Patent Publication Number: US-2018038309-A1

Title: Piston of internal combustion engine or method for processing surface of piston of internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to a piston of an internal combustion engine or a method for processing a surface thereof. 
     BACKGROUND ART 
     There is known a piston of an internal combustion engine that includes a film on an outer peripheral surface of a skirt portion slidably movable relative to an inner wall of a cylinder (for example, PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Public Disclosure No. 2010-216362 
     SUMMARY OF INVENTION 
     Technical Problem 
     Conventionally, no consideration is given to a frictional coefficient under fluid lubrication of the outer peripheral surface of the skirt portion. 
     Solution to Problem 
     According to one aspect of the present invention, a piston preferably includes at least one electrodeposited film layer on an outer peripheral surface of a skirt portion. 
     Therefore, the fluid lubrication frictional coefficient on the outer peripheral surface of the skirt portion can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrate a side and a cross-section of a piston according to an embodiment. 
         FIG. 2  illustrates cross-sections of a cylinder and the piston according to the embodiment. 
         FIG. 3  schematically illustrates a cross-section of a skirt portion outer peripheral surface according to a first embodiment. 
         FIG. 4  illustrates an electrodeposition coating apparatus (a first example) according to the embodiment. 
         FIG. 5  illustrates how electrodeposition coating is carried out by the electrodeposition coating apparatus according to the first example. 
         FIG. 6  illustrates an electrodeposition coating apparatus (a second example) according to the embodiment. 
         FIG. 7  illustrates a liquid passage member of the electrodeposition coating apparatus according to the second example. 
         FIG. 8  illustrates experiment data indicating a relationship between a height of a streak and a fluid lubrication friction coefficient on the skirt portion outer peripheral surface. 
         FIG. 9  schematically illustrates a cross-section of the skirt portion outer peripheral surface in a case where one film layer is formed without use of the electrodeposition coating. 
         FIG. 10  schematically illustrates a cross-section of a skirt portion outer peripheral surface according to a second embodiment. 
         FIG. 11  schematically illustrates a cross-section of the skirt portion outer peripheral surface in a case where two film layers are formed without use of the electrodeposition coating. 
         FIG. 12  schematically illustrates a cross-section of a skirt portion outer peripheral surface according to a third embodiment. 
         FIG. 13  illustrates a cross-section of the skirt portion outer peripheral surface in a result of an experiment according to the third embodiment. 
         FIG. 14  schematically illustrates a cross-section of a skirt portion outer peripheral surface according to a fourth embodiment. 
         FIG. 15  schematically illustrates a cross-section of a skirt portion outer peripheral surface according to a fifth embodiment. 
         FIG. 16  illustrates a cross-section of the skirt portion outer peripheral surface in a result of an experiment according to the fifth embodiment. 
         FIG. 17  schematically illustrates a cross-section of a skirt portion outer peripheral surface according to a sixth embodiment. 
         FIG. 18  illustrates a cross-section of the skirt portion outer peripheral surface in a result of an experiment according to the sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following description, how to implement a piston and a method for processing a surface thereof according to one embodiment of the present invention will be described with reference to the drawings. 
     First Embodiment 
     First, a configuration will be described.  FIG. 1  illustrates a piston  1  of an internal combustion engine (hereinafter referred to as an engine) according to the present embodiment as viewed from a direction in which a central axis P of a piston pin hole  111  extends. A right half illustrates a cross-section of the piston  1  in a plane containing a central axis O of the piston  1 . The engine is, for example, a four-cycle gasoline engine but is not limited thereto. The piston  1  is reciprocatably contained in a cylinder  2  of the engine.  FIG. 2  illustrates cross-sections of the piston  1  and the cylinder  2  coupled to a connecting rod (con rod)  4  taken along a plane containing the central axis O and extending perpendicularly to the central axis P. The cylinder  2  is formed into a cylinder block, and an inner wall  20  thereof (a wall surface inside the cylinder  2 ) is generally cylindrical. In other words, the inner wall  20  of the cylinder  2  (hereinafter referred to as the cylinder inner wall  20 ) is generally circular in a plane perpendicular to a central axis of the cylinder  2 . A cylinder head is mounted on the cylinder block. The cylinder head closes an opening of the cylinder  2 . A combustion chamber is defined between a crown surface  104  of the piston  1 , the cylinder inner wall  20 , and the cylinder head. The piston  1  is coupled to one end side (a small end portion  40 ) of the con rod  4  via a piston pin  3 . The other end side (a large end portion) of the con rod  4  is coupled to a crankshaft. 
     The piston  1  is formed by casting or the like with use of aluminum alloy (for example, Al—Si AC8A) as a parent material (a base material). The piston  1  has a bottomed cylindrical shape, and includes a head portion  10 , apron portions  11 , and skirt portions  12 . The head portion  10  includes the crown surface  104  at a crown portion thereof. The head portion  10  is hollow on an inner peripheral side of a portion other than the crown portion. Three ring grooves  101 ,  102 , and  103  extend on an outer peripheral surface of the head portion  10  in a circumferential direction of the piston  1  (a direction around the central axis O). Compression rings  51  and  52  are placed in the ring grooves  101  and  102  located closer to the crown surface  104 , respectively, and an oil ring  53  is placed in the ring groove  103  located farther away from the crown surface  104 . The apron portions  11  and the skirt portions  12  are located on an opposite side from the crown surface  104  with respect to the ring grooves  101  to  103  in a direction in which the central axis O of the piston  1  extends (a central-axis direction). The skirt portions  12  and the apron portions  11  are hollow on inner peripheral sides thereof. The pair of apron portions  11  are provided on both radial sides of the piston  1  with the central axis O sandwiched therebetween. Each of the apron portions  11  includes a pin boss  110 . Each of the pin bosses  110  includes the piston pin hole ill. The piston pin hole  111  extends in the radial direction of the piston  1  while penetrating through the pin boss  110 . An end of the piston pin  3  is fitted to the piston pin hole  111 . An outer peripheral surface of the apron portion  11  is located closer to the central axis O than an outer peripheral surface of the head portion  10  (the skirt portions  12 ) is. The pair of skirt portions  12  are provided on the both radial sides of the piston  1  with the central axis O sandwiched therebetween. The skirt portions  12  include a thrust-side skirt portion  121  and an opposite thrust-side skirt portion  122 . The skirt portions  12  are sandwiched by both the apron portions  11  in the circumferential direction of the piston  1 . The skirt portions  12  are thinner than the apron portions  11 . An outer peripheral surface  120  of each of the skirt portions  12  (hereinafter referred to as the skirt portion outer peripheral surface) has a circular-arc shape in the plane perpendicular to the central axis O. Twice a distance from the central axis O to the skirt portion outer peripheral surface  120  (an outer diameter of the skirt portions  12 ) is slightly larger than an outer diameter of the head portion  10 , and is slightly smaller than a diameter of the cylinder inner wall  20  (an inner diameter of the cylinder  2 ). 
       FIG. 3  illustrates a (partial) cross-section of the outer peripheral side of the skirt portion  12  in the plane containing the central axis O. The skirt portion outer peripheral surface  120  is covered by at least one film portion  13 . In the present embodiment, the film portion  13  includes only a single layer that is electrodeposited film  130 . The base material  100  of the piston  1  is covered by the single layer that is electrodeposited film  130  on the skirt portion outer peripheral surface  120 , and this electrodeposited film  130  is exposed on the skirt portion outer peripheral surface  120  (in other words, the electrodeposited film  130  directly faces the cylinder inner wall  20 , the meaning of being exposed also applies hereinafter). The piston  1  includes streaks  14  on the skirt portion outer peripheral surface  120 . The steaks  14  are streaked grooves extending in the circumferential direction of the piston  1 . The plurality of streaks  14  are lined up adjacent to each other in the central-axis direction of the piston  1 . Each of the streaks  14  includes a streak  140  on the base material  100  of the piston  1 . The streak  140  is spirally formed around the central axis O by, for example, turning processing with use of a turning tool or rolling processing with use of a roller. A cross-sectional shape of the streak  140  (a shape in the plane containing the central axis O) is a U shape or a V shape in conformity to a shape of a blade edge of a processing tool. The cross-sectional shape of the streak  140  may be a stepped shape due to, for example, the processing performed partially redundantly. A shape of a portion between the adjacent streaks  140  (hereinafter referred to as a top) may be a protruding shape (having a pointed tip) or a planer shape (having a flat tip). A distance between the adjacent streaks  140  (a pitch of the streak) is a predetermined value within a range from several dozen μm to several hundred μm (for example, approximately 250 μm). A height or a depth of the streak  140  is a predetermined value within a range of several μm to several dozen μm (for example, approximately 10 μm). Now, the height (the depth) of the streak  14  refers to a distance from a lowermost portion to an uppermost portion (the top) of this streak  14  in a normal direction of the skirt portion outer peripheral surface  120  (the radial direction of the piston  1 ). A direction in which the streak  140  extends along the skirt portion outer peripheral surface  120  may be any direction as long as this direction is angled with respect to the central-axis direction of the piston  1 , and the streak  140  may, for example, extend obliquely with respect to the circumferential direction of the piston  1 . The streak  140  may snake in a wavelike manner instead of extending linearly. The shape and the size of the streak  140  may be changed according to the position and the range of the skirt portion  12 . 
     The electrodeposited film  130  is a film formed by electrodeposition coating. More specifically, the electrodeposited film  130  is formed by electrodepositing an electrodeposition coating material on the skirt portion outer peripheral surface  120 . The electrodeposited film  130  functions as a decorative layer for improving smoothness of the skirt portion outer peripheral surface  120 . The electrodeposited film  130  contains a resin (a binder resin) as a binding agent (a binder) having an adhesion property to another material. A resin having an excellent heat resistance property and abrasion resistance property, such as a polyamide-imide resin (hereinafter referred to as PAI), is used as the binder resin. The electrodeposited film  130  may contain, as the binder resin, another binder resin, such as at least one of a polyimide resin (hereinafter referred to as PI) or an epoxy resin (hereinafter referred to as EP), together with or instead of PAI. PI has an excellent heat resistance property and abrasion resistance similarly to PAI. PAI, PI, and EP also have an excellent adhesion property. Further, the electrodeposited film  130  may contain an additive other than the binder resin. The electrodeposited film  130  does not contain a conductive solid lubricant, such as graphite and molybdenum disulfide. 
     The film portion  13  (the electrodeposited film  130  in the present embodiment) is formed so as to cover the skirt portion outer peripheral surface  120  in processing of the surface of the piston  1 . A method for processing the surface of the piston  1  includes a process for forming the electrodeposited film. In the process for forming the electrodeposited film, an electrodeposition coating process, a water washing process, a burning process, and a cooling process are performed in this order. Processing for, for example, removing oil and contamination from the skirt portion outer peripheral surface  120  of the piston  1  (the base material  100 ), which is a coating target, may be performed before the electrodeposition coating process to, for example, improve the adhesiveness of the film portion  13 . In the electrodeposition coating process, the skirt portion  12  of the piston  1 , which is the coating target, is dipped in an aqueous electrodeposition coating material, and the electrodeposition coating material side is formed as an opposite electrode. For example, in a case where an anionic coating material is used, the skirt portion  12  is formed as an anode and the electrodeposition coating material side is formed as a cathode. The polarity may be reversed with use of a cationic coating material. The electrodeposition coating material is prepared by, for example, dissolving or dispersing (into a colloid state) the binder resin serving as a base in water. An organic solvent, a neutralizer, an additive, and/or the like are added as necessary. The electrodeposition can be carried out by a constant current method or a constant voltage method. A direct-current voltage is applied between the electrodes to cause a direct current to pass therethrough under predetermined electrodeposition conditions. The electrodeposition conditions are, for example, a voltage of several dozen V to several hundred V and a processing time (a power supply time) of several seconds to several dozen seconds. When the current passes through, the water in the electrodeposition coating material is electrolytically decomposed and coating material particles including the binder resin are ionized, and these coating material particles are electrophoresed toward one side where the skirt portion  12  as the electrode is located. The coating material particles precipitated on the skirt portion outer peripheral surface  120  are fused due to Joule heat generated from an electric resistance, and are deionized and become insoluble. As a result, an electrically insulating (exhibiting a non-conductive resistance) film (the electrodeposited film  130 ) covering the base material  100  is formed on the skirt portion outer peripheral surface  120 . A thickness (a film thickness) of the electrodeposited film  130  can be appropriately adjusted according to the electrodeposition conditions. 
