Effects of Thermal Cycle and Ultraviolet Radiation on 3D Printed

aerospace

Article Eﬀects of Thermal Cycle and Ultraviolet Radiation on 3D Printed Carbon Fiber/Polyether Ether Ketone Ablator

Farhan Abdullah 1,* , Kei-ichi Okuyama 2, Akito Morimitsu 1 and Naofumi Yamagata 1

1 Department of Applied Science for Integrated Systems Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata, Kitakyushu, Fukuoka 804-8550, Japan; [email protected] (A.M.); [email protected] (N.Y.) 2 Department of Aerospace Engineering, College of Science and Technology, Nihon University, 7-24-1 Narashinodai, Funabashi, Chiba 274-8501, Japan; [email protected] * Correspondence: [email protected]

Received: 27 May 2020; Accepted: 6 July 2020; Published: 8 July 2020

Abstract: The extreme heating environment during re-entry requires an efficient heat shield to protect a spacecraft. The current method of manufacturing a heat shield is labor intensive. The application of 3D printing can reduce cost and manufacturing time and improve the quality of a heat shield. A 3D printed carbon fiber/polyether ether ketone (CF/PEEK) composite was proposed as a heat shield material. The aim was to develop a heat shield and the structural member as a single structure while maintaining the necessary recession resistance. Test samples were exposed to thermal cycles and ultraviolet (UV) radiation environment. Subsequently, a tensile test was performed to evaluate the effect of thermal cycle and UV radiation on the mechanical properties. The sample’s recession performance and temperature behavior were evaluated using an arc heated wind tunnel. Exposure to thermal cycle and UV radiation have limited effect on the mechanical properties, recession behavior and temperature behavior of 3D CF/PEEK. Results from the arc heating test showed an expansion of the sample surface and better recession resistance than other existing ablator materials. Overall, 3D CF/PEEK has excellent recession resistance while maintaining mechanical properties when exposed to high temperature, thermal cycle and UV radiation.

Keywords: 3D printing; additive manufacturing; ablation; arc-heated; carbon ﬁber; heat shield; polyether ether ketone (PEEK); re-entry; thermal cycle; ultraviolet

1. Introduction Spacecraft are exposed to an extreme heating environment when entering Earth or another planetary atmosphere. An efficient thermal protection system (TPS), hereafter referred to as a heat shield, must be used to minimize heat conducted into the spacecraft. Moreover, a heat shield needs to have excellent specific strength, specific rigidity and high resistance to shear loads caused by aerodynamic loading to the surface [1]. Heat shields can be divided into reusable and ablative heat shields [2]. Composite ablative heat shield materials consist of a resin and reinforcing material such as carbon fiber. The ablative heat shield provides thermal protection through the process of ablation. The resin in the ablator undergoes pyrolysis reaction resulting in the release of pyrolysis gas and the carbonization of the resin while the carbon remains. A porous char layer is formed due to the carbonized resin. Pyrolysis gas percolates through the char surface and block incoming heat at the surface. At the same time, heat is absorbed when the pyrolysis gas percolates through the char layer. A thermochemical

Aerospace 2020, 7, 95; doi:10.3390/aerospace7070095 www.mdpi.com/journal/aerospace Aerospace 2020, 7, 95 2 of 28 process such as oxidation, sublimation, melting or vaporization and a mechanical process such as spallation cause the recession of the char layer [3]. The ablation process occurs in three modes. The first mode is the rate-controlled oxidation which occurs approximately below 1230 ◦C. The term rate refers to the rate for the carbon and oxygen chemical reaction. The second mode occurs between 1230 and 2730 ◦C and is termed as diffusion-controlled oxidation. The degradation rate is limited by the oxygen diffusion rate into the surface. The third mode occurring above approximately 2730 ◦C is termed as sublimation. The sublimation of carbon occurs during the third mode [4]. The current method of manufacturing heat shield is labor intensive, resulting in high manufacturing cost, long manufacturing time and quality issues. Moreover, most heatshields have gaps and seams which can increase the risk of accidents during re-entry. Recently, 3D printing or additive manufacturing (AM) technology has matured enough for application in the aerospace industry [5]. The European Space Agency has manufactured 3D printed PEEK structures for CubeSat. The unique feature was the embedded electrical lines inside the PEEK structure [6]. The Group of Astrodynamics for the Use of Space Systems in Italy developed the TuPOD 3U CubeSat. TuPOD was manufactured completely by 3D printing using Windform X2, a proprietary material by CRP USA [7]. The application of 3D printing in the manufacturing of heat shield has the potential to reduce cost and manufacturing time and improve quality by including high accuracy in manufacturing [8]. Moreover, 3D printing allows the ability to manufacture monolithic or one-piece heat shield, thus reducing gaps and seams. Recently, the National Aeronautics and Space Administration (NASA) conducted preliminary research regarding 3D printed TPS. NASA applied thermoset resin mixture printing to manufacture the test pieces [8]. Before re-entry, the long duration exposure of the ablator materials to the low Earth orbit (LEO) environment may affect the thermal and mechanical performance of the ablator. Exposure to thermal cycles causes the temperature to vary between 160 and +120 C when a satellite passes from direct − ◦ sunlight into Earth shadow [9]. This process repeats continuously throughout the satellite mission life. The thermal cycle can affect the mechanical properties of composites such as tensile strength and Young’s modulus [10]. The ultraviolet (UV) wavelength in LEO is between 0.1 and 0.4 µm (100–400 nm) [11]. The shorter is the wavelength, the greater is the UV energy. The mean energy possesses by vacuum ultraviolet (VUV) and UV-C is 214.5 and 122.6 Kcal/mole, respectively. Therefore, VUV and UV-C have the potential to break several chemical bonds [12]. A previous study described that UV radiation with a spectrum less than or equal to 250 nm is the dominant contributor for material degradation [13]. UV radiation causes both chain scission and crosslinking of polymers, thus affecting the mechanical properties [14]. However, there is limited information on the effect of thermal cycle and UV radiation on the thermal performance of composite ablators. In this work, an ablator material using 3D printed carbon fiber/polyether ether ketone (CF/PEEK), hereafter referred to as 3D CF/PEEK, is proposed as a new material for a heat shield. The aim is to propose a heat shield that can incorporate the structural member and heat shield section as a single structure while maintaining the necessary recession resistance. Test samples were exposed to thermal cycles and ultraviolet (UV) radiation environment. The effects of thermal cycle and UV radiation on tensile strength and Young’s modulus of the samples were evaluated using a tensile test. Further, the 3D CF/PEEK was exposed to a high-temperature environment in an arc heated wind tunnel for evaluation of surface and in-depth temperature response and recession resistance. A comparison with existing ablator materials was performed to determine the performance of 3D CF/PEEK compared to other ablator materials.

2. Materials and Methods

2.1. Materials A 3D CF/PEEK composite developed by AGC Inc. (Tokyo, Japan) was used as the ablator material. CF/PEEK composite is a high-performance carbon ﬁber composite made of carbon ﬁber as reinforcement Aerospace 2020, 7, 95 3 of 28 Aerospace 2020, 7, x FOR PEER REVIEW 3 of 29

CF/PEEKmaterial withcan be PEEK an ideal as the material matrix. for PEEK use is as a a semi-crystalline spacecraft material thermoplastic due to low polymer. outgassing CF/PEEK properties, can be goodan ideal resistance material to for space use sourced as a spacecraft radiation material and good due totoughness low outgassing property properties, [9]. The 3D good CF/PEEK resistance was to printedspace sourced using Arevo radiation laser-based and good direct toughness energy propertydeposition [9]. (DED) The 3D 3D CF printer/PEEK (Arevo, was printed CA, USA). using Laser- Arevo basedlaser-based DED uses direct a laser energy to depositionmelt deposited (DED) material 3D printer in the (Arevo, form of CA, wire USA). or filament. Laser-based The DEDfilament uses is a thenlaser compressed to melt deposited using materiala compaction in the roller form ofto wirebond or it ﬁlament.to the previous The ﬁlament layer or is thenbuild compressed plate. The DED using processa compaction allows rollerin-situ to consolidation bond it to the of previousparts. Figure layer 1 orshows build the plate. laser-based The DED DED process process. allows Arevo in-situ 3D printerconsolidation is capable of parts.of depositing Figure1 materials shows the in laser-based any orientation DED by process. using a Arevo six-axis 3D 3D printer printing is capable platform of [15].depositing materials in any orientation by using a six-axis 3D printing platform [15].

Figure 1. Illustration of the laser-based DED process [15]. Figure 1. Illustration of the laser-based DED process [15]. Continuous carbon fiber impregnated with PEEK resin was used to make the filament. The fiber Continuous carbon fiber impregnated with PEEK resin was used to make the filament. The fiber volume fraction is 56.5%. Two types of samples were manufactured: one for tensile test and another volume fraction is 56.5%. Two types of samples were manufactured: one for tensile test and another for arc heating test. The tensile test samples were cut from panels with (0/90/0) fiber layup direction. for arc heating test. The tensile test samples were cut from panels with (0/90/0) fiber layup direction. The arc heating test samples were cut from panels with (0/90)S fiber layup direction. The type and The arc heating test samples were cut from panels with (0/90)S fiber layup direction. The type and dimension of the samples are listed in Table1. Figures2 and3 show the computer-aided drawing dimension of the samples are listed in Table 1. Figures 2 and 3 show the computer-aided drawing (CAD) and actual picture of tensile and arc heating test samples, respectively. (CAD) and actual picture of tensile and arc heating test samples, respectively. Table 1. Sample types and dimensions for tensile and arc heating test. Table 1. Sample types and dimensions for tensile and arc heating test. Type of Test Length (mm) Width/Diameter (mm) Thickness (mm) Fiber Lay-up Type of Test Length (mm) Width/Diameter (mm) Thickness (mm) Fiber Lay-up TensileTensile test 150 150 20 20 2 2 (0/90/0) (0/90/0) Arc heating test - 20 30 (0/90)S Arc heating test - 20 30 (0/90)S

Aerospace 2020, 7, x FOR PEER REVIEW 4 of 29 Aerospace 2020, 7, x FOR PEER REVIEW 4 of 29 Aerospace 2020, 7, 95 4 of 28

(a) (a)

(b) (b) Figure 2. CAD drawing of test samples: (a) arc heating test sample with the position of holes for Figureplacement 2. CADCAD of thermocouples; drawing drawing of of test test and samples: samples: (b) tensile ( (aa) test) arc arc sample.heating heating test test sample sample with with the the position position of of holes holes for for placement of thermocouples; and ( b) tensile test sample.

