C-SiC exposed to Highly Dissociated Oxygen and Nitrogen Flows

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Original Article

Oxidation of PM1000 and C/C-SiC exposed to Highly Dissociated Oxygen and Nitrogen Flows Bartomeu Massuti-Ballestera,1,*, Georg Herdricha,2, Martin Frießb,3 a b

Institute of Space Systems (IRS), University of Stuttgart, Pfaffenwaldring 29, 7569-Stuttgart, Germany German Aerospace Center (DLR), Pfaffenwaldring 38-40, 70569-Stuttgart, Germany

A B S T R A C T

The present work investigates the formation and stability of different oxides on PM1000, an Oxide dispersion-strengthened (ODS) Ni-Cr superalloy, and on SiC coated C/C-SiC after being exposed to highly dissociated nitrogen and oxygen flows in a Plasma Wind Tunnel (PWT). PM1000 and C/C-SiC are candidate materials for reentry vehicles Thermal Protection System (TPS) and were used e.g. on the European eXPErimental Re-entry Testbed (EXPERT). The surface characterisation covers the identification of the oxide layers formed on the specimen's surface via X-ray Diffraction (XRD) and Energy-Dispersive X-ray spectroscopy (EDX). Grain sizes are assessed from XRD patterns and different topographical features are identified for Cr2O3 and NiO on PM1000 samples from Secondary Electron Microscopy (SEM). Surface spectral and total normal emissivities of the different oxides are assessed showing a significant dependency on the oxide, which is verified with previous investigations on ground and with flight data from the FOTON capsule.

1. Introduction Atmospheric entry manoeuvres are critical phases of a mission. Due to the high relative velocity between the entering body and the atmosphere a huge amount of kinetic energy needs to be dissipated, absorbed by the atmosphere and/or by the Thermal Protection System (TPS) of the spacecraft in the form of heat. The atmospheric gas particles in front of the vehicle, especially molecular nitrogen and oxygen, for Earth re-entry, are rapidly compressed, gaining thermal energy and leading to their dissociation and ionisation. The physical and chemical interactions between the gas and the TPS surfaces become important at high temperatures. These gas-surface interactions determine the amount of heat received by the TPS and define the capability thereof to withstand reentry manoeuvres. Atomic recombination processes at the surface are commonly associated with heterogeneous catalysis. In this case, the vehicle TPS surface behaves as a catalyst and the gas species are the reactants. Atomic species impinging onto the surface also modify the surface, forming different oxides and nitrides, which influence the surface emissivity and the catalytic processes [1]. For the investigation of the reaction mechanisms in ground test facilities, material samples are tested under single gas conditions, e.g. for an Earth reentry manoeuvre pure oxygen and pure nitrogen conditions are used [2]. Material testing is done under a variation of the

wall temperature and the number flux of impinging atoms onto the surface. The latter is not a control parameter for those tests and is a result of the total pressure and the dissociation degree of the gas. In order to identify the type of oxides formed, the samples tested in the Plasma Wind Tunnel (PWT) are examined in an X-ray diffractometer at the German Aerospace Center (DLR) in Stuttgart. Complementary, they are tested in the Emissivity Measurement Facility (EMF) [3] at IRS in order to obtain spectral and effective emissivity changes between pre and post test samples. SiC coated C/C-SiC and pre-oxidised PM1000 were selected for the ESA experimental re-entry capsule EXPERT for the nose cap and the lateral fairing respectively [4] and they are the major motivation of the current study. The facilities an measurement techniques used are first described. Information about the PWT flow conditions are included for each sample. Results are shown and eventually compared and discussed with existing data from literature. 2. Measurement techniques 2.1. Plasma wind tunnel facility IRS features a large PWT facility PWK3 shown in Fig. 1, which is driven by the inductively-heated plasma generators (IPG 3-4) [5],

⁎

Corresponding author. PhD student, Institute of Space Systems. 2 Head of Plasma Wind Tunnels and Electric Propulsion, Institute of Space Systems. 3 Researcher, German Aerospace Center. 1

https://doi.org/10.1016/j.jeurceramsoc.2020.01.053 Received 26 March 2019; Received in revised form 18 January 2020; Accepted 20 January 2020 0955-2219/ © 2020 Elsevier Ltd. All rights reserved.

Please cite this article as: Bartomeu Massuti-Ballester, Georg Herdrich and Martin Frieß, Journal of the European Ceramic Society, https://doi.org/10.1016/j.jeurceramsoc.2020.01.053

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Fig. 1. Schematic of the PWK3 facility and the IPG's resonant circuit.

