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] The present 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 automobile 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 peripheral surface of a cylinder member in order to stabilize the friction resistance of the shock absorber.

[0003] Conventionally, a hexavalent chromium plating bath using hexavalent chromium as a chromium component has been used to form a hard chromium plating film. However, in recent years, hexavalent chromium has been designated as a substance of high environmental concern in environmental regulations such as the REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) regulation and wastewater regulations, and its use is desired to be reduced worldwide. In light of this background, development of a hard chromium plating film using a less toxic trivalent chromium compound is being promoted as an alternative technology to a hard chromium plating film using hexavalent chromium. For example, Patent Document 2 discloses a hard trivalent chromium plating film that can obtain the same hardness and wear resistance as a hard hexavalent chromium plating film with the aim of replacing a hexavalent chromium plating bath. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 61-21439 [Patent Document 2] Special Publication No. 2010-540781 Summary of the Invention [Problem to be solved by the invention]

[0005] For example, sliding parts such as the suspension rod disclosed in Patent Document 1 need to have both high hardness and sliding properties such as frictional force. It is known that hard trivalent chromium plating film can obtain 800 HV or more even after plating, which is equivalent to hard hexavalent chromium plating, and can obtain 1000 HV or more by further performing heat treatment because chromium carbide is formed. However, properties other than hardness and wear resistance have not been evaluated much so far, and further study is required to improve sliding properties such as frictional force required for sliding parts such as suspension rods.

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

[0007] One aspect of the present invention for achieving the above-mentioned object is a shock absorber including a cylinder filled with hydraulic oil, a piston rod movable inside the cylinder, and an oil seal fixed to the cylinder and sliding against the piston rod, wherein the shock absorber has a hard layer on the surface of the piston rod obtained from a plating bath containing trivalent chromium as a main component, and the hard layer contains both crystalline and amorphous parts in XRD analysis, and contains additives other than chromium.

[0008] In another embodiment of the present invention for achieving the above object, the method for manufacturing a shock absorber of the present invention described above further comprises a plating step of forming a plating film made of a hard layer mainly composed of trivalent chromium on the surface of the piston rod, the plating step being carried out in a plating bath not containing hexavalent chromium salt, but containing trivalent chromium salt, carboxylic acid, a pH buffer and a conductive salt, and having a pH of 0.1 or less, with a cathode current density of 100 A / dm 2This is a manufacturing method for a shock absorber, which is characterized by being carried out as described above.

[0009] More specific configurations of the present invention are described in the claims. Effect 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 achieves both high hardness and low friction at a high level by using trivalent chromium instead of hexavalent chromium, which is a concern for its impact on the human body and the environment.

[0011] Problems, configurations and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of a shock absorber according to the present invention. [Diagram 2] Graph showing the relationship between the frictional force of the test pieces of Examples 1 to 3 and Comparative Examples 1 and 2 and the content of additives in the chrome plating film [Diagram 3] Graph showing the relationship between the surface roughness Rz of the test pieces of Examples 1 to 3 and Comparative Examples 1 and 2 and the additive content in the chrome plating film [Figure 4] Graph showing the relationship between the crystallization rate and the additive content of the chromium plating film of the test pieces of Examples 1 to 3 and Comparative Examples 1 and 2 [Diagram 5] Graph showing XRD analysis results of Example 2 and Comparative Examples 1 and 2 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] A shock absorber and a manufacturing method thereof according to an embodiment of the present invention will be described below. Note that the present invention is not limited to this embodiment, and improvements can be made based on the knowledge and findings of a person skilled in the art without departing from the scope of the present invention.

[0014] [Basic concept of the present invention] Generally, hexavalent chromium plating films used as hard layers do not contain many additives other than chromium metal salts in the plating bath, so the additive content in the plating film is low and it tends to be crystalline. In addition, since the friction coefficient of 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, trivalent chromium plating films, which are being considered as an alternative technology to hard hexavalent chromium plating films, contain various additives such as complexing agents, conductive salts, pH buffers, and crystallizing agents in addition to trivalent chromium compounds in order to improve the deposition stability of the plating bath and the hardness of the film. Therefore, unlike hexavalent chromium plating films, which are crystalline, trivalent chromium plating films are basically amorphous because they contain many additives other than chromium, such as carbon, nitrogen, oxygen, and hydrogen. For this reason, it is more difficult to improve the smoothness of trivalent chromium plating films by polishing after plating compared to hexavalent chromium plating films, and it has been found that the frictional force is higher than that of hard hexavalent chromium plating films.

