buffer

Abstract

To provide a buffer that does not use hexavalent chromium, which is of concern for its effects on a human body and an environment, but uses trivalent chromium to achieve a high level of both high hardness and low friction and a method for manufacturing the same.SOLUTION: A buffer (100) comprises a cylinder (1) filled with hydraulic oil (3), a piston rod (2) movable inside the cylinder (1), and an oil seal (8) fixed to the cylinder (1) and sliding against the piston rod (2). The surface of the piston rod (2) has a hard layer obtained from a plating bath mainly composed of trivalent chromium. The hard layer contains both crystalline and amorphous parts in XRD analysis, and contains additives other than chromium.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a shock absorber and a method for manufacturing a shock absorber. [Background technology]

[0002] Hard chrome plating films have excellent properties such as wear resistance, corrosion resistance, and sliding properties, and are therefore widely used in various industrial fields, including automotive parts such as suspension rods, piston rings, and brake pistons, as well as shafts for hydraulic equipment and gravure rolls for printing equipment. For example, Patent Document 1 discloses a shock absorber in which a hard surface layer such as a chrome plating film or a nickel plating film is provided on the inner circumferential surface of the cylinder member for the purpose of stabilizing the frictional resistance of the shock absorber.

[0003] Traditionally, hexavalent chromium plating baths, using hexavalent chromium as the chromium component, have been used to form hard chromium plating films. However, in recent years, hexavalent chromium has been designated as a substance of high environmental concern under environmental regulations such as REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) and wastewater regulations, and there is a global demand for its reduction. Against this backdrop, development of hard chromium plating films using less toxic trivalent chromium compounds is progressing as an alternative technology to hard chromium plating films using hexavalent chromium. For example, Patent Document 2 discloses a hard trivalent chromium plating film that can obtain hardness and wear resistance equivalent to that of a hard hexavalent chromium plating film, with the aim of replacing hexavalent chromium plating baths. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 61-21439 [Patent Document 2] Special Publication No. 2010-540781 [Overview of the project] [Problems that the invention aims to solve]

[0005] For example, sliding parts such as suspension rods disclosed in Patent Document 1 require a high level of both hardness and sliding properties such as frictional force. Hard trivalent chromium plating films are known to achieve a hardness of 800 HV or more even after plating, comparable to hard hexavalent chromium plating, and can achieve 1000 HV or more by further heat treatment to form chromium carbides. However, properties other than hardness and wear resistance have not been well evaluated until now, and further research is needed to improve the sliding properties such as frictional force required for sliding parts such as suspension rods.

[0006] In view of the above circumstances, the object of the present invention is to provide a shock absorber and a method for manufacturing a shock absorber that achieve both high hardness and low friction at a high level, using trivalent chromium instead of hexavalent chromium, which is of concern for its impact on the human body and the environment. [Means for solving the problem]

[0007] One aspect of the present invention for achieving the above objective is a buffer comprising a cylinder filled with hydraulic fluid, a piston rod movable inside the cylinder, and an oil seal fixed to the cylinder and sliding with respect to the piston rod, wherein the surface of the piston rod has a hard layer obtained from a plating bath mainly composed of trivalent chromium, and the hard layer contains both crystalline and amorphous materials in XRD analysis and also contains additives other than chromium.

[0008] Furthermore, another aspect of the present invention for achieving the above objectives is a method for manufacturing the buffer of the present invention described above, comprising a plating step of forming a plating film consisting of a hard layer mainly composed of trivalent chromium on the surface of the piston rod, wherein the plating step is performed in a plating bath that does not contain hexavalent chromium salt but contains trivalent chromium salt, carboxylic acid, pH buffer and conductive salt, and has a pH ≤ 0.1, and the cathode current density is 100 A / dm 2A method for manufacturing a shock absorber, characterized by implementing as described above.

[0009] A more specific configuration of the present invention is described in the claims.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide a shock absorber and a method for manufacturing a shock absorber that do not use hexavalent chromium, which raises concerns about the impact on the human body and the environment, and that achieve both high hardness and low friction at a high level using trivalent chromium.

[0011] Problems, configurations, and effects other than those described above will be clarified by the description of the following embodiments.