(a) (a)

(b) (b) Figure 3. Actual test samples: ( (aa)) side side view view of of arc arc heating heating test test sample; sample; and and ( (b) top view of the tensile Figuretest sample. 3. Actual test samples: (a) side view of arc heating test sample; and (b) top view of the tensile test sample. 2.2. Exposure to Thermal Cycle 2.2. Exposure to Thermal Cycle 2.2. ExposureLong duration to Thermal exposure Cycle to thermal cycles can aﬀect the properties of 3D CF/PEEK. The large Long duration exposure to thermal cycles can affect the properties of 3D CF/PEEK. The large number of thermal cycles that the satellite will experience during a typical mission makes it diﬃcult numberLong of durationthermal cyclesexposure that tothe thermal satellite cycles will experience can affect duringthe properties a typical of mission 3D CF/PEEK. makes itThe difficult large to perform real-time testing. The solution is to perform thermal cycle exposure at accelerated levels. number of thermal cycles that the satellite will experience during a typical mission makes it difficult

Aerospace 2020, 7, x FOR PEER REVIEW 5 of 29 Aerospace 2020, 7, 95 5 of 28 to perform real-time testing. The solution is to perform thermal cycle exposure at accelerated levels. According to previous studies, thermal cycles can make a crack in carbon fiberfiber reinforced plastic (CFRP) and carbon fiberfiber reinforcedreinforced thermoplasticthermoplastic (CFRTP), which leads to a changechange inin mechanicalmechanical properties [[16,17].16,17]. This change can aaffectffect ablator performanceperformance duringduring re-entryre-entry intointo thethe atmosphere.atmosphere. The CoCoffin–Mansonffin–Manson modelmodel isis used to relate fieldfield usageusage to accelerated test conditions [[18,19].18,19]. This is aa simple simple model model used used for estimatingfor estimating the temperature the temperature cycle acceleration cycle acceleration factor. Reasonably factor. Reasonably estimating theestimating acceleration the acceleration factor depends factor on thedepends failures on being the failures caused bybeing fatigue, caused subject by fatigue, to the Co subjectffin–Manson to the lawCoffin–Manson for cyclic strain law versusfor cyclic the strain number versus of cycles the number to failure of ascycles shown to infailure Equation as shown (1) [18 in,20 Equation]: (1) [18,20]: α ∆εPN = c (1) ∆f = (1) wherewhere∆εP is ∆ the is plastic the plastic strain amplitude,strain amplitude,Nf is the Nf number is the number of cycles of to cycles failure, to αfailure,is the fatigueα is the ductility fatigue exponentductility exponent and c is the and material c is the constant. material Whenconstant. applied When to app anlied accelerated to an accelerated thermal cycling thermal sequence, cycling Equationsequence, (1)Equation can be re-written(1) can be tore-written define the to acceleration define the acceleration factor of the factor test as of Equation the test (2)as Equation [18,20]: (2) [18,20]: m AF = (∆Ttest/∆Tuse) (2) = (∆⁄)∆ (2) where AF is the acceleration factor, ∆T is the temperature cycle test range, ∆T is the nominal where AF is the acceleration factor,test ∆Ttest is the temperature cycle test range, ∆Tuseuse is the nominal temperature changechange inin the field field and m isis the the Coffin–Manson Coffin–Manson exponent. According According to to previous previous studies, studies, the CoCoffin–Mansonffin–Manson exponent forfor carboncarbon fiberfiber compositescomposites isis approximatelyapproximately 66 [[20].20]. There were two types of samples exposed to thermalthermal cycle. The first first type was for the tensile test while the secondsecond type waswas forfor arcarc heatingheating test.test. The “Despatch 935E-1-4-120” (Despatch Industries, Minneapolis, USA) was used to create the thermal cycle conditions.conditions. Figure 4 4 shows shows thethe thermalthermal cyclecycle chamber. Table2 2 shows shows the the specification specification of of the the thermal thermal cycle cycle chamber. chamber.

Figure 4. The thermal cycle chamber used for exposing the tensile test and arc heating test samples to various numbers of thermal cycles.

Aerospace 2020, 7, x FOR PEER REVIEW 6 of 29

Table 2. Main specification of the thermal cycle chamber. Aerospace 2020, 7, 95 6 of 28 Specification Description Size (mm) 440 width × 480 depth × 500 height UltimateTable Vacuum 2. Main (Pa) specification of the thermal Room cycle chamber.pressure ShroudSpecification temperature (℃) −190 Description to 200 ℃ Maximum test sample size (mm) 350 width × 250 depth × 200 height Size (mm) 440 width 480 depth 500 height 40−100 ℃: 6 ℃/min× , 40−200× ℃: 19 ℃/min UltimateHeating Vacuum rate (Pa) Room pressure Shroud temperature ( C) 40−260190 ℃ to: 31 200 ℃/minC ◦ − ◦ Maximum test sample size (mm) 350 width 250 depth 200 height × × 40 100 C: 6 C/min, 40 200 C: 19 C/min An accelerated thermalHeating ratecycle condition was performed− ◦ for◦ a temperature− ◦ range◦ between −70 and 40 260 ◦C: 31 ◦C/min 140 °C. The thermal cycle involved heating in air and cooling− using liquid nitrogen. The ∆Ttest is −70 to 140 °C and the ∆Tuse is −40 to 50 °C. The ∆Tuse was based on a previous microsatellite external structureAn accelerated temperature thermal measurements cycle condition [21]. The was equivalent performed number for a temperature of thermalrange cycles between in LEO orbit70 andwas − 140calculated◦C. The based thermal on cycle the assumption involved heating that the in orbita air andl period cooling was using equivalent liquid nitrogen. to the International The ∆Ttest is Space70 − toStation 140 ◦C (ISS) and orbital the ∆ Tperioduse is of40 appr to 50oximately◦C. The 90∆T minuse was[22]. basedThe number on a previous of thermal microsatellite cycles for the external ground − structuretest was temperaturecalculated using measurements Equation (2). [21 The]. The calculat equivalented number number of ofthermal thermal cycles cycles is shown in LEO in orbit Table was 3. calculatedTable 3 also based shows on the number assumption of samples that the exposed orbital periodto each was thermal equivalent cycle condition. to the International The heating Space and Stationcooling (ISS) rate orbital or ramp period rate of was approximately approximately 90 min 6 °C/min. [22]. The The number soaking of thermal period cycleswas 7 for min the to ground allow testuniform was calculated distribution using of heat Equation over (2).each The test calculated sample. The number profile of thermalfor one cyclesthermal is showncycle is in shown Table 3in. TableFigure3 also 5. shows the number of samples exposed to each thermal cycle condition. The heating and cooling rate or ramp rate was approximately 6 ◦C/min. The soaking period was 7 min to allow uniform distributionTable 3. of The heat number over each of tensile test sample. and arc The heating profile test for samples one thermal exposed cycle to different is shown thermal in Figure cycle5. conditions. Table3. The number of tensile and arc heating test samples exposed to different thermal cycle conditions. Number of Samples Per Mission Life NumberNumber of Thermal of EquivalentEquivalent Number Number of Number of of SamplesTest Per Type Test Type Mission Life (months) CyclesThermal in Ground Cycles in Test ThermalThermal Cycles Cycles in in Orbit Tensile ArcArc Heating Heating (months) Tensile Test Ground Test Orbit Test TestTest 66 1717 2800 2800 22 2 2 99 2626 4200 4200 22 2 2 1212 3535 5600 5600 22 2 2

Figure 5. Temperature proﬁle for thermal cycle exposure.

Aerospace 2020, 7, 95 7 of 28

2.3. Exposure to Ultraviolet Radiation Two types of samples were exposed to different amounts of UV fluence. The first type was for tensile test purposes while the second type was for arc heating test. A UV chamber without a thermal vacuum was used for UV irradiation (WorldJB, Taito, Japan). A fluorescent lamp was used to provide UV irradiation with a wavelength of 253.7 nm, which is in the UV-C wavelength range (G6T5, Sankyo-Denki, Kanagawa, Japan). The test condition is shown in Table4. A previous study has shown that UV degradation can be affected by temperature [23]. Therefore, the sample temperature was set to 50.0 ◦C to simulate UV degradation in space, which is the maximum temperature based on a thermal simulation of a small satellite in LEO. Table5 shows the list of samples exposed to UV radiation.

Table 4. UV irradiation conditions.

Type of Test UV Fluence (ESD) Wavelength (nm) 253.7 UV ﬂuence (ESD) 15, 30, 45 UV intensity (W/m2) 20.5 Pressure in UV chamber (Pa) 1.013 105 × Sample temperature (◦C) 50.0

Table 5. List of samples exposed to thermal cycle.

Number of Samples Per Test Type UV Fluence (ESD 1) Tensile Test Arc Heating Test 15 2 2 30 2 2 45 2 2 1 ESD refers to Equivalent Solar Day.

2.4. Tensile Test Method Tensile tests were conducted after the test samples were exposed to the thermal cycle and UV radiation. The tensile test was performed by Agne Technical Center Co. Ltd. (Tokyo, Japan). The tensile properties were measured using a universal testing machine at room temperature. The tensile test procedure was based on the ASTM D3039 standard. The tensile test was conducted using 14 test samples. Two samples were pristine samples referred to as the base sample. Six samples were previously exposed to the thermal cycle while the remaining six samples were previously exposed to UV radiation. Table6 shows the number of samples for each exposure condition. The applied displacement rate was 1 mm/min. The grip distance was 50 mm. The amount of strain was measured using the KFGS-5-120-C1-11 (Kyowa, Tokyo, Japan) strain gauge with a gauge length of 5mm. The Young’s modulus calculation range was between 0.05% and 0.25% of the longitudinal strain.

Table 6. List of sample types for tensile tests.

Environment Number of Test Samples Base sample 2 2800 thermal cycles 2 4200 thermal cycles 2 5600 thermal cycles 2 15 ESD UV fluence 2 30 ESD UV fluence 2 45 ESD UV fluence 2 Aerospace 2020, 7, 95 8 of 28