[0016] The present inventors have conducted extensive research with the aim of achieving both high hardness and low friction in a trivalent chromium plating film. As a result, they have found that by adjusting the ratio of amorphous components to crystalline components in a hard trivalent chromium plating film and including additives other than chromium, the surface roughness and friction 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] The shock absorber of the present invention will be described in detail below.

[0018] [Summary of the shock absorber] FIG. 1 is a schematic cross-sectional view showing an example of a shock absorber of the present invention. FIG. 1 shows an example of a shock absorber, which is a hydraulic shock absorber for an automobile. As shown in FIG. 1, a shock absorber 100 of the present invention is of a double-cylinder type, and includes a cylinder 1 consisting of a cylindrical outer cylinder 11 having a bottom and an inner cylinder 12 provided coaxially with the outer cylinder 11 inside the outer cylinder 11, a piston rod 2 movable relative to the cylinder 1, and an oil seal 8 fixed to the cylinder 1 and sliding with the piston rod 2. Hydraulic device oil 3 and gas for volume compensation are injected into a 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, one end of which is inserted into the inner cylinder 12 to which a piston 5 is fixed, and the other end is disposed so as to protrude from the cylinder 1. The outer periphery of the piston 5 slidably fits into 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 for slidably guiding 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, and the piston rod 2 is inserted into this through hole. 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 via the oil seal 8. The oil seal 8 fits into the piston rod 2, and the oil seal 8 prevents hydraulic oil 3 for hydraulic equipment from leaking out of the cylinder 1 through a gap between the piston rod 2 and the rod guide 7.

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

[0021] [Hard layer] The hard layer 4 of the present invention is a plating film that contains trivalent chromium as a main component, contains both crystalline and amorphous, and contains additives other than chromium. By using such a chromium plating film, it is possible to reduce the surface roughness in the polishing process after plating, and to reduce the friction coefficient. In other words, by providing the hard layer 4 made of a smooth, hard trivalent chromium plating film with a low friction coefficient on the surface of the piston rod 2, it is possible to realize an automobile hydraulic shock absorber 100 with low friction force.

[0022] [Additives for hard trivalent chromium plating] Hard trivalent chromium plating films contain additives such as hydrogen, carbon, nitrogen and oxygen. These are presumed to originate from organic components such as carboxylates, pH buffers and conductive salts contained in the plating bath. It was also confirmed that the additive content varies depending on the plating treatment conditions, with the pH and current density conditions contributing significantly in particular. This is presumed to be because the ability of additives to be absorbed when chromium is deposited affects the current density (reduction rate) and pH (complex formation).

[0023] The content of the additives contained in the plating film is preferably 2.3% by mass or more and 4.3% by mass or less. By containing 2.3% by mass or more of the additives, the effect of reducing the friction coefficient can be obtained. This is presumably because the additives undergo chemical adsorption with the oil in the shock absorber, improving the oil retention of the plating film surface. On the other hand, it has been confirmed that the surface roughness Rz after polishing after plating tends to increase as the additive content increases. This is presumably because the additives contained in the plating film inhibit the polishing process and reduce the smoothness. Therefore, the additive content of the hard trivalent chromium plating film of the present invention is preferably 2.3% by mass or more and 4.3% by mass or less.

[0024] [Crystallization rate of hard trivalent chromium plating film] The crystallization rate of the hard trivalent chromium plating film calculated by the following formula is preferably 5 to 26%. A correlation with the additive content of the plating film was observed, and it was confirmed that the crystallization rate tends to decrease as the additive content increases. This is presumed to be because the additives reduce crystallinity. As a result of the experiment, from the correlation between the additive content and the crystallization rate, the crystallization rate of the hard trivalent chromium plating film that can obtain the above-mentioned low friction force was 5 to 26% when the additive content was 2.3 mass% or more and 4.3 mass% or less. Crystallinity ratio (peak integrated intensity ratio) = (crystalline / (crystalline + amorphous)) x 100%... (Equation 1) The integrated intensity ratio in the above formula (1) is a value determined in an XRD (X-ray diffraction) analysis under the conditions of 2θ: 30 to 140°, half-width of crystalline material: <3°, and half-width of amorphous material: ≧3°.

