Effect of TiC/C graded coatings on performance of graphite as thermal stress relieving transitions

Kunjie Wanga, b, Quangui Guoa, Guobing Zhanga, b, Jingli Shia, Lang Liua, *, 1 a Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, 030001, Taiyuan, P.R. China b Graduate University, Chinese Academy of Sciences, Beijing, 100039, PR China Abstract

To relieve the thermal stresses at interface of TiC and graphite, compositionally graded TiC/C transition layers were prepared by chemical vapor deposition. Scanning electron microscopy (SEM), energy dispersion spectrometer (EDS) and X-ray diffraction (XRD) results indicated that the coatings were characterized by a gradient in morphologies and compositions. For further study, the effect of graded TiC/C coatings on the thermo-mechanical properties was evaluated. Results showed that the thermal conductivity of coated graphite decreased, and mechanical strength was increased. For practical use, thermal recycles between 298K and 1273K were carried out. Results indicated that the thermal shock resistance of TiC coatings was significantly improved by the transitions. To clarify the mechanism, finite element calculations were carried out and results suggest that thermal stress shifts at interface of graphite and TiC coatings were effectively relieved by the transition layers. Keywords: titanium carbide; chemical vapor deposition; functionally graded materials; thermal stresses. 1. Introduction

Carbon material exhibits excellent properties such as high thermal conductivity, mechanical properties at high temperatures and resistance against irradiation, so it is widely used as plasma face components (PFCs) in current Tokamak facilities [1]. But the enhanced erosion due to chemical sputtering and radiation-enhanced sublimation have been known as major drawbacks, which limited its broad applications at high temperature [2]. As the fusion nuclear technology continues to develop, many investigations have been carried out aiming to fulfill the requirements of PFCs for quasi-steady state operation in next generation of fusion devices [3].

The use of low-Z coatings is still considered an effective approach to improve the surface properties of PFCs. As a mixture of covalent, metallic and ionic components [4], titanium carbide (TiC) exhibits advantageous properties such as high melting temperature, good thermal shock resistance, and good chemical and physical sputtering resistance under bombardment by highly energetic particles [5]. These unique combined properties have made titanium carbide of particular interest as protective coatings of graphite and graphite based materials [6].

Numerous investigations have been made on TiCx/a-C:H coatings. A.A. Voevodin and S. Veprek reported its potential as high strength materials [7,8]. Then investigations by G. Li, T. Zehnder and X. Ding studied the composition of different TiC/C composites [9, 10, 11, 12], and results indicated the potential to prepare TiC/C compositional graded coatings. Further investigations by T. Sonoda etl. [13] illustrated the Ti/C compositional gradient coatings on Ti-alloy. However, when the structures of substrates and coating materials are not a good match, the latter becomes distorted, and internal stresses, dislocations, and other defects appear in it. Meanwhile, due to the mismatch of coefficient of thermal expansion (CTE) between different materials, integration of stresses at sharp interfaces may cause cracks, poor adhesion, or delamination of TiC coatings. To extend the lifetime of coated-graphite, the concept of

Corresponding author. Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of

Sciences, Taiyuan, Shanxi 030001, PR China. Fax: +86 351 408 4106.

a new coating system based on gradient microstructures received much attention [14]. However, up to now little systematic work on this subject has been reported. The objective of the present work demonstrates compositions, microstructures and structural characterization of TiC/C composites in broad compositions prepared by chemical vapor deposition (CVD). On the basis of this, thermal shock resistances as well as performance evolutions of the gradient TiC/C composites such as thermal conductivity, mechanical strength, and porosities are investigated. 2. Experimental 2.1. Deposition of graded TiC/C coatings on graphite

Doped graphite (1%B4C, 2.5%Si and 7.5% Ti) with the size of 10×10×10 mm was polished, and then ultrasonically cleaned in distilled water, ethanol and subsequently acetone. During deposition of TiC/C composites, Ar (99.99%) and H2 (99.99%) were used as protective and reaction/carrier/diluent gas, respectively. Moreover, titanium tetrachloride (99.9%) imbedded in an evaporator and acetylene (99.9%) were used as sources of titanium and carbon, respectively. And the amount of TiCl4 transferred into the reactor was controlled by the flow rate of carrier gas H2 and the temperature of the evaporator.

Compositions of different TiC/C inter-layers were conducted by adjusting the temperature and flow rates of the TiCl4 and C2H2 gas at intervals from 0 to 35 sccm and 100 to 65 sccm, respectively.

2.3. Properties testing and microstructure characterization The open porosity and pore size distribution were analyzed by Hg adsorption with a mercury porosimeter. Compression tests were carried out to determine the compressive strength (δc) of species, and the specific values were calculated by the following equation: δc = P/(a×b), where P was the breaking force of the sample, a and b were the width and length of the sample, respectively. The microstructure and composition were characterized by backscattered electron (BE), SEM and EDS, respectively. The residual thermal stresses were calculated by finite element method.

