JPH0369830A - Coil spring and manufacture thereof - Google Patents

Abstract

PURPOSE:To remarkably enhance abrasion resistance without deteriorating creep resistance and fatigue resistance in a coil spring by covering an element ire made of titanium or titanium alloy with one or more kinds of ceramics films in mono- or multilayers. CONSTITUTION:An element wire 11 made of titanium alloy is covered with a TiN film 12 as a ceramics film. beta-type titanium alloy is used for the element wire 11. Shot peening is performed so as to secure fatigue strength, and a compressive residual stress is applied onto the surface of the element wire 11. The element wire 11 is covered with the ceramics film 12 in order to provided abrasion resistance for a coil spring formed of the element wire 11. Fine TiN crystals are laminated in a direction of film thickness, obtaining the TiN ceramics film 12. With the resultant fine polycrystalline structure in multilayers, a stress inside the ceramics film 12 can be decreased.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a coil spring used in various devices, etc.
In particular, the present invention relates to a coil spring using a cable made of titanium or a titanium alloy, and a method for manufacturing the same.

[Prior Art] There is a strong desire to reduce the weight of coil springs used in various devices, including coil springs used in parts for vehicles such as automobiles, from the viewpoint of energy conservation. As a means to reduce the weight of a coil spring, the guidelines are to increase the strength of the material, increase the usable stress of the spring, or use a material with high specific strength (strength/weight).

Titanium or a titanium alloy is known as a material with high specific strength. However, coil springs made of titanium and titanium alloys are prone to wear, so when the spring is bent, the wear progresses significantly at the parts where the wires come in contact with each other, the end faces of the end turns, etc., and the cross-sectional shape of the wire changes quickly. At present, it has not been put into practical use as a coil spring because it has serious drawbacks such as the spring constant changing and fatigue failure occurring due to wear.

Therefore, as a means of improving wear resistance, the cable surface can be modified by diffusion treatments such as nitriding or carburizing, which have been conventionally applied to steel springs, hard Cr plating, electroless N
It has also been considered to apply surface coating methods such as i-plating to titanium springs or titanium alloy springs.

In addition, it is essential that coil springs not only have wear resistance, but also that they do not undergo creep deformation even when subjected to repeated loads, and that they do not cause fatigue failure even when subjected to repeated loads of 67 times. In the case of coil springs made of titanium or titanium alloys, as with conventional steel springs, it is possible to increase the creep resistance by so-called processing heat treatment, which is a combination of cold drawing of the cable wire and aging treatment. Furthermore, it is also possible to improve fatigue resistance by well-known shot peening.

[Problems to be Solved by the Invention] When a titanium or titanium alloy spring is surface modified by diffusion treatment such as nitriding or carburizing in order to improve its wear resistance, the adhesion to the wire is good, but , the diffusion rate of nitrogen or carbon atoms in titanium or titanium alloys is significantly slower than in steel, so in order to obtain a diffusion layer deep enough to exhibit sufficient wear resistance, the solution temperature of titanium or titanium alloys (approximately 800 must be processed at high temperatures, such as ~900°C).

For this reason, when diffusion treatment is applied after coiling the strands, even if the strength is improved by the heat treatment, the effect of the heat treatment is lost due to the high temperature associated with the diffusion treatment, resulting in poor creep resistance. A problem arises. Furthermore, if a diffusion treatment is performed after shot peening, the creep property and fatigue resistance properties will be significantly impaired due to the softening of the material and the relaxation of compressive residual stress.

Furthermore, if the diffusion treatment is performed after the cable wire is drawn, coiling becomes impossible because the material surface has already been work hardened.

For these reasons, surface modification by diffusion treatment is not a suitable method for titanium or titanium alloys.

On the other hand, surface coating methods using hard Cr plating and electroless Ni plating do not usually involve heat treatment and therefore do not cause material softening or residual stress release problems, but they do not cause problems with material softening or residual stress release. Therefore, it is difficult to apply plating with sufficient adhesion.

Therefore, an object of the present invention is to provide a coil spring made of titanium or a titanium alloy that has excellent wear resistance while maintaining material strength and fatigue strength that are essential for a coil spring.

[Means for Solving the Problems] The coil spring developed by the present inventors in order to achieve the above-mentioned purpose has an intermediate layer or multiple layers of one or more types of ceramic films on the surface of a wire made of titanium or a titanium alloy. It is characterized by overturning him.

Although oxide-based, nitride-based, and carbide-based ceramic films can be used, ceramics containing Ti are particularly recommended from the viewpoint of adhesion to titanium and titanium alloys. For example, TiO2.

TiN, Tic, Tin A11v N2. TixA
llvcN2. TiXA11v Cz, TiwA
RxV y N z , T i WA (l x
V y CN z , T l wAN x Vy
Ti oxides, nitrides, or carbides such as Cz are suitable.

If the ceramic film is too thin, it will not be able to exhibit sufficient wear resistance. On the other hand, an excessively thick ceramic film is likely to peel off during use due to the brittleness of the ceramic itself. Therefore, the thickness of the ceramic film is 0.5
It is desired that the film thickness be in the range of ~10 μm, and more preferably, it is recommended that the film thickness be in the range of 1 to 4 μm.

Regarding the film structure, although a single layer film structure can achieve the intended purpose of the present invention, a fine polycrystalline structure laminated in the film thickness direction is superior in terms of adhesion and wear resistance. This is because the film stress that occurs during film formation is reduced due to the fine crystal structure of ceramics, and the film quality is tougher, making it less likely that cracks will occur in the film even when subjected to distortion caused by deflection when using a spring. It's for the benefit. Moreover, this fine polycrystalline structure is effective in terms of adhesion and film toughness even when multiple types of ceramic films are laminated in the film thickness direction.

The method for coating the ceramic film is sputtering, which allows film formation at lower temperatures.

Physical vapor deposition method (PVD treatment) such as ion plating,
Alternatively, promising results can be obtained by plasma CVD. Further, the film forming temperature is recommended to be 300° C. or lower because it is desirable that the temperature does not soften the material and does not reduce the compressive residual stress obtained by shot peening. In addition, in the case of titanium alloys, it is necessary that the temperature is below the aging treatment temperature.

[Function] In the coil spring of the present invention, since the surface of the titanium or titanium alloy wire is strongly coated with the ceramic film described above, pure film stress acts on the surface like a coil spring, and the wire Even when used under harsh conditions such as repeated contact between wires, it does not suffer from problems such as peeling and maintains excellent durability and wear resistance, and the wire itself has a high specific strength. It becomes a lightweight coil spring. Furthermore, it is possible to form the film using a process that does not adversely affect creep resistance or fatigue resistance.

[Embodiment 1] A first embodiment of the present invention will be described below with reference to FIGS. 1 to 4. As shown schematically in cross section in FIG. 1, the coil spring for a valve spring of this embodiment has a TiN film 1 as a ceramic film on the outside of a strand 11 made of titanium alloy.
2 is coated.

A β-type titanium alloy is used for the wire 11, and its composition (weight %) is 13%V, 11%C, 3%A11.
). Using this titanium alloy, a coil spring is obtained through the manufacturing process shown in FIG. Table 1 shows the main specifications of the coil spring of this example.

Table 1 In the manufacturing process illustrated in FIG. 2, cold drawing using a die is performed to impart creep resistance to the material treated at the solution temperature.

In this area-reducing process, the strand 11 is subjected to large plastic working with an area reduction rate of about 80%, and then coiling is performed while the strand 11 maintains its ductility. After coiling, 427
The tensile strength is maintained at about 170 kg f/mm 2 by aging at ℃ x G 2 hr.

After the aging treatment, shot peening is performed to ensure fatigue strength, and compressive residual stress is applied to the surface of the wire 11. After that, low-temperature annealing at 180° C. x 10 m1 to remove strain is performed.

Further, in order to impart wear resistance to the coil spring using this wire 11, a ceramic film 12 is coated on the surface of the wire 11. The ceramic film 12 of this embodiment is made of TiN and is formed to a thickness of about 2 μm. Moreover, this TiN ceramic film 12 has fine TiN crystals laminated in the film thickness direction, as schematically shown in FIG. That is, it was made into a fine polycrystalline body of TiN. Such a multi-layered fine polycrystalline structure suppresses the stress drop inside the ceramic film 12 and overcomes the weakness of ceramic socks, which have brittle properties. Even if it is used for something, the adhesion of the ceramic film 12 to the wire 11 will be further improved, and the ceramic film 12 will be less likely to crack during use.

An arc discharge type ion plating device is used as an example of a device for obtaining the ceramic film 12 having the fine polycrystalline structure. In order to obtain the above-mentioned fine polycrystalline body by ion plating, when depositing two TiN films on the surface of the wire 11 as a processed product, the wire 11 is kept until a predetermined film thickness is reached. The film formation process is divided into multiple steps and the film formation and cooling are repeated until a predetermined film thickness is reached without taking the film out of the apparatus. That is, as shown by the solid line in FIG. 4, by providing a sufficient interval between the film forming processes divided into a plurality of parts, the product to be processed is cooled between the film forming processes. By performing the film-forming process in multiple steps in this manner, the ceramic film 12 having a multilayered film, that is, a fine polycrystalline structure, can be obtained even when the ceramic film is of the same type.

Incidentally, in this example, the predetermined thickness of 2 .mu.m was achieved by forming the film in four steps of approximately 0.5 .mu.m each. Another important advantage of this process is that by reducing the thickness of the film formed in the -step process, it is possible to suppress the rise in temperature of the wire 11 that occurs during film formation. In this example, the temperature rise during ion plating is approximately 3
It is suppressed to 00'C. By doing this, it is possible to prevent the strands 11 from softening, and also to prevent the compressive residual stress imparted to the strands 11 by shot peening before film formation from being released, improving not only wear resistance but also resistance. A titanium alloy coil spring with excellent creep resistance and fatigue resistance can be obtained.

FIG. 5 shows the results of a fatigue test conducted to examine the wear resistance of this product. The test conditions were an average stress of 36 kg.
f/m+s 2, stress amplitude 16.4 kg f/
It is mm2. The untreated product (a) in the figure is an uncoated titanium alloy coil spring, but the abrasion weight reached approximately 1.35 g after only a few repetitions of aXlOb bending. This untreated product weighs 10.7K L before wear.
Because of this 1.35 fr wear, the spring constant decreases significantly, and the spring no longer fulfills its intended function.

In contrast to such an untreated product, the example product shown in (b) in the figure is coated with a single layer of TiN to a thickness of 0.5 μm, and the amount of wear is 3×106 times, which is approximately The amount of wear is only 0.75g, which is a significant reduction compared to the untreated product. Furthermore, the product of this example shown in (C) in the figure is made of TiN.
The film has the aforementioned fine polycrystalline structure (see Figure 3), and even after repeated bending of 3X106, the amount of wear is only 0.4g, and it has a remarkable wear resistance effect compared to untreated products. It was confirmed that there is.

[Example 2] FIG. 6 schematically shows a part of the cross section of the product of this example. The coil spring of this embodiment has T as a base on the surface of the wire 11.
A first ceramic film 15 made of iAj7N is coated, and a second ceramic film 16 of TiA is coated thereon.
It is coated with jilCN film.

A sputtering process is employed as one means for obtaining such a multilayer film structure. That is, TiAj
TiAIN was deposited on the titanium alloy by magnetron sputtering with a target of 750:50 (atomic ratio) in an argon atmosphere and by reactive sputtering, which formed a nitride film by introducing nitrogen gas as a reactive gas.
After that, acetylene gas was gradually increased to a predetermined flow rate, and TiAg
Top coat with a CN film (film thickness approximately 0.5.czm).

The TiAgN film 15 has a hardness comparable to that of the TiN film 1 and 2 described above, has excellent adhesion to the wire 11, and has excellent oxidation resistance.

Furthermore, since a film with excellent adhesion can be formed under a bias voltage lower than that of a TiN film, it has the advantage of being able to prevent excessive temperature rise of the processed product, which often occurs during coating. For example, in the magnetron sputtering equipment used this time, ordinary TiN
Where the film requires a bias voltage of -150V, the Ti
A, in forming the pN film 15-, a dense film can be deposited with a bias voltage of about -100V.

The TiANCN film 16 was top coated on the outermost film surface 6.
This is because this film 16 has high hardness and excellent wear resistance, and has a small coefficient of friction. In other words, TiN and Ti
Vickers hardness of Aj7N film is 2000-2700kg
/m+s2, whereas the TiAl1CN film has a hardness of 2800-3600kg/m112,
Moreover, the friction coefficient of TiA, &CN film is about 0. It's as small as I5. Therefore, in cases where repeated loads are applied and compression deformation is repeated, such as in the case of compression coil springs, it is effective in reducing the frictional force acting in the longitudinal direction of the strands 11, and in turn has the effect of reducing the amount of wear. I can demonstrate it. In addition, TiA, 1
If the 7cN film 16 is directly formed on the surface of the titanium alloy, the adhesion will be poor, so as in this example, the TiA, 17N film 1
A TiA, QCN film 16 was formed through the film 5.

The process for obtaining the film structure of this example may be similar to that for forming the TiN film in Example 1 described above, but as a more preferable example, the film formation process as shown by the two-dot chain line in FIG. The number of divisions was made larger than in Example 1. An example of the apparatus used in this film forming process is shown in FIG.

The β-type titanium alloy strand 11 as a processed product is set inside a cylindrical vacuum chamber 20 . That is, a plurality of pairs of magnetron cathodes 21.2 are provided in the circumferential direction.
The target 24 is placed in the width of the plasma zone 23 created by the second discharge, and the wire 11 is housed therein.

By moving the wire 11 in the circumferential direction of the vacuum chamber 20, the wire 11 enters and exits the plasma zone 23 intermittently, and film formation and cooling are repeated until a predetermined film thickness is reached. Therefore, like the polycrystalline structure of the above-mentioned embodiment, the rolled ceramic film is a fine polycrystalline body stacked in layers. In the case of this example, by increasing the number of film forming processes until a predetermined film thickness is reached, the temperature rise of the wire 11 during film formation could be suppressed to about 250'C.

Since the outermost surface of this example product is coated with a TiA, QCN film 16 that is effective in reducing the coefficient of friction, it exhibits excellent wear resistance.

As shown in FIG. 5(d), after repeated bending of about 3×10 6 times, the amount of wear was zero, and the gloss of the surface only increased, and no wear was observed at all. By the way, even after repeated bending up to 2x107 times, the amount of wear is 0.
005 g, and almost no wear was observed.

[Effects of the Invention] According to the present invention, the wear resistance of a coil spring using a wire made of titanium or a titanium alloy with high specific strength can be greatly improved without impairing the creep resistance and fatigue resistance. can.

[Brief explanation of drawings]

Fig. 1 is a sectional view schematically showing a coil spring according to a first embodiment of the present invention, Fig. 2 is a process explanatory drawing showing the manufacturing process of the coil spring shown in Fig. A cross-sectional view schematically showing a polycrystalline body, Figure 4 is a diagram showing the relationship between film thickness and temperature when the film formation process is performed in multiple steps, and Figure 5 is a fatigue test to investigate wear resistance. FIG. 6 is a sectional view schematically showing a part of a film structure of another example of the present invention, and FIG. 7 is a sectional view schematically showing an example of a film forming apparatus. 11...Element wire, 12,15.16...Ceramics film.

Claims (6)

[Claims] (1) A coil spring characterized in that the surface of a wire made of titanium or a titanium alloy is coated with a single layer or multiple layers of one or more types of ceramic film. (2) The coil spring according to claim 1, wherein the ceramic film is selected from among oxide-based, nitride-based, and carbide-based ceramics. (3) The coil spring according to claim 2, wherein the ceramic film is selected from among oxide-based, nitride-based, and carbide-based ceramics containing Ti. (4) The coil spring according to claim 1, wherein the ceramic film is a fine polycrystalline body laminated in the thickness direction. (5) The surface of the wire made of β-type titanium alloy is coated with a TiAlN film as a base, and then Ti_XA
The coil spring according to claim 1, wherein the coil spring is coated with a film of l_YCN_Z. (6) A step of reducing the area of a wire made of titanium or a titanium alloy to a predetermined wire diameter, a step of coiling the wire after the area reduction, a step of shot peening the wire after coiling, 1. A method for manufacturing a coil spring, comprising the step of forming a ceramic film made of fine polycrystalline material on the surface of a wire by ion plating or sputtering.