[0025] [Hardness of hard trivalent chromium plating film] The hardness of the hard trivalent chromium plating film is preferably 750HV or more, which is equivalent to industrial hexavalent chromium (JIS (Japanese Industrial Standards) H8615), but in order to improve the wear resistance of the piston rod, 800HV or more is preferable. Note that the hard trivalent chromium plating film of the present invention can be further hardened by subjecting it to a 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 making the surface roughness Rz after polishing 0.6 or less. There is no particular limitation on the polishing method, but super-finish processing using a polishing film is preferable.

[0027] [Shock absorber manufacturing method] In the manufacturing method of the shock absorber of the present invention, the piston rod 2 of the shock absorber is plated with a hard layer 4 made of the above-mentioned hard trivalent chromium plating film. As described above, the plating bath for the hard trivalent chromium plating film contains a trivalent chromium salt as a main component, and additives such as a complexing agent, a pH buffer, and a conductive salt. As the trivalent chromium salt, chromium chloride, chromium sulfate, basic chromium sulfate, etc. can be used, with chromium chloride being particularly preferred. As the complexing agent, glycine, formic acid, oxalic acid, acetic acid, etc., which are carboxylates, can be used, with glycine being particularly preferred. As the pH buffer, boric acid and citric acid can be used, with boric acid being particularly preferred. As the conductive salt, ammonium chloride, ammonium sulfate, ammonium sulfamate, etc. can be used, with ammonium chloride being particularly preferred.

[0028] The pH of the plating bath should be strongly acidic, preferably below pH 0.1. If the pH is above 4.0, a plating film containing both crystalline and amorphous materials cannot be obtained. The higher the current density, the better, preferably above 100 A / dm. 2 Current density of 50A / dm or more is preferable. 2 In the following cases, even at a pH of 0.1, it is not possible to obtain a plating film that contains both crystalline and amorphous materials. As mentioned above, this is presumably because the crystallization rate is affected by the current density (reduction rate) and pH (complex formation).

[0029] The plating bath temperature and stirring method are not particularly limited, and the temperature can be 10 to 90° C., and the stirring method can be air, a stirrer, etc. As for the anode material, Pt, Ti, Ir, carbon, etc., which have good insolubility, can be used.

[0030] In the above embodiment, the hydraulic shock absorber for an automobile having a hard trivalent chromium plating film on the surface of the piston rod 2 has been described, but a two-layer structure can be formed by providing a nickel plating film between the piston rod 2 and the hard trivalent chromium plating film, depending on the corrosion resistance specifications required for the product. 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 preferable to provide it on the surface of the cylinder inner tube 21, which is the sliding surface for the piston 5. EXAMPLES

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

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

[0033] A Watts bath was used to form the nickel plating film. A plating bath containing trivalent chromium salt, complexing agent, pH buffer and conductive salt as main components was used to form the hard trivalent chromium plating film. The plating conditions were pH 0.1, current density 100-400 A / dm 2 The plating temperature was 50°C. Pt / Ti was used as the anode material. After plating, the piston rod test pieces were subjected to super-finish processing using a polishing film.

[0034] [Comparative Example 1] A test piece was prepared by forming a hard hexavalent chromium plating film on the surface of a carbon steel substrate. A test piece was prepared by forming a hard hexavalent chromium plating film 20μm thick on the surface of a copper plate for additive analysis and crystallization rate measurement. In addition, a test piece was prepared by forming a hard hexavalent chromium plating film 20μm thick on the surface of a carbon steel piston rod for hardness measurement, surface roughness measurement after polishing, and friction force evaluation.

[0035] A HEEF bath was used to form the hard hexavalent chromium plating film. The plating conditions were a current density of 60 A / dm 2 The temperature was 60°C. A lead alloy was used as the anode material. After plating, the piston rod test pieces were subjected to super-finish processing using a polishing film.

[0036] [Comparative Example 2] In Comparative Example 2, a test piece was prepared using a conventional hard trivalent chromium plating film in the same manner as in Example 1. A plating bath containing trivalent chromium salt, a complexing agent, a pH buffer, and a conductive salt as main components was used to form the conventional hard trivalent chromium plating film. The plating treatment conditions were pH 5.5, current density 40 A / dm 2 The plating temperature was 50°C. Pt / Ti was used as the anode material. After plating, the piston rod test pieces were subjected to super-finish processing using a polishing film.

