CERAMICSINTERNATIONAL

Available online at www.sciencedirect.com

http://dx.doi.org/0272-8842/& 20

nCorrespondeNWPU, YouyiTel.: þ86 29 88

E-mail addrermwang@nwpu.

(2015) 13751–13758

Ceramics International 41 www.elsevier.com/locate/ceramint

Sublimation and oxidation zone ablation behaviorof carbon/carbon composites

Shameel Farhana, Rumin Wanga,n, Kezhi Lib, Chuang Wangb

aSchool of Science, Northwestern Polytechnical University, Xi’an 710072, ChinabSchool of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

Received 21 July 2015; received in revised form 31 July 2015; accepted 7 August 2015Available online 18 August 2015

Abstract

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate moduluscarbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltrationprocess followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preformarchitecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature(4750 K) sublimation to high-temperature (2467 K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. Theablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions whilethe back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased withthe rise in temperature till 1173 K and later on became fairly flat till 1523 K and onwards. In the compression test, 4750 K exposed specimenshowed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467 K exposed specimen showedend brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: E. Thermal applications; Carbon/carbon composites; Plasma arc-heater; Ablation; Oxidation

1. Introduction

Carbon/carbon (C/C) composites are materials of choice inaerospace industry due to their outstanding thermostability andthermomechanical properties at a very high temperature [1–5].C/C composites were originally used on ablative structures(e.g., rocket nozzles, re-entry nose tips & leading edges) inwhich the material has to endure high thermal gradient and ismainly ablated by some gasification processes like oxidationand sublimation [6]. The re-entry conditions vary with the typeof mission: temperature up to 5000 K and pressure up to100 bar, and a heat flux received by the protection ranging

10.1016/j.ceramint.2015.08.04315 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

nce to: Department of Applied Chemistry, School of Science,West Road 127, Xi’an 710072, Shaanxi, China.

492947.sses: shameelfarhan@yahoo.com (S. Farhan),edu.cn (R. Wang).

from 0.1 to 500 MW/m2 [7]. Under such conditions, aconsiderable part of the heat flux is consumed by masstransfer, which has two principal forms: oxidation andsublimation. These phenomena are grouped under the genericname of ablation [8]. By definition, ablation is a process ofelimination of a large amount of thermal energy by sacrificinga portion of mass of the ablative material. General physio-chemical reactions containing this process are phase changes,conduction, convection, radiation, diffusion and exothermic/endothermic chemical reactions [9]. Chemical reactions play asignificant role in establishing ablation rates of most of theresultant material-environment combinations. Ultra high tem-perature ceramics are the potential candidates for thermalprotection of reentry vehicles because of their very highmelting points. The major hurdle in the development of thesematerials is that their manufacturing techniques are not matureuntil now [10]. High density of the material as well asmachining problems is another reason to search for alternative

S. Farhan et al. / Ceramics International 41 (2015) 13751–1375813752

materials overcoming aforementioned problems with addi-tional advantages apart from the uniform thermo-mechanicalproperties, low erosion and high thermal stability. The materialstudied in this research is three-dimensional and four-directional (4D) in structure. The 4D C/C composites are wellknown for their excellent thermo-physical and thermo-mechanical properties in short-term, high-temperature applica-tions [11–14]. Due to the high interlaminar shear strength, theyexhibit high performance with respect of shape stability andanti-ablation [15].

Many experimental characterizations of C/C compositeshave been reported in literature [16–19]. The oxyacetyleneflame ablation and plasma arc ablation are the most widelyused methods to study the ablation behavior of C/C composites[20–24]. Oxyacetylene flame does not simulate the actualreentry conditions due to limited heat flux and flow species[25]. Only plasma arc heater can produce nearly similar reentryconditions. Moreover, these experiments include micro, mesoand macroscopic observations of the ablated surface andmeasurements of the recession rates. The ablated surfacemorphology generally consists of fiber denudation and blockdenudation and the recession rates can be expressed in terms ofthe linear erosion rate and the mass ablation rate. The testingtemperature is around 3200 K similar to the temperature in thenozzle of solid rocket motor.

