Microstructure and Residual Stress of TiC/C Functionally Graded Materials for Irradiation Protection of Carbon-based Composites

TIAN Wei , LI Xuhua , ZHANG Hongliang , WANG Kunjie , ZHANG Zhaofu

(1. Xi’an Aerospace Composites Research Institute, Xi’an 710025, Shaanxi, China; 2. Academy of Aerospace Solid Propulsion

Technology, Xi’an 710025, Shaanxi, China; 3. The 8th Military Representative Office of Air Force Equipment Department in Xi’an, Xi’an 710065, Shaanxi, China)

Abstract: To improve the erosion resistance of carbon-based facing materials during plasma instabilities, a series of TiC/C composites were prepared on C/C substrates via chemical vapor deposition (CVD). Microstructure and composition of the samples after long pulse plasma discharge in HT-7 tokamak were characterized by using scanning electron microscopy (SEM)and X-ray photoelectron spectroscopy (XPS). Distinct eroded pits were observed in the C-rich TiC/C coatings, but no cracks were present. On the contrary, the eroded pits were ambiguous in the case of pure TiC coating, but large cracks were observed. On the basis of these results, TiC/C functionally graded materials were further prepared, whose residual thermal stresses and deformation were then analyzed by using finite element calculation. It was found that the introduction of TiC/C composites as transition layer had significantly decreased the stress shifts, with the largest shift to as low as 474 MPa. Additionally, extent of deformation was largely decreased. This is beneficial to avoiding formation of large cracks and preventing impurities from entering plasma environment.

Key words: titanium carbide coating; pyrocarbon; functionally graded materials; finite element calculation

1 Introduction

In the last couple of years, the application of carbon/carbon composites has been extended to international thermonuclear experimental reactor(ITER) plasma facing components[1]. However, the next generation of fusion device will be operated at quasi-steady state condition[2]. Higher heat load and stronger energetic particle bombardment will be exerted on the surface of plasma facing components(PFCs). As a consequence, more carbon impurity from the plasma-facing materials will be released into the plasma, which will induce the loss of plasma energy and influence properties of the plasma confinement. To overcome the limits of carbon-based facing materials, processes associated with the erosion have been focused in terms of plasmamaterial interaction[3].

To improve the erosion resistance of carbon-based facing materials during plasma instabilities, the application of coatings is an effective way[4]. Due to the combined properties of high melting point, strong thermal shock resistance and high chemical/physical sputtering resistance under bombardment by highly energetic particles[5,6],chemical vapor deposited TiC coating is considered as a promising candidate of coating materials to withstand the high-energy and particles fluxes[7,8].However, intense stresses originated from mismatch of coefficients of thermal expansion (CTEs) between the coatings and substate materials may result in cracks in the coatings. Then, the cracks will provide channels for deuterium, thus leading to erosion of the substrates. Therefore, TiC/C transition layers with gradient CTEs may have potential to relieve the CTE-induced residual stresses. However, up to now, only thermodynamic calculations on co-deposition of TiC and pyrocarbon were carried out[9,10,11]. Almost no systematic work on properties of TiC/pyrocarbon functionally graded materials (FGMs), especially irradiation resistance property, has been reported.

In this study, a series of TiC/C composites were prepared on C/C substrates by using chemical vapor deposition. The distribution of TiC particles in the pyrocarbon was examined by using transmission electron microscopy (TEM). For further study, the as-prepared coatings were exposed to long pulse plasma discharge in HT-7 tokamak, while the microstructure and composition changes after long pulse plasma discharge in HT-7 tokamak were characterized by using scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy (XPS).On the basis of these results, TiC/C functionally graded materials were further prepared and used as limiter tiles that were exposed to pulse plasma discharge. Residual thermal stresses and deformation of the materials were analyzed by using finite element calculation to clarify the prevention of cracks in the FGM coatings.

