FRICTION MATERIAL COMPOSITIONS AND ASSOCIATED BRAKE PAD

Abstract

A friction material composition and associated brake pad for a vehicle having reduced or zero tendency for both stiction and creep growth, the composition comprising an organic binder, an inorganic filler, a lubricant, a hard abrasive having a Mohs hardness greater than 7 having a rounded shape consisting of, but not limited to, alumina, corundum, silicon carbide, tungsten carbide, zirconium carbide, zirconium silicate, boron nitride, a soft abrasive having a Mohs hardness less than 7, a carbon-based material, and a metal or mixture of metals, excluding Cu, in an amount of less than 7 volume % calculated on the total volume of the composition, wherein the ratio between the rounded hard abrasive and the carbon-based material is 1:3, the ratio between the rounded hard abrasive and the soft abrasive is 1:6, and the ratio between the metal or mixture of metals and the carbon-based material is 1:6.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority to Italian Patent Application No. 102021000008807, filed April 8, 2021, the entire disclosure of which is incorporated herein by reference. [Technical field]

[0002] The present invention relates to a friction material composition that is particularly suitable for the manufacture of friction layers / friction blocks for friction elements such as brake pads or brake shoes for vehicles. The present invention also relates to associated brake pads made with this friction material composition and that are particularly, but not exclusively, suitable for equipping the rear axle of a vehicle.

[0003] The friction material composition of the present invention is asbestos-free, in particular, but not exclusively, of the so-called "low steel" type, and has excellent properties with respect to the phenomena known as "stiction" and "creep growth", i.e. the occurrence of such phenomena is limited or non-existent. [Background technology]

[0004] Friction material compositions for producing brake pads can be classified as semi-metallic materials having more than 50% ferrous metals (iron powder, steel fibers), low-metallic materials or LS (low steel) having 5-35% ferrous metals, and non-asbestos organic materials (NAO) that contain substantially no ferrous materials.

[0005] It is known that, especially under harsh operating and / or environmental conditions, this can result in the "sticking" of one or more of the brake pads of a vehicle to the associated brake disc made of cast iron or steel in use. This is generally due to corrosion phenomena that affect the brake disc and result in corrosion products that, during braking, combine with the friction material of the brake pads to cause the accidental adhesion of the brake pads to the brake disc, this adhesion being temporarily maintained even when the vehicle brakes are deactivated. This phenomenon of adhesion is known by the English technical term "stiction", which is derived from the contraction and fusion of the terms "static" and "friction", i.e. the term "static friction".

[0006] Obviously, the occurrence of stiction in vehicles during use is accompanied by various drawbacks such as shock / distortion during braking and subsequent release of the brakes, increased energy consumption, and in extreme cases, breakdown of the friction material of the brake pads leading to vehicle failure and / or premature replacement of the brake pads. Thus, stiction is well known in the field of brake pads and also affects vehicles that are constantly driven on paved roads when exposed to wet weather conditions.

[0007] Another phenomenon that occurs in brake pads, especially when made with LS friction material, is the so-called "creep groan". This phenomenon consists of low-frequency chassis vibrations in the vehicle, which are encountered at very low brake pad pressures and at extremely low speeds, causing a very unpleasant sensation for the driver. This is a classic example of the so-called "stick-slip" effect, i.e. self-excited braking vibrations caused by repeated "sticking" and subsequent slipping of the brake pad on the brake disc, or repeated transitions between dynamic and static friction.

[0008] Various solutions are known in the art that attempt to reduce these phenomena, but they either do not completely solve the problem or present additional drawbacks.

[0009] EP0959262 discloses a disc brake pad capable of reducing creep growth using a composition containing a fibrous base material, a binder, and a friction modifier, excluding asbestos, in which the binder is composed entirely or partially of a modified silicone resin, and in combination, the friction material composition contains 0.5-20% by volume of zeolite as part of the friction adjusting agent, and the modified silicone resin is contained in the friction material composition in an amount of 3% by volume to 30% by volume of the total composition. The modified silicone resin is obtained by reacting oil or silicone rubber with a novolac-type phenolic resin. This results in expensive materials and difficult production.

[0010] US 2005 / 004258 describes a friction lining material which is copper-free but contains a significant amount of stainless steel fibres. The filler material used in US 2005 / 004258 is graphite in an amount of 7-15% by volume.

[0011] WO2011 / 131227 discloses a low steel (LS) friction material for brake pads, which is substantially copper-free. The carbon component is contained in the composition described therein in an amount of 36-51% by volume. The low steel friction lining composition is designed for high pressure, high temperature applications and has good material properties at high speeds. However, the known low steel (LS) friction lining material mixtures / compositions are disadvantageous in terms of braking comfort.

[0012] WO2019120648 discloses a hybrid friction lining material and a brake pad made therefrom, in which the positive properties of a steel low friction lining material (so-called low steel (LS) friction lining or friction lining material) and an asbestos-free organic friction lining material (so-called non-asbestos organic (NAO) friction lining or friction lining material) are attempted to be combined. In a preferred embodiment, such hybrid friction material contains 15-22%, particularly 17-20%, of at least one binder, 5-11% of an organic fiber or a mixture of organic fibers, 1-20%, particularly 8-14%, of at least one further organic compound, 0 or 8-16% of an inorganic fiber or a mixture of inorganic fibers, 10-40% of at least one inorganic oxide, 6-12% of at least one inorganic silicate, 13-15% of sulfur or at least one inorganic sulfur compound, 10-16% of carbon or at least one material essentially consisting of carbon, particularly selected from the group consisting of natural graphite, synthetic graphite, petroleum coke, desulfurized petroleum coke, carbon black, and any mixtures thereof, 1-1.5% of at least one filler selected from the group of inorganic hydroxides, particularly calcium hydroxide, and 0-1% of at least one metal, particularly iron or an iron alloy.

[0013] Such hybrid materials, however, result in compromised behavior in terms of braking performance and comfort, which may not be optimal or may be less than optimal for many applications. Summary of the Invention

[0014] It is an object of the present invention to provide an embodiment of a friction material designed, in particular, but not exclusively, to produce friction blocks or layers for brake pads, where the friction material has a formulation that allows for reducing or eliminating both the phenomena of stiction and creep groan by reducing the tendency of the friction blocks to stitch against the surface of the friction mating with which they cooperate.