Brief Description of the Drawings

[0012] [Figure 1] Schematic cross-sectional view showing an example of the shock absorber of the present invention [Figure 2] Graph showing the relationship between the friction force of the test pieces of Examples 1 to 3 and Comparative Examples 1 to 2 and the content of additives in the chromium plating film [Figure 3] Graph showing the relationship between the surface roughness Rz of the test pieces of Examples 1 to 3 and Comparative Examples 1 to 2 and the additive content in the chromium plating film [Figure 4] Graph showing the relationship between the crystallization rate of the chromium plating film of the test pieces of Examples 1 to 3 and Comparative Examples 1 to 2 and the additive content [Figure 5] Graph showing the XRD analysis results of Example 2 and Comparative Examples 1 to 2

Modes for Carrying Out the Invention

[0013] Hereinafter, a shock absorber and a method for manufacturing a shock absorber according to an embodiment of the present invention will be described. Note that the present invention is not construed as being limited thereto, and modifications can be made based on the knowledge and findings of those skilled in the art without departing from the scope of the present invention.

[0014] [Basic Idea of the Present Invention] Generally, the hexavalent chromium plating film used as a hard layer contains few additives other than chromium metal salts in the plating bath components, so the additive content in the plating film is also low and it tends to be crystalline. Also, since the friction coefficient as a chromium metal is small, it has been widely used as a hard plating film with high hardness and excellent wear resistance.

[0015] On the other hand, the trivalent chromium plating film being considered as an alternative technology to the hard hexavalent chromium plating film contains various additives such as complexing agents, conductive salts, pH buffers, and crystallization agents in addition to the trivalent chromium compound in order to improve the precipitation stability and film hardness of the plating bath. Therefore, unlike the crystalline hexavalent chromium plating film, since it contains many additives such as carbon, nitrogen, oxygen, and hydrogen other than chromium in the plating film, it is basically amorphous. For this reason, it has been found that the trivalent chromium plating film is difficult to improve the smoothness by polishing after plating compared to the hexavalent chromium plating film, and the frictional force is higher than that of the hard hexavalent chromium plating film.

[0016] The present inventor has intensively studied aiming to achieve both high hardness and low frictional force at a high level in the trivalent chromium plating film. As a result, it has been found that in the hard trivalent chromium plating film, by adjusting the ratio of the amorphous component and the crystalline component and including additives other than chromium, the surface roughness and frictional force of the plating film can be reduced, and a plating film suitable for a shock absorber can be obtained. The present invention is based on this finding.

[0017] Hereinafter, the shock absorber of the present invention will be described in detail.

[0018] [Outline of Shock Absorber] Figure 1 is a schematic cross-sectional view showing an example of a shock absorber of the present invention. Figure 1 shows an automotive hydraulic shock absorber as an example of a shock absorber. As shown in Figure 1, the shock absorber 100 of the present invention is a twin-tube type and comprises a cylinder 1 consisting of a cylindrical outer cylinder 11 having a bottom and an inner cylinder 12 provided inside the outer cylinder 11 coaxially with the outer cylinder 11, a piston rod 2 that is movable relative to the cylinder 1, and an oil seal 8 fixed to the cylinder 1 and sliding with the piston rod 2. Hydraulic fluid 3 for the hydraulic system and gas for volume compensation are injected into the reservoir chamber C inside the inner cylinder 12 and between the inner cylinder 12 and the outer cylinder 11.

[0019] The surface of the piston rod 2 has a hard layer 4, and a piston 5 is fixed to one end of the piston rod 2, which is inserted into the inner cylinder 12, while the other end is positioned to protrude from the cylinder 1. The outer circumference of the piston 5 is slidably fitted to the inner surface of the inner cylinder 12. A bottom valve 6 is fixed to the lower end of the inner cylinder 12, and a rod guide 7, which slidably guides the piston rod 2, is fixed to the upper end of the inner cylinder 12. A through hole is provided in the center of the rod guide 7, into which the piston rod 2 is inserted. An oil seal 8 is fixed to the upper part of the outer cylinder 11, and the inside of the outer cylinder 11 is sealed through the oil seal 8. The oil seal 8 and the piston rod 2 are fitted together, and the oil seal 8 prevents hydraulic fluid 3 for the hydraulic system from leaking to the outside of the cylinder 1 through the gap between the piston rod 2 and the rod guide 7.

