FIELD OF INVENTION
The present invention relates to a composite plating film of chromium with near spherical shape with regular crystal of diamond particles with lower level of impurities in network-shaped micro-cracks, and a piston ring having its plating film and a method of machining for producing the piston ring.
BACKGROUND OF THE INVENTION
Future development of Diesel engine suggest that with the increase in mechanical and thermal load , for piston rings having improved scuffing resistance and wear resistance vis-a-vis cylinder liners are being demanded, as higher-power , higher fuel-efficiency internal combustion engines are required .
To fulfil the above requirement of such demand, piston rings are coated with composite chromium plating film.
In past the composite plating of chromium with embedded hard particles like micron aluminium oxide , diamond particles etc , increases the scuffing resistance and wear resistance capabilities compared to the composite plating of chrome with only the hard particles like aluminium oxide (A1203).

However the increasing trend of thermal and mechanical load on the piston ring and cylinder surface, more work need to be done for further improvement in scuffing resistance & wear resistance of the piston ring. Thus the proposal in present invention is to provide a composite plating of chromium film having excellent wear resistance and scuffing resistance with reduction in attacking ability to a mating surface. The increasing conflict between longer life of a piston ring in a higher temperature cylinder environment and a reduced availability of lubrication is one of the motivations of development in this context.
These imminent tribologically demanding operating conditions mean that a new coating system is clearly required which is superior to the conventional composite plating of chrome.
For instance, EP 1 719 827 Bl teaches a composite plating film of chromium containing hard particles in network-shaped micro-cracks, said micro-cracks having a surface-occupying ratio of 10-20% by area and a distribution density of 1,200 to 2,500/cm, and the amount of said hard particles being 1 to 15% by mass per 100% by mass of the entire plating film. The micro-cracks are preferably distributed over the entire thickness of the plating film. The hard particles are selected from the group consisting of A1203, SiC, Si3N4, and diamond. The plating film preferably comprises at least two layers.
US 6,503,642 Bl discloses an electrodeposited hard-chromium coating, particularly for a piston ring, the coating having cracks therein and having diamond particles

having a size ranging from 0.25 to 0.4 urn embedded in said cracks. In this coating, compared with former chromium coatings with embedded aluminum oxide particles, ring wear was reduced by half and corrosion resistance was improved greater than 20%. Furthermore, it is taught that this coating demonstrates a unique property improved due to transformation from diamond to graphite even when it is subjected to high thermal loads.
However, in the severe environment of increased thermal and mechanical loads, it is in fact hard to say that the above mentioned coating is a robust coating achieving the sufficient wear resistance and scuffing resistance.
OBJECTIVE OF THE INVENTION
In view of the above limitations, the primary objective of the present invention is to provide a composite plating film with improved diamond particle shape (nearly spherical with lesser impurities having length to width ratio of 1 to 2 comprising 95% or more by ratio) having excellent wear resistance and scuffing resistance with little attacking ability to a mating member coated with such plating film and its production method.
Further object of the present invention is to provide the piston rings having a composite plating film with improved diamond particle shape (nearly spherical with lesser impurities having length to width ratio of 1 to 2 comprising 95% or more by ratio) having excellent wear resistance and scuffing resistance with little attacking ability to a mating member coated with such plating film and its production method.

Further object of the present invention is to provide the method of production of the composite plating film with improved diamond particle shape (nearly spherical with lesser impurities having length to width ratio of 1 to 2 comprising 95% or more by ratio) having excellent wear resistance and scuffing resistance with little attacking ability to a mating member coated with such plating film and its production method.
Yet further object of the present invention is to provide the machining process of the composite plating film with improved diamond particle shape (nearly spherical with lesser impurities having length to width ratio of 1 to 2 comprising 95% or more by ratio) having excellent wear resistance and scuffing resistance with little attacking ability to a mating member coated with such plating film and its production method.
SUMMARY OF THE INVENTION
In order to obviate the drawbacks of the prior known coatings, the present invention provides a composite plating film of chromium having excellent wear resistance and scuffing resistance with reduction in attacking nature to a mating surface (a sliding member) by incorporating nearly spherical & regular shaped crystal diamond micron particles with minimum impurities level of Al, Fe ,Na, K & Si in network-shaped micro-cracks and by controlling crack network ratio and an average crack-opening width of the micro-cracks which function as oil-reservoirs and finding suitable manufacturing process to reduce the piston ring outer peripheral roughness value to further reduce the attacking nature of mating surface.

