Sliding component, method for producing sliding component, and device for producing sliding component

A piston shoe as a sliding component includes a base section, which is made of steel, and a sliding section having a sliding surface, which is made of copper alloy and joined to the base section. The base section and the sliding section are joined, with a base section joint region being formed in the base section, the base section joint region including a base section joint surface that is a surface joined to the sliding section and having smaller grains than other regions in the base section.

TECHNICAL FIELD

The present invention relates to sliding components, methods for producing sliding components, and devices for producing sliding components. More specifically, the present invention relates to a sliding component which includes a sliding section made of copper alloy and a base section made of steel or cast iron and joined to the sliding section, a method for producing the sliding component, and a device for producing the sliding component.

BACKGROUND ART

As a sliding component which slides with respect to another component, one having a structure in which a sliding section made of copper alloy and having a sliding surface is fixed to a base section made of steel or cast iron may be used. For example, as a piston shoe of a hydraulic pump or a hydraulic motor, one having a base section made of steel to which a sliding section made of copper alloy is fixed is known. As a piston shoe of this type, one in which the sliding section is fixed to the base section by caulking may be used.

In order for the sliding section to be fixed to the base section by caulking, however, the sliding section needs to be machined to a predetermined shape enabling the caulking, before being attached to the base section. This increases the production cost of the sliding component due to the expense required for machining the sliding section. On the other hand, a piston shoe in which the sliding section is fixed to the base section by pressing the sliding section against the base section so that the sliding section is deformed and thus engaged with the base section has been proposed (see, for example, Japanese Patent Application Laid-Open No. H10-89241 (Patent Literature 1)).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

With the structure of the piston shoe described in the Patent Literature 1 above, however, the sliding section is merely fixed to the base section by engagement. If the piston shoe receives an impact, for example, the fixed state of the sliding section to the base section may become unstable.

The present invention has been accomplished to solve the above-described problem, and an object of the present invention is to provide a sliding component having the sliding section stably fixed to the base section.

Solution to Problem

A sliding component according to the present invention is a sliding component that has a sliding surface. The sliding component includes a base section made of steel or cast iron, and a sliding section having the sliding surface, made of copper alloy, and joined to the base section. The base section and the sliding section are joined, with a base section joint region being formed in the base section, the base section joint region having smaller grains than other regions in the base section and including a base section joint surface that is a surface joined to the sliding section.

In the sliding component of the present invention, the base section and the sliding section are joined while the base section joint region including the base section joint surface and having smaller grains than other regions in the base section is formed. That is, the sliding component of the present invention has a structure in which the sliding section is joined with the base section joint region which is excellent in toughness because of the small grains therein. As a result, even when the sliding component receives an impact, for example, the sliding section is stably fixed to the base section. Therefore, according to the sliding component of the present invention, it is possible to provide the sliding component having the sliding section stably fixed to the base section.

In the above-described sliding component, the base section joint region may have a thickness, in a direction perpendicular to the base section joint surface, that is greater in a region including a surface of the sliding component than in the inside of the sliding component. This ensures that, in the vicinity of the joint surface, high toughness is more reliably imparted to the surface region of the base section from which cracking may start.

In the above-described sliding component, a sliding section joint region may be formed in the sliding section, the sliding section joint region having a lower hardness than other regions in the sliding section and including a sliding section joint surface that is a surface joined to the base section. This can relieve the strain in the joint portion between the sliding section and the base section.

In the above-described sliding component, the sliding section joint region may have a thickness of 0.2 mm or less in a direction perpendicular to the sliding section joint surface. By making the sliding section joint region no thicker than necessary, it is possible to impart sufficient strength to the sliding section.

In the above-described sliding component, the copper alloy may be high-strength brass. The high-strength brass is a material which has high strength and excellent sliding characteristics, and is suitable as the material constituting the sliding section.

In the above-described sliding component, the high-strength brass constituting the sliding section may include precipitates having a higher hardness than a matrix, and the precipitates in the sliding section joint region may be smaller in size than the precipitates in other regions in the sliding section. This can improve the toughness of the sliding section in the vicinity of the joint portion.

In the above-described sliding component, a precipitate aggregate as an aggregate of the precipitates may be formed in a region, within the sliding section joint region, that is in contact with the sliding section joint surface. The aggregate of fine precipitates formed in the vicinity of the sliding section joint surface can improve the strength in the vicinity of the sliding section joint surface, without significantly decreasing toughness.

In the above-described sliding component, the sliding section joint region may have a higher volume fraction of α phase than other regions in the sliding section. This can improve the toughness of the sliding section in the vicinity of the joint portion.

