Nathan S. Jacobson

Lewis Research Center, Cleveland, Ohio

Space Shuttle Pinhole Formation

Mechanism Studies

NASA/TM—1998-208659

November 1998

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Nathan S. Jacobson

Lewis Research Center, Cleveland, Ohio

Space Shuttle Pinhole Formation

Mechanism Studies

NASA/TM—1998-208659

National Aeronautics and

Space Administration

Lewis Research Center

November 1998
Acknowledgments

It is pleasure to acknowledge the many individuals who assisted with the exposure and analyses of these specimens. We thank Donald Humphrey of NYMA/NASA Lewis Group for the furnace exposure of the RCC and SiC

materials. For specimen preparation, we thank Todd Leonhardt, formerly of NYMA/NASA Lewis. For the SEM

study, we thank Pat Dickerson, formerly of NYMA/NASA Lewis Group, David Hull of NASA Lewis, and

Terry McCue of NYMA/NASA Lewis Group. For the EPMA study, we thank Bill Blackburn of

Binghamton University, Binghamton, NY. The support and guidance of Dr. Donald Curry,

NASA-JSC, is very much appreciated.

Available from

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Trade names or manufacturers’ names are used in this report for

identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.
NASA/TM—1998-208659 1

SPACE SHUTTLE PINHOLE FORMATION MECHANISM STUDIES

Nathan S. Jacobson

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

SUMMARY

Pinholes have been observed to form on the wing leading edge of the space shuttle after about 10 to 15 flights.

In this report we expand upon previous observations by Christensen (ref. 1) that these pinholes often form along

cracks and are associated with a locally zinc-rich area. The zinc appears to come from weathering and peeling paint

on the launch structure. Three types of experimental examinations are performed to understand this issue further:

(a) Detailed microstructural examination of actual shuttle pinholes (b) Mass spectrometric studies of coupons containing actual shuttle pinholes and (c) Laboratory furnace studies of ZnO/SiC reactions and ZnO/SiC protected carbon/carbon reaction. On basis on these observations we present a detailed mechanism of pinhole formation due to

formation of a corrosive ZnO-Na2O-SiO2 ternary glass, which flows into existing cracks and enlarges them.

INTRODUCTION

Material Description

The wing leading edge and nose cap of the space shuttle orbiter is composed of a reinforced carbon/carbon

(RCC) material. The location of these panels on the orbiter is shown in figure 1. A diagram of the RCC material is

shown schematically in figure 2 and described in more detail in reference 1. It consists of layers of a graphitized

rayon fabric, which are repeatedly impregnated with a liquid carbon precursor. Pyrolysis then converts the precursor

to carbon. A diffusion conversion coating of silicon carbide is then grown on the surface at high temperatures by a

pack siliconizing cementation technique. Next the material is vacuum infiltrated with tetraethyl orthosilicate (TEOS)

to fill cracks and fissures. On heating, the TEOS decomposes to silica. The outer layer is a proprietary sealant of

‘Type A’ glass, which is primarily sodium silicate. This is intended to flow into the cracks which develop in the SiC.

An important issue with this system is the thermal expansion mismatch between the SiC coating and the carbon/

carbon. Cracks develop in the SiC coating during processing and also during service. Figure 3 and the associated

oxygen map illustrate these two types of cracks. The crack which developed during processing is filled with pure

silica, as indicated by the oxygen map. No other elements were detected in this crack. The crack which developed

during testing is visible in the image as an open crack.

At this point it is appropriate to discuss the oxidation of SiC and the effect of a sodium silicate glass coating.

Oxidation of SiC has been extensively studied and recently reviewed in detail (ref. 2). The process is illustrated in

figure 4(a). Oxygen permeates a growing SiO2 film and reacts with SiC at the SiC/SiO2 interface, according to:

SiC s O g SiO s CO g () ( ) () ( ) () + =+ 32 1 2 2

The rate controlling step is the permeation of oxygen inward through the SiO2 network. As the reaction proceeds CO(g) must escape from the reaction interface, either by a permeation process through the SiO2 scale or, in

some cases, by forming bubbles in the SiO2. Generally bubbles in SiO2 indicate a large amount gas escaping and

hence a vigorous reaction.

Small amounts of foreign cations, such as Na or K, tend to act as network modifiers, breaking up the SiO2

network and allowing faster oxygen transport and hence faster oxidation rates (ref. 2). Controlled studies of cation

modified SiO2 scales indicate that these cations can accelerate oxidation rates by up to 10X (ref. 2). Larger amounts

of cations, as added oxides (e.g., Na2O) may react with the SiO2 to form low melting eutectic liquids which have

rapid transport rates and allow for rapid oxidation. Figure 5 is the Na2O-SiO2 binary phase diagram (ref. 3). It

follows that when a Na2O.x(SiO2) liquid is applied to a SiO2 -forming ceramic, the equilibrium situation is illustrated in figure 4(b). The composition of the liquid silicate at the silicate/SiO2 interface is given by the liquidus
NASA/TM—1998-208659 2

(liquid/tridymite + liquid line) in figure 5 (refs. 2 and 4). In this case with the RCC material, when Na2O.x(SiO2) is

added to the a pre-existing SiO2 film on SiC, one would expect the SiO2 to dissolve in the Na2O.x(SiO2) until the

region adjacent to the SiO2 film has the liquidus composition (SiO2 saturated) and the morphology shown in

figure 4(b).

