Materials Science and Engineering A 420 (2006) 150–154

Sol–gel alumina environmental barrier coatings for SiC grit

H. Kim, M. Chen, Q. Yang, T. Troczynski ∗UBCeram at Materials Engineering Department, University of British Columbia, 309-6350 Stores Road, Vancouver, BC, Canada V6T 1Z4

Received 26 July 2005; received in revised form 4 January 2006; accepted 17 January 2006

Abstract

SiO2 surface film is insufficient to protect SiC from the oxidation at widely varying partial pressures of oxygen, in particular in the presence ofwater vapor (e.g. in gas turbines) and also in other environments, e.g. during brazing for hard “tipping” of turbine blades. This work demonstratesthat sol–gel alumina, coated on ∼0.5 mm coarse SiC grit, may form an acceptable, up to ∼10 �m thick “environmental barrier coating” EBC forsome of these applications. The sol–gel has advantages over other methods (such as CVD) is the simplicity and low cost. We have used NH4OHpre-treatment to hydroxylate surface of SiC prior to applying alumina coating. Such modified SiC/SiO2 surface helped to deposit the positivelycharged alumina sol, and thus allowed to build thick coatings on the SiC grit. There is some indication that these coatings partially convert tomullite through reaction at the interface with the native silica on SiC. Oxidation resistance tests at 1200 ◦C were performed to show effectivenessof such coated SiC grit.©

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eywords: SiC; Sol–gel; EBC; Alumina; Oxidation resistance

. Introduction

SiC is one of the best high-temperature structural ceramicsecause of its high thermal shock resistance due to the highhermal conductivity and low thermal expansion coefficient [1].owever, the use of the SiC in high temperature application

uch as hot section of gas turbine engines has been limited duets corrosion. In this environment, silica scale (SiO2) on the sur-ace of SiC reacts with water vapor in a gas turbine, and volatileilicon hydroxide (Si(OH)4) forms [2–5], “opening the door”or further corrosion. “Environmental barrier coatings (EBC)”2–7] were proposed as a possible solution. One of the firstBC of mullite/zirconia has failed due to the large differencef thermal expansion coefficients (zirconia: 10.5 × 10−6 ◦C−1,iC: 4.3 × 10−6 ◦C−1) [6,8]. The most successful EBC usesi/mullite/BSAS ((1 − x)BaO·xSrO·Al2O3·2SiO2) layer on SiC2]. However, controlling the glassy phase due to reactionetween silica and BSAS is required to avoid spallation of theoating from SiC substrate. Many other methods have beenroposed to deposit EBCs, such as thermal spray [6], metal infil-ration (MI) [2,3], CVD/CVI [3,4], and sol–gel [5,7].

The sol–gel method is the economical way to developthe desired performance of EBC system, although the coat-ings reported so far were relatively thin, e.g. <1 �m. Stud-ies of surface/interface phenomena of SiC in aqueous systemsadvanced our understanding of this system [11–15]. Zeta poten-tial [10,12,15] and the viscosity of SiC water suspensions havebeen studied in the past [13,14]. SiC suspension was found stableat pH 4. It has been also shown [9,10] that, at 3.5 < pH < 5.5, SiCsurface in water is negatively charged and boehmite sol clustersare positively charged, thus the coating forms simply by elec-trostatic interaction of the two. Yang et al. [13,14] investigatedthe effect of pH on the boehmite coating on the SiC and gela-tion, consolidation, and rheology of such aqueous suspensions.Adding acid to deagglomerate alumina sol in the SiC suspensionlead to homogeneous boehmite coating on SiC at pH 4.

Surface condition of the surface silica significantly affectsthe coating process. For example, in this work we have appliedsurface treatment with NH4OH in order to generate substantialhydroxylation of the “natural” silica coating on SiC, thus modi-fying the deposition of alumina sol layer. The following reactionis expected [16]:

∗ Corresponding author. Tel.: +1 604 822 2612; fax: +1 604 822 3619.E-mail address: troczyns@interchange.ubc.ca (T. Troczynski).

SiO2 + 2NH4OH → Si(OH)4 + 2NH3(g)

It is believed that attraction of positively charged boehmitesol clusters to the negatively charged hydroxylated SiO2/SiC

921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.01.087

H. Kim et al. / Materials Science and Engineering A 420 (2006) 150–154 151

surface is the key in formation of uniform thick films on com-plex surfaces, such as SiC grit shape. During heat treatmentof such films, boehmite decomposes to alumina which par-tially reacts with the silica present on the surface of the SiC,forming mullite at the interface. The strongly bonded mulliteinterface plays a significant role in maintaining the integrity ofthe alumina/mullite/silica/SiC system during thermal cycling,thus forming a good EBC. We demonstrate that such modifiedSG technology, including the alkaline surface treatment of SiC,is able to provide ∼5–10 �m thick EBCs on SiC.

