Patent Publication Number: US-2023160473-A1

Title: Seal structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage application of International Patent Application No. PCT/JP2021/012995 filed on Mar. 26, 2021, which claims the benefit of Japanese Patent Application No. 2020-065182, filed on Mar. 31, 2020. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a seal structure. 
     Description of the Related Art 
     Seal structures have a function of preventing leakage of a liquid or a gas from the inside of a machine or the like to the outside, and a function of preventing the intrusion of dust and the like into the inside of the machine or the like from the outside. Such seal structures can be roughly divided into dynamic seals such as oil seals and mechanical seals, and static seals (i.e., fixing seals) such as gaskets. 
     A dynamic seal needs to serve both the sealing and lubrication functions on a sliding surface. As a technique that can achieve both excellent sealing performance and excellent lubricity on a sliding surface, a technique described in International Publication No. WO 2018/198876 is known. International Publication No. WO 2018/198876 describes that a sliding surface of a mechanical seal is provided with a polymer brush layer so that when a sliding member is rotated at the boundary between different environments, such as environments of different pressures or environments of different gases, on the opposite sides of the seal, it is possible to improve the sealing performance and achieve low frictional properties at the boundary or in a mixed lubrication region. 
     However, in the seal structure described in International Publication No. WO 2018/198876, as illustrated in  FIG.  2    thereof, for example, when the sliding member rotates in a sealed state under environments of different pressures, a plane to which a load is applied due to the pressure difference is parallel with the rubbing surface (i.e., a contact surface between a mating ring  24  (i.e., a seal member  1 ) and a seal ring  25  (i.e., a seal member  2 )) during the rotation, and thus, the applied load acts directly on the rubbing surface. Therefore, since a load (i.e., a contact load) is continuously applied to the polymer brush layer, there is concern that the wear of the polymer brush layer may proceed. The wear of the polymer brush layer proceeds significantly when the sliding member rotates in a sealed state under environments of different pressures. In addition, such wear is also seen when the sliding member rotates in a sealed state under environments of different gases. If the polymer brush layer wears, it will become unable to achieve the effects of improving the sealing performance and providing low frictional properties. 
     Meanwhile, International Publication No. WO 2018/199181 describes, as illustrated in  FIGS.  1  and  2    thereof, a technique for achieving the effects of improving the sealing performance and providing low frictional properties with a rubbing surface by forming a polymer brush layer on the outer peripheral surface of a shaft (i.e., a shaft member, a rod specimen  3 ) that rubs on the inner peripheral surface of a shaft hole of a bush (i.e., a ring specimen  2 ). International Publication No. WO 2018/199181 describes that the seal structure described therein can achieve the effects of improving the sealing performance and providing low frictional properties when the outer peripheral surface of the shaft slides on the inner peripheral surface of the shaft hole of the bush in a reciprocating manner. At this time, a plane on which the polymer brush layer is formed is parallel with the direction in which the shaft slides on the bush in a reciprocating manner. Thus, there is no possibility that a load (i.e., a contact load) in a perpendicular direction or the like will be continuously applied to the polymer brush layer. 
     However, International Publication No. WO 2018/199181 fails to describe that the shaft rotates with respect to the bush. Thus, when the seal structure described in International Publication No. WO 2018/199181 is applied to a configuration in which the shaft rotates with respect to the bush, there is concern that if the shaft becomes eccentric with respect to the bush, the sealing performance may degrade or wear may proceed due to an increase in friction. 
     Since each of the aforementioned two related techniques is applicable only to a movement pattern in which a shaft rotates or reciprocates, it would be difficult to achieve the effects of satisfactorily improving the sealing performance and providing low frictional properties for a long term for a seal of a structure in which a shaft rotates and reciprocates at the same time (e.g., a vacuum seal of a semiconductor manufacturing apparatus). 
     It should be noted that even for the movement pattern in which a shaft only rotates without reciprocating or a shaft only reciprocates without rotating, it is desired to achieve even greater effects of excellently improving the sealing performance and providing low frictional properties. 
     SUMMARY 
     The present disclosure relates to the aforementioned problems. 
     A seal structure of one aspect of the present disclosure is a seal structure for sealing a gap between an opening provided in a housing and a shaft member inserted through the opening, including a tubular bush provided between the shaft member and the housing and arranged around the shaft member, the tubular bush being relatively movable with respect to the shaft member in an axial direction and/or a circumferential direction; a pressure-receiving member having a facing surface facing an end surface of the bush on one side in the axial direction; and an elastic member that presses the bush against the pressure-receiving member from another side in the axial direction, in which a first coating layer is provided between an outer peripheral surface of the shaft member and an inner peripheral surface of the shaft hole in the bush, and a second coating layer is formed between the end surface of the bush on the one side in the axial direction and the facing surface of the pressure-receiving member. 
     In one aspect of the present disclosure, the first coating layer and the second coating layer may be swollen with a liquid substance. 
     An inner surface region of the housing surrounding the opening may form the facing surface of the pressure-receiving member. 
     Alternatively, a flat ring member may be provided between an inner surface region of the housing surrounding the opening and the end surface of the bush on the one side in the axial direction, and the ring member may form the pressure-receiving member. 
     When the ring member forms the pressure-receiving member, an elastic member for the ring member may be further provided between the inner surface region of the housing and the ring member, and the elastic member for the ring member may press the ring member against the bush from the one side in the axial direction. 
     In the seal structure of one aspect of the present disclosure, the elastic member may be provided between an inner surface region of the housing surrounding, of the opening, the opening on the other side in the axial direction and an end surface of the bush on the other side in the axial direction. In such a case, one end side of the elastic member along a direction of expansion and contraction may be connected to the bush, and another end side of the elastic member may be connected to an inner surface of the housing. 
     A plurality of seal units may be arranged in parallel along an axial direction, the seal units each having the aforementioned seal structure of the present disclosure and sharing the housing for a shaft member. Orientations of at least one pair of adjacent seal units among the plurality of seal units may be inverted with respect to each other in the axial direction. In such a case, as another aspect of the present disclosure, a seal structure may be exemplarily illustrated in which the housing has a pair of through-holes at opposite positions along the axial direction, the pair of through-holes being adapted to pass the shaft member, the number of the seal units is two, and one of the through-holes corresponds to an opening of one of the pair of seal units, and another through-hole corresponds to an opening of another seal unit. 
     In such a case, the housing may include a protruding portion protruding from an inner peripheral surface of the housing toward the shaft member at a center of the housing along the axial direction, and in each of the two seal units, the elastic member may be provided between a surface of the protruding portion on a side of the seal structure and an end surface of the bush facing the protruding portion. 
     In each of the two seal units, one end side of the elastic member along the direction of expansion and contraction may be connected to the bush, and another end side of the elastic member may be connected to the protruding portion. 
     Orientations of at least one pair of adjacent seal units among the plurality of seal units may be the same along the axial direction. 
     In addition, another seal unit may be provided adjacent to one side of one of the plurality of seal units along the axial direction. 
     Further, an internal space of the housing may be filled with a liquid substance that swells the first coating layer and the second coating layer in each of the two seal units. 
     According to one aspect of the present disclosure, a seal structure can be provided that can achieve the effects of excellently improving the sealing performance and providing low frictional properties for a gap between a shaft member and a bush. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a seal structure according to a first embodiment as an exemplary aspect of the present disclosure. 
         FIG.  2    is a partially enlarged schematic cross-sectional view of a cross-section of a polymer brush layer and a substrate (i.e., a bush and a ring member). 
         FIG.  3    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a seal structure according to a second embodiment as an exemplary aspect of the present disclosure. 
         FIG.  4    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a seal structure according to a third embodiment as an exemplary aspect of the present disclosure. 
         FIG.  5    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a seal structure according to a fourth embodiment as an exemplary aspect of the present disclosure. 
         FIG.  6    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a seal structure according to a fifth embodiment as an exemplary aspect of the present disclosure. 
         FIG.  7    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a seal structure according to a sixth embodiment as an exemplary aspect of the present disclosure. 
         FIG.  8    is a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a prototype device produced in an Example. 
         FIG.  9    is an A-A cross-sectional view of  FIG.  8    for illustrating the schematic configuration of the prototype device produced in the Example. 
     
    
    
     DETAILED DESCRIPTION 
     A seal structure and a seal structural body according to an embodiment as an exemplary aspect of the present disclosure will be described with reference to the drawings. 
     Hereinafter, for convenience&#39;s sake of description, the direction of an arrow (a) along the direction of the axis x (see  FIGS.  1  and  3  to  5   ) shall be assumed as the upper side (a), and the direction of an arrow (b) along the direction of the axis x (see  FIGS.  1  and  3  to  5   ) shall be assumed as the lower side (b). 
     First Embodiment 
       FIG.  1    is a cross-sectional view of a cross-section along the axis x for illustrating the schematic configuration of a seal structure  10  according to a first embodiment as an exemplary aspect of the present disclosure. The configuration of the seal structure  10  according to the present embodiment will be described with reference to  FIG.  1   . 
     The seal structure  10  according to the present embodiment is a seal structure for sealing an annular gap between a shaft  13 , which is a rotary shaft member, and a shaft hole  11   c  of a cylindrical bush  11  through which the shaft  13  is adapted to be fitted and inserted. The seal structure  10  is used to seal the gap between the shaft member and the shaft hole that is formed in the bush, a housing, or the like and through which the shaft member is adapted to be inserted in a semiconductor manufacturing apparatus, a general-purpose machine, or a vehicle, for example. It should be noted that the target of application of the seal structure  10  according to the embodiment of the present disclosure is not particularly limited. 
     As illustrated in  FIG.  1   , the seal structure  10  according to the present embodiment includes the rotary shaft  13 , the bush  11 , a flat ring member (i.e., a pressure-receiving member)  16 , springs (i.e., elastic members)  14 , a housing  15 , and a seal ring  18 . 
     The housing  15  has openings  15   a  and  15   b , which are adapted to pass the shaft  13 , in its opposite end surfaces along the direction of the axis x, and houses therein the bush  11 , the elastic members  14 , and the ring member  16 . 
     The bush  11  has the shape of a thick-walled cylinder with a shaft hole  11   c  through which the shaft  13  is adapted to be fitted and inserted, and has flat ring-shaped end surfaces  11   a  and  11   b  on its opposite sides (i.e., the upper side (a) and the lower side (b)) along the direction of the axis x. 
