Effects of Porosity in Graphite Materials on Ablation in Arc-heated Jets

7/27/2019 Effects of Porosity in Graphite Materials on Ablation in Arc-heated Jets

1/6

V O L . 14, NO. 1, J A N U A R Y 1 977 J. SPACECRAFT 19

Effects of Porosity in Graphite Materialson Ablation in Arc-Heated JetsI rv ing Auerbach ,* Mor ton L. L iebe rman ,* K a t h e r y n E . Lawson, t and Hugh O. P ierson*

SandiaLaboratories, Albuquerque,N. Mex.T he ablat ion performance of chemical ly vapor-deposited carbon on a carbon felt made from polyacrylonitr i le

fiber, ATJ-S , an d ATJ-S percursor graphites w as studied in an arc jet at stagnation pressures an d bulk gas en-thalpies of 2.2 to 10.3 MPa and 4700 to 5800 J/g, respectively. T he initial nonlinear ablat ion kinetics isassociated wit h the attainment of thermal an d chemical equil ibrium. In the subsequent steady-state period, th eablation rate is constant . Ablation was modeled wit h an exponent ia l rate relationship and the steady-state rateconstants were shown to be related exponentially to porosi ty . A scanning electron microscope examination ofablated surfaces showed that nonuniform surface ablation can be correlated with porosi ty , f iber densi ty, and thef iber-matrix interfacial spacing.

IntroductionT H E abla t ion of a thermal protect ive mater ia l on a re-en t r y vehic le , e .g. , gr aphi te , ph enol ic composi tes, e tc ., i sa su r face phenomenon and is , therefore , subject to theporosi ty of the protect ive mater ia l . This study addresses theeffect of porosi ty on ab la t i on pe r fo rmance for two types ofgraphi t ic mater ia ls: 1) ATJ-S and i t s precursors, and 2) a car-bon/carbon composi te prepared by chemical ly vapor-deposi t ing carbon on a polyacr ylon i t r i le precursor felt( C V D / P A N ) . A l t h o u g h carbon/carbon composi tes preparedfrom r ayon p recur so r felts (CVD/rayon) have been used foraf t heat shields on re-ent ry vehic les, th e improved the rma lstress resistance of the CVD/PAN composite suggested that itcould be used for nose t ips.I t was ant ic ipated that porosi ty would be an important fac-tor in the abla t ion performance of the CVD/PAN composi te .For th is reason, ATJ-S g r a p h i t e an d ATJ-S precur so r s , one o fwhich had received n o i m p r e g n a t i o n and the other of w h i c hhad rece ived one impregnat ion, were inc luded in the testmatr i x for compar ison. This paper provides the resul ts of thiss tudy . A more deta i led repor t is g i v e n in Ref. 1.Experimental

Test Facil it iesT he Sandia 2 M W 2 and the M cDon ne l l Douglas HIP3 archeaters were used to study the abla t ion performance of themater ia ls inc luded in this paper . The Sandia facility consists

of a L i n d e N-1000 arc hea t er w hich ex haus ts t h rough a supe r -sonic nozzle (Mach 1 .7 and thr oat diameter of 0 .70 cm ) and a1 .02 cm exi t into a test chamber main ta ined a t p ressures below6500 Pa. The M cD onne l l Douglas H IP facility uses th e M D C -200 arc heater which has a M ach 1.7 nozzle , a 0.953-cm diamthroat , an d an ex i t diameter of 1.14 cm w hich ex haus t s i n to anambient envi ronment . In both systems, models were injec tedinto th e heat exhaust by a Fe rguson In t e rmi t t e r a t a fixedposit ion of 0.25 cm from th e nozzle exit . A m a x i m u m of fivemodels was tested in a s ingle test run. Tw o injec to r a rms helda pi tot probe and a nul l point ca lor imeter for pressure andheat-flux measurements. Bulk enthalpies were obta ined f romenergy balance measurements on the arc heater . T hestagnat ion pressures used in the Sandia and HIP fac i l i t ieswere 2.3 to 2.5 and 10.3 MPa, respectively. T he bulk gas en-Received A u g .27, 1976.I ndex ca tegor ies : Mate r ia l Abla t ion , Therm ochem is t ry andChemical K ine t ics .'Member of the Technica l Staf f .tMember o f the Technica l Staf f ; presen t address: B end ix Cor-pora t ion Resea rch Labora to r ies , Sou thf ie ld , Mich .