[0037] [Crystallinity measurement] XRD analysis was used to measure the crystallization rate of the hard chrome plating film. The measurement conditions were Cu X-ray source and diffraction angle 20≦2θ≦140°. The crystallization rate was calculated as the ratio of the total integrated intensity of the crystalline peak to the total integrated intensity of all peaks from the peak integrated intensity ratio found in the range of 30≦2θ≦140° (see formula 1). Crystalline and amorphous were distinguished by the half-width of the peak, with a half-width <3° being crystalline and a half-width ≧3° being amorphous. Crystallinity ratio (peak integrated intensity ratio) = (crystalline / (crystalline + amorphous)) x 100... (Equation 1).

[0038] [Additive analysis] The additives in the chromium plating film were analyzed by semi-quantitative analysis using glow discharge optical emission spectroscopy (GD-OES). The analysis conditions were high-frequency power 35 W, pulse frequency 100 Hz, and Ar gas type.

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

[0040] [Hardness measurement] The hardness was measured using a Vickers hardness tester under the following measurement conditions: load 25 gf, load holding time 15 s.

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

[0042] [Characteristics evaluation results] Table 1 shows the additive contents, crystallization rates, hardness, surface roughness after polishing, and frictional force of Examples 1 to 3 and Comparative Examples 1 and 2.

[0043] [Table 1]

[0044] (Additive ingredients and additive content) The main additive components in Examples 1 to 3 were hydrogen (H), carbon (C), nitrogen (N) and oxygen (O), and the additive content was 2.3 to 4.3 mass%. On the other hand, the main additive components in Comparative Examples 1 and 2 included sulfur (S) in addition to hydrogen, carbon, nitrogen and oxygen, and the additive content was 0.4 mass% in Comparative Example 1 and 2, and 21.4 mass% in 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. It was also found that the additive components of the hard trivalent chromium plating film of the present invention did not contain sulfur components, unlike the hard hexavalent chromium plating film and the conventional hard trivalent chromium plating film.

[0045] (Crystallization and crystallinity) FIG. 5 is a graph showing the XRD analysis results of Example 2 and Comparative Examples 1 and 2. As shown in FIG. 5, in Comparative Example 1, only a sharp peak derived from chromium was detected, so it is presumed to be crystalline. In addition, since the (222) peak is prominent, it is presumed to have an orientation with a preferred orientation of (111). In Comparative Example 2, only a broad peak was detected, so it is presumed to be amorphous. In Example 2, a sharp peak derived from chromium was detected in addition to the broad peak, so it is presumed to contain both crystalline and amorphous. In addition, the crystallization rate was 5 to 26% in Examples 1 to 3, 100% in Comparative Example 1, and 0% in Comparative Example 2. As a result, it was found that the hard trivalent chromium plating film of the present invention is a plating film containing both crystalline and amorphous, unlike the hard hexavalent chromium plating film and the conventional hard trivalent chromium plating film.

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

[0047] (hardness) The hardness was 827 to 894HV in Examples 1 to 3, 974HV in Comparative Example 1, and 919HV in 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 800HV or more, which is sufficient to ensure the wear resistance of the piston rod.

[0048] (frictional force) The frictional force was 33.4 to 39.5 N in Examples 1 to 3, 47.3 N in Comparative Example 1, and 70.6 N in Comparative Example 2. As a result, it was found that the frictional force of the hard trivalent chromium plating film of the present invention was 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 the various characteristic evaluations of Examples 1 to 3 and Comparative Examples 1 and 2 will be considered. Figure 2 is a graph showing the relationship between frictional force and the content of additives in the chrome plating film. As a result, it can be confirmed that there is a correlation between frictional force and the content of additives, with the minimum value being observed when the content of additives is approximately 2 to 3 mass%. In other words, this suggests that there is an optimal value for the content of additives in order to obtain low frictional force.

[0050] Figure 3 shows the surface roughness R of the shock absorber after polishing. z The graph shows the relationship between the amount of additives in the chrome plating film and the surface roughness (R z ) increases in proportion to the additive content. From the results in Figures 2 and 3, it can be seen that the friction force tends to decrease as the additive content increases, but on the other hand, as the additive content increases, the surface roughness (R z ) increases, and so, taken together with Figure 2, it is inferred that there is an optimum value for the additive content with respect to friction force.

[0051] The reason for the reduction in frictional force due to the inclusion of additives is believed to be that the additives undergo chemical adsorption with oil, improving the oil retention of the plating film surface. Also, the reason for the decrease in surface roughness after polishing due to an increase in the additive content is believed to be that the additives inhibit the polishing process and reduce smoothness.