Fig. 2. Cross sectional view of the IPG4 (left). IPG4 includes a 50 mm throat convergent nozzle, which enlarges the induction chamber by 35 mm length and stabilises the discharge. Oxygen plasma plume seen from the vacuum chamber, a so called 50 mm ESA-standard material probe is placed inside the flow (right).

at different positions inside the generated flow. Windows on the laterals and at the top of the tank enable optical access for optical measurement techniques. With more than 30 channels for data acquisition, including monitoring data of operational parameters and experimental data, which can be simultaneously recorded. The material samples are placed at the front side of the PWT material probe as shown in Fig. 3. The geometry of the probe is defined by the shape of the SiC cap, which is also used as a holder for the samples. They are not directly in contact and an insulation ring is used in order to reduce the heat losses. It is made out of a silicon nitride ceramic that has been successfully tested at temperatures of up to about 2000 K. The attachment of the cap to the cold structure is done indirectly trough three ZrO2 pins inserted from the lateral surface. In order to minimise the heat conduction towards the inner part of the holder, the samples are held by three ZrO2 pointed rods with compression springs and the entire cap is filled with an aluminium oxide silicate foam that insulates the front side of the probe head. The material probe head is instrumented with a miniaturised pyrometer MP3 [1] that allows for assessing the sample's rear side temperature independently of the material's surface emissivity. A cylindrical tube made out of SiC serves as the optical path between the material sample and the optical instrumentation and it is used to rise the effective emissivity of the

shown in Fig. 2. This facility allows for the generation of high enthalpy flows simulating Earth reentry manoeuvres with practically no contamination, even with pure chemically reactive gases such as oxygen or carbon dioxide. The IPG is mounted on a flat lid to a 2.6 m length and 1.8 m diameter vacuum tank connected to a centralised vacuum system, capable of extracting more than 70 m3/s at a base pressure of 10 Pa. A power supply runs a vacuum tube triode with nominal anode power of 150 kW and a 75 % efficiency feeding a resonant RLC circuit, with a variable frequency between 0.5 and 1.5 MHz. A capacitor bank with variable number of capacitors k (up to 7) of 6 nF each, and an inductance coil of 5.5 turns (nc = 5.5), 100 mm diameter, and 130 mm length form the resonant circuit. The inductance coil is wrapped around a water-cooled thin-walled quartz tube with 88 mm outer diameter and 2.3 mm thickness, which comprises the induction chamber where plasma is ignited. Cold gas is injected, from the injector head, tangentially to the quartz wall, in swirl-like motion for a proper energy coupling [5]. Free electrons present in the gas accelerate in the presence of the alternate magnetic flux produced by the coil and are capable to ionise other neutral gas species increasing the temperature and conductivity of the flow, which is thermally and electromagnetically accelerated and expanded into the testing chamber. Probes are mounted on a motor-controlled bench and can be placed

2

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Fig. 3. Material probe and european-standard material sample geometry.

sample's rear surface. The temperature and heat flux distributions inside the material probe are obtained from finite element methods supported by experimental data for the definition of the boundary conditions at the respective conditions. The effective emissivity of the sample's rear surface, as determined from these numerical investigations, shows that under thermal steady-state conditions εeff > 0.995. The test chamber is first evacuated to a base pressure of about 10 Pa before any gas is injected. The plasma source ignites at base pressures before the control parameters, i.e. electric power, gas flow rate and tank pressure, are adjusted to achieve the target conditions. During this process, which takes up to 3 minutes, the samples are placed at a certain distance from the hot plasma in order to minimise pre-heating effects and surface changes. The test starts by shifting the material probe at the centre line of the plasma jet at a certain distance from the IPG exit. Material samples experience very high temperature gradients, which depend on the flow conditions they are exposed to. In Fig. 4 two front surface temperature histories for PM1000 under O2 flows at different flow conditions are shown. These profiles are representative of a standard PWT test procedure, lasting between a few seconds and over 30 min depending on the heating rates and purpose of the test. For the present work, the tests lasted between 14 and 22 min. The PWT facility as well as the test procedure are extensively described in Ref. [2].

scheme of the measurement setup used in this work is presented in Fig. 5. The X-ray source uses Cu emitting at K − α1 band at 0.15406 nm, K − α2 at 0.154439 nm and at K − β at 0.13922 nm. In order to have a monochromatic radiation source, and since K − α1 and K − α2 have much higher intensities than K − β emission band, the latter is removed via a 12 μm Ni filter. The generator voltage used for the present work is 40 kV and the measurement surface is about 100mm2 varying slightly with the incident angle. The so-called locked-coupled method is used, where measurements are taken for emitter and detector angles 2θ between 20 and 100 degrees, at discrete steps of 0.02 degrees every 10 s. The complete scan for a single probe exceeds the 11 hours and accumulates 4000 shots per angular position. From the broadening of peaks in a diffraction pattern the mean size τ of the crystalline domains can be obtained by making use of Scherrer equation,

τ=

KλK − α1 β cos θ

(1)

where β is the line broadening at half maximum intensity, θ is the Bragg angle, λK−α1 the working X-ray beam wavelength and K a dimensionless shape factor, here K = 0.9. The actual grain size may be equal or larger than τ.