tha lpies were nominal ly 5200 to 5800 and 4700 J/g, respec-t ive ly.Materials

Five types of graphi t ic mater ia ls were studied: CVD/PAN,commercia l ATJ-S, and three graphi tes specia l ly prepared byUnion Ca rb ide , nam e ly , non impregn a ted and s i ng ly im-pregnated ATJ-S precur so r s an d fully impregna t ed ATJ-S ob-ta ined f rom these precursors. T he CVD/PAN ca rbon/ca rboncomposi tes were fabr ica ted by carbonizat ion of the PAN felt ,i n f i l t r a t ion of the carbon matr ix by pyrolysis of methan e, andgraphi t iza t ion of the mater ia l a t 3000C. The models used inth e Sandia test facil i ty were obtained f rom two frustadesignated 4A and 6A. Details of the fabr ica t ion condi t ionsemployed in the i r prepara tion are given in Ref . 4 . Thematerial tested in the HIP facility was obta ined f rom pla tesprepared unde r s imi lar condi t ion s.T h e non impregn a ted , s ing ly impregn a t ed , an d fully im -pregnated ATJ-S, whi ch had been obta ined f ro m a singleprecursor bi l le t an d w h i c h had been g raph i t i zed unde r th esame condi ti ons , p rov ided the op por tun i t y t o s tudy the effectof poros i t y on ab l a t i on w i th ma te r i a ls possess i ng min ima l d i f -ferences. Sect ions w ere cu t dur ing each step of the process an dg r ap h i t i zed . C o m m e r c i a l ATJ-S models w ere inc luded in eachrun as cont rols , a long wi th CVD/PAN and the speciallyprocessed ATJ-S and i t s p recur so r s .

Fig. 1 Models an d model holders. Al l dimensions in centimeters, a)Model used fo r C V D / P A N 6A series, ATJ-S, an d ATJ-S graphiteprecursors, b) Model used fo r C V D / P A N 4A series an d ATJ-Sgraphite, c) Model holder fo r model in Fig . Ib which interfaces withholder in Fig . Id . d) Phenol ic-Refrasi l model holder wh kh interfaceswith arc-heater assembly.


2/6

20 A U E R B A C H , L I E B E R M A N , L A W S O N , A N D P I E R S O N J. SPACECRAFTTest Mode l s

Two test -model designs were used. The design in Fig. law as used for mater ia l f rom f rus tum 6A and the plates . To at -tain th e d i mens i o ns in this f igure , th e models were cut in thel ong i tud ina l di rect ion of the f r u s t u m . T o s tudy th e ablat ionpe r fo rmance of surfaces normal to the radia l di rec t ion, sho r -te r sect ions wi th dimens ions in Fig. I b were cut f rom the cen-ter of the i n n e r and outer surfaces of f rus tum 4A where thedensi t i es were m o r e u n i f o r m . These models were in ter facedwith the Ferguson In termi t ter in ject ion system t h r o u g h theATJ-S graph i t e and phenolic-Refras i l holders (Figs. I c andId) . The models were selected from several sections of thef rus tum to obta in a rang e of porosi t ies.Data A c q u i s i t i o n

Model recession w as pho tog raphed at 200 f rames/sec onmicrof i lm and a Boscar Fi lm Reader was used to measurerecession as a fun c t ion of f r a m e n u m b e r and t ime.P o ro s i ty M e asure m e n ts

