SLIDING PART AND METHOD FOR MANUFACTURING THE SAME

Abstract: An object of the present invention is to provide a sliding part having a crowning configuration by effecting surface hardening at a portion constituted by steel and a method for manufacturing the same part. The configuration and volume of crowning can be adjusted by heat treatment and processing after the surface hardening, and the sliding part is constituted by a single steel unit or includes at least one portion of a member forming a sliding surface formed into the crowning configuration through surface hardening is jointed to or fitted in a steel sliding part main body and made of ceramics.

Description: Technical Field: This invention relates to sliding components having a plurality of sliding faces, for which wear resistance is requisite, such as a tappet, a rocker arm and other engine components, bearings, and so forth, and a production method of such sliding components. Background Art: In order to prevent uneven contact due to poor alignment, one of a pair of sliding faces of a mechanical sliding component generally is not a flat face but has a convexed crowning shape such that its center portion is slightly higher than its outer edge portion (by several to dozens of microns). This crowning shape is formed by various methods such as machining (polishing), a method described in Japanese Patent Laid-Open No. 63-289306 which fits metal over ceramic so as to cause elastic deformation of the ceramic by its fastening force, a method described in Japanese Patent Laid-Open No. 63-225728 which heats and joins ceramic that form a sliding face to a metal as a main body and utilizes the difference of their thermal expansion coefficients, and a method which shapes in advance a calcined body into a crowning shape, then sinters this calcined body and utilizes the as-sintered face as the sliding face ["Automobile Technology", Vol. 39, No. 10, (1985) p1184], and so forth. Further on, JP-A-61/103 057 discloses a gear wheel having crowning surfaces formed on tooth flanks by induction hardening. However, since the crowning shape is a three-dimensional shape, formation of this shape by machining requires an enormous cost of production. According to the method which fits metal over ceramic or the method which utilizes the difference of thermal expansion coefficients between ceramic and metal, the crowning quantity is limited once the structure, the heating temperature, etc, are decided. On the other hand, the method which shapes in advance the calcined body into the crowning shape, then sinters it and utilizes the as-sintered face as the sliding face is not free from the problem that the face shaped into the crowning shape undergoes deformation due to shrinkage at the time of sintering, and dimensional accuracy drops. In view of the problems of the prior art described above, the present invention aims at providing a sliding component having improved utility and a method of producing such a sliding component. Disclosure of the Invention: The sliding component provided by the present invention for accomplishing the object described above is defined in claim 1. The method of producing such a sliding component is defined in claim 9. It is more preferred to use ceramics for the member for forming the sliding face which is formed by joining or fitting. In the sliding component according to the present invention, the crowning shape is formed on the sliding face of at least one portion by partially applying surface quenching to a steel which constitutes the sliding component and is hardenable. In other words, deformation is partially generated by utilizing volume expansion due to martensitic transformation or so-called quenching distortion at the time of surface quenching, and the crowning shape is imparted to at least one arbitrary sliding face in the sliding component. The portion to which surface quenching is applied is appropriately selected in accordance with the position of the sliding face to which crowning is imparted, or with the crowning quantity. Crowning by surface quenching is imparted by utilizing the phenomenon described above. Accordingly, it is more efficient to apply surface quenching to the portion or portions near the joined portion or portions in a broader range. Incidentally, the total surface area of surface quenched is preferably at least 30% of the surface area as the difference obtained by subtracting the surface area of the portion, which is shaped into the crowning shape, from the entire surface of the component. The crowning quantity to be imparted can be broadly controlled in accordance with the means and methods (heating, cooling time, etc) of surface quenching, with the kinds of steel materials used and so on. The portion to which surface quenching is applied is hardened and has low wear and high durability. At the same time, it plays the role of the sliding portion. There is no limitation to the kind of the steel to which surface quenching treatment is applied, so long as the steel undergoes hardening by the surface quenching treatment. From