JP2008025018A - Sintering friction material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sintering friction material which is preferable in the face of environmental protection since it comprises no chemical substances designated by a PRTR (Pollution Release and Transfer Register) law at all, and further, by retaining the matrix structure thereof to ferrite, has excellent high temperature properties and corrosion resistance, and also, has performance upon braking, e.g., of reducing the wear amount of the mating material. <P>SOLUTION: The sintering friction material is, as a base, composed of powder formed from a reduced iron having a high melting point instead of the conventional friction material essentially consisting of copper powder. The material is obtained by adding an element(s) having a high graphitization tendency to the reduced iron powder, and sintering the same, and the matrix structure is retained to ferrite. By adding the element(s) having a high tendency of increasing the ratio at which carbon in iron is made to independently exist as graphite (i.e., a graphitization tendency) upon the sintering, the matrix structure is held to ferrite, and the sintering friction material having excellent high temperature properties and corrosion resistance can be obtained. Further, it is possible that its attackability against the mating material such as a brake disk is reduced, and the wear amount of the mating material can be reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a sintered friction material as a friction material for a brake used in a braking device for an automobile, a motorcycle, a railway vehicle, an industrial machine or the like.

  Conventionally, as a sintered friction material for brakes, copper is the main component, and tin or sometimes a metal added with iron, nickel, zinc, antimony, chromium, lead, etc. is used as a base material, and alumina, mullite, zirconia, etc. Ceramic friction materials and sintered friction materials to which lubricants such as graphite and molybdenum disulfide are added are used. This kind of sintered friction material is heavier than resin-based friction material, is expensive, and has the points to be improved such as the possibility of generating brake noise. Even under low conditions, it does not cause fade phenomenon (a phenomenon in which the friction coefficient during braking is greatly reduced at high temperatures), provides stable performance, and has excellent strength and wear resistance. However, it has been widely used for brakes that require high friction performance.

  An example of a bronze-type dry sintered friction material that aims to stably obtain a high friction coefficient has been proposed (Patent Document 1). This dry-sintered friction material is 60-80% copper, 3-20% tin, 3-20% alumina and / or silica, 3-10% graphite, 1-5% molybdenum disulfide and 15 manganese. %, Which is composed as a matrix component, stabilizes the friction coefficient during braking, forms a hard copper oxide film on the surface by heat generation with the mating plate, water fading phenomenon and heat It has resistance to fading phenomenon and aims to obtain a stable friction surface. Alumina and silica are added for the purpose of withstanding high loads and high-temperature frictional sliding. Graphite and molybdenum disulfide are added for the purpose of improving lubricity. Manganese reduces and oxidizes oxide films of other metals during sintering. It is added for the purpose of improving the properties.

  As another example of the sintered friction material, an iron-based sintered friction material having a perforated main body portion made of an iron-based sintered body and an alkaline substance in which an aqueous solution fixed in the hole of the main body portion exhibits alkalinity. Has been proposed (Patent Document 2). The metal base material used as the skeleton of the friction material is a material mainly composed of iron, and can be a general iron-based metal such as stainless steel or cast iron, a mixture thereof, or a mixture with other metals. Examples of the lubricant include graphite and molybdenum disulfide.

  As another example of the sintered friction material, a sintered friction material containing copper or a copper alloy as a matrix and containing 2 to 20% by weight of stabilized zirconia has been proposed (Patent Document 3). According to this sintered friction material, by adopting stabilized zirconia in copper-based or iron-based sintered friction material, adaptability to a wide range of braking conditions is good, a stable friction coefficient is obtained, and wear resistance is improved. We aim to obtain a sintered friction material that is good in heat resistance and heat resistance, and has little attack on the mating material.

  However, in recent years, the PRTR Law (Act on the Promotion of Improvement in Management and Management of Specific Chemical Substances Emissions from the Environmental Protection Perspective) has been enacted, and materials used as brake friction materials are also considered for environmental protection. However, it has been demanded not to use designated chemical substances stipulated by the law. However, among the above-mentioned additive materials such as copper, lead, ceramics, and lubricants that have been used as raw materials for sintered friction materials, materials other than iron, ceramics, and graphite are designated by the PRTR method. It is set as a chemical substance and it is hoped that it will not be used as much as possible.

