, observed lines and bands are then associated with identified species, the labels of which are also colour-coded to indicate their earliest discernible appearance. Due to the limited scope of this article, a focus is put on material-specific species. Accordingly, the appearance of species associated with the free stream are not discussed in detail but shown in the Figures. 4.1

Silicon Carbide

Fig. 4 displays the temperature history of SSiC subjected to the high heat flux condition for a total of 970 seconds. A steady state is attained within four minutes of commencing the experiment, and slightly altered following a small increase of the ambient pressure around 450 s into the test. As can clearly be seen in the associated spectra (Fig. 5), no material-specific species

RESULTS AND DISCUSSION

In the following, one selected result per candidate material is presented and discussed. The temperature history for the respective sample subjected to the given condition is shown both for the rear and the front surface, in the latter case the temperature values are depicted without prior emissivity correction as well as following the a correction with temperature-dependent emissivities determined for specimens both in a virgin and a post-test state. Front-face temperatures were obtained using the LP3 linear pyrometer, while back face pyrometry was conducted with the IRS Mini-PYREX. Colour-coded markers in each temperature history diagram point out the time and presumptive surface temperature at which recorded and evaluated spectra are represented in the correspondingly presented spectral diagram. These are shown alongside a freestream

582 s / 1520 K

PWK4 / RB3 HHF condition

8 s / 966 K

RB3 shutdown

materials, with spectra recorded from burning hydrocarbons and carbon stars, which include the Swan bands [9, 10, 11].

Figure 4: Heating history of SSiC sample subjected to high heat flux condition

Figure 5: Normalised emission spectra of silicon carbide SSiC subjected to high heat flux condition at different heating phases (see Fig. 4) with tentatively identified atomic and diatomic species and superimposed freestream spectrum (independently normalised). Note that no material-specific species could be identified in the measurement range.

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can be identified at any time within the boundary layer either during the initial heating phase or following the achievement of a steady state, indicating that no significant ablation occurs at this or less volatile experimental conditions for SSiC. 4.2

Type 316L Stainless Steel

The results of the type 316L stainless steel test shown were obtained by subjecting the specimen to the high heat flux condition. Fig. 6 depicts the recorded spectra. It is noted that, while the sample eventually proceeded to melt (see also Fig. 7), the onset of apparent demise, which is further reflected by a sudden increase in the front surface temperature history (see Fig. 8) occurred no earlier than 180 s into the experiment. It is believed that a thick oxide layer, formed on the front surface of the material sample during initial heating, provided both thermal and mechanical protection, allowing the exposed surface to heat beyond the melting point of the virgin alloy material, curbing heat conduction into the pristine phase, improving heat dissipation through an increase in both the emissivity and effective melting temperature, and possibly providing a mechanical barrier delaying the outflow of a molten

183 s / 1702 K

PWK4 / RB3 HHF condition

RB3 shutdown

34 s / 1419 K

liquidus (1673 K) solidus (1650 K)

Figure 6: Heating history of Type 316L Stainless Steel sample subjected to high heat flux condition

phase. Visual observations both during and following the experiment support this assumption, depicting a “superheated” uniform surface breaking up, starting at a “hot spot” near the stagnation point, and gradually releasing a molten phase. A visual post-test inspection of the specimen indicates a prevalence of both iron(III) oxide and iron nitride (see also Fig. 7). In Fig. 6, spectra are depicted reflecting the initial heating phase as well as a state immediately preceding the visible onset of melt. During the initial heating phase, strongly characteristic emission lines attributed to chromium are identified in the boundary layer, along with lines associated with nickel and, to a lesser part, manganese. While the relative apparent intensity of these lines is slightly reduced over time, additional nickel lines appear towards the end of the experiment and are further complemented by lines which are identified as stemming from iron and molybdenum in particular. 4.3

