Thin Solid Films 519 (2011) 6969–6973

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Characterization of ceramic plasma-sprayed coatings, and interaction studiesbetween U–Zr fuel and ceramic coated interface at an elevated temperature

Ki Hwan Kim a,⁎, Chong Tak Lee a, Chan Bock Lee a, R.S. Fielding b, J.R. Kennedy b

a Recycled Fuel Technology Development Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Republic of Koreab Nuclear Fuel and Materials Division, Idaho National Laboratory, P.O. BOX 1625, Idaho Falls, ID 83415, USA

⁎ Corresponding author. Tel.: +82 42 868 2308; fax:E-mail address: khkim2@kaeri.re.kr (K.H. Kim).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.01.209

a b s t r a c t

a r t i c l e i n f o

Available online 22 January 2011

Keywords:Metallic fuelCeramic cruciblePlasma-sprayed coatingThermal cyclingInteraction

Candidate coating materials for re-usable metallic nuclear fuel crucibles, HfN, TiC, ZrC, and Y2O3, were plasma-sprayed onto niobium substrates. The coating microstructure and the thermal cycling behavior werecharacterized, and U–Zr melt interaction studies carried out. The Y2O3 coating layer had a uniform thicknessand was well consolidated with a few small pores scattered throughout. While the HfN coating was not wellconsolidated with a considerable amount of porosity, but showed somewhat uniform thickness. Thermalcycling tests on the HfN, TiC, ZrC, and Y2O3 coatings showed good cycling characteristics with nointerconnected cracks forming even after 20 cycles. Interaction studies done on the coated samples by dippinginto a U–20wt.%Zr melt indicated that HfN and Y2O3 did not form significant reaction layers between the meltand the coating while the TiC and the ZrC coatings were significantly degraded. Y2O3 exhibited the mostpromising performance among HfN, TiC, ZrC, and Y2O3 coatings.

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1. Introduction

Traditionally to prevent melt/material interactions metallic nuclearfuels, such as the U–Zr alloy system used in or proposed for sodium-cooled fast reactors (SFR), have been melted and cast in slurry-coatedgraphite crucibles and slurry-coated quartz molds. Application of thesecoatings in a hot cell environment is labor-intensive and operator-dependent, and can introduce additional waste streams. Also, melt/coating interactions and porous coatings can be a source of meltcontamination and fuel losses, respectively. Furthermore, slurry appliedcoatings must be recoated after every batch. Thermal plasma-sprayedcoatings of refractory materials can be applied to develop a re-usablecrucible coating for metallic fuel. The plasma-sprayed coating canprovide the crucible with a denser, more durable, coating layer,comparedwith themore friable coating layer formed by slurry-coating.Plasma-sprayed coatings are consolidated bymechanical interlocking ofthemolten particles impacting on the substrate and are densified by theheat applied by the plasma [1]. The increased coating density isadvantageous because it should not require frequent recoating andU–Zrmelt penetration through theprotective layer ismore difficult in a densecoating than in a porous coating.

To investigate permanent coatings for re-usable crucibles formelting and casting of metallic fuel, the refractory coating wasplasma-sprayed onto niobium substrates. HfN, TiC, ZrC, and Y2O3wereselected as promising candidate coating materials through a literature

search [2,3]. These materials have been used as coatings in thermalbarrier, high-reflectivity, and electronic device applications [4–8].Niobium was selected as a substrate because of its refractory natureand the coefficient of thermal expansion (CTE) is similar to that ofmany of the candidate materials.

After the HfN, TiC, ZrC, and Y2O3 coatings were applied theresultingmicrostructure was characterized. Thermal cycle testing wasperformed to investigate the effect of repeated heating and cooling[9,10]. Interaction studies between molten U–Zr and the plasma-sprayed coatings were also carried out.

2. Experimental

HfN, TiC, ZrC, and Y2O3 powders, ranging from 10 μm to 45 μm insize, were plasma-sprayed onto 99.8% pure niobium discs and rods6.3 mm in diameter. To provide a rough surface finish to enhance theadhesion of the coating layer the niobium substrate was grit blastedwith alumina and cleaned using a standard ultrasonicator. Approxi-mately 100 μm thick coatingswere deposited controlling the number ofthe coating layers, with a torch input power of about 15 KW, an arccurrent of about 750 A, and a plasma gas of an argon and heliummixture. Thehighermeltingpointmaterials, TiC, ZrC, andHfN required ahigher helium input as opposed to Y2O3. Thermal cycling of the coateddiscs was conducted in a programmable vacuum furnace. Thermal cycletemperature and holding time were based on the melting and castingcondition of U–10wt.%Zr metallic fuels. So, the thermal cycle consistedof heating of the specimens to 1450 °C at a rate of 20 °C/min andholdingat temperature for 30 min, followed by furnace cooling to near-roomtemperature. Coating/melt interaction was studied by immersing the

