Abstract:
A wear resistant coating for protecting surfaces undergoing sliding contact is disclosed. The wear resistant coating is applied by high velocity plasma process deposition of a powdered blend of the coating constituents. The powdered blend includes a nickel-chromium alloy, chromium carbide, and molybdenum. The molybdenum powder has a particle size of less than about 45 microns. The disclosed coating should find use as a bearing surface on piston rings, cylinder liners, and other components of a power cylinder assembly of an internal combustion engine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Provisional Application 60/573,968, filed on May 24, 2004, the contents of which are hereby incorporated herein in their entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to materials and methods for protecting surfaces subject to frictional forces, heat, and corrosion, and more particularly, to wear-resistant coatings that can be applied to piston rings and cylinder liners of internal combustion engines.  
       BACKGROUND OF THE INVENTION  
       [0003]     A power cylinder assembly of an internal combustion engine generally comprises a reciprocating piston disposed within a cylindrical cavity of an engine block. One end of the cylindrical cavity is closed while another end of the cylindrical cavity is open. The closed end of the cylindrical cavity and an upper portion or crown of the piston, define a combustion chamber. The open end of the cylindrical cavity permits oscillatory movement of a connecting rod, which joins a lower portion of the piston to a crankshaft, which is partially submersed in an oil sump. The crankshaft converts linear motion of the piston (resulting from combustion of fuel in the combustion chamber) into rotational motion.  
         [0004]     The power cylinder assembly typically includes one or more piston rings and a cylindrical sleeve or cylinder liner, which is disposed within the engine block and forms the side walls of the cylindrical cavity. The piston rings are disposed in grooves formed in the lateral walls of the piston, and extend outwardly from the piston into an annular space delineated by the piston wall and the cylinder liner. During movement of the piston within the cylindrical cavity, the piston rings bear against the cylinder liner. The piston rings have two main functions. First, they inhibit gas flow from the combustion chamber into the oil sump through the annular space between the piston and the cylinder liner. Second, they minimize oil flow from the oil sump into the combustion chamber.  
         [0005]     To improve their durability, wear and scuff resistance, the piston rings, and in some cases the cylinder liner, are coated with relatively hard materials such as chromium hard plate and alloys containing chromium carbide. Although such coatings have met with considerable success, they have been found inadequate for newer engine technologies, including diesel engines employing exhaust gas recirculation (EGR).  
         [0006]     For high firing pressure diesel applications, known plasma spray thermal coatings either exhibit insufficient ring wear or excessive bore wear to meet established durability requirements. Also, current hexavalent chrome plating has problems with scuffing in highly loaded engines along with environmental impact issues such as increased waste streams.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides coatings that offer improved wear and scuff resistance for demanding applications such as piston rings and cylinder liners of internal combustion engines. In one embodiment, a wear resistant coating is applied with a high velocity plasma process. The coating is a powder coating and the powder includes about 13 wt. % to about 43 wt. % of a nickel-chromium alloy, about 25 wt. % to about 64 wt. % chromium carbide, and about 15 wt. % to about 50 wt. % molybdenum, wherein chromium from the nickel-chromium alloy is at least 7.2 wt % of the blend.  
         [0008]     In another embodiment, a piston ring having a wear resistant coating is provided, where the coating includes a blended powder comprising a pre-alloyed chrome carbide powder and a metallic molybdenum powder, the is coating applied by subjecting the powder to a high velocity plasma process.  
         [0009]     In yet another embodiment, a method for forming a wear resistant coating to a piston ring is provided where the method includes combining a powder with a pre-alloyed chrome carbide and a powder of a metallic molybdenum to form a blended powder and applying the blended powder to the piston ring using a high velocity plasma process.  
         [0010]     The invention is also directed to a chemistry and prealloyed microstructure in a chrome carbide/nickel chrome constituent plus the addition of molybdenum utilizing a unique plasma spray thermal process. The process has the advantage of lower investment and operational costs than competing technologies such as physical vapor deposition (PVD), high-velocity oxy-fuel (“HVOF”), and advanced chrome plating.  
