BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a friction pair that exhibits satisfactory wear resistance, noise properties, vibration properties and braking properties.

2. Description of the Related Art

[0002] The brake pads, brake linings, clutch linings and ^* other friction materials used in industrial machinery, railroad cars, cargo vehicles, automobiles and the like need to have high reliability and increasingly high performance in order to ensure safety. More specifically, because friction materials convert kinetic energy to heat through friction, they must have sufficient heat resistance against the frictional heat generated during braking. In addition, from the viewpoint of running stability, the friction materials must also demonstrate frictional properties that remain constant under variations in temperature and weather conditions, have superior wear resistance with little changes in properties over long period of time, and not generate noise, such as squealing during braking or vehicle vibrations. In particular, noise and vibrations attributable to frictional vibration of the friction material are considered to be an important technical issue from the viewpoint of product γalue and quietness of vehicles.

[0003] In order to satisfy these requirements, friction materials are generally formed by combining several types of components. For example, various combinations of fiber base materials for retaining the shape of the friction material, binders that bind components such as fiber base materials, and fillers for adjusting various properties of friction materials (such as adjustment and stabilization of wear resistance, heat resistance or friction coefficient) are used. Friction materials are produced by curing a raw material mixture, which is obtained by mixing these components with a mixer, with hot-pressing followed by molding, grinding as necessary and sizing. Hard particles of high hardness and highly effective in increasing the friction coefficient of the friction material are blended into the friction material in order to enhance the braking properties of the friction material. Although braking properties may be enhanced by increasing the amount of hard particles, the counterpart material may suffer wear by the hard particles. Local wear caused by the hard particles result in uneven wear of the friction surface of the counterpart material, ^' while wear debris from the worn counterpart material may remain on the friction surface, thereby exacerbating wear of the friction surfaces of the friction pair. As a result, the friction material may have reduced wear resistance, while noise and vibration are more likely to occur. Thus, it becomes difficult to preserve high braking properties in a friction material while ensuring excellent wear resistance, noise properties and vibration properties.

[0004] In order to solve the above problems, for instance Japanese Patent Application Publication No. 2003-268352 (JP-A-2003-268352) describes a friction material containing substrate fibers, a binder and a friction modifier. The friction modifier contains a highly elastic abrasive material composed of porous hard particles and a highly elastic substance that is fixed in the pores of the porous hard particles.

[0005] The invention described in JP-A-2003-268352 relates to one of the friction materials that makes up a friction pair. However, the braking properties and the wear resistance of a friction material are determined by the material combination in the friction pair that constitutes the friction surface. It is therefore difficult to obtain a friction pair that can fully deliver the intended performance by improving only the properties of one of the friction materials that make up the friction pair. Specifically, a friction material having excellent wear resistance, noise properties, vibration properties and braking properties can be obtained through material design focusing on the friction surfaces of both a first friction material and a second friction material that make up the friction pair. Design of only one of the friction materials necessitates separate design of a counterpart friction material that conforms to the foregoing friction material, in a process that requires trial and error in terms of repeated prototyping and evaluation. Therefore, joint design of the friction materials that make up a friction pair would allow cutting costs and time in the development of a friction pair, and would allow predicting the resulting gains in the performance of the friction pair. SUMMARY OF THE INVENTION

[0006] The invention provides to a friction pair that possesses wear resistance, noise properties, vibration properties and braking properties.

[0007] A friction pair according a first aspect of the invention generates frictional force through mutual frictional sliding, and includes: a first friction material that contains first hard particles and a resin with a lower Mohs hardness than the first hard particles, wherein the resin surrounds the entire surface of the first hard particles; and a second friction material that contains second hard particles and either a metallic material or an inorganic material with a lower Mohs hardness than the second hard particles and a higher Mohs hardness than the resin, wherein the metallic material br an inorganic material forms a matrix of the second friction material in which the second hard particles are embedded.

[0008] The friction pair possesses a stress distribution mechanism whereby even if stress acts ^" on the friction pair, the stress applied to the first hard particles or the second hard particles is sufficiently absorbed by the resin, the metallic material or the inorganic material. As a result, the first hard particles or the second hard particles are less likely to be subjected to stress beyond yield stress. Therefore, the friction pair can generate high frictional force, by way of the first hard particles and the second hard particles, while achieving excellent frictional force stability and suppression of noise and vibration.

[0009] In the above friction pair, the first friction material may further contain an elastic material that forms a matrix of the first friction material in which resin-coated hard particles in which the first hard particles are coated by the resin are embedded. The friction pair may also satisfy at least one of following first to fourth conditions:

1. the first friction material further contains first inorganic particles having a Mohs hardness lower than the first hard particles, and a first ratio τjt _t of the mean diameter r ₀ of the resin-coated hard particles relative to the mean diameter r _f of the first inorganic particles is equal to at least 0.2;

2. the second friction material further contains second inorganic particles having a Mohs hardness lower than the second hard particles, and a second ratio RA/R _F of the mean diameter RA of the second hard particles relative to the mean diameter R _F of the second inorganic particles is equal to at least 0.2;

3. the first friction material satisfies formula (1), below; and

4. the first friction material and the second friction material satisfy formula (2), below. r 'E S ^≤ 2{E _m -E _b ) ^foimula W

In formula (1), s is the average coating thickness of the resin in the resin-coated hard particles, E _b is the elastic modulus of the resin, E _m is the elastic modulus of the elastic material, and r _a is the mean diameter of the first hard particles, such that Em>E _t ,.

I °n formula) (2), C _a } is thτe conc ^≤ en ⁵ tration (vol%) o ² f ⁾ the first hard particles in the first friction material, CA is the concentration (vol%) of the second hard particles in the second friction material, r _a is the mean diameter of the first hard particles in the first friction material, R _A is the mean diameter of the second hard particles in the second friction material, σi is the yield stress of the first friction material, and σ ₂ is the yield stress of the second friction material, such that σi=10 to 100 MPa, σ ₂ =100 to SOO MPa and C _A =0.1 to 95vol%.