According to one of the embodiment the present invention provides the composite plating film of chromium with hard particles in network-shaped micro-cracks, said micro-cracks having a surface-occupying ratio of 5 -18% by area, said hard particles comprising diamond particles (nearly spherical and with lesser impurities). Said micro-cracks preferably have a surface-occupying ratio of 5% or more and less than 18%.
According to another embodiment of the present invention provides Said hard particles preferably comprise second hard particles selected from the group consisting of oxide, carbide and nitride of Al, Si and Ti, and cubic boron nitride, in addition to said diamond particles (nearly spherical and with lesser impurities).
According to another embodiment of the present invention average crack width of a second hard particle preferably larger than an average crack-opening width of a diamond particle containing area of said micro-cracks.
According to another embodiment of present invention said micro-cracks preferably have a distribution density of 400-1,300/cm.
According to another embodiment of present invention the amount of said diamond particles is preferably 0.25-10 % by mass, and the amount of said second hard particles is preferably 0.5-12 % by mass. Said second hard particles preferably comprise aluminum oxide (A1203) particles.

According to another embodiment of present invention said composite plating film of chromium preferably comprises at least two (2) layers. Further, the piston ring of the present invention is a piston ring having the composite plating film of chromium mentioned above formed on at least a sliding surface of a piston ring base material.
According to another embodiment of present invention the method for producing the piston ring plating and machining of the present invention is a method comprising conducting at least one cycle comprising (a) forming a hard chromium plating layer on said sliding surface of said piston ring base material, and (b) subjecting the resultant hard chromium plating layer to an inverse voltage treatment, in a state where said piston ring base material is immersed in a chromium-plating bath, (c) applying special technique to do machining (Grinding operation & Lapping operation) to achieve required shape and surface finish less than Rz 2.50.
According to another embodiment of present invention in the production procedure of the piston ring for plating and machining operation, the hard chromium plating layer is deposited on the piston ring base material as a cathode. After deposition of the hard chromium plating layer having a predetermined thickness, network-shaped micro-cracks are formed and extended in the hard chromium plating layer by using an inverse voltage treatment where polarity of the piston ring base material and the counter electrode is reversed. The micro-cracks formed by using the inverse voltage treatment are opened on a surface, and the hard particles dispersed in the chromium-plating bath and charged negative are captured in the micro-cracks. By repeating a cycle of the deposition of the hard chromium plating layer and the

formation of the micro-cracks and the capture of the hard particles using the inverse voltage treatment, a multi-layered coating film can be obtained. The deposition of the hard chromium plating layer is preferably conducted at the current density of 30-80 A/dm and a plating bath temperature of 40-70°C. The inverse voltage treatment is preferably conducted on the condition of a current density of 5-70 A/dm and a plating bath temperature of 50-70°C. A current density of the inverse voltage treatment is more preferably 15-65 A/dm . Hard particles primary & secondary are uniformly distributed in the plating bath.
According to another embodiment of present invention for the galvanic process the electroplating bath were prepared with help of Chromic acid (Cr03), Sulfuric acid (H2S04), Sodium hexa-fluorosilicate (Na2SiF6), Methane sulfonic acid (CH3SO3H), Anionic surfactant (trade name "Ftergent 110"), Micron Diamond particles (Nearly spherical shape with low impurities) & A1203 particles .
According to another embodiment of present invention in the production procedure of the piston ring for plating and machining operation by applying special technique to machining the piston ring plating layer esp. special grinding operation and lapping operation shape is controlled & surface finish preferably kept < Rz 2.50.
Because the composite plating film of Chromium of the present invention contains diamond particles of nearly spherical shape with lesser impurities of 0.25 to 10% by mass and distributed uniformly along with second hard particle in network-shaped micro-cracks on the surface of plated rings with distribution density

of 400 to 1300/cm and a controlled surface occupying ratio of 5 ~ 18% by area with controlled surface roughness less than Rz 2.50 by specific machining operation like special grinding and lapping the plating film on piston rings demonstrates excellent scuffing and wear resistance . Further, because micro-cracks, which function as oil-reservoirs, have a predetermined surface-occupying area ratio 5 to 18%, the attacking nature to the mating member can be suppressed to low as a result of a high lubricating function. By using the second hard particles, e.g., alumina particles, having less attacking nature to the mating member than diamond particles, as a part of hard particles, it makes it possible to set a wide distribution width of the hard particles, i.e., to increase a surface-occupying ratio and controlled amount of primary and secondary hard particles 0.25 to 10 % by mass and 0.5 to 12% by mass respectively without increasing attacking nature to the mating member, resulting in further improving in lubricating characteristics. Therefore, the composite plating film of chromium of the present invention can be used to cope with required fuel efficiency and the new emission norms as a highly robust coating in the severe environment of increased thermal and mechanical loads.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and further objects, features and advantages of the present subject matter will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements.
It is to be noted, however, that the appended drawings illustrate only typical