the aspects of strength, and the costs of material and machinability, however, carbon steels widely used as the steels for machine structural use and alloy steels containing Ni, Cr and Mo as the alloy elements are preferred. According to the present invention, the crowning quantity is changed by applying heat-treatment to the sliding component subjected to the surface quenching treatment. This utilizes release of the residual stress occurring due to the surface quenching or the change of an unstable structure formed by quenching such as martensite. Heat-treatment may be applied either wholly or partially, and is selected in accordance with the position, the quantity and the shape of the crowning which is to be changed. Suitable hardness and toughness in accordance with the object of use can be provided by carrying out this heat-treatment as tempering treatment of the hardened portion. Since the residual stress can be removed, the change of the crowning quantity with aging and the crack of the hardened portion can be prevented. In the sliding component according to the present invention, the crowning quantity is changed by applying machining to the steel portion after the surface quenching treatment. The sliding component keeps its crowning shape because various residual stresses such as quenching distortion balance with one another. Therefore, this balance is lost by changing the rigidity by machining or removing the residual stress layer, and the crowning quantity can be changed in this way. The machining position is suitably selected in accordance with the position and the quantity of crowning to be changed. This machining may be used as machining for forming the sliding portion for which high dimensional accuracy as well as surface roughness are naturally required. A member having excellent sliding characteristics may be joined or fitted to the sliding component main body for the portion for which sliding characteristics are particularly required. In this case, release of the residual stress occurring by joining or fitting is encountered in heat-treatment or machining after the quenching. Therefore, the change quantity of crowning can be made over a broad range. The member which is fitted to the sliding component main body and forms the sliding face is particularly preferably a ceramic material having excellent sliding characteristics and high heat resistance. Ceramic materials having high strength such as aluminum oxide (Al 2 O 3 ), zirconium oxide (ZrO 2 ), silicon nitride (Si 3 N 4 ), etc, are more preferred. These ceramic materials must have a four-point flexural strength of at least 50 kg/mm 2 according to JIS standard and a thermal shock resistance to a temperature difference (thermal shock resistance temperature difference) of at least 400°C. Particularly preferred among them is the Si 3 N 4 ceramic material which exhibits excellent performance. Further preferably, silicon nitride type ceramics having a strength value at room temperature of at least 100 kg/mm 2 for test pieces for four-point flexural test according to the JIS standard and a thermal shock resistance against to a temperature difference of at least 800°C are used. When the ceramics and the steel are joined at the portion near the portion of surface quenching treatment, the treatment condition is adjusted by, for example, reducing the temperature of the joined portion to a lower temperature than the temperature at the time of joining so as to keep the joined state and the joining strength, but there is the case where the temperature of the joined portion rises near to the joining temperature due to the restrictions such as the shapes. Therefore, in order to avoid deterioration of the strength after thermal impact due to cooling (oil cooling, etc), the ceramics should have a thermal shock resistance withstanding a temperature difference of at least 400°C, most reliably at least 800°C. When silicon nitride type ceramics having a strength of at least 100 kg/mm 2 , preferably at least 130 kg/mm 2 are selected as such high strength ceramics, the ceramics can withstand the stress occurring thereinside and the occurrence of cracks can be easily prevented even when surface quenching treatment is applied to the portion near the joining portion. Next, the production method of the sliding component according to the present invention will be explained. The surface quenching treatment is carried out by using known quenching methods by radio frequency, flame, laser beam, electron beam, and so forth. Where toughness must be secured at the portion to be quenched, a steel main body which is in advance subjected to carburization treatment may be employed. Heat-treatment after the surface quenching is carried out at a temperature within the range of 100 to 700°C. If the temperature is lower than 100°C, the change of crowning hardly occurs and if it is higher than 700°C, an austenite structure will develop and will break the structure generated by quenching. The temperature range is more preferably 150 to 600°C. Machining of the steel portion after