2.2. X-ray Diffractometer

2.3. Emissivity Measurement Facility

The facility used for the investigation of the surface crystallography is a commercially available system, AXS D8 Advance from Brucker. A

The samples tested in PWK3 are mounted in the Emissivity Measurement Facility (EMF), which is qualified for the measurement of total and spectral emissivities of ceramic and metallic materials. The EMF has strong similarities to a variable black body source, see Fig. 6. The cavity consists of a graphite rod (εGraphite = 0.9) with a high lengthto-diameter ratio, and quasi isothermal over a length of 4 times its diameter, which results in an apparent emissivity of at least 0.999 [3]. A vessel to protect the graphite tube and the material sample from oxidation is evacuated and filled with an argon atmosphere of about 125 kPa cyclically. In order to guarantee a free oxygen atmosphere the vessel is evacuated down to 1 Pa and refilled three times with 5.0 argon, with impurities of ≤ 0.001%. The geometrical setup of the cavity permits a heating of itself and the sample in an initial position inside the cavity, whereas the resulting black body temperature is measured using multiple optical measuring devices to target the specimen simultaneously. In this work, the Linear pyrometer LP3 [6] measuring at 958.1 nm, an NIR spectrometers (NQ512/2.2 from Ocean Optics) recording between 900 and 2500 nm with a resolution of 4 nm and a modified broadband thermopile of the product line ”GIRL” from the company PB Messtechnik [7]) sensitive between 400 and 8000 nm are used. The latter covers over 90 % of the radiation emitted by the target

Fig. 4. PM1000 front surface temperature histories under different O2 conditions. Changes on the heat flux balance during the heating process can be observed in both tests with sudden increases of the temperature histories. In the case of PM1000 this is caused by a change of the surface oxide protection layer as explained below. 3

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Fig. 5. Bragg Brentano Theta/Theta setup.

carbide (SiC). During the thermal pre-treatments, the fibre/matrix bonds are reduced gradually in transverse direction resulting in a continuous increase of the SiC formation from the bulk to the surface. Due to the higher coefficient of thermal expansion of the outer SiC-rich layers, transverse cracks occur when cooling down from the sintering temperature [10]. For the C/C-SiC specimens investigated the transverse cracks are few microns wide and they are separated over 0.5 mm from each other. The C/C-SiC samples are coated with SiC by chemical vapor deposition (CVD) as oxidation protection system (OPS) and before the PWT tests they are pre-oxidised into a furnace in order to create a passivation layer of SiO2. The samples are brought at 1573 K for 90 min under 1 atm and cooled down inside the furnace for several hours. All samples lost mass through this process, i.e. between 1.2 and 2.0 % of the initial weight, and formed a thin SiO2 layer. PM1000 is a Nickel-based ODS-superalloy with 20 wt% Cr, strengthened with nanoparticles of yttrium oxide Y2O3 and other 3% Fe, 0.5% Ti and 0.3% Al, manufactured by Plansee. PM1000 has a silvery colour which darkens with increasing roughness. The material easily oxidises above 1100 K forming a stable greenish Cr(III) oxide Cr2O3 that protects the material from further oxidation. Emissivity investigations of PM1000 show that with a pre-oxidation treatment the sample's total emissivity increases significantly turning it a well suited TPS candidate material. The PWT samples in this work are pre-oxidised into a furnace at atmospheric pressure and 1173 K for 90 min and are cooled down inside the furnace for several hours. In this process, a stable Cr2O3 oxide layer is built. Mechanical properties of PM1000 and of similar ODS-superalloys were extensively investigated by Nganbe [11] while oxidation at high temperatures were assessed by Martinz et al. [12]. Surface analysis of pre-oxidised PM1000 exposed to highly dissociated air flows using the diffusion reactor MESOX were preformed by Balat-Pichelin [13] and using both a PWT arc-jet and a Side-Arm Reactor by Stewart [14].

for a Planck distribution between 500 and over 2000 K, and detects at a frequency of 5 Hz. At the temperature of interest, the sample is rapidly shifted outside the cavity through the use of a piston. A sufficiently fast transition (less than 0.2 s), allows the assumption of constant temperature of the sample between the two positions and wherein its grey body emissivity can be determined. The total emissivity at a certain temperature Tref is,

εeff =

4 Tapp 4 Tref

(2)

being Tref the temperature measured by the pyrometers, set with ε = 1, when the sample is inside the black body cavity, and Tapp is the apparent temperature measured by the pyrometers once the sample is shifted outside the cavity. Two devices are used for spectral emissivity. A thermographic imaging camera LumaSense MCS640 [8] records at a wavelength of 960 nm with a total bandwidth of approximately 10 nm. The spectral emissivity for a given effective wavelength can be then determined by applying,

1 1 ⎞⎞ ⎛C ελ = exp ⎜ 2 ⎜⎛ − ⎟ λ ⎝ Tref, λ Tapp, λ ⎠ ⎟ ⎝ ⎠

(3)

where C2 is a constant given as C2 = 1.4388 · 10−2 mK. 3. Material samples and Test conditions 3.1. Sample preparation C/C-SiC samples, manufactured by DLR-Stuttgart, are highly porous C/C preform laminates infiltrated with liquid silicon (Si) [9].The specimen's manufacturing fundamental process is shown in Fig. 7 and is described in Ref. [10]. The most important matrix components are carbon fibres and amorphous carbon, residual Si and crystalline silicon

Fig. 6. Emissivity Measurement Facility (EMF). 4

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Fig. 7. Overview of the fundamental processing steps for the manufacture of C/C-SiC components [10].