[0015] In particular, it is an object of the present disclosure to provide embodiments of such friction materials as an integral part of or integrated into brake pads designed to cooperate in use with brake discs made of steel or cast iron.

[0016] A further object of the present invention is to provide an embodiment of a friction material having a composition that does not contain copper and / or copper alloys, except for impurities, aiming to obtain braking behavior of LS (low steel) friction composition type, but where the formulation aims to obtain a low tendency towards stiction and creep groan behavior comparable or even better compared to that of NAO (non-asbestos organic) friction compositions.

[0017] The present disclosure thus relates to embodiments of the so-called "copper-free" type of friction material and related brake pads, as defined in the appended claims, which are formulated / made to reduce or eliminate the tendency of the friction block to stitch against the surface of its friction counterpart, such as a brake disc made of steel or cast iron, or an integral part of a clutch disc for vehicles or any other application.

[0018] Here and below, the expression "copper-free" should be understood to imply a copper content of less than or equal to 0.5% mass (weight) and / or a content of copper-containing materials such as copper alloys.

[0019] The present disclosure further relates to embodiments of brake pads equipped with, or having as an integral component thereof, such friction blocks or layers made of the friction material according to the present invention.

[0020] The present disclosure also relates to embodiments of methods for manufacturing brake pads for vehicles having reduced or zero tendency toward both stiction and creep groin.

[0021] In the embodiments of the invention disclosed herein below, the asbestos-free friction material composition is designed to be molded into a friction block or layer, preferably, but not exclusively, to equip a braking element, such as a brake pad or brake shoe.

[0022] The asbestos-free friction material composition comprises as its compositional ingredients at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive, at least one soft abrasive, a carbon-based material, and optionally at least one metal or mixture of metals.

[0023] Here and below, a hard abrasive is to be understood as an inorganic substance having a Mohs hardness of 7 or more, whereas a soft abrasive is to be understood as an inorganic substance having a Mohs hardness of less than 7.

[0024] In an embodiment of the invention according to the present disclosure, the hard abrasives having a Mohs hardness greater than 7 consist exclusively of hard abrasives having rounded shapes.

[0025] Here and below, a "rounded shape" must be understood to be a shape of a substance in particle, e.g. powder, form, for which the ratio R / S between its circularity and its sphericity can be calculated and has a value comprised between 0.6 and 0.8, inclusive of the lower and upper limits of the said interval.

[0026] Circularity and sphericity are quantities mathematically defined according to Krumbein and Sloss (1963), as described, for example, in the publication "Oil Sand Characterization for Standalone Screen Design and Large-Scale Laboratory Testing for Thermal Operations" - Mahadi Mahmoudi et al. - SPE Thermal Well Integrity and Design Symposium - Banff, Alberta, Canada, 23-25 ​​November 2015.

[0027] The rounded shaped hard abrasives (i.e., having a Mohs hardness of greater than 7) suitable for use in accordance with the present invention may be preferably, but not exclusively, selected from the group consisting of alumina, corundum, silicon carbide, tungsten carbide, zirconium carbide, zirconium silicate, boron nitride, any mixture thereof.

[0028] Carbon-based materials that may be used in embodiments according to the present disclosure may be preferably, but not exclusively, selected in the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene, and mixtures thereof.

[0029] Carbon-based materials that may be used in embodiments according to the present disclosure may be included in the asbestos-free friction material composition in an amount less than 22% vol ("% vol" means "percentage by volume") calculated on the total volume of the asbestos-free friction material composition.

[0030] At least one metal or mixture of metals may be contained in the asbestos-free friction material composition in an amount of less than 7% by volume, calculated on the total volume of the asbestos-free friction material composition, preferably not consisting of copper and / or copper alloys, except for impurities (copper content being maintained below 0.5% by weight in any case), and may be selected preferably, but not exclusively, in the group consisting of iron, steel, stainless steel, tin, zinc, metal alloys in powder or fiber form, steel fibers, stainless steel fibers.

[0031] The at least one lubricant may preferably comprise a sulfide-based lubricant selected in the group consisting of metal sulfides of Sn, Zn, Fe, Mo, and mixtures thereof, and the sulfide-based lubricant may be contained in the asbestos-free friction material composition in an amount comprised between 6% vol and 18% vol, calculated on the total volume of the asbestos-free friction material composition.

[0032] At least one soft abrasive having a Mohs hardness of less than 7 may be contained in the asbestos-free friction material composition in an amount comprised between 26% vol and 38% vol calculated on the total volume of the asbestos-free friction material composition, and may be preferably, but not exclusively, selected in the group consisting of magnesia, chromite, zirconia, magnetite, hematite, quartz, zinc oxide, tin oxide, barium sulfate, silicates, fluorides, any mixture thereof.

[0033] The at least one organic binder may be any one of phenolic resin, epoxy resin, silicone resin, modified phenolic resin, melamine resin, polyimide resin, and mixtures thereof, and the organic binder may be present in the asbestos-free friction material composition alone or together with organic fibers in an amount of 20% vol to 30% vol calculated on the total volume of the asbestos-free friction material composition. Suitable organic fibers may be preferably, but not exclusively, selected from the group consisting of polyacrylic fibers, polyaramid fibers, aramid fibers, cellulose fibers, and any mixtures thereof.

[0034] The content of organic fibres present in the composition is calculated as a part, i.e. as a fraction, of the total content of organic binder.

[0035] At least one inorganic filler may be present in the asbestos-free friction material composition in an amount comprised between 8% vol and 20% vol calculated on the total volume of the asbestos-free friction material composition, and may be preferably, but not exclusively, selected in the group consisting of mineral fibers, glass fibers, rock wool, phyllosilicates (mica, vermiculite, talc), titanates, inorganic hydroxides of Ca, Mg, K, any mixture thereof.

[0036] According to one aspect of the invention, embodiments of the present disclosure may not include any hard abrasives with a Mohs hardness greater than 7, except for impurities, having an angular shape as defined according to Krumbein and Sloss (1963), i.e., a grain size with a ratio R / S (roundness / sphericity) of less than 0.6.

[0037] Moreover, embodiments of the present disclosure may be free of any copper or copper alloys in either powder or fiber form, except for impurities.

