Abstract:
A lightweight brake disk is made of a Titanium alloy and coated with a coating material that is hard and wear resistant. The aesthetically pleasing, wear resistant coating overlays wear surfaces and portions of the brake disk that will be visible when the brake disk is installed on the vehicle. The coating includes a first layer of a metal, such as amorphous Titanium metal, and a second layer that preferably includes a Nitride, Boride, Carbide or Oxide of the metal used in the first layer. The coating is preferably applied using a physical vapor deposition source such as a cathodic arc source with a controlled gas atmosphere.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to coated brake disks and rums and methods for coating brake disks and drums. More particularly, the resent invention pertains to brake disks and drums that are coated with a hard, wear resistant coating. The present invention is particularly, but not exclusively, useful as a lightweight brake disk for use on motorcycles and automobiles.  
       BACKGROUND OF THE INVENTION  
       [0002]     There are a number of reasons why it is important to minimize the weight of a brake disk (also sometimes referred to as a brake rotor). First, the weight of the brake disk contributes to the overall weight of the vehicle, and in this respect, affects the vehicle&#39;s fuel efficiency and performance. Additionally, the weight of the brake disk adds to the vehicles “unsprung weight” which is generally considered to be the primary source of vehicle noise and vibration. In addition, less energy is required to rotate a relatively light brake disk during vehicle travel, and accordingly, a reduction in brake disk weight is generally accompanied by an increase in the vehicle&#39;s fuel efficiency and performance. A final consideration, which is especially important for brake disks used on motorcycles, is the effect of brake disk weight on the motorcycle&#39;s handling characteristics. In greater detail, gyroscopic inertia is generated when a brake disk is rotated. Specifically, for a given rotation velocity, a heavy brake disk generates more gyroscopic inertia that a light brake disk. This gyroscopic inertia, in turn, must be overcome by the rider to steer the motorcycle. The result is that a heavier brake disk adversely affects a motorcycle&#39;s handling characteristics.  
         [0003]     During braking, hydraulic energy is used to press the vehicle&#39;s brake pads against the rotating brake disk. The friction resulting from the moving contact between brake pad and brake disk slows the rotation of the brake disc and decreases the speed of the vehicle. This frictional contact generates heat and causes the contact surfaces on the brake pad and brake disk to wear unevenly. Excessive wear can cause the brake disk to become thin and weak. In some cases, the thinning of the brake disk becomes so severe that the brake disk is no longer able to support the stresses and heat generated during braking. The result is typically a warped brake disk that can cause undesirable brake chattering.  
         [0004]     Conventional brake disks have typically been made of cast iron. Cast iron is relatively inexpensive, machines freely and has adequate strength and wear resistance at the relatively high brake system service temperatures. On the other hand, cast iron brake disks are relatively heavy due to the high density of cast iron. For example, the density, ρ, of cast iron is approximately 7.4 gms/cc compared to light metals such as aluminum (ρ≅2.7 gms/cc) and Titanium (ρ≅4.5 gms/cc). Thus, a significant weight reduction could be achieved by using a light metal in place of cast iron. In this respect, aluminum has been considered for use in brake disks, however, aluminum&#39;s high temperature properties are inadequate for most brake rotor applications.  
         [0005]     On the other hand, Titanium and its alloys have relatively low densities and maintain good mechanical properties at the elevated temperatures seen in most brake disk applications. Titanium alloys are, however, relatively soft and susceptible to wear and galling at the contact surfaces. As indicated above, this wear and galling can lead to thinning, weakening and warpage of the brake disk. It follows that Titanium alloys would be suitable for brake disk application if the contact surfaces could be modified or coated to prevent wear and galling at these surfaces.  
         [0006]     A final factor that must be considered when designing brake rotors is aesthetics. Modern racing motorcycles have rather large diameter brake disks that are plainly visible, especially the front disk. Because of this visibility, the color and surface appearance of a brake disk can add to or detract from the overall look of the motorcycle. These considerations can affect a purchaser&#39;s decision when buying a new motorcycle and when retrofitting a motorcycle with a new brake system.  
