Fusion Engineering and Design 58–59 (2001) 731–735

Self-healing electrical insulating coating processes forvanadium alloy–lithium systems

A.V. Vertkov *, V.A. Evtikhin, I.E. LyublinskiState Enterprise ‘Red Star’-‘Prana-Center’ Co 1A, Electrolitniy Proezd, 115230 Moscow, Russia

Abstract

The existing technological approaches for the formation of nitride- and oxide-based self-healing electrical insulatingcoatings for vanadium alloy–lithium systems are considered. The results of the property study of coatings appliedfrom liquid lithium containing Al, N, Si, B additions on various modes are considered. The formation conditions ofAlN-based coatings with scale specific electrical resistivity (�50 � m) on the V–4Ti–4Cr vanadium alloy aredetermined. The results of formation and stability research of coatings on the V–4Ti–4Cr vanadium alloy inconvectional and forced circulating lithium with Al and N additions in the homogeneous and heterogeneous lithiumsystems are discussed. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Lithium; Vanadium alloys; Coating

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1. Introduction

The influence of strong magnetic fields on theliquid metal system availability is a critical factorin liquid metal application for tokamak fusionreactor blanket and divertor. The most radicalsolution of this problem is believed to be theapplication of electrical insulating coatings tochannel inner surfaces by ‘in situ’ method.

The choice of electrically insulating coating ma-terials in a vanadium–lithium reactor concept isprimarily connected with meeting of the basicrequirements [1,2], available applying technologyand self-healing at low concentration of reaction

components in liquid metal. The experimentaldata given in [2,3] allow for concluding, that AlN,BN, CaO, MgO, Y2O3, ThO2 and materials ontheir base can be the most resistant to lithium.Therefore, the best material will be such one,which will retain the insulating properties at areasonable level under operation conditions, ap-plying technology of which is the most acceptableunder reactor conditions and gives stable result,rather than possesses the highest insulatingcharacteristics.

Proceeding from the available experimental andcalculated evaluations, the nitride-base coatingsare more resistant to lithium environment thanoxide-base ones at a rather low content of addi-tives in lithium [2]. Therefore, the primary effortsof the others were directed towards creating theAlN-base nitride coatings as the most advanced.

* Corresponding author. Tel.: +7-095-978-4443; fax: +7-095-284-5843.

E-mail address: evtikhin@protein.bio.msu.ru (A.V.Vertkov).

0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0920 -3796 (01 )00548 -8

A.V. Vertko� et al. / Fusion Engineering and Design 58–59 (2001) 731–735732

Experience in application of ‘in situ’ method forAlN-base coating applying is currently rather lim-ited [4]. The most stable results of applying acoating by this method obtained by the authorsare given in work [2,5].

2. Technology of electrical insulating coatingapplying

The aluminum, boron and silicon nitride basecoating applying technique was elaborated understatic isothermal conditions in the ampoules fromV–8Cr alloy. The following vanadium alloys ofV–Ti–Cr system were employed as a substratematerial: V–10Cr–15Ti–0.05Y, V–7Cr–15Ti–1Zr–0.1C, V–8Cr–5Ti–1Zr–0.1C, V–7Cr–14Ti,V–4Cr–4Ti.

The coatings were formed under two treatmentmodes. The first mode consisted in exposure tolithium with additives at 500–700 °C. The secondmode is an exposure to lithium with additiveswith stepwise variations of temperature from 600to 800 °C for up to 200 h. The environmentcomposition for coating applying is given in Ta-bles 1 and 2. The nitrogen concentration inlithium was provided by the addition of ratedquantity of stoichiometric Li3N. Boron was addedby ferroboron with 30 wt.% B which served as ansource.

When coating applying on mode 2 the simulta-neous and successive additions of constituents

Table 2Environment composition for nitride coating applying onmode 2

Additive concentration in lithium, wt.%Number

Al NaSiB

b – – –1b 5–132 – –

3 –5–13 –––4 b 5–15 5–15

7–15b –5 1–66 b – 5–15 1–67 1–61–105–15b

–– N2 (gas)–8c

– –9 – 1–6– 5–1510 – 0.5–5

–11 – 0.5–51–5

a From Li3N.b Source: ferroboron with 30 wt.% B.c Gaseous nitrogen at P=250 Pa, T=750 °C, �=5 h.

took place on consecutive exposure to environ-ments of proper composition. Proceeding fromthe experimental data, it has been found that thebest results on the formation and properties havebeen achieved for AlN-base coatings according tomode 2 in successive addition of constituents. Thecoatings with boron and silicon in different com-bination either did not be formed or cracked andpeeled off and did not possesses the stability ofelectrical properties.

