JPH08135703A - Brake lining for rotor made of stainless steel - Google Patents

Abstract

PURPOSE: To ensure high friction coefficient at the time of high load. CONSTITUTION: A friction material containing a fiber base material, a resin bonding agent and a filling agent contains a cupper system fiber consisting of at least one kind out of cupper fiber and brass fiber with 30-60weight% for an entire friction material and the fiber or powder made of stainless steel with 3-10weight% and graphite with 2-10weight% for an entire friction material as a metal component. As this friction material contains comparatively lots of cupper system fiber, its thermal conductivity is favorable and fade can be restrained at the time of high load in the case of that a partner material is a rotor made of stainless steel with low thermal conductivity. As the fiber or powder made of stainless steel is combined with graphite at an adequate rate, a low partner material attack property is maintained and also high friction coefficient is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a friction material for disc brake pads, linings, clutch facings, etc. used in vehicles, industrial machines and the like.
In particular, the present invention relates to a friction material for a rotor made of stainless steel, which is preferably used for a mating member made of a rotor made of stainless steel.

[0002]

Friction materials such as disc brake pads and linings used in vehicles such as automobiles and motorcycles are
It plays an important role in converting kinetic energy into heat energy by frictionally engaging with the disc rotor and brake drum, which are its counterparts. Therefore, not only is the friction material required to have excellent wear resistance, but it is also necessary to have a sufficiently high coefficient of friction, and moreover, heat is constantly generated during braking, and the temperature is high. Stable friction characteristics with little change in friction coefficient are required. Furthermore, there is no aggression to the other material, and abnormal noise (squeaking) occurs during friction.
It is also necessary to prevent the occurrence of friction, etc., and the characteristics required of the friction material cover many items.

Therefore, conventionally, in order to satisfy these various characteristics, the friction material is formed as a composite material.
That is, the friction material is composed of a fiber base material forming a skeleton such as aramid fiber and potassium titanate fiber, a resin binder such as a phenol resin for binding and holding the fiber base material, and a matrix of these fibers and the binder. It is generally formed from various fillers dispersed in and filled in to adjust the friction performance. And as this filler, barium sulfate,
Body fillers such as calcium carbonate, solid lubricants such as graphite and molybdenum disulfide, abrasive agents such as cashew dust and silica, and other additives for friction adjustment are used.

Here, the fiber base material forms the skeleton of the friction material as described above, and is the most main material among the friction material materials. Asbestos fibers (asbestos) have been used as the fiber base material in the past. However, this asbestos fiber has been found to be unhealthy, and therefore fiber materials other than asbestos fiber are now used as the fiber base material for friction materials.

As an example thereof, there is a so-called semi-metallic friction material using steel (carbon steel) fiber as the main material of the fiber base material. Regarding this, for example, Japanese Patent Laid-Open No. 2-117
There is one disclosed in Japanese Patent Publication No. 985, and mainly, graphite as a solid lubricant and a metal sulfide are blended in a specific ratio and a specific ratio to improve brake squeal. A semi-metallic friction material using such a steel fiber as a fiber base material has high hardness because the steel fiber has a high hardness, and thus can have a relatively high friction coefficient. However, on the other hand, it has a problem that it is heavy, rusts, and damages the mating material.

Therefore, in recent years, so-called non-steel type friction materials, which do not use steel fibers as a fiber base material, have become the mainstream of friction materials instead of semi-metallic type friction materials. In this non-steel friction material, as the fiber base material, heat-resistant organic fibers such as aramid fibers, glass fibers, inorganic fibers such as potassium titanate fibers, copper fibers, non-ferrous metal fibers such as brass fibers are used. Are used in combination. Examples of such a friction material include those described in JP-A-2-298575 and JP-A-3-185030, and the former is specifically It relates to the type and composition of solid lubricants for suppressing the fluctuation of the friction coefficient of the friction material due to the change of the water content, and the latter is specifically the addition of zirconium oxide powder for stabilizing the friction coefficient. It is related to In this non-steel type friction material, metal fibers such as copper fibers and brass fibers are used in a relatively small proportion, for example, 10% by weight in the one disclosed in JP-A-3-185030.