     The shaft  13  is adapted to rotate counterclockwise (i.e., in the direction of an arrow T) and also move up and down between the upper side (a) and the lower side (b) in the direction of the axis x (indicated by an arrow R) as illustrated in  FIG.  1   . That is, rotational sliding caused by a counterclockwise rotation (i.e., in the direction of the arrow T) as well as reciprocating sliding in the direction of the axis x occur between an outer peripheral surface of the shaft  13  and an inner peripheral surface  11   d  of the shaft hole  11   c  of the bush  11 . 
     The ring member  16  is provided between an inner surface region  15   d  of the housing  15  surrounding, of the openings  15   a  and  15   b  of the housing  15 , the opening  15   b  on one side (i.e., the lower side (b)) in the direction of the axis x and an end surface  11   b  of the bush  11  on one side (i.e., the lower side (b)) in the direction of the axis x. In addition, the ring member  16  has a facing surface  16   a  facing the end surface  11   b  of the bush  11  on one side (i.e., the lower side (b)) in the direction of the axis x. 
     The inner peripheral surface  11   d  of the shaft hole  11   c  of the bush  11  has a first coating layer  12 A formed thereon. The first coating layer  12 A fills a gap between the inner peripheral surface  11   d  of the shaft hole  11   c  and the outer peripheral surface of the shaft  13 . 
     In addition, the facing surface  16   a  of the ring member  16  has a second coating layer  12 B formed thereon. 
     The springs  14  are provided between an inner surface region  15   c  of the housing  15  surrounding, of the openings  15   a  and  15   b  of the housing  15 , the opening  15   a  on the other side (i.e., the upper side (a)) in the direction of the axis x and an end surface  11   a  of the bush  11  on the other side (i.e., the upper side (a)) in the direction of the axis x. The springs  14  press the bush  11  against the ring member  16  from the other side (i.e., the upper side (a)) in the direction of the axis x. Thus, the end surface  11   b  of the bush  11  is pressed against the facing surface  16   a  of the ring member  16  having the second coating layer  12 B formed thereon due to the elastic action (i.e., the restoring action in the extension direction) of the springs  14 . 
     Further, one end side (i.e., the lower side (b)) of each spring  14  along its direction of expansion and contraction (which is the same as the direction of the axis x) is connected to the bush  11 , and the other end side (i.e., the upper side (a)) of each spring  14  is connected to the inner surface (i.e., the inner surface region  15   c ) of the housing  15 . Although  FIG.  1    schematically illustrates a pair of right and left springs  14 , three springs are, in practice, arranged at equiangular (or rotationally symmetric) positions (that is, at positions of a circumference angle of 120°) along the circumferential direction of the annular end surface  11   a  of the bush  11  as seen from the other end side (i.e., the upper side (a)) in the direction of the axis x (see  FIG.  9    described later). 
     The internal space of the housing  15  is filled with a swelling liquid (i.e., a liquid substance)  17 . The internal space of the housing  15  is sealed except the opening  15   a , and the swelling liquid  17  stays in the internal space. 
     The swelling liquid  17  is in contact with the first coating layer  12 A and the second coating layer  12 B. The swelling liquid  17  is a liquid with a property of swelling the first coating layer  12 A and the second coating layer  12 B, and an ionic liquid is used, for example. The details of the swelling liquid  17  will be described later. 
     As illustrated in  FIG.  1   , an edge of the opening  15   b  of the housing  15  on the side of the ring member  16  (i.e., the upper side (a)) is provided with a step portion. In addition, an edge of the central hole of the ring member  16  on the side of the housing  15  (i.e., the lower side (b)) is also provided with a step portion with the same shape. The seal ring  18  is arranged so as to be housed within the two step portions. 
     In the seal structure  10  according to the present embodiment, the upper portion in  FIG.  1    is at a predetermined pressure P 1 , and the lower portion is at a predetermined pressure P 2 . The seal structure  10  seals the boundary between the two pressures. For example, when the seal structure  10  according to the present embodiment is applied to a vacuum seal of a semiconductor manufacturing apparatus, P 1  is the atmospheric pressure AP and P 2  is the vacuum pressure V, and the seal structure  10  isolates the different pressures from each other. 
     It should be noted that in the seal structure  10  of the present embodiment, the predetermined pressures P 1  and P 2  are not limited to different pressures like the atmospheric pressure AP and the vacuum pressure V, and the seal structure  10  can be suitably applied to a seal under environments of different pressures having other pressure relationships. Further, the seal structure  10  of the present embodiment can be suitably applied to a seal at the boundary between different environments, such as environments of different gases, regardless of whether or not the predetermined pressures P 1  and P 2  are different pressures (hereinafter, the same is true of the predetermined pressures P 1  and P 2  in the other embodiments). 
     The swelling liquid  17  that has entered the contact surface between an inner peripheral surface  15   e  of the housing  15  and the outer periphery of the ring member  16  reaches the contact surface between the inner surface region  15   d  of the housing  15  and a rear surface  16   b  of the ring member  16 , but is blocked and sealed by the seal ring  18 . As the seal ring  18 , a common O-ring can be used as appropriate as long as it is resistant to the swelling liquid  17 . 
     The first coating layer  12 A and the second coating layer  12 B are the same coating layers. 
     In this specification, the term “coating layer” simply refers to the first coating layer  12 A or the second coating layer  12 B without distinction, and may be assigned a reference sign “12.” In addition, in this specification, a region around a surface on which the coating layer  12  is formed may be referred to as a “seal member,” and in such a case, a reference sign “1” is assigned thereto. 
     Hereinafter, coating layers that can form the first coating layer  12 A and the second coating layer  12 B will be described. 
     Examples of the applicable coating layers include coating layers made of conventionally known various organic thin films or inorganic thin films. 
     Examples of inorganic materials applicable to the coating layers include chromium nitride (CrN, Cr 2 N), chromium carbide (Cr 3 C 2 ), hard chromium plating, diamond-like carbon (DLC), diamond, titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), titanium aluminum nitride (TiAlN), alumina, alumite (Al 2 O 3 ), silicon carbide (SiC), graphite (C), and carbon black. 
     Examples of organic materials applicable to the coating layers include polybutadiene, polyurethane, polyisocyanate, acrylic resin, and silicone resin (in this specification, silicone resin is regarded as a type of organic materials). 
     The thickness of each coating layer made of an inorganic thin film or an organic thin film differs depending on the thickness suitable for each of various thin films or a thickness that is possible, and it is acceptable as long as the thickness is appropriately adjusted according to each material or various other conditions. 
     Among such inorganic thin films or organic thin films, the thin film is preferably a thin film in a swollen state, or more preferably, an organic thin film in a swollen state. 
     In the present disclosure, a polymer brush layer is particularly preferably used as each of the coating layers that can form the first coating layer  12 A and the second coating layer  12 B. 
     Described hereinafter is an example of the seal member  1  in which a polymer brush layer is used as each coating layer  12 . 
       FIG.  2    illustrates a partially enlarged schematic cross-sectional view of a cross-section of the coating layer  12  (hereinafter, a polymer brush layer formed as the coating layer  12  may be particularly expressed as the “polymer brush layer  12 ”) and the substrate (i.e., the bush  11  and the ring member  16 ; hereinafter, the two shall be collectively referred to as the “substrate” and will be described with a reference sign “ 19 ” assigned thereto). As illustrated in  FIG.  2   , the seal member  1  has a seal surface  1   a  as its surface  1 A. 
     Herein, the seal surface  1   a  is at least a part of the surface  1 A of the seal member  1 , and faces the outer peripheral surface of the shaft  13  or the end surface  11   b  of the bush  11  (hereinafter collectively referred to as a “surface to be sealed”). The seal surface  1   a  means a surface capable of forming a sealed state between the seal surface  1   a  and the surface to be sealed. In addition, the seal member  1  includes the substrate  19  and the polymer brush layer  12  formed on the surface  11 A of the substrate  19  as illustrated in  FIG.  2   . 
     The formation state of the polymer brush layer  12  is not particularly limited, and may be appropriately selected according to the shape, material, and surface texture of the substrate  19  and the usage pattern of the seal member  1 , for example. 
     For example, the polymer brush layer  12  may be formed directly or indirectly on a substrate surface  11 A. 
     A case where the polymer brush layer  12  is formed indirectly on the substrate surface  11 A is, for example, a configuration in which surface treatment is applied to the substrate  19  to form another layer on the substrate surface  11 A, and the polymer brush layer  12  is formed on the surface of the other layer. Herein, examples of the other layer include a silica coat layer described below. 
     The polymer brush layer  12  need not necessarily cover the entire substrate surface  11 A completely. The seal member  1  may have a portion in which the polymer brush layer  12  is not formed on the substrate surface  11 A within the range that the effects of the present disclosure are not hindered, or the polymer brush layers  12  having a given area may be scattered on the substrate surface  11 A. It should be noted that even when the coating layer  12  is a layer other than a polymer brush layer, the coating layer  12  need not still completely cover the entire substrate surface  11 A. 
     Further, in the seal member  1 , the polymer brush layer  12  may be formed beyond the substrate surface  11 A, for example, so as to cover the entire substrate  19 . That is, as long as the seal member  1  has the polymer brush layer  12  on the substrate surface  11 A, the seal surface  1   a  exhibits excellent sealing performance. Even when the coating layer  12  is a layer other than a polymer brush layer, the coating layer  12  may be similarly formed beyond the substrate surface  11 A. 
     From the viewpoint of obtaining higher sealing performance with the seal surface  1   a , the polymer brush layer  12  is preferably formed on the entire substrate surface  11 A. In such a case, the seal surface  1   a  corresponds to a surface  12   a  of the polymer brush layer  12 . 
     The polymer brush layer  12  is a layer obtained by covalently immobilizing a plurality of polymer graft chains  121  on the substrate surface  11 A as illustrated in  FIG.  2   . The surface  12   a  of such a polymer brush layer  12  is a surface in which the tips of the polymer graft chains  121  on the side not immobilized on the substrate  19  thicken in a brush form, and has a surface texture like the surface of a brush. 
     In the seal member  1  having such a polymer brush layer  12  as the seal surface  1   a , the surface texture (in particular, the surface roughness) of the substrate surface  11 A corresponding to the seal surface  1   a  is eased by the polymer brush layer  12 , whereby the surface texture hardly influences the flatness of the seal surface  1   a . This makes it unnecessary to subject the substrate surface  11 A corresponding to the seal surface  1   a  to precise surface work. 