[0020] As can be seen from Figure 1, in the automotive hydraulic shock absorber 100, the piston 5 separates the inside of the inner cylinder 12 into oil chamber A and oil chamber B. The bottom valve 6 separates the reservoir chamber C between the inner cylinder 12 and the outer cylinder 11 from oil chamber A. The piston 5 is equipped with a damping force generating mechanism 9 with a valve. When the piston rod 2 moves in the extension direction (the piston rod 2 moves upward in Figure 1), the valve of the damping force generating mechanism 9 opens, forming a small cross-sectional area flow path between oil chamber A and oil chamber B, thereby restricting the flow of hydraulic fluid 3 for the hydraulic system and generating a predetermined damping force. The bottom valve 6 is equipped with a damping force generating mechanism 10. The damping force generating mechanism 10 is composed of a small cross-sectional area flow path formed in the bottom valve 6. When the piston rod 2 moves in the contraction direction (the piston rod 2 moves downward in Figure 1), the hydraulic fluid 3 for the hydraulic system in oil chamber A flows through the small cross-sectional area flow path to reservoir chamber C, generating a damping force.

[0021] [Hard layer] The hard layer 4 of the present invention is a plating film characterized by having trivalent chromium as its main component, containing both crystalline and amorphous materials, and also containing additives other than chromium. By using such a chromium plating film, it is possible to reduce surface roughness during polishing after plating, thereby reducing the coefficient of friction. In other words, by providing the surface of the piston rod 2 with a hard layer 4 consisting of a smooth, low-friction hard trivalent chromium plating film, a low-friction automotive hydraulic shock absorber 100 can be realized.

[0022] [Additives for hard trivalent chromium plating films] Hard trivalent chromium plating films contain additives such as hydrogen, carbon, nitrogen, and oxygen. These are presumed to originate from organic components in the plating bath, such as carboxylates, pH buffers, and conductive salts. Furthermore, the additive content varies depending on the plating conditions, and it has been confirmed that it significantly contributes to pH and current density conditions in particular. This is presumed to be because the ability of chromium to incorporate additives during deposition affects the current density (reduction rate) and pH (complex formation).

[0023] The additive content in the plating film is preferably between 2.3% by mass and 4.3% by mass. Including 2.3% by mass or more of additives has the effect of reducing the coefficient of friction. This is presumed to be because the additives chemically adsorb with the oil in the buffer, improving the oil retention of the plating film surface. On the other hand, it was observed that increasing the additive content tended to increase the surface roughness Rz after polishing following plating. This is presumed to be because the additives in the plating film inhibit the polishing process and reduce smoothness. Therefore, the additive content of the hard trivalent chromium plating film of the present invention is preferably between 2.3% by mass and 4.3% by mass.

[0024] [Crystallization rate of hard trivalent chromium plating film] The crystallinity of the hard trivalent chromium plating film, as calculated by the following formula, is preferably 5-26%. A correlation was observed with the additive content of the plating film, and a tendency for the crystallinity to decrease with increasing additive content was confirmed. This is presumed to be because the additives reduce crystallinity. From the experimental results, based on the correlation between additive content and crystallinity, the crystallinity of the hard trivalent chromium plating film that yields the low frictional force described above was 5-26% when the additive content was between 2.3% by mass and 4.3% by mass. Crystallinity (peak integral intensity ratio) = (crystalline / (crystalline + amorphous)) × 100% ... (Equation 1) The integrated intensity ratio in (Equation 1) above is the value obtained in XRD (X-ray diffraction) analysis, where 2θ: 30~140°, full width at half maximum for crystalline materials: <3°, and full width at half maximum for amorphous materials: ≥3°.

[0025] [Hardness of hard trivalent chromium plating film] A hardness of 750 HV or higher, equivalent to industrial hexavalent chromium (JIS (Japanese Industrial Standards) H8615), is desirable for the hardness of the hard trivalent chromium plating film, but 800 HV or higher is preferable to improve the wear resistance of the piston rod. Furthermore, the hard trivalent chromium plating film of the present invention can be made even harder by heat treatment after plating.

