Abstract:
A method of enhancing wear resistance of a metallic substrate includes applying a coating of an interstitial element to a surface of a substrate. A laser beam is directed onto a localized area of the metallic substrate coated with the interstitial element locally raising a temperature of the metallic substrate to a temperature causing the interstitial element to diffuse into the substrate. A layer of alloy including the interstitial element is generated onto the localized area of the metallic substrate. A focal point of the laser beam is disposed at a location spaced from the surface of the substrate for optimizing a power density of the laser beam at the surface of the substrate. The coating of interstitial element not diffused into the substrate is removed exposing the layer of alloy including the interstitial element.

Description:
PRIOR APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/813,297, filed Apr. 18, 2013. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates generally toward an improved process for increasing hardness of a soft metallic substrate. More specifically, the present invention relates toward the use of a laser to assist interstitial alloying of a soft metallic substrate. 
       BACKGROUND 
       [0003]    A dichotomy exists when selecting metallic substrate for use in industrial processes or mechanical devices that are subject to frictional forces. During a fabricating or forming process, it is preferable to select a soft material for ease of forming. However, a selection of soft material substrates results in poor durability, particularly when the device is subject to frictional forces. Therefore, when durability of a mechanical device is desired, a hard metallic substrate is selected, which is problematic when fabricating or forming the device. 
         [0004]    Various attempts have been made to coat soft metallic substrates to improve wear resistance and related material loss known to cause adverse dimensional changes to the substrate. For example, plasma coatings and weld overlays have been used, but offer limited durability and significantly increase the cost of fabricating due to requisite post-machining operations. Vapor deposition has also been used to increase surface hardness. However, mechanical bonds between the coating and the substrate are weak causing the coating to degrade or lose adhesion causing vapor deposition to be of limited use. 
         [0005]    Diffusion of interstitial elements having higher a hardness value than a soft alloy substrate has been experimented with, but has not achieved significant industrial use. Various attempts to improve control over an interstitial alloying have not proven affective. Therefore, it would be desirable to provide an enhanced process for increasing a hardness of a substrate by way of diffusion of an interstitial alloy. 
       SUMMARY 
       [0006]    A method of enhancing wear resistance of a metallic substrate includes applying a coating including an interstitial element to a surface of the substrate. A laser beam is directed onto a localized area of the metallic substrate coated with the interstitial element. The laser beam locally raises a temperature of the metallic substrate to a temperature causing the interstitial element to diffuse into the substrate providing a layer of alloy including the interstitial element onto the localized area of the metallic substrate. A focal point of a laser beam is positioned at a spaced location from the surface of the substrate to optimize a power density of the laser beam at the surface of the substrate. The coating of the interstitial element not diffused into the substrate is removed exposing a layer of alloy including the interstitial element. 
         [0007]    The present inventive method provides an enhanced ability to control excitation of substrate molecules to control diffusion of interstitial elements into a soft metallic substrate. By controlling the focal point relative to the surface of the substrate an optimum energy beam and energy configuration is achieved to enhance control over the diffusion process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detail description when considered in connection with the accompanying drawings, wherein: 
           [0009]      FIG. 1  shows a metallic substrate; 
           [0010]      FIG. 2  shows a metallic substrate with a localized application of a coating including an interstitial element; 
           [0011]      FIG. 3  shows a laser heating a localized area of the soft metallic substrate having a coating including an interstitial element; 
           [0012]      FIG. 4  shows an alternative method of locally raising a temperature of the soft metallic substrate; 
           [0013]      FIG. 5  shows a cylindrical component being subject to the method of the present invention; 
           [0014]      FIG. 6  shows a process of diffusing an inside of a tubular component using a galvanometer to redirect the laser beam of the present application; and 
           [0015]      FIG. 7  shows a chart of experimental hardness of a substrate being subject to the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , a metallic substrate in the form of a planar component is generally shown at  10 . The metallic substrate  10  is contemplated to be formed from metals, such as, for example, various steels, nickel alloys, cobalt alloys, aluminum alloys, and copper alloys. It is anticipated that the substrate  10  is machined or formed into a final shape through grinding, machining, or turning as is known to those of skill in the art. The substrate  10  is contemplated by the inventor to be any substrate  10  subject to frictional or other mechanical forces known to degrade the geometry and function of the substrate  10 . 