     In addition, the surface  12   a  of the polymer brush layer  12  in which the polymer graft chains  121  thicken in a brush form has moderate flexibility, and thus exhibits excellent followability with respect to the surface to be sealed when contacting the surface to be sealed. Therefore, the influence of the surface texture (in particular, the surface roughness) of the surface to be sealed is also eased by the polymer brush layer  12  of the seal surface  1   a , and good adhesion is obtained between the seal surface  1   a  and the surface to be sealed. 
     According to such a seal member  1  having the polymer brush layer  12  as the seal surface  1   a , the seal surface  1   a  exhibits excellent sealing performance without being influenced by the surface textures of the substrate surface  11 A corresponding to the seal surface  1   a  and the surface to be sealed. 
     The thickness of the polymer brush layer  12  formed as the seal surface  1   a  is not particularly limited, but the thickness is preferably greater than 0 nm and less than or equal to 10000 nm from the viewpoint of obtaining good sealing performance with the seal surface  1   a , and more preferably greater than or equal to 100 nm and less than or equal to 2000 nm from the practical viewpoint. 
     It should be noted that the thickness of the coating layer  12  including the polymer brush layer  12  can be measured by measuring its dry thickness using ellipsometry. A specific measurement method is as described in the pages of Examples of Patent Literature 1. In addition, a more specific description of the polymer brush layer  12  is also seen in Patent Literature 1. 
     The material of the substrate  19  can be appropriately selected according to the application and usage pattern of the seal member  1  and the method of forming the polymer brush layer  12 , for example. For example, hard ceramics, such as alumina and boron carbide, rubber, and plastics can be selected. The surface texture of the substrate  19  is not particularly limited. It is acceptable as long as the substrate surface  11 A corresponding to the seal surface  1   a , in particular, has moderate flatness and smoothness, and thus, the substrate surface  11 A need not be subjected to precise surface work. 
     In a suitable example of the seal structure of the present disclosure, the polymer brush layer  12  is provided as the seal surface  1   a . Therefore, the surface roughness of the substrate surface  11 A corresponding to the seal surface  1   a  of the seal member  1  is eased by the polymer brush layer  12  even if the substrate surface  11 A is a somewhat coarse surface, whereby the surface roughness hardly influences the sealing performance of the seal surface  1   a.    
     Hereinafter, a method of forming the polymer brush layer  12  will be described with reference to a case where the polymer brush layer  12  is formed directly on the substrate surface  11 A such as the one illustrated in  FIG.  2    as an example. 
     The polymer brush layer  12  can be formed using a surface-initiated living radical polymerization method, for example. The surface-initiated living radical polymerization method is a method including the following to form the polymer graft chains  121 : 
     (I) introducing a polymerization initiating group into the substrate surface  11 A serving as the starting point of the polymer graft chains  121 , and 
     (II) performing the surface-initiated living radical polymerization method with the polymerization initiating group as the starting point. 
     Specifically, as such a surface-initiated living radical polymerization method, it is possible to use the method described in Arita, T., Kayama, Y., Ohno, K., Tsujii, Y. and Fukuda, T., “High-pressure atom transfer radical polymerization of methyl methacrylate for well-defined ultra high molecular-weight polymers, “Polymer, 49, 2008, 2426-2429 (hereinafter referred to as Literature P), Japanese Patent Laid-Open No. 2009-59659 (hereinafter referred to as Literature Q), Japanese Patent Laid-Open No. 2010-218984 (hereinafter referred to as Literature R), or Japanese Patent Laid-Open No. 2014-169787 (hereinafter referred to as Literature S), for example. 
     (I) Immobilization of Polymerization Initiating Group on Substrate Surface 
     Examples of the method of introducing the polymerization initiating group into the substrate surface  11 A include, but are not particularly limited to, a method of dissolving or dispersing the polymerization initiator in a solvent to prepare a polymerization initiator solution, and immersing the substrate  19  in the prepared polymerization initiator solution. 
     The polymerization initiator is not particularly limited, but is preferably a compound containing a group capable of binding to the substrate surface  11 A and a radical generating group. For example, the polymerization initiator disclosed in Literature P, Literature R, or Literature S can be widely used. Among these, the polymerization initiator is preferably an atom transfer radical polymerization (ATRP)-based polymerization initiator, and more preferably, (3-trimethoxysilyl)propyl 2-bromo-2-methylpropionate. 
     It should be noted that the substrate surface  11 A is desirably cleaned as appropriate before the polymerization initiating group is introduced thereinto. The substrate surface  11 A can be cleaned using a known method according to the material, shape, and the like of the substrate  19 . 
     (II) Synthesis of Polymer Graft Chains Through Surface-Initiated Living Radical Polymerization 
     The method of forming the polymer graft chains on the substrate surface  11 A into which the polymerization initiating group has been introduced is not particularly limited. First, various components required for a polymerization reaction, such as a predetermined monomer and various low-molecular free initiators (i.e., radical initiators), are dissolved or dispersed in a solvent to prepare a polymerization reaction solution. Thereafter, the substrate  19  into which the polymerization initiating group has been preliminarily introduced is immersed in the prepared polymerization reaction solution, followed by pressurization and heating as appropriate, whereby the polymer graft chains  121  containing a predetermined monomer as a polymerization unit can be formed on the substrate surface  11 A. 
     The method of preparing the polymerization reaction solution is not particularly limited. For example, the methods described in Literature P and Literature S can be suitably used, and monomers, low-molecular free initiators, and the like described in these literatures can be widely used. Among these, the polymerization reaction solution is preferably prepared using the method described in Literature P, and the monomer is preferably methyl methacrylate (hereinafter, MMA). In addition, ethyl 2-bromo-2-methylpropionate is preferably used as the low-molecular free initiator. 
     The reaction conditions of the surface-initiated living radical polymerization are not particularly limited. For example, the surface-initiated living radical polymerization can be performed under the conditions of Literature P and Literature S. Among these, the polymerization reaction is preferably performed using the method described in Literature P, and particularly preferably performed under pressurized conditions (for example, about greater than or equal to 400 MPa and less than or equal to 500 MPa) and heated conditions (for example, about greater than or equal to 50° C. and less than or equal to 60° C.). If the polymerization reaction is performed during pressurization, it is possible to form the polymer brush layer  12  that is denser (i.e., has a higher graft density of the polymer graft chains  121 ) and has a greater thickness (i.e., has a longer average molecular chain length). 
     The surface occupation rate σ* (i.e., the occupation rate per polymer cross-sectional area) of the polymer graft chains  121  formed on the substrate surface  11 A with respect to the area of the substrate surface  11 A is preferably greater than or equal to 10%, more preferably, greater than or equal to 15%, and further preferably, greater than or equal to 20%. The surface occupation rate σ* can be calculated by determining the polymer cross-sectional area from the repeating unit length of the polymer in a fully stretched state and the bulk density of the polymer, and then multiplying the resultant cross-sectional area by the graft density. More specifically, the surface occupation rate σ* can be determined according to the following formula. That is, the surface occupation rate σ* means the area rate of a graft point (i.e., a first monomer) in the substrate surface  11 A (100% in closest packing, grafting cannot be performed so as to exceed 100%). 
       σ*=(polymer cross-sectional area)×graft density σ
 
       (polymer cross-sectional area=(volume per monomer of graft chain portion)/(repeating unit length of polymer in fully stretched state) 
       volume per monomer of graft chain portion=[{(molecular weight of monomer of graft chain portion)/(Avogadro&#39;s number)}/(bulk density of polymer)]). 
     The graft density σ represents the number of the polymer graft chains  121  that are present per nm 2  (chain/nm 2 ). Specifically, the graft density σ can be calculated from the absolute value of the number-average molecular weight (Mn) of the graft chains, the amount of a polymer grafted (i.e., the dry thickness of the polymer brush layer  12 ), and the surface area of the substrate surface  11 A. In particular, a polymer brush having a graft density σ of greater than or equal to 0.1 chain/nm 2  is defined as a dense polymer brush. It should be noted that when the surface occupation rate σ* is 100% (i.e., closest packing), the upper limit of the theoretical graft density σ is 1.79 (chain/nm 2 ) if PMMA is used. 
     The average molecular chain length Lp (that is, the polymer brush length) of the polymer graft chains  121  forming the polymer brush layer  12  is preferably greater than 0 nm and less than or equal to 10000 nm, and more preferably, within the range of 100 nm to 2000 nm from the practical viewpoint. If the average molecular chain length Lp of the polymer graft chains  121  is too short, the sealing performance tends to degrade. The average molecular chain length Lp of the polymer graft chains  121  can be adjusted by adjusting the polymerization conditions, for example. 
     The average molecular chain length Lp of the polymer graft chains  121  can be determined from, for example, the results of measuring the number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the polymer graft chains  121 . For example, the number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the polymer graft chains  121  are measured by cutting out the polymer graft chains  121  from the substrate  19  through treatment with hydrofluoric acid, and performing measurement with the cut-out polymer graft chains  121  using a gel permeation chromatography method. Alternatively, it has been known that a free polymer to be produced during polymerization has the same molecular weight as the polymer graft chains  121  introduced onto the substrate  19 . Thus, it is also possible to use a method of measuring the number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the free polymer using a gel permeation chromatography method, and using the resultant values as they are. 
     The molecular weight distribution (Mw/Mn) of the polymer graft chains  121  in the polymer brush layer  12  forming the seal surface  1   a  is preferably close to 1, suitably, less than or equal to 1.3, more preferably, less than or equal to 1.25, and still more preferably, less than or equal to 1.20, and particularly preferably, less than or equal to 1.15. 
     In the seal structure  10  according to the present embodiment, a silica coat layer may be provided between the substrate surface  11 A and the polymer brush layer  12 . The silica coat layer can be provided by, for example, performing silica coating treatment on the substrate surface  11 A with a sol-gel method using alkoxysilane. 
     The types and conditions of the silica coating treatment performed with the sol-gel method using alkoxysilane are not particularly limited as long as the silica coat layer can be provided. Examples of the silica coating treatment performed with the sol-gel method using alkoxysilane include a method of dissolving or dispersing alkoxysilane, such as tetraalkoxysilane, and an alkaline aqueous solution, such as 28 mass % of ammonia water, in a solvent to prepare a reaction solution, and immersing the substrate  19  in the prepared reaction solution. In the silica coating treatment performed with the sol-gel method using alkoxysilane, alkoxysilane is converted into silica (SiO2) through hydrolysis, for example. 