To the authors’ knowledge, no one has done such kind ofwork in which operating parameters of an arc heater werechanged with regard to chemical reactivity of C/C composites.The purpose of this research is to differentiate the two mainchemical reaction regimes: oxidation dominant and sublima-tion dominant and compare the effects of these regimes on theablation behaviour of a 4D C/C composite. The temperaturewas adjusted by controlling the mass flow rate of thesecondary air (plasma cooling gas) so that the oxygen-richhigh temperature (�2500 K) and oxygen-lean ultra-hightemperature (� 4500 K) were created in the plasma flowfield. Moreover, the effect of plasma stream temperatures,stagnation enthalpies, gaseous velocities, and cooling (second-ary) air flow rate on the ablation behavior of the 4D C/Ccomposite was also studied. The stagnation-point ablation rateswere measured with different mix ratios of primary andsecondary air working as the test gas. Air was chosen as atesting media for the simulation of the reentry condition. Airhaving a mixture of O2 and N2, dissociates into N, O, NO,when it passes through the shock wave created arc in theplenum chamber of arc heater. The composite studied wasprepared by a hybrid process and six samples of similardensity and fiber orientation were machined out for testing. Toexamine the effect of hostile environment on the virginmaterial, back-face temperature and the residual compressivestrength of the two specimens that experienced extremeconditions were also tested. The two extreme conditions wereultra-high temperature (4750 K) oxygen-lean and high tem-perature (2467 K) oxygen-rich environment respectively. Theresulting back-face temperature, crushing behavior and load–displacement plot were also discussed in relation to theablation test conditions.

2. Experimental

2.1. Preform fabrication

A 4D preform was fabricated by the insertion of stiffenedcarbon rods in four directions in an orthogonal plane. TC 42Scarbon fiber (Tairyfil Co., China) bundles with 5.7 μmdiameter, 5150 MPa ultimate tensile strength and 290 GPaYoung’s modulus were used in the dry weaving process.Multiple fiber bundles were combined and passed through a8–15% binder solution in a standard pultrusion machine. Thebinder can be any kind of organic polymer like polyvinylchloride, polyvinyl alcohol, epoxies and resins that are used inthe manufacture of advanced fiber reinforced composites. Heresolid novolac dissolved in organic propanol was chosen as anovel choice. The excessive resin was squeezed by passingthrough a series of nozzles and the coated rods were dried in atube-heating oven. The maximum fiber preheating and thepultruded rods post-treatment temperatures were 653 K (opti-mum for removing coatings from the carbon fiber) and 523 K(curing temperature of novolac), respectively. The diameter ofthe pultruded rod was controlled within 1.0 to 1.30 mmdepending upon the number of tows and weaving geometryrequirements. After rigidization, novolac binder in the pultrur-ods was about 5% by weight. Fiber volume fraction (Vf) wasmaintained in the range of 8–9% in the XY-plane and 13–14%in the Z-axis. Total Vf of the preform was about 40% and thebulk density was almost 0.69 g/cm3. Further details about thepreform employed can be found in [26,27].

2.2. Densification process

The preform was densified using a hybrid process which is acombination of two processes. A pre-densification step with athermal-gradient chemical vapor infiltration (TCVI) processand a high-densification using a high pressure pitch impreg-nation and carbonization (HiPIC) process. The preform wasfirst high temperature (HT) treated at 1873 K to convert thefiber coating and resin into the carbon. The HT process wasconducted at a very slow rate with the aid of graphitic holdingplates to avoid the collapse of the preform. Pyrolytic carbonfrom the natural gas (98% CH4, 0.3% C3H8, 0.3% C4H10,0.4% other hydrocarbon, 1% N2) was deposited on the carbonfibers in a TCVI furnace at temperature range of 1350–1450 K[28]. At a deposition rate of 0.5 mm/h, a bulk density of1.70 g/cm3 was achieved in 80 h. It was further increased to1.86 g/cm3 by three successive HiPIC cycles. Intermediate heattreatment at 2500 K after TCVI process was used for poreopening, graphitization and strength enhancement [29]. Highsoftening (HS) pitch (399 K softening point, 1.30 g/cm3