6970 K.H. Kim et al. / Thin Solid Films 519 (2011) 6969–6973

coated rods inU–20wt.%Zr alloymelt contained in an inductively heatedY2O3-slurry coated graphite crucible under vacuum. Samples werelowered into the melt at 1550 °C for 5 min, and cooled in place withoutwithdrawing from the melt, or the coated rods were lowered into themelt at 1550 °C for 15 min, and withdrawn and cooled outside thecrucible in the vacuumatmosphereof the induction furnace. The coatingmicrostructure before and after testing was characterized using ascanning electron microscope (SEM). The chemical compositions of thecoated specimens were measured by energy-dispersive spectroscope(EDS). Coating phases were analyzed by X-ray diffraction (XRD) usingCu K α1 radiation with a graphite monochrometer.

3. Results and discussion

Cross-sectional SEM micrographs of the coated discs are shown inFig. 1. The HfN coating layer showed fairly uniform thickness but poorconsolidation with a considerable amount of porosity in the form ofsmall closed pores. The TiC and the ZrC coating layers also had fairlyuniform thickness with poor consolidation characterized by largeclosed pores. The Y2O3 coating layer had a uniform thickness, was wellconsolidated with only a few small closed pores being visible, andexhibited good interface contact between the coating layer and Nbsubstrate. It is thought that the Y2O3 coating layer showed goodconsolidation with few small pores due to its relatively low meltingtemperature of 2440 °C; compared to, the poorly consolidated HfN,TiC, and ZrC layers with melting temperatures in excess of 3000 °C.

Samples underwent X-ray diffraction in the as coated and afterannealing at 1600 °C for 2 h. The resulting diffraction patterns areshown in Fig. 2. The major phases of the HfN, TiC, and ZrC discs werecubic HfN, cubic TiC, and cubic ZrC with some oxide phases thatformed during the high temperature plasma spraying process. TheY2O3 disc showed only cubic Y2O3 phase. The cubic phases of HfN, TiC,and ZrC were more dominant after an annealing treatment comparedto those of the as-plasma-sprayed disc. These slight changes are

(a)

(c)

Fig. 1. Cross-sectional SEM micrographs showing the coating layer plasma-sp

thought to be related to residual stress-relieving and annealing in thecoatings during the heat-treatment.

Cross-sectional SEMmicrographs of the plasma-sprayed discs after20 thermal cycles at 1450 °C are shown in Fig. 3. The coated discs wereexamined for signs of coating deterioration. The HfN coated discsshowed some cracking after 5 cycles, likely due to slight difference ofthermal expansion with the niobium substrate, with the cracksrunning parallel to the substrate surface; however, even after 20cycles, the cracks were not interconnected. Similarly, some parallelcracking could be observed in the ZrC coated discs after 5 cycles, witha small number of cracks becoming interconnected after 20 cycles.The TiC coatings were much more porous than the other samples andno cracks were formed in these samples after 20 cycles due to highporosity. The Y2O3 coated discs showed microcracking formed due tothermal shock after 5 cycles; however, the micro-cracks did notpropagate further or join after 20 cycles. Hence, thermal cycling testsof the ceramic plasma-sprayed coatings showed that HfN, TiC, ZrC,and Y2O3 have good thermal cycling characteristics with very fewinterconnected cracks even after 20 cycles.

After exposure to the U–20wt.%Zr melt and cooling outside of themelt, the HfN coated rod showed a discrete coating interface betweenHfN coating layer and U–Zr alloy. Some penetration of U–Zr meltthrough the grain boundaries around the interface also occurred asshown in Fig. 4 (a). The TiC coated rod showed an indiscrete coatingboundary, and significant penetration of U–Zr melt into the coatinglayer through grain boundaries after exposure to themelt, as shown inFig. 4 (b). Fig. 4 (c) shows that the discrete ZrC coating layerdisappeared due to the eutectic reaction of U–Zr alloywith the coatinglayer. Significant degradations occurred on HfN, TiC, and ZrC coatinglayers after dipping into U–Zr melt. Transition metals such as Hf, Ti,and Zr elements, have considerable reactivitywith uranium, and showeutectic or eutectoid reaction with uranium even at low temperatures[11]. It is thought that U–Zr melt penetrates and reacts along the grainboundaries, and dissolves the coating layer at the interface. Finally, in

(b)

(d)

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rayed on the niobium substrate; (a) HfN, (b) TiC, (c) ZrC, and (d) Y2O3.