         [0007]     In light of the above, it is an object of the present invention to provide lightweight brake disks. It is another object of the present invention to provide lightweight brake disks that are coated with a material that is hard and wear resistant. Another object of the present invention is to provide methods for coating brake discs with a hard, wear resistant coating that is aesthetically pleasing.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to coated brake disks and methods for coating brake disks. For the present invention, a typical brake disk is disk-shaped having a central hole (or in some cases multi-holes) to allow the brake disk to be positioned over a hub. The brake disk is further formed with a pair of flat annular surfaces that extend from the central hole to the periphery of the brake disk. These flat surfaces are provided for contact with the brake pads during braking and constitute the wear surfaces for the brake disk.  
         [0009]     In accordance with the present invention, the brake disk is made of a Titanium alloy such as Titanium-6 Aluminum-4 Vanadium or Titanium-6 Aluminum-2 Tin-4 Molybdenum-2 Zirconium, and accordingly is relatively lightweight as compared to a similarly sized brake disk made of cast iron. Importantly, a portion of the Titanium alloy brake disk including the wear surfaces is coated with a coating material that is hard and wear resistant. Further, the grey, aesthetically pleasing coating material is preferably deposited on portions of the brake disk that will be visible when the brake disk is installed on the vehicle. In one implementation of the present invention, the coating is deposited on nearly the entire brake disk.  
         [0010]     In one aspect of the present invention, the coating includes a first layer of a material having an amorphous structure (i.e. a non-crystalline structure). In a particular embodiment, the amorphous material is a metal such as Titanium, Chromium, Zirconium, Aluminum or an alloy thereof. The first layer is applied directly on the Titanium alloy brake disk. The coating further includes a second layer that overlays and contacts the first layer. The second layer preferably includes one or more Metal Nitrides, Metal Borides, Metal Carbides and Metal Oxides. More preferably, the second layer includes one or more Nitrides, Borides, Carbides or Oxides of the metal used in the first layer. For example, for a coating having Titanium as the first layer, the second layer can be Titanium Nitride (TiN). Note; the abbreviations (e.g. TiN) are used herein as a shorthand rather than an exact chemical label, and do not suggest that the stoichiometry of the indicated compound must be exactly as stated in the abbreviation.  
         [0011]     In accordance with the present invention, both layers of the coating are preferably applied using a physical vapor deposition source such as a cathodic arc source with a controlled gas atmosphere. Other operable techniques such as unbalanced magnetron sputtering may also be used. During coating deposition, the brake disks are positioned on a fixture and the fixture is rotated in a planetary movement about a central axis. In greater detail, the fixture includes three parallel poles that are mounted on a plate and arranged wherein each pole is spaced at an equal distance from the other two poles. A plurality of brake disks can be stacked on each pole, with spacers to separate adjacent disks within each stack. The poles are spaced from each other to allow the brake disks on one pole to overlap the brake disks on an adjacent pole. The spacers prevent brake disks on one pole from contacting the brake disks on an adjacent pole. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0013]      FIG. 1  is a perspective view of a motorcycle having a disk brake system;  
         [0014]      FIG. 2  is a perspective view of a coated disk brake;  
         [0015]      FIG. 3  is an enlarged cross-sectional view of a portion of the coated disk brake shown in  FIG. 2  as seen along line  3 - 3  in  FIG. 2  showing the coating layers;  
         [0016]      FIG. 4  is a front elevation view of a fixture for supporting the disk brakes during the coating process;  
         [0017]      FIG. 5  is a top plan view of a fixture for supporting the disk brakes during the coating process;  
         [0018]      FIG. 6  is a schematic plan view and control diagram of a deposition apparatus for use in the invention;  
         [0019]      FIG. 7  is a schematic perspective view of a detail of the deposition apparatus of  FIG. 5 ; and  
         [0020]      FIG. 8  is a schematic cross-sectional view of a preferred cathodic arc source, taken along lines  8 - 8  of  FIG. 7 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]     Referring to  FIG. 1 , motorcycle  10  is shown that includes a disk brake system. As shown, the disk brake system includes a brake disk  12  that is attached to the front wheel  14  of the motorcycle  10  for rotation therewith. The brake system further includes a caliper  16  having a pair of brake pads that can be selectively applied against the brake disk  12  using hydraulic pressure to slow the rotation of the brake disk  12  and wheel  14 . In a typical setup, the hydraulic pressure is provided by the motorcycle operator using a hand lever mounted on the handlebars of the motorcycle  10 .  
