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Multilayer ceramic coatings appear tooffer the best oxidation protection forcarbon-carbon composites that make upthe structure of future hypersonic spacevehicles.

Erica L. Corral*University of ArizonaTucson, Arizona

Ultra high temperature ceramics (UHTCs)have been identified as a class of ma-terials with the potential to withstand ex-treme aerothermal heating environments.

For example, diboride and carbide-based ceramicspossess melting temperatures in excess of 3000C.Table 1 lists a range of candidate high-temperatureceramics that have decomposition or melting tem-peratures above 1600C.

Current work at the University of Arizona in col-laboration with Sandia National Laboratories fo-cuses on adherent, continuous coatings of SiC andZrB2 for enhanced oxidation protection of carbon-carbon composites. ZrB2 was chosen based on itshigh melting point, and SiC can provide oxidationprotection at lower temperatures. The combinationof both materials would synergistically provide ox-idation protection over a wide range of tempera-tures. Based on previous research, no single coatingmaterial appears to be sufficiently protective. There-fore, multilayer ceramic coatings engineered to pro-vide necessary oxidation resistance for C-C com-posites and high temperature oxidation testing will

be the focus of this article.*Member of ASM International

Hypersonic flight vehiclesAdvanced thermal protection systems are needed

to mitigate effects of aerothermal heating that other-wise limit the performance of hypersonic flight ve-hicles. Avariety of mission profiles are being pro-posed with different thermal loads and temperatures.For example, a single use, unmanned vehicle that hasa flight profile lasting only minutes can reach a max-imum temperature of ~ 2800C. Figure 1 shows hy-personic vehicle designs that are being researchedfor NASAand the United States Air Force.

However, hypersonic vehicles require coatingsthat will protect structures against temperatures inexcess of 2800C, enabling them to maintain tensilestrength and oxidation resistance. Carbon-carboncomposites provide high strength and low density,

but they oxidize in air at temperatures >500C, andneed thermal protection systems (TPS) to surviveaerothermal heating.

Oxidation protectionApplying UHTC coatings to carbon-carbon com-

posites for oxidation protection is a viable approach

to TPS design if the coating can be engineered tohave the following qualities:

30 ADVANCED MATERIALS & PROCESSES/OCTOBER 2008

Table 1 Thermal and physical properties of UHTCs and refractory ceramicsDecomposition and melting Oxidation products

Ceramic material temperature Td and Tm, C (F) Density, g/cm3 in air

SiC Td=1600 (2910)Tm=2730 (4950) 3.22 SiO2 + CO2

ZrB2 Tm=3200 (5790) 6.08 ZrO2 + B2O3

HfB2 Tm=3250 (5880) 10.50 HfO + B2O3

HfC Tm=3890 (7030) 12.22 HfO + CO2

ZrC Tm=3530 (6390) 6.73 ZrO2 + CO2

Fig. 1 Illustrations of re-entry and hypersonic test vehiclesresearched at NASA and USAF. Images are from NASADryden Flight Research Center and Air Force Office ofScientific Research.

ULTRA-HIGH TEMPERATURE

CERAMIC

COATINGS

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tion of ceramic coatings by controlling the physicaland chemical properties that affect coating infiltra-tion and adhesion. Heat treatments up to 1600Cfor four hours under vacuum are usually required toaffect fiber size and crystallinity, and to influenceadhesion properties.

Evaluating oxidationLow cost, reliable testing methods for evaluating

oxidation-protection materials are needed to ad-vance TPS materials development and the under-standing of coating materials. The multilayer coat-ings of ZrB2 and SiCr were tested at the National

Solar Thermal Testing Facility at Sandia NationalLaboratories, Albuquerque, N.M.