embodiments of the present subject matter, and are therefore, not to be considered for limiting of its scope, for the subject matter may admit to other equally effective embodiments. The figures depict a simplified structure only showing some elements and functional entities, all being logical units whose implementation may differ from what is shown.
Fig. 1(a) shows a secondary electron image of a surface of the composite plating
film of chromium of the present invention observed by a scanning electron
microscopy.
Fig. 1(b) shows a SEM-CL image of a surface of the composite plating film of
chromium of the present invention observed by a scanning electron microscopy.
Fig. 2(a) shows a nearly spherical diamond particle in the micro-cracks of the
composite plating film of chromium of the present invention observed by a
scanning electron microscope.
Fig. 2(b) shows an alumina particle containing area of the micro-cracks of the
composite plating film of chromium of the present invention observed by a
scanning electron microscope.
Fig. 3 shows particle image analysis system results of length/width (LAV) of nearly
spherical diamond particles
Fig. 4 shows a schematic view of a wear test machine.
Fig. 5 shows the comparative data of Wear test Rig data w.r.t. Comp. Exp. 2
Fig. 6 shows outer peripheral surface profiles of the piston rings of Example 1 (1st
cylinder, 3nd cylinder 4th cylinder and 6th cylinder) observed after the engine test
(350 hours), compared with results of Comp. Exp. 1 (2nd cylinder & 5th cylinder)

Fig. 7 shows appearance photographs of liner wear profile of Exp. 1 (1st cylinder, 3rd cylinder, 4th cylinder, and 6th cylinder) and of Comp. Exp. 1 (2nd cylinder & 5th cylinder) after 350 hours engine test.
DETAILED DESCRIPTION OF THE INVENTION
The following presents a detailed description of various embodiments of the present subject matter with reference to the accompanying drawings. However, the present subject matter is not limited to these embodiments which are only provided to explain more clearly the present subject matter to a person skilled in the art of the present disclosure. In the accompanying drawings, like reference numerals are used to indicate like components.
The specification may refer to "an", "one", "different" or "some" embodiment(s) in several locations. This does not necessarily imply that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments.
As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes", "comprises", "including" and/or "comprising" 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. As used herein, the term "and/or" includes any and all combinations and arrangements of one or more of the associated listed items.
The composite plating film of chromium of the present invention has network-shaped micro-cracks in which the hard particles including diamond particles are embedded. The network-shaped micro-cracks are formed and extended when an electrodeposited hard chromium plating layer having a predetermined thickness is subjected to an inverse voltage treatment. The hard particles dispersed in a chromium-plating bath are penetrated into the micro-cracks opened during the inverse voltage treatment, forming a so-called composite plating film of chromium with hard particles. When stopping the inverse voltage treatment, though the opened micro-cracks are going to close, closure of the micro-cracks is restricted due to the presence of the hard particles, and thus the micro-cracks have a predetermined average crack-opening width. In this way, the micro-cracks opening to the film surface function as effective oil reservoirs, improving sliding characteristics.
The composite plating film of chromium of the present invention contains nearly spherical diamond particles with lesser impurities in network-shaped micro-cracks. The diamond particles with lesser impurities of Al, Fe, Na, K, & Si have higher modulus and higher hardness than other hard particles such as A1203. Particularly, near spherical mono-crystal diamond particles significantly improve the wear resistance of the composite plating film of chromium. However, the Industrial diamond particles which have a problem which increases the attacking nature to the