[0010] In a friction pair having the above features, the elastic material in the first friction material ensures that the entire friction surfaces of the friction materials contact each other. This affords to generate friction force between the entire friction surfaces. When the friction pair having the above features satisfies the first condition, the first ratio r^/rf is equal to at least 0.2. As a result, compressive stress generated in the first friction material is exerted also on the resin-coated hard particles, whereby stress is transmitted uniformly throughout the first friction material, even if the resin-coated hard particles are disposed at the center of a close-packed structure formed by four first inorganic particles. If the friction pair having the above features satisfies the second condition, the second ratio RA/R _F is equal to at least 0.2. As a result, compressive stress generated in the second friction material is exerted also on the hard particles, whereby stress is transmitted uniformly throughout the second friction material, even if the second hard particles are disposed at the center of a close-packed structure formed by four second inorganic particles. When the friction pair having the above features further satisfies the third condition, stress is concentrated in the first hard particles by setting an appropriate coating thickness s. Also, Occurrence of yield in first hard particles and the second hard particles may be suppressed when the friction pair having the above features further satisfies the fourth condition.

[0011} The friction pair may also satisfy all the first to fourth conditions,

[0012] In the above friction pair, at least one of the first hard particles and the second hard particles may have a Mohs hardness of at least 4.5.

[0013] A friction pair having the above features can generate even higher frictional forces while achieving excellent frictional force stability and suppression of noise and vibration. A friction pair having the above features uses hard particles that have a high yield stress. As a result, yield do not occur in the hard particles readily, because stress acting on the first and second friction materials elicits only displacement of the components that surround the hard particles at the outermost surface of the materials. This reduces wear and improves wear resistance of the friction materials. ^

[0014] Above friction pair may satisfy the third condition, and the elastic modulus E _b of the resin may be at least 1 GPa.

[0015] In the above-described friction pair a resin having a sufficient elastic modulus prevents the first hard particles from sloughing off from the first friction material when the first friction material is abraded. In addition, the friction pair achieves more adequate stress to which the first hard particles are subjected and more adequate amount of protrusion of the first hard particles from the first friction surface. The friction pair also suppresses occurrence of yield in the first hard particles.

[0016] In the above friction pair, the resin may contain at least one of non-crystalline resin selected from the group consisting of polyimides, polyamideimides, polycarbonates, polyphenylene ether, polyallylates, polysulfones and polyether sulfones.

[0017] The selection of an appropriate non-crystalline resin yields even more appropriate stress to which the first hard particles are subjected and even more adequate amount of protrusion of the first hard particles from the first friction surface. [0018] Above friction pair may satisfy at least one of the first condition and the second condition, and at least one of the first inorganic particles and the second inorganic particles may have a Mohs hardness that does not exceed 4.

[0019] The friction pair having the above features uses at least one of the first inorganic particles and the second inorganic particles having low yield stress. Therefore, at least one of the first hard particles and the second hard particles in the resin-coated hard particles do not fracture when the first and the second friction materials are subjected to stress. This allows reducing wear, and achieving a desired wear resistance, in the friction materials.

[0020] Above friction pair may satisfy the third condition, and the elastic modulus E ₀₁ of the elastic material may be at least 1 GPa.

[0021] In the friction pair having the above features, an elastic material having a sufficient elastic modulus achieves more adequate stress to which the first hard particles are subjected, and more adequate amount of protrusion of the first hard particles from the first friction surface.

[0022] The elastic material may contain at least one of resin selected from the group consisting of phenolic resins, modified phenolic resins, amino resins, furan resins, unsaturated polyester resins, diallylphthalate resins, alkyd resins, epoxy resins, thermosetting polyamideimide resins, thermosetting polyimide resins and silicone resins.

[0023] In the friction pair having the above features, selecting an appropriate elastic material achieves more adequate stress to which the first hard particles are subjected, and more adequate amount of protrusion of the first hard particles from the first friction surface.

[0024] Above friction pair may satisfy at least one of the first condition and the second condition, and at least one of the first ratio rjx _t or the second ratio R _A /R- _F ∞ay be at least 0.3. Alternatively, both the first condition and the second condition may be satisfied, and both the first ratio and the second ratio may be equal to or above 0.3.

[0025] The friction pair having the above features affords at least one of the following effects. With a first ratio equal to at least 0.3, stress may be generated more reliably in the resin-coated hard particles when a compressive stress in the first friction material occurs, even if the resin-coated hard particles are disposed at the center of a close-packed structure formed by four first inorganic particles. With a second ratio of at least 0.3 stress may be generated more reliably in the second hard particles when a compressive stress in the second friction material occurs, even if the second hard particles are disposed at the center of a close-packed structure formed by four second inorganic particles.

[0026] The first friction material may contain a total of at least 5 vol% of the resin-coated hard particles and the elastic material, and the volume ratio of the resin-coated hard particles to the elastic material may range from 2:1 to 1:50.

[0027] In the friction pair having the above features, excellent factional force stability and noise and vibration suppression may be achieved when the first friction material includes the resin-coated hard particles and the elastic material in more adequate amounts. In the above-described friction pair, moreover, excellent frictional force stability and noise and vibration suppression may be achieved if the volume ratio of the resin-coated hard particles and the elastic material fall within an appropriate range.

[0028] In the above friction pair, the elastic modulus of the first friction material may be 100 to 300 MPa.

[0029] The friction pair having the above features, prevents the first hard particles from sloughing off from the first friction material under abrasion. In addition, the friction pair achieves more adequate stress to which the first hard particles are subjected, and more adequate amount of protrusion of the first hard particles from the first friction surface. The friction pair having the above features also suppresses occurrence of yield in the first hard particles.

[0030] In the above friction pair, the surface roughness of the friction surface of the second friction material may be not greater than 10 μrn.

[0031] The friction pair having the above features affords good initial break-in,, and allows suppressing variation of frictional force and wear increases. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The features, advantages, and technical and industrial significance of this invention will be described in the following detailed description of example embodiments of the invention with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a schematic diagram illustrating resin-coated hard particles that occupy the voids in a close-packed structure formed by inorganic particles in a first friction material;

FIG 2 is a graph illustrating schematically a range defined by condition (4), wherein the X-axis represents (TJR _A ) ² and the Y-axis represents (C _B /C _A );

FIG 3 is a diagram illustrating a typical example of a friction pair of the invention;

FIGS. 4A and 4B are cross-sectional schematic diagrams illustrating typical examples of a friction pair before and after stress application; and

FIG 5 is a cross-sectional schematic diagram illustrating comparatively situations before and after occurrence of strain in resin-coated hard particles (c) at a friction surface portion, and in an elastic material (m) that makes up the matrix of a friction material, when stress acts on the friction material.