     The polymer brush layer  12  can be provided on the surface of the silica coat layer as in the case where the polymer brush layer  12  is provided directly on the substrate surface  11 A. 
     The substrate surface  11 A is desirably cleaned as appropriate before the silica coat layer is provided thereon as in the case where the aforementioned polymer brush layer  12  is provided. The substrate surface  11 A can be cleaned using a known method according to the material, shape, and the like of the substrate  19 . 
     The polymer brush layer  12  is preferably a layer obtained by swelling the polymer graft chains  121  formed on the substrate surface  11 A using a liquid substance (i.e., a swelling liquid). The liquid substance for swelling the polymer graft chains  121  is not particularly limited as long as it is a compound that exhibits swelling properties to the polymer graft chains  121 , but is preferably an ionic liquid from the viewpoint of high affinity thereof for the polymer graft chains  121 . It should be noted that even when the coating layer  12  is a layer other than a polymer brush layer, it is preferable to use as the liquid substance a compound that exhibits swelling properties to the material forming such a layer, and is more preferable to use an ionic liquid. 
     Examples of an ionic liquid that is preferable as the liquid substance for swelling the polymer brush layer  12  include those described in Literature S. Among these, N,N-diethyl-N-methyl-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (hereinafter also referred to as “DEME-TFSI”) and methoxyethylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (hereinafter also referred to as “MEMP-TFSI”) are preferable as the ionic liquid. 
     Examples of the method of swelling the polymer graft chains  121  formed on the substrate surface  11 A using a liquid substance include, but are not particularly limited to, a method of applying a liquid substance to the polymer graft chains  121  formed on the substrate surface  11 A and leaving it, or a method of immersing the substrate  19  having the polymer graft chains  121  formed thereon in a liquid substance. 
     It should be noted that in the present embodiment, as illustrated in  FIG.  1   , the internal space of the housing  15  is filled with the swelling liquid  17 . The swelling liquid  17  filling the internal space of the housing  15  is in contact with the end portions of the first coating layer  12 A and the second coating layer  12 B. Thus, the first coating layer  12 A and the second coating layer  12 B can be impregnated with the swelling liquid  17  and thus can be swollen. 
     In the seal structure  10  of the present embodiment, the internal space of the housing  15  is always filled with the swelling liquid  17  unless the swelling liquid  17  leaks from the opening  15   a  on the other side (i.e., the upper side (a)) in the direction of the axis x. Thus, the first coating layer  12 A and the second coating layer  12 B are always supplied with the swelling liquid  17 , and the swollen states of such layers are maintained. 
     As described above, the seal member  1  used for the seal structure  10  according to the present embodiment has the coating layer  12  as the seal surface  1   a , whereby the seal surface  1   a  exhibits excellent sealing performance without being influenced by the surface textures of the substrate surface  11 A corresponding to the seal surface  1   a  and the surface to be sealed. 
     Usually, in a seal structure of a mechanical seal, hard materials, such as hard ceramics like alumina and silicon carbide, are widely used for a substrate from the viewpoints of wear resistance and heat resistance, for example. In addition, usually, a surface to be contacted by a seal surface (i.e., a surface to be sealed) is commonly machined to have high flatness and high smoothness from the viewpoint of securing sufficient sealing performance (in particular, static sealing performance). 
     To machine the substrate surface as such a seal surface to allow it to have high flatness and high smoothness, high-precision surface work is required. However, since the aforementioned hard materials have poor workability, it would be difficult to subject the substrate surface as the seal surface to high-precision surface finishing, with the result that leakage is disadvantageously likely to occur. 
     However, in the seal member  1 , even if the substrate surface corresponding to each seal surface is not subjected to precise surface finishing, at least one of the seal surfaces has a coating layer provided thereon. Thus, the presence of such a coating layer eases the influence of the surface texture of each substrate surface, and each seal surface can thus secure high sealing performance. This can effectively prevent the occurrence of leakage. 
     In the seal structure  10  of the present embodiment, first, the first coating layer  12 A is formed on the inner peripheral surface  11   d  of the shaft hole  11   c  of the bush  11  as the substrate  19  (which corresponds to the substrate surface  11 A in  FIG.  2   ), so that the seal member  1  described with reference to  FIG.  2    is formed. Therefore, excellent sealing performance can be secured between the inner peripheral surface  11   d  of the shaft hole  11   c  of the bush  11  and the outer peripheral surface of the shaft  13 , which rubs the inner peripheral surface  11   d  when the shaft  13  rotates counterclockwise (i.e., in the direction of the arrow T). This can effectively prevent the occurrence of leakage. 
     In the present embodiment, one side (i.e., the lower side (b)) in the direction of the axis x is at P 2  (for example, a vacuum pressure V), and the other side (i.e., the upper side (a)) is at P 1  (for example, an atmospheric pressure AP). Thus, a load is applied to one side (i.e., the lower side (b)) in the direction of the axis x due to the pressure difference (P 1 -P 2 ). In addition, as the shaft  13  slides in a reciprocating manner by moving between the upper side (a) and the lower side (b) in the direction of the axis x, a load is applied to the other side (i.e., the upper side (a)) and one side (i.e., the lower side (b)) in the direction of the axis x. In any case, the load is applied in the axial direction, and no contact load in the direction perpendicular to the first coating layer  12 A or the like is applied. Thus, since a load applied to the first coating layer  12 A is small, wear of the first coating layer  12 A can be suppressed. 
     In addition, in the present embodiment, since the springs  14  are adapted to press the bush  11  against the ring member  16  from the other side (i.e., the upper side (a)) in the direction of the axis x, the bush  11  is allowed to have a posture inclined to a certain degree from the state in which the axis of the bush  11  coincides with the axis x. The upper side (a), which is connected to the springs  14 , of the bush  11  with its posture having changed following the inclination of the shaft  13  moves slightly in the direction intersecting the axis x due to the flexibility of the springs  14 , and the lower side (b) of the bush  11  moves like a pendulum with the upper side (a) as the base point. 
     Since the end surface  11   b  of the bush  11  on one side (i.e., the lower side (b)) in the direction of the axis x is pressed against the ring member  16 , which is a pressure-receiving member, with an elastic extension force of the springs  14 , the end surface  11   b  is allowed to move in a direction within the contact surface of the facing surface  16   a  of the ring member  16 . 
     According to such a configuration, even when the shaft  13  becomes eccentric with respect to the axis x or is inclined into a swinging state, the bush  11  can change its posture and follow the eccentric position or inclination of the shaft  13 . Thus, even when the shaft  13  becomes eccentric or is inclined with respect to the axis x, it is possible to achieve the effects of excellently improving the sealing performance and providing low frictional properties. 
     In the seal structure  10  of the present embodiment, the second coating layer  12 B is further formed on the facing surface  16   a  of the ring member  16  as the substrate  19  (which corresponds to the substrate surface  11 A in  FIG.  2   ), so that the seal member  1  described with reference to  FIG.  2    is formed. In addition, since the end surface  11   b  of the bush  11  is pressed against the second coating layer  12 B with an elastic extension force of the springs  14 , a contact load is applied to the second coating layer  12 B. 
     Therefore, even when the bush  11  has changed its posture following the inclination of the shaft  13 , and the end surface  11   b  of the bush  11  thus contacts the facing surface  16   a  of the ring member  16  and slightly slides thereon in a direction within the contact surface, it is possible to achieve the effects of excellently improving the sealing performance and providing low frictional properties due to the function of the second coating layer  12 B. 
     That is, in the present embodiment, a contact load is not applied between the inner peripheral surface  11   d  of the bush  11 , which always slides on the shaft  13  during the rotation of the shaft  13 , and the outer peripheral surface of the shaft  13 . Therefore, it is possible to suppress the load applied to the first coating layer  12 A, and thus achieve the effects of satisfactorily improving the sealing performance and providing low frictional properties for a long term. 
     Meanwhile, a contact load is actively applied between the end surface  11   b  of the bush  11 , which does not slide or slides only slightly and not frequently, and the facing surface  16   a  of the ring member  16 , by the springs  14 . Therefore, it is possible to secure excellent sealing performance while utilizing the effects of low frictional properties obtained with the second coating layer  12 B, and thus effectively prevent the occurrence of leakage. 
     Second Embodiment 
       FIG.  3    is a cross-sectional view of a cross-section along the axis x for illustrating the schematic configuration of a seal structure  20  according to a second embodiment as an exemplary aspect of the present disclosure. The configuration of the seal structure  20  according to the present embodiment will be described with reference to  FIG.  3   . 
     The seal structure  20  according to the second embodiment has the same shape and configuration as those of the seal structure  10  according to the first embodiment except some structures. Accordingly, in  FIG.  3    of the present embodiment, members with the same configurations as those of the first embodiment are denoted by the same reference signs as those of the first embodiment, and the detailed description thereof will be omitted. 
     See the description in the first embodiment mainly made with reference to  FIG.  2    for a configuration around a surface on which a coating layer  12  is formed (i.e., a seal member  1 ) in the seal structure  20  according to the second embodiment (the same is true of seal structures  30  and  40  in the following embodiments). 
     In the present embodiment, between an inner surface region  15   d  of a housing  15  and a ring member  26  serving as a pressure-receiving member, springs (i.e., elastic members for the ring member)  24  for a ring member  26  are further provided. The springs  24  push the ring member  26  against a bush  11  from one side (i.e., the lower side (b)) in the direction of the axis x. 
     The springs  24  are provided between the inner surface region  15   d  of the housing  15  surrounding, of openings  15   a  and  15   b  of the housing  15 , the opening  15   b  on one side (i.e., the lower side (b)) in the direction of the axis x and an end surface  11   b  of the bush  11  on one side (i.e., the lower side (b)) in the direction of the axis x. The springs  24  press the ring member  26  against the bush  11  from one side (i.e., the lower side (b)) in the direction of the axis x. Thus, a facing surface  26   a  of the ring member  26  having a second coating layer  12 B formed thereon is pressed against the end surface  11   b  of the bush  11  due to the elastic action (i.e., the restoring action in the extension direction) of the springs  24 . 