ENTRY CONDITIONS

The chemical reactions which lead to pinhole formation are expected to occur when the shuttle wing leading

edges are hottest—during launch and re-entry (ref. 5). Launch lasts about eight minutes and involves temperatures

only to about 755 K (900 °F). Re-entry involves temperatures up to 1922 K (3000 °F) and lasts about 30 min. The

time to reach temperature is 5 to 8 min and the highest temperatures last about 7 to 8 min. Possible trans-Atlantic

(TAL) abort trajectories for the Shuttle involve temperatures to 2095 K. The re-entry atmosphere is essentially a

reduced pressure of air—0.005 to 0.010 atm and contains a complex mixture of molecules, atoms, and ions. There is

a good deal of information on this atmosphere (ref. 5). Simulating this environment for ground based studies with

controlled variables has always been a problem. However an arc-jet is generally accepted as the best approach

(refs. 6 and 7).

IMPURITY DEPOSITS

The orbiter may sit on the launch pad for periods of up to a month before launch. The sea-salt laden air of the

Florida coast is a corrosive environment to both the launch structure and the vehicle. Recently Gordon (ref. 8) has

completed a series of chemical analyses to determine the protected RCC contamination. Most of the impurities

found are from the weathering paint. These include Fe, Ca, Si, Zn, Al, Mg.

CONDITIONS FOR PINHOLE FORMATION

After about 10 to 15 flights small pinholes have been observed primarily on the wing leading edge. Detailed

descriptions of these pinholes have been reported in several reports (refs. 1 and 9). Here we shall summarize the

observations critical to the mechanism. Initially pinholes were discovered on the wing leading edge on each of the

four orbiters. Later a small number of pinholes were discovered on the nose-cap. The wings are exposed on the pad,

whereas the nose-cap is covered. The orbiter, OV-102 (Columbia), has the largest number of pinholes. This is the

oldest of the shuttles and generally flies hotter.

Figure 6 shows six different pinholes from OV-102. Some are from a panel removed after 15 flights and some

from a panel removed after 19 flights. For each pinhole, a surface view and a polished cross section is presented. In

all cases the surface view is a backscattered electron image (BSE). In some cases a BSE image of the polished cross

section is presented; in other cases an optical micrograph of the polished cross section is presented. From these micrographs and other observations, a number of generalizations about pinholes can be made.

In a BSE image, the phases with heavier elements tend to appear brighter. Elemental identification of these

bright phases indicated a large amount of zinc. The source of this zinc is likely the flaking paint from the launch

structure. Thus pinholes are always associated with zinc. As pointed out by Christensen (ref. 1), the glass adjacent to

the pinhole has a more frothy and bubbly appearance than the glass which normally covers the RCC. The BSE images of the cross sections indicate that the zinc-rich glass flows into the pinholes. In addition the pinholes are nearly

always along craze cracks in the SiC. These are formed due to the thermal expansion mismatch between the carbon/

carbon and the SiC coating. The cross sections all suggest that the pinhole is simply an enlargement of upper portion

of one of these cracks.

PROPOSED MECHANISMS

A number of mechanisms have been proposed to explain the pinhole formation (refs. 1, 9 to 11). To date, no

mechanism fully explains all of the observations associated with pinholes. The proposed mechanisms have been
NASA/TM—1998-208659 3

nicely summarized in the report by Christensen (ref. 1). Therefore we shall only briefly discuss the key issues of

each mechanism, with an emphasis on the high temperature chemical reactions in each process.

Several proposed mechanisms are based on internal effects (refs. 1, 9 and 11). These are shown schematically in

figure 7(a) to (c) and listed below:

(a) Pinholes are defects in the SiC coating formed during processing. As the outer layer of glass volatilizes during each mission, these defects are exposed.

(b) Interaction of SiC (or C) and SiO2 to generate gases:

SiC SiO SiO g CO g a += + 23 2 2 () () ( )

C SiO SiO g CO g b += + 2 () () ( ) 2

(c) Formation of SiO(g) due to low partial pressures at the bottom of pinhole and subsequent re-oxidation to

SiO2(s) at the top.

The basic problem with each of these is that they do not account for the concentration of zinc noted at each pinhole. In addition, one would expect mechanism (a) to be operative in laboratory furnace tests. Despite extensive

testing of the coated RCC material in the laboratory, no pinholes have been observed.

A sodium chloride induced mechanism was proposed in reference 10. This is shown schematically in figure 8.

In this case the NaCl is proposed to collect in a crack or fissure where the oxygen potential is low. This reacts with

the SiC to form SiClx species—primarily SiCl2. These diffuse to the top and are re-oxidized to SiO2, which would

create an additional glassy area near the pinhole. As will be shown, sodium chloride is present on the RCC material,

very likely from the sea-salt laden air of the cape. It has been shown that this is a thermochemically viable mechanism for pinhole formation and that diffusion can account for the material removal necessary to create these pits

(ref. 10). However this mechanism does not account for the concentration of zinc associated with each pinhole and

sodium chloride/RCC laboratory tests were not able to reproduce the pinholes or any actual pinhole features. However, non-representative pinholes were formed after treating a coated RCC coupon with sea salt and testing in a

NASA/JSC arc-jet (refs. 1 and 13).

The concentration of zinc associated with each pinhole cannot be ignored. It is likely that the zinc plays a role in

pinhole formation. Christensen (ref. 1) suggests that formation of various zinc silicate products may indeed lead to

consumption of the SiC coating and pinhole formation. In this report we shall build on this idea. A series of microstructures will be examined and discussed to develop a detailed, thermochemically sound mechanism.

EXPERIMENTAL

This report consists of three experimental sections: (1) Detailed microstructural observations of RCC material

removed from the Shuttle. (2) Mass spectrometric studies of the RCC material removed from the Shuttle and (3)

Laboratory furnace studies of the RCC material and microstructural examination.