Pore vo lu mes of the mater ia ls wer e measured b y q u a n -t i ta t ive te lev is ion mic roscopy. The Quant ime t 720 imageana lyz ing compute r ( t r ade name of I M A N C O , L t d . ,M elbourne , England) , which di f ferent iates the pores f rom themat r i x mate r i a l b y differences in opt ica l ref lec t iv i t ies , w asused for the measurement s . A detailed descr ipt ion of the in-s t r u men t is g iv en in Ref. 5. Brief ly, an image f rom a specimenor a p h o t o g r a p h is focused o n t o a te lev is ion camera w h i c hconver ts th e opt ica l i m a g e i n to an electronic image fo r detec-t ion . Fea tu re s o f interest a re d i s t i ngu i sh ab le f rom u n w a n t e dfeatures on the basis of gray- level cr i ter ia . The o u t p u t of thedetector then i s fed in to a compute r module fo r c o m p u t a t i o nof th e desi red p a r a m e t e r .T w o techniques were used wi th th i s i n s t r u men ta t i o n . Fo rth e full-sized g r ap h i te models (Fig. l a ) , the top surfaces wereprojected wi th a Lei tz Orthop lan mic roscope opera ted at124 x . For the small-sized models (Fig. I b ) , 10 x p h o t o g r a p h swere m a d e and examined u s i ng an epidiascope f i t ted w i t h a7x lens as image source , g iv in g a second magn i f i ca t i o n of70 x . Typ i ca l ly , the pores, as sh o wn in Fig. 2, appear as da rkfeatures and in suff icient cont ra s t aga ins t the l igh ter graph i t ematr ix to i so late th e pore bounda r i e s . For an a ssumedspher ical geometry of the pores , the lower l imits of detect ionfo r the two image sources (microscope and epidiascope) are 2and 4 / im, respect ive ly.Direct microscopic observat ion is prefer red because abroader r a n g e of mag n i f i ca t ion i s avai lab le. Wi th theep id i a scope , pho tograph ic magn i f i ca t i o n becomes thel imi t i n g factor and t h i s , in tu rn , i s l imi ted by the mode l andp r i n t size. Wh en th e CVD/PAN 4 A mo dels were s tud ied ,however , the epidiascope was the only ope ra t iona l modeava i lab le on the Q u a n t i m e t at that t i m e .

F i g . 2 Representative graphite surfaces showi ng porosity variat ion.U p p er left: A T J - S precursor , not impregnated; upper right: ATJ-Sgraphite; l o w e r le f t : CVD/PAN 6 A .

F o r t h e C V D / P A N 6 A mode l s , both the epidiascope andmicroscope wer e used to measure the porosi t ies. Data fromboth t ech n i qu es prov ided a me thod fo r compar ing t he relat iveporosi t i es of models wi th pore sizes greater than 2 and 4 / im.In addi t i on to us in g bo th techniques for pretest or v i r g i n -mater ia l measurements in the 6A models, post - test porosi t ieswere measured microscopically a b o u t 5 mm below the abla tedsurface so that a scal ing factor could be ob ta ined to conver tthe porosi t ies of the 4A series of models to comparab leporosi t i es for the 6A series an d th e r eb y permit a composi teplot of all of the models studied. The lack of def in i t ion in the10 x p h o t o g r a p h s m a d e th i s conve r s ion ques t ionab le .

Resul t s an d DiscussionPorosity

Percent porosi ty sh o u ld be re la ted to densi ty t h rough thefol lowing r e la t i o nsh i p :porosi ty = [1 -(density/2.26)] x 100 (1 )

Table 1 Com parative measured an d c ompute d porosit iesM e a s u r e d d e ns i ty Meas ured poros i t y Computed poros i t y

Mater i a lC V D / P A N6A seriesATJ-S precur s or sN o i m p r e g n a t i o n s

O n e impregna t ionATJ-S f rom a b o v e

N u m b e r o fmodels6351

Beforet e s t ing ,g/cm3

1.818

Af te r5a t e s t i ng ,g/cm3

0.0098 1.8111.5191.7521.830

s Beforetes t i ng , 7 o0.0170 16.80.0143 ...0.0047 ...

Afte r5 tes t ing, %2.32 31.8

29.821.120.6

s

5.313.832.43

Beforetest ing, 7 o19.6

Af te rt e s t i ng , %

19.932.822.519.0precur s or s(Two i m p r e g n a t i o n s )C o m m e r c i a l ATJ-S 1.845 0.0117 ... 19.4 3.78 18 . 3

s = S tan da rd dev ia t ion .


