SLIDING MEMBER AND METHOD OF MANUFACTURING THEREOF

Abstract: ABSTRACT A sliding member (12) is disclosed that has superior slidability and durability at a low cost. A top sliding surface (12a) of the sliding member (12) is formed by a nitriding process that creates a compound layer (14) and a diffusion layer (15) on a base metal (16) of the sliding member (12). The buff polishing process is thinly performed on an outermost layer portion (14a) of the compound layer (14), such that a portion of the compound layer (14b and/or 14c) remains.

Description: DESCRIPTION SLIDING MEMBER AND METHOD OF MANUFACTURING THEREOF BACKGROUND OF THE INVENTION Technical Field The present invention generally relates to a sliding member such as valve lifter in an internal combustion engine, and a method of manufacturing a sliding member. Description of Related Art Japanese Laid-Open Utility Model Publication H4-121404 discloses a valve lifter having a shim that slideably contacts , a cam for driving an intake/exhaust valve of an internal combustion engine. The shim ofthe valve lifter needs to have a sliding surface whose surface roughness is sufficiently small to minimize friction. At the same time, the sliding surface has to have a sufficient hardness in order to prevent excessive wear in the sliding surface, and also to prevent an increase in friction due to the increase in surface roughness ofthe sliding surface from the wear. Therefore, it has been known to smooth the base metal ofthe sliding member, such as a valve lifter, with high precision through a lapping process, and thereafter to create a hard material protection coating such as a titanium nitride through physical vapor deposition (PND) on its top sliding surface. There exists a need for a sliding member that has low friction and superior durability at a low cost in comparison ofthe above mentioned prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure. SUMMARY OF THE INVENTION It has been discovered that when a hard material coating is created on a sliding surface through physical vapor deposition, it is necessary to perform the process using a vacuum furnace. Accordingly, only a limited number of pieces can be processed at a time. Therefore, manufacturing cost ofthe sliding member or cam follower becomes very expensive. The present invention has been conceived in view ofthe aforementioned problem. One ofthe objects ofthe present invention is to provide a sliding member that has low friction and superior durability at a low cost. In accordance with one aspect ofthe present invention, a sliding member is produced that comprises a base metal, a diffusion layer, and a compound layer. The diffusion layer has a first predetermined depth and overlies the base metal. The compound layer has a second predetermined depth and overlies the diffusion layer. The diffusion layer and the compound layer are formed on the base metal through a nitriding process. The second predetermined depth ofthe compound layer is formed by a polishing process on an outermost layer portion ofthe compound layer such that an original depth ofthe compound layer formed by the nitriding process is reduced in depth to the second predetermined depth ofthe compound layer so that a smooth top sliding surface remains. These and other objects' features, aspects and advantages ofthe present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment ofthe present invention. BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the attached drawings which form a part of this original disclosure: Figure 1 is a partial diagrammatic view of a valve actuator assembly for an internal combustion engine having a valve lifter (sliding member) manufactured in accordance with one embodiment ofthe present invention; Figure 2 is a cross sectional view of a valve lifter (sliding member) manufactured in accordance with one embodiment o he present invention; Figure 3 is an enlarged partial cross sectional view of a selected portion ofthe valve lifter before a buff polishing process has been performed on the top sliding surface ofthe valve lifter; Figure 4 is an enlarged partial cross sectional view of a selected portion ofthe valve lifter after a buff polishing process has been performed on the top sliding surface of the valve lifter; Figure 5 an enlarged partial cross sectional view of a selected portion ofthe valve lifter that illustrates the diffusion layer and the compound layer created by the gas nitrocarburizing process performed on the top sliding surface ofthe valve lifter; and Figure 6 is a property characteristics chart showing the hardness ofthe valve lifter based on nitrogen concentration in relation to the depth ofthe top sliding surface ofthe valve lifter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Selected embodiments ofthe present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following description ofthe embodiments ofthe present invention is provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Referring initially to Figure 1, a portion of a valve actuator assembly 10 for an internal combustion engine (not shown) is diagrammatically illustrated to explain a first embodiment ofthe present invention. The valve