(1) Observations from Shuttle

The pinhole cross sectioning technique, illustrated in figure 6, was particularly useful for this study. This has

been described in a previous publication (ref. 12) and will only be briefly summarized here. The region around the

pinhole was cut with a diamond sectioning saw and then gently polished to reveal a cross section of the pinhole.

Samples were examined with both optical and scanning electron microscopy (SEM). As mentioned, backscattered

electron (BSE) images were particularly useful in identifying regions containing zinc. A newer SEM with a field

emission gun (FEG) was particularly valuable for high magnification examination of the glass/SiC interface.

Elemental analysis was done with energy dispersive spectroscopy (EDS). One issue with EDS is the overlapping

of the Zn and Na peaks. Additional elemental analyses were therefore done with a wavelength dispersive spectrometer (WDS), which allowed separation of the Na and Zn peaks.
NASA/TM—1998-208659 4

(2) Mass Spectrometric Studies of the RCC Shuttle Material

When an RCC panel from the shuttle was tested in the NASA JSC arc-jet facility, a red-orange gas emerged

from the pinholes was observed (ref. 13). This is shown in a video captured photograph in figure 9. Identifying this

gas has proven to be a challenge. Classical methods of gas identification (e.g., mass spectrometry and UV/VIS/IR

based spectroscopies) are simply not possible in an arc-jet. Caution should be taken in intrepreting the colors in

figure 9, however the red-orange appearance is strongly suggestive of a sodium flame spectra.

We examined these same types of panels in our molecular beam mass spectrometer at NASA Lewis. This

unique instrument allows direct mass spectrometric sampling of a process occurring at one atmosphere with a high

sensitivity quadrupole mass spectrometer. This is accomplished by a free-jet expansion of the gases to be sampled

into a large, rapidly pumped vacuum chamber. This leads to a well-defined molecular beam with limited molecule/

molecule and molecule/wall collision. The beam is directed to a mass spectrometer in an ultra-high vacuum chamber

and chemical and dynamic integrity of the sampled gas is preserved. A block diagram of this system is shown in

figure 10 and more details on the system can be found in reference 14. This particular configuration is well-suited to

identifying high temperature, condensable species such as NaCl(g), Zn(g), and Na2O(g). The sensitivity is better

than one ppm.

(3) Laboratory Furnace Tests

Although duplicating the complex conditions of vehicle re-entry in a laboratory furnace is not possible, we can

examine some possible RCC/ZnO/entry atmosphere reactions at elevated temperatures in the laboratory. It is important to note the Zn from the paint would not exist in elemental form, but rather as a binary oxide or complex oxide.

For these studies we used ZnO. Table I lists the various reactants and furnace exposures used. In order to put ZnO

on a SiC or RCC coupon, a water-based slurry of ZnO was made. Drops of this slurry were put on the coupon and

the coupon then dried in a drying oven at 100 °C for several hours. Then the specimen was heat treated under the

conditions given in table I. After exposure, samples were analyzed in several ways. SiC samples were chemically

analyzed after treating in warm 10 percent HF for 2 hr to remove the reaction products. These products were then

analyzed with atomic emission spectroscopy. In addition, standard scanning electron microscopy was used to examine the sample surfaces and cross sections. Electron microprobe analysis (EPMA—JEOL Superprobe, Binghamton

University, Binghamton, NY) was used for determining elemental distributions.

RESULTS

Pinhole Microstructures

In this section, we present detailed microstructural examinations of the pinholes shown in figure 6. Figure 11 is

a high magnification micrograph and associated EDS of figure 6(a). This was taken in the backscattered mode with

our FEG-SEM. Note that the Na-Zn-Si-O glass only extends to the bottom of the enlarged crack. Below this, only a

Na-Si-O glass is observed in the crack. Figure 12 is a 7000X magnification of a region of the glass in the enlarged

part of figure 6(f). Three distinct layers are present in the glass and associated EDS spectra show a gradient in Zn.

The inner most layer is nearly pure silica. The EDS spectra indicate a gradient in Zn concentration through the layers with the largest concentration on the outside. As discussed, Na and Zn peaks overlap in EDS spectra. In order to

resolve these, WDS was taken of this glassy region. The SEM with the WDS attachment did not allow the three

layers to be resolved. However a WDS spectrum of the entire glass region showed both Na and Zn (fig. 13). Typically such SiO2 dissolution microstructures (ref. 2) show a pure layer of SiO2 adjacent to the SiC and a decreasing

gradient in Na from the gas/solid interface. Further, in this case, it is likely that both Na and Zn show similar gradients, decreasing toward the SiC/glass interface.

Mass Spectrometry of Shuttle RCC Panels

As discussed, we could not attach a mass spectrometer directly to an arc-jet. However, we could very roughly

approximate the conditions of the arc-jet in our high pressure sampling mass spectrometer. To do this, we used a
NASA/TM—1998-208659 5

mixture of 1 percent oxygen/Ar and heated the sample to 1100 °C. Interestingly, the mass spectrum showed a lot of

Zn and NaCl vapors. This is shown in figure 14. These vapors were observed for about 5 to 10 min at temperature.

Unfortunately, we were not able to view the samples during this test, so we cannot unequivocally say that the

orange-red gas was Zn(g) and/or NaCl(g). However this test indicated there was a substantial amount of Zn and

NaCl deposited on the RCC panels.

Furnace Tests of SiC and RCC Panels

A limited number of furnace tests were done to try to understand some of the high temperature reactions. The

conditions are listed in table I. The results of a simple test of ZnO and SiC are illustrated in figure 15 and listed in

table II. It appears that ZnO enhances the oxidation of SiC by a factor of two. This is very likely by the Zn doping

the SiO2 network and leading to faster transport rates. Since the ZnO-SiO2 eutectic is high (fig. 16—1705 K), no

liquid is formed under these conditions. A solid system exhibits much less attack than a liquid system.