         [0011]     The present invention is an improvement of the invention disclosed in U.S. Pat. No. 6,562,480, the contents of which are incorporated by reference. The present invention is also an improvement on co-pending application Ser. No. 10/804,332, the contents of which are incorporated by reference herein in their entirety. The present invention is an improvement on co-pending application Ser. No. 10/255,814, the contents of which are incorporated by reference herein in their entirety.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The figure is a sectional side view of a portion of a power cylinder assembly illustrating a piston ring with a wear resistant coating made in accordance with an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     Referring to the figure, a sectional side view of a portion of a power cylinder assembly  10  of an internal combustion engine is illustrated. The power cylinder assembly  10  includes a piston  12 , which can move linearly within a cylindrical cavity  14  that is defined by an inner wall  16  of a cylinder liner, or a cylindrical sleeve,  18 . The cylinder liner  18  is disposed within a cylindrical bore  20  formed in an engine block  22 .  
         [0014]     The power cylinder assembly  10  also includes a combustion chamber  24 , which is defined by an upper portion  26  of the cylinder liner  18  and a top portion or crown  28  of the piston  12 . During engine operation, fuel combustion in the combustion chamber  28  generates gas pressure that pushes against the crown  28  of the piston  12 , driving the piston  12  downward.  
         [0015]     In addition to the crown  28 , the piston  12  includes a first groove  30 , a second groove  32 , and third groove  34  formed in a lateral wall  36  of the piston  12 . Each of the grooves  30 ,  32 ,  34  are sized to accept, respectively, first  38  and second  40  piston (compression) rings, and an oil ring assembly  42 . The oil ring assembly  42  includes a pair of rails  44 ,  46 , and a sinusoidal expander  48 , which pushes the rails  44 ,  46  outward from the lateral wall  36  of the piston  12 . The expander  48  also includes a drain slot  50  (shown by hidden lines) that channels oil away from the inner wall  16  of the cylinder liner  18  to an oil sump via a conduit (not shown) within the piston  12 . As can be seen in the figure, a first land  52 , a second land  54 , and a third land  56  separate each of the grooves  30 ,  32 ,  34  and help retain the pistons rings  38 ,  40  and the oil ring assembly  42  in their respective grooves  30 ,  32 ,  34 . The piston  12  also includes a lower skirt  58 , which reduces lateral movement of the piston  12  during the combustion cycle.  
         [0016]     As shown in the figure, the first  38  and second  40  piston rings, and the rails  44 ,  46  of the oil ring assembly  42 , contact the inner wall  16  of the cylinder liner  18 . The rings  38 ,  40  and rails  44 ,  46  act as sliding seals that prevent fluid flow through an annular region  60  formed by the lateral wall  36  of the piston  12  and the inner wall  16  of the cylinder liner  18 . Thus, the first piston ring  38 , and to some extent the second piston ring  40  and the oil ring assembly  42  rails  44 ,  46 , reduce gas flow from the combustion chamber  24  to the oil sump region of the engine. Similarly, the rails  44 ,  46  of the oil ring assembly  42  and the second  40  piston ring (and to less extent the first  38  piston ring), help prevent oil in the sump from leaking into the combustion chamber  24 .  
         [0017]     In the embodiment illustrated, a coating  62  is disposed on a radial periphery  64  of the first piston ring  38  to improve durability, wear resistance and scuff resistance of the first piston ring  38  and the cylinder liner  18 . As can be seen, the radial periphery  64  of the first piston ring  38  includes a radial groove  66 , which improves the adhesion of the coating  62  to the first piston ring  38 . The coating  62  may also be applied to other surfaces of the power cylinder assembly  10  that are subject to frictional forces (bearing surfaces), heat, or corrosion. Such surfaces include, but are not limited to, the inner wall  16  of the cylinder liner  18 , and radial peripheries  68 ,  70 ,  72  of the second piston ring  40  and the rains  44 ,  46  of the oil ring assembly  42 .  
