Compressors

Abstract

Compressors may preferably include a swash plate coupled to a drive shaft, so that the swash plate will rotate in response to rotation of the drive shaft. A piston is preferably disposed within a cylinder bore. A shoe preferably couples the piston to the swash plate, so the piston will reciprocate within the cylinder bore in order to compress a refrigerant in response to rotation of the swash plate. The shoe preferably comprises a main body, a metallic hard layer at least partially disposed on the main body and an anti-friction layer at least partially disposed on the metallic hard layer. The main body preferably comprises a spherical surface portion and a substantially flat surface portion and comprises aluminum. The metallic hard layer preferably has a hardness of at least HV 300 based upon the Vickers hardness scale and is disposed on at least one of the spherical surface portion and the substantially flat surface portion. The anti-friction layer preferably comprises a metal and a solid lubricating agent. The solid lubricating agent may be, for example, molybdenum disulfide (MoS2), boron nitride (BN), tungsten disulfide (WS2), graphite or polytetrafluoroethylene. The metal within the anti-friction layer may be, for example, nickel phosphorus, nickel boron, nickel phosphorus boron tungsten, cobalt phosphorus or hard chromium plating

Description

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to compressors having a plurality of shoes that connect a swash plate to a plurality of pistons, which pistons reciprocate in response to rotation of the swash plate in order to compress a fluid and more particularly, relates to compressors having highly durable and low-friction shoes.

[0003] 2. Description of the Related Art

[0004] Within a swash plate type compressor, fluid is drawn into the compressor and then compressed by a reciprocating piston disposed within a cylinder bore. A shoe couples the piston to the swash plate in order to convert rotational movement of the swash plate into linear reciprocation of the piston. Thus, when the swash plate rotates, the piston reciprocates within the cylinder bore and fluid drawn into the cylinder bore is compressed and discharged from the compressor.

[0005] An example of a known shoe is disclosed in Japanese Laid-open Patent Publication No. 10-205442. This known shoe is made of aluminum and has a tin-plated layer. A coating layer that includes a solid lubricating agent is provided on the tin-plated layer in order to reduce friction between the shoe and another functional member, such as the rotating swash plate and the reciprocating piston.

SUMMARY OF THE INVENTION

[0006] It is one object of the present teachings to provide improved compressors. In one aspect of the present teachings, improved shoes are taught that are preferably highly durable and relatively low friction. Therefore, the shoes provide improved performance when connecting a piston to a swash plate.

[0007] In one embodiment of the present teachings, compressors may preferably include a drive shaft, a swash plate, a piston and a shoe. The swash plate may be coupled to the drive shaft so as to rotate in response to rotation of the drive shaft. The piston is preferably disposed within a cylinder bore of the compressor. The shoe preferably connects an end portion of the piston to a peripheral edge of the swash plate. Thus, the piston will linearly reciprocate within the cylinder bore and compress a fluid or refrigerant gas disposed within the cylinder bore in response to rotation of the inclined swash plate.

[0008] In another embodiment of the present teachings, the shoe preferably includes a main body, a metallic hard layer and an anti-friction layer. The main body may include first surface portion that is substantially spherical in cross-section and a second surface portion that is substantially flat in cross-section. Preferably, the main body comprises aluminum or an aluminum alloy. The metallic hard layer is preferably disposed around the main body and has a hardness that is greater than HV 300 on the Vickers hardness scale. For example, the metallic hard layer may be provided on the spherical surface portion and/or the flat surface portion. The anti-friction layer may be disposed around the metallic hard layer and preferably comprises a mixture of a metal and a solid lubricating agent. For example, the anti-friction layer may be provided over the metallic hard layer on the spherical surface portion and/or the flat surface portion.

[0009] According to this embodiment of the present teachings, relatively lightweight shoes can constructed, because the main body is made of or comprises aluminum. Further, the shoes may exhibit high durability and good anti-friction performance, because the metallic hard layer is provided on the main body and the anti-friction layer is provided on the surface of the metallic hard layer.

[0010] Other objects, features and advantage of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a representative compressor.

[0012] FIG. 2 shows a front cross-sectional view of a first representative shoe that is adapted for use with the representative compressor.