Against this background, research and development have been carried out on sintered friction materials having a formulation that uses as little PRTR-designated chemical substances as possible. However, in the case of a sintered friction material mainly composed of an iron-based material that does not use copper or tin as the main component until now, the wear amount of the friction material or the counterpart material (for example, brake disc. In other words, the amount of wear of a low-alloy steel, stainless steel, or other iron-based material is greatly increased, and the required coefficient of friction cannot be ensured. In addition, materials such as aluminum, magnesium, and titanium that are not specified chemical substances in the PRTR method other than ferrous materials have many problems as the main component of sintered friction materials, and practical application of sintered friction materials excellent in environmental protection is quite easy. It was difficult. The present applicant has invented an iron-based sintered friction material based on cast iron powder or reduced iron powder, and has obtained performance equivalent to that of a copper-based sintered friction material (for example, Japanese Patent Application No. 2005-300817). . However, in the evaluation under high load conditions, it has been found that it has not reached the same level as the copper-based sintered friction material.
Japanese Examined Patent Publication No. 63-15976 (second column, second line to fourth column, first line) JP 2002-181095 A (paragraphs [0022] to [0026]) Japanese Patent No. 2958493

  Therefore, as a raw material of the sintered friction material, not only hexavalent chromium compounds and nickel compounds which are specified first class designated chemical substances of the PRTR method, but also first class designated chemical substances zinc, antimony, copper, tin, By not using any material such as lead or molybdenum, it contributes to environmental protection, keeps the base structure of the sintered friction material in ferrite with high temperature characteristics and corrosion resistance, and prevents similar friction of iron There is a problem to be solved in terms of reducing the amount of wear.

  The object of the present invention is that it is preferable from the viewpoint of environmental protection because it does not contain any PRTR-designated chemical substances, and it is excellent in high-temperature characteristics and corrosion resistance by maintaining the base structure of the sintered friction material in ferrite, and also has a brake. An object of the present invention is to provide a sintered friction material having excellent performance at the time of brake braking, such as reducing the amount of wear of the counterpart material by reducing the aggression to the counterpart material such as a disk.

  The sintered friction material according to the present invention is sintered by adding an element having a high graphitization tendency to high melting point reduced iron powder as a main component, and the base structure is kept in ferrite.

  This sintered friction material is configured based on a powder formed from reduced iron having a high melting point, instead of a friction material mainly composed of conventional copper powder. At the time of sintering, the graphite of the lubricant is dissolved in reduced iron powder having a high melting point to become (Fe-C). Although (Fe-C) increased in hardness, the melting point decreased and the heat resistance decreased. Therefore, by suppressing the solid solution and adding an element having a large tendency to increase the proportion of carbon in iron as graphite (graphite) independently (ie, graphitization tendency), the base structure becomes ferrite (body). A solid solution of up to 0.02% carbon (C) in α-iron with a center cubic lattice.

  In this sintered friction material, the graphitization accelerating element may be one or more elements selected from the element group of aluminum, silicon, and titanium. Examples of elements having a large tendency to graphitize include aluminum (Al), silicon (Si), and titanium (Ti). The graphitization tendency is Al> Si> Ti> C in descending order.

  In the sintered friction material, the aluminum 1 to 9 vol%, the silicon 1 to 8 vol%, or the titanium 0.5 to 8 vol% can be added in a total range of 0.5 to 20 vol%. By determining the ratio of each of the graphitization promoting elements aluminum, silicon or titanium and the total amount in this way, solid solution of the graphite, which is a lubricant, with respect to the reduced iron powder during sintering is suppressed, and the base structure is reduced. In addition, since the melting point is prevented from being lowered and the high temperature characteristics are excellent and the material is relatively soft, it is possible to keep the ferrite having a low attacking property on the counterpart material. It has been found that each graphitization accelerating element cannot keep the base structure in ferrite below each lower limit addition amount, and generates carbide containing the added metal above each upper limit addition amount. When silicon is added, it is preferable to add ferrosilicon instead of metal silicon in order to prevent the formation of carbides. Although some pearlite is partly dissolved by sintering, the added metal can keep most of the matrix structure in ferrite.

