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  Friction material and manufacturing method thereofThe Patent Description & Claims data below is from USPTO Patent Application 20070148428. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001]This application claims foreign priority from Japanese Patent Application No. 2005-379127, filed Dec. 28, 2005, the entire contents of which are hereby incorporated by reference. BACKGROUND OF THE INVENTION [0002]1. Field of the Invention [0003]The present invention relates to a friction material of a brake and a clutch, used, for example, in automobiles and industrial machines, and a manufacturing method thereof. More specifically, the invention relates to a friction material with a scarce change in the friction coefficient and capable of ensuring a stability of the friction coefficient at the initial stage of using new friction materials, as well as a manufacturing method thereof. [0004]2. Related Art [0005]In recent years, along with improvement in the performance and increase in the power of automobiles, thermal stability has been demanded also for a friction material such as a brake lining used, for example, in drum brakes, disc brakes, etc. of automobiles. Specifically, mechanical strength, structural stabilities and wear-resistance at elevated temperatures are required, as well as stable friction characteristics such as less change of friction coefficient during a frequent change of temperatures have been demanded. [0006]Generally, the friction material is composed of various ingredients. That is, they include matrix fibers such as glass fibers and aramid fibers, lubricants such as graphite and molybdenum disulfide, fillers, and binders such as phenolic resin. [0007]Further, as the fillers, materials adjusting friction characteristics such as organic dust, metal powder, inorganic compound have been used in addition to the lubricant such as graphite and molybdenum disulfide. As other fillers, materials so-called abrasives such as alumina having 7 or more Mohs hardness, i.e., are used for increasing the friction coefficient of the friction materials high. [0008]In addition to the abrasives described above, those capable of remarkably raising the friction coefficient of the friction materials include monoclinic zirconia. For example, JP-A-62-020581 discloses a friction material composition, for use in a brake lining, containing a zirconium oxide powder by 0.5 to 10% by weight. JP-A-03-185030 discloses an on-asbestos type friction material incorporated with zirconium oxide as at least one kind of fillers. Further, abrasives improving the friction characteristic at high temperature include stabilized zirconia. For example, JP-A-09-031440 discloses a friction material containing zirconium oxide in which the zirconium oxide is stabilized with at least one of calcia (CaO), yttria (Y.sub.2O.sub.3), and magnesia (MgO). JP-A-2000-160135 discloses compositions of friction materials containing 1 to 20% by weight of partially stabilized or fully stabilized zirconium oxide having a Mohs hardness of 7 or more and an average particle size of from 0.2 to 70 micron (.mu.m). JP-A-09-031440 discloses that the amount of wear can be reduced by using stabilized zirconia instead of unstabilized monoclinic one. JP-A-2000-160135 discloses that it is possible to prevent generation of uncomfortable sound called as groan by the use of the monoclinic zirconia. In each of them, the problem caused by structural phase transition of monoclinic zirconia at high temperatures (800 to 1200.degree. C.) from a monoclinic crystal phase to a tetragonal one is solved by the use of stabilized zirconia which does not cause any structural phase transitions and, therefore, maintains cubic crystals at the high temperatures. [0009]However, in the existent friction materials using the stabilized zirconia, after a brake is used under conditions at which so-called "fading phenomena" occur or under other high load conditions at an initial stage of using a new friction materials, the friction coefficient changes greatly to result in a problem such as occurrence of various kinds of noise. Although to improve the problems, various studies on the amount of addition, particle sizes, and/or stabilized ratio of stabilized zirconia have been attempted in the prior art, the problems have not been solved till now. SUMMARY OF THE INVENTION [0010]In view of the foregoings, the present invention intends to provide a friction material having stable friction characteristics such as less change of friction coefficient at the initial stage of using new friction materials even after the friction materials were used under severe braking conditions in which such as fading phenomena occur. [0011]The inventors have noted on the lattice constant of stabilized zirconia contained in the friction materials which had not considered in the above-mentioned Japanese Patent publications and the values of the lattice constant also thereof have not yet been analyzed. As a result of their studies, it has been found that the lattice constant of stabilized zirconia contained in the friction material changed greatly before and after the use of the friction material. Then, the invention has been accomplished based on the finding that the friction coefficient at the initial stage can be stabilized with a friction material containing stabilized zirconia having a lattice constant within a predetermined range. That is, in accordance with one or more embodiments of the present invention, a friction material is provided with a matrix fiber, a binder and a filler and utilizing a stabilized zirconia having a lattice constant in a range from 99.93 to 99.95% as compared with a lattice constant of a single crystal of stabilized zirconia. The stabilized zirconia having the lattice constant in the range from 99.93 to 99.95% as compared with that of the single crystal of stabilized zirconia may be a calcia-stabilized zirconia having a lattice constant of from 5.1253 to 5.1263 .ANG.. Further, in accordance with one or more embodiments of the present invention, a friction material is provided with a matrix fiber, a binder, and a filler in which a stabilized zirconia having a lattice constant in a range from 99.95 to 99.97% as compared with a lattice constant of a single crystal of stabilized zirconia is used as a compound of the filler. The stabilized zirconia may be a calcia-stabilized zirconia having a lattice constant in a range from 5.1263 to 5.1275 .ANG.. Further, in accordance with one or more embodiments of the present invention, a method of manufacturing a friction material is provided with steps of uniformly mixing a matrix fiber, a binder, and a filler, and molding a mixture of them under heating and pressing, in which a stabilized zirconia having a lattice constant in a range from 99.95 to 99.97% compared with a lattice constant of a single crystal of stabilized zirconia is used as a compound in the filler. The stabilized zirconia may be calcia-stabilized zirconia having a lattice constant in a range from 5.1263 and 5.1275 .ANG.. [0012](Note) .ANG.: angstrom (10.sup.-1 nm) is not the SI unit (The International System of Units), however since this is still customarily used often in the field of crystallography to which the invention is concerned, this is used as a unit of the lattice constant throughout the description. [0013]As described above, change of the lattice constant of stabilized zirconia and that of the friction coefficient are in a close relation. The friction material containing stabilized zirconia having its lattice constant in a certain range changes its constant scarcely even after a friction of the friction material under a high load condition. Therefore, the friction coefficient of such a friction material also scarcely changes after the test. In the consequence stability of the friction coefficient at the initial stage of using a new friction material can be ensured by using the friction material of this invention. [0014]Other aspects and advantages of the invention will be apparent from the following description and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS [0015]FIG. 1 is a graph showing a relation between an annealing time and a lattice constant; [0016]FIG. 2 is a graph showing the result of X-ray diffraction of calcia-stabilized zirconia; [0017]FIG. 3 is a graph showing the result of X-ray diffraction of silicon (Si); [0018]FIG. 4 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 311 peak of Si in FIG. 3; [0019]FIG. 5 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 400 peak of Si in FIG. 3; [0020]FIG. 6 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 331 peak of Si in FIG. 3; [0021]FIG. 7 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 422 peak of Si in FIG. 3; Continue reading... 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