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Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
Cwbonvol. 17,pp. 157-174 0 Pergamon Press Ltd, 1979. Printed in Great Britain
lmL6223/79/040l-0l57/$02.al/0
OXIDATION EFFECTS ON THE TENSILE STRENGTH OF ATJS GRAPHITE AND VITREOUS CARBON-t T. C. PENG McDonnell Douglas Research Laboratories, McDonnell Douglas Corporation, St. Louis, MO 63166,U.S.A. (Received
3 July 1978)
Abstract-Tensile strength reductions from the oxidation of ATJS graphite and vitreous carbon were measured in a high-temperature material facility. The oxidation tests were conducted on resistively heated samples (- 2 mm dia. or - 2 x 2 mm2 cross section) at temperatures up to 2500K. Oxygen flowed around the samples at mass flow rates from 0.64 to 80 g/hr for oxidation times of 2 - 60 min. The pre- and post-oxidation sample surfaces were examined for microstructure deterioration by optical and scanning-electron microscopy. The results indicate that (I), oxidation thresholdtemperaturesfor significant strength reduction are 850 K for ATJS graphite and 1000K for vitrous carbon; and (2), both mass loss and microstructure deterioration are major causes of stength reduction for ATJS graphite, and mass loss appears to be the primary cause of strength reduction in vitrous carbon.
micropores (0.1-10 pm dia.). Optical and electron microscopy[l l-131 allow direct observations on the binders, fillers and pores, and also the changes of these microstructures caused by oxidation. Microstructure deterioration is difficult to describe quantitatively because its mechanism involves complicated surface reactions between the carbon/graphite surface and oxygen atoms/molecules. It is expected, therefore, that the type and the amount of microstructure deterioration will vary according to the form of carbon and the type of oxidizer used. In previous studies [5-131, emphasis has been placed on (l), air, CO, and Hz0 as the oxidizer; (2), nuclear-reactor graphites; and (3), roomtemperature stress measurements before and after oxidation. This paper describes the changes in tensile strength resulting from oxidation of carbon/graphite at elevated temperatures. Samples of polycrystalline ATJS graphite and amorphous vitreous carbon were resistively heated to a high temperature, exposed to a controlled oxygen flow for a specific period, and subjected to a gradually increasing tensile stress until sample failure. Sample surface before and after oxidation were examined by optical and scanning electron microscopy.
1. INTRODUCTION
Low density and high-temperature strength make carbon/graphite useful in reentry vehicles[l, 21, rocket nozzles[3], and aircraft brakes[4]. However, carbon/graphite is vulnerable to oxidation, especially at elevated temperatures. Generally, oxidation of carbon/graphite at temperatures higher than 800 K results in mass loss and microstructure deterioration (e.g. crack development, enlargement of pores, and debounding between material components). Both mass loss and microstructure deterioration reduce the mechanical strength of carbon/graphite material. Thus, based on the data before and after oxidation, the percentage of strength reduction can exceed that of mass loss. For graphites used in nuclear reactors, the strength reduction is initially much greater than the mass loss[5,6]. A correlation between the mechanical strength, S, and the mass loss (expressed in terms of the specific gravity, p, of oxidized graphite) is given by
s -=
so
0;o’ -
”
where the subscript 0 refers to the unoxidized state and n is - 8.2 for tensile strength[71. According to this correlation, 1,5 and 10% mass loss corresponds to 8, 34 and 50% strength reduction, respectively. While mass loss can be determined quantitatively by weighing, the microstructure deterioration often is described qualitatively. Both gas permeability and electrical resistivity[‘l, 81 increase exponentially with oxidation, indicating the growth of interconnected pores and the loss of structural contacts within the material. Statistical analyses of the pore size distribution, surface area, and effective gas diffusivity[8-101 provide further insight on initially closed micropores and the generation of new
tThis research was conducted under the McDonnell Douglas Independent Research and Development Program.
2. EXPERIMENTAL
The oxidation experiments are shown schematically in Fig. 1. Three different systems were used. The indirect sample heating (Fig. la) has the advantage of studying any material regardless of its electrical properties. In addition, the sample temperature of the blackbody-like thermal cell is independent of the material’s emissivity. However, the oxidation limit of the vitreous carbon oven restricts this system to sample temperatures below 1200K[14]. Direct sample heating (Fig. lb) removes the 1200K limitation, but requires an electrically conductive sample (e.g. carbon/graphite) with known emissivity. This system was used for sample temperatures up to 1800K. At temperatures above 1800K, the heated sample holders affect the performance of the linear variable differential transformer (LVDT) displacement sensor and
157
T. C. PENG
158
Vitreous carbon tube oven
tube
7
7
(A) Oven heating
(B)
(C)
Sample heating
Sample heating
Fig. 1. Experimental arrangements for carbon/graphite oxidation studies.
the load spring. Thus, a water-cooled heat shield and sample holders were assembled in the third system (Fig. lc). In this system, the sample temperature routinely reaches 3000 K and is limited only by sublimation of the carbon/graphite samples. Evaluation of the sample surface temperature in the direct heating systems (Figs. lb and c) requires an emissivity value. For carbon/graphite, the emissivity varies from 0.77 for a polished surface to 0.9 for a rough surface[l5,16]; an average value of 0.83 was used. Within a range of 1000-2500 K, the surface temperatures determined using a 0.77 emissivity are 40-100 K higher than those determined using a 0.9 emissivity. Thus, the uncertainty in temperature resulting from the use of an average emissivity of 0.83 is 2 2%. All three heating systems were used inside a vacuum chamber maintained at 5 Pa. The sample and the thermal cell were flushed with argon and the vacuum chamber was reevacuated to -5 Pa. The resistive heating was initiated by passing an electrical current through the tube oven (Fig. la) or the sample (Figs. lb and c) fastened between two water-cooled electrodes. The sample temperature was monitored by an infrared pyrometer (Ircon 300 CH) or an optical pyrometer (Pyromicro-optical, model 95) through two quartz windows. The first window was attached to the vacuum chamber wall, and the second window was placed near the oxygen inlet. The cold gas flow over the second quartz window prevents
material vapor and particulate deposition on the window surface. The dumbbell-shaped samples were heated to maintain a relatively uniform temperature over the middle test section (- 2 mm dia. or - 2 x 2 mm2 cross section and a length to diameter ratio of -8). The temperature calibration over the test section indicated a 2-4% reduction from the center to the ends. When the sample reached the desired temperature, oxygen flowed around the sample at a predetermined rate for a specified period. A steady-state condition of temperature and oxygen flow rate was maintained for the oxidation process in the thermal cell. After the oxidation phase was complete, the sample (still at the elevated temperature) was placed under tension with a known load increment through the calibrated load spring. At each load increment, there was an instant elongation response followed by l-5% additional increase. Most of this additional increase in elongation was caused by readjustment within the sample holders. Generally, a period of - 2 min was allowed for sample elongation to reach a steady value as indicated by the displacement reading from the LVDT sensor. The tensile load was increased by increments until the sample broke. Scanning electron microscopy (SEM) was used to study the surface and the substructure just below the surface. SEM micrographs illustrated the surface roughness conditions, microstructure heterogeneity,
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
159
bonds[23,24,28]. Under non-oxidizing conditions, vitreous carbon retains its microstructure up to 2800K and t~nsforms into ~aphitic structure when heated above 28OOK.Vitreous carbon has small pores (2-10x 10e3pm[24,26], 3 1.2x low3pm[28]) which are mostly 3. MATIERIAL isolated and inaccessibleto gas penetration as evidenced The samples investigated were ATJS graphite and by the low values of porosity and permeability(Table If. Both ATJS graphite and vitreous carbon sampleswere vitreous carbon. Some of the material properties are listed in Table 1. ATJS graphite (Union Carbide) is fabricated from the material blocks as received and the a polycrystailine substance made from calcined sample surfaces were kept free of dust and grease, but petroleum coke (filler)and coal tar pitch (binder)[l9]. It were not treated by special techniques. has an average grain size of - 70 iu,m[27]and a pore size 4. REsurB range (as measured by mercury porosimetry) of 0.07The stanch-temperature relation of ATJS graphite in 100,umdia.: % of the pore volume per unit mass was argon (Fig. 2) demons~ates the consistency of all three l-5 pm pores. Vitrous carbon (Atomergic Chemetals, Inc.) has a experiment ~angements (Fig. 1) operated within basic structure consisting of randomty distributed small proper temperature limits.These results compare favor packets of graphite crystals that are cross linked by C-C ably with those reported by Southern Research Institute
grain shapes and sizes, pores and inter-grainboundaries. The electron microscope(JEtX ModelJSM-2)employed has a resolution of 0.025pm and accepts a maximum samplesize of 2.5cm dia. and 1cm thick.