2.5. Arc Heating Test Method

Aerospace 2020, 7, x FOR PEER REVIEW 8 of 29 ArcAerospace heating 2020, 7 test, x FOR was PEER conducted REVIEW after ablator samples were exposed to the thermal cycle8 of 29 and UV radiation.2.5. Arc The Heating ablator Test samples Method were exposed to high-temperature flow using an arc heated wind tunnel2.5. located Arc Heating in Japan Test Method Aerospace Exploration Agency (JAXA) Institute of Space and Aeronautical Arc heating test was conducted after ablator samples were exposed to the thermal cycle and UV Science (ISAS)Arc heating in Sagamihara, test was conducted Japan. Figure after ablator6 shows samples the arc were heated exposed wind to the tunnel thermal in ISAS. cycle Theand UV distance radiation. The ablator samples were exposed to high-temperature flow using an arc heated wind betweenradiation. the wind The nozzleablator andsamples the samplewere exposed surface to washigh-temperature 100 mm. The flow heat using flux an varied arc heated between wind 5.0 and tunnel located in Japan Aerospace Exploration Agency (JAXA) Institute of Space and Aeronautical tunnel 2located in Japan Aerospace Exploration Agency (JAXA)2 Institute of Space and Aeronautical 2 14.2 MWScience/m . (ISAS) The heating in Sagamihara, duration Japan. was Figure 10 s for6 show 14.2s the MW arc/m heatedheat wind flux tunnel and 20in ISAS. s for 5.0The MWdistance/m heat Science (ISAS) in Sagamihara, Japan. Figure 6 show2 s the arc heated wind tunnel in ISAS. The distance flux. Thebetween lower the heating wind nozzle duration and forthe sample 14.2 MW surface/m was was due100mm. to aThe conservative heat flux varied approach between for 5.0 the and ablator between the wind nozzle and the sample surface was 100mm. The heat flux varied between 5.0 and test. To14.2 our MW/m knowledge,2. The heating this duration is the first was time10s for that 14.2 aMW/m 3D CF2 heat/PEEK flux and material 20s for was 5.0 MW/m evaluated2 heat flux. as a heat 14.2 MW/m2. The heating duration was 10s for 14.2 MW/m2 heat flux and 20s for 5.0 MW/m2 heat flux. The lower heating duration for 14.2 MW/m2 was due to a conservative approach for the ablator test. shieldThe material, lower heating thus being duration exposed for 14.2 to MW/m high-temperature2 was due to a flow. conservative Therefore, approach the initial for the plan ablator was test. to have a To our knowledge, this is the first time that a 3D CF/PEEK material was evaluated as a heat shield preliminaryTo our assessmentknowledge, ofthis the is etheffects first of time high that heat a 3D flux CF/PEEK on 3D CF material/PEEK was as a evaluated potential as heat a heat shield shield material. material, thus being exposed to high-temperature flow. Therefore, the initial plan was to have a Futurematerial, test plans thus under being consideration exposed to high-temperature will incorporate flow. a gradual Therefore, increase the initial in the plan heating was to duration. have a The preliminary assessment of the effects of high heat flux on 3D CF/PEEK as a potential heat shield gas flowpreliminary used during assessment the heating of the testeffects was of air. high Table heat7 fluxshows on the3D CF/PEEK test conditions as a potential for the heat arc heatingshield test. material. Future test plans under consideration will incorporate a gradual increase in the heating material. Future test plans under consideration will incorporate a gradual increase in the heating The surfaceduration. temperature The gas flow was used measured during the using heating an infrared test was thermometerair. Table 7 shows or pyrometer.the test conditions The thermometer for the duration. The gas flow used during the heating test was air. Table 7 shows the test conditions for the has a sensorarc heating that test. detects The surface the infrared temperature radiation was measur on theed sample using an surface. infrared thermometer A type-K thermocouple or pyrometer. was arc heating test. The surface temperature was measured using an infrared thermometer or pyrometer. used toThe measure thermometer the internal has a sensor temperature that detects of thethe sample.infrared radiation The internal on the temperature sample surface. was A measured type-K at The thermometer has a sensor that detects the infrared radiation on the sample surface. A type-K thermocouple was used to measure the internal temperature of the sample. The internal temperature 5, 10 andthermocouple 20 mm from was theused sample to measure surface. the internal Figure temperature7 shows the of the location sample. of The each internal thermocouple temperature within was measured at 5, 10 and 20 mm from the sample surface. Figure 7 shows the location of each the sample.was measured A Bakelite at 5, casing10 and acts20 mm as afrom heat the insulator sample surface. to house Figure each 7 sample.shows the The location Bakelite of each tube was thermocouple within the sample. A Bakelite casing acts as a heat insulator to house each sample. The wrappedthermocouple with glass within cloth the to reduce sample. lateral A Bakelite heating casing of ac thets as sample a heat andinsulator simulate to house one-dimensional each sample. The heating. Bakelite tube was wrapped with glass cloth to reduce lateral heating of the sample and simulate one- The purposeBakelite oftube the was one-dimensional wrapped with glass heating cloth to was reduce to facilitate lateral heating future of comparison the sample and between simulate ground one- test dimensional heating. The purpose of the one-dimensional heating was to facilitate future comparison dimensional heating. The purpose of the one-dimensional heating was to facilitate future comparison and a one-dimensionalbetween ground test numerical and a one-dimensional analysis of the ablatornumerical performance. analysis of the The ablator density performance. of each sample The was between ground test and a one-dimensional numerical analysis of the ablator performance. The calculateddensity based of each on thesample sample was masscalculated and based dimensions on the sample before mass exposure and dimensions to heat. The before mass exposure and thickness to density of each sample was calculated based on the sample mass and dimensions before exposure to of eachheat. sample The mass were and measured thickness before of each andsample after were each measured test to before evaluate and theafter surface each test recession to evaluate and the mass heat. The mass and thickness of each sample were measured before and after each test to evaluate the loss ratesurface for each recession sample. and mass Figure loss8 showsrate for theeach completed sample. Figure sample 8 shows assembly the completed before sample the arc assembly heating test. surface recession and mass loss rate for each sample. Figure 8 shows the completed sample assembly before the arc heating test. Figure 9 shows the sample during the heating test. Figurebefore9 shows the thearc heating sample test. during Figure the 9 shows heating the test. sample during the heating test.

Figure 6. Arc heated wind tunnel facility located in JAXA ISAS. FigureFigure 6. Arc6. Arc heated heated wind wind tunneltunnel facility facility located located in inJAXA JAXA ISAS. ISAS.

Figure 7. Location of thermocouples within sample including the direction of heat ﬂow. TC refers to

the thermocouple. Aerospace 2020, 7, 95 9 of 28

Table 7. Test conditions for arc heating test.

Density Heat Flux Heating Stagnation Group Model Environment (kg/m3) 1 (MW/m2) Duration (s) Pressure (kPa) G1 1395.3 5.0 20 12.46 G2 1411.2 14.2 10 60.76 A H1 1359.2 5.0 20 12.46 H2Base sample 1405.9 14.2 10 60.76 A1 1412.2 5.0 20 12.46 A2 1413.3 5.0 20 12.46 A3 1415.4 5.0 20 12.46 A4 1419.7 5.0 20 12.46 B1 2800 thermal cycles 1405.9 5.0 20 12.46 B2 2800 thermal cycles 1414.4 5.0 20 12.46 C1 4200 thermal cycles 1412.2 5.0 20 12.46 C2 4200 thermal cycles 1411.7 5.0 20 12.46 B D1 5600 thermal cycles 1409.1 5.0 20 12.46 D2 5600 thermal cycles 1412.2 5.0 20 12.46 E1 15 ESD UV fluence 1409.1 5.0 20 12.46 E2 15 ESD UV fluence 1413.3 5.0 20 12.46 F1 30 ESD UV fluence 1410.6 5.0 20 12.46 Aerospace 2020,F2 7, x FOR PEER 30 ESD REVIEW UV fluence 1410.6 5.0 20 12.469 of 29 G1 45 ESD UV fluence 1411.2 5.0 20 12.46 Figure 7.G2 Location 45 of ESD thermocouples UV fluence within 1419.7sample including 5.0the direction of 20heat flow. TC refers 12.46 to the thermocouple.1 The mean (M) was 1408.60 kg/m3 with a standard deviation (SD) of 12.40.

(a) (b)

FigureFigure 8. 8.Completed Completed sample sample wrapped wrapped inin glassglass cloth and Bakelite Bakelite attached attached to to the the sample sample holder: holder: (a) (a) frontfront view view of of the the sample; sample; and and ( b(b)) side side view view ofof thethe samplesample attached to to the the sample sample holder. holder.

FigureFigure 9. 9.View View of of ablator ablator samplesample insideinside the arc heating heating chamber chamber during during the the test. test.

Table 7. Test conditions for arc heating test.

Aerospace 2020, 7, 95 10 of 28

3. Results This section summarizes the results of the tensile test and arc heating test for the 3D CF/PEEK ablator samples.

3.1. Tensile Test Results During re-entry, the heatshield of a spacecraft must endure aerodynamic heating and loading. This section focuses on the area of aerodynamic loading. The 3D CF/PEEK serves both as an ablative heat shield and as a structural member of the TPS. The ablator material needs to have specific strength and specific rigidity [1]. Both of the mentioned properties are dependent on tensile strength and Young’s modulus [24]. Figure 10 shows the effect of the different numbers of thermal cycles and UV fluence on the average tensile strength. The tensile strength of the samples decreased with an increase in the number of thermal cycles. The tensile strength decreased as much as 8.5% after 5600 thermal cycles as compared to the base sample. However, a different behavior was observed for samples exposed to UV radiation. There was an increase in tensile strength until 30 ESD before the tensile Aerospacestrength 2020 decrease., 7, x FOR PEER The increaseREVIEW was as much as 6.7% compared to the base sample. 11 of 29

Figure 10. Effect of variation in the number of thermal cycles and UV fluence on the tensile strength of Figure3D CF 10./PEEK. Effect of variation in the number of thermal cycles and UV fluence on the tensile strength of 3D CF/PEEK. Figure 11 shows the effect of thermal cycles and UV fluence on Young’s modulus. A decrease in averageFigure 11 Young’s shows the modulus effect of was thermal also observed cycles and with UV anfluence increase on Young’s in the number modulus. of thermalA decrease cycles. in averageThe maximum Young’s decreased modulus was also 3.2% observed compared with to the an baseincrease sample. in the However, number samplesof thermal exposed cycles.to The UV maximumradiation experienceddecreased was a small 3.2% but compared gradual increaseto the ba ofse 2.5% sample. in the However, value of Young’s samples modulus exposed compared to UV radiationto the base experienced sample. Overall, a small tensile but gradual strength increase incurred of more 2.5% changes in the compared value of toYoung’s Young’s modulus modulus comparedwhen exposed to the to base the sample. different Overall, numbers tensile of thermal strength cycles incurred and UV more fluence. changes compared to Young’s modulus when exposed to the different numbers of thermal cycles and UV fluence.

Figure 11. Effect of variation in the number of thermal cycles and UV fluence on the Young’s Modulus of 3D CF/PEEK.

Aerospace 2020, 7, x FOR PEER REVIEW 11 of 29

Figure 10. Effect of variation in the number of thermal cycles and UV fluence on the tensile strength of 3D CF/PEEK.

Figure 11 shows the effect of thermal cycles and UV fluence on Young’s modulus. A decrease in average Young’s modulus was also observed with an increase in the number of thermal cycles. The maximum decreased was 3.2% compared to the base sample. However, samples exposed to UV radiation experienced a small but gradual increase of 2.5% in the value of Young’s modulus compared to the base sample. Overall, tensile strength incurred more changes compared to Young’s Aerospace 2020, 7, 95 11 of 28 modulus when exposed to the different numbers of thermal cycles and UV fluence.

Figure 11. Eﬀect of variation in the number of thermal cycles and UV ﬂuence on the Young’s Modulus Figure 11. Effect of variation in the number of thermal cycles and UV fluence on the Young’s Modulus Aerospaceof 3D2020 CF, 7/,PEEK. x FOR PEER REVIEW 12 of 29 of 3D CF/PEEK. 3.2. Surface Surface Expansion Expansion Behavior

Figure 12 shows sample A1 before and after the heating test. The charred surface of the sample after the test can be seen in Figure 1212b.b. The length of all samples increased after completion of the test. The The length measurement waswas performedperformed using using a a caliper. caliper. The The average average increase increase in in length, length, which which is isdenoted denoted as as negative negative surface surface recession, recession, ranges ranges between between 0.60 0.60 and and 1.80 1.80mm mm for for all all samples, samples, as as shown shown in inFigure Figure 13 .13. A A comparison comparison between between base base samples samples and and thermally thermally cycled cycled showedshowed aa decreasedecrease inin sample expansion as the number of thermalthermal cyclescycles increased.increased. The same behavior was shown by samples irradiated with with UV. UV. The The sample sample expansion expansion decreased decreased wi withth an an increase increase in inUV UV fluence. ﬂuence. However, However, the amountthe amount of decrease of decrease is less is lesscompared compared to samples to samples exposed exposed to thermal to thermal cycle cycle conditions. conditions.

(a) (b) (c)

Figure 12. External view of ablator sample before and after arc heating test: ( a)) front view of sample before arc heating heating test; test; (b (b) )front front view view of of the the sample sample after after test; test; and and (c) (sidec) side view view of the of thesample sample after after the test.the test.

Figure 13. Relationship between surface recession with maximum surface temperature of 3D CF/PEEK.

3.3. Surface and Internal Temperature Behavior

Aerospace 2020, 7, x FOR PEER REVIEW 12 of 29

3.2. Surface Expansion Behavior Figure 12 shows sample A1 before and after the heating test. The charred surface of the sample after the test can be seen in Figure 12b. The length of all samples increased after completion of the test. The length measurement was performed using a caliper. The average increase in length, which is denoted as negative surface recession, ranges between 0.60 and 1.80mm for all samples, as shown in Figure 13. A comparison between base samples and thermally cycled showed a decrease in sample expansion as the number of thermal cycles increased. The same behavior was shown by samples irradiated with UV. The sample expansion decreased with an increase in UV fluence. However, the amount of decrease is less compared to samples exposed to thermal cycle conditions.

