FIELD OF INVENTION
5 [0001] The present invention relates to a composite plating film of chromium with
diamond particles in network-shaped micro-cracks, and a piston ring having its plating film
and a method for producing the piston ring.
BACKGROUND OF THE INVENTION
10 [0002] Modern day engine development demands, meeting the new emission norms and
increased fuel efficiency to reduce CO2 emission. Achieving these higher fuel efficiency
or emission norms require downsizing of engines by increasing peak cylinder pressure and
temperature. This change in in-cylinder conditions, coupled with demand for longer life
of engine components, calls for highly robust outer periphery coating on piston rings.
15 [0003] For years, the composite plating of chrome has proved to provide an answer to
these situations of engine development. The composite plating of chrome is based on
ceramic particles of aluminum oxide, etc., which are embedded in the hard chromium
matrix during a multi-stage electrochemical process. Compared to plain chromium
coatings, the ceramic particles content increases the scuff resistance while at the same time
20 reduces wear of the coating and mating components.
[0004] Since the future development of diesel engines suggests that the thermal and
mechanical load on the tribological system of a piston ring and a cylinder surface will
continue to increase, eventually, even the composite plating of chrome with alumina hard
particles will reach its limits, which can result in a tribological escalation with wearing and
25 ultimately even seizure. 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.
[0005] These imminent tribologically demanding operating conditions mean that a new
coating system is clearly required which is superior to the conventional composite plating
30 of chrome.
[0006] For instance, EP 1 719 827 B1 teaches a composite plating film of chromium
containing hard particles in network-shaped micro-cracks, said micro-cracks having a
2
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 Al2O3,
5 SiC, Si3N4, and diamond. The plating film preferably comprises at least two layers.
[0007] US 6,503,642 B1 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 μm embedded in said cracks. In this
coating, compared with former chromium coatings with embedded aluminum oxide
10 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.
[0008] However, in the severe environment of increased thermal and mechanical loads,
15 it is in fact hard to say that the above mentioned coating is a robust coating achieving the
sufficient wear resistance and scuffing resistance.
SUMMARY OF INVENTION
OBJECTIVE OF THE INVENTION
20 [0009] In view of the above circumstances, the objective of the present invention is to
provide a composite plating film of chromium having excellent wear resistance and
scuffing resistance with little attacking nature to a mating surface (a sliding member), and a
piston ring having the composite plating film of chromium and a method for producing the
piston ring.
25
DISCLOSURE OF THE INVENTION
[0010] As a result of intensive research on composite plating films of chromium in
order to solve the above mentioned problems, the present inventors have succeeded in
obtaining a composite plating film of chromium having excellent wear resistance and
30 scuffing resistance with little attacking nature to a mating surface (a sliding member) by
incorporating diamond particles having a predetermined size in network-shaped
micro-cracks and by controlling a surface-occupying ratio and an average crack-opening
3
width of the micro-cracks which function as oil-reservoirs, and have completed the present
invention.
[0011] That is, the composite plating film of chromium of the present invention is a
composite plating film of chromium with hard particles in network-shaped micro-cracks,
5 said micro-cracks having a surface-occupying ratio of 1-20% by area and an average
crack-opening width of 0.6-3.0 μm, and said hard particles comprising diamond particles.
Said micro-cracks preferably have a surface-occupying ratio of 1% or more and less than
10%.
[0012] Said hard particles preferably comprise second hard particles selected from the
10 group consisting of oxide, carbide and nitride of Al, Si and Ti, and cubic boron nitride, in
addition to said diamond particles.
[0013] Further, an average crack-opening width of a second hard particle containing
area of said micro-cracks is preferably larger than an average crack-opening width of a
diamond particle containing area of said micro-cracks. Specifically, said diamond particle
15 containing area of said micro-cracks preferably has an average crack-opening width of
0.5-1.1 μm, and said second hard particle containing area of said micro-cracks preferably
has an average crack-opening width of 1.2-4.5 μm.
[0014] Further, said micro-cracks preferably have a distribution density of
600-1,900/cm.
20 [0015] Further, the amount of said diamond particles is preferably 0.5-12.0% by mass,
and the amount of said second hard particles is preferably 0.5-15.0% by mass. Said
second hard particles preferably comprise alumina (Al2O3) particles.