Table 1. Materials characteristics of Material
ATJSgraphite and vitreous carbon at 300K
Density ~cm3)
1.83’[37,181
Impurity (ppm) Porosity (St)
- 193[18] 19: (6.8-8.5 (0))[17] (6-7 (O), 6-7 K))’ (16.2-17.6)[19]16.gd 0.00045(AG) 0.0005(WG) Darcys’
Permeability (cm’ls) Electrical resistivity (rnil cm) Thermal conductivity (W&cm0 Thermal expansion (10-6/K) Sonic velocity (IO*cm/s) Elastic modulus~ (10’ MPa)
Ultimate tensile strength (MPa)
Vitreouscarbon (Atomergie chemetals)
ATJS graphite (Union carbide)
0.87 (WG), 1.16(AG)[17] 0.83 (WG), 1.09(AG)[lI] (0.95-1.0~[19],0.86[151 1.46(WC), 1.11(AG)[lfl 1.79(WG), 1.47(AG)[lS] 1.40(WG), 1.0 (AG)[l9] 2.16 (WG), 3.10 (AG)[l7] 2.09 (WG), 3.17 (AG)[I8] 3.97 (WG), 5.11 (AG)[19] 2.71 (WG), 2.28 (AG)[17] 2.83 (WG), 2.34 (AG)[l9] 1.15(WG), 0.8 (AG)[17l 1.17(WG), 0.86 (AG)[l8] 1.2 (WC), 0.68 (AG)[19] 1.0 (WG), 0.69 (AG)[21] 36.5 (WG), 30 (AG)[17] 35 (WC), 28 (AG)[l8] 43.4 (WG), 34.7 (AG)fl9] 33.4 (WC), 27.6 (AG)[Zlj 34.8 (WG), 29.6 (AG)[22]
1.47[23],(1.3-1.55)[24], (1.5-1.55)[25] f30-3000)[24],2001231,< 50[25] (< O.OS)[23], (0.2~5)[24], 0 (apparent ~rosity)[~] < 2.5 X tO_” (He)[23], (10-9-10-‘2)[24], ( < 10-9[25] 3-8[23], 3.5-20[24],4.5[25]
(0.~2~.0~)[23], (O&42-0.25)[24],0.251251 (2.2-3.2)[23],(2-3.4)[24] 3.2 at lwC[25]
WWI (1.37-3.24)[24] 2.16[25] - 39[24],(41-90)[26]
tThe initial slope of tensile stress-strain curve near zero level. *Selected samples were weighed in air and their volumes were measured by immersing the samples in water. bCaIculated value given by Porosity (%) = (1 - ~)
X 100
where 2.26g/cm3 is the density of a graphite crystal. ‘Private communication on porosity with C. Pratt, AFML. 0 = open porosity, C = closed porosity. ‘Mercury porosimeter meas~ement for open and cracked-closed pores at 4.14~ l@ kPa (- 60,ooapsi). “Ref. [20]. “Darcy” is a unit of gas permeability through porous material such as graphite, and is defined according to the form of Darcy’s law. The values quoted in this table are supplied by Union Carbide (Ref. [18]). AG = Across Grain, WG = With Grain.
T. C. PENC
160 ATJS graphite
strength in argon
0 Exposed to 02 flow (0.55 g/h) for 1 h
0
Oven heating,
Fig. la
0
Sample heating,
Fig. 1 b
A
Sample heating,
Fig. lc >
0
Southern
0 Exposed to 02 flow (0.64 g/h) for 20 min MDRL
Research data (Ref.
data
17)
MDR L average -
-
80
-
Manufacture’s
I
I
data (Ref.
18) With oxidation
I
I
With oxidation
1.0
0.6
0
0
I
I
I
I
0.5
1.0
1.5
2.0
Sample temperature
1.8 x 103
1.4
Sample temperature
(KO
Fig. 3. Strength reduction by oxidation (ATJS graphite with grain). -
2.5 x 103
(K)
8oL
Fig. 2. Strength-temperature relations (ATJS graphite with grain). (SoRI)[17]
and the manufacturer[l8]
(below
1400 K).
Above 1400K, the data are consistent with those of SoRI but are 15% higher than the m~ufacturer’s value at 25OOK. Although the temperature in the third system (Fig. Ic) can go as high as 3400K (5660°F), the data presented herein are limited to ~2500 K (4Q40°F) since graphite sublimination at low pressures (- 2 kPa or 15 Torr) becomes appreciable at temperatures greater than 2500 K. ATJS graphite and vitreous carbon samples were tested for ultimate tensile strength under a variety of oxidizing conditions. Strength reduction is generally observed whenever sig~ficant oxidation occurs. For ATJS graphite, the effects of sample temperature, oxygen mass flow, and oxidation time are shown in Figs. 3-5. Similar effects for vitreous carbon are shown in Figs. 6-8. The ultimate tensile stress based on the sample cross-sectional area before oxidation was evaluated. The crosssectional area remained unchanged at a low level of oxidation and became badly damaged and too irregular to be defined after severe oxidation. Thus, the strength reduction reported reflects the reduction of the applied force during oxidation. The data scatter observed in Figs. 2-8 is caused primarily by the non-un~ormity of material properties either from different billets or difterent sections of the same billet. Vitreous carbon is also sensitive to small flaws or cracks. Hence, the scatter of vitreous carbon
0
0
10
20
30
40
50
60
70
80
Oxygen mass flow (g/h) Fii. 4. Effects
of oxygen flow rate on ultimate (ATJS graphite with grain).
tensile
stench
data (Figs. 7 and 8) is greater than that for ATJS graphite (Figs. 4 and 5). The physical characteristics of the surface and the cross sections before and after oxidation were examined using scanning electron and ,optical microscopy (Figs. 9-24). In these illustrations, surface A refers to the side of a square sample facing the oxygen flow. Surface B is the side of the square sample parallel to the oxygen flow. Surface C is the fracture surface when the sample failed under tension. For ATJS graphite, both surfaces A and B were machined, whereas only the B surface of vitreous carbon was machined with diamond tools. The A surface
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon 1oa
l-r
2500
I
I
161
I
I
K, 29 g/h 02 flow 80
l-
in Ar at 1575
Strength
K
1510-1610 K (1575 K average) 20 mln oxidation time
0 0
20 Oxidation
40
60
time (min) 20
Fig. 5. Effects of oxidation time on ultimate tensile strength (ATJS graphite with grain). 80
I
I
I
0
t
2
3
4
5
6
7
Oxygen mass flow (g/h)
Fig. 7. Effects of oxygen flow rate on ultimate tensile strength (vitreous carbon).
.J, 5 w 0 z E 20 ._ C
-
0
Without
oxidation
for 20 min 01 0.2
0.6
1.0
Sample temperature
1.4
1.8 x lo3
(K)
Fig. 6. Strength reduction by oxidation (vitreous carbon).
of vitreous carbon was the original “glassy” surface as received from the supplier. Surfaces A and B were exposed to oxidation; fracture surface C, however, was not exposed to oxidation because it was produced during the stress phase following completion of the oxidation phase. 5. DISCUSSION The strength of carbon/graphite is reduced significantly above a threshold temperature. For ATJS graphite at 0.55-0.64 g/hr oxygenflow rate, the threshold temperature is -850K after either 20 or 60min of oxygen exposure (Fig. 3). For vitrous carbon at - 6.4glhr oxygen flow rate (10 times that for ATJS graphite) and after 20min of oxygen exposure, the threshold temperature is - 1008K (Fig. 6). For the oxidizing conditions used in this study (Figs. 3-6), the temperature ranges for the strength reduction are 850CAR Vol 17, No. 2-E
20
0
0
4 Oxidation
8
12
time (mln)
Fig. 8. Effects of oxidation time on ultimate tensile strength (vitreous carbon).
