Brake disc and method for producing same

A brake disc, in particular for a motor vehicle, includes a base disc of a first material and a wear-reducing coating of a second material. The first material is a lightweight metal and the second material is an oxide layer.

This application is a 35 U.S.C. § 371 National Stage Application of PCT/EP2017/059291, filed on Apr. 19, 2017, which claims the benefit of priority to Serial No. DE 10 2016 207 876.3, filed on May 9, 2016 in Germany, the disclosures of which are incorporated herein by reference in their entirety.

The disclosure relates to a brake disc, in particular for motor vehicles, having a base disc composed of a first material and having a wear-reducing coating composed of a second material.

The disclosure furthermore relates to a braking device for a motor vehicle, having a brake disc which can be connected/is connected to a wheel and having a brake caliper, which is assigned to the brake disc and carries at least one actuable braking element.

The disclosure furthermore relates to a method for producing a brake disc of this kind.

BACKGROUND

Conventional brake systems of motor vehicles have at least one brake disc assigned to each wheel, which can be clamped between two brake pads of a brake caliper to produce a braking torque. During the clamping process, friction arises between the brake pads and the brake disc, generating heat. In this case, the heat generated is stored at least partially in the brake disc. In extreme cases, e.g. in the case of braking from high speeds, the temperature of the brake disc can reach very high values as a result, thereby potentially compromising the strength of the brake disc. Brake discs are therefore usually produced from gray cast iron, which has a high temperature stability. This relative wind furthermore generally leads to rapid cooling of the brake disc.

As an alternative, there is a known practice of producing wear-resistant brake discs which are formed as fully ceramic brake discs or gray cast iron brake discs with a hard metal layer. Brake discs of this kind are relatively expensive to produce and have the disadvantage that, owing to the different coefficients of expansion of the materials of the base disc and the wear-reducing coating, cracks can form in the coating under high temperature loads. Moreover, the brake disc is thermally stressed during the known coating processes, potentially leading to oxidation of the wear-reducing layer.

SUMMARY

The brake disc according to the disclosure has the advantage that it can be produced in a relatively advantageous way and yet offers sufficient strength and wear reduction, while the abovementioned disadvantages are nevertheless overcome. According to the disclosure, this is achieved by virtue of the fact that the first material or base disc is manufactured from a lightweight metal and that the second material or coating is an oxide layer. An oxide layer, in particular a hard anodized layer, is produced by hard anodizing, also referred to as hard anodic coating or hard coating, or by plasma electrolytic oxidation (PEO). In this case, electrolytic oxidation of the base disc takes place, giving rise to an abrasion-resistant layer on the base disc. For this purpose, the base disc is preferably dipped into an electrolyte and connected up as the anode. During this process, the surface of the base disc is oxidized, with the result that an oxide layer forms on the lightweight metal. This layer has a high wear-reducing effect and high load-bearing capacity and strength, even at high temperatures which can occur during the operation of the brake disc. In particular, this eliminates the abovementioned disadvantages in respect of different coefficients of thermal expansion. It is furthermore possible to implement the brake disc according to the disclosure at low cost.

Overall, there is the advantage here that manufacturing methods for shaping which were previously not possible for gray cast iron brake discs because the lightweight metals melted at relatively low temperatures are available for the production of the brake disc. Thus, it is possible, for example, to use casting, metal powder injection molding, 3-D printing, deep drawing or the like. A combination of these methods is also possible. Moreover, simpler processes or more accurate process control procedures can be employed in the joining processes, e.g. laser welding or ultrasonic welding, for lightweight metal brake discs. The hard anodizing for the production of the hard anodized layer converts the initial micrometers of the lightweight metal surface into a very hard and abrasion-resistant oxidation layer. At the same time, the chemical bonding of the oxide layer formed during this process to the lightweight metal substrate is very good, in particular better than with a hard metal coating on a gray cast iron brake disc. The use of an electric field in the coating process produces a continuous oxide layer, which prevents flaking or corrosion damage. The oxide layer is itself resistant to cleaning agents and has a uniform layer thickness, thereby further minimizing wear. In comparison with hard coating a gray cast iron brake disc, hard anodizing or plasma-electrolytic oxidation is economical. Intermediate layers are not necessary with hard anodizing or PEO methods.