2 Experimental

C/C composites were polished and then ultrasonically cleaned in distilled water, ethanol and acetone. TiC/C composites were prepared on dried C/C composites by using thermal CVD. During deposition, H2(99.99%, Xianyang Qinhong Gas Manufacturing Co., Ltd.) and Ar(99.99%, Xianyang Qinhong Gas Manufacturing Co., Ltd.) were used as reaction and protective/carrier/diluent gases, respectively. Moreover,titanium tetrachloride (TiCl4, 99.9%, Sinopharm Chemical Reagent Co., Ltd.) imbedded in an evaporator and acetylene (C2H2, 99.9%, Xianyang Qinhong Gas Manufacturing Co., Ltd.) were used as the sources of titanium and carbon, respectively.The amount of TiCl4transferred into the reactor was regulated by the flow rate of carrier gas (H2)and the temperature of the evaporator. The composition of the TiC/C composites was controlled by adjusting the flow rates of the TiCl4and C2H2gas at intervals from 0 to 35 sccm and 100 to 65 sccm, respectively. The deposition temperature was controlled in the range of 1000-1200 °C.

Irradiation resistances were tested under continuous plasma bombardment, while the surface temperature, voltage and current parameters were 1200 K, 10-12 keV and 1 A,respectively. Microstructures and compositions of the coatings after the irradiation tests were examined by using SEM and XPS, respectively.

3 Results and Discussion

3.1 TEM observation of TiC/C composites

Fig. 1 shows a representative TEM image of the TiC particles and pyrocarbon. It is seen that the TiC particles are imbedded in the pyrocarbon matrix, forming a uniform composite. Moreover,the change in volume of the TiC/C composites was lower than that of pure TiC based on the mixture rule. Generally, due to the different volume change caused by CTE mismatch, large cracks could be easily formed at the surface of pure TiC coated carbon substrate. However, no cracks were observed on the surface of the C-rich coatings, because the large expansion of the TiC particles during the thermal cycles has been relieved by the pyrocarbon. In this case, mismatch of CTEs in the TiC/C composites would have been relieved, so that penetrating cracks on the surfaces might be avoided. Therefore, the TiC/C composites could be used as coatings on carbon-based substrates.

Fig.1 TEM image of the TiC/C composites (mole ratio of TiC︰C = 2︰1)

3.2 Microstructure and composition of the samples after plasma discharge

As PFCs, irradiation resistance was one of the most important properties. Herein, different TiC/C coatings were irradiated by plasma in the HT-7 tokamak facilities. As shown in Fig. 2,microstructure changes induced by the discharge were observed in the SEM images. As shown in Fig. 2(a), the original C-rich coatings were coarse and the material at edges of the cavities was loosely bound to the surface. Removal of loose particles as well as erosion of excessive pyrocarbon might become the source of dust to enter the plasma environment. As a consequence,eroded pits were distinct, as shown in Fig. 2(b).On the other hand, the presence of excessive pyrocarbon lowered the CTE of the coatings,which would facilitate to lower the concentration of the residual thermal stresses at the interfaces of the two materials. Therefore, cracks might be absent at the interfaces. However, for pure the TiC coating shown in Fig. 2(c-d), although the eroded pits were ambiguous, large cracks were observed.These cracks could act as channels for hydrogen ion to diffuse into the substrates, thus resulting in erosion of the substrates and impurities into the plasma environment. Due to the respective advantages of pure TiC and C-rich coating,combination of the two could be a solution, i.e.,making TiC/C graded materials. Fig. 2(e) shows SEM image of the TiC/C graded sample before plasma discharge. The coatings are highly dense,without the observation of pores. After plasma discharge, unlike the morphologies as shown in Fig. 2(b) and Fig. 2(d), there were no large cracks and deep eroded pits on the surface of the sample,as shown in Fig. 2(f).

