稀有金属(英文版) 2021,40(01),202-211

A comparative study of rhenium coatings prepared on graphite wafers by chemical vapor deposition and electrodeposition in molten salts

Jiang-Fan Wang Shu-Xin Bai Yi-Cong Ye Li-An Zhu Hong Zhang

College of Aerospace Science and Engineering,National University of Defense Technology

作者简介：Li-An Zhu e-mail:mr_zla@163.com;

收稿日期：13 June 2018

基金：financially supported by the National Natural Science Foundation of China (No.51501224);

A comparative study of rhenium coatings prepared on graphite wafers by chemical vapor deposition and electrodeposition in molten salts

Jiang-Fan Wang Shu-Xin Bai Yi-Cong Ye Li-An Zhu Hong Zhang

College of Aerospace Science and Engineering,National University of Defense Technology

Abstract：

The purity,preferred orientation,microstructure,microhardness,bonding strength,thickness uniformity and thermal stability of rhenium(Re) coatings prepared on graphite wafers by chemical vapor deposition(CVD) and electrodeposition(ED) in molten salts were comparatively studied in this paper.It was found that carbon(0.0140 wt%) and oxygen(0.0067 wt%) were the primary impurities for CVD and ED Re coatings,respectively.The diffusion of carbon into CVD Re coating caused higher microhardness near the substrate and helped to improve the bonding strength at the same time.The preferred orientation,microstructure and microhardness of ED Re coating were all susceptible to oxygen.The coating deposition uniformity of ED Re is obviously better than that of CVD Re coating,due to its intrinsic characteristics.The(002)-oriented,coarse columnar CVD Re coating exhibited better thermal stability compared with that of the <110>-oriented,fiber-like columnar ED Re coating,while the ED Re grains grew remarkably and the microstructure evolved toward the similar structure of CVD Re after annealing treatment.The persity of Re coatings in microstructure could be attributed to the mobility of grain boundaries(affected by temperature and impurity) during deposition processes.

Keyword：

Rhenium coating; Graphite wafer; Chemical vapor deposition; Electrodeposition; Molten salt; Grain boundary mobility;

Received： 13 June 2018

1 Introduction

As one of the most promising high-temperature materials,rhenium (Re) has attracted much attention for its exclusive combination of properties including high melting point(3180℃),high modulus,high strength and ductility at elevated temperatures [ 1, 2] .Besides,Re is resistant to a variety of harsh environments and immune to most combustion gases,with the exception of oxygen [ 3] .Therefore,Re is an attractive material for missile propulsion and space system.For example,Re applied in nozzles for both solid and liquid rocket engines has demonstrated tremendous advantages,allowing operation at higher temperature and gas pressure.Re also provides erosion resistance to components in high-temperature rocket engines and hot gas valves,serving as a liner [ 3] .However,Re is one of the heaviest materials (21.04 g·cm^-3) and extremely rare [ 4] ,which limits its widespread applications.Therefore,coatings of Re should be characterized to enable industrial use in the most efficient manner.

Re is the only refractory metal that does not form carbides,and the Re/carbon interface is strong enough at high temperatures due to the diffusion of carbon atoms [ 5] .Consequently,as a lower cost and lighter weight alternative,Re-coated graphite is believed to be a promising substitute for solid Re.Currently,Re can be mainly manufactured by powder metallurgy (PM),chemical vapor deposition (CVD),spray forming,electron beam-physical vapor deposition (EB-PVD) and electrodeposition (ED) [ 6, 7, 8, 9, 10, 11, 12, 13, 14] .The effect of manufacturing methods on the tensile properties has been the research focus of a number of investigations [ 15, 16, 17] .However,to the best of our knowledge,far too little attention has been paid to compare the persity of Re coatings by different preparation methods,which is the intrinsic reason for property differences of Re.Up to now,the most mature and widely used manufacturing processes for Re coatings are electrodeposition (ED) in molten salts and chemical vapor deposition(CVD) [ 10, 12, 13, 18] .Therefore,it is of great interest to compare the characteristics of Re coatings fabricated by these two different methods.In this paper,a general comparison of purity,preferred orientation,microstructure,microhardness,bonding strength,thickness uniformity and thermal stability of Re coatings prepared on graphite wafers by CVD and ED in molten salts was investigated.