         [0022]     A better appreciation of a brake disk  12  can be obtained with reference to  FIG. 2 . As shown, the brake disk  12  is disk-shaped having a central hole  18  to allow the brake disk  12  to be positioned over the hub of the wheel  14  (shown in  FIG. 1 ). The brake disk  12  is further formed with flat annular surfaces  20   a,b  (see also  FIG. 4 ) that extend from the central hole  18  to the periphery  22  of the brake disk  12 . As shown, flat surface  20   a  is parallel with and opposed to flat surface  20   b  on the brake disk  12 . These flat surfaces  20   a,b  are provided for contact with the brake pads during braking and constitute the wear surfaces for the brake disk  12 .  
         [0023]     Referring now to  FIG. 3 , a coating  24  is shown applied to a brake disk substrate  26 . For the brake disk  12 , the brake disk substrate  26  is made of a Titanium alloy such as Titanium-6 Aluminum-4 Vanadium or Titanium-6 Aluminum-2 Tin-4 Molybdenum-2 Zirconium, and accordingly is relatively lightweight as compared to a similarly sized and shaped brake disk made of cast iron. As further shown in  FIG. 3 , the coating  24  includes a first layer  28  of a material having an amorphous structure (i.e. a non-crystalline structure). In a particular embodiment, the amorphous material is a metal such as Titanium, Chromium, Zirconium, Aluminum or an alloy thereof.  
         [0024]     Continuing with  FIG. 3 , the coating  24  further includes a second layer  30  that overlays and contacts the first layer  28 . The second layer  30  preferably includes one or more Metal Nitrides, Metal Borides, Metal Carbides and Metal Oxides. More preferably, the second layer includes one or more Nitrides, Borides, Carbides or Oxides of the metal used in the first layer. In a particular embodiment of a coating  24 , amorphous Titanium constitutes the first layer  28  and a Titanium Nitride (TiN, Ti 2 N, etc.) constitutes the second layer  30 . With this cooperation of structure, a coating  24  having a service life to exceed approximately 12,000 vehicle miles can be obtained. Note: the abbreviations (e.g. TiN, Ti 2 N, etc.) are used herein as a shorthand rather than an exact chemical label, and do not suggest that the stoichiometry of the indicated compound must be exactly as stated in the abbreviation.  
         [0025]     Referring now with cross-reference to  FIGS. 4 and 5 , a fixture  34  is shown for holding the brake disk substrates  26  during coating. Although the fixture  34  is shown holding five brake disk substrates  26   a - e , it is to be appreciated that the fixture  34  is merely exemplary and that fewer or more brake disk substrates  26  could be positioned on a fixture  34 . As shown, the fixture  34  includes three parallel poles  36 ,  38 ,  40  that are mounted on and extend from a base plate  42 . The parallel poles  36 ,  38 ,  40  are arranged on the base plate  42  with each pole  36 ,  38 ,  40  spaced at an equal distance from the other two poles  36 ,  38 ,  40 . With this cooperation of structure, a plurality of brake disk substrates  26  can be stacked on each pole  36 ,  38 ,  40 . For example, as shown, brake disk substrates  26   a  and  26   d  are stacked on pole  36 , brake disk substrate  26   c  is stacked pole  38  and brake disk substrates  26   b  and  26   e  are stacked on pole  40 .  
         [0026]     Continuing with cross-reference to  FIGS. 4 and 5 , it can be seen that spacers  44   a - e  can be used to selectively separate adjacent brake disk substrates  26  on each pole  36 ,  38 ,  40 . For the implementation shown, each spacer  44   a - e  includes a tube  46  and flange  48  allowing each spacer  44   a - e  to be slid over a respective pole  36 ,  38 ,  40  and positioned as desired. In the implementation shown in  FIGS. 4 and 5 , the spacing between poles  36 ,  38  is established to allow the brake disk substrates  26  on one pole  36 ,  38 ,  40  to overlap the brake disk substrates  26  on an adjacent pole  36 ,  38 ,  40 . Also for the implementation shown in  FIGS. 4 and 5 , the spacers  44   a - e  have been sized to prevent brake disk substrates  26  on one pole  36 ,  38 ,  40  from contacting the brake disk substrates  26  on an adjacent pole  36 ,  38 ,  40 .  
