Brake disc with protective layer and making method

Abstract

A brake disc (1) and a method for making it are described. The brake disc (1) comprising a braking band (2) extending around an axis of rotation (X-X) and comprising two side braking surfaces (4) adapted to cooperate with a related brake caliper to exert a braking action on the vehicle on which said brake disc (1) is installed; the surfaces comprise at least one anti-wear layer and at least one intermediate layer (6) arranged between said at least one anti-wear layer (5) and said side braking surfaces (4); hard material particles are embedded in said layers.

Description

[0001] “Brake disc with protective layer and making method”

[0002] Field of the invention

[0003] The present invention relates to a brake disc for disc braking systems and a method for making it. In particular, the invention relates to brake discs for vehicles equipped with a braking band whose braking surfaces, that is, adapted to come into contact with a brake caliper in a braking system of a motor vehicle, are coated with a double protective layer and the respective making method.

[0004] Known art

[0005] The following documents describe some examples of braking systems equipped with protective coatings applied to brake discs: US4715486 and WO2014 / 097187.

[0006] Summary of the invention

[0007] As it is well known, the so-called brake discs are used in braking systems of a vehicle. In particular, brake discs which comprise a braking band equipped with braking surfaces adapted for cooperating with friction elements (brake pads), which are housed in at least one caliper body straddling such braking band and integral with a non-rotating component of the vehicle suspension, are known.

[0008] Generally, the brake disc is made of grey cast iron or steel. Indeed, this material allows to achieve good braking performances at relatively low cost. Furthermore, brake discs made of carbon or carbon-ceramic materials which offer significantly higher performance, but at much higher costs, are known.

[0009] The limitations of traditional cast iron or steel brake discs are related to wear and corrosion. Indeed, regarding grey cast iron discs, a drawback is related to excessive surface oxidation, resulting in rust formation. This aspect affects both the performance of the brake disc and its appearance, as rust on the brake disc is aesthetically unacceptable to the user.

[0010] Regarding wear, grey cast iron or steel discs with a layer of protective coating deposited on the braking surface of the disc are known.

[0011] Protective layers, i.e. anti -wear layers, are known to use hard material particlesembedded in a surface layer that is deposited on the braking surfaces of the disc. Hard materials are, for example but not only, metal carbides, oxides, nitrides, borides, silicides or mixtures thereof.

[0012] The presence of hard particles in an anti-wear metal layer is considered to be an element that leads to a decrease in the corrosion resistance of the cast iron part of the disc. For this reason, providing an intermediate layer free of hard particles between the anti-wear layer provided with hard particles and the main element so as to serve, in particular, as a bonding agent and anti-corrosion layer between the anti-wear layer and the main element is known.

[0013] Therefore, the Applicant was faced with the problem of making a brake disc equipped with at least one anti-wear layer with improved corrosion and wear resistance.

[0014] Therefore, in a first aspect, the invention relates to a brake disc comprising a braking band extending around an axis of rotation of said disc; said braking band having two side braking surfaces;

[0015] at least one anti-wear layer containing hard material particles, which is applied to said side surfaces; and

[0016] at least one intermediate layer arranged between said at least one anti-wear layer and said side braking surfaces;

[0017] characterized in that said at least one intermediate layer consists of at least one layer of a metal material, preferably a ferrous alloy, embedding at least one type of hard material particles;

[0018] wherein the percentage by volume of hard material particles in said intermediate layer is less than the percentage by volume of hard material particles in said anti-wear layer.

[0019] The present invention, in one or more preferred aspects, may comprise one or more of the following features.

[0020] In a preferred implementation, the hard material particles are selected from metal carbides, oxides, nitrides, borides, silicides and mixtures thereof.

[0021] Advantageously, the anti -wear layer has a thickness greater than or equal to 30pm and less than or equal to 1500 pm.