3. Results and discussion 3.1. Microstructure, composition and structural characterization of TiC/C composites

As is shown in Fig.1a, b and c, TiC/C composites are characterized by the presence of a large number of TiC/C particles and the morphologies are depending on the composition. TiC/C coatings at C/Ti ratio of 0.3 were showed by fine grains, which developed into columnar grains with decreasing carbon content. Moreover, with decreasing carbon content the roughness and the number of cavities increased, which suggests that non-stoichiometric composition lead to coarse pored structures. In literature, it is suggested that moderate cavities existing in the interlayer are helpful to reduce the thermal stresses [15]. As is known, evolution of TiC crystallites is governed by a competitive reaction between metallic titanium and amorphous carbon. According to Hertz-Knudsen formula [16], growth-driving force in vapor phase can be expressed as follows:

σ =(p−pe)/pe

where σ, p and pe are growth driving-force, pressure and equilibrium vapor pressure, respectively.

Therefore, growth-driving force of carbon is higher than that of titanium and titanium carbide at high C2H2 flux, and deposition of amorphous carbon is the dominant reaction. The amorphous carbon may hinder the growth of TiC crystallite and leads to grain refinement in chemically vapor deposited coatings [6]. While decrease in a-C:H content will result in insufficient obstacles for growth of TiC crystals, and the crystallite size sharply increases. Further investigations carried out in Fig.2 indicated that the growth of TiC crystals was accompanied by the formation of small islands on surfaces. EDS results showed that the small islands and crystals are different in Ti content, which will be useful to illustrate the deposition process in future.

a Fig.1a C:Ti = 68:32 Fig.1b C:Ti=56:44

Fig. 1c C:Ti=49:50

Fig. 1. SEM micrographs of TiC/C composites with different amorphous carbon contents.

Fig. 2a. SEM micrograph of TiC/C composite.

island

crystal

a b

c

Fig. 2b. Compositions of small islands. Fig. 2c. Compositions of crystals.

Fig. 2. SEM micrograph and EDS analyses of TiC crystals. Substrates with non-graded TiC coatings are shown in Fig. 3. Although TiC/C transition layers exist

due to diffusion of titanium and infiltration of TiCl4, large cracks can be observed in the surface of coatings. The cracks may become channels for hydrogen ion to diffuse into substrates, and result in erosion of substrate and contamination into plasma environment. To fulfill requirements of PFCs, TiC/C transition layers are needed to introduce into the coating system.

Fig. 3. Microstructures of substrates with non-graded TiC coatings (NG).

(a: cross-section; b: surface; c: transition layer) The cross-section micrographs of the TiC/C graded coatings with the thickness of about 120 um are

shown in Fig. 4, and each layer was labeled L-1, L-2, L-3, respectively. It is obvious that the formation of graded TiC/C coatings is dense and no cracks originated from mismatch of CTEs between coatings and matrixes are observed. A dense and crack-free coating exposed to plasma against degeneration is expected, so lack of cracks in TiC/C coatings is especially advantageous to inhibit the diffusion of hydrogen.

a b

c

Fig. 4. BE and EDS analyses of the cross-section of the transition layers. For further study, X-ray spectrums of different TiC/C layers are given in Fig. 5. At low carbon content,

the TiC crystallites exhibit a preferred orientation, which is indicated by the peak corresponding to (111) in the TiC structure. With increasing carbon content, the slight preferred orientation in (111) changes to (200). Moreover, calculation from the width of XRD peaks was carried out, and results suggested that the crystal size of TiC amounts to about 6 um at 46 at. % amorphous carbon contents, which is increased with decreasing carbon content. The phenomenon is similar to the investigation of Yeon-Gil Jung etl. [17], which is considered to be resulted from the difference of deposition rate.

123467

L-1L-2L-3

1 2 3 4 5 6 7

0

20

40

60

80

100

Con

tent

s of

am

orph

ous

carb

on (%

)

N um bers o f lines

3.2. Effect of graded TiC/C coatings on mechanical properties of graphite For practical use, reasonable mechanical properties and porosities are needed. Many works have been made to study the mechanical strength of graphite [18, 19]. However, no literature has been found that reports the effect of TiC/C graded coatings on properties of graphite. In the study, effect of the coatings on substrates was investigated and the properties were listed in Table 1, where NG means the substrate with non-graded TiC coatings, NC and GC mean substrates and substrates with TiC/C graded coatings, respectively.