[0013] FIG. 3 shows a partially magnified front cross-sectional view of the first representative shoe.

[0014] FIG. 4 shows a front cross-sectional view of a second representative shoe.

[0015] FIG. 5 shows a front cross-sectional view of a third representative shoe.

[0016] FIG. 6 shows a partially magnified front cross-sectional view of a fourth representative shoe.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferably, the main body of a compressor shoe may be made of aluminum or an aluminum alloy in order to reduce the weight of the shoe. The metallic hard layer may be provided on the spherical surface portion and/or the flat surface portion of the main body. Further, the metallic hard layer may preferably have a hardness that is greater than HV 300 on the Vickers hardness scale in order to increase the durability of the shoe.

[0018] In addition, the anti-friction layer may be made of or comprise a mixture of a metal and a solid lubricating agent. The anti-friction layer may be provided on the spherical surface portion and/or the flat surface portion of the main body of the shoe and is preferably disposed over the metallic hard layer. The anti-friction layer preferably imparts low-friction properties to the surface. Therefore, when rotational movement of the swash plate is converted to linear reciprocal movement of the piston, the shoes of the present teachings permit the piston to slide more freely relative to the swash plate.

[0019] Preferably, the swash plate may inclinably be coupled to the drive shaft and the inclination angle of the swash plate may be changed during operation in order to change the compressor output discharge capacity.

[0020] In another embodiment of the present teachings, the solid lubricating agent may preferably be selected from one or more of molybdenum disulfide (MoS2), boron nitride (BN), tungsten disulfide (WS2), graphite or polytetrafluoroethylene. Furthermore, the metal of the anti-friction layer may preferably be selected from one or more of nickel boron (Ni—B), nickel phosphorus boron tungsten (Ni—P—B—W), cobalt phosphorus (Co—P) or a hard chromium plated layer.

[0021] In another embodiment of the present teachings, the hard metallic layer also may preferably comprise at least one solid lubricating agent in order to further improve the anti-friction properties of the shoe. For example, if the anti-friction layer wears out, the lubricating material (e.g., the solid lubricating agent) within the hard metallic layer will emerge to the surface and maintain the anti-friction performance of the shoe.

[0022] Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide improved compressors and air conditioning systems and methods for making and using such compressors and air conditioning systems. Representative examples of the present invention, which examples utilize many of these additional features and method steps in conjunction, will now be described in detail with reference to the drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe some representative examples of the invention, which detailed description will now be given with reference to the accompanying drawings.

[0023] A first representative embodiment will now be described in further detail with reference to FIGS. 1 to 3. The representative compressor 1 shown in FIG. 1 is known as a variable displacement compressor and may include a compressor housing defined by a front housing 16, a cylinder block 10 and a rear housing 18. The front housing 16 is coupled to the front end of the cylinder block 10. The rear housing 18 is coupled to the rear end of the cylinder block 10. A valve plate 20 is provided between the cylinder block 10 and the rear housing 18.

[0024] A crank chamber 86 is defined by a space within the front housing 16. A drive shaft 50 is rotatably supported within the crank chamber 86. At the center of the cylinder block 10, a bearing is disposed within a bearing storing chamber 56 and one end of the drive shaft 50 is supported by the bearing within the bearing storing chamber 56. Although not particularly shown in the drawings, the drive shaft 50 is preferably connected to an automotive engine via an electromagnetic clutch. In that case, the engine will cause the drive shaft 50 to rotate when clutch mechanism couples the driving force of the engine to the drive shaft 50.

[0025] Within the crank chamber 86, a rotating swash plate 60 is inclinably and slidably coupled to the drive shaft 50 via a rotor 62. The rotor 62 is coupled to the drive shaft 50 and can rotate together with the drive shaft 50. The rotor 62 is rotatably supported to the front housing 16 by means of a thrust bearing 64. The drive shaft 50 extends through a penetration hole 61 defined in the center of the swash plate 60. A hinge mechanism 66 is disposed between the rotor 62 and the swash plate 60 in order to transmit torque from the drive shaft 50 to the swash plate 60. Preferably, the swash plate 60 can rotate at a variety of inclination angles.

