Development of Self-Healing Coating for Tritium Permeation

DEVELOPMENT OF SELF-HEALING COATING FOR TRITIUM PERMEATION BARRIERS

Jinping Suo, Jifeng Gao, Dawei Liu

Tel: (+86)02787558055; fax: (+86)02787558055.

E-mail address: [email protected].

State Key Laboratory of Mould Technology, Institute of Materials Science

and Engineering, Huazhong University of Science and Technology,

Wuhan 430074, PR China

ABSTRACT

In fusion reactor, ceramic coatings on metal substrate are

usually performed to decrease the tritium loss from the

pipeline to the air environment. However, pores and cracks

often exist in the thick ceramic coatings. A new kind of

composited ceramic coating with self healing ab ility is

developed. The porosity of the coating was 4.43% before heat

treatment while 0.46% after being maintained at 600 °C for 30

hours period. The evaluation of electrochemical performance

in 3.5 wt% NaCl solution by electrochemical impedance

spectroscopy shows that the corrosion resistance of the whole

coating after being maintained at 600°C for 18 hours is much

better than the other samples, which indicates holes and cracks

in the whole coating have been decreased. The adhesive

strength of the coatings is higher than 9 MPa after being

maintained at 600 °C for 6 hours. The thermal shock cycles

were 300, 210 and 123 at 600°C, 700°C and 800°C

respectively. Energy dispersive X-ray analysis indicates that

the oxygen content at the crack location increases significantly

after being maintained at 600 °C for 30 hours. The results

suggest that this self healing coating can meet the requirement

of application under the actual temperature conditions.

1. INTRODUCTION

The technological accomplishment of a fusion power

source is crucially dependent on the successful development of

high-performance materials [1]. The reduction of trit ium

permeat ion through cooling tubes and blanket structural

materials is one of the key problems in development of HCLL

(Helium Cooled Lead Lithium) Test Blanket Module [2] for

ITER. The reduction can be achieved by using a trit ium

permeat ion barrier (TPB) fabricated by means of chemical

vapor deposition (CVD), physical vapor deposition (PVD),

electrochemical deposition or other methods.

Ceramic materials composed of TiC, TiN, Al2O3, SiC,

SiO2 and Er2O3, have been introduced into the TPB [2-6] and

proved to be good candidates. Among the available

technologies in preparation ceramic coatings, thermal spraying

is a commercially viab le technique and has been used by

researchers [7-9]. However, the cracks and pores can be easily

developed from the manufacturing process or high temperature

[10, 11] and tend to tritium loss. To a large extent, the efficiency

of these coatings depends on how to reduce the porosity and the

1 Copyright © 2012 by ASME

Proceedings of the 2012 20th International Conference on Nuclear Engineering collocated with the

ASME 2012 Power Conference ICONE20-POWER2012

July 30 - August 3, 2012, Anaheim, California, USA

ICONE20-POWER2012-54178

cracks.

It is known that Al2O3 reinforced by SiC particles have an

interesting ability of crack-healing [12]. The crack-healing

occurs main ly due to the oxidation of SiC in the composite

which flow sufficiently and fill the cracks. Cracks and pores in

the ceramic coatings caused by thermal spraying may be healed

if this ability is applied. So far, TiC has been incorporated into

the composite coatings for its proper oxidation temperature and

contribution to the reduction of tritium permeation. The aim of

the present research is to obtain a thermally sprayed coating

with self-healing ability and mechanical integrity at high

temperature which might be used as TPB for fusion reactor.

Composite layers of TiC+mixture (TiC/Al2O3)+Al2O3 are

deposited by atmosphere plasma spraying on a certain

martensitic steel substrate. The microstructure and thermal

shock resistance of the samples are studied. The coating

porosity is measured by image analysis software (Image Pro

Plus) on the scanning electron microscopy (SEM) images. The

results indicate that, the TiC+mixture (TiC/Al2O3)+Al2O3

composite layer exh ibits an efficient mechanical integrity and

significant self-healing ability. This mechanism might

overcome the disadvantage of high porosity in

thermally -sprayed coatings and be used to develop efficient

TPB.

2. EXPERIMENT DETAILS

2.1. Sample preparation

The martensitic steel matrix was cut into several s mall

cuboids with dimensions of 45 mm×30 mm×4 mm and the

chemical composition of this steel is shown in Table 1.