     Further, the other end side (i.e., the upper side (a)) of each spring  24  along its direction of expansion and contraction (which is the same as the direction of the axis x) is connected to the ring member  26 , and one end side (i.e., the lower side (b)) of each spring  24  is connected to the inner surface (i.e., the inner surface region  15   d ) of the housing  15 . Although  FIG.  3    schematically illustrates a pair of right and left springs  24 , three springs are, in practice, arranged at equiangular (or rotationally symmetric) positions (that is, at positions of a circumference angle of 120°) along the circumferential direction of the surface of the annular ring member  26  as seen from one end side (i.e., the lower side (b)) in the direction of the axis x. 
     A seal ring  28  is attached to the outer periphery of the ring member  26 . Since the ring member  26  is movable in the direction of the axis x, it would be impossible to arrange the seal ring between the edge of the central hole of the ring member  16  on the side of the housing  15  (i.e., the lower side (b)) and the inner surface region  15   d  of the housing  15  like the seal ring  18  of the first embodiment. 
     Thus, in the present embodiment, the ring member  26  is allowed to move in the direction of the axis x as the seal ring  28  attached to the outer periphery of the ring member  26  frictionally slides on an inner peripheral surface  15   e  of the housing  15 . The seal ring  28  blocks and seals a swelling liquid  17 , and thus prevents the swelling liquid  17  from passing across the boundary between the inner peripheral surface  15   e  of the housing  15  and the outer periphery of the ring member  26 . 
     In the present embodiment, a configuration substantially similar to that of the first embodiment is provided. Thus, functions and effects similar to those of the first embodiment can be achieved. In the present embodiment, not only the functions and effects similar to those of the first embodiment, but also functions and effects obtained with the provision of the springs  24  as the elastic members for the ring member can be expected to be achieved. 
     That is, since the bush  11  and the ring member  26  that abut each other with the second coating layer  12 B interposed therebetween are held so as to be sandwiched by the springs  14  and the springs  24  from the opposite sides in the direction of the axis x, the bush  11  and the ring member  26  can be stably supported. In addition, since the bush  11  and the ring member  26  are held by the elastic springs  14  and the elastic springs  24  on the opposite sides in the direction of the axis x, the degree of freedom of the posture of each of the bush  11  and the ring member  26  is increased, and followability thus improves. Thus, it is possible to achieve the effects of improving the sealing performance and providing low frictional properties at a higher level when the shaft  13  becomes eccentric or is inclined with respect to the axis x. 
     Third Embodiment 
       FIG.  4    is a cross-sectional view of a cross-section along the axis x for illustrating the schematic configuration of a seal structure  30  according to a third embodiment as an exemplary aspect of the present disclosure. The configuration of the seal structure  30  according to the present embodiment will be described with reference to  FIG.  4   . 
     The seal structure  30  according to the third embodiment includes two seal units  10 A and  10 B sharing a single housing  35  for a single shaft  13 . The two seal units  10 A and  10 B each have a structure substantially similar to that of the seal structure  10  according to the first embodiment (i.e., an aspect of the seal structure of the present disclosure), and are arranged in a mutually inverted state. 
     Thus, each of the two seal units  10 A and  10 B has the same shape and configuration as those of the seal structure  10  according to the first embodiment except some structures. Accordingly, in  FIG.  4    of the present embodiment, members of the seal unit  10 B with the same configurations as those of the first embodiment are denoted by the same reference signs as those of the first embodiment, and the detailed description thereof will be omitted. In addition, members of the seal unit  10 A with the same configurations as those of the first embodiment are denoted by the same reference signs as those of the first embodiment with “′ (prime)” added thereto, and the detailed description thereof will be similarly omitted. 
     The housing  35  includes a cup member  35   f  partially forming the seal unit  10 A, a cup member  35   i  partially forming the seal unit  10 B, and a disk member  35   e  having an opening  35   m  for passing the shaft  13  at the center. The cup members  35   f  and  35   i  respectively have flange portions  35   g  and  35   h  extending outward from the edges of the openings of the cups. 
     The cup member  35   f  and the cup member  35   i  are arranged such that the openings of the cups face each other and the flange portion  35   g  and the flange portion  35   h  are fastened together with fasteners  35   j  each including a vis and a nut, for example, while sandwiching the disk member  35   e  therebetween. Accordingly, a single housing  35  is formed as a whole. 
     The internal space of the housing  35  is a hermetically sealed space that allows the side of the seal unit  10 A and the side of the seal unit  10 B to communicate with each other through the opening  35   m . The hermetically sealed space (i.e., the internal space) is filled with a swelling liquid  17  for swelling first coating layers  12 A and  12 A′ and second coating layers  12 B and  12 B′ in the seal unit  10 A and the seal unit  10 B. 
     In the seal unit  10 B, the housing  15  of the seal structure  10  according to the first embodiment is replaced with the aforementioned housing  35 . The housing  35  has openings (i.e., through-holes)  35   a  and  35   b  for passing the shaft  13  in its opposite end surfaces along the direction of the axis x. In the seal unit  10 B, the opening  35   b  of the housing  35  closer to the ring member  16  corresponds to the opening  15   b  on one side (i.e., the lower side (b)) in the direction of the axis x in the seal structure  10  according to the first embodiment (see  FIG.  1   ). 
     In the seal unit  10 A also, the housing  15  of the seal structure  10  according to the first embodiment is replaced with the aforementioned housing  35 . It should be noted that in the seal unit  10 A arranged in a position vertically inverted from the seal unit  10 B, the opening  35   a  of the housing  35  closer to the ring member  16 ′ corresponds to the opening  15   b  on one side (i.e., the lower side (b)) in the direction of the axis x in the seal structure  10  according to the first embodiment (see  FIG.  1   ). 
     In other words, the opening (i.e., the through-hole)  35   a  of the housing  35  on the other side in the direction of the axis x in one seal unit  10 B corresponds to the opening on one side in the direction of the axis x (which corresponds to the opening  15   b  in  FIG.  1   ) in the other seal unit  10 A. 
     The disk member  35   e  is located at the center of the housing  35  along the direction of the axis x. In addition, a region of the disk member  35   e  closer to the axis x than is the region sandwiched by the flange portions  35   g  and  35   h  is a disk-like (flat) protruding portion  35   k  protruding from the inner peripheral surface of the housing  35  toward the shaft  13 . 
     In the two seal units  10 A and  10 B, springs (i.e., elastic members)  14 ′ and  14  are respectively provided between surfaces  35   ka  and  35   kb  of the protruding portion  35   k  on the sides of the respective seal structures and end surfaces  11   a ′ and  11   a  of the bushes  11 ′ and  11  facing the protruding portion  35   k.    
     In addition, in the two seal units  10 A and  10 B, one end sides (i.e., the upper side (a) in the seal unit  10 A and the lower side (b) in the seal unit  10 B) of the springs  14 ′ and  14  along their directions of expansion and contraction (which are the same as the direction of the axis x) are respectively connected to the bushes  11 ′ and  11 , and the other sides of the springs  14 ′ and  14  are connected to the protruding portion  35   k.    
     It should be noted that the shape of the protruding portion  35   k  is not limited to a disk. For example, only the portions of the protruding portion  35   k  to which the springs (i.e., the elastic members)  14 ′ and  14  abut or are fixed may have a shape protruding from the inner peripheral surface of the housing  35 . In such a case, the shape of the protruding portions is preferably flat, but any shape is acceptable as long as the springs (i.e., the elastic members)  14 ′ and  14  can abut the protruding portions or can be fixed thereto as appropriate. 
     In the present embodiment, the seal units  10 A and  10 B each having a configuration substantially similar to that of the first embodiment are provided. Thus, functions and effects similar to those of the first embodiment can be achieved. In the present embodiment, not only the functions and effects similar to those of the first embodiment, but also functions and effects unique to the present embodiment can be expected to be achieved. 
     That is, the present embodiment provides a unique configuration in which the two seal units  10 A and  10 B sharing a single housing  35  are provided for a single shaft  13 , and thus, there is no possibility of leakage of the swelling liquid  17  filling the internal space, which is a hermetically sealed space, of the housing  35 . Thus, no matter how the seal structure  30  according to the present embodiment is applied such that its top and bottom sides are inclined or tipped over, the sealing effect is not influenced. This can significantly ease the restrictions on the applicable conditions, such as the applicable range, applicable environments, and applicable places. 
     In addition, since the internal space of the housing  35  filled with the swelling liquid  17  is a hermetically sealed space, there is no concern that foreign matter may intrude into the space from outside, or the swelling liquid  17  may evaporate and thus may have a reduced volume. Therefore, with the seal structure  30  according to the present embodiment, it is possible to reduce the concern that the effects of the sealing performance and low frictional properties may decrease due to degradation of the swelling liquid  17  resulting from foreign matter that has intruded into the space, damage to the interior members, or shortage of the swelling liquid  17 , and thus reduce the labor of replenishing the space with the swelling liquid  17  or replacing the swelling liquid  17 . Thus, with the seal structure  30  according to the present embodiment, it is possible to maintain the effects of excellently improving the sealing performance and providing low frictional properties for a long term in a wide applicable range. 
     Fourth Embodiment 
       FIG.  5    is a cross-sectional view of a cross-section along the axis x for illustrating the schematic configuration of a seal structure  40  according to a fourth embodiment as an exemplary aspect of the present disclosure. The configuration of the seal structure  40  according to the present embodiment will be described with reference to  FIG.  5   . 
     The seal structure  40  according to the fourth embodiment includes two seal units  20 A and  20 B sharing a single housing  35  for a single shaft  13 . The two seal units  20 A and  20 B each have a structure substantially similar to that of the seal structure  20  according to the second embodiment (i.e., an aspect of the seal structure of the present disclosure), and are arranged in a mutually inverted state. 
     Thus, each of the two seal units  20 A and  20 B has the same shape and configuration as those of the seal structure  20  according to the second embodiment except some structures. Accordingly, in  FIG.  5    of the present embodiment, members of the seal unit  20 B with the same configurations as those of the second embodiment are denoted by the same reference signs as those of the second embodiment, and the detailed description thereof will be omitted. In addition, members of the seal unit  20 A with the same configurations as those of the second embodiment are denoted by the same reference signs as those of the second embodiment with “′ (prime)” added thereto, and the detailed description thereof will be similarly omitted. 
     In addition, the single housing  35  shared by the two seal units  20 A and  20 B has the same shape and configuration as those of the seal structure  30  according to the third embodiment. Accordingly, in  FIG.  5    of the present embodiment, the housing  35  and its constituent members or related members (i.e., those including a reference sign  35 ) are denoted by the same reference signs as those of the third embodiment, and the detailed description thereof will be omitted. 