         [0018]     The coating  62  comprises an alloy of one or more base metals, a hard ceramic material, and molybdenum. The base metal serves as a binder for the hard ceramic material. Suitable base metals include nickel, chromium, and, preferably, mixtures of nickel and chromium. A useful base metal is a nickel-chromium alloy containing from about 40 wt. % to about 60 wt. % nickel. The base metal generally comprises about 13 wt. % to about 43 wt. % of the coating  62 , and more particularly, about 18 wt. % to about 35 wt. % of the coating  62 . An especially useful coating  62  includes about 28 wt. % of a nickel-chromium alloy containing about 50 wt. % nickel.  
         [0019]     The hard ceramic material, which imparts wear resistance, ordinarily should remain substantially solid throughout application of the coating  62 . Examples of hard ceramic materials include chromium carbide, vanadium carbide, and tungsten carbide. Of these, chromium carbide is especially useful. The hard ceramic materials are available as finely divided powders ranging in size from about 15 microns to about 45 microns. Useful forms of chromium carbide include Cr 3 C 2 , Cr 7 C 3 , and Cr 23 C 6 , among others, and a mixture of Cr 7 C 3 , and Cr 23 C 6  is particularly advantageous. The hard ceramic material generally comprises about 25 wt. % to about 64 wt. % of the coating  62 , and more particularly, about 35 wt. % to about 53 wt. % of the coating  62 . When the chromium carbide level is less than about 25 wt. %, the abrasion or wear resistance of the coating  62  is inadequate for power cylinder applications, and when the chromium carbide level is greater than about 64 wt., the coating  62  is too brittle. A particularly useful coating  62  comprises about 42 wt. % chromium carbide, which includes about 50 wt. % Cr 7 C 3  and about 50 wt. % Cr 23 C 6 .  
         [0020]     While the powder may include various components, in a preferred embodiment it consists of two components. The first component is a pre-alloyed chrome carbide (predominantly Cr 7 C 3  and Cr 7 C 3  and Cr 23 C 6 ) nickel chrome (approximately 60/40 ratio and more preferably a 60/40 ratio) such as that available from Praxair Surface Technologies Inc. The second component is essentially pure molybdenum. The two powder components are mechanically blended to approximately a 70/30 ratio (CRC-NiCr/Mo) ratio. The actual ratio ranges that can be used are discussed in greater detail in the &#39;480 patent.  
         [0021]     The method of applying the coating  62  includes employing a spraying technique. The spraying technique utilizes a high velocity plasma process, which is a low oxidation thermal spraying technique. The technique results in a higher deposit efficiency than HVOF and has improved wear performance over traditional thermal spray plasma techniques.  
         [0022]     In thermal spraying processes such as a high velocity plasma process that utilize a carrier gas, flight time and oxidation can be decreased by increasing flow rate of the carrier gas. In a plasma process, increasing the flow rate of the plasma can be accomplished by using a greater volume of fuel gas in a given time period, increasing the voltage and/or the amperage used to create the electric arc, and/or using different fuel gas mixture to generate the plasma flame. For example, typically fuel gas is used at a volume of around 100 standard cubic feet/hour (cfh). Increasing the volume of fuel gas to more than 200 cfh will decrease oxidation. Increasing the voltage and amperage from the typical 30 volts and 600 amps to 50-70 volts and 800-1000 amps has the effect of decreasing oxidation. Preferably, a voltage of about 60 volts is used in combination with amperage of about 900 amps. Indeed, a fuel gas of argon and helium allows less oxidation than a fuel gas of argon and hydrogen. In a preferred method, an argon/helium fuel gas is used at a volume of 200 cfh of argon and a volume of 30 cfh helium. Obviously, using more than one of these techniques may have a synergistic effect on the reduction of oxidation of the coating material.  
         [0023]     Finally, it has been found that particle size is unexpectedly very important to the proper creation of the wear coating. In the prior art, the CrC/NiCr size=15 to 45 microns while the Molybdenum size=45 to 74 microns. Molybdenum of 45 to 74 microns was found to have inadequate fusion.  