3/6

J A N U A R Y 1 9 7 7 POROSITY EFFECTS ON ABLATION IN GRAPHITE MATERIALS 21wh er e 2 . 2 6 is the densi ty of n o n p o r o u s s ingle crystal graph i t e .Table 1 compares th e computed an d measured porosi t ies fo rmost of the materials studied. The correlat ion is good, wi ththe exception of the C V D / P A N 6A graphi tes. In t h i s case, thecomputed poros i t y , 1 9 . 6 to 19.9%, i s comparab le wi th th emeasured pre test porosi ty of 16.8% but not wi th the post-testvalue of 3 1 . 8 % . Ob vi o u s ly , the mate r i a l j u s t below th eabla ted surface (about 5 mm) has become more porous t hanthat in the b u lk . This p h e n o m e n o n w as observed only in the6A m ater ia l . The post- test porosi t ies of the A T J - S and p recur -sor graphi tes below the abla ted surfaces compared f avo r ab lywi th thei r calculated porosi t ies (Table 1). Porosi t ies of pretestor v i r g i n mater ial were, therefore, no t measured .Ablat ion

The ablat ion results of the C V D / P A N 6A , ATJ-S, an dATJ-S precursor models obta ined with the larger test modeldesign ( F i g . la ) provided a broade r spectrum of mater ial typesand porosi t ies and more precise data. Typical recession vst ime curves ar e shown in F i g . 3. These curves always show anin i t ia l nonl i nea r kinet ics phase , du r i ng wh i ch t he rma l andchemical equi l ib r ium condi t i ons are app roached , and a s u b -sequent l inear or steady-sta te phase, in which the recessionrate appears to be cons tan t . Devia t ions f rom l inear i ty af terth e steady-sta te condi t ion w as a t t a i ned were observed whererecession exceeded 0.5 cm. This was due to the model leavingt he cons t an t e n v i r o n m e n t zone in the arc j e t . The steady-sta tecondi t ion general ly provides a conven ien t source foreva lu a t i ng a rate cons t an t i n compara t i ve s tud i e s . Howeve r ,th e high-porosi ty mater ia ls tended to develop grosslyroughened surfaces (v isual to the e y e ) du r i ng th e latter por t ionof th e in i t ia l phase. Thus, a compar i son of ra t e cons t an t s inthe steady-state phase is a comparison of models with d i f -feren t gross surface areas. I t w a s , therefore, desi rable to ob-tain an est imate of the in i t ia l abla t ion ra tes w h e n th e grosssurface areas wer e equ i va len t , as well as the steady-sta te rates ,to see how the in tr ins ic porosi ty affected th e ablat ion rate of asmooth surface. To evaluate ra te constants tha t gove rned thein i t ia l nonsteady-sta te phase before roughness developed, anexponent ia l r e la t i o nsh i p , g iven below, w as assumed .Al though the relat ionship i s empir ica l , i t s co nf o r mi ty wi th ex-per imenta l results jus t i f ies c o m p a r i s o n s of i ts paramete rs w i t hmaterial propert ies and the computat ion of extrapolatedabla t ion ra tes

r ) t (2 )wh er e 5 = recession, cm; kl steady-sta te ra te cons tan t ,cm/sec; k2 nons t eady-s t a t e ra te cons t an t , s e c " 1 ; and/ = t i me , s e c .Equa t ion (2) is approx ima te because kj is expressed as afun c t ion of k2 an d t i me r a the r t han mode l t empe ra tu re , wh i chi s p r ima r i ly responsible for the ab la t i on r a t e an d m e c h a n i s m .Tempera tu re , i n tu r n , i s dependent on the heat f lux and themater ia l proper t ies. Because temperature and heat flux datawere n ot a lways avai lable , k2 w as used as an a l te rnatepa rame te r fo r co r r e la t i o ns i n v o lv in g mater ia l type andporos i t y .The ra te cons t an t s wer e eva lu a ted in the fol lowing m a n n e r .Values fo r k j were obtained f rom s/t values a t extended t imeswh en e"k approached ze ro and s/t became cons tan t . Valu esfo r k2 th en wer e obta ined f rom th e fol lowing r ea r r anged formo f E q . ( 2 )

-( s / t ) } =- (3)Plots of In [k , ( s / t ) } v s / were l inear an d thei r slopesprovided values fo r k2.The adequacy of Eq. (2) for mode l i ng ablat ion u n d e r th econdi t ions of th i s s tudy i s shown in sample plots in F i g . 4. Alog-log format i s used to emphasize the correlat ion over theent i re r a n g e of t ime and recession covered .

O C V D / P A N 6A A T J - S

F i g . 3 Representative recession vs t ime plot for ATJ-S an dCVD/PAN 6 A showi ng initial an d steady-state phases.

.0 1.008.006

A T J - S F-129A T J - S P r e c u r s o r s

No I m p r e g n a t i o n N T - 3S i n g l e I m p r e g n a t i o n A 5 A - 1

F i g . 4 Correlat ion of computed recess ion from Eq. (2) (curve) an dexperimental values (symbo ls) .

