Patent Application
20240044382
This invention relates to a braking band for a brake disc for disc brakes, a brake disc, and a method for manufacturing the braking band and the brake disc.
A brake disc of a disc braking system of a vehicle comprises an annular structure, or braking band, and a central fixing element, known as a bell, by means of which the disc is fixed to the rotating part of a vehicle suspension, for example a hub. The braking band is provided with two opposed braking sides (or braking surfaces) suitable for cooperating with friction elements (brake pads) which are housed in at least one caliper body that is placed astride said braking band and is integral with a non-rotating component of the vehicle suspension. The controlled interaction between the opposing brake pads and the opposing braking surfaces of the braking band creates, by means of friction, a braking action which decelerates or stops the vehicle.
Generally, the brake disc is made of gray cast iron or steel. In fact, this material allows good braking performance (especially in terms of limited wear) to be obtained at a relatively low cost. Discs made of carbon or carbon-ceramic materials offer much higher performance, but at a much higher cost.
The limits of traditional discs made of cast iron or steel are linked to excessive weight, excessive wear and excessive corrosion. As far as gray cast iron discs are concerned, another very negative aspect is linked to excessive surface oxidation, with the consequent formation of rust. This aspect affects both the performance of the brake disc and its appearance, as the rust on the brake disc is aesthetically unacceptable to the user. An attempt was made to address these problems by manufacturing the discs in gray cast iron or steel with a protective coating. The protective coating reduces the wear of the disc, and protects the gray cast iron base from surface oxidation, thus avoiding the formation of a layer of rust. The protective coatings available today and applied to discs, although offering resistance to wear, are however subject to flaking that causes said coatings to detach from the disc itself.
A protective coating of this type is described for example in U.S. Pat. No. 4,715,486, which relates to a low-wear disc brake. The disc, made in particular of cast iron, has a coating made with a particulate material deposited on the disc by means of a high kinetic energy impact technique. Traditional protective coatings tend to detach from discs of this kind made of gray cast iron or steel; microbubbles form inside the coating, which microbubbles may prevent sufficient adhesion between the coating and the disc, thus facilitating the detachment of said coating.
A further example of a cast iron brake disc that has a protective coating is also described in EP3658798 A1. This type of disc requires adhesion layers or surface treatments between the cast iron and the coating in order to make adhesion more effective.
It is clear from the above that discs which are made of gray cast iron or steel and provided with protective coatings are not suitable for the high-performance braking system sector.
In the sector, there is therefore a great need to resolve the disadvantages cited with reference to the prior art. In particular, there is a need to provide discs which are capable of increasing resistance to wear and corrosion but which are at the same time as light as possible.
The need to provide discs provided with coatings which are not subject to flaking or which do not generally detach, which guarantee resistance to wear and corrosion over time, and which have as contained a weight as possible, is satisfied by a method for manufacturing a braking band and a brake disc, by a braking band and by a brake disc for a disc brake according to the accompanying independent claims.
Further features and advantages of this invention will become more apparent from the following detailed description of preferred, non-limiting embodiments thereof, in which:
Elements or parts of elements common to the embodiments described hereinafter will be indicated with the same reference signs.
With reference to the above figures, reference sign 1 globally denotes a brake disc according to this invention.
According to a general embodiment of the invention, shown in the accompanying figures, the brake disc 1 comprises a braking band 2 which is provided with two opposed braking surfaces 2a and 2b, each of which defines, at least in part, one of the two main sides (or faces) of the brake disc.
The braking band 2 consists of a base band 20 having an upper face 20a and a lower face 20b arranged on the opposite side to, i.e. opposed to, the upper face 20a. The lower face 20b and upper face 20a each define, at least in part, one of the two sides of the brake disc 1.
The base band 20 is mainly or entirely made of titanium or titanium alloy, preferably a titanium (Ti) and aluminum (Al) alloy, even more preferably a titanium (Ti), aluminum (Al) and vanadium (V) alloy, preferably a Ti6Al4V alloy.
According to an embodiment, the base band 20 is mainly or entirely made of Ti6242 (Ti-6Al-2Sn-4Zr-2Mo).