Grade 5 Titanium Ti-6Al-4V

Grade 5 titanium specimens subjected to the high heat flux condition are found to not demise, however the resulting front and rear surface temperature histories in particular reveal a highly dynamic internal thermal behaviour (see also Fig. 9), most likely in the course of various phase creation and transformation processes such as oxidation, nitration and beta phase transition. On the surface, a liquid oxide film is formed (see also Fig. 11), which through its behaviour and appearance is identified as consisting primarily of vanadium(V) oxide, also known as vanadium pentoxide. This compound has its boiling point at 963 K and is yellow in appearance when solid [13]. A quasi-steady state is attained after approximately 500 s. Following the termination of the test, the re-solidified outer oxide layer cools down rapidly and is quickly removed through shearing due its thermal contraction, compared to the less rapidly cooling phase beneath. Similar to the behaviour of stainless steel, it appears that a minimum of two distinct (solid and liquid) oxide and possibly nitride phases create a thermal barrier on the

Figure 7: From left to right: Type 316L stainless steel specimen subjected to high heat flux condition at onset of test, 120 s into the test, after 190 s, after 225 s immediately preceding test termination. Far right: Close-up microscope image of post-test surface.

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Figure 8: Normalised emission spectra of type 316L stainless steel subjected to high heat flux condition at different heating phases (see Fig. 8) with tentatively identified atomic and diatomic species and superimposed freestream spectrum (independently normalised).

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liquidus (1933 K)

RB3 shutdown

718 s / 1906 K

solidus (1877 K) 30 s / 1205 K

32 s / 1547 K

PWK4 / RB3 MHF Condition

PWK4 / RB3 HHF condition

20 s / 1049 K

liquidus (908 K)

beta transus (1253 K)

RB3 shutdown

10 s / 980 K

2s/ 737 K

Figure 9: Heating history of Grade 5 Ti-6Al-4V titanium sample subjected to high heat flux condition front face of the exposed material, allowing front face temperatures to exceed the critical temperatures of the virgin alloy without resulting in a demise of the sample. However, especially with regards to the liquid oxide layer, it must be considered whether the potential effect of increased shear forces due to higher stagnation pressures in actual re-entry scenarios would inhibit the creation of such oxide/nitride phases and thus reduce their protective influence or even remove it entirely. The highest activity in the boundary layer spectrum (see also Fig. 10) is observed during the primary heating phase above 1500 K, during which primarily emission lines associated with vanadium can be seen, with some evidence of titanium. A comparison with emission spectra measured during the airborne re-entry observation campaign of the object WT1190F [14] indicates that TiO occurs during the primary heating phase around the 32-s-mark. However, currently this cannot be confirmed, as the emission bands constitute an imperfect match with the reference measurements. Upon attainment of a quasi-steady state, the general intensity of the spectrum is reduced, and the remaining visible emission lines are dominated by vanadium, most likely stemming directly from the liquid oxide layer.

solidus (750 K)

Figure 10: Heating history of aluminium alloy 7075T651 sample subjected to moderate heat flux condition 4.4

Aluminium Alloy 7075

Fig. 12 depicts the temperature history of an Al7075T651 specimen subjected to the moderate heat flux condition. The already existing thin, barely visible thermally resistive oxide layer on the surface was thickened during exposure, acting as a temporary mechanical barrier, which can more fittingly be described as a pouch, briefly retaining and thus delaying the spill of the liquefying bulk material until the 33-smark. Following the spill-out, surface temperatures increased rapidly before the test was terminated. While very few lines associated with relevant alloy species are identified (see also Fig. 14), these are quite pronounced and first become visible preceding the visible onset of surface melt by a few seconds. At the 20second-mark, atomic magnesium lines are first observed as well as lines associated with what is at this point believed to be Zn+ ions. Only after approximately 23 seconds, which coincides with the formation of a superficial molten film, is a single emission line identified that is correlated with atomic aluminium.

Figure 11: From left to right: Grade 5 titanium Ti-6Al-4V specimen subjected to high heat flux condition at onset of test, 180 s into the test, immediately following test termination at approx. 720 s, after explosion of outer solidified oxide layer. Far right: Close-up microscope image of post-test surface.

