Abstract:
The invention provides a method for processing a ferrous work-piece. The method includes the step of coating a surface of a ferrous work-piece with a predetermined material. The method also includes the step of generating a plasma at the surface with a laser by at least partially vaporizing the predetermined material to release electrons and ions. Different materials can be selected as the coating material to promote different surface changes. For example, carbon can be selected as the coating material for improved hardness. Phosphate can be selected as the coating material for enhanced tribological properties.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a process for changing a property of a ferrous work-piece and more particularly to subjecting a ferrous work-piece to a laser to change a surface condition of the metal work-piece.  
         [0003]     2. Description of Related Art  
         [0004]     Lasers can rapidly heat a surface of a work-piece for adjusting properties of the surface. An absorptive coating can be applied to the surface to be heated to enhance the energy transfer from the laser to the work-piece. By using a laser to quickly heat a surface, conventional quenching by a gas or a liquid is unnecessary since only the shallow surface area is heated. The part will actually self-quench, due to the extremely high heat differential between the surface layer heated by the laser and the remainder of the work-piece. This is in sharp contrast to carburizing or induction heating, where the part must be heated in one operation, and then is required to be quickly quenched by a gas or a liquid. Laser radiation can be generated by CO 2 , Excimer or Nd-YAG lasers, which can achieve intensities of more than 10 6  watt/cm 2 . A sample work-piece  10  treated with laser radiation according to the prior art is shown in  FIG. 1 . The depth  12  of treatment is approximately 2 microns.  
       SUMMARY OF THE INVENTION  
       [0005]     The invention provides a method for processing a ferrous work-piece. The method includes the step of coating a surface of a ferrous work-piece with a predetermined material. The method also includes the step of generating a plasma at the surface with a laser by at least partially vaporizing the predetermined material to release electrons and ions. Different materials can be selected as the coating material to promote different surface changes. For example, carbon can be selected as the coating material for improved hardness. A metal phosphate can be selected as the coating material for enhanced tribological properties. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     Advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein:  
         [0007]      FIG. 1  is a cross-section a work-piece after laser treatment according to the prior art;  
         [0008]      FIG. 2  is a simplified flow chart illustrating the steps for performing the exemplary embodiment of the invention;  
         [0009]      FIG. 3  is a cross-section a work-piece prior to laser treatment according to the exemplary embodiment of the invention;  
         [0010]      FIG. 4  is a cross-section a work-piece after laser treatment according to the exemplary embodiment of the invention;  
         [0011]      FIG. 5  is a first photograph of a reaction occurring during the exemplary embodiment of the invention along a short axis of a laser wherein an initiating flame is shown above the surface of the work-piece;  
         [0012]      FIG. 6  is a second photograph of the reaction taken along the long axis of the laser and occurring just after flame initiation wherein the reaction is changing to a plasma;  
         [0013]      FIG. 7  is a third photograph of the reaction taken along the long axis of the laser and occurring after a stable plasma has been generated; and  
         [0014]      FIG. 8  is a fourth photograph of the reaction taken along the short axis of the laser and occurring after a stable plasma has been generated. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     A process according to a first exemplary embodiment of the invention is shown in  FIG. 2 . The process starts at step  26 . At step  28 , a work piece  14 , shown in  FIGS. 3 and 4 , is formed from low carbon steel. Typically, low carbon steel is steel having about 0.1% carbon or less. The exemplary work piece  14  can be formed from 1010 steel. Prior to step  28 , the process could also include the step of applying an initial surface finish to the work-piece, such as by machining, grinding to a given grit size, or polished to a given diamond finish.  
         [0016]     The process continues to step  30  and a surface  16  of the work piece  14  is coated with a material  18 . Different coatings can be applied to the surface  16  to achieve different desired results. For example, in a first exemplary embodiment of the invention, the material  18  can be carbon for improving the hardness and wear resistance of the work piece  14 . The carbon can be applied in the form of hairspray sprayed on the surface  16 . In another embodiment, electrodag can be applied to the surface  16 . Electrodag can be acquired from Acheson Colloids in Port Huron, Mich. The carbon can be applied to a depth of five microns. Alternatively, carbon can be applied to the surface  16  such that 0.9 milligrams of carbon are disposed per square centimeter of the surface  16 . In another embodiment in the invention, one to two microns of carbon can be disposed on the surface  16  by vacuum deposit.  