2 Experimental

2.1 Preparation of rhenium coatings

The schematic diagrams of the apparatus for preparation of Re coatings by CVD and ED in molten salts are presented in Fig.1.The experimental details were previously described in Refs. [ 13, 18] .Φ18 mm×10 mm graphite wafers (R8710,SGL Carbon Company,Germany) were used as substrates in this study.The graphite wafers were polished,carefully ultrasonically cleaned in acetone for15 min and subsequently dried at 150℃for 5 h.The ED Re coating was prepared in NaCl-KCl-CsCl-K₂ReCl₆molten salts at 800℃with a cathodic current density of50 mA·cm^-2.The rotation speed of cathode was 10r-min^-1.The deposition conditions for CVD Re coating are presented in Table 1.High-temperature annealing treatments of Re coatings were carried out in a molybdenum sheet furnace at 1200℃in hydrogen atmosphere.

Fig.1 Schematic diagrams of apparatus for preparation of Re coatings by a CVD and b ED in molten salts

2.2 Testing

The cross-sectional microhardness (Vickers hardness) of Re coating was measured by a microhardness tester (HXD-1000TC) with a load of 0.2452 N for dwell time of 15 s.The bonding strength of Re/graphite interface was measured by the coating-pull-off test (Fig.2) with a tensile rate of 0.5 mm·min^-1 according to ISO4624:2002(E).

2.3 Characterization

The surface and fracture morphologies of Re coatings were observed by JSM-6490LV scanning electron microscope(SEM).The metallographic structure of Re coating was observed under Hirox KH-3000 optical microscope (OM).The phase composition and preferred orientation of Re coatings were determined by X-ray diffractometer (XRD,Rigaku D/Max 2550VB⁺) using Ni-filtered Cu Kαradiation at a scanning rate of 6 (°)·min^-1 with 2θfrom 30°to80°.To obtain the metallographic microstructure,cross section of Re coating was polished with abrasive papers and diamond pastes followed by chemical etching using a freshly prepared etchant (10 g K₃Fe (CN)6+10 g KOH+100 ml distilled water).Re coatings were detached from the graphite substrates,polished with abrasive papers to remove residual graphite and ultrasonically cleaned in ethanol for 15 min before impurity testing.The oxygen content in as-deposited Re coating detached from the graphite substrate was measured using a Leco TCH-600O/N/H analyzer.The carbon content was measured using a Leco CS-600 C/S analyzer.

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Table 1 Deposition conditions for CVD Re coating

Td_eposition—deposition temperature,—flow rate of chlorine,F_Ar—flow rate of argon,P_total—total pressure,R—rotation speed of sample

Fig.2 Schematic diagram of coating-pull-off test

3 Results and discussion

3.1 Purity

Since impurities,mainly carbon and oxygen [ 9, 13, 19] have a great influence on the mechanical properties of Re,we only measured carbon and oxygen in this study.As shown in Table 2,the carbon content was relatively high but oxygen content was low in CVD Re coating,while results were reversed for ED Re coating.As the raw materials used in CVD process were pure enough (Re powder>99.999 wt%,Ar>99.999 wt%,Cl₂>99.999wt%),the most possible carbon source was from the diffusion of graphite substrate.The incorporation mechanism of oxygen into ED Re coating has been proposed by the Russian scientists in Ref. [ 10] .The ED Re coating is susceptible to oxygen existing in ambient environment,whether it comes from the molten salts or the atmosphere.Thus,the discrepancy in Table 2 could be attributed to the differences in coating growth environment of different processes.The effect of impurities on coating properties will be discussed below.