A method for producing a sliding component according to the present invention includes the steps of: preparing a base member made of steel or cast iron and a sliding member made of copper alloy; heating a region, within the base member, including a base member contact surface that is a surface of the base member coming into contact with the sliding member to a temperature not lower than the A1transformation point by causing the base member brought into contact with the sliding member to slide relatively to the sliding member to generate frictional heat; and joining the base member and the sliding member by letting the region including the base member contact surface cooled to a temperature lower than the A1transformation point in the state where the heated base member is held in contact with the sliding member.

In the method for producing a sliding component of the present invention, the base member is brought into contact with the sliding member and caused to slide relatively to the sliding member to generate frictional heat, thereby heating the region including the base member contact surface to a temperature not lower than the A1transformation point. Thereafter, the base member and the sliding member are cooled while being held in contact with each other, whereby the base member and the sliding member are joined, with the grains being made finer in the region including the base member contact surface. With this configuration, it is readily possible to produce the inventive sliding component described above having the sliding section stably fixed to the base section.

In the above-described method for producing a sliding component, in the step of heating the region including the base member contact surface to a temperature not lower than the A1transformation point, the base member may rotate relatively to the sliding member while being pressed against the sliding member, without changing its position relative to the sliding member.

With this configuration, the frictional heat can be generated without changing the positional relationship between the sliding member and the base member. This facilitates joining of the sliding member and the base member in a desired positional relationship.

In the above-described method for producing a sliding component, in the step of heating the region including the base member contact surface to a temperature not lower than the A1transformation point and in the step of joining the base member and the sliding member, the sliding member may be restrained on an outer peripheral side of a sliding member contact surface that is a surface of the sliding member coming into contact with the base member.

With this configuration, the deformation amount of the softened sliding member is restricted. As a result, the work amount in the machining work after the joining is reduced, leading to an improved yield of the material of the sliding member. Even in the case where the sliding member is small in thickness, the plastically deformed region in the sliding member is prevented from being exposed to the sliding surface of the sliding component, ensuring stable sliding characteristics of the sliding component.

In the above-described method for producing a sliding component, the copper alloy may be high-strength brass. The high-strength brass is a material having high strength and excellent sliding characteristics, and is thus suitable as a material constituting the sliding member.

The above-described method for producing a sliding component may further include the step of forming, in a region, within the sliding member, that is in contact with the sliding member contact surface, a region having a higher volume fraction of α phase than other regions in the sliding member by heating the sliding member in the state where the base member and the sliding member are joined. This can improve the toughness in the vicinity of the joint portion.

A device for producing a sliding component according to the present invention is a device for producing a sliding component by joining a base member made of steel or cast iron and a sliding member made of copper alloy. This device for producing a sliding component includes: a spindle that is rotatable about an axis; a base portion that is arranged spaced apart from the spindle in the axial direction; and a spacing adjusting portion that adjusts a spacing between the spindle and the base portion. The spindle includes a first holding portion that holds one of the base member and the sliding member so as to face the base portion. The base portion includes a second holding portion that holds the other of the base member and the sliding member so as to face the first holding portion. The first holding member and the second holding member are arranged such that, in the state where the base member and the sliding member are brought into contact with each other with the spacing between the spindle and the base portion adjusted by the spacing adjusting portion, the first or second holding portion that holds the sliding member surrounds an outer periphery of a sliding member contact surface that is a surface of the sliding member coming into contact with the base member.

The sliding component of the present invention can readily be produced by using the device for producing a sliding component of the present invention to carry out the above-described method for producing a sliding component. More specifically, while the spindle is caused to rotate about the axis in the state where the base member and the sliding member are held by one and the other of the first and second holding portions, the base member is pressed against the sliding member with the spacing between the spindle and the base portion adjusted by the spacing adjusting portion, to thereby heat the base member and the sliding member. The base member and the sliding member are then cooled in the state where they are in contact with each other, whereby the base member and the sliding member are joined.

Here, in the device for producing a sliding component of the present invention, the outer periphery of the sliding member contact surface is surrounded by the first or second holding portion in the state where the base member and the sliding member are in contact with each other. Therefore, upon joining of the base member and the sliding member, the softened sliding member is restrained on the outer peripheral side. As a result, the deformation amount of the softened sliding member is restricted, and the work amount in the machining work after the joining is reduced, leading to an improved yield of the material of the sliding member. Even in the case where the sliding member is small in thickness, the plastically deformed region in the sliding member is prevented from being exposed to the sliding surface of the sliding component, ensuring stable sliding characteristics of the sliding component. As such, according to the device for producing a sliding component of the present invention, it is possible to produce the sliding component of the present invention while improving the yield of the material of the sliding member and stabilizing the sliding characteristics of the sliding component.