     In the present embodiment, between inner surface regions  15   d ′ and  15   d  of the single housing  35  shared by the two seal units  20 A and  20 B and ring members  26 ′ and  26  each serving as a pressure-receiving member, springs (i.e., elastic members for the ring members)  24 ′ and  24  for the ring member  26 ′ and  26  are further provided, respectively. The springs  24 ′ and  24  respectively press the ring members  26 ′ and  26  against bushes  11 ′ and  11  from one side (i.e., the upper side (a) in the seal unit  20 A and the lower side (b) in the seal unit  20 B) in the direction of the axis x. 
     The springs  24 ′ and  24  are respectively provided between inner surface regions  35   c  and  35   d  of the housing  35  surrounding openings  35   a  and  35   b  on one side (i.e., the upper side (a) in the seal unit  20 A and the lower side (b) in the seal unit  20 B) in the direction of the axis x and end surfaces  11   b ′ and  11   b  of the bushes  11 ′ and  11  on one side (i.e., the upper side (a) in the seal unit  20 A and the lower side (b) in the seal unit  20 B) in the direction of the axis x. 
     The springs  24 ′ and  24  respectively press the ring members  26 ′ and  26  against the bushes  11 ′ and  11  from one side (i.e., the upper side (a) in the seal unit  20 A and the lower side (b) in the seal unit  20 B) in the direction of the axis x. Thus, opposed surfaces  26   a ′ and  26   a  of the ring members  26 ′ and  26  having second coating layers  12 B′ and  12 B formed thereon are respectively pressed against the end surfaces  11   b ′ and  11   b  of the bushes  11 ′ and  11  due to the elastic action (i.e., the restoring action in the extension direction) of the springs  24 ′ and  24 . 
     Further, the other end sides (i.e., the lower side (b) in the seal unit  20 A and the upper side (a) in the seal unit  20 B) of the springs  24 ′ and  24  along their directions of expansion and contraction (which are the same as the direction of the axis x) are respectively connected to the ring members  26 ′ and  26 , and one end sides (i.e., the upper side (a) in the seal unit  20 A and the lower side (b) in the seal unit  20 B) of the springs  24 ′ and  24  are respectively connected to the inner surfaces (i.e., the inner surface region  35   c  in the seal unit  20 A and the inner surface region  35   d  in the seal unit  20 B) of the housing  35 . It should be noted that in  FIG.  5   , three springs  24 ′ and  24  are respectively arranged along the circumferential direction of the surfaces of the annular ring members  26 ′ and  26  as with the springs  24  of the second embodiment. 
     Seal rings  28 ′ and  28  are respectively attached to the outer peripheries of the ring members  26 ′ and  26 . The ring members  26 ′ and  26  are allowed to move in the direction of the axis x as the seal rings  28 ′ and  28  respectively frictionally slide on inner peripheral surfaces  35   n  and  35   p  of the housing  35  as in the second embodiment. The seal rings  28 ′ and  28  block and seal a swelling liquid  17 . Thus, the internal space of the housing  35  is a hermetically sealed space that allows the side of the seal unit  20 A and the side of the seal unit  20 B to communicate with each other through the opening  35   m.    
     In the present embodiment, a configuration substantially similar to that of the first embodiment is provided. Thus, functions and effects similar to those of the first embodiment can be achieved. In the present embodiment, not only the functions and effects similar to those of the first embodiment, but also functions and effects obtained with the provision of the springs  24 ′ and  24  as the elastic members for the ring members can be expected to be achieved. 
     That is, since the bushes  11 ′ and  11  and the ring members  26 ′ and  26  that respectively abut each other with the second coating layers  12 B′ and  12 B interposed therebetween are held so as to be sandwiched by the springs  14 ′ and  14  and the springs  24 ′ and  24  from the opposite sides in the direction of the axis x, the bushes  11 ′ and  11  and the ring members  26 ′ and  26  can be stably supported. In addition, since the bushes  11 ′ and  11  and the ring members  26 ′ and  26  are respectively held by the elastic springs  14 ′ and  14  and the elastic springs  24 ′ and  24  on the opposite sides in the direction of the axis x, the degree of freedom of the posture of each of the bushes  11 ′ and  11  and the ring members  26 ′ and  26  is increased, and followability thus improves. Thus, it is possible to achieve the effects of improving the sealing performance and providing low frictional properties at a higher level when the shaft  13  becomes eccentric or is inclined with respect to the axis x. 
     Further, the present embodiment provides a unique configuration in which the two seal units  20 A and  20 B sharing a single housing  35  are provided for a single shaft  13 , and thus, there is no possibility of leakage of the swelling liquid  17  filling the internal space, which is a hermetically sealed space, of the housing  35 . Thus, no matter how the seal structure  40  according to the present embodiment is applied such that its top and bottom sides are inclined or tipped over, the sealing effect is not influenced. This can significantly ease the restrictions on the applicable conditions, such as the applicable range, applicable environments, and applicable places. 
     In addition, since the internal space of the housing  35  filled with the swelling liquid  17  is a hermetically sealed space, there is no concern that foreign matter may intrude into the space from outside, or the swelling liquid  17  may evaporate and thus may have a reduced volume. Therefore, with the seal structure  40  according to the present embodiment, it is possible to reduce the concern that the effects of the sealing performance and low frictional properties may decrease due to degradation of the swelling liquid  17  resulting from foreign matter that has intruded to the space, damage to the interior members, or shortage of the swelling liquid  17 , and thus reduce the labor of replenishing the space with the swelling liquid  17  or replacing the swelling liquid  17 . Thus, with the seal structure  40  according to the present embodiment, it is possible to maintain the effects of excellently improving the sealing performance and providing low frictional properties for a long term in a wide applicable range. 
     Fifth Embodiment 
       FIG.  6    is a cross-sectional view of a cross-section along the axis x for illustrating the schematic configuration of a seal structure  50  according to a fifth embodiment as an exemplary aspect of the present disclosure. The configuration of the seal structure  50  according to the present embodiment will be described with reference to  FIG.  5   . 
     The seal structure  50  according to the fifth embodiment includes two seal units  10 B and  10 C sharing a single housing  55  for a single shaft  13 . The two seal units  10 B and  10 C each have a structure substantially similar to that of the seal structure  10  according to the first embodiment (i.e., an aspect of the seal structure of the present disclosure), and the orientations of the pair of the adjacent seal units  10 B and  10 C are the same along the direction of the axis x. 
     Thus, each of the two seal units  10 B and  10 C has the same shape and configuration as those of the seal structure  10  according to the first embodiment except some structures. Accordingly, in  FIG.  6    of the present embodiment, members of the seal unit  10 B with the same configurations as those of the first embodiment are denoted by the same reference signs as those of the first embodiment, and the detailed description thereof will be omitted. In addition, members of the seal unit  10 C with the same configurations as those of the first embodiment are denoted by the same reference signs as those of the first embodiment with “C” added thereto, and the detailed description thereof will be similarly omitted. Besides, some of the structures of the housing  55  have the same shapes and configurations as those of the housing  35  according to the third embodiment. Accordingly, such members are denoted by the same reference signs as those of the third embodiment, and the detailed description thereof will be omitted. 
     The housing  55  includes disk members  55   n  and  55   e  partially forming the seal unit  10 C and respectively having openings  55   a  and  55   m  for passing the shaft  13  at the center, a cylindrical portion  55   f  partially forming the seal unit  10 C as well and having flange portions  55   p  and  55   g  at its opposite ends along the direction of the axis x, and a cup member  35   i  partially forming the seal unit  10 B. 
     The flange portion  55   g  of the cylindrical portion  55   f  and a flange portion  35   h  of the cup member  35   i  are fastened together with fasteners  35   j  while sandwiching the disk member  55   e  therebetween. In addition, the disk member  55   n  is fastened to the upper side (a) of the other flange portion  55   p  of the cylindrical portion  55   f  with fasteners  55   j  each including a vis and a nut, for example. Accordingly, a single housing  55  is formed as a whole. 
     In the present embodiment, springs  14  of the seal unit  10 B are provided between a surface  55   kb  on the lower side (b) of a protruding portion  55   k  of the disk member  55   e , which corresponds to the inner surface region of the housing  55 , and an end surface  11   a  of a bush  11 . The springs  14  are connected to the surface  55   kb  and the end surface  11   a.    
     Springs  14 C of the seal unit  10 C are provided between a surface  55   c  on the lower side (b) of the disk member  55   n , which corresponds to the inner surface region of the housing  55 , and an end surface  11   a C of a bush  11 C. The springs  14 C are connected to the surface  55   c  and the end surface  11   a C. 
     In the present embodiment, there is no member like the ring member  16  between the bush  11 C and a surface  55   ka  of the protruding portion  55   k  of the disk member  55   n , which corresponds to the inner surface region of the housing  55 , in the seal unit  10 C. 
     Therefore, a facing surface that faces an end surface  11   b C of the bush  11 C on one side (i.e., the lower side (b)) in the direction of the axis x is the surface  55   ka , and the housing  55  having the surface  55   ka  forms the pressure-receiving member as referred to in the present disclosure. In addition, a second coating layer  12 BC of the seal unit  10 C is formed on the surface  55   ka.    
     The springs  14 C press the bush  11 C against the surface  55   ka  of the housing  55  from the other side (i.e., the upper side (a)) in the direction of the axis x. Thus, the end surface  11   b C of the bush  11 C is pressed against the surface  55   ka  having the second coating layer  12 BC formed thereon due to the elastic action (i.e., the restoring action in the extension direction) of the springs  14 C. 
     In the housing  55 , the internal space on the side of the seal unit  10 B communicates with the side of the seal unit  10 C through the opening  55   m , but is sealed by a first coating layer  12 AC and the second coating layer  12 BC in the seal unit  10 C on the upper side (a) immediately above it. Therefore, the internal space on the side of the seal unit  10 B is a hermetically sealed space. 
     Meanwhile, the internal space on the side of the seal unit  10 C communicates with the outside on the upper side (a) through the opening  55   a , but is sealed by the first coating layer  12 AC and the second coating layer  12 BC on the lower side (b). Therefore, the internal space on the side of the seal unit  10 C is separated from the internal space on the side of the seal unit  10 B, and thus can be filled with a swelling liquid (i.e., a liquid substance)  57  of a different type from that of the swelling liquid  17  filling the internal space on the side of the seal unit  10 B. 
