Patent Publication Number: US-2016222501-A1

Title: Post-treatment method of film-coated member

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2015-0014407, filed on Jan. 29, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a post-treatment method of a film-coated member and, more particularly, to a post-treatment method of a film-coated member as a component facing a nuclear fusion reactor. 
     2. Description of the Related Art 
     About 80% or more of world energy consumption currently depends on fossil fuels and no significant change thereof is expected for the next few decades. According to a report of BP (British Petroleum), there are only about 1.3331 trillion barrels of oil (45.7 years), about 187.5 trillion m 3  of natural gas (62.8 years), and about 826 billion tons of coal (119 years) in recoverable deposits based on 2009. Besides, uranium which is the most commonly used to generate nuclear power will be exhausted after about 70 years. In spite of the resources close to exhaustion, global energy demand will continuously increase. Particularly, a rapid increase in energy demand accompanied with economic growth is expected in large countries such as China, India, and Brazil. 
     Meanwhile, Korea highly depends on fossil fuels to generate electricity and imports crude oil mostly from the Middle East. Since crude oil has instable supply and demand and high price fluctuations depending on the international situation, a solution for stably securing energy is deeply needed. Furthermore, fossil fuels are regarded as the cause of an environmental disaster in terms of carbon dioxide emissions. Development of new clean energy sources for replacing the fossil fuels is highly demanded, and new renewable energy such as solar heat/light energy, biomass energy, and wind energy are be actively developed. 
     However, the new renewable energy has a low energy conversion efficiency, is highly instable depending on natural conditions, and thus is not suitable for the demand. Nuclear fusion energy is regarded as one of solutions thereof. The nuclear fusion energy uses seawater, which is hardly exhausted, as a fuel and is safe compared to nuclear fission. However, a reactant for nuclear fusion should be made to plasma state, and heating is needed to an ultra-high temperature above about 100 million ° C. to generate high-performance plasma and to effectively calculate the amount of output energy. 
     In addition, a technology related to an extreme material capable of withstanding an ultra-high temperature and a high vacuum state is needed first. Tungsten attracts people&#39;s attention as a plasma-facing material in a tokamak which is a device for sealing plasma therein. However, tungsten is brittle at a low temperature, and has a high density in terms of structural load and thus should be used together with another structural material. 
     PRIOR ART 
     Patent Document 
     (Patent document 1) Korean Patent No. 10-1459051 (Oct. 31, 2014) 
     SUMMARY 
     The present invention provides a post-treatment method of a film-coated member to improve mechanical properties thereof. However, the scope of the present invention is not limited thereto. 
     According to an aspect of the present invention, there is provided a post-treatment method of a film-coated member. The method includes applying a pressure to a member including a substrate and an electrically conductive film on at least a surface of the substrate; and heating the film by applying a high-frequency direct current pulse to the film, wherein the pressure is applied in a direction having a component along a direction of a normal vector to a layer on which the film is disposed. 
     The film may be formed by thermal spraying. 
     The thermal spraying scheme may include at least one of gas flame thermal spraying, arc thermal spraying, plasma thermal spraying, detonation thermal spraying, vacuum plasma spraying, and high velocity oxy-fuel spraying. 
     The film may include tungsten. 
     The heating may include heating the film to a temperature of about 900° C. to about 1100° C. 
     The heating may include applying a pulsed current of up to 4000 A and 20 kHz. 
     The substrate may be an electrical conductor, and electrodes for applying the high-frequency direct current pulse may be provided in such a manner that one of the electrodes contacts the substrate and the other of the electrodes contacts the film. 
     The heating may include applying a pulsed current having a current density from 50 A/mm 2  to 250 A/mm 2 , to the film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic diagram of a post-treatment apparatus used in a post-treatment method of a film-coated member, according to an embodiment of the present invention; 
         FIG. 2  is a flowchart of a post-treatment method of a film-coated member, according to an embodiment of the present invention; and 
         FIGS. 3, 4 and 5  are images showing results of analyzing samples implemented using a post-treatment method of a film-coated member, according to test examples and comparative examples of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. However, embodiments are not limited to the embodiments illustrated hereinafter, and the embodiments herein are rather introduced to provide easy and complete understanding of the scope and spirit of embodiments. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. 
