CN118911864A - Piston ring coating - Google Patents

Abstract

A piston ring and method of forming the same, the piston ring comprising: a base portion formed of a metallic material; an outer contact surface and an inner contact surface extending between a first face of the piston ring and a second face opposite the first face; a chromium coating on the second face, the outer contact surface and the inner contact surface; a diamond-like carbon (DLC) layer disposed over the chromium coating on the outer contact surface; and a phosphate layer on the first side.

Description

Piston ring coating

Cross Reference to Related Applications

The present application claims priority and benefit from U.S. provisional application No.63/464675 filed on 5/8 of 2023, the contents of which are hereby incorporated by reference in their entirety.

Technical Field

Exemplary technical fields of the present disclosure may relate to, for example, pistons and piston rings, and in particular, to a coating for a piston ring.

Background

A power cylinder assembly for an internal combustion engine typically includes a reciprocating piston disposed within a barrel cavity of an engine block. One end of the cylindrical cavity is closed and the other end of the cylindrical cavity is open. The closed end of the barrel cavity and the upper portion or crown of the piston define a combustion chamber. The open end of the cylindrical cavity allows an oscillating movement of a connecting rod that joins the lower portion of the piston to a crankshaft that is partially submerged in the oil pan. The crankshaft converts linear motion of the piston (caused by combustion of fuel in the combustion chamber) into rotational motion.

The power cylinder assembly typically includes one or more piston rings and a cylindrical sleeve or liner disposed within the engine cylinder and forming a sidewall of a 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 defined by the piston walls and the cylinder liner. During the movement of the piston in the cylindrical chamber, the piston rings are supported on the cylinder liner. The piston ring has at least two functions. First, they prevent gas from flowing from the combustion chamber into the oil pan through the annular space between the piston and the cylinder liner. Second, they minimize the flow of oil from the sump into the combustion chamber.

Piston rings must typically withstand the extreme temperatures and pressures generated by the combustion cycle. The outer surface of a piston ring supported on the surface of a cylinder liner or bore typically includes a hard surface coating or is otherwise treated to produce a hardened outer surface that is more durable than the untreated surface.

Thus, some known power cylinder assemblies include piston rings having chrome plated lateral sides, i.e., sides of the ring positioned to interface with the piston ring groove surface, with Diamond-like Carbon (DLC) coated over the chrome. Chromium is located on the lower surface of the piston ring, while DLC is located on the outer radial surface of the piston ring.

DLC, however, has the ability to bond to the face above chromium, and bonding has generally been accomplished, particularly at the corner areas of the piston ring and where the underside intersects the outer radius. Thus, running-in of the piston ring (during early operation of the newly manufactured engine) may present challenges and DLC may overheat and fail, during which initial operation the DLC running surface may crack.

Thus, there is a need for improved piston ring designs.

Disclosure of Invention

The present disclosure relates to a piston ring and a method of forming a piston ring.

According to a first aspect, a piston ring comprises: a base portion formed of a metallic material; an outer contact surface and an inner contact surface extending between a first face of the piston ring and a second face opposite the first face; a chromium coating on the inner contact surface, the outer contact surface, and the second face; a diamond-like carbon (DLC) layer positioned on the chromium coating of the outer contact surface.

According to one embodiment, a phosphate layer is provided on the first face of the piston ring. The phosphate layer may be applied directly to the metallic material of the base portion. The phosphate layer may have a thickness of 3 μm or less. The phosphate layer may overlap the DLC portion at an upper outer corner area of the base portion, and may overlap the chromium coating portion at an upper inner corner area of the base portion. The phosphate layer may have a gradually decreasing thickness along the upper outer corner region and the upper inner corner region.

Additionally or alternatively, the DLC may have an approximately constant thickness along the outer contact surface and a gradually decreasing thickness at the lower outer corner region of the base portion and the upper outer corner region of the base portion. The DLC may have a thickness of 2 μm to 25 μm, at least in a portion of approximately constant thickness.

Additionally or alternatively, the chromium coating may have an approximately constant thickness along the outer contact surface, the lower outer corner region, the second face, the lower inner corner region of the base portion, and the inner contact surface. Further, the chromium coating may have a gradually decreasing thickness at the upper outer corner region and the upper inner corner region.

The chromium coating may have a thickness between 10 μm and 20 μm, the thickness variation being equal to or less than 5 μm. The chromium coating may have a flatness of 2 μm or less along at least the second face and the inner contact surface.

Additionally or alternatively, a physical vapor deposition (PVD: physical Vapor Deposition) chromium interlayer may be interposed between the DLC and the chromium coating along the outer contact surface. The chromium intermediate layer may have a thickness of less than or equal to 1 μm.

Additionally or alternatively, a polymer layer covering the DLC may be provided.

According to a second aspect, a method of forming a piston ring includes providing a base portion formed of a metallic material, the base portion having a first surface and a second surface opposite the first surface; applying a chromium coating on the first face, the second face, the outer contact surface and the inner contact surface; and depositing a diamond-like carbon (DLC) layer over the chromium coating on the outer contact surface.

According to one embodiment, the method includes trapezoid grinding the second face to planarize an outer surface of the chromium coating to a flatness variation of 2 μm or less.

The method may include brushing (brushing) the second face after the trapezoidal grinding to remove nodules from the chromium coating.

Additionally or alternatively, the method may include trapezoid grinding the first face to remove substantially all of the chromium coating from the first face to expose the base portion and roughen an outer surface of the base portion; and depositing a phosphate layer on the first face. The phosphate layer may be applied directly to the metallic material of the base portion by phosphating. Furthermore, the phosphate layer may at least partially cover the DLC at an upper corner area of the base part and partially overlap the chromium coating at an upper inner corner area of the base part.

According to one embodiment, the method includes trapezoid grinding the second face prior to applying the chromium coating.

Additionally or alternatively, the method may include Outer Diameter (OD) grinding the chromium coating along the outer contact surface prior to depositing the DLC; gap grinding the chromium coating at piston ring gap prior to depositing the DLC; brushing an Inner Diameter (ID) of the chrome-coated layer along the inner contact surface after the gap grinding and before depositing the DLC; and gap finishing the chromium coating at the piston ring gap after depositing the DLC and before trapezoid grinding the first face. A first or initial gap grinding may be performed prior to applying the chromium coating.

According to one embodiment, the trapezoid grinding is performed after the OD grinding and before the gap grinding. Alternatively, the trapezoid grinding may be performed after the clearance finish grinding.