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Table 2 Carbon and oxygen contents in as-deposited CVD and ED Re coatings

3.2 Preferred orientation

Very often polycrystalline films have what is referred to as a crystallographic texture,in which crystals tend to exhibit specific crystallographic directions normal to the plane of the film [ 20, 21] .Figure 3 shows XRD patterns of Re coatings prepared for different periods.For CVD Re coatings,both the XRD patterns remain almost the same and exhibit only one strongest peak at 40.58°,corresponding to (002) plane of Re.As for ED Re coatings,the(110) peak increases remarkably and other peaks decrease with deposition time increasing.It could be inferred that CVD Re coating has a dominating<002>preferred orientation developed at the early stage,while the ED Re coating reveals an increasing (110) preferred orientation.The (002) plane is the close-packed plane for HCP metals with the lowest surface free energy;therefore,the CVD process performed at a higher deposition temperature(1200℃) is near the thermodynamic equilibrium condition.

The (002)-oriented ED Re coating with higher surface free energy is believed to be a result of non-equilibrium deposition considering its much lower deposition temperature (800℃).It has been demonstrated in the literature that the growth texture of ED Re coating would change as follows:(100)→(110)→(10X)→(001) with the concentration of oxygen-containing impurities in chloride molten salts decreasing [ 10] .The above sequence corresponds to a decrease in surface energy of the corresponding plane.Since the transition from one texture to another depends strongly on the surface adsorption of impurities [ 22] ,it seems that oxygen took part in changing its growth mode.

3.3 Microstructure and morphology

As can be seen from Fig.4,morphology and micros trueture vary remarkably with different processing methods although the deposition rates are almost the same(一40μm·h^-1).The CVD Re coating has a flat-topped surface and a coarse columnar structure with well-crystallized grains extending throughout the coating (Fig.4a-c).A further observation shows that the CVD Re coating consists of two sub-layers,i.e.,an inner nucleation layer of fine equiaxed grains and an outer layer composed of upright columnar grains (Fig.4c).As shown in Fig.4d-f,ED Re coating exhibits a fiber-like columnar structure and a ridge-like surface.There are a large number of fine grains on the bottom side.As the thickness increases,the grains gradually become longer and larger.To clearly reveal themicro structure of Re coatings,metallographic images are presented in Fig.5.

Fig.3 XRD patterns of Re coatings prepared for different periods by a CVD and b ED in molten salts compared with the standard PDF card

Fig.4 SEM images of Re coatings deposited for 1.00 h on graphite:a,b CVD Re surfaces,c CVD Re fracture surface,d ED Re surface and e,f ED Re fracture surfaces

Fig.5 Cross-sectional OM images of as-deposited a CVD and b ED Re coatings on graphite substrates after chemical etching

The persity in preferred orientation,morphology and micro structure of the CVD and ED Re coatings can be explained by the nucleation and growth mechanisms of coating on an indifferent (inert) substrate.Therefore,texture formation may alternatively be considered to arise from the two basic processes,i.e.,nucleation (initial texture) and growth of competing crystal surface (growth texture),which depends on the relative surface energy [ 23] .

As for the nucleation process,temperature affects the critical radius of crystal nucleus,only above which the crystal nucleus is stable.It is generally found that the critical radius of crystal nucleus increases and the nucleation rate decreases correspondingly with the temperature increasing [ 24, 25] .Therefore,the critical radius of crystal nucleus of CVD Re is much larger than that of ED Re.Fewer but larger crystal nucleus could survive and continue to grow.Consequently,the difference in Re coatings near the substrate (Figs.4c;5e) could be mainly attributed to the different nucleation processes occurring at different temperatures.At higher temperature,the mobility of grain boundary is also high due to the high diffusivity of atoms [ 21] .The grain structure evolves continuously during the nucleus coalescence and coating thickening processes,resulting in the coarse and well-crystallized grains throughout the thickness of CVD Re coating.The macrosteps at the edge of surface grains indicate that the grains adopted a lamellar growth mode,which is usually performed by two-dimensional (2D) nucleation [ 26] .2D nucleation generates a new plane on the substrate surface,where adsorbed atoms incorporate one after another into microsteps without forming new nuclei [ 27] .New steps can be reproduced by 2D nucleation [ 28] .Multiplying of these microsteps (which is called"bunching") leads to the formation of macrosteps [ 29] .Besides,it has been confirmed by XRD result that most CVD Re crystals were oriented with the densely populated (002) plane parallel to the substrate.Hence,it can be inferred that the hexagonal,flattopped crystal plane is parallel to (002) plane of the HCP Re crystal.As the grains of the columnar structure extend throughout the coating and the grain boundaries are almost straight,it could be inferred that (002)-preferred texture was formed and developed at the early stage of the deposition.