[0038] The ratio between the volume content of the rounded, hard abrasive and the volume content of the soft abrasive may be 1:5, and in combination, the ratio between the volume content of the lubricant and the volume content of the total abrasives (soft and hard, rounded) may be 1:4.

[0039] The ratio of the volume content of at least one hard abrasive having a Mohs hardness greater than 7 with a rounded shape to the volume content of the carbon-based material may be 1:3.

[0040] The ratio between the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material may be 1:6, and in combination, the ratio between the volume content of the at least one metal or mixture of metals and the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 with a rounded shape may be 1:2.

[0041] Brake pads, including friction material blocks made with the friction material composition, can be molded and cured in any suitable known manner using the friction material composition described above.

[0042] In an embodiment of a method for manufacturing a brake pad for a vehicle having reduced or zero tendency for both stiction and creep groin, the method comprises the steps of preparing an asbestos-free friction material composition as described above, molding the friction material composition into a friction material block or friction layer that is applied onto a metal substrate, and curing the friction material block or friction layer so obtained.

[0043] Preferred but non-limiting embodiments will now be described in more detail with reference to some actual examples of its implementation, which are intended only to disclose in a non-exhaustive and non-limiting manner the features that are part of the subject matter of the present disclosure, and with reference to the accompanying drawing figures. [Brief description of the drawings]

[0044] [Figure 1] 1 shows a SEM (scanning electron microscope) micrograph of an embodiment of a friction material composition according to the present invention after molding and curing. [Diagram 2] 1 illustrates, in a schematic and concise manner, how to classify irregularly shaped solid particles as rounded or non-rounded in shape. [Diagram 3] 1 shows an SEM micrograph of a hard abrasive (zirconium silicate) having a rounded shape according to the definition given in this disclosure. [Figure 4]1 shows an SEM micrograph of a hard abrasive (silicon carbide) that does not have a rounded shape according to the definition given in this disclosure. [Diagram 5] 1 shows an SEM micrograph of a raw material (vermiculite-phyllosilicate) used as a filler in an embodiment of the present disclosure before being mixed with other compositional materials of an embodiment of the friction material of the present disclosure. [Figure 6] FIG. 6 shows an SEM micrograph of the same material (vermiculite-phyllosilicate) as in FIG. 5, used as a filler in embodiments of the present disclosure, in a friction material block after mixing, molding, and curing (see grey elongated particles). [Figure 7] 1 shows a graph reporting the results of SQuadriga triaxial accelerometer testing in amplitude (m / s2) versus time (s) for a friction material block made with LS friction material according to a standard formulation. [Figure 8] 13 shows a graph reporting the results of the same SQuadriga triaxial accelerometer test in amplitude (m / s2) versus time (s) for a friction material block made with a friction material according to the present invention. [Figure 9] 8 shows a comparison between the corresponding fading portions of the graphs showing the test results of the same AK-Master efficiency test performed on two friction material blocks formed in the LS friction material with standard formulation of FIG. 7 (left graph) and the friction material according to the present invention of FIG. 8 (right graph), respectively. Description of the Preferred Embodiments

[0045] Asbestos-free friction material compositions designed to be formed into friction blocks or layers, preferably but not exclusively, to equip braking elements such as brake pads or brake shoes, were prepared and tested using standard mixing techniques.

[0046] It was prepared and tested using a standard formulation of an asbestos-free LS friction material composition as a reference material, which was formulated according to possible embodiments of the present invention, but for the remaining materials, friction material compositions having formulations substantially corresponding to the formulation of the reference material were also prepared and tested.

[0047] All formulations of the asbestos-free friction material compositions (reference and inventive) prepared and tested comprise as their constituent ingredients at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive having a Mohs hardness greater than 7, at least one soft abrasive having a Mohs hardness less than 7, a carbon-based material, and optionally at least one metal or mixture of metals.

[0048] The at least one lubricant may preferably comprise a sulfide-based lubricant selected in the group consisting of metal sulfides of Sn, Zn, Fe, Mo, and mixtures thereof, and in an exemplary embodiment of the present invention, the sulfide-based lubricant may be contained in the asbestos-free friction material composition in an amount comprised between 6% vol and 18% vol, calculated on the total volume of the asbestos-free friction material composition.

[0049] At least one soft abrasive having a Mohs hardness of less than 7 may be preferably, but not exclusively, contained in the composition in an amount comprised between 26% vol and 38% vol, calculated relative to the total volume of the composition.

[0050] The soft abrasive may preferably be selected from the group consisting of, but not limited to, magnesia, chromite, zirconia, magnetite, hematite, quartz, zinc oxide, tin oxide, barium sulfate, silicates, fluorides, and any mixtures thereof.

[0051] The at least one organic binder may preferably be comprised of, but is not limited to, any one of phenolic resins, epoxy resins, silicone resins, modified phenolic resins, melamine resins, polyimide resins, and mixtures thereof.

[0052] The organic binder may be present in the asbestos-free friction material composition in an amount comprised between 20% vol and 30% vol, calculated on the total volume of the asbestos-free friction material composition.

[0053] The friction material composition according to the present invention may also comprise organic fibers, which may be preferably, but not exclusively, selected from the group consisting of polyacrylic fibers, polyaramid fibers, aramid fibers, cellulosic fibers, and any mixtures thereof.

[0054] Organic fibers may be preferably, but not exclusively, included in the friction material composition of the present disclosure as part of the organic binder, since the organic fibers may have a primary purpose of increasing the strength of the brake pad / shoe that may be produced from the friction material composition of the present disclosure under operational working conditions.

[0055] Therefore, the content of organic fibers present in the composition of the present disclosure is selected as a part of the content of organic binder, e.g., as a part of a total amount of 20% vol to 30% vol calculated with respect to the total volume of the composition, depending on the mechanical properties to be realized in the final friction material block molded from the raw material composition and / or the operating conditions, e.g., mechanical and thermal loads, of the brake pad / shoe.

[0056] At least one inorganic filler may be present in the asbestos-free friction material composition of the present disclosure in an amount comprised between 8% vol and 20% vol calculated on the total volume of the asbestos-free friction material composition, and the at least one inorganic filler is preferably, but not limited to, selected in the group consisting of mineral fibers, glass fibers, rock wool, phyllosilicates (mica, vermiculite, talc), titanates, inorganic hydroxides of Ca, Mg, K, any mixture thereof.