actuator assembly 10 includes a cam 11 of a camshaft operatively contacting a cam follower (sliding member) in the form of a valve lifter 12 that moves an intake/exhaust valve 13. As seen in Figure 2, the valve lifter 12 as a finished product has a cylindrical shape with an open bottom. The valve lifter 12 is coupled to the intake/exhaust valve 13 in a conventional manner. The valve lifter 12 is placed in between the intake/exhaust valve 13 and the cam 11 of the camshaft that rotates together with a crankshaft (not shown). The valve lifter 12 has a top sliding surface 12a functioning as a cam sliding surface that slideably contacts the cam 11 of the camshaft. A surface finishing process is performed on this top sliding surface 12a as described below. As seen in Figures 3 and 5, the sliding member or valve lifter 12 manufactured according to the present invention includes a top sliding surface 12a formed of a compound layer 14 and a diffusion layer 15 overlying the base metal 16. In particular, the top sliding surface 12a is preferably formed by a nitriding process on the base metal 16 of the valve lifter 12. The compound layer 14 and the diffusion layer 15 have original predetermined depths that are initially created by the nitriding process on the base metal 16 ofthe valve lifter 12. The original predetermined depth ofthe compound layer 14 is indicated as "to" in Figure 3. After performing the nitriding process on the base metal 16, a. polishing process is thinly performed on an outermost layer portion 14a ofthe compound layer 14, such that only layer portions 14b and 14c ofthe compound layer 14 remains. In other words, the outermost layer portion 14a ofthe compound layer 14 is completely removed by the polishing process. Accordingly, the original depth "t 0 " ofthe compound layer 14 (Figure 3) formed by the nitriding process is reduced in depth to the finished predetermined depth "t" ofthe compound layer 14 (Figure 4) so that the smooth sliding surface 12a remains. Thus, the top sliding surface 12a is formed by thinly polished the compound layer 14 in a manner that conforms to the contour ofthe top sliding surface 12a so that a uniform finish is obtained. The above-described nitriding process is a method by which nitrogen is diffused onto the base metal 16, thereby hardening the outer surface. Some ofthe nitriding processes contemplated by the present invention include pure nitriding in which only nitrogen is permeated, and nitrocarburizing in which nitrogen and carbon are permeated at the same time. More specifically, gas nitriding with ammonia gas, salt bath nitriding using salt bath with cyanide salt and cyanic acid type salt bath, liquid nitriding using cyanic acid, gas nitrocarburizing using ammonia gas and carburizing gas, and ion nitriding in which ionized nitrogen collides into the base metal at a high speed. In particular, gas nitrocarburizing is a pollution free processing method since it does not produce cyan. Also, gas nitrocarburizing can be processed in a stable and continuous manner. Accordingly, manufacturing cost can be kept low. Therefore, gas nitrocarburizing is well suited for the present invention. Through such nitriding process shown in Figure 5, the diffusion layer 15 and the compound layer 14 are formed in a layered manner on the base metal 16. From this nitriding process, the nitrogen (N) concentration in the diffusion layer 15 is relatively low, while the nitrogen (N) concentration in the compound layer 14 is relatively high. Since the hardness ofthe material increases as the nitrogen concentration increases, the hardness ofthe compound layer 14 is greater than that ofthe diffusion layer 15. Thus, the hardness ofthe sliding surface 12a decreases in the depth of penetration, since the nitrogen concentration decreases as graphically shown in Figure 6. However, since the original depth "to" ofthe compound layer 14 is very small (preferably 5μm to 15μm), if a conventional lapping process was performed to uniformly smoothen the top sliding surface 12a, then all ofthe compound layer 14 may be removed such that the diffusion layer 15 may be partially exposed. Therefore, in this invention, only the outermost layer portion 14a ofthe compound layer 14 is polished, such that the portions 14b and 14c ofthe compound layer 14 remain. In other words, the surface ofthe compound layer 14 is thinly polished in a manner that conforms to the contour ofthe sliding surface 12a. Accordingly, the remaining compound layer 14 can function as a protection film having a high hardness. Accordingly, a valve lifter 12 having superior slideability and durability can be obtained at a low cost. As the base metal 16, various steel materials can be utilized such as carbon steel, alloy steel, toll steel, and steel materials. Typically, a chromium molybdenum steel is utilized that has been carburizing, quenching, and tempering. An appropriate grinding and/or polishing process is performed beforehand on the outer surface on which the nitiriding process is to be performed. In the preferred embodiment, the base metal 16 is preferably a forged steel (SCM420H) formed by forging, carburizing, quenching, and tempering processes that are performed