According to one embodiment, the method includes depositing a chromium intermediate layer over the chromium coating along the outer contact surface such that the chromium intermediate layer is interposed between the chromium coating and the DLC. The chromium intermediate layer may be deposited via physical vapor deposition and the chromium coating may be applied electrochemically. The chromium intermediate layer may be deposited in the same step as the DLC. Additionally or alternatively, the chromium interlayer may be deposited at a low temperature, for example, at a temperature of about 200 ℃ or less.

According to another embodiment, a polymer layer is deposited on the outer surface of the DLC layer.

Various other features and advantages will become apparent from the following detailed description and the accompanying drawings.

Drawings

Referring now to the drawings, illustrative examples are shown in detail. Although the drawings represent the exemplary illustrations described herein, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain an innovative aspect of the exemplary illustrations. Furthermore, the exemplary illustrations described herein are not intended to be exhaustive or otherwise limit or restrict the precise forms and configurations shown in the drawings and disclosed in the following detailed description. An exemplary illustration of the invention is described in detail below by referring to the drawings:

FIG. 1 is a view of an exemplary piston assembly;

FIG. 2A is a view of a ring of the piston assembly of FIG. 1;

FIG. 2B is a cross-sectional view of a piston ring in a groove of a piston;

FIG. 3A is a cross-sectional view of a piston ring according to one example;

FIG. 3B is a close-up view of the piston ring of FIG. 3A;

FIG. 4 shows the steps of a method of producing a piston ring according to FIG. 3A;

FIG. 5 is a cross-sectional view of a piston ring according to another example;

FIG. 6A is a close-up view of the piston ring of FIG. 5;

FIG. 6B is a close-up view of the piston ring of FIG. 5;

FIG. 6C is a close-up view of the piston ring of FIG. 5;

FIG. 7 shows a flow chart of an exemplary method for producing a piston ring according to FIG. 5;

FIG. 8 illustrates steps of a method according to the present disclosure;

FIG. 9 illustrates steps of a method according to the present disclosure;

FIG. 10 illustrates steps of a method according to the present disclosure;

FIG. 11 illustrates steps of a method according to the present disclosure; and

Fig. 12 shows a cross-sectional view of a piston ring according to another example.

Detailed Description

Reference in the specification to "an example illustration," "example," or similar language means that a particular feature, structure, or characteristic described in connection with the example method is included in at least one illustration. The appearances of the phrase "in a drawing" or similar type of language in various places in the specification are not necessarily all referring to the same drawing or example.

Various exemplary illustrations are provided herein that include a piston assembly having two or more rings that provide various functions during piston operation when positioned within a groove of the piston assembly. In one example, the piston assembly may have two rings including a combustion ring and a wiper ring. The combustion ring typically prevents a majority of the combustion gases from passing through the piston assembly of the internal combustion engine such that little or no gases are transferred to the crank. The wiper ring provides a wiping motion during a downward movement of the piston assembly and a sliding motion during an upward movement of the piston assembly.

In another example, a three-ring piston assembly includes a combustion ring, an oil control ring, and a wiper ring positioned between the combustion ring and the oil control ring. A third or lower oil control ring controls the supply of oil to the liner, while lubricating the piston skirt and other rings. In some example methods, a piston assembly may include a piston head having a first compression ring groove and a second compression ring groove, a compression ring within the groove, an oil control ring groove, and an oil control ring assembly.

According to various exemplary illustrations, a piston ring may include a base portion formed of a metallic material, an outer (radial) contact surface and an inner (radial) contact surface extending between a first face or surface of the piston ring and a second face or surface opposite the first face. A chromium coating is disposed on the inner contact surface, the outer contact surface, and the second face. The phosphate layer may be disposed on the first face. A diamond-like carbon coating (DLC) is deposited on the radially outermost surface over the chromium coating. A chromium interlayer may be interposed between the DLC and the chromium coating.

DLC is a class of amorphous carbon materials that exhibit properties such as very low coefficient of friction, high hardness, excellent wear resistance, and very low wear. DLC coatings may be deposited using cathodic arc Physical Vapor Deposition (PVD) processes. Cathodic arc PVD coating processes involve the use of a cathodic arc source or cathodic arc target (negatively charged), such as graphite or other suitable carbon material, and depositing a thin film over a substrate or anode (positively charged). The target material may be placed in a vacuum chamber together with the substrate to be coated or filled with an inert gas such as argon. Once the chamber is evacuated, a high voltage pulse is applied to the target material. The voltage pulse causes an arc discharge that evaporates the target material. The evaporated material then condenses onto the substrate, forming a thin film of DLC coating. The DLC coating formed is amorphous and may include a combination of sp3 hybridized carbon atoms and fillers (e.g., sp2 carbon atoms), up to 3% hydrogen, and trace amounts of other metals to impart desired properties to the material. According to one embodiment, the DLC coating used herein may exhibit characteristics including a ratio of sp ³ to sp ² ranging from 0.42 to 2.33 and a distribution of Rmr (0.2-0.5) greater than at least 25% and/or a distribution of Rmr (0.3-0.5) greater than at least 50%, as described in commonly owned patent application Ser. No.15/289097, commonly owned at 7 of 10 months 2016, now U.S. patent application Ser. No.10400895B2, the contents of which are incorporated herein by reference in their entirety. According to one embodiment, the DLC coating used herein may also exhibit the characteristics wherein the DLC coating is over an intermediate coating, wherein the DLC coating comprises a first DLC layer having a first hardness and a first porosity, and a second DLC layer having a second hardness lower than the first hardness and a second porosity greater than the first porosity, as described in commonly owned patent application No.17/488942 filed at 9/29 2021, the contents of which are hereby incorporated by reference in their entirety.

Typically, the wear environment between the lateral surface of the piston ring, such as the lower radially extending surface, and the receiving ring groove is different than the typical wear environment of the generally perpendicular radially outermost surface of the ring and the mating cast iron surface of the cylinder liner. For example, the material properties of machined steel piston ring grooves that engage machined piston rings provide a distinction. The second difference relates to the reciprocating vertical movement of the piston ring relative to the groove surface as the piston moves up and down. Furthermore, the ring flexes radially inward and radially outward, rubbing against the piston ring groove surface during piston operation. Accordingly, a method of forming a piston ring is disclosed, the method comprising providing a base portion formed of a metallic material, applying DLC over a radially outward surface, and applying a chromium layer to a lateral surface and/or an Inner Diameter (ID) surface of the base portion, the lateral surface and the ID surface being configured to be surface-butted with a piston ring groove.