Stagna t ion pressu re and b u lk en th a lp y va r i ed f rom test totest over a smal l bu t s igni f icant r ange . I t w a s , therefore ,necessary to normalize kj an d k2 fo r these var ia t ions beforethey were used fo r corre la t ions wi th poros i t y . The n o r -malizat ion re la t ionship is given below

wh er eAs(k)

k=As(k)-Pl.H02/r* (4)

- norm al ized recession ra te cons t an t ;fo r kn the uni t s are (cm 37 2 g 2 ) /(sec Pa J 2)fo r & 2, the units are (cm l/2 g 2 ) / s e c Pa J2)= stagnation pressure, Pa= bulk gas entha lpy , J/g- model radius , cm

In addi t ion to the recession rate constant As(k,), a mass-t ransfer rate cons tan t w as calculated from th e followingre la t ionshipAm(k1)=As(ki)-p (5)


4/6


5/6

JANUARY 1977 POROSITY EFFECTS ON ABLATION IN GRAPHITE MATERIALS 23

2 .0 - O C V P / P A N - 4A Ser iesA T J - S - One standarddev iat ion rangeTi

' 0 2 4 6 8 1 0 1 2P o r o s i t y ( p e r c e n t )

Fig. 7 Relationship between the normalized recession rate As(k3)and porosi ty fo r CVD/PAN 4A material .

As(k}) an d porosi ty within the groups is much smaller thanbetween groups and the exper imenta l errors mask th ecorre la t ions.T he values fo r As(k2) in Table 2 are essentially cons tan t ;var ia t ions are within on e s tandard devia t ion. This observat ionsuggests that those proper t ies tha t are responsible for at-ta in ing thermal and rate equil ibra t ion are s imilar . Similar i tyin these mater ials , except fo r porosi ty, also is suppor ted byFigs. 4 and 5 where all of the normalized ra te cons tan ts con-form with the log re la t ionships.A correlat ion between abla t ion and porosi ty for ATJ-S an dit s precursors is not surprising, since they conta in a c o m m o nfi l ler-binder formulat ion made f rom petroleum and coal-tarpitches. The presence of C V D / P A N in this same g r o u p showsthat th e carbon source (in this case, PA N fibers and pyrolyt iccarbon) is not of major impor tance . Instead, s t ructuralproper t ies such as porosi ty outweigh formulat ion con-siderations.A s noted before, pr io r to the steady-state phase th e highlyporous graphi tes develop very rough surfaces which persistduring the steady-state condition. A larger surface area couldbe responsible , at least in pa r t , for the variation in the valuesf o r / : / .An examinat ion of Eq. (2) and Table 2 shows tha t th is isnot cor rect , since the te rm (1 - e ~ k2 ') , which would be respon-sible for va r ia t ions in the initial rates, does not differ in value ,within experimental e r ro r , for the mater ials tested. Thus, th einit ial as well as the steady-state rates are dependent on k}which , i n tu rn , i s dependent on porosity.The mater ia l in f rus tum 4A was not as u n i f o r m as tha t inf rustum 6A a n d , therefore, provided the opportuni ty toexamine th e effect of a f ivefold var ia t ion in porosi ty onabla t ion per fo rmance within a single mater ia l . Unfo r tuna te ly ,the abla t ion data obtained in this test series were not as preciseor as extensive in time or recession as those for the 6A seriesor the other mater ials . Values fo r /: / or k2 could n o t ,therefore, be evaluated.A ra te constant that could be used fo r corre la t ion wi thporosity for the 4A materials w as obta ined f rom a plot of thelog of the recession vs t ime. This plot was linear for the firstfew seconds of ab la t ion , which extended into th e steady-statephase, al though less precisely than for the cases of k! and k2.The slope, k3, when no rmal ized fo r stagnation pressure an dbulk g as en tha lpy , As(k3), also was exponent ial ly related toporosity for the 4A models (Fig. 7), as well as the othermater ia ls . The s t raight l ine is a least-squares fit of the 4Adata.No epidiascope porosi ty data were avai lable fo r ATJ-S. T ocompare the 4A data with ATJ-S , mercu ry porosimetry datawere converted to total porosities (open an d closed) greaterthan 4 ^m. The mean abla t ion and porosi ty data f rom 14 and11 models, respectively, and their standard devia t ion ranges

Fig. 8 Representative ablated surfaces . Up per left: ATJ-S precursor,not impregnated; upper right: CVD/PAN; lower le ft : ATJ-S .