As for the ED Re coating,the nucleation and thickening processes were developed at a much lower temperature.The critical radius of the crystal nucleus is much smaller than that of CVD Re,and thus,the nucleation rate is correspondingly very high.As the grain boundaries are immobile due to the low diffusivity of atoms,the grain structure co-determined by the nucleation,growth and coalescence processes is retained at the base of the coating,resulting in a fine nucleation layer in the ED Re coating(Fig.4e).As the thickness increases,the grains gradually become longer and larger (Fig.5).On the bottom side of Re coating,there should be a competitive growth between differently oriented grains.Some grew continuously,and others were buried by more competitive neighboring grains.According to XRD results,the preferentially growing grains are those having{110}crystallographic planes parallel to the substrate.Based on the theory by Pangarov [ 30] ,the ED Re coating adopts an outward-type growth with the most densely populated atom row<110>in the most densely populated atom plane perpendicular to the substrate.

As discussed above,temperature plays a crucial role in determining the nucleation and growth processes of Re coatings by affecting the mobility of grain boundary(diffusivity of atoms).Given that the oxygen and carbon are the primary impurities in Re coatings,the effect of impurities on the microstructure of Re coatings should not be ignored.The diffusion of carbon atoms is mainly along the grain boundaries of Re [ 31] ;therefore,the carbon appears to have little influence on the as-built microstructure of CVD Re coating.While the incorporation of oxygen is coinstantaneous with the normal deposition process of Re,oxygen probably plays an important role in modifying the microstructure of ED Re coating.As known,the addition of even a minute quantity of impurity to an otherwise pure metal can dramatically decrease the boundary mobility (by several orders of magnitude) [ 32] .Impurities in coating can lead to grain boundary drag and even grain growth stagnation [ 33, 34, 35] .So,incorporation of impurities during coating growth can suppress grain growth during coalescence,leading to smaller as-deposited grain sizes [ 21] .Given that the grains of ED Re prepared in the literature with lower oxygen content (<1×10^-6) are much larger than that of ED Re prepared in this study [ 9] ,it could be inferred that the fiber-like columnar structure of as-deposited ED Re coating may be a result of the oxygeninduced suppression of grain growth during coating thickening process.

To sum up,the persity in preferred orientation,morphology and microstructure of Re coatings prepared by CVD and ED can be attributed to the differences in nucleation and growth processes.Compared with the CVD Re coating,the fine-grained structure of ED Re coating can be attributed to the grain growth suppression by low temperature and impurity.

3.4 Microhardness and bonding strength

Figure 6 shows the indentation images of Re coatings after microhardness test.It is found that the microhardness of CVD Re coating is evidently location dependent along the thickness direction.The coating near the substrate (smaller indentation) is harder than that close to the surface (larger indentation).The result is consistent with that obtained from Re coating prepared by CVD on C/C composite [ 36] .On the contrary,the microhardness of ED Re coating is relatively homogeneous along the thickness direction.Isobe et al. [ 37] investigated the annealing treatment of Re coating prepared at 700℃by thermal decomposition of Re₂(CO)10 on graphite substrate.It was found that the lattice parameter apparently increased after annealing at1100℃,with microhardness increasing from 254 to429 kg-mm^-2 at the same time,while the lattice parameter remains almost unchanged by annealing at 800℃.It is suggested that the diffusion of carbon atoms could be neglected during the ED process (800℃) and would aggravate during the CVD process in this study (1200℃).As a result,the diffusion of carbon atoms is the responsible factor for the increase in the microhardness of CVD Re coating near the substrate,while the higher microhardness of ED Re coating prepared in this study can be attributed to the effect of oxygen,compared with that(～300 kg·mm^-2) of Re prepared by electroforming in the literature with lower oxygen content (<<1×10^-6) [ 8] .From this point of view,the microhardness of Re coating was a sensitive indicator of its purity and even the impurity distribution.

