Brake disc with at least one braking surface and a wear-resistant coating applied to the braking surface

Abstract

Brake disc, with at least one braking surface and a wear-resistant coating applied to the braking surface, wherein the wear-resistant coating is thermally sprayed onto the braking surface and has a ground surface, characterized in that the wear-resistant coating applied by high-speed flame spraying comprises an oxide-free ceramic material in the form of tungsten carbide (WC), has a hardness between 1200 and 1300 HV01 and a ground surface, and that the ground surface has a surface roughness Rz of less than 2 µm and a profile bearing area tp of more than 95%, preferably more than 99%.

Description

[0001] The invention relates to a brake disc, with at least one braking surface and a wear-resistant coating applied to the braking surface.

[0002] DE 698 12 344 T2 already discloses a method for coating a brake element with a friction material in the form of a wear-resistant ceramic-metal composite material, in which a ceramic preform made of B4C, AlB2, Al4BC, Al3B 48 C2, AlB 12 and / or AlB 24 C4 is infiltrated with metal, and the preform is then subjected to heat treatment before being laminated onto the brake element, for example by bonding. However, the known method is considered disadvantageous because it is very time-consuming and also very expensive.

[0003] A brake disc according to the preamble of claim 1 is already known from DE 100 56 161 A1. There, the brake disc is provided with a metallic, non-ceramic coating, preferably made of an unalloyed or alloyed steel that is harder than the metal of the brake disc. Although this increases the hardness of the braking surface, unalloyed or alloyed steels still have a comparatively low hardness, are prone to scoring as metals, and are subject to wear-related abrasion during operation, which generates metallic brake dust. This contributes to the particulate matter problem in road traffic and can also lead to troublesome brake dust build-up on chrome-plated surfaces near the brakes.

[0004] From US patent 2006 / 0272909A1, a brake disc with a coating that is gradually worn away during operation is known. The coating comprises a first component, which has a more negative electrode potential than the brake disc, and a second component, which is harder than the first component. The components are applied together, for example by thermal spraying.

[0005] DE 44 13 306 C1 discloses a method for reinforcing a component, which can be a brake disc. The component can be provided with a reinforcing layer containing reinforcing particles by high-speed flame spraying. The reinforcing particles can consist of a ceramic material such as silicon carbide, aluminum oxide, boron carbide, magnesium oxide and / or titanium boride.

[0006] DE 10 2005 008 569 A1 discloses a method for manufacturing a brake disc in which a coating of a melt-in alloy, which may also contain hard particles such as oxides, carbides, borides, silicides or nitrides, is applied to a base body, e.g. by high-speed flame spraying. The base body can be roughened before coating and the coating can be ground flat after coating.

[0007] German patent DE 196 08 952 A1 discloses a brake disc with a wear layer applied by thermal spraying. Ceramic materials, such as aluminum oxide, titanium oxide, and mixtures thereof, are mentioned as coating materials, while flame spraying is given as an example of thermal spraying. US patent 5,612,110 A discloses a brake disc consisting of several layers.

[0008] In R. Schwetzke and H. Kreye's article, "Microstructure and Properties of Tungsten Carbide Coatings Sprayed with Various High-Velocity Oxygen Fuel Spray Systems," several experiments using high-velocity flame spraying systems to investigate the properties of several sprayed non-oxide ceramic materials containing tungsten carbide (WC) are described. The sprayed ceramic materials exhibited a relatively high porosity.

[0009] US Patent 4,180,622 A describes a wear-resistant coating for disc-shaped machine parts made of aluminum or aluminum alloys, applied by a thermal spraying process. The coating consists of two components: one component is pure aluminum oxide or aluminum oxide mixed with another oxide, and the other component is aluminum, an aluminum-silicon alloy, or an aluminum-magnesium alloy.

[0010] Document US 5,612,110 A shows a composite brake disc with multiple flanges. A central flange is separated from an outer flange by a heat shield, the outer flange having a ceramic coating.

[0011] DE 30 33 139 A1 discloses a device with a friction pair, in particular a friction brake or a friction clutch, wherein a first friction element consisting of organic binders and friction materials is connected to a more durable friction element made of an iron material.

[0012] In JP 2006-152 433 A, a brake disc is disclosed in which a coating of WC-Co and Fe-C is applied to a cast iron brake disc by means of flame spraying.

[0013] JP 2001-317 573 A describes a titanium brake disc coated with WC-Co or Fe-C by flame spraying. The coating thickness is preferably between 50 µm and 500 µm.

[0014] Based on this, the invention aims to improve a brake disc of the type mentioned above in such a way that the wear and abrasion resistance of the brake disc can be improved, the coated friction surface of the brake disc has good grip and no noticeable scoring is observed even after a longer period of use.

[0015] This problem is solved according to the invention by the features in the characterizing part of claim 1.

[0016] It has been shown that the friction surfaces of friction elements for brakes, and especially brake discs, can be coated by thermal spraying with a coating that ensures good grip in conjunction with the friction surface of another friction element of the brake, such as a brake pad, with no noticeable scoring occurring in the braking surfaces of the brake disc even after a longer period of use, and with sufficient heat dissipation through the coating even under heavy load to ensure excellent braking properties.

[0017] Due to the high hardness and wear resistance of the coating, the friction elements according to the invention exhibit virtually no wear-related abrasion. Thus, practically no brake dust is generated, which not only contributes to reducing the fine dust problem in road traffic, but also, especially on motorcycles, prevents annoying brake dust build-up on chrome-plated surfaces near the brakes.

[0018] If, according to a preferred embodiment of the invention, the friction element is a conventional brake disc, which normally has two opposing, relatively smooth braking surfaces to which thermally sprayed coatings adhere poorly, the invention provides for roughening these braking surfaces before applying the coating in order to improve the coating's adhesion. The roughening of the braking surfaces is advantageously carried out by blasting the brake disc with an abrasive material in the form of a hard granulate, preferably using aluminum corundum granulate with a grain size of approximately 180 mesh.The blasting nozzle used for blasting is preferably moved linearly across the braking surfaces at a distance of approximately 150 to 200 mm from the surfaces to be roughened and at an angle of approximately 10 to 20 degrees, preferably approximately 15 degrees, to the axis of rotation of the brake disc, while the hard abrasive granules are ejected from the spray nozzle by means of compressed air. To prevent undesirable oxidation of the blasted brake disc, the compressed air used as the carrier gas during blasting, which is advantageously at a pressure of 5 to 9 bar, is preferably pre-cleaned to completely remove dust, oil, or moisture.

[0019] The type and grain size of the blasting media and the pressure of the compressed air are appropriately matched so that the braking surfaces have a surface roughness Rz between 20 and 30 µm and a profile bearing area t after blasting. ppossessing 70 to 90%, which ensures optimal adhesion of most thermally sprayed coatings.

[0020] Another measure to improve the adhesion of the coating to the metallic friction element is to appropriately heat the latter before applying the coating, preferably to a temperature between 120 and 150 °C, in order to expand the grain structure.

[0021] The thermal spraying process used for the brake disc according to the invention is high-speed flame spraying, in which a burner system is supplied with a powdered material used for coating in a carrier gas and with a combustible gas mixture, preferably an acetylene-oxygen mixture. The carrier gas with the powdered material it carries and the combustible gas mixture are fed into the burner system in such a way that the hot combustion gases generated when the combustible gas mixture burns, together with the carrier gas, accelerate the powdered material to supersonic speeds and simultaneously cause it to partially melt, so that when the molten material particles impact the roughened braking surface, a mechanical interlock occurs between the material particles and the friction element to be coated.

[0022] According to the invention, the material used for high-speed flame spraying of brake discs comprises an oxide-free ceramic material in the form of tungsten carbide (WC) and is preferably a ceramic-metal composite material consisting of the oxide-free ceramic component and a metal component, the latter expediently being either a cobalt or nickel alloy or pure cobalt (Co) or nickel (Ni). While cobalt (Co) improves the wear resistance of the ceramic-metal composite material, nickel (Ni) improves the thermal conductivity, adhesion to the friction element, and adhesion within the applied ceramic-metal composite material. Overall, the ceramic-metal composite material preferably consists of more than 80% ceramic and less than 20% metal.

[0023] Since coatings applied by thermal spraying have a very high surface roughness, which, due to an insufficient bearing area of ​​the surface, is unsuitable for brake friction elements, the surface roughness must be reduced before the friction elements are used. For this purpose, the coating, applied with a layer thickness of 0.4 to 0.6 mm, is expediently ground down to approximately half this thickness, i.e., to about 0.2 to 0.4 mm. This improves heat dissipation through the coating into the friction element and thus reduces the risk of unwanted cracking or flaking of the coating due to differing coefficients of thermal expansion between the friction element and the coating.

[0024] According to a further preferred embodiment of the invention, the coating is expediently ground with a diamond-containing abrasive, for example diamond grinding paste, until its surface has a surface roughness Rz of less than 2 µm, which can be achieved by using a diamond grinding paste with the finest grain size, for example D-126, in the final grinding step.

[0025] Investigations of the friction surfaces of finished steel brake discs coated by thermal spraying, as previously described, revealed that the brake discs possessed very good thermal shock resistance. After heating in a vacuum furnace to over 1000°C and subsequent quenching in cold water, no cracks or flaking of the coating were observed. Furthermore, these investigations showed that, according to the invention, the friction surfaces exhibited a hardness between 1200 and 1300 HV01 after coating and grinding, which remained essentially unchanged even after heating and quenching during the thermal shock test.

[0026] Furthermore, the grinding of the coating according to the invention increases the profile bearing area t compared to an uncoated metallic brake disc with the same surface roughness. pThe braking surface according to DIN is increased from approximately 50% to almost double that, namely according to the invention to more than 95%, preferably to more than 99% and best to approximately 99.9%.

[0027] If the brake disc is made of steel and is intended for operating conditions where the brake disc is heated to high temperatures, it may be advantageous to provide the brake disc with perforations in the area of ​​the friction surfaces before applying the coating, for example by laser cutting into the brake disc.

[0028] The following are some examples of implementation explained in more detail: Exemplary embodiment 1 (not according to the invention)

[0029] The braking surfaces of a standard steel motorcycle brake disc were first roughened by blasting with aluminum corundum particles with a grain size of 180 mesh to improve the subsequent adhesion of the coating. Surface roughening was carried out by blasting each of the two braking surfaces sequentially with the aluminum corundum particles from an injector nozzle while the rotating brake disc was clamped in place. The particles were ejected from the nozzle using pre-cleaned compressed air, free of oil, dust, and moisture, at a working pressure of 5 to 9 bar. Pre-cleaning the compressed air prevented surface oxidation. During blasting, the nozzle was held at a distance of 150–200 mm and held at an angle of approximately 15 degrees to the brake disc's axis of rotation, moving linearly across the braking surface to be coated.After blasting, the braking surface was measured to determine the surface roughness according to DIN 4777 and DIN, whereby a value of Rz = 20 to 30 µm was found for the surface roughness and t for the profile bearing area. p a value of 70 to 90% was obtained.

[0030] Subsequently, the two braking surfaces of the brake disc were coated sequentially in a thermal coating system using an arc spraying process with Fe 18 Cr 8 Ni 2 Mn. For this purpose, two Fe 18 Cr 8 Ni 2 Mn wires were used, which were energized and joined at their free ends to generate an electric arc that melted the wire ends. The melting temperature was approximately 3500–4000 °C. During arc generation, compressed air was applied to the free wire ends from the side facing away from the brake disc to detach molten particles from the wire ends and accelerate them to approximately 700–800 m / s. These particles, in a liquid or semi-liquid state, were then sprayed onto the braking surface to be coated, resulting in mechanical bonding of the molten particles to the brake disc material.The coating was applied to the brake disc in a layer thickness of 0.6 - 0.7 mm.

[0031] After the brake disc cooled, the raised coating on both braking surfaces was successively ground down on a surface grinding machine using a diamond or corundum-containing abrasive to a layer thickness of 0.3 to 0.4 mm each, in order to achieve a high degree of flatness on the braking surfaces. A D-126 diamond paste was used for the final grinding. Subsequently, the surface roughness Rz was determined using a roughness measuring instrument according to DIN 4777, and the surface profile and profile area tp of the ground braking surface were determined using the stylus method according to DIN 4768. A consistently very low value of Rz < 2 µm was determined for the surface roughness, while a very high value of 99.9% was determined for the profile area tp, which is approximately twice as high as the profile area tp of an uncoated braking surface with the same surface roughness.

[0032] Subsequently, an adhesion test according to DIN EN 582 was carried out, which resulted in an adhesion of the coating of 2 275 N / cm2 or of 3 300 psi.

[0033] The brake disc's thermal shock resistance was then tested. For this test, it was heated in a vacuum oven to 1050 °C, i.e., until red-hot, and subsequently quenched in a container with 10 liters of water at a water temperature of 15 °C. A subsequent inspection revealed no cracks or other surface defects. Exemplary embodiment 2 (not according to the invention)

[0034] The braking surfaces of a standard steel motorcycle brake disc were pretreated as in embodiment 1 and coated using an arc spraying process under the same conditions, but using two Fe13Cr0.5Si wires. After application, the coating was ground down to a layer thickness of 0.3 to 0.4 mm. After the final grinding, the braking surfaces exhibited a surface roughness of Rz < 2 µm and a profile bearing area tp of 99.9%.

[0035] A subsequent adhesion test and a test of the thermal shock resistance of the brake disc, as previously described, led to similar results as with the brake disc from embodiment 1. Exemplary embodiment 3 (not according to the invention)

[0036] The braking surfaces of a standard steel motorcycle brake disc were pretreated as in embodiments 1 and 2 and coated using an arc spraying process under the same conditions, but using two wires made of alloyed carbon steel with a carbon content of approximately 0.35% and other alloying elements in the form of Si, Ni, Mn, P (traces), and sulfur (traces). The applied coating was then ground down to a layer thickness of 0.3 to 0.4 mm to provide the braking surfaces with high flatness and a surface roughness of Rz < 2 µm and a profile bearing area tp of 99.9%.

[0037] A subsequent adhesion test and a test of the thermal shock resistance of the brake disc, as previously described, led to similar results as with the brake disc from embodiments 1 and 2. Exemplary embodiments 4 to 7 (according to the invention)

[0038] In a further series of tests, the braking surfaces of several commercially available steel brake discs were coated with various powdered materials by high-speed flame spraying using the HVOF (High Velocity Oxy Fuel) process, after they had previously been pretreated in the same way as in the preceding embodiments by blasting with an aluminum corundum hard aggregate granulate to ensure a surface roughness of Rz = 20 to 30 µm and a profile bearing area tp of 70 to 90%.

[0039] For coating, the powdered material was fed into a burner system in a carrier gas together with an acetylene-oxygen mixture in order to accelerate the material to supersonic speed by the carrier gas and the hot combustion gases generated when burning the gas mixture, while simultaneously ensuring partial melting of the material itself.

[0040] In all tests, the coating was applied in a layer thickness of 0.3 - 0.4 mm and then ground down to a layer thickness of 0.2 - 0.25 mm to create a flat braking surface with high planarity.

[0041] Here too, a diamond grinding paste with the finest grain size was used in the final grinding step to achieve a uniform surface roughness of Rz between 0.5 and 1 µm across the entire braking surface, as well as a high profile bearing area t. p to achieve 99.9%.

[0042] After grinding the coating, its hardness in the area of ​​the braking surfaces was measured before and after a thermal shock test, in which the brake discs were heated to 1200 °C in a vacuum oven and then quenched in water with a water temperature of 15 °C. Example 4

[0043] One of the pre-treated brake discs was coated with WC Co 10 4 Cr powder, which, after prior heating of the brake disc to 120 to 150 °C, was sprayed onto the pre-treated brake surfaces at a speed of more than 2.5 to 3 Mach and at temperatures of more than 3 000 °C in a layer thickness of 0.3 to 0.4 mm. Example 5

[0044] Another brake disc, intended for use in a motorcycle wheel brake, was also coated with WC Co 12 after heating, with the process parameters and the sprayed layer thickness corresponding to those in the preceding embodiment 4.

[0045] The hardness of the WC Co 12 coating measured before and after the thermal shock test was 1250 HV01, with no noticeable decrease in hardness as a result of the thermal shock test. Example 6

[0046] Another brake disc, intended for use in a passenger car wheel brake, was coated with WC / Ni after heating, with the process parameters and the sprayed layer thickness corresponding to those in the preceding embodiments 4 and 5.

[0047] The hardness of the WC / Ni coating measured before and after the thermal shock test was 1210 HV01, with no noticeable decrease in hardness as a result of the thermal shock test. Example 7

[0048] Another brake disc, intended for use in a passenger car wheel brake, was coated with agglomerated WC / Ni powder instead of WC / Ni, with the process parameters and the sprayed layer thickness corresponding to those in the preceding embodiments 4 to 6.

[0049] The hardness of the WC / Ni coating measured before and after the thermal shock test was 1280 HV01, with no noticeable decrease in hardness as a result of the thermal shock test.

[0050] The coatings applied by high-speed flame spraying exhibited higher thermal shock resistance compared to the coatings applied by arc spraying, as the thermal shock test prior to quenching allowed heating to temperatures of up to 1200 °C without any negative effect on the coating.

[0051] Brake discs with these coatings are therefore expected to have a longer lifespan when used in short-course racing or under similar operating conditions with high temperatures and / or temperature fluctuations.

[0052] However, all of the brake discs described above are well suited for use under normal operating conditions in road traffic.

[0053] Tests with the coated brake discs showed that these brake discs, in combination with commercially available brake pads, provided good grip and excellent braking conditions, that even after prolonged use under harsh conditions no noticeable scoring was observed, and that the heat dissipation through the coating into the brake disc was sufficient to prevent overheating leading to chipping or damage to the coating, even under heavy use of the brake disc.

[0054] Furthermore, even after extended use, virtually no abrasion was observed on the coated brake discs, meaning that practically no brake dust is produced. This is a great advantage, especially considering the growing importance of particulate matter pollution in road traffic, and also leads to a significant reduction in the build-up of hot brake dust on high-gloss chrome-plated surfaces, particularly on motorcycles near the brakes.

Claims

[1] Brake disc, comprising at least one braking surface and a wear-resistant coating applied to the braking surface, wherein the wear-resistant coating is thermally sprayed onto the braking surface and has a ground surface, characterized by , that the wear-resistant coating applied by high-speed flame spraying comprises an oxide-free ceramic material in the form of tungsten carbide (WC), has a hardness between 1200 and 1300 HV01 and a ground surface, and that the ground surface has a surface roughness Rz of less than 2 µm and a profile bearing area tp of more than 95%, preferably more than 99%. [2] Brake disc according to claim 1, characterized by , that the wear-resistant coating has a layer thickness between 0.15 and 0.3 mm. [3] Brake disc according to one of claims 1 or 2, characterized bythat the coating consists of the oxide-free ceramic material and at least one metal.