       FIG. 4  schematically illustrates a first example of an apparatus  5  for performing the electrodeposition coating process. The apparatus  5  includes an electrodeposition coating material reservoir unit  6 , an electrodeposition coating material delivery unit  7 , a power supply unit  8 , and a control unit  9 . The electrodeposition coating material reservoir unit  6  includes a liquid tank  60 . The liquid tank  60  reserves the electrodeposition coating material therein. The electrodeposition coating material delivery unit  7  includes masking plates  71 , nozzles  72 , a pipe conduit  73 , and a pump  74 . The pair of masking plates  71  are disposed on both the radial sides of the piston  1 . A masking plate  711 , which is one of the making plates  71 , covers a region other than the thrust-side skirt portion  121  on the outer peripheral surface of the piston  1  on one radial side. A masking plate  712 , which is the other of the masking plates  71 , covers a region other than the opposite thrust-side skirt portion  122  on the outer peripheral surface of the piston  1  on the other radial side. A seal member (an O-ring)  713  is disposed between the making plates  71  and the outer peripheral surface of the piston  1  so as to surround the skirt portion  12 . The pair of nozzles  72  are disposed on both the radial sides of the piston  1 . A nozzle  721 , which is one of the nozzles  72 , faces the thrust-side skirt portion  121 . A nozzle  722 , which is the other of the nozzles  72 , faces the opposite thrust-side skirt portion  122 . These nozzles  721  and  722  are connected to the pump  74  via pipe conduits  731  and  732 , respectively. The pump  74  is connected to the liquid tank  60  via a pipe conduit  730 . The pump  74  introduces the electrodeposition coating material from the liquid tank  60  therein, and discharges the electrodeposition coating material toward the nozzles  72  side. 
     The power supply unit  8  includes electrodes, electrodeposition wirings  83  and  84 , and a power source  85 . The electrodes include an anode  81  and a cathode  82 . The anode  81  is disposed the piston  1  side. The anode  81  has a rod-like shape, and a distal end thereof faces the crown surface  104  of the piston  1 . The cathode  82  has a cylindrical shape, and is disposed on an inner peripheral side of each of the nozzles  72  (the electrodeposition coating material side). An inner peripheral side of the cathode  82  is in contact with the electrodeposition coating material. An outer peripheral side of the cathode  82  is covered by an insulating member  720  (refer to  FIG. 5 ). The anode  81  is connected to the power source  85  via the electrodeposition wiring  83 . The cathode  82  is connected to the power source  85  via the electrodeposition wiring  84 . The control unit  9  includes a console  90 , actuators, and control wirings  92  to  94 . The actuators include an anode driving actuator  91 , a masking plate driving actuator  92 , and a pump driving actuator. The anode driving actuator  91  can press the anode  81  against the crown surface  104  of the piston  1  and separate the anode  81  from the crown surface  104  of the piston  1 . The masking plate driving actuator  92  can press the masking plate  71  against the outer peripheral surface of the piston  1  and separate the masking plate  71  from the outer peripheral surface of the piston  1 . The pump driving actuator drives the pump  74 . These actuators are connected to the console  90  via the control wirings  92 ,  93 , and  94 , respectively. The power source  85  is connected to the console  90  via a control wiring  95 . Positions of the masking plate  71  and the anode  81 , an activation state of the pump  74 , and a state of power supply to the electrodes  81  and  82  are controlled by the console  90 .  FIG. 5  schematically illustrates how the electrodeposition coating process is performed by the apparatus  5 .  FIG. 5  illustrates a cross-section of the piston  1  and the like in the plane containing the central axis O of the piston  1  as viewed from the direction in which the central axis P of the piston pin hole  111  extends. Peripheries of the respective outer peripheral surfaces  120  of the skirt portions  121  and  122  are masked by the masking plates  711  and  712 , respectively. The electrodeposition coating material discharged from the pump  74  is injected toward the skirt portions  121  and  122  from the individual nozzles  721  and  722 , respectively, hits the skirt portion outer peripheral surfaces  120 , and then falls due to the force of gravity to return to inside the liquid tank  60 . The binder resin in the electrodeposition coating material is electrophoresed during the injection from one side where the inner peripheral surface of the cathode  82  disposed in the nozzle  72  is located toward the skirt portion outer peripheral surface  120  serving as the anode. 
       FIG. 6  is a cross-sectional view schematically illustrating a second example of the apparatus  5 . The electrodeposition coating material delivery unit  7  includes liquid passage members  75  ( 751  and  752 ). The actuators include liquid passage member driving actuators  95  ( 951  and  952 ). The pair of liquid passage members  75  and the pair of liquid passage member driving actuators  95  are disposed on both the radial sides of the piston  1 .  FIG. 7  is a perspective view schematically illustrating the liquid passage member  75  and the liquid passage member driving actuator  95 . The liquid passage member  75  is made from a conductive material, and includes an opening portion  76  shaped generally similarly to the skirt portion outer peripheral surface  120 . The opening portion  76  faces the skirt portion outer peripheral surface  120 . A seal member (an O-ring)  753  is disposed between a portion around the opening portion  76  of the liquid passage member  75  and the outer peripheral surface of the piston  1  so as to surround the skirt portion outer peripheral surface  120 . The liquid passage member  75  forms a liquid passage including the skirt portion outer peripheral surface  120  and the inner peripheral surface of the liquid passage member  75  as a part thereof. More specifically, the liquid passage member  75  includes two opening portions  77  and  78  in addition to the opening portion  76 . The opening portion  77 , which is one of these two opening portions, is connected to the pump  74  via a pipe conduit  731  or  732 . The opening portion  78 , which is the other of these two opening portions, is connected to the liquid tank  60  via a pipe conduit  733  or  734 . The liquid passage connecting the pump  74  and the liquid tank  60  to each other via the liquid passage member  75  uses the inner peripheral surface of the liquid passage member  75  and the skirt portion outer peripheral surface  120  as a part thereof. A drain pipe conduit  735  is connected to the pipe conduit  731  connecting the pump  74  and the liquid passage member  75  to each other. The pipe conduit  735  is opened to the liquid tank  60 . A drain valve is provided in the pipe conduit  735 . The cathode  82  is set on the liquid passage member  75 . The liquid passage member driving actuator  95  can press the liquid passage member  75  against the outer peripheral surface of the piston  1  and separate the liquid passage member  75  from the outer peripheral surface of the piston  1 . A position of the liquid passage member  75  is controlled by the console  90 . Other configurations of the apparatus  5  are similar to the first example. The pump  74  introduces the electrodeposition coating material therein from the liquid tank  60  via the pipe conduit  730 , and discharges the electrodeposition coating material toward the liquid passage member  75  side. The electrodeposition coating material discharged from the pump  74  returns to the liquid tank  60  by passing through the liquid passage defined by the liquid passage member  75 . When passing through the liquid passage defined by the liquid passage member  75 , the electrodeposition coating material is in contact with the skirt portion outer peripheral surface  120 . The binder resin in the electrodeposition coating material is electrophoresed from one side where the inner peripheral surface of the liquid passage member  75  serving as the cathode is located toward the skirt portion outer peripheral surface  120  serving as the anode. 
     A third example of the apparatus  5  includes a liquid tank, electrodes, electrodeposition wirings, and a power source. An anode is set on the crown surface  104  of the piston  1 , and a cathode is set in the electrodeposition coating material in the liquid tank. These electrodes are connected to the power source via the electrodeposition wirings, respectively. The piston pin hole  111  of the piston  1  is plugged for the masking. Power is supplied with a portion of this piston  1  other than the head portion  10  (the skirt portions  12  and the apron portions  11 ) dipped in the electrodeposition coating material in the liquid tank. The first example is most preferable and the second example is second most preferable from the view point of efficiently forming the electrodeposited film  130  on the skirt portion outer peripheral surface  120 . The third example is preferable from the view point of simplification of the apparatus  5 . 
     In the water washing process, remaining liquid is removed by washing the skirt portion  12  (the electrodeposited film  130 ) after the electrodeposition coating process, with water. This process improves a finished quality and overcoatability. The water washing process may be omitted. After that, when the skirt portion  12  is dried by heating (baking drying), a solvent is volatilized and the resin is also polymerized and cured on the electrodeposited film  130 . The electrodeposited film  130  is cured, and also adheres to the skirt portion outer peripheral surface  120 . In the burning process, the skirt portion  12  (the electrodeposited film  130 ) after the water washing process is burned under predetermining burning conditions. Burning the skirt portion  12  improves the hardness and adhesiveness of the electrodeposited film  130 , compared to simple baking drying (for example, 90 degrees Celsius to 120 degrees Celsius without a holding time). The burning conditions are, for example, 180 degrees Celsius and 30 minutes. The skirt portion  12  can be burned even under a low temperature as low as 200 degrees Celsius or lower, and therefore the burning process can be easily applied even in the case where the base material  100  of the piston  1  is aluminum alloy: In the cooling process, the skirt portion  12  (the electrodeposited film  130 ) after the burning process is cooled. The skirt portion  12  may be cooled naturally (by just being left unattended) without being forcibly cooled. 
     Next, functions and effects will be described. A rotational motion of the crankshaft is converted into a reciprocating motion of the piston  1 . When the piston  1  reciprocates inside the cylinder  2 , the outer peripheral surface  120  in the surface of the skirt portion  12  is slidably moved relative to the cylinder inner wall  20 . This movement prevents or reduces an oscillation operation of the piston  1  around the central axis P of the piston pin  3  inside the cylinder  2 , thereby smoothing the reciprocating motion of the piston  1  and also preventing or reducing hitting noise. In the present specification, the slidable movement includes both a movement of the skirt portion outer peripheral surface  120  (even partially) relative to the cylinder inner wall  20  while contacting this inner wall  20  as a contact between solid objects without intervention of an oil membrane of engine oil, and a movement of the skirt portion outer peripheral surface  120  relative to the cylinder inner wall  20  in a state facing the cylinder inner wall  20  via the oil membrane (i.e., without causing the contact between the solid objects). The con rod  4  is inclined with respect to the central axis of the cylinder  2  according to a crank angle. In an expansion stroke (a combustion stroke) or a compression stroke, a pressure is applied from one side where the crown surface  104  of the piston  1  is located. Balance between forces causes the thrust-side skirt portion  121  to be pressed against the cylinder inner wall  20  (a thrust side) when the piston  1  is stroked toward a bottom dead center side in the expansion stroke. When the piston  1  is stroked toward a top dead center side in the compression stroke, the opposite thrust-side skirt portion  122  is pressed against the cylinder inner wall  20  (an opposite thrust side). A force by which the skirt portion outer peripheral surface  120  is pressed against the inner wall (a surface pressure on a surface slidably moved relative to the cylinder inner wall  20 ) is stronger on the thrust-side skirt portion  121 , which is in pressure contact with the cylinder inner wall  20  by receiving a combustion pressure, than on the opposite thrust-side skirt portion  122 . 