[0026] The hinge mechanism 20 preferably includes a support arm 67, which serves as a guide member disposed on the rotor 62, and a guide pin 69 disposed on the swash plate 60. Thus, the guide pin 69 is disposed within a guide hole 68 of the support arm 67. Further, the support arm 67 and the guide pin 69 are mutually engaged to connect the swash plate 60 with the rotor 62.

[0027] The cylinder block 10 preferably includes six cylinder bores 12 in which six pistons 14 are respectively disposed. However, FIG. 1 only shows one piston 14 for purposes of illustration. Each piston 14 is reciprocally and slidably supported within each cylinder bore 12. Each piston 14 has a piston head 72 and an engaging portion 70. The engaging portion 70 is substantially U-shape in cross-section and further includes arms 120, 122 and a connector 124. The arms 120 extend in parallel in a direction perpendicular to the center axis of the piston head 72. The connector 124 connects both the arms 120 and 124. A concave-shaped spherical surface 128 is provided on each side of the arms 120, 122. A shoe 76 is coupled to the engaging portion 70 by contacting each spherical surface 128. The shoe 76 is also coupled to the peripheral portion of the swash plate 60. Thus, the piston 14 is coupled to the swash plate 60 via the shoe 76. The rotational movement of the swash plate 60 is converted into reciprocating movement of the pistons 14 via the shoe 76.

[0028] A suction chamber 22 and a discharge chamber 24 are respectively defined by spaces within the rear housing 18. A suction port 32, a suction valve 34, a discharge port 36, and a discharge valve 38 are preferably disposed on the valve plate 20. The suction chamber 22 has an inlet 26 and the discharge chamber 24 has an outlet 28. Both the inlet 26 and the outlet 28 are connected to an air conditioning system disposed outside the representative compressor 1. Although the air conditioning system is not particularly shown in the drawings, the air conditioning system may include a cooling circuit. When the piston 14 reciprocates, refrigerant in the suction chamber 22 is drawn into the cylinder bore 12 from the suction port 32 via the suction valve 34. Then, the refrigerant is compressed and the compressed refrigerant is discharged from the discharge port 36 to the discharge chamber 24 via the discharge valve 38.

[0029] The crank chamber 86 preferably communicates with the discharge chamber 24 via a capacity control passage 80. The capacity control passage 80 is opened and closed by a capacity control valve 90. The pressure state within the crank chamber 86 is controlled by opening and closing the capacity control passage 80. The opening and closing of the capacity control passage 80 is controlled by a solenoid 92 disposed at the capacity control valve 90. The exciting and non-exciting of the solenoid 92 to open and close the capacity control valve 90 are controlled by a controller in accordance with the working-load of the air conditioning system. In addition, a bleed passage 100 preferably connects the crank chamber 86 and the suction chamber 22 via a bleed port 104.

[0030] The cylinder block 10 and the piston 14 are typically made of an aluminum alloy. Preferably, a fluoro-resin may be coated on the outer peripheral surface of the piston 14. By coating the piston 14 with fluoro-resin, the aluminum piston 14 can be prevented from directly contacting the aluminum cylinder block. As a result, seizing between the cylinder block 10 and the piston 14 can be avoided and the clearance between the cylinder block 10 and the piston 14 can be minimized. Naturally, materials other than fluoro-resin also may preferably be utilized to coat the piston 14.

[0031] As shown in FIG. 2, the shoe 76 may include a spherical surface portion 132 and a substantially flat surface portion 138. In this representative embodiment, the substantially flat surface portion 138 has a slightly curved surface. However, due to relatively the large radius of curvature, the curved surface may be substantially defined as a “flat surface.” The outer periphery of the substantially flat surface portion 138 has tapered surface. A rounded portion having a relatively small radius of curvature is provided at the boundary between the substantially flat surface portion 138 and the spherical surface portion 132. A pair of shoes 76 is supported such that the spherical surface portion 132 of each shoe 76 is slidably coupled to the concave spherical surface 128 of the piston 14. The substantially flat surface portion 138 contacts the sliding surface 140, 142 of the outer periphery of the swash plate 60 such that the pair of shoes 76 holds the outer periphery of the swash plate 60 from both sides.