(a) (b) (c)

Figure 12. External view of ablator sample before and after arc heating test: (a) front view of sample Aerospacebefore2020 arc, 7 ,heating 95 test; (b) front view of the sample after test; and (c) side view of the sample after the12 of 28 test.

FigureFigure 13. 13. RelationshipRelationship betweenbetween surface surface recession recession with wi maximumth maximum surface surface temperature temperature of 3D CF of/PEEK. 3D CF/PEEK. 3.3. Surface and Internal Temperature Behavior 3.3. SurfaceTable 7and described Internal Temperature the test conditions Behavior to observe the surface and internal temperature behavior. Figure 14 shows the relationship between heat flux and maximum surface temperature. The average maximum surface temperature increased from 2634.6 to 3107.1 ◦C when the heat flux increased from 2 5 to 14.2 MW/m . A comparison of the in-depth temperature–time histories between heat fluxes of 5 and 14.2 MW/m2 is shown in Figure 15. The in-depth temperature for the sample exposed to 5 MW/m2 heat flux is comparatively higher than the samples exposed to 14.2 MW/m2. The reason for the higher in-depth temperature for lower heat flux compared to higher heat flux was due to heating duration, thermal conductivity and thermal capacity of the test material. The samples were exposed to 14.2 MW/m2 for 10 s while the heating time was 20 s for 5 MW/m2. As a result, the higher heat flux samples were not able to reach a higher temperature. Figure 16 shows the time history of the surface temperature for base, thermally cycled and UV irradiated samples. Initially, there was a sharp increase in temperature, as shown in Figure 16a. The initial sharp increase happened when the ablator sample entered the plasma flow. As heating time increases, the change in surface temperature decreases and is nearly constant. The temperature then decreased back to the ambient temperature of the plasma due to the sample exiting the plasma jet. The same overall pattern can be observed in Figure 16b. The near-constant surface temperature might arise from the blocking action by pyrolysis gas. The product of the pyrolysis reaction in the PEEK resin produces gas that forms a protective layer from the hot stream over the char surface [25]. Moreover, when the surface temperature rises above 1500 K or 1230 ◦C, nearly all oxygen is consumed by the reaction at the surface. As a result, the reaction rate is limited, resulting in the near-constant surface temperature [26]. Table8 shows the average maximum surface temperature for each ablator sample. Comparison in time history of surface temperature and maximum surface temperature showed no significant difference between base sample, thermally cycled and UV irradiated samples. Aerospace 2020, 7, x FOR PEER REVIEW 13 of 29

Table 7 described the test conditions to observe the surface and internal temperature behavior. Figure 14 shows the relationship between heat flux and maximum surface temperature. The average maximum surface temperature increased from 2634.6 to 3107.1 °C when the heat flux increased from 5 to 14.2 MW/m2. A comparison of the in-depth temperature–time histories between heat fluxes of 5 and 14.2 MW/m2 is shown in Figure 15. The in-depth temperature for the sample exposed to 5 MW/m2 heat flux is comparatively higher than the samples exposed to 14.2 MW/m2. The reason for the higher in-depth temperature for lower heat flux compared to higher heat flux was due to heating duration, thermal conductivity and thermal capacity of the test material. The samples were exposed to 14.2 MW/m2 for 10s while the heating time was 20s for 5 MW/m2. As a result, the higher heat flux samples were not able to reach a higher temperature. Figure 16 shows the time history of the surface temperature for base, thermally cycled and UV irradiated samples. Initially, there was a sharp increase in temperature, as shown in Figure 16a. The initial sharp increase happened when the ablator sample entered the plasma flow. As heating time increases, the change in surface temperature decreases and is nearly constant. The temperature then decreased back to the ambient temperature of the plasma due to the sample exiting the plasma jet. The same overall pattern can be observed in Figure 16b. The near-constant surface temperature might arise from the blocking action by pyrolysis gas. The product of the pyrolysis reaction in the PEEK resin produces gas that forms a protective layer from the hot stream over the char surface [25]. Moreover, when the surface temperature rises above 1500 K or 1230 °C, nearly all oxygen is consumed by the reaction at the surface. As a result, the reaction rate is limited, resulting in the near-constant surface temperature [26]. Table 8 shows the average maximum surface temperature for each ablator

Aerospacesample.2020 Comparison, 7, 95 in time history of surface temperature and maximum surface temperature13 of 28 showed no significant difference between base sample, thermally cycled and UV irradiated samples.

. Aerospace 2020, 7, x FOR PEER REVIEW 14 of 29 Figure 14. Relationship between maximum surface temperature and heat ﬂux. Figure 14. Relationship between maximum surface temperature and heat flux.

Figure 15. Comparison in time history of in-depth temperature between sample exposed to 5 and Figure 15. Comparison in time history of in-depth temperature between sample exposed to 5 and 14.2 14.2 MW/m2:(a) temperature at a depth of 5 mm; (b) temperature at a depth of 10 mm; and (c) MW/m2: (a) temperature at a depth of 5mm; (b) temperature at a depth of 10mm; and (c) temperature temperature at a depth of 20 mm. at a depth of 20mm.

Figure 16. Time history of surface temperature during arc heating test: (a) comparison between the base sample and thermally cycled samples; and (b) comparison between the base sample and UV irradiated samples.

Table 8. The average maximum surface temperature for difference ablator samples during arc heating test.

Aerospace 2020, 7, x FOR PEER REVIEW 14 of 29

Figure 15. Comparison in time history of in-depth temperature between sample exposed to 5 and 14.2 AerospaceMW/m20202,: 7(,a 95) temperature at a depth of 5mm; (b) temperature at a depth of 10mm; and (c) temperature14 of 28 at a depth of 20mm.

Figure 16. Time history of surface temperature during arc heating test: ( a)) comparison between the the base sample and thermally cycled samples; and (b) comparison between the base sample and UV base sample and thermally cycled samples; and (b) comparison between the base sample and UV irradiated samples. irradiated samples. Table 8. The average maximum surface temperature for diﬀerence ablator samples during arc Tableheating 8. test.The average maximum surface temperature for difference ablator samples during arc heating test. Sample Type Average Maximum Surface T (◦C) A (Base sample) 2640.78 B (2800 thermal cycles) 2662.25 C (4200 thermal cycles) 2621.75 D (5600 thermal cycles) 2613.38 E (15 ESD) 2588.48 F (30 ESD) 2612.83 G (45 ESD) 2566.44

Figure 17 shows the maximum temperature at different distances from the heated surface for all types of ablator samples. Thermocouples were located inside each sample at distances of approximately 5, 10 and 20 mm from the sample surface to measure inside temperature. The slight variation in thermocouple position for the same depth affected the maximum temperature. For the 5-mm reference point, the position of the thermocouples varied between 4.1 and 6.6 mm. The variation in position was due to error during the placement of thermocouple. Thermocouples with location of 4.1 and 4.2 mm recorded higher temperatures between 650 and 1050 ◦C. Most of the recorded temperatures were within 300–500 ◦C for the 5-mm distance, as shown in Figure 17a. The plot for the maximum temperature as a function of distance from the surface is not linear but a gradual downward curve. The temperature was approximately 250 ◦C at 10 mm before reaching 150 ◦C at 20 mm from the surface. There was a small difference in maximum temperature between different sample types near the surface, as shown in Figure 17a. The temperature difference gradually diminishes for 10- and 20-mm distance from the surface. Figure 17b shows a detailed internal temperature around the 5-mm reference point. Thermally cycled samples show a slightly higher maximum temperature compared to the base samples between 4.5 and 6 mm. Aerospace 2020, 7, x FOR PEER REVIEW 15 of 29

Sample Type Average Maximum Surface T (°C) A (Base sample) 2640.78 B (2800 thermal cycles) 2662.25 C (4200 thermal cycles) 2621.75 D (5600 thermal cycles) 2613.38 E (15 ESD) 2588.48 F (30 ESD) 2612.83 G (45 ESD) 2566.44 Figure 17 shows the maximum temperature at different distances from the heated surface for all types of ablator samples. Thermocouples were located inside each sample at distances of approximately 5, 10 and 20mm from the sample surface to measure inside temperature. The slight variation in thermocouple position for the same depth affected the maximum temperature. For the 5- mm reference point, the position of the thermocouples varied between 4.1 and 6.6mm. The variation in position was due to error during the placement of thermocouple. Thermocouples with location of 4.1 and 4.2mm recorded higher temperatures between 650 and 1050 °C. Most of the recorded temperatures were within 300–500 °C for the 5-mm distance, as shown in Figure 17a. The plot for the maximum temperature as a function of distance from the surface is not linear but a gradual downward curve. The temperature was approximately 250 °C at 10mm before reaching 150 °C at 20mm from the surface. There was a small difference in maximum temperature between different sample types near the surface, as shown in Figure 17a. The temperature difference gradually diminishes for 10- and 20-mm distance from the surface. Figure 17b shows a detailed internal temperature around the 5-mm reference point. Thermally cycled samples show a slightly higher maximum temperature compared Aerospace 2020, 7, 95 15 of 28 to the base samples between 4.5 and 6mm.

Figure 17. Comparison of the maximum temperature at diﬀerent depths for all types of 3D CF/PEEK samples: (a) overall internal maximum temperature; and (b) detailed internal maximum temperature near 5-mm reference point.

3.4. Recession Behavior Two parameters were considered in analyzing the recession behavior of the 3D CF/PEEK ablator. The parameters were as follows: 1. Surface recession. 2. Mass loss rate. The following sections discuss the effect of the thermal cycle, UV radiation and different heat flux on the two mentioned parameters.

. L L = (3) t . where L is the surface recession rate (m/s), L is the amount of surface recession (m) which is the length or thickness of the sample before and after heating and t is the heating duration (s). Negative values were observed for the surface recession rate of all samples. The negative values implied that the surface expanded instead of decreased, as discussed in Section 3.2. Surface recession ranges between 0.00003 − and 0.00015 m/s. The surface recession rate decreased with an increase in surface temperature due − to increased heat ﬂux. In other words, the surface expansion rate increased with increased surface Aerospace 2020, 7, x FOR PEER REVIEW 16 of 29

Figure 17. Comparison of the maximum temperature at different depths for all types of 3D CF/PEEK samples: (a) overall internal maximum temperature; and (b) detailed internal maximum temperature near 5-mm reference point.

3.4. Recession Behavior Two parameters were considered in analyzing the recession behavior of the 3D CF/PEEK ablator. The parameters were as follows: 1. Surface recession. 2. Mass loss rate. The following sections discuss the effect of the thermal cycle, UV radiation and different heat flux on the two mentioned parameters.

3.4.1. Surface Recession Rate Figure 18 shows the relationship between surface recession rate and maximum surface temperature for all samples. The surface recession was obtained using Equation (3) [1]. = (3) where is the surface recession rate (m/s), L is the amount of surface recession (m) which is the length or thickness of the sample before and after heating and t is the heating duration (s). Negative values were observed for the surface recession rate of all samples. The negative values implied that Aerospacethe surface2020, 7, 95expanded instead of decreased, as discussed in Section 3.2. Surface recession ranges16 of 28 between −0.00003 and −0.00015 m/s. The surface recession rate decreased with an increase in surface temperature due to increased heat flux. In other words, the surface expansion rate increased with temperature. The surface recession rate for the thermally cycled sample and UV irradiated sample increased surface temperature. The surface recession rate for the thermally cycled sample and UV varyirradiated near the sample base sample vary near with the an base increase sample in with surface an increase temperature. in surface temperature.

FigureFigure 18. 18.The The relationship relationship between between maximummaximum surface temp temperatureerature and and surface surface recession recession rate rate of of3D 3D CFCF/PEEK./PEEK.

