Abstract:
A piston that maintains excellent lubricating performance even when the internal combustion engine is operated in severe environments. A primer layer including a resinous material is disposed on the sliding surface of the skirt of a piston, and solid lubricating parts, preferably including silver (Ag), a silver alloy, copper (Cu), or a copper alloy are disposed on the primer layer. In the primer layer and the solid lubricating parts, a fibrous filler including metallic fibers, etc., is present so as to extend across the boundary between the primer layer and the solid lubricating parts.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a piston for an internal combustion engine, the piston being movable back and forth in a cylinder of the internal combustion engine. 
       BACKGROUND ART 
       [0002]    Automobiles travel by causing tires to rotate with a rotational drive force, which is converted from a drive force that is generated by an internal combustion engine supplied with a fuel. Recently, various attempts have been made to improve the fuel consumption ratio (gas mileage) of internal combustion engines on such automobiles. Since an improved fuel consumption ratio reduces the amount of fuel consumed, energy savings and protection of the global environment can be realized. 
         [0003]    One such attempt is directed toward reducing the resistance to sliding movement between inner wall surfaces of cylinders (inner wall surfaces of bores or sleeves) of the internal combustion engine and pistons that move back and forth in the cylinders. If resistance to sliding movement is reduced, the pistons move back and forth more easily in the cylinders. Therefore, the drive force applied to move the pistons back and forth is reduced, resulting in a reduction in the amount of fuel consumed. 
         [0004]    It is known in the art to deposit a layer including a lubricant-rich material on the inner wall surfaces of cylinders or piston skirts in order to reduce resistance to sliding movement of the pistons, for improving the lubrication properties of the inner wall surfaces of the cylinders or the piston skirts. For example, as disclosed in International Publication No. WO 2011/115152, the present applicant has proposed providing ridges on the sliding surface of a piston skirt, and covering the ridges with a lubricating film made of silver, silver alloy, copper, or copper alloy. 
         [0005]    As disclosed in International Publication No. WO 2011/115152, it is preferable to interpose an intermediate layer made of a heat-resistant resin material between the film and the piston skirt, in order to ensure that the film is firmly bonded to the piston skirt by the intermediate layer. Specific examples of such a heat-resistant resin material include polyimide resin, polyamide-imide resin, epoxy resin, nylon-6 resin, and nylon-6,6 resin, etc. 
         [0006]    The existence of the film on the piston of the internal combustion engine is effective to suitably maintain a lubricant between the inner wall surface of the cylinder, e.g., the inner wall surface of the sleeve, and the piston skirt. The existence of the film also serves to spread or transfer frictional heat quickly, so that the piston skirt and the inner wall surface of the cylinder can be prevented from becoming adhered to each other. 
       SUMMARY OF INVENTION 
       [0007]    Vehicles that travel in severe environments, such as racing cars or the like, which are driven at high speeds over a long period of time, are required to be powered by a highly durable internal combustion engine as compared to general vehicles. For example, the piston used in the internal combustion engine disclosed in International Publication No. WO 2011/115152 desirably makes the film less liable to come off the piston skirt insofar as possible, thereby preventing the piston skirt and the inner wall surface of the cylinder from becoming adhered to each other over a long period of time. 
         [0008]    The present invention has been made in connection with the technology disclosed in International Publication No. WO 2011/115152. A major object of the present invention is to provide a piston for use in an internal combustion engine, which is capable of making a solid lubricator less liable to come off for thereby suitably maintaining a lubricant between a piston skirt and the inner wall surface of the piston. 
         [0009]    Another object of the present invention is to provide a piston for use in an internal combustion engine, which is capable of preventing the inner wall surface of a cylinder and a piston skirt from becoming adhered to each other. 
         [0010]    According to an embodiment of the present invention, there is provided a piston for use in an internal combustion engine, which is movable back and forth in a cylinder of the internal combustion engine, comprising:
       a base layer disposed on a sliding contact surface of a piston skirt, the base layer containing a resin material;   a solid lubricator disposed on the base layer; and   fibrous fillers that reside within and extend between the base layer and the solid lubricator.       