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Figure 12: Normalised emission spectra of grade 5 titanium Ti-6Al-4V subjected to high heat flux condition at different heating phases (see also Fig. 8) with tentatively identified atomic and diatomic species and superimposed freestream spectrum (independently normalised)

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Figure 15: Normalised emission spectra of aluminium alloy Al7075-T651 subjected to moderate heat flux condition at different heating phases (see also Fig. 9) with tentatively identified atomic and diatomic species and superimposed freestream spectrum (independently normalised). Due to a slight misalignment of the optical setup incorporating a fraction of the material probe surface into the measurement volume, a superimposed partial Planck curve is visible.

Figure 14: From left to right: aluminium alloy Al7075 sample #10 surface after 10s at moderate heat flux condition, after 23 s, after 25 s, after 30 s, and at approx. 34 s 40 s / 1905 K

4.5 94 s / 1925 K

375 s / 1773 K

First perforation

PWK4 / RB3 HHF Condition

RB3 shutdown

5s/ 1814 K

Figure 13: Heating history of CFRP EX-1515/M55J sample subjected to high heat flux condition

Carbon Fibre-Reinforced Polymer

The CFRP EX-1515/M55J specimen subjected to the high heat flux condition was subject to gradual ablation, forming a crater-like depression with perforation around the stagnation point initially observed around 377 s into the test shot. Aided by relatively mild shear forces associated with a comparatively low stagnation pressure, the design of the sample holder system largely prevented delamination, with the exception of a few isolated events clearly represented in the front surface temperature history (see Fig. 13), approximately emulating the context of a closed structure. The temperature difference between the front and rear surfaces of the specimen remains large up to the point at which the sample is fully perforated. Prior to perforation and the subsequent termination of the test, the steady reduction in the specimen’s thickness is reflected by a gradual convergence of the respective surface

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Figure 16: Normalised emission spectra of CFRP EX-1515/M55J subjected to high heat flux condition at different heating phases (see also Fig. 12) with tentatively identified atomic and diatomic species and superimposed freestream spectrum (independently normalised). Due to a slight misalignment of the optical setup incorporating a fraction of the material probe surface into the measurement volume, a superimposed partial Planck curve is visible. temperatures. Spectroscopy shows CN bands dominating the spectrum and Swan (C2) bands appearing, only to gradually diminish towards the end of the experiment (Fig. 16). A blue, fluorescent gas is observed in the first ten seconds of the experiment (see also Fig. 17), which may be associated with the C2 band briefly emitting within the 460 nm to 470 nm range, and is potentially associated with the outgassing of water. A CH band is observed towards the end of the test. The later spectra are superimposed with a Planck radiation curve, implying a partial obstruction of the measurement volume, resulting from a slight misalignment of the optical setup. At the 375 s mark, an emission line associated with copper is identified, depicting an isolated event in which copper stemming from the plasma generator briefly contaminated the plasma.

The line visible at around 589 nm is identified as sodium, believed to possibly stem from impurities in the material’s resin. As documented in Refs. [15, 16], the same emission line and associated species had been identified during the re-entry observation campaigns for the Stardust and Hayabusa sample return mission capsule re-entries conducted using carbon-phenolic heat shields of a comparable overall species composition as CFRP. In both cases, the precise origin of sodium was not unambiguously attributed to the heat shield material, however the present observations may lend some credibility to this assumption. 5

CONCLUSIONS

This article discusses experimental findings on the boundary layer composition of five specific aerospace materials at selected relevant simulated atmospheric entry conditions as well as observations made with regards to demise-relevant effects resulting from thermochemical and physical in-situ modifications of the

Figure 17: From left to right: CFRP EX-1515/M55J specimen subjected to high heat flux condition at 1 s into the test, after 30 s, delamination event after 83 s, and after 300 s.

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exposed surface and bulk materials. Where possible, links between thermochemical processes at the exposed surface and the occurrence of specific species in the boundary layer were drawn.

oxidation under actual re-entry conditions may be subject to shear forces, which may be considered comparatively mild at the stagnation pressure levels provided under the given test conditions.