Fig.6 Indentation images of Re coatings after microhardness test:a CVD and b ED

The experimental results of the pull-off test can be pided into the followings:if the rupture position is at the substrate or the adhesive,it means that the bonding strength of the coating/substrate interface is higher than the rupture strength;if the rupture position is at the coating/substrate interface,it means that the bonding strength is equal to the rupture strength.The rupture surfaces of Re/graphite samples after pull-off test are shown in Fig.7.For CVD Re coating,the test is finished with the rupture of graphite substrate itself,leaving two fractured rough graphite surfaces (Fig.7a).The rupture strength is as high as22.96 MPa,indicating that the bond strength of CVD Re/graphite interface is even higher.While for ED Re coating,the specimen was just detached at the interface between Re coating and graphite,showing two smooth cleavage surfaces (Fig.7b),the rupture strength is only10.20 MPa,indicating a poor interface bond between ED Re coating and graphite substrate.The better adhesion of CVD Re/graphite interface could also be attributed to the massive diffusion of carbon atoms into Re coating during deposition process [ 31] .

In conclusion,carbon atoms diffused into the CVD Re coating,causing higher microhardness near the substrate and helping to improve the bonding strength at the same time,while no obvious diffusion happened during the ED process,causing a poor bonding strength at the Re/graphite interface.The microhardness of Re coating is sensitive to impurities,either from the inner substrate (carbon) or the outer environment (oxygen).

3.5 Thickness uniformity

Thickness uniformity was evaluated for Re coatings prepared for both short-and long-time deposition,as it is important to obtain uniform Re coating on a complexshaped component in practical applications.Figure 8depicts cross-sectional and surface images of CVD and ED Re coatings deposited for different periods for comparison.The thickness uniformity of Re coatings also varies considerably with different processing methods.The CVD Re coating exhibits poor thickness uniformity even for 1.00-h deposition.With deposition time increasing,the coating at the edge of substrate has an accelerated growth rate and the thickness uniformity aggravates remarkably.Besides,the crystal grains of CVD Re coating at corner grow outward and become much larger than those at the adjacent position.It is demonstrated that the CVD Re coating has an obvious preferential growth at the edge than other positions.

Fig.7 The rupture surfaces of a CVD Re/graphite sample and b ED Re/graphite sample after pull-off test

Fig.8 Cross-sectional and surface SEM images of CVD and ED Re coatings deposited for different periods:a CVD 1.00 h cross section,b CVD 3.00 h cross section,c CVD 3.00 h surface,d ED 1.00 h cross section,e ED 12.00 h cross section and f ED 12.00 h surface

The gas velocity magnitude at the edge is much larger than other positions,which will supply Re source more quickly.What is more,the edge position with a thicker Re coating would be heated to a higher temperature by induction coils,which will in turn promote the preferential growth again.Therefore,the deposition process is prone to be controlled by surface-reaction kinetics and gradually changes to mass transport process from the edge to the center of substrate.As a result,the Re coating at the edge has an accelerated growth rate and the thickness distribution aggravates over time.

It is found that the growth rate of ED Re coating can be perfectly kept constant (～40μm·h^-1),i.e.,the thickness of Re coating deposited for 12.00 h is 11-12 times thicker than that deposited for 1.00 h.Since the current distribution is heterogeneous on an intricate-shaped substrate during the ED process,the edge of the substrate often has a higher current density (higher growth rate) than other positions [ 38] ,and the ED Re coating still exhibits good thickness uniformity for both short-and long-term deposition.The conventional electrodeposition process in molten electrolytes tends to be diffusion controlled [ 39] .In that case,concentration polarization would gradually aggravate,surface roughening and even dendrite or powder would often occur under the experimental conditions (inert anode condition and long-term deposition).Therefore,the ED process in this study is believed to be surface reaction kinetics controlled by the rotation of cathode and good thickness uniformity is obtained despite the heterogeneous current distribution.