     It will be understood that when an element, such as a layer, a region, or a substrate, is referred to as being “on,” “connected to” or “coupled to” another element, it may be directly on, connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like reference numerals refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of embodiments. 
     Spatially relative terms, such as “above,” “upper,” “beneath,” “below,” “lower,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “above” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Embodiments of the invention are described herein with reference to schematic illustrations of idealized embodiments (and intermediate structures) of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, the embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein, but are to include deviations in shapes that result, for example, from manufacturing. 
       FIG. 1  is a schematic diagram of a post-treatment apparatus  1000  used in a post-treatment method of a film-coated member, according to an embodiment of the present invention. 
     Referring to  FIG. 1 , in the post-treatment apparatus  1000  according to an embodiment of the present invention, a thermal-sprayed circular specimen  100  may be put in a mold  110  and then two electrodes  210  may individually contact top and bottom surfaces of the specimen  100 . The mold  110  is formed of, for example, a graphite carbon material, and has a cylindrical hole to put the specimen  100  therein. The specimen  100  may be produced by providing a film  20  on a substrate  10 . The electrodes  210  may be formed of, for example, the same material as the mold  110 , i.e., the graphite material, and may have a size to be put in the hole of the mold  110 . The mold  110  and the electrodes  210  are cylindrical in  FIG. 1  but may have another shape depending on the shape of the specimen  100 . 
     In addition, after the specimen  100  is put in the mold  110 , one of the electrodes  210  may contact the substrate  10 , the other of the electrodes  210  may contact the film  20 , electrode pressers  200  of a spark plasma sintering device  900  may apply pressure thereto based on each condition, and a power supplier  300  may apply a high-frequency direct current pulse to the specimen  100  to heat the film  20  of the specimen  100 . For example, a pulsed current having a current density from 50 A/mm 2  to 250 A/mm 2  may be applied to the film  20  to heat the film  20 . 
       FIG. 2  is a flowchart of a post-treatment method of a film-coated member, according to an embodiment of the present invention. 
     Referring to  FIG. 2 , the post-treatment method according to an embodiment of the present invention includes forming a film on a substrate using plasma thermal spraying (S 100 ), and heating the film by applying pressure and a high-frequency direct current pulse to a member including the film (S 200 ). 
     A detailed description is now given of the post-treatment method. A member including an electrically conductive film may be produced using a thermal spraying scheme on an electrical conductor substrate. The thermal spraying scheme may use at least one of, for example, gas flame thermal spraying, arc thermal spraying, plasma thermal spraying, detonation thermal spraying, vacuum plasma spraying, and high velocity oxy-fuel spraying. 
     The film may be heated by applying the high-frequency direct current pulse to the film while applying pressure to the member including the electrically conductive film produced using the above method. In this case, the pressure may be applied in a direction having a normal-vector-direction component of a layer of the film. Herein, the normal-vector-direction component refers to a non-zero component. Assuming that the film is in a horizontal layer, the direction of applying the pressure may be a direction perpendicular to the layer of the film. 
     That is, for example, a specimen may be produced by plasma-thermal-spraying tungsten powder on a graphite substrate using helium and hydrogen as a plasma induced gas. The substrate may use a graphite substrate. The specimen may be put in a mold to contact electrodes, mounted in a spark plasma sintering device, and then pressed with a pressure of about 2 kN to 3 kN. 
     In this case, one of the electrodes for applying a high-frequency direct current pulse contacts the graphite substrate and the other contacts the tungsten film. As such, a pulsed current of up to 4000 A, 20 kHz may be applied to the tungsten film and thus the tungsten film may be heated to a temperature of about 900° C. to 1100° C. After reaching a set temperature, the tungsten film may be maintained at the set temperature for a certain period of time and then cooled. 