The specimens were machined, ground, polished,

ultrasonically cleaned in acetone and dried in air. Then they

were blasted at a blasting pressure of 3 MPa by Al2O3 grit

with the part icle size of 80 μm to improve the adherence

between the substrate and the ceramic coating.

Nanosized TiC and α-Al2O3 powder ， which were

delivered by Kaier Nanotechnology & Development

Corporation, Anhui, China and Wanjing New Material

Corporation, Hangzhou, China，were employed as spraying

materials. The grain size of the TiC particles was 40 nm while

the size of the α-A l2O3 particles was 30 nm.

2.2. Spraying granulation

Before spraying, the nanosized TiC and α-A l2O3 were

slurried and then dried to form micrometer sized granules.

Meanwhile, the TiC and α-A l2O3 mixture (1:1 vol.%) was

granulated to form the transition layer. As can be seen from

Fig. 1, the agglomerated particles are spherical with the size

varying from 5 μm to 40 μm. Moreover, from the fracture

morphology of the granules shown in Fig. 2, we can see that

the nanosized particles were compactly arranged with few

defects. Also, the energy dispersive X-ray (EDX) regional

analysis shows the presence of aluminum, carbon, titanium

and oxygen (28.42, 15.45, 28.4 and 27.42 wt .%, respectively),

which indicates that the TiC and A l2O3 nanoparticles

composed in the granules are uniformly mixed.

TABLE 1

CHEMICAL COMPOSITION OF THE STEEL EMPLOYED IN THIS EXPERIMENT IN WT.%.

C Mn P S Ni Cr V Mo Si Fe

0.12 0.53 0.018 0.005 0.06 9.11 0.08 0.01 0.12 Balance

FIG. 1. MICROSCOPIC FEEDSTOCK PARTICLES FORMED BY THE SPRAYING GRANULATION OF

NANOSTRUCTURED PARTICLES OF (A) TIC. (B) MIXTURE (TIC/AL2O3). (C) AL2O3.


2.3. Spraying procedure

The spraying was performed by atmospheric plasma

spraying (APS) system with a spraying gun directed against

the surface to be coated. The stand-off distance was kept

constant at 80 mm while the powder feeding rate was 30 g

min−1

throughout the experiments. N2 with a flow rate of 3 L

min−1

was the carrier gas. It also acted as the primary gas with

a rate of 45 L min−1

. The corresponding secondary gas was

Ar, which was maintained at a rate of 30 L min−1

. The

spraying current and voltage were 450 A and 80 V

respectively.

2.4. Microstructure observation and analysis

X-ray diffraction (XRD) was used to perform the phase

analysis. The cross-section and the surface morphology were

examined by scanning electron microscopy (SEM/EDS) after

the specimen was cleaned by ultrasonic in ethanol. The

coating porosity was measured by using the Image Pro

software on the SEM images with the following three steps: (1)

binary conversion, (2) subtracting the pores and (3) area

calculation.

2.5. Thermal shock resistance test

The thermal shock specimens were cut into s mall

cuboids with surface area o f 20 mm×15 mm. They were first

heated to 800 °C for 10 min in an electric furnace and then

rapidly quenched in ambient temperature water. The heating

was repeated until the coatings failed due to cracking or

peeling. The thermal shock resistance of the samples was

evaluated by the number of thermal shock cycles until failure.

2.6 Corrosion resistance test

Polarizat ion curve and elect rochemical impedance

spectroscopy (EIS) were used to evaluate the electrochemical

properties of the samples. The side without coatings of the

samples were ground, polished on which the copper wires

were welded. Then all the superficies of the samples without

coatings were coated by the 703 insulating silica gel and lay

aside for more than 6 hours to be solidified. The experiment

was done by CS-350 electrochemical workstation with the

coated samples as working elect rode, Pt as an auxiliary

FIG. 2. INTERNAL STRUCTURE AND EDS RESULT OF THE MIXTURE (TIC/AL2O3) PARTICLE.

FIG.3. CROSS-SECTION MICROSTRUCTURE OF THE TIC+ MIX(AL2O3/TIC)+AL2O3 (A)BEFORE HEAT-TREATMENT

(B)AFTER HEAT-TREATMENT.


electrode and a saturated calomel electrode (SCE) as reference.