mating member when the manufacturing impurities of diamond particles increased. In that sense, the diamond particles preferably have lesser impurities particularly Al, Fe, Na, K, & Si.
Since the micro-cracks opening on the film surface function as effective lubricating oil reservoirs, the composite plating film of chromium has micro-cracks having a controlled surface-occupying ratio of 5-18% by area distribution density of 400 ~ 1300/cm considering the sufficient oil retention on the film surface and also the sufficient strength of the film. The surface-occupying ratio is preferably 5-11% by area, more preferably 6-10% by area, most preferably 7-9% by area.
Furthermore, in order to control the average length and width of the film the hard particles preferably contain second hard particles having less attacking nature to the mating member than diamond particles, in addition to the diamond particles. The second hard particles are preferably selected form the group consisting of oxide, carbide and nitride of Al, Si and Ti, and cubic boron nitride. Specifically, A1203, AIN, Si02, SiC, Si3N4, Ti02, TiC , TiN, c-BN, etc. are preferable. Since the second hard particles have less attacking nature to the mating member than diamond particles, the hard particles having a larger particle size than diamond particles can be used to have a surface-occupying ratio and a crack-opening width of the micro-cracks large, resulting in further improving in lubricating characteristics. The second hard particles preferably have an amount 0.5 to 12% by mass. The second hard particles are preferably alumina (A1203) particles.

Fig. 1(a) and Fig. 1(b) show micro-crack network films on a film surface of the composite plating film of chromium with diamond particles further containing alumina particles as the second hard particles. The SEM-CL detector of Fig. 1(b) & 2 (a) shows white shinny nearly spherical hard particles embedded in the micro-cracks (1). On the other hand, in the Fig. 2(b) second hard particle A1203 embedded.
Fine diamond particles and a little bit larger A1203 particles are relatively uniformly dispersed in the micro-cracks.
Though the network-shaped micro-cracks are exclusively defined in terms of a surface-occupying ratio w.r.t. machining improvement in Rz value in the present invention, attention may be paid to a distribution density of micro-cracks which is conventionally known as a characteristic parameter of the composite plating film of chromium with hard particles.
A distribution density of micro-cracks is preferably 400/cm or more. An upper limit of the distribution density is preferably 1,300/cm or less. The distribution density of micro-cracks is more preferably 600-1,100/cm, most preferably 700-1,100/cm.
In the composite plating film of chromium of the present invention, the amount of diamond particles is preferably 0.25-10.0% by mass per 100% by mass of the entire plating film, more preferably 0.25-5.0% by mass, most preferably 0.30-2.0% by mass.

The amount of diamond particle is measured by specific method by Carbon-Sulpher Analyzer.
Further, the amount of second hard particles is preferably 0.5-12.0% by mass per 100% by mass of the entire plating film, more preferably 0.5-8.0% by mass, most preferably 0.5-5.0% by mass. The second hard particle measurement is done by X-ray Fluoroscopy method.
The hard particles comprising nearly spherical diamond particles with lesser level of impurities having length to width ratio of 1 to 2 comprising 95% or more by ratio , more preferably 1 to 1.80 and most preferably 1 to 1.70. Fig 3 is the method of measurement of the length to width ratio of above said diamond particle by the particle shape analysis. The above ratio is measured by using the most scientific statistics of the number of the particle and the number of pixels occupied by each particle (Fig. 3). Further the area of equivalent circle and the volume of equivalent sphere are also obtained, therefore diameter of equivalent circle area and length by width ratio are obtained.
Since it is difficult to deposit the composite plating film of chromium, in which a surface-occupying ratio and an average distribution density of the network-shaped micro-cracks are controlled within a predetermined range, up to a thickness more than 50 urn, it is preferable to laminate several of the composite plating layer of chromium having a thickness of about 5-20 urn, forming a multi-layered film, in

order to form the composite plating film of chromium having several hundred um as a whole.
When the composite plating film of chromium of the present invention is applied to an outer peripheral surface of a piston ring for an internal combustion engine, it effectively exhibits excellent wear resistance and scuffing resistance with little attacking nature to a mating member with the controlled outer peripheral surface shape and roughness value preferably less than Rz 2.50.
A method for producing a piston ring having the composite plating film of chromium of the present invention comprises conducting at least one cycle comprising (a) forming a hard chromium plating layer on a sliding surface of a piston ring base material, and (b) subjecting the resultant hard chromium plating layer to an inverse voltage treatment, in a state where the piston ring base material is immersed in a chromium-plating bath, the chromium-plating bath containing nearly spherical fine diamond particles with lesser impurities.
In the production method of the present invention, the hard chromium plating layer is deposited on the piston ring base material as a cathode. After deposition of the hard chromium plating layer having a predetermined thickness, network-shaped micro-cracks are formed and extended in the hard chromium plating layer by using an inverse voltage treatment where polarity of the piston ring base material and the counter electrode is reversed. The micro-cracks formed by using the inverse voltage treatment are opened on a surface, and the hard particles dispersed in the