     A description is now given of test examples to which the above-described technical idea is applied. However, the following test examples are given only for a better understanding of the present invention and the present invention is not limited thereto. 
       FIGS. 3 to 5  are images showing results of analyzing surface microstructures and hardness of samples implemented using a post-treatment method of a film-coated member, according to test examples and comparative examples of the present invention. 
     Referring to  FIGS. 3 to 5 , samples  1  to  8  correspond to the comparative examples of the present invention, and samples  9  to  14  correspond to the test examples of the present invention. Specimens produced by plasma-thermal-spraying a tungsten film to a thickness of about 200 μm on a graphite substrate are used in all cases. 
     Specifically, sample  1  is a reference specimen produced by plasma-thermal-spraying tungsten powder on a graphite substrate without performing post-treatment, and optical microscope, scanning electron microscope, and hardness tests are performed thereon to check basic information of the reference specimen. Sample  2  is a specimen produced by performing vacuum heat treatment while maintaining a vacuum level of 10 −4 torr at about 900° C. for about 1 hour. Samples  3  to  8  are specimens produced by applying pressures of 2 kN and 3 kN in a deposition direction at about 900° C., 1000° C., and 1100° C. and maintaining the temperatures for about 10 minutes before cooling. Pressure and heat treatment is performed on the specimens by setting a heating rate to about 100° C./min at about 1 atmosphere of an argon (Ar) gas. 
     Meanwhile, samples  9  to  14  are specimens produced by applying a high-frequency direct current pulse to the samples to generate resistance heat in the specimens to perform heat treatment. A spark plasma sintering device is used to form an environment for allowing the pulsed current to flow through the specimens, and pressures of about 2 kN and 3 kN are individually applied to allow electrodes to contact the specimens to flow the current therethrough. In this case, final temperatures of the specimens are about 900° C., 1000° C., and 1100° C., and are maintained for about 10 minutes before cooling. 
       FIG. 3  shows results of analyzing microstructures of surfaces of the samples using an optical microscope,  FIG. 4  shows results of analyzing microstructures of surfaces of the samples using a scanning electron microscope. Referring to  FIGS. 3 and 4 , based on characteristics of thermal spray coating, splats of molten powder are sequentially accumulated on the substrate due to a spray gas and thus cracks are shown between the splats, which are called splat boundaries. 
     Since the splat boundaries inhibit overall properties of the tungsten film, heat treatment is performed to eliminate the same. The splat boundaries may be observed using an optical microscope and may also be clearly viewed using a scanning electron microscope. When tungsten splats are accumulated on the substrate, due to a large difference in temperature between the substrate and the molten tungsten splats, the speed at which a melting point is transferred to the inside is less than the speed at which crystal grains grow. Accordingly, a crystal growth direction is determined and the crystal structure inside one splat forms a columnar structure. Since solidification occurs instantaneously and rapidly due to contact with the substrate, a sufficient time to grow crystals having a preferred orientation may not be secured. 
     Furthermore, a microstructure of sample  1  is compared to those of the other samples in terms of splat boundaries. When a rate of splat boundaries corresponding to empty spaces between splats is qualitatively compared, sample  2  produced by performing vacuum heat treatment at about 900° C. for 1 hour has a microstructure similar to that of sample  1 . Samples  3  to  8  produced by performing pressure and heat treatment as another post-treatment method, and samples  9  to  14  produced based on post treatment performed by applying a direct current pulse show that regions observed as splat boundaries on the optical images are considerably reduced. 
     Meanwhile, referring to samples  3  to  8  and samples  9  to  14  shown in  FIG. 3 , when pressure and heat treatment is equally performed as post treatment, regions of splat boundaries are reduced and gaps between splats are also reduced if the temperature is higher or if the applied pressure is higher under the same temperature condition. In addition, when the microstructures are compared between the simple pressure and heat treatment method and the post-treatment method for applying a direct current pulse to generate heat under the same temperature and pressure condition, splat boundaries are eliminated a lot from the microstructure produced using the post-treatment method for applying a direct current pulse. 