The experiments were performed in 3.5 wt.% NaCl solution at

25 ℃. The polarizat ion curves were scanned from −1V to

+1V vs the open circuit potential (OCP) at a scan rate of 1 mV

s−1

after a steady-state of the working electrode in the solution

had been established. The impedance measurements were

performed at open-circuit potentials with 10 mV sinusoidal

perturbations in frequency range from100 kHz to 10 MHz.

2.7. Adhesive strength tensile test

Adhesive strength tensile test was carried out to evaluate

the tensile strength between coatings and substrate. The

substrate was cemented to stretching die while the coating was

cemented to another stretching die by epoxy resin. Then they

were stretched continuously on the tensile testing machine till

they separated from each other.

3. RES ULTS AND DISCUSSION

3.1. S EM observations before heat-treatment

Fig.3(a) shows the cross section morphology of the

samples before heat treatment. The thickness of the

TiC+mixture (TiC/Al2O3)+Al2O3 composite coating was

about 80 μm. The Al2O3 top coating was continuous with a

depth of 40 μm. It appeared compact at its lower part despite

of some pores at its upper side. The middle layer, which was

made up of TiC/Al2O3 mixture, appeared loose and the

presence of cavities was detected there. The bottom TiC layer

was difficult to be d istinguished from the middle layer, and

loosely combined with the substrate.

3.2. Phase determination via XRD

As shown in the typical XRD patterns in Fig. 4(a),

theAl2O3 powders before spraying main ly consisted of

rhombohedral phase which corresponded to α-Al2O3. No

peaks of γ-Al2O3 and β-Al2O3 were found despite of few

impurities which were introduced during the manufacturing

process of the raw materials. Panels (b) of Fig. 4 are typical

X-ray diffraction patterns for the samples. The characteristic

peaks of Al2O3 were seen clearly and no spectrum of the steel

substrate or the transition layer was seen, indicating that a

rather thick and compact Al2O3 layer was fo rmed on the

surface. The morphology of the Al2O3 film on the mixture

(TiC/Al2O3) layers in the coatings also showed that the

surface was compact with laminate structure due to the

collision process between the melting or partially melt ing

Al2O3 and the substrate. In comparison with the Al2O3 powder

before spraying, a few cubic phase (γ-Al2O3) appeared, which

was transformed from rhombohedral phase (α-Al2O3) during

the rapid cooling procedure after spraying.

FIG. 4. THE XRD PATTERNS OF THE SAMPLES. (A) Α-AL2O3 POWDERS AFTER GRANULATION (B) TIC+MIXTURE

FIG. 6. EDS RESULT OF THE BOTTOM TIC FILM IN TIC+MIXTURE (TIC/AL2O3)+AL2O3 BEFORE HEAT-TREATMENT.

(TIC/AL2O3)+AL2O3.


3.3. Thermal shock resistance

Thermal shock tests were carried out to detect the

mechanical integrity and durability of the coatings at high

temperature. Fig. 5 shows the number of thermal shock cycles

resulting in cracking or peeling at different temperature. The

TiC+mixture (TiC/Al2O3)+Al2O3 coating exh ib ited a high

resistance to thermal shock due to the transition mixture layer

as well as the good match between the TiC bottom film and the

matrix.

FIG. 5. COMPARISON OF THERMAL SHOCK OF

COATINGS IN DIFFERENT TEMPERATURE.

3.4. S EM observation after heat-treatment

Fig.3(b) shows the cross section morphology of the

samples after heat treatment. In the TiC+mixture

(TiC/Al2O3)+Al2O3 coating, the bottom TiC film and the

transition mixture (TiC/Al2O3) film were combined into a more

compact mixed layer with good adherence to the substrate. It

can be exp lained in terms of the self-healing ability due to the

expansion of TiO2 oxid ized from TiC. The oxidation products

filled the cracks and the “thermal expansion coefficient”

resulted from the gradual oxidation in the reaction well

matched the substrate. Meanwhile, the TiC might contribute to

the decrease of the grain size and the melt ing point of A l2O3

[13], which favored the formation of compact coating. In Fig. 6,

the energy dispersive X-ray (EDX) regional analysis shows

that, before healing heat treatment, the bottom layer in the

TiC+mixture (TiC/Al2O3)+Al2O3 coating mainly consisted of

TiC. But after the self healing heat treatment, as is shown in Fig.