[0026] [Surface roughness of hard trivalent chromium plating film] The surface roughness (Rz) of the hard trivalent chromium plating film is preferably 0.6 μm or less. The hard trivalent chromium plating film of the present invention can reduce the frictional force to that of hard hexavalent chromium or less by achieving a surface roughness Rz of 0.6 or less after polishing. While there are no particular limitations on the polishing method, a super-finish process using a polishing film is preferred.

[0027] [Method of manufacturing a shock absorber] The present invention relates to a method for manufacturing a buffer, which involves plating the piston rod 2 of the buffer with a hard layer 4 made of the hard trivalent chromium plating film described above. The plating bath for the hard trivalent chromium plating film contains, as described above, a trivalent chromium salt as the main component, and additives such as a complexing agent, a pH buffering agent, and a conductive salt. As the trivalent chromium salt, chromium chloride, chromium sulfate, and basic chromium sulfate can be used, but chromium chloride is particularly preferred. As the complexing agent, carboxylate salts such as glycine, formic acid, oxalic acid, and acetic acid can be used, but glycine is particularly preferred. As the pH buffering agent, boric acid and citric acid can be used, but boric acid is particularly preferred. As the conductive salt, ammonium chloride, ammonium sulfate, and ammonium sulfamate can be used, but ammonium chloride is particularly preferred.

[0028] A strongly acidic pH is preferable for the plating bath, with a pH of 0.1 or lower being ideal. If the pH is 4.0 or higher, it is not possible to obtain a plating film containing both crystalline and amorphous materials. A higher current density is preferable, ideally 100 A / dm². 2 The above is preferable. Current density 50 A / dm 2 In the following cases, even with a pH of 0.1, it is not possible to obtain a plating film containing both crystalline and amorphous materials. As mentioned above, this is presumed to be because the crystallization rate is influenced by the current density (reduction rate) and pH (complex formation).

[0029] There are no particular limitations on the plating bath temperature or stirring method; the temperature can be 10-90°C, and stirring methods such as air and agitators can be used. Furthermore, materials with good insolubility, such as Pt, Ti, Ir, and carbon, can be used as anode materials.

[0030] In the above-described embodiment, an automotive hydraulic shock absorber was explained in which a hard trivalent chromium plating film is provided on the surface of the piston rod 2. However, depending on the corrosion resistance specifications required for the product, a two-layer structure can also be adopted in which a nickel plating film is provided as an intermediate layer between the piston rod 2 and the hard trivalent chromium plating film. Furthermore, the hard trivalent chromium plating film of the present invention can also be provided on sliding parts other than the surface of the piston rod 2. For example, it is also suitable to provide it on the surface of the cylinder inner cylinder 21, which is a sliding surface against the piston 5. [Examples]

[0031] An example in which a hard layer 4 of a hard trivalent chromium plating film was formed on the surface of the piston rod 2 described above, and the crystallinity, surface roughness Rz, hardness, and frictional force were evaluated is described below.

[0032] [Examples 1-3] In Examples 1-3, test specimens were prepared by forming a 20 μm thick hard trivalent chromium film on the surface of a copper plate for additive analysis and crystallization rate measurement. In addition, test specimens were prepared by forming a 10 μm thick nickel plating film on the surface of a carbon steel piston rod, and then forming a 10 μm thick hard trivalent chromium plating film on top of the nickel plating film, for hardness measurement, surface roughness measurement after polishing, and friction force evaluation.

[0033] A Watt bath was used to form the nickel plating film. For the formation of the hard trivalent chromium plating film, a plating bath mainly composed of trivalent chromium salt, complexing agent, pH buffer, and conductive salt was used. The plating conditions were a pH of 0.1 and a current density of 100-400 A / dm². 2 The temperature was set to 50°C. Pt / Ti was used as the anode material. The piston rod test specimens were plated and then subjected to a super-finish process using a polishing film.