     In addition to the above optical microscope image analysis results, the scanning electron microscope image analysis results of  FIG. 4  also show that a heat treatment method for directly applying a high-frequency direct current pulse to a specimen is effective to reduce spaces of splat boundaries. 
     Meanwhile, compared to sample  1 , most post-treated specimens show that no preferred orientation is generated or no change in crystal distribution occurs in the tungsten film, because a temperature range of about 900° C. to 1100° C. is not sufficiently high to cause re-crystallization or recovery of tungsten. In general, it is known that a re-crystallization temperature of metal is located between ⅓ and ½ of a melting point thereof and that a re-crystallization temperature of tungsten is located in a temperature range of about 1300° C. to 1500° C. As such, since the temperature condition range of the current test examples and the comparative examples does not reach a re-crystallization temperature, a noticeable change in crystal distribution does not occur in electron backscattered diffraction analysis. 
     Furthermore, it is reported that a spark plasma sintering scheme is more effective to sinter a powdered material compared to a general sintering scheme or a hot pressing scheme because localized necking occurs on fine contact surfaces between powder grains due to Joule heating and thus densification of the material is caused within a short time. 
     In the current test examples, since the tungsten film of the specimen is provided in the form of accumulation of splats, the splats may be regarded as powder grain entities. In this case, pressure and DC electricity are applied to provide a high-frequency pulsed current, temperature is locally concentrated on small contact areas between splats due to Joule heating to reduce the area of splat boundaries, and thus the microstructure of the film becomes more dense. 
       FIG. 5  is a graph showing results of measuring hardness of the samples. Referring to  FIG. 5 , due to continuous reception of plasma particles and heat, a plasma-facing component has a high probability of corrosion and thus the life thereof is shortened. Various mechanical properties such as toughness, ductility, and hardness may be used to evaluate resistance against exposure to plasma particles for a long life. 
     However, the current tests are performed to aim a reference hardness value recommended for an international thermonuclear experimental reactor (ITER) and a demonstration power plant (DEMO). The recommended hardness value is HV 30 and a load of about 300N should be applied. However, since the height of a tungsten film coated using a currently optimized plasma thermal spraying scheme to have durability is about 200 um, if the load of about 300N is applied, the load far exceeds this thickness and thus is not appropriate to evaluate properties of only the tungsten film. As such, the current tests apply a load of up to about 0.5N in consideration of the size of indentations to compare all samples. 
     The measured hardness is 122 HV (a standard deviation is 27). Although the hardness of a tungsten block other than the thermal-sprayed tungsten film is variable depending on a scheme of producing the block or the type of tungsten powder used to produce the block, the hardness of a block produced using raw material powder of the tungsten film of the reference specimen is about 502 HV, and the hardness of the tungsten film of the reference specimen of a plasma-facing component may be about ¼ compared to the bulk block. 
     The reason why the hardness of the plasma thermal-sprayed tungsten film is lower than that of bulk tungsten may include the influence of splat boundaries. Dispersion of splat boundaries on a microstructure exerts a bad influence on overall mechanical properties of the film and considerable amounts of bubbles and cracks are observed to reduce density. 
     The hardness value of sample  1  is compared to those measured after various post-treatment processes. Micro indentation tests are performed on each specimen at an interval equal to or greater than about 100 um after polishing a cross section of the specimen, and results thereof are as described below. Average hardness values of the tungsten films of samples  1  and  2  are 134.66 HV and 134.97 HV, and standard deviations thereof are 13.68 HV and 9.95 HV, respectively. 