         [0027]      FIGS. 6 and 7  depict a preferred deposition apparatus  50  for coating the brake disk substrates  26 , although other operable deposition apparatus may be used. The deposition apparatus  50  includes a chamber  52  having a body  54  and a door  56  that may be opened for access to the interior of the chamber  52  and which is hermetically sealed to the body  54  when the chamber  52  is in operation. The interior of the chamber  52  is controllably evacuated by a vacuum pump  58  pumping through a gate valve  60 . The vacuum pump  58  includes a mechanical pump and a diffusion pump operating together in the usual manner. The interior of the chamber  52  may be controllably backfilled to a partial pressure of a selected gas from a gas source  62  through a backfill valve  64 . The gas source  62  typically includes several separately operable gas sources. The gas source  62  usually includes a source  62   a  of an inert gas such as argon and a source  62   b  of Nitrogen gas, each providing gas selectively and independently through a respective selector valve  65   a  or  65   b . Other types of gas can also be provided as desired, such as gases required to produce borides, oxides and/or carbides.  
         [0028]     The pressure within the chamber  52  is monitored by a vacuum gage  66 , whose output signal is provided to a pressure controller  68 . The pressure controller  68  controls the settings of the gate valve  60  and the backfill valve  64  (and, optionally, the selector valves  65 ), achieving a balance of pumping and backfill gas flow that produces a desired pressure in the chamber  52  and thence pressure reading in the vacuum gauge  66 . Thus, the gaseous backfilled atmosphere within the chamber  52  is preferably a flowing or dynamic atmosphere.  
         [0029]     At least two, and preferably four as shown, linear deposition sources  70  are mounted within the interior of the chamber  52  in a circumferentially spaced-apart manner. In  FIG. 6 , the four deposition sources are identified as distinct sources  70   a ,  70   b ,  70   c , and  70   d , as they will be addressed individually in the subsequent discussion. The four deposition sources  70  are generally rectangular bodies having a greatest rectilinear dimension elongated parallel to a source axis  72 . This type of deposition source is distinct from either a stationary point source or a point source that moves along the length of the substrate  26  during deposition procedures.  
         [0030]     A support  74  is positioned in the chamber  52 . The support  74  produces a compound rotational movement of a fixture  34  mounted thereon. The preferred support  74  includes a rotational carriage  76  that rotates about an axis  78 , driven by a rotational drive motor  80  below the rotational carriage  76 . Mounted on the rotational carriage  76  are at least one and preferably six, as shown, planetary carriages  82 . The planetary carriages  82  are rotationally driven about a rotational axis  84  by a planetary drive motor  86  below the planetary carriages  82 . The speeds of the rotational drive motor  80  and the planetary drive motor  86  are controlled by a rotation controller  88 . The rotation controller  88  preferably rotates the rotational carriage  76  at a rate of about 1 revolution per minute (rpm).  
         [0031]     Continuing with  FIGS. 6 and 7 , for deposition processing of brake disk substrates  26 , a fixture  34  as described above can be mounted on the planetary carriage  82 , as shown. For commercial operations, a fixture  34  having a plurality of brake disk substrates  26  is typically mounted on each planetary carriage  82  in the manner described, as illustrated for one of the planetary carriages  82  in  FIG. 6 .  
         [0032]     The temperature in the chamber  52  during deposition is controlled using a heater  92  that extends parallel to the deposition sources  70  on one side of the interior of the chamber  52 . The heater  92  is preferably a radiant heater operating with electrical resistance elements. The temperature of the heating array is monitored by a temperature sensor  94  such as an infrared sensor that views the interior of the chamber  52 . The temperature measured by the sensor  94  is provided to a temperature control circuit  96  that provides the power output to the heater  92 . Acting in this feedback manner, the temperature controller  96  allows the temperature of the heating array to be set. In the preferred processing, the heating array is heated to a temperature of from about 1000° F. to about 1700° F.  
