Brake disc for a friction brake of a motor vehicle and method for manufacturing a brake disc

Abstract

One aspect of the invention relates to a brake disc (1) for a friction brake of a motor vehicle, the brake disc comprising a friction portion (6) having at least one friction face (6a, 6b) and a fastening portion (3) for fastening to the wheel of the motor vehicle, wherein the friction portion (6) and the fastening portion (7) are formed on a main body (2) made of grey cast iron or steel, wherein a wear protection layer (8) is thermally applied to the friction portion (6) as the friction face (6a, 6b), and wherein an adhesion promoter layer (7) is arranged between the wear protection layer (8) and the main body (2) and is used to attach the wear protection layer (8) to the main body (2). Colour pigments (10) are applied to the side of the adhesion promoter layer (7) facing the wear protection layer (8) or colour pigments (10) are embedded into the adhesion promoter layer (7). A further aspect of the invention relates to a method for manufacturing a brake disc.

Description

[0001] Description

[0002] Brake disc for a friction brake of a motor vehicle and method for producing a brake disc

[0003] The invention relates to a brake disc for a friction brake of a motor vehicle, comprising a friction section with at least one friction surface and a fastening section for wheel-side fastening to the motor vehicle, wherein the friction section and the fastening section are formed from a base body made of gray cast iron or steel, wherein a wear protection layer is thermally applied to the friction section as a friction surface, and wherein an adhesion-promoting layer is arranged between the wear protection layer and the base body, via which adhesion-promoting layer the wear protection layer is bonded to the base body.

[0004] Furthermore, the invention relates to a method for producing a brake disc for a friction brake of a motor vehicle, comprising providing a base body made of gray cast iron or steel, which forms a friction section and a fastening section for wheel-side fastening to the motor vehicle, applying an adhesion-promoting layer to the base body by a thermal application process, and applying a wear-protection layer to the adhesion-promoting layer to provide a friction surface on the friction section by a thermal application process.

[0005] Brake discs for friction brakes in motor vehicles are typically made of gray cast iron. Gray cast iron is characterized by a high volumetric heat capacity and good thermal shock resistance. Furthermore, gray cast iron is a cost-effective material, and the production of brake discs from gray cast iron is technically manageable. However, the material's high tendency to corrosion and its high level of wear during vehicle operation are problematic. Corrosion leads to visual defects, as red rust can develop within a very short time, which is directly visible through frequently used open rims. The material's strong tendency to corrosion can lead to increased surface deterioration in economical driving or in electric and hybrid vehicles with high levels of recuperation due to the infrequent braking, necessitating early replacement of the brake disc.

[0006] Furthermore, brake discs made of ceramic materials are known, which, although they have a long service life and excellent stability, have not yet been able to establish themselves for large-scale production due to their very high price.

[0007] As an alternative, it has already been proposed to provide a gray cast iron base body with a wear-resistant coating in the area of ​​the friction surfaces. For example, DE 10 2018 112 035 B3 discloses a wear-resistant coating in the form of a hard coating made of tungsten carbide. Such solutions also achieve a significantly longer service life and durability compared to pure gray cast iron brake discs, but are significantly more cost-effective than brake discs made entirely of ceramic materials.

[0008] Pure corrosion protection coatings such as temporary paint, zinc, or powder coatings, however, are unsuitable as wear protection layers, as they are abraded on the friction surface during the first braking applications and are unable to provide lasting corrosion protection. Such corrosion protection coatings are only suitable for protecting non-frictionally stressed sections of a brake disc and, if necessary, for temporarily preserving an attractive surface appearance of the friction surface during delivery of new vehicles.

[0009] To improve the adhesion of the wear protection layer, it is also known to arrange an adhesion-promoting layer between it and the gray cast iron base body. A generic brake disc with a wear protection layer and an adhesion-promoting layer, as well as a generic manufacturing method for the same, are known from WO 2020 / 234144 A1.

[0010] However, determining the individual layer thicknesses is difficult. Typically, magnetic inductive coating thickness gauges are used for this purpose. However, these only work satisfactorily with non-magnetic coating materials. If ferromagnetic alloys are to be used, the layer thicknesses must be determined by other methods. The invention is based on the object of remedying this problem. WO 2021 / 008744 A1 discloses a brake disc with a base body made of gray cast iron, to which a diffusion layer formed from iron, aluminum, and silicon is thermally applied in the area of ​​a friction surface. This is expressly intended to eliminate the more expensive hard material coating based on carbides. Heat treatment triggers a diffusion process which creates a graded layer system of iron and aluminum in which the aluminum content increases towards the surface and in which silicon is homogeneously distributed.The aluminum boundary layer is finally removed mechanically. In this context, WO 2021 / 008744 A1 proposes incorporating coloring substances into the diffusion layer.

[0011] The coloring substances are added before or during the coating process to create color markings within the diffusion layer. The aim is to create color shades in the diffusion layer, particularly in the area of ​​the friction surfaces, which could be used, for example, as wear indicators. It is also possible, however, that the coloring substances could serve as temperature indicators to provide information about the operating temperature of the brake disc at the friction surfaces.

[0012] However, the mechanism of action remains diffuse in WO 2021 / 008744 A1. Furthermore, dispersing color-imparting substances in the diffusion layer does not satisfactorily solve the problem of determining layer thickness.

[0013] The invention is based on the object of simplifying the determination of the layer thickness in a brake disc of the type mentioned at the outset and of specifying a corresponding manufacturing method for such a disc.

[0014] This object is achieved by a brake disc according to patent claim 1. The brake disc according to the invention for a friction brake of a motor vehicle comprises a friction section with at least one friction surface and a fastening section for wheel-side fastening to the motor vehicle, wherein the friction section and the fastening section are formed on a base body made of gray cast iron or steel, wherein a wear protection layer is thermally applied to the friction section as a friction surface, and wherein an adhesion-promoting layer is arranged between the wear protection layer and the base body, via which adhesion-promoting layer the wear protection layer is bonded to the base body. It is characterized in that color pigments are applied to the side of the adhesion-promoting layer facing the wear protection layer or are embedded in the adhesion-promoting layer.

[0015] This creates a defined depth mark between the wear protection layer and the adhesion-promoting layer, which allows the layer thickness of both layers to be determined, assuming that the thickness of the substrate before coating and its total thickness after coating are known. The transition between the two applied layers can be visually detected and thus used to determine the layer thickness.

[0016] Special embodiments of a brake disc according to the invention are the subject of further patent claims.

[0017] According to a special embodiment, the color pigments comprise those made of chromium(III) oxide. The majority of color pigments lose their color or become unstable at high temperatures, such as those encountered during thermal coating. Tests have surprisingly shown that chromium(III) oxide is colorfast up to approximately 1000°C for at least about 30 minutes and can also withstand temperatures of 1600°C for short periods, so the color pigments are not affected by the thermal application processes used to apply the friction surface.

[0018] According to another special design, the bonding layer is made of stainless steel. This protects the gray cast iron base body from corrosion. This bonding layer also serves to prevent any cracks from developing in the wear-resistant layer. Ductile, rust-proof stainless steels are therefore preferred. Austenitic stainless steels with the designation 316L-AISI (steel designation 1.4404), ferritic stainless steels with the designation 430 AISI (steel designation 1.4016), and duplex steels with an austenitic / ferritic structure have proven particularly suitable.

[0019] According to another special design, the wear protection layer has embedded hard material particles with a maximum diameter of 10 to 60 pm. The wear protection layer basically serves to create a functional tribological system together with a brake pad and, thanks to the embedded hard material particles, to exhibit low wear. A characteristic of the hard material particles is that they are significantly harder than the material of the base body. Ceramic hard material particles are primarily suitable for this purpose. Carbides, nitrides, oxides and borides are particularly suitable examples. With these particle sizes, in combination with a brake pad, high braking performance combined with low wear on the brake disc can be achieved. Alternatively, however, a coating made of such a material can also be provided as a wear protection layer.

[0020] According to another special design, the wear protection layer comprises a matrix made of stainless steel or an iron alloy, into which the hard material particles are embedded. This allows for sufficient ductility with regard to the thermal expansion behavior of the brake disc and low crack formation combined with high wear resistance. The same materials as for the adhesion promoter can be used as matrix materials, particularly stainless steels of the designations 316L and 430, as well as duplex steels.

[0021] Preferably, the proportion of the matrix is ​​60 to 70 volume percent based on the wear protection layer, with the remainder being formed by the hard material particles.

[0022] According to another special embodiment, the adhesion-promoting layer is applied to the base body by laser deposition welding.

[0023] According to another special embodiment, the wear protection layer is applied to the adhesion-promoting layer by laser deposition welding.

[0024] In laser cladding, the welded layer is directly bonded to the underlying material. Unlike thermal spraying processes such as flame spraying, laser cladding slightly melts the underlying material, resulting in a particularly close bond. Surface pretreatment of the substrate, as required in thermal spraying processes, which only result in a mechanical bond, is unnecessary with laser cladding. The material to be welded can be supplied as a powder, but optionally also in the form of a wire or a rigid electrode. The above-mentioned object is further achieved by a method for producing a brake disc for a friction brake of a motor vehicle according to patent claim 8. The method according to the invention comprises the following steps:

[0025] Providing a base body made of grey cast iron or steel, which extends over a friction section and a fastening section for wheel-side fastening to the motor vehicle;

[0026] Applying an adhesion-promoting layer to the base body by a thermal application process,

[0027] Applying a wear protection layer to the adhesion-promoting layer to provide a friction surface at the friction section by a thermal application process; and

[0028] Application and / or introduction of color pigments such as chromium(III) oxide, wherein the color pigments are introduced into the adhesion-promoting layer during its application to the base body and / or are applied to the adhesion-promoting layer after its application and before application of the wear-resistant layer.

[0029] This makes it easy to produce a brake disc with high wear resistance, which also allows easy control of the layer thickness.

[0030] In a special version of the process, the color pigments are applied to the adhesion-promoting layer using a suspension or solution. The liquid component of the suspension or solution is then evaporated, leaving only the color pigments.

[0031] In another special version of the process, the color pigments are incorporated into a stainless steel powder, which is welded onto the base body as an adhesion-promoting layer. The color pigments must be sufficiently temperature-resistant and must not lose their color. Chromium(III) oxide, for example, is suitable for this purpose. However, other color pigments can also be used that remain stable for at least 30 minutes at 1000°C and can withstand temperatures up to 1600°C for a few seconds.

[0032] The invention will be explained in more detail below with reference to an embodiment shown in the drawing. The drawing shows: Figure 1 is a schematic sectional view of a brake disc according to an embodiment of the invention, and

[0033] Figure 2 shows a detailed view of the friction surface in section to illustrate the layer structure.

[0034] The embodiment shows a brake disc 1 for a friction brake of a motor vehicle, in particular a passenger car or light commercial vehicle.

[0035] The brake disc 1 has a base body 2 which is made of grey cast iron or a cast steel alloy.

[0036] This base body 2 initially extends over a fastening section 3 of the brake disc 1, via which the brake disc 1 can be fastened to the wheel side of the vehicle.

[0037] Without limitation, the fastening section 3 can be cup-shaped, as shown by way of example in Fig. 1. At the bottom 4 of this cup shape, one or more through-openings 5 ​​are formed, via which the brake disc 1 can be fixed, in particular screwed, to a wheel hub, for example.

[0038] Furthermore, the base body 2 extends into the area of ​​a friction ring of the brake disc 1, which is also referred to below as friction ring section 6.

[0039] In the illustrated embodiment, two friction surfaces 6a and 6b are provided on the friction ring section 6, which are each annular and lie opposite one another.

[0040] The friction surfaces 6a and 6b interact on a disc brake when the same is actuated in a manner known per se with brake pads attached to a brake calliper to generate braking power.

[0041] The friction surfaces 6a and 6b are preferably offset parallel to the plane of the pot bottom 4.

[0042] However, within the scope of the present invention, brake discs 1 with only one friction surface 6a are also possible. Furthermore, the base body 2 can optionally be constructed in two or more parts, with friction surfaces 6a, 6b being arranged on different parts of the base body.

[0043] In the present case, the friction surfaces 6a and 6b consist of a different material or material system than the base body 2, in particular a material which has a higher hardness and wear resistance on the surface facing the brake pads than the material of the base body 2.

[0044] In the embodiment shown in Figs. 1 and 2, the friction surfaces 6a and 6b consist of a thermally applied layer system comprising at least two layers of different materials. Two layers are shown here. However, a larger number of layers is also possible.

[0045] The layer system comprises at least one adhesion-promoting layer 7, which is applied to the material of the base body 2, and at least one wear-protection layer 8, which forms the surface of the friction surface 6a or 6b for contact with the brake pads.

[0046] In Figs. 1 and 2, a wear protection layer 8 is thermally applied to the friction section 6 as a friction surface 6a or 6b. However, the wear protection layer 8 is not located directly on the base body 2 made of gray cast iron or steel. Rather, the adhesion-promoting layer 7 is arranged between the wear protection layer 8 and the base body 2. In the illustrated embodiment, the wear protection layer 8 is bonded to the base body 2 via the adhesion-promoting layer 7. If necessary, additional intermediate layers can be provided between the adhesion-promoting layer 7 and the wear protection layer 8.

[0047] The adhesion-promoting layer 7 can be made of stainless steel, for example. Ductile, rust-proof stainless steels are preferred. This protects the base body 2 from corrosion. Furthermore, such an adhesion-promoting layer 7 serves to stop any cracks from the wear-resistant layer 8 before they can reach the base body 2.

[0048] The wear-resistant layer 8 comprises a metallic matrix and hard material particles 9 embedded therein. Ceramic hard material particles 9 are preferably used in this case. Carbides, nitrides, oxides, and borides are particularly suitable as examples. A preferred material for the hard material particles 9 is tungsten carbide.

[0049] The hard material particles 9 preferably have a maximum diameter in the range of 10 to 60 pm, preferably 15 to 55 pm. With these particle sizes, in conjunction with a brake pad, high braking performance combined with low wear on the brake disc 1 can be achieved.

[0050] Hard material particles 9 in spherical form are preferably used.

[0051] The metallic matrix preferably consists of a stainless steel or an iron alloy in which the hard material particles 9 are embedded. This enables, on the one hand, sufficient ductility with regard to the thermal expansion behavior of the brake disc 1 and, on the other hand, low crack formation combined with high wear resistance.

[0052] Preferably, the proportion of the matrix is ​​60 to 70 volume percent based on the wear protection layer 8, the remainder being formed by the hard material particles 9.

[0053] According to a further special embodiment, the adhesion-promoting layer 8 is applied to the base body 2 by laser deposition welding.

[0054] Alternatively, however, the wear protection layer 8 can also be provided as a coating made purely of a ceramic material, which was presented above for the hard material particles 9.

[0055] Both the adhesion-promoting layer 7 and the wear-resistant layer 8 can be applied by laser cladding. In this process, the welded layer is directly bonded to the underlying material. Unlike thermal spraying processes such as flame spraying, laser cladding slightly melts the underlying material, resulting in a particularly tight bond. Surface pretreatment of the substrate, as required with thermal spraying processes, which only create a mechanical bond, is unnecessary with laser cladding. The material to be welded can be applied as a powder, but also, if necessary, in the form of a wire or a rigid electrode.

[0056] Tests have shown that the wear of a brake disc 1 of the type described above is considerably lower than that of simple gray cast iron brake discs. However, it is difficult to clearly determine at what point the wear protection layer 8 on the brake disc 1 is worn and replacement of the brake disc 1 becomes necessary.

[0057] One possibility is to determine layer thicknesses non-destructively using a magnetic inductive layer thickness gauge. This method works in that an approximate layer thickness can be determined after the first coating layer. After applying the second coating layer, i.e.

[0058] Wear protection layer 8, the layer thickness is measured again so that it can be determined how thick the wear protection layer 8 is after a subsequent grinding process.

[0059] However, the measurement accuracy depends heavily on the surface roughness after coating and thus exhibits a certain degree of inaccuracy. Another problem is that the coating thickness measurement only works with the stainless steel-based coating system specified above. The stainless steel used is non-magnetic due to a high nickel content in the alloy, so this measurement method is functional. The inspection and measurement of the residual coating thickness of the brake disc can only be performed during maintenance in workshops. However, by using color-based wear detection, even a layperson can easily determine at any time whether the wear limit has been reached.

[0060] However, cheaper alloys such as 430 alloys are often ferromagnetic, so that the above-mentioned measuring method cannot be used.

[0061] The problem of determining the layer thickness is addressed here by a targeted embedding of suitable color pigments 10.

[0062] For this purpose, color pigments 10 are applied during the coating process to the side of the adhesion-promoting layer 7 facing the wear-protection layer 8. It is also possible to embed these color pigments 10 in the adhesion-promoting layer 7. This provides precise information about the position of the layer boundary between the wear-protection layer 8 and the adhesion-promoting layer 7.

[0063] This creates a way to visualize the wear limit of the wear-resistant layer 8. Preferably, the natural inorganic color pigment chromium(III) oxide is used for this purpose, which has a greenish color within a specific particle size range. This color pigment 10 is both high-temperature stable and high-temperature color-stable, so that during normal color determinations (approximately 30 minutes holding time), the color pigments 10 retain their color up to approximately 1000°C. Due to the thermal application, the color pigments 10 are heated to approximately 1600°C for a few seconds in this case, and it has been shown that the color pigments 10 made of chromium(III) oxide remain stable and colored even here. Other, preferably inorganic, color pigments 10 with similar properties can also be used in this case.

[0064] During the production of the brake disc 1, the base body 2 made of grey cast iron or steel is first prepared.

[0065] The adhesion-promoting layer 7 is then applied to the base body 2 by a thermal deposition process, for example laser deposition welding, preferably limited to the area of ​​the friction surface section 6.

[0066] The color pigments 10 can be incorporated into the adhesion-promoting layer 7 during its application to the base body 2. For this purpose, the color pigments 10 can be added to the material to be welded, which can be in powder form, for example.

[0067] Alternatively or additionally, the color pigments 10 are applied to the adhesion-promoting layer 7 after the application of the adhesion-promoting layer 7 and before the application of the wear-resistant layer 8.

[0068] For the firm, residue-free application of the color pigments 10 to the adhesion-promoting layer 7, a suspension or solution can be used in which the color pigments 10 are present in a residue-free vaporizing solvent, such as isopropanol, and are applied to the surface of the adhesion-promoting layer 7. Other possibilities would be to prepare pastes that adhere better to the surface.

[0069] This variant offers very easy handling of the color pigments 10 and hardly affects the cycle times when applying the layer system.

[0070] Furthermore, due to the roughness of the adhesion-promoting layer 7, the color pigments 10 are not located on a single plane, but are scattered in an interval. This means that once the wear limit is reached, the color marking on the brake disc remains for a longer period of time. With an ideal layer, the color particles would lie in a single plane and be worn off after just a few braking actions. The existing surface roughness therefore prolongs the visibility of the color marking.

[0071] Thereafter, the wear protection layer 8 is finally applied to the adhesion-promoting layer 7, including the color pigments 10, to provide a friction surface 6a or 6b on the friction section 6 by a thermal application process, for example laser deposition welding.

[0072] Finally, the surface of the friction surface 6a or 6b can be ground.

[0073] The existing color pigments 10 cause a color change as soon as the wear-resistant layer 8 is worn and the color pigments 10 emerge to the surface. The incorporation of the color pigments 10 does not impair the bonding quality of the wear-resistant layer 8 to the underlying, color-pigmented adhesion-promoting layer 7.

[0074] The above-described incorporation of color pigments 10 makes it possible, for example, to very easily determine the layer thickness ratios on the friction surfaces 6a and 6b during the development phase up to series production. This provides an opportunity to test the performance of the wear protection layer 8 to its extreme limits and thus draw conclusions about the service life of a coated brake disc 1. In series production, this allows brake disc wear to be determined visually. In a modification of the above-described embodiment, it is also possible to omit the adhesion-promoting layer 7.The color pigments 10 are then applied directly to the base body 2 before the wear protection layer 8 with embedded ceramic hard material particles 9, in particular made of spherical tungsten carbide, or a wear protection layer 8 made purely of a ceramic material is thermally applied, in particular laser-welded, to the base body.

[0075] The invention has been explained in more detail above using an exemplary embodiment and further modifications. In particular, individual technical features explained above in the context of further individual features can be implemented independently of these and in combination with further individual features, even if not expressly described, as long as this is technically possible. The invention is therefore expressly not limited to the described exemplary embodiment and modifications, but encompasses all configurations defined by the patent claims.

[0076] List of reference symbols

[0077] brake disc

[0078] Basic body

[0079] Floor mounting section

[0080] passage opening

[0081] Friction surface section a Friction surface b Friction surface

[0082] Adhesion layer

[0083] Wear protection layer

[0084] Hard material particles 0 color pigments

Claims

Claims: Brake disc (1) for a friction brake of a motor vehicle, comprising a friction section (6) with at least one friction surface (6a, 6b) and a mounting section (3) for wheel-side attachment to the motor vehicle, wherein the friction section (6) and the mounting section (7) are formed on a base body (2) made of gray cast iron or steel, wherein a wear-resistant layer (8) is thermally applied to the friction section (6) as the friction surface (6a, 6b), and wherein an adhesion-promoting layer (7) is arranged between the wear-resistant layer (8) and the base body (2), via which the wear-resistant layer (8) is connected to the base body (2), characterized in that color pigments (10) are applied to or embedded in the adhesion-promoting layer (7) on the side of the adhesion-promoting layer (7) facing the wear-resistant layer (8).Brake disc according to claim 1, characterized in that the color pigments (10) comprise chromium(III) oxide. Brake disc according to claim 1 or 2, characterized in that the adhesion-promoting layer (7) is stainless steel. Brake disc according to any one of claims 1 to 3, characterized in that the wear-resistant layer (8) comprises embedded hard particles (9) with a maximum diameter of 10 to 60 pm or embedded hard particles (9) made of a carbide, in particular globulitic tungsten carbide, with a maximum diameter of 10 to 60 pm. Brake disc according to any one of claims 1 to 4, characterized in that the wear-resistant layer (8) comprises a matrix made of stainless steel or an iron alloy. Brake disc according to claim 5, characterized in that the matrix comprises 60 to 70 percent by volume of the wear-resistant layer (8) and the remainder is formed by hard particles (9). Brake disc according to any one of claims 1 to 6, characterized in that the adhesion promoter layer (7) is applied to the base body (2) by laser cladding and / or the wear-resistant layer (8) is applied to the adhesion promoter layer (7) by laser cladding. Method for manufacturing a brake disc (1) for a friction brake of a motor vehicle, comprising: Providing a base body (2) made of grey cast iron or steel, which extends over a friction section (6) and a fastening section (3) for wheel-side attachment to the motor vehicle, Applying an adhesion promoter layer (7) to the base body (2) by a thermal application process, Applying a wear-resistant layer (8) to the adhesion-promoting layer (8) to provide a friction surface (6a, 6b) on the friction section (6) by a thermal application process, characterized in that Color pigments (10) are introduced into the adhesion promoter layer (7) during its application to the base body (2) or applied to the adhesion promoter layer (7) after its application and before the application of the wear-resistant layer (8). The method according to claim 8, characterized in that the color pigments (10) are applied to the adhesion promoter layer (7) by means of a suspension or a solution, and the liquid component of the suspension or solution is evaporated after application. The method according to claim 8, characterized in that the color pigments (10) are incorporated into a stainless steel powder, which is welded onto the base body (2) as an adhesion promoter layer (7).

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