     The hardness values of the tungsten films of sample  1  produced without performing post-treatment, sample  2  produced by performing vacuum heat treatment at about 900° C. for 1 hour, and samples  3  to  8  produced by applying pressures of 2 kN and 3 kN in a deposition direction of the specimens at about 900° C., 1000° C., and 1100° C. are shown in  FIG. 5( a ) . 
     In consideration of the size of the samples, the forces of 2 kN and 3 kN correspond to pressures of 25.47 MPa and 38.21 MPa, respectively. It is shown that the hardness increases if the heat treatment temperature is high and if the applied pressure is high. When the heat treatment temperature is 900° C., the hardness values corresponding to the applied pressures of 25.47 MPa and 38.21 MPa are 167.83 HV and 167.87 HV, and standard deviations thereof are 30.74 HV and 22.02 HV, respectively. When the heat treatment temperature is 1000° C., the hardness values corresponding to the applied pressures are 259.26 HV and 276.15 HV, and standard deviations thereof are 46.25 HV and 54.32 HV, respectively. When the heat treatment temperature is 1100° C., the hardness values corresponding to the applied pressures are 282.02 HV and 286.95 HV, and standard deviations thereof are 59.32 HV and 66.22 HV, respectively. Although the ranges of the standard deviations overlap between the processing conditions, it is shown that an increase in temperature or pressure is related to an increase in hardness based on the tendency of average values. 
     In addition, the hardness values of the tungsten films of sample  1  produced without performing post-treatment, sample  2  produced by performing vacuum heat treatment at about 900° C. for 1 hour, and samples  9  to  14  produced by applying pressures of 2 kN and 3 kN in a deposition direction of the specimens and applying a high-frequency direct current pulse at about 900° C., 1000° C., and 1100° C. are shown in  FIG. 5( b ) . 
     Similarly to the case of pressure and heat treatment, the hardness increases if the heat treatment temperature is high and if the applied pressure is high. When the heat treatment temperature is about 900° C., the hardness values measured at the pressures of 25.47 MPa and 38.21 MPa are 212.12 HV and 247.35 HV in average, and standard deviations thereof are 56.75 HV and 32.02 HV, respectively. When the heat treatment temperature is 1000° C., the average hardness values are 340.25 HV and 358.52 HV, and standard deviations thereof are 41.80 HV and 56.52 HV, respectively. When the heat treatment temperature is 1100° C., the average hardness values are 363.16 HV and 363.11 HV, and standard deviations thereof are 46.90 HV and 23.87 HV, respectively. 
     When the post-treatment method for performing pressure and heat treatment is compared to the post-treatment method for applying a high-frequency direct current pulse using a spark plasma sintering device, if post-treatment is performed at the same temperature by applying the same pressure for the same period of time, the hardness of the tungsten film is increased more in the method for applying a high-frequency direct current pulse compared to the post-treatment method for performing pressure and heat treatment using external heating. In this case, an average hardness value of bulk tungsten is measured to about 501 HV when a load of up to about 0.5N is applied as in the above tungsten film test. More than 70% of the hardness of bulk tungsten may be achieved as the hardness of the tungsten film using the post-treatment method for directly applying a direct current pulse to a specimen. 
     By slowing down corrosion of a component facing a nuclear fusion reactor to be exposed to various plasma particles, a cycle of replacing a tungsten film may be increased and the amount of tungsten by-products generated due to corrosion and serving as impurities in reaction plasma may be reduced. 
     A film member implemented using the above-described post-treatment method of a film-coated member, according to an embodiment of the present invention, is merely an example of a component facing a nuclear fusion reactor, and the technical idea of the present invention is not limited thereto. Accordingly, due to an excellent corrosion resistance and a high hardness value, the film member is applicable as a film member used in a highly corrosive environment as well as the component facing a nuclear fusion reactor. 
     As described above, a post-treatment method of a film-coated member, according to an embodiment of the present invention, may densify a microstructure of the film and may improve mechanical properties of a component facing a nuclear fusion reactor. However, the scope of the present invention is not limited to the above effect. 
     While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.