In the above-described method for producing a sliding component, at least one of the spindle and the base portion may be provided with a load sensor that detects a contact load between the base member and the sliding member. This facilitates adjustment of the contact load between the base member and the sliding member to an appropriate range.

Effects of the Invention

As is clear from the above description, according to the sliding component and the method for producing a sliding component of the present invention, it is possible to provide the sliding component having the sliding section stably fixed to the base section. Further, according to the device for producing a sliding component of the present invention, it is readily possible to produce the sliding component of the present invention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be described. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated.

FIG. 1is a schematic cross-sectional view showing the structure of a piston shoe as a sliding component according to an embodiment of the present invention. Referring toFIG. 1, the piston shoe1is a component which is connected to a piston body (not shown) of a hydraulic pump or a hydraulic motor and slides with respect to a swash plate. The piston shoe1includes a base section2, which is made of steel, and a sliding section3having a sliding surface31, which is made of copper alloy and joined to the base section2. For the steel constituting the base section2, for example, alloy steel for machine structural use (such as JIS SCM440) or carbon steel for machine structural use that has undergone thermal refining, or, quenching and tempering can be adopted. The base section2includes a spherical portion21of a spherical shape, which is to be swingably connected to the piston body, and a disk portion22of a stepped disk shape, which is connected to the spherical portion21.

The spherical portion21is swingably held by a holding portion (not shown) having a spherical inner wall, formed in the piston body. At an end of the spherical portion21opposite to the side connected to the disk portion22, a planar flat part21A is formed. At an end of the disk portion22opposite to the spherical portion21side, a planar base section joint surface23is formed.

The base section joint surface23is joined with the sliding section3, which is of a disk shape and smaller in thickness than the disk portion22. The sliding section3is joined, at one main surface constituting a sliding section joint surface32, to the base section joint surface23of the disk portion22. The other main surface of the sliding section3constitutes the sliding surface31. This sliding surface31slides with respect to a swash plate (not shown) of a hydraulic pump, for example. The sliding section3, made of copper alloy having excellent sliding characteristics, restricts the frictional force between the swash plate and the piston shoe1. For the copper alloy constituting the sliding section3, brass such as high-strength brass, as well as bronze such as aluminum bronze, can be adopted. In the present embodiment, the sliding section3is made of high-strength brass. Further, the sliding surface31has a plurality of annular grooves31A formed concentrically. These grooves31A hold a proper amount of oil, thereby further restricting the frictional force between the swash plate and the piston shoe1.

The piston shoe1has a shape which is symmetrical about the central axis A. The piston shoe1has a linear center hole29which is formed in the region including the central axis A to penetrate through the piston shoe1from the flat part21A of the spherical portion21to the sliding surface31of the sliding section3. The center hole29includes a first region29A, a second region29B, a third region29C, and a fourth region29D. The first region29A extends from the flat part21A in the direction of the sliding surface31. The second region29B is connected to the first region29A, and has a cross section perpendicular to the longitudinal direction (along the axis A) smaller in area than that of the first region29A. The third region29C is connected to the second region29B, and has a cross section perpendicular to the axial direction increasing in area as it approaches the sliding surface31. The fourth region is connected to the third region29C, and has a cross section perpendicular to the axial direction larger in area than that of the third region29C.

The structure near the joint portion between the base section2and the sliding section3will now be described.FIG. 2is an enlarged schematic cross-sectional view of the region II inFIG. 1. Referring toFIG. 2, the base section2is directly joined to the sliding section3, with a base section joint region24being formed in the base section2, the base section joint region24including the base section joint surface23and having smaller grains than other regions in the base section2.

In the piston shoe1of the present embodiment, the sliding section3is directly joined to the base section2in which the base section joint region24having smaller grains than other regions in the base section2has been formed. That is, the piston shoe1has a structure in which the base section joint region24excellent in toughness due to the small grains therein and the sliding section3are joined directly. Thus, the sliding section3is stably fixed to the base section2. As such, the piston shoe1is a sliding component having the sliding section3stably fixed to the base section2.

Here, referring toFIG. 2, the thickness t2of the base section joint region24in the region including a surface1A of the piston shoe1may be greater than the thickness t1of the base section joint region24in the inside. This ensures that, in the vicinity of the joint surface, high toughness is more reliably imparted to the surface region of the base section2from which cracking may start. In the present embodiment, the base section joint region24has its thickness increasing gradually as it approaches the surface1A of the piston shoe1.