Some tests were also done on the RCC material + ZnO. In this case extensive reaction was noted, as shown in

the optical micrographs in figure 17. This bubbly, friable glass was reminiscent of the glass observed on the shuttle

specimens. This glass was further examined with the EPMA. First a region which appeared to be Zn-rich glass on

SiC was examined. This is shown in figures 18 and 19. These figures indicate a thick layer of Zn-rich glass, with a

thin layer of SiO2 adjacent to the sample. The large bubble indicates extensive gas generation. Figure 19 shows an

enlarged region with what are very likely SiO2 crystallites adjacent to the SiC. This is reminiscent of the glass observed on the shuttle specimens.

Figure 20 a region where the Zn-rich glass was adjacent to a SiO2 filled region. This SiO2 region was likely

formed in processing by the TEOS vacuum infiltration/decomposition to silica step. The significant point here is that

the zinc rich glass maintains a distinct layer from the silica.

In order to explore the effects of both ZnO and NaCl on the RCC material, a coating of NaCl was put on the

Zn-rich glass. The EPMA results are shown in figure 21. Generally most of the NaCl remains on top of the ZnO-rich

glass with little interaction. However the elemental maps suggest that some Cl flows down a crack.

DISCUSSION

It is appropriate to summarize what we know about pinholes at this point:

A. Shuttle:

1. Pinholes form on primarily on the wing leading edges, but not necessarily on the hottest parts.

2. A minimal amount of pinholes form on the nosecap, which is covered while on the launch pad.

3. Pinholes appear after 10-15 flights.

4. The orbiter may sit on the pad up to a month, collecting various impurities from flaking paint and the sea salt

laden air. The frequent appearance of zinc appears to be highly significant.

5. Mass spectrometric evidence indicates Zn and NaCl collect on the coated RCC material.

6. Pinholes are associated with a region of zinc rich glass.

7. The zinc rich glass appears frothy and more fluid than the Type A glass.

8. Pinholes occur along the cracks in the SiC caused by CTE mismatch between the RCC and the SiC coating.

9. Pinholes appear to be an enlargement of the outermost regions of the cracks in the SiC.

10. Detailed microstructural examination of the glass inside the pinholes indicates an Na-Zn-Si-O glass with

multiple composition layers. There is a decreasing gradient in Zn and Na toward the glass/SiC interface with nearly

pure SiO2 crystallites adjacent to the SiC.

B. Laboratory Experiments:

1. ZnO does not react appreciably with pure SiC.

2. ZnO appears to cause a vigorous reaction with the protected RCC material (Type A glass). This is evidenced

by the formation of entrapped bubbles and a thick glassy layer.
NASA/TM—1998-208659 6

3. The glassy Zn-rich layer is not in direct contact with the SiC, rather there is layer of pure SiO2 crystallites

between the Zn-rich glass and the SiC coating on the RCC.

4. NaCl deposits on the glass do not appear to interact with the glass directly.

5. NaCl deposits may collect in internal cracks.

These observations support a zinc oxide based pinhole mechanism and to a less extent a NaCl-based mechanism

discussed in an earlier report (ref. 10). Consider the zinc oxide based mechanism. The key observation here is in

figure 17. The presence of ZnO on the protected RCC material leads to the formation of a thick Zn-Na-Si-O glass,

with a lot of entrapped bubbles. The additional silica in this glass cannot be from the silica which is already a part of

the RCC material, but rather must come from oxidation of the SiC layer. This leads to consumption of the layer and

also accounts for the extensive gas and resultant bubble formation. Thus a sequence of reactions is likely:

Na O x SiO y ZnO s Na O y ZnO x SiO 22 2 2 . ( ) ( ) . ( ). ( ) ( ) ( ) l l + = ( ) 3

SiC s O g SiO s CO g () ( ) () ( ) ( ) + =+ 32 4 2 2

Na O y ZnO x SiO SiO s Na O y ZnO x SiO 2 2 22 2 . ( ). ( ) ( ) . ( ).( ) ( ) ( ) ( ) l l += +1 5 ( )

Reaction (3) is significant since it leads to melt with a higher capacity to dissolve silica. The Na-Zn-Si-O compound is likely to be even lower melting than Na-Si-O melts, as can be seen in the Na2O-ZnO-SiO2 ternary phase

diagram (fig. 22). This lower melting point explains the observed increased fluidity of the glass. Unfortunately due

to the formation of glasses of varying stoichiometry, it is not possible to write exact reactions. Note that at the temperatures of reaction, the glass must be molten. Although post-examination has revealed zinc silicates (ref. 1), it is

likely that these crystallized out of the melt on cooling.

Figure 23 is a schematic of a proposed mechanism. ZnO particles lodge near existing cracks. Then a Na2O.

y(ZnO).x(SiO2) glass forms due to reaction of the ZnO with the sodium silicate sealant via reaction (3). This reactive glass can dissolve the stable SiO2 layer on the SiC in the crack via reaction (5). Further rapid oxidation of SiC

can occur via transport through the Na2O-ZnO-SiO2 liquid via reaction (4). This oxidation and dissolution effectively leads to consumption of SiC and enlargement of the crack. In time the Na2O-ZnO-SiO2 liquid will become

saturated in SiO2 and the reaction will stop. This type of oxidation and dissolution has been discussed in the introduction and is commonly observed in high temperature Na2O-induced corrosion reactions (ref. 2 and fig. 4).

However dissolution/oxidation cannot account for the enlargement of cracks to the point of 0.1 mm pinholes.