The optimal procedure of AlN coating applyingconsisted in pre-saturating of vanadium alloy sur-face in lithium with aluminum additive within thelimits of 5–15 wt.% at 800 °C for the aluminidesurface layer formation. The subsequent exposureof pre-saturated alloys to lithium with 1–5 wt.%of Al and 0.5–5 wt.% of N with stepwise varia-tions of temperature (�T=50 °C) from 600 to800 °C resulted in AlN-base coating continuousfilm formation (�2–4 �m) without dependencyon alloy composition. The micro-X-ray spectrumanalysis of metallic element distribution at samplesection pointed to a high content of Al (60–80wt.%) in the film area and lack of change invanadium alloy composition in the under-filmarea. The microhardness distribution measure-ments at section of samples with coating are

Table 1Environment composition for nitride coating applying onmode 1

Number Additive concentration in lithium, wt.%

B Al Si Na

b –1 – –2 b – – 0.1–1

b 7–133 – –b 7–134 – 0.1–1

0.1–15–105–10b5– 0.1–17–13–6

– –7 7–13 0.1–1

a From Li3N.b Source: ferroboron with 30 wt.% B.

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indicative of the formation of hardened near-sur-face layer with a thickness of 10–100 �m depend-ing on the applying environment and basematerial composition. This seems to be connectedwith the penetration of nonmetallic impuritiesinto a depth of base material.

After the formation procedure the samples wereexposed to thermocycling tests (up to 103 cycles at�T=50 °C in the range of 400–600 °C). Elec-tron microscope studies after tests have revealedno cracking and peeling coatings.

The electric resistance of coatings after thermo-cycling tests was measured in lithium at 300 °Cby four-probe method. Results of the measure-ments are given in Table 3.

A subsequent step was the AlN-base coatingtechnology development for dynamic nonisother-mal lithium systems. The coating applying to theV–4Ti–4Cr alloy was conducted in the loop facil-ity from 316-type stainless steel at a flow velocityof 3–5 m/s and temperature drop over the loop of�20 °C. The stepwise temperature change in therange of 450–700 °C and successive additions ofelements have been used. The concentrations ofaluminum and nitrogen in lithium were equal to 5and 0.5 wt.%, respectively. The total duration ofprocess was 200 h.

Only chromium and vanadium nitrides havebeen revealed on the coating surface. The enrich-ment of vanadium alloy surface with chromium(�70 wt.%), iron (�8 wt.%) and nickel (�2wt.%) is detected and explained by the dissolution

process in lithium and steel component transfer tothe vanadium alloy. The formation of chromiumnitrides and, possibly, carbides (carbon from steelin lithium has a tendency to transfer to vanadium[3]) on the surface of alloy seems to inhibit theformation of vanadium aluminide layer and thenAlN. Hence, the coating applying on the vana-dium alloys in the dynamic heterogeneous systemfrom chromium-nickel steel is inappropriate. Forsuch systems the formation of AlN-base coating isfeasible on our estimates at temperatures notabove 350oC when the dissolution and transfer ofsteel components will be negligible.

On subsequent step the coating applying pro-cess to the V–4Cr–4Ti alloy on optimal modewas conducted in a convection ampoule fabri-cated from niobium alloy. The lithium convectionflow velocity was 10–15 cm/s. The specimens wereplaced in the hot spot of the ampoule. The alu-minide layer was formed in lithium with 3–7 wt.%of aluminum at 700–800 °C with a total exposureof 100 h. The nitride coating was formed inlithium with 1 wt.% of nitrogen at 550–600 °C.The stage duration was 150 h. The temperaturedrop in the ampoule amounted to 120 °C. Finallya coating of 4–6 �m thick was formed and com-prises AlN with only traces of chromium andvanadium nitrides. The coating was continuousand had a homogeneous surface with fine relief.The coating specific resistance measured inlithium at 300 °C was 40–45 � m. This confirmsthe feasibility of selected technology of AlN coat-ing applying to the vanadium alloy in flowinglithium.