20 30 40 50 60 70 80 90 1000

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tetragonal-HfO2

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s)

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HfN (Plasma-coated)

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Fig. 2. X-ray diffraction patterns of the as-plasma-sprayed discs and the as-annealed discs after heating to 1600 °C for 2 h; (a) HfN, (b) TiC, (c) ZrC, and (d) Y2O3.

(a) Crack

(b)

(c) (d)

100 µm

(a) Crack

Fig. 3. Cross-sectional SEM micrographs showing the ceramic coating layer after 20 thermal cycles at 1450 °C for 30 min; (a) HfN, (b) TiC, (c) ZrC, and (d) Y2O3.

6971K.H. Kim et al. / Thin Solid Films 519 (2011) 6969–6973

(a) (b)

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Nb substrate

Nb substrateNb substrate

Nb substrate

U-20Zr

U-20Zr

U-20Zr

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TiCC

200 µm

Fig. 4. Cross-sectional BSE micrographs showing the interface between U–20wt.%Zr and ceramic coating layer after dipping at 1550 °C for 15 min; (a) HfN, (b) TiC, (c) ZrC, and (d) Y2O3.

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Fig. 4-(d), no reaction layer formation or penetration into the coatingwas observed in the Y2O3 coating. The Y2O3 coating layer is free fromdissolution in spite of its lower melting temperature. It is known thatY2O3 is the most stable compound among HfN, TiC, ZrC, and Y2O3

coatings in the standpoint of thermodynamics [11]. Yttrium doesn'thave any eutectic or eutectoid reaction with uranium in phasediagram [3,12]. Hence, it is supposed that the Y2O3 coating layershows a good thermal compatibility with U–Zr melt.

Results were similar for longer exposure times. The HfN coated rodformed a reaction layer of about 50 μm in thickness between HfNcoating andU–Zr alloy,with somepenetration of U–Zrmelt into theHfNcoating occurring after exposure to the melt for 15 min and cooling inthe melt. No reaction layer formation and penetration into the coatingwere observed in the Y2O3 coating during the same test. Hence,interaction studies bymelt dipping tests of the plasma-sprayed coatingsindicated that HfN and Y2O3 do not form a significant reaction layerbetween U–20wt.%Zr melt and the coating layer. The Y2O3 coatingexhibited the most promising performance among HfN, TiC, ZrC, andY2O3 coatings.

4. Conclusions

Plasma-sprayed coatings of HfN, TiC, ZrC, and Y2O3, were appliedand characterized. Thermal cycling and U–20wt.%Zr melt interactionstudies were also carried out. The HfN coating showed somewhatuniform thickness and poor consolidationwith a considerable numberof small pores. The TiC and the ZrC coatings also had fairly uniformthickness and low density with a large amount of large closed pores,showing poor consolidation due to higher melting temperature inexcess of 3000 °C. The Y2O3 coating layer had uniform thickness, and

high density with only a few small closed pores showing goodconsolidation due to its lower melting temperature of 2440 °C. TheY2O3 layer also exhibited good interface contact between the coatinglayer and Nb substrate.

The major phase of the HfN, TiC, and ZrC discs was cubic HfN, cubicTiC, and cubic ZrC phases, with some oxide phases due to oxidationduring the plasma-spraying process; however, the plasma-sprayed Y2O3

specimen showed only cubic Y2O3 phase. Thermal cycling tests of thecoatings showed that HfN, TiC, ZrC, and Y2O3 had good cyclingcharacteristics with only a few interconnected cracks developing in theZrC and no interconnected cracks in the other materials even after 20cycles, showinga good thermal shock resistance.Melt dipping tests of theplasma-sprayed coatings also indicated that HfN and Y2O3 didn't formsignificant reaction layers between U–20wt.%Zr melt and the coatinglayer. The Y2O3 coating exhibited the most promising performanceamong HfN, TiC, ZrC, and Y2O3 coatings for re-usable metallic fuelcrucibles due to good thermal compatibility with U–20Zr melt.

Acknowledgement

This study was supported by the Fuel Cycle Research andDevelopment Program funded by the U.S. Department of Energy andthe National Nuclear R&D Program of the Ministry of Science andTechnology (MOST) of Korea.

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