It is proposed during re-entry, fresh surfaces SiC are continually exposed for attack by the Na2O-ZnO-SiO2 liquid.

The volatile species escaping from the crack (eg. CO(g), NaCl(g), Na2O(g)) may push out the SiO2-saturated

Na2O-ZnO-SiO2 liquid. This process may in fact be the eruptions observed (figure 9 and ref. 13) in the arc-jet testing. In addition, the thermal expansion mismatch between the SiC and the RCC material creates stresses which keep

the crack opening and exposing SiC surfaces. Then SiO2-depleted Na2O-ZnO-SiO2 liquid may attack these SiC

surfaces. This continual process of oxidation and dissolution (reactions 4 and 5), leads to effective crack enlargement into a pinhole.

RECOMMENDATIONS

It appears that the pinhole formation is a result of external corrosive elements. Most of the evidence points

toward the deleterious effects of Zn although NaCl may still play a role. The aging launch structure in the corrosive

marine environment is the problem here. Thus the obvious solutions are physical protection of the wings and improvement of the paint on the launch structure.

The current refurbishment procedure involves sanding off the glass/SiO2 outer layers on the protected RCC,

re-infiltrating with TEOS, decomposing the TEOS to SiO2, and sealing with Type A glass. This should be adequate

for near-term repairs as it will remove the Zn deposits. However, in the long term, pinholes will continue to form

and existing pinholes may grow and simply be filled with SiO2 as long as there is a source of Zn contamination.
NASA/TM—1998-208659 7

This will gradually increase the volume fraction of SiO2 in the SiC coating, eventually leading to an inadequate

coating. Kinetic data would be helpful in assessing how quickly these pinholes form and grow.

In terms of solutions, the removal of the Zn contamination is clearly a needed step—by improved coatings of

the launch structure. It is recommended that the quality of the SiC coating should be monitored and the entire RCC

material should be replaced periodically. Also development of more crack-resistant coatings (e.g., graded SiC) and

corrosion resistant sealants should be beneficial.

SUMMARY

In this report we explore a number of issues related to the observed pinholes in the wing leading edge of the

space shuttle. Experimentally this involves (1) Detailed microstructural examination of the pinholes from the shuttle.

(2) Mass spectrometric examination of the protected RCC material removed from the shuttle and (3) Laboratory

furnace studies to understand ZnO/protected RCC interactions. Most of the evidence points toward a critical role of

Zn in creating the pinholes. A mechanism is proposed based on the formation of a ZnO-Na2O-SiO2 ternary glass.

This glass runs into cracks and attacks the SiC walls. This effectively enlarges the uppermost portions of the cracks,

leading to the observed pinholes. The features of this mechanism are consistent with most of the observations

regarding pinholes.

REFERENCES

1. S.V. Christensen, “Reinforced Carbon/Carbon Pin Hole Formation through Zinc Oxide Attack,” Rockwell

International Internal Letter, RDW-96-057, May 1996.

2. N.S. Jacobson, “Corrosion of Silicon-Based Ceramics in Combustion Environments,” J. Am. Ceram. Soc., 76

[1], pp. 3–28, 1993.

3. E.M. Levin, C.R. Robbins, and H.F. McMurdie, Phase Diagrams for Ceramists, The American Ceramic Society,

Columbus, OH, 1964, p. 94.

4. M.I. Mayer and F L. Riley, “Sodium-Assisted Oxidation of Reaction Bonded Silicon Nitride,” J. Mater. Sci. 13,

pp. 1319–28 (1978).

5. Leading Edge Structural Subsystems (LESS) Entry Heating Study—Final Report, Volume—Methodology and

Results, Prepared by Lockheed Engineering and Sciences Company, Houston, TX, Contract NAS 9–17900,

LESC-30453, May 1993.

6. S.D. Williams, D.M. Curry, D. Chao, and V.T. Pham, “Analysis of the Shuttle Orbiter Reinforced CarbonCarbon Oxidation System, NASA TM–104792, June 1994.

7. S.D. Williams, D.M. Curry, D.C. Chao, and V.T. Pham, “Ablation Analysis of the Shuttle Orbiter Oxidation

Protected Reinforced Carbon-Carbon,” Paper AIAA 94–2084, presented at the 6th AIAA/ASME Joint

Thermophysics and Heat Transfer Conference, June 20-23, 1994, Colorado Springs, CO.

8. M. Gordon, private communication

9. D.W. Johnson, Final Report—RCC Pin-hole/Sealant Loss Invesitigation, Lockheed Martin 221RP10558,

September 1996.

10. N.S. Jacobson and R.A. Rapp, “Thermochemical Degradation Mechanisms for the Reinforced Carbon/Carbon

Panels on the Space Shuttle,” NASA TM–106793, January 1995.

11. J.A. Orench, “A Mechanism of Pinhole Formation in the Reinforced Carbon-Carbon Thermal Protection System

of the Spase Shuttle Orbiter,” MS Thesis, College of Engineering, University of Central Florida, 1996.

12. N.S. Jacobson, T.A. Leondhardt, D.M. Curry, and R. A. Rapp, “Oxidative Attack of Carbon/Carbon Substrates

through Coating Pinholes,” Carbon in press.

13. D.M. Curry, private communication.

14. C.A. Stearns, F.J. Kohl, G.C. Fryburg, and R.A. Miller, “A High Pressure Modulated Molecular Beam Mass

Spectrometric Sampling System,” NASA TM–73720, 1977.

15. Op. Cit. Levin, p. 120.

16. E.M. Levin and H.F. McMurdie, Phase Diagrams for Ceramists 1975 Supplement, The American Ceramic Society, Columbus, OH, 1975, pp. 206–207.
NASA/TM—1998-208659 8

TABLE I. LABORATORY SAMPLES AND EXPOSURE CONDITIONS.