3. Electrical insulating coating resistance tolithium

For estimation of AlN-base coating availabilitythe V–4Ti–4Cr specimens with coating appliedon optimal mode have been tested for 1100 hunder static isothermal conditions at 525–550 °C.The initial specific resistance of coating measuredin lithium at 300 °C constituted 40–50 � m. Onehalf the specimens was placed in ampoules fromV–4Ti–4Cr alloy filled with lithium containing 10wt.% of aluminum and 1 wt.% of nitrogen and the

Table 3Specific resistance of nitride coatings on vanadium alloysmeasured in lithium at 300 °C

Base material Specific resistanceMode//environmentnumber (� m)

39.02/2, 11V–10Cr–15Ti–0.05Y25.2V–4Cr–4Ti21.0V–7Cr–14Ti31.0V–7Cr–15Ti–1Zr–0.1C33.0V–10Cr–15Ti–0.05Y

2/2, 8, 11V–7Cr–14Ti 8.6V–7Cr–15Ti–1Zr–0.1C 10.5

1.3V–8Cr–5Ti–1Zr–0.1C1/6V–10Cr–15Ti–0.05Y 1.8

V–7Cr–14Ti 1.9

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Fig. 1. Weight change of V–4Ti–4Cr alloy samples and alloywith AlN coating after test in lithium with the various contentsof nitrogen and aluminium at 700 °C for 100 h.

The coating resistance to convection flowinglithium was tested for 50 h in the temperaturerange of 750–900 °C at a temperature drop of150 °C. The coating and vanadium alloy dissolu-tion and transfer of dilute material to ampoulecold spot have been found to occur even withaddition of aluminum and nitrogen to lithium.

An investigation of the resistance of nitridecoatings to flowing lithium was conducted in atest loop facility from V–4Cr–4Ti alloy [6]. Thespecimens of V–4Cr–4Ti alloy with AlN–basecoating applied by the above-mentioned technol-ogy were tested in flowing high purity lithium(0.005–0.008 wt.% N, 0.005 wt.% O, 0.001 wt.%C) in the range of 370–700 °C and at a flowvelocity of 0.4–1.0 m/s. The temperature dropover the loop was 250 °C. The test time was 1000h. From these results it has been evident [5] thatthe AlN does not possess reasonable stability inflowing high purity lithium and coating on itsbase is dissolved. It confirms the calculation dataon nitride coating stability and data of tests con-ducted under static isothermal conditions [2].

To assess the radiation stability of AlN-basecoatings a radiation test program has been devel-oped and realized [7]. It has been determined thatthe coating electric resistance is stable (is varied inthe range of 0.9–1 of reference value) on irradia-tion and also the rupture of coating integrity andradiation-induced damage are lacking. The typicalparameter of coating resistance (the product ofspecific resistance by coating thickness) made up(1.0–1.6)×10−4 � m2.

4. Conclusion

The realistic approach in evaluating therequirement to coating electrical resistance in aliquid metal system can be based on assumptionof MHD pressure drop excess, for example 10wt.%, in case of final resistance in relation to thesystem with homogeneous, flawless coating havinghigh specific resistance. According to ourestimates, for realistic liquid metal system versionwith rectangular channels the coating minimalrequired resistance can be provided at a specificresistance of �102–104 � m with a coatingthickness of �1 �m.

other half-with lithium containing 1wt.% of alu-minum and 0.1 wt.% of nitrogen. The measure-ment of electrical resistance in lithium showedthat it did not depend on the environment compo-sition and was 35–45 � m. The coating thicknessequal to 3–4 �m in the initial state ranged from 2to 4 �m after tests. The AlN was noted on thespecimen surface.

The results of nitride coating tests in lithiumwith nitrogen additive of �0.005–0.2 wt.% andaluminum additive of 0.003–3 wt.% for 100 h at700 °C are given in Fig. 1. As seen the AlNcoating is dissolved in lithium with a minimalcontent of aluminum. Increasing the nitrogen con-centration in lithium with a minimal content ofaluminum neutralizes partially the dissolutionprocess of nitride coating. The AlN coatingproved to be stable at the aluminum and nitrogenconcentrations in lithium exceeding the predictedequilibrium concentrations. An increase in con-centrations of nitrogen and aluminum in lithiumup to 0.2 and 3 wt.%, respectively, has resulted ininitiation of available coating layer building-up onthe specimens and formation of AlN coating onthe initial V–4Ti–4Cr alloy.

Thus, the addition of soluble elements tolithium, such as aluminum, nitrogen, which inter-act with each other and vanadium-base alloy un-der certain conditions may result in formation ofhomogeneous, continuous, resistant to thermalcycling electrical insulating coatings with goodoperating properties.

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In spite of the not high value of AlN-base coatingspecific resistance but close to the required, to a firstapproximation, the reproducibility of its propertiesmakes it possible to consider the above coatingapplying technology as a principal base for thefusion reactor vanadium– lithium systems. Com-plex of coating properties can be improved byoptimization of their formation modes and condi-tions. Hence, the feasibility of applying the AlN-base self-healing electrical insulating coating to theV–Cr–Ti system alloy by the ‘in-situ’ method asunder static conditions so in flowing lithium, whichsatisfies the requirements on stability and electricalparameters, has been confirmed.

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