TableTable9 shows 9 shows the the average average surface surface recession recession for for didifferentﬀerent 3D 3D CF/PEEK CF/PEEK samples. samples. The The base base sample sample hashas the the lowest lowest surface surface recession recession rate whilerate while the thermally the thermally cycled cycled sample sample has the has highest the highest surface surface recession rate. A gradual increase in surface recession rate can be observed with an increase in the number of thermal cycles. The surface recession rate was near-constant with a small increase in samples irradiated with 45 ESD. The mentioned results imply a marginally lower surface expansion rate for thermally cycled samples compared to base samples. However, the surface expansion rate for UV radiated samples was almost identical to base samples. Based on the previous observations, it can be concluded that the thermal cycle and UV radiation have a minor impact on the surface recession rate properties of 3D CF/PEEK ablator in LEO.

Table 9. The average surface recession for diﬀerence ablator samples after arc heating test.

Sample Type Average Surface Recession Rate (m/s) A (Base sample) 0.000071 − B (2800 thermal cycles) 0.000064 − C (4200 thermal cycles) 0.000050 − D (5600 thermal cycles) 0.000035 − E (15 ESD) 0.000066 − F (30 ESD) 0.000066 − G (45 ESD) 0.000052 −

3.4.2. Mass Loss Rate The degradation mode during ablation depends on the surface temperature [27]. Figure 16 shows 2 that surface temperature was predominantly around 2600 ◦C for the 5 MW/m heat flux. Coupled with a modest heating rate of 5 MW/m2, the surface ablation was by the diffusion-controlled oxidation mode. The mass-loss rate in diffusion-controlled oxidation mode is governed by Equation (4) [1]. Aerospace 2020, 7, 95 17 of 28

. p mD = C0 Pe/RB (4)

. 2 where mD is the mass loss rate (kg/m /s), C0 is the diffusion-controlled mass-transfer constant 3/2 1/2 (kg/m s Pa ), Pe is the stagnation pressure (Pa) and R is the radius of the sample (m). The front · · B of the sample is flat instead of hemispherical. Therefore, the value of RB is obtained by multiplying the sample diameter by 2.463. The mass-loss rate can also be calculated based on measured data using Equation (5). . m f mi m = − (5) A t · . 2 where m is the mass loss rate (kg/m /s), mf is the sample mass after heating (kg), mi is the initial sample mass before heating (kg), t is the heating duration (s) and A is the frontal area of the sample (m2). In this experiment, the frontal area of the sample is equivalent to the area of a circle. Figure 19 shows the mass-loss rate to surface temperature for all samples. Mass loss rate increased with an increase in surface temperature due to an increase in heat flux. Samples exposed to 45 ESD of UV radiation showed the highest difference in mass loss rate compared to base samples. The other samples showed a mass loss rate ranging between 0.095 and 0.12 kg/m2/s. There is no significant difference between all samples except for the samples exposed to higher heat flux. Aerospace 2020, 7, x FOR PEER REVIEW 18 of 29

FigureFigure 19. 19.The The relationship relationship between between mass-loss mass-loss rate rate and maximumand maximum surface surface temperature temperature of 3D CFof /PEEK.3D CF/PEEK. 4. Discussion 4. Discussion 4.1. Changes in Mechanical Properties 4.1.In Changes the space in Mechanical environment, Properties the temperature can vary between 160 and +120 C when a satellite − ◦ passesIn from the directspace environment, sunlight into the Earth temperature shadow [ 9can]. This vary process between repeats −160 and continuously +120 °C when throughout a satellite the satellitepasses missionfrom direct life sunlight and is termed into Earth as thermal shadow cycling. [9]. This Figure process 10 repeats shows thatcontinuously there is a throughout small decrease the in tensilesatellite strength mission after life and 5600 is thermal termed as cycles. thermal However, cycling. Young’sFigure 10 modulus shows that did there not is change a small significantly decrease afterin tensile 5600 thermal strength cycles. after 5600 This thermal trendoccurred cycles. Howe becausever, Young’s the thermal modulus cycle did can not have change limited significantly degradation effafterect on 5600 tensile thermal strength cycles. and This Young’s trend occurred modulus. beca Theuse thermal the thermal cycle cycle primarily can have aff ectslimited matrix degradation dominated propertieseffect on tensile such as strength compression and Young’s and coe modulus.fficient Th ofe thermal thermal expansioncycle primarily (CTE) affects [17, 28matrix]. Tensile dominated strength andproperties Young’s such modulus as compression are dominated and coefficient by the fiber of thermal properties, expansion thus are(CTE) not [1 severely7,28]. Tensile affected strength by the and Young’s modulus are dominated by the fiber properties, thus are not severely affected by the thermal cycle [29]. The results for the thermally cycled samples were consistent with previous studies whereby 500 thermal cycles from approximately −150 to +93 °C did not significantly change tensile strength and Young’s modulus for CF/PEEK composites [10]. About UV radiation, both chain scission and crosslinking can affect the PEEK resin [14]. Chain scission creates weak bonds in polymers by cutting molecular chains. Crosslinking caused embrittlement of the polymer by limiting the movement of the molecular chain [30]. An increase in UV exposure duration can increase the crystallinity of the polymer [31]. Crystallization leads to tighter packing of the polymer chains thus resulting in stronger intermolecular bonding forces between the chains [32]. An increase in the percentage of crystallinity can improve the tensile properties of polymers in composites. Hence, as shown in Figure 10, the tensile strength initially increases until the point of 30 ESD. The initial increase in tensile strength suggests the increase in the crystallization of PEEK. The tensile strength subsequently decreases after 30 ESD. After 30 ESD the degradation by chain scission is more dominant than the crystallization of PEEK resulting in the decrease of tensile strength. However, further investigation should be devoted to determining the percent crystallinity at various UV fluence. However, Figure 11 shows an increasing Young’s modulus but at a decreasing rate as the UV fluence increases to 45 ESD. The increasing trend in Young’s modulus suggests that PEEK became embrittled due to crosslinking. Moreover, the increased crystallinity due to the longer duration to UV can increase Young’s modulus [31,33].

Aerospace 2020, 7, 95 18 of 28 thermal cycle [29]. The results for the thermally cycled samples were consistent with previous studies whereby 500 thermal cycles from approximately 150 to +93 C did not significantly change tensile − ◦ strength and Young’s modulus for CF/PEEK composites [10]. About UV radiation, both chain scission and crosslinking can affect the PEEK resin [14]. Chain scission creates weak bonds in polymers by cutting molecular chains. Crosslinking caused embrittlement of the polymer by limiting the movement of the molecular chain [30]. An increase in UV exposure duration can increase the crystallinity of the polymer [31]. Crystallization leads to tighter packing of the polymer chains thus resulting in stronger intermolecular bonding forces between the chains [32]. An increase in the percentage of crystallinity can improve the tensile properties of polymers in composites. Hence, as shown in Figure 10, the tensile strength initially increases until the point of 30 ESD. The initial increase in tensile strength suggests the increase in the crystallization of PEEK. The tensile strength subsequently decreases after 30 ESD. After 30 ESD the degradation by chain scission is more dominant than the crystallization of PEEK resulting in the decrease of tensile strength. However, further investigation should be devoted to determining the percent crystallinity at various UV fluence. However, Figure 11 shows an increasing Young’s modulus but at a decreasing rate as the UV fluence increases to 45 ESD. The increasing trend in Young’s modulus suggests that PEEK became embrittled due to crosslinking. Moreover, the increased crystallinity due to the longer durationAerospace to 2020 UV, 7 can, x FOR increase PEER REVIEW Young’s modulus [31,33]. 19 of 29

4.2.4.2. Recession Recession Behavior Behavior IncreaseIncrease in thein the length length of of sample sample or or surface surface expansionexpansion was was observed observed in inall all 3D 3D CF/PEEK CF/PEEK samples samples afterafter the the arc arc heating heating test. test. Previous Previous tests tests usingusing traditionaltraditional CF/PEEK CF/PEEK that that was was not not 3D3D printed, printed, hereafter hereafter referredreferred to as to CFas /CF/PEEK,PEEK, showed showed the the same samebehavior. behavior. In contrast, contrast, previous previous tests tests involving involving Lightweight Lightweight AblatorAblator Series Series for for Transfer Transfer Vehicle Vehicle System System (LATS)(LATS) ablatorablator showed showed surface surface recession recession or decreased or decreased in in the lengththe length of ablatorof ablator samples samples [34 [34].]. LATS LATS is is aa lightweightlightweight CFRP CFRP ablator ablator material material made made of carbon of carbon fiber ﬁber impregnated with phenolic resin [35]. A lightweight ablator based on LATS was used in the recently impregnated with phenolic resin [35]. A lightweight ablator based on LATS was used in the recently recovered H-II transfer vehicle (HTV) small re-entry capsule on November 27, 2018 [36]. A recovered H-II transfer vehicle (HTV) small re-entry capsule on November 27, 2018 [36]. A comparison comparison between the surface recession between 3D CF/PEEK, CF/PEEK and LATS ablator betweenmaterials the surfaceis shown recession in Figure between 20. The surface 3D CF /expansPEEK,ion CF is/PEEK represented and LATS by a ablatornegative materials surface recession is shown in Figurein Figure 20. The 20. surface expansion is represented by a negative surface recession in Figure 20.

FigureFigure 20. 20.Comparison Comparison of of surface surface recession recession betweenbetween 3D 3D CF/PEEK, CF/PEEK, CF/PEEK CF/PEEK and and LATS. LATS. The The negative negative surfacesurface recession recession denotes denotes a surfacea surface expansion. expansion.

A comparison of surface recession rate also showed the same behavior as the surface recession, as shown in Figure 21. The surface recession rate for 3D CF/PEEK and CF/PEEK shows a negative value, implying that the surface expanded. However, the surface recession rate for LATS showed a positive value due to surface recession.

Aerospace 2020, 7, 95 19 of 28

Figure 21. Comparison of surface recession rate between 3D CF PEEK, CF PEEK and LATS. The Figure 21. Comparison of surface recession rate between 3D CF/PEEK,/ CF/PEEK/ and LATS. The negative surface recession rate denotes a surface expansion rate. negative surface recession rate denotes a surface expansion rate. Various processes happened during the ablation of a heat shield as discussed in Introduction. The Various processes happened during the ablation of a heat shield as discussed in Introduction. primary recession mechanism is through the thermochemical process [37]. However, the process of The primary recession mechanism is through the thermochemical process [37]. However, the process delamination also occurred during the ablation of 3D CF/PEEK in addition to the thermochemical of delamination also occurred during the ablation of 3D CF/PEEK in addition to the thermochemical reaction. Figure 22 shows the side view of the base, thermally cycled and UV irradiated samples after reaction. Figure 22 shows the side view of the base, thermally cycled and UV irradiated samples after arc heating test. The pyrolysis reaction of the PEEK resin released pyrolysis gas which creates pressure arc heating test. The pyrolysis reaction of the PEEK resin released pyrolysis gas which creates in the out-of-plane direction inside the sample structure. Delamination occurs when the pressure due pressure in the out-of-plane direction inside the sample structure. Delamination occurs when the to the pyrolysis gas exceeds the interlaminar strength. In a previous study, pressure from pyrolysis pressure due to the pyrolysis gas exceeds the interlaminar strength. In a previous study, pressure gas was identiﬁed as the cause of delamination in CFRP ablators [38]. Therefore, during the ablation from pyrolysis gas was identified as the cause of delamination in CFRP ablators [38]. Therefore, of 3D CF/PEEK, the thermochemical process caused surface recession while the delamination and during the ablation of 3D CF/PEEK, the thermochemical process caused surface recession while the material expansion cause surface expansion. Note that the recession process is dependent on the delamination and material expansion cause surface expansion. Note that the recession process is surface temperature whereby the surface recession and expansion rate increase with an increase in dependent on the surface temperature whereby the surface recession and expansion rate increase surface temperature as shown in Figure 21. with an increase in surface temperature as shown in Figure 21. Therefore, it is suspected that the surface recession rate is less than the surface expansion rate, Therefore, it is suspected that the surface recession rate is less than the surface expansion rate, resulting in surface expansion being the dominant process during the ablation of 3D CF/PEEK. As a resulting in surface expansion being the dominant process during the ablation of 3D CF/PEEK. As a result, the surface of 3D CF/PEEK samples expanded, as shown in Figure 20. The lower surface result, the surface of 3D CF/PEEK samples expanded, as shown in Figure 20. The lower surface recession rate is due to the higher activation energy (E) of 3D CF/PEEK compared to CFRP used recession rate is due to the higher activation energy (E) of 3D CF/PEEK compared to CFRP used in in LATS. LATS.