In the continuation of the present discussion, it is understood that references to titanium alloy are also intended to encompass the titanium alloys mentioned above.
It is clear that titanium and aluminum alloy is therefore intended to mean all of the possible alloys that are suitable for being used in drawing processes and subsequent mechanical processing or in casting processes and subsequent mechanical processing.
According to an embodiment, the whole brake disc 1 is made of titanium or titanium alloy.
The braking band 2 is provided with an upper coating layer 3 which is joined to the base band 20 along the upper face 20a, i.e. preferably directly joined to the base band 20.
The braking band 2 is provided with a lower coating layer 3′ which is joined to the base band 20 along the lower face 20b, i.e. preferably directly joined to the base band 20.
The upper coating layer 3 and the lower coating layer 3′ consist of a mixture of ceramic and metal and/or intermetallic particles.
According to one embodiment, the variant in which the upper coating layer 3 and/or the lower coating layer 3′ are directly joined to the base band 20, the mixture of ceramic and metal particles consists of chromium carbide (CrC) and nickel-chromium (NiCr), or a nickel-chromium superalloy.
According to one embodiment, the variant in which the upper coating layer 3 and/or the lower coating layer 3′ are directly joined to the base band 20, the mixture or ceramic and metal particles consists of chromium carbide and iron (Fe), chromium (Cr) and aluminum (Al).
In other alternative embodiments, the chromium carbide in all of the preceding variants may be substituted with a carbide selected from the group including: tungsten carbide (WC), silicon carbide (SiC), niobium carbide (NbC) and titanium carbide (TiC).
In other alternative embodiments, together with the nickel-chromium (NiCr), or together with the mixture of iron (Fe), chromium (Cr) and aluminum (Al), a mixture of one or more of the following carbides may be used: chromium carbide (CrC), tungsten carbide (WC), silicon carbide (SiC), niobium carbide (NbC) and titanium carbide (TiC).
The synergistic combination between the base band made of titanium or titanium alloy and the upper or lower coating layer 3, 3′ provided with ceramic and metal particles surprisingly makes it possible to ensure an extremely high corrosion potential of greater than −400 mV with respect to the SCE (saturated calomel electrode) (the greater the corrosion potential, the less the manufactured product corrodes). Consequently, the synergistic combination between the base band made of titanium or titanium alloy and the upper or lower coating layer 3, 3′ makes it possible to ensure a greater resistance to corrosion than the solutions which are known today and commonly used to manufacture brake discs.
According to an advantageous embodiment, the upper coating layer 3 and the lower coating layer 3′ consist of a mixture of particles comprising at least one transition metal carbide and at least one metal or an intermetallic compound (alloy).
More particularly, according to an even more advantageous embodiment, the upper coating layer 3 and the lower coating layer 3′ consist of tungsten carbide (WC), iron (Fe), chromium (Cr) and aluminum (Al) and are obtained by depositing tungsten carbide (WC), iron (Fe), chromium (Cr) and aluminum (Al) in particulate form directly on the base band 20 by means of a suitable depositing technique. The depositing technique is preferably a spraying technique, for example a HVOF (high velocity oxygen fuel) or HVAF (high velocity air fuel) or KM (kinetic metallization) technique. Moreover, the depositing technique is preferably a plasma deposition technique, for example APS (atmospheric plasma deposition), or a laser deposition technique, for example a laser cladding technique.
According to a further aspect of the invention, as shown schematically in
The protective base coating 30 consists of a material in particulate form consisting of:
The protective base coating 30 is therefore attached directly to the titanium or to the titanium alloy which forms the base band 20 of the braking band 2.
The protective base coating 30 preferably consists of:
The upper coating layer 3 or lower coating layer 3′ preferably consists of 75% to 87% by weight of tungsten carbide (WC) and the remainder iron (Fe), chromium (Cr) and aluminum (Al). The upper coating layer 3 or lower coating layer 3′ even more preferably consists of 75% to 87% by weight of tungsten carbide (WC) and the remaining portion iron (Fe), chromium (Cr) and aluminum (Al).
Advantageously, the protective base coating 30 has a thickness between 10 μm and 250 μm, preferably between 15 and 150 μm and even more preferably between 20 μm and 90 μm, while the upper coating layer 3 or lower coating layer 3′ has a thickness between 10 μm and 250 μm, preferably between 15 μm and 150 μm, even more preferably between 20 and 90 μm.
It was possible to verify that, surprisingly, the presence of titanium and a coating layer formed of ceramics and metals, i.e. of particles of ceramic compounds dispersed in a metal and/or intermetallic matrix, makes it possible to significantly reduce the corrodibility the brake disc, while simultaneously preventing the occurrence of phenomena of the coatings flaking or generally detaching.
The thickness of the protective base coating 30 and of the upper coating layer 3 and lower coating layer 3′ is defined so as to prevent thicknesses that are too thin, which could be abraded during braking (or produce less of an anti-corrosive effect), and thicknesses that are too thick, which could cause detachment problems. With high thicknesses, the detachment may be caused by the different behavior of titanium with respect to the material of the coatings: a thick rigid layer might not imitate the deformations of the disc and thus cause the protective base or upper and lower coating layers to fracture and detach. In other words, the thickness of the protective base coating 30 or of the upper coating layer 3 and lower coating layer 3′ may create more or less significant mechanical stresses on the interface between the coating and underlying material, which risks causing the detachment of the coating itself.
Moreover, it has surprisingly been experimentally verified that the combination between the base band 20 made of titanium alloy and the coating layer preferably formed of tungsten carbide and iron, chromium and aluminum ensures a corrosion potential of greater than −370 mV vs SCE.
It has also been experimentally verified that a braking band of this kind ensures high mechanical strength and a lower brittleness than discs made of cast iron/steel.
Moreover, the anticorrosive action is in any case further enhanced by the presence of the base coating layer 30. By virtue of the composition of this protective base coating 30 (Cr3C2 and NiCr, or NiCr, Fe, Mo, Co, Mn and Al) and the deposition method, this coating 30 also performs an anticorrosive action on the braking surface of the brake disc.
The anticorrosive action is also beneficial to the integrity of the upper and lower coating layer 3, 3′ and to the adhesion thereof to the brake disc, although it is not indispensable.
In the embodiment in which the protective base coating 30 is formed of Cr3C2 and NiCr, or NiCr, Fe, Mo, Co, Mn and Al, this base coating 30 also performs a mechanical function of matching the thermal expansions of the titanium, thus reducing the risk of localized weakening.
For simplicity of discussion, the braking band 2 and the brake disc 1 will now be described in conjunction with the method according to this invention.
The brake disc 1 comprises a braking band 2 as described herein and a bell 5 connected to said braking band 2.
According to one embodiment, the bell 5 is connected in one piece to the braking band 2 and consists of a titanium or titanium alloy co-casting with the base band 20.
According to one embodiment, the bell 5 is connected in one piece to the braking band 2 and consists of an aluminum alloy co-casting with the base band 20 made of titanium or titanium alloy.
It is clear that the braking band 2 according to this invention may also be connected to a bell 5 which is not co-cast (or made in one piece), but may be connected by means of bell-band connection means according to known techniques (assembling, interference fitting, riveting and the like).
The braking band 2 is preferably, but not necessarily, manufactured using the method according to the invention which will now be described.
According to a general embodiment of the method according to the invention, the method comprises the following operating steps:
According to one embodiment, in order to manufacture the braking band 2 of step a), therefore before step b), the following steps are optionally provided:
In the variant embodiment of the brake disc 1, manufactured by co-casting a bell made of aluminum, the method comprises the steps of:
It is clear that, even in this variant embodiment, the braking band 2 consists of a base band 20 made of titanium that is obtained by blanking or by casting or by machining or by forging.
According to a variant embodiment of the brake disc 1, manufactured by co-casting a bell in aluminum, the above steps s1), s2) and s3) are therefore carried out before step b) of creating the upper coating layer 3 and/or lower coating layer 3′, so that the braking band 2 which has not yet been coated is positioned in the co-casting mold.
According to another variant embodiment of the brake disc 1, manufactured by co-casting a bell in aluminum, the above steps s1), s2) and s3) are carried out after step b) of creating the upper coating layer 3 and/or lower coating layer 3′, so that the braking band 2 which has already been coated is positioned in the co-casting mold. In this variant, therefore, step s1) includes positioning the braking band 2 obtained after step a) and step b) have been carried out in a mold for aluminum casting. Steps s2) and s3) are then carried out after step s1). The processing carried out in step b) preferably comprises the step of directly depositing, above at least the upper face 20a and/or the lower face 20b, a material in particulate form consisting of tungsten carbide (WC), iron (Fe), chromium (Cr) and aluminum (Al), by a deposition technique, preferably a spray deposition technique, for example HVOF (high velocity oxygen fuel), or by a HVAF (high velocity air fuel) technique or by a KM (kinetic metallization) technique, thus forming an upper coating layer 3 and/or lower coating layer 3′ suitable for withstanding the braking action of the calipers of a disc brake, i.e. forming at least one of the two braking surfaces 2a, 2b of the braking band 2.
According to a second aspect of the method according to the invention, before directly depositing a material in particulate form consisting of ceramic and metal particles above at least the upper face 20a and/or lower face 20b, step b) may include a material in particulate form consisting of:
As already mentioned above, the titanium alloy of the base band 20 is preferably a titanium (Ti) and aluminum (Al) alloy, more preferably a titanium (Ti), aluminum (Al) and vanadium (V) alloy, even more preferably a Ti6Al4V alloy.
The material in particulate form deposited in the depositing step b) to obtain the protective base coating 30 preferably consists of 65% to 95% by weight of chromium carbide (Cr3C2) and the remainder nickel-chromium (NiCr).
The material in particulate form deposited in the depositing step b) to obtain the upper or lower coating layer (3, 3′) preferably consists of 75% to 87% by weight of tungsten carbide (WC) and the remainder iron (Fe), chromium (Cr) and aluminum (Al), preferably consisting of 10% to 17% by weight of iron (Fe), 2.5% to 5.8% by weight of chromium (Cr), 0.6% to 2.2% by weight of aluminum (Al) and the remainder tungsten carbide (WC), even more preferably of 85% by weight of tungsten carbide (WC) and 15% by weight of iron (Fe), chromium (Cr) and aluminum (Al).
Advantageously, the brake disc 1 is provided with a portion suitable for fixing the disc to a vehicle, which portion consists of an annular portion 4 arranged centrally to the disc 1 and concentrically to the braking band 2. The fixing portion 4 supports the connecting element 5 to the wheel hub (i.e. the bell). The bell may be made in one piece with the annular fixing portion 4 or may be made separately and then fixed to the fixing portion by means of suitable connecting elements.
The annular fixing portion 4 may be made of the same material as the braking band 2, i.e. titanium or titanium alloy, or steel or aluminum. The bell 5 may also be made of titanium or titanium alloy or of another suitable material, for example steel or aluminum, this latter ensuring it is possible to keep the weight of the disc low. In particular, the whole disc (i.e. braking band, fixing portion and bell) may be made of titanium or titanium alloy.
The braking band 2 is preferably made by casting. Similarly, the fixing portion 4 and/or the bell 5 may be made by casting.
The annular fixing portion 4 may be made in one piece with the braking band 2 or may be made as a separate piece and mechanically connected to the braking band.
As already stated, both the material which forms the protective base coating 30 (chromium carbide (Cr3C2) dispersed in nickel-chromium (NiCr), or nickel-chromium (NiCr), iron (Fe), molybdenum (Mo), cobalt (Co), manganese (Mn) and aluminum (Al)), and the material which forms the upper or lower coating layer 3, 3′ (for example tungsten carbide dispersed in iron, chromium and aluminum intermetallic) are deposited in particulate form, preferably using an HVOF technique or HVAF technique or KM technique or APS technique or laser cladding technique.
These three deposition techniques are well known to a person skilled in the art, and will therefore not be described in detail in the following.
HVOF (high velocity oxygen fuel) is a powder spray deposition technique which uses a spray device provided with a mixing and combustion chamber and a spray nozzle. The chamber is supplied with oxygen and fuel. The hot combustion gas, which forms at pressures close to 1 MPa, passes the powdered material through the convergent-divergent nozzle, reaching hypersonic speeds (i.e. higher than MACH 5). The powder material to be deposited is injected into the hot gas stream, where it melts rapidly and is accelerated to speeds of the order of 1000 m/s. Once impacted on the deposition surface, the molten material cools rapidly and, due to the impact with high kinetic energy, forms a very dense and compact structure.
The HVAF (high velocity air fuel) deposition technique is similar to the HVOF technique. The difference is that, in the HVAF technique, the combustion chamber is fed with air instead of oxygen. The temperatures involved are therefore lower than those of HVOF. This allows for greater control of the thermal alteration of the coating.
The KM (kinetic metallization) deposition technique is a solid-state deposition process in which metal powders are sprayed through a two-phase sonic deposition nozzle which accelerates and electrostatically charges metal particles within an inert gas stream. Thermal energy is supplied to the transport stream. The process transforms the internal energy of the compressed inert gas and the thermal energy supplied into the kinetic energy of the powders. Once accelerated to high speed and electrically charged, the particles are directed against the deposition surface. The high-speed collision of the metal particles with this surface causes a large deformation of the particles (approximately 80% in the direction normal to the impact). This deformation results in a huge increase in the surface area of the particles. Upon impact, the effect is therefore intimate contact between the particles and the deposition surface, which leads to the formation of metal bonds and a coating having a very dense and compact structure.
Advantageously, as an alternative to the three deposition techniques listed above which are all high kinetic energy impact deposition techniques, it is also possible to use other techniques which exploit different deposition methods but which are able to generate coatings having a very dense and compact structure.
The combination of the HVOF or HVAF or KM deposition technique and the chemical components used to form the coatings 3 and base coating 30 allows coatings to be obtained which have a high bond strength to the lower material on which they are deposited.
The uniqueness of the APS (atmospheric plasma deposition) technique resides in its ability to develop complex films at relatively low substrate temperatures. These processes make it possible to widely vary the properties of the film by controlling the conditions of the plasma, for example its electron density, energy and distribution function. In particular, the APS technique makes it possible to obtain extremely high deposition speeds that comply with continuous flow processes, but without requiring any vacuum system, thus allowing a production process that is cheap and relatively efficient to implement on pre-existing production lines. Moreover, the APS technique is highly versatile in terms of the plasma sources which may be used to clean the surface before deposition, and has a surprising capability for developing modelled or nanostructured surfaces.
The additive laser welding technique, also known as laser cladding, may also be carried out using powders. The laser beam used locally heats the surface of the substrate, generating a weld pool thereon, to which the powdered coating material is simultaneously conducted in order to be welded by the laser. The time of action is short and, in any case, only causes minimal deformations; the cooling process takes place fast. As a result, a coating is obtained that is joined to the base material by means of a metallurgical procedure. This coating is stronger than those which are produced by thermal spraying techniques, and thus has a lower environmental impact.
The laser cladding technique therefore has the advantage of joining the material of the coating to the material of the base by means of bonds that are stronger than those produced by thermal spraying, and at the same time with optimal surface quality and few deformations, while also ensuring better energy efficiency.
As shown in the accompanying figures, the disc 1 is preferably provided with an upper and lower coating layer 3, 3′, which layers cover both of the faces 20a, 20b and thus create both of the braking surfaces 2a and 2b of the braking band 2.
It is clear that, in one variant, just the upper coating layer 3 or just the lower coating layer 3′ may be provided, so as to cover the braking band on only one braking surface.
According to embodiments not shown in the appended figures, the upper coating layer 3 or lower coating layer 3′ may also extend to other parts of the disc 1 such as the annular fixing portion 4 and the bell 5, up to covering the entire surface of the disc 1. In particular, the upper coating layer 3 or lower coating layer 3′, as well as the protective base coating 30, may cover—in addition to the braking band—only the fixing portion or only the bell.
According to a particularly preferred embodiment of the method, the depositing step b) for forming the protective base coating 30 comprises two or more distinct stages of depositing the material in particulate form on the same surface to form the protective coating.
In greater detail, the aforesaid depositing step b) comprises:
As will be explained in the following, the second finishing layer makes it possible to set the surface finish of the protective base coating 3.
Dividing the depositing step b) into two or more stages makes it possible in particular to differentiate at least the granulometry of the material in particulate form used in the different stages. This makes the depositing step b) more flexible.
According to a particularly preferred embodiment of the method, the step b) of depositing the particulate material (WC+Fe+Cr+Al) which forms the upper coating layer 3 or lower coating layer 3′ comprises two or more distinct stages of depositing the particulate material on the same surface in order to form the protective coating, in a manner similar to that described for the protective base coating 30.
As may be appreciated from what has been described, the braking band, the brake disc and the method for manufacturing the braking band and the brake disc according to the invention make it possible to overcome the disadvantages of the prior art.
In fact, the braking bands and the coated brake discs manufactured according to the invention are more resistant to wear and corrosion than the cast iron/steel discs of the prior art.
Moreover, in a particularly innovative manner, the braking band and the disc manufactured according to this invention do not require particular treatments to guarantee effective adhesion of the coating layers on the base band made of titanium, by contrast with the brake discs made of cast iron/steel.
In other words, there is no need for intermediate layers or interlayers, mechanical activations or other pre-treatments, with the exception of possible cleaning and grease removal procedures, before the step of depositing the mixture of ceramic and metal particles. This makes it possible to obtain a braking band and a coated brake disc that are easy to manufacture and reliable.
Moreover, synergistically, at the same time a particularly light disc is obtained which has a high mechanical strength and is suitable for withstanding mechanical stresses even at high temperatures, and which also has a lower tendency to accumulate mechanical and thermal deformations by comparison with cast iron discs of the prior art, again as a result of the low coefficient of thermal expansion.
As a result of the synergistic combination of titanium alloy and a coating preferably made of tungsten carbide, iron, chromium and aluminum, a brake disc and a braking band are obtained which have a limited thermal expansion, which makes the manufactured product more stable in temperature and guarantees that the coating will not detach and, consequently, guarantees an improved resistance to corrosion that, as a result of the combined effect of the titanium alloy and the coating layer, achieves a strength comparable to that of stainless steel.
Furthermore, a brake disc that is made of coated titanium ensures that the thermal limit for the use of titanium for structural applications (typically 550/580° C.), which limit is caused by the phenomena of oxidation and heat which affect titanium and its alloys, is exceeded. The coating layer ensures that the base material (titanium) is protected from oxidation phenomena which would otherwise make it impossible to create a disc that is able to guarantee the lightness specifications that are required of high-performance discs, for example brake discs for sports cars. In other words, without a coating layer, it would be necessary to design discs which are heavy and have relatively large dimensions and a high thermal inertia, i.e. which do not have any advantage over standard materials already in use for manufacturing discs.
It has also been possible to verify that the coated brake disc according to the invention has better performance in terms of resistance to environmental conditions (thermal shocks and saline attacks).
The brake disc 1 is also generally relatively cheap to manufacture, as a result of simplifying the process for manufacturing the coating layers.
Moreover, by contrast with the sintering techniques of the prior art, in which the titanium and ceramic particles are simultaneously sintered together in a mix in one single stage, all being subjected to pressures between 800° C. and 1200° C., the manufacturing method according to this invention makes it possible to deposit the upper coating layer (3) and/or lower coating layer (3′) at much lower temperatures (at substantially “cold” temperatures). As a result, the microstructure and the mechanical properties of the base band (20) are better preserved and in particular are disassociated from the mechanical properties of the ceramic particles (and therefore of the coating layers). Therefore, as a result of the present invention, the rigidity and breaking load of the base band (20) are optimized independently from the hardness, wear and corrosion resistance properties of the coating layers.
Moreover, depositing the coating layer directly on the base band made of titanium, without intermediate layers, advantageously makes the deposition process more efficient. It should be noted that titanium has a reduced coefficient of thermal expansion by comparison with the typical coefficient of thermal expansion of cast iron. This allows more effective matching of the thermal expansions and thus makes it possible to not have to use an intermediate layer between the base band and the coating layer for bonding.
A person skilled in the art, in order to satisfy contingent and specific needs, may make numerous modifications and variations to the invention described above, which modifications and variations are all contained within the scope of the invention as defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102020000031880 | Dec 2020 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/062155 | 12/22/2021 | WO |