162
T. C. PENG
Fig. 9. Unoxidized ATJS graphite surface (A) at room temperature. Fig. 10. Fractured ATJS graphite surface(C) at room temperature. 1300 K for ATJS graphite and 1000-1600 K for vitreous
carbon, respectively. The thermogravimetric analysis (TGA) data of Shapiro1291show that oxidation mass loss in air starts at - 930 K and reaches - 90% of the initial weight at 1100K for ATJS graphite. Furthermore, he found that the mass loss in air for “glassy” carbon (Lockheed), a material similar to the vitreous carbon used here, begins at - 1050K and reaches - 90% of the initial weight at 1350K. The mass loss data of Shapiro and the strength reduction data reported here agree on the temperature required for oxidation effects, thereby suggesting that mass loss is an important contribution to strength reduction.
Using the cross-sectional area change before and after oxidation (Figs. 22-24) as an indicator of the mass loss at the failure plane, the data show that the estimated ATJS graphite area decreases of -1, -25 and -68% resulted in 24-36, 52 and 90% strength reductions. These results support the conclusions of other investigators [5-71 that 1,5 and 10% mass losses cause 8,34 and 50% reductions in tensile strength. This finding holds true in spite of different sample sizes used: a sample of - 2 mm dia. or - 2 x 2 mm2 cross section was used in this study, and a sample of 9-25 mm dia. was used in the other investigations[6,7]. Thus, for ATJS graphite, the dis-
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
1600X
1 cii?UX
Fig.If. Oxidized ATJS graphite surface (A) at 1541K, OZflow at 0.64glhr for 20 min.
Fig. 12. Oxidized ATJS graphite surface (A) at 1530K, O2flow at 6.4 g&r for 20 min.
proportions high percentage of strength reduction over that of mass loss indicates the major importance for microstructure deterioration caused by oxidation. For vitreous carbon, the fracture cross-section is defined with difficulty because the material often shatters at failure. The cross-sectional data (Fig. 24) showed that an approximately 50% area decrease was accompanied by a 60% reduction in strength. Hence, the microstructure change should be a factor in predicting the oxidationcaused strength reduction. However, the available evidence is not sufficient to determine the proper role of the microst~~true effects.
While temperature is the ~ontroll~g parameter for the initiation of significant mass loss and strength reduction, oxygen flow rate, oxidation time, and temperature are factors controlling the rate and the amount of strength reduction. Thus, for ATJS graphite at 1150K (Fig. 3) and an oxygen flow rate of 0.55 g/hr, a 60 min oxidation time reduced tensile strength by 63%, whereas 0.64g/hr and 20min oxidation time yield only 1% loss. In Fig. 4, the 2 x 2mm ATJS sample is nearly burned out for - 7g/hr oxygen flow at - 1.550K and 20 min exposure, yet a similar sample suffered only 60% strength reduction for - 80 g/hr oxygen tlow at 2500 K
T. C. PENG
Fig. 13. Fractured ATE graphite surface (C) at 1530K.
for 2 min exposure. Thus, it appears from the data shown in Figs. 3 and 4 that oxidation time is a primary variable in assessing oxidation damage. However, Fig. 5 clearly shows the marked influence of the oxygen flow rate and sample temperature on the loss of tensile strength. Hence, oxygen flow rate, oxidation time and sample temperature interact to produce the overall oxidation damage. The effects of flotield are shown in Figs. 3 and 4. For a low oxygen Bow rate (0.64 g/hr) and a 20 min exposure, the tensile strength reduction is - 27% at 14OOK and then becomes constant up to 1sOOK (Fig. 3). Since the oxidation rate usuahy increases with temperature, the constant strength r~uction between 1400 and BOOK may indicate partial blocking of the material surface by
less reactive oxidation products. A possible alternative to the flow field expl~ation would be that the increasing graphite strength with temperature and the decreasing load support area from oxidation balance each other. However, this balance, if it occurred, would be a coincidence, and is unlikely to hoid for a temperature range of 1400-18OOK. Furthermore, the 60% strength reduction at 2.500K and 2 ruin exposure also becomes constant for oxygen flow rates greater than SOg/hr (Fig, 4). A possible ~o~eld explanation for this behavior is that the boundary layer prevents the oxidation products from leaving the material surface readily and thereby limits further increases in oxidation damage. This ~o~eld explanation is speculative since the flow around a sample with square cross section is not well
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
Originat
“gtassy” surface (Al
Fig. 14. Unoxidized vitreous carbon surfaces, 200x .
16.5
166
T. C. PENG
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
167
Fig. 16. t3xidized vitreous carbon surface (A) at 1020K, 02 flow at 6.4g/hr for 20
defined. However, the apparent flo~eld effects shown in Figs. 3 and 4 suggest that experiments with well defined flotileds (e.g. a cylindrical cross section ) should be conducted to investigate co~elations between strength reductions from sample tempe~ture, oxygen flow rate, and oxidation time. In the case of vitreous carbon, the effects of sampfe temperature, oxygen ffow rate, and oxidation time on strength reduction are shown in Figs. 6-8. Generally, vitreous carbon is more oxidation resistant that ATJS graphite. For example, the oxidation threshold temperature for vitreous carbon is - WOOK compared with -850K for ATJS graphite, and the strength reductions for vitreous carbon are 1%20% less than those of ATJS graphite (Fig. 25). These data corroborate Shapiro’s weight loss data1291 and Lewis’ oxi-
dation rate constants [30]. However, the general behavior of oxidation for both ATJS graphite and vitreous carbon is similar as shown by (I), parallel slopes in strength~oxygen-flow diagrams (Fig. 25); and (2), comparison of strength~oxidation-time curves (2500 K curve in Figs. 5 and 8). On a relative basis, the oxidation behavior of these two materials with large differences in microstructure is similar (Fig. 26). This result suggests that the effect of oxygen exposure on strength in relative terms does not depend markedly on material structure. Micrographs of unoxidized ATJS graphite show similar heterogenous compositions on all three surfaces (A, B and C as defined previously, Figs. 9 and 10). The straight lines at 150Xresulted from machining. At 1600X, the outline of filler particles and pores can be recognized. However, the presence of paste-like binder surrounding
168
T. C. PENG
Fig. 17. Oxidized vitreous carbon surface (B) at 1020K, O2 flow at 6.4 glhr for 20 min.
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
169
1600X Fig. 18. Oxidized vitreous carbon surface (A) at 1230K, Oz flow at 6.4 glhr for 20 min.
the filler particles makes the sizing of graphite grains diEcult. The effects of oxidation are illustrated in Figs. 1l-13. At a low oxygen flow rate @.64gfhr), the paste-like binder appearance persists to a temperature of 1550K, whereas at a high oxygen flow rate (6.4g/hr) and a similar temperature (1530K), the binder disappears and only the filler particles (graphite grains) with sharp edges remain. Based on similar observations, Maahs and Schryer[31] concluded that the oxidation rate of the binder is higher than that of filler. At 10 times the oxygen flow, the oxidation rate per unit surface is increased sufficiently to consume a11binder near the surface. Surface C is not affected greatly by oxidation since tensile fracture occurred only after the oxidation phase was finished (Fig. 13). However, when oxidation CAR Vol. Il. No 2-F
Fig. 19. Oxidizedvitreous carbon surface (B) at 1230K, O2flow at 6.4g/br for 20min. penetrated the material interior, then clearly definable filler particles were observed, indicating the preferential removal of binders by their higher rate of oxidation. Scanning electron microscopy observations of unoxidized and oxidized vitreous carbon samples are shown in Figs. 14-21. Before oxidation, surface A, the original glassy surface of vitreous carbon, is smooth and homogeneous at 1600X. Surface B was machined with a diamond tool and exhibits a lame&r structure. Surface C, the fracture surface, shows a smooth surface with small pit marks, presumably micropores. Both surfaces B and C reveal isolated pores of 2-20 pm dia. and granules (l-3pm, occasionally 30pm) around the pores. This
T. C. PENG
Fig. 20. Oxidized vitreous carbon surface (A) at 1440K, O2flow at 6.4 glhr for 20 min.