density, 490% carbon contents, 4.5% hydrogen, 455%carbon yield) was poured on the pre-densified C/C compositeunder vacuum and stabilized for 5 h at 573 K. As shown bystudies [30] on wetting and impregnation of carbon fibers byHS pitches, no impregnation of the inter-filamentary porosityoccurs even at high temperature and therefore, pressure turnsout to be necessary for the densification of C/C composites. It

S. Farhan et al. / Ceramics International 41 (2015) 13751–13758 13753

was further carbonized at 1100–1200 K and 80 MPa isostaticpressure.

2.3. Characterization

2.3.1. Density and porosityThe geometric density of 4D C/C specimens

(ø¼22 mm� 50 mm) after final HT treatment was measuredby the ratio of mass of the sample to the total apparent volume.True density and apparent porosity of carbon foam wereexamined by helium gas displacement pycnometer (Pentapyc5200e Quantachrome Instruments, Florida, USA). True densityis the mass per unit volume of material, which exclude allvoids or pores. Open porosity was calculated using thefollowing expression:

%P¼ ρt�ρaρt

� �� 100 ð1Þ

where %P is the bulk open porosity, ρt and ρa are true andapparent densities of the samples respectively. The true densityof the composite was found to be 1.9070.01 g/cm3 while thefinal open porosity of the composite was 3.2570.25%.

2.3.2. Electric arc plasma testingThe 4D C/C composite block was cut along the Z-axis of the

fiber architecture forming a cylindrical geometry with outerdiameter of 22.5�0.5 mm and length of 50 mm. The length (orthickness) of the specimens is not so important because onlythe blunt face was exposed to air plasma and thus, the exposedarea was kept fixed at 3.97� 10�4 m2. Fibers in the XY-planewere oriented in 01 (U-direction), þ601 (V-direction) and�601 (W-direction) angles whereas Z-directional fibers werealong the Z-axis. In the testing, Z-axis fibers were at 901 to theblunt face (face exposed to plasma stream). The ablationexperiments were performed on a Huels type arc heater. Twotandem cylindrical electrodes were separated by a centralcopper tube and insulated from each other. Primary air wasinjected tangentially to the wall of the heater from fourdifferent points in a manner to encircle the plasma jet alongthe centerline and to stabilize it with the minimum contact withthe inner surfaces. A magnetic coil was used on the anode torotate the arc termination and prevent arc backfire to the rearplug. The heater was ignited by a high-voltage breakdown withAr gas filling the chamber. Within one second, after the arcbreakdown, the gas supply was switched from Ar to air. Theplasma expanded as it passed through a converging–divergingnozzle to a low supersonic Mach number. The secondary airwas injected into the upstream end of the constrictor at variousflow rates for the adjustment of final temperature and velocityof spurting plasma stream. To ensure quasi-one dimensionalheat flow, a silica/phenolic holder was used to hold and protectthe samples from side-wall heating at a length of 15 mm fromthe exposed face. The planar surface of the specimen and theholder were flushed and grinded using SiC emery papers with360 down to 1200 grit. The following parameters listed inTable 1 were either fixed or varied for the study of their effecton the ablation behavior of the 4D C/C composite.

A small hole (ø¼3 mm� 35 mm) was drilled in the speci-mens from the backside for the insertion of a thermocouple.Because C/C composites have good electrical conductivity, thecompacted mineral-insulated metal-sheathed, K type thermo-couple with a range of �73 to 1550 K was used followingASTM E235 M-23 test method. The distance between thethermocouple and the exposed surface was fixed at almost15 mm. The centers of the plasma and the specimen werealigned horizontally. A microbalance (1 mg precision) and a0.1 mm precise caliper gauge were used for the pre- and post-test evolution of samples. After attaining the required condi-tions of static chamber pressure and heat flux, the sample wasmoved into the sputtering plasma flow with the help ofpneumatically-drive trolley. After the test, ablated mass wasdivided by the exposed surface area and test duration and theresults were expressed with a unit of kg/m2-s. Fig. 1 shows theschematic of the experimental set up fitted with a specimen anda back-face thermocouple.