Material Designation Deposit Gas

atmosphere

Temperature, oCPressure,

atm

Times,

hr

SiC SiC 4, 5 Air 1000 0.007 1

SiC/ZnO 2,3 ZnO Air 1000 0.007 1

CVD SiC 1, 2, 3 Air 1200 0.007 1

SiC/ZnO-12-1, 2, 3 ZnO Air 1200 0.007 1

CVD SiC A, B, C Air 1450 0.007 1

CVD/ZnO A, B, C ZnO Air 1450 0.007 1

Protected RCC PC-1 ZnO Air 1200 1 27x 0.5 hr cycles

C-1 ZnO

NaCl added

after four

cycles

Air 1200 1 4x 0.5 hr cycles

CC-ZnO-12-C-1 ZnO Air 1200 0.092 2x 0.5 hr cycles

CC-ZnO-1-2 ZnO Air 1000 0.092 2x 0.5 hr cycles

CC-ZnO-2-1 ZnO Air 1200 0.092 1x 0.5 hr cycles

Table II. XRD and chemical analysis results for ZnO + SiC study.

Sample XRD Results Weight of

products,

mg

Si,

mg

Zn ,

mg

SiC Oxidation at 1000oC

SiC 4 0.2 20

SiC 5 0 15

SiC + ZnO Oxidation at 1000oC

SiC/ZnO 2 SiC, ZnO, Zn2SiO4 18.3 40 15.74

SiC/ZnO 3 SiC, ZnO, Zn2SiO4 18.8 40 16.18

SiC Oxidation at 1200oC

SiC 1 SiC

SiC 2 0.1 55

SiC 3 0.2 27.5

SiC + ZnO Oxidation at 1200oC

SiC/ZnO-12-1 SiC, SiO2 , Zn2SiO4

SiC/ZnO-12-2 1.4 145 1.1

SiC/ZnO-12-3 2.3 130 1.96

SiC Oxidation at 1450oC

SiC A SiC, SiO2 (α-cristobalite)

SiC B 0.2 127.5

SiC C 0.3 155

SiC + ZnO Oxidation at 1450oC

SiC/ZnO A SiC, SiO2 (α-cristobalite)

SiC/ZnO B 0.4 265 0.5

SiC/ZnO C 0.2 247.5 0.25
NASA/TM—1998-208659 9

Figure 1.—Schematic of shuttle showing RCC panel location.

RCC Seal strip

Wing L.E. RCC 

panels

Nose cap

Figure 2.—Diagram of material.

Short SiC

fibers, 

particles

Carbon/carbon - 2 dimensional lay-up

Cracks

SiC

Sodium 

silicate 

glass

~50 μm

~0.5 mm
NASA/TM—1998-208659 10

Figure 3.—Cross section of material, BSE image and associated oxygen WDS map, illustrating a crack formed 

 as-fabricated (filled with SiO2) and in-service (not filled).

(a)

CP Level

(b)

(c) (d)

50 μm O 50 μm

50 μm Na Si 50 μm

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Si Level

1013

950

886

823

760

696

633

570

507

443

380

317

253

190

127

63

0

O Level

151

142

132

123

113

104

94

85

76

66

57

47

38

28

19

9

0

Na Level

30

20

27

25

23

21

19

17

15

13

11

10

8

6

4

2

0
NASA/TM—1998-208659 11

Figure 4.—Diagram of (a) SiC oxidation process and

 (b) thermochemically stable Na2O·x (SiO2) coating

 on SiC.

SiC SiO2

(a)

(b)

O2

CO, CO2

SiC SiO2

Na2O·x (SiO2) liquid

SiO2-saturated liquid
NASA/TM—1998-208659 12

Figure 5.—Na2O·SiO2 binary phase diagram (ref. 3).

Liquid

Cristobalite

+ Liquid

Tridymite

+ Liquid

1470°

707°

678°

867°

Quartz 

+ Liquid

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

30 40 50 60 70 80 90 100

Na2O

Na2O

+ Liquid

2Na2O·SiO2

+ Liquid Na2O·SiO2

+ Liquid

Na2O·2SiO2

 + Liquid

Na2O·2SiO2 + Na2O·SiO2

2Na2O·SiO2 + Na2O·2SiO2

Mol % SiO2

I

II

III

2Na2O·SiO2

Na2O·SiO2

Na2O-2SiO2

T, °C
NASA/TM—1998-208659 13

Figure 6.—(a) Pinhole from OV-102, Wing Leading Edge Panel 12 RH, 15 flights. 

 Cross-sectional view is a BSE micrograph.
NASA/TM—1998-208659 14

Figure 6.—Continued. (b) Pinhole from OV-102, Wing Leading Edge Panel 12 RH, 

 15 flights. Cross-sectional view is an optical micrograph.

0.16 mm
NASA/TM—1998-208659 15

Figure 6.—Continued. (c) from OV-102, Wing Leading Edge Panel 10 LH, 19 

 flights. The cross-sectional view is a BSE micrograph.
NASA/TM—1998-208659 16

Figure 6.—Continuing. (d) from OV-102, Wing Leading Edge Panel 10 LH, 

 19 flights. The cross-sectional view is a BSE micrograph.
NASA/TM—1998-208659 17

Figure 6.—Continued. (e) from OV-102, Wing Leading Edge Panel 10 LH, 

 19 flights. The cross-sectional view is an optical micrograph.