such that the surface hardness is equal to or greater than 58H R C with an effective depth is 0.7-1.1 mm. Then, a surface polishing process is performed such that the surface roughness ofthe outer surface is approximately Ra 0.02. Thereafter, a gas nitrocarburizing process is performed such that the surface hardness ofthe outer surface is equal to or greater than 660Hv, and that the depth ofthe compound layer 14 is equal to or greater than 7μm. In this manner, as shown in Figure 3, the diffusion layer 15 and the compound layer 14 have original predetermined thicknesses that are formed on the base metal in a layered manner. Next, as shown in Figure 4, the buff polishing process is performed such that the surface roughness ofthe finished top sliding surface 12a is equal to or less than Ra 0.02, and that the depth "t" ofthe remaining compound layer 14 is preferably equal to or greater than 2.5μm. In this buff polishing process, the polishing is performed in a manner that conforms to the contour ofthe top surface 12a, such that the compound layer 14 has a remaining or finished depth "t" of about 2.5μm to lOμm. Accordingly, only the outermost portion ofthe compound layer 14 is thinly and uniformly polished. In other words, the amount ofthe compound layer 14 removed by the buff polishing process is very small, approximately 3μm to 5μm. In the valve lifter 12 manufactured in accordance with the present invention, the hard compound layer 14 is left on the base metal 16 to form the sliding surface 12a. Therefore, in comparison with a case where a hard film is separately created by PVD after the lapping process, the manufacturing cost can be reduced to approximately half, while securing the substantially same friction reduction effect and durability. Also, by performing the buff polishing process on the top surface 12a ofthe valve lifter 12, the edges ofthe periphery ofthe top surface 12a are adequately rounded. Accordingly, there is no need to separately perform a chamfering process. One ofthe surface processing methods that can conform to the contour ofthe surface is buff polishing process. The buff polishing is a surface finishing process that utilizes particles as in lapping process. However, the buff polishing utilizes a buff that is made of a cloth, felt, or leather having a soft elasticity, instead of a hard metal lap. Therefore, as described above, it is possible to thinly polish only the outermost layer portion so as to conform to the contour ofthe surface. Accordingly, the buff polishing process is suited for the present invention. In other words, if the lap polishing process is performed on the aforementioned compound layer 14, although the surface can be smoothened properly, it is difficult to leave a thin uniform layer of compound layer 14. Therefore, the effects ofthe present invention cannot be obtained. Referring back to Figure 5, an ε phase (Fe 2 N-Fe 3 N) is created in the outermost layer portion 14a ofthe compound layer 14 by the nitriding process, while an ε+γ' phase and an γ* phase are formed inside the ε phase by the nitriding process. The ε phase ofthe compound layer 14 has a lower toughness than the remaining layer portions 14b and 14c ofthe compound layer 14. Thus, the outermost layer portion 14a ofthe compound layer 14 is not preferable as the sliding surface 12a ofthe valve lifter 12. Accordingly, in the present invention, the aforesaid polishing process adequately removes this outermost layer portion 14a. As a result, the layer portions 14b and 14c having the ε+γ' phase and the γ 1 phase are exposed. Therefore, no negative effect results from the ε phase that was formed by the nitriding process. If the original depth "t 0 " ofthe compound layer 14 before the polishing process is smaller than 5μm, it is difficult to secure the thickness ofthe processed material layer after the polishing process. If the original depth "to" ofthe compound layer 14 exceeds 15μm, a porous layer with porosity may be created. Accordingly, the original depth ' " ofthe compound layer 14 by the nitriding process should be preferably 5μm to 15μm before the polishing process. Also, if the finished predetermined depth "t" ofthe compound layer 14 after the polishing process is less than 2μm, the compound layer 14 may wear out during use. The aforesaid ε phase may also be left. If the finished predetermined depth "t" ofthe compound layer 14 after the polishing process exceeds lOμm, a porous layer may result at the time of creating the compound layer 14, as described above. Therefore, the finished predetermined depth "t" ofthe compound layer 14 after the polishing process should be preferably 2μm to lOμm. If the surface roughness ofthe surface 12a ofthe compound layer 14 after the polishing process is less than Ra 0.01, it is difficult to perform the process on a mass- production scale. On the other hand, if the surface roughness is greater than Ra 0.05, sufficient friction reduction effect cannot be obtained. Therefore, the surface roughness of the compound layer 14 after the polishing process should be preferably Ra 0.01-0.05. Of course, it will be apparent to those skilled in the art from this disclosure that the scope ofthe present