granular structure may arise from the preferential the pores during heat around graphitization treatment [32]. Oxidation at 6.4glhr oxygen flow rate and 20 min oxidation time produced different microstructure deteriorations on the surface depending on the temperature and original surface conditions (Figs. 16-21). Thus, oxidation at 1020K produced numerous pits in the originally surface A and visible cracks or channels on the machined surface B (Figs. 16 and 17). Oxidation at 1230K resulted in large, round depressions on surfaces A and B. In addition, - 2 pm dia. particles also appeared on surface B (Figs. 18 and 19). As the oxidation temperature was increased to 1440K; large, round holes appeared on surface A and a uniform laminar grain structure emerged. Large craters are present on surface B,
1600X
Fig. 21. Oxidized vitreous carbon surface (B) at 1440K, O2 flow at 6.4 g/hr for 20 min.
and oxidation-induced pores (20-40 pm across) penetrated the sublayer beneath the surface (Figs. 20 and 21). Previously observed - 2 pm dia. particles disappeared, thereby indicating that these particles are transient products of oxidation. Optical microscopy, because of its small depth of field, provides valuable insight on oxidation penetration from the surface into the substructure via cross-sectional view. For ATJS graphite, oxidation at low oxygen flow rates and certain combinations of temperature and exposure time (Fig. 22a and 23b) creates significant microstructure
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
a)
%mPk surfam oxidized
180X
993 stteadv OXYWMflow of O&5 g/h. Fig.
at
K fur 1 h by
b)
sPrmr;c-lswcf oxidkmdat1139Kforl steady axyg%n flow of 0.55 glh.
22. Cross-sectional views of ATJS rod samples cut diametrically from a cylindrical billet
h&y
172
T. C. PENG
bl 1541 I(, 02 flow 0.64 g/h, 20 min
e!
1530 K 02 flow 8.4 g/h, 20 min
Fig. 23. Cross-sectional views of both unoxidized and oxidized ATJS graphite samples, 50x.
Oxidation effects on the tensile strength of ATJS graphite and vitreous
carbon
173
Vlrreous carbon 1510-1610 K (1 575 K average) 20 m,n oxidation
1500-l 20 mln
Room
temperature 0
608
oxldatlon
t,mc
K
time
1
I
I
I
I
I
1
2
3
4
5 (g/h)
6
Oxygen
mass flow
rate
Fig. 25. Relative effects of oxygen flow
rate
7
for different
materials.
from scanning electron microscopy (Figs. 16-21) indicate surface-layer damage only. This result also supports the previous observation that the strength reduction for vitreous carbon is lower than that of ATJS graphite at the same oxidizing conditions (Fig. 25). Hence, in the case of vitreous carbon, the primary cause for strength reduction appears to be mass loss. The microstructure damage within the surface layer also contributes to strength reduction by providing material flaws where local stress concentrations produce material failure. 6. CONCLUSIONS 1440
K, 02
flow
at
6.4
g/h,
20 mm
Fig. 24. Cross-sectional view of both unoxidized and oxidized vitreous carbon samples, 35x. damage without significant mass removal. This damage is indicated by numerous enlarged pores and visible chan-
nels leading from the oxidized surface to the material interior; yet, the cross-sectional area remains unchanged. Oxidation at higher temperatures (Fig. 22b) or higher flow rates (Fig. 23~) produces large mass loss with similar microstructure damage. Thus, the optical micrograph data (Figs. 22 and 23) indicate that the initial strength reduction of ATJS graph by oxidation is primarily caused by microstructure damage, and subsequent strength reductions resulg from the large mass loss. Optical micrographs of oxidized vitreous carbon (Fig. 24) do not show deep oxidation penetration of microstructure. The observed pit marks, holes and depressions
A threshold temperature for oxidation exists above which the strength reduction becomes significant. For ATJS graphite and vitreous carbon, the threshold temperatures are 850 and lOOOK, respectively. These threshold temperatures are close to those determined from weight loss data by thermogravimetric analysis. However, the progress and the amount of oxidation are controlled by the combined effects of material temperature, oxygen flow rate, oxidation time flowfield configuration. For ATJS graphite, both mass loss and microstructure deterioration from oxidation are major factors producing strength reduction. microstructure Furthermore, deterioration is the primary cause for initial strength reduction. For vitreous carbon, the microstructure deterioration is confined to a thin surface later, and strength reduction is caused primarily by mass loss and to a lesser extent by local material flaws resulting from oxidation.
T. C. PENG
174
I
I
2.
I
3. 4. 5.
80
6.
W. E. Nicolet, J. T. Howe and S. A. Mezines, AIAA Paper No. 74-701 (1974). M. R. Wool, H. Tong and R. J. Bedard, CIPA Publ. 266 3, 385 (1975). F. P. Kirkhart, Proc. 12fh Carbon Conf. 279 (1975). W. Karcher, R. Krefeld and P. Glaude, Proc. 9th Carbon Conf. 67 (1%9). J. A. Board and R. L. Squaires, Proc. 2nd London Conf.
Carbon Graphite 289 (1%5). 7. C. Rounthwaite, G. A. Lyons and R. A. Snowdon, Proc. 2nd London Conf. Carbon Graphite 299 (1%5). 8. R. H. Knibbs and J. B. Morris. Proc. 3rd London Conf. Carbon Graphite 297 (1970). 9. W. M. Kalback, L. F. Brown and R. E. West, Carbon 8, 117
from curve in Fig. 5 2500 K, 29 g/h 02 flor
(1970). 10. E. T. Turkdogan, R. G. Olson and J. V. Vinters, Carbon 8, 545 (1970). 11. R. D. Reiswig, L. S. Levinson and T. D. Baker, Carbon 5, 603 (1%7). 12. S. S. Jones Carbon 8,673 (1970). 13. R. Stevens, Carbon 9,573 (1971). 14. T. C. Peng, Proc. 22nd Conf. Mass Spectrometry Allied Topics 384 (1974). 15. C. L. Mantel, In Carbon and Graphite Handbook, Chap. 19, p. 344. Interscience-Wiley, New York (1%8). 16. R. E. Tavlor and W. D. Kimbroueh. Carbon 8.665 (1970). 17. H. S. Starrett and C. D. Pears, A&ML-TR-73-14, Voi. 1 (Feb.
20
Vitreous carbon, from curve in Fig. 6 iO0 K, 29 g/h 02 flow
0 0
40
20
% burnout
60
80
100
time
Fig. 26. Relative effects of oxidation time for different materials. Although materials with variable microstructures respond to oxidation differently, the relative correlation between strength reduction and oxidation time depends only weakly on microstructure differences. A similar relation also appears applicable to the oxygen flow rate.