     Generally, it is difficult to keep even a distribution of the force (the surface pressure or a load) by which the skirt portion outer peripheral surface  120  is pressed against the cylinder inner wall  20 . Normally, a high surface pressure region where the surface pressure is relatively high and a low surface pressure region where the surface pressure is relatively low are generated on the skirt portion outer peripheral surface  120 . A partial range in the skirt portion outer peripheral surface  120  (this will be hereinafter referred to as a first range) has a larger bump and dent than a thickness of the oil membrane between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20 , thereby forming the high surface pressure region. On the other hand, a range other than the first range in the skirt portion outer peripheral surface  120  (this will be hereinafter referred to as a second range) has a smaller bump and dent than the thickness of the oil membrane between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20 , thereby forming the low surface pressure region. 
     While the formation of the oil membrane is easily impeded between the first range and the cylinder inner wall  20 , the oil membrane is easily formed between the second range and the cylinder inner wall  20 , in the skirt portion outer peripheral surface  120 . There is such a situation where, after an operation of the engine is started, the first range (even partially) is slidably moved while contacting the cylinder inner wall  20  as the contact between the solid objects without the intervention of the oil membrane. For example, around the top dead center of the piston  1 , the oil membrane is especially difficult to be formed between the cylinder inner wall  20  and the first range due to, for example, a reduction in a speed of the piston  1  and an increase in the load. When the thickness of the oil membrane reaches or falls below surface roughness of them (a lubrication gap reaches or falls below a lower limit value), boundary lubrication is established as the lubrication therebetween, so that a solid contact further frequently occurs therebetween. More specifically, depending on the scene, because a parameter based on conditions such as the load and the speed in a Stribeck curve is located in a boundary lubrication region between the cylinder inner wall  20  and the first range, a frictional coefficient may increase and a scuff may be generated therebetween. On the other hand, the second range is slidably moved relative to the cylinder inner wall  20  with the oil membrane formed between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  in many scenes, while the engine is in operation. In other words, the thickness of the oil membrane exceeds the surface roughness of them (the lubrication gap exceeds the lower limit value), and fluid lubrication is established as the lubrication therebetween, so that no solid contact occurs therebetween. In other words, in many scenes, the above-described parameter in the Stribeck curve is located in a fluid lubrication region between the cylinder inner wall  20  and the second range, so that the frictional coefficient is reduced and the scuff less likely occurs therebetween. 
     In the present embodiment, an exposure of the base material  100  of the piston  1  on the skirt portion outer peripheral surface  120  including the first range is prevented or reduced by the electrodeposited film  130 . A contact between the base material  100  and the cylinder inner wall  20  as the contact between the soil objects is prevented or reduced in the first range in the skirt portion outer peripheral surface  120 , which improves a scuff resistance property of the piston  1 . PAI and PI have the excellent abrasion resistance property and heat resistance property, and therefore detachment of the electrodeposited film  130  from the base material  100  is prevented or reduced. Therefore, in the case where the electrodeposited film  130  contains PAI or PI as the binder resin, the above-described effects are improved. The electrodeposited film  130  does not contain the solid lubricant while containing the binder resin (the electrodeposited film  130  is a layer containing the resin alone). Therefore, the electrodeposited film  130  exerts a strong adhesion force, and excellent adhesiveness is achieved between the electrodeposited film  130  and the base material  100 . Further, the electrodeposited film  130  is cured by being burned. As a result, the detachment of the electrodeposited film  130  from the base material  100  is prevented or reduced. 
     The electrodeposited film  130  (the electrodeposition coating material) may contain a solid lubricant that is an electric insulator, such as a fluoropolymer (polytetrafluoroethylene, hereinafter referred to as PTFE). In this case, even when the electrodeposited film  130  is in contact with the cylinder inner wall  20  in the first range, the above-described solid lubricant contributes to a reduction in the strength of the frictional force between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20 . In the present embodiment, the electrodeposited film  130  is located on an uppermost layer, and is exposed on the skirt portion outer peripheral surface  120 . Therefore, the above-described function of reducing the frictional force (lubricity) can be acquired at a further early stage after the operation of the engine is started. In this case, the abrasion property and the adhesiveness of the electrodeposited film  130  can be adjusted by adjusting contained amounts of the solid lubricant and the binder resin in the electrodeposited film  130 . For example, this adjustment will be described, supposing that the contained amount of the solid lubricant is 50% by weight (hereinafter referred to as wt %) or more and 95 wt % or less and the contained amount of the binder resin is 5 wt % or more and 50 wt % or less, in the electrodeposited film  130 . The electrodeposited film  130  is easily abraded since the contained amount of the solid lubricant is 50 wt % or more (the contained amount of the binder resin is 50 wt % or less). Therefore, initial conformability when the skirt portion outer peripheral surface  120  is moved slidably relative to the cylinder inner wall  20  is improved. Further, the adhesion force of the electrodeposited film  130  is secured to some degree since the contained amount of the binder resin is 5 wt % or more (the contained amount of the solid lubricant is 95 wt % or less). Therefore, a reduction in the adhesiveness between the electrodeposited film  130  and the base material  100  is prevented or cut down. On the other hand, in a case where the contained amount of the solid lubricant is more than 0 wt % and 50 wt % or less and the contained amount of the binder resin is 50 wt % or more and less than 100 wt % in the electrodeposited film  130 , the electrodeposited film  130  exerts a strong adhesion force since the contained amount of the binder resin is 50 wt % or more (the contained amount of the solid lubricant is 50 wt % or less). Therefore, excellent adhesiveness is achieved between the electrodeposited film  130  and the base material  100 . 
     Generally, the streak is formed and caused to exert a lubrication function on the skirt portion outer peripheral surface to, for example, prevent or reduce the generation of the scuff. More specifically, the engine oil (hereinafter referred to as the oil) is stored in the streak and is held in the streak even while the engine is out of operation. The held oil is supplied to between the skirt portion outer peripheral surface and the cylinder inner wall as appropriate. For example, a loss of the oil is prevented or reduced when the engine is started from a stopped state or when the piston reciprocates rapidly. This effect prevents or reduces the generation of the scuff between the skirt portion outer peripheral surface and the cylinder inner wall and smooths the reciprocating motion of the piston due to a reduction in the frictional force therebetween. On the other hand, the present inventor has found out the following fact. That is, the frictional coefficient under the fluid lubrication between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  (hereinafter referred to as a fluid lubrication frictional coefficient) has a close relationship with the height of the streak  14  on the skirt portion outer peripheral surface  120 , and the fluid lubrication frictional coefficient reduces as the height of the streak  14  reduces (as the skirt portion outer peripheral surface  120  is further smoothed). As the height of the streak  14  reduces, a shear resistance of the oil membrane between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  reduces, and it is considered that this contributes to the reduction in the fluid lubrication frictional coefficient. The above-described height of the streak  14  on the skirt portion outer peripheral surface  120  refers to the height of the streak  140  processed on the base material  100  in a case where the film portion  13  does not cover the base material  100  of the piston  1 , and refers to a height of a groove (the steak  14 ) on this film portion  13  that is formed (consequently) at a portion corresponding to the streak  140  processed on the base material  100  in a case where the film potion  13  covers the base material  100 . 
     An experiment was conducted to study the relationship between the height of the streak  14  on the skirt portion outer peripheral surface  120  and the fluid lubrication frictional coefficient. In this experiment, a chip (block)-on-disk type friction/abrasion testing machine was used. A surface of a testing piece (made from ΔC8A) imitating the skirt portion  12  where a streak was formed was placed in abutment with a surface (surface roughness: average 0.5 μmRa (min. 0.42 to max. 0.66)) of a disk (made from FC250) imitating the cylinder inner wall  20 . The disk was rotated at a predetermined speed with a predetermined load (surface pressure) and lubricant oil provided thereto. A direction in which the surface of the disk was slidably moved relative to the surface of the testing piece was a direction generally perpendicular to a direction in which the streak extended. A supply amount of the lubricant oil and other conditions were adjusted so as to establish the fluid lubrication. The frictional coefficient (the fluid lubrication frictional coefficient) at the time of the slidable movement was measured while the height of the streak was changed a plurality of times.  FIG. 8  illustrates a result of measuring the frictional coefficient with respect to each height of the streak when the testing conditions were set to the surface pressure: 2.8 Mpa, the speed: 2.0 m/sec, the supply amount of the lubricant oil (a grade thereof was 5W-30): 40 ml/min. A plurality of pieces of data (a plurality of points in a graph) indicating the measurement result is contained in a region of a predetermined range centered at one straight line L. The above-descried straight line L indicates that the above-described frictional coefficient is changed proportionally to the height of the streak. This experiment result reveals that the fluid lubrication frictional coefficient is lower when the streak  14  on the skirt portion outer peripheral surface  120  is short than when the streak  14  is tall. 
     Due to the electrodeposited film  130 , the smoothness of the skirt portion outer peripheral surface  120  is improved. More specifically, a bottom of the streak  140  is buried by the electrodeposited film  130  in the process for forming the electrodeposited film, by which the height of the streak  14  reduces (the streak shape is smoothed) as schematically illustrated in  FIG. 3 . After the skirt portion outer peripheral surface  120  is covered by the electrodeposited film  130 , the smoothness of the skirt portion outer peripheral surface  120  is improved compared to before the skirt portion outer peripheral surface  120  is covered by the electrodeposited film  130 . The improvement of the smoothness leads to a reduction in the frictional coefficient (the fluid lubrication frictional coefficient) when the skirt portion outer peripheral surface  120  contacts the oil and the fluid lubrication is established. Therefore, engine efficiently (fuel efficiency) is improved. While the engine is in operation, the friction (and an energy loss due to this friction) between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  is mainly constituted by a friction under the boundary lubrication and a friction under the fluid lubrication. Then, the latter friction occupies a considerably larger percentage of the whole than a percentage of the whole that is occupied by the former friction regardless of the operation condition and the stroke of the piston  1 . The latter friction (the friction under the fluid lubrication) reduces due to the electrodeposited film  130 , by which the friction (and the energy loss) as a whole generated on the skirt portion outer peripheral surface  120  can efficiently reduce. The film thickness of the electrodeposited film  130  can be appropriately selected within a range that can acquire the above-described smoothing effect. 
     Then, the reduction in the height of the streak  14  (the improvement of the smoothness) due to the electrodeposited film  130  has the following meaning. That is, a new streak  141  is formed (consequently) at the portion corresponding to the streak  140  processed on the base material  100  (the portion covering this streak  140 ) on the surface of the film portion  13  covering the base material  100  of the piston  1 . The height of the streak  141  is shorter on the electrodeposited film  130  than on a common film portion  13  (not formed by the electrodeposition). This will be specifically described with reference to the drawing.  FIG. 9  schematically illustrates the cross-section of the outer peripheral side of the skirt portion  12  similarly to  FIG. 3 . Assume that a0 represents the height of the streak  140 . Assume that a1 represents a height of the steak  141  formed on the film portion  13  directly covering the base material  100 . Assume that b1 represents a distance from the lowermost portion of the streak  140  to a lowermost portion of the streak  141 . Assume that c1 represents a distance from the uppermost portion (the top) of the streak  140  to an uppermost portion (a top) of the streak  141 . The following equation (1) is established. 
         a 1= a 0+ c 1− b 1  (Equation 1)
 