  In the sintered friction material, it is preferable that 30% or more of the ferrite is present in the matrix structure. It is preferable that the abundance ratio of ferrite in the matrix structure (percentage of ferrite / matrix structure) is 30% or more, since there is little decrease in heat resistance.

  The sintered friction material contains 20 to 45 vol% of the reduced iron, and further contains 0 to 15 vol% of fine alumina having an average particle size of 0.3 to 2 μm, 0 to 10 vol% of magnesia having an average particle size of 50 to 250 μm, and an average particle 2-10 vol% of alumina having a diameter of 5-20 μm and 30-45 vol% of graphite can be added. However, the total of the magnesia and the alumina is preferably in the range of 5 to 15 vol%. Here, the reason for setting the range of each component is that when the reduced iron is less than 20 vol%, the friction material wear amount rapidly increases due to insufficient binding force in the friction material, and when it exceeds 45 vol%, other components are insufficient and there is a problem. Arise. When magnesia + alumina is less than 5 vol%, the friction coefficient decreases, and when it exceeds 15 vol%, the increase in the wear amount of the mating material becomes significant. Since the particle size of alumina is small, even with alumina (range of 5 to 15 vol%) alone, the aggressiveness is suppressed, and the friction material and the counterpart material (iron-based material) wear during braking are reduced, but in combination with magnesia, It becomes easy to achieve both the securing of the friction coefficient and the suppression of the partner material aggression. When the graphite powder is less than 30 vol%, the lubrication effect decreases, so both the friction material and the counterpart material wear amount increase, and when it exceeds 45 vol%, the friction coefficient decreases greatly. Each graphitization accelerating element for keeping the matrix structure in ferrite cannot keep the matrix structure in ferrite below each lower limit addition amount, and if it exceeds each upper limit addition amount, carbide containing the added metal is generated.

  In the sintered friction material, the relative density as a percentage of the sintered body density after sintering by a pressure sintering method such as discharge plasma sintering or hot pressing can be 80% or more.

  The iron-based sintered friction material having a ferrite base structure with high heat resistance described above achieved performance superior to that of the copper-based sintered friction material, but because the reduced iron powder has a high melting point, the sintering temperature is high, and the friction coefficient It is difficult to obtain desired physical properties such as wear resistance and wear resistance. Among them, even if a high physical property value is obtained, a spark is generated during high load braking, which causes a problem. Therefore, fine iron powder was added to the reduced iron powder, which is the main component, to make the iron powder a combination of reduced iron powder and fine iron powder. The fine iron powder is an iron powder having an average particle diameter of 1/10 or less of the reduced iron powder. By mixing the iron powder as the main component with reduced iron powder having a high melting point and relatively soft iron powder, the hardness (HRS) can be softened without lowering the relative density. In addition, the contact property with a mating material such as a brake disk can be improved.

  In the sintered friction material in which fine iron powder is mixed with the reduced iron powder, the mixing ratio of the fine iron powder to the reduced iron powder is preferably in the range of 10 to 30 vol%. By selecting the addition ratio of the fine iron powder to such a ratio, the participation in the sintering of the fine iron powder is increased and the sinterability can be improved. Therefore, the sintered friction material can be densified without reducing the pores of the reduced iron powder itself with many voids by sintering, and the hardness can be reduced. The fine iron powder is selected from powders with an average particle size of 1/10 or less than the reduced iron powder. improves.