Referring to FIG. 1, an exemplary piston assembly 100 is shown. The piston assembly 100 includes a piston head 102, the piston head 102 including an upper compression ring 104 positioned within an upper compression ring groove 106 and an oil control ring 108 positioned within a lower compression ring groove 110. The piston head 102 also includes a wiper ring 112 positioned within the intermediate recess 114. The rings 104, 108, and 112 seal against the cylinder bore surfaces during reciprocation of the piston assembly 100 within the cylinder bore. The piston head 102 moves in a first or upward direction 116 during an upstroke phase of the piston assembly 100 and moves in a second or downward direction 118 during a downstroke phase of the piston assembly 100.

Referring now to fig. 2A, a piston assembly 200 for an internal combustion engine includes a cylinder bore 202, the cylinder bore 202 having a central axis 204 (e.g., a reciprocation axis) and a cylinder wall 206 equidistant from the central axis 204. The piston assembly 200 includes a radial axis 208 that is orthogonal to the central axis 204. The piston assembly 200 includes a piston 210 having two or more ring grooves, the piston 210 including a combustion ring and groove 212, an oil control ring and groove 214, and an oil scraper ring 216 positioned within an oil scraper ring groove 218.

A cross-sectional detail of the scraper ring 216 is shown in fig. 2B. Referring to fig. 2B, a groove 218 is formed between an upper groove surface 220 and a lower groove surface 222. The oil scraper ring 216 is positioned within the groove 218 such that when the piston 210 is moved in the downward direction 118 (corresponding to fig. 1 during which scraping occurs), the oil scraper ring 216 is constrained against the upper groove surface 220 and when the piston 210 is moved in the upward direction 116, the oil scraper ring 216 is constrained against the lower groove surface 222. In one exemplary method as shown herein, the oil scraper ring 216 is positioned within the groove 218 between the grooves 212 and 214.

Typically, when there are three ring grooves, the rings in groove 212 are typically combustion rings that are primarily used to prevent combustion gases from crossing over the piston 210. The ring in groove 214 generally serves primarily as an oil control ring that controls the distribution of oil for lubrication purposes. Thus, if following common practice, ring 216 may be placed in intermediate groove 218. Furthermore, although the oil scraper ring 216 is shown in a three-ring design, it is contemplated that the oil scraper ring 216 may also be implemented in other multi-groove designs, such as a double-ring groove mechanism having only the combustion ring and the oil scraper ring 216, where the oil scraper ring 216 also serves as an oil control ring. Furthermore, the oil scraper ring is shown in a somewhat non-rectangular arrangement, wherein the surface 224 is non-orthogonally positioned relative to the lower surface 226 and the upper surface 232. Corner 242 is formed between surface 224 and surface 226.

Any one or all three rings of grooves 212, 214, and 218 may benefit from rings made in accordance with the present disclosure. Thus, any or all of the piston rings of fig. 2A may include the features of the piston rings and their coatings described herein.

Fig. 3A is a cross-sectional view of a piston ring 300 according to the present invention, and fig. 3B is a close-up view of a corner of the piston ring 300 of fig. 3A. The piston ring 300 includes a base portion 302 formed of a metallic material. The metallic material may be composed of steel, for example, carbon steel and/or non-nitrided steel and/or chrome silicon steel wire. The base portion 302 includes an outer contact surface or contact surface (hereinafter referred to as an outer contact surface) 304 extending between a first face or side or upper side or surface (hereinafter referred to as a first face) 306 and a second face or side or bottom surface (hereinafter referred to as a second face) 308 opposite the first face 306 of the piston ring 300 about a central axis of the piston ring and/or piston and/or cylinder bore. According to the illustrated example, the first face 306 is a top surface and the second face 308 is a bottom surface. Corner region 310 is disposed at the intersection of outer contact surface 304 and second face 308. Corner region 310 may be a relatively sharp corner having a radius of a few hundred μm (or microns) to a millimeter or more. A diamond-like carbon (DLC) layer 312 is positioned on the outer contact surface 304. DLC 312 has an approximately constant thickness over outer contact surface 304 and a tapered thickness 318 in corner region 310, tapering to zero thickness 320 at or beyond corner region 310 (because DLC 312 wraps around corner region 310 and extends to zero thickness 320 with tapered thickness 318). A chromium overcoat 314 is formed or positioned on the second face 308, the chromium overcoat 314 is formed over the tapered thickness 318 of the DLC layer 312, and the chromium overcoat 314 itself has a tapered thickness 316 in the corner areas 310. A phosphate layer 322 may be disposed along the first face 306. The phosphate layer 322 may at least partially overlap or cover the DLC layer 312 in the (upper) corner region in a similar manner as the chrome coating 314 at the (lower) corner region 310.

This reduced thickness 316 layer of chromium overcoat 314 is affected by the DLC and chromium formation process and the electrical and mechanical properties, according to the present disclosure. For example, DLC layer 312 is formed by known processes such as ion beam deposition, sputtering, cathodic arc, electron beam, laser, or RF plasma deposition, as examples, and DLC 312 may be formed according to any known process of the present disclosure. Due to the nature of DLC 312 formation, DLC 312 (as described above) generally has a constant thickness over outer contact surface 304, but in the corner areas, the thickness tapers as shown by taper thickness 318. Thus, the tapered thickness 318 of the DLC 312 in the corner region 310 may be due to the nature of the manner in which the DLC is formed, and may not be due to any post-processing (e.g., forming the tapered thickness). However, the tapered thickness 318 may also be formed by further post-processing, such as grinding or other techniques for removing material in the corner region 310 and around the circumference of the base portion 302 (the base portion 302 represents a circular piston ring as described).

Once DLC 312 is formed and has a gradually decreasing thickness 318, a chromium layer is applied to the surface of the piston ring. In accordance with the present disclosure and still referring to fig. 3A and 3B, a chromium coating 314 is deposited using an electrochemical process. Typically, this process depends on the conductive properties of the base material on which the chromium is plated, in which case the base portion 302 represents the base material for the piston ring. However, while DLC is generally considered an electrical insulator, it should be understood that although an insulator, there is some electrical conduction that occurs through DLC, in this case DLC 312. The electrical conductivity of an object such as an insulator also depends on the thickness of any layer through which current passes.

Thus, electrical conduction through DLC, while very limited, is more pronounced in the region of the tapered thickness 318 of DLC 312 and around corner region 310. During deposition of the chromium coating 314 on the second face 308 of the base portion 302, a varying degree of electrical conduction through the DLC occurs through the tapered thickness 318 of the DLC 312, forming a tapered thickness 316 of the chromium coating 314 over the DLC 312.