2.3.3. Thermal diffusivityThe thermal diffusivity of 4D C/C composite cut into disk

geometry (ø¼12.5 mm� 2.5 mm) was measured by the flashdiffusivity method, which basically consists of subjecting oneside of a sample to a single laser flash and then monitoring thetransient temperature response on the other side. A NetzschLFA 457 Micro-Flash instrument with the guidelines of ASTME-1461 standard was operated from room temperature to1250 1C under Ar atmosphere. The uncertainty of the mea-surements was 75%.

2.3.4. Compression after ablation (CAA)The compression-after-ablation (CAA) [26] of the material

was studied using the SANS CMT 5105 (100 kN) mechanicaltesting machine. The vertical moving speed of the crossheadwas set at 0.5 mm/min with the load and displacement beingrecorded and were in the direction parallel to Z-axis carbonfiber rods. The specimens were cut using a diamond saw to aheight of about 30 mm. To create a smooth surface, thin layersof ablated surface were removed using 600 sand papers. Thespecimens were dried in a drying oven at 110 1C for 120 minbefore the compressive tests. The compressive strength wascalculated according to the following equation:

σc ¼Ρ

Αð2Þ

where P was the maximum loading of fracture (N); A was thecross-sectional area of the specimen (mm2).

3. Results and discussion

3.1. Electric arc plasma testing

Fig. 2 shows the macro-morphologies of the samples afterexposure to high-energy reactive air plasma test in an ambientatmosphere. The recession rates along with different arc heaterparameters are summarized in Table 2. The ablation rate andtemperature as a function of the ratio of secondary to primary

Table 1Arc-heater parameters during sublimation and oxidation zone ablation testing of 4D C/C composites.

Fixed parameters Variable parameters

Primary air flowrate (g/s)

Distance ofspecimen (mm)

Ablationtime (s)

Secondary air flowrate (g/s)

Hot chamberpressure (bar)

Thermal enthalpy(kJ/kg)

Velocity of plasmastream (m/s)

Temperature(K)

15.8570.10 1070.10 15 15.85–38.70 4.2–5.6 3200–5766 574–628 4750–2500

Holder Thermocouple

SpecimenPlasma Stream

Arc HeaterW-direction V-direction

U-directionZ-direction

Fig. 1. Arc heater testing; (a) Schematic of experimental set up and (b) test specimen along Z-axis with reinforcement directions highlighted by bold lines.

5 mm5 mm

Fig. 2. Macro-morphologies of specimens after plasma arc heater test; speci-men 1 (sublimation regime) to 6 (oxidation regimes) from left to right.

0.8 1.2 1.6 2.0 2.4 2.82.0

2.2

2.4

2.6

2.8 Ablation rateTemperature

Mass flow ratio (Sec. Air / Pri. Air)

Abl

atio

n ra

te (k

g/m

2 -s)

2400

3000

3600

4200

4800

Tem

pera

ture

(K)

Fig. 3. Effect of mass flow ratio on ablation rate and exposure temperature ofplasma arc heater (sublimation to oxidation regimes).

S. Farhan et al. / Ceramics International 41 (2015) 13751–1375813754

air are plotted in Figs. 3 and 4. The ablation rate came out to be2.17 kg/m2-s for the initial flow ratio of 1.01, and increased to2.62 kg/m2-s when the ratio exceeded to 2.62. Such anincrease was attributed to the strong oxidizing condition inthe plasma flow resulting from the increase in mass flow rate ofthe secondary air. The flow enthalpy of the plasma stream alsodecreased with the increase in secondary air flow rate. Theback-face temperatures of the specimen 1 and 6 were recordedduring the ablation testing to see the effect of ultra-hightemperature (4750 K) and high temperature (2467 K) environ-ments on the back-face temperature. From specimen 1 to 6, thetemperature and stagnation enthalpy gradually decreased from