         [0017]    Knives, mechanical parts, such as, for example, piston heads, other internal combustion elements and any metallic component subject to wear are all believed to be enhanced by the process of the present invention. After processing, the substrate  10 , it is desirable to include a surface roughness having an Ra value of less than about 20 microns and an Rt value of less than about 100 microns. As set forth above, the part geometry includes a flat knife blade, a rotary knife blade, an engine cylinder liner, or a piston ring for an engine. It should be understood by those of ordinary skill in the art that any metallic substrate subject to durability requirements is included within the scope of this invention. 
         [0018]      FIG. 2  shows the metallic substrate  10  having a coating  12  applied over an area of interest known to be subject to frictional forces. The coating includes an interstitial element having an atomic size known to allow diffusion into a lattice structure of an alloy. More specifically, the coating includes at least one of hydrogen, boron, carbon, or nitrogen. Additionally, combinations of these interstitial elements are included within the scope of this invention to further enhance wear resistance of the metallic substrate  10 . 
         [0019]    The coating  12  is applied either as a powder, or a liquid, in which case, a solvent is used to liquefy and suspend the interstitial element of choice. The solvent is either water or organic, but is selected to flash from the surface of the substrate  10  without requiring significant amount of time or heat. When a liquid coating  12  is applied to the substrate  10 , the substrate  10  is preheated in an oven to a temperature of about 240° C. for about 20 minutes so that the substrate (or component) receives a uniform temperature. It should be understood by those of ordinary skill in the art that the temperature selected to flash the solvent from the coating  12  is below the melting temperature of the substrate  10  alloy to prevent adversely affecting the dimensional configuration of the component. After preheating, the component is removed from an oven and a coating including carbon black powder is applied, or other interstitial element, using an aerosol or atomizing spray method. The coating includes a uniform thickness over the surface requiring improved wear resistance. In the alternative, a tape comprising an interstitial element is applied to an area of interest that requires enhanced wear protection. 
         [0020]    Referring now to  FIG. 3 , a laser  14  is shown projecting a laser beam  16  (or energy beam) onto an area of interest  18  that has received a coating  12  including an interstitial element. The laser comprises a CO 2  laser, a diode laser, a fiber optic laser, or any equivalent energy source, capable of directing the laser beam  16  to a localized area of interest  18  of the substrate. The laser beam  16  defines a laser focal point  20  that is located at a position spaced from the surface of the substrate  10  determined to optimize the power density of the laser beam at the surface of the substrate  10 . For example, it is believed that locating the focal point on the surface of the substrate  10  or too close to the surface of a substrate results in generating a cast iron surface known not to provide durable property achieved by proper diffusion of an interstitial element. Therefore, the location of the focal point  20  is predetermined to provide a proper amount of energy to excite the lattice structure of the substrate alloy material known to allow diffusion of the interstitial element to a proper depth. 
         [0021]    In one embodiment, the laser beam is a divergent laser beam where the focal point  20  is spaced above the surface  22  of the substrate  10 . It is within the scope of the invention that the laser beam is a convergent laser beam where the focal point  20  would be positioned below the surface  22  of the substrate  10 . 