         [0017]     The use of carbon for the coating material  18  can result in improved hardness and wear resistance in the work-piece  14 . In alternative embodiments of the invention, the material  18  can be selected to result in different material property changes at the surface  16 . For example, the material  18  can be a phosphate to enhance the surface lubrication of the work piece  14 . Generally, the phosphate can be applied to the surface  16  in greater quantities than the quantities set forth above with respect to carbon.  
         [0018]     After step  30 , the process continues to step  32  and the coated work piece  14  is disposed in a controlled atmosphere  24 , best shown in  FIG. 3 . The controlled atmosphere can be air or nitrogen or a combination of air and nitrogen. Other possible gasses for use in the controlled atmosphere  24  include methane and argon. Nitrogen can be used if a nitride surface is desired. Also, oxygen should be excluded from the atmosphere  24  if a nitride surface is desired. The controlled atmosphere  24  is maintained at a pressure slightly higher than atmospheric in the exemplary process.  
         [0019]     The process continues to step  34  and the diode laser  20  is directed at the coated surface  16 . The exemplary laser  20  is a 4 kilowatt diode laser and emits a beam  22  at the material  18 . As a result of the application of energy by the laser  20 , the surface  16  and the material  18  form a chemical mixture containing reactive species. It has been observed that a plasma is created at the surface  16  fed by the energy from the chemical reactions between the material  18  and the surface  16  as well as the energy provided by the laser  20 . As a result, after the application of the laser  20  to the surface  16 , the work piece  14  has been treated to a depth  36 . The depth can be between five microns and 20 microns. The depth  36  can be substantially greater than the depth  12  produced by prior art laser treatments. The process ends at step  38 .  
         [0020]      FIGS. 5-8  are photographs that illustrate conditions at the surface  16  of the work-piece  14  during the exemplary method. Graphs have been applied to provide perspective. In  FIG. 5 , a flame is generated when the laser  20  is first applied to the surface  16 . It is believed that the flame occurs as a result the vaporization of carbon in the material  18 .  FIG. 6  shows that the flame diminishes and  FIGS. 7 and 8  shows the flame replaced with a bright, well controlled zone of plasma. The zone cannot be safely observed by the naked eye. It has also been observed that a popping noise occurs initially during the process; the popping being evidence that some reaction other than the flame is occurring at the surface  16 .  
         [0021]     Generally, it is believed that an energy level above 10 6  watts/cm 2  is required to generate plasma in air. However, it is known that the presence of vaporized metal or water can reduce the energy required to generate plasma since free electrons and/or ions are contained with the vaporized metal/water. It is believed that the carbon material  18  is vaporized during initial application of the laser  20  and that the vaporized carbon facilitates the creation of plasma.  
         [0022]     The above-described first exemplary process according to the invention can be used to harden particular surfaces of work-pieces such as pistons and pistons rings, or any other work-piece formed from low carbon steel. In a second exemplary embodiment of the invention, a work-piece having a relatively higher quantity of carbon can be hardened. For example, a cast iron work-piece, such as a brake rotor, can be hardened.  
         [0023]     The process for hardening the cast iron work-piece can be similar to the process for hardening the low-carbon steel work-piece. Preferably, the cast iron work-piece is placed in a controlled environment and subjected to a 4 kilowatt diode laser. A relatively thin layer of coating material is placed on the surface of the cast iron work-piece. In one example, a black marker was used to color the surface to be treated with black ink. The ink of the marker is absorptive of the wavelength of the laser. The surface of the cast iron work-piece is positioned at the focus of the beam emitted by the diode laser. Preferably, the laser is moved along the surface at a scan speed of 3.5 meters/minute. The laser will vaporize the chemicals in the ink, including the carbon, and generate a plasma. The scan speed will prevent the work-piece from absorbing an undesirable amount of energy and overheat. Also, the scan speed substantially eliminates the need to preheat the work-piece to avoid cracking at the surface. A thin layer of the cast iron work-piece is melted, about 50-100 microns. The excess graphite in the cast iron is converted to a hard carbide structure, resulting in surface with enhanced hardness and wear resistance.  
         [0024]     As set forth more fully above, the invention can be used with work-pieces having relatively higher quantities of carbon, such as cast iron, to improve hardness as well as with work-pieces having relatively lower quantities of carbon, such as 1010 steel, to improve hardness. In the case of a cast iron work-piece, the carbon coating acts like a “plasma propellant” to promote the conversion of contained graphite to hard carbide. In the case of a low carbon steel work-piece, the carbon coating acts like a plasma propellant and combines with the steel to improve hardness.  
         [0025]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.