Fig. 9 Scanning e lectron photomicrographs of a CVD/PANmaterial. Left: valley on the ablated surface where maximum ablationoccurred showing greater porosi ty, lower f iber density , and sm aller in-terfacial spacing between th e fiber and matrix . Right : hill on theablated surface where minimum ablation occurred showing lowerporosi ty, higher fiber density , and larger spacing between f iber an dmatrix.

ar e given in Fig. 7. They show, as in the case of the data inFig. 3, tha t ATJ-S an d CVD/PAN const i tute a single class ofablators wi th respect to th e i r ab la t i on dependence onporosi ty . Both f igures suggest tha t a reduc t ion in theCVD/PAN porosi ty to that in ATJ-S would p rov ide anequivalent ab la to r .The ablat ion pe r fo rmance of CVD g r ap h i t i c mater ia ls wasassessed in two additional studies. One involved a C V D / P A Nmater ia l prepared in pla te ra ther than f rus tum f o r m . I ts den-sity was 1.83 g/cm3, and it was tested at the McDonnel lDouglas HIP facility a t a h ighe r s t agna t ion pressu re .T he other abla t ion study was conducted by Sheldahl an dW r i g h t6 in a channe l test device at a lower s tagna t ion pressurewith CVD/rayon. The rat ios of the abla t ion ra tes for theCVD mate r i a ls an d ATJ-S are shown in Table 3. The rat io forth e C V D / P A N 4A series is based on the lowest As ( k 3 ) value(Fig. 7). The va r i a t i on in the rat ios l imits th e possibi li ty ofdefinit ive conclusions concerning th e abla t ion pe r fo rmance ofCVD/fe l t mater ia ls re la t ive to ATJ-S, since the graphi tes andtest conditions var ied . However , the data imply again that th eformer have th e potent ia l to per fo rm as well as ATJ-S.Morpho logy of Ablated Surfaces

A dis t inguish ing feature between th e more abla t ion-res is tant mater ia ls an d those less resistant was the gross


6/6

24 A U E R B A C H , L I E B E R M A N , L A W S O N , A N D P I E R S O N J. SPACECRAFTTable 3 Com parat ive ablat ion performance of CVD/fe l t an d ATJ-S materials

Test fac i l i tySandia 2-MW impactSandia 2-MW i mpac tM c D o n n e l l DouglasSandia 2-MW c h a n n e ltest device

FiberPAN(4A series)P A N(6 A series)P A NR a y o n

PaxlO' 6

2.3-2.42.2-2.5

10.34.7

J / g x l O 3 Recession ra tesCVD/ f e l t / ATJ - S5.2-5.45.0-5.8

4.6520.1

1.03.21.31.0

Fig . 10 Scanning electron photomicrographs of polished ATJ-S an dprecursor graphite surfaces. Top l ef t : 0 im pre gn at io n s , 33.3%po ro s i ty ; to p right: 1 i mpreg na t i on , 24.8% poros i ty ; bo t to m le f t : 2i mpreg na t i ons , 20.6% poros i ty .

roughness of the abla ted surface. In the previous sect ion, itwas suggested that porosi ty r a the r t han gross surfaceroughness was responsib le for the var iat ion. I t was, th e r e f o r e ,of in terest to ex amine th e surfaces microscopically to gain fur-ther ins ight on how s t ruc tu re relates to abla t ion p e r f o r m a n c e .Both scanning electron microscopy (SEM) and l ightmicroscopy wer e used.Figure 8 shows representa t ive ablated surfaces of threemodels in which oblique l i g h t in g w as used to enhance surfacer o u gh ness . I t i s ev ident t ha t r o u gh ness increases wi thporosi ty. To determine the causes, hills an d val leys wh er em i n i m u m an d m a x i m u m abla t ion occur red wer e examinedwith S E M . For the CVD/PAN models, three factors weref o u nd which cor relate w i t h n o n u n i f o r m ab la t i on : poros i t y ,fiber dens i ty , and the f iber-matr ix in terfac ia l spac ing .Figure 9 (left) is a pho tomicrog raph of a ra re valley inwhich all three factors occur, i.e., high porosi ty , lo w fiberdensi ty, and smal l f iber -matr ix in ter facial spacing. Figure 9(r ight) shows a representat ive hill wh er e mi n i mu m abla t ionoccurred for compar ison. For the ATJ-S and i t s precursor-type graphi tes , poros i ty appears to be the predominant factorin in f luen c in g abla t ion ra te an d n o n u n i f o r m ab la t ion . Figure10 shows SE M examples of ablated surfaces from ATJ-S an dit s precur so r s .