[0032] As shown in FIG. 2, the shoe 76 preferably includes a main body 146, a hard metallic layer 150 and an anti-friction layer 152, which preferably includes a solid lubricating agent 158. The main body 146 of the shoe 76 is preferably made of an aluminum silicon alloy and the swash plate 60 may be made of steel. The sliding surfaces 140, 142 are thermally sprayed with a material that includes aluminum and an anti-friction layer of synthetic resin. In the alternative, the sliding surfaces 140, 142 may be quench-hardened and thermal spraying of aluminum material may be omitted. As another alternative, the swash plate 60 can be made of a material that primarily comprises aluminum and both sliding surfaces 140, 142 can be thermally sprayed with an iron material. Naturally, the material for the swash plate 60 is not limited to the above-described materials. For example, copper materials, such as lead bronze and brass, may also be utilized. Additionally, the swash plate 60 may preferably be made by thermally spraying a copper material onto the surface of an iron material or may preferably be coupled by sintering. Materials other than those described above can be used in combination, as well.

[0033] The hard layer 150 is provided over the outer surface of the main body 146 of the shoe 76. Then, the anti-friction layer 152 is formed over the entire outer surface of the hard layer 150. In FIG. 2, the thickness of the hard layer 150 and the anti-friction layer 152 have been exaggerated for the purpose of illustration. The hard layer 150 may preferably comprise a metal as the main component and have a hardness that is greater than 300 HV based upon the Vickers hardness scale. Preferably, the hard layer 150 may include at least 70% by weight of metal.

[0034] The hard layer 150 may comprise, for example, nickel phosphorus (NiP), nickel boron (NiB), nickel phosphorus boron tungsten (NiPBW), cobalt phosphorus (CoP) or hard chromium plating. Preferably, a plate or layer of NiP is utilized for the hard layer 150. The NiP layer preferably includes 90 to 92% by weight of nickel and 8 to 10% by weight of phosphorus. The Vickers hardness of the NiP layer is preferably about 400 to 550 Hv.

[0035] As shown in FIG. 3, a layer of metal that includes the solid lubricating agent 158 preferably defines the anti-friction layer 152. For example, the metal in the anti-friction layer 152 may preferably be selected from one or more of nickel boron (Ni—B), nickel phosphorus boron tungsten (Ni—P—B—W), cobalt phosphorus (Co—P) or a hard chromium plated layer. Further, the solid lubricating agent may preferably be selected from one or more of molybdenum disulfide (MoS2), boron nitride (BN), tungsten disulfide (WS2), graphite or polytetrafluoroethylene. By incorporating the solid lubricating agent 158 into the metal, wear-resistance can be improved and the coefficient of friction can be reduced. The anti-friction layer 152 may preferably be obtained by dispersing or mixing the solid lubricating agent 158 into a metal, such as Ni—B, Ni—P—B—W, Co—P or hard chromium. Within this representative embodiment, a NiB plated layer, which includes the solid lubricating agent 158, is utilized as the anti-friction layer 152. The Ni—P plated layer, Ni—B plated layer, Ni—P—B—W plated layer and the Co—P plated layer may preferably be a non-electrolytic plated layer (chemically plated layer). Consequently, two plated layers having uniform thickness can be easily plated onto the main body 146. Preferably, the Ni—P plated layer and the Ni—B plated layer may respectively have thickness of 25 &mgr;m (micro meter).

[0036] By utilizing the anti-friction layer 152, the anti-friction performance of the spherical surface portion 132 and the substantially flat surface portion 138 of the shoe 76 can be increased with respect to the sliding movement against the swash plate 60 and piston 14. Therefore, the sliding friction of the shoe 76 can be reduced and the spherical surface portion 132 can be prevented from seizing with the concave spherical surface 128 of the piston 14. In addition, seizing between the substantially flat surface portion 138 and the sliding surfaces 140, 142 of the swash plate 60 also can be avoided or alleviated. Further, by incorporating the metallic hard layer 150 between the main body 146 and the anti-friction layer 152, the anti-friction layer 152 can be securely adhered to the main body 146. Therefore, the hard layer 150 may also function as an adhesive for the anti-friction layer 152. As a result, peeling of anti-friction layer 152 from the main body 146 (shoe 76) can be avoided or minimized.

