Patent Publication Number: US-2017362965-A1

Title: Boron doped ta-c coating for engine components

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to components for internal combustion engines, such as coated piston rings, and methods of forming the same. 
     2. Related Art 
     Components for a reciprocating engine, such as an internal combustion engines, are subjected to high wear and thus are oftentimes coated to prolong service life. For example, a piston of the internal combustion engine typically includes piston rings disposed in grooves along the outer diameter of the piston. The piston rings facilitate guiding of the piston during reciprocation in a cylinder bore. The piston rings also seal combustion gases and inhibit the upward passage of oil. However, the piston rings are subject to wear as they move along the cylinder bore due to gas load and their own inherent load. Accordingly, the outer diameter surfaces of the piston rings are coated or treated to enhance wear resistance. For example, the piston rings may be nitrided, coated with chromium, or coated with a ceramic, which may be applied by physical vapor deposition (PVD) or chemical vapor deposition (CVD). 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides a component for an engine, for example a piston ring, which is coated to enhance wear resistance. The component includes a base body presenting an outer surface, and the coating is applied to the outer surface of the base body. The coating includes tetrahedral amorphous carbon (ta-C), and the carbon of the coating includes sp 3  hybrid orbitals. The coating further includes boron in an amount of 0.1 wt. % to 4.0 wt. %, based on the total weight of the coating. 
     Another aspect of the invention provides a method of manufacturing a coated component for an engine, for example a piston ring. The method includes applying a coating to an outer surface of a base body. The coating includes tetrahedral amorphous carbon (ta-C), the carbon of the coating includes sp 3  hybrid orbitals, and the coating includes boron in an amount of 0.1 wt. % to 4.0 wt. %, based on the total weight of the coating. 
     The doped boron makes the coating less sensitive to ion energy and improves adhesion of the coating to the component. In addition, the doped boron is expected to reduce compressive stress in the coating, which typically limits the thickness of ta-C coatings. Thus, the boron-doped ta-C coating can be applied to a greater thickness compared to ta-C coatings without the doped boron. The improved adhesion and greater thickness of the coating will increase the service life of the coated component. In addition, there is a strong indication that the addition of boron in the amount of 0.1 wt. % to 4.0 wt. % will allow the coating to maintain a high level of sp 3  bonded carbon and a high microhardness, without an unacceptable amount of internal or compressive stress. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a coated piston ring according to an embodiment of the invention; and 
         FIG. 2  is an enlarged cross-sectional view of a portion of the coated piston ring of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     One aspect of the invention provides a coated component, for example a piston ring  10 , for reciprocating engine applications, such as internal combustion engines. Alternatively, the component could be another component of the engine which is subject to wear, such as a piston pin, crank shaft, tappet, etc. A coating  12  including tetrahedral amorphous carbon (ta-C) is applied to the piston ring  10  to improve wear resistance. An example of the coated piston ring  10  is shown in  FIGS. 1 and 2 . 
     The piston ring  10  includes a base body  14  extending circumferentially around a center axis A. The base body  14  is formed of a metal material, such as cast iron, steel, or cast steel. The base body  14  presents an outer surface  16  facing opposite the center axis A and an inner surface  18  facing and surrounding the center axis A. 
     The coating  12  is applied to the outer surface  16  of the base body  14 , which slides along the inner surface of a cylinder liner (not shown) during use in the internal combustion engine. Thus, the coating  12  prevents wear caused by the friction between the piston ring  10  and cylinder liner. The coating  12  includes diamond-like carbon (DLC), and thus is referred to as a DLC coating. 
     More specifically, the coating  12  applied to the base body  14  is homogenous and includes tetrahedral amorphous carbon (ta-C). The ta-C is also known as the toughest form of diamond-like carbon. The carbon of the coating  12  includes a mixture of carbon including sp 2  hybrid orbitals and carbon including sp 3  hybrid orbitals. The ratio of sp 2  to sp 3  hybrid orbitals present in the coating  12  ranges from 1:99 to 99:1. The amount of sp 2  and sp 3  hybrid orbitals depends on the desired properties of the coating  12 . The sp 3  hybrid orbitals increase the hardness of the coating  12  but also increase the compressive or internal stress of the coating  12 . According to one embodiment, greater than 50% of the carbon atoms present in the coating  12  include sp 3  hybrid orbitals, for example 50% to 99%, or 65% to 90%, or 70% to 85% of the hybrid orbitals can be sp 3  hybrid orbitals. The carbon atoms including sp 3  hybrid orbitals are bonded to other carbon atoms including sp 3  hybrid orbitals. The coating  12  is also free of hydrogen. 