Further, in the sliding section3, a sliding section joint region34having a lower hardness than other regions in the sliding section3may be formed to include the sliding section joint surface32, which is the surface joined to the base section2. This can relieve the strain in the joint portion between the sliding section3and the base section2.

It is preferable that the sliding section joint region34has a thickness of 0.2 mm or less in the direction perpendicular to the sliding section joint surface32. By making the sliding section joint region34no thicker than necessary, it is possible to impart sufficient strength to the sliding section3.

Further, the high-strength brass constituting the sliding section3may include precipitates having a higher hardness than the matrix, and the precipitates in the sliding section joint region34may be smaller in size than the precipitates in other regions in the sliding section3. This can improve the toughness of the sliding section in the vicinity of the joint portion.

Further, a precipitate aggregate as an aggregate of the precipitates may be formed in a region, within the sliding section joint region34, that is in contact with the sliding section joint surface32. The aggregate of fine precipitates formed in the vicinity of the sliding section joint surface32can improve the strength in the vicinity of the sliding section joint surface32, without significantly decreasing toughness.

Furthermore, the sliding section joint region34may have a higher volume fraction of α phase than other regions. This can improve the toughness of the sliding section3in the vicinity of the joint portion.

A method for producing the above-described piston shoe1will now be described.FIG. 3is a flowchart schematically illustrating the method for producing a piston shoe.FIG. 4is a schematic diagram showing the structure of a device for producing a piston shoe.FIG. 5is a schematic cross-sectional view showing the operation of the device for producing a piston shoe.FIG. 6is a schematic plan view showing the structure of a restraint jig included in the device for producing a piston shoe.

Referring toFIG. 3, in the method for producing the piston shoe1in the present embodiment, first, a formed-members preparation step is carried out as a step S10. In this step S10, referring toFIG. 5, a base member4, made of thermally refined alloy steel for machine structural use, and a disk-shaped sliding member5, made of high-strength brass, are prepared. The base member4includes a disk portion4B of a disk shape, and a cylindrical portion4C smaller in outer diameter than the disk portion, which is connected to the disk portion4B. At an end of the disk portion4B opposite to the cylindrical portion4C side, a base member contact surface4A is formed which is a flat surface to be joined to the sliding member5. One main surface of the sliding member5constitutes a sliding member contact surface5A which is a flat surface to be joined to the base member4.

Next, a cleaning step is carried out as a step S20. In this step S20, the base member4and the sliding member5prepared in the step S10are cleaned. More specifically, the base member4and the sliding member5are cleaned using methanol, ethanol, acetone, or other liquid. This removes any foreign matters attached to the base member4or the sliding member5during the cutting, machining, or other processes for preparing the base member4and the sliding member5. In the method for producing the piston shoe1in the present embodiment, precision finish work on the sliding member contact surface5A may be omitted; the sliding member contact surface5A may be left as cut, for example.

Next, referring toFIG. 3, an enclosed friction welding step is carried out. This enclosed friction welding step includes a joining preparation step, a friction step, and a cooling step. Here, a device for producing a piston shoe (sliding component) which produces the piston shoe by conducting enclosed friction welding will be described.

Referring toFIG. 4, an enclosed friction welding device9as the device for producing a piston shoe includes: a spindle95which is rotatable about an axis α, a base portion98disposed spaced apart from the spindle95in the axis α direction, a spacing adjusting portion97which adjusts the spacing between the spindle95and the base portion98, and a frame90which supports the spindle95and the base portion98.

The spindle95is provided with a chuck94which is a first holding portion for holding the base member4to face the base portion98. The spindle95is connected with a spindle motor95B which rotatively drives the spindle95about the axis α. The spindle95is further provided with a load sensor96which detects a contact load between the base member4and the sliding member5. The load sensor96detects the contact load between the base member4and the sliding member5from the magnitude of the contact reaction force between the base member4and the sliding member5that is applied to the chuck94. The load sensor96is not an indispensable component of the enclosed friction welding device9but, when provided, facilitates adjustment of the contact load between the base member4and the sliding member5to an appropriate range.

The base portion98is provided with a restraint jig93which is a second holding portion for holding the sliding member5to face the chuck94. More specifically, referring toFIGS. 4 and 5, the base portion98includes a base body91, a jig holder92, and the restraint jig93. The base body91is disposed on the frame90. The jig holder92is fixed onto the base body91. The restraint jig93is fixedly fitted in a jig holding portion92A which is a recessed portion formed in the jig holder92. The restraint jig93can be separated into two parts99,99, as shown inFIG. 6. Further, the restraint jig93has a holding portion93A which is a region where the sliding member5is held. In a planar view (as seen in the direction along the axis α), the holding portion93A has a polygonal shape, specifically a hexagonal shape, which circumscribes the outer peripheral surface of the disk-shaped sliding member5.