Thus, the piston ring may comprise: a base portion formed of a metallic material; an outer contact surface extending between a first face of the piston ring and a second face opposite the first face, a corner region being formed at an intersection of the outer contact surface and the second face; a diamond-like carbon (DLC) layer positioned on the outer contact surface; and a chromium coating on the second face, the chromium coating being positioned over the DLC layer in the corner area. According to one embodiment, a phosphate layer may be provided on the first face.

DLC has an approximately constant thickness of DLC on the outer contact surface. The DLC has a tapered thickness of DLC in the corner areas, which tapers from an approximately constant thickness of DLC to zero thickness of DLC at or beyond the corner areas. The chromium coating is of approximately constant thickness on the second face. The chromium coating is of approximately constant thickness on the second face. The chromium coating has a hardness of at least 8.5 mohs hardness. DLC is an amorphous carbon material.

Further in accordance with one aspect, a method of forming a piston ring includes providing a base portion formed of a metallic material, the base portion having a first side and a second side opposite the first side, an outer contact surface of the base portion extending between the first side and the second side; depositing a diamond-like carbon (DLC) layer on the outer contact surface; and depositing a chromium coating on the second (bottom) side and over the DLC layer in the corner areas at the intersection of the outer contact surface and the second side. According to one embodiment, the method may further comprise applying a phosphate layer on the first (top) side of the base portion.

In one example, the chromium coating has a hardness of at least 8.5 mohs hardness. In one example, the DLC is an amorphous carbon material.

At a high level, and with reference to fig. 4, a method of forming a piston ring 400 begins 402 and includes applying DLC to an outer radial surface 404 of the piston ring and applying a chromium layer or chromium-based layer to a bottom surface 406 of the piston ring such that a chromium coating is positioned over the DLC layer in a corner region between the outer radial surface and the bottom surface.

Furthermore, while the disclosed method is described with respect to a ring in the middle groove of the three grooves shown in fig. 1 and 2A, it is contemplated that the disclosed method and apparatus may be used with any ring groove where wear occurs due to frictional wear of the ring in the groove or where improved flatness is desired to better seal the ring with the piston to control oil and/or combustion gas blowby.

The method includes depositing DLC having an approximately constant thickness on the outer contact surface. The method includes depositing DLC having a tapered thickness of DLC in the corner areas, starting from an approximately constant thickness of DLC, tapering to zero thickness of DLC at or beyond the corner areas. The method includes depositing a chromium coating having an approximately constant thickness of the chromium coating on the bottom surface. The method includes depositing a chromium coating having a progressively decreasing thickness in the corner regions, starting from an approximately constant thickness of the chromium coating, progressively decreasing to zero thickness of the chromium coating at or beyond the corner regions. The method includes depositing a DLC layer using a Physical Vapor Deposition (PVD) process.

Fig. 5 is a cross-sectional view of a piston ring 100 according to the present disclosure, and fig. 6A, 6B, and 6C are close-up views of the corners of the piston ring of fig. 5. The piston ring 500 includes a base portion 502 formed of a metallic material, such as steel, for example, carbon steel and/or non-nitrided steel and/or chrome silicon steel wire. The base portion 502 includes an outer contact surface 504 extending between a first face 506 (e.g., an upper surface or side) and a second face 508 (e.g., a lower surface or side) opposite the first face 506 of the piston ring 500, an upper outer corner region 518 being formed at the intersection of the outer contact surface 504 and the first face 506, and a lower outer corner region 520 being formed at the intersection of the outer contact surface 504 and the second face 508. The base portion 502 includes an inner contact surface 510 extending between the first face 506 and the second face 508 of the piston ring 500, a first (upper) inner corner region 522 being formed at the intersection of the inner contact surface 510 and the first face 506, and a second (lower) inner corner region 524 being formed at the intersection of the inner contact surface 510 and the second face 508.

The piston ring 500 includes a chromium coating 514 disposed on one or more of the outer contact surface 504, the second (lower) face 508, and the inner contact surface 510. In the example shown, a chromium coating 514 is disposed on each of the outer contact surface 504, the second (lower) face 508, and the inner contact surface 510. The chromium coating 514 may have a thickness of 10 μm to 20 μm, with a thickness variation of 5 μm or less. The chromium coating may have a flatness of 2 μm or less along at least the second face 508 and the inner contact surface 510. DLC 512 is disposed on chrome coating 514 along outer contact surface 504, e.g., DLC 512 covers chrome coating 514 to form an outer sliding layer. DLC may have a thickness of 2 μm to 25 μm. A chromium interlayer 532 may be interposed between the chromium coating 514 and the DLC 512 along the outer contact surface 504 to improve adhesion between the DLC 512 and the chromium coating 514. The chromium interlayer 532 may have a thickness of less than or equal to (+.1 μm). The phosphate layer 516 is disposed on the first (upper) face 506 and may be applied directly to the base portion 502 along the first face 506 at least in some areas and may also partially overlap the DLC 512 in the upper outer corner area 518. The phosphate layer 516 may have a thickness of 3 μm or less. The phosphate layer 516 may promote improved wear and lubrication characteristics and help prevent rust and/or oxidation.

Fig. 6A, 6B, and 6C are close-up views of a lower outer corner region 520, an upper outer corner region 518, and an upper inner corner region 522, respectively. Corner regions 518, 520, 522, 524 may be relatively sharp corners having a radius of a few hundred μm to one millimeter or more. The chromium coating 514 may be formed or positioned on the second face 508, the inner contact surface 510, and the outer contact surface 504 with a substantially constant thickness (e.g., greater than 5 μm, particularly 10-20 μm) on the second face 508, the inner contact surface 510, and the outer contact surface 504. As used herein, an approximately constant thickness of the chromium coating 512 means a thickness variation of less than 5 μm.

Referring to fig. 6A, the chrome coating 514 may have a substantially constant thickness on the second face 508 and remain substantially constant when the chrome coating 514 is wrapped around the lower outer corner region 520 to the outer contact surface 504. DLC 512 is disposed over chrome coating 514 along outer contact surface 504 and may form the outermost sliding layer. DLC 512 may have an approximately constant thickness over outer contact surface 504 and may have a tapered thickness 602 in lower outer corner region 520 that tapers to zero thickness 604 at or beyond lower outer corner region 520 (as DLC 512 wraps around lower outer corner region 520 and extends to zero thickness 604 with tapered thickness 602). According to one embodiment, the chromium intermediate layer 532 is interposed between the DLC 512 and the chromium coating 514. The chrome interlayer 532 may have a thickness of 1 μm or less. The chrome intermediate layer 532 may be formed in a similar manner to DLC 512, e.g., with approximately constant thickness along the outer contact surface 504, and gradually decreasing thickness at the top and bottom outer corner regions 518, 520.