Fig.2 Microstructure changes of the composite coating (TiC︰C=1︰2) (a, b), TiC coating (c, d) and TiC/C graded coating(e, f) before (a, c, e) and after (b, d, f) plasma discharge

Fig. 3 shows XPS spectra of the sample with mole ratio of TiC : C=1 : 2. Two peaks located at binding energies of 455.0 and 461.3 eV are assigned to titanium carbide[12]. Other peaks belonging to Ti (IV) oxide and oxidized titanium with intermediate oxidation states were also detected. As shown in Fig. 3(b), the intensity of the pyrocarbon became to be lower than that of TiC. Further calculation according to the XPS spectra suggests that the content of pyrocarbon was greatly decreased from 63.3% to 35.3% after the discharge experiments. On the contrary, the content of Ti was increased from 31.7% to 59.8%.Therefore, it can be concluded that the irradiation changed the stoichiometric ratio of C and Ti during the discharge experiments. As a consequence, after irradiation, the C/Ti became to be 1.7, due to the depletion of C[7].

Fig.3 XPS spectra of the TiC︰C=1︰2 sample before and after plasma discharge

3.3 Residual thermal stresses and deformation of the coatings

Based on above results, TiC/C coating systems were designed to improve the irradiation resistance. In this system, C/C substrate mainly maintained the structural integrity in the high-temperature environment, pure TiC coatings retarded the reaction of atomic hydrogen with the substrates, and TiC/C composites acted as the transition layers. Due to the lower CTE of the TiC/C composites, the transition layers might be able to effectively relieve the residual thermal stresses at the interfaces and avoid the occurrence of cracks. Microstructure difference shown in Fig.2 indicated the advantageous of TiC/C transition layers to protect the substrates against the plasma erosion. To clarify the relation between cracks and residual thermal stresses originated during the thermal cycles between 1373 K and 298 K, finite element calculation was conducted.

Since TiC particles possess a significantly higher stiffness than C/C composites, a change in stiffness and internal stresses would be present,which may be propagated towards the surface of the coating, thus varying the stress state of the coating materials. Residual thermal stresses between the adjacent layers may result in cracks at the interfaces of the two materials. In addition, the intense stresses may result in cracks in the coating itself and deformation of the materials. Kim et al.[13]developed a finite element method to calculate the residual thermal stress distribution in carbon/ceramic FGM materials. Herein, the method was used to study the deformation of the TiC/C FGM cylinder. The elastic moduli and CTEs of TiC/C composites were calculated as follows[14,15]:

whereEmandEcare elastic modulus of the two phases, whileVmandVcare volume fractions of the two components. Specifically, EC= 10 GPa[16]and ETiC= 450 GPa[17]were used for calculation. Moreover, αm, Kmand K are CTE, bulk modulus of m component and that of graded layer, respectively. During calculation, αTiC= 7.2×10-6/K[18]and αC= 4.35×10-6/K[19].Additionally, Poisson’s ratio was set to be 0.21.Results are shown in Fig. 4.

Fig.4 (a) Residual thermal stress shifts and (b) deformation of different coatings

It was found that the stress shift of pure TiC-coated C/C substrate reached 1230 MPa, but the lowest stress of C-rich coating was just 564 MPa. Moreover, according to the dimension deformations denoted as a, b and c, the buckling deformation extent of single layer coatings was increased with increasing content of TiC.Introduction of the TiC/C composites as transition layer could significantly decrease the stress shifts, with the largest shift to be as low as 474 MPa. Additionally, the extent of deformation was largely decreased, which is beneficial to avoiding the formation of large cracks and preventing impurities from entering the plasma environment.

4 Conclusions

TiC/C composites with TiC particles to be imbedded in pyrocarbon were successfully fabricated by using CVD. No large cracks and deep eroded pits were observed on the surface of the TiC/C composites after plasma discharge.These TiC/C composites could act as transition layer in TiC/C coating systems to form TiC/C functionally graded materials (FGMs). The as-prepared TiC/C FGMs exhibited largely enhanced irradiation resistance, because the residual thermal stresses were reduced due to the application of the TiC/C transition layer.Additionally, extent of deformation was largely decreased.