4750 to 2467 K and 5750 to 2819 kJ/kg, respectively, byincreasing the mass flow rate of the secondary air. Thesecondary air was injected upstream before the nozzle andacted as a cooling gas. The ablation rate increased withdecreasing the temperature of plasma stream, which wasnearly in a reverse order. The highest ablation rate of2.62 kg/m2-s was observed in case of the lowest temperatureof 2467 K, the highest air mass flow rate (15.8þ41.42 g/s) andthe plasma stream velocity of 630 m/s. Virtually all oxygen inthe plasma stream reacted with the carbon atoms, irrespectiveof the heating rate, to form CO2 and CO when the temperaturewas r3000 K. The enthalpies of these species are negativewith respect to that of un-dissociated air. In specimen 6, theconditions were favorable for oxidation reaction dominance(excess secondary air) and mechanical denudation due to theshear forces of high plasma velocity. Macro-morphology of thespecimen 6 also revealed excessive oxidation pits on the

3000 4000 5000 60002.0

2.2

2.4

2.6 Ablation rate Temperature

Enthalphy (KJ/Kg)

Abl

atio

n ra

te ( K

g/m

2 -s)

2400

3200

4000

4800

Tem

pera

ture

(K)

Fig. 4. Effect of stagnation enthalpy on ablation rate of 4D C/C compositesand exposure temperature of plasma arc heater.

Table 2Summary of arc-heater parameters and resulting recession rates of 4D C/C composites.

Sr.no

Recession rate (kg/m2-s)

Temperature(K)

Back facetemperature (K)

Enthalpy(kJ/kg)

Primary air flowrate (g/s)

Secondary air flowrate (g/s)

Flowratio

Plasma velocity(m/s)

1 2.17 4750 1094 5750 15.89 16.00 1.01 5702 2.25 4127 – 4936 15.83 22.50 1.42 5873 2.31 3531 – 4198 15.80 27.28 1.73 6004 2.40 3389 – 4033 15.79 29.69 1.88 6075 2.50 2967 – 3480 15.80 34.86 2.21 6176 2.62 2467 1075 2819 15.80 41.42 2.62 630

S. Farhan et al. / Ceramics International 41 (2015) 13751–13758 13755

surface and specially at the peripheral area which is compara-tively weaker against the strong shear forces.

C(s)þO2-CO2þ395 kJ/mol (3)

CO2þC(s)-2CO–142.5 kJ/mol (4)

CO formed is further reacted with dissociated oxygen whichis very excess in this case, forming CO2 as following:

COþO-CO2þ520.9 kJ/mol (5)

N2 also begins to undergo heterogeneous reactions directlywith the surface to form CN as following:

1/2N2þC(s)-CN–460.5 kJ/mol (6)

which is further reacted with dissociated oxygen atoms asfollowing:

CNþ2O-CO2þ1/2N2þ1364 kJ/mol (7)

The resulting energy change due to combustion is thereforeexothermic, giving a positive contribution to the heat trans-ferred to the surface. Above 3000 K, carbon also startedsublimation in addition to the oxidation [31]. In the specimen1, the plasma stream temperature was the highest (4750 K)with a minimum air mass flow (15.89þ16.0 g/s). At such ahigh temperature, the oxidation reaction of carbon wasgoverned by the restricted diffusion rate because of the limitedamount of oxygen [32]. The surface of the specimen 1 was

roughly smooth with a few oxidation pits. It is to be noted thatthe plasma stream temperature was 4750 K based on thetesting parameters, the temperature of the ablated surfacewas a little lower due to the heat transfer and dynamic thermalbalance existing among the plasma, the specimen and theenvironment [33]. We can define the sublimation regime as therange of conditions where the mass loss due to vaporizationexceeds the diffusion controlled oxidation mass loss rate. It isnoted that at the high surface temperatures, not only dochemical reactions occur between carbon and oxygen, but alsonitrogen reacts with carbon to form cyanogen (CN) and thecyano radical CNn [34]. As the surface temperature rises, thevaporization rate of atomic and molecular carbon species, suchas C, C2, C3, C4 and C5, all increase exponentially. Note that,the sublimation process yields appreciably greater amounts oftriatomic carbon gas than monatomic carbon [35].The major sublimation reactions can be summarized as

following:

