The present invention relates to a frictional material for use in a means for arbitrarily controlling the rotation or movement of various machines, namely as a frictional material advantageously used in a magnetic rail brake.

A frictional material generally used in a magnetic rail brake is obtained by mixing metal powder forming a matrix with hard particles and sintering the resultant mixture. Examples of conventional frictional materials for magnetic rail brake include a sintered material comprised of a wear-resistant material comprised of at least one member selected from the group consisting of Al₂O₃, ZrO₂, Al₂TiO₅, Y₂O₃, SiC, Si₃N₄, WC, Cr₃C₂, and TiC, and powder of at least one member selected from the group consisting of nodular cast iron, graphite, iron sulfide, manganese sulfide, lead, and molybdenum sulfide (see, for example, Patent document 1 - Japanese Unexamined Patent Publication No. Hei 11-78882).

Furthermore JP 59 197549A discloses an iron sintered material for electromagnetic coupling.

When a magnetic rail brake operates, a frictional material is pressed against a rail and, simultaneously, the magnetic force causes tension between the frictional material and the rail, so that the friction of the frictional material against the rail causes the train to slow down. In a magnetic field generated from an electromagnet in the magnetic rail brake, the higher the magnetic flux density of the frictional material, the stronger the magnetic force, or the stronger the tension between the frictional material and the rail, that is, the magnetic rail brake increases in damping force. The frictional material has a high coefficient of friction and hence, when the frictional material is unlikely to be melted and deposited, the frictional material achieves higher frictional force, thus increasing the damping force. Further, when the frictional material has excellent wear resistance, the frequency of replacement of the frictional material by another can be reduced. Therefore, it is desired that the frictional material for use in magnetic rail brake has excellent frictional properties evaluated in terms of a wear resistance and a coefficient of friction, excellent resistance to melting and depositing, and excellent magnetic properties evaluated in terms of a magnetic flux density. Accordingly, an object of the present invention is to provide a frictional material having excellent frictional properties and excellent resistance to melting and depositing as well as excellent magnetic properties.

The present inventors have made extensive and intensive studies on frictional materials. As a result, they have succeeded in obtaining a frictional material having excellent frictional properties and excellent resistance to melting and depositing as well as excellent magnetic properties. Specifically, the present invention is directed to a frictional material comprising a metal matrix and hard particles, wherein the frictional material contains: 1 to 5% by weight of Cu; 0.1 to 1.0% by weight of P; 1 to 5% by weight of at least one soft metal selected from the group consisting of Bi, Sb, In, and Ag; 1 to 6% by weight of hard particles of at least one member selected from the group consisting of zircon (ZrSiO₄) and zirconia (ZrO₂); and the balance comprising Fe and an unavoidable impurity.

The frictional material of the present invention comprises a metal matrix comprised mainly of Fe containing Cu, P, and Bi or total Bi, Sb and Ag, and hard particles of at least one member selected from zircon and zircon and zirconia.

In the frictional material of the present invention, when the Cu content is less than 1% by weight, the resistance to melting and depositing is lowered. On the other hand, when the Cu content is more than 5% by weight, the coefficient of friction is lowered, and further the relative Fe content is reduced, so that the magnetic flux density is lowered. Therefore, the Cu content must be 1 to 5% by weight. It is preferred that the Cu content is 2 to 4% by weight.

In the frictional material of the present invention, when the P content is less than 0.1% by weight, the resistance to melting and depositing is lowered. On the other hand, when the P content is more than 1.0% by weight, the coefficient of friction is lowered, and further the relative Fe content is reduced, so that the magnetic flux density is lowered. According to the present invention, the P content must be 0.3 to 0.5% by weight.

When the content of at least one soft metal selected from the group consisting of Bi or total of Bi, Sb, and Ag in the frictional material of the present invention is less than 1% by weight, the resistance to melting and depositing is lowered. On the other hand, when the soft metal content is more than 5% by weight, the coefficient of friction is lowered, and further the relative Fe content is reduced, so that the magnetic flux density is lowered. Therefore, the soft metal content must be 1 to 5% by weight. It is preferred that the soft metal content is 1.5 to 3.0% by weight.

When the content of hard particles of at least one member selected from the group consisting of zircon or zircon and zirconia in the frictional material of the present invention is less than 1% by weight, both the coefficient of friction and the resistance to melting and depositing are lowered. On the other hand, when the hard particle content is more than 6% by weight, the relative Fe content is reduced, so that the magnetic flux density is lowered. Therefore, the hard particle content must be 1 to 6% by weight. It is preferred that the hard particle content is 2 to 4% by weight.