     Generally, the coating material contains a volatile component such as the solvent, and a solid component such as the resin. The solid component remains as the film even after the coating material is dried (the volatile component is volatilized). The volatile component is volatilized from the coating material, and does not remain as the film. Hypothetically suppose that, after the coating process, the surface of the film portion  13  before the volatile component in the coating material is volatilized is a completely smooth surface without any bump and dent formed thereon regardless of whether the surface is the portion corresponding to the streak  140  as indicated by an alternate long and short dash line in  FIG. 9 . Assume that x represents a distance from the uppermost portion (the top) of the streak  140  to the surface of the film portion  13  before the above-described volatilization. A distance from the lowermost portion of the streak  140  to the surface of the film portion  13  before the above-described volatilization is (a0+x). Assuming that a represents a ratio of a volume of the solid component to a volume of the entire coating material (a sum of a volume of the volatile component and the volume of the solid component), the following equations (2) and (3) are established. 
         b 1=( a 0+ x )α  (Equation 2)
 
         c 1=α x   (Equation 3)
 
     The ratio α ranges from 0.3 to 0.5 for a normal coating material, but ranges from 0.1 to 0.6 if also containing special types. The following equation (4) is established from the above-described equations (1) to (3). 
         a 1= a 0+α x −α( a 0+ x )=(1−α) a 0  (Equation 4)
 
     In other words, even if the coated film is flat at the portion corresponding to the streak  140  in the state before the volatile component in the coating material is volatized, the streak  141  is formed on the surface of the film portion  13  due to the volatilization of the volatile component from this coated film. (1−α)a0 is the height of this streak  141 . The streak  141  of this film portion  13  becomes shorter than the original streak  140  by an amount corresponding to the volume ratio α of the solid component in the coating material of the film portion  13 . The reduction in the height of the streak  14  (the improvement of the smoothness) due to the electrodeposited film  130  means that a1 becomes shorter than (1−α)a0, and therefore is expressed by the following equation (5). 
         a 1&lt;(1−α) a 0  (Equation 5)
 
     In other words, the streak  141  of the electrodeposited film  130  becomes shorter than the original streak  140  by a larger amount than the amount corresponding to the volume ratio α of the solid component in the electrodeposition coating material. 
     As indicated by the above-described equation (4), due to the volatilization of the volatile component of the coating material from the coating material of the film portion  13 , the streak  14  of this film portion  13  is formed so as to have a height as tall as the height obtained by the original streak  14  being multiplied by the volume ratio (1−α) of the above-described volatile component. In other words, by the amount corresponding to the volume ratio α of the solid component in the coating material of the film portion  13 , the streak  14  of this film portion  13  becomes shorter than the original streak  14 . Therefore, stacking several common films that are not the electrodeposited film  130  allows the skirt portion outer peripheral surface  120  to be smoothed to a similar level to the smoothness achieved by the virtue of the electrodeposited film  130 . In the present embodiment, the use of the electrodeposited film  130  makes the streak  14  shorter by the larger amount than the amount corresponding to the volume ratio α, as indicated by the above-described equation (5). The skirt portion outer peripheral surface  120  is efficiently smoothed with use of the electrodeposited film  130  alone without requiring several films to be stacked. Therefore, the process for manufacturing the piston  1  can be simplified. The skirt portion  12  may include not only one electrodeposited film  130  but also a plurality of electrodeposited films  130 . The skirt portion  12  according to the present embodiment includes only one electrodeposited film  130 . The present embodiment does not require several electrodeposited films  130  to be stacked, and therefore can simplify the process for manufacturing the piston  1 . 
     One conceivable mechanism of smoothing the skirt portion outer peripheral surface  120  (reducing the height of the streak) due to the electrodeposited film  130  is, for example, the following mechanism. In the electrodeposition coating process, the coating material precipitated on the skirt portion outer peripheral surface  120  during the power supply loses the conductivity. In other words, the electric resistance increases and the flow of the current is stopped. Therefore, the growth of the film is stopped, so that a thin film even in thickness should be formed on the skirt portion outer peripheral surface  120 . However, a heat release state of the Joule heat is different between the bottom (a groove) and the top (a ridge) of the streak  14 . It is considered that this contributes to the smoothness. More specifically, a heat release performance is excellent on the top of the streak  14  but the heat is confined on the bottom of the streak  14 . Therefore, the film growth due to the fusion of the coating material particles is more sped up on the bottom. On the other hand, even after the coating material is precipitated on the skirt portion outer peripheral surface  120 , the film becomes porous due to gas generated from the electrolysis, so that the electric resistance of the film does not increase so much actually and the flow of the current continues. Therefore, the film continues growing and the film grows more quickly on the bottom of the streak  14 . As a result, the formation of the even film thickness is impeded, and the film is formed so as to fill the bottom of the streak  14 . The film in the porous state is formed into a continuous film by being melted and flowing in the next burning process (the baking drying). 
     The piston  1  does not necessarily have to include the streak  140 . In other words, the electrodeposited film  130  may be provided on the piston  1  unequipped with the streak  140  on the base material  100  on the skirt portion outer peripheral surface  120 . In the present embodiment, the base material  100  includes the streak  140  on the skirt portion outer peripheral surface  120 . This configuration leads to an increase in a surface area of the base material  100  on the skirt portion outer peripheral surface  120 , and thus an increase in a contact area between the base material  100  and the electrodeposited film  130 , thereby improving the adhesion force therebetween. Therefore, the detachment of the electrodeposited film  130  from the base material  100  is prevented or reduced, so that the solid contact between the base material  100  and the cylinder  2  in the first range is further reliably prevented or reduced. Even if the electrodeposited film  130  is detached from the base material  100 , this detachment leads to an exposure of the streak  140  formed on the base material  100  on the skirt portion outer peripheral surface  120 , allowing the streak  140  to exert the lubrication function. 
     It is important to employ a less conductive material as the solid component (the coating material particles) in the electrodeposition coating material to improve the smoothness of the skirt portion outer peripheral surface  120  due to the electrodeposited film  130 . More specifically, if a conductive solid component is added to the electrodeposition coating material, a conductive film is formed on the skirt portion outer peripheral surface  120  by the power supply. This film is less electrically resistive, thereby resulting in a reduction in the generation of the Joule heat. Therefore, the difference in the growth speed of the film due to the fusion of the coating material particles reduces between the bottom and the top of the streak  14 . As a result, the formation of the even film thickness is facilitated, making it difficult for the film to be formed so as to fill the bottom of the streak  14  (the achievement of the smoothness is impeded). The above-described conductive solid component contains a conductive solid lubricant, such as graphite (hereinafter referred to as C) and molybdenum disulfide (hereinafter referred to as MoS 2 ). On the other hand, the electrodeposition coating material according to the present embodiment does not contain the conductive solid lubricant. Therefore, the electrodeposition coating material becomes further less conductive, which improves the above-described smoothing effect due to the electrodeposited film  130 . It is allowable that a small amount of the conductive solid component (solid lubricant) is added or mixed in the electrodeposition coating material as long as the non-conductivity of the electrodeposition coating material is maintained to some degree and the above-described smoothing effect due to the electrodeposited film  130  can be acquired. Further, it is also allowable that the solid lubricant as the electric insulator (for example, PTFE) is contained in the electrodeposition coating material. In other words, even when the non-conductive solid lubricant is added to the electrodeposition coating material, the electrodeposition coating material when power is supplied exhibits a similar behavior to when the solid lubricant is not added at all, and the electrodeposited film  130  is formed so as to fill the bottom of the streak  14 . The solid lubricant contained in the electrodeposited film  130  may be any material that is less conductive than C and MoS 2 , and is not limited to PTFE. 
     Another film is not interposed between the base material  100  and the electrodeposited film  130 . In the electrodeposition coating process, the base material  100  is exposed to the electrodeposition coating material on the skirt portion outer peripheral surface  120 . Therefore, the skirt portion outer peripheral surface  120  can easily function as the electrode, and the electrodeposited film  130  can be easily formed. 
     Second Embodiment 
     First, a configuration will be described.  FIG. 10  illustrates a cross-section of the outer peripheral side of the skirt portion  12  in the plane containing the central axis O of the piston  1  according to the present embodiment. The film portion  13  includes the electrodeposited film  130  and also includes one film layer in addition to the electrodeposited film  130 . More specifically, the skirt portion  12  of the piston  1  includes two film layers. The above-described film other than the electrodeposited film  130  is a lubrication film  131 . The film portion  13  includes the electrodeposited film  130  and the lubrication film  131  in this order from one side where the base material  100  of the piston  1  is located. The electrodeposited film  130  covers the base material  100 . The composition of the electrodeposited film  130  is similar to the first embodiment. The lubrication film  131  covers the electrodeposited film  130 , and is exposed on the skirt portion outer peripheral surface  120 . The lubrication film  131  contains a solid lubricant and a binder resin. The solid lubricant is C. The lubrication film  131  may contain, as the solid lubricant, another solid lubricant, such as at least one of MoS 2  and PTFE, together with or instead of C. The binder resin has a function of fixing the solid lubricant to a coating target object, and, for example, PAI is used as the binder resin. The lubrication film  131  may contain, as the binder resin, another binder resin, such as at least one of PI and EP, together with or instead of PAI. In the lubrication film  131 , a contained amount of the solid lubricant is 50 wt % or more and 95 wt % or less, and a contained amount of the binder resin is 5 wt % or more and 50 wt % or less. 
     The method for processing the surface of the piston (the present processing) includes the process for forming the electrodeposited film and a process for forming the lubrication film. In the present processing, the process for forming the electrodeposited film and the process for forming the lubrication film are performed in this order. The procedure of the process for forming the electrodeposited film is similar to the first embodiment. The process for forming the lubrication film includes execution of a so-called drying and baking method that forms a dried film by applying a coating material in which the solid lubricant is distributed in a solution of the binder resin to a surface of a target object, and drying and baking that. In the process for forming the lubrication film, a coating process, a drying process, and a cooling process are performed in this order. Processing for, for example, removing oil and contamination from the skirt portion outer peripheral surface  120  (the electrodeposited film  130 ), which is the coating target, may be performed to, for example, improve adhesiveness of the lubrication film  131  before the coating process. In the coating process, the coating material is applied to the skirt portion outer peripheral surface  120  (the electrodeposited film  130 ) by screen printing. The coating material may be applied by printing other than screen printing, spraying the coating material with use of a spray or the like, or dipping the skirt portion outer peripheral surface  120  in the coating material. The coating material can be prepared by, for example, blending the binder resin and the solid lubricant in an organic solvent, adding an additive and hard particles to this solvent as necessary, and mixing and distributing it with use of a bead mill or the like. The coating material may be diluted with use of the organic solvent as necessary. The coated film covering the electrodeposited film  130  is formed on the skirt portion outer peripheral surface  120  by the coating process. In the drying process, the coated film is baked and dried under a drying condition such as heating it at 90 degrees Celsius to 120 degrees Celsius (a holding time is unnecessary). The drying process is ended when, for example, the coating material is dried enough not to stain a hand. A volatile component (the organic solvent) is removed from the above-described coated film by the drying, and along therewith, the solid lubricant is fixed on the skirt portion outer peripheral surface  120  via the binder resin by the baking, by which the lubrication film  131  is formed. In the cooling process, the skirt portion  12  (the lubrication film  131 ) after the burning process is cooled. The skirt portion  12  may be cooled naturally without being cooled forcibly. 
     The drying process in the process for forming the lubrication film may be changed to the burning process, and the burning process in the processing for forming the electrodeposited film may be changed to the drying process (similar to the drying process in the process for forming the lubrication film). There are four possible combinations as the process for forming the entire (multilayered) film depending on which is carried out for each of the films, the drying or the burning. However, it is preferable to carry out the burning at least once or more (burn at least one film) in the process for forming the entire film to improve strength of the film as a whole. Therefore, there are three possible combinations as the process for forming the entire film depending on which is carried out for each of the films, the drying or the burning, excluding a combination in which the burning is not carried out even once (the drying is carried out for all of the films). The present embodiment employs the combination in which the process for forming the electrodeposited film includes the burning process and the process for forming the lubrication film includes the drying process, as a representative process for forming the entire film, by way of example. 
     Next, functions and effects will be described. Due to the lubrication film  131 , the friction reduces between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  at a relatively early (initial) stage after the operation of the engine is started. More specifically, even when the oil membrane is not formed between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  in the first range, since the lubrication film  131  is exposed on the skirt portion outer peripheral surface  120 , the strength of the frictional force reduces therebetween due to the solid lubricant in the lubrication film  131 . In the lubrication film  131 , the contained amount of the solid lubricant may be 50 wt % or less (the contained amount of the binder resin is 50 wt % or more). In this case, the lubrication film  131  exerts a strong adhesion force, and excellent adhesiveness is achieved between the lubrication film  131  and the electrodeposited film  130 . In the present embodiment, in the lubrication film  131 , the contained amount of the solid lubricant is 50 wt % or more (the contained amount of the binder resin is 50 wt % or less). The lubrication film  131  is easily abraded by containing a large amount of the solid lubricant (a small amount of the binder resin). Therefore, the initial conformability when the outer peripheral surface of the piston  1  is slidably moved relative to the cylinder inner wall  20  is improved. More specifically, the first range (the surface of the lubrication film  131 ) is abraded and then smoothed early, and starts to conform to the cylinder inner wall  20  quickly. The smooth sliding surface is swiftly formed in the first range, and the lubrication gap exceeds the lower limit value. As a result, the lubrication in the first range is changed from the boundary lubrication to the fluid lubrication. Further, the improvement of the smoothness causes a reduction in the fluid lubrication frictional coefficient in the first range (and the second range as will be described below). These effects contribute to the reduction in the strength of the frictional force between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20 , and thus the reduction in the frictional loss on the skirt portion  12 . 
     In the lubrication film  131 , the contained amount of the binder resin is 5 wt % or more (the contained amount of the solid lubricant is 95 wt % or less). Therefore, the adhesion force of the lubrication film  131  is secured to some degree, and a reduction in the adhesiveness between the lubrication film  131  and the electrodeposited film  130  is prevented or cut down. The lubrication film  131  is not burned, and is more easily abraded than when being burned. Therefore, the initial conformability due to the lubrication film  131  can be easily acquired. A material having a low abrasion resistance property may be used as the binder resin contained in the lubrication film  131  to improve the initial conformability. 
     Due to the electrodeposited film  130 , the streak shape on the base material  100  is smoothed (the streak  14  becomes shorter due to the electrodeposited film  130 ) at least in the second range of the skirt portion outer peripheral surface  120 . The lubrication film  131  is formed so as to cover the smoothed skirt portion outer peripheral surface  120  (the electrodeposited film  130 ), by which the surface of the lubrication film  131  is also smoothed (consequently). In other words, the skirt portion outer peripheral surface  120  (at least the second range) covered by the lubrication film  131  is smoothed. As a result, the fluid lubrication frictional coefficient reduces in the second range. Conventionally, there has been known a piston including a film containing the solid lubricant and also having a high abrasion property on the skirt portion outer peripheral surface. In this piston, the film is abraded and starts conform to the inner wall of the cylinder early in the first range of the skirt portion outer peripheral surface, by which the lubrication therebetween is changed from the boundary lubrication to the fluid lubrication, and the fluid lubrication frictional coefficient reduces along therewith. However, in the second range other than the first range, the film is not moved while directly contacting the inner wall of the cylinder, so that the lubrication therebetween remains the fluid lubrication, and the height of the streak formed on the film in the second range is not changed along therewith. Therefore, the second range, which is the surface in contact with the oil, has low smoothness and exhibits a high friction. On the other hand, in the piston  1  according to the present embodiment, the streak  14  becomes shorter due to the electrodeposited film  130  in the second range, which is the surface in contact with the oil, so that the smoothness is improved. Therefore, not only in the first range but also in the second range, the friction reduces under the fluid lubrication. In other words, the frictional coefficient (the frictional force) reduces under the fluid lubrication throughout the entire skirt portion outer peripheral surface  120  (including the first range and the second range). 
     The meaning of the reduction in the height of the streak  14  due to the electrodeposited film  130  (the improvement of the smoothness) on the skirt portion outer peripheral surface  120  covered by the two films  130  and  131  can be understood in a similar manner to the first embodiment. This will be described now with reference to  FIG. 11  illustrating a schematic cross-section similar to  FIG. 9 . In  FIG. 11 , a0, a1, b1, and c1 are similar to  FIG. 9 . A groove (a streak  142 ) is formed (consequently) on the surface of the lubrication film  131  covering the electrodeposited film  130  at the portion corresponding to the streak  141 . Assume that a2 represents a height of the streak  142 . Assume that b2 represents a distance from the lowermost portion of the streak  141  to a lowermost portion of the streak  142 . Assume that c2 represents a distance from the uppermost portion (the top) of the streak  141  to an uppermost portion (a top) of the streak  142 . The following equation (6) is established. 
         a 2= a 1+ c 2− b 2  (Equation 6)
 