DETAILED DESCRIPTION OF EMBODIMENTS

[0033] A friction pair according to a first aspect of the invention generates factional force through mutual frictional sliding, and includes a first friction material, which contains hard particles (a) and a resin (b) that surrounds the entire surface of the hard particles (a); and a second friction material, which contains hard particles (A) and either a metallic material or an inorganic material (M) that forms a matrix of the second friction material in which the hard particles (A) are embedded. The resin (b) of the first friction material has a lower Mohs hardness than the hard particles (a), and the metallic or inorganic material (M) of the second friction material has a lower Mohs hardness than the hard particles (A) and higher Mohs hardness than the resin (b).

[0034] In an example of a friction pair that specifically implements the first aspect of the invention, the first friction material preferably contains resin-coated hard particles (c) in which the resin (b) coats the hard particles (a), and an elastic material (m) that forms the matrix of the first friction material in which the resin coated particles are embedded; and the second friction material contains the hard particles (A) and either the metallic material or the inorganic material (M) (hereinafter, "matrix material") that forms the matrix of the second friction material, wherein the friction pair satisfies at least one of fllowing conditions (1) to (4).

(1) The first friction material further contains inorganic particles (f) having a Mohs hardness lower than the hard particles (a), and a ratio r _c /r _f of the mean diameter r ₀ of the resin-coated hard particles (c) relative to the mean diameter r _f of the inorganic particles (f) is equal to at least 0.2.

(2) The second friction material further contains inorganic particles (F) having a Mohs hardness lower than the hard particles (F), and a ratio IWRF of the mean diameter R _A of the hard particles (A) relative to the mean diameter R _F of the inorganic particles (F) is equal to at least 0.2.

(3) The first friction material satisfies formula (1), below.

(4) the first friction material and the second friction material satisfy formula (2), below.

s ^≤ w^k> ^formula(1)

•In formula (1), s is the average coating thickness of the resin (b) in the resin-coated hard particles (c), E _b is the elastic modulus of the resin (b), E _m is the elastic modulus of the elastic material (m), and r _a is the mean diameter of the hard particles (a), such that E _m >E _b .

I °n- ² fo ^≤ rmula (2), C _a is thte con ^≤ ce ⁵ ntration (vol%) ² o>f the hard particles (a) in the first friction material, CA is the concentration (vol%) of the hard particles (A) in the second friction material, r _a is the mean diameter of the hard particles (a) in the first friction material, R _A is the mean diameter of the hard particles (A) in the second friction material, σj is the yield stress of the first friction material, and σ ₂ is the yield stress of the second friction material, such that σi=l 0 to 100 MPa, σ ₂ =100 to 800 MPa and C _A =0.1 to 95vol%. [0035] The friction pair includes a first friction material and a second friction material. Specific applications of the friction pair include, for example, disc brakes, wherein the friction pair has a pad as the first friction material and a rotor as the second friction material.

[0036] The "Mohs hardness" is a typical hardness index for expressing the hardness of mineral resources, and ranges from 10 for diamond to 1 for talc.

[0037] The hardness and hardness ratios of the hard particles in the friction materials, the mean diameter and mean diameter ratios of the hard particles, and the addition amounts and addition amount ratios of the hard particles have not been approached scientifically in conventional friction pairs. Therefore, the problems of wear resistance and noise and vibration suppression have persisted regardless of the frictional force of the friction material. Conversely, a friction pair may be obtained that fails to elicit a high frictional frictional force, regardless of how high the wear resistance and noise and vibration suppressing effect of the friction pair may be. It has been thus difficult simultaneously achieve wear resistance and noise and vibration suppression while preserving a high frictional frictional force.

[0038] Hard particles are an important component for increasing the friction coefficient of the friction surface and for ensuring the braking properties of the friction material. The hard particles have high yield stress, and hence yield does not occur readily in the hard particles when the friction material is subjected to stress.

[0039] The hardness of the hard particles may cause excessive wear in the counterpart material. If the counterpart material also contains hard particles, the hard particle in one of the friction material are rubbed against the hard particles and components other than the hard particles in the counterpart material, alternately and intermittently. This may cause friction variation of the friction pair. Or, friction between the hard particles may break the hard particles in at least one of the materials. The broken hard particles remaining in the friction surface may in turn promote wear in at least one of the friction material and the counterpart material.

[0040] Therefore, merely developing a friction material on its own, as in conventional friction materials, is not sufficient in order to achieve simultaneously wear resistance and noise and vibration suppression while preserving a high frictional force. Instead, friction materials must be designed as combinations of materials in a friction pair, and the component ratios of hard particles and so forth in the respective friction materials must be controlled.

[0041] A friction pair according to a first aspect of the invention has a first friction material and a second friction material, having each hard particles and an elastic material other than hard particles. In the friction pair, moreover, a stress distribution mechanism and a material design method have been established to prevent the hard particles from being subjected to stress beyond yield stress.

[0042] Establishing such a stress distribution mechanism and material design method confines local breaks in the elastic material of the friction material, other than the hard particles, to strain the outermost surface layer of the friction material, thereby making local breaks less likely to propagate into the friction material. Thus, suitable wear resistance and noise and vibration suppression may be achieved while preserving a high frictional force.

[0043] Specifically, the above stress distribution mechanism is a dynamic mechanism that amount of protrusion of the hard particles from the friction surface is sufficiently reduced by the distortion of elastic material surrounding hard particles in each friction material. If hard particles protrude from the friction surface, friction between hard particles takes place not only between tops of the protruded portions in the first and second friction materials, but also at areas other than the top of the of the protruded portions. Therefore, substantial wear cannot be avoided, no matter how resistant to wear the hard particles may be assumed to be. The above-described stress distribution mechanism solves this problem. The described mechanism controls the stress that acts on the respective hard particles in the first and second friction materials, so that the hard particles are not subjected to stress beyond yield stress, and so that the hard particles rub against one another only at the top of the protruded portion. As a result it becomes possible to simultaneously improve wear resistance and noise and vibration suppression while preserving a high fπctional force.

[0044] Specifically, the above material design method is a scientific method that involves adjusting the mean diameters, blending amounts, elastic moduli and so forth of the hard particles and elastic materials other than the hard particles, as well as other materials, that are present in the respective friction materials. Although the above-described stress distribution mechanism solves the above- described problems, the mechanism by itself involves trial and error in terms of repeated prototyping and evaluation of friction pairs, while the gain in performance afforded by the mechanism is difficult to predict. Developing a friction pair with an optimal combination of friction materials becomes then a time-consuming endeavor. The development time of friction pairs can be shortened, and the gain in friction pair performance can be predicted, by scientifically designing various parameters, such as mean diameters, blending amounts, elastic moduli and so forth, of the respective materials present in the first and second friction materials.