As an example of a method for producing the frictional material of the present invention, there can be mentioned the following method.
Commercially available Fe powder having an average particle size of 60 to 140 µm, Cu powder having an average particle size of 30 to 70 µm, Fe-P powder having an average particle size of 60 to 140 µm, Cu-P powder having an average particle size of 60 to 140 µm, Bi powder having an average particle size of 40 to 100 µm, Sb powder having an average particle size of 40 to 100 µm, In powder having an average particle size of 50 to 150 µm, Ag powder having an average particle size of 5 to 15 µm, ZrSiO₄ powder having an average particle size of 100 to 400 µm, ZrO₂ powder having an average particle size of 400 to 800 µm, MoS₂ powder having an average particle size of 3 to 10 µm, C powder having an average particle size of 50 to 350 µm, and SiC powder having an average particle size of 2 to 8 µm are prepared. These powders are weighed and mixed to achieve a desired composition. The resultant mixture is subjected to cold molding under a pressure of 196 and 490 MPa. The cold-molded mixture is placed in a sintering furnace and sintered in a hydrogen gas atmosphere under conditions such that the sintering temperature is 900 to 1,100°C and the sintering time is 0.5 to 2 hours, thus producing the frictional material of the present invention. When the sintering is performed while uniaxial pressing under a pressure of 0.49 to 3.92 MPa, the resultant frictional material advantageously has an improved wear resistance.

With respect to the frictional material of the present invention, for increasing the tension between the frictional material and a rail, it is preferred that the frictional material has a magnetic flux density of 1.38 to 1.42 T in a magnetic field at a strength of 50 kA/m. Further, with respect to the frictional material of the present invention, for further increasing the tension between the frictional material and a rail, it is more preferred that the frictional material has a magnetic flux density of 2.05 to 2.09 T in a magnetic field at a strength of 400 kA/m.

Examples of uses of the frictional material of the present invention include means for arbitrarily controlling the rotation or movement of various machines, such as machine tools, construction machineries, agricultural machineries, automobiles, two-wheeled vehicles, railroads, aircrafts, or marine structures, specifically, so-called clutches and brakes. Of these, more preferred are uses of the frictional material as a magnetic rail brake or an electromagnetic clutch, which fully utilize excellent magnetic properties of the frictional material, i.e., high magnetic flux density, and especially, the most preferred is use of the frictional material as a magnetic rail brake.

The frictional material of the present invention has excellent frictional properties including wear resistance and coefficient of friction, excellent resistance to melting and depositing, and excellent magnetic properties including magnetic flux density. When the frictional material of the present invention is used as a frictional material for clutch or brake in various machines, such as machine tools, construction machineries, agricultural machineries, automobiles, two-wheeled vehicles, railroads, aircrafts, or marine structures, the frictional material not only exhibits excellent damping properties but also extends the use life. The frictional material of the present invention has excellent magnetic properties and hence, when used as a frictional material for magnetic rail brake or electromagnetic clutch, the frictional material exhibits especially excellent damping properties and extends the use life.