1. V. DiCristina, Final Summary Report, USAF SAMSO Rep. TR-76-12, (Dec. 1975).
1973). 18. Union Carbide Technical Information Bulletin No. 463201 H 1, Carbon Product Division, 270 Park Ave., New York, NY 10017. 19. S. G. Bapat, Carbon 11,511 (1973). 20. R. E. Nightingale (Ed.), In Nuclear Graphite, Chap. 6, pp. 176-186. Academic Press, New York (1%2). 21. J. E. Zimmer and R. A. Meyer, Proc. 12th Carbon Conf. 95 (1975). 22. E. Y. Robinson, Proc. 12th Carbon Conf. 101(1975). 23. F. C. Cowlard and J. C. Lewis, I. Mat. Sci. 2,507 (1%7). 24. T. Noda, M. Inagaki and S. Yamada, J. Noncrystalline Solids 1, 285 (1%7). 25. Atomergic Chemetals Company Product Bulletin. 584 Mineola Ave., Carle Place, L.I., NY 11514. 26. S. Yamada, Defense Ceramic Information Center (DUG’) Rep. 68-2. Battelle Memorial Institute, Columbus, OH (April 1968). 27. J. S. Evangelides, Proc. 12th Carbon Conf. 91 (1975). 28. G. D. Wienal and C. J. Pines. Carbon 12.51 (1974). 29. I. Shapiro, Proc. 10th Car& Conf. 83 (i971). 30. J. C. Lewis, Proc. 2nd London Conf. Carbon Graphite 258 (1%5). 31. H. G. Maahs and D. K. Schryer, AIAA I. 7,2178 (1%9). 32. K. Kamiya and K. Suzuki, Carbon 13,317 (1975).
lmL6223/79/040l-0l57/$02.al/0
OXIDATION EFFECTS ON THE TENSILE STRENGTH OF ATJS GRAPHITE AND VITREOUS CARBON-t T. C. PENG McDonnell Douglas Research Laboratories, McDonnell Douglas Corporation, St. Louis, MO 63166,U.S.A. (Received
3 July 1978)
Abstract-Tensile strength reductions from the oxidation of ATJS graphite and vitreous carbon were measured in a high-temperature material facility. The oxidation tests were conducted on resistively heated samples (- 2 mm dia. or - 2 x 2 mm2 cross section) at temperatures up to 2500K. Oxygen flowed around the samples at mass flow rates from 0.64 to 80 g/hr for oxidation times of 2 - 60 min. The pre- and post-oxidation sample surfaces were examined for microstructure deterioration by optical and scanning-electron microscopy. The results indicate that (I), oxidation thresholdtemperaturesfor significant strength reduction are 850 K for ATJS graphite and 1000K for vitrous carbon; and (2), both mass loss and microstructure deterioration are major causes of stength reduction for ATJS graphite, and mass loss appears to be the primary cause of strength reduction in vitrous carbon.
micropores (0.1-10 pm dia.). Optical and electron microscopy[l l-131 allow direct observations on the binders, fillers and pores, and also the changes of these microstructures caused by oxidation. Microstructure deterioration is difficult to describe quantitatively because its mechanism involves complicated surface reactions between the carbon/graphite surface and oxygen atoms/molecules. It is expected, therefore, that the type and the amount of microstructure deterioration will vary according to the form of carbon and the type of oxidizer used. In previous studies [5-131, emphasis has been placed on (l), air, CO, and Hz0 as the oxidizer; (2), nuclear-reactor graphites; and (3), roomtemperature stress measurements before and after oxidation. This paper describes the changes in tensile strength resulting from oxidation of carbon/graphite at elevated temperatures. Samples of polycrystalline ATJS graphite and amorphous vitreous carbon were resistively heated to a high temperature, exposed to a controlled oxygen flow for a specific period, and subjected to a gradually increasing tensile stress until sample failure. Sample surface before and after oxidation were examined by optical and scanning electron microscopy.
1. INTRODUCTION
Low density and high-temperature strength make carbon/graphite useful in reentry vehicles[l, 21, rocket nozzles[3], and aircraft brakes[4]. However, carbon/graphite is vulnerable to oxidation, especially at elevated temperatures. Generally, oxidation of carbon/graphite at temperatures higher than 800 K results in mass loss and microstructure deterioration (e.g. crack development, enlargement of pores, and debounding between material components). Both mass loss and microstructure deterioration reduce the mechanical strength of carbon/graphite material. Thus, based on the data before and after oxidation, the percentage of strength reduction can exceed that of mass loss. For graphites used in nuclear reactors, the strength reduction is initially much greater than the mass loss[5,6]. A correlation between the mechanical strength, S, and the mass loss (expressed in terms of the specific gravity, p, of oxidized graphite) is given by
s -=
so
0;o’ -
”
where the subscript 0 refers to the unoxidized state and n is - 8.2 for tensile strength[71. According to this correlation, 1,5 and 10% mass loss corresponds to 8, 34 and 50% strength reduction, respectively. While mass loss can be determined quantitatively by weighing, the microstructure deterioration often is described qualitatively. Both gas permeability and electrical resistivity[‘l, 81 increase exponentially with oxidation, indicating the growth of interconnected pores and the loss of structural contacts within the material. Statistical analyses of the pore size distribution, surface area, and effective gas diffusivity[8-101 provide further insight on initially closed micropores and the generation of new
tThis research was conducted under the McDonnell Douglas Independent Research and Development Program.
2. EXPERIMENTAL
The oxidation experiments are shown schematically in Fig. 1. Three different systems were used. The indirect sample heating (Fig. la) has the advantage of studying any material regardless of its electrical properties. In addition, the sample temperature of the blackbody-like thermal cell is independent of the material’s emissivity. However, the oxidation limit of the vitreous carbon oven restricts this system to sample temperatures below 1200K[14]. Direct sample heating (Fig. lb) removes the 1200K limitation, but requires an electrically conductive sample (e.g. carbon/graphite) with known emissivity. This system was used for sample temperatures up to 1800K. At temperatures above 1800K, the heated sample holders affect the performance of the linear variable differential transformer (LVDT) displacement sensor and
157
T. C. PENG
158
Vitreous carbon tube oven
tube
7
7
(A) Oven heating
(B)
(C)
Sample heating
Sample heating
Fig. 1. Experimental arrangements for carbon/graphite oxidation studies.
the load spring. Thus, a water-cooled heat shield and sample holders were assembled in the third system (Fig. lc). In this system, the sample temperature routinely reaches 3000 K and is limited only by sublimation of the carbon/graphite samples. Evaluation of the sample surface temperature in the direct heating systems (Figs. lb and c) requires an emissivity value. For carbon/graphite, the emissivity varies from 0.77 for a polished surface to 0.9 for a rough surface[l5,16]; an average value of 0.83 was used. Within a range of 1000-2500 K, the surface temperatures determined using a 0.77 emissivity are 40-100 K higher than those determined using a 0.9 emissivity. Thus, the uncertainty in temperature resulting from the use of an average emissivity of 0.83 is 2 2%. All three heating systems were used inside a vacuum chamber maintained at 5 Pa. The sample and the thermal cell were flushed with argon and the vacuum chamber was reevacuated to -5 Pa. The resistive heating was initiated by passing an electrical current through the tube oven (Fig. la) or the sample (Figs. lb and c) fastened between two water-cooled electrodes. The sample temperature was monitored by an infrared pyrometer (Ircon 300 CH) or an optical pyrometer (Pyromicro-optical, model 95) through two quartz windows. The first window was attached to the vacuum chamber wall, and the second window was placed near the oxygen inlet. The cold gas flow over the second quartz window prevents
material vapor and particulate deposition on the window surface. The dumbbell-shaped samples were heated to maintain a relatively uniform temperature over the middle test section (- 2 mm dia. or - 2 x 2 mm2 cross section and a length to diameter ratio of -8). The temperature calibration over the test section indicated a 2-4% reduction from the center to the ends. When the sample reached the desired temperature, oxygen flowed around the sample at a predetermined rate for a specified period. A steady-state condition of temperature and oxygen flow rate was maintained for the oxidation process in the thermal cell. After the oxidation phase was complete, the sample (still at the elevated temperature) was placed under tension with a known load increment through the calibrated load spring. At each load increment, there was an instant elongation response followed by l-5% additional increase. Most of this additional increase in elongation was caused by readjustment within the sample holders. Generally, a period of - 2 min was allowed for sample elongation to reach a steady value as indicated by the displacement reading from the LVDT sensor. The tensile load was increased by increments until the sample broke. Scanning electron microscopy (SEM) was used to study the surface and the substructure just below the surface. SEM micrographs illustrated the surface roughness conditions, microstructure heterogeneity,
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
159
bonds[23,24,28]. Under non-oxidizing conditions, vitreous carbon retains its microstructure up to 2800K and t~nsforms into ~aphitic structure when heated above 28OOK.Vitreous carbon has small pores (2-10x 10e3pm[24,26], 3 1.2x low3pm[28]) which are mostly 3. MATIERIAL isolated and inaccessibleto gas penetration as evidenced The samples investigated were ATJS graphite and by the low values of porosity and permeability(Table If. Both ATJS graphite and vitreous carbon sampleswere vitreous carbon. Some of the material properties are listed in Table 1. ATJS graphite (Union Carbide) is fabricated from the material blocks as received and the a polycrystailine substance made from calcined sample surfaces were kept free of dust and grease, but petroleum coke (filler)and coal tar pitch (binder)[l9]. It were not treated by special techniques. has an average grain size of - 70 iu,m[27]and a pore size 4. REsurB range (as measured by mercury porosimetry) of 0.07The stanch-temperature relation of ATJS graphite in 100,umdia.: % of the pore volume per unit mass was argon (Fig. 2) demons~ates the consistency of all three l-5 pm pores. Vitrous carbon (Atomergic Chemetals, Inc.) has a experiment ~angements (Fig. 1) operated within basic structure consisting of randomty distributed small proper temperature limits.These results compare favor packets of graphite crystals that are cross linked by C-C ably with those reported by Southern Research Institute
grain shapes and sizes, pores and inter-grainboundaries. The electron microscope(JEtX ModelJSM-2)employed has a resolution of 0.025pm and accepts a maximum samplesize of 2.5cm dia. and 1cm thick.