     In the present embodiment, the seal units  10 B and  10 C each having a configuration substantially similar to that of the first embodiment are provided. Thus, functions and effects similar to those of the first embodiment can be achieved. In the present embodiment, not only the functions and effects similar to those of the first embodiment, but also functions and effects unique to the present embodiment can be expected to be achieved. 
     That is, in the present embodiment, another seal unit  10 B is provided adjacent to one side (i.e., the lower side (b)) of the seal unit  10 C along the direction of the axis x. The gap between one side (i.e., the lower side (b)) of the seal unit  10 C along the direction of the axis x and the adjacent seal unit  10 B is sealed. 
     Therefore, the swelling liquids  17  and  57  can respectively fill different rooms on the sealed side (i.e., the side of the seal unit  10 B) and the open side (i.e., the side of the seal unit  10 C), and thus, optimum swelling liquids can be selectively used according to the use conditions. In addition, optimum layers can also be selectively used as the coating layers  12  in the seal unit  10 B and the seal unit  10 C according to the use conditions. 
     Sixth Embodiment 
       FIG.  7    is a cross-sectional view of a cross-section along the axis x for illustrating the schematic configuration of a seal structure  60  according to a sixth embodiment as an exemplary aspect of the present disclosure. The configuration of the seal structure  60  according to the present embodiment will be described with reference to  FIG.  7   . 
     The seal structure  60  according to the sixth embodiment includes three seal units  10 A,  10 D, and  10 B sharing a single housing  65  for a single shaft  13 . The three seal units  10 A,  10 D, and  10 B each have a structure substantially similar to that of the seal structure  10  according to the first embodiment (i.e., an aspect of the seal structure of the present disclosure). 
     The pair of the adjacent seal units  10 A and  10 D are arranged in a mutually inverted state. Meanwhile, the orientations of another pair of the seal units  10 D and  10 B are the same along the direction of the axis x. 
     It should be noted that the seal structure  60  according to the present embodiment is considered as a structure in which the seal unit  10 D is sandwiched between the seal unit  10 A and the seal unit  10 B in the seal structure  30  according to the third embodiment. Accordingly, in  FIG.  7    of the present embodiment, members of the seal unit  10 A and the seal unit  10 B with the same configurations as those of the third embodiment are denoted by the same reference signs as those of the third embodiment, and the detailed description thereof will be omitted. 
     In addition, members of the seal unit  10 D with the same configurations as those of the first embodiment are denoted by the same reference signs as those of the first embodiment with “D” added thereto, and the detailed description thereof will be similarly omitted. 
     The housing  65  includes a cup member  35   f  partially forming the seal unit  10 A, a cup member  35   i  partially forming the seal unit  10 B, a disk member  35   e  having an opening  35   m  for passing the shaft  13  at the center, and members forming the seal unit  10 D. The members of the housing  65  forming the seal unit  10 D include a cylindrical portion  65   g  having flange portions  65   r  and  65   s  at its opposite ends along the direction of the axis x, and a disk member  65   u  having an opening  65   w  for passing the shaft  13  at the center. Such members are sandwiched between the disk member  35   e  and the cup member  35   i.    
     The flange portion  35   h  of the cup member  35   i  among the three members fastened together with the fasteners  35   j  in the third embodiment is replaced with the flange portion  65   r  of the cylindrical portion  65   g  in the present embodiment. That is, in the present embodiment, the flange portion  35   g  of the cup member  35   f  and the flange portion  65   r  of the cylindrical portion  65   g  are fastened together with the fasteners  35   j  while sandwiching the disk member  35   e  therebetween. 
     In addition, the flange portion  65   s  of the cylindrical portion  65   g  and the flange portion  35   h  of the cup member  35   i  are fastened together with fasteners  65   t  each including a vis and a nut, for example, while sandwiching the disk member  65   u  therebetween. 
     Through the fastening with the fasteners  35   j  and the fasteners  65   t , a single housing  65  is formed as a whole. 
     In the present embodiment, springs  14  of the seal unit  10 B are provided between a surface  65   vb  on the lower side (b) of a protruding portion  65   v  of the disk member  65   u , which corresponds to the inner surface region of the housing  65 , and an end surface  11   a  of a bush  11 . The springs  14  are connected to the surface  65   vb  and the end surface  11   a.    
     Springs  14 D of the seal unit  10 D are provided between a surface  35   kb  on the lower side (b) of a protruding portion  35   k  of the disk member  35   e , which corresponds to the inner surface region of the housing  65 , and an end surface  11   a D of a bush  11 D. The springs  14 D are connected to the surface  35   kb  and the end surface  11   a D. 
     In the present embodiment, there is no member like the ring member  16  between the bush  11 D and a surface  65   va  of the protruding portion  65   v  of the disk member  65   u , which corresponds to the inner surface region of the housing  65 , in the seal unit  10 D. 
     Therefore, a facing surface that faces an end surface  11   b D of the bush  11 D on one side (i.e., the lower side (b)) in the direction of the axis x is the surface  65   va , and the housing  65  having the surface  65   va  forms the pressure-receiving member as referred to in the present disclosure. In addition, a second coating layer  12 BD of the seal unit  10 D is formed on the surface  65   va.    
     The springs  14 D press the bush  11 D against the surface  65   va  of the housing  65  from the other side (i.e., the upper side (a)) in the direction of the axis x. Thus, the end surface  11   b D of the bush  11 D is pressed against the surface  65   va  having the second coating layer  12 BD formed thereon due to the elastic action (i.e., the restoring action in the extension direction) of the springs  14 D. 
     The internal space of the housing  65  is a hermetically sealed space that allows the region of the seal unit  10 A and the region of the seal unit  10 D to communicate with each other through the opening  35   m . The hermetically sealed space (i.e., the internal space) is filled with a swelling liquid  17  for swelling the first coating layers  12 A′ and  12 AD and the second coating layers  12 B′ and  12 BD in the seal unit  10 A and the seal unit  10 D. 
     Meanwhile, in the internal space of the housing  55 , the internal space of the region of the seal unit  10 B communicates with the side of the seal unit  10 D through the opening  65   w , but is sealed by the first coating layer  12 AD and the second coating layer  12 BD in the seal unit  10 D on the upper side (a) immediately above it. Therefore, the internal space of the region of the seal unit  10 B is separated from the internal space of the region of the seal units  10 A and  10 D, and thus can be filled with a swelling liquid (i.e., a liquid substance)  67  of a different type from that of the swelling liquid  17  filling the internal space of the region of the seal units  10 A and  10 D. 
     In the present embodiment, the seal units  10 A,  10 D, and  10 B each having a configuration substantially similar to that of the first embodiment are provided. Thus, functions and effects similar to those of the first embodiment can be achieved. In the present embodiment, not only the functions and effects similar to those of the first embodiment, but also functions and effects unique to the present embodiment can be expected to be achieved. 
     That is, in the present embodiment, another seal unit  10 B is provided adjacent to one side (i.e., the lower side (b)) of the seal unit  10 D along the direction of the axis x. The gap between one side (i.e., the lower side (b)) of the seal unit  10 D along the direction of the axis x and the adjacent seal unit  10 B is sealed. 
     Therefore, the swelling liquids  17  and  67  can respectively fill different rooms of the two seal regions, that is, the region of the seal unit  10 B and the region of the seal units  10 A and  10 D, and thus, optimum swelling liquids can be selectively used according to the use conditions. In addition, optimum layers can also be selectively used as the coating layers  12  in the seal unit  10 B, the seal unit  10 A, and the seal unit  10 D according to the use conditions. 
     In addition, in the present embodiment, each of the region of the seal unit  10 B and the region of the seal units  10 A and  10 D is a hermetically sealed space. That is, although the two rooms in the internal space of the housing  65  are filled with the different swelling liquids  17  and  67 , there is no possibility of leakage of the swelling liquids  17  and  67  since each of the two rooms is a hermetically sealed space. Thus, no matter how the seal structure  60  according to the present embodiment is applied such that its top and bottom sides are inclined or tipped over, the sealing effect is not influenced. This can significantly ease the restrictions on the applicable conditions, such as the applicable range, applicable environments, and applicable places. 
     Further, in the present embodiment, three seal units are arranged successively. Thus, even when leakage has occurred in any of the three seal units, the other seal units can serve as backups. Thus, the durability of the seal structure as a whole significantly improves. In particular, when the seal structure is used under environments of different pressures, for example, when the opposite ends (i.e., the upper side (a) and the lower side (b)) of the seal structure are exposed to greatly different environments or conditions, loads due to such environments or conditions would be applied to the seal units on the opposite ends. Thus, to reduce the load applied to each seal unit, it would be effective to increase the number of seal units. 
     The effect of improving durability based on such backups is greater when two seal units are provided (as in the third to fifth embodiments) than when one seal unit is provided (as in the first and second embodiments). The effect is even greater when three seal units are provided as in the present embodiment. Needless to say, four or more seal units may be provided as appropriate. 
     Although six preferred embodiments of the seal structure of the present disclosure have been described above, the seal structure of the present disclosure is not limited to the configurations of the seal structures  10 ,  20 ,  30 ,  40 ,  50 , and  60  according to the aforementioned embodiments. For example, although each of the aforementioned embodiments has illustrated an example in which coiled springs are used as the elastic members and the elastic members for the ring member, the elastic members and the elastic members for the ring member of the present disclosure are not limited to coiled springs, and various elastic bodies, such as leaf springs and rubber, can also be used. 
     In addition, although each of the aforementioned embodiments has illustrated an example in which the springs  14 ′,  14 ,  14 C,  14 D,  24 ′, or  24  are fixed to members on the opposite sides thereof, it is acceptable as long as each of the elastic members and the elastic members for the ring member of the present disclosure has a function of pressing a target member to be pressed (i.e., the bush, ring member, or housing) in a predetermined direction. Further, each of the elastic members and the elastic members for the ring member may be arranged at any position as long as it has such a function. Needless to say, each spring is preferably located between target members to be pressed and fixed to the target members on the opposite sides from the viewpoint of stabilizing the support and pressure. 