the surface quenching is made by known machining methods such as cutting. Particularly when a quenched sliding portion is employed, a surface layer called a mill scale must be removed and deformation due to quench distortion must be eliminated so as to conduct high precision machining. When a surface roughness is adjusted suitably to a lower level, polishing may be employed. When the member for forming the sliding face is fitted to the sliding component main body, joining and fitting may be employed. Known joining methods such as heat-joining, e.g. brazing or diffusion joining, welding, pressure joining, etc, may be utilized. The temperature of heat-joining is most preferably at least 800°C so as to eliminate the influences of the temperature rise at the time of surface quenching treatment. In other words, the position of surface quenching is preferably selected so as not to exceed the temperature at the time of heat-joining, and in the case of quenching using the electron beam or the laser beam having less heat diffusion at the time of surface quenching, quenching can be applied to the portion near the joined portion, and the area that can be surface-quenched can be increased. In the case of flame hardening and induction hardening, on the other hand, the heat affected portions become greater. Therefore, it becomes difficult to apply hardening to the portion near the joined portion. In the case of induction hardening, for example, the hardening range is preferably spaced apart by several millimeters from the joined portion, though it varies depending on the heating time and the frequency. When the member to be joined is ceramic, joining by brazing is effected. When the ceramic is directly joined to the metal, the brazing material is a Ti-containing silver brazing such as an Ag-Cu-Ti type, an Ag-Ti type, etc. When the member is metallized on the joined face side of the ceramic, an Ag-Cu type brazing is preferred. The brazing atmosphere is preferably a non-oxidizing atmosphere (vacuum and Ar, N 2 , H 2 and their mixed gases). Fitting may be carried out by known methods such as press fit, shrinkage fit, and so forth. Brief Description of Drawings: Fig. 1 is a longitudinal sectional view of a tappet. Fig. 2 is a longitudinal sectional view of a tappet. Fig. 3 is a longitudinal sectional view of a tappet main body. Fig. 4 is a longitudinal sectional view of a tappet. Fig. 5 is a longitudinal sectional view of a tappet. Fig. 6 is a longitudinal sectional view of a valve lifter. Explanation of Reference Numerals: A: upper limit of quenching range 2: tappet main body 3: sliding member 4: valve lifter main body 5: sliding member 10: sliding face 11: outer peripheral face 12: hemispherical face 14: outer peripheral face of neck portion Best Mode for Carrying Out the Invention: Example 1: Fig. 1 shows a tappet produced as an example of the sliding components according to the present invention. An alloy steel nickel-chromium steel SNC836 for machine structural use (JIS G4102) was used for the tappet main body 2. The dimensions of this sliding component included a diameter of 30 mm, a hollow portion of 25 mm in an inner diameter and a total height of 40 mm. A commercially available silicon carbide (SiC) ceramic and a cemented carbide having a diameter of 30 mm and a thickness of 1.5 mm were used for a sliding member 3 that formed the sliding face 10 according to the present invention, and the face 10 as the sliding face was machined into a flatness of 5 µm and a surface roughness of not greater than 1.6 µm (ten-point mean roughness). Joining of the sliding member 3 to the tappet main body 2 was carried out by holding them in vacuum at 860°C for 30 minutes through an Ag-Cu-Ti type brazing material having a thickness of 50 µm. The outer peripheral face 11 was heated by an electron beam at an accelerated voltage of 6 kV and quenched. The crowning quantity of the spherical shape of the center portion with respect to the outer peripheral edge portion (25 mm) increased by 9 and 4 µm, respectively, as the mean of twenty samples due to the surface quenching treatment in both SiC and the cemented carbide in the shape of the face 10, and the total crowning quantity was 29 µm and 22 µm. Example 2: A tappet having the same shape as that of the tappet of Example 1 was produced in the following way. An alloy steel chromium steel SCr440 (JIS G4104) for machine structural use was used for the tappet main body 2, and the sliding member 3 made of Si 3 N 4 was produced in the following way. To commercially available Si 3 N 4 powder were added 5 wt.