Table 1. Materials characteristics of Material
ATJSgraphite and vitreous carbon at 300K
Density ~cm3)
1.83’[37,181
Impurity (ppm) Porosity (St)
- 193[18] 19: (6.8-8.5 (0))[17] (6-7 (O), 6-7 K))’ (16.2-17.6)[19]16.gd 0.00045(AG) 0.0005(WG) Darcys’
Permeability (cm’ls) Electrical resistivity (rnil cm) Thermal conductivity (W&cm0 Thermal expansion (10-6/K) Sonic velocity (IO*cm/s) Elastic modulus~ (10’ MPa)
Ultimate tensile strength (MPa)
Vitreouscarbon (Atomergie chemetals)
ATJS graphite (Union carbide)
0.87 (WG), 1.16(AG)[17] 0.83 (WG), 1.09(AG)[lI] (0.95-1.0~[19],0.86[151 1.46(WC), 1.11(AG)[lfl 1.79(WG), 1.47(AG)[lS] 1.40(WG), 1.0 (AG)[l9] 2.16 (WG), 3.10 (AG)[l7] 2.09 (WG), 3.17 (AG)[I8] 3.97 (WG), 5.11 (AG)[19] 2.71 (WG), 2.28 (AG)[17] 2.83 (WG), 2.34 (AG)[l9] 1.15(WG), 0.8 (AG)[17l 1.17(WG), 0.86 (AG)[l8] 1.2 (WC), 0.68 (AG)[19] 1.0 (WG), 0.69 (AG)[21] 36.5 (WG), 30 (AG)[17] 35 (WC), 28 (AG)[l8] 43.4 (WG), 34.7 (AG)fl9] 33.4 (WC), 27.6 (AG)[Zlj 34.8 (WG), 29.6 (AG)[22]
1.47[23],(1.3-1.55)[24], (1.5-1.55)[25] f30-3000)[24],2001231,< 50[25] (< O.OS)[23], (0.2~5)[24], 0 (apparent ~rosity)[~] < 2.5 X tO_” (He)[23], (10-9-10-‘2)[24], ( < 10-9[25] 3-8[23], 3.5-20[24],4.5[25]
(0.~2~.0~)[23], (O&42-0.25)[24],0.251251 (2.2-3.2)[23],(2-3.4)[24] 3.2 at lwC[25]
WWI (1.37-3.24)[24] 2.16[25] - 39[24],(41-90)[26]
tThe initial slope of tensile stress-strain curve near zero level. *Selected samples were weighed in air and their volumes were measured by immersing the samples in water. bCaIculated value given by Porosity (%) = (1 - ~)
X 100
where 2.26g/cm3 is the density of a graphite crystal. ‘Private communication on porosity with C. Pratt, AFML. 0 = open porosity, C = closed porosity. ‘Mercury porosimeter meas~ement for open and cracked-closed pores at 4.14~ l@ kPa (- 60,ooapsi). “Ref. [20]. “Darcy” is a unit of gas permeability through porous material such as graphite, and is defined according to the form of Darcy’s law. The values quoted in this table are supplied by Union Carbide (Ref. [18]). AG = Across Grain, WG = With Grain.
T. C. PENC
160 ATJS graphite
strength in argon
0 Exposed to 02 flow (0.55 g/h) for 1 h
0
Oven heating,
Fig. la
0
Sample heating,
Fig. 1 b
A
Sample heating,
Fig. lc >
0
Southern
0 Exposed to 02 flow (0.64 g/h) for 20 min MDRL
Research data (Ref.
data
17)
MDR L average -
-
80
-
Manufacture’s
I
I
data (Ref.
18) With oxidation
I
I
With oxidation
1.0
0.6
0
0
I
I
I
I
0.5
1.0
1.5
2.0
Sample temperature
1.8 x 103
1.4
Sample temperature
(KO
Fig. 3. Strength reduction by oxidation (ATJS graphite with grain). -
2.5 x 103
(K)
8oL
Fig. 2. Strength-temperature relations (ATJS graphite with grain). (SoRI)[17]
and the manufacturer[l8]
(below
1400 K).
Above 1400K, the data are consistent with those of SoRI but are 15% higher than the m~ufacturer’s value at 25OOK. Although the temperature in the third system (Fig. Ic) can go as high as 3400K (5660°F), the data presented herein are limited to ~2500 K (4Q40°F) since graphite sublimination at low pressures (- 2 kPa or 15 Torr) becomes appreciable at temperatures greater than 2500 K. ATJS graphite and vitreous carbon samples were tested for ultimate tensile strength under a variety of oxidizing conditions. Strength reduction is generally observed whenever sig~ficant oxidation occurs. For ATJS graphite, the effects of sample temperature, oxygen mass flow, and oxidation time are shown in Figs. 3-5. Similar effects for vitreous carbon are shown in Figs. 6-8. The ultimate tensile stress based on the sample cross-sectional area before oxidation was evaluated. The crosssectional area remained unchanged at a low level of oxidation and became badly damaged and too irregular to be defined after severe oxidation. Thus, the strength reduction reported reflects the reduction of the applied force during oxidation. The data scatter observed in Figs. 2-8 is caused primarily by the non-un~ormity of material properties either from different billets or difterent sections of the same billet. Vitreous carbon is also sensitive to small flaws or cracks. Hence, the scatter of vitreous carbon
0
0
10
20
30
40
50
60
70
80
Oxygen mass flow (g/h) Fii. 4. Effects
of oxygen flow rate on ultimate (ATJS graphite with grain).
tensile
stench
data (Figs. 7 and 8) is greater than that for ATJS graphite (Figs. 4 and 5). The physical characteristics of the surface and the cross sections before and after oxidation were examined using scanning electron and ,optical microscopy (Figs. 9-24). In these illustrations, surface A refers to the side of a square sample facing the oxygen flow. Surface B is the side of the square sample parallel to the oxygen flow. Surface C is the fracture surface when the sample failed under tension. For ATJS graphite, both surfaces A and B were machined, whereas only the B surface of vitreous carbon was machined with diamond tools. The A surface
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon 1oa
l-r
2500
I
I
161
I
I
K, 29 g/h 02 flow 80
l-
in Ar at 1575
Strength
K
1510-1610 K (1575 K average) 20 mln oxidation time
0 0
20 Oxidation
40
60
time (min) 20
Fig. 5. Effects of oxidation time on ultimate tensile strength (ATJS graphite with grain). 80
I
I
I
0
t
2
3
4
5
6
7
Oxygen mass flow (g/h)
Fig. 7. Effects of oxygen flow rate on ultimate tensile strength (vitreous carbon).