[0034] [Comparative Example 1] Test specimens were prepared using carbon steel as the base material, with a hard hexavalent chromium plating film formed on its surface. For additive analysis and crystallinity measurement, test specimens were prepared with a 20 μm thick hard hexavalent chromium plating film formed on the surface of a copper plate. In addition, for hardness measurement, surface roughness measurement after polishing, and friction force evaluation, test specimens were prepared with a 20 μm thick hard hexavalent chromium plating film formed on the surface of a carbon steel piston rod.

[0035] A HEEF bath was used to form the hard hexavalent chromium plating film. The plating conditions were a current density of 60 A / dm². 2 The temperature was set to 60°C. A lead alloy was used as the anode material. The piston rod test specimens were plated and then subjected to a super-finish process using a polishing film.

[0036] [Comparative Example 2] Comparative Example 2 involved preparing test specimens of a conventional hard trivalent chromium plating film in the same manner as in Example 1. The conventional hard trivalent chromium plating film was formed using a plating bath primarily composed of trivalent chromium salt, complexing agent, pH buffer, and conductive salt. The plating conditions were pH 5.5 and a current density of 40 A / dm². 2 The temperature was set to 50°C. Pt / Ti was used as the anode material. The piston rod test specimens were plated and then subjected to a super-finish process using a polishing film.

[0037] [Crystallinity measurement] XRD analysis was used to measure the crystallinity of hard chromium plating films. The measurement conditions were a Cu X-ray source and a diffraction angle of 20 ≤ 2θ ≤ 140°. The crystallinity was calculated as the ratio of the sum of the integrated intensities of crystalline peaks to the sum of the integrated intensities of all peaks, obtained in the range of 30 ≤ 2θ ≤ 140° (see Equation 1). Crystalline and amorphous materials were distinguished by the full width at half maximum (FWHM) of the peaks, with FWHM < 3° indicating crystalline and FWHM ≥ 3° indicating amorphous. Crystallinity (peak integral intensity ratio) = (crystalline / (crystalline + amorphous)) × 100 ... (Equation 1).

[0038] [Additive analysis] For the analysis of additives in the chromium plating film, semi-quantitative analysis was performed using glow discharge emission spectroscopy (GD-OES). The analytical conditions were a high-frequency power of 35W, a pulse frequency of 100Hz, and the gas species Ar.

[0039] [Surface roughness measurement] For measuring the surface roughness of the plated film after polishing, a surface roughness measuring instrument (manufactured by TDS Corporation, product name: Surfcom 1500 DX) was used. The measurement conditions followed JIS B0633, with a measurement distance of 4 mm.

[0040] [Hardness measurement] A Vickers hardness tester was used to measure the hardness. The measurement conditions were a load of 25 gf and a load holding time of 15 s.

[0041] [Friction force evaluation] A micro-amplitude vibrator test was used to evaluate the frictional force between the piston rod and the oil seal. The test setup consisted only of the piston rod and oil seal, in an upright position. The test conditions were gas and oil present, a sliding width of ±1 mm, and a frequency of 10 Hz.

[0042] [Characteristic evaluation results] Table 1 shows the additive content, crystallinity, hardness, surface roughness after polishing, and frictional force for Examples 1-3 and Comparative Examples 1 and 2.

[0043] [Table 1]

[0044] (Additive ingredients and additive content) The main additive components of Examples 1-3 were hydrogen (H), carbon (C), nitrogen (N), and oxygen (O), with additive content ranging from 2.3 to 4.3% by mass. On the other hand, the main additive components of Comparative Examples 1 and 2 included sulfur (S) in addition to hydrogen, carbon, nitrogen, and oxygen, with additive content of 0.4% by mass for Comparative Example 1 and 21.4% by mass for Comparative Example 2. As a result, it was found that the additive content of the hard trivalent chromium plating film of the present invention was higher than that of the hard hexavalent chromium plating film and lower than that of the conventional trivalent chromium plating film. Furthermore, it was found that the additive components of the hard trivalent chromium plating film of the present invention differ from those of the hard hexavalent chromium plating film and the conventional hard trivalent chromium plating film in that they do not contain sulfur.