 
         [0014]    The fibrous fillers exist across a boundary between the base layer and the solid lubricator, such that the fibrous fillers extend from the base layer into the solid lubricator. In other words, the fibrous fillers have ends that are embedded in the solid lubricator and other ends that are embedded in the base layer. Hence, the fibrous fillers develop an anchoring effect in both the base layer and the solid lubricator. Therefore, the base layer and the solid lubricator are firmly joined to each other via the fibrous fillers. As a result, it is difficult for the solid lubricator to peel off and separate away from the base layer. 
         [0015]    The internal combustion engine in which the piston is incorporated remains highly durable even if the internal combustion engine is used in cars that travel in severe environments, such as racing cars or the like. 
         [0016]    Assuming that a weight of the resin material is given as 100% by weight, the proportion of the fibrous fillers lies within a range from 10% to 65% by weight. The proportion of the fibrous fillers, which is 10% or greater by weight, allows the fibrous fillers to develop a sufficient anchoring effect, thereby making it possible to sufficiently increase the bonding strength between the base layer and the solid lubricators. The proportion of the fibrous filler, which is 65% or less by weight, is effective to cause the resin material to sufficiently hold the solid lubricator on the piston skirt. Stated otherwise, the fibrous fillers, which are contained in the resin material in the above range, make it possible to prevent the solid lubricator from coming off, suitably maintain the lubricant between the inner wall surface of the cylinder and the piston skirt, and are capable of avoiding adhesion from occurring between the inner wall surface of the cylinder and the piston skirt. 
         [0017]    The solid lubricator preferably comprises at least one of silver, silver alloy, copper, or copper alloy. Each of such materials exhibits an excellent lubricating capability when the piston skirt is held in sliding contact with the inner wall surface of the cylinder. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a perspective view showing in its entirety a piston according to an embodiment of the present invention; 
           [0019]      FIG. 2  is a side elevational view of the piston shown in  FIG. 1 ; 
           [0020]      FIG. 3  is a fragmentary cross-sectional view of a surface layer region of a piston skirt of the piston; 
           [0021]      FIG. 4  is an enlarged cross-sectional view of a boundary region between a base layer and a solid lubricator, which are deposited on a sliding contact surface in a surface layer region of the piston skirt; 
           [0022]      FIG. 5  is a side elevational view of a piston according to another embodiment of the present invention; 
           [0023]      FIG. 6  is an enlarged cross-sectional view of a surface layer region of a piston skirt of a piston according to yet another embodiment of the present invention; 
           [0024]      FIG. 7  is a view showing a test piece according to an Inventive Example and the result of a peel test; 
           [0025]      FIG. 8  is a view showing a test piece according to a Comparative Example 1 and the result of a peel test; 
           [0026]      FIG. 9  is a view showing a test piece according to a Comparative Example 2 and the result of a peel test; 
           [0027]      FIG. 10  is a view showing a test piece according to a Comparative Example 3 and the result of a peel test; and 
           [0028]      FIG. 11  is a view showing a test piece according to a Comparative Example 4 and the result of a peel test. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    Pistons for use in internal combustion engines (hereinafter referred to simply as “pistons”) according to preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. 
         [0030]      FIG. 1  is an overall perspective view showing the entirety of a piston  10  according to an embodiment of the present invention.  FIG. 2  shows the piston  10  in side elevation. The piston  10  includes a pair of piston skirts  12 ,  12  in a lower portion thereof, and a pair of walls  14 ,  14 , which extend substantially vertically and are disposed between the piston skirts  12 ,  12 . The walls  14 ,  14  have respective pin bosses  16 ,  16  that project horizontally. The pin bosses  16 ,  16  have respective piston pin holes  17 ,  17  defined respectively therethrough for insertion of a non-illustrated piston pin. The piston pin extends through a penetrating hole, which is defined in a smaller end of a non-illustrated connecting rod, thereby pivotally supporting the connecting rod on the piston  10 . 