% of Y 2 O 3 powder and 2 wt. % of Al 2 O 3 powder as sintering aids, and they were mixed in ethanol by using a ball mill for 96 hours. After drying, the resulting powder mixture was press-molded and further subjected to CIP. Thereafter, it was sintered at 1,710°C for 4 hours in a nitrogen atmosphere of 2 atms, and was next subjected to HIP treatment at 1,660°C for 1 hour in the nitrogen gas atmosphere of 1,000 atms. The resulting sintered body had an alpha (α) percentage of 11% and 155 crystal grains per a 50 µm length as a linear crystal grain density. The alpha (α) percentage was determined from a peak intensity ratio, that is, α[(102) + (210)]/{α[(102) + (210)) + β[(101) + (210)]}, wherein (102) + (210) and (101) + (210) are peak intensities of (α-silicon nitride and α'-sialon), (β-silicon nitride and β'-sialon), respectively in X-ray diffraction patterns. The mechanical properties of the sintered body are shown in Table 1. mechanical characteristics flexural strength 145 kg/mm 2 thermal shock resistance temperature difference 860°C A blank having a diameter of 30 mm and a thickness of 1 mm was cut out from the resulting sintered body, and the face 10 as the sliding face was machined into a flatness of 5 µm and a surface roughness of not greater than 1.6 µm (ten-point mean roughness). The blank was then brazed to the tappet main body 2 by holding them in vacuum at 1,000°C for 30 minutes through an Ag-Ti type brazing material having a thickness of 50 µm. The surface of the outer peripheral face 11 of the tappet so brazed was heated from the open portion to the A portion (25 mm from the open portion) by the radio frequency (400 kHz ) in the same way as in Example 1, and the whole tappet was immediately thereafter cooled with water. Subsequently, the hemispherical face 12, too, was quenched ( heating time: 5 seconds) by radio frequency and was then cooled with water. After the surface quenching treatment, the spherical crowning quantity (the change quantity of crowning) of the center portion with respect to the outer edge portion (25 mm) of the sliding face increased by 8 µm as the mean of twenty samples when only the face 11 was quenched, and the total crowning quantity was 32 µm. When the face 12 was quenched, too, the crowning quantity further increased by 12 µm. Example 3: In Example 3, the quenching range of the outer peripheral face 11 was changed to 5, 15, 25 and 30 mm in terms of the distance from the open portion. As a result, the change quantity of crowning due to the quenching of the outer peripheral face became as tabulated in Table 2. quenching range (mm) surface quenching area percentages (%) crowning change quantity (µm) 5 7 0 15 21 0 25 35 8 30 42 11 Example 4: In Example 4, quenching of the hemispherical face 12 was carried out by changing the heating time to 3, 7 and 9 seconds. As a result, the change quantity of crowning after the quenching of the outer peripheral face was 16, 5 and -2 µm, respectively, as the mean of twenty samples. Example 5: The tappet of Example 2, which had been induction hardened, was heat-treated (tempered) in an oil bath at 200°C. As a result, the change quantity of crowning after the hardening of the outer peripheral face 11 was 5 µm as the mean of twenty samples. Example 6: Fig. 2 shows a tappet produced as an example of the sliding components according to the present invention. An alloy steel nickel-chromium steel SCM435 (JIS G4105) for machine structural use was used for the tappet main body 2. The dimensions of the sliding component included a diameter of 31 mm, a hollow portion of 27 mm in an inner diameter, and a total height of 55 mm. The silicon nitride produced in Example 3 was machined into a diameter of 30 mm and a thickness of 1.3 mm to obtain a sliding member 3. The face 10 as the sliding face was polished into a flatness of 3 µm and a surface roughness of not greater than 0.8 µm (ten-point mean roughness). Joining of the sliding member 3 to the tappet main body 2 was carried out by holding them in vacuum at 880°C for 40 minutes through an Ag-Cu-Ti type brazing material having a thickness of 50 µm. The surface of the outer peripheral face 11 of the tappet so brazed was heated from its open portion to the A portion by radio frequency in the same way as in Example 2, and the whole tappet was cooled thereafter immediately with water. Subsequently, the hemispherical face 12, too, was hardened by radio frequency and was cooled with water. After tempering was conducted in an oil bath at 150°C, the tappet main body 2 was machined into 30.5 mm by centerless grinding. As a result, the change quantity of crowning after tempering was 6 µm as the mean of twenty samples. Incidentally, crowning was measured as a difference in level between the center portion and the outer peripheral portion (25 mm). Example 7: Fig. 3 shows a tappet main body 2 produced as an example of the sliding components according to the present invention. An alloy steel nickel-chromium steel SNC631 (JIS G4102) for machine structural use was used as the material. The dimensions of the sliding component included a diameter of 25.5 mm, a hollow portion of 22 mm in an inner diameter and a total height of 45 mm. The silicon nitride produced in Example 2 was machined into a sliding member having a diameter of 24.5 mm and a thickness of 1.2 mm, and the face 10 as the sliding face was polished into a flatness of 3 µm and a surface roughness of not greater than 0.8 µm (ten-point mean roughness). Joining of the sliding member 3 to the tappet main body 2 was carried out by holding them in a