  In the sintered friction material in which fine iron powder is mixed with the reduced iron powder, the graphitization promoting element is one or more elements selected from the element group of aluminum, silicon, and titanium, 9 vol%, the silicon 1-8 vol%, and the titanium 0.5-8 vol% can be added in a total range of 0.5-20 vol%. By determining the proportion of each of the graphitization accelerating elements aluminum, silicon or titanium and the total amount in this way, solid solution of the graphite of the lubricant to the reduced iron powder during sintering is prevented, and the base structure is made of ferrite ( Reduced iron powder base organization). It has been found that each graphitization accelerating element cannot keep the base structure in ferrite below each lower limit addition amount, and generates carbide containing the added metal above each upper limit addition amount. In addition, when adding silicon, it is preferable to add ferrosilicon instead of metal silicon in order to prevent the formation of carbides.

  In the sintered friction material in which fine iron powder is mixed with the reduced iron powder, the reduced iron contains 20 to 45 vol% and the fine iron powder is 2 to 14 vol% which is 30 vol% or less with respect to the reduced iron. Further, the graphitization accelerating element is one or more elements selected from the group consisting of aluminum, silicon and titanium, the aluminum being 1 to 9 vol%, the silicon being 1 to 8 vol%, and the titanium being 0.5 -8 vol%, and the total of them is added in the range of 0.5 to 20 vol%. Furthermore, 0-15 vol% of fine alumina having an average particle size of 0.3-2 µm, and magnesia 0, having an average particle size of 50-250 µm, are added. 10 vol%, 2-10 vol% of alumina having an average particle diameter of 5-20 μm, and 30-45 vol% of graphite, and a total of 5-15 vol% of the magnesia and the alumina. To, can be added. Here, the reason for setting the range of each component is that when the reduced iron is less than 20 vol%, the friction material wear amount rapidly increases due to insufficient binding force in the friction material, and when it exceeds 45 vol%, other components are insufficient and there is a problem. Arise. When fine iron exceeds 30% with respect to reduced iron, the wear material hardness increases and becomes hard and brittle, so both the friction material wear amount and the counterpart material wear amount increase. When magnesia + alumina is less than 5 vol%, the friction coefficient decreases, and when it exceeds 15 vol%, the increase in the wear amount of the mating material becomes significant. Since the particle size of alumina is small, even with alumina (range of 5 to 15 vol%) alone, the aggressiveness is suppressed, and the friction material and the counterpart material (iron-based material) wear during braking are reduced, but in combination with magnesia, It becomes easy to achieve both the securing of the friction coefficient and the suppression of the partner material aggression. When the graphite powder is less than 30 vol%, the lubrication effect decreases, so both the friction material and the counterpart material wear amount increase, and when it exceeds 45 vol%, the friction coefficient decreases greatly. Each graphitization accelerating element for keeping the matrix structure in ferrite cannot keep the matrix structure in ferrite below each lower limit addition amount, and if it exceeds each upper limit addition amount, carbide containing the added metal is generated. In addition, when adding silicon, it is preferable to add ferrosilicon instead of metal silicon in order to prevent the formation of carbides.

  Furthermore, even in sintered friction materials in which fine iron powder is mixed with reduced iron powder, the relative density after sintering by pressure sintering methods such as spark plasma sintering and hot pressing (sintering resistance density / true density (Percentage) can be 80% or more.

  The sintered friction material according to the present invention is mainly composed of an iron-based material, and the other compounding materials are ceramics as an abrasive, graphite as a lubricant, and metal powder. Therefore, at the time of sintering, the graphite of the lubricant is dissolved in reduced iron powder having a high melting point to form iron carbide (Fe—C). Although (Fe-C) increased in hardness, the melting point decreased and the heat resistance decreased. Therefore, by adding an element having a large graphitization tendency (Al> Si> Ti> C), the matrix structure can be kept in ferrite. As a result, it is possible to obtain a sintered friction material having a small decrease in the melting point of the main component and excellent in high temperature characteristics and corrosion resistance. Since ferrite is softer than (Fe—C) in which graphite is dissolved, the amount of wear of the counterpart material can be reduced.