Commercially available Fe powder having an average particle size of 100 µm, Cu powder having an average particle size of 49 µm, Fe-P powder having an average particle size of 100 µm, Cu-P powder having an average particle size of 100 µm, Bi powder having an average particle size of 70 µm, Sb powder having an average particle size of 70 µm, Ag powder having an average particle size of 10 µm, ZrSiO₄ powder having an average particle size of 200 µm, ZrO₂ powder having an average particle size of 600 µm, MoS₂ powder having an average particle size of 7 µm, C powder having an average particle size of 200 µm, and SiC powder having an average particle size of 5 µm were prepared. These raw material powders were weighed and mixed to achieve compositions shown in Table 1. The resultant mixture was subjected to cold molding under a pressure of 392 MPa, and then placed in a sintering furnace. The mixture was sintered using a hot press in a hydrogen gas atmosphere at a sintering temperature of 1,000°C under a pressure for uniaxial pressing of 1.5 MPa for one hour as a sintering time. Composition (wt%) Fe Cu P Bi Sb Ag ZrSiO₄ ZrO 2 MoS₂ C SiC Total Example 1 91.7 2.9 0.5 2.0 - - 3.0 - - - - 100 Example 2 93.6 1.0 0.5 2.0 - - 3.0 - - - - 100 Example 3 89.8 4.8 0.5 2.0 - - 3.0 - - - - 100 Example 6 92.6 2.9 0.5 1.0 - - 3.0 - - - - 100 Example 7 88.8 2.8 0.5 5.0 - - 3.0 - - - - 100 Example 8 88.8 2.8 0.5 3.0 1.0 1.0 3.0 - - - - 100 Example 9 93.6 2.9 0.5 2.0 - - 1.0 - - - - 100 Example 10 88.8 2.8 0.5 2.0 - - 6.0 - - - - 100 Example 11 91.7 2.9 0.5 2.0 - - 1.0 2.0 - - - 100 Comparative Example 1 94.5 - 0.5 2.0 - - 3.0 - - - - 100 Comparative Example 2 86.9 7.6 0.5 2.0 - - 3.0 - - - - 100 Comparative Example 3 92.2 2.9 - 2.0 - - 3.0 - - - - 100 Comparative Example 4 90.3 2.9 1.9 2.0 - - 3.0 - - - - 100 Comparative Example 5 93.6 2.9 0.5 - - - 3.0 - - - - 100 Comparative Example 6 85.9 2.7 0.4 8.0 - - 3.0 - - - - 100 Comparative Example 7 94.6 2.9 0.5 2.0 - - - - - - - 100 Comparative Example 8 86.9 2.7 0.5 2.0 - - 8.0 - - - - 100 Comparative Example 9 77.0 3.0 - 3.0 - - - - 3.0 7.0 10.0 100 Comparative Example 10 93.5 5.0 - - - - - - 0.5 0.5 0.5 100

A sample obtained by sintering was machined into a test specimen having a size: 10 mm x 10 mm x 8 mm, and a magnetic flux density of the sample was measured using a magnetic property tester (BH analyzer). The results are shown in Table 2. Further, an abrasion test shown below was conducted to measure a coefficient of friction, an abrasion wear, and an amount of melting and deposition. The results are also shown in Table 2. Abrasion test Test machine: Inertia abrasion tester Moment of inertia: 7.35 kgm² Speed: 33 m/s Contact pressure: 980 kPa Test specimen size: 25mm × 25mm × 10 mm Brake starting temperature: 100°C or lower Magnetic properties Frictional properties Resistance to melting and depositing Overall judgment (Good, Bad) Magnetic flux density (T) Coefficient of friction (-) Abrasion wear (mm) Amount of melting and deposition (Large, Medium, Small, None) In magnetic field at strength of 50 kA/m In magnetic field at strength of 400 kA/m Example 1 1.41 2.09 0.40 0.339 Small Good Example 2 1.42 2.09 0.39 0.345 Small Good Example 3 1.40 2.08 0.40 0.320 Small Good Example 6 1.41 2.08 0.40 0.350 Small Good Example 7 1.38 2.06 0.40 0.426 None Good Example 8 1.39 2.07 0.40 0.402 Small Good Example 9 1.39 2.08 0.39 0.389 Small Good Example 10 1.40 2.09 0.38 0.418 Small Good Example 11 1.38 2.06 0.40 0.311 None Good Example 12 1.41 2.09 0.40 0.352 Small Good Comparative Example 1 1.39 2.09 0.39 0.366 Large Bad Comparative Example 2 1.36 2.02 0.36 0.388 Large Bad Comparative Example 3 1.40 2.07 0.38 0.335 Large Bad Comparative Example 4 1.39 2.03 0.37 0.412 Medium Bad Comparative Example 5 1.41 2.09 0.40 0.338 Large Bad Comparative Example 6 1.34 2.01 0.35 0.456 Small Bad Comparative Example 7 1.41 2.07 0.34 0.433 Large Bad Comparative Example 8 1.35 2.00 0.40 0.310 Small Bad Comparative Example 9 1.23 1.85 0.40 0.341 Small Bad Comparative Example 10 1.42 2.09 0.32 0.620 Large Bad

Table 2 indicates: that the higher the magnetic flux density, the more excellent the magnetic properties; that the higher the coefficient of friction or the smaller the abrasion wear, the more excellent the frictional properties; and that the smaller the amount of melting and deposition, the more excellent the welding resistance. As can be seen from Table 2, in the Examples of the present invention, the balance between the magnetic properties, frictional properties, and resistance to melting and depositing is excellent and therefore, overall judgments better than those of the Comparative Examples were obtained.