         [0022]    The surface  22  of the substrate  10  does not melt under optimum circumstances. The avoidance of a eutectic reaction which would result in the interstitial element reacting with the substrate  10  alloy is desirable. The optimum effect of the laser (or energy) beam  16  on the substrate is to merely excite the molecular lattice of the substrate  10  alloy. As such, an optimum laser power and speed is predetermined for each application based upon the substrate alloy and the desired depth of diffusion of the interstitial element. In one embodiment, a CO 2  laser provides an adequate amount of energy to the substrate  10 . In other embodiments, a diode laser is preferable. Additionally, the laser  14  is modified to project an alternatively shaped laser beam  16  onto the area of interest of the substrate  10 . In some application, a rectangular shaped laser beam  16  is preferable, such as, for example a 12×1 millimeter or 20×1 millimeter shape laser beam. Further applications make use of a round spot of 2 millimeters or 4 millimeters diameter, or an oval shape. In this regard, a shaping nozzle  36  ( FIG. 6 ) is used. 
         [0023]    In some applications, rapid diffusion of the interstitial element into the substrate  10  required a serpentine path  24  be established. This is best represented in  FIG. 4  where the laser beam zig zags to cover more surface area than capable by a single pass across an area interest of the metallic substrate  10 . An optimum path of travel is determined based upon a level of energy required to diffuse the interstitial element into the substrate  10 , which will dictate a size of the laser beam  16  at the surface  22  of the substrate  10 . It should be understood by those of ordinary skill in the art that either the laser  14  or the substrate  10  is movable so that the laser beam  16  provides an adequate amount of excitation energy to the substrate  10 . 
         [0024]      FIG. 5  shows the ability of the present inventive method to diffuse an interstitial element into components having various three dimensional configurations. In this instance, a cylindrical element, such as, for example, a piston rotates relative to the laser beam  16  to provide a single circumferential band  24  around an exterior surface  26  of the component. It is contemplated by the inventor that either circular tool path or rectangular tool path provides an adequate level of excitation energy to the substrate  10 . 
         [0025]    To further control diffusion of the interstitial element, the laser  14  interfaces with a computer aided design (CAD) data to adjust the location of the focal point of the laser beam  16  to maintain a constant distance from the surface of a three dimensional substrate  10 . The CAD data is used to direct the laser to either adjust a physical location relative to the substrate  10  or adjust the focal point  20  by way of a controller (not shown). Alternatively, the substrate  10  is moved relative to the laser  14  by the controller. 
         [0026]    A still further embodiment is shown at  FIG. 6  where interstitial diffusion into a substrate  10  is desired on an interior surface  28  of a tubular component  30 . In this embodiment, a laser beam  32  is directed toward a galvanometer-controlled mirror  34  to redirect the laser beam  32 . Once redirected, the laser beam  32  passes through a shaping nozzle  36  directing the divergent beam  38  onto an area of interest  40  on the inner surface  28  of the tubular component  30 . 
         [0027]    Tests have shown that the diffusion of the interstitial element ranges between a depth of 30 microns and 500 microns. The table shown in  FIG. 7  provides the test results where significant hardness improvement is achieved up to 10 millimeters from an edge of a knife blade (not shown). In this example, 1018 steel was coated with carbon powder and subject to excitation by way of a laser beam  16 ,  38  as explained above. Maximum hardness of around 900 VHS is achieved to 9 millimeters indicating the density of the interstitial carbides similar or equal to the density of interstitial carbides at the surface. Hardness requirements of a given application are achieved by adjusting the strength and speed of the laser treatment of the area of interest on the substrate  10 . The range of depth from the knife edge where hardness drops from above 800 VHS to that of the un-alloyed substrate, or in this example around 300 VHS is identified as the transition zone. At 11 millimeters the hardness drops that of the unalloyed substrate. 
         [0028]    Following treatment of the component, the surface  22  of the metallic substrate  10  is polished to remove interstitial element not diffused into the substrate  10 . In one embodiment, the surface is cleaned and polished with a diamond paste having 0.3 micron sized diamond particles mixed into a kerosene solution. However, it should be understood by those of ordinary skill in the art that alternative polishing methods will suffice. 
         [0029]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The foregoing invention has been described in accordance with the relevant legal standards; thus, the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.