  In this sintered friction material, in order to keep the base structure in ferrite (base structure of reduced iron powder), the upper and lower limits of the addition amount of the graphitization promoting element aluminum, silicon, or titanium are determined. The solid solution can be suppressed and the base structure can be kept in ferrite. Each graphitization accelerating element cannot keep the base structure in ferrite below the respective lower limit addition amount, and carbide including the added metal is generated above each upper limit addition amount.

  In this sintered friction material, the abundance of ferrite (percentage of ferrite / matrix structure) is 30% or more, and there is little deterioration in heat resistance. be able to.

  In this sintered friction material, by setting the range of each component for reduced iron, magnesia, alumina, magnesia + alumina, and graphite, ensuring the friction coefficient and suppressing the friction material wear amount and the wear amount of the counterpart material Can be achieved. Also for graphite powder, if the amount is less than the lower limit, the lubrication effect decreases, so both the friction material and the mating material wear amount increase, and if the upper limit is exceeded, the friction coefficient decreases greatly. Each graphitization accelerating element for keeping the matrix structure in ferrite cannot keep the matrix structure in ferrite below each lower limit addition amount, and generates carbide containing the added metal above each upper limit addition amount.

  In this sintered friction material, when the pressure sintering method is used, the relative density after sintering (percentage of sintered body density / true density) is set to 80% or more so that the bonding force between the iron-based materials is increased. Since it is strong, a sintered friction material having excellent strength and wear resistance can be obtained.

  Furthermore, according to the sintered friction material mainly containing reduced iron powder, the hardness can be reduced without reducing the relative density by mixing fine iron powder with the reduced iron powder, and contact with the counterpart material. As a result, the temperature rise and uneven wear due to local contact with the counterpart material such as a brake disk can be suppressed, and the friction coefficient and wear resistance can be improved. By making the amount of fine iron powder added in the range of 10 to 30 vol% with respect to the reduced iron powder, sintering is possible at a lower sintering temperature than when the main component is only reduced iron powder, and the fine iron powder However, it is possible to enhance the sinterability by greatly participating in the sintering. Further, by mixing fine iron powder with reduced iron powder, the relative density (sintered body density / true density) of the sintered friction material after being sintered by a pressure sintering method such as discharge plasma sintering or hot pressing. (Percentage of density) is 80% or more, the bonding strength between the iron-based materials can be increased, and a sintered friction material excellent in strength and wear resistance can be obtained.

Hereinafter, the sintered friction material according to the present invention will be described in more detail with reference to examples.
First, reduced iron powder having an average particle size of about 160 μm as raw materials, aluminum powder having an average particle size of about 20 μm and / or silicon having an average particle size of about 24 μm (ferrosilicon), which is an element for promoting graphitization to keep the base structure in ferrite. And / or titanium powder having an average particle size of about 10 μm, fine alumina powder having an average particle size of about 1.2 μm, magnesia powder having an average particle size of about 190 μm, natural graphite powder having an average particle size of about 170 μm, and average particles An artificial graphite powder having a diameter of about 240 μm was prepared.

Next, after weighing each of the above raw materials into each of the blends C shown in Table 1, a 4% methanol was added to the mixture for 10 minutes in order to prevent segregation during mixing using a stirring mill crusher (manufactured by Ishikawa Factory). A mixed powder was prepared by mixing. In C1 to C6, an element having a large tendency to graphitize is added, and the base structure is kept in ferrite having high heat resistance. C7 to C9 keep the base structure in ferrite by adding two or three elements having a large tendency to graphitize. In addition, a mixed powder of copper-based sintered material A currently mass-produced as a comparative material and a mixed powder of representative example B having a composition not containing an element having a large tendency to graphitize were also prepared.

  Further, each mixed powder is filled into a graphite mold having a cavity of 23 mm × 35 mm, and a discharge plasma sintering apparatus (manufactured by Sumitomo Coal Mining Co., Ltd., model SPS-515S) is used. Sintering was performed under the conditions of a sintering temperature of 1150 ° C. and a holding time of 5 minutes. In addition, since A compound material is produced on the same conditions as mass production material, it sinters also with a batch-type sintering furnace (temperature increase rate 10-20 degreeC / min, pressurization 0.7MPa), and it is a discharge plasma sintering apparatus Compared to the sintered one.