chromium-plating bath and charged negative are captured in the micro-cracks. By repeating a cycle of the deposition of the hard chromium plating layer and the formation of the micro-cracks and the capture of the hard particles using the inverse voltage treatment, a multi-layered coating film can be obtained. The deposition of the hard chromium plating layer is preferably conducted on the condition of a current density of 30-80 A/dm and a plating bath temperature of 40-70°C. The inverse voltage treatment is preferably conducted on the condition of a current density of 5-70 A/dm and a plating bath temperature of 50-70°C. A current density of the inverse voltage treatment is more preferably 15-65 A/dm .
Although the invention has been described with reference to specific embodiments, the present description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore, contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined. EXAMPLE
(1) Wear Test: To evaluate the wear resistance and the attacking nature to a mating member, wear tests were conducted. Fig. 4 shows a schematic view of a wear test machine. A cut piece of the piston ring, on which the composite plating film of chromium is coated was prepared as a test piece (5), The electrolytic bath was prepared with chromic acid (Cr03) ; 254 g/L , sulpuric acid (H2S04) ; 1.1 g/L , Sodium hexa-fluorosilicate ( NaSiF6), : 3.62 g/L , methane sulfonic acid

( CH3S03H): 6.96 g/L ,anionic surfactant (tradename " Ftergent 110" : 500 ppm by mass) Surface tension of the plating bath was 48.6 mN/m , nearly spherical shape diamond particle having length to width ratio of 1 to 2 comprising 95% or more by ratio . A drum made of FC250 material was prepared as a mating member (7). The test piece (5) and the mating member (7) were contacted each other so that the ring axis was parallel to the drum axis. Tests were conducted by applying a load of 490 N to the test piece (5), and rotating the mating member (7) at a peripheral speed of 0.5 m/sec for 4 hours, while supplying SAE #30 lubricant oil (6) at a rate of 0.15 ml/min. Here the mating member (7) was heated to the surface temperature of 180°C. The wear of the plating film and the mating member were respectively evaluated in terms of a wear depth of the plating film and a wear area of the cross section of the mating member from the observation of the cross sectional profile. The comparative Study of above said coating with diamond particle (nearly spherical shape and less impurities ) Exp. 1 shows the comparative wear ratio 0.44 compare to base value (1.00) of Comp. Exp.2 & 0.67 of the mating surface with respect to the base value (1.00) of Comp. Exp. 2. The said Comp. Exp. 2 is the coating with only aluminium oxide (A1203) in the micro -cracks as explained above. (Fig. 5)
Further, the comparative Study of above said coating with diamond particle (non-spherical shape & without control of Impurities) Comp. Exp. 1 shows the comparative wear ratio 0.56 compare to base value (1.00) of Comp. Exp.2 & 1.28 of the mating surface with respect to the base value (1.00) of Comp. Exp. 2 (Table 1 & Fig. 5). The said Comp. Exp. 2 is the coating with only aluminum oxide (A1203) in

the micro cracks as explained above. Table 1: Results comparison

Evaluation items Ex.1 Comp. Ex. 1 Comp. Ex. 2
Shape of Diamond Particle Nearly Spherical Non-Spherical ~
Shape of A1203 particles Irregular shape Irregular shape Irregular shape
Impurities level in Diamond Micron Powder * Minimum level of Al , Fe , K , Na & Si as Impurities Control No Control
Micro-cracks
Distribution density (number /cm) 821 1079 1020
Hard particles
The amount of diamond particles
(%) 1.10 1.60 -
The amount of A1203 particles (%) 3.33 1.44 2.89
Wear resistance, Attacking nature to mating member, Scuffing resistance
Wear depth of plating film (Comparative ratio) 0.44 0.56 1.00**
Wear area of mating member (Comparative ratio) 0.67 1.28 1.00**
* Manufacturer test certificates ** Base value (2) Engine Test
In a 6 cylinder heavy duty turbo diesel engine for truck application, piston rings of Example 1 were installed in 1st cylinder 3 rd cylinder , 4th cylinder & 6th cylinder and piston rings of Comparative Example 1 were installed in 2nd cylinder and 5th . An endurance test of 350 hours was conducted by using an engine dynamometer. The amount of wear of the piston rings were determined by comparing the outer peripheral surface profiles of the same position after finishing 350 hours with the profile before the testing. Results are shown in Table 2. The average amount of