.J, 5 w 0 z E 20 ._ C
-
0
Without
oxidation
for 20 min 01 0.2
0.6
1.0
Sample temperature
1.4
1.8 x lo3
(K)
Fig. 6. Strength reduction by oxidation (vitreous carbon).
of vitreous carbon was the original “glassy” surface as received from the supplier. Surfaces A and B were exposed to oxidation; fracture surface C, however, was not exposed to oxidation because it was produced during the stress phase following completion of the oxidation phase. 5. DISCUSSION The strength of carbon/graphite is reduced significantly above a threshold temperature. For ATJS graphite at 0.55-0.64 g/hr oxygenflow rate, the threshold temperature is -850K after either 20 or 60min of oxygen exposure (Fig. 3). For vitrous carbon at - 6.4glhr oxygen flow rate (10 times that for ATJS graphite) and after 20min of oxygen exposure, the threshold temperature is - 1008K (Fig. 6). For the oxidizing conditions used in this study (Figs. 3-6), the temperature ranges for the strength reduction are 850CAR Vol 17, No. 2-E
20
0
0
4 Oxidation
8
12
time (mln)
Fig. 8. Effects of oxidation time on ultimate tensile strength (vitreous carbon).
162
T. C. PENG
Fig. 9. Unoxidized ATJS graphite surface (A) at room temperature. Fig. 10. Fractured ATJS graphite surface(C) at room temperature. 1300 K for ATJS graphite and 1000-1600 K for vitreous
carbon, respectively. The thermogravimetric analysis (TGA) data of Shapiro1291show that oxidation mass loss in air starts at - 930 K and reaches - 90% of the initial weight at 1100K for ATJS graphite. Furthermore, he found that the mass loss in air for “glassy” carbon (Lockheed), a material similar to the vitreous carbon used here, begins at - 1050K and reaches - 90% of the initial weight at 1350K. The mass loss data of Shapiro and the strength reduction data reported here agree on the temperature required for oxidation effects, thereby suggesting that mass loss is an important contribution to strength reduction.
Using the cross-sectional area change before and after oxidation (Figs. 22-24) as an indicator of the mass loss at the failure plane, the data show that the estimated ATJS graphite area decreases of -1, -25 and -68% resulted in 24-36, 52 and 90% strength reductions. These results support the conclusions of other investigators [5-71 that 1,5 and 10% mass losses cause 8,34 and 50% reductions in tensile strength. This finding holds true in spite of different sample sizes used: a sample of - 2 mm dia. or - 2 x 2 mm2 cross section was used in this study, and a sample of 9-25 mm dia. was used in the other investigations[6,7]. Thus, for ATJS graphite, the dis-
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
1600X
1 cii?UX
Fig.If. Oxidized ATJS graphite surface (A) at 1541K, OZflow at 0.64glhr for 20 min.
Fig. 12. Oxidized ATJS graphite surface (A) at 1530K, O2flow at 6.4 g&r for 20 min.
proportions high percentage of strength reduction over that of mass loss indicates the major importance for microstructure deterioration caused by oxidation. For vitreous carbon, the fracture cross-section is defined with difficulty because the material often shatters at failure. The cross-sectional data (Fig. 24) showed that an approximately 50% area decrease was accompanied by a 60% reduction in strength. Hence, the microstructure change should be a factor in predicting the oxidationcaused strength reduction. However, the available evidence is not sufficient to determine the proper role of the microst~~true effects.
While temperature is the ~ontroll~g parameter for the initiation of significant mass loss and strength reduction, oxygen flow rate, oxidation time, and temperature are factors controlling the rate and the amount of strength reduction. Thus, for ATJS graphite at 1150K (Fig. 3) and an oxygen flow rate of 0.55 g/hr, a 60 min oxidation time reduced tensile strength by 63%, whereas 0.64g/hr and 20min oxidation time yield only 1% loss. In Fig. 4, the 2 x 2mm ATJS sample is nearly burned out for - 7g/hr oxygen flow at - 1.550K and 20 min exposure, yet a similar sample suffered only 60% strength reduction for - 80 g/hr oxygen tlow at 2500 K
T. C. PENG
Fig. 13. Fractured ATE graphite surface (C) at 1530K.
for 2 min exposure. Thus, it appears from the data shown in Figs. 3 and 4 that oxidation time is a primary variable in assessing oxidation damage. However, Fig. 5 clearly shows the marked influence of the oxygen flow rate and sample temperature on the loss of tensile strength. Hence, oxygen flow rate, oxidation time and sample temperature interact to produce the overall oxidation damage. The effects of flotield are shown in Figs. 3 and 4. For a low oxygen Bow rate (0.64 g/hr) and a 20 min exposure, the tensile strength reduction is - 27% at 14OOK and then becomes constant up to 1sOOK (Fig. 3). Since the oxidation rate usuahy increases with temperature, the constant strength r~uction between 1400 and BOOK may indicate partial blocking of the material surface by
less reactive oxidation products. A possible alternative to the flow field expl~ation would be that the increasing graphite strength with temperature and the decreasing load support area from oxidation balance each other. However, this balance, if it occurred, would be a coincidence, and is unlikely to hoid for a temperature range of 1400-18OOK. Furthermore, the 60% strength reduction at 2.500K and 2 ruin exposure also becomes constant for oxygen flow rates greater than SOg/hr (Fig, 4). A possible ~o~eld explanation for this behavior is that the boundary layer prevents the oxidation products from leaving the material surface readily and thereby limits further increases in oxidation damage. This ~o~eld explanation is speculative since the flow around a sample with square cross section is not well
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
Originat
“gtassy” surface (Al
Fig. 14. Unoxidized vitreous carbon surfaces, 200x .
16.5
166
T. C. PENG
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
167
Fig. 16. t3xidized vitreous carbon surface (A) at 1020K, 02 flow at 6.4g/hr for 20
defined. However, the apparent flo~eld effects shown in Figs. 3 and 4 suggest that experiments with well defined flotileds (e.g. a cylindrical cross section ) should be conducted to investigate co~elations between strength reductions from sample tempe~ture, oxygen flow rate, and oxidation time. In the case of vitreous carbon, the effects of sampfe temperature, oxygen ffow rate, and oxidation time on strength reduction are shown in Figs. 6-8. Generally, vitreous carbon is more oxidation resistant that ATJS graphite. For example, the oxidation threshold temperature for vitreous carbon is - WOOK compared with -850K for ATJS graphite, and the strength reductions for vitreous carbon are 1%20% less than those of ATJS graphite (Fig. 25). These data corroborate Shapiro’s weight loss data1291 and Lewis’ oxi-
dation rate constants [30]. However, the general behavior of oxidation for both ATJS graphite and vitreous carbon is similar as shown by (I), parallel slopes in strength~oxygen-flow diagrams (Fig. 25); and (2), comparison of strength~oxidation-time curves (2500 K curve in Figs. 5 and 8). On a relative basis, the oxidation behavior of these two materials with large differences in microstructure is similar (Fig. 26). This result suggests that the effect of oxygen exposure on strength in relative terms does not depend markedly on material structure. Micrographs of unoxidized ATJS graphite show similar heterogenous compositions on all three surfaces (A, B and C as defined previously, Figs. 9 and 10). The straight lines at 150Xresulted from machining. At 1600X, the outline of filler particles and pores can be recognized. However, the presence of paste-like binder surrounding
168
T. C. PENG
Fig. 17. Oxidized vitreous carbon surface (B) at 1020K, O2 flow at 6.4 glhr for 20 min.