vacuum at 1,100°C for 20 minutes through an Ag-Ti type brazing material having a thickness of 50 µm. The surface of the outer peripheral face 11 of the tappet so brazed was heated from the open portion to the A portion by radio frequency in the same way as in Example 2 and immediately thereafter, the whole tappet was cooled with water. Subsequently, the hemispherical face 12, too, was quenched by radio frequency and was then cooled with water. After the tappet was tempered in an oil bath at 150°C, the tappet steel portion was machined to 25.0 mm by centerless grinding. Thereafter, the portion near the joined portion was machined and finished to 24.75 mm as in Fig. 4. As a result, crowning of the samples, which were machined at the portion near the joined portion, increased by 5 µm than those which were not machined, as the mean of twenty samples. Incidentally, crowning was measured as the difference in level between the center portion and the outer edge portion (25 mm). Example 8: Fig. 5 shows a tappet produced as an example of the sliding components according to the present invention. The sliding member had a dimension of an umbrella portion having a diameter of 30 mm, a neck portion having a diameter of 17 mm and a total height of 45 mm. The silicon nitride produced in Example 2 was machined into the sliding member 3 having a diameter of 30 mm and a thickness of 1.2 mm. The flatness of the face 10 and its surface roughness were the same as those of Example 2. An alloy steel nickel-chromium-molybdenum steel SNCM616 (JIS G4103) for machine structural use, which had been subjected to carburizing treatment (carburization depth: 0.5 mm) was used for the tappet main body 2. However, the carburizing layer on the joined face with the sliding member 3 was removed by machining. Joining to the sliding member 3 was carried out by holding the tappet main body 2 and the sliding member 3 in vacuum at 860°C for 10 minutes through an Ag-Cu-Ti type brazing material having a thickness of 70 µm. On the other hand, a commercially available cemented carbide was machined in the same way as the silicon nitride, and was joined to the tappet main body at 1,050°C by diffusion joining. The outer periphery 14 of the neck portion of the tappet so brazed was heated by radio frequency, and the entire tappet was cooled immediately thereafter with water. As a result, crowning increased by 10 µm and 7 µm, respectively, due to the quenching as the mean of twenty samples in the silicon nitride and the cemented carbide. Example 9: Fig. 6 shows a valve lifter produced as an example of the sliding components according to the present invention. An alloy steel nickel-chromium-molybdenum steel for machine structural use SNCM439 (JIS G4103) was used for the valve lifter main body 4. The dimensions of the sliding component included a diameter of 30 mm and a total height of 40 mm. The sliding face 10 is formed according to the present invention. A commercially available silicon nitride ceramic, a cemented carbide and the silicon nitride ceramic produced in Example 2, each having a diameter of 27.5 mm and a thickness of 6 mm, were used for the sliding member 5, and each was fitted with a press-in margin of 50 µm. The face 10 as the sliding face was machined in the same way as in Example 1. The outer peripheral face 11 was heated by an electron beam at an accelerated voltage of 7 kV for quenching. The shape of the sliding face 10 spherically swelled out by 7, 5 and 8 µm at the center portion in comparison with the outer edge portion (23 mm) as the mean of twenty samples due to the quenching treatment in each of the commercially available silicon nitride, the cemented carbide and the silicon nitride produced in Example 2, respectively, and the total crowning quantities are 14, 10 and 15 µm, respectively. Industrial Applicability: The present invention forms a crowning shape by applying a known surface quenching treatment to a portion made of the steel in a sliding component, changes this crowning shape by heat-treatment or machining of the steel portion after the surface quenching, forms at least one of the sliding faces forming a crowning shape by a member, preferably by a silicon nitride type ceramic having excellent flexural strength and high thermal shock resistance, and joins or fits this member to the sliding component. Therefore, the present invention provides the following effects. 1) Since the crowning shape is imparted by surface quenching treatment, and the heat-treatment and machining of the steel portion after the surface quenching, the portion to which this crowning shape is to be imparted and the quantity of crowning can be controlled. 2) The shape of the member before machining to be joined or fitted to the portion requiring sliding performance is a flat face, so that three-dimensional pre-machining is not necessary. Therefore, the sliding components can be economically provided. 3) Since the ceramics are joined or fitted as the sliding member to the portion requiring sliding performance, the sliding components can be provided economically.