3C(s)-C3–753.5 kJ/mol (8)

3C3þ3N2-6CN–1297 kJ/mol (9)

which is further reacted with limited supply of dissociatedoxygen as following:

CNþO-COþ1/2N2þ849.5 kJ/mol (10)

The overall energy change in this zone is endothermic. Theeffect of these exothermic and endothermic regimes wasfurther noted in the back-face temperature of the specimensduring ablation testing. The back-face temperature of thespecimen 1 and 6 was 1094 K and 1075 K, respectively. Thedifference was not very large as compared with the very largedifference between the hot-face plasma stream temperatures.The temperature profile is plotted in Fig. 5. The experimentalconditions in the specimen 1 corresponded to the sublimationregime and in this case, chemical heat flux was negative, whilein the specimen 6, it was positive into the solid which gavenearly the same back face temperature as that in case ofsublimation regime. In the specimen 1, the effect of ultra-hightemperature on the back-face temperature was lowered by thedominant sublimation reaction (cooling) whereas in the speci-men 6, oxidation reaction (heating) reheated the high tempera-ture conditions. The back-face temperature rose exponentiallyat the beginning of the experiment and attained the maximumgrowth rate at 4�5 s when a small wiggle in the rising curve

0 12 24 36 48 60300

600

900

1200

Tem

pera

ture

(K)

Time (s)

Specimen 6

Specimen 1

Back-face absolute temperatureDistance from hot face = 15 mm

Fig. 5. Back-face temperature profile of sublimation (specimen 1) andoxidation (specimen 6) regime ablation testing.

Fig. 6. Effect of temperature on thermal diffusivity and heat capacity of 4D C/Ccomposites along Z-axis.

0.0 0.2 0.4 0.60

7

14

21

28

Load

(kN

)

Dispalcement (mm)

Specimen 6Specimen 1

Fig. 7. Residual load–displacement curves of specimen 1 (sublimation regime)and 6 (oxidation regime) after plasma arc heater testing.

S. Farhan et al. / Ceramics International 41 (2015) 13751–1375813756

occurred. After that, it started rising with a decreasing growthrate for the 15 s of test duration and continued 5�7 s after thecompletion of the tests. The cooling profiles were different inthe two cases: slow in oxidation leading ablation and fast insublimation leading ablation. The small wiggle in the growthcurve occurred at about 373 K due to the moist air superheatedand trapped in the specimens. The melting and sublimation ofthe specimen 1 were considered because these phenomena aresignificant at a surface temperature greater than 4000 K [36].At this higher surface temperature, the vaporizing species of Creact with nitrogen into the boundary layer and some of CNevolved diffuses back to the condensed phase. Also, as before,CO diffuse back to the surface. All these endothermic reactionsnear the surface will reduce the back face temperature [37].Temperature overshoot from the exothermic reaction occursfurther out in the gas phase.

3.2. Thermal diffusivity

The thermal diffusivity represents the ability of a material toconduct thermal energy relative to its ability to store thermalenergy and make the temperature uniform in the materials [38].The thermal diffusivity value of the 4D C/C compositedecreased with the rise in temperature till 1173 K and lateron became fairly flat till 1523 K and onwards as shown inFig. 6. It may be noted that the thermal diffusivity of all theconstituents of C/C composites decreases with the rise intemperature [16]. In this case, the thermal diffusivity decreasedby 44 mm2/s when the temperature from room temperaturewas increased to 1100 K which is almost the back-facetemperature of the specimens in arc heater testing. In C/Ccomposites, carbon fibers are the main channels of heattransmission and hence their direction and distribution has amajor impact on thermal diffusivity. Their thermal diffusivityvaries from 60 to 19 mm2/s in the temperature range of500�1500 K. In all the specimens, the Z-axis fibers are alongthe test direction and their volume percentage is 13.5, which