         [0031]    The piston  10  includes an oil ring groove  18 , a first piston ring groove  20 , and a second piston ring groove  22 , which are defined above the piston skirts  12 ,  12  and arranged successively upward in this order. The oil ring groove  18 , the first piston ring groove  20 , and the second piston ring groove  22  extend circumferentially around a head portion of the piston  10 . 
         [0032]    The piston  10 , which is constructed in the foregoing manner, is made of an aluminum alloy such as AC2A, AC2B, AC4B, AC4C, AC4D, AC8H, or A1100 (aluminum alloys defined according to JIS), an Al—Cu alloy, or the like. 
         [0033]    As shown at an enlarged scale in  FIGS. 3 and 4 , each of the piston skirts  12  has a sliding contact surface formed as a smooth surface, and a base layer  24  that is fixed to the smooth sliding contact surface. The base layer  24  covers the entirety of the sliding contact surface of each of the piston skirts  12  and has a substantially uniform thickness. 
         [0034]    The base layer  24  contains a heat resistant resin material  26 , which increases the bonding strength between solid lubricators  30 , to be described below, and the piston skirts  12 . Preferred examples of the resin material  26  include polyimide resin, polyamide-imide resin, epoxy resin, nylon-6 resin, and nylon-6,6 resin, etc. 
         [0035]    The base layer  24  also contains fibrous fillers  28  in the resin material  26 . The fibrous fillers  28  are in the form of metal fibers, the lengths of which lie within a range from several tens to several hundreds μm, for example, and have ends that project from the surface of the base layer  24 . A specific example of the metal fibers is Fe whiskers, although fibers of Fe—Ni—Cr alloy or fibers of tin (Sn) may be used. Alternatively, for example, the fibrous fillers  28  may be in the form of ceramic fibers made of silicon carbide (SiC) or the like, carbon nanotubes, or fibrous graphite. 
         [0036]    Assuming that the weight of the resin material  26  is given as 100% by weight, the proportion of the fibrous fillers  28  preferably lies within a range from 10% to 65% by weight. The proportion of the fibrous fillers  28 , which is 10% or greater by weight, allows the fibrous fillers  28  to be embedded suitably in the base layer  24  and the solid lubricators  30 , thereby making it possible to sufficiently increase the bonding strength between the base layer  24  and the solid lubricators  30 . The proportion of the fibrous fillers  28 , which is 65% or less by weight, is effective to cause the resin material  26  to sufficiently hold the solid lubricators  30  on the piston skirts  12 . 
         [0037]    Stated otherwise, the fibrous fillers  28 , which are contained in the resin material  26  in the above range, make it possible to prevent the solid lubricators  30  from coming off, maintain a lubricant suitably between the inner wall surface of a cylinder and the piston skirts  12 , and avoid the occurrence of adhesion between the inner wall surface of the cylinder and the piston skirts  12 . 
         [0038]    Although the base layer  24  may contain only the resin material  26  and the fibrous fillers  28 , additionally, the base layer  24  may include a solid lubricant. The solid lubricant may be of a known nature. Preferred examples of the solid lubricant include molybdenum disulfide (MoS 2 ), boron nitride (BN), and graphite (C), etc. 
         [0039]    The solid lubricators  30 , which extend in a linear manner circumferentially around the piston skirts  12 , are disposed on the base layer  24  (see  FIGS. 1 and 2 ). Each of the solid lubricators  30  is raised horizontally from the base layer  24 . Therefore, each of the linearly shaped solid lubricators  30  is shaped in the form of a ridge. 