  After sintering, the relative density (apparent density of sintered body / percentage of true density of sintered body) and hardness of each sintered body were measured. Moreover, the high temperature brake performance test of 500 degreeC and 600 degreeC was implemented, and the friction coefficient and the other party material abrasion amount were calculated | required. The apparent density of the sintered body was calculated from the weight in the air and water, and the true density was calculated from the true density of the raw materials and the blending ratio. Hardness was measured with the S scale (HRS) of a Rockwell hardness tester. The brake performance test was conducted using a 1/10 scale tester tester owned by our company. Table 1 shows the sintering conditions, the relative density, the hardness, the average friction coefficient in the brake performance test, and the wear amount of the counterpart material.

  As understood from Table 1, it can be seen that all of the products of the present invention have a stable and high coefficient of friction at high temperatures, and the amount of wear of the counterpart material is smaller than that of the comparative material B. C1 to C6 contain elements having a large tendency to graphitize, and Comparative Materials A and B have an average coefficient of friction of 0.22 to 0.23 at 600 ° C., whereas 0.25 to 0.28. C7 to C9 are preferably combined with two or more elements having a large tendency to graphitize, and the average coefficient of friction at 600 ° C. is as high as 0.27 to 0.29.

  FIG. 1 is a photomicrograph showing an example of a base structure of the sintered friction material C1 according to the present invention, and FIG. The base tissue was etched with 5% nital for 10 seconds. From FIG. 2, it can be seen that the base structure of the comparative material B generates a lot of pearlite. When silicon is added, ferrosilicon is added instead of metal silicon to prevent the formation of carbides. The abundance of ferrite was about 20% for comparative material B, while about 45% for inventive material Cl.

Next, another embodiment of the sintered friction material according to the present invention will be described.
First, a reduced iron powder having an average particle size of about 160 μm as a raw material and an average particle size of about 5 μm whose size (average particle size) is 1/10 or less, which is a kind of the reduced iron powder. Fine iron powder, an aluminum powder having an average particle size of about 20 μm and / or silicon (ferrosilicon) having an average particle size of about 24 μm and / or an average particle size of about 10 μm, which is an element for promoting graphitization to keep the base structure in ferrite Titanium powder, fine alumina powder with an average particle size of about 1.2 μm, magnesia powder with an average particle size of about 190 μm, natural graphite powder with an average particle size of about 170 μm, and artificial graphite powder with an average particle size of about 240 μm did.

Next, after weighing each of the above raw materials into the blend D shown in Table 2, 4% methanol was added to the mixture for 10 minutes in order to prevent segregation at the time of mixing using a stirring mortar (produced by Ishikawa Factory). A mixed powder was prepared by mixing. In Table 2, as a comparative material, samples A1 and A2 that are already mass-produced as a mixed powder of the copper-based sintered material A in Table 1 and the heat resistance of the present invention shown in Table 1 are shown. Representative examples C8 and C9 of iron-based sintered materials having a high ferrite matrix structure are shown again.

  The sintering type and the discharge plasma sintering apparatus, and the sintering conditions such as temperature, pressure, and time, or the sintering of the compound A may be the same as in the previous embodiment, and these will be described again. Is omitted. In addition, the relative density (apparent density and true density) of each sintered body after sintering, the measurement of hardness, the friction coefficient in the high-temperature brake performance test, and the measurement of the wear amount of the counterpart material are also the same as in the previous example The same conditions may be used, and repeated description is omitted.