0.16 mm
NASA/TM—1998-208659 18

Figure 6.—Continued. (f) from OV-102, Wing Leading Edge Panel 10 LH, 

 19 flights. The cross-sectional view is a BSE micrograph.
NASA/TM—1998-208659 19

Figure 6.—Concluded. (g) Figures from OV-102, Wing Leading Edge Panel 10 LH,

 19 flights. The cross-sectional view is a BSE micrograph.
NASA/TM—1998-208659 20

Figure 7.—Schematic of proposed pinhole formation mechanisms which

 are intrinsic to the material.

C/C

SiO2

SiC

SiC + 2SiO2 = 3 SiO (g) + CO (g)

SiC + O2 (g) = SiO (g) + CO (g)

SiO (g)

CO (g)

(b)

C/C SiC

Na2O - x (SiO2)

Na2O·x (SiO2)

Vaporize C/C SiC

(a)

C/C

SiC SiC + 1/2 O2 (g) = SiO2

SiO (g)

(c)
NASA/TM—1998-208659 21

Figure 8.—Schematic of the proposed NaCl pinhole

 formation mechanism (ref 10).

SiC

NaCl

C/C

(a)

O2

N2

SiO

NaCl(v)

Na Cl2 (v)

SiO2

SiCl2 (v)

SiO(v)

C/C SiC

(b)

SiC

N2(g)

SiO2

O2

N2

SiO2

SiO2

SiCl2 (v)

Cl(g)

C/C C

(c)
NASA/TM—1998-208659 22

Figure 9.—Photograph (video captured) of orange-red gas emerging from pinholes in an arc-jet test.
NASA/TM—1998-208659 23

Figure 10.—Schematic of molecular beam mass spectrometer.

RCC sample
NASA/TM—1998-208659 24

Figure 11.—(a) High magnification of photo of glass/SiC interface in a pinhole (full view is in Figure 6(a)) and 

 associated EDS traces with regions (b) and (c). Note that Zn only extents to bottom of enlarged portion 

 of crack.

C

O

Zn,

Na

Zn,

Na

Zn,

Na

Si

Si

C

O

Zn Zn

Zn Zn

C

O

Zn Zn (c)

(a)

(c)

(b)

(c)

(b)

(b)

Si
NASA/TM—1998-208659 25

C

O

Zn, Na

Si

C

O

Zn, Na

Si

Figure 12.—High magnification photo of the glass/SiC interface in a pinhole (Figure 6(f)), showing composition

 at various points within the pinhole.

C

O

Zn, Na

Si

C

O

Zn, Na

Si
NASA/TM—1998-208659 26

Figure 13.—WDS of the pinhole glass, indicating it is rich in both Na and Zn.

 This resolves the signal EDS peak in Figure 11 & 12.

500

400

300

200

100

0

Na La

11.90

Counts

11.95 12.00 12.05 12.10

eV

12.15 12.20 12.25 12.30 12.35

Zn La

Figure 14.—Mass spectrometer trace showing gases coming off 

 protected RCC removed from the shuttle. The trace indicates

 the presence of both NaCl(g) and Zn(g). 54 58 60 81 83 84

85

65 

Zn+

NaCl+

 Na2Cl+

Intensity

Mass to charge ratio
NASA/TM—1998-208659 27

Figure 15.—CVD SiC with ZnO, 1 hr in 0.05 atm air. (a) Surface view showin

 ZnO and Zn2SiO4 on surface after 1000 °C treatment. (b) Surface product

 in 14(a) removed by treatment in aqueous HF solution to reveal limited 

 surface attack after 1200 °C treatment.

(a)

(b)

(a)

(b)
NASA/TM—1998-208659 28

Figure 16.—ZnO-SiO2 binary phase diagram (ref. 15).

Liquid B

Cristobalite

+ Liquid B

Tridymite + Liquid B

Zn2SiO4

+ Liquid B

ZnO

+ Liquid B

Cristobalite

+ Liquid A

Liquid B

+ Liquid A

Tridymite

+ Zn2SiO4

1695°

1470°

1432°

1507°

1078°

1512°

1700

1600

1500

1400

1300 0 40 60 80 100 20

SiO2 Mol, % ZnO

ZnO+Zn2SiO4

T, °C
NASA/TM—1998-208659 29

Figure 17.—Protected RCC material and ZnO (a) Before reaction. (b) After 1 cycle

 in 0.092 atm air for 0.5 hr at 1200 °C.

(a)

(b)
NASA/TM—1998-208659 30

Figure 17.—Concluded. (c) After 4 cycles in 1 atm air. (d) After 27 cycles in 1 atm air.

 These cycles are 0.5 hr at 1200 °C.

(c)

(d)
NASA/TM—1998-208659 31

Figure 18.—Protected RCC material reacted with ZnO for 27 0.5 hr, cycles at 1200 °C in 1 atm air, region showing

 ZnO-Na2O-SiO2 glass adjacent to SiC. EPMA results—BSE image and associated elemental maps.

(a) (b)

(c) (d)

20 μm C 20 μm

Cl 20 20 μm Na μm

CP Level

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Cl Level

12

11

11

10

9

8

8

7

6

5

5

4

3

2

2

1

0

C Level

887

827

772

717

662

606

551

496

441

386

331

276

221

165

110

55

0

Na Level

37

35

32

30

28

25

23

21

19

16

14

12

9

7

5

2

0
NASA/TM—1998-208659 32

Figure 18.—Concluded.

(e) (f)

(g) (h)

20 μm O 20 μm

Si 20 20 μm Zn μm

CP Level

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Si Level

967

907

846

786

725

665

604

544

404

423

363

302

242

181

121

60

0

O Level

130

129

121

112

104

95

86

78

69

60

52

43

35

26

17

9

0

Zn Level

30

28

26

24

23

21

19

17

15

13

11

9

8

6

4

2

0
NASA/TM—1998-208659 33

Figure 19.—Protected RCC material reacted with ZnO for 27 0.5 hr, cycles at 1200 °C

 in 1 atm air, region showing ZnO-Na2O-SiO2 glass adjacent to SiC. High magnification

 photograph showing glass/SiC interface—note intermediate layer of SiO2 crystallites.