         [0040]    Ends of the fibrous fillers  28  are embedded in the solid lubricators  30  and project from the base layer  24  in the vicinity of a contact surface, which is held in contact with the base layer  24 . In other words, the fibrous fillers  28  are contained in such a manner that the fibrous fillers  28  lie within and extend between the base layer  24  and the solid lubricators  30 . Since ends of the fibrous fillers  28  are embedded in the solid lubricators  30  and other ends thereof are embedded in the base layer  24 , the fibrous fillers  28  develop an anchoring effect both in the solid lubricators  30  and in the base layer  24 . Therefore, the base layer  24  and the solid lubricators  30  are firmly joined to each other. As a result, it is difficult for the solid lubricators  30  to peel off or become separated from the base layer  24 . 
         [0041]    According to the present embodiment, the solid lubricators  30  are made of any one of silver, silver alloy, copper, and copper alloy. Each of such materials exhibits an excellent lubricating capability when the piston skirts  12  are held in sliding contact with the inner wall surface of a bore in a cylinder block or the inner wall surface of a cylinder sleeve. Preferred examples of silver alloy include Ag—Sn alloy and Ag—Cu alloy. Preferred examples of copper alloy include Cu—Sn alloy, Cu—Zn alloy, and Cu—P alloy, etc. 
         [0042]    If the solid lubricators  30  are made of silver or silver alloy, the purity of silver preferably is 60% by weight or greater. If the purity of silver is less than 60% by weight, the thermal conductivity of the solid lubricators  30  is slightly low, and hence the solid lubricators  30  cannot easily form a smooth wearing surface, resulting in a tendency to lessen the ability to reduce the frictional loss (Psf) of the internal combustion engine. More preferably, the purity of silver is 80% by weight or greater. 
         [0043]    If the solid lubricators  30  are made of copper or copper alloy, the purity of copper preferably is 70% by weight or greater, for the same reasons as described above, and more preferably, is 80% by weight or greater in particular. 
         [0044]    The purity of silver is defined as the “% by weight of silver contained in the solid lubricators  30 ”. For example, if the solid lubricators  30  are made of silver alloy, the purity of silver is determined as the % by weight of silver contained in the solid lubricators  30 . If the solid lubricators  30  are in the form of sintered bodies produced from a paste after being coated with silver particles, the purity of silver is defined as the proportion of the silver particles in the paste. The purity of copper is defined similarly. 
         [0045]    It is not required that all of the solid lubricators  30  are made of the same metal. The solid lubricators  30  may be made of different metals, for example, in such a manner that one of the solid lubricators  30  is made of silver, while another of the solid lubricators  30  adjacent thereto is made of copper alloy. 
         [0046]    The solid lubricators  30  are not limited to having a particular thickness. However, if the thickness of the solid lubricators  30  is excessively small, the solid lubricators  30  become worn in a relatively short period of time. Conversely, if the thickness of the solid lubricators  30  is excessively large, the solid lubricators  30  become so heavy that a large driving force is required to move the piston  10  back and forth. In order to avoid such problems, the thickness of the solid lubricators  30  preferably lies within a range from 0.5 to 100 μm. 
         [0047]    When the internal combustion engine, which is equipped with such a piston  10 , is assembled and operated, the solid lubricators  30  essentially are held in sliding contact with the inner wall surface of the cylinder (the inner wall surface of the cylinder bore or the inner wall surface of the cylinder sleeve) with a lubricating oil interposed therebetween. If the solid lubricators  30  are held in sliding contact with the inner wall surface of a sleeve that is made of FC (gray cast iron) or Al, for example, the sum of the thermal conductivity of the solid lubricators  30  and the thermal conductivity of the sleeve of FC or Al is determined to be 350 W/m·K or greater. In addition, the absolute value of the difference between the Young&#39;s moduli of the solid lubricators  30  and the sleeve of FC or Al is 10 GPa or greater. 