Referring toFIG. 4, inside the frame90, a shaft90A is disposed to extend in parallel with the axis α. This shaft90A supports a spindle support portion90C which supports the spindle95, so as to allow the spindle support portion90C to move in the direction in which the shaft90A extends. A spindle moving motor90B for driving the shaft90A is connected to the shaft90A. As the shaft90A is driven by the spindle moving motor90B, the spindle95supported by the spindle support portion90C moves in the axis α direction. This enables adjustment of the spacing between the spindle95and the base portion98. The shaft90A, the spindle support portion90C, and the spindle moving motor90B constitute the spacing adjusting portion97.

The chuck94and the restraint jig93are arranged such that, in the state (as shown inFIG. 5) where the base member4and the sliding member5are brought into contact with each other with the spacing between the spindle95and the base portion98adjusted by the spacing adjusting portion97, the restraint jig93serving as the second holding portion surrounds the outer periphery of the sliding member contact surface5A, which is the surface of the sliding member5coming into contact with the base member4. Stated from another point of view, referring toFIG. 5, the holding portion93A of the restraint jig93has a height in the axis α direction that is greater than the thickness of the sliding member5.

A specific procedure of the enclosed friction welding step will now be described.FIG. 7shows changes over time of the rotational speed of the spindle95, the contact load (pressing load) between the base member4and the sliding member5, and the temperature of the joint portion between the base member4and the sliding member5, during the enclosed friction welding step. Referring toFIGS. 4 and 5, in the joining preparation step carried out as a step S30, the base member4is held by the chuck94at the outer peripheral surface of the cylindrical portion4C, and the sliding member5is set in the holding portion93A of the restraint jig93. At this time, the base member4and the sliding member5are arranged such that the base member contact surface4A faces the sliding member contact surface5A and that the central axes of the base member4and the sliding member5agree with the rotational axis α of the chuck94.

Next, the friction step is carried out as a step S40. In this step S40, the spindle95is driven by the spindle motor95B to rotate about the axis α, and it is also driven by the spindle moving motor90B to approach the base portion98. Consequently, the chuck94approaches the restraint jig93while rotating about the axis α. At this time, referring toFIG. 7, the rotational speed of the spindle95, which started rotating at time S0, reaches a desired rotational speed at time S1, and is maintained at the desired rotational speed thereafter. Further, at time S2, the base member contact surface4A comes into contact with the sliding member contact surface5A, as shown inFIG. 5. Thus, the base member4rotates with respect to the sliding member5, while being pressed against the sliding member5with load L, without changing its position relative to the sliding member5. As a result, as shown inFIG. 7, the temperature of the contact portion (joint portion) between the base member4and the sliding member5increases due to the heat of friction. Then, at time S3, the pressing load (contact load between the base member contact surface4A and the sliding member contact surface5A) detected by the load sensor96reaches a desired level, and is maintained at the desired level thereafter. During this time, the temperature of the contact portion between the base member4and the sliding member5continues to increase.

Then, at time S4, the temperature of the contact portion between the base member4and the sliding member5reaches a temperature that is not lower than the A1transformation point and lower than the solidus temperature. As a result, a region, within the base member4, that includes the base member contact surface4A is heated to a temperature not lower than the A1transformation point and lower than the solidus temperature, and the steel constituting that region attains the austenite state including no liquid phase.

On the other hand, the heated sliding member5softens and deforms to fill in gaps93B between the sliding member5and the restraint jig93(seeFIG. 6). As a result, even when the base member4rotates about the axis α, the sliding member5will not rotate in accordance therewith.

Next, the cooling step is carried out as a step S50. In this step S50, first, the rotational speed of the spindle95is lowered, and the rotation is stopped at time S5. Thereafter, the pressing load detected by the load sensor96is decreased. During this time, the contact portion between the base member4and the sliding member5is cooled, with the base member4and the sliding member5being maintained in the state of pressing each other. Accordingly, the base member4and the sliding member5are joined. Then, at time S6, the pressing load is set to zero, and the structural body configured with the base member4and the sliding member5joined to each other is taken out from the enclosed friction welding device9.

Here, the region within the base member4including the base member contact surface4A, which was heated to a temperature not lower than the A1transformation point in the step S40, is cooled to a temperature lower than the A1transformation point in the step S50. In such a region that was once heated to a temperature not lower than the A1transformation point and then cooled to a temperature lower than the A1transformation point, the grains become finer. As a result, the base section joint region24having smaller grains than the other regions is formed (seeFIG. 2). The enclosed friction welding step is completed through the above-described procedure.