ConclusionsA compara t ive study of the ablat ion performance ofCVD/PAN, ATJ-S, an d ATJ-S precur so r g raph i t es in an arc

je t env i ronmen t provided th e fol lowing conclus ions:1) Ablat ion kinetics can be characterized by an init ialnonl inear phase , dur ing which the rmal and chemicalequilibrium are approached, and a subsequent steady-statephase, dur ing which th e abla t ion ra te is cons tan t .2) T he kinetics can be modeled with a relat ionship in whichth e init ial phase is exponent ial and the subsequent phase isl inear .3) The ra te constants for the linear phases of all of thepreceding mater ia ls are related exponent ia l ly to theirporosities. The ra te constants for the nonl inear phases areconstant . T he latter result shows that the propert ies on whichthermal and chemical equilibrium are dependen t are verysimilar for the preceding materials .4) The densities of these mater ia ls below the ablated sur-faces (-5 m m ) correspond to thei r bulk densities, with the ex-ception of CVD/PAN 6. This mater ia l apparently decreasesin densi ty from an in itial value of 1.811 to 1.541 g/cm3.5) Gross surface roughness or n o n u n i f o r m abla t ion isrelated to porosi ty. Although the abla t ion ra te m i g h t be ex-pected to be r elated to gross surface roughness , th e measuredablation ra te appears to be more highly dependent onporosi ty .6) A microscopic in vest igat ion of surfaces before an d afterablation shows that surface roughness correlates wi th porosi tydis t r ibut ion, fiber densi ty , and the fiber-matrix interfacialspacing.7) CVD/fe l t appears to have the potent ial for improvedablation performan ce i f i ts poros i ty can be reduced.

AcknowledgmentsThis work w as suppor ted by the U.S. Ene rgy Research andDevelopment Adminis t r a t ion . The au tho rs gratefullyacknowledge th e cont r ibu t ions of G.T. Noles and J.F.Smatana for the prepara t ion of the C V D / P A N composites,K.L. Coin an d G.F. W r i g h t , Jr. for the ablation tes t ing , S.F.Duliere and J.F. Maurin for the scanning e lec t ronmicroscopy, J.B. Duran for the metallography, and A.E.M clntyre fo r compu te r p rog ramming .

References' Au e r b a c h , I., Lieberman , M. L . , Lawson , K .E . , and Pierson ,H . W . , "Effects of Porosity and Structure of Graphite Materials onAblation in Arc Genera ted Plasmas," Sandia Laborator ies ,Albuquerque , N. Mex. , SAN D 76-0176, Ju ly 1976.2G o i n , K . L . , "Performance an d Operation of the High PressureArc Heater of the Sandia 2-Megawatt Plasma-Jet Faci li ty," SandiaLaborator ies , Albuquerque, N . Mex. , SC-DR-72-0521, Dec. 1972.3Wil l iams , R . R . , "High Impact Pressure (HIP) Facility TestStream Calibration Results ," McDonne l l Douglas ResearchLaboratories, St. Louis, Mo. , MDC-Q-0467, M ay 1972.4Lieberman, M.L . , Curlee, R.M., Braaten, F.H., and Noles, G.T.,"CVD/PAN Felt Carbon/Carbon Composi tes , " Journal of Com-

positeMaterials, Vol. 9, Oct. 1975, pp . 337-346.5F i s h e r , C.F., "The N ew Quant imet 720," Microscope, Vol. 19.1971, pp.1-20.6Sheldahl, R.E. and Wright , G.F., Jr., "MTV Antenna Jo in tAblation an d Heat-Transfer Rate Study in a Conf ined Channel,"Sandia Laboratories , Albuquerque, N. Mex. , SC-DR-70-298, June1970.

Documents

Effects of Porosity in Graphite Materials on Ablation in Arc-heated Jets