[0045] In a concrete example of a friction pair provided with the above-described stress distribution mechanism and designed in .accordance with the above-described material design method, the friction pair includes a first friction material that contains resin-coated hard particles (c) in which hard particles (a) are coated with a resin (b), and an elastic material (m) other than the resin-coated hard particles, that forms the matrix of the first friction material; and a second friction material that contains hard particles (A) and matrix material (M), wherein the friction pair satisfies at least one of conditions (1) to (4) described below The hard particles (A) in the second friction material may also be coated with resin.

[0046] In condition (1), "the first friction material further contains inorganic particles (f) having lower Mohs hardness than the hard particles (a), and a ratio (T ₀ ZTt) of the mean diameter r _c of the resin-coated hard particles (c) relative to the mean diameter rf of the inorganic particles (f) is not below 0.2". In condition (2), "the second friction material further contains inorganic particles (F) having lower Mohs hardness than the hard particles (A) ₇ and a ratio (RA/R _F ) of the mean diameter R _A of the hard particles (A) relative to the mean diameter RF of the inorganic particles (F) is not below 0.2".

[0047] FIG 1 is a schematic diagram illustrating a resjn-coated hard particle's that occupy the voids in a close-packed structure formed by inorganic particles in a first friction material. Inorganic particles (f) 61, drawn in solid lines and inorganic particle (f) 62, drawn in dotted lines and lying closer to the front of the paper than the former, form a close-packed structure. The resin-coated hard particle (c) 63 occupy the voids in the close-packed structure. The inorganic particles 61 and 62 and the resin-coated hard particle 63 are assumed to be spherical. In FIG. 1, the inorganic particle 62 is depicted in a see-through fashion in order to show the resin-coated hard particle 63.

[0048] When the resin-coated hard particles 63 fit exactly in the voids of the close-packed structure (tetrahedral voids), the diameter of the resin-coated hard particles 63 is 0.225r _f ∞0,2r _f , wherein r _f is the diameter of the inorganic particles (f) 61 and 62. Therefore, if the resin-coated hard particles 63 have a diameter equal to or greater than the above, i.e. when the ratio r _c relative to r _f (r _o /r _f ) is equal to or greater than 0.2, where r _c is the diameter of the resin-coated hard particles (c), any compressive stress generated in the first friction material is exerted not only on the inorganic particles (f) but also on the resin-coated hard particles (c), whereby the stress can be transmitted uniformly throughout the first friction material, even if the resin-coated hard particles (c) are disposed at the center of a close-packed structure formed by four inorganic particles (f).

[0049] The relationship between inorganic particles (F) and hard particles (A) in the second friction material is similar. Specifically, when the ratio (R _A /RF) of RA relative to RF is equal to or greater than 0.2, where R _F is the mean diameter of the inorganic particles (F) and RA is the mean diameter of the hard particles (A), any compressive stress 'generated in the second friction material is exerted not only on the inorganic particles (F) but also on the hard particles (A), whereby stress can be transmitted uniformly throughout the second friction material, even if the hard particles (A) are disposed at the center of a close-packed structure formed by four inorganic particles (F).

[0050] In conditions (1) or (2), the inorganic particles (fj or (F) have an inorganic substance that functions as, for instance, a friction modifier of the respective friction material. Specific materials that can be used for the inorganic particles (f) or (F) may include, for instance, carbon, ceramics, oxides such as iron oxide or copper oxide, inorganic fillers such as barium sulfate or calcium carbonate, metal powders such as copper powder or brass powder, or a solid lubricant such as graphite or molybdenum disulfide.

[0051] In condition (1) the Mohs hardness of the inorganic particles (f) and inorganic particles (F) preferably does not exceed 4, which is sufficient to provide higher frictional forces while achieving excellent frictional force stability and suppression of noise and vibration. If inorganic particles having a Mohs hardness in excess of 4 are used in both the first and the second friction materials, the inorganic particles have high yield stress, and hence the hard particles (a, A) of at least one of the resin-coated hard particles or the second friction material may more likely to break if the first and the second friction materials are subjected to stress when the resin-coated hard particles (c) are in contact with the inorganic particles (f) and when the hard particles (A) are in contact with the inorganic particles (F). Wear increases as a result in each friction material, whereby the desired wear resistance may fail to be achieved. In particular, the Mohs hardness of at least one of the above inorganic particles more preferably does not exceed 3.5, and most preferably does not exceed 3.

[0052] Preferably, rjt _f in condition (1) is at least 0,3, and/or R _A /R _F in condition (2) is at least 0.3. A ratio ijτ _f of at least 0.3 allows the generation of stress more reliably in the resin-coated hard particles (c) when a compressive stress occurs in the first friction material, even if the resin-coated hard particles (c) are disposed at the center of a close-packed structure formed by four inorganic particles (f). A ratio RA/RF of at least 0.3 generates stress more reliably in the hard particles (A) when a compressive stress occurs in the above-described second friction material, even if the hard particles (A) are disposed at the center of a close-packed structure formed by four inorganic particles (F).

[0053] The mean diameter r _f of the inorganic particles (f), the mean diameter Rp of the inorganic particles (F), the mean diameter r _c of the resin-coated hard particles (c) and the mean diameter RA of the hard particles (A) may be measured, for instance, using laser diffraction or a scattering method (microtrack method).

[0054] In condition (3), the first friction material satisfies formula (1) below.

In formula (1), s is the average coating thickness of the resin (b) in the resin-coated hard particles (o), E _b is the elastic modulus of the resin (b), E _m is the elastic modulus of the elastic material (m), and r _a is the mean diameter of the hard particles (a), such that E _m >Et,.

[0055] FIG 5 is a cross-sectional schematic diagram illustrating comparatively the situations before and after occurrence of strain in the resin-coated hard particles (c) at the friction, surface portion, and in the elastic material (m) that forms the matrix of the friction material, when stress acts on the friction material. In the schematic diagram, the cross section of the resin-coated hard particles (c) is represented in the form of cutaways of small regions of great circles in the stress application direction. The cross section of the elastic material (m) is likewise represented as cutaways of a portion having the same length as the resin-coated hard particles (c). In order to emphasize strain, the cross sections are depicted as bands. In FIG. 5, an elastic material (m) 71m and resin-coated hard particles (c) 73c are placed on a friction surface 70, before the friction material is subjected to stress. The elastic material 71m and the particles 73c in FIG 5 are depicted with dotted lines. An elastic material (m) 72m and resin-coated hard particles (c) 74c are depicted to illustrate the situation after the elastic material 71m and the particles 73c are acted upon by stresses σ _m and σ _θ5 generated by friction, in the direction indicated by arrows in FIG. 5, In FIG. 5, the elastic material 72m and the particles 74c are depicted with solid lines. The particles 73c contain the hard particles (a) 73 a and the coating resin (b) 73 b, and the particles 74c contain the hard particles (a) 74a and the coating resin (b) 74b. As illustrated in FIG. 5, r _a is the mean diameter of the hard particles 73a and s is the average coating thickness of the coating resin 73b.

[0056] When a stress σ _c acts on the hard particles 74c, a stress σ _a acts on the hard particles 74a and a stress σ _b acts on the coating resin 74b. Because the resin-coated hard particles 74c, the hard particles 74a and the coating resin 74b are disposed coaxially, the stresses can be expressed as (formula (Ia)). In formula (Ia), ε _a and Sb denote the strain in the hard particles 74a and the coating resin 74b respectively, and E ₈ and E _b denote the elastic moduli of the hard particles 74a and the coating resin 74b respectively. Generally, Et _> «E _a , and hence it is assumed that ε _a =εb-(Eb/E _a )∞0. Therefore, the strain (the amount of displacement) % of the hard particles 74a is negligible in the resin-coated hard particles 74c, and hence it is possible to express ε _b as wherein ε _c represents the strain of the resin-coated hard particles 74c and γ represents the variation of the size of the resin-coated hard particles 74c.

[0057] The strain of the elastic material 72m can be represented as ε _m =γ/(r _a +2s)(formula (Ic)), assuming the mean diameter of the elastic material 72m and the mean diameter of the resin-coated hard particles 73c are substantially identical, as illustrated in FIG. 5. In formula (Ic), the variation of the size of the resin-coated hard particles 74c and the variation of the elastic material 72m are assumed to be substantially identical, as illustrated in FIG. 5.

[0058] The relationship σ _m ≤σ ₀ must hold true in order for stress concentrate in the hard particles 74a within the resin-coated hard particles 74c. According to formula (Ia) above, σb=σ _c is true, and hence the relationship σ _m <σ ₀ can be expressed as ε _m -E _m ≤εb-Eb (formula (Id)). Substituting formulas (Ib) and (Ic) in formula (Id), we obtain (E _m -γ)/(r _a +2s)<(E _b ^> γ)/(2s)(forrnula (Ie)). Transforming formula (Ie) ₅ we obtain formula (1) above, in which E _m >Eb.

[0059] In condition (3), Eb is preferably equal to or above 1 GPa, because a resin (b) having a sufficient elastic modulus prevents the hard particles (a) from sloughing off from the first friction material during friction, and achieves adequate stress to which the hard particles (a) are subjected, and adequate amount of protrusion of the hard particles (a) from the friction surface, which are result in suppression of occurring yield in the hard particles (a). IfEb is below 1 Gpa, the slough-preventing effect and the yield-suppressing effect on the hard particles (a) may be lost. In particular, E _b is preferably not below 2 GPa, and most preferably not below 3 GPa.

[0060] In condition (3), E _n , is preferably not below 1 GPa, because an elastic material (m) having sufficient elastic modulus results in adequate stress to which the hard particles (a) are subjected, and adequate amount of protrusion of the hard particles (a) from the friction surface. In particular, E _m is preferably not below 2 GPa, and most preferably not below 3 GPa.

[0061] The method for calculating the average coating thickness s may involve, for instance, determining the thickness of the coating resin by subtracting the diameter of the hard particles before resin coating from the diameter of the hard particles after resin coating. The method for measuring the diameter of the hard particles before and after resin coating may be, for instance, laser diffraction or a scattering method (microtrack method).

[0062] The elastic moduli E _b and E _m may be measured and calculated, for instance, through testing in accordance with the method which is defined by Japanese Industrial Standard JIS K 7181.

[0063] The method for measuring the mean diameter r _a of the hard particles (a) may be the same method used for measuring the mean diameter rf of the inorganic particles

(f).

[0064] In condition (4), the first friction material and the second friction material satisfy formula (2) below.

°- ^2S In formulla (2), C÷ ₃ is th ^≤ e ⁵ concentration ( ² v ⁾ ol%) of the hard particles (a) in the first friction material, C _A is the concentration (vol%) of the hard particles (A) in the second friction material, r _a is the mean diameter of the hard particles (a) in the first friction material, R _A is the mean diameter of the hard particles (A) in the second friction material, σi is the yield stress of the first friction material, and σ ₂ is the yield stress of the second friction material, such that to 800 MPa and C _Λ =0.1 to 95vol%.

[0065] Formula (2) is derived as follows. In the friction pair in the first embodiment of the invention, the factional forces occur mainly in the hard particles. When friction occurs between the first friction material and the second friction material, a frictional force F ₁ received by the first friction material from the second friction material becomes concentrated in the hard particles (a) at the outermost surface layer of the first friction material. The frictional force Fi must be withstood by the hard particles (a) in the outermost surface layer and by the components that surround the hard particles (a) (namely, the elastic material (m), resin (b) that coats the hard particles, and inorganic particles (f)). The first friction material includes the hard particles (a)' and the above-described surrounding components. Therefore, the yield stress on the material that supports the hard particles (a) is the yield stress σj of the first friction material. Accordingly, the frictional force F] may be expressed as Fiocσi-r _a ² -C _a (formula (2a)), wherein C _a is the concentration of the hard particles (a) in the first friction material, and r _a is the mean diameter of the hard particles (a) in the first friction material.

[0066] Similar to the case of the first friction material, a frictional force F ₂ received by the second friction material from the first friction material may be expressed as F2°CCT ₂ -RA ² -CA (formula (2b)), wherein CA is the concentration of hard particles (A) in the second friction material, RA is the mean diameter of the hard particles (A) in the second friction material, and σ ₂ is the yield stress of the second friction material. Although Fj should be ideally equal to F ₂ by virtue of the action-reaction law, formula (2) is obtained by assuming 0.2<(F ₂ /Fi)<5, when contact probability is factored in.

[0067] Besides satisfying formula (2), condition (4) requires also that to 100 MPa ₅ σ ₂ =100 to 800 MPa and CA=O.1 to 95vol%. Sufficient frictional force cannot be generated if the yield stress σj of the first friction material is below 10 MPa. If the σi value exceeds 100 MPa, the second friction material may suffer excessive wear. Preferably, σi is below 22 MPa, and more preferably not below 24 MPa. Also, σi preferably does not exceed 38 MPa, and more preferably does not exceed 36 MPa,

[0068] Sufficient frictional force cannot be generated if the yield stress σ ₂ of the second friction material is below 100 MPa. If the σ ₂ value exceeds 800 MPa, the first friction material may suffer excessive wear. Preferably, σ ₂ is not below 110 MPa, and more preferably not below 120 MPa. Also, σ ₂ preferably does not exceed 790 MPa, and more preferably does not exceed 780 MPa.

[0069] The yield stress σj of the first friction material and the yield stress σ ₂ of the second friction material may be measured and calculated by determining the yield stress based on testing in accordance with the above-mentioned method which is defined by JIS K 7181.

[0070] Sufficient fiictional force cannot be generated if the concentration CA of hard particles (A) in the second friction material is below 0.1vol%. If the value of CA exceeds 95vol%, the first friction material may suffer excessive wear. CA is preferably at least 0.15vol%, and more preferably at least 0.2vol%. Also, C _A preferably does not exceed 92.5vol%, and more preferably does not exceed 90vol%.

[0071] The inequality l≤fø/σO≤δO (formula (2c)) arises from the conditions CT ₁ =I 0 to 100 MPa and σ ₂ =100 to 800 MPa. The inequality of formula (2d) below can be derived from formula (2c) and formula (2).

0.2 ≤ -^y -S=- I < 400 formula (2d) CA K. ^R A J

[0072] FIG 2 is a graph illustrating schematically a range defined by condition (4), wherein the X-axis represents OVRA) ² and the Y-axis represents (C ₈ /CA). The range represented by formula (2d) is delimited by the curve (C _a /C _A Xr _a /R _A ) ^2:=: 0.2 and the curve (C _a /C _A )-(r _a /R _A ) ² =400. Because C _A =0.1 to 95vol%, the upper limit y! of (C ₈ ZCA) is set at yi( ⁼ C _a /0.1voI%), and the lower limit at y ₂ (=C _a /95vol%). The range represented by condition (4) is thus the area with oblique hatching in the graph.

[0073} In the friction pair of the first embodiment of the invention, preferably at least condition (1) is satisfied, from among conditions (1) to (4) above, more preferably at least conditions (1) and (3) are satisfied, and most preferably all conditions (1) to (4) are satisfied, from the viewpoint of the above-described stress distribution mechanism and material design method.

[0074] A detailed explanation follows next on the first friction material, the resin-coated hard particles (c) in the first friction material and in which the hard particles (a) are coated with the resin (b), and the elastic material (m) other than the resin-coated hard particles, that makes up the matrix of the first friction material. A detailed explanation follows next as well as on the second friction material, the hard particles (A) in the second friction material, and the matrix material (M) having lower Mohs hardness than the hard particles (A) and higher Mohs hardness than the resin (b), and in which the hard particles (A) are embedded.

[0075] The hard particles (a) and (A) used in the first friction material and the second friction material have high hardness and are made of a material that is the main agent responsible for the friction of the friction material. Specific examples of such a material is, for instance, ceramic materials. The ceramic materials include carbides such as silicon carbide, tungsten carbide, boron carbide, titanium carbide, zirconium carbide, tantalum carbide, iron carbide or chromium carbide; oxides such as alumina, zirconia, titania, chromia or silicon oxide; nitrides such as silicon nitride, titanium nitride, boron nitride or zirconium nitride; or boron compounds such as titanium boride or iron boride. In addition to the ceramic materials listed above, hard intermetallic compounds such as FeAl may also be used as the hard particles (a) and the hard particles (A). The hard particles (a) and (A) may both be the same material among those listed above. Alternatively, hard particles (a) and (A) may be different materials. Likewise, mixtures of two or more different materials from the above materials may be used for the hard particles (a) and the hard particles (A).

[0076] To increase ftictional force while achieving excellent firictional force stability and suppression of noise and vibration, the Mohs hardness of at least one from among the hard particles (a) and the hard particles (A) is preferably not below 4.5. The hard particles have low yield stress when using hard particles having a Mohs hardness below 4.5 in both the first and the second friction materials, as a result of which yield may be likelier to occur in the hard particles when both the first and second material are subjected to stress. Wear in each friction material itself increases as a result, which may preclude achieving the desired wear resistance. In particular, the Mohs hardness of at least one of the above hard particles is preferably not below 4.75, and most preferably not below 5.

[0077] The resin (b) used in the first friction material is an elastic material having an appropriate elastic modulus. The resin (b) is mainly responsible for sufficiently reducing the amount of protrusion of the hard particles (a) from the friction surface by being distorted during friction. Specifically, the resin (b) is preferably at least one selected from non-crystalline resin from among polyimid.es, polyamideimides, polycarbonates, poly phenylene ether, polyallylates, polysulfones and polyether sulfones. By selecting an appropriate non-crystalline resin, results in more adequate stress to which the hard particles (a) are subjected, and more adequate amount of protrusion of the hard particles (a) from the friction surface during friction, and better suppression of yielding of the hard particles. The resin (b) may be a mixture of two or more different materials from among the above materials.

[0078] The resin-coated hard particles (c) used in the first friction material are completed through total coating of the hard particles (a) by the resin (b). By coating the hard particles with an elastic resin, the resin to fulfill its function of sufficiently reducing the amount of protrusion of the hard particles from the friction surface, while also allowing the hard particles to elicit sufficiently high frictional force, unlike in the case where the hard particles are not coated by the elastic resin.

[0079] The method for coating the hard particles with the elastic resin may be, for instance, a dipping method wherein the hard particles are coated with the elastic resin through immersion, a coating method in which the elastic resin is coated, in the form of a spray or the like, onto the hard particles; a method in which the coating resin and the hard particles are mechanically kneaded together into pellets; or a method wherein the coating resin is formed as a fluid layer and the hard particles, heated to a temperature equal to or higher than the softening point of the resin, are fed into the fluid layer. The optimal coating method may be selected in accordance with the material that is actually used. In particular, a coating method is preferably selected from among the above-described methods, as it allows designing the thickness of the elastic resin coating (for instance, the average coating thickness s in condition (3) above) with more precision.

[0080] In addition to the resin-coated hard particles (c), the first friction material according to a first aspect of the invention further includes an elastic material (m) other than the resin-coated hard particles. The elastic material also forms the matrix of the first friction material.

[0081] Preferably, the elastic material (m) contains at least one elastic material selected from among phenolic resins, modified phenolic resins, amino resins, furan resins, unsaturated polyester resins, diallylphthalate resins, alkyd resins, epoxy resins, thermosetting polyamideimide resins, thermosetting polyimide resins and silicone resins. By selecting an appropriate elastic material results in more adequate stress to which the hard particles (a) are subjected, and more adequate amount of protrusion of the hard particles (a) from the friction surface during friction.

[0082] The matrix material (M) forms the matrix of the second friction material. Specific examples of the matrix material (M) include, for instance, a metallic material such as iron, cobalt, nickel, chromium, titanium, copper, aluminum, or alloys having one of these metals as a main component; or an inorganic material such as carbon, a composite of carbon and carbon fibers, or a composite of carbon and a ceramic.

[0083] The first friction material according to a first aspect of the invention may further use a base material. The base material used is preferably a material that does not deform upon heating, specifically organic fibers such as aramide fibers, nylon, cellulose or the like, or inorganic fibers such as steel fibers, copper fibers, ceramic fibers, glass fibers, rock wool or the like. The proportion of base material ranges preferably from 5 to 50vol% of the entire friction material.

[0084] Preferably, the first friction material contains a total of no less than 5vol% of the resin-coated hard particles (c) and the elastic material (m), and the volume ratio of the resin-coated hard particles (c) to the elastic material (m) falls within the range of 2:1 to 1 :50. In terms of the above-described stress distribution mechanism, the combined effects of frictional force stability and noise and vibration suppression may fail to be sufficiently brought out if the total concentration of resin-coated hard particles (c) and elastic material (m) is less than 5vol%. Wear of the second friction material relative to that of the first friction material may increase if the concentration of the resin-coated hard particles (c) exceeds a concentration at which the volume ratio of the resin-coated hard particles (c) to the elastic material (m) is 2:1. On the other hand, the frictional force may decrease if the concentration of elastic material (m) exceeds the concentration at which the volume ratio of the resin-coated hard particles (c) to the elastic material (m) is 1 :50. In particular, the total concentration of resin-coated hard particles (c) and elastic material (m) is preferably not below 6vol%, and the volume ratio of the resin-coated hard particles (c) to the elastic material (m) preferably falls within the range of 1:1 to 1:30. Most preferably, the total concentration of resin-coated hard particles (c) and elastic material (m) is not below 7vol%, and the volume ratio of the resin-coated hard particles (c) to the elastic material (m) ranges

[0085] Preferably, the elastic modulus of the first friction material ranges from 100 to 300 MPa. The elastic modulus of the friction material as a whole may become too low if the elastic modulus of the first friction material is belowlOO MPa. This may preclude achieving more adequate stress to which the hard particles (a) included in the first friction material are subjected, and more adequate amount of protrusion of the hard particles (a) from the friction surface . The elastic modulus of the first friction material as a whole may be too high if the elastic modulus of the first friction material exceeds 300 MPa. As a result, the hard particles (a) in the first friction material may slough off during friction. In particular, the elastic modulus of the first friction material is preferably not below 120 MPa, and most preferably not below 140 MPa, In addition, the elastic modulus of the first friction material preferably does not exceed 280 MPa, and most preferably does not exceed 260 MPa. The method for measuring the elastic modulus of the first friction material may be the same as the method for measuring the elastic modulus of the above-described elastic resin, or the elastic modulus of the elastic material.

[0086J Preferably, the surface roughness of the friction surface' of the second friction material does not exceed 10 μm, because a surface roughness exceeds 10 μrn may impair initial break-in, which in turn may result in frictional force variation as well as in increased wear. The surface roughness is defined in accordance with the ten-point average roughness (Rz JIS) according to Japanese Industrial Standard JIS B 0601. In particular, the surface roughness of the friction surface of title second friction material is at most 9 μm, and most preferably does not exceed 8 μm. The above-described effects may be sufficiently brought out when the surface roughness of the friction surface of the second friction material is at least 0.01 μm. [0087] The materials that comprise the first friction material explained thus far may be mixed using a conventional method, for instance dry mixing using a vertical mixer, a horizontal mixer or the like; or a method in which wet mixing is carried out in the presence of water or an organic solvent, using the above mixers or the like, followed by vacuum deaeration or heating deaeration to remove the solvent. The frictional material may be molded using a method in which the mixture obtained using the above-described mixing methods is charged into a heated mold and is pressed, or a method in which a mixture obtained using the above mixing methods is bonded to a base material. The friction material may be shaped to any suitable shape, such as a wire, rod, plate or sheet.

[0088] The the second friction material may be prepared using various methods, such as mixing the hard particles (A) with particles of the matrix material (M), and also with inorganic particles (F), as the case may require, using a ball mill or the like, followed by sintering of the resulting mixture; joining a sintered body to a friction surface portion of a structure base member, by mechanical fastening or welding such as electric welding or laser welding; spraying a mixed material powder onto the friction surface of a structure base member, by plasma spraying or the like; or plating a metallic material (M), in which hard particles (A) or hard particles (A) and inorganic particles (F) are dispersed by particle dispersion plating, on the friction surface of a structure base member. A casting method may also be used, so long as hard particles segregate during the solidification process. Cast iron may be the second friction material, provided that the alloy composition and the casting conditions are controlled so as to cause cementite (iron carbide) to segregate.

[0089] FIG. 3 is a cross-sectional schematic diagram illustrating a typical example of a friction pair of the invention. The friction pair 100 of the invention has a first friction material 1 and a second friction material 2, such that a friction surface 41 of the first friction material 1 abuts a friction surface 43 of the second friction material 2. The first friction material 1 has resin-coated hard particles (c) 13 in which hard particles (a) 11 are coated with a resin (b) 12, and an elastic material (m) 14 other than the particles (c) 13, that forms the matrix of the first friction material. The second friction material 2 has hard particles (A) 21 and a matrix material (M) 24. Preferably, the friction pair 100 satisfies at least both (A) 21 and a matrix material (M) 24. Preferably, the friction pair 100 satisfies at least both conditions (1) and (3) described above.

[0090] FIGS. 4A and 4B are cross-sectional schematic diagrams illustrating typical examples of the above friction pair before and after stress application. FIG, 4A illustrates the situation before application of stress to a typical example of the friction pair, i.e. the situation before frictional sliding. The protruded portion 50 of hard particles (a) 11 and (A) 21 that is protruded from the friction surface of the respective friction material is one factor that the stress applied to the friction pair stress exceeds yield stress in the hard particles. FIG. 4B illustrates the situation after application of stress to a typical example of the friction pair, i.e. the situation during frictional sliding. As illustrated in the figure, the resin (b) 12 that coats the hard particles (a) 11, and the matrix material (M) 24 become distorted upon stress application. As a result, the friction pair 100 possesses a stress distribution mechanism whereby excessive stress imparted to the hard particles (a) 11, (A) 21 is sufficiently absorbed by distortion of the resin (b) 12 or the matrix material (M) 24 when the friction pair 100 is subjected to stress. This makes the hard particles less likely to be subjected to stress beyond yield stress. Moreover, the elastic material (m) 14 in the first friction material ensures a contact surface 15 over the entire friction surface. This affords effective friction over the entire area between friction surfaces.