Next, a machining step is carried out as a step S60. In this step S60, the structural body obtained in the step S50is subjected to machining. Specifically, referring toFIG. 1, the outer peripheral surface of the sliding member5is machined to form the disk-shaped sliding section3. Further, the cylindrical portion of the base member4is machined to form the spherical portion21. The center hole29, the flat part21A, and the grooves31A are also formed in this step.

Next, a gas nitrocarburizing step is carried out as a step S70. In this step S70, referring toFIG. 1, the gas nitrocarburizing processing is carried out in the state where the spherical portion21formed in the step S60is fitted in a holding portion (not shown) having a spherical inner wall, formed in a piston body prepared separately. Specifically, while being heated within an atmosphere including ammonia gas to a temperature lower than the A1transformation point, nitrided layers are formed in the surface portions of the base member4(base section2) and the piston body (not shown). At this time, with the heating for the gas nitrocarburizing processing, a region having a higher volume fraction of α phase than other regions is formed in a region, within the sliding member5, that is in contact with the sliding member contact surface5A. Accordingly, referring toFIG. 2, the volume fraction of the α phase in the sliding section joint region34becomes higher than in the other regions.

Next, a finishing step is carried out as a step S80. In this step, the base member4, the sliding member5, and the piston body (not shown), which have undergone the gas nitrocarburizing processing in the step S70, are subjected to finishing processing as required. Through the above procedure, the piston shoe1in the present embodiment is completed in the state of being combined with the piston body.

As described above, according to the method for producing a piston shoe in the present embodiment, the piston shoe1of the present embodiment described above can be produced. Here, the friction step performed as the step S40can be carried out for example by causing the base member4to reciprocate relatively to the sliding member5. However, causing the base member4to rotate without changing its position relative to the sliding member5facilitates joining of the sliding member5and the base member4in a desired positional relationship.

Further, referring toFIG. 6, as the base member4rotates, in the step S40, without changing its position relative to the sliding member, the circumferential velocity of the base member4with respect to the sliding member5increases with increasing distance from the axis α. Therefore, the heat produced by friction increases on the outer peripheral side of the base member4. As a result, the region within the base member4where the temperature exceeds the A1transformation point due to the frictional heat becomes larger in thickness on the outer peripheral side of the base member4. Accordingly, referring toFIG. 2, the base section joint region24in which the grains are smaller than in the other regions can be made thicker on the outer peripheral side, i.e. in the region including the surface1A of the piston shoe1, than in the inside. Further, referring toFIG. 6, the cylindrical portion4C of the base member4in the present embodiment described above has an outer diameter smaller than that of the disk portion4B. This makes it difficult for the frictional heat produced in the outer peripheral portion of the base member contact surface4A to be transmitted to the cylindrical portion4C. As a result, the region within the base member4where the temperature exceeds the A1transformation point due to the frictional heat becomes still larger in thickness on the outer peripheral side of the base member4. Therefore, according to the method for producing the piston shoe1in the present embodiment, it is readily possible to increase the thickness of the base section joint region24on the outer peripheral side, i.e. in the region including the surface1A of the piston shoe1, than in the inside.

Further, in the method for producing the piston shoe1in the present embodiment, referring toFIG. 5, the height of the holding portion93A in the axis α, direction is greater than the thickness of the sliding member5. As a result, in the steps S40and S50, the state where the sliding member5is restrained on the outer peripheral side of the sliding member contact surface5A is maintained. This can restrict the deformation amount of the softened sliding member5. More specifically, in the piston shoe1produced, the sliding section joint region34formed by deformation of the sliding member5can be made to have the thickness of 0.2 mm or less in the direction perpendicular to the sliding section joint surface32. As a result, the work amount in the machining work after the joining is reduced, leading to an improved yield of the material of the sliding member5. Even in the case where the sliding member5is small in thickness, the plastically deformed region in the sliding member5is prevented from being exposed to the sliding surface31of the piston shoe1, ensuring stable sliding characteristics of the sliding section3. Further, the sliding section joint region34having a low hardness is made no thicker than it needs to be, whereby sufficient strength can be imparted to the sliding section3.

Here, referring toFIG. 6, the softened sliding member5deforms to fill in the gaps93B between the sliding member5and the restraint jig93. This means that adjusting the gaps93B to an appropriate size can restrict the deformation amount of the sliding member5.

While the case where the base member moves (rotates) while the sliding member is fixed has been described in the above embodiment, the method for producing a sliding component in the present invention is not limited thereto; the sliding member may move (rotate) while the base member is fixed, or both members may move (rotate) so that one slides relatively to the other.