ZnO - Na2O - SiO2 glass

SiO2

SiC

5 μm
NASA/TM—1998-208659 34

Figure 20.—Protected RCC material reacted with ZnO for 27 0.5 hr, cycles at 1200 °C in 1 atm air, region showing

 ZnO-Na2O-SiO2 glass adjacent to a portion of the SiC layer, rich in SiO2. EPMA results—BSE image and 

 associated elemental maps.

(a) (b)

(c) (d)

10 μm C 10 μm

Cl 10 10 μm Na μm

CP Level

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Cl Level

12

11

11

10

9

8

8

7

6

5

5

4

3

2

2

1

0

C Level

1020

956

893

829

765

701

638

574

510

446

383

319

255

191

120

64

0

Na Level

31

29

27

25

23

21

19

17

16

14

12

10

8

6

4

2

0
NASA/TM—1998-208659 35

Figure 20.—Concluded. 

(e) (f)

(g) (h)

10 μm O 10 μm

Si 10 10 μm Zn μm

CP Level

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Si Level

1009

946

883

820

757

694

631

568

505

441

378

315

252

189

126

63

0

O Level

144

135

126

117

108

99

90

81

72

63

54

45

36

27

18

9

0

Zn Level

28

26

25

23

21

19

18

16

14

12

11

9

7

5

4

2

0
NASA/TM—1998-208659 36

(a) (b)

(c) (d)

50 μm C 50 μm

Cl 50 50 μm Na μm

CP Level

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Si Level

77

73

68

63

58

53

48

44

39

34

29

24

19

15

10

5

0

C Level

935

877

818

760

701

643

584

526

468

409

351

292

234

175

117

58

0

Na Level

54

51

47

44

41

37

34

30

27

24

20

17

14

10

7

3

0

Figure 21.—Protected RCC material reacted with ZnO for five 0.5 hr, cycles at 1200 °C in 1 atm air, region 

 showing ZnO-Na2O-SiO2 glass adjacent to SiC. After the four cycles a film of NaCl was sprayed on this 

 sample and the sample was cycled one more time. EPMA results—BSE image and associated elemental 

 maps. Note NaCl in primarily on top but some appears to have penetrated the crack.
NASA/TM—1998-208659 37

Figure 21.—Concluded.

(e) (f)

(g) (h)

50 μm O 50 μm

Si 50 50 μm Zn μm

CP Level

2040

1913

1785

1658

1530

1403

1225

1148

1020

893

765

638

510

383

255

128

0

Si Level

1012

949

886

822

759

696

633

569

506

443

380

316

253

190

127

63

0

O Level

935

877

818

760

701

643

584

526

468

409

351

292

234

175

117

58

0

Zn Level

11

10

10

9

8

8

7

6

6

5

4

3

3

2

1

1

0
NASA/TM—1998-208659 38

Figure 22.—ZnO-Na2O-SiO2 Ternary Phase Diagram (ref. 16).
NASA/TM—1998-208659 39

Figure 23.—Schematic of proposed pinhole formation mechanism, based on ZnO-Na2O-SiO2 crack enlargment.

 Steps (c)-(e) repeat, enlarging crack.

SiC

(b) ZnO reacts with Na2O·x (SiO2).

C/C

Na2O·x (SiO2)

ZnO

(a) ZnO deposits in crack.

SiC

C/C

Na2O·y (ZnO)·x (SiO2)

CO (g)

(c) Na2O·y (ZnO)·x (SiO2) dissolves SiO2, thereby

 attacking SiC.

(d) SiO2 rich Na2O·y (ZnO)·(x+1) SiO2 is pushed 

 out by escaping gases, exposing fresh SiC.

SiC

C/C

Na2O· 

y(ZnO)(x+1)·(SiO2)

CO (g)

C/C

(f) Crack is gradualy enlarged. Final microstructure 

 is shown below.

(e) SiO2, depleted Na2O·y (ZnO)·x (SiO2) flows 

 into crack, attacking SiC.

SiO2 Crystallites

C/C

SiC

Na2O·y (ZnO)·(x + 1) SiO2

CO (g)
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Unclassified Unclassified

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National Aeronautics and Space Administration

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National Aeronautics and Space Administration

Washington, DC 20546–0001

November 1998

NASA TM—1998-208659

E–11351

WU–260–10–0G–00

45

A03

Space Shuttle Pinhole Formation Mechanism Studies

Nathan S. Jacobson

Carbon/carbon; Shuttle; Corrosion

Unclassified -Unlimited

Subject Category: 27 Distribution: Nonstandard

Responsible person, Nathan S. Jacobson, organization code 5160, (216) 433–5498.

Pinholes have been observed to form on the wing leading edge of the space shuttle after about 10–15 flights. In this report

we expand upon previous observations by Christensen (1) that these pinholes often form along cracks and are associated

with a locally zinc–rich area. The zinc appears to come from weathering and peeling paint on the launch structure. Three

types of experimental examinations are performed to understand this issue further: (A) Detailed microstructural examination of actual shuttle pinholes (B) Mass spectrometric studies of coupons containing actual shuttle pinholes and (C)

Laboratory furnace studies of ZnO/SiC reactions and ZnO/SiC protected carbon/carbon reaction. On basis of these

observations we present a detailed mechanism of pinhole formation due to formation of a corrosive ZnO–Na2O–SiO2

ternary glass, which flows into existing cracks and enlarges them.