[0016] Further, said composite plating film of chromium preferably comprises at least
two (2) layers.
25 [0017] 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.
[0018] Further, the method for producing the piston ring of the present invention is a
method comprising conducting at least one cycle comprising (a) forming a hard chromium
30 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
4
bath comprising diamond particles having a median diameter of 0.5-1.0 μm, particles
having particle sizes of more than 0.2 μm and 2.0 μm or less being 95% or more by mass.
Said chromium-plating bath preferably comprises second hard particles having a median
diameter of 1.2-4.0 μm, particles having particle sizes of more than 0.5 μm and 8.0 μm or
5 less being 95% or more by mass.
EFFECT OF THE INVENTION
[0019] Because the composite plating film of chromium of the present invention
contains diamond particles having a predetermined particle distribution as a hard particle, it
10 demonstrates excellent scuffing and wear resistance. Further, because micro-cracks,
which function as oil-reservoirs, have a predetermined surface-occupying ratio and an
average crack-opening width, 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
15 particles, and a larger particle size 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 an average crack-opening width of the micro-cracks without
increasing attacking nature to the mating member, resulting in further improving in
lubricating characteristics. Therefore, the composite plating film of chromium of the
20 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
25 [0020] 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.
[0021] Fig. 1(b) shows a backscattering electron image of a surface of the composite
plating film of chromium of the present invention observed by a scanning electron
microscopy.
30 [0022] Fig. 1(c) shows an element mapping image of carbon (C) of the composite
plating film of chromium of the present invention analyzed by an electron probe
micro-analyzer.
5
[0023] Fig. 1(d) shows an element mapping image of aluminum (Al) of the composite
plating film of chromium of the present invention analyzed by an electron probe
micro-analyzer.
[0024] Fig. 2(a) shows a diamond particle containing area of the micro-cracks of the
5 composite plating film of chromium of the present invention.
[0025] Fig. 2(b) shows an alumina particle containing area of the micro-cracks of the
composite plating film of chromium of the present invention.
[0026] Fig. 3 shows a secondary electron image of a cross section of the composite
plating film of chromium of the present invention observed by a scanning electron
10 microscopy.
[0027] Fig. 4 shows a schematic view of a wear test machine.
[0028] Fig. 5 shows a schematic view of a scuffing test machine.
[0029] Fig. 6 shows outer peripheral surface profiles of the piston rings of Example 1
(1st cylinder (#1) , 2nd Cylinder (#2) and 3rd Cylinder (#3) observed after the engine test
15 (350 hours) , compared with those observed before the test.
[0030] Fig. 7 shows appearance photographs of the outer peripheral surfaces of piston
rings observed after the engine test (350 hours).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
20 [0031] 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
25 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.
30 In this way, the micro-cracks opening to the film surface function as effective oil reservoirs,
improving sliding characteristics.
[0032] The composite plating film of chromium of the present invention contains
6
diamond particles in network-shaped micro-cracks. The diamond particles have higher
modulus and higher hardness than other hard particles such as Al2O3. Particularly,
mono-crystal diamond particles significantly improve the wear resistance of the composite
plating film of chromium. However, the diamond particles have a problem which
5 increases the attacking nature to the mating member when the particle sizes are increased.
In that sense, the diamond particles preferably have a median diameter of 0.5-1.1 μm, and
more preferably have a median diameter of 0.6-0.9 μm.
[0033] Considering that the micro-cracks opening to the film surface function as
effective lubricating oil reservoirs, the composite plating film of chromium of the present
10 invention has micro-cracks having a surface-occupying ratio of 1-20% by area and an
average crack-opening width of 0.6-3.0 μm. When the micro-cracks have a
surface-occupying ratio less than 1% by area or an average crack-opening width less than
0.6 μm, oil retention on the film surface is insufficient. On the other hand, when the
micro-cracks have a surface-occupying ratio more than 20% by area or an average
15 crack-opening width more than 3.0 μm, the composite plating film of chromium is made to
be brittle, resulting in insufficient strength. The surface-occupying ratio is preferably
1-10% by area, