METHOD FOR MANUFACTURING FRICTION UNITS

Description

[0001] The present invention relates to a method for producing friction units by coating a supporting body made of a C / SiC material.

[0002] DE 44 38 455 C1 describes a method for producing a friction unit with good cooling properties for frictional engagement with a counter-body, in particular for producing a brake or clutch body. A porous carbon body corresponding to the final contour of the friction unit is provided, and the pores of this carbon body are infiltrated with liquid silicon. By initiating a chemical reaction, the body is ceramized to form silicon carbide. Before the silicon infiltration, the porous carbon body is structured in such a way that cavities and / or recesses are formed in defined inner and / or outer areas for cooling and / or stiffening the friction unit. These cavities and / or recesses retain their shape and size after ceramization.

[0003] EP 2 058 546 B1 describes a method for producing friction discs having a supporting body and at least one friction layer. The friction layer is produced from a silicon carbide slurry containing phenolic resins dissolved in a solvent and a mass fraction of carbon fibers or fiber bundles in the solids of the slurry of 0.1% to 10%. Alcoholic solutions (C1 to C4 alcohols) or a mixture thereof with water for diluting the alcoholic solution are preferred solvents. The friction layer produced from the slurry is applied to a supporting body in the form of a cylindrical ring in the CFRP state. In a temperature step, the friction layer produced from the slurry is first hardened. This is followed by a pyrolysis step in which the CFRP is converted into a C / C. This C / C is finally converted to C / SiC in a further temperature treatment using liquid silicon.

[0004] CN 113847365 A describes a carbon-ceramic brake disc with a multi-layer structure designed to compensate for thermal stresses caused by the difference in the thermal expansion coefficient between the uppermost SiSiC friction layer and the supporting body. The supporting body is made of carbon / ceramic, which is coated multiple times with slurries of different compositions. The slurries used consist of silicon particles, resin (binder), silicon carbide particles, and the slurry for the uppermost friction layer also contains chopped carbon fibers. The liquid carrier is not specified. Due to a high resin content of up to 70 wt.%, the coated carbon / ceramic composite material is crosslinked at 160-220°C and then pyrolyzed at temperatures of 800-1000°C.

[0005] CN 111960860 A describes the production of a wear-resistant Si-SiC coating on the surface of a C / C-SiC brake material. However, the coating is applied to a C / C composite material. The slurry used consists of the powdered solids carbon and SiC and a liquid carrier consisting of an aqueous solution with polyvinyl alcohol. Because PVA acts as a binder component, a temperature treatment between 160 and 300°C in an Ar atmosphere is required. The coating on the C / C composite material is then converted into the final SiSiC friction layer using liquid siliconization.

[0006] US 2014 / 0272249 A1 relates to a method for identifying damaged areas in a ceramic matrix composite and repairing the damaged areas by filling the damaged area with a slurry. The slurry comprises a liquid carrier and a restoration coating material. A solvent or water, for example, is listed as a liquid carrier. The restoration coating material consists of at least one binder, a dispersant, a fugitive material, or a combination thereof. Graphite or polymers in general are proposed as the fugitive material. The restoration coating material also consists, for example, of at least one mullite, a rare earth silicate, or a rare earth oxide, as well as other materials.Other examples recommend the use of a binder to increase carbon yield. This binder also binds the slurry components and potentially reacts with the binder components in a high-temperature step. Other examples suggest the addition of additives such as carbon (e.g., powder, nanotubes, or chopped fibers). Furthermore, the possible addition of various preceramic polymers is discussed. In particular, the use of preceramic polymers and fugitive materials is discussed. In a heat treatment, the fugitive material is burned out, and a glass phase is formed through reaction with other components.

[0007] CN 113045324 A describes a process for repairing and recycling a C / SiC brake disc worn at the end of its service life using a slurry. The slurry is made from alcohol, a resin, chopped fibers, and hexamethylenetetramine. This slurry is applied to the brake disc after grinding the worn surface and subjected to a heat treatment. The coated C / C-SiC brake disc is then thermally treated in an N2 atmosphere at 900°C to 1200°C and infiltrated with liquid silicon in a further temperature step between 1420°C and 1600°C to create a C / C-SiC brake disc with a repaired layer.

[0008] CN 106986664 A describes a repair process for a carbon fiber-reinforced silicon carbide composite material. The area to be repaired is first cleaned. The defect is repaired using carbon fiber layers tailored to the defect, which are impregnated with a slurry consisting of the preceramic polymer polycarbosilane and the organic solvent dimethylbenzene. Drying and sintering then follow. Repeated vacuum-assisted reinfiltrations with a mixture of polycarbosilane and xylene, as well as repeated heat treatments, are carried out until the weight gain caused by reinfiltration is below 1%. Consequently, preceramic polymers are used to obtain the densest possible material, and it must be re-infiltrated and thermally treated. Carbon fabric layers, binders, and solvents are also used.

[0009] CN 111455375 A describes another repair method for a carbon fiber-reinforced silicon carbide composite material. The damaged area is filled with a repair mixture consisting of silicon powder, a portion of carbon powder, and a portion of silicon carbide powder in an ethyl alcohol solvent and dried. The filled area is then treated locally with a CO2 laser.

[0010] The invention is based on the object of providing a method which enables a simple, rapid and cost-effective restoration of worn or defective friction layers of ceramic supporting bodies and is based on a slip which is free of solvents, preceramic polymers or binders.

[0011] This object is achieved by a method having the features of claim 1. Preferred developments of the method are specified in the dependent claims.

[0012] The inventive method for producing friction units comprises coating support bodies made of a C / SiC material with a SiSiC friction layer or simply repairing damaged parts of such a friction layer. In this method, the support body to be coated or repaired is first cleaned on the surface to be machined. This cleaning process can be performed by grinding or abrasive blasting to remove residues and weakly bonded support material. A water-based slurry containing SiC and carbon particles is then prepared and applied to the support body or the damaged area of ​​the support body. The slurry can be applied by brushing, spraying, pouring, or dipping.

[0013] This coated support body is then preferably subjected to drying at room temperature of approximately 20°C, which removes the water from the slip. This gentle drying process and the defined composition of the slip prevent cracks in the layer. This simultaneously creates open porosity in the layer formed by the slip. The support body, with its dried, open-pore, and crack-free structure, then undergoes a high-temperature treatment. Cracks are prevented by an adjusted, low carbon content. In particular, the carbon particle size, SiC particle size, and the solid content are taken into account. The coating is applied by adding liquid silicon to the open porosity at temperatures above 1420°C. This process step is continued until a carbon-free friction layer consisting exclusively of SiC and silicon is present.It is essential for the process that the composition of the slurry is selected such that the SiC content (volume fraction) in the friction layer is at least 70 vol.%, with the remaining volume fraction consisting of elemental silicon. The final SiSiC friction layer exhibits not only high wear resistance but also excellent thermal conductivity, which is a particular advantage for applications in the brake sector.

[0014] The finished friction layer is usually finished by abrasive post-processing. Pre-grinding to near-net-shape while dry can reduce the finishing effort.

[0015] This process allows ceramic friction units to be refurbished, even multiple times, resulting in significant cost savings in the maintenance of ceramic friction units, such as ceramic brake discs and ceramic clutch discs. The friction layers of such ceramic discs can be repaired over the entire surface or only in the area of ​​localized defects. The process can also be used to create a friction layer on ceramic carrier bodies that previously lacked discrete friction layers.

[0016] Particular advantages of the process according to the invention are that only one thermal treatment is required for the process, namely a high-temperature treatment in which the liquid silicon is introduced into the open porosity of the friction layer to be produced. During this high-temperature step, the SiSiC friction layer forms, which forms a strong bond with the supporting body due to the SiC formation at the interface, resulting in very good adhesion of the friction layer.

[0017] The water-based slurry is solvent-free, which makes it easier to handle. Crucially, it doesn't require a binder. This has the advantage of eliminating an additional crosslinking / curing step and time-consuming pyrolysis.

[0018] For the new or repaired friction unit, no expensive semi-finished carbon fiber products, such as nonwovens, woven fabrics, or short fibers, are used. Furthermore, the process according to the invention enables the creation of friction layers as thick as 2 mm in a single processing step. It has also been demonstrated that crack-free drying can be achieved even for thick coatings.

[0019] Since the process according to the invention, unlike prior art processes, does not require crosslinking or pyrolysis steps, the manufacturing process for SiSiC friction layers is shortened, resulting in significant energy savings. The friction layers and the corresponding friction units produced using the process according to the invention are cost-effective compared to processes that use carbon semi-finished products and / or preceramic polymers for the production of the friction layers. Of particular note is the friction layer thickness of up to 2 mm that can be produced in a single process step using the process according to the invention. Such layer thicknesses require multiple repetitions of the individual coating processes according to the prior art.

[0020] Before a support body is provided with a friction layer or before its friction layer is reconditioned, i.e. renewed or repaired, the support body should preferably be cleaned by abrasive removal in order to achieve a good bond between the friction layer and the support body.

[0021] In order to achieve the formation of a friction layer in which the SiC is homogeneously distributed, the SiC particles of the slip should be in the size range of 1 µm to 100 µm, preferably in the size range of 3 µm to 50 µm.

[0022] It has also been shown that a slurry in which 50% of the SiC particles have an average particle size of 40 µm to 50 µm, 25% of the SiC particles have an average particle size of 10 µm to 15 µm and 25% of the SiC particles have an average particle size of 3 µm to 5 µm leads to particularly good properties of the friction layer in terms of SiC particle distribution. At the very least, a bimodal particle size distribution, i.e. at least two size fractions, should be present in order to achieve the highest possible SiC content. The use of several size fractions supports a denser particle arrangement and also has a positive effect on drying. Care must, however, be taken to ensure that the particles used are not too fine. SiC particles in the slurry with a size of < 3 µm lead to a cracked layer after drying. An excessively high proportion of the fine SiC fraction has the same effect.

[0023] The carbon used in the slip preferably consists of soot with an average particle size of 10 nm to 25 nm, more preferably with a particle size of about 16 nm. Here, too, the particle size has an influence on the drying behavior and very small particles of, for example, 10 nm can lead to cracks in the layer due to their overall high surface area and the associated faster drying.

[0024] To ensure that the slip components form a homogeneous suspension, a dispersant is added to the slip. The dispersant content can be between 1% and 3% by weight, based on the solids content of the slip, allowing for high solids contents. A dispersant content of 2% by weight is preferred.

[0025] Particularly good properties of the friction layer in terms of processability, drying behavior, and final composition are achieved when the slip has a solids content of 60 wt.% to 70 wt.% and a water content of 40 wt.% to 30 wt.%. Such a slip has a very low viscosity and is therefore also very easy to apply by spraying. Pre-drying the slip also allows for the creation of a pasty repair compound, which is ideal for repairing defects and applying thick layers.

[0026] The solids content in the slurry preferably consists of 85 wt% to 95 wt% SiC particles and 15 wt% to 5 wt% carbon. A solids content of 90 wt% SiC and 10 wt% carbon is particularly suitable.

[0027] The process allows the water-based slip to be applied to the support body by brushing, spraying, dipping or pouring, as the slip has a low viscosity.

[0028] After the substrate has been coated with the slip, it is subjected to drying in order to specifically remove the water from the slip and create open porosity. This type of drying can also be carried out at room temperature to save energy. Drying a slip coating up to 2 mm thick, even at room temperature at around 20 °C, results in a visually crack-free friction layer. However, care should be taken with the selected proportions of SiC and C as well as the particle sizes used; excessively fine particles or an excessively high proportion of these significantly increases the tendency for cracking and thus the tendency for the dried layer to detach from the substrate.

[0029] The dried, open-pored, and crack-free coating is subjected to a high-temperature treatment with liquid silicon being injected into the open porosity. The high-temperature treatment should be carried out under vacuum at temperatures of 1500 °C to 1700 °C with liquid silicon being injected into the open porosity. The high-temperature treatment can be carried out at a maximum temperature T max for a period of 0.5 to 2 hours until a final, carbon-free friction layer with a SiC content of at least 70 vol.% has formed.

[0030] Where the term "water-based slip" is used, this means that only water was used as the liquid carrier and no organic binder was used.

[0031] The invention is explained in more detail below with reference to the drawing. Figur 1a flow chart with the individual process steps for carrying out the process according to the invention and Figuren 2B bis 2C schematically different cross sections of friction units, the friction layers of which can be produced by the method according to the invention, wherein Figur 2A a friction unit with a support body provided with a friction layer on both sides, Figur 2B represents a friction unit with a reconditioned friction surface on one side and Figur 2C shows a friction unit with a repaired friction layer on one side.

[0032] Like the one in Figur 1 As shown in the process flow, a friction unit selected in step 100 is subjected to an analysis of the condition of its friction surface in step 101. This analysis may reveal that the friction lining or the carrier body of the friction unit is irreparably damaged, leading to disposal of the friction unit.

[0033] If the analysis in step 101 concludes that the friction lining is worn or has repairable damage in a local area, the surface of the friction lining is cleaned entirely or locally in step 102. This cleaning can be performed abrasively using corundum blasting or by grinding. This also roughens the surface, which increases the contact area between the slurry and the supporting body and leads to better adhesion. Subsequently, in step 103, the cleaned surface of the remaining friction lining is coated with a previously prepared slurry, which may be adapted to the composition of the remaining friction lining, to a thickness sufficient to restore the worn friction lining to its original thickness. Due to the low viscosity of the slurry, the uneven surface or local defects are comprehensively wetted.

[0034] If it is determined that the coating performed in step 103 does not result in the required thickness of the friction lining, the coating process of step 103 is repeated, possibly several times, via step 104. Between individual coating processes via step 104, intermediate drying can preferably be performed by briefly drying the previously applied slurry to create a stable base for the subsequent coating process. However, it is also possible to apply a coating wet.

[0035] After the coating process of step 103 and, if applicable, step 104 is completed, the applied slip layer is subjected to drying at ambient temperature (approx. 20°C) or forced drying at a temperature of up to 80°C. Since the applied, water-based slip has very good flow properties, a flat or smooth layer results after drying. If the resulting surface of the dried slip layer still has unevenness that does not meet the application-specific specifications, the process proceeds to step 105, in which the slip layer is pre-ground. If the slip layer resulting from step 104 meets the flatness specifications, the process proceeds to step 106, in which the friction layer is subjected to siliconization, without pre-grinding the layer in step 105.For this siliconization step, the coated friction unit is infiltrated with the addition of silicon at temperatures above 1420 °C. The silicon is introduced into the friction layer via at least three carbon wicks, resulting in the complete reaction of the carbon in the friction layer with the silicon, resulting in the formation of SiC. The open porosity of the friction layer is completely filled with elemental silicon. When the friction layer cools, the different expansion coefficients between the supporting body and the friction layer lead to tensile stress cracks in the SiSiC friction layer. These cracks are typical for friction layers on C / SiC materials but do not negatively affect the adhesion of the layer. After siliconization, the friction layer is subjected to a final processing, for example by grinding, in step 107.

[0036] In the Figuren 2A bis 2Cvarious possibilities are shown for which the method according to the invention can be used.

[0037] The Figur 2A shows the cross-section of a friction unit, for example, a brake disc, with a supporting body 1 coated on both sides with a friction lining 2. The supporting body 1 is a C / SiC material, meaning it consists of carbon fibers in a matrix of carbon, SiC, and residual silicon. The respective friction lining 2 is obtained according to Example 1 below. Example 1:

[0038] To produce friction linings for a supporting body, its surface is first inspected for defects. This is followed by an abrasive cleaning step using corundum blasting, which also roughens the surfaces. The slip is then evenly applied to the surface using spraying, dipping, pouring, or a brush and dried. This process is repeated until the desired layer thickness is achieved, depending on the application method. After drying, the coated friction unit can go directly to high-temperature treatment, during which the friction layer is finally formed with the addition of liquid silicon. The liquid silicon is applied directly to the friction layer using carbon wicks. Mechanical processing to the final size can be achieved, for example, by grinding.

[0039] The Figur 2B shows a friction unit in which the friction lining 2 is worn on one side, so that it is less than the friction lining 2 of the Figur 2Ahas a smaller thickness. For this reason, the Figur 2B The right friction lining 2 was reconditioned by recoating the surface of the remaining friction lining 2 with an additional friction lining layer 3 to restore the original thickness of the friction lining 2. The procedure for this is described in Example 2 below. Example 2:

[0040] The friction unit, including the friction linings, are first checked for wear and defects. If the friction linings are worn, the entire surface of the friction linings is cleaned and roughened using abrasive corundum blasting. The slurry is then evenly applied to the surface to be repaired using a spraying process and dried. This process is repeated until the desired layer thickness is achieved. After drying, the newly coated friction unit goes directly to the high-temperature treatment, during which the friction layer is finally formed with the addition of liquid silicon. The liquid silicon is fed directly to the friction layer using carbon wicks. Mechanical processing to the final size can be achieved, for example, by grinding. A slurry with somewhat finer SiC particles (e.g. 25% SiC particle content with 3 µm) and a minimal amount more carbon (e.g. ≥ 10 wt.-% in solids content), since the layer thicknesses are usually significantly less than 1 mm per application.

[0041] The Figur 2C schematically shows a friction unit whose friction lining 2 has two damages 4 on one friction surface. Such damages 4 of a friction unit can be repaired according to the following example 3. Example 3:

[0042] The defects on the friction lining of the friction unit are first identified and analyzed. The friction lining is masked around the defect before abrasive cleaning or covered with a stencil so that only the defect area is machined. Corundum blasting removes loose components from the defect area and roughens it. The slurry is then applied locally, for which the casting process is particularly suitable. After drying, the filled area can be directly subjected to high-temperature treatment, during which the friction layer is finally formed with the addition of liquid silicon. The silicon should be introduced directly into the area to be repaired using a carbon wick. Through mechanical processing, the height of the repaired defect can be adjusted to the height of the friction lining.

[0043] In addition to introducing the silicon melt, the carbon wicks also serve to support the friction unit during the siliconizing process, preventing it from being directly immersed in the melt. Suitable wicks include pyrolyzed beechwood rods or graphite felt. Both have a sufficiently high open porosity to feed the liquid silicon to the friction layer at temperatures above 1420 °C.

[0044] Slips with slightly finer SiC particles are suitable for spraying, dipping, and brushing, as the layer thicknesses are generally significantly less than 1 mm per application. The fine fraction of the smallest SiC fraction can have an average diameter of 3 µm. Three SiC particle fractions are used for this purpose, averaging 44.5 µm (50% share), 12.8 µm (25% share), and 3 µm (25% share), which together represent 90% of the solids content. A carbon content of 10 wt.% and carbon particles with a diameter of 16 nm are advantageous. A solids content of 66 wt.% in the water-based slip is recommended. Using the application methods mentioned, even very thin layers < 100 µm can be produced.

[0045] A slip with slightly larger SiC particles (e.g. 25% SiC particle proportion with 4.5 µm) and slightly less carbon (e.g. ≤ 9 wt.% solids content) than with spraying is suitable for casting. By casting, layers approximately 2 mm thick can be produced in a single pass, which is particularly suitable for repairing local defects. To prevent drying cracks, the carbon content should be 9 wt.% and the average particle size should not be less than 16 nm. The finest SiC particle fraction should have an average particle diameter of at least 4.5 µm. Three SiC particle fractions are used, with an average of 44.5 µm (50% proportion), 12.8 µm (25% proportion) and 4.5 µm (25% proportion), which together represent 91% of the solids content. A solids content of 65 wt.% in the water-based slip is recommended. Drying the slip may also offer advantages for thicker layer application. This allows, for example,Edge breakouts can be modeled without the slurry running off the defective area.

[0046] The solids content can also be used to control the viscosity and the final SiC content. A lower solids content, for example, 60 wt.%, results in a lower viscosity and a higher volume fraction of silicon in the friction layer. In contrast, a higher solids content, for example, 70%, can increase the viscosity of the slurry and the SiC content of the final friction layer. If the carbon content is too high and the particle size is very fine, the slurry cannot be homogenized to higher solids contents.

Claims

1. A process for the production of friction units by coating a support body made of a C / SiC material, wherein the support body is cleaned on its surface to be coated in a first step, a water-based slurry containing particles of SiC and carbon is then applied to the support body, the support body thus coated is subjected to drying in order to remove the water from the slurry and to produce an open porosity, and then the support body with the dried, open-pored and crack-free coating is subjected to a high-temperature treatment with the introduction of liquid silicon into the open porosity, and then the support body with the dried, open-pored and crack-free coating is subjected to a high-temperature treatment with the addition of liquid silicon into the open porosity until a carbon-free friction layer consisting exclusively of SiC and silicon is formed, whereby the composition of the slurry being selected such that the SiC content in the friction layer is at least 70 by vol.-%.

2. Process according to claim 1, characterized in that the supporting body is cleaned by abrasive removal.

3. Process according to claim 1 or 2, characterized in that the SiC particles of the slurry are in the size range from 1 µm to 100 µm.

4. Process according to claim 3, characterized in that the SiC particles are in the size range from 3 µm to 50 µm.

5. Process according to claim 4, characterized in that 50 % of the SiC particles have an average particle size of 40 µm to 50 µm, 25 % of the SiC particles have an average particle size of 10 µm to 15 µm and 25 % of the SiC particles have an average particle size of 3 µm to 5 µm.

6. Process according to one of claims 1 to 5, characterized in that the carbon in the slurry consisting of carbon black has an average particle size of 10 nm to 25 nm.

7. Process according to claim 6, characterized in that the carbon from carbon black contains a particle size of about 16 nm.

8. Process according to one of claims 1 to 7, characterized in that the slurry has a solids content of 60 wt.% to 70 wt.% and a water content of 40 wt.% to 30 wt.%.

9. Process according to one of claims 1 to 8, characterized in that the solids content in the slurry is formed from 85% by weight to 95% by weight of SiC particles and 15% by weight to 5% by weight of carbon.

10. Process according to one of claims 1 to 9, characterized in that the slurry is applied to the supporting body by brushing, spraying, dipping or casting.

11. Process according to one of claims 1 to 10, characterized in that the coated support body is subjected to drying in order to remove the water from the slurry and produce an open porosity.

12. Process according to claim 11, characterized in that the drying takes place at room temperature.

13. Process according to one of claims 1 to 12, characterized in that the dried, open-pored and crack-free coating is subjected to a high-temperature treatment with liquid silicon being fed into the open porosity.

14. Process according to claim 13, characterized in that the high-temperature treatment is carried out at 1500 °C to 1700 °C in a vacuum with liquid silicon being fed into the open porosity.

15. Process according to claim 13 or 14, characterized in that the high-temperature treatment is carried out at a maximum temperature Tmax for a period of 0.5 to 2 hours until a final friction layer with a SiC content of at least 70 vol-% has been formed.