         [0048]    According to an intensive study by the inventors, in this case, the lubricating oil is retained suitably in the small clearance between the sleeve and the piston skirts  12 , thereby preventing adhesion from taking place between the sleeve and the piston skirts  12 . Therefore, the sleeve and the piston skirts  12  are effectively prevented from suffering from seizure, whereby the frictional loss of the internal combustion engine is significantly reduced. 
         [0049]    According to the present embodiment, furthermore, the solid lubricators  30  and the base layer  24  are firmly joined to each other as a result of the fibrous fillers  28  that are interposed therebetween. 
         [0050]    Consequently, it is difficult for the solid lubricators  30  to peel off from the base layer  24 . Stated otherwise, the solid lubricators  30  are held on the sliding contact surfaces of the piston skirts  12  over a long period of time. Therefore, due to the existence of the solid lubricators  30 , the piston  10  can maintain the above-described advantages over a long period of time. 
         [0051]    Since it is difficult for the solid lubricators  30  to peel off from the base layer  24 , the above advantages are obtained by the action of the solid lubricators  30 , even if the piston  10  is moved back and forth intensively in the cylinder. More specifically, the internal combustion engine in which the piston is incorporated remains highly durable, even if the engine is used in cars that travel in severe environments, such as racing cars including Formula 1 type racing cars or the like, for example. 
         [0052]    According to the present embodiment, the piston requires only the addition of a plurality of linear solid lubricators  30 . The above-described solid lubricant and the resin material  26  are inexpensive and lightweight. Even though the sliding contact surfaces of the piston skirts  12  overall are covered with the base layer  24  having the solid lubricators  30  disposed thereon, the piston  10  is prevented from becoming high in cost or excessively heavy. In other words, the piston  10  is capable of carrying out a sufficient lubricating action, even though the weight of the piston  10  is prevented from increasing. 
         [0053]    Even if a sleeve of Al, which tends to experience seizure in comparison with a sleeve of FC, is used in combination with the piston  10 , which is made of aluminum alloy, the piston  10  effectively avoids seizure and is capable of significantly reducing frictional loss in the internal combustion engine. Further, if the base layer  24  contains a solid lubricant, the solid lubricant can ensure a lubricating capability. 
         [0054]    The base layer  24  and the solid lubricators  30  can be provided on the sliding contact surfaces of the piston skirts  12  in the following manner. 
         [0055]    First, the resin material  26 , which is to be made into the base layer  24 , is prepared and melted. The fibrous fillers  28  are mixed with the melted material. In the resin material  26 , the content of the fibrous fillers  28  preferably lies within a range from 10% to 65% by weight. A solid lubricant may also be added to the resin material  26  and the fibrous fillers  28 . 
         [0056]    Next, the melted material is supplied to the sliding contact surfaces of the piston skirts  12 . The melted material may be sprayed onto the sliding contact surfaces of the piston skirts  12 , or alternatively, the sliding contact surfaces of the piston skirts  12  may be coated with the melted material. The melted material preferably is applied so that the sliding contact surfaces of the piston skirts  12  are covered entirely with the melted material. It is easier and simpler to cover the sliding contact surfaces of the piston skirts  12  entirely with the melted material. Stated otherwise, rather than selectively coating portions of the sliding contact surfaces of the piston skirts  12  with the melted material, the base layer  24  can be formed with greater ease. 
         [0057]    The melted material, which has been supplied as described above, is cooled and solidified in a state in which the contained fibrous fillers  28  project from the surface of the material. In this manner, the base layer  24  is formed on the sliding contact surfaces of the piston skirts  12 . 
         [0058]    Meanwhile, fine particles of silver, silver alloy, copper, or copper alloy, preferably having an average particle diameter in a range from 1 to 80 nm, and more preferably from 30 to 80 nm, or stated otherwise, nanoparticles of silver, silver alloy, copper, or copper alloy, are dispersed in a dispersion medium in order prepare a paste. Preferred examples of the dispersion medium are polar solvents including aromatic alcohols such as benzylic alcohol, propylene glycol monomethyl ether acetate (PEGMEA), polyethylene glycol monomethacrylate (PEGMA), terpineol, etc. An unsaturated fatty acid ester may be added as a dispersant to such polar solvents. 