         [0024]     For the present invention, the CrC/NiCr size=15 to 45 microns while the Molybdenum size=15 to 45 microns. The smaller particle size of Molybdenum provides appropriate fusion when applied in accordance with the teachings of the present invention. In fact, while having a particle size of less than 45 microns is important, having the smallest possible size results in a lack of improvement over the overall fusion. In particular, Molybdenum powder of 15 to 25 microns was found to perform no better than powder of slightly greater size than 25 microns.  
         [0025]     Although the base metal and the hard ceramic component of the coating  62  can be dry-blended, it is advantageous to pre-alloy the components prior to application. Suitable alloying techniques include liquid and gas atomization, which generate particles having substantially uniform concentrations of the base metal and the hard ceramic component. For example, a pre-alloyed mixture of chromium carbide and nickel-chrome, which is produced by atomization, is available under the trade designation CRC-291 from Praxair Inc. The pre-alloyed mixture comprises about 60 wt. % chromium carbide, primarily as Cr 7 C 3  and Cr 23 C 6 , and about 40 wt. % of a nickel-chrome alloy. The chromium carbide portion of the mixture contains about equal amounts (by weight) of Cr 7 C 3  and Cr 23 C 6 , and the nickel-chrome alloy contains about equal amounts (by weight) of nickel and chromium. The pre-alloyed mixture has a maximum particle size less than about 53 microns. For a description of liquid atomization, see U.S. Pat. No. 5,863,618, “Method for Producing a Chromium Carbide-Nickel Chromium Atomized Powder,” which is herein incorporated by reference.  
         [0026]     In addition to the base metal and the hard ceramic component, the coating  62  also includes molybdenum, which imparts scuff resistance. Here, scuffing refers to binding or grabbing that may occur when two surfaces, such as the piston rings  38 ,  40  and the cylinder liner  18 , are in sliding contact. In extreme cases of scuffing, the intense heat generated by friction may cause the two surfaces to weld together. The molybdenum component of the coating  62  may include a few weight percent impurities, such as metal oxides, and generally ranges in particle size from about 105 microns to less than about 45 microns. For power cylinder applications, molybdenum should comprise between about 15 wt. % and 50 wt. % of the coating 62-molybdenum levels less than about 15 wt. % result in coatings  62  having inadequate scuff resistance, and molybdenum levels greater than about 50 wt. % result in coatings  62  having inadequate wear resistance. A particularly useful coating  62  comprises about 30 wt. % molybdenum.  
         [0027]     The application of coating  62  involves the spraying of a metallic powder using a plasma spray thermal process onto the outer periphery of a piston ring body. The ensuing coating is designed to improve the wear and scuff characteristics of the piston ring. Preferably, the coating is deposited in at least a peripheral groove in the ring.  
         [0028]     Prior to application, the powders that comprise the coating 62—base metal, hard ceramic component, molybdenum—are mixed in a dry state using a v-cone blender, a ball mill, and the like. Once blended, the coating  62  constituents are applied to the first piston ring  38 , cylinder liner  18 , or other bearing surfaces of the power cylinder  10 . To efficiently coat a piston ring&#39;s radial periphery, a group of piston rings are stacked on an arbor having a controllable rotation rate. A nozzle, which propels the coating  62  constituents against the outer periphery of each of the rings, is mounted on a translation stage, which can control the position of the nozzle relative to the stack of piston rings. Prior to coating, the translation stage adjusts the standoff distance from the thermal spray nozzle tip to the stack of piston rings. To coat the rings, the arbor rotates the piston rings at a desired angular velocity while the translation stage moves the nozzle between the ends of the stack along the arbor&#39;s axis at a desired speed. For a given powder feed rate, one can adjust the coating thickness by adjusting the angular velocity of the arbor and the translation speed of the nozzle. Preferably, one can adjust the coating thickness by changing the number of nozzle translations over the arbor. Following application of the coating  62 , the stack of piston rings are separated and finished by grinding.  
         [0029]     While the invention has been described with respect to specific examples including preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.