     Hypnotically suppose that the surface of the lubrication film  131  after the coating process for forming the lubrication film  131  is performed and before the volatile component in the coating material is volatilized is a completely smooth surface without any dent and bump formed thereon as indicated by an alternate long and two short dashes line in  FIG. 11 . Assume that y represents a distance from the uppermost portion (the top) of the streak  141  to the surface of the lubrication film  131  before the above-described volatilization. The distance from the lowermost portion of the streak  141  to the surface of the lubrication film  131  before the above-described volatilization is (a1+y). Assume that β represents a ratio of a volume of the solid component to a volume of the entire coating material in the lubrication film  131 . The solid component is the solid lubricant, the resin, or the like. The following equations (7) and (8) are established. 
         b 2=( a 1+ y )β  (Equation 7)
 
         c 2=β  (Equation 8)
 
     The ratio β ranges from 0.3 to 0.5 for the normal coating material, but ranges from 0.1 to 0.6 if also containing special types. The following equation (9) is established from the above-described equations (6) to (8). 
         a 2= a 1+β y −β( a 1+ y )=(1−β) a 1  (Equation 9)
 
     (1−β)a1 is the height of the streak  142  formed on the surface of the lubrication film  131  due to the volatilization of the volatile component of the coating material from the coating material of the lubrication film  131 , based on a1. The above-described equation (9) is rewritten into the following equation (10) with use of the above-described equation (4). 
         a 2=(1−α)(1−β) a 0  (Equation 10)
 
     In other words, by an amount corresponding to the volume ratios α and β of the solid components in the respective coating materials of the films  130  and  131 , the streak  142  on the plurality of layers as these films  130  and  131  becomes shorter than the original streak  140 . The reduction in the height of the streak  142  due to the electrodeposited film  130  (the improvement of the smoothness) means that a2 becomes shorter than (1−α)(1−β)a0, and therefore is expressed by the following equation (11). 
         a 2&lt;(1−α)(1−β) a 0  (Equation 11)
 