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
169
1600X Fig. 18. Oxidized vitreous carbon surface (A) at 1230K, Oz flow at 6.4 glhr for 20 min.
the filler particles makes the sizing of graphite grains diEcult. The effects of oxidation are illustrated in Figs. 1l-13. At a low oxygen flow rate @.64gfhr), the paste-like binder appearance persists to a temperature of 1550K, whereas at a high oxygen flow rate (6.4g/hr) and a similar temperature (1530K), the binder disappears and only the filler particles (graphite grains) with sharp edges remain. Based on similar observations, Maahs and Schryer[31] concluded that the oxidation rate of the binder is higher than that of filler. At 10 times the oxygen flow, the oxidation rate per unit surface is increased sufficiently to consume a11binder near the surface. Surface C is not affected greatly by oxidation since tensile fracture occurred only after the oxidation phase was finished (Fig. 13). However, when oxidation CAR Vol. Il. No 2-F
Fig. 19. Oxidizedvitreous carbon surface (B) at 1230K, O2flow at 6.4g/br for 20min. penetrated the material interior, then clearly definable filler particles were observed, indicating the preferential removal of binders by their higher rate of oxidation. Scanning electron microscopy observations of unoxidized and oxidized vitreous carbon samples are shown in Figs. 14-21. Before oxidation, surface A, the original glassy surface of vitreous carbon, is smooth and homogeneous at 1600X. Surface B was machined with a diamond tool and exhibits a lame&r structure. Surface C, the fracture surface, shows a smooth surface with small pit marks, presumably micropores. Both surfaces B and C reveal isolated pores of 2-20 pm dia. and granules (l-3pm, occasionally 30pm) around the pores. This
T. C. PENG
Fig. 20. Oxidized vitreous carbon surface (A) at 1440K, O2flow at 6.4 glhr for 20 min.
granular structure may arise from the preferential the pores during heat around graphitization treatment [32]. Oxidation at 6.4glhr oxygen flow rate and 20 min oxidation time produced different microstructure deteriorations on the surface depending on the temperature and original surface conditions (Figs. 16-21). Thus, oxidation at 1020K produced numerous pits in the originally surface A and visible cracks or channels on the machined surface B (Figs. 16 and 17). Oxidation at 1230K resulted in large, round depressions on surfaces A and B. In addition, - 2 pm dia. particles also appeared on surface B (Figs. 18 and 19). As the oxidation temperature was increased to 1440K; large, round holes appeared on surface A and a uniform laminar grain structure emerged. Large craters are present on surface B,
1600X
Fig. 21. Oxidized vitreous carbon surface (B) at 1440K, O2 flow at 6.4 g/hr for 20 min.
and oxidation-induced pores (20-40 pm across) penetrated the sublayer beneath the surface (Figs. 20 and 21). Previously observed - 2 pm dia. particles disappeared, thereby indicating that these particles are transient products of oxidation. Optical microscopy, because of its small depth of field, provides valuable insight on oxidation penetration from the surface into the substructure via cross-sectional view. For ATJS graphite, oxidation at low oxygen flow rates and certain combinations of temperature and exposure time (Fig. 22a and 23b) creates significant microstructure
Oxidation effects on the tensile strength of ATJS graphite and vitreous carbon
a)
%mPk surfam oxidized
180X
993 stteadv OXYWMflow of O&5 g/h. Fig.
at
K fur 1 h by
b)
sPrmr;c-lswcf oxidkmdat1139Kforl steady axyg%n flow of 0.55 glh.
22. Cross-sectional views of ATJS rod samples cut diametrically from a cylindrical billet
h&y
172
T. C. PENG
bl 1541 I(, 02 flow 0.64 g/h, 20 min
e!
1530 K 02 flow 8.4 g/h, 20 min
Fig. 23. Cross-sectional views of both unoxidized and oxidized ATJS graphite samples, 50x.
Oxidation effects on the tensile strength of ATJS graphite and vitreous
carbon
173
Vlrreous carbon 1510-1610 K (1 575 K average) 20 m,n oxidation
1500-l 20 mln
Room
temperature 0
608
oxldatlon
t,mc
K
time
1
I
I
I
I
I
1
2
3
4
5 (g/h)
6
Oxygen
mass flow
rate
Fig. 25. Relative effects of oxygen flow
rate
7
for different
materials.
from scanning electron microscopy (Figs. 16-21) indicate surface-layer damage only. This result also supports the previous observation that the strength reduction for vitreous carbon is lower than that of ATJS graphite at the same oxidizing conditions (Fig. 25). Hence, in the case of vitreous carbon, the primary cause for strength reduction appears to be mass loss. The microstructure damage within the surface layer also contributes to strength reduction by providing material flaws where local stress concentrations produce material failure. 6. CONCLUSIONS 1440
K, 02
flow
at
6.4
g/h,
20 mm
Fig. 24. Cross-sectional view of both unoxidized and oxidized vitreous carbon samples, 35x. damage without significant mass removal. This damage is indicated by numerous enlarged pores and visible chan-
nels leading from the oxidized surface to the material interior; yet, the cross-sectional area remains unchanged. Oxidation at higher temperatures (Fig. 22b) or higher flow rates (Fig. 23~) produces large mass loss with similar microstructure damage. Thus, the optical micrograph data (Figs. 22 and 23) indicate that the initial strength reduction of ATJS graph by oxidation is primarily caused by microstructure damage, and subsequent strength reductions resulg from the large mass loss. Optical micrographs of oxidized vitreous carbon (Fig. 24) do not show deep oxidation penetration of microstructure. The observed pit marks, holes and depressions
A threshold temperature for oxidation exists above which the strength reduction becomes significant. For ATJS graphite and vitreous carbon, the threshold temperatures are 850 and lOOOK, respectively. These threshold temperatures are close to those determined from weight loss data by thermogravimetric analysis. However, the progress and the amount of oxidation are controlled by the combined effects of material temperature, oxygen flow rate, oxidation time flowfield configuration. For ATJS graphite, both mass loss and microstructure deterioration from oxidation are major factors producing strength reduction. microstructure Furthermore, deterioration is the primary cause for initial strength reduction. For vitreous carbon, the microstructure deterioration is confined to a thin surface later, and strength reduction is caused primarily by mass loss and to a lesser extent by local material flaws resulting from oxidation.
T. C. PENG
174
I
I
2.
I
3. 4. 5.
80
6.
W. E. Nicolet, J. T. Howe and S. A. Mezines, AIAA Paper No. 74-701 (1974). M. R. Wool, H. Tong and R. J. Bedard, CIPA Publ. 266 3, 385 (1975). F. P. Kirkhart, Proc. 12fh Carbon Conf. 279 (1975). W. Karcher, R. Krefeld and P. Glaude, Proc. 9th Carbon Conf. 67 (1%9). J. A. Board and R. L. Squaires, Proc. 2nd London Conf.
Carbon Graphite 289 (1%5). 7. C. Rounthwaite, G. A. Lyons and R. A. Snowdon, Proc. 2nd London Conf. Carbon Graphite 299 (1%5). 8. R. H. Knibbs and J. B. Morris. Proc. 3rd London Conf. Carbon Graphite 297 (1970). 9. W. M. Kalback, L. F. Brown and R. E. West, Carbon 8, 117
from curve in Fig. 5 2500 K, 29 g/h 02 flor
(1970). 10. E. T. Turkdogan, R. G. Olson and J. V. Vinters, Carbon 8, 545 (1970). 11. R. D. Reiswig, L. S. Levinson and T. D. Baker, Carbon 5, 603 (1%7). 12. S. S. Jones Carbon 8,673 (1970). 13. R. Stevens, Carbon 9,573 (1971). 14. T. C. Peng, Proc. 22nd Conf. Mass Spectrometry Allied Topics 384 (1974). 15. C. L. Mantel, In Carbon and Graphite Handbook, Chap. 19, p. 344. Interscience-Wiley, New York (1%8). 16. R. E. Tavlor and W. D. Kimbroueh. Carbon 8.665 (1970). 17. H. S. Starrett and C. D. Pears, A&ML-TR-73-14, Voi. 1 (Feb.
20
Vitreous carbon, from curve in Fig. 6 iO0 K, 29 g/h 02 flow
0 0
40
20
% burnout
60
80
100
time
Fig. 26. Relative effects of oxidation time for different materials. Although materials with variable microstructures respond to oxidation differently, the relative correlation between strength reduction and oxidation time depends only weakly on microstructure differences. A similar relation also appears applicable to the oxygen flow rate.
1. V. DiCristina, Final Summary Report, USAF SAMSO Rep. TR-76-12, (Dec. 1975).
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