Referring now to fig. 6B, the chrome coating 514 may have an approximately constant thickness over the outer contact surface 504 and may have a tapered thickness 606 in the upper outer corner region 518 that tapers to a zero thickness 614 at or beyond the upper outer corner region 518 (as the chrome coating 514 wraps around the upper outer corner region 518 and extends to the zero thickness 614 with the tapered thickness 606). DLC 512 is wrapped around chrome coating 514 on outer contact surface 504 at an approximately constant thickness. The DLC 512 may then have a tapered thickness 608 in the upper outer corner region 518, reducing to zero thickness 616 at or beyond the upper outer corner region 518 (as the DLC 512 wraps around the upper outer corner region 518 and extends to zero thickness 616 with the tapered thickness 608). The phosphate layer 516 is disposed along the first (upper) face 506 and may have an approximately constant thickness along the first face 506 and a tapered thickness 610 in the upper outer corner region 518, tapering to a zero thickness 612 at or beyond the upper outer corner region 518 (as the phosphate layer 516 wraps around the upper outer corner region 518 and extends to the zero thickness 612 with the tapered thickness 610). Thus, the phosphate layer 516 may partially overlap or cover the DLC 512 in the upper outer corner region 518. According to one embodiment, the phosphate layer 516 has a thickness of 3 μm or less.

Referring now to fig. 6C, the chrome coating 514 may have an approximately constant thickness on the inner contact surface 510 and a tapered thickness 620 in the upper inner corner region 522, tapering to a zero thickness 624 at or beyond the upper inner corner region 522 (as the chrome coating 514 wraps around the upper inner corner region 522 and extends to the zero thickness 624 with the tapered thickness 620). The phosphate layer 516 has an approximately constant thickness on the first face 506 and a tapered thickness 618 in the upper inner corner region 522, tapering to a zero thickness 622 at or beyond the upper inner corner region 522 (as the phosphate layer 516 wraps around the upper inner corner region 522 and extends to a zero thickness 624 with the tapered thickness 618). Thus, the phosphate layer 516 may at least partially overlap the chromium coating 514 in the upper inner corner region 522.

As shown in fig. 5, the chrome coating 512 may have an approximately constant thickness along the second (lower) face 508, the lower inner corner region 524, and the inner contact surface 510, and then gradually decrease in thickness 620 in the upper inner corner region 522 to zero thickness 624.

Without wishing to be bound by theory, the chromium coating 514 having a decreasing thickness 606 in the upper outer corner region 518 may be affected by the DLC and chromium forming process as well as the electrical and mechanical properties. For example, DLC 512 may be formed by known processes such as Physical Vapor Deposition (PVD) including ion beam deposition, sputtering, cathodic arc, electron beam, laser, or RF plasma deposition. Due to the nature of DLC 512 formation, DLC 512 (as described above) typically has a constant thickness over outer contact surface 504, but in upper outer corner region 518 and lower outer corner region 520, the thickness gradually decreases as shown by gradually decreasing thickness 608 and gradually decreasing thickness 602, respectively. Thus, in accordance with the present disclosure, the tapered thickness 608 of the DLC 512 in the upper outer corner area 518 and the tapered thickness 602 of the DLC 512 in the lower outer corner area 520 are due to the nature of the manner in which the DLC 512 is formed, and may not be due to any post-processing (e.g., forming the tapered thickness). However, in accordance with the present disclosure, the tapered thicknesses 608 and 602 may also be formed by further post-processing, such as grinding or other techniques for removing material in the upper outer corner region 518 and lower outer corner region 520 and around the circumference of the base portion 502 (the base portion 502 represents the depicted circular piston ring).

An electrochemical process (e.g., electroplating) may be used to deposit the chromium coating 514. Typically, this process depends on the conductive properties of the base material on which the chromium is plated, in which case the base portion 502 represents the base material for the piston ring.

Referring to fig. 7, a method of forming a piston ring 700 is shown according to one example. At block 702, a metal base portion is provided. At block 704, the outer diameter is formed into a preferred profile for the piston ring. The chromium coating is electrochemically applied 706 to the periphery of the base portion, including the first face, the second face, the outer contact surface, and the inner contact surface. A chromium intermediate layer is applied 708 to the outer contact surface over the chromium coating. The chromium intermediate layer (also referred to as a chromium strike layer) may be applied by a PVD process at low temperatures (e.g., about 200 ℃) to prevent a reduction in the hardness of the chromium coating. The chromium intermediate layer may have a thickness of about 1 μm or less and may generally be softer than the chromium coating to help provide adhesion and break-in characteristics. At block 710, DLC is then applied (e.g., via PVD) to the external contact surface such that the DLC layer is positioned over the chromium intermediate layer and the chromium overcoat layer. The first face is treated by grinding or another technique to remove the chromium coating prior to applying 712 the phosphate layer to the first face.

The application of a chromium coating on the second face protects the ring and the undercut. The process of manufacturing the piston ring is modified such that the first face is ground after the chromium coating is applied so that the first face can be layered with phosphate. Chromium thicknesses generally greater than 5 μm at least in areas of constant thickness (e.g., along the outer contact surface, the second (bottom) surface, and the inner contact surface) are notoriously difficult to control in a production environment. Applying a chromium coating on the second face and a phosphate on the first face allows the ring to maintain a desired piston ring gap in the production setting.

Fig. 8-11 illustrate a further development of the method 700 of fig. 7. In an exemplary method for forming a piston ring, it should be appreciated that the second face (bottom side) is "keystone" ground prior to full chrome plating (e.g., applying a chrome coating). The trapezoid may be in the shape of a ring, wherein the first (top) face and the second (bottom) face are wedge-shaped. Trapezoid grinding or trapezoid processing (keystoned) may refer to the process of grinding a face into a trapezoid shape, and may also be referred to as side grinding. After the trapezoidal treatment of the second face, chromium is applied to the periphery of the base portion including the first face, the second face, the outer contact surface and the inner contact surface. A chromium intermediate layer may be applied to the chromium coating along the outer contact surface. DLC is then applied to the outer contact surface over the chromium intermediate layer and/or the chromium cladding. Since the thickness of chromium is difficult to control, the first face (top surface) is subjected to a trapezoidal treatment after DLC is applied. The bottom trapezoid is used to determine how much of the trapezoid treatment is needed for the top surface to maintain the desired gap. Once the top surface is trapezoidal treated, a phosphate layer is deposited on the top surface. This process ensures the desired thickness and dimensions of the piston ring. The intermediate gap grinding may take place before or after complete chrome plating.