         [0059]    For forming the solid lubricators  30 , the base layer  24  is coated with the paste containing the dispersion medium, using a known coating process such as a screen printing process, a pad printing process, or the like. Thereafter, the paste together with the piston  10  is heated to a temperature preferably within a range from 160° C. to 240° C. The dispersion medium in the paste is volatilized and the nanoparticles are fused together. In other words, the nanoparticles are sintered, thereby producing the solid lubricators  30  in the form of sintered bodies made up of nanoparticles. 
         [0060]    The solid lubricators  30  are obtained by coating the base layer  24  with a paste, at a location where the ends of the fibrous fillers  28  project from the surface of the base layer  24 . Consequently, the solid lubricators  30  have ends of the fibrous fillers  28  embedded therein, in the vicinity of a contact surface that is held in contact with the base layer  24 . Accordingly, the fibrous fillers  28  are contained in such a manner that the fibrous fillers  28  lie within and extend between the base layer  24  and the solid lubricators  30 . 
         [0061]    As described above, the solid lubricators  30  are obtained by a coating process, such as a screen printing process, a pad printing process, or the like. Since the coating process is carried out after the melted material has been cooled and solidified into the base layer  24 , the printing plate is prevented from becoming clogged with the melted material. In other words, the solid lubricators  30  can be obtained in an efficient manner. 
         [0062]    If the solid lubricators  30  are formed from nanoparticles, the solid lubricators  30  are sintered in a relatively low temperature range from 160° C. to 240° C., thereby producing a coating. Therefore, the piston skirts  12 , which are made of an aluminum alloy, are prevented from being heated to a high temperature, and the mechanical strength thereof, etc., is prevented from being adversely affected. 
         [0063]    The present invention is not limited to the embodiment described above. Various changes may be made to the embodiment without departing from the scope of the invention. 
         [0064]    For example, although according to the present embodiment, the solid lubricators  30  are provided in a linear shape, as shown in  FIG. 5 , the solid lubricators  30  may be provided in a dot shape. Recesses, which are defined between the dot-shaped solid lubricators  30 , are effective to fulfill a role of maintaining the lubricating oil. 
         [0065]    According to the arrangement shown in  FIG. 5 , the amount of paste used to form the solid lubricators  30 , i.e., the amount of metal (silver, silver alloy, copper, or copper alloy) used, is reduced. Thus, the cost of the piston is further reduced and the weight of the piston  10  is prevented from increasing. 
         [0066]    The base layer  24  may be formed selectively only on portions of the piston skirts  12  where the solid lubricators  30  are to be formed. Alternatively, the entire sliding contact surfaces of the piston skirts  12  may be coated with the base layer  24 , together with the entirety of the base layer  24  being coated with the solid lubricators  30 . 
         [0067]    A plurality of linear marks may be provided on the sliding contact surfaces of the piston skirts  12 . In addition, the base layer  24  may be provided selectively on the linear marks, whereas the solid lubricators  30  may be provided selectively only on the base layer  24 . Alternatively, as shown in  FIG. 6 , a plurality of protrusive linear ridges  32 , which extend around the sliding contact surfaces, may be provided on the base layer  24 , and the solid lubricators  30  may be provided in a linear shape or a dot shape on the ridges  32 . 
         [0068]    In the above embodiment, the base layer  24  is formed by supplying the melted material to the sliding contact surfaces of the piston skirts  12 , and then cooling and solidifying the melted material, after which the base layer  24  is coated with the paste in order to form the solid lubricators  30 . However, the present invention is not limited to such a process. Alternatively, before the melted material is cooled and solidified, the melted material may be coated with the paste in order to form the solid lubricators  30 . 