Claims: A sliding component characterized in that at least one of members forming a crowning-shaped sliding face is joined or fitted to a portion made of steel and the crowning-shaped sliding face is formed by partially applying surface quenching treatment to said portion made of steel. A sliding component according to claim 1, wherein the surface area to which said surface quenching is applied is at least 30% with the respect to the area obtained by subtracting the portion shaped into the crowning shape from the entire surface of said component. A sliding component according to claim 2, wherein a difference in level between the center portion and an outer edge portion of said sliding face or in other words, a so-called "crowning quantity", is increased by applying the surface quenching treatment. A sliding component according to claim 2, wherein the crowning quantity is reduced by applying the surface quenching treatment. A sliding component according to any of claims 1 through 4, wherein the crowning quantity is increased by applying heat-treatment after the surface quenching treatment. A sliding component according to any of claims 1 through 5, wherein the crowning quantity is increased by machining a part, or the whole, of the steel portion after the surface quenching treatment. A sliding component according to any of claims 1 to 6, wherein at least one of the members forming the crowning-shaped sliding face by surface quenching treatment is made of a ceramic. A sliding component according to any of claims 1 to 7, wherein at least one of the members forming the crowning-shaped sliding face by surface quenching treatment is made of a silicon nitride type ceramic, and its strength at room temperature.and its temperature difference representing thermal shock resistance are at least 100 kg/mm 2 and at least 800°C, respectively. A production method of a sliding component characterized in that at least one of members forming a crowning-shaped sliding face is joined or fitted to a portion made of steel and the crowning-shaped sliding face is formed by applying partially surface quenching treatment to said portion made of steel. A production method of a sliding component according to claim 9, wherein the surface area to which the surface quenching is applied is at least 30% of the area obtained by subtracting the portion at which crowning is shaped from the entire surface of the sliding component. A production method of a sliding component according to claim 10, wherein the crowning quantity is increased at a certain portion by applying the surface quenching treatment. A production method of a sliding component according to claim 10, wherein the crowning quantity is decreased at a certain portion by applying the surface quenching treatment. A production method of a sliding component. according to any of claims 9 through 12, wherein the crowning quantity is increased by heat-treatment after the surface quenching treatment. A production method of a sliding component according to claim 13, wherein the temperature range of said heat-treatment is 100 to 700°C. A production method of a sliding component according to any of claims 9 through 14, wherein the crowning quantity is increased by machining a part, or the whole, of the steel portion after the surface quenching treatment. A production method of a sliding component according to claim 15, wherein said machining method is polishing. A production method of a sliding component according to any of claims 9 to 16, wherein at least one of the members forming the crowning-shaped sliding face by the surface quenching treatment is made of a ceramic. A production method of a sliding component according to any of claims 9 to 17, wherein at least one of the members forming the crowning-shaped sliding face by the surface quenching treatment is made of a silicon nitride type ceramic, and its strength at room temperature and its temperature difference representing thermal shock resistance are at least 100 kg/mm 2 and at least 800°C respectively.

Inventors: NISHIOKA TAKAO, TAKEUCHI HISAO, YAMAGIWA MASAMICHI, YAMAKAWA AKIRA, NISHIOKA, TAKAO, TAKEUCHI, HISAO, YAMAGIWA, MASAMICHI, YAMAKAWA, AKIRA, NISHIOKA, Takao, Itami Works, TAKEUCHI, Hisao, Itami Works, YAMAGIWA, Masamichi, Itami Works, Yamakawa, Akira, Itami Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1996
Main classification: F01L1
IPC codes: F01L1