Further, in the above embodiment, the enclosed friction welding device9(device for producing the sliding component) was explained as the structure in which the spindle is movable in the axial direction. The device for producing a sliding component in the present invention, however, is not limited thereto; the base portion may be movable in the axial direction.

Furthermore, in the above embodiment, the case where the holding portion93A of the restraint jig93is of a hexagonal shape in a planar view (as seen in the direction along the axis α) was explained. The restraint jig adoptable, however, is not limited thereto; the holding portion may be of another polygonal shape (for example, octagonal shape), or it may be of a circular shape having a diameter slightly larger than that of the sliding member5.

In the above embodiment, the piston shoe was explained as an example of the sliding component. The sliding component of the present invention, however, is not limited thereto; the present invention is applicable to a variety of sliding components configured with a base section made of steel or cast iron and a sliding section made of copper alloy joined together. Further, while the base member (base section) was made of steel in the above embodiment, the base member (base section) may be made of cast iron.

EXAMPLES

A base member and a sliding member having shapes similar to those of the above embodiment were prepared (seeFIG. 5), and the procedure in the above embodiment except for combining with a piston body was carried out to create a test piece (piston shoe) having the base member and the sliding member joined by enclosed friction welding. The test piece was then subjected to experiments for measuring the structure and hardness thereof. The results of the experiments are as follows.

FIG. 8are photographs showing the metallic structure near the joint portion between the base section2(steel) and the sliding section3(high-strength brass). More specifically,FIG. 8shows the state where the obtained test piece was cut in a cross section perpendicular to the joint surface and etched with a ferric chloride solution.FIG. 9are optical micrographs showing, in enlarged view, the regions A-1to A-4, B-1to B-4, C-1to C-4, and D-1to D-4inFIG. 8. The numerical value at the upper right of each micrograph inFIG. 9indicates the grain size (in μm) in that micrograph.

Referring toFIG. 8, it is confirmed that the base section2and the sliding section3have been joined favorably over the whole area. Further, there is a deeply etched region, shown in dark color, in the base section2along the joint interface.

Referring toFIG. 9, the grain size in each of the regions A-3, B-3, C-3, and D-3corresponding to the dark-colored region is smaller than the grain size in each of the regions A-4, B-4, C-4, and D-4corresponding to the region other than the dark-colored region.

From this, it is confirmed that in the piston shoe produced in a similar manner as in the above embodiment, a base section joint region is formed in the base section, which includes the base section joint surface and has smaller grains than other regions in the base section. Further, it is confirmed that the thickness of the base section joint region (dark-colored region) in the direction perpendicular to the joint surface becomes greater in the region including the surface than in the inside of the piston shoe which is the sliding component (see the photograph in the upper part ofFIG. 8).

The hardness of each of the regions A, B, C, and D inFIG. 8was measured in the direction perpendicular to the joint interface. The measurement results are shown inFIG. 10. InFIG. 10, the horizontal axis represents distance from the joint interface. The point where the value on the horizontal axis is zero corresponds to the joint interface. InFIG. 10, the region where the value on the horizontal axis is negative corresponds to the base section2(steel), and the region where the value is positive corresponds to the sliding section3(high-strength brass). Referring toFIG. 10, the hardness is discontinuous at the joint interface as the boundary. From this, it is confirmed that the steel constituting the base section2and the high-strength brass constituting the sliding section3have been directly joined favorably, without the intermediary of any other substance.

FIG. 11shows hardness distribution in the sliding section3in the vicinity of the joint interface. Referring toFIG. 11, in the region where the distance from the joint interface is 100 μm or less, the hardness decreases as the distance from the joint interface decreases. That is, it is confirmed from the measurement results inFIG. 11that in the sliding section, a sliding section joint region has been formed which includes the sliding section joint surface and has a lower hardness than other regions in the sliding section. The correspondence between the sliding section joint region, which is the low-hardness region, and the metallic structure will be described below.

FIG. 12is an optical micrograph illustrating the structure of the copper alloy.FIG. 13are optical micrographs showing the metallic structure in the regions A-1, A-2, B-1, B-2, C-1, C-2, D-1, and D-2inFIG. 8, or, the metallic structure of the high-strength brass constituting the sliding section. Referring toFIG. 12, the high-strength brass constituting the sliding section includes α phase71and β phase72constituting a matrix, and precipitates73having a higher hardness than the matrix. Referring toFIG. 13, it is seen that the precipitates in the sliding section joint region (in the vicinity of the joint interface in A-2, B-2, C-2, and D-2) are smaller in size than the precipitates in other regions (for example, in A-1, B-1, C-1, and D-1) in the sliding section. That is, the sliding section joint region which is a low-hardness region corresponds to the region in which the size of the precipitates is small. Further, as apparent fromFIG. 13, the thickness of the sliding section joint region is 0.2 mm or less.