Referring to fig. 8, an exemplary method 800 for producing a piston ring is shown according to one embodiment. Method 800 includes providing a base portion that may include coiling 802 a base material (e.g., steel), stress relief 804 (e.g., annealing and/or tempering), first gap grinding 806 of a piston ring gap, PIP marking 808 (e.g., marking a top side of a piston ring), first side grinding 810, light grinding 812, shaping the base portion 814 by Outer Diameter (OD) profile grinding and optional chamfering, and OD brushing 816.

After the base portion has been provided by one or more of steps 802-816, a first trapezoid grinding 818 is performed on a second (bottom) face of the base portion to shape and/or roughen the second face. At step 820, chrome is plated on at least the second (bottom) surface and the inner contact surface to form a chrome coating. According to one embodiment, the base portion comprising the outer contact surface, the inner contact surface, the first (top) surface and the second (bottom) surface is completely chrome plated. The chrome plating is electrochemical and may include at least 3 rotations of the base portion (cathode) during electroplating to ensure proper thickness and quality. Next, OD milling 822 is performed to planarize the chrome coating along the outer contact surface, followed by second or mid-gap milling 824, inner Diameter (ID) brushing 826, and dry blasting 828.

At step 830, DLC is applied to the outer contact surface over the chrome (plating) coating via PVD. Optionally, a chromium interlayer is applied to the chromium overlay via PVD in step 830, and then DLC is applied to the chromium interlayer. It should be appreciated that the chromium intermediate layer may be applied in a separate step from the DLC, for example, after dry blasting 1028 and before application of DLC coating 1030. After applying the DLC, an OD polish 832 (e.g., diamond sandpaper) is performed on the DLC, then a third or gap finish grind 834 is performed, and a side brush or trapezoid brush is performed on the second (bottom) face.

At step 838, a second trapezoidal grinding is performed on the first (top) side to remove the chrome plating along the first side, exposing the material of the base portion, and then a phosphate coating is applied via phosphating along the first side at step 840. The piston ring is inspected 842 and the process ends.

Fig. 9 illustrates an exemplary method 900 for producing a piston ring according to another embodiment. Fig. 9 differs from fig. 8 only in the order of the mid-gap grinding 924 and the OD grinding 922, wherein the mid-gap grinding 924 occurs before the OD grinding 922, for example, between the OD profile grinding 914 and the OD brush grinding 916, and the OD grinding 922 is performed after the OD brush grinding 916 and before the first side (trapezoid) grinding of the second (bottom) face. In other respects, the process is identical to that in fig. 8, as indicated by the same reference numerals.

Referring to fig. 10, an exemplary method 1000 for producing a piston ring is shown in accordance with another embodiment. The example method 1000 includes providing a base portion of a piston ring, which may include one or more of forming the base portion or coil 1002 of a base material (e.g., steel), stress relief 1004 (e.g., tempering/annealing or other heat treatment at a temperature below its critical temperature for a sufficient time so that the base metal may recrystallize to relieve internal stresses), first gap grinding 1006 (e.g., circumferential grinding of a gap between a first end and a second end of the coil/base portion), PIP marking 1008 (e.g., marking a top side of the piston ring), first side grinding 1010 grinding (e.g., rough grinding) the first and/or second sides, lightly grinding 1012, shaping the base portion 1014 by grinding an Outer Diameter (OD) profile (e.g., an outer contact surface) to a desired profile and optional chamfer, and OD brushing 1016.

At step 1018, the base portion undergoes further shaping via a first trapezoid grinding or side grinding (if not shaped as a trapezoid) on the second face, such that the second (bottom) face is ground to form a trapezoid surface. Trapezoid grinding 1018 is performed prior to chrome plating to improve the accuracy and tolerance control of the piston ring. An optional side brushing 1044 may be performed after the first trapezoid grinding on the second face and/or the first face and before the chrome plating is applied to break up any nodules on the ring surface.

At step 1020, the base portion (cathode) is electrochemically chromed by fully immersing the base portion in a chromium-containing electrolyte to provide a fully electroplated chromed on the base portion to provide a chromium coating on the inner and outer contact surfaces, the first and second faces, and the corner regions. For example, the anode may be chromium metal. The base portion is rotated one or more times, and in one embodiment at least three (3) times, to ensure a complete, robust coating with a satisfactory thickness (e.g., between 10 and 20 μm as measured by X-ray fluorescence) and to help facilitate a reduction in thickness variation. It will be appreciated that after the electrodeposition process, the thickness of the chrome plating may be slightly greater than a desired or predetermined thickness (e.g., 20 μm) which is then reduced to a predetermined thickness between 10 and 20 μm by grinding and/or grinding steps, wherein the thickness variation on the inner, outer and second/bottom surfaces is less than or equal to 5 μm and the flatness is less than or equal to 2 μm.

At step 1022, an Outer Diameter (OD) grind is then performed along the outer contact surface of the base portion to remove excess chromium material and planarize the surface to achieve a predefined chromium coating thickness variation, which may be less than or equal to 5 μm according to one embodiment. An optional side brushing 1044 may be performed after the OD milling to break up nodules on the chromium surface but without removing material.

After planarizing the chromium coating to a predetermined thickness and thickness variation along the outer diameter of the piston ring, the method 1000 may include performing a second or mid-gap grinding 1024 on the piston ring gap, further planarizing any chromium buildup at the ring/skirt ends. At step 1026, an Inner Diameter (ID) brush, e.g., of the inner contact surface, is performed to remove nodules on the chrome surface, and then the frit (e.g., the outer contact surface) is dry blasted 1028.

In step 1030, a running or sliding surface may be formed on the outer contact surface over the chrome coating. Step 1030 includes forming a chromium interlayer overlying the chromium coating along the outer contact surface, such as by PVD. The PVD chromium intermediate layer is softer than the electrochemical chromium coating and improves adhesion between the chromium coating and the DLC coating. The PVD chrome intermediate layer can form a thin film (e.g., +.1 μm) over the electrochemical chrome plating along some or all of the outer contact surface and the upper and lower outer corner regions of the base portion. The DLC coating is then formed via PVD applied to the outer contact surface, for example over a chrome intermediate layer, to form the outer sliding surface of the piston ring. The DLC coating may have a thickness of 2 μm to 25 μm. The chromium intermediate layer and DLC coating may be applied at low temperatures, e.g., about 200 ℃ or less, to help reduce hardness degradation that may be associated with higher application temperatures. The chromium intermediate layer may be formed in a single step during the PVD process for DLC. Alternatively, the chromium intermediate layer may be applied in a separate step prior to DLC.