the variation of the heat flux and the mass flow rate of the injected gas [15]. The parameter q˙ CuOx in Table 2 is the measured heat flux by the copper oxide calorimeter of the probe when no cold gas is injected. The flow velocity u∞ is obtained from the flow enthalpy and total and static pressure measurements via the energy conservation in subsonic conditions

3.2. PWT plasma conditions The target conditions are defined by three different stagnation pressures around 100 Pa, 500 Pa and 2000 Pa, in which the sample's wall temperature is maintained, and the plasma mass specific enthalpy varied between 23.3 and 5.4 MJ/kg for oxygen and between 27.8 and 7 MJ/kg for nitrogen flows. Flow parameters are strongly coupled in PWK3-IPG3 and in order to achieve all target pressures at a constant wall temperature, adjustments on the operational parameters, such as gas flow rate, electric power and tank pressure are required. In Table 1 the operational configurations of PWK3 including the mass flow rate m˙ O2 or m˙ N2 , tank pressure p∞, electric power Panode, number of capacitors k, number of coil turns nc and quartz tube thickness thQ are presented. A total of twelve flow conditions are characterised for the investigation of six C/C-SiC and six PM1000 samples. An overview of the flow conditions and the tested samples are shown in Table 2. The parameter Tw indicates the sample's surface temperature at steady-state conditions, which is achieved late in the test and represents de maximum temperature achieved. The tests are performed at constant enthalpy h∞ and total pressure p0 of the flow, which leads to a particular heating rate depending on the surface properties such as emissivity and catalysis. The total pressure is measured with a Pitot probe and the massspecific enthalpy is determined making use of the enthalpy probe developed by Löhle et al. [15]. This probe allows for the assessment of the mass-specific enthalpy of the flow by measuring the heat flux variations onto a water-cooled copper oxide calorimeter when injecting small amounts of cold gas into the boundary layer at the stagnation region. An analytical solution of the energy equation for the boundary layer is then used to fit the enthalpy of the gas in the free flow with respect to

κ

ptot κ − 1 Mi (2 − Φi ) 2 ⎞ κ − 1 v∞ ⎟ , = ⎜⎛1 + p∞ 2κ RTeq ⎝ ⎠ and via the Rayleigh-Pitot formulation in supersonic conditions κ

O#01 O#02a O#02b O#03 N#01 N#02a N#02b N#03a N#03b

1

ptot κ + 1 Mi (2 − Φi ) 2 ⎞ κ − 1 ⎛ 2 Mi (2 − Φi ) 2 κ − 1 ⎞1−κ v∞ ⎟ v∞ − = ⎜⎛ ⎜ ⎟ p∞ 2 κ T κ 1 T κ + 1⎠ R R + eq eq ⎝ ⎠ ⎝ (5) where Mi is the atomic mass of the gas specie i and R the universal gas constant. The effective isentropic exponent κ, the degree of dissociation Φi, and the gas temperature in thermal equilibrium Teq are obtained assuming local thermodynamic equilibrium LTE. The heat flux on the sample front surface q˙tot is assessed during the material testing via pyrometry to determine the total emitted flux q˙rad and calculated according to Stefan-Boltzmann's law,

q˙rad = εσTw4

(6)

where ε is the surface total hemispherical emissivity, σ the StefanBoltzmann constant and Tw the surface temperature measured with the LP3 pyrometer. Additionally, the structural heat losses in the sample holder are estimated. Radiation is the main dissipation mechanism for the tested samples where the structural heat losses are below 10 % of q˙tot for the tested specimens and at the tested conditions as investigated in Ref. [2].

Table 1 Operational parameters of PWK3. Set up

(4)

4. Surface analysis

m˙ gas

gas

p∞

Panode

[g/s]

type

[Pa]

[kW]

3.21 4.82 4.82 3.75 2.1 6.0 7.8 3.0 3.3

O2 O2 O2 O2 N2 N2 N2 N2 N2

40 160 160 2000 40 190 200 1400 1600

120 110 100 120 150 100 90 140 140

The surface analysis on the pre-oxidised SiC coated C/C-SiC and PM1000 samples includes optical and electron microscopy, EDX and XRD. Surface total normal and spectral emissivities of these two materials are investigated making use of the EMF. Additionally, as a verification activity for the determination of the surface emissivity of the SiC coated C/C-SiC samples, the miniaturised pyrometer embedded into the PWT material probe is used together with the LP3 linear pyrometer to determine the spectral emissivity at 958.1 nm of multiple sintered SiC (SSiC) which are made of the same material composition as the SiC coating applied to the C/C-SiC samples. A relation of the tested samples with respect to the surface treatment, and PWT test conditions is shown

Observations

k = 4; nc = 5.5; thQ = 2.3mm; No nozzle (IPG 3) k = 5; nc = 5.5; thQ = 2.3mm; Convergent nozzle (IPG 4) with dthroat = 50mm

5

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Table 2 Overview of the flow characteristics and the tested sample. Material

C/C-SiC #4 C/C-SiC #5 C/C-SiC #6 PM1000 #4 PM1000 #5 PM1000 #6 C/C-SiC #1 C/C-SiC #2 C/C-SiC #3 PM1000 #1 PM1000 #2 PM1000 #3

Set up

O#01 O#02a O#03 O#01 O#02b O#03 N#01 N#02a N#03a N#01 N#02b N#03b

x-dist.

Tw

p0

u∞

h∞

q˙ tot

q˙ CuOx

[mm]

[K]

[Pa]

[m/s]

[MJ/kg]

[kW/m2]

[kW/m2]

228 355 235 620 535 300 226 430 370 364 510 415

1398 1401 1398 1519 1507 1498 1398 1396 1392 1502 1515 1503

155 535 2110 150 480 2070 130 550 2000 85 510 1995

2632 1888 374 1990 1560 475 2526 2014 1080 1684 1706 736

23.31 14.39 9.63 15.55 10.14 5.35 27.84 15.28 11.74 17.61 10.44 7.03

208 210 208 244 237 232 208 207 205 268 265 257

710 985 870 390 545 475 625 620 910 300 405 680

Table 3 Overview of the tested specimens including the type of surface treatment, the test conditions, surface temperature and mass losses during PWT tests, as well as the type of post-test analyses conducted on each sample. OM stands for optical microscopy. Specimen ID.

Surface treatment

PWT cond.

Tw,PWT [K]

Mass loss rate [mg m-2s-1]

Post-test analysis

C/C-SiC#1 C/C-SiC#2 C/C-SiC#3 C/C-SiC#4 C/C-SiC#5 C/C-SiC#6 PM1000#1 PM1000#2 PM1000#3 PM1000#4 PM1000#5 PM1000#6

SiO2 passivation: at 1573 K for 90 min at 1 atm air

N#01 N#02a N#03a O#01 O#02a O#03 N#01 N#02b N#03b O#01 O#02b O#03

1398 1396 1392 1398 1401 1398 1502 1515 1503 1519 1507 1498

7.4 ± 1.8 3.6 ± 1.3 0 ± 3.0 6.7 ± 2.6 5.0 ± 2.1 18.8 ± 3.3 4.4 ± 1.3 0.7 ± 1.8 5.7 ± 2.2 5.6 ± 2.2 −5.4 ± 3.7 4.2 ± 1.8

EMF, XRD, OM, XRD, OM XRD, OM EMF, XRD, OM, OM, EDX XRD, OM, EDX EMF, XRD, OM, EMF, XRD, OM EMF, XRD, OM EMF, XRD, OM, XRD, OM XRD, OM

Cr2O3 passivation: at 1173 K for 90 min at 1 atm air

EDX

EDX

EDX

EDX

condition a light coloration suggests that the oxide layer is not completely removed. The analysis of the surface oxides via X-Ray Diffraction (XRD) using a 2θ pattern as shown in Fig. 9 fails on detecting the composition of the oxide layer, probably due to its small thickness. The main features observed are associated with the crystalline SiC in the bulk, which is primarily a β-SiC structure according to COD 1010995 from the Crystallography Open Database [16], with a lesser concentration of α-SiC. The presence of a small peak at 22° for O#01a condition reveals the presence of Cristobalite SiO2 (AMCSD 0001629) at the surface. Amorphous carbon and C-fibres diffraction patterns were expected to be found since the material is built from C/C preform laminate. However, these are not detected, which means that near the surface only SiC is present. The grain sizes are obtained for both β-SiC, and SiO2 using Eq. (1). This yields a grain diameter of at least 18 nm and 24 nm, respectively.

in Table 3. Information on the steady-state surface temperature and the mass lost during PWT tests as well as the type of post-test analyses conducted on each sample is also indicated. The results obtained for each of the material types are discussed below. 4.1. SiC-coated C/C-SiC The mass losses of C/C-SiC samples during PWT tests are very small and the variations between test conditions are in the order of the measurement uncertainties. However, samples experienced significant changes on the surface. Samples tested under oxygen flows maintain the coating with significant differences in coloration, as shown in Fig. 8. The oxide layer is more homogeneous at high pressures with a greenyellowish colour, whereas for lower pressures a purple hue appears at the top. Samples tested under nitrogen flows experience a severe degradation of the oxide layer and only for the low-pressure nitrogen

Fig. 8. Optical (left) and electron (right) microscopy of C/C-SiC samples before and after PWT tests. 6

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Fig. 9. XRD patterns of C/C-SiC samples after PWT tests. Table 4 Total normal and spectral (at 958.1 nm) emissivities obtained with the EMF for raw and tested C/C-SiC specimens at 1400 K. Material

Condition

εeff

(SiC) C/C-SiC raw (SiC) C/C-SiC (ox.) (SiC) C/C-SiC (SiC) C/C-SiC

Before Pre-ox. Before PWT N2 flows O2 flows

0.86 (0.90 [17]) − (0.80 [17]) 0.86 0.84

ελ=958.1nm

Oxide type

− − 0.81 0.78

− SiO2 − SiO2

The 35.8° β-SiC|[111] peak overlaps with several peaks of the α-SiC. Thus, a mean value of the FWHM of the three other larger peaks (βSiC|[220], β-SiC|[311], and β-SiC|[222] ) is used to determine the grain size for β-SiC. EDX analyses show a consistent 55 % Si and 45 % O of apparent concentration for C/C-SiC#4 and #6, which proves the existence of an SiOx oxide layer on the samples tested in oxygen plasmas. To detect species, EDX penetrates deeper than the oxide layer thickness and in fact, the high concentration of Si in the bulk artificially increases the concentration of Si in the measurements leading to an apparent ratio of 1:1 between Si and O concentration. Since the oxidation of silicon monoxide SiO into SiO2 is irreversible for temperatures above 700 K, only the latter is expected on the samples tested in PWT. No other elements such as carbon are detected. The investigation of the surface emissivity includes the assessment of the spectral and total normal emissivities. The results obtained for different SiC coated C/C-SiC samples tested at the EMF at different conditions, i.e. before the pre-oxidation at the furnace, before being tested in the PWT and after the exposure to nitrogen and oxygen flows in the PWT, are shown in Table 4. The spectral emissivity at 958.1 nm is obtained with the linear pyrometer LP3 and the total emissivity with the broadband pyrometer ”GIRL”. The surface oxide has a small influence on the surface total normal emissivity but has a stronger influence on the spectral emissivity as shown in Fig. 10 for the specimen C/C-SiC#4 tested under oxygen flows. The spectral emissivity in this case are obtained with the NIRspectrometer and compared with the emissivities of C/C-SiC and SiO2coated SiC from a FOTON post-flight investigation by Neuer et al. [17]. The results from different samples with and without an oxide layer are consistent and a reduction of the total normal emissivity is observed when an SiO2 layer is present. The same trend is valid for the spectral emissivity within the investigated wavelength range between 1 and 3 μm. Here, the obtained spectral emissivity of the C/C-SiC#4 sample lies in between the two FOTON samples, where C/C-SiC (without an oxide layer), exhibits the highest emissivity in the investigated wavelength range, and SiO2-coated SiC shows the lowest. Variations of the surface emissivity were observed by Liedtke et al.

Fig. 10. Spectral emissivity of C/C-SiC at 1400 K with different SiO2 coatings (top) and total normal emissivities of C/C-SiC and SiC-coated C/C-SiC (bottom).

Fig. 11. Surface temperature of SSiC specimen under O#01 conditions over multiple tests, each one with an approximated duration 20 min. Unless specified, the test location is at x = 230mm distance to the generator.

[18] for two C/SiC variants coated with SiC using a sol-gel technique after being tested in PWT. These variations were related to changes in the OPS coatings although there is no clear evidence supporting it. In Fig. 11, surface steady state temperatures from four different SSiC samples are shown together with the reference C/C-SiC#004 sample. 7

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Fig. 12. Optical (left) and electron (right) micrographs of PM1000 samples before and after PWT tests.

Spectral emissivities between 0.78 and 0.95 are obtained for SSiC samples with this approach, which is in line with the emissivity measurements using the EMF for SiC coated samples as shown in Fig. 10. At the position of 260 mm lower emissivity values are obtained with respect to the ones at 230 mm. Considering that the stagnation pressure in all three positions is practically the same, the lower surface temperature at 260 mm leads to the formation of a more stable oxide, which grows faster than at higher temperatures and is therefore more stable. Variations in the temperatures obtained between tests are caused by the formation of the oxide layer which changes the emissivity as well as other surface properties such as the surface catalysis. The lower catalytic behaviour of SiO2 with respect to SSiC [19] competes with the reduction of the surface emissivity for the change of the heat flux balance and in turn the resulting steady-state temperatures. This makes the evaluation of the individual parameters complicated.

SSiC samples are tested multiple times under the same and slightly different oxygen flow conditions in order to compare the variability of the steady-state temperatures achieved after each PWT test at the reference condition O#01. The x-axis corresponds to the number of times one and the same sample is tested. Unlike the C/C-SiC samples, which are pre-oxidised before the PWT test, all SSiC samples have no preoxidation treatment. The first three tests for SSiC#110 are conducted 30 mm further away from the generator than the target position for condition O#01a, i.e. x = 230 + 30mm, thus resulting in lower surface temperatures. The sample SSiC#115 is initially tested twice at a position that is 50 mm closer to the generator exit than the target position of O#01a, resulting in temperatures above 1500 K. In all tests, samples are exposed to the oxygen plasma for about 20 minutes and a stable SiO2 layer is formed as can be derived from the surface coloration after the test which is identical to the one observed for the C/C-SiC#004 sample. From the steady state temperature difference between consecutive tests, it can be observed that at the 180 and 260 mm positions the measured surface temperature respectively decreases. By contrast, the surface temperature increases at the reference position of x = 230mm. It must be noted that the spectral emissivity used for the temperature correction of the LP3 measurements is the same for all data points and ελ=958.1nm = 0.78. This is taken from spectral emissivity measurements conducted at the EMF and shown in Fig. 10. For the samples SSiC#110 and SSiC#114, temperatures measured with LP3 are also given for ελ=958.1nm = 1. For these two samples, temperature measurements from the back surface assessed with the MP3 are also available. Since the temperature measurements with MP3 are emissivity-independent at steady-state conditions [1], having temperature measurements from both LP3 and MP3 allows for the investigation of the emissivity variations due to changes in the oxide layer.