[0091] The friction pair in the above typical example has thus a stress distribution mechanism whereby excessive stress imparted to the hard particles (a), (A) upon application of stress to the friction pair is sufficiently absorbed by the resin (b) or by the matrix material (M), as a result of which the hard particles (a), (A) is less likely to be subjected to stress beyond yield stress. Therefore, the friction pair generates high frictional force, by way of the hard particles (a), (A), while achieving excellent frictional force stability and suppression of noise and vibration.

[0092] 1. Preparation of resin-coated hard particles by coating hard particles (a) with a resin (b)

Silicon carbide (SiC , Mohs hardness 9.3, Green Densic® (GC) ₅ by Showa Denko) was used as the hard particles (a), A polyamideimide (hereafter PAI for short, Molykote ® (PA-744) by Dow Corning Toray) was used as the resin (b). To coat the hard particles (a) with the resin (b), SiC was mixed with PAI dissolved in a solvent, to an equivolumetric ratio (1:1) of PAI and SiC (solids in the solvent). The solvent was evaporated thereafter to form the resin-coated hard particles. The particle size of SiC before and after resin coating was measured using a laser diffraction particle size analyzer. The thickness of the resin coating (s) was then calculated based on the difference between the mean diameter of SiC after resin coating and the mean diameter of SiC before resin coating.

[0093] 2. Production of a friction pair

A first friction material (brake pad) according to an example of the invention was produced by mixing the materials given in Table 1 below, in the proportions (vol%) indicated. The term "SiC (PAI-coated)" in Table 1 denotes resin-coated silicon carbide obtained in the above-described preparation of resin-coated hard particles. The particle sizes set forth in Table 1 below were measured using the above-described laser diffraction particle size analyzer. Of the materials set forth in Table 1, the Mohs hardness of the SiC (hard particles (a)) is 9.3, and the mean diameter of the resin-coated SiC is 26 μm. The Mohs hardness of mica (inorganic particles (f), mean diameter 15 μm) is 2.5 to 3.0. The Mohs hardness of barium sulfate (inorganic particles (f), mean diameter 10 μm) is 3.5. Therefore, the first friction material of the example satisfies condition (1). The details of the manufacturing method were as follows. Firstly, the various starting materials were mixed uniformly for 5 minutes in a vertical mixer, to yield a friction material starting mixture. Ih a subsequent heat molding step, the friction material starting mixture was charged into a mold heated at 150 ⁰ C, and was pressed for 10 minutes at 200 kg/cm ² . Curing was carried out thereafter at 200 ⁰ C for 2 hours, to yield the first friction material. The second friction material (rotor) was manufactured in accordance with the method below. Tungsten carbide (WC, Mohs hardness 9) having a mean diameter of 3 μm was used as the hard particles (A) ₅ while cobalt (Mohs hardness 5.5) was used as the matrix material (M). The second friction material was obtained by plasma-spray of the WC on a cast-iron rotor, using cobalt as a binder. The WC concentration in the spray layer was 90 vol%. The first friction material (brake pad) and the second friction material (rotor) were combined to yield the friction pair of the example.

[0094] A first friction material (brake pad) of a comparative example was prepared by mixing the materials listed in Table 1 below, in the proportions (vol%) given in the column of comparative examples in Table 1. The term "SiC (uncoated)" in Table 1 denotes silicon carbide (SiC, Mohs hardness 9.3, Green Densic® (GC) ₃ by Showa Denko) used as-is, without any resin coating. The particle sizes set forth in Table 1 below were measured using the above-described laser diffraction particle size analyzer. In the materials set forth in Table 1 below, SiC has no resin coating by definition, and hence the first friction material of the present comparative example does not satisfy condition (1). The details of the manufacturing method were as follows. Firstly, the various starting materials were mixed uniformly for 5 minutes in a vertical mixer, to yield a friction material starting mixture. In a subsequent heat molding step, the friction material starting mixture was charged into a mold heated at 150 ⁰ C, and was pressed for 10 minutes at 200 kg/cm ² . Curing was carried out thereafter at 200 ⁰ C for 2 hours, to yield the first friction material. The second friction material (rotor) was manufactured using the same method as that used to manufacture the second friction material of the friction pair according to the example. The first friction material (brake pad) and the second friction material (rotor) were combined to yield the friction pair of the comparative example. Table 1

[0095] 3. Measurement and evaluation of the factional properties of the friction pair

Test samples of the friction pairs according to the example and the comparative example were placed under a load of 200 N at temperatures of 100 ⁰ C or 300°C, and were slid against each other at a speed of 1 m/s for 1 hour, followed by braking from 1 m/s to 0 m/s. Table 2 compares the friction coefficient (μ), the friction coefficient variation (Δμ) during braking, and amounts of wear from before to after braking. Table 2

[0096] As Table 2 shows, the friction coefficient μ and the amount of wear were the same in the friction pairs of the example and the comparative example, at both 100 ⁰ C and 300 ⁰ C. In contrast, the friction coefficient variation Δμ remained smaller in the friction pair of the example. The results of the above frictional property measurements indicate that the friction pair of the example of the invention, which satisfies condition (1), possesses, in particular, superior frictional force stability as compared with a conventional friction pair that does not satisfy condition (1).

[0097] 4. Measurement and evaluation of squealing of the friction pair The number of occurrences of squealing, as well as the loudness of that squealing, were measured for the friction pairs of the example and the comparative example, during simulated urban cruising of an automobile equipped with the respective friction pair (100 cycles of braking at a speed of 40 km/h, deceleration of 0.1 to 1.5 m/s ² and temperature of 50 to 15O ⁰ C). Table 3 summarizes comparatively the number of squeals and the loudness thereof. Table 3 [0098] As Table 3 shows, the friction pair of the example emitted fewer squeals, and of a lower loudness, than the friction pair of the comparative example. The measurement results of the frictional properties ^" relating to squealing indicate that the friction pair of the example of the invention, which satisfies condition (1), suppresses brake squeal to a greater extent than a conventional friction pair that does not satisfy condition (1).