[0022] In turn, the intermediate layer 6 preferably has a thickness greater than or equal to 20 pm, preferably greater than or equal to 30pm, and less than or equal to 1000 pm, more preferably a thickness greater than or equal to 60 pm and less than or equal to 300pm.

[0023] The term "thickness" of the intermediate layer or outer anti-wear layer means the average thickness of said layer.

[0024] In an implementation, the anti-wear layer has a thickness D greater than or equal to the thickness d of the intermediate layer, preferably has a thickness D greater than or equal to 1.2 times the thickness d of the intermediate layer. The advantage of this configuration is that the outer anti-wear layer remains anchored to the surface of the brake body even under high stress conditions, in use.

[0025] In an implementation, the anti-wear layer has a thickness D less than the thickness d of the intermediate layer.

[0026] Conveniently, the outer anti-wear layer has a thickness D less than or equal to 0.8 times the thickness d of the intermediate layer. In an implementation, the thickness d of the intermediate layer is 0.6 times the thickness D of the anti-wear layer; ratio values less than 0.6 such as 0.5 are also possible.

[0027] In this implementation, there is the advantage that since the intermediate layer contains hard material particles, said intermediate layer can take over from the outer anti-wear layer and work as a replacement anti-wear layer in case the outer anti-wear layer is worn out or, for other reasons, is not present. In particular, since the hard material particles in the intermediate layer are present in smaller quantities (by volume) than in the outer anti-wear layer, the thickness of the intermediate layer is advantageously greater than that of the outer layer, to compensate for the lower antiwear effect resulting from the smaller quantity of hard material particles.

[0028] In an implementation, the hard material particles are embedded in the anti-wear layer in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume and the hard material particles embedded in the intermediate layer are in an amount greater than or equal to 0.3% by volume and less than or equal to 10%by volume.

[0029] Preferably, the average particle size in the at least one intermediate layer is smaller than the average particle size of the hard material particles in the anti-wear layer. Advantageously, the average size of the particles embedded in the intermediate layer is equal to or less than 30 pm, more preferably in the range between 1 pm and 20 pm.

[0030] In a preferred implementation, the metal alloy of the intermediate layer and the anti-wear layer is the same or the same type. Advantageously, the metal alloy is a ferritic steel, where the term ferritic steel means a steel with a predominantly ferritic matrix. In particular, the presence of other phases that may develop during a laser deposition process of the layers cannot be excluded; for example, phases other than ferritic may form at the interface between the intermediate layer and the surface of the cast iron body of the disc as a result of the laser deposition technique. In a preferred implementation, ferritic steel comprises Cr in a percentage between 11% and 19% by weight, preferably with a percentage greater than or equal to 15%. A preferred ferritic steel has a Nickel content of less than 5% by weight, preferably less than 0.5% by weight, and more preferably less than 0.1% by weight, even more preferably, the Nickel content is not detectable by standard analytical tests.

[0031] Thus, the Applicant has achieved a new implementation of brake disc that offers the advantages of excellent adhesion of the anti-wear layer to the intermediate layer and of the intermediate layer to the braking faces of the disc body, the presence of an intermediate layer that has itself also anti-wear properties, as well as corrosion resistance of the friction braking element.

[0032] Indeed, the Applicant surprisingly found that, in the implementations of the present invention, the presence of hard material particles in the intermediate layer does not result in a source of corrosion of the brake disc, in particular, no cracking in said intermediate layer was observed.

[0033] Furthermore, in the implementations where ferritic steel is used, the environmental and human impact of abrasion during use of the anti-wear layer is reduced.Indeed, the presence of hard material particles included in the substantially or totally nickel-free steel allows to impart adequate mechanical strength and wear resistance, while eliminating the toxicity of the micro-particulate dispersed by abrasion during braking action.

[0034] For the purposes of the present invention, the following definitions apply: "Ferritic steel" means a stainless steel that contains a large proportion of Chromium (typically ranging between 11% and 19 %), but without the addition of Nickel and other elements capable of forming austenite. In particular, this refers to stainless steel that should it contain a percentage of Nickel, that percentage would be negligible, by way of example less than or equal to 5% by weight, preferably less than 0.5% by weight, more preferably less than 0.1% by weight.

[0035] Therefore, according to another aspect, the invention relates to a method for making a brake disc of the aforementioned type, comprising the following operational steps:

[0036] a) preparing a brake disc, comprising a braking band equipped with two side braking surfaces, said braking band being made of grey cast iron or steel;

[0037] b) after step a), depositing, on at least one of the two side braking surfaces, at least one intermediate layer of a metal alloy containing hard material particles which are embedded in said intermediate layer, said metal alloy being preferably an ironbased alloy constituted by ferritic steel;

[0038] c) after step b), depositing an anti-wear layer of a metal alloy containing hard material particles which are embedded in said anti-wear layer, said metal alloy being preferably an iron-based alloy;

[0039] preferably the amount of hard material particles embedded in said anti-wear layer being greater than the amount of hard material particles embedded in said at least one intermediate layer;

[0040] said depositing step being implemented by laser deposition technique which leads to melting said metal alloy and embedding said particles of hard material in said metal alloy.

[0041] In an implementation, the anti-wear layer is deposited so that its thickness onthe finished brake disc is greater than the thickness of the intermediate layer.

[0042] Conveniently, the anti-wear layer is deposited so that its thickness D on the finished brake disc is greater than or equal to 1.2 times the thickness d of the intermediate layer. Typically, D / d is between 1.0 and 1.4, preferably between 1.2 and 1.4.

[0043] In an implementation, the anti-wear layer is deposited so that its thickness on the finished brake disc is less than the thickness of the intermediate layer.

[0044] Conveniently, the anti-wear layer is deposited so that its thickness D on the finished brake disc is less than or equal to 0.8 times the thickness d of the intermediate layer; typically, the value of D / d is between 0.4 and 0.8, preferably 0.6 and 0.8.

[0045] In a preferred implementation, during step b) the hard material particles are embedded in an anti-wear layer in an amount greater than or equal to 0.3% by volume and less than or equal to 10% by volume.

[0046] Advantageously, during step c) the particles of hard material are embedded in an anti -wear layer in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume.

[0047] Advantageously, during step b), preferably also during step c), the laser deposition technique is selected from LMD (Laser Metal Deposition) technique, or HSLMD (High Speed Laser Metal Deposition) technique, or HSLC - high speed laser cladding technique, or EHLA - Extreme High Speed Laser Application technique, or Top Speed Cladding technique.

[0048] In an implementation, at least part of the hard material particles are iron carbides and / or chromium carbides as obtained or obtainable in the laser deposition step of the intermediate layer in step b). In other words, in an implementation, the required hard material particles are produced by means of the laser deposition technique; in a version of this implementation, preferably no more hard material particles are added to the intermediate layer.

[0049] Further characteristics and advantages of the invention will become clearer from the detailed description of some preferred, but not exclusive, embodiments of a brake disc equipped with a protective layer and a method form making it according tothe present invention.

[0050] Brief description of the drawings

[0051] This description will be set forth hereinafter with reference to the attached drawings provided for illustration purposes only and without limitation, in which:

[0052] - figure 1 shows a schematic, sectional view of a first embodiment of the brake disc with a protective layer according to the invention;

[0053] - figure 2 is an enlarged view of a portion, in particular of the protective layer, of the brake disc in figure 1;

[0054] - figure 3 is a schematic view of one step of the method for making the brake disc according to the present invention.

[0055] Detailed description of embodiments of the invention

[0056] With reference to the attached figures, a brake disc according to the present invention is generally denoted by 1.

[0057] The brake disc 1 comprises a braking band 2 extending around a central axis of rotation denoted by X-X.

[0058] The braking band 2 is equipped with two side braking surfaces denoted by 4, which are adapted to be engaged by a brake caliper (not shown) of a disc brake system so as to exert a braking action to the vehicle on which said brake disc 1 is installed, if necessary.

[0059] The braking band 2 is made of grey cast iron or steel.

[0060] Preferably, the braking band 2 is made of grey cast iron. In particular, almost the entire disc, except for the protective layer, is made of grey cast iron. Thus, in the description that follows, reference will be made to a disc made of grey cast iron, anyway without ruling out the possibility that it may be made of steel.

[0061] The disc 1 is equipped with at least one anti-wear layer 5 covering at least one, preferably both, the braking surfaces 4 of the braking band 2 and at least one intermediate layer 6 arranged between said at least one anti-wear layer 5 and the side braking surfaces 4.

[0062] Preferably, as visible in the embodiment shown in the figures, the brake disc 1 is equipped with an anti-wear layer 5 covering the two side braking surfaces 4 of thebraking band 2 and an intermediate layer 6 arranged between the two anti-wear layers 5 and the two side braking surfaces 4.

[0063] The intermediate layer 6 and the anti-wear layer 5 both comprise a plurality of hard material particles; preferably these particles are present in different quantities and / or sizes. In particular, the amount of particles contained, that is, embedded, in the layer 5 is greater than the amount of particles contained, that is, embedded, in the intermediate layer 6.

[0064] According to an aspect of the present invention, the aforesaid at least one antiwear layer 5 is constituted by ferritic steel embedding hard material particles. In a preferred implementation, also the intermediate layer 6 is constituted by ferritic steel embedding hard material particles.

[0065] In order to make a brake disc with extended product service life, that is, increased resistance to wear and corrosion and adhesion between the braking surfaces 4 and the anti-wear layer 5 over time, in an implementation the latter has a thickness D greater than or equal to the thickness d of the intermediate layer 6.

[0066] Preferably, the anti -wear layer 5 has a thickness D greater than or equal to 1.2 times the thickness d of the intermediate layer 6. Typically, D / d is between 1.0 and 1.4, preferably between 1.2 and 1.4.

[0067] In an implementation, the anti-wear layer has a thickness D on the finished brake disc less than the thickness d of the intermediate layer 6.

[0068] Conveniently, the thickness D of the layer 5 on the finished brake disc is less than or equal to 0.8 times the thickness d of the intermediate layer; typically, the value of D / d is between 0.4 and 0.8, preferably 0.6 and 0.8.

[0069] In the embodiment shown in figures, the anti-wear layer 5 has a thickness D greater than or equal to 30 pm and less than or equal to 1500 pm.

[0070] Preferably, the anti-wear layer 5 has a thickness D greater than or equal to 80 pm and less than or equal to 1000 pm.

[0071] The intermediate layer 6 has a thickness d greater than or equal to 20 pm, preferably greater than or equal to 30pm, and less than or equal to 1000 pm.Preferably, the intermediate layer 6 has a thickness d greater than or equal to 60 pm and less than or equal to 300 pm.

[0072] According to a further aspect of the present invention, the intermediate layer 6 is constituted by ferritic steel having a Nickel content of less than or equal to 5% by weight, preferably equal to or less than 0.5% by weight, more preferably less than 0.1% by weight. Preferably, that intermediate layer 6 is totally nickel-free, that is, the Nickel content is not detectable by standard analytical methods. In particular, suitable methods of analysis are ASTM E1473-22. In other words, a ferritic steel as defined here has a Nickel content that is undetectable according to the ASTM E1473-22 standard.

[0073] This helps to limit, if not even to avoid, dispersion of nickel particles throughout the lifetime of the brake disc 1.

[0074] To allow for a greater corrosion resistance, the ferritic steel used for the intermediate layer has an amount of chromium Cr in a percentage greater than or equal to 15%.

[0075] According to the present invention, the intermediate layer 6 consists of at least one layer formed by a metal alloy, preferably an iron-based alloy, more preferably a ferritic steel as described above with reference to layer 5.

[0076] The intermediate layer 6 itself also comprises particles of at least one hard material, 7a; the percentage by volume of said particles 7a of hard material in the intermediate layer is less than the percentage by volume of particles 7 of hard material in said anti -wear layer 5.

[0077] The particles 7 and 7a can be of the same type or they can be different from each other. In particular, it is advantageous that the average size of the particles 7a in said at least one intermediate layer 6 is less than the average size of said particles 7 of hard materials in said anti -wear layer 5.

[0078] In a preferred implementation, said average size of the particles 7a embedded in the intermediate layer is preferably equal to or less than 30 pm, more preferably in the range between 1 pm and 20 pm.Particles 7a of hard materials suitable for use in the present invention are selected from carbides, oxides, nitrides, borides, silicides of metals and mixtures thereof.

[0079] As seen above, in an implementation, the anti-wear layer 5 has a thickness D greater than or equal to the thickness d of the intermediate layer 6, preferably has a thickness D greater than or equal to 1.2 times the thickness d of the intermediate layer 6. Typically, D / d is between 1.0 and 1.4, preferably between 1.2 and 1.4.

[0080] In an implementation, the anti-wear layer 5 has a thickness D less that the thickness d of the intermediate layer 6. Preferably, the thickness D of the layer 5 is less than or equal to 0.8 times the thickness d of the intermediate layer; typically, the value of D / d is between 0.4 and 0.8, preferably 0.6 and 0.8.

[0081] In an implementation, the anti-wear layer 5 has a thickness D greater than or equal to 30 pm and less than or equal to 1500 pm and the intermediate layer 6 (or the sum of the intermediate layers if there are more than one) has a thickness greater than or equal to 30 pm and less than or equal to 1000 pm, preferably a thickness greater than or equal to 60 pm and equal to or less than 300pm.

[0082] In an implementation, the hard material particles embedded in the anti-wear layer 5 are present in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume, whereas the hard material particles embedded in the intermediate layer 6 are in an amount greater than or equal to 0.5% by volume and less than or equal to 10% by volume.

[0083] The intermediate layer 6, as well as the anti-wear layer 5, is produced by deposition technique using a laser beam, for example, by LMD (Laser Metal Deposition) technique, or HSLC - high speed laser cladding technique, or HSLMD (High Speed Laser Metal Deposition) or HSLC - high speed laser cladding technique, or EHLA - Extreme High Speed Laser Application technique or Top Speed Cladding technique.

[0084] The anti-wear layer 5 is deposited on top of the intermediate layer 6 so as to completely cover it.As previously mentioned, the anti-wear layer 5 is preferably constituted by ferritic steel embedding hard material particles.

[0085] The ferritic steel used is the same as that used for the intermediate layer, that is, a ferritic steel having a Nickel content of less than or at most 5% by weight.

[0086] Preferably, the ferritic steel of the anti-wear layer 5 is totally nickel-free.

[0087] This helps to limit, if not even to avoid, dispersion of nickel particles throughout the lifetime of the brake disc 1.

[0088] Even in this case, to allow for a greater wear resistance, the ferritic steel used for the anti-wear layer 5 has a relatively high amount of chromium Cr, that is, a percentage greater than or equal to 15%.

[0089] The particles 7 of hard material are embedded in the anti-wear layer 5 and in particular in the ferritic steel, in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume.

[0090] Preferably, the particles 7 of hard material are embedded in the anti -wear layer 5 and in particular in the ferritic steel, in an amount greater than or equal to 15% by volume and less than or equal to 35% by volume.

[0091] The hard material particles in the layer 5 preferably have an average size of less than 60 pm.

[0092] Preferably, the particles 7 of hard material are selected from particles of carbides, oxides, nitrides, borides, silicides or a mixture thereof.

[0093] In the case of carbides, in particle form, the latter are preferably selected from tungsten carbide (WC), chromium carbide, niobium carbide (NbC), titanium carbide (TiC) and / or mixtures thereof. As mentioned above, the required particles can be obtained partly or totally by carbide formation following laser deposition on ferritic steel; in particular, these carbides are, for example, iron (Fe) carbides and chromium (Cr) carbides and mixtures thereof.

[0094] Preferably, the carbides in particulate form that are voluntarily deposited on the surface of the disc, are selected from chromium carbide, titanium carbide (TiC) and / or mixtures thereof.The anti-wear layer 5 is obtained by deposition technique using a laser beam, for example, LMD (Laser Metal Deposition) technique, or HSLC - high speed laser cladding technique, or EHLA - Extreme High Speed Laser Application technique, or TSC - Top Speed Cladding technique.

[0095] The brake disc 1 is preferably, but not necessarily, made by the method according to the invention that will now be described.

[0096] In a first step, a brake disc 1 comprising a braking band 2 equipped with two side braking surfaces 4 is prepared. The braking band 2 is made of grey cast iron or steel, preferably the braking band 2 is made of grey cast iron.

[0097] Next, at least one side braking surface 4 of the braking band 2, preferably both, is preferably pre-machined or pre-treated mechanically or thermally, in particular to roughen or geometrically adapt the surface on which the intermediate layer 6 has to be applied.

[0098] In the next step, the intermediate layer 6 including the hard particles 7a is then applied directly to the side braking surfaces 2 by laser welding, such as by LMD (Laser Metal Deposition) technique, or by HSLC - high speed laser cladding technique, or by EHLA - Extreme High Speed Laser Application technique, or by TSC - Top Speed Cladding technique.

[0099] The bond resulting from laser welding ensures that the intermediate layer 6 adheres well to the side braking surfaces 2. The resulting intermediate layer 6 preferably covers the side braking surfaces 2 entirely.

[0100] The intermediate layer 6 which is obtained comprises, or is constituted by, hard material particles embedded in a metal alloy, preferably an iron-based metal alloy, more preferably ferritic steel having a Nickel content of less than or at most 5% by weight. Preferably, this intermediate layer 6 has a Nickel content of less than 0.1%, more preferably it is nickel-free.

[0101] The deposited intermediate layer 6 has a thickness d greater than or equal to 30 pm and less than or equal to 1000 pm, that is, the thickness d is in the range between 30 pm and 1000 pm inclusive. Preferably, the intermediate layer 6 has a thickness d greater than or equal to 60 pm and less than or equal to 300 pm.Then, after the intermediate layer 6 has solidified or cooled down, the antiwear layer 5 is deposited directly onto the intermediate layer 6.

[0102] Because ferritic steel was chosen for the intermediate layer 6, cooling is relatively rapid, benefiting a significant reduction in overall cycle times.

[0103] In the next step, the anti-wear layer 5 is then applied directly on the intermediate layer 6 by laser welding.

[0104] The anti-wear layer 5 is deposited so that the thickness of anti-wear layer 5 that is formed on the finished brake disc 1 is greater than that of the intermediate layer 6.

[0105] In particular, the anti-wear layer 5 is deposited so that the thickness of the antiwear layer 5 that is formed on the finished brake disc 1 is greater than or equal to 1.2 times the thickness d of the intermediate layer 6. Typically, D / d is between 1.0 and 1.4, preferably between 1.2 and 1.4.

[0106] In an implementation, the anti-wear layer is deposited so that its thickness on the finished brake disc is less than the thickness of the intermediate layer.

[0107] Conveniently, the anti-wear layer is deposited so that its thickness D on the finished brake disc is less than or equal to 0.8 times the thickness d of the intermediate layer; typically, the value of D / d is between 0.4 and 0.8, preferably 0.6 and 0.8.

[0108] The anti-wear layer 5 that is formed has a thickness D greater than or equal to 30 pm and less than or equal to 1500 pm.

[0109] Preferably, the anti-wear layer 5 has a thickness D greater than or equal to 80 pm and less than 1000 pm.

[0110] As previously mentioned, the anti-wear layer 5 that is formed is constituted by ferritic steel embedding hard material particles.

[0111] The ferritic steel of the anti-wear layer 5 that is formed has a Nickel content of less than or at most 5%.

[0112] Preferably, the ferritic steel of the anti-wear layer 5 that is formed has a Nickel content of less than 0.1%, more preferably is completely nickel-free.

[0113] This helps to limit, if not even to avoid, dispersion of nickel particles throughout the lifetime of the brake disc 1.The particles 7 of hard material that are embedded in the anti-wear layer 5, and in particular in ferritic steel, are embedded in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume.

[0114] Preferably, the particles 7 of hard material are selected from particles of carbides, oxides, nitrides, borides, silicides or a mixture thereof.

[0115] The hard material particles in the layer 5 preferably have an average size of less than 60 pm.

[0116] The anti-wear layer 5 is obtained by deposition technique using a laser beam, for example, LMD (Laser Metal Deposition) technique, or HSLC - high speed laser cladding technique, or EHLA - Extreme High Speed Laser Application technique.

[0117] The anti -wear layer 5 may be subjected to mechanical or thermal postprocessing, in particular grinding, to ensure the desired surface roughness for interaction with the friction brake pad.

[0118] Various modifications may be made to the embodiments described in detail, all anyhow remaining within the scope of protection of the invention, as defined by the following claims.

Claims

CLAIMS1. Brake disc (1) comprising a braking band (2) extending around an axis of rotation (X-X) of said disc; said braking band (2) having two side braking surfaces (4) adapted to cooperate with a respective brake caliper to exert braking action on a vehicle on which said brake disc (1) is installed;at least one anti-wear layer made of a metal alloy, preferably an iron-based alloy, containing particles of hard material, applied to said side surfaces (4); and at least one intermediate layer (6) made of a metal alloy, preferably an ironbased alloy, arranged between said at least one anti-wear layer (5) and said side braking surfaces (4);characterized in that said at least one intermediate layer consists of at least one layer made of a metal alloy, preferably an iron-based alloy, embedding particles of at least one hard material; wherein the percentage by volume of said particles of hard material in said intermediate layer is less than the percentage by volume of particles of hard material in said anti-wear layer.

2. Brake disc (1) according to claim 1, characterized in that said particles of hard materials are selected from metal carbides, oxides, nitrides, borides, silicides and mixtures thereof.

3. Brake disc (1) according to claim 1 or 2, characterized in that said anti-wear layer has a thickness (D) greater than or equal to the thickness (d) of said intermediate layer (6), preferably has a thickness (D) greater than or equal to 1.2 times the thickness (d) of the intermediate layer (6), or in that the anti-wear layer (5) has a thickness (D) on the finished brake disc less than the thickness (d) of said intermediate layer (6), preferably said thickness (D) of the anti-wear layer (5) is less than or equal to 0.8 times the thickness (d) of said intermediate layer (6).

4. Brake disc (1) according to any one of claims 1 to 3, characterized in that said anti -wear layer (5) has a thickness (D) greater than or equal to 30 pm and less than or equal to 1500 pm, preferably a thickness (D) greater than or equal to 80 pm and less than or equal to 1000 pm.

5. Brake disc (1) according to any one of claims 1 to 4, characterized in thatsaid intermediate layer (6) has a thickness greater than or equal to 20 pm and less than or equal to 1000 pm, preferably a thickness greater than or equal to 60 pm and equal to or less than 300qm.

6. Brake disc (1) according to any one of claims 1 to 5, characterized in that said particles of hard material embedded in said anti-wear layer (5) are in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume and in that said particles of hard material embedded in said intermediate layer (6) are in an amount greater than or equal to 0.5% by volume and less than or equal to 10% by volume.

7. Brake disc according to one of the preceding claims, characterized in that the average size of said particles in said at least one intermediate layer is less than the average size of said particles of hard materials in said anti-wear layer, said average size of said particles embedded in said intermediate layer being preferably equal to or less than 30 pm, more preferably in the range between 1 pm and 20 pm.

8. Brake disc (1) according to any one of claims 1 to 7, characterized in that said metal alloy is a ferritic steel, preferably said ferritic steel comprises Cr in a percentage greater than or equal to 15%, more preferably said ferritic steel having a Nickel content less than 5% by weight, more preferably less than 0.1% by weight or said Nickel content being not detectable.

9. Friction brake for motor vehicles, comprising at least one brake disc (1) having two braking surfaces adapted to cooperate with a respective brake caliper to exert braking action on a vehicle on which said brake disc (1) is installed and at least one movable pad combined with the brake disc (1) and said brake caliper, characterized in that said brake disc (1) is made as a friction braking element according to any one of claims 1 to 8.

10. Method for making a brake disc (1) according to one of claims 1 to 8, comprising the following operational steps:a) preparing a brake disc (1), comprising a braking band (2) equipped with two side braking surfaces (4), said braking band (2) being made of grey cast iron or steel;b) after step a), depositing, on at least one of the two side braking surfaces (4), at least one intermediate layer (6) of a metal alloy containing particles of hard material which are embedded in said intermediate layer, said metal alloy being preferably an iron-based alloy;c) after step b), depositing an anti -wear layer (30) of a metal alloy containing particles of hard material which are embedded in said anti-wear layer, said metal alloy being preferably an iron-based alloy;preferably the amount of particles of hard material embedded in said anti-wear layer (1) being greater than the amount of particles of hard material embedded in said at least one intermediate layer (6);said depositing step being implemented by laser deposition technique which leads to melting said metal alloy and embedding said particles of hard material in said metal alloy.

11. Method according to claim 10, characterized in that said metal alloy is constituted by ferritic steel and / or in that said particles of hard materials are selected from metal carbides, oxides, nitrides, borides, silicides and mixtures thereof.

12. Method for making a brake disc (1) according to claim 10 or 11, characterized in that the anti-wear layer (5) is deposited so that its thickness on the finished brake disc 1 is greater than the thickness (d) of the intermediate layer (6), preferably so that its thickness (D) on the finished brake disc (1) is greater than or equal to 1.2 times the thickness (d) of said intermediate layer (6) or in that the antiwear layer (5) is deposited so that its thickness (D) on the finished brake disc is less than the thickness (d) of the intermediate layer (6) preferably so that its thickness (D) is less than or equal to 0.8 times the thickness (d) of said intermediate layer (6).

13. Method for making a brake disc (1) according to one of claims 10 to 12, characterized in that the average size of the particles of hard material embedded in said anti-wear layer (1) is greater than the average size of particles of hard material embedded in said at least one intermediate layer (6).

14. Method for making a brake disc (1) according to any one of claims 10 to 13, characterized in that, during step b), said particles of hard material areembedded in said intermediate layer in an amount greater than or equal to 0.5% by volume and less than or equal to 10% by volume and in that, during said step c), said particles of hard material are embedded in said anti-wear layer (5) in an amount greater than or equal to 10% by volume and less than or equal to 40% by volume.

15. Method for making a brake disc (1) according to any one of claims 10 to 14, characterized in that, during step b), said deposition, by using a laser beam, is implemented with a technique selected from LMD (Laser Metal Deposition) technique, or HSLMD (High Speed Laser Metal Deposition) technique, or HSLC -high speed laser cladding technique, or EHLA - Extreme High Speed Laser Application technique, or Top Speed Cladding technique.

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