         [0033]      FIG. 8  illustrates a cathodic arc source  100  used in the preferred form of the deposition source  70 . The cathodic arc source  100  includes a channel-shaped body  102  and a deposition target  104 . The deposition target  104  is in the form of a plate that is hermetically sealed to the body  102  using an O-ring  106 , forming a water-tight and gas-tight hollow interior  108 . The interior  108  is cooled with cooling water flowing through a water inlet  110  and a water outlet  112 . Two spirally shaped (only sections of the spirals are seen in  FIG. 8 ) permanent magnets  114  extend parallel to the source axis  72 . Positioned above the deposition target  104  exterior to the body  102  is a striker electrode  118 . A voltage V ARC  is applied between the striker electrode  118  and the deposition target  104  by an arc source power supply  120 . V ARC  is preferably from about 10 to about 50 volts.  
         [0034]     The metallic material that forms the deposition target  104  is deposited onto the brake disk substrate  26  together with, if desired, gas atoms producing gaseous species from the atmosphere of the chamber  52 . For the embodiment describe herein, the deposition target  104  is made of Titanium (Ti) metal.  
         [0035]     To accomplish the deposition, an arc is struck between the striker electrode  118  and the deposition target  104 , locally heating the deposition target  104  and causing Titanium atoms and/or ions to be ejected from the deposition target  104 . (The deposition target  104  is therefore gradually thinned as the deposition proceeds.) The striking point of the arc on the deposition target  104  moves in a racetrack course along the length of the deposition target  104 . A negative bias voltage V BIAS  is applied between the deposition target  104  and brake disk substrate  26  by a bias power supply  122 , so that any positively charged ions are accelerated toward the brake disk substrate  26 .  
         [0036]     V BIAS  is preferably from about −30 to about −600 volts. The value selected for V BIAS  determines the energy of ionic impact against the surface of the substrates, a phenomenon termed ion peening. In a typical case, V BIAS  is initially selected to be a relatively large negative voltage to achieve good adherence of the metallic first layer  28  (see  FIG. 3 ) to the brake disk substrate  26 . V BIAS  is subsequently reduced (made less negative) when the overlying hard layer is deposited, to achieve a uniform, fine microstructure in the overlying layer. The values of V BIAS  are desirably maintained as low as possible, consistent with obtaining an adherent coating  24 . V BIAS  is more positive than −600 volts, and most preferably more positive than −400 volts. If V BIAS  is too negative, corona effects and backsputtering may occur at some regions of the brake disk substrate  26 . Thus, while higher V BIAS  voltages may be used in some instances, generally it is preferred that V BIAS  be more positive than −600 volts. The cathodic arc source  100  is preferred, but other types of sources, such as sputtering sources, may also be used.  
         [0037]     The cooperative selection of the material of the deposition target  104  and the gases introduced into the deposition chamber  52  from the gas source  62  allows a variety of coatings  24  to be deposited onto the brake disk substrate  26 , within the constraints discussed previously. The total thickness of the coating  24  is preferably from about 1 to about 10 micrometers. If the coating thickness is less than about 1 micrometer, the physical properties of the coating  24  are insufficient to produce the desired results. If the coating thickness is more than about 10 micrometers, the coating  24  has a high internal stress that leads to a tendency for the coating  24  to crack and spall away from the brake disk substrate  26  during deposition or during service.  
         [0038]     These general principles are applied in preparing the coatings  24  of interest, as described previously in relation to  FIG. 3 . The coating  24  of  FIG. 3  includes an amorphous metallic first layer  28 , such as amorphous metallic Titanium, that contacts and overlays the surface of the brake disk substrate  26 . The amorphous metallic first layer  28  is deposited by backfilling the deposition chamber  52  with a small partial pressure of about 5 microns of an inert gas, such as flowing argon (flowing at a rate of about 200-450 standard cubic centimeters per minute (sccm) in the apparatus used by the inventors), and then depositing metal, such as Titanium, from the deposition target  104  with V BIAS  about −400 volts. Because the argon does not chemically react with the metal, an amorphous metallic first layer  28  is deposited.  
         [0039]     As shown in  FIG. 3 , a second layer  30 , which for the embodiment described herein is a metal Nitride, overlies the amorphous metallic first layer  28 . The second layer  30  is deposited by backfilling the deposition chamber  52  with a small partial pressure of about 5 microns of flowing Nitrogen (flowing at a rate of about 150-500 seen in the inventors&#39; apparatus), and then depositing metal, such as Titanium, from the deposition target  104  with V BIAS  about −50 volts. The metal combines with the Nitrogen to produce the metal Nitride in the second layer  30 .  
         [0040]     While the particular brake disks and methods for coating as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.