4.2. Oxidised PM1000 The mass of the PM1000 samples does not vary significantly during PWT tests. By contrast, their surface appearance drastically changes between plasma conditions. Samples tested in nitrogen plasmas present the smallest alteration, and only a slightly change of the coloration into a darker green is observed for the lower pressure condition (see the optical microscope images in Fig. 12). In oxygen flows though, the surface colour changes towards light brown, being more uniform across the surface at lower pressures. The change of the surface morphology between samples exposed to highly dissociated nitrogen and oxygen plasmas can be seen in Fig. 12, where electron microscope images of the samples PM1000#1 and PM1000#4 after the respective PWT tests are shown. 8

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easy to remove and only remains adherent to the sample due to the low mechanical stresses the specimens experience at the selected conditions. The stability of the Cr2O3 layer on PM1000 is explained by Panerai et al. [20] via the volatility diagrams of CrO3 and NiO for atomic and molecular oxygen atmospheres. With the presence of oxygen, the solid Cr2O3 forms CrO3, which is a liquid at 1500 K and is very volatile at the tested conditions, where the vapor pressure for CrO3 is about two orders of magnitude larger than the atomic oxygen partial pressures of 74.5, 147 and 103 Pa at the respective conditions O#01, O#02b, and O#03. On the contrary, NiO has a vapor pressure about four orders of magnitude lower than the atomic oxygen partial pressures, which makes it very stable at all conditions. The Cr(III) oxide is formed in column-like shapes with a diameter between 1 and 4 μm and distributed irregularly along the surface. In regions of high density the gap between the columns is smaller than 0.3 μm, whereas in low-density regions they are spaced at more than 2 μm distance. In the low-density regions, nickel from the substrate can be detected by EDX, whereas it is barely detected in the high-density regions. An EDX mapping of Ni, Cr, O, Fe, and Ti species on the surfaces of PM1000 #1 and #4 samples is shown in Fig. 14. The samples tested in oxygen plasmas form the Ni(II) oxide in flake-like shapes distributed heterogeneously over the surface. A diffusion of the oxygen atoms between the Cr2O3 column structures towards the substrate (≈80% Ni) could be the explanation of the formation of NiO on PM1000 samples. Variations in grain size are small between the tested conditions and the crystal types. For instance, the minimum grain size for cubic Ni is about 30 nm, while for Cr2O3, Cr2NiO4, and NiO, the minimum grain sizes are 23 - 33 nm, 20 - 28 nm and 28 - 38 nm, respectively. A slight increase of the Cr2O3 grain size is observed for increasing pressures. By contrast, the formation of NiO and Cr2NiO4 layers show higher grain sizes in lower pressure conditions. The FWHM averaged value of all non-overlapping peaks from the 2θ patterns is used. The peaks used in the averaging of the FWHM are listed in Table 5. From the investigation of several samples covered by different types of oxides, a correlation between the surface oxide layer and emissivity is noticeable. A significant increment of the emissivity is observed with the formation of the Cr2O3 in the oven with respect to the non-oxidised PM1000. After exposure to dissociated N2 flows, the emissivity of the material slightly reduces as the Cr2O3 layer is modified. Although Cr2O3 appears to degrade faster for the higher dissociated flows, the resulting emissivity reduces more for the less dissociated flows. On the other hand, the formation of NiO and Cr2NiO4 under O2 flows forces the emissivity to fall significantly from 0.85 for Cr2O3 down to 0.77 for NiO. An overview of spectral and total emissivities obtained via the EMF facility, is shown in Table 6 as well as in Fig. 15. The change of the surface oxide under oxygen plasmas from Cr2O3 to NiO is accompanied by a change of the surface catalysis as observed with the temperature histories of PM1000 samples and exemplary shown in Fig. 4 for the O#01 condition. The reduction of the surface emissivity with the formation of the NiO is not sufficient to explain the increase of the heating rates. Thus, an increase of the surface catalysis with the formation of NiO is expected. The higher surface catalysis of NiO with respect to Cr2O3 is in agreement with Balat-Pichelin et al. [13], where recombination coefficients for PM1000 and sintered Cr2O3 exposed to air plasmas are assessed.

Fig. 13. XRD patterns of PM1000 samples after PWT tests in (top) N2 and (bottom) O2 flows. Offsets are applied in order to better identify and compare peaks between samples.

Investigations on the surface oxidation of PM1000 using the 2θ patterns from X-Ray Diffraction (XRD), presented in Fig. 13, and Energy-Dispersive X-ray (EDX) spectroscopy, show that there is a transition from the Cr(III) oxide Cr2O3 into an Ni(II) oxide NiO, which forms relatively fast when exposed to oxygen plasmas. Unlike the SiO2 and Cr2O3, NiO oxide layer does not stick to the surface and can be easily removed. However, the low aerodynamic forces at the PWT test conditions lead to low mechanical erosion and negligible mass loss for the PM1000 samples with a NiO oxide layer. Samples tested in pure nitrogen plasmas maintain the Cr(III) oxide without formation of NiO. The XRD measurements reveal an abundant Ni CCP4 from the bulk and further the existence of a relatively thick Cr2O3 oxide layer stemming from the passivation treatment conducted prior the PWT testing. The stability of the oxide layer differs between highly dissociated N2 and O2 flows. In general, the Cr2O3 layer is very stable under N2 conditions, although it has been observed that it degrades faster at lower pressures. The presence of nitrides, if any, can not be resolved with XRD either because they form very thin layers or are not stable at high temperatures. Under O2 flows, the degradation of the Cr2O3 shows a similar behaviour as in N2 flows, but is also accompanied by an oxidation of Ni, either from oxygen diffusion to the bulk or a reduction of the Cr2O3 layer. Two types of crystallographic structures are observed, NiO and Cr2NiO4. These are built more uniformly along the sample surface at lower pressure regimes and form a fine powder, which is very

5. Conclusions The surface oxidation changes on C/C-SiC and PM1000 specimens tested at high enthalpy oxygen and nitrogen flows have been investigated by means of XRD and EDX analysis as well as surface emissivity measurements. The techniques used in this work performed better for the investigation of the oxides on PM1000 than for SiC coated C/C-SiC and in the latter the crystallographic structure of SiO2 could not be measured.

4 Cubic Close-Packed (CCP) and Face-Centred Cubic (FCC) are different names for the identical unit cell arrangement of the lattice.

9

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Fig. 14. EDX mapping of Ni, Cr, O, Fe, and Ti species on the surfaces of PM1000 #1 (top) and #4 (bottom) samples after being exposed to nitrogen and oxygen plasmas respectively. Table 5 Mean size of the crystalline domains τ of the different oxides and peaks of the 2Θ patterns utilised for the calculation of τ using the Scherrer equation, Eq. (1). Type of Crystal AMCSD code

Ni|CCP 0011153 Plane [111] [200] [311] [222]

τ, nm

NiO 0011371 2θ 44.3° 51.6° 92.2° 97.6°

30

Plane [111] [200] [220] [222]

Cr2NiO4 0012951 2θ 37.3° 43.3° 62.9° 79.5°

28-38

Plane [220] [511]

Table 6 Total hemispherical and spectral (at 958.1 nm) emissivities obtained with the EMF for raw and tested PM1000 specimens at 1500 K.

Cr2O3 0004566 2θ 30.4° 57.6°

20-28

Plane 2θ [012] 24.5° [104] 33.6° [113] 41.4° [116] 54.8° [214] 63.4° 23-33

Material

Condition

εeff

PM1000 raw PM1000 (ox.) PM1000 PM1000 PM1000 PM1000

Before Pre-ox. Before PWT N#01b N#02b N#03b O2 flows

0.71 0.89 0.85 0.85 0.82 0.77

ελ=958.1nm

Oxide type

− − 0.85 0.85 0.82 0.77

− Cr2O3 Cr2O3 Cr2O3 Cr2O3 NiO

via XRD. The effect of the SiO2 layer on the surface emissivity has been observed and the measurements obtained in this work are in good agreement with Neuer et al. [17] and validated against FOTON flight data. Another successful verification activity conducted during this work to prove the effect of the surface oxidation on the surface emissivity is the use of an emissivity independent technique such as the miniaturised pyrometer mounted in side the material probe. In addition, with this activity, the surface emissivities obtained with the EMF

However, knowing the type of species on the surface as identified with EDX measurements and the possible oxides that could be formed, was sufficient to adequately identify the surface characteristics. The formed SiO2 layer on C/C-SiC samples is very stable under oxygen conditions. However it is rapidly removed under high enthalpy nitrogen flows. The oxide layers are very thin and could not be observed 10

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Fig. 15. Spectral (left) and total normal (right) emissivities of PM1000. Total emissivities are obtained from EMF tests within this work and from Massuti-Ballester et al. [21] and spectral emissivities from Stewart [22].

facility have been verified. The impact of the surface structure with PM1000 oxides on the emissivity is proved to be significantly large. In fact, the selection of oxidised PM1000 as TPS candidate material due to the high surface emissivity and oxidation resistance of Cr2O3 is not recommended. The passivation treatment with Cr2O3 is not stable under oxygen plasmas and the oxide reduces rapidly to NiO. This oxidation change is accompanied by a reduction of the emissivity as proved in this work and an increase of the surface catalysis, both undesirable for TPS. Cr2O3 and NiO oxide layers are thick enough to be identified with XRD analysis. Cristal size in the order of 30 nm were measured for Cr2O3, Cr2NiO4, and NiO with small variation between test conditions. Future investigations on the characteristics of the oxide layers built on C/C-SiC and PM1000 samples exposed to high enthalpy flows should be extended to a wider range of surface temperatures and stagnation pressures, since they both have a significant influence on the chemical stability of the oxide formed. Investigations on the adhesion of the oxide layers is important in order to identify for which flow conditions mechanical erosion becomes relevant. For that, flow conditions with higher dynamic pressure should be assessed.

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