Coatings were tested for thermal shock resistanceat high temperatures and severe heat-flux conditionsin Sandias Solar Furnace. The furnace can accuratelycontrol a variable heat-flux up to 800 W/cm2 whilecontinuously monitoring the specimen surface tem-perature up to 2600C. This test does not include allthe hypersonic environmental conditions, which can

be achieved only in an actual test flight. However,these test conditions are suitable for studying oxida-tion resistance at extreme temperature and heat-fluxconditions, and are meant to serve as a screening test

prior to evaluation at more complex and expensivefacilities. Alist of ground-based testing methods isshown in Table 2 for comparison.

Analysis of multilayer coatingsSolar Furnace testing directly duplicates the se-

vere high heat-flux (680 W/cm2) and high-temper-ature (>2600C) conditions for short periods of time(15 seconds), similar to a hypersonic flight profile.

Figures 4 (a) and (b) show SEM micrographs rep-resentative of solar-tested surfaces of C-C compos-ites coated with SiC/ZrB2 that provided oxidationprotection and those that did not, respectively.Figure 4 (a) shows a C-C composite surface that is

coated with SiC and decorated with islands thatcontain boron and are presumed to be B2O3 parti-

cles. Spot elemental analysis of these small particlesshowed they contain boron, and that the sur-rounding area is covered with silicon.

X-ray diffraction analysis on the tested surface ofthe specimen also revealed intense graphite reflec-tions, as well as peaks for SiC, B2O3, and SiO2, indi-cating that the multilayer coating protected the C-C composite from oxidation. Figure 2 (b) shows thecross-section of the tested specimen that survivedsolar testing, and shows a significant amount of SiCwithin the C-C composite. It also shows a SiC-rich

band of material that formed after high-tempera-ture testing just one mm below the surface. No

boron or zirconium-rich particles were detected inthe cross-section. It is most likely that the ZrB2 ma-terial decomposed to B2O3 and ZrO2 at the surface,where the ZrB2 was exposed to the highest temper-ature.

Figure 4 (c) shows a C-C composite surface thatdid not provide oxidation protection. It shows ex-posed weaves of oxidized C-C fibers with agglom-erated SiC particles embedded between the layers.The coating composition was most likely not opti-mized in the specimen. The failed coating resultedfrom processing variations that did not produce acontinuous and protective coating. Spot elemental

analysis of the particles confirmed that they containsilicon.X-ray diffraction analysis also showed very low

intensity peaks for graphite due to the oxidation ofthe C-C, and low intensities for the SiC peaks dueto the failure and depletion of the coating duringsolar testing.

Figure 4 (d) shows the cross-section of the samespecimen that did not provide oxidation protection.The top region shows a thick layer of oxidized C-Cmaterial and residual SiC further from the surfaceof the C-C composite.

These results clearly show that multilayer coat-ings with SiC and ZrB2 provide oxidation protec-

tion for C-C composites under severe heat-flux andhigh-temperature conditions.

For moreinformation:

Erica L. Corral,University of

Arizona,Department of

Materials Science& Engineering,

1235 James E.Rogers Way,Tucson, AZ

85721;

elcorral@email.arizona.edu;www.arizona.

edu.

32 ADVANCED MATERIALS & PROCESSES/OCTOBER 2008

Fig. 4 SEM micrographs of solar tested SiC/ZrB2 coated specimens show (a) protective and (c) non-protective coating on the surface. Incross-section (b), the protective coating retains the initial shape of the carbon-carbon composite. Part (d) shows that when it is notoxidative-protective and it fails, a large section of the composite near the surface is oxidized.

a b c d

Table 2 Ground based testing facilities comparisonMaximum

temperature, Maximum OxygenTesting method Heating mode C (F) heat flux mass flux Atomic oxygen Shear flow

High-temp furnace Radiative & convective 2000 (3630) n/a Poor Poor n/a

Oxyacetylene torch Convective >2000 (3630) >150 W cm-2 Poor Poor n/a

Solar Radiative >2600 (4710) 250-800 W cm-2 Fair Poor n/a

Laser subsonic tunnel Radiative w/convective cooling 2000 (3630) >250 W cm-2 Good Poor Fair

Arcjet Convective w/ some radiative >2200 (3990) >250 W cm-2 Excellent Excellent Excellent