       EXAMPLES  
     Inventive Example  
       [0069]    A test piece  34  shown in  FIG. 7  was fabricated, and a peel test was conducted on the test piece  34 . The test piece  34  had a laminated body  42  made of a base layer  38  and a solid lubricator  40 . The laminated body  42  was disposed on the surface of an aluminum alloy sheet  36 , which was formed in a sheet-like shape having a length of 25 mm, a depth of 25 mm, and a height of 5 mm. An aluminum alloy sheet  46 , which was formed in the same manner as the aluminum alloy sheet  36 , was joined to the laminated body  42  by an interposed adhesive  44 . 
         [0070]    More specifically, a melted material, which was produced by melting a resin material  48  of polyamide imide (PAI), was mixed with fibrous fillers  50  made of iron. At this time, the content of the fibrous fillers  50  in the resin material  48  was 10% by weight. 
         [0071]    The surface of the aluminum alloy sheet  36  was treated by shot peening, and thereafter, the surface was coated with the melted material made up of the mixture of the resin material  48  and the fibrous fillers  50 , which was supplied by spray coating. Using radiative cooling, the melted material was solidified into the base layer  38 . 
         [0072]    A paste, which was prepared by dispersing fine particles of silver in benzylic alcohol containing an unsaturated fatty acid ester as a dispersant, was supplied to the base layer  38  by screen printing, after which the entire piece was sintered at 210° C. for 2 hours. Thus, the laminated body  42 , in which the base layer  38  and the solid lubricator  40  were joined together by the fibrous fillers  50 , was obtained. The thickness of the base layer  38  was 10 μm, whereas the thickness of the solid lubricator  40  was 9μm. 
         [0073]    The solid lubricator  40  of the laminated body  42  was coated with the adhesive  44 , and the aluminum alloy sheet  46  was joined thereto, thereby fabricating the test piece  34 . 
         [0074]    The peel test was conducted by applying forces in the directions of the arrows X 1  and X 2  in  FIG. 7  to the aluminum alloy sheets  36 ,  46  of the test piece  34 , and confirming which one of the layers between the aluminum alloy sheets  36 ,  46  was peeled off. As a result, as indicated by the broken line in  FIG. 7 , it was confirmed that peel-off occurred between the solid lubricator  40  and the adhesive  44 , whereas the base layer  38  and the solid lubricator  40  remained suitably joined to each other. 
       Comparative Example 1  
       [0075]    As shown in  FIG. 8 , a test piece  52  was fabricated, and a peel test was conducted on the test piece  52 , in the same manner as the peel test that was performed on the test piece  34 . In  FIG. 8  and subsequent figures, components which are identical to those shown in  FIG. 7  are denoted by identical reference characters, and such features will not be described in detail below. 
         [0076]    The test piece  52  included a lubricating layer  54  instead of the laminated body  42  of the test piece  34 . In other words, the test piece  52  was fabricated in the same manner as in the Inventive Example, except for the process of fabricating the lubricating layer  54 . The lubricating layer  54  was obtained using a melted material, which was produced by melting a resin material of polytetrafluoroethylene (PTFE) and PAI, mixing the melted material with a solid lubricant of MoS 2  and C, supplying the mixed melted material to the aluminum alloy sheet  36  in the same manner as in the Inventive Example, and thereafter sintering the entire piece at 190° C. for 90 minutes. The content of the solid lubricant in the resin material was 10% by weight, and the thickness of the lubricating layer  54  was 22 μm. 
         [0077]    A peel test was conducted on the test piece  52 . As indicated by the broken line shown in  FIG. 8 , peel-off occurred between the lubricating layer  54  and the adhesive  44 . 
       Comparative Example 2  
       [0078]    As shown in  FIG. 9 , a test piece  56  was fabricated, and a peel test was conducted on the test piece  56  in the same manner as the peel test described above. More specifically, shot peening was not performed on the surface of the aluminum alloy sheet  36 , and a melted material, which was produced by melting a PAI resin, was supplied to the aluminum alloy sheet  36  by screen printing, thereby producing a base layer  58 . Next, a paste of fine particles of silver and a dispersant, which was prepared in the same manner as in the Inventive Example, was supplied to the base layer  58  by screen printing. Thereafter, the test piece  56  was fabricated by the same process used in the Inventive Example. The thickness of the base layer  58  was 3 μm. 
         [0079]    A peel test was conducted on the test piece  56 . As indicated by the broken line shown in  FIG. 9 , peel-off occurred between the base layer  58  and the solid lubricator  40 . 
       Comparative Example 3  
       [0080]    As shown in  FIG. 10 , a test piece  60  was fabricated, and a peel test was conducted on the test piece  60  in the same manner as the peel test described above. The test piece  60  included a base layer  62  instead of the base layer  38  of the test piece  34 . The test piece  60  was fabricated in the same manner as the test piece  34 , except for the process of forming the base layer  62 . More specifically, in order to form the base layer  62  of the test piece  60 , a melted material produced by melting a resin material of PAI was mixed with a solid lubricant of MoS 2  and C, and thereafter, the mixture was supplied to the surface of the aluminum alloy sheet  36 , which had been treated by shot peening. The content of the solid lubricant in the resin material was 10% by weight. 
         [0081]    A peel test was conducted on the test piece  60 . As indicated by the broken line shown in  FIG. 10 , peel-off occurred between the base layer  62  and the solid lubricator  40 . 
       Comparative Example 4  
       [0082]    As shown in  FIG. 11 , a test piece  64  was fabricated, and a peel test was conducted on the test piece  64  in the same manner as the peel test described above. The test piece  64  included a base layer  66  instead of the base layer  38  of the test piece  34 . The test piece  64  was fabricated in the same manner as the test piece  34 , except for the process of forming the base layer  66 . More specifically, in order to form the base layer  66  of the test piece  64 , a melted material produced by melting a resin material of PAI was mixed with a solid lubricant of C, and thereafter, the mixture was supplied to the surface of the aluminum alloy sheet  36 , which had been treated by shot peening. The content of the solid lubricant in the resin material was 10% by weight. 
         [0083]    A peel test was conducted on the test piece  64 . As indicated by the broken line shown in  FIG. 11 , peel-off occurred between the base layer  66  and the solid lubricator  40 . 
         [0084]    In peel tests performed on the Inventive Example and on Comparative Examples 1 through 4, shear strengths upon the occurrence of peel-off were measured. The shear strengths of the Inventive Example and Comparative Examples 2 through 4, which included the base layer and the solid lubricator, were substantially twice the shear strength of Comparative Example 1, which included PTFE but did not include a base layer. 
         [0085]    As can be understood from a comparison of  FIGS. 8 through 11 , Comparative Examples 2 through 4, which are free of fibrous fillers between the base layer and the solid lubricator, exhibited the occurrence of peal-off between the base layer and the solid lubricator, whereas in the Inventive Example, which includes the fibrous fillers  50 , the base layer  38  and the solid lubricator  40  remained in a suitably joined condition while exhibiting substantially the same shear strength as in Comparative Examples 2 through 4. 
         [0086]    Accordingly, it was confirmed that the bonding strength between the base layer  38  and the solid lubricator  40  is increased by the fibrous fillers  50 , which are provided between the base layer  38  and the solid lubricator  40 . Such a feature implies that the existence of the fibrous fillers  50  fortifies the bond between the base layer  38  and the solid lubricator  40 , so that it is extremely difficult for interlayer peel-off to occur between the base layer  38  and the solid lubricator  40 . 
         [0087]    From the foregoing description, it is clear that the fibrous fillers  50 , which lie within and extend between the base layer  38  and the solid lubricator  40 , make it less likely for the solid lubricator to come off from the piston skirts, and as a result, the lubricant can be maintained suitably between the inner wall surface of the cylinder and the piston skirts.