[0045] (crystallinity and crystallinity) Figure 5 is a graph showing the XRD analysis results for Example 2 and Comparative Examples 1-2. As shown in Figure 5, Comparative Example 1 is presumed to be crystalline because only a sharp peak derived from chromium was detected. Furthermore, the prominent (222) peak suggests that it has an orientation with a preferred orientation of (111). Comparative Example 2 is presumed to be amorphous because only a broad peak was detected. In Example 2, a sharp peak derived from chromium was detected in addition to the broad peak, suggesting that it contains both crystalline and amorphous materials. The crystallinity rates were 5-26% for Examples 1-3, 100% for Comparative Example 1, and 0% for Comparative Example 2. These results show that the hard trivalent chromium plating film of the present invention differs from hard hexavalent chromium plating films and conventional hard trivalent chromium plating films in that it contains both crystalline and amorphous materials.

[0046] (Surface roughness after polishing) Surface roughness after polishing (R z The surface roughness (R) of the hard trivalent chromium plating film of the present invention after polishing was 0.52 to 0.58 μm for Examples 1 to 3, 0.42 μm for Comparative Example 1, and 0.79 μm for Comparative Example 2. As a result, the surface roughness (R) of the hard trivalent chromium plating film of the present invention after polishing was z It was found that the cost was higher than that of hard hexavalent chromium plating films, but lower than that of conventional hard trivalent chromium plating films.

[0047] (Hardness) The hardness values were 827 - 894 HV for Examples 1 - 3, 974 HV for Comparative Example 1, and 919 HV for Comparative Example 2. As a result, it was confirmed that the hardness of the hard trivalent chromium plating film of the present invention is slightly lower than that of the hard hexavalent chromium plating film and the conventional hard trivalent chromium plating film, but is 800 HV or more, which can ensure the wear resistance of the piston rod.

[0048] (Frictional force) The frictional forces were 33.4 - 39.5 N for Examples 1 - 3, 47.3 N for Comparative Example 1, and 70.6 N for Comparative Example 2. As a result, it was found that the frictional force of the hard trivalent chromium plating film of the present invention is lower than that of the hard hexavalent chromium plating film and the conventional hard trivalent chromium plating film.

[0049] [Discussion of results] The results of various property evaluations of Examples 1 - 3 and Comparative Examples 1 and 2 will be discussed. Figure 2 is a graph showing the relationship between the frictional force and the content of the additive in the chromium plating film. As a result, it was confirmed that the frictional force has a correlation with the additive content, and the lowest value is shown when the additive content is around 2 - 3 mass%. That is, it is suggested that there is an optimum value for the additive content to obtain a low frictional force.

[0050] Figure 3 is a graph showing the relationship between the surface roughness R z after polishing of the shock absorber and the content of the additive in the chromium plating film. As a result, it was confirmed that the surface roughness (R z ) after polishing increases in proportion to the additive content. From the results of Figure 2 and Figure 3, although the frictional force tends to decrease as the additive content increases, on the other hand, as the additive content increases, the surface roughness (R z ) after polishing increases. Therefore, in combination with Figure 2, it is inferred that there is an optimum value for the additive content regarding the frictional force.

[0051] Furthermore, it is presumed that the reason for the reduction in frictional force due to the inclusion of additives is that the additives chemically adsorb with the oil, improving the oil retention of the plating film surface. In addition, it is presumed that the reason for the decrease in surface roughness after polishing due to the increase in additive content is that the additives inhibit the polishing process and reduce smoothness.

[0052] Figure 4 is a graph showing the relationship between the crystallinity rate and additive content of a chromium plating film. From these results, it was found that the crystallinity rate decreases with increasing additive content, and becomes amorphous when the additive content exceeds approximately 5% by mass. In other words, it is inferred that there is a correlation between the crystallinity rate and the additive content in the range of 5% by mass or less. From these results, it can be confirmed that the crystallinity rate of a hard trivalent chromium plating film (additive content 2.3-4.3% by mass), which can obtain lower friction force than conventional hard hexavalent chromium, is 5-26%. It is inferred that the reason for the correlation between the crystallinity rate and additive content is that the additives reduce the crystallinity.

