Abstract:
A method for improving corrosion resistance in FNC cast iron substrates without the need for additional coating or painting. The exemplary methods remove a portion of the FNC coating applied to a cast iron substrate, preferably through polishing, to expose the epsilon phase portion of the compound area. The epsilon phase portion is thought to provide improved corrosion protection as compared to non-polished FNC cast iron substrates. One exemplary product that may be provided with improved corrosion protection according to the above method is a brake rotor having a FNC treatment.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 12/574,940 filed Oct. 7, 2009. 
     
    
     TECHNICAL FIELD 
       [0002]    The field to which the disclosure generally relates to methods for making ferritic nitrocarburized cast iron substrates more corrosion resistant. 
       BACKGROUND 
       [0003]    Motor vehicle disc brake systems utilize a disc brake rotor at each respective wheel, wherein the disc brake rotor typically includes a rotor hat for connecting to an axle hub of a rotatable axle of the motor vehicle, and at least one annular rotor cheek connected to the rotor hat, wherein the at least one rotor cheek has a pair of mutually opposed braking surfaces onto which brake pads are selectively applied when braking is desired. 
         [0004]    Typically, brake rotors are either made solid or are provided with internal ventilation. There are usually cast from iron-based alloys and especially cast iron such as grey cast iron a (G3000) and damped cast iron (G1800). Cast iron rotors are casted to near shape and machined to shape after casting. The disadvantage of cast iron rotors is that they exhibit insufficient corrosion resistance compared to other conventional materials. Winter climate and using the salt on roads can make the situation worse. 
         [0005]    To remedy corrosion issues with cast iron rotors, a ferritic nitrocarburizing (FNC) method to prevent the friction surface from corrosion during operation has been developed. However, the as-received FNC surface on non-frictional surface may still be prone to corrosion after exposure to a humid atmosphere. 
       SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
       [0006]    The exemplary embodiments provide a method for improving corrosion resistance in FNC cast iron substrates without the need for additional coating or painting. The exemplary methods remove a portion of the FNC coating applied to a cast iron substrate, preferably through polishing, to expose the epsilon phase portion of the compound area. The epsilon phase portion is thought to provide improved corrosion protection as compared to non-polished FNC cast iron substrates. 
         [0007]    One exemplary product that may be provided with improved corrosion protection according to the above method is a brake rotor having a FNC treatment. 
         [0008]    Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0010]      FIG. 1  illustrates a perspective view of a brake rotor according to one exemplary embodiment; 
           [0011]      FIG. 2  is a section microscopic view illustrating the ferritic nitrocarburized treatment applied to a portion of the brake rotor as in  FIG. 1 ; and 
           [0012]      FIG. 3  is a section microscopic view of a portion of the brake rotor of  FIG. 2  with a portion of the ferritic nitrocarburized treatment removed in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0013]    The following description of the embodiment(s) is merely exemplary (illustrative) in nature and is in no way intended to limit the invention, its application, or uses. 
         [0014]    The exemplary embodiments provide a method for improving corrosion resistance on cast iron substrates that include a ferritic nitrocarburized (FNC) surface treatment. Two specific exemplary products having FNC treated cast iron substrates include solid and vented brake rotors. 
         [0015]    Referring now to  FIG. 1 , a brake rotor  20  may be illustrated according to one exemplary embodiment as having a hat portion  22  with a rotor cheek  24  extending about the periphery thereof. The rotor cheek  24  may be generally referred to as a friction surface of the rotor  20  that engages the caliper and other brake parts to slow a vehicle during use, while the hat portion  22  may be generally referred to as a non-frictional surface that does not participate in the slowing of a vehicle through frictional engagement and disengagement. 
         [0016]    The shape of the brake rotor  20  as illustrated, and specifically the respective shapes and relative dimensions of the hat  22  and rotor cheek  24 , are but one specific example of a potentially infinite variety of possibilities or shapes and dimensions of brake rotors and are thus not limited as illustrated in  FIG. 1 . 
         [0017]    The brake rotor  20  may be formed from an iron-based alloy or steel, and especially cast iron such as grey cast iron a (G3000) and damped cast iron (G1800). 
         [0018]    A surface treatment  28  may be applied to the outer surface  26  of brake rotor  20  and provides the outer surface  26  with a degree of friction resistance and with a degree of corrosion resistance. 