piston ring wear at 350 hours of Example 1 was 1.21 um which was 30% of that of Comparative Example 1, 4.0 um and Fig.6 show the outer peripheral surface profiles of the piston rings of Example 1 and Comparative Example 1 after finishing 350 hours, in contrast to the profiles before the testing.
The amounts of wear of the piston rings of Comparative Example 1 (2nd cylinder and 5th cylinder) were considerably larger than the amounts of wear of the piston rings of Example 1 (1st cylinder, 3rd cylinder, 4th cylinder and 6th cylinder) and liner wear is less than 1% effected. Fig. 7 show the Liner wear profile of Comparative Example 1 (2nd cylinder and 5th cylinder) and Liner wear profile of Example 1 (1st cylinder, 3rd cylinder, 4th cylinder and 6th cylinder) after 350 hours engine test. Table 2: Results comparison

Operating Time The Amount of Wear (um)

Ex. 1 Comp. Ex. 1

Avg. Ring Wear Avg. Liner Wear Avg. Ring Wear Avg. Liner Wear
350 hours 1.21 7.03 4.00 6.15
REFERENCE SIGNS LIST
1 Micro-cracks
2 Diamond particles embedded in micro-cracks
3 Diamond particles (nearly spherical)
4 A1203 Particles
5 Test piece
6 Lubricating oil
7 Mating member (drum)

CLAIM:
1. A composite plating film of chromium with nearly spherical hard particles with lesser impurities in network-shaped micro-cracks, said micro-cracks having a surface-occupying ratio of 5-18% by area and an average distribution density 600 to 1300 /cm, said hard particles comprising diamond particles with lesser impurities of Al, Fe , Na , K & Si.
2. The composite plating film of chromium as claimed in claim 1, wherein said micro-cracks have a surface-occupying ratio of 1% or more and less than 10%.
3. The composite plating film of chromium as claimed in claim 1 or 2, wherein said hard particles comprise second hard particles selected from the group consisting of oxide, carbide and nitride of Al, Si and Ti, and cubic boron nitride, in addition to said nearly spherical diamond particles with lesser manufacturing impurities of Al, Fe , Na , K & Si.
4. The composite plating film of chromium as claimed in claim 3, wherein an average crack-opening width of a second hard particle containing area of said micro-cracks is larger than an average crack-opening width of a nearly spherical diamond particle with lesser impurities of Al , Fe , Na , K & Si containing area of said micro-cracks.
5. The composite plating film deposited on piston ring of chromium as claimed in claims 4, wherein surface roughness of the outer peripheral surface less than Rz 2.50 after special grinding and lapping operation subsequently after plating for better wear & scuff resistance properties.

6. The composite plating film of chromium as claimed in any of claims 1-5, wherein said micro-cracks have a distribution density of 600-1,300/cm.
7. The composite plating film of chromium as claimed in any of claims 1-6, wherein the amount of said nearly spherical diamond particles is 0.25-10.0% by mass.
8. The composite plating film of chromium as claimed in any of claims 1-7, wherein the amount of said second hard particles is 0.5-12.0% by mass.
9. The composite plating film of chromium as claimed in any of claims 1-8, wherein said second hard particles comprise alumina (A1203) particles.
10. The composite plating film of chromium as claimed in any of claims 1-9, wherein said plating film comprises at least two layers.
11. A piston ring having the composite plating film of chromium recited in any of claims 1-10 formed on at least a sliding surface of a piston ring base material.
12. A method for producing the piston ring recited in claim 11, comprising conducting at least one cycle comprising:

(a) forming a hard chromium plating layer on said sliding surface of said piston ring base material, and
(b) subjecting the resultant hard chromium plating layer to an inverse voltage treatment, in a state where said piston ring base material is immersed in a chromium plating bath,
said chromium plating bath comprising nearly spherical shape diamond particle having length to width ratio of 1 to 2 comprising 95% or more by ratio.

13. A method for producing the piston ring recited in claim 11, comprising conducting
(a) at least one cycle comprising forming a hard chromium plating layer on said sliding surface of said piston ring base material,
(b) subjecting the resultant hard chromium plating layer to an inverse voltage treatment, in a state where said piston ring base material is immersed in a chromium-plating bath, said chromium-plating bath comprising nearly spherical diamond particles with lesser impurities of Al , Fe , Na , K & Si, and
(c) subsequent special grinding and lapping operation to achieve outer
peripheral surface of piston ring surface roughness less than Rz 2.5.