[0057] The carbon-based material used in the friction material composition of the present disclosure may be preferably, but not limited to, selected from the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene, and mixtures thereof.

[0058] According to a first aspect of the present invention, the hard abrasives having a Mohs hardness greater than 7 that may be used in the friction material compositions of the present disclosure may consist exclusively or nearly exclusively of rounded hard abrasives, i.e., hard abrasives formed by solid particles having a rounded shape (i.e., having a Mohs hardness greater than 7), where "rounded" is defined as described below with reference to FIG. 2.

[0059] Here and below, "hard abrasives having a Mohs hardness greater than 7 and having a rounded shape" are to be understood as hard abrasives which may comprise abrasive particles having exclusively or almost exclusively a ratio R / S (roundness / sphericity) within the following intervals: 0.6≦R / S≦0.8 Here, R and S are calculated according to Krumbein and Sloss (1963), as for example mentioned in the paper "Oil Sand Characterization for Standalone Screen Design and Large-Scale Laboratory Testing for Thermal Operations" - Mahadi Mahmoudi et al. - SPE Thermal Well Integrity and Design Symposium - Banff, Alberta, Canada, 23-25 ​​November 2015.

[0060] Here and hereinafter, "almost exclusively" is to be understood as an amount of particles which is at or approaching 90% by volume of the total volume of the hard abrasive material under consideration.

[0061] The calculation of the values ​​R and S is carried out using the following formulas (also reported in FIG. 2), the coefficients introduced in the formulas being shown in a non-limiting visual example given in the right-hand part of FIG.

[0062]

number

[0063]

number

[0064] That is, an irregularly shaped particle can be diagrammed as shown on the right side of FIG. 2, and such a particle can have several rounded edges, as shown in FIG. 2, with each of the radii being r1, r2, ..r i The particle may be inscribed in a circle, and the inscribed circle of the smallest radius may have a radius "r min-cir " and the maximum radius rounded edge has a radius "r max-in " and even if no rounded edges are found, "r max-in " would then be the radius of the largest circle that can be inscribed within the grain boundary.

[0065] In the table shown on the left side of Figure 2, the boundary shapes of some "real" particles are visually diagrammed, and the values ​​of R and S that can be calculated as explained above are given. As can be seen, even a particle identified as P1, which has only sharp edges, can have a high sphericity (0.9), but (not surprisingly) a very low circularity (0.1).

[0066] Therefore, the ratio R / S can always be calculated, and as can be seen from the diagram on the left of FIG. 2, the particles that appear to have the smoothest, rounded boundaries are all within the intervals listed above: 0.6≦R / S≦0.8

[0067] According to the definition given above, the hard abrasive having a rounded shape and a Mohs hardness greater than 7 may be preferably, but not exclusively, selected from the group consisting of alumina, corundum, silicon carbide, tungsten carbide, zirconium carbide, zirconium silicate, boron nitride, any mixture thereof.

[0068] As a further example of what the visual appearance of a hard abrasive raw material might be that may be classified as a "rounded shape hard abrasive", an SEM micrograph of zirconium silicate is presented in Figure 3. The appearance of this material in Figure 3 should be compared with that of the material shown in Figure 4, which is a micrograph having the same magnification as Figure 3, of a hard abrasive with a Mohs hardness greater than 7 but that does not have a rounded shape, namely silicon carbide in the example shown.

[0069] 1 shows the embodiment envisioned by the feedstock of FIG 3 in a friction material composition according to the present invention after curing, i.e., in a friction block of friction material ready (or already applied) to a metal substrate to form a brake pad. As clearly shown, the large rounded particles dispersed in the matrix of the cured friction material are particles of zirconium silicate, i.e., particles of a hard abrasive of rounded shape having a Mohs hardness of greater than 7, as can be identified by spectrophotometric photomicrograph analysis.

[0070] Also, the selection of the correct filler(s) can be very important in the friction material composition of the present disclosure. In particular, FIG. 5 shows a micrograph of a raw material consisting of a preferred filler, namely, vermiculite-phyllosilicate. FIG. 6 shows the embodiment of the same filler in a cured friction material composition according to the present invention. Considering the embodiment of the starting raw material, the vermiculite-phyllosilicate is pressed into elongated particles embedded in the matrix of the friction material, which can impart higher strength to the cured friction material composition.

[0071] According to a further aspect of the invention, the carbon-based material as mentioned above may be contained in the composition according to the present disclosure in an amount of less than 22% (by volume) vol, calculated relative to the total volume of the composition, i.e. in a limited, or in any case restricted, amount.

[0072] According to a further aspect of the invention, the at least one metal or mixture of metals, when present in the composition, does not consist of copper and / or any copper alloy, except for impurities (in any case the total copper content should be at most equal to or preferably less than 0.5% mass, i.e. % by weight), and must in any case be contained in the composition in small amounts, i.e. less than 7% by volume, calculated relative to the total volume of the composition.

[0073] Also, the ratio between the selected pairs of component materials in the friction material composition according to the present invention may be of utmost importance to achieve the goal of solving the desired technical problem as described at the beginning of this disclosure, i.e., to reduce or eliminate the tendency of the final friction material to suffer from both the phenomena of creep growth and stiction.

[0074] The ratio between the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material is preferably 1:6 and should in any case be within the range of 5:100 to 88:100.

[0075] In combination with the above characteristics, the ratio between the volume content of at least one metal or mixture of metals and the volume content of at least one hard abrasive having a rounded shape and a Mohs hardness of more than 7 should preferably be 1:2 and in any case be within the range of 11:100 to 233:100.

[0076] According to an embodiment of the present invention, the friction material composition of the present disclosure does not contain, except for impurities or in any substantial amount, any hard abrasive having a Mohs hardness greater than 7, having an angular shape as defined according to Krumbein and Sloss (1963), i.e., having a grain size with a ratio R / S (roundness / sphericity) of less than 0.6.

[0077] Thus, for example, compositions according to the present invention will not include, or at least will not include in substantial amounts, silicon carbide as a hard abrasive.

[0078] Here and hereinafter, a "substantial amount" is to be understood as an amount less than 10% vol of the total volume of the composition.

[0079] The ratio between the volume content of rounded / rounded hard abrasives (Mohs hardness greater than 7) and the volume content of soft abrasives is preferably 1:6 and in any case is within the range of 8:100 to 35:100.

[0080] In combination with the above features, the ratio between the volume content of the lubricant and the volume content of the total of the abrasives (soft and hard, rounded) is preferably 1:5 and in any case within the range of 13:100 to 62:100.

[0081] It is important that the ratio between the volume content of at least one hard abrasive with a Mohs hardness greater than 7 having a rounded shape and the volume content of the carbon-based material is maintained at approximately 1:3, and in any case within the range of 15:100 to 113:100.

[0082] The present invention will now be better disclosed with reference to the following examples, which are intended to be non-limiting and non-exhaustive in any way.

[0083] Examples and comparative examples are given herein below by way of illustration and therefore are not intended to limit the present invention. Example 1

[0084] Two formulations, marked "Standard" and "Innovative", were prepared according to the table below: The "Standard" formulation corresponds to a well-known friction material formulation of the LS (low steel) category that is typically used to manufacture friction material blocks for vehicle brake pads, and is used herein as the "reference" material.

[0085] [Table 1]

[0086] The ingredients shown in Table 1, which indicate the values ​​in volume % relative to the total volume of the mixture / blend, are mixed homogeneously in a horizontal mixer (e.g. a mixer of the Loedige type), molded in a mold under a pressure of 20 tons for 3 minutes at a temperature of 160° C., then cured by heat treatment at 400° C. for 10 minutes to produce friction materials according to the invention, indicated as “innovative”, and reference materials according to known technology, indicated as “standard”, which are used for subsequent comparative tests; each block of friction material so obtained is integrated with an identical metal support consisting of a flat steel plate (back plate) to form a vehicle brake pad. Example 2

[0087] Brake pads produced in the manner described in Example 1 were mounted on a vehicle and subjected to the following tests. Stiction Testing - Step 1

[0088] Bedding → 100 stops from 50 to 0 kph at 20% g · Conditioning with tap water Parking mode with parking brake applied Indoor overnight parking Release the parking brake and record the sound level (dB) Repeat the above steps for 10 days (excluding weekends) The lowest possible sound pressure is required and the sound pressure is detected in dB via an audio and acoustic handheld analyzer type XL2 from NTI Audio provided with an omnidirectional, pre-polarized condenser, free-field microphone, with a frequency range of 5Hz to 20kHz and a typical sensitivity of 1kHz to 27.5dBV / Pa ±2dB (42mV / Pa).

[0089] The results obtained are reported in Tables 2 and 3 below.

[0090] Note that 43 dB corresponds to the sound level of the vehicle (car) with the engine on (i.e., background noise). Stiction noise levels below 43 dB are therefore not perceived as they are covered by the background noise of the car. Hence, a value of (43) dB is reported in the table when no noise is recorded.

[0091] [Table 2]

[0092] [Table 3]

[0093] As can be seen from a comparison of the noise values ​​in Tables 2 and 3 above, the dB noise produced for the low steel "innovative" formula is much lower than the "standard" formula (baseline). Stiction Testing - Step 2

[0094] 200 stops from 80km / h to 20km / h at 0.3g Spray the disk with 5% NaCl solution -Clamping force according to project parameters Park the car outside for 3 days (first and second tests) and 10 days (third test) - Torque measurement using a torque wrench Results are expressed in Nm

[0095] The test results are reported in Table 4 below.

[0096] [Table 4]

[0097] As shown in Table 4, the stiction results for the low steel innovative formulation are much lower than the standard formulation. Creepgrown Vehicle Instructions - Description and Evaluation:

[0098] Bedding → 40br.-30bar-100~50km / h-every 1.5km Brake squeal evaluation steps 1-5: Step 1 - Morning - After parking the vehicle outside overnight, drive up to the ramp without braking - 12% ramp - Engine on - [Forward] Step 2 - Repeat step 1 [back] Step 3 - Flat road Step 4 - Repeat step 1 after warming up at 30°C [Forward] Step 5 - Repeat step 2 after warming up at 30°C [Back] Repeat steps 1-5 for 3 days. - Consider humidity RH [%] and temperature [℃] Evaluate CG (Creep Groove) taking into account: Strength / Reproducibility - Subjective Index Min: 4; Index Max: 10

[0099] The results obtained are reported in Table 5 below.

[0100] [Table 5]

[0101] As can be seen, the low steel innovative formulation exhibits significantly better creep growth behavior than the baseline (standard) formulation, and is consistent with the creep growth behavior of known NAO materials.

[0102] The vibration in function of time was also measured during brake application by a SQuadriga triaxial accelerometer: Amplitude (m / s 2 ) versus time (s). The results obtained are reported in Figures 7 and 8. Figure 7 shows the behavior of the reference (standard) formulation and Figure 8 shows the behavior of the innovative formulation. It is clear that the amplitude and frequency of the detected vibrations are dramatically lower in the friction material composition according to the invention (innovative). Example 3

[0103] Brake pads produced in the manner described in Example 1 were mounted on a vehicle and subjected to efficiency tests according to the AK-Master standard, including settling braking, braking at different fluid pressures, low temperature braking (<50°C), simulated highway braking, and two series of high energy braking interspersed with a series of recovery braking (FADE tests).

[0104] The results obtained are reported in FIG. 9, which compares the graphs for the FADE section of the test for the standard (reference) formulation - graph on the left - and for the innovative (according to the invention) formulation - graph on the right.

[0105] Of particular importance is the circled area in both graphs: the innovative formulation exhibited an overall nominal friction level comparable to the standard low steel formulation (0.41 vs. 0.44), but the fading performance of the innovative formulation was higher and more stable than that of the standard formulation along all fade sections. Example 4

[0106] Ten formulations were prepared by randomly varying the relative amounts of the friction material components listed in Table 6 below, within the ranges set forth in Table 6 itself.

[0107] [Table 6]

[0108] Then, using 10 different formulations of the friction material composition prepared according to the contents of Table 6, corresponding friction pads were prepared operating similarly to that in Example 1. The tests described and reported in Examples 2 and 3 were then repeated on the new brake pads, always with results comparable to those previously described (the behavior of the compositions was limited to 10% more or less variability than the innovative composition of Example 1).

[0109] From the above and from the preceding examples, it is apparent that the present invention also extends to brake pads comprising friction material blocks made from the friction material composition according to the present disclosure.

[0110] Moreover, the present invention also extends to a method for manufacturing a brake pad for a vehicle having reduced or zero tendency towards both stiction and creep groan, the method comprising the steps of preparing an asbestos-free friction material composition having as its compositional ingredients at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive having a Mohs hardness greater than 7, at least one soft abrasive having a Mohs hardness less than 7, a carbon-based material, and optionally at least one metal or mixture of metals, shaping the asbestos-free friction material composition into a friction material block or friction layer applied onto a metal substrate and curing the friction material block or friction layer so obtained; i. the hard abrasives having a Mohs hardness greater than 7 contained in the asbestos-free friction material composition consist exclusively or almost exclusively of hard abrasives having rounded shapes (within the meaning defined above); ii. the carbon-based material contained in the asbestos-free friction material composition is maintained in an amount of less than 22% (by volume) vol, calculated on the total volume of the asbestos-free friction material composition; iii. At least one metal or mixture of metals contained in the asbestos-free friction material composition does not consist of Cu except for impurities and may be selected in the group consisting of iron, steel, stainless steel, tin, zinc, metal alloys except Cu alloys in powder or fiber form (unless there are impurities), steel fibers, stainless steel fibers, mixtures thereof, and is contained in the asbestos-free friction material composition in an amount of less than 7% by volume calculated on the total volume of the asbestos-free friction material composition.

[0111] In the method of the present invention, the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 with a rounded shape and the volume content of the carbon-based material may be selected in a ratio of 1:3 with respect to the total of the friction material composition.

[0112] Moreover, the carbon-based material may preferably be selected in the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene, and mixtures thereof.

[0113] Finally, the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material in the friction material composition may be selected in a ratio of 1:6, and the volume content of the at least one metal or mixture of metals and the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 and a rounded shape in the friction material composition may be selected in a ratio of 1:2. conclusion

[0114] From the above examples and disclosure, it is evident that friction material compositions prepared according to the present disclosure, particularly within the relative composition values ​​listed in Table 6, are significantly less susceptible to both stiction and creep groin phenomena than similar standard LS compositions while maintaining comparable or even better braking performance than the standard LS compositions, and the creep groin behavior is substantially comparable to that of NAO friction materials without the innovative materials, which further exhibits the notoriously poor braking performance of NAO materials compared to LS materials.

[0115] Thus, all objectives of the present disclosure are met. Certain terminology

[0116] Although certain braking devices, systems, and methods are disclosed in the context of certain illustrative embodiments, it will be understood by those skilled in the art that the scope of the disclosure extends beyond the specifically disclosed embodiments to the use of other alternative embodiments and / or embodiments, as well as certain modifications and equivalents thereof. Use with any structure is clearly within the scope of the invention. Various features and aspects of the disclosed embodiments can be combined with or substituted for one another to form various modes of assembly. The scope of the disclosure should not be limited by the specific disclosed embodiments described herein.

[0117] Conditional language such as "can," "could," "might," or "may," unless otherwise expressly stated or understood otherwise within the context as it is used, is generally intended to convey that a particular embodiment includes or does not include certain features, elements, and / or steps. Because of this, such conditional language is generally not intended to imply that features, elements, and / or steps are in some way required for one or more embodiments.

[0118] Unless otherwise indicated, the terms "approximately," "about," and "substantially," as used herein, refer to an amount close to the recited amount that still performs the desired function or achieves the desired result. For example, in some embodiments, as the context may dictate, the terms "approximately," "about," and "substantially" may refer to an amount within 10% or less of the recited amount. Similarly, the term "generally," as used herein, refers to a value, amount, or characteristic that primarily includes or tends toward a particular value, amount, or characteristic.

[0119] The present disclosure expressly contemplates that various features and aspects of the disclosed embodiments can be combined with or substituted for one another, and therefore the scope of the present disclosure should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the following claims along with their full scope of equivalents. The invention as originally claimed in the present application is set forth below. [1] Preferably, but not exclusively, an asbestos-free friction material composition designed to be formed into a friction block or layer for equipping a braking element such as a brake pad or brake shoe, having a copper content of less than 0.5% by mass, said asbestos-free friction material composition comprising as its constituent materials at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive having a Mohs hardness greater than 7, at least one soft abrasive having a Mohs hardness less than 7, a carbon-based material, and optionally at least one metal or mixture of metals, in combination with: The hard abrasive having a Mohs hardness of more than i.7 is exclusively composed of hard abrasives having a rounded shape; ii. the carbon-based material is present in the asbestos-free friction material composition in an amount of less than 22% (by volume) vol, calculated on the total volume of the asbestos-free friction material composition; iii. at least one of the metals or mixture of metals does not consist of copper and / or copper alloys, except for impurities, and is contained in the asbestos-free friction material composition in an amount of less than 7% by volume, calculated on the total volume of the asbestos-free friction material composition; 1. An asbestos-free friction material composition comprising: [2] The asbestos-free friction material composition according to [1], wherein the at least one lubricant comprises a sulfide-based lubricant selected from the group consisting of metal sulfides of Sn, Zn, Fe, Mo, and mixtures thereof, and the sulfide-based lubricant is contained in the asbestos-free friction material composition in an amount of 6% vol. to 18% vol. calculated based on a total volume of the asbestos-free friction material composition. [3] The asbestos-free friction material composition according to [1] or [2], characterized in that the at least one soft abrasive having a Mohs hardness of less than 7 is contained in the asbestos-free friction material composition in an amount of 26% to 38% by volume calculated on the total volume of the asbestos-free friction material composition, and the soft abrasive is preferably selected from the group consisting of magnesia, chromite, zirconia, magnetite, hematite, quartz, zinc oxide, tin oxide, barium sulfate, silicates, fluorides, and any mixture thereof. [4] The hard abrasive having a rounded shape and a Mohs hardness greater than 7 includes, in particular, abrasive particles having a ratio R / S (roundness / sphericity) within the following interval: 0.6≦R / S≦0.8 The asbestos-free friction material composition according to any one of [1] to [3], wherein R and S are calculated according to Krumbein and Sloss (1963). [5] The asbestos-free friction material composition according to [4], wherein the hard abrasive having a rounded shape and a Mohs hardness of greater than 7 is selected from the group consisting of alumina, corundum, silicon carbide, tungsten carbide, zirconium carbide, zirconium silicate, boron nitride, and any mixture thereof. [6] The asbestos-free friction material composition according to any one of [1] to [5], wherein the at least one organic binder is any one of a phenolic resin, an epoxy resin, a silicone resin, a modified phenolic resin, a melamine resin, a polyimide resin, and mixtures thereof, and the organic binder is present in the asbestos-free friction material composition in an amount of 20% vol to 30% vol, calculated with respect to a total volume of the asbestos-free friction material composition. [7] The asbestos-free friction material composition according to any one of [1] to [6], characterized in that the at least one inorganic filler is present in the asbestos-free friction material composition in an amount of 8% vol to 20% vol calculated on the total volume of the asbestos-free friction material composition, and the at least one inorganic filler is preferably selected from the group consisting of mineral fibers, glass fibers, rock wool, phyllosilicates (mica, vermiculite, talc), titanates, inorganic hydroxides of Ca, Mg, K, and any mixture thereof. [8] The asbestos-free friction material composition according to any one of [1] to [7], characterized in that, except for impurities, it does not contain any hard abrasives having a Mohs hardness greater than 7, which have an angular shape as defined according to Krumbein and Sloss (1963), i.e., a particle size in which the ratio R / S (roundness / sphericity) is less than 0.6. [9] The asbestos-free friction material composition according to any one of [1] to [8], characterized in that the ratio between the volume content of the rounded, hard abrasives and the volume content of the soft abrasives is comprised between 8:100 and 35:100, preferably 1:6, and in combination, the ratio between the volume content of the lubricant and the volume content of the total of the abrasives (soft and hard, rounded) is comprised between 13:100 and 62:100, preferably 1:5.

[10] The asbestos-free friction material composition according to any one of [1] to [9], characterized in that the ratio between the volume content of the at least one hard abrasive having a rounded shape and a Mohs hardness greater than 7 and the volume content of the carbon-based material is comprised between 15:100 and 113:100, preferably 1:3, and the carbon-based material is preferably selected from the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene, and mixtures thereof.

[11] The asbestos-free friction material composition according to any one of [1] to

[10] , further comprising organic fibers selected from the group consisting of polyacrylic fibers, polyaramid fibers, aramid fibers, cellulose fibers, and any mixtures thereof, wherein the content of the organic fibers present in the asbestos-free friction material composition is selected as, for example, a portion of, the content of the organic binder.

[12] The asbestos-free friction material composition according to any one of [1] to

[11] , characterized in that the at least one metal is selected from the group consisting of iron, steel, stainless steel, tin, zinc, metal alloys except Cu alloys in powder or fiber form, steel fibers, stainless steel fibers, and the ratio between the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material is comprised between 5:100 and 88:100, preferably 1:6, in combination, the ratio between the volume content of the at least one metal or mixture of metals and the volume content of the at least one hard abrasive having a Mohs hardness of more than 7 with a rounded shape is comprised between 11:100 and 233:100, preferably 1:2.

[13] A brake pad comprising a friction material block made of the asbestos-free friction material composition according to any one of [1] to

[12] .

[14] A method for manufacturing a brake pad for a vehicle having reduced or zero tendency towards both stiction and creep groan, the method comprising the steps of preparing an asbestos-free friction material composition having as its constituent ingredients at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive having a Mohs hardness greater than 7, at least one soft abrasive having a Mohs hardness less than 7, a carbon-based material, and optionally at least one metal or mixture of metals; shaping the asbestos-free friction material composition into a friction material block or friction layer applied onto a metal support, and curing the friction material block or friction layer so obtained, i. the hard abrasives having a Mohs hardness greater than 7 contained in the asbestos-free friction material composition consist exclusively of hard abrasives having rounded shapes; ii. the carbon-based material present in the asbestos-free friction material composition is maintained in an amount of less than 22% (by volume) vol, calculated on a total volume of the asbestos-free friction material composition; iii. The method of claim 1, wherein the at least one metal or mixture of metals contained in the asbestos-free friction material composition is selected from the group consisting of iron, steel, stainless steel, tin, zinc, metal alloys except Cu alloys in powder or fiber form, steel fibers, stainless steel fibers, and mixtures thereof, and is present in the asbestos-free friction material composition in an amount of less than 7% by volume calculated on the total volume of the asbestos-free friction material composition.

[15] The method according to

[14] , wherein the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 and having a rounded shape and the volume content of the carbon-based material are selected in the asbestos-free friction material composition in a ratio comprised between 15:100 and 113:100, preferably in a ratio of 1:3, the carbon-based material being preferably selected in the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene and mixtures thereof, the volume content of the at least one metal or mixture of metals in the asbestos-free friction material composition and the volume content of the carbon-based material are selected in a ratio comprised between 5:100 and 88:100, preferably in a ratio of 1:6, and the volume content of the at least one metal or mixture of metals in the asbestos-free friction material composition and the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 and having a rounded shape and having a Mohs hardness greater than 7 are selected in a ratio comprised between 11:100 and 233:100, preferably in a ratio of 1:2.

Claims

1. An asbestos-free friction material composition designed to be formed into a friction block or layer for equipping a braking element and having a copper content of less than 0.5% by mass, said asbestos-free friction material composition comprising as its constituent materials at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive having a Mohs hardness greater than 7, at least one soft abrasive having a Mohs hardness less than 7, a carbon-based material, and optionally at least one metal or mixture of metals; i. the hard abrasive having a Mohs hardness greater than 7 consists exclusively of hard abrasives having a rounded shape; ii. the carbon-based material is present in the asbestos-free friction material composition in an amount of less than 22% (by volume) calculated on the total volume of the asbestos-free friction material composition; iii. the at least one metal or mixture of metals does not consist of copper and / or copper alloys, except for impurities, and is contained in the asbestos-free friction material composition in an amount of less than 7% by volume, calculated based on the total volume of the asbestos-free friction material composition; iv. The hard abrasive having a Mohs hardness greater than 7 and a rounded shape comprises only abrasive particles having a ratio R / S (roundness / sphericity) within the following interval: 0.6≦R / S≦0.8 R and S are calculated according to Krumbein and Sloss (1963).

1. An asbestos-free friction material composition comprising:

2. 2. The asbestos-free friction material composition of claim 1, wherein the at least one lubricant comprises a sulfide-based lubricant selected from the group consisting of metal sulfides of Sn, Zn, Fe, Mo, and mixtures thereof, and the sulfide-based lubricant is contained in the asbestos-free friction material composition in an amount comprised between 6% vol and 18% vol, calculated with respect to a total volume of the asbestos-free friction material composition.

3. 2. The asbestos-free friction material composition of claim 1, wherein the at least one soft abrasive having a Mohs hardness of less than 7 is contained in the asbestos-free friction material composition in an amount comprised between 26% vol and 38% vol calculated with respect to the total volume of the asbestos-free friction material composition, and the soft abrasive is selected from the group consisting of magnesia, chromite, zirconia, magnetite, hematite, quartz, zinc oxide, tin oxide, barium sulfate, silicates, fluorides, and any mixture thereof.

4. 2. The asbestos-free friction material composition of claim 1, wherein said hard abrasive having a rounded shape and a Mohs hardness greater than 7 is selected from the group consisting of alumina, corundum, silicon carbide, tungsten carbide, zirconium carbide, zirconium silicate, boron nitride, and any mixture thereof.

5. 2. The asbestos-free friction material composition of claim 1, wherein the at least one organic binder comprises any one of a phenolic resin, an epoxy resin, a silicone resin, a modified phenolic resin, a melamine resin, a polyimide resin, and mixtures thereof, and the organic binder is present in the asbestos-free friction material composition in an amount comprised between 20% vol and 30% vol calculated with respect to a total volume of the asbestos-free friction material composition.

6. 2. The asbestos-free friction material composition of claim 1, wherein the at least one inorganic filler is present in the asbestos-free friction material composition in an amount comprised between 8% vol and 20% vol calculated with respect to the total volume of the asbestos-free friction material composition, and the at least one inorganic filler is selected in the group consisting of mineral fibers, glass fibers, rock wool, phyllosilicates (mica, vermiculite, talc), titanates, inorganic hydroxides of Ca, Mg, K, and any mixture thereof.

7. 2. The asbestos-free friction material composition according to claim 1, characterized in that it does not contain, except for impurities, any hard abrasives with a Mohs hardness greater than 7 having an angular shape as defined according to Krumbein and Sloss (1963), i.e. a particle size with a ratio R / S (roundness / sphericity) of less than 0.

6.

8. 2. The asbestos-free friction material composition of claim 1, characterized in that the ratio between the volume content of the rounded, hard abrasives and the volume content of the soft abrasives is comprised between 8:100 and 35:100, and in combination, the ratio between the volume content of the lubricant and the volume content of the total of the abrasives (soft and hard, rounded) is comprised between 13:100 and 62:

100.

9. The asbestos-free friction material composition of claim 8, wherein the ratio between the volume content of the rounded hard abrasive and the volume content of the soft abrasive is comprised between 1:

5.

10. 2. The asbestos-free friction material composition of claim 1, characterized in that the ratio between the volume content of at least one hard abrasive having a rounded shape and a Mohs hardness greater than 7 and the volume content of the carbon-based material is comprised between 15:100 and 113:100, and the carbon-based material is selected in the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene, and mixtures thereof.

11. 2. The asbestos-free friction material composition of claim 1, further comprising organic fibers selected from the group consisting of polyacrylic fibers, polyaramid fibers, aramid fibers, cellulose fibers, and any mixture thereof, wherein the content of organic fibers present in the asbestos-free friction material composition is selected as a fraction of the content of the organic binder.

12. 2. The asbestos-free friction material composition according to claim 1, characterized in that the at least one metal is selected in the group consisting of iron, steel, stainless steel, tin, zinc, metal alloys except Cu alloys in powder or fiber form, steel fibers, stainless steel fibers, and the ratio between the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material is comprised between 5:100 and 88:100, and in combination, the ratio between the volume content of the at least one metal or mixture of metals and the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 with a rounded shape is comprised between 11:100 and 233:

100.

13. A brake pad comprising a friction material block made of the asbestos-free friction material composition of any one of claims 1 to 12.

14. 1. A method for manufacturing a brake pad for a vehicle having reduced or zero tendency towards both stiction and creep groan, the method comprising the steps of preparing an asbestos-free friction material composition having as its constituent ingredients at least one organic binder, at least one inorganic filler, at least one lubricant, at least one hard abrasive having a Mohs hardness greater than 7, at least one soft abrasive having a Mohs hardness less than 7, a carbon-based material, and optionally at least one metal or mixture of metals; shaping the asbestos-free friction material composition into a friction material block or friction layer applied onto a metal substrate and curing the friction material block or friction layer so obtained, the method comprising the steps of: i. the hard abrasives having a Mohs hardness greater than 7 contained in the asbestos-free friction material composition consist exclusively of hard abrasives having a rounded shape; ii. the carbon-based material present in the asbestos-free friction material composition is maintained in an amount of less than 22% (by volume) vol. calculated on the total volume of the asbestos-free friction material composition; The at least one metal or mixture of metals contained in the asbestos-free friction material composition is selected from the group consisting of iron, steel, stainless steel, tin, zinc, metal alloys except Cu alloys in powder or fiber form, steel fibers, stainless steel fibers, and mixtures thereof, and is contained in the asbestos-free friction material composition in an amount of less than 7% by volume calculated based on the total volume of the asbestos-free friction material composition; iv. The hard abrasive having a Mohs hardness greater than 7 and a rounded shape comprises only abrasive particles having a ratio R / S (roundness / sphericity) within the following interval: 0.6≦R / S≦0.8 R and S are calculated according to Krumbein and Sloss (1963); A method comprising:

15. 15. The method of claim 14, wherein the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 and a rounded shape and the volume content of the carbon-based material are selected in the asbestos-free friction material composition in a ratio of 15:100 to 113:100, the carbon-based material being selected from the group consisting of graphite, graphitized coke, petroleum coke, desulfurized petroleum coke, carbon black, graphene, and mixtures thereof, the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material in the asbestos-free friction material composition are selected in a ratio of 5:100 to 88:100, and the volume content of the at least one metal or mixture of metals and the volume content of the at least one hard abrasive having a Mohs hardness greater than 7 and a rounded shape are selected in a ratio of 11:100 to 233:

100.

16. The method of claim 15, wherein the volume content of the at least one metal or mixture of metals and the volume content of the carbon-based material in the friction material composition are selected in a ratio consisting of 1:6.

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