[0037] The NiP plated hard layer 150 is preferably provided between the main body 146 and the NiB plated anti-friction layer 152. The hard layer 150 also may function as an adhesive so as to tightly adhere the anti-friction layer 152 to the main body 146. That is, the NiB plated layer can be prevented from peeling off from the main body 146. If the hardness of the NiB plated layer is greater than the hardness of the NiP plated layer, the NiB plated anti-friction layer can have high anti-wear performance. Further, because processes for forming the NiB plated layer can be relatively expensive, the thickness of the NiP plated layer may preferably be greater than the thickness of the NiB plated layer in order to reduce manufacturing costs.

[0038] The NiP plated hard layer 150 also preferably functions as a shock absorbing layer when a force is exerted from the outside. Thus, peeling and chipping of the hard layer 150 and the anti-friction layer 152 from the main body 146 can be avoided or alleviated. As a result, the sliding performance and the durability of the shoe 76 can be enhanced.

[0039] If the representative compressor 1 is operated for a long time, the anti-wear capability of the anti-friction layer 152 of the shoe 76 may possibly be reduced. Thus, long-time operation of the compressor may possibly result in an insufficient supply of lubricating oil, thereby causing the anti-friction layer 152 to partially wear. If the anti-friction layer 152 wears out, the hard layer 150 will be exposed, but the main body 146 will not be exposed to the outside. However, even if the hard layer 150 becomes exposed, the shoe 76 can be prevented from being exposed and seizing due to friction, because the hard layer 150 will strongly adhere to the main body 146 and has good anti-wear and anti-friction properties.

[0040] Further, because the force (pressure) exerted onto the surfaces of the shoe 76 varies periodically in response to the rotation of the swash plate 60, the anti-friction layer 152 is supplemented from the surrounding of the anti-friction layer 152 to the area where the anti-friction layer 152 is worn. Thus, the wear will be repaired when the surface pressure of the shoe 76 decreases. A representative process for repairing the anti-friction layer 152 may occur as follows. Due to the frictional force and heat generated when the shoe 76 moves with respect to the swash plate 60 and the piston 14, the anti-friction layer 152, which includes solid lubricating agent 158, will flow to the area where the anti-friction layer 152 has been worn out or peeled off. As a result, the anti-friction layer 152 will be repaired. Therefore, anti-friction performance will not be reduced, even if the compressor is operated for a long time.

[0041] Further, even if the supply of lubricating oil is reduced due to leakage of refrigerant gas to outside the compressor, the shoe 76 can maintain high anti-friction performance, thereby preventing seizing of the shoes 76 with respect to the swash plate 60 and the piston 14. Thus, the service life of the shoe 76 can be extended. For example, the above-described representative compressor was operated in a dry condition (i.e., no lubrication) and the swash plate 60 was driven at a rotational rate of 1000 rpm (revolutions per minute). In this state, the shoe 76 seized against the swash plate 60 and the piston 14 after 72 seconds when the anti-friction layer 152 was provided over the entire hard layer 150 and after 69 seconds when the anti-friction layer 152 is provided only over the spherical surface portion 132. On the other hand, the show of the known compressor (described above in the Background section) sized against the swash plate and the piston after an average of 49 seconds (44 seconds at the first measurement and 54 seconds at the second measurement).

[0042] The anti-friction layer 152 may preferably be provided only on one side of the spherical surface portion 132 and the substantially flat surface portion 138. For example, FIG. 4 shows a second representative embodiment in which the anti-friction layer 152 is provided only over the spherical surface portion 132. Further, FIG. 5 shows a third representative embodiment in which the anti-friction layer 152 is provided only over the substantially flat surface portion 138.

[0043] Further, the hard layer 150 may preferably be defined by a metallic layer that comprises a solid lubricating agent. The solid lubricating agent may preferably be selected from the materials described above with respect to the first representative embodiment. Because the hard layer 150 includes the solid lubricating agent, the anti-friction performance of the shoe 76 can be maintained even if the anti-friction layer 152 has worn out. As a result, seizing of the shoe 76 against the piston 14 and the swash plate 60 can be further avoided or alleviated.

[0044] Further, the shoe 76 as described above may preferably be utilized in a fixed capacity type swash plate compressor, such as a swash plate compressor that includes a double-headed piston.