Compared to NC, the bulk density, compressive strength of NG and GC was increased, and the open

(111

(200)

(220)

20 30 40 50 60 70 802 the ta

(311)

(222) (002)

(101)

L-1

L-2

L-3

Fig. 5. XRD patterns of each TiC/C layer( :TiC; :carbon).

porosity decreased to 13.1% and 12.7%, respectively. Further investigation shown in Fig. 6 indicated that the pore diameter becomes smaller. As is known, formation of dense and high strength TiC/C composites over the substrates could contribute to the strength of materials. In addition, TiC and amorphous carbon were formed by diffusion of TiCl4 and C2H2 into the interior of substrates through pores. Especially, due to the reaction of TiCl4 with substrates, “bridgework effect” may also contribute to the strength of the material. All the processes are favorable to form less pores and improve the density and mechanical strength of substrates. On the other hand, too many TiC formed near the substrate may cause cracks during thermal recycles, so it is very important to control the content of TiC in each interlayer. NC

Fig. 6. Pore size distribution of NC and GC.

3.3 influence of TiC/C graded coatings on thermal shock resistance of TiC coatings Effect of TiC/C transitions on thermal shock properties of the coatings was evaluated and the thermal

cycles were performed in flowing high purity argon between 298K and 1273K. As is shown in Fig. 7, the TiC/C transition layers significantly increased thermal shock resistances of TiC coatings. Without thermal cycles, cracks occurred in the surface of NG. On the contrary, no cracks were observed in the surface of GC. At 60 thermal cycles, large cracks about 4 um occurred at interface of TiC and substrate in NG. As for GC, cracks only 1 um or so were observed in TiC/C interlayer. Moreover, width of cracks occurred during cycles were listed in Fig. 8. As for functionally graded coatings, the continuous variations of composition, microstructure, porosity, young’s modulus and CTE all contribute to obtaining good bonding strength and integrated coatings.

To reveal the mechanism, finite element calculation was carried out. As for GC, the outermost layer was prepared at 1423K, which was higher than deposition temperature of other layers. And conventional properties of TiC and C were calculated by the improved rule of mixture [20]. The CTEs of TiC/C composites were calculated as follows [21]:

ii

iii

EvEvaa

ΣΣ=

where αi, νi and Ei are the CTE, volume fraction and elastic modulus of the ith component, respectively. αTiC=7.2×10-6/K [18], and αC=4.35×10-6/K [19] were used for calculation. Poisson’s ratio was set at 0.21.

Log

diff

eren

tial i

nstr

usio

n (m

L/g

)

Pore size diameter (nm) Pore size diameter (nm)

Log

diff

eren

tial

inst

rusi

on

(mL

/g)

NCGC

And the elastic modulus was calculated by a rule of mixture using ETiC=450GPa [22], and EC=10GPa [19], respectively. Results were shown in Fig. 8. The maximum stress shift in graded coatings is only 130 MPa compared to that in non-graded one 250 MPa. Obviously, stress distribution is varied with changing the composition of TiC/C inter-layers. And thermal stress shifts also exist in TiC/C graded coatings, but the values are much lower than that in non-graded coatings. That is stress shifts are significantly reduced by the transition layers and the thermal stresses in the coatings are balanced between adjacent layers.

Fig. 7a. Surface micrographs of NG and GC before thermal cycles.

Fig. 7b. Surface micrographs of NG and GC after 60 thermal cycles.

Fig. 7c. Cross-section of NG and GC after 60 thermal cycles.

Fig. 7. Effect of TiC/C transition layers on thermal shock resistances of coated graphite.

0 10 20 30 40 50 60

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

number of thermal cyclesav

erag

e w

idth

of c

rack

s bet

wee

n la

yers

(um

)

GC NG

Fig. 8a. Relations between crack size and number of thermal cycles.

-100 -50 0 50 100 150

-50

0

50

100

150

200

250

300

Cal

cula

ted

ther

mal

stre

sses

(MP

a)

distance from substrates (um)

NG GC

Fig. 8b. Distribution of calculated residual thermal stresses in the layer systems.

4. Conclusions It has been shown that TiC/C coatings can be grown on the surface of graphite by chemical vapor

deposition. Morphologies of TiC/C coatings vary with C/Ti ratio, which showed a smooth topology for stoichiometric samples (Ti:C~1:1). Further observation indicated that growth of TiC crystals was initiated by small islands, in which sufficient titanium is needed. XRD results suggest that with increasing a-C:H content the crystal size was decreased. Mechanical tests show that compressive strength of graphite with graded coatings was increased by 35.4%, and the open porosity as well as thermal conductivity was reduced by 33.5% and 16.2%, respectively. Further investigations show that the decrease of thermal stress shifts may contribute to the thermal shock resistances.

Acknowledgements The authors acknowledge the financial support of Fund of ICCCAS (Institute of Coal Chemistry,

Chinese Academy of Sciences) for Youth and Fund of Key Laboratory in Shanxi Province.

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