         [0019]    In the exemplary embodiment as shown in  FIGS. 1 and 2 , the surface treatment  28  may be a ferritic nitrocarburized (FNC) coating  28  applied to a depth of between 10 and 20 microns extending from the outer surface  26 , and more preferably about 15 microns. The ferritic nitrocarburizing surface treatment  28  may enhance surface hardness and corrosion resistance in the brake rotor  20 , as well as providing increased friction for portions of the rotor  20  that engage the caliper and other brake parts, including the rotor cheek  24 , to aid in slowing the vehicle to which they are applied. 
         [0020]    The process for applying the FNC surface treatment  28  may be carried out at temperatures between about 525 and 650 degrees Celsius (975 and 1200 degrees Fahrenheit); the preferred process temperature may be approximately 565 degrees Celsius (1050 degrees Fahrenheit) to achieve the desired coating of about 10 to 20 microns. 
         [0021]    Upon application, as best shown in  FIG. 2 , a portion of the FNC coating  28  may diffuse into the outer surface  26  of the brake rotor  20  to form a diffusion layer  30 , while the remaining portion of the FNC coating  28  above the surface  26  may be referred to as the compound layer  32 . The compound layer  32 , as stated above, may preferably have a depth of between 10 and 20 microns, and more preferably about 15 microns, extending from the outer surface  26 . 
         [0022]    The diffusion layer  30  may contain a mix of the phases, including epsilon-Fe2-3(N,C) (the “epsilon phase” or “hexagonal phase”) and gamma-prime Fe4(N,C) (the “gamma phase”) and a ferrite phase that results from details of the process parameters such as temperature, heat treatment time, and gas composition and pressure. As shown in  FIG. 2 , the ferrite phase may become more predominant further away from the compound layer  32  and outer surface  26 . 
         [0023]    The compound layer  32  may also contain a specific mix of the phases, including the epsilon phase, the gamma phase, and a ferrite phase that results from details of the process parameters such as temperature, heat treatment time, and gas composition and pressure. 
         [0024]    The compound layer  32  may further be characterized as having an inner portion  33  closer to the outer surface  26  of the hat  22  (and diffusion area  30 ), and an outer surface portion  34 . 
         [0025]    The inner portion  33  may be considered substantially in the epsilon phase, also known as the dominant epsilon phase portion  33 . The outer surface portion  34  may contain a mix of the gamma phase, epsilon phase as well as oxides such as Fe 3 O 4 . 
         [0026]    Next, as best shown in  FIG. 3 , the hat  22 , or other non-frictional surfaces of the brake rotor  20  (not shown), may be treated to remove the outer surface portion  34  and expose the underlying inner portion  33  of the compound area  32 . More specifically, the treatment removes enough of the outer surface portion  34  of the compound layer  32  to expose the dominant epsilon phase portion  33  there within. In one exemplary embodiment, for a surface treatment  28  in which the total compound layer  32  depth is between about 10 and 20 microns, the treatment may remove about 2 and 6 microns of the outer surface portion  34  to expose the interior portion  33 . 
         [0027]    The exposure to the epsilon phase portion  33  is believed to provide improved corrosion resistance to the non-frictional surfaces of the brake rotor  20  as compared with a non-polished surface treatment (i.e. where the outer surface portion  34  remains intact and may include primarily the gamma phase and oxides are described above). 
         [0028]    In one exemplary embodiment, the treatment may consist of grinding, conditioning or polishing, preferably with a diamond paste of 1 micron particles, of the outer coating surface  34  inward to a depth of between about 2 and 6 microns to expose the dominant epsilon phase  33  portion of the compound area  32 . Experimental testing of rotors  20  according to this treatment confirm that samples having the exposed dominant epsilon phase portion  33  in the hat  22  exhibited less corrosion compared to the rotors  20  in which the outer coating  34  within the hat  22  remained unpolished. 
         [0029]    While the above method for improving the corrosion resistance was specifically discussed with respect to brake rotors  20  in the exemplary embodiments as described above, a similar improvement in corrosion resistance may be expected in any cast iron substrate in which an FNC surface treatment has been utilized. Thus, the exemplary method for improving corrosion resistance may be equally applicable to any FNC treated cast iron substrate. 
         [0030]    The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.