[0007]

By the way, the disc rotor (brake disc) of the disc brake is generally made of cast iron or steel (carbon steel) because it can quickly absorb and release frictional heat generated during braking. It is said that. However, in the case of a motorcycle or a vehicle equipped with spoke-shaped aluminum wheels, the disc brake is exposed to the outside or is visible from the outside, and recently, in order to improve the appearance due to upsizing, the disc rotor is It is often made of stainless steel that has high brightness and does not rust.
Even in the case of disc brakes and drum brakes that cannot be seen from the outside, rusting on those disc rotors and brake drums can cause brake judder. It is also considered to be made.

As described above, the rotor such as the disk rotor, which is the counterpart of the friction material, is often made of stainless steel in recent years. On the other hand, the braking conditions are becoming more and more severe as the speed of the vehicle is increased by improving the performance of the vehicle and improving the road environment. Then, for example, when braking is performed from a high speed of 100 km / h, or when such severe braking is repeated, a large amount of frictional heat causes a fade or a phenomenon similar to a fade, and the friction coefficient decreases. Tended to. In this fade, organic components such as cashew dust and resin binder contained in the friction material decompose under high heat to generate gas, and this decomposed gas is present in the friction engagement surface between the friction material and the mating material. This is due to the gas lubrication effect. The tendency for the friction coefficient to decrease at high load is particularly because when the rotor as the mating member is made of stainless steel, the stainless steel has a smaller thermal conductivity than cast iron or the like, and frictional heat is likely to be accumulated. It was remarkable.

Such a decrease in the friction coefficient at the time of high load braking can be compensated by increasing the operating force of the brake, but it is not preferable from the viewpoint of stability of brake operation. So I want to suppress the fade,
In order to maintain a high friction coefficient at high load, it is necessary to reduce the organic matter that causes the fade,
Cashew dust and the like are important components for improving friction performance, and there is a limit to reducing them. It is also generally known that the porosity of the friction material may be increased by weakening the pressure at the time of molding the friction material so that the decomposed gas is easily released and released from the friction engagement surface. However, the friction material formed in this way also has low strength, so that the wear resistance and the like are reduced. Therefore, in this situation, even when the mating member is a stainless steel rotor, it is necessary to suppress a decrease in the friction coefficient at the time of high load such as high-speed braking and to secure a high friction coefficient. There has been a strong demand for a friction material capable of achieving the above.

Therefore, an object of the present invention is to provide a friction material for a rotor made of stainless steel, which can secure a high friction coefficient under high load.

[0011]

The friction material for a rotor made of stainless steel according to the present invention is a friction material containing a fiber base material, a resin binder, and a filler. On the other hand, it contains 30 to 60% by weight of a copper-based fiber consisting of at least one kind of copper fiber and brass fiber, and 3 to 10% by weight of stainless steel fiber or powder, and
The content of graphite is 2 to 10% by weight based on the whole friction material.

Here, the copper-based fiber imparts a sufficiently high thermal conductivity to the friction material, quickly absorbs and diffuses the friction heat generated at the time of frictional engagement with the mating material, and suppresses fade. Used for. Therefore, this copper-based fiber is mixed in a relatively large amount as a main material of the fiber base material, and is mixed in a ratio of 30 to 60% by weight with respect to the entire friction material. That is, when the mixing ratio of the copper-based fiber is less than 30% by weight, practically sufficient thermal conductivity cannot be obtained, and therefore the fade cannot be sufficiently suppressed, and therefore the friction coefficient under high load is also high. Cannot be maintained. However, the thermal conductivity is not saturated even if the copper-based fiber is added in an excessively large amount, and if the content exceeds 60% by weight, it is generally difficult to form the friction material,
Further, it becomes difficult to mix other friction material components such as a filler. Therefore, the mixing ratio of the copper-based fiber is practically 30%.
-60% by weight.

As the copper-based fiber, at least one of copper fiber and brass fiber is used. These have high thermal conductivity, especially copper fibers. Therefore, among these copper-based fibers, it is more preferable to mainly use copper fibers. Bronze fiber may be used as the copper-based fiber and may be used as a fiber base material of the friction material. However, this bronze fiber is not used here because of its relatively low thermal conductivity. Furthermore, in terms of only thermal conductivity, those copper-based fibers may be powders, but when used as powders, it is necessary to use a large amount of resin binder that is a causative agent of fading. Therefore, it is used here as a fibrous fiber base material.

Another stainless steel fiber or powder, which is a metal component, is used to increase the friction coefficient of the friction material due to friction between metals of the same kind and the stainless steel rotor, which is a mating material. Then, the stainless fiber or powder is mixed in a ratio of 3 to 10% by weight with respect to the entire friction material. If it is less than 3% by weight, the friction coefficient of the friction material cannot be sufficiently increased. Also, the more this is added, the more the friction coefficient of the friction material can be increased, but at the same time, the aggressiveness against the stainless steel rotor, which is the other material, is also increased, increasing the amount of wear and causing brake judder. Or Therefore, it is practically preferable to limit the compounding ratio of the stainless steel fiber or powder to 10% by weight in order to keep the aggressiveness of the mating material sufficiently low. To be done.

The stainless steel for increasing the friction coefficient of the friction material may be in the form of fiber or powder, but the form of fiber is more preferred. That is, although stainless steel has poor adhesiveness to resin, if it is in the form of fibers, it can be firmly bonded together with the fiber base material by the resin binder and can be held well by the friction material. When used as a fiber in this way, the same applies to the case of the above-mentioned copper-based fiber, but in general, as a fiber having a diameter of 5 to 1000 μm, and a fiber length of 0.1 to 0.1 depending on the molding method of the friction material. 1
It can be used as 0 mm short fibers or long fibers.

Further, in order to improve the wear resistance and the like of the friction material, graphite which is a solid lubricant is generally blended, and this graphite is the fiber or powder of the above stainless steel here. It is also an important ingredient for suppressing the aggressiveness of the body's opponent material. It is preferable that the graphite is mixed in a proportion of 2 to 10% by weight with respect to the entire friction material. If it is less than 2% by weight, the attacking property of the mating member cannot be sufficiently suppressed, and the abrasion resistance of the friction material cannot be sufficiently improved. However, if it is added in an excessively large amount, its lubricity lowers the friction coefficient, which hinders the action of the stainless fiber or powder to increase the friction coefficient. Therefore, it is practically preferable to limit the blending ratio of this graphite to 10% by weight, and therefore, it is used in the range of 2 to 10% by weight. The graphite as the solid lubricant may be either natural or artificial.

The other components of the friction material according to the present invention are the same as conventional ones.

As the fibrous base material, any fibrous material can be used as long as it is a non-metallic fiber in combination with the above-mentioned copper-based fiber. Examples of such non-metal fibers include organic fibers such as aramid fibers, novoloid fibers, nylon fibers, rayon fibers, silicate fibers, alumina fibers, potassium titanate fibers, rock wool, slag wool, carbon fibers, and glass fibers. Inorganic fibers can be mentioned. Any one or more of these can be used, but at least one of the organic fibers and at least one of the inorganic fibers are used in combination with the above copper-based fibers. Is preferred. Since this friction material is formed as a so-called non-steel friction material, steel fiber is not used. No metal fiber is used other than the above copper-based fiber (and stainless fiber).

As the resin binder, phenol resin, melamine resin, epoxy resin, polyimide resin, polyester resin, or rubber such as SBR can be used. However, among these, the phenol resin or a modified product thereof is the most commonly used one from the past and is preferable.

As the filler, in addition to the above graphite, a body filler such as barium sulfate or calcium carbonate, mainly an organic dust such as cashew dust or other organic polymer powder for stabilizing the friction coefficient, Alternatively, other additives for adjusting friction can be used as appropriate. In addition to graphite, other types of solid lubricants such as molybdenum disulfide and antimony trisulfide can also be used as appropriate as long as the friction coefficient can be maintained sufficiently high. Further, an inorganic abrasive agent such as silica is not necessary because the stainless steel fiber or powder is blended, but if necessary, it can be used in combination within a range in which the opponent material aggressiveness can be kept sufficiently low. . The metal powder for improving the thermal conductivity is not used here.

In the friction material according to the present invention, for example, the above copper-based fibers and other fiber base materials, resin binders, stainless steel fibers or powders, and other fillers are mixed, and this mixture is mixed. It can be produced by a usual thermoforming method in which the material is preformed and then heated and pressed.

[0022]

As described above, when the mating member is a stainless steel rotor, since the stainless steel has a low thermal conductivity, frictional heat generated during frictional engagement is likely to accumulate on the frictional engagement surface, resulting in high speed. At the time of high load such as braking, a large amount of frictional heat causes a fade or a phenomenon similar thereto, and the friction coefficient tends to decrease.

In the friction material for a rotor made of stainless steel according to the present invention, the copper-based fiber composed of at least one of copper fiber and brass fiber is 30 to 60 with respect to the entire friction material.
Since they are contained in a high percentage by weight, the thermal conductivity of the friction material is sufficiently increased by those copper-based fibers having a high thermal conductivity. Therefore, the frictional heat generated at the time of frictional engagement with the mating member can be quickly absorbed and diffused, and the fade can be suppressed. In addition, stainless steel fibers or powder, together with an appropriate proportion of graphite of 2 to 10% by weight with respect to the entire friction material, and 3 to the entire friction material.
Since it is contained in an appropriate ratio of 10% by weight, the attacking property of the mating material is kept sufficiently low, while the friction coefficient of the friction material is sufficiently made by the friction between the similar metals with the stainless steel rotor which is the mating material. Can be increased. Therefore, it is possible to secure a high friction coefficient under high load.

[0024]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

[Preparation of Friction Material (Pad)] Friction materials of Examples 1 to 7 of the present invention were prepared with the compounding composition (% by weight) shown in FIG. Further, for comparison with these, the friction materials of Comparative Examples 1 to 3 were also produced. In addition,
The friction materials of these examples and comparative examples are specifically embodied as disc brake pads used for disc brakes of motorcycles in which the disc rotor is made of stainless steel.

As shown in FIG. 1, the friction materials (disc brake pads) of these Examples and Comparative Examples are composed of a fiber base material,
It is formed by including a resin binder and a filler such as a friction modifier.

Here, as the fiber base material, there are aramid fiber, potassium titanate fiber, ceramic (calcium silicate) fiber which are non-metal fibers, and copper fiber and brass (brass) fiber which are copper-based metal fibers. This copper-based fiber is used in a relatively large amount. And, here, steel (carbon steel) fibers are not used, and therefore the friction material is formed as a non-steel friction material. In addition to these fiber base materials, stainless steel fibers are also used. The stainless fiber may be in the form of powder and is used to improve the friction coefficient of the friction material by friction between the same kind of metal and the stainless disk rotor. Therefore, this stainless fiber is not used as a fiber base material, but rather as a friction modifier.

A phenol resin is used here as the resin binder. Further, as the filler, mainly cashew dust for stabilizing friction performance, barium sulfate as a body filler, and graphite as a solid lubricant are used. This graphite is
It is used to improve the wear resistance and the like of the friction material due to its lubricity and to suppress the mating property of the stainless fiber against the mating material.

Among these components, aramid fiber, potassium titanate fiber and ceramic fiber, which are non-metal fibers, are used in the same mixing ratio in each of Examples and Comparative Examples, and each is mixed by 5% by weight. did. The same applies to the phenolic resin and cashew dust, which are resin binders, and 10% by weight and 5% by weight, respectively, were blended. On the other hand, the copper fiber, the brass fiber, the stainless fiber, and the graphite were used by changing the compounding ratio in each Example and Comparative Example and adjusting the compounding ratio of barium sulfate accordingly.

That is, in Example 1, copper fiber was 30% by weight, stainless fiber was 5% by weight, graphite was 5% by weight, and barium sulfate was 30% by weight. In Example 2, the amount of copper fiber was increased to 40% by weight, and the amount of barium sulfate was reduced by that amount to 20% by weight. Furthermore,
In Example 3, the compounding of barium sulfate was omitted, and 30% by weight of brass fiber was added together with 30% by weight of copper fiber. Therefore, in this Example 3, the total amount of the copper-based fibers made of copper fibers and brass fibers was 60% by weight.

In Example 4, the copper fiber content was 40% by weight, the graphite content was 5% by weight, the mixing ratio of the stainless fiber content was slightly reduced to 3% by weight, and the balance was barium sulfate. On the contrary, in Example 5, the mixing ratio of the stainless fiber was increased to 10% by weight.
Furthermore, in Example 6, the copper fiber content was 40% by weight, the stainless fiber content was 5% by weight, and the graphite content was reduced to 2% by weight. On the contrary, in Example 7, the blending ratio of graphite was increased to 10% by weight.
And

With respect to these Embodiments 1 to 7,
In Comparative Example 1, the content of the copper-based fiber was reduced, and the copper fiber was 20% by weight, the stainless fiber was 5% by weight, the graphite was 5% by weight, and the barium sulfate was 40% by weight.
In Comparative Example 2, a relatively large amount of stainless fiber was blended, and copper fiber was 40% by weight, stainless fiber was 15% by weight, graphite was 5% by weight, and barium sulfate was 10% by weight. Further, in Comparative Example 3, the content of the copper-based fiber was reduced and the content of the graphite was relatively increased. Copper fiber was 20% by weight, stainless fiber was 5% by weight, graphite was 15% by weight, and barium sulfate was used. Remains 3
0% by weight.

The friction materials (disc brake pads) of these Examples and Comparative Examples were manufactured by the usual thermoforming method, specifically as follows.

That is, the friction material raw material containing a relatively large amount of copper-based fibers and also stainless steel fibers was thoroughly and uniformly mixed with a V-type blender, and then this powdery mixture was put into a preforming die. Then, at room temperature, a pressure of 200 kg / cm ² was applied for 1 minute to preform. This friction material preform is set in a thermoforming die together with a backing metal whose surface has been coated with a phenolic resin adhesive in advance, and 400 kg / cm
Thermoforming was performed at a pressure of ² and a temperature of 160 ° C. for 10 minutes.
Then, this is further heat-treated at 250 ° C. for 120 minutes,
A friction material was obtained.

[Evaluation Test] Next, regarding each friction material (disk brake pad) of these produced Examples and Comparative Examples, their friction performance when the mating material (disk rotor) is made of stainless steel, and its friction performance An evaluation test was conducted on the attacking property of the stainless steel mating material.

Regarding friction performance, JASO-C406
According to -82, at the time of the second effect after rubbing (stable period)
The high-speed braking test was conducted and the friction coefficient μ at that time was measured. The test conditions are as follows. Caliper brake model used: PD48-11S (stainless steel) Inertia: 3.5 kgf · m · s ² Initial speed: 100 km / h Braking fluid pressure: 80 kgf / cm. The rubbing is 65 km / h × 40 kgf / cm ²
It was performed at × 120 ° C × 200 times.

Further, in addition to this, in order to evaluate the anti-fade performance, the first fade test was carried out under the above conditions, and the minimum friction coefficient μ at the first fade was measured. The braking interval is 35 seconds, and the number of brakings is 10.

Further, in order to evaluate the aggressiveness to the mating member during braking, the wear amount (μm) of the stainless disk rotor after the first fade test was measured.
The amount of wear is measured at the maximum wear portion.

The friction coefficient at the time of high speed braking, "friction coefficient at high speed braking", and the minimum friction coefficient at the first fade test "minimum friction coefficient at first fade μ" were measured for each friction material of Examples and Comparative Examples. FIG. 1 shows the wear amount “rotor wear amount” (μm) of the stainless disk rotor after the completion of the first fade test, together with their compositions.

[Test Results] As shown in FIG. 1, friction of Examples 1 to 7 in which a relatively large amount of copper fibers (and brass fibers) were blended and stainless steel fibers and graphite were blended in an appropriate ratio The material (disc brake pad) maintains a sufficiently high coefficient of friction not only when the stainless rotor is the mating material but also during the first fade as well as during high speed braking. Moreover, the amount of wear of the rotor is sufficiently small, and the aggression against the mating material is small.

The effects of these examples are more apparent from comparison with the comparative examples. That is, in Comparative Example 1 in which the amount of copper fibers (or copper-based fibers including brass) blended is small, the friction coefficient during high-speed braking is low, and particularly, the friction coefficient during the first fade is low. Compared to Comparative Example 1, in Examples 1 to 3 in which a large amount of copper-based fiber was mixed, the amounts of stainless fiber and graphite were the same, but the friction coefficient during high-speed braking, especially during the first fade, was Has been greatly improved. Therefore, from this fact, by adding a relatively large amount of copper-based fibers, it is possible to increase the thermal conductivity of the friction material and suppress the fade so that the friction coefficient can be maintained high even under high load such as high speed braking. I see what I can do. Further, in this case, it is also understood from Examples 1 to 7 that the mixing ratio of the copper-based fiber is preferably 30 to 60% by weight.

In addition, Example 4 (3 wt%), Example 2 (5 wt%), and Example 5 (1
0% by weight) and Comparative Example 2 (15% by weight), the higher the blending ratio, the higher the friction coefficient obtained, and this stainless fiber is also an important component for maintaining the high friction coefficient under high load. It turns out that However, in Comparative Example 2 in which the mixing ratio of the stainless fiber is large, the aggressiveness to the disk rotor made of stainless steel also tends to be high, and the rotor wear amount is large. Therefore, the mixing ratio of this stainless fiber is 3-1 which is the range of the example.
It can be seen that 0% by weight is preferred.

Further, with regard to graphite as a solid lubricant, Example 6 (2% by weight), Example 2 (5% by weight), Example 7 (10% by weight), and Comparative Examples with different blending ratios were used. 3 (15% by weight), the higher the blending ratio, the more the aggressiveness against the mating material is improved and the amount of rotor wear tends to decrease. At the same time, however, the lubricity also reduces the friction coefficient. In No. 3, although it is also because the amount of copper-based fibers is small,
The friction coefficient at the time of fading both decreased considerably. Therefore, it is understood that the blending ratio of graphite is preferably 2 to 10% by weight, which is the range of the embodiment, because a high friction coefficient can be secured and the amount of rotor wear can be sufficiently reduced.

As described above, according to the friction materials (disc brake pads) of Examples 1 to 7, even if the mating member is a rotor made of stainless steel having low heat conductivity, the high friction coefficient at high load is obtained. Can be secured. Further, in this case, the aggressiveness to the mating material can be kept sufficiently low. The friction materials of these examples do not rust because they contain copper-based fibers and stainless steel as metal components and no steel (carbon steel) fibers. Therefore, this friction material can be particularly suitably used for a disk brake of a motorcycle or a vehicle equipped with a spoke-shaped aluminum foil, which has a stainless disk rotor whose appearance is also important. However, due to its excellent fade resistance and the like, it can be similarly suitably used for a disc brake or the like equipped with a usual cast iron or steel rotor.

Although the disc brake pad has been described as an example, the present invention is not limited to the disc brake pad, and various friction members such as a lining of a drum brake or a clutch facing can be used. It can be applied as a friction material for a compounding device. Further, the types and blending ratios of non-metallic fiber base materials other than copper-based fibers, the types and blending ratios of fillers other than stainless steel fibers and graphite are not limited to this example, and may be changed variously. You can

[0046]

As described above, the friction material for a rotor made of stainless steel according to the present invention is a friction material containing a fiber base material, a resin binder, and a filler. On the other hand, it contains 30 to 60% by weight of copper fibers and at least one of copper fibers and brass fibers, and 3 to 10% by weight of stainless steel fibers or powders. To 2 to 10% by weight.

Therefore, according to this stainless steel rotor friction material, at least one of copper fiber and brass fiber is used.
Since it contains a relatively large amount of seed-based copper fibers,
The heat conductivity of the friction material can be made sufficiently high by those copper fibers having high heat conductivity. As a result, the frictional heat generated during frictional engagement with the mating member can be quickly absorbed and diffused, and fade can be suppressed. Also, since the stainless steel fibers or powder are contained together with graphite in an appropriate ratio, the attacking property of the mating material is kept sufficiently low, while the friction of metal of the same kind with the mating stainless steel rotor causes friction. The friction coefficient of the material can be sufficiently increased. Therefore, there is an effect that a high friction coefficient can be secured even under a high load such as during high speed braking.

[Brief description of drawings]

FIG. 1 is a table showing compositions (% by weight) of friction materials (disk brake pads) for stainless steel rotors of Examples and Comparative Examples of the present invention and results of evaluation tests thereof.

Claims (1)

[Claims] 1. A friction material comprising a fibrous base material, a resin binder and a filler, the metal component being 30 to 60% by weight based on the whole friction material.
2-10 weight% with respect to the whole friction material, containing copper-based fiber consisting of at least one kind of copper fiber and brass fiber, and 3-10 wt% of stainless steel fiber or powder. % Friction material for rotors made of stainless steel.