invention is not limited to a valve lifter, but rather the present invention can be used with other types of sliding members. Thus, the scope ofthe invention is not limited to the disclosed embodiments. Some other examples of other sliding member include a shim that is slideably positioned adjacent a cam of an intake/exhaust valve, a cam follower such as a rocker arm, a piston ring, and various bearing members. However, since the polishing is thinly performed in a manner that conforms to the contour ofthe surface ofthe sliding member in the present invention, the present invention is particularly suitable for sliding members such as cam followers. Specifically, the reduction ofthe surface roughness of the sliding surface, rather than the smoothness ofthe sliding surface is more important for cam followers. In any event, with the present invention, it is possible to provide a sliding member at a low cost that has also superior slideability and durability. The terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation ofthe modified term such that the end result is not significantly changed.. For example, these terms can be construed as including a deviation of at least ± 5% of the modified term if this deviation would not negate the meaning of the word it modifies. This application claims priority to Japanese Patent Application No. 2000-286497- The entire disclosure of Japanese Patent Application No. 2000-286497 is hereby incorporated herein by reference. While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope ofthe invention as defined in the appended claims. Furthermore, the foregoing description ofthe embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope ofthe invention is not limited to the disclosed embodiments.

Claims: CLAIMS 1. A sliding member comprising: a base metal; a diffusion layer with a predetermined depth overlying said base metal; and a compound layer with a second predetermined depth overlying said diffusion layer, said diffusion layer and said compound layer being formed on an outer surface of said base metal through a nitriding process, and said second predetermined depth of said compound layer being formed by a polishing process on an outermost layer portion of said compound layer such that an original depth of sai compound layer formed by said nitriding process is reduced in depth to said second predetermined depth of said compound layer so that a smooth sliding surface remains. 2. The sliding member as set forth in claim 1, wherein said second predetermined depth of said compoun layer formed by said polishing process has a substantially uniform depth in that said polishing process conforms to a contour of said outer surface of said base metal. 3. The sliding member as set forth in claim 1 or 2, wherein said polishing process forming said smooth sliding surface is a buff polishing process. 4. The sliding member as set forth in one of claims 1-3, wherein said sliding member is a cam follower that is slideably adjacent a cam that drives an intake valve or exhaust valve of an internal combustion engine. 5. . The sliding member as set forth in one of claims 1 -4, wherein said original depth ofthe compound layer before performing said polishing process is Sμm to 15μm. 6. The sliding member as set forth in one of claims 1-5, wherein said second predetermined depth of said compound layer after performing said polishing process is 2μm to lOμm. 7. The sliding member as set forth iri one of claims 1-6, wherein said smooth sliding surface of said compound layer after performing said polishing process has a surface roughness of Ra 0.01-0.05. 8. A method of manufacturing a sliding member, comprising: creating a diffusion layer and a compound layer having predetermined depths on a base metal of said sliding member through a nitriding process; and performing a polishing process on an outermost layer portion of said compound layer, such that said predetermined depth of said compound layer is reduced in depth so that a portion of said compound layer remains to create a smooth sliding surface on said sliding member. 9. The method of manufacturing as set forth in claim 8, wherein . said polishing process is a buff polishing process. 10. The method of manufacturing as set forth in claim 8 or 9, wherein said sliding member is a cam follower that is slideably adjacent a cam that drives an intake valve or exhaust valve of an internal combustion engine. 11. The method of manufacturing as set forth in one of claims 8-10, wherein a depth of said compound layer before performing said polishing process is 5μm to 15μm. 12, The method of manufacturing as set forth in one of claims 8-11, wherein said predetermined depth of said compound layer after performing said polishing process is 2μm to lOμm. 13. The method of manufacturing as set forth in one of claims 8-13, wherein said smooth sliding surface of said compound layer after performing said polishing process has a surface roughness of Ra 0.01-0.05.

Inventors: ISHIHARA MOTOKATA, ISHIHARA, MOTOKATA
Applicants: ISHIHARA MOTOKATA, NISSAN MOTOR, ISHIHARA, MOTOKATA, NISSAN MOTOR CO., LTD.
Application year: 2001
Main classification: F01L1
IPC codes: C23C8, F01L1, F01L3, F16H53