  Table 2 shows the sintering conditions, the relative density, the hardness, the average friction coefficient in the brake performance test, and the wear amount of the counterpart material. As is understood from Table 2, D1 to D5 contain fine iron in vol% of the numerical values in the table (the content rate increases from D1 to D5), and elements having a large tendency to graphitize are contained. It has been added. It can be seen that all of the products (D1 to D5) of the present invention have a stable and high coefficient of friction at high temperatures, and the amount of wear of the counterpart material is less than that of the comparative material. That is, with respect to the relative density and the hardness, compared with the material C of the present invention, the hardness is brought close to the sintered friction material in the case of the mass production type sample A without lowering the relative density (the numerical value is low). And soft). Regarding the coefficient of friction, the comparative material A has an average friction coefficient of 600 ° C. of 0.22 to 0.23, whereas the average friction coefficient of the material C of the present invention is equal to or equal to 0.27 and 0.29. The coefficient is higher. Furthermore, the wear amount of the counterpart material is also improved by improving the wear amount of the counterpart material C of the present invention C and approaching the sintered friction material in the case of the mass production type sample A (the numerical value is small and the counterpart material wear amount is small) ) Is obtained.

It is a microscope picture which shows an example of the base structure | tissue of the sintered friction material C1 by this invention. It is a microscope picture which shows an example of the base structure of the comparative material B.

Claims (11)

  Sintered friction material consisting of high-melting point reduced iron powder as a main component, which is sintered by adding an element that has a high tendency to graphitize, and the base structure is kept in ferrite.   The sintered friction material according to claim 1, wherein the graphitization promoting element is one or more elements selected from the group consisting of aluminum, silicon, and titanium.   The sintering according to claim 2, wherein the aluminum is added in an amount of 1 to 9 vol%, the silicon is 1 to 8 vol%, or the titanium is 0.5 to 8 vol% in a total range of 0.5 to 20 vol%. Friction material.   The sintered friction material according to claim 3, wherein the ferrite is present in the matrix structure at 30% or more.   Alumina 2 containing 20 to 45 vol% of the reduced iron, 0 to 15 vol% of fine alumina having an average particle diameter of 0.3 to 2 μm, 0 to 10 vol% of magnesia having an average particle diameter of 50 to 250 μm, and 5 to 20 μm of average particle diameter. The sintered friction material according to claim 3, wherein 10 to 10% by volume and 30 to 45% by volume of graphite are added so that the total of the magnesia and the alumina is in the range of 5 to 15% by volume.   6. The relative density as a percentage of the sintered body density after sintering by a pressure sintering method such as spark plasma sintering or hot pressing is 80% or more. 6. Sintered friction material.   The sintered friction material according to claim 1, wherein fine iron powder having an average particle size of 1/10 or less is added to the reduced iron powder and sintered.   The sintered friction material according to claim 7, wherein a mixing ratio of the fine iron powder to the reduced iron powder is in a range of 10 to 30 vol%.   The graphitization accelerating element is one or more elements selected from the group consisting of aluminum, silicon and titanium, and the aluminum is 1 to 9 vol%, the silicon is 1 to 8 vol%, and the titanium is 0.5 to 8 vol%. The sintered friction material according to claim 7 or 8, wherein is added in a range of 0.5 to 20 vol% in total.   20% to 45% by volume of the reduced iron and 2% to 14% by volume of the fine iron that is 30% by volume or less with respect to the reduced iron, and the graphitization promoting element is selected from an element group of aluminum, silicon, and titanium. 1 to 9 vol% of the aluminum, 1 to 8 vol% of the silicon, and 0.5 to 8 vol% of the titanium are added in a total range of 0.5 to 20 vol%, Furthermore, 0 to 15 vol% of fine alumina having an average particle size of 0.3 to 2 μm, 0 to 10 vol% of magnesia having an average particle size of 50 to 250 μm, 2 to 10 vol% of alumina having an average particle size of 5 to 20 μm, and 30 to 45 vol of graphite. The sintered friction according to claim 7, wherein% is added so that the total of the magnesia and the alumina ranges from 5 to 15 vol%. .   The relative density as a percentage of the sintered body density after sintering by a pressure sintering method such as electric discharge plasma sintering or hot pressing is 80% or more. Sintered friction material.

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