3.6 Thermal stability

As Re is generally served at high-temperature applications,thermal stability of Re coatings with different microstructures and preferred orientations needs to be evaluated.Figures 9 and 10 show the fracture morphologies and XRD patterns of as-annealed Re coatings compared with the asdeposited Re coatings.For CVD Re coating,the grains grow slightly and XRD pattern remains almost the same after annealing treatment at 1200℃for 3.00 h.On the contrary,the grains of ED Re coating grow remarkably and the microstructure evolves toward the similar structure of CVD Re coating.Besides,the preferred orientation of ED Re coating changes from (110) to (101) after annealing treatment at 1200℃for 1.00 h and retains (101) after longer annealing treatment (3.00 h).It can be concluded that the CVD Re coating exhibits better thermal stability compared with ED Re coating.

As discussed in Sect.3.2,CVD process is near the thermodynamic equilibrium condition,which can be also demonstrated by the good thermal stability upon annealing,while ED Re coating with higher surface free energy is a result of non-equilibrium deposition at a lower temperature.The suppression of grain growth during deposition (by both low temperature and impurity) leads to a rapid increase in the grain boundary energy per volume,which is thermodynamically unstable at higher temperature.Upon annealing,the microstructure and orientation of ED Re coating will be rearranged to the configuration of lower energy (lower surface energy and interface energy).Besides,the heat treatment is necessary to improve the bonding strength with the graphite substrate and eliminate the oxygen in as-deposited ED Re coating.The rupture strength of ED Re-coated samples is also higher than the strength of graphite after annealing at 1200℃for 1.00 h,as all the tests were finished with the rupture of substrate itself.The result is consistent with the report in our previous studies [ 18, 20] .The C and O contents of Re coatings after annealing at 1200℃for 3.00 h are also given in Table 3.The change of C content and bonding strength proves the speculation of the strengthen effect of C by diffusion.

Fig.9 Fracture SEM images of as-deposited and as-annealed Re coatings:a as-deposited,CVD,b 1200°C,3.00 h,CVD,c as-deposited,ED and e 1200℃.3.00 h,ED

Fig.10 XRD patterns of as-deposited and as-annealed Re coatings:a CVD and b ED

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Table 3 Carbon and oxygen contents in as-annealed CVD and ED Re coatings

Based on the persity of Re coatings prepared in this study,it can be concluded that grain structure evolution from nucleation to thickening process occurs in two fundamentally different ways,as schematically illustrated in Fig.11.If grain boundaries are mobile (CVD),the Re coating will exhibit a well-crystallized,coarse columnar structure with most grain boundaries traverse the thickness of the coating.If the grain boundaries are immobile (ED),the Re coating will exhibit a fine-grained columnar structure.The mobility of grain boundary can be suppressed either by low temperature or impurity (oxygen).Heating of such a coating will improve the boundary mobility and eliminate the effect of oxygen,leading to observable grain growth and evolution toward the similar structure of CVD Re coating.

Fig.11 Overview of grain structure evolution during deposition of Re coating

4 Conclusion

The (002) preferentially oriented CVD Re coating revealed a coarse columnar structure and flat-topped surface.The diffusion of carbon (0.0140 wt%) into CVD Re coating caused higher microhardness near the substrate and helped to improve the bonding strength at the same time.The CVD Re coating had poor thickness uniformity but good thermal stability.In conclusion,CVD process is best suit for applications that require thin Re coatings with good adherence.

The (110) preferentially oriented ED Re coating had a fiber-like columnar structure and ridge-like surface.The preferred orientation,microstructure and microhardness of ED Re coating were all susceptible to oxygen (0.0067wt%).The growth rate was relatively homogeneous,and the thickness uniformity was easy to be obtained on the wafer-shaped substrate.In conclusion,ED process is best suit for applications that require uniform Re coatings or thin-walled Re components,as the mandrel is easy to be removed.

The microstructure of ED Re coating was less thermodynamically stable than CVD Re coating.The grains of ED Re coating grew remarkably,and the microstructure evolved toward the similar structure of CVD Re coating.

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