After step 1030, an Outer Diameter (OD) polish 1032 is performed on the DLC coating using diamond sandpaper, including applying a chrome interlayer and/or applying a DLC coating in embodiments where no chrome interlayer is used. In step 1034, a third gap grinding or gap finishing grinding is performed on the piston ring gap such that the first and second ends of the ring/disc stock are finished to a desired roughness.

At step 1046, a trapezoid (or side) grind is performed on one or both of the sides of the base portion to remove portions of the chromium material and planarize the surface of the cladding. Trapezoid grinding 1046 may planarize the chromium coating along the second (bottom) surface by removing an outer portion of the chromium material to form a chromium coating having a desired thickness (10 μm to 20 μm), thickness variation (+.5 μm), and flatness (+.2 μm). Additionally or alternatively, the trapezoidal shaped grinding 1046 may remove portions of the chromium material along the first (top) face, e.g., at the upper inner corner regions, in order to planarize and shape the chromium coating. At step 1036, a trapezoidal brushing is then performed on the second face to reduce or remove nodules on the chromium surface.

At step 1038, a second trapezoid grinding 1038 is performed on the first (top) face to remove all or substantially all of the chromium material along the first face from the upper inner corner region to the upper outer corner region, expose the material of the base portion, and roughen the exposed surface of the base material. The second trapezoid grinding 1038 may also assist in shaping the upper outer corner region to provide a tapered thickness of the DLC and/or shaping the upper inner corner region to provide a tapered thickness of the chrome coating. In step 1040, a phosphate coating is applied to the first (top) face by phosphorylation to form a phosphate coating having a thickness of approximately +.3 μm (e.g., between 2 μm and 3 μm), which coating may be applied directly to the base portion along the first face (via the second trapezoid grinding 1038) and at least partially overlapping the DLC on the upper outer corner region and at least partially over the chrome coating on the upper outer corner region. Phosphate coatings help to improve corrosion resistance and promote lubrication and wear resistance. The phosphate coating may include, for example, magnesium phosphate. A final check 1042 may be made and the process ends.

Fig. 11 illustrates an exemplary method 1100 for producing a piston ring according to another embodiment. Fig. 11 differs from fig. 10 in the order of the trapezoidal grinding 1146, wherein, according to the method 1100, the trapezoidal grinding 1146 is performed prior to the second/mid-gap grinding 1124 (e.g., after the OD grinding 1122) and may facilitate a gradual reduction in the thickness of the chromium coating formed at the upper outer corner region. Trapezoid grinding 1146 planarizes the outer surface of the chromium coating along the second (bottom) surface by some of the chromium material to form a chromium coating having a desired thickness (10 μm to 20 μm), thickness variation (+.5 μm), and flatness (+.2 μm). The OD grind 1122 and trapezoid grind 1144 together shape and planarize the outer contours (e.g., outer contact surface, first/top surface, and second/bottom surface) of the chrome-plated ring. The method 1100 may also include an optional side brushing 1144 on the first and/or second face after the trapezoid grinding 1146 to break up nodules on the chromium surface. In other respects, the process remains the same as in fig. 10, as indicated by the same reference numerals.

It should be appreciated that grinding is different from lapping, where lapping helps to increase smoothness and flatness (flatness), while grinding removes more material and produces a rougher surface than lapping.

Thus, according to the present disclosure and the methods of fig. 5 and 6A-6C, and according to the methods of fig. 7-11, the chromium coating 514 is deposited on the outer surface 530 of the base portion 502 at steps 706, 820, 920, 1020, 1120. The outer surface 530 surrounds the outer periphery of the base portion 502, including on the first face 506, the second face 508, the outer contact surface 504 and the inner contact surface 510 of the side facing the ring 500. After shaping at least the outer diameter of the base portion 502 along the outer contact surface 504 (e.g., by OD grinding 822, 922, 1022, 1122 and/or grinding) to remove excess chromium material, DLC 512 is deposited (e.g., via PVD) on the outer surface of the chromium coating 514 on the outer contact surface 504 facing the side of the ring 500 at steps 710, 830, 930, 1030, 1130. According to one embodiment, a chromium intermediate layer 532 may also be deposited (e.g., via PVD) on the outer surface of the chromium coating 514 along the outer contact surface 504 to improve adhesion between the chromium coating 514 and the DLC 512. Thereafter, the first face 506 is ground 838, 938, 1038, 1138 such that the chromium coating 514 is removed from the first face 506 and the outer surface 530 is exposed on the first face 506 facing the side of the ring 500. The chromium coating 514 remains on the outer surface 530 on the second face 508, the outer contact surface 504 and the inner contact surface 510 of the side facing the ring 500. At steps 712, 840, 940, 1040, 1140, a phosphate layer 516 is deposited on the first face 506 by phosphating such that the phosphate layer 516 is deposited on the outer surface 530 from which the chromium coating 514 was removed. The phosphate layer 516 may additionally be deposited on the outer corner surface 536 in the upper outer corner region 518. The outer corner surface 536 is the outer surface of the DLC layer 512 as the DLC layer 512 extends from the tapered thickness 608 to zero thickness 610. Thus, the phosphate layer 516 may partially cover/overlap the DLC 512 on the outer corner surface 536 so as to wrap around the upper outer corner area 518 and be located on the outer surface 536 of the DLC layer 512. A phosphate layer 516 is also deposited on the inner corner surface 534 in the upper inner corner region 522. The inner corner surface 534 is the outer surface of the chromium coating 514 as the chromium coating 514 extends from the tapered thickness 620 to the zero thickness 624. The phosphate layer 516 may partially cover/overlap the chrome coating 514 on the inner corner surface 534 so as to wrap around the upper inner corner region 522 and be located on the outer surface 534 of the chrome coating 514.