(a) (b)

Aerospace 2020, 7, x FOR PEER REVIEW 20 of 29

Figure 21. Comparison of surface recession rate between 3D CF/PEEK, CF/PEEK and LATS. The negative surface recession rate denotes a surface expansion rate.

Various processes happened during the ablation of a heat shield as discussed in Introduction. The primary recession mechanism is through the thermochemical process [37]. However, the process of delamination also occurred during the ablation of 3D CF/PEEK in addition to the thermochemical reaction. Figure 22 shows the side view of the base, thermally cycled and UV irradiated samples after arc heating test. The pyrolysis reaction of the PEEK resin released pyrolysis gas which creates pressure in the out-of-plane direction inside the sample structure. Delamination occurs when the pressure due to the pyrolysis gas exceeds the interlaminar strength. In a previous study, pressure from pyrolysis gas was identified as the cause of delamination in CFRP ablators [38]. Therefore, during the ablation of 3D CF/PEEK, the thermochemical process caused surface recession while the delamination and material expansion cause surface expansion. Note that the recession process is dependent on the surface temperature whereby the surface recession and expansion rate increase with an increase in surface temperature as shown in Figure 21. Therefore, it is suspected that the surface recession rate is less than the surface expansion rate, resulting in surface expansion being the dominant process during the ablation of 3D CF/PEEK. As a result, the surface of 3D CF/PEEK samples expanded, as shown in Figure 20. The lower surface Aerospacerecession2020 rate, 7, 95is due to the higher activation energy (E) of 3D CF/PEEK compared to CFRP used20 of in 28 LATS.

Aerospace 2020, 7, x FOR PEER REVIEW 21 of 29 (a) (b)

Figure 22. SideSide view view of of ablator ablator sample sample used in arc heating test: (a)) base base sample sample before before the test; ( b) base sample after test; (c) thermally cycled sample;sample; andand ((d)) UVUV irradiatedirradiated sample.sample.

Activation energy can be calculated using Equation (6)(6) [[4].4]. The same equation is used to to calculate calculate . 2 2 the mass-loss rate, mR (kg (kg/m/m /s),/s), in in the the rate-controlled rate-controlled oxidation oxidation region region [ 4[4].].

. p ⁄ = E/RTW (6) mR = k0 X0Pee− (6) ⁄ where is the mass loss rate (kg/m2/s), k0 is collision frequency (⁄ ∙ ∙ ), X0 is the . 2 2 1/2 where mR is the mass loss rate (kg/m /s), k0 is collision frequency (kg/s m ηPa ), X0 is the mole mole fraction of oxygen in the air (0.21), Pe is the stagnation pressure (Pa),· E is the activation energy fraction of oxygen in the air (0.21), Pe is the stagnation pressure (Pa), E is the activation energy (J/mol), (J/mol), R is the universal gas constant (8.318 J/mol/K) and TW is wall temperature (K). Taking the Rlogarithmis the universal of both gassides constant resulted (8.318 in Equation J/mol/K) (7) and [4].TW is wall temperature (K). Taking the logarithm of both sides resulted in Equation (7) [4]. ln( ) = − +ln( ) (7) . E p ln mR = + ln k X Pe (7) −RT 0 0 Figure 23 shows the relationship between theW logarithmic mass loss rate and the reciprocal of the surfaceFigure temperature23 shows the for relationship all types of between samples. the Ba logarithmicsed on Figure mass 23, lossthe slope rate and or E/R the for reciprocal 3D CF/PEEK of the issurface 15,319 temperature K. The slope for value all types was multiplied of samples. with Based the on R-value Figure to 23 obtain, the slope the activation or E/R for energy 3D CF/ PEEKof 127.4 is 15,319kJ/mol. K. The The E slope value value for was3D multipliedCF/PEEK is with about the R-value10% more to obtain compared the activation to 115.6 energykJ/mol of of 127.4 CF/PEEK. kJ/mol. TheHowever, E value the for E 3D value CF/ PEEKis much is about higher 10% compared more compared to the CFRP to 115.6 ablator kJ/mol using of CF phenolic/PEEK. However,resin which the is E value83.3 kJ/mol is much [39]. higher Therefore, compared the higher to the E CFRP value ablator for 3D usingCF/PEEK phenolic compared resin whichto CFRP is 83.3ablators kJ/mol caused [39]. Therefore,the lower surface the higher recession E value rate. for The 3D minor CF/PEEK difference compared between to CFRP activation ablators energy caused of the3D CF/PEEK lower surface and CF/PEEKrecession rate.is due The to minorthe resin diﬀ erencecontent. between The activati activationon energy energy is of dependent 3D CF/PEEK on andresin CF content/PEEK is[1]. due The to CF/PEEKthe resin content.used in Thethe previous activation heating energy test is dependent has a resin on content resin content of 63% [compared1]. The CF /toPEEK 44% usedfor the in the3D previousCF/PEEK. heating The collision test has frequency a resin content is then of calculated 63% compared by substituting to 44% for the the surface 3D CF /massPEEK. loss The rate collision of 3D CF/PEEK,frequency the is then material calculated surface by temperature, substituting thethe surfacestagnation mass pressure loss rate measured of 3D CF /duringPEEK, thetest materialand the activationsurface temperature, energy of the3D stagnationCF/PEEK into pressure Equation measured (6). The during average test and collision the activation frequency energy based of 3Don ⁄ measuredCF/PEEK intoexperimental Equation (6).values The was average 0.331 collisionkg⁄ s ∙ m frequency∙Pa . based on measured experimental values was 0.331 kg/s m2 Pa1/2. · ·

Aerospace 2020, 7, 95 21 of 28 Aerospace 2020, 7, x FOR PEER REVIEW 22 of 29

Figure 23. The relationship between reciprocal of surface temperature and the logarithm of mass loss Figure 23. The relationship between reciprocal of surface temperature and the logarithm of mass loss rate of 3D CF/PEEK and CF/PEEK. rate of 3D CF/PEEK and CF/PEEK.

Based on the mass-loss rate calculated using Equation (5), the C0 was obtained using the linear Based on the mass-loss rate calculated using Equation (5), the C0 was obtained using the linear relationship between √Pe/RB and the mass loss rate shown in Equation (4) [39]. The C0 value aﬀects relationship between ⁄ and the mass loss rate shown in Equation (4) [39]. The C0 value affects the maximum value of the mass-loss rate. Figure 24 shows the relationship between the mass-loss the maximum value of the mass-loss rate. Figure 24 shows the relationship between the mass-loss rate of 3D CF/PEEK and √Pe/RB based on measured data. The slope in Figure 24 or C0 value of 3D rate of 3D CF/PEEK and4 ⁄3/ 2 based1/ on2 measured data. The slope in Figure 24 or C0 value of 3D CF/PEEK is 1.996 10− (kg/m ⁄ s Pa ),⁄ which is lower than the C0 value for CF/PEEK. An increase CF/PEEK is 1.996× × 10−4 (⁄ · ∙∙· ), which is lower than the C0 value for CF/PEEK. An in C0 meant an increase in the mass loss rate [1]. Therefore, 3D CF/PEEK has a lower mass-loss rate increase in C0 meant an increase in the mass loss rate [1]. Therefore, 3D CF/PEEK has a lower mass- compared to CF/PEEK. lossAerospace rate compared2020, 7, x FOR to PEER CF/PEEK. REVIEW 23 of 29 Figure 25 shows a comparison in the relationship between mass-loss rate and maximum surface temperature for 3D CF/PEEK, CF/PEEK and LATS. Based on Figure 25, 3D CF/PEEK has the lowest mass loss rate comparative to CF/PEEK and LATS. The lower mass-loss rate of 3D CF/PEEK can be attributed to the higher activation energy compared to LATS and CF/PEEK. The small difference in mass loss rate between 3D CF/PEEK and CF/PEEK is attributed to higher activation energy and lower C0 value in 3D CF/PEEK compared to CF/PEEK. In conclusion, the higher activation energy caused 3D CF/PEEK to be less susceptible to surface recession and mass loss but more affected by surface expansion due to delamination and thermal expansion.

Figure 24. The relationship between the mass-loss rate and √Pe/RB of 3D CF/PEEK and CF/PEEK. Figure 24. The relationship between the mass-loss rate and ⁄ of 3D CF/PEEK and CF/PEEK.

Figure 25. The relationship between mass-loss rate and mass loss rate of 3D CF/PEEK, CF/PEEK and LATS.

Figure 13 shows that an increase in the amount of thermal cycle caused the amount of surface expansion to be decreased. Samples exposed to 5600 thermal cycles, equivalent to one year, showed a marginally lower surface expansion rate compared to base samples. Thermal cycles can affect the CTE of a sample due to the advent of microcracks within the sample structure. It is suspected that the CTE value decreases when the number of thermal cycles increased. The reason is that the damage to the matrix by microcracking caused the CTE to be dominated by the fiber, thus decreasing the CTE value. Studies have shown that fiber has a lower CTE compared to the matrix [40]. Moreover, another

Aerospace 2020, 7, x FOR PEER REVIEW 23 of 29

Aerospace 2020, 7, 95 22 of 28

Figure 25 shows a comparison in the relationship between mass-loss rate and maximum surface temperature for 3D CF/PEEK, CF/PEEK and LATS. Based on Figure 25, 3D CF/PEEK has the lowest mass loss rate comparative to CF/PEEK and LATS. The lower mass-loss rate of 3D CF/PEEK can be attributed to the higher activation energy compared to LATS and CF/PEEK. The small diﬀerence in mass loss rate between 3D CF/PEEK and CF/PEEK is attributed to higher activation energy and lower C0 value in 3D CF/PEEK compared to CF/PEEK. In conclusion, the higher activation energy caused

3D CF/PEEK to be less susceptible to surface recession and mass loss but more aﬀected by surface expansionFigure due 24. The to delamination relationship between and thermal the mass-loss expansion. rate and ⁄ of 3D CF/PEEK and CF/PEEK.

FigureFigure 25. 25. TheThe relationship relationship between between mass-loss mass-loss rate rate and and mass mass loss lossrate rateof 3D of CF/PEEK, 3D CF/PEEK, CF/PEEK CF/PEEK and LATS.and LATS.

Figure 13 shows that an increase in the amount of thermal cycle caused the amount of surface Figure 13 shows that an increase in the amount of thermal cycle caused the amount of surface expansion to be decreased. Samples exposed to 5600 thermal cycles, equivalent to one year, showed a expansion to be decreased. Samples exposed to 5600 thermal cycles, equivalent to one year, showed marginally lower surface expansion rate compared to base samples. Thermal cycles can affect the CTE a marginally lower surface expansion rate compared to base samples. Thermal cycles can affect the of a sample due to the advent of microcracks within the sample structure. It is suspected that the CTE CTE of a sample due to the advent of microcracks within the sample structure. It is suspected that value decreases when the number of thermal cycles increased. The reason is that the damage to the the CTE value decreases when the number of thermal cycles increased. The reason is that the damage matrix by microcracking caused the CTE to be dominated by the fiber, thus decreasing the CTE value. to the matrix by microcracking caused the CTE to be dominated by the fiber, thus decreasing the CTE Studies have shown that fiber has a lower CTE compared to the matrix [40]. Moreover, another study value. Studies have shown that fiber has a lower CTE compared to the matrix [40]. Moreover, another showed that pores at the interface between consequently deposited layers in a 3D printed CF/PEEK can lead to multiple crack formation. The cracks can affect the CTE value compared to CF/PEEK [41]. Therefore, it is suspected that the decrease in CTE value restricts the surface from expanding thus decreasing the surface expansion rate.

4.3. Surface and Internal Temperature Behavior Figure 26 shows the variation in maximum surface temperature with heat flux for different ablator materials. The results for 3D CF/PEEK whereby surface temperature increases with an increase in heat flux is consistent with previous tests involving LATS and CF/PEEK. Figure 17 shows that maximum temperature as a function of distance from the surface is not linear but a gradual downward curve. The nature of the curve plot is due to the dependence of the thermal conductivity of ablator material on the density of resin [1]. The thermal decomposition of resin near the surface during the arc heating Aerospace 2020, 7, x FOR PEER REVIEW 24 of 29 study showed that pores at the interface between consequently deposited layers in a 3D printed CF/PEEK can lead to multiple crack formation. The cracks can affect the CTE value compared to CF/PEEK [41]. Therefore, it is suspected that the decrease in CTE value restricts the surface from expanding thus decreasing the surface expansion rate.

4.3. Surface and Internal Temperature Behavior Figure 26 shows the variation in maximum surface temperature with heat flux for different ablator materials. The results for 3D CF/PEEK whereby surface temperature increases with an increase in heat flux is consistent with previous tests involving LATS and CF/PEEK. Figure 17 shows that maximum temperature as a function of distance from the surface is not linear but a gradual downward curve. The nature of the curve plot is due to the dependence of the thermal conductivity of ablator material on the density of resin [1]. The thermal decomposition of resin near the surface during the arc heating test resulted in a charred surface. The porous charred surface and near-surface area are less dense than the virgin material inside the ablator [42]. As a result, the thermal conductivity decreases due to the porosity and decrease in density [43]. Based on Figure 16, the maximum surface temperature was on average approximately 2600 °C. The temperature than sharply decreased to 500 °C at 5 mm from the surface. The sharp drop can be explained by the porous region near the surface area which has low thermal conductivity. A comparison of in-depth temperature between different sample types shows a slightly higher trend in temperature for thermally cycled samples. The difference is mainly in the region between 4.5 and 6mm, as shown in Figure 17. The primary effect of thermal cycling is to induce microcracks Aerospaceinside the2020 structure, 7, 95 of a material [7]. Previous studies have shown that the orientation of a crack23 ofcan 28 affect the thermal conductivity of a material [43,44]. It is suspected that microcracks in the near- surface area affect the thermal conductivity property of the ablator material. However, the magnitude testof the resulted difference in a charredin temperature surface. Thevalue porous between charred thermally surface cycled and near-surface samples and area base are lesssamples dense is than not thesignificantly virgin material large. insideTherefore, the ablatorexposure [42 to]. Asthe ather result,mal thecycle thermal has no conductivity significant effect decreases on the due internal to the porositytemperature and behavior decrease inof densityablator samples. [43]. Based on Figure 16, the maximum surface temperature was on averageFigure approximately 27 shows that 2600 the◦ C.surface The temperature temperatures than of 3D sharply CF/PEEK, decreased CF/PEEK to 500 and◦C LATS at 5 mm were from nearly the surface.identical Thewhen sharp exposed drop to can a heat be explained flux of approximately by the porous 5.0 region MW/m near2. The the in-depth surface areatemperature which has for low 3D thermalCF/PEEK conductivity. is almost identical to CF/PEEK and lower than LATS at 20 mm from the surface.

Figure 26. 26. TheThe relationship relationship between between maximum maximum surface surface temp temperatureerature and heat and flux heat for ﬂux different for di ablatorﬀerent ablatormaterials. materials.

A comparison of in-depth temperature between different sample types shows a slightly higher trend in temperature for thermally cycled samples. The difference is mainly in the region between 4.5 and 6 mm, as shown in Figure 17. The primary effect of thermal cycling is to induce microcracks inside the structure of a material [7]. Previous studies have shown that the orientation of a crack can affect the thermal conductivity of a material [43,44]. It is suspected that microcracks in the near-surface area affect the thermal conductivity property of the ablator material. However, the magnitude of the difference in temperature value between thermally cycled samples and base samples is not significantly large. Therefore, exposure to the thermal cycle has no significant effect on the internal temperature behavior of ablator samples. Figure 27 shows that the surface temperatures of 3D CF/PEEK, CF/PEEK and LATS were nearly identical when exposed to a heat flux of approximately 5.0 MW/m2. The in-depth temperature for 3D CF/PEEK is almost identical to CF/PEEK and lower than LATS at 20 mm from the surface. Aerospace 2020, 7, 95 24 of 28 Aerospace 2020, 7, x FOR PEER REVIEW 25 of 29

Figure 27. Comparison of maximum surface and in-depth temperature between 3D CF/PEEK, LATS and FigureCF/PEEK 27. Comparison ablator samples: of maximum (a) maximum surface surface and temperature;in-depth temperature and (b) maximum between in-depth3D CF/PEEK, temperature LATS andfor 20CF/PEEK mm from ablator surface. samples: (a) maximum surface temperature; and (b) maximum in-depth temperature for 20mm from surface. 4.4. Future Works 4.4. Future Works Previously discussed results concerning the effect of thermal cycle and UV radiation were limited to surfacePreviously recession discussed rate, results surface concerning temperature the and effect in-depth of thermal temperature cycle and behavior. UV radiation The next were step limitedwill be to to surface determine recession the eff rate,ect of surface the thermal temperat cycleure and and UV in-depth radiation temperature on the activation behavior. energy The next and stepdiff usion-controlledwill be to determine mass-transfer the effect constant.of the ther Bothmal parameterscycle and UV can radiation affect the on surface the activation recession rateenergy and andmass diffusion-controlled loss rate [1]. Arc heating mass-trans test conductedfer constant. in Both a wider parameters range of heatcan affect flux and the heatingsurface recession duration usingrate andthermally mass loss cycled rate and [1]. UVArc irradiatedheating test samples conducted can facilitate in a wider study range on of the heat effects flux of and thermal heating cycle duration and UV usingradiation thermally on the cycled mentioned and UV parameters. irradiated samples can facilitate study on the effects of thermal cycle and UVAccurate radiation prediction on the mentioned of the recession parameters. and temperature behavior of 3D CF/PEEK material during the re-entryAccurate environment prediction of is the necessary recession for and the temper designature of the behavior TPS. Previously, of 3D CF/PEEK various material ablation analysisduring thecodes, re-entry mainly environment one-dimensional is necessary ablation for the simulation design of codes, the TPS. were Previously, developed various to cater ablation for high analysis density codes,and low-density mainly one-dimensional CFRP-based ablator ablation materials simulation [3,45 codes,]. However, were thedeveloped suitability to cater of the for existing high density ablation andcodes low-density has not yet CFRP-based been confirmed ablator for materials 3D CF/PEEK. [3,45]. To However, improve the the suitability existing codes, of the the existing following ablation may codesprove has to benot useful: yet been confirmed for 3D CF/PEEK. To improve the existing codes, the following may prove to be useful: 1. Execution of further arc heating test using different heat flux and heating duration, which among 1. Execution of further arc heating test using different heat flux and heating duration, others can confirm the surface expansion behavior in different heating environments. which among others can confirm the surface expansion behavior in different heating 2. Determinationenvironments. of the density relation between virgin layer and char layer of 3D CF/PEEK. 3. Measurement2. Determination of thermal conductivityof the density and relation specific heatbetween variation virgin of 3Dlayer CF /PEEKand char with layer temperature. of 3D CF/PEEK. 5. Conclusions 3. Measurement of thermal conductivity and specific heat variation of 3D CF/PEEK In this research,with temperature. a new heat shield material made of 3D printed CF/PEEK was evaluated using tensile and arc heating test. Young’s modulus did not significantly change after exposure to an 5.increasing Conclusions number of thermal cycles and UV fluence. However, tensile strength showed a small decreasedIn this afterresearch, exposure a new to heat the thermal shield material cycle but made no significant of 3D printed change CF/PEEK after exposure was evaluated to UV radiation. using tensileThe length and arc of theheating samples test. increased Young’s ormodulus the surface did not expanded significantly during change the heating after test,exposure resulting to an in increasinga negative number surface of recession thermal rate. cycles The and increased UV flue surfacence. However, temperature tensile increased strength the showed mass-loss a small rate. decreased after exposure to the thermal cycle but no significant change after exposure to UV

Aerospace 2020, 7, 95 25 of 28

The surface expansion rate was more than the surface recession rate for 3D CF/PEEK resulting in an expanded surface. The lower surface recession rate was due to a higher activation energy for 3D CF/PEEK compared to CFRP based ablator such as LATS. The activation energy for 3D CF/PEEK was marginally higher than CF/PEEK due to difference in resin content. Moreover, 3D CF/PEEK has the lowest mass loss rate compared to LATS and CF/PEEK attributed to a higher activation energy. Therefore, 3D CF/PEEK high activation energy makes it less susceptible to surface recession and mass loss but more affected by surface expansion due to delamination and material expansion. The surface expansion decreased when the number of thermal cycles increased. Moreover, the surface expansion rate for samples exposed to the thermal cycle was marginally lower than other samples. It is suspected that a decrease in CTE value due to microcracks restricts surface expansion, thus decreasing surface expansion rate. However, the mass-loss rate was almost identical for base samples and samples exposed to the thermal cycle and UV radiation. The increased surface temperature of 3D CF/PEEK due to increasing heat flux was consistent with LATS and CF/PEEK. Exposure to similar heat flux yielded almost identical surface temperatures among 3D CF/PEEK, CF/PEEK and LATS. In-depth temperature for 3D CF/PEEK was almost identical to CF/PEEK but lower than LATS when exposed to similar heat flux. Samples exposed to the thermal cycle exhibited a marginally higher temperature in the near-surface region compared to other samples. However, the temperature difference between base samples and samples exposed to the thermal cycle and UV radiation gradually diminishes with an increase in depth. Overall, the thermal cycle and UV radiation have no significant effect on the surface and in-depth temperature behavior of samples. The new 3D CF/PEEK material has demonstrated excellent recession resistance while maintaining mechanical properties when exposed to high temperature, thermal cycle and UV radiation. Consequently, 3D CF/PEEK can be considered as a viable heat shield material for the re-entry flight.