     In the example embodiment, the coating  12  is applied to the base body  14  of the piston ring  10  by physical vapor deposition (PVD). This process typically includes forming a cathode comprising a mixture, and applying a gas including positive ions to the cathode so that the mixture of the cathode deposits on the outer surface  16  and forms the coating  12 . 
     The coating  12  applied to the base body  14  also includes boron in an amount of 0.1 wt. % to 4.0 wt. %, based on the total weight of the coating. In one example embodiment, boron particles are mixed with graphite or carbon particles to form the cathode, and the cathode is then subjected to the ion energy of the physical vapor deposition process such that the mixture of the cathode deposits on the outer surface  16  of the base body  14  and forms the coating  12 . In another example embodiment, the cathode is formed of graphite powder, and the boron is provided in the gas to form the coating  12 . Alternatively, some of the boron can be provided in the solid cathode, and some can be provided in the gas to form the coating  12 . 
     The boron makes the coating  12  less sensitive to the ion energy during the physical vapor deposition process and improves adhesion of the coating  12  to the outer surface  16  of the piston ring  10 . In addition, the doped boron is expected to reduce compressive or internal stress in the coating  12 , which typically limits the thickness of ta-C coatings. Thus, the boron-doped ta-C coating  12  can be applied to a greater thickness compared to ta-C coatings without the doped boron. The improved adhesion and greater thickness of the coating  12  will increase the service life of the coated piston ring  10 . In addition, there is a strong indication that the addition of boron in the amount of 0.1 wt. % to 4.0 wt. % will allow the coating  12  to maintain a high level of sp 3  bonded carbon and a high microhardness, with an acceptable level of internal or compressive stress and thickness. 
     In the example embodiment, the thickness of the coating  12  is greater than the thickness of other known ta-C coatings without doped boron. For example, the coating  12  can have a thickness of 1 to 60 microns. The coating  12  also maintains a high microhardness, due to the sp 3  hybrid orbitals. The coating  12  also has a friction coefficient which contributes to the improved wear resistance, for example a friction coefficient of 0.01 to 0.30. 
     Optionally, the piston ring  10  can include an adhesive layer  24  disposed between the outer surface  16  and the coating  12 , and/or a finish layer  22  applied to the coating  12 . The adhesive layer  24  is typically a metal layer, for example a layer formed of chromium, titanium, chrome nitride, or another hard material or compound. The outermost surface of the piston ring  10  can be formed by the finish layer  22 , when present, or by the wear resistant coating  12 , when the finish layer  22  not present. 
     Another aspect of the invention provides a method of manufacturing the coated piston ring  10 . The method includes applying the coating  12  to the outer surface  16  of a base body  14 . As discussed above, the coating  12  includes tetrahedral amorphous carbon (ta-C), the carbon of the coating  12  includes sp 3  hybrid orbitals, and the coating  12  includes boron in an amount of 0.1 wt. % to 4.0 wt. %, based on the total weight of the coating  12 . 
     In the example embodiment, the step of applying the coating  12  to the outer surface  16  of the base body  14  includes physical vapor deposition (PVD). Various different types of physical vapor deposition can be used, but in the example embodiment, the physical vapor deposition step includes a plasma-assisted high vacuum process. Other methods that can be used to apply the coating  12  include laser arc vapor deposition, magnetically enhanced arc vapor deposition, filtered arc vapor deposition, and magnetron sputtering, or another process capable of re-condensing the mixture of tetrahedral amorphous carbon (ta-C) and boron on the base body  14 . In the example embodiment, the process includes forming a cathode by mixing carbon or graphite and the boron particles in an amount of 0.1 wt. % to 4.0 wt. %, based on the total weight of the mixture, and the step of applying the coating  12  to the outer surface  16  includes applying a gas including positive ions to the cathode so that the mixture of the cathode deposits on the outer surface  16  and forms the coating  12 . Alternatively, the boron can be provided in the gas used to form the coating  12 , or some of the boron can be provided in the cathode, and some of the boron can be applied in the gas. The gas typically includes argon, and argon ions of plasma atomize or vaporize the mixture and cause the mixture to deposit on the outer surface  16  of the base body  14 . In the example embodiment, the coating  12  is applied to a thickness of 1 to 60 microns. 
     As discussed above, according to the example embodiment, the carbon of the coating  12  is diamond-like carbon (DLC). The coating  12  is also homogeneous and free of hydrogen. The carbon of the coating  12 , which includes the sp 3  hybrid orbitals, is in the form of atoms including sp 3  hybrid orbitals bonded one another. 
     The method can optionally include applying the adhesive layer  24  to the outer surface  16  of the base body  14  prior to applying the coating  12 . The adhesive layer  24  is typically a metal layer, for example a layer formed of chromium, titanium, chrome nitride, or another hard material or compound. The method can also optionally include applying the finish layer  22  to the coating  12 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the following claims.