During none of the test conditions implemented in this campaign did tests with silicon carbide indicate any material diffusion into the boundary layer within the spectrum of 300 to 880 nm for which the calibrated OES measurement setup was sensitive. This reflects the thermal steady state attained by the sample, with no indication of demise.

For the exposed CFRP EX-1515/M55J specimen, diatomic species were observed in the boundary layer, which may be considered typical for ablating organic materials, including CH, CN, NH and C2, the latter manifesting itself through the characteristic Swan bands. As the only identified atomic species believed to emanate from the test subject itself, sodium was observed, although its exact origin could not be determined. With delamination largely inhibited due to a constricting sample holder design emulating the context of a closely wrapped structure, as well as comparatively low shear forces, the ablation process was observed up to a full perforation of the sample.

Spectra recorded for type 316L stainless steel reflected a particularly dominant emission from chromium, which was accompanied in particular by nickel and manganese during the primary heating phase of the material sample. Nearing the demise of the sample, additional nickel and molybdenum lines appear together with such associated with iron. The demise behaviour of stainless steel strongly suggests that oxidation and nitration processes may have a demise-prolonging effect on some metallic alloys under certain environmental conditions, as the formation of an oxide/nitride layer creates both a thermal as well as a mechanical barrier, considerably increasing both the emissivity and melting point at the surface, reducing thermal conduction into the material’s interior, and further maintaining the structural stability of the sample even after a liquid phase forms within its volume. This is supported through visual observations for all three metallic samples. Accordingly, for grade 5 titanium Ti-6Al-4V, the exposed front surface ultimately exceeded the critical temperature of the alloy in its pristine state. Visual and thermal measurements indicate the formation of at least one solid and one liquid phase of oxides and/or nitrides with a comparatively low combined thermal conductivity. The liquid phase was identified as being primarily composed of vanadium(V) oxide, which appears to relate to the overall dominance of vanadium observed in the optical emission spectrum. Evidence of TiO emission was found, however this was not fully conclusive. The thermal response of Ti-6Al-4V subjected to high-enthalpy air flows, as measured on both sides of the exposed sample coin, has proven to be of a highly dynamic nature, subject not only to surface oxidation and possibly nitration, but also most likely to be influenced by the beta phase transition. The behaviour of aluminium 7075 was observed to be similar to the other metallic alloys, albeit more straightforward. A “pouch” composed of aluminium oxide allowed for an apparent superheating of the solid surface to take place while briefly retaining the liquefying bulk material, thus essentially delaying the specimen’s demise. Few, but distinct emission lines associated with the alloy’s species composition are identified. The relevance of the demise-prolonging effect of surface

Supporting analyses are being conducted using the IRS plasma radiation assessment software PARADE (Plasma Radiation Database), which will provide some verification of the observed spectra. A further crossexamination of the data generated in the course of this activity with emission spectra obtained in the course of (destructive) re-entry observation campaigns [15, 17] may prove valuable towards a deeper interpretation of the latter. In analogy to the experiments discussed here, three additional materials, including Aluminium-Lithium alloy 2099, GLARE, and a CFRP variant featuring a PEEK matrix have been subjected to plasma wind tunnel testing at identical conditions, the evaluation of which will be disseminated at a later time. A further variant of CFRP was investigated under analogous conditions in the course of a joint experimental campaign with JAXAISAS, which will be presented separately. 6

ACKNOWLEDGEMENTS

The experiments discussed were performed at the Institute of Space Systems at the University of Stuttgart in the state of Baden-Württemberg, Germany. The authors would like to gratefully acknowledge funding of these research efforts by the European Space Agency (ESA) under contract 4000109981/13/NL/CP. The authors at IRS would further like to thank their colleagues Tobias Hermann, Edgar Schreiber, Dr. Fabian Zander, and Dr. Stefan Löhle for their valuable advice and assistance. 7

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7th International Workshop on Radiation of High Temperature Gases in Atmospheric Entry 21st-25th November 2016, Stuttgart, Germany

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7th International Workshop on Radiation of High Temperature Gases in Atmospheric Entry 21st-25th November 2016, Stuttgart, Germany