Author Contributions: Conceptualization, F.A., A.M. and N.Y.; methodology, F.A., A.M., N.Y. and K.-i.O.; validation, F.A., N.Y. and K.-i.O.; formal analysis, F.A., A.M. and N.Y.; investigation, F.A., A.M. and N.Y.; resources, F.A., A.M., N.Y. and K.-i.O.; data curation, F.A., A.M. and N.Y.; writing—original draft preparation, F.A.; writing—review and editing, F.A. and K.-i.O.; visualization, F.A. and N.Y.; supervision, K.-i.O.; project administration, K.-i.O.; and funding acquisition, K.-i.O. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research was performed in collaboration with AGC Inc. This study could not be completed without the effort and co-operation from the Okuyama Lab team members. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Abbreviations The following abbreviations were used in this manuscript: AM Additive Manufacturing CAD Computer-Aided Drawing CF Carbon Fiber CFRP Carbon Fiber Reinforced Plastic CFRTP Carbon Fiber Reinforced Thermoplastic CTE Coeﬃcient of Thermal Expansion DED Direct Energy Deposition ESD Equivalent Solar Day FDM Fused Deposition Modelling HTV H-II Transfer Vehicle ISAS Institute of Space and Aeronautical Science ISS International Space Station JAXA Japan Aerospace Exploration Agency Aerospace 2020, 7, 95 26 of 28

LATS Lightweight Ablator Series for Transfer Vehicle System LEO Low Earth Orbit NASA National Aeronautics and Space Administration PBF Powder Bed Fusion PEEK Polyether Ether Ketone SLA Stereolithography SLS Selective Laser Sintering TPS Thermal Protection System UV Ultraviolet VUV Vacuum Ultraviolet

References

1. Okuyama, K.; Zako, M. A Study on Recession Characteristics of Completely Carbonized CFRP. SPACE Technol. Jpn. Jpn. Soc. Aeronaut. SPACE Sci. 2004, 3, 35–43. [CrossRef] 2. Beck, R.A.S. Ablative Thermal Protection Systems Fundamentals; Thermal and Fluids Analysis Workshop: Huntsville, AL, USA, 2017. 3. Kato, S.; Okuyama, K.; Gibo, K.; Miyagi, T.; Suzuki, T.; Fujita, K.; Sakai, T.; Nishio, S.; Watanabe, A. Thermal Response Simulation of Ultra Light Weight Phenolic Carbon Ablator by the Use of the Ablation Analysis Code. Trans. Jpn. Soc. Aeronaut. SPACE Sci. Aerosp. Technol. Jpn. 2012, 10, 31–39. [CrossRef] 4. Metzger, J.W.; Engel, M.J. Oxidation and Sublimation of Graphite in Simulated Re-Entry Environments. AIAA J. 1967, 5, 10. [CrossRef] 5. Hacopian, E.; Bouslog, S. Technical Challenges with 3D Printing Heat Shields; Advanced Manufacturing & Carbon Materials Workshop: Houston, TX, USA, 2018. 6. ESA. 3D printing Cubesat bodies for cheaper, faster missions. Available online: http://www.esa.int/Enabling_ Support/Space_Engineering_Technology/3D_printing_CubeSat_bodies_for_cheaper_faster_missions (accessed on 20 June 2020). 7. Di Roberto, R. TUPOD. Available online: https://www.gaussteam.com/satellites/gauss-latest-satellites/tupod/ (accessed on 4 April 2020). 8. Bouslog, S. 3D Printing Heat Shields; Additive Manufacturing & Advanced Materials Workshop: Houston, TX, USA, 2017. 9. Cogswell, F.N. Thermoplastic Aromatic Polymer Composites; Butterworth-Heinemann: Oxford, UK, 1992; ISBN 978-0-7506-1086-5. 10. Funk, J.G.; Sykes, G.F. The Effects of Simulated Space Environmental Parameters on Six Commercially Available Composite Materials; National Aeronautics and Space Administration: Hampton, VA, USA, 1989; p. 34. 11. Slemp, S. Ultraviolet Radiation Effects; NASA, Scientific and Technical Information Office: Hampton, VA, USA, 1989; p. 22. 12. Tennyson, R.C. Atomic Oxygen and Its Effect on Materials. In The Behavior of Systems in the Space Environment; DeWitt, R.N., Duston, D., Hyder, A.K., Eds.; Springer Netherlands: Dordrecht, The Netherlands, 1993; pp. 233–257. ISBN 978-94-010-4907-8. 13. Johnson, R.H.; Montierth, L.D.; Dennison, J.R.; Dyer, J.S.; Lindstrom, E.R. Small-Scale Simulation Chamber for Space Environment Survivability Testing. IEEE Trans. Plasma Sci. 2013, 41, 3453–3458. [CrossRef] 14. Nakamura, T.; Fujita, O. Effects of LEO Environment on Mechanical Properties of Peek Films under Tensile Stress. In Proceedings of the Proc. of International Symposium on “SM/MPAC&SEED Experiment”, Tsukuba, Japan, 10–11 March 2008; p. 8. 15. Zhang, D.; Rudolph, N.; Woytowitz, P. Reliable Optimized Structures with High Performance Continuous Fiber Thermoplastic Composites from Additive Manufacturing (AM). In Proceedings of the SAMPE 2019, Charlotte, NC, USA, 20–23 May 2019. 16. Kim, R. Dimensional stability of composite in a space thermal environment. Compos. Sci. Technol. 2000, 60, 2601–2608. [CrossRef] 17. Stewart, M.C. The Dependence of the Change in the Coefficient of Thermal Expansion of Graphite Fiber Reinforced Polyimide IM7-K3B on Microcracking due to Thermal Cycling; NASA: Hampton, VA, USA, 1995. Aerospace 2020, 7, 95 27 of 28

18. Crowe, D.; Feinberg, A. Design for Reliability; CRC Press: Boca Raton, FL, USA, 2001; p. 24. ISBN 978-0-8493-1111-6. 19. Escobar, L.A.; Meeker, W.Q. A Review of Accelerated Test Models. Stat. Sci. 2006, 21, 552–577. [CrossRef] 20. Unigovski, Y.B.; Grinberg, A.; Gutman, E.M.; Shneck, R. Low-cycle Fatigue of Thermally-Cycled Carbon-Epoxy Composite. J. Met. Mater. Miner. 2012, 22, 21–30. 21. Abdullah, F.; Okuyama, K.; Fajardo, I.; Urakami, N. In Situ Measurement of Carbon Fibre/Polyether Ether Ketone Thermal Expansion in Low Earth Orbit. Aerospace 2020, 7, 35. [CrossRef] 22. Peat, C. ISS Orbit. Available online: https://www.heavens-above.com/orbit.aspx?satid=25544 (accessed on 15 April 2020). 23. Mori, K.; Ishizawa, J. Protection of Materials and Structures from the Space Environment; Springer Berlin Heidelberg: Heidelberg, Germany, 2013; Volume 32, ISBN 978-3-642-30228-2. 24. Wijker, J. Jaap. Spacecraft Structures; Springer Berlin Heidelberg: Heidelberg, Germany, 2008; ISBN 978-3-642-09477-4. 25. Natali, M.; Torre, L. Wiley Encyclopedia of Composites; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012; p. weoc045. ISBN 978-1-118-09729-8. 26. Potts, R.L. Application of integral methods to ablation charring erosion - A review. J. Spacecr. Rockets 1995, 32, 200–209. [CrossRef] 27. Miller, I.M.; Sutton, K. An Experimental Study of the Oxidation of Graphite in High-Temperature Supersonic and Hypersonic Environments; National Aeronautics and Space Administration: Hampton, VA, USA, 1966; p. 45. 28. Paillous, A.; Pailler, C. Degradation of multiply polymer-matrix composites induced by space environment. Composites 1994, 25, 287–295. [CrossRef] 29. Chawla, K.K. Composite Materials; Springer New York: New York, NY, USA, 2012; pp. 137–195. ISBN 978-0-387-74364-6. 30. Russell, D.A.; Connell, J.W.; Fogdall, L.B. Electron, Proton, and Ultraviolet Radiation Effects on Thermophysical Properties of Polymeric Films. J. Spacecr. Rockets 2002, 39, 833–838. [CrossRef] 31. Mahat, K.B.; Alarifi, I.; Alharbi, A.; Asmatulu, R. Effects of UV Light on Mechanical Properties of Carbon Fiber Reinforced PPS Thermoplastic Composites. Macromol. Symp. 2016, 365, 157–168. [CrossRef] 32. Callister, W.D.; Rethwisch, D.G. Materials Science and Engineering, 10th ed.; Wiley: Hoboken, NJ, USA, 2018. 33. Chivers, R.A.; Moore, D.R. The effect of molecular weight and crystallinity on the mechanical properties of injection moulded poly(aryl-ether-ether-ketone) resin. Polymer 1994, 35, 110–116. [CrossRef] 34. Kato, S.; Matsuda, S.; Okuyama, K.; Gibo, K.; Oya, H.; Watanabe, A.; Shimada, N.; Sakai, S. Study of the Effects of Heat Load, Ablator Density and Backup Structure upon the Thermal Protection Performance of Heat Shield Systems Consisting of Phenolic Carbon Ablators. Trans. Jpn. Soc. Aeronaut. SPACE Sci. Aerosp. Technol. Jpn. 2016, 14, 95–104. [CrossRef] 35. Szasz, B.A.; Okuyama, K. A New Method for Estimating the Mass Recession Rate for Ablator Systems. Int. J. Mech. Mechatron. Eng. 2014, 8, 5. [CrossRef] 36. Recovered HTV Small Re-entry Capsule was opened for media at JAXA TKSC. Available online: https: //iss.jaxa.jp/en/htv/mission/htv-7/181205_interview.html (accessed on 13 January 2020). 37. Yin, T.; Zhang, Z.; Li, X.; Feng, X.; Feng, Z.; Wang, Y.; He, L.; Gong, X. Modeling ablative behavior and thermal response of carbon/carbon composites. Comput. Mater. Sci. 2014, 95, 35–40. [CrossRef] 38. Kubota, Y.; Miyamoto, O.; Aoki, T.; Ishida, Y.; Ogasawara, T.; Umezu, S. New thermal protection system using high-temperature carbon fibre-reinforced plastic sandwich panel. Acta Astronaut. 2019, 160, 519–526. [CrossRef] 39. Okuyama, K.; Kato, S.; Yamada, T.; Zako, M. Determination of thermo-chemical parameters affecting the oxidation recession of carbonized CFRP surface. Tanso 2004, 213, 128–133. 40. Barnes, J.A.; Simms, I.J.; Farrow, G.J.; Jackson, D.; Wostenholm, G.; Yates, B. Thermal expansion characteristics of PEEK composites. J. Mater. Sci. 1991, 26, 2259–2271. [CrossRef] 41. Stepashkin, A.A.; Chukov, D.I.; Senatov, F.S.; Salimon, A.I.; Korsunsky, A.M.; Kaloshkin, S.D. 3D-printed PEEK-carbon fiber (CF) composites: Structure and thermal properties. Compos. Sci. Technol. 2018, 164, 319–326. [CrossRef] 42. Duffa, G. Ablative Thermal Protection Systems Modeling; American Institute of Aeronautics and Astronautics: Reston, VA, USA, 2013; ISBN 978-1-62410-171-7. Aerospace 2020, 7, 95 28 of 28

43. Hasselman, D. Eﬀect of Micro-Cracking on Thermal Conductivity: Analysis and Experiment. In Thermal Conductivity 16; Larsen, D.C., Ed.; Springer: Boston, MA, USA, 1983; pp. 417–431. 44. Siebeneck, H.J.; Hasselman, D.P.H.; Cleveland, J.J.; Bradt, R.C. Eﬀects of Grain Size and Microcracking on the

Thermal Diﬀusivity of MgTi2O5. J. Am. Ceram. Soc. 1977, 60, 336–338. [CrossRef] 45. Kato, S.; Okuyama, K.; Nishio, S.; Sakata, R.; Hama, K.; Inatani, Y. Numerical Analysis of Charring Ablation for Ablative Materials of Re-Entry Capsules. J. Jpn. Soc. Aeronaut. SPACE Sci. 2002, 50, 255–263. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).