According to a preferred development of the disclosure, it is envisaged that the lightweight metal is aluminum. As a result, an aluminum oxide layer with the abovementioned advantages is obtained in the hard anodic coating process.

As an alternative, provision is preferably made for the lightweight metal to be titanium. Moreover, it is also conceivable to produce the base disc from some other suitable lightweight metal, e.g. magnesium.

Provision is furthermore preferably made for the first material to be applied to a support body manufactured from plastic. The base disc is thus not produced from a solid casting but is of multi-part design, wherein the base disc is mounted on the support body. The base disc can be embodied as a fully cast element or as an MIM component.

According to another embodiment of the disclosure, provision is preferably made for the base disc itself to be mounted on a support body. In this case, provision is made, for example, for the base disc to be manufactured from a formed metal sheet composed of an alloy, which is/has been processed by hard anodic coating and then mounted on the support body, as described above. In particular, the support body is designed as a heat accumulator or as a support body with a high thermal conductivity.

According to a preferred embodiment of the disclosure, provision is furthermore made for the base disc or the support body to comprise a phase change material. The specific heat capacity of the brake disc is thereby increased, thus enabling the brake disc to withstand even high temperature loads during operation.

According to an advantageous development of the disclosure, provision is made for the base disc and/or the support body to have at least one chamber, in which the phase change material is arranged. In particular, the chamber is of closable or closed design, thus preventing the phase change material from escaping. In this case, it is ensured that the heat absorption capacity of the brake disc is maintained over the long term. The base disc and/or the support body expediently have a plurality of chambers containing phase change material arranged in a uniformly distributed manner over the circumference, thus ensuring that no unbalanced locations arise during the operation of the brake disc.

The braking device according to the disclosure is characterized by the embodiment of the brake disc in accordance with the disclosure. The abovementioned advantages are thereby obtained.

Provision is preferably made for at least one heat conducting element that can be brought into or is in direct contact with the brake disc to be arranged on the brake caliper. The direct contact between the heat conducting element and the brake disc ensures that heat stored or produced in the brake disc is dissipated from the brake disc via the heat conducting element. In particular, the direct contact is between the base disc and the heat conducting element. In this case, the direct contact furthermore exists particularly in a region in which the base disc is provided with the hard anodic coating in order to avoid wear.

In particular, provision is made for the heat conducting element to be a contact brush, which, in particular, is in continuous direct contact with the brake disc. For this purpose, the contact brush is preferably assigned to one face of the brake disc and is pressed axially against the face, in particular with a preloading force, thus ensuring the continuous direct contact. Heat is then dissipated from the brake disc to the heat conducting element by virtue of the rubbing contact.

As an alternative, provision is preferably made for the heat conducting element to be a rotatably mounted contact wheel, which rolls on the brake disc, in particular on one face of the brake disc. This minimizes the wear between the heat conducting element and the brake disc and ensures advantageous heat dissipation. According to a first embodiment, the heat conducting element is advantageously continuously in direct contact with the base disc. According to a second embodiment, provision is preferably made for an actuator to be assigned to the heat conducting element, said actuator feeding the heat conducting element to the brake disc when required in order to make direct contact with said disc and to dissipate heat. In particular, provision is made for the current operating temperature of the brake disc to be monitored, e.g. measured or calculated, and for the heat conducting element to be fed to the brake disc in accordance with the operating temperature determined in order to avoid overheating of the brake disc.

According to a preferred development of the disclosure, provision is made for the heat conducting element to be thermally connected to a heat sink. This enables the heat given off by the brake disc to the heat conducting element to be fed to the heat sink, with the result that the heat conducting element does not serve as an additional store of heat but rather as a heat conductor which feeds the heat produced in the brake disc to the heat sink. Continuous advantageous cooling of the brake disc is thereby ensured. The method according to the disclosure leads to a brake disc with the abovementioned advantages. According to the disclosure, provision is made, for this purpose, for a base disc composed of a lightweight metal to be supplied in a first step. The base disc is then provided with a hard anodic coating by hard anodizing. The advantages already mentioned are thereby obtained. In particular, provision is made for the base element to be manufactured from titanium or aluminum. Manufacture from magnesium is also conceivable.

Provision is furthermore preferably made for the base disc to be mounted on a support body, wherein the support body is produced, in particular, from a material with a high specific heat capacity. In particular, a phase change material is used for this purpose.

According to a preferred embodiment of the disclosure, provision is made for the base disc to be produced as a sheet-metal part and to be provided with the hard anodic coating and finally mounted on a support body or on the support body.

Further advantages and preferred features will emerge, in particular, from what has been described above and from the claims.

DETAILED DESCRIPTION

FIG. 1shows a brake disc1for a motor vehicle (not shown) in perspective. The brake disc1has a brake ring3, which is formed by a base disc2. The brake ring3has brake surfaces4and5, respectively, on both faces. In the center, the brake disc1has a support boss6for fastening the brake disc1on a wheel bearing or a wheel of the motor vehicle. In terms of materials, the support boss6can differ from the brake ring and can be manufactured from aluminum, for example. This reduces the unsprung mass, and more advantageous heat dissipation to the hub and rim is achieved. A plurality of holes for screwed joints for fastening are formed on the support boss6. The braking surfaces A, b are subject to wear during operation, leading to the necessity of replacing the brake disc1at regular intervals. Current brake discs are usually produced from gray cast iron and, in some cases, also from ductile cast iron or are cast from a suitable steel alloy and machined by turning. Brake discs for motorcycles are preferably manufactured from corrosion-resistant steels. In some cases, more wear-resistant ceramic brakes are also employed, although these lead to high costs. For greater braking power, higher wear resistance and better insensitivity to fading combined with low weight, silicon carbide reinforced with carbon fibers and carbon-fiber-reinforced plastic are also used for brake discs in racing and in aircraft construction. Vehicle brake discs can also be punched out of sheet metal.

The base disc2or brake ring3has two brake rings3′ and3″, which are arranged spaced apart and each form one of the braking surfaces4and5, respectively. Arranged between the brake rings3′ and3″ are spacers in the form of ribs1, which form a plurality of cavities between the brake rings3′,3″. In this case, an air flow from the inside to the outside between the brake rings3′,3″ arises from the centrifugal force that occurs during driving, with the result that the brake disc1is actively cooled. As an option, ventilation holes can also be formed in the respective brake ring3′,3″ in order to improve the air cooling, with the air flow also passing through the ventilation holes and thereby producing turbulence which optimizes the heat transfer between the brake disc and the air.

The base disc2or brake rings3′,3″ and optionally also the ribs7are manufactured from a lightweight metal, in particular titanium, aluminum or magnesium. The braking surfaces4,5are furthermore provided with an oxide layer8,9.

In this regard,FIG. 2shows an enlarged detail view of the brake disc1in a simplified sectional illustration. According to the illustrative embodiment under consideration, both braking surfaces4,5are provided with an oxide layer8,9, which, in particular, extends 100 nm to 10 μm into the material of the brake rings3′,3″. In particular, the oxide layer is produced by hard anodizing or plasma electrolytic oxidation of the base disc2or brake rings3′,3″. If the base disc2is manufactured from aluminum, the braking surfaces are converted into an aluminum oxide layer or provided with such a layer by the methods mentioned.

The brake disc is produced at low cost by virtue of manufacture from a lightweight metal and, by virtue of the advantageous oxide layer, ensures low wear and high temperature stability. The formation of the wear-reducing oxide layer8,9ensures optimum chemical bonding of the protective layer to the base disc2. Moreover, the oxide layer is produced uniformly and airtightly, thus preventing corrosion of the base disc2.

FIG. 3shows, in a simplified illustration, a braking device10which has a brake caliper11, which has two movably mounted brake pads12,13, between which the brake disc1is situated, brake pad12thus being assigned to braking surface4having oxide layer8, and brake pad13being assigned to braking surface5having oxide layer9. If the brake pads12,13are moved toward one another, the brake disc1situated therebetween is clamped between them and adhesion occurs between them, exerting a decelerating effect on the rotating brake disc1, as with conventional braking devices.

In the present case, the inner brake ring31′ has an annular region14which is free from the oxide layer9. This annular region14is assigned a heat conducting element15, which in the present case is designed as a heat conducting brush16and is pressed against the brake disc1in the region of region14by means of a preloading force by a brush holder17. The brush16is manufactured from metal and serves for heat dissipation. The heat conducting element15is furthermore connected to a heat sink18, to which the heat absorbed by the heat conducting element15is dissipated. The heat sink18can be a radiator, a heat storage element or a coolant line of a cooling circuit of the motor vehicle, for example.

Region14, which in the present case is situated radially to the inside of the oxide layer9, can also be embodied as a groove in the brake ring3″, thereby making possible improved guidance of the heat conducting element15. In an alternative embodiment, provision is preferably made for region14to be coated with a hard metal. The maximum temperature of the brake disc1is thus limited by the design of the direct contact between the heat conducting element15and the brake disc1and by the thermal heat dissipation to the heat sink18.

As an alternative to embodiment as a brush16, provision is made, according to another illustrative embodiment shown inFIG. 3Awherein like numbers refer to like elements, for the heat conducting element15to be designed as a contact wheel19, which is mounted rotatably on the brake caliper11by means of a bearing20, in the present case a rolling element bearing. In this case, the bearing20is formed in thermal contact with the heat sink18. Like the alternative brush16, the contact wheel19is in continuous direct contact with the brake ring3″ in order to dissipate heat from the brake disc1to the heat sink18by means of the direct contact. In this arrangement, the axis of rotation of the contact wheel19is aligned radially with respect to the axis of rotation of the brake disc1.

FIG. 4shows another illustrative embodiment of the brake disc1in a simplified sectional illustration. In contrast to the previous illustrative embodiments, provision is now made for the brake disc1to have a heat storage element21. The heat storage element21is situated between the brake rings3′ and3″ and is connected thermally thereto. In particular, the heat storage element21is formed integrally with the brake rings3′,3″ and the ribs7. In the interior, the heat storage element21, which thus forms a heat storage ring, has one or more chambers22, in which a material with a high specific heat capacity, e.g. oil, is arranged or held. As a particular preference, the material23is designed as a phase change material, e.g. a paraffin, which stores heat by changing its state of aggregation from solid to liquid, for example. Heat is thereby stored in the brake disc1by means of a phase change in accordance with the specific heat capacity of the phase change material, in addition to the increase in temperature, and hence thermal overloading of the brake disc1, in particular of the brake rings3′ and3″, is avoided.

It is advantageous if the base disc2is manufactured by metal powder injection molding, 3-D printing and/or deep drawing, thus enabling the production of the brake disc1to be carried out at low cost and with little effort.

As an alternative to the illustrative embodiment under consideration, one or more such chambers22are formed in the brake rings3′ and3″ themselves, as shown inFIG. 3. It is also conceivable to provide additional chambers22in the brake rings3′ and3″ in the illustrative embodiment inFIG. 4. As already described above, the chambers22are filled with a material that has a high specific heat capacity and/or a phase change material. As a final step, openings24used to fill the chambers are closed, in particular welded.

The brake disc1according to the illustrative embodiments under consideration is in each case of single-part design. According to an alternative embodiment, however, provision is advantageously made for the brake disc to be of multi-part design, wherein the parts are connected to one another by suitable joining technologies, e.g. ultrasonic welding or laser welding. In particular, it is conceivable to produce the brake disc1in three parts, wherein parting lines are in each case preferably situated in a central plane of one of the brake rings3′ and3″, as shown by dashed lines inFIG. 3. In this way, it is also possible to produce the chambers22at low cost during the manufacturing process.

In order, in general terms, to improve the decrease in strength at high temperatures and the reduction in weight through the avoidance of metal mass, the maximum temperature is lowered by increasing the heat capacity C. For the same heat energy Q to be stored, this results in a lower temperature difference, i.e. a lower maximum temperature Tmax. A reduction in temperature can furthermore be achieved when using phase change materials. This is explained below by means of basic physical equations:

The heat capacity C is influenced by three parameters, the material-dependent, specific heat capacity c, the volume V and the density ρ. The latter can be combined in the weight m, although this should be kept as low as possible:
C=c·ρ·V,C=c·m

To enable as much heat as possible to be stored with a low weight in comparison with gray cast iron brake discs, the primary choice is for a high specific heal capacity. Materials with a phase change, e.g. paraffin, are suitable for this purpose. The additional heat that can be stored in the case of a phase transition is:
Q=m·L
where L is the enthalpy of melting.