2. Experimental procedures

Two-molar alumina sol in distilled water (pH 4.0) was pre-pared using boehmite (AlOOH < 20 nm, SOL-2PK, Condea,Germany). SiC particles (500 �m large SiC grit, Micro Abra-sives Corp., USA) were pre-oxidized at 1100 ◦C for 10 h inair to form the silica scale <0.5 �m thick (as estimated fromthe typical ∼0.2% weight gain during the treatment). The pre-

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oxidized SiC grit was stirred for 3 h at room temperature inthree-molar NH4OH solution, followed by a 65 ◦C oven dry, andthen exposed to alumina sol for 1 min through dipping (referredto as “dip-coated” SiC) or 2 h stirred in the sol (“stirred” SiC)at room temperature. For comparison, the pre-oxidized SiC gritwas also coated with alumina sol without the NH4OH treatment,following the same coating procedure. Such coated SiC parti-cles were centrifuged to remove excess of alumina sol, driedat 65 ◦C and heat treated at 1450 ◦C for 2 h in air. SEM imag-ing and Energy Dispersive X-Ray Spectroscopy (EDX) analysis(Hitachi S-3000N, Japan) as well as X-ray diffraction (Multi-flex, Rigaku Co., Japan) have been performed to characterizethe coatings. Simple oxidation tests (i.e. through monitoringweight gain during isothermal hold at 1200 ◦C for up to 72 h)and thermal cycling tests, including furnace-heating to 1200 ◦C,isothermal holding at 1200 ◦C for 24 h in air then cooling to

ig. 1. Comparison of the surfaces morphology between the (a) as-received andb) pre-oxidized and NH4OH-treated SiC grit.

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ig. 2. Line scan of the pre-oxidized SiC grit at 1100 ◦C for 10 h. The presencef oxygen is indicative of formation of silica scale on the surface of SiC.

152 H. Kim et al. / Materials Science and Engineering A 420 (2006) 150–154

room temperature per cycle, for up to 8 cycles, have beenperformed.

3. Results

Fig. 1 compares the morphology of the as-received and pre-oxidized SiC after NH4OH treatment. The as-received SiC,Fig. 1a, has smooth surface with the distinctive cleavage stepsresulting from the manufacturing (crushing) process. In Fig. 1b,micro-rough surface is observed due to formation of silica scaleon SiC by oxidation at 1100 ◦C for 10 h. The formation of thesilica scale was verified by line scan EDX (Fig. 2). NH4OHtreatment of such pre-oxidized SiC grit was to induce the nega-tively charged surface by formation of –OH hydroxyl bonds andthus fast gelation and uniform coating of the positively chargedboehmite sol. Somewhat similar surface treatment has been usedpreviously; however a molten Na2CO3 [2] has been utilized andthe main purpose was to introduce roughness to SiC surface.The main problem with using sodium-including agents is that

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Na reacts with silica and forms low-melting point glass phase(sodium silicate) at the interface.

Figs. 3a and 4a represent stir-coated SiC and Figs. 3b and 4bdip-coated SiC, in terms of surface morphology (Fig. 3) and thecross-section (Fig. 4). The coating morphology is similar for thetwo different processing methods. However, the cross-sectionsshow that the dip-coating provides more uniform and thickercoating than the stirred one, although the sharp edges of SiC gritreceived thinner coating in both methods.

Fig. 5 suggests that the alumina-coated layer is possiblycomposed of silica and mullite (moving away from the surfaceof SiC) due to the simultaneous presence of Si, O, and Al insome sections of the coated layer. X-ray diffraction data, Fig. 6,seem to confirm this hypothesis, as silica and mullite (i.e. thetwo strongest peaks for mullite at 2θ = 26.3◦ and 26.0◦) wereobserved (in addition to majority SiC), while no alumina peakswere found. However, the intensity of all the peaks of silicaand mullite were very low due to the low fraction (estimatedat ∼3.7 vol.%) of the coated layer versus the total volume ofmaterial.

ig. 3. SEM images of (a) stir and (b) dip-alumina-coated SiC grit, sinteredt 1450 ◦C for 2 h. In both cases, SiC particles are uniformly coated withoutpallation.

Fig. 4. Cross-sections of (a) stir and (b) dip-coated SiC.

H. Kim et al. / Materials Science and Engineering A 420 (2006) 150–154 153

Fig. 5. Line scans of the cross-section of the dip-alumina-coated SiC grit.

Fig. 6. Comparison of X-ray diffraction spectra for (a) alumina-coated and (b)as-received SiC. Peaks between 20◦ and 30◦ are magnified show the mullitepeaks at 26.0◦ and 26.3◦.

Fig. 7. Weight gain due to oxidation for as-received and coated SiC at 1200 ◦C.