Claims

1. A compressor comprising:

a drive shaft,

a swash plate coupled to the drive shaft, wherein the swash plate rotates in response to rotation of the drive shaft,

a piston disposed within a cylinder bore, the piston having an engage portion, and

a shoe coupling the engage portion of the piston to a peripheral edge of the swash plate, whereby the piston will reciprocate within the cylinder bore in order to compress a refrigerant in response to rotation of the swash plate, wherein the shoe comprises a main body, a metallic hard layer at least partially disposed on the main body and an anti-friction layer at least partially disposed on the metallic hard layer, wherein the main body comprises a spherical surface portion and a substantially flat surface portion and comprises aluminum, the metallic hard layer has a hardness of at least HV 300 based upon the Vickers hardness scale and is disposed on at least one of the spherical surface portion and the substantially flat surface portion, and the anti-friction layer comprises a metal and a solid lubricating agent.

2. A compressor according to claim 1, wherein the swash plate is inclinably coupled to the drive shaft and the inclination angle of the swash plate can be changed to change the compressor output discharge capacity.

3. A compressor according to claim 1, wherein the solid lubricating agent comprises at least one of molybdenum disulfide (MoS2), boron nitride (BN), tungsten disulfide (WS2), graphite or polytetrafluoroethylene.

4. A compressor according to claim 1, wherein the metal within the anti-friction layer comprises at least one of nickel phosphorus, nickel boron, nickel phosphorus boron tungsten, cobalt phosphorus or hard chromium plating.

5. A compressor according to claim 1, wherein the piston and the swash plate are arranged and constructed to contact the anti-friction layer of the shoe during operation of the compressor.

6. A compressor according to claim 1, wherein the metallic hard layer further comprises a solid lubricating agent.

7. A compressor according to claim 1, wherein the substantially flat surface portion is slightly curved.

8. A compressor comprising:

a drive shaft,

a swash plate coupled to the drive shaft, wherein the swash plate rotates in response to rotation of the drive shaft,

a piston disposed within a cylinder bore,

means for coupling the piston to the swash plate so that the piston will reciprocate within the cylinder bore in order to compress a refrigerant in response to rotation of the swash plate, wherein the coupling means comprises a main body, a metallic hard layer at least partially disposed on the main body and an anti-friction layer at least partially disposed on the metallic hard layer, wherein the main body comprises a spherical surface portion and a substantially flat surface portion and comprises aluminum, the metallic hard layer has a hardness of at least HV 300 based upon the Vickers hardness scale and is disposed on at least one of the spherical surface portion and the substantially flat surface portion, and the anti-friction layer comprises a metal and a solid lubricating agent.

9. A compressor according to claim 8, wherein the swash plate is inclinably coupled to the drive shaft and the inclination angle of the swash plate can be changed to change the compressor output discharge capacity.

10. A compressor according to claim 9, wherein the solid lubricating agent comprises at least one of molybdenum disulfide (MoS2), boron nitride (BN), tungsten disulfide (WS2), graphite or polytetrafluoroethylene.

11. A compressor according to claim 9, wherein the metal within the anti-friction layer comprises least one of nickel phosphorus, nickel boron, nickel phosphorus boron tungsten, cobalt phosphorus or hard chromium plating.

12. A compressor according to claim 11, wherein the piston and the swash plate are arranged and constructed to contact the anti-friction layer of the coupling means during operation of the compressor.

13. A compressor according to claim 12, wherein the metallic hard layer further comprises a solid lubricating agent.

14. A compressor according to claim 13, wherein the substantially flat surface portion is slightly curved.

15. A compressor according to claim 14, wherein the metallic hard layer comprises least one of nickel phosphorus, nickel boron, nickel phosphorus boron tungsten, cobalt phosphorus or hard chromium plating.

16. A compressor according to claim 8, wherein the main body comprises an aluminum silicon alloy, the metallic hard layer comprises nickel phosphorus and the anti-friction layer comprises nickel boron.

17. A compressor according to claim 16, wherein the nickel phosphorus of the metallic hard layer comprises about 90-92% by weight of nickel and about 8-10% of phosphorus, the metallic hard layer having a hardness of about 400-500 Hv on the Vickers hardness scale.