provides more continuous channels for the phonon transmis-sion. After final densification with coal tar pitch, the residualopen porosity of the pre-densification step was further reducedto 3.25% and the composite became more compact. Theseentire factors contributed in higher thermal diffusivity at roomtemperature. However, as the test temperature went up, thephonon vibration frequency also increased resulting in scatter-ing or decrease in the mean free path of the phonon leading toa rapid decrease in thermal diffusivity. It also has an influenceon the decreasing trend of back-face temperature duringablation testing.

3.3. Compression after ablation (CAA)

Fig. 7 shows the load–displacement curves of the specimen1 and 6 after the ablation test. The linear region, representingthe true elastic response of the material was used to calculate

10 mmFig. 8. Macro-morphologies of specimen 1 (left) and 2 (right) after compres-sion after ablation test.

S. Farhan et al. / Ceramics International 41 (2015) 13751–13758 13757

the compressive modulus. The specimen 1 showed a residualcompressive strength of 75.63 MPa with a Young’s modulusof 6.04 GPa. The specimen 6 showed a bilinear response in thelinear portion. The peak portion between 0.2 to 0.45 mmdisplacements was the ultimate compressive load. The result-ing strength and modulus were 69.15 MPa and 4.02 GPa,respectively. These lower values were due to the deeppenetration of the oxidative plasma into the interfacial areasof the composite. The load displacement curves represented atypical brittle behavior without a catastrophic failure. Thespecimen 1 failed on the non heat-effected side with both theends crushed and shear-failure in the central portion. The endcrushing is typical in this configuration of compression testingand it is due to the buckling of carbon rods and matrix failure(Fig. 8). When the load was applied on the specimen, shearstress was induced along the interface between the specimenend and the loading platen [39]. From the macro images(Fig. 2) of the ablated specimens, a sequential difference in thesurface morphology can be observed. The surface of thespecimen 1 is smooth and flat with a few pinholes while deepmesh like pits with surface undulation are visible on the wholesurface of the specimen 6. From the specimen 1 to 6, there is atransition from smooth to rough surface. The material becametougher at the heat-affected zone because of more gasificationand failed on the opposite side in the compression testing. Thespecimen 6 sowed end brushing at the plasma heat-effectedzone. This behavior along with macro image (Fig. 2) con-firmed the deep penetration of plasma into the intra rod spaceswith oxidation of matrix carbon. The 4D C/C composites showextremely low interfacial strength between the fiber and thematrix [40]. The strong oxidizing conditions and high-temperature further reduced the strength and the carbon rodsfailed due to macro-buckling.

4. Conclusions

4D C/C composite was fabricated using an intermediatemodulus carbon fiber and a hybrid processing method using aTCVI and HiPIC processes resulting in a final density of1.86 g/cm3 and a fiber volume fraction of 13.5% in Z-axis.

Plasma arc-heater testing was conducted to see the ablationbehavior. Arc-heater parameters like temperature, enthalpy,velocity and flow rate were adjusted to create the ultra high-temperature (4750 K) sublimation and high-temperature(2467 K) oxidation regimes. The ablation rate increased withthe transition from oxygen-lean sublimation to oxygen-richoxidation environment. The back-face temperature showed asimilar but not so rapid rise in temperature as compared withthe large variation in the plasma stream temperatures at the hotface. The endothermic sublimation reaction lowered the back-face temperature while the exothermic oxidation reactionreheated the specimen so that the net effect on the back-facein both the cases was similar. In the compression test afterablation, the ultra-high temperature exposed specimen showedtoughening at the plasma-affected zone, failed in end crushingand shear mode from the opposite side while the hightemperature exposed specimen showed end brushing in plasmaheat affected zone with a lower residual strength and modulus.

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