Oxidation of the coated SiC grit has been investigated at1200 ◦C in air, for 24–72 h. Weight gain of the coated SiCgrit with/without NH4OH treatment and by stirring or dipping(and as-received SiC particles) after exposure to air at 1200 ◦C

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ig. 8. Comparison of the surface morphology of the dip-alumina-coated SiCrit between (a) without and (b) with NH4OH pre-treatment, after 8 thermalycles.

154 H. Kim et al. / Materials Science and Engineering A 420 (2006) 150–154

has been measured. About four times lower weight gain wasobserved for the coated SiC as compared to the as-received SiC,Fig. 7, with no significant difference in between the coatingtechniques.

Fig. 8 illustrates the comparison of the coating morphologyof the SiC grit (a) without and (b) with NH4OH treatment aftercompletion of 8 thermal cycles (24 h at 1200 ◦C followed by aircooling). Some cracks were found in both NH4OH treated andnon-treated SiC. Spallation and recession of surface by oxidationof SiC coated without NH4OH treatment is however seen clearlyin Fig. 8a, whereas the NH4OH treated and coated SiC doesnot seem to show significant coating deterioration, Fig. 8b. SiCcoated after NH4OH treatment shows only few cracks rather thanspallation or boundary separation, possibly due to the strongerbonding of the coating on SiC surface through the mullite inter-layer.

4. Summary

We have introduced novel sol–gel based processing techniquefor environmental barrier coatings for non-oxide ceramics. Thiswork demonstrates that coarse and irregular SiC grit (∼0.5 mmparticle size) can be successfully coated by relatively thick (up to∼10 �m) and uniform coatings via sol–gel method. In order toachieve such thick coatings, SiC surface pre-oxidation followedby hydroxylation treatment using NH OH. Such hydroxylatedsswaslTat

SiC. Oxidation film morphology, after thermal cycling, indicatesthat the coatings on the hydroxylated surface substantially avoidspallation. We conclude that the proposed process could leadto economical method for deposition of environmental barriercoatings on non-oxide ceramics.

Acknowledgements

The authors would like to thank to Sulzer Ltd. and the Nat-ural Sciences and Engineering Research Council of Canada(NSERC), for supporting the project.

References

[1] S.R. Levine, T.P. Herbell, Engineered Metals Handbook, vol. 4, ASM,1991, pp. 1003–1006.

[2] K. Lee, D.S. Fox, J.I. Eldridge, D. Zhu, R.C. Robinson, N.P. Bansal,R.A. Miller, J. Am. Ceram. Soc. 86 (8) (2003) 1299–1306.

[3] P.F. Tortorelli, K.L. More, J. Am. Ceram. Soc. 86 (8) (2003) 1249–1255.[4] K.L. More, P.F. Tortorelli, L.R. Walker, N. Miriyala, J.R. Price, M.V.

Roode, J. Am. Ceram. Soc. 86 (8) (2003) 1272–1281.[5] E.J. Opila, J. Am. Ceram. Soc. 86 (8) (2003) 1238–1248.[6] H.E. Eaton, G.D. Linsey, J. Eur. Ceram. Soc. 22 (2002) 2741–2747.[7] S. Li, Z. Lu, J. Gao, Z. Jin, Mater. Chem. Phys. 78 (2003) 655–659.[8] S. Parola, M. Verdenelli, C. Sigala, J.P. Scharff, K. Velez, C. Veytizou,

J.F. Quinson, J. Sol–Gel Sci. Tech. 26 (2003) 803–806.[9] Q. Yang, PhD Thesis, Department of Materials Engineering, The Uni-

versity of British Columbia, 1999.[[[

[[

[

[

4urface, exposed to positively charged boehmite (in pH 4 wateruspension) appears to build several microns thick sol film,hich is subsequently converted to a dense film after sintering

t 1450 ◦C for 2 h. It appears that during this process aluminaol reacts with the silica scale on SiC grits to form mullite ateast in some section of the coating adjacent to the silica scale.he coating may therefore consist of two oxide films of silicand mullite. In 1200 ◦C oxidation test, the coated SiC had fourimes better oxidation resistance as compared to the as received

10] Q. Yang, T. Troczynski, J. Am. Ceram. Soc. 82 (7) (1999) 1928–1930.11] Q. Yang, T. Troczynski, J. Am. Ceram. Soc. 83 (4) (2000) 958–960.12] Q. Huang, P. Chen, M. Gu, Y. Jin, K. Sun, Mater. Lett. 56 (2002)

546–553.13] C. Yang, W. Shih, J. Am. Ceram. Soc. 82 (2) (1999) 436–440.14] C. Yang, W.Y. Shih, W.H. Shih, J. Am. Ceram. Soc. 83 (8) (2000)

1879–1884.15] K. Prabhakaran, J. James, C. Pavithran, J. Eur. Ceram. Soc. 23 (2003)

379–385.16] F.J. Pearson Jr., Nagra themochemical data base. Wettingen, Switzerland,

1991.