     This equation (11) is also derived from the above-described equations (5) and (9). 
     As indicated by the above-described equation (9), due to the volatilization of the volatile component of the coating material from the coating material of the film portion  13 , the streak  14  is formed on the lubrication film  131  so as to have the height a2 that is obtained by the height a1 of the streak  141  on the electrodeposited film  130  being multiplied by the volume ratio (1−β) of the above-described volatile component. In other words, by the amount corresponding to the volume ratio β of the solid component in the coating material of the lubrication film  131 , the streak  142  of the lubrication film  131  becomes shorter than the original streak  141 . Comparing the above-described equations (5) and (11), the streak  142  becomes further shorter than the first embodiment (the streak  141 ) by an amount corresponding to β, due to the lubrication film  131 . Therefore, the skirt portion outer peripheral surface  120  is more efficiently smoothed than when the electrodeposited film  130  is used alone. 
     The electrodeposited film  130  does not contain the solid lubricant while containing the binder resin. Therefore, the electrodeposited film  130  exerts a strong adhesion force, and the excellent adhesiveness is achieved between the electrodeposited film  130  and the base material  100 . Further, excellent adhesiveness is achieved between the electrodeposited film  130  and the lubrication film  131 . Further, the electrodeposited film  130  is cured by being burned. As a result, the detachment of the electrodeposited film  130  from the base material  100  is prevented or reduced. The electrodeposited film  130  may contain the solid lubricant that is the electric insulator similarly to the first embodiment. In this case, the present embodiment can acquire the smoothing effect due to the electrodeposited film  130  similarly to the first embodiment, and also acquire lubricity and the like due to the electrodeposited film  130  containing the solid lubricant when the electrodeposited film  130  is exposed on the skirt portion outer peripheral surface  120  due to, for example, the abrasion of the lubrication film  131 . 
     Third Embodiment 
     First, a configuration will be described.  FIG. 12  illustrates a cross-section of the outer peripheral side of the skirt portion  12  in the plane containing the central axis O of the piston  1  according to the present embodiment. The skirt portion  12  includes two film layers similarly to the second embodiment. The films include the lubrication film  131  and the electrodeposited film  130  in this order from the base material  100  side. The lubrication film  131  covers the base material  100 . The lubrication film  131  contains C as the solid lubricant. The solid lubricant may contain MoS 2  together with or instead of C. In the lubrication film  131 , a contained amount of the solid lubricant is more than 0 wt % and 50 wt % or less, and a contained amount of the binder resin is 50 wt % or more and less than 100 wt %. The electrodeposited film  130  covers the lubrication film  131 , and is exposed on the skirt portion outer peripheral surface  120 . The composition of the electrodeposited film  130  is similar to the first embodiment. 
     In the method for processing the surface of the piston  1  (the present processing), the process for forming the lubrication film and the process for forming the electrodeposited film are performed in this order. In the process for forming the lubrication film, the coating process, the burning process, and the cooling process are performed in this order. In the burning process, the skirt portion  12  (the lubrication film  131 ) after the coating process is burned under burning conditions such as burning it at 180 degrees Celsius for 30 minutes and burning it at 200 degrees Celsius for 20 minutes. This process improves the hardness and the adhesiveness of the lubrication film  131  compared to the simple baking drying. The other processes in the process for forming the lubrication film are similar to the second embodiment. In the process for forming the electrodeposited film, the electrodeposition coating process, the water washing process, the drying process, and the cooling process are performed in this order. In the drying process, the coated film is baked and dried under a drying condition such as heating it at 90 degrees Celsius to 120 degrees Celsius (the holding time is unnecessary). The drying process is ended when, for example, the coating material is dried enough not to stain a hand. The other processes in the process for forming the electrodeposited film are similar to the first embodiment. The water washing process may be omitted. 
     Next, functions and effects will be described. Due to the electrodeposited film  130  and the lubrication film  131 , the exposure of the base material  100  on the skirt portion outer peripheral surface  120  is prevented or reduced. Therefore, the scuff resistance property of the piston  1  is improved similarly to the first embodiment. 
     Due to the electrodeposited film  130 , the smoothness of the skirt portion outer peripheral surface  120  is improved. As a result, the fluid lubrication frictional coefficient reduces at least in the second range. The electrodeposited film  130  does not contain the solid lubricant. Therefore, the electrodeposited film  130  exerts the strong adhesion force, and the excellent adhesiveness is achieved between the electrodeposited film  130  and the lubrication film  131 . The electrodeposited film  130  may contain the solid lubricant that is the electric insulator similarly to the first embodiment. In this case, the present embodiment can acquire the smoothing effect due to the electrodeposited film  130  and also acquire the lubricity and the like due to the electrodeposited film  130 . In the present embodiment, the electrodeposited film  130  does not contain the solid lubricant, and therefore has a higher abrasion resistance property than when containing the solid lubricant. On the other hand, the electrodeposited film  130  is not burned, and is more easily abraded than when being burned. Therefore, in the first range, the electrodeposited film  130  is abraded and then smoothed early, and easily starts to conform to the cylinder inner wall  20  quickly. Further, this makes it easy for the lubrication film  131  covered by the electrodeposited film  130  to be exposed on the skirt portion outer peripheral surface  120 . As a result, the merits of the lubrication film  131  (the lubricity and the initial conformability) can be easily acquired. A material having a low abrasion resistance property may be used as the binder resin contained in the electrodeposited film  130  to allow the electrodeposited film  130  to be easily abraded. Further, in the case where the electrodeposited film  130  contains the solid lubricant that is the electric insulator, the contained amount of the solid lubricant may increase (in other words, the contained amount of the binder resin may reduce). 
     The lubrication film  131  contains the solid lubricant (the contained amount of the solid lubricant is more than 0 wt %). Therefore, when the lubrication film  131  is exposed on the skirt portion outer peripheral surface  120  due to, for example, the abrasion of the electrodeposited film  130  in the first range, the strength of the frictional force reduces between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  due to the solid lubricant similarly to the second embodiment. In the lubrication film  131 , the contained amount of the solid lubricant may be 50 wt % or more. In this case, the lubrication film  131  is easily abraded. Therefore, when the lubrication film  131  is exposed on the skirt portion outer peripheral surface  120  due to, for example, the abrasion of the electrodeposited film  130  in the first range, the surface of the lubrication film  131  is abraded and then smoothed early, by which the initial conformability is improved, similarly to the second embodiment. In the present embodiment, in the lubrication film  131 , the contained amount of the binder resin is 50 wt % or more (the contained amount of the solid lubricant is 50 wt % or less). Therefore, the lubrication film  131  exerts a high adhesion force, and excellent adhesiveness is achieved between the lubrication film  131  and the base material  100 . Further, the excellent adhesiveness is achieved between the lubrication film  131  and the electrodeposited film  130 . Further, the lubrication film  131  is cured by being burned. Therefore, the detachment of the lubrication film  131  from the base material  100  is prevented or reduced. The base material  100  includes the streak  140  on the skirt portion outer peripheral surface  120 . Due to that, the lubrication film  131  further securely adheres to the base material  100 . The solid lubricant contained in the lubrication film  131  is C or MoS 2 , and is conductive. Therefore, in the electrodeposition coating process, the skirt portion outer peripheral surface  120  covered by the lubrication film  131  easily functions as the electrode, and the electrodeposited film  130  is easily formed on the lubrication film  131 . 
       FIG. 13  illustrates a cross-section of the outer peripheral side of the skirt portion  12  in the plane containing the central axis O of the piston  1  in a result of an experiment when the electrodeposition conditions in the electrodeposition coating process were set to 60 V and 5 seconds. A table 1 indicates the film thickness of each of the films and the height of the streak  14  formed in this experiment. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Height of Streak 
                 Film Thickness 
               
               
                   
                 Layer 
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
                 Electrodeposition Film 
                 1.7 
                 2.0 
               
               
                   
                 Lubrication Film 
                 6.5 
                 6.9 
               
               
                   
                 Base Material of Piston 
                 9.5 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     Now, the film thickness refers to the distance from the uppermost portion (the top) of the streak  14  on the film on some layer to the uppermost portion (the top) of the streak  14  on the film on a next layer covering the film on some layer in the normal direction of the skirt portion outer peripheral surface  120 . A reference position for measuring the film thickness is not limited to the top of the streak  14 . Each of numerical values indicating the film thicknesses and the height of the streak  14  indicates an average for a plurality of streaks  14 . The height a0 of the streak  140  processed on the base material  100  was 9.5 μm. The film thickness of the lubrication film  131  was 6.9 μm. The height a1 of the streak  141  formed on the lubrication film  131  was 6.5 μm. The film thickness of the electrodeposited film  130  was 2.0 μm. The height a2 of the streak  142  formed on the electrodeposited film  130  was 1.7 μm. The meaning of the reduction in the height of the streak  14  due to the electrodeposited film  130  (the improvement of the smoothness) can be understood in a similar manner to the second embodiment. The height a1 (=6.5 μm) corresponds to “the height of the streak  141  formed due to the volatilization of the volatile component of the coating material from the coating material of the lubrication film  131  based on the height a0 of the streak  140 , (1−β)a0.” The height a2 (=1.7 μm) corresponds to a height shorter than “the height of the streak  142  formed due to the volatilization of the volatile component of the electrodeposition coating material from the electrodeposition coating material based on the height a1 of the streak  141 , (1−α)a1=(1−α)(1−β)a0.” 
     According to the graph (the straight line L) illustrated in  FIG. 8 , the fluid lubrication frictional coefficient is approximately 0.012, approximately 0.009, and approximately 0.004 when the height of the streak  14  on the skirt portion outer peripheral surface  120  is a0 (=9.5 μm), a1 (=6.5 μm), and a2 (=1.7 μm), respectively. Therefore, it can be understood that, in the piston  1  according to the present embodiment, the fluid lubrication frictional coefficient reduces to approximately one-third due to the reduction in the height of the streak  14  from a0 to a2, compared to a piston in which the streak  140  of the base material  100  is exposed on the skirt portion outer peripheral surface  120  (hereinafter referred to as a comparative example 1). Further, it can be understood that the fluid lubrication frictional coefficient reduces to approximately four-ninths due to the reduction in the height of the streak  14  from a1 to a2, compared to a piston in which the skirt portion outer peripheral surface  120  (the base material  100 ) is covered by only the lubrication film  131  (hereinafter referred to as a comparative example 2). According to the graph illustrated in  FIG. 8 , the fluid lubrication frictional coefficient is approximately 0.0045 or lower when the height of the streak  14  on the skirt portion outer peripheral surface  120  is 2.0 μm or shorter. It can be understood that, at this time, the fluid lubrication frictional coefficient reduces to approximately three-eighths or lower compared to the comparative example 1 and reduces to approximately half or lower compared to the comparative example 2. 
     A table 2 indicates an experiment result indicating the height a2 of the streak  142  when the electrodeposition conditions were changed. The heights a0 and a1 of the streaks  140  and  141  and the film thickness of the lubrication film  131  were the same as the experiment result when the electrodeposition conditions were set to 60 V and 5 seconds. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Processing Time (sec) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 5 
                 10 
                 15 
                 30 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 voltage (V) 
                 40 
                 5.0 
                 6.3 
                 1.1 
                 1.3 
               
               
                   
                   
                 60 
                 1.7 
                 1.6 
                 1.0 
                 0.9 
               
               
                   
                   
                 80 
                 — 
                 1.5 
                 — 
                 — 
               
               
                   