7, the oxygen content increased dramatically from 7.23 wt .% to

27.25 wt.%, indicating that the oxidation occurred as follows

2TiC+3O2=2TiO2+2CO (1)

FIG. 7. EDS RESULT OF THE BOTTOM TIC FILM IN TIC+MIXTURE (TIC/AL2O3)+AL2O3 BEFORE HEAT-TREATMENT.

FIG. 8. PROCESSED SEM IMAGES OF CROSS -SECTION. (A) BEFORE HEAT-TREATMENT. (B) AFTER

HEAT-TREATMENT.

0

100

200

300

400

the

rmal

sh

ock

cy

cle

s/ti

me

s

diffierent temperature

thermal shock cycles


While the density of TiC and TiO2 were 4.93 g cm−3

and

4.26 g cm−3

respectively, after the complete reaction, the

volume change could be calculated by the formula as follows

（） (2)

Hence, after heat-treatment, the TiC material could

expand by 53% at most. Fig. 8 shows the processed SEM

images of the coating by Image Pro software, the black areas

are pores after processing while the gray region represents

compactness. As can be calculated from the images by the

software, the porosity of the coating declined from 4.43% to

just 0.46%, indicat ing that the cracks were significantly

reduced.

3.5. Resistant to corrosion

Fig . 9. shows the potentiodynamic anodic po larizat ion

curves of four samples. The sample without coatings exhib ited

lowest corrosion potential and highest corrosion current. The

corrosion potential rose and the corrosion current declined with

the extension of thermal oxidation time. Fig. 10. shows the

electrochemical impedance spectra of the coating samples and

the bare subst rate. The corros ion res istance is pos it ive

FIG.9. POTENTIODYNAMIC ANODIC POLARIZATION CURVES OF (A)SUBS TRATE AND TIC+(AL2O3/TIC)+AL2O3

COATINGS IN 3.5 WT.% NACL SOLUTION AFTER (B)0H(C)3H(D)18H THERMAL OXIDATION AT 600 ℃

FIG.10. ELECTROCHEMICAL IMPEDANCE SPECTRA OF THE COATING SAMPLES AND THE BARE SUBSTRATE


correlated to the radius of the electrochemical impedance

spectra. All the samples with coatings were much better than

the one without coating, and the best one was the sample

having been heated for 18 hours. They can both be explained

by the decrease of the holes and cracks in the coating.

3.6. The adhesive strength of the coatings

Fig.11. shows the tensile strength of the coatings after

different time of self healing heat treatment at 800℃ . After 2

hours’ self healing heat treatment, the tensile strength rose from

10.4 MPa to 11.4 MPa. It may be because of the decrease of

holes and cracks. But when the time of self healing heat

treatment became longer, the tensile strength declined

obviously. This may be because of the growing up of crystal

grain of the coating or the excessive expansion of the vo lume.

FIG.11. TENSILE STRENGTH OF THE COATINGS

AFTER DIFFERENT TIME OF SELF HEALING HEAT

TREATMENT AT 800℃

4. CONCLUS ION

How to reduce pores and cracks in ceramic coatings is one

of the key prob lems for developing effective trit ium

permeat ion barrier in fusion reactor. The self-healing ability of

carbide/oxide composite may be utilized to heal cracks

automatically at certain temperature. In this work, TiC+mixture

(TiC/Al2O3)+Al2O3 was fabricated by means of APS. The

performance and morphology of the samples were analyzed by

thermal shock test, SEM/EDS and XRD, respectively. The

results indicated that, this composite coating exh ibited a high

resistance to thermal shock. After 30 hours of healing heat

treatment, the porosity in the coating declined by 90%

compared with the coating before healing heat treatment. It also

exhibited good corrosion resistance in 3.5wt% NaCl solution.

The tensile strength was reached 11.5 MPa after 2 hours of

healing heat treatment. The oxidation products of TiC filled the

cracks and pores introduced during the thermal spraying

process. Therefore, this coating with self healing ability in

normal atmosphere may be a good candidate for TPB in fusion

reactors. The combination of crack-healing and thermal

spraying will be useful for developing efficient TPB. Further

tests are necessary to evaluate the efficiency of TPB made of

the proposed coating.

ACKNOWLEDGEMENTS

This work is supported by National Magnetic

Confinement Fusion Program

(2011GB108009).

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Development of Self-Healing Coating for Tritium Permeation