In another example, as shown in fig. 12, a polymer layer 1218 is provided. The piston ring 1200 includes a base portion 1202 formed of a metallic material such as steel. The base portion includes an outer contact surface 1204 extending between a first face 1206 (e.g., an upper surface) and a second face 1208 (e.g., a lower surface) opposite the first face 1206 of the piston ring 1200. The base portion includes an inner contact surface 1210 extending between the first face 1206 and the second face 1208 of the piston ring 1200. A chromium coating 1214 is formed or positioned on the second face 1208, the outer contact surface 1204, and the inner contact surface 1210. A DLC layer 1212 is formed on the outer contact surface 504 over the chrome layer 1014, and a chrome interlayer (not shown) may be interposed between the DLC layer 1212 and the chrome overcoat 1214. A phosphate layer 1216 is formed on the first face 1206. A polymer layer 1218 is formed over the DLC layer 1212 such that the polymer layer 1218 covers the outer surface of the DLC layer 1212. The high hardness of DLC layer 1212 makes it difficult for piston ring 1200 to wear in during the early part of engine life. The polymer layer 1218 provides a solid lubricant to the outer contact surface 1204 of the piston ring 1200 to allow for faster wear of engine components and to improve the durability of the piston ring 1200. The polymer layer 1218 is a resin bonded coating and may include, in particular, resin bonded Polyamideimide (PAI) or resin bonded Polytetrafluoroethylene (PTFE) with molybdenum disulfide added. For example, the piston ring 1200 may be formed according to the method shown in fig. 7-11, with the addition of a polymer layer 1218 applied to the outer contact surface 1204 over the DLC layer 1212. Thus, it should be appreciated that the polymer layer 1218 may be applied to the DLC 512 shown in fig. 5-6C, for example, without departing from the scope of the present disclosure.

In one example, the phosphate layer 1016 is approximately 3 μm or less, e.g., approximately 2-3 μm, and the polymer layer 1018 is approximately 5-15 μm, e.g., approximately 10 μm. The chrome coating 1014 is approximately 10-20 μm with a thickness variation of 5 μm or less and a flatness of 2 μm or less (at least on the second face 508 and the inner contact surface 510 including the corner regions 520, 524). The chromium interlayer may be approximately 1 μm or less. The DLC layer may be approximately 2-25 μm.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many applications other than the examples provided will follow upon reading the above description. The scope of the disclosure should be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. It is anticipated and intended that future developments will occur in the arts discussed herein, and that the disclosed systems and methods will be incorporated into such future embodiments. In summary, it is to be understood that the present disclosure is capable of modification and variation and is limited only by the following claims.

All terms used in the claims are intended to be given their broadest reasonable constructions and their ordinary meanings as understood by those skilled in the art unless an explicit indication to the contrary is made herein. In particular, use of the singular articles such as "a," "an," "the," and the like should be understood to recite one or more of the indicated elements, unless a claim recites an explicit limitation to the contrary.

It should be understood that references to individual elements are not necessarily limited thereto, and may include one or more of such elements. Any directional references (e.g., positive, negative, up, down, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader's understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use of the examples/embodiments.

"One or more" includes functions performed by one element, functions performed by more than one element, e.g., in a distributed fashion, functions performed by one element, functions performed by multiple elements, or any combination of the above.

It will be further understood that, although the terms first, second, etc. may be used herein to describe various elements in some instances, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the various described embodiments. The first element and the second element are both elements, but they are not the same element.

The terminology used in the description of the various described embodiments herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described embodiments and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Claims (20)

1. A piston ring, comprising:

a base portion formed of a metallic material;

an outer contact surface and an inner contact surface extending between a first face of the piston ring and a second face opposite the first face;

A chromium coating on the inner contact surface, the outer contact surface, and the second face;

A diamond-like carbon DLC layer disposed over the chromium coating on the outer contact surface.

2. The piston ring of claim 1, wherein the piston ring further comprises a phosphate layer on the first face of the piston ring.

3. The piston ring of claim 2, wherein the phosphate layer overlaps the DLC portion at an upper outer corner area of the base portion and overlaps the chromium coating portion at an upper inner corner area of the base portion.

4. A piston ring according to claim 3, wherein the phosphate layer has a gradually decreasing thickness along the upper outer corner region and the upper inner corner region.

5. The piston ring of claim 1, wherein the piston ring further comprises a physical vapor deposition PVD chromium intermediate layer along the outer contact surface between the DLC and the chromium coating.

6. The piston ring of claim 1, wherein the piston ring further comprises a polymer layer covering the DLC.

7. The piston ring of claim 1, wherein the DLC has a substantially constant thickness along the outer contact surface and a gradually decreasing thickness at a lower outer corner region of the base portion and an upper outer corner region of the base portion.

8. The piston ring of claim 7, wherein:

The chrome coating has a substantially constant thickness along the outer contact surface, the lower outer corner region, the second face, the lower inner corner region of the base portion, and the inner contact surface; and

The chromium coating has a progressively decreasing thickness at an upper outer corner region and an upper inner corner region of the base portion.

9. The piston ring of claim 1, wherein the chromium coating has a thickness between 10 and 20 μιη with a thickness variation of equal to or less than 5 μιη, and wherein the chromium coating has a flatness of equal to or less than 2 μιη along the second face and the inner contact surface.

10. A method of forming a piston ring, comprising:

Providing a base portion formed of a metallic material, the base portion having a first surface, a second surface opposite the first surface, an outer contact surface, and an inner contact surface;

applying a chromium coating on the first face, the second face, the outer contact surface and the inner contact surface;

A diamond-like carbon DLC layer is deposited over the chromium coating on the outer contact surface.

11. The method of claim 10, further comprising trapezoid grinding the second face to planarize an outer surface of the chromium coating to a flatness variation of 2 μιη or less.

12. The method of claim 11, further comprising brushing the second face after the trapezoid grinding to remove nodules from the chromium coating.

13. The method of claim 11, wherein the method further comprises:

Trapezoid grinding said first face to remove substantially all of said chromium coating from said first face, thereby exposing said base portion and roughening an outer surface of said base portion; and

A phosphate layer is deposited on the first face.

14. The method of claim 13, further comprising trapezoid grinding the second face prior to applying the chromium coating.

15. The method of claim 13, wherein the method further comprises:

Performing an OD grinding of the chromium coating along the outer contact surface prior to depositing the DLC;

gap grinding the chromium coating at piston ring gap prior to depositing the DLC;

brushing the inner diameter ID of the chromium coating along the inner contact surface after the gap grinding and before depositing the DLC; and

The chromium coating is gap finish ground at the piston ring gap after the DLC is deposited and before trapezoid grinding the first face.

16. The method of claim 16, wherein the trapezoid grinding is performed after the OD grinding and before the gap grinding.

17. The method of claim 10, further comprising depositing a chromium intermediate layer over the chromium overcoat along the outer contact surface, the chromium intermediate layer being interposed between the chromium overcoat and the DLC.

18. The method of claim 17, wherein the chromium coating is applied electrochemically and the chromium intermediate layer is deposited via physical vapor deposition.

19. The method of claim 17, wherein the method has at least one of the following features:

the chromium intermediate layer is deposited in the same step as the DLC; and

The chromium interlayer is deposited at a temperature of about 200 ℃ or less.

20. The method of claim 11, wherein the method further comprises depositing a polymer layer on an exterior surface of the DLC layer.