[0052] FIG. 4 is a graph showing the relationship between the crystallization rate and the additive content of the chrome plating film. From this result, it was found that the crystallization rate decreases with an increase in the additive content, and when the additive content exceeds about 5 mass%, the film becomes amorphous. In other words, it is presumed that there is a correlation between the crystallization rate and the additive content when the additive content is 5 mass% or less. From this result, it can be confirmed that the crystallization rate of the hard trivalent chrome plating film (additive content 2.3 to 4.3 mass%), which provides lower frictional force than conventional hard hexavalent chrome, is 5 to 26%. It is presumed that the correlation between the crystallization rate and the additive content is due to the additives reducing crystallinity.

[0053] [Examples 4 to 5 and Comparative Examples 3 to 6] In this example, two types of hard trivalent chromium plating baths with different pH values ​​were used to study the crystallinity of the plating film when the current density was changed. Table 2 shows the plating treatment conditions for Examples 4 to 5 and Comparative Examples 3 to 6. In Examples 4 and 5 and Comparative Example 3, a plating bath containing glycine and having a pH of 0.1 was used, and the current density was 40 to 400 A / dm 2 In Comparative Examples 4 to 6, the plating bath contained formic acid and had a pH of 5.5, and the current density was 40 to 400 A / dm 2 In this experiment, a copper plate was used as the substrate. The results are shown in Table 2.

[0054] [Table 2]

[0055] As a result, plating films containing both crystalline and amorphous phases were obtained only in Examples 4 and 5. That is, in a plating bath containing glycine and having a pH of 0.1, the current density was 100 A / dm 2It was found that if the above conditions were not met, a plating film containing both crystalline and amorphous portions could not be formed.

[0056] These results are discussed below. Conventional hard trivalent chromium plating films tend to become amorphous due to the large amount of additives contained therein. It is generally believed that additives are less likely to be mixed into the plating film at higher current densities. Therefore, it is presumed that the additive content of hard trivalent chromium plating films is lower under high current density conditions, and the crystallization rate is improved. However, in the plating baths of Comparative Examples 4 to 6, which contain formic acid and have a pH of 5.5, the current density of 100 A / dm 2 The crystallization rate was amorphous even at temperatures above this range. This suggests that the crystallization rate is affected not only by the current density conditions, but also by the pH and the additive components of the plating bath. The reason for this is presumably that the current density is a factor in the reduction rate of chromium ions, and the pH is a factor in the formation of a complex with chromium ions.

[0057] As described above, it has been demonstrated that the present invention can provide a shock absorber and a manufacturing method for a shock absorber that achieves both high hardness and low friction at a high level by using trivalent chromium instead of hexavalent chromium, which is a concern for its effects on the human body and the environment.

[0058] The present invention is not limited to the above-mentioned embodiment, and includes various modified examples. The above-mentioned embodiment is for explaining the present invention in an easy-to-understand manner, and is not necessarily limited to the embodiment having all the configurations explained. It is also possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to add, delete, or replace a part of the configuration of each embodiment with another configuration. [Explanation of symbols]

[0059] 100... shock absorber, 1... cylinder, 2... piston rod, 3... hydraulic oil for hydraulic system, 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 chamber, C... reservoir chamber.

Claims

1. A cylinder filled with hydraulic oil; A piston rod movable inside the cylinder; A shock absorber including an oil seal fixed to the cylinder and sliding with the piston rod, 1. A shock absorber comprising: a hard layer formed on a surface of the piston rod from a plating bath containing trivalent chromium as a main component; the hard layer containing both crystalline and amorphous matter in an XRD analysis; and containing additives other than chromium.

2. 2. The shock absorber according to claim 1, wherein the hard layer contains the additive in an amount of 2.3 mass % to 4.3 mass %.

3. 3. The shock absorber according to claim 1, wherein the hard layer has a ratio of crystalline material to the total of the crystalline material and the amorphous material of 5 to 26%.

4. 4. The shock absorber according to claim 1, wherein the additive includes at least one element of hydrogen, carbon, nitrogen, and oxygen.

5. 5. The shock absorber according to claim 1, wherein the trivalent chromium salt of the plating bath is chromium chloride.

6. 6. The shock absorber according to claim 1, further comprising a nickel plating film as an intermediate layer between the surface of the piston rod and the hard layer.

7. 7. The shock absorber according to claim 1, wherein the hard layer has a Vickers hardness HV of 750 HV or more.

8. 8. The shock absorber according to claim 1, wherein the hard layer has a surface roughness Rz of 0.6 μm or less after polishing.

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