Further, referring toFIG. 13, it is confirmed that a precipitate aggregate73A as an aggregate of precipitates has been formed in the region, within the sliding section joint region, that is in contact with the sliding section joint surface. Referring also toFIG. 13, it is confirmed that there is almost no β phase in the sliding section joint region (in the vicinity of the joint interface in A-2, B-2, C-2, and D-2) and that the volume fraction of the α phase in that region is higher than in other regions. Such a structure of the sliding section is considered to be formed in the following manner.

FIG. 14is a schematic diagram showing the distribution state of precipitates within the sliding section of the piston shoe. In the friction step in the enclosed friction welding explained in the above embodiment, the base member4rotates while being pressed against the sliding member5. Referring toFIG. 14, there exist large-sized precipitates73B and small-sized precipitates73C in the sliding member5made of high-strength brass. Before initiation of the friction step, the large-sized precipitates73B and the small-sized precipitates73C are mixed and distributed uniformly. At the initial stage of the friction step, the region in the vicinity of the contact surface is heated by frictional heat.

Thereafter, a contact-surface vicinity region82of the sliding member5corresponding to the sliding section joint region softens by the frictional heat over time. As a result, the contact-surface vicinity region82starts deformation because of the contact load between the base member4and the sliding member5. As the deformation proceeds, the large-sized precipitates73B are broken into small-sized precipitates73C. Consequently, in the contact-surface vicinity region82, the large-sized precipitates73B decrease, and the volume fraction of the small-sized precipitates73C increases. At this time, part of the deformed sliding member5is adhered to the outer peripheral surface of the base member4as a small amount of burr59.

As the time further passes, the contact-surface vicinity region82that has softened by the frictional heat deforms completely and, as shown inFIG. 14, the small-sized precipitates73C are closely packed in the deformed contact-surface vicinity region82. It is thus considered that the precipitate aggregate73A as the aggregate of fine precipitates is formed in that region within the sliding section joint region which is in contact with the sliding section joint surface (in the vicinity of the joint interface in A-2, B-2, C-2, and D-2). It is also considered that, as the plastically deformed contact-surface vicinity region82is heated again in the nitrocarburizing step, the sliding section joint region having a higher volume fraction of α phase than other regions is formed.

A test piece was prepared by joining a base member made of steel (JIS SCM440H) and a sliding member made of high-strength brass by enclosed friction welding, through the procedure similar to that in the present embodiment. The test piece was subjected to a shear test to perform an experiment for confirming the strength of the joint portion. The test was conducted in the following manner.

FIG. 15is a schematic cross-sectional view illustrating the method for testing shear strength. Referring toFIG. 15, a test piece50has a structure in which a base member51made of steel and a sliding member52made of high-strength brass are joined by enclosed friction welding, as in the above embodiment. A shear testing device60includes a main body61having a test-piece holding portion61A which is a recessed portion for holding a test piece, and a load applying portion62which applies load to the test piece50. The test-piece holding portion61A has a shape that receives the base member51alone, causing only the sliding member52to protrude from the test-piece holding portion61A. The load applying portion62can be lowered in the direction indicated by an arrow γ, to apply load in the direction along the joint interface between the base member51and the sliding member52. Two test pieces were prepared, and the stress (shear stress) at the time point when the respective one of the test pieces was broken was calculated (Examples A and B). Further, for comparison, test pieces were also prepared by placing a powder compact between a base member51and a sliding member52and sinter-bonding the base member51and the sliding member52. The test pieces were subjected to a similar test (Comparative Examples A and B).

FIG. 16shows the results of the shear strength test. Referring toFIG. 16, while the shear strength of the comparative examples having the structure similar to that of a conventional piston shoe was 25.2 kgf/mm2, the shear strength of the examples produced by enclosed friction welding was 29.2 to 34.4 kgf/mm2. Such strength is far greater than that of a conventional one, and is comparable to that of the base material. From the experimental results described above, it has been confirmed that according to the method for producing a sliding member in the present invention, it is possible to provide a sliding component having the sliding section firmly fixed to the base section.

INDUSTRIAL APPLICABILITY

The sliding component, the method for producing a sliding component, and the device for producing a sliding component of the present invention are applicable particularly advantageously to the sliding component having a base section made of steel or cast iron and a sliding section made of copper alloy and to the production thereof.

DESCRIPTION OF REFERENCE NUMERALS