[0053] [Examples 4-5 and Comparative Examples 3-6] In this example, the crystallinity of the plated film was investigated when the current density was varied using two types of hard trivalent chromium plating baths with different pH levels. Table 2 shows the plating conditions for Examples 4-5 and Comparative Examples 3-6. For Examples 4, 5, and Comparative Example 3, a glycine-containing plating bath with a pH of 0.1 was used, and the current density was 40-400 A / dm². 2 Comparative Examples 4-6 used a plating bath with formic acid at pH 5.5, and the current density was 40-400 A / dm². 2 This was the result. In this experiment, a copper plate was used as the base material. The results are shown in Table 2.

[0054] [Table 2]

[0055] As a result, only Examples 4 and 5 yielded plating films containing both crystalline and amorphous materials. Specifically, in a plating bath with pH 0.1 containing glycine, a current density of 100 A / dm² was obtained. 2It was found that unless the above conditions are met, a plating film containing both crystalline and amorphous materials cannot be formed.

[0056] The results are discussed below. Conventional hard trivalent chromium plating films tend to be amorphous due to the high additive content. Also, it is generally thought that additives are less likely to be incorporated into the plating film at higher current densities. Therefore, it is presumed that even in hard trivalent chromium plating films, the additive content will be lower and the crystallization rate will improve under high current density conditions. However, in the formic acid-containing plating baths with a pH of 5.5 in Comparative Examples 4-6, the current density was 100 A / dm 2 The material remained amorphous even under these conditions. This suggests that the crystallinity is influenced not only by the current density but also by the pH and the additive components of the plating bath. This is presumed to be because the current density is a factor in the reduction rate of chromium ions, and pH is a factor in the complex formation of chromium ions.

[0057] As explained above, the present invention has demonstrated that it is possible to provide a shock absorber and a method for manufacturing a shock absorber that achieve both high hardness and low friction at a high level, using trivalent chromium instead of hexavalent chromium, which is a concern for its impact on the human body and the environment.

[0058] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are included. The embodiments described above are for illustrative purposes only and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to replace parts of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add configurations from other embodiments to the configuration of one embodiment. In addition, it is possible to add, delete, or replace parts of the configuration of each embodiment with other configurations. [Explanation of symbols]

[0059] 100... shock absorber, 1... cylinder, 2... piston rod, 3... hydraulic fluid for hydraulic systems. 4...Hard layer, 5...Piston, 6...Bottom valve, 7...Rod guide, 8...Oil seal, 9, 10... Damping force generating mechanism, 11... Outer cylinder, 12... Inner cylinder, A, B... Oil chambers, C... Reservoir chamber.

Claims

1. A cylinder filled with hydraulic fluid, A piston rod that can move inside the cylinder, A shock absorber comprising an oil seal fixed to the cylinder and sliding with the piston rod, The surface of the piston rod has a hard layer obtained from a plating bath mainly composed of trivalent chromium. The buffer is characterized in that the hard layer contains both crystalline and amorphous materials as determined by XRD analysis, and contains additives other than chromium in an amount of 2.3% by mass or more and 4.3% by mass or less.

2. A cylinder filled with hydraulic fluid, A piston rod that can move inside the cylinder, A shock absorber comprising an oil seal fixed to the cylinder and sliding with the piston rod, The surface of the piston rod has a hard layer obtained from a plating bath mainly composed of trivalent chromium. The buffer is characterized in that the hard layer contains both crystalline and amorphous materials as determined by XRD analysis, and contains additives other than chromium, with the ratio of crystalline material to the total of crystalline and amorphous materials being 5 to 26%.

3. The buffer according to claim 1, characterized in that the ratio of crystalline material to the total of crystalline and amorphous material in the hard layer is 5 to 26%.

4. The buffer according to any one of claims 1 to 3, characterized in that the additive contains at least one element among hydrogen, carbon, nitrogen, and oxygen.

5. The buffer according to any one of claims 1 to 4, characterized in that the salt of trivalent chromium in the plating bath is chromium chloride.

6. The shock absorber according to any one of claims 1 to 5, characterized in that it has a nickel plating film as an intermediate layer between the surface of the piston rod and the hard layer.

7. The shock absorber according to any one of claims 1 to 6, characterized in that the Vickers hardness HV of the hard layer is 750 HV or more.

8. The buffer according to any one of claims 1 to 7, characterized in that the surface roughness Rz of the hard layer after polishing is 0.6 μm or less.

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