     Further, although each of the aforementioned embodiments has illustrated an example in which the first coating layer  12 A is formed on the inner peripheral surface  11   d ′,  11   d ,  11   d C, or  11   d D of the shaft hole  11   c ′,  11   c ,  11   c C, or  11   c D of the bush  11 ′,  11 ,  11 C, or  11 D, it is acceptable as long as the first coating layer of the present disclosure is formed on one or each of the outer peripheral surface of the shaft  13  and the inner peripheral surface  11   d ′,  11   d ,  11   d C, or  11   d D of the shaft hole  11   c ′,  11   c ,  11   c C, or  11   c D of the bush  11 ′ or  11 . As long as the first coating layer is formed between the inner peripheral surface  11   d ′,  11   d ,  11   d C, or  11   d D of the shaft hole  11   c ′,  11   c ,  11   c C, or  11   c D and the shaft  13 , the gap between them can be filled. 
     Each of the first to fourth embodiments has illustrated an example in which the second coating layer  12 B or  12 B′ is formed on the facing surface  16   a ′,  16   a ,  26   a ′, or  26   a  of the ring member  16 ′,  16 ,  26 ′, or  26 . However, in the present disclosure, it is acceptable as long as the second coating layer is formed on one or each of the end surface of the bush  11 ′ or  11  on one side in the direction of the axis x and the facing surface  16   a ′,  16   a ,  26   a ′, or  26   a  of the ring member  16 ′,  16 ,  26 ′, or  26 . 
     Each of the fifth and sixth embodiments has illustrated an example in which the second coating layer  12 B,  12 BC, or  12 BD is formed on each of the facing surface  16   a ′ or  16   a  of the ring member  16 ′ or  16  and the surface  55   ka  or  65   va  that is the inner surface region of the housing  55  or  65 . 
     However, in the present disclosure, it is acceptable as long as the second coating layer is formed on one or each of the end surface of the bush  11 ′,  11 ,  11 C, or  11 D on one side in the direction of the axis x and the facing surface  16   a ′ or  16   a  of the ring member  16 ′ or  16  or the surface  55   ka  or  65   va  that is the inner surface region of the housing  55  or  65 . 
     It should be noted that “one side of/along the direction of the axis x” corresponds to the lower side (b) of the seal units  10 B and  20 B in the first and second embodiments and the third and fourth embodiments, and corresponds to the upper side (a) of the seal units  10 A and  20 A in the third and fourth embodiments. Likewise, “one side of/along the direction of the axis x” corresponds to the lower side (b) of the seal units  10 D and  10 B in the fifth and sixth embodiments, and corresponds to the upper side (a) of the seal unit  10 A in the sixth embodiment. 
     Although each embodiment has illustrated an example in which the ring member or the housing (or the inner surface region thereof) forms the pressure-receiving member, the member forming the pressure-receiving member of the present disclosure may be those with other shapes or structures besides the ring member or the housing. In addition, the ring member is not an essential component, and thus, the configuration of each of the first to fourth embodiments may be replaced with the configuration of each of the fifth and sixth embodiments in which a ring member is not provided and the housing forms the pressure-receiving member. Needless to say, even in the fifth and sixth embodiments, the housing may form the pressure-receiving member or a ring member may form the pressure-receiving member in each of the seal units, for example. 
     For example, in the first embodiment described with reference to  FIG.  1   , when the inner surface region of the housing forms the facing surface of the pressure-receiving member, the ring member  16  and the seal ring  18  are removed, and thus, the inner surface region  15   d  of the housing  15  surrounding the opening  15   b  on one side (i.e., the lower side (b)) in the direction of the axis x corresponds to the “opposed surface of the pressure-receiving member” as referred to in the present disclosure. 
     In addition, it is acceptable as long as the second coating layer is formed on one or each of the end surface  11   b  of the bush  11  on one side (i.e., the lower side (b)) in the direction of the axis x and the inner surface region  15   d  of the housing  15 . This is also true of the seal units  10 A and  10 B of the third embodiment. 
     Besides, one of ordinary skill in the art can modify the seal structure of the present disclosure as appropriate based on the conventionally known finding. Needless to say, such a modified seal structure is also encompassed by the present disclosure as long as it has the configuration of the present disclosure. 
     Example 
     Described hereinafter is a test conducted by actually producing a prototype device with the seal structure of the present disclosure and confirming its sealing performance and durability. It should be noted that the present disclosure is not limited to the configuration of an Example described below. 
       FIG.  8    illustrates a cross-sectional view of a cross-section along an axis for illustrating the schematic configuration of a prototype device produced in an Example.  FIG.  9    illustrates an A-A cross-sectional view of the prototype device in  FIG.  8   . It should be noted that in  FIG.  9   , the illustration of a swelling liquid (i.e., a liquid substance)  117  described below is omitted. It should be also noted that the following description includes a case where the top-down relationship of the prototype device is described based on the top-down relationship in  FIG.  8   . 
     The prototype device has a configuration similar to that of the first embodiment illustrated in  FIG.  1   . 
     The present prototype device mainly includes, as illustrated in  FIGS.  8  and  9   , a shaft (i.e., shaft member)  113 , a bush  111 , a ring member (i.e., a pressure-receiving member)  116 , a housing  115 , and springs (i.e., elastic members)  114   a ,  114   b , and  114   c , a support portion  122 , bearings  120  and  121 , and a rotary drive device (not illustrated). 
     The shaft  113  includes a small-diameter portion  113   a  on the lower side and a large-diameter portion  113   b  on the upper side. 
     The housing  115  has openings  115   a  and  115   b , which are adapted to pass the small-diameter portion  113   a  of the shaft  113 , in its opposite end surfaces along the up-down direction. 
     The bush  111  has the shape of a thick-walled cylinder with a shaft hole  111   c  through which the shaft  113  is adapted to be fitted and inserted, and has flat ring-shaped end surfaces  111   a  and  111   b  on its upper side and lower side. The outer peripheral surface of the bush  111  is provided with an arc-shaped cutout portion  111   e  extending linearly in the up-down direction. 
     The ring member  116  is provided between an inner surface region  115   d  of the housing  115  surrounding the opening  115   b  and the lower end surface of the bush  111 . In addition, the ring member  116  has a facing surface  116   a  facing the lower end surface of the bush  111 . 
     A seal ring  118  is provided between a region around the central opening of the ring member  116  and the inner surface region  115   d  of the housing  115 . 
     A first coating layer  112 A is formed on the outer peripheral surface of the shaft  113  and the inner peripheral surface of the shaft hole  111   c  of the bush  111 . In addition, a second coating layer  112 B is formed on the facing surface  116   a  of the ring member  116 . 
     The internal space (which is not a hermetically sealed space) of the housing  115  is filled with the swelling liquid  117 . 
     The springs  114   a ,  114   b , and  114   c  are provided between an inner surface region  115   c  of the housing  115  surrounding the opening  115   a  and the upper end surface of the bush  111 . The springs  114   a ,  114   b , and  114   c  press the bush  111  against the ring member  116  from the upper side. 
     Further, the lower side of each spring  114  along its direction of expansion and contraction is connected to the bush  111 , and the upper side of each spring  114  is connected to the inner surface region  115   c  of the housing  115 . 
     A cylindrical whirl-stop rod  119  extends downward in the vertical direction from the inner surface region  115   c  of the housing  115 . The whirl-stop rod  119  is fitted in the cutout portion  111   e  of the bush  111 , and regulates the movement of the bush  111  in the direction of rotation (i.e., the direction of the arrow T). 
     The shaft  113  is rotatably supported above the present prototype device as the large-diameter portion  113   b  of the shaft  113  is fixed to the inner rings of the bearings  120  and  121  whose outer rings are fixed to the support portion  122 . 
     The shaft  113  is configured to rotate counterclockwise (i.e., in the direction of the arrow T) as the upper end of the shaft  113  receives a rotary driving force from the rotary drive device (not illustrated). 
     In the present prototype device, the housing  115  and its internal structure form the seal structure of the present disclosure. 
     For such a prototype device, each polymer brush layer was formed under the following conditions. 
     The dry thickness of each polymer brush layer: the first coating layer  112 A (on the outer peripheral surface of the shaft  113 ): 440 nm, the first coating layer  112 A (on the inner peripheral surface of the shaft hole  111   c  of the bush  111 ): 1120 nm, and the second coating layer  112 B: 460 nm 
     Molecular Structure of Each Polymer Brush Layer: 
     
       
         
         
             
             
         
       
         
         
           
             The surface occupation rate σ* of the polymer graft chains forming each polymer brush layer: 
           
         
       
    
     the first coating layer  112 A (on the shaft  113 ): the graft density σ=0.33 
     the first coating layer  112 A (on the bush  111 ): the graft density σ=0.33 
     the second coating layer  112 B: the graft density σ=0.31
         The average molecular chain length Lp of the polymer graft chains forming each polymer brush layer:       

     the first coating layer  112 A (on the shaft  113 ): the average molecular chain length Lp=2.4 μm 
     the first coating layer  112 A (on the bush  111 ): the average molecular chain length Lp=6.1 μm 
     the second coating layer  112 B: the average molecular chain length Lp=2.9 μm
         The molecular weight distribution (Mw/Mn) of the polymer graft chains forming each polymer brush layer:       

     the first coating layer  112 A (on the shaft  113 ): the molecular weight distribution (Mw/Mn)=1.09 
     the first coating layer  112 A (on the bush  111 ): the molecular weight distribution (Mw/Mn)=1.18 
     the second coating layer  112 B: the molecular weight distribution (Mw/Mn)=1.09 
     As the swelling liquid  117 , an ionic liquid (N-(2-methoxyethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide) (hereinafter abbreviated as “MEMP-TFSI”) was used. The member having the polymer brush layer formed thereon was immersed in MEMP-TFSI under a reduced-pressure environment (5000 Pa) for 48 hours to have the polymer brush layer swollen, and was then used for assembling the prototype device. 
     When the prototype device was assembled, the pressing force of the springs  114  was adjusted by controlling the attached states of the various components so as to allow a load of 20 N to be applied to the facing surface  116   a  of the ring member  116  from the bush  111 . 
     The upper side of the prototype device was released to be at an atmospheric pressure AP, while the lower side thereof was set to a vacuum state V. 
     In such a state, the shaft  113  of the prototype device was steadily rotated at 100 rpm (revolutions per minute), and a continuous durability test for about 2000 hours was conducted. Consequently, neither leakage of the swelling liquid  117  from the side of the atmospheric pressure AP to the side of the vacuum state V nor a decrease in the degree of vacuum was confirmed during the durability test. Thus, the sealed state was maintained.