                   
                 100 
                 — 
                 1.5 
                 — 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     With the processing time (the power supply time) set to 5 seconds, a2 was 5.0 μm and 1.7 μm when the voltage was 40 V and 60 V, respectively. With the processing time set to 10 seconds, a2 was 6.3 μm, 1.6 μm, and 1.5 μm when the voltage was 40 V, 60 V, and 80 V and 100 V, respectively. Under such conditions that a0 and a1 were 7.5 μm and 5.3 μm, respectively, with the processing time set to 15 seconds, a2 was 1.1 μm and 1.0 μm when the voltage was 40 V and 60 V, respectively. Under the same conditions, with the processing time set to 30 seconds, a2 was 1.3 μm and 0.9 μm when the voltage was 40 V and 60 V, respectively. When the voltage was 40 V, a2 did not reduce so much from the height of the streak  141  with the processing time set to 10 seconds or shorter. When the voltage was 60 V, a2 reduced significantly from the height of the streak  141  to reach or fall below 2.0 μm even with the processing time set to 10 seconds or shorter. In this manner, even when the voltage increased to higher than 60 V, a2 did not change so much with the processing time set to 10 seconds. When the voltage was 60 V, even with the processing time set to 5 seconds, a2 did not change so much from the value when the processing time was set to 10 seconds. Therefore, it can be understood that the electrodeposition at 60 V for 5 seconds was enough as the electrodeposition conditions for keeping the voltage low, shortening the processing time, and reducing a2 to 2.0 μm or shorter. The lubrication film  131  was interposed between the base material  100  and the electrodeposited film  130 . The electrodeposition was carried out by the flow of the current through the lubrication film  131  and the precipitation of the electrodeposition coating material on the surface of the lubrication film  131 . The lubrication film  131  was less conductive than the base material  100 , and the base material  100  was covered by the film thickness of 6.9 μm. It can be understood that, even in this case, the electrodeposited film  130  including the streak  142  having the height a2 as short as 2.0 μm or shorter was formed under the electrodeposition conditions of 60V and 5 seconds as described above. 
     When the electrodeposition conditions were set to 60 V and 5 seconds, the film thickness of the electrodeposited film  130  was 2.0 μm. This value is considerably smaller than the film thickness of the lubrication film  131  (6. 9 μm). The increase in the film thickness of the electrodeposited film  130  is prevented or cut down in this manner, which makes it easy for the electrodeposited film  130  to be abraded. As a result, the initial conformability and the lubricity are improved as described above. To improve the initial conformability and the like, the film thickness of the electrodeposited film  130  is not limited to 2.0 μm, and may be, for example, slightly thicker than approximately 3.0 μm and is preferably 3 μm or thinner (thicker than 0 μm). 
     Fourth Embodiment 
     First, a configuration will be described.  FIG. 14  illustrates a cross-section of the outer peripheral side of the skirt portion  12  in the plan containing the central axis O of the piston  1  according to the present embodiment. The film portion  13  includes the electrodeposited film  130 , and also includes a lower layer film (a first film)  132  and an upper layer film (a second film)  133  in addition to the electrodeposited film  130 . In other words, the skirt portion  12  of the piston  1  includes three film layers. The lower layer film  132  and the upper layer film  133  correspond to the lubrication film containing the binder resin and the solid lubricant that is multilayered. The film portion  13  includes the electrodeposited film  130 , the lower layer film  132 , and the upper layer film  133  in this order from the base material  100  side. The electrodeposited film  130  covers the base material  100 . The composition of the electrodeposited film  130  is similar to the first embodiment. The lower layer film  132  is located on one side closer to the base material  100  than the upper layer film  133  is, i.e., a lower layer side, and covers the electrodeposited film  130 . The lower layer film  132  contains PAI as the binder resin. The binder resin may contain at least one of PI and EP together with or instead of PAI. The lower layer film  132  contains C as the solid lubricant. The solid lubricant may contain at least one of MoS 2  and PTFE together with or instead of C. In the lower layer film  132 , a contained amount of the solid lubricant is more than 0 wt % and 50 wt % or less, and a contained amount of the binder resin is 50 wt % or more and less than 100 wt %. The upper layer film  133  is located on another side farther away from the base material  100  than the lower layer film  132  is, i.e., an upper layer side. The upper layer film  133  covers the lower layer film  132 , and is exposed on the skirt portion outer peripheral surface  120 . The upper layer film  133  contains PAI as the binder resin. The binder resin may contain at least one of PI and EP together with or instead of PAI. The upper layer film  133  contains MoS 2  as the solid lubricant. The solid lubrication may contain at least one of C and PTFE together with or instead of MoS 2 . In the upper layer film  133 , a contained amount of the solid lubricant is 50 wt % or more and 95 wt % or less, and a contained amount of the binder resin is 5 wt % or more and 50 wt % or less. 
     The method for processing the surface of the piston  1  (the present processing) includes the process for forming the electrodeposited film, a process for forming the lower layer film, and a process for forming the upper layer film. In the present processing, the process for forming the electrodeposited film, the process for forming the lower layer film, and the process for forming the upper layer film are performed in this order. A procedure of the process for forming the electrodeposited film is similar to the first embodiment. Procedures of the process for forming the lower layer film and the process for forming the upper layer film are similar to the procedure of the process for forming the lubrication film according to the second embodiment. The burning process in the process for forming the electrodeposited film may be changed to the drying process. The drying process in the process for forming the lower layer film may be chanted to the burning process, and the drying process in the process for forming the upper layer film may be changed to the burning process. There are eight possible combinations as the process for forming the entire film depending on which is performed for each of the films, the drying or the burning. There are seven possible combinations as the process for forming the entire film, excluding a combination in which the burning is not carried out even once (the drying is carried out for all of the films). The present embodiment employs the combination in which the process for forming the electrodeposited film includes the burning process and the process for forming the lower layer film and the process for forming the upper layer film include the drying process, as a representative process for forming the entire film, by way of example. 
     Next, functions and effects will be described. The film portion  13  includes the lower layer film  132  and the upper layer film  133  in this order. Due to the upper layer film  133 , the present embodiment can acquire similar functions and effects to the lubrication film according to the second embodiment (the initial conformability and the like). The lower layer film  132  includes the solid lubricant (the contained amount of the solid lubricant is more than 0 wt %). Therefore, when the lower layer film  132  is exposed on the skirt portion outer peripheral surface  120  due to, for example, abrasion of the upper layer film  133 , in the first range, the strength of the frictional force reduces between the skirt portion outer peripheral surface  120  and the cylinder inner wall  20  due to the solid lubricant in the lower layer film  132  similarly to the second embodiment. In the lower layer film  132 , the contained amount of the binder resin is 50 wt % or more (the contained amount of the solid lubricant is 50 wt % or less). Therefore, the lower layer film  132  exerts a strong adhesion force, and excellent adhesiveness is achieved between the lower layer film  132  and the electrodeposited film  130 . Further, excellent adhesiveness is achieved between the lower layer film  132  and the upper layer film  133 . 
     Due to the electrodeposited film  130 , the present embodiment can acquire similar functions and effects to the electrodeposited film  130  according to the second embodiment (the improvement of the smoothness and the like). More specifically, the skirt portion outer peripheral surface  120  covered by the lubrication film (the upper layer film  133 ) is smoothed. The meaning of the reduction in the height of the steak  14  on the skirt portion outer peripheral surface  120  covered by the three film layers due to the electrodeposited film  130  (the improvement of the smoothness) can be understood in a similar manner to the first and second embodiments. For example, this can be confirmed by making a similar calculation to the second embodiment, assuming that β represents a ratio of a volume of the solid component to a volume of the entire coating material in the lower layer film  132 , and γ represents a ratio of a volume of the solid component to a volume of the entire coating material in the upper layer film  133 . The skirt portion outer peripheral surface  120  is further efficiently smoothed by forming the plurality of (two) films  132  and  133  in addition to the electrodeposited film  130 . The electrodeposited film  130  may contain the solid lubricant that is the electric insulator, similarly to the second embodiment. 
     Fifth Embodiment 
     First, a configuration will be described.  FIG. 15  illustrates a cross-section of the outer peripheral side of the skirt portion  12  in the plane containing the central axis O of the piston  1  according to the present embodiment. The skirt portion  12  includes three film layers similarly to the fourth embodiment. The film portion  13  includes the lower layer film  132 , the electrodeposited film  130 , and the upper layer film  133  in this order from the base material  100  side. The lower layer film  132  covers the base material  100 . The composition of the lower layer film  132  is similar to the lubrication film  131  according to the third embodiment. The electrodeposited film  130  covers the lower layer film  132 . The composition of the electrodeposited film  130  is similar to the first embodiment. The upper layer film  133  covers the electrodeposited film  130 , and is exposed on the skirt portion outer peripheral surface  120 . The composition of the upper layer film  133  is similar to the fourth embodiment. In the method for processing the surface of the piston  1  (the present processing), the process for forming the lower layer film, the process for forming the electrodeposited film, and the process for forming the upper layer film are performed in this order. The procedure of the process for forming the lower layer film is similar to the procedure of the process for forming the lubrication film according to the third embodiment. The procedure of the process for forming the electrodeposited film is similar to the third embodiment. The procedure of the process for forming the upper layer film is similar to the fourth embodiment. 
     Next, functions and effects will be described. Due to the upper layer film  133 , the present embodiment can acquire similar functions and effects to the lubrication film  131  according to the second embodiment (the initial conformability and the like). Due to the electrodeposited film  130 , the skirt portion outer peripheral surface  120  (at least the second range) covered by the lubrication film (the upper layer film  133 ) is smoothed similarly to the electrodeposited film  130  according to the second embodiment. The electrodeposited film  130  does not contain the solid lubricant. Therefore, the electrodeposited film  130  exerts a strong adhesion force, and excellent adhesiveness is achieved between the electrodeposited film  130  and the lower layer film  132 . Further, excellent adhesiveness is achieved between the electrodeposited film  130  and the upper layer film  133 . The electrodeposited film  130  may contain the solid lubricant that is the electric insulator, similarly to the second embodiment. The electrodeposited film  130  is not burned, and therefore is abraded and then smoothed early (together with the upper layer film  133 ) in the first range, thereby easily starting to conform to the cylinder inner wall  20  quickly, similarly to the electrodeposited film  130  according to the third embodiment. Further, this makes it easy for the lower layer film  132  covered by the electrodeposited film  130  to be exposed on the skirt portion outer peripheral surface  120 , so that the merits (the lubricity and the initial conformability) of the lower layer film  132  can be easily acquired. The electrodeposited film  130  may contain the binder resin having a low abrasion resistance property or may contain a large amount of the solid lubrication that is the electric insulator, similarly to the third embodiment. Due to the lower layer film  132 , the present embodiment can acquire similar functions and effects to the lubrication film  131  according to the third embodiment. 
       FIG. 16  illustrates a similar cross-section to  FIG. 13  in a result of an experiment when the electrodeposition conditions were set to 60 V and 5 seconds. A table 3 indicates the film thickness of each of the films and the height of the streak  14  formed in this experiment. The heights a0, a1, and a2 of the streaks  140 ,  141 , and  142 , and the film thicknesses of the lower layer film  132  and the electrodeposited film  130  were the same as the result of the experiment according to the third embodiment ( FIG. 13  and the table 1). 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Height of Streak 
                 Film Thickness 
               
               
                   
                 Layer 
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
                 Upper Layer Film 
                 1.2 
                 4.0 
               
               
                   
                 Electrodeposition Film 
                 1.7 
                 2.0 
               
               
                   
                 Lower Layer Film 
                 6.5 
                 6.9 
               
               
                   
                 Base Material of Piston 
                 9.5 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     The film thickness of the upper layer film  133  was 4.0 μm. The height a3 of the streak  143  formed on the upper layer film  133  was 1.2 μm. The height a3 corresponds to “the height of the streak  143  formed due to the volatilization of the volatile component of the coating material from the coating material of the upper layer film  133  based on the height a2 of the streak  142 , (1−γ)a2.” It can be understood that the height a3 of the streak  143  on the skirt portion outer peripheral surface  120  was shorter than the third embodiment (the height a2 of the streak  142 =1.7 μm) by an amount corresponding to being covered by the upper layer film  133 . 
     According to the graph (the straight line L) indicated by the graph illustrated in  FIG. 8 , the fluid lubrication frictional coefficient is approximately 0.0037 when the height of the streak  14  on the skirt portion outer peripheral surface  120  is a3 (=1.2 μm). Therefore, it can be understood that, in the piston  1  according to the present embodiment, the fluid lubrication frictional coefficient reduces to approximately 30% compared to the above-described comparative example 1 due to the reduction in the height of the streak  14  from a0 to a3. Further, it can be understood that the fluid lubrication frictional coefficient reduces to approximately 40% compared to the above-described comparative example 2 due to the reduction in the height of the streak  14  from a1 to a3. 
     The film thickness of the electrodeposited film  130  was 3 μm or shorter. The increase in the film thickness of the electrodeposited film  130  is prevented or cut down in this manner, which makes it easy for the electrodeposited film  130  to be abraded. As a result, the initial conformability and the lubricity are improved as described above. The film thickness of the electrodeposited film  130  may be, for example, slightly thicker than approximately 3.0 μm to improve the initial conformability and the like. 
     Sixth Embodiment 
     First, a configuration will be described.  FIG. 17  illustrates a cross-section of the outer peripheral side of the skirt portion  12  in the plane containing the central axis O of the piston  1  according to the present embodiment. The skirt portion  12  includes three film layers similarly to the fourth embodiment. The film portion  13  includes the lower layer film  132 , the upper layer film  133 , and the electrodeposited film  130  in this order from the base material  100  side. The lower layer film  132  covers the base material  100 . The composition of the lower layer film  132  is similar to the lubrication film  131  according to the third embodiment. The upper layer film  133  covers the lower layer film  132 . The composition of the upper layer film  133  is similar to the fourth embodiment. The upper layer film  133  contains MoS 2  as the solid lubricant. The solid lubricant may contain C together with or instead of MoS 2  but does not contain PTFE. The electrodeposited film  130  covers the upper layer film  133 , and is exposed on the skirt portion outer peripheral surface  120 . The composition of the electrodeposited film  130  is similar to the first embodiment. In the method for processing the surface of the piston  1  (the present processing), the process for forming the lower layer film, the process for forming the upper layer film, and the process for forming the electrodeposited film are performed in this order. The procedure of the process for forming the lower layer film and the procedure of the process for forming the upper layer film are similar to the procedure of the process for forming the lubrication film according to the third embodiment. For example, after the burning process in the process for forming the lower layer film, the coating process in the process for forming the upper layer film is performed when the temperature of the piston  1  is 50 degrees Celsius to 120 degrees Celsius. The procedure of the process for forming the electrodeposited film is similar to the third embodiment. 
     Next, functions and effects will be described. Due to the electrodeposited film  130 , the present embodiment can acquire similar functions and effects to the electrodeposited film  130  according to the third embodiment (the smoothness of the outer peripheral surface  120  and the like). When the upper layer film  133  is exposed on the skirt portion outer peripheral surface  120  due to, for example, abrasion of the electrodeposited film  130  in the first range, the strength of the frictional force reduces due to the solid lubricant in the upper layer film  133  and the upper layer film  133  is abraded early along therewith, by which the initial conformability is improved. When the lower layer film  132  is exposed on the skirt portion outer peripheral surface  120  due to, for example, the abrasion of the electrodeposited film  130  and the upper layer film  133  in the first range, the strength of the frictional force reduces due to the solid lubricant in the lower layer film  132 . 
     In the lower layer film  132 , the contained amount of the binder resin is 50 wt % or more (the contained amount of the solid lubricant is 50 wt % or less). Therefore, the lower layer film  132  exerts a strong adhesion force, and excellent adhesiveness is achieved between the lower layer film  132  and the base material  100 . Further, excellent adhesiveness is achieved between the lower layer film  132  and the upper layer film  133 . The solid lubricants contained in the lower layer film  132  and the upper layer film  133  are C or MoS 2 , and are conductive. Therefore, in the electrodeposition coating process, the skirt portion outer peripheral surface  120  covered by the lower layer film  132  and the upper layer film  133  can easily function as the electrode, and the electrodeposited film  130  can be easily formed on the upper layer film  133 . 
       FIG. 18  illustrates a similar cross-section to  FIG. 13  in a result of an experiment when the electrodeposition conditions were set to 100 V and 10 seconds. A table 4 indicates the film thickness of each of the films and the height of the streak  14  formed in this experiment. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Height of Streak 
                 Film Thickness 
               
               
                   
                 Layer 
                 (μm) 
                 (μm) 
               
               
                   
                   
               
             
            
               
                   
                 Electrodeposition Film 
                 1.9 
                 3.2 
               
               
                   
                 Upper Layer Film 
                 5.2 
                 4.0 
               
               
                   
                 Lower Layer Film 
                 5.6 
                 6.0 
               
               
                   
                 Base Material of Piston 
                 8.8 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     The height a0 of the streak  140  was 8.8 μm. The film thickness of the lower layer film  132  was 6.0 μm. The height a1 of the streak  141  formed on the lower layer film  132  was 5.6 μm. The film thickness of the upper layer film  133  was 4.0 μm. The height a2 of the streak  142  formed on the upper layer film  133  was 5.2 μm. The film thickness of the electrodeposited film  130  was 3.2 μm. The height a3 of the streak  143  formed on the electrodeposited film  130  was 1.9 μm. The height a1 (=5.6 μm) corresponds to “the height of the streak  141  formed due to the volatilization of the volatile component of the coating material from the coating material of the lower layer film  132  based on the height a0 of the streak  140 , (1−β)a0.” The height a2 (=5.2 μm) corresponds to “the height of the streak  142  formed due to the volatilization of the volatile component of the coating material from the coating material of the upper layer film  133  based on the height a1 of the streak  141 , (1−γ)a1=(1−β)(1−γ)a0.” 
     The height a3 (=1.9 μm) corresponds to a shorter height than “the height of the streak  143  formed due to the volatilization of the volatile component of the electrodeposition coating material from the electrodeposition coating material based on the height a2 of the streak  142 , (1−α)a2=(1−α)(1−β)(1−γ)a0.” 
     According to the graph (the straight line L) indicated by the graph illustrated in  FIG. 8 , the fluid lubrication frictional coefficient is approximately 0.011, 0.008, and approximately 0.004 when the height of the streak  14  on the skirt portion outer peripheral surface  120  is a0 (=8.8 μm), a2 (=5.2 μm), and a3 (=1.9 μm), respectively. Therefore, it can be understood that, in the piston  1  according to the present embodiment, the fluid lubrication frictional coefficient reduces to approximately 40% compared to the above-described comparative example 1 due to the reduction in the height of the streak  14  from a0 to a3. Further, it can be understood that the fluid lubrication frictional coefficient reduces to approximately half compared to the example in which the skirt portion outer peripheral surface  120  (the base material  100 ) is covered by only the lubrication films  132  and  133  due to the reduction in the height of the streak  14  from a2 to a3. 
     A table 5 indicates an experiment result indicating the height a3 of the streak  143  when the electrodeposition conditions were changed. The heights a0 to a2 of the streaks  140  to  142  and the film thicknesses of the lubrication films  132  and  133  were the same as the above-described experiment result when the electrodeposition conditions were set to 100 V and 10 seconds. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 5 
               
             
            
               
                   
                   
               
               
                   
                 Processing Time (sec) 
                   
               
            
           
           
               
               
               
            
               
                   
                 5 
                 10 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 voltage (V) 
                 60 
                 5.0 
                 4.4 
               
               
                   
                   
                 80 
                 4.4 
                 3.9 
               
               
                   
                   
                 100 
                 3.4 
                 1.9 
               
               
                   
                   
                 120 
                 3.8 
                 5.0 
               
               
                   
                   
               
            
           
         
       
     
     With the processing time set to 5 seconds, a3 was 5.0 μm, 4.4 μm, 3.4 μm, and 3.8 μm when the voltage was 60 V, 80 V, 100 V, and 120 V, respectively. With the processing time set to 10 seconds, a3 was 4.4 μm, 3.9 μm, 1.9 μm, and 5.0 μm when the voltage was 60 V, 80 V, 100 V, and 120 V, respectively. In this manner, when the voltage was 100 V or lower, a3 reduced as the voltage increased regardless of the processing time. When the voltage was 120 V, a3 was taller than when the voltage was 100 V regardless of the processing time. When the voltage was 100 V or lower, a3 was shorter when the processing time was 10 seconds than when the processing time was 5 seconds. Under 60 V and 5 seconds, a3 was 5.0 μm, and hardly reduced from a2 (=5.2 μm). It can be understood that the generation of the electrodeposited film to reduce a3 was insufficient under 60 V and 5 seconds. It can be understood that a3 was able to reduce to 2.0 μm or shorter under 100 V and 10 seconds. The lubrication films (the lower layer film  132  and the upper layer film  133 ) were interposed between the base material  100  and the electrodeposited film  130 . The electrodeposition was carried out by the flow of the current through the lubrication films  132  and  133  and the precipitation of the electrodeposition coating material on the surface of the upper layer film  133 . The lubrication films  132  and  133  were less conductive than the base material  100 , and the base material  100  was covered by a film thickness of 10.0 μm (=6.0 μm+4.0 μm). It can be understood that, even in this case, the electrodeposition film  130  including the streak  143  having the height a3 as short as 2.0 μm or shorter was formed under the electrodeposition conditions of 100V and 10 seconds as described above. 
     When the electrodeposition conditions were set to 100 V and 10 seconds, the film thickness of the formed electrodeposited film  130  was 3.2 μm. This value is considerably smaller than the film thicknesses of the lubrication films  132  and  133  (10. 0 μm). In this manner, the increase in the film thickness of the electrodeposited film  130  is prevented or cut down, which makes it easy for the electrodeposited film  130  to be abraded. As a result, the initial conformability and the lubricity are improved as described above. The electrodeposited film  130  may contain the binder resin having a low abrasion resistance property or may contain a large amount of the solid lubrication that is the electric insulator, similarly to the third embodiment. 
     Other Embodiments 
     Having described embodiments for implementing the present invention based on the exemplary embodiments thereof, the specific configuration of the present invention is not limited to the exemplary embodiments, and the present invention also includes a design modification and the like thereof made within a range that does not depart from the spirit of the present invention. For example, the base material of the piston is not limited to aluminum alloy and may be iron or the like. PAI, PI, and EP can be applied to not only the base material but also another material because they can achieve excellent adhesiveness. Further, the individual components described in the claims and the specification can be arbitrarily combined or omitted within a range that allows them to remain capable of achieving at least a part of the above-described objects or producing at least a part of the above-described advantageous effects. 
     The present application claims priority to Japanese Patent Application No. 2015-059906 filed on Mar. 23, 2015. The entire disclosure of Japanese Patent Application No. 2015-059906 filed on Mar. 23, 2015 including the specification, the claims, the drawings, and the abstract is incorporated herein by reference in its entirety. 
     REFERENCE SIGN LIST 
     
         
           1  piston 
           100  base material 
           12  skirt portion 
           120  outer peripheral surface 
           13  film 
           130  electrodeposited film 
           132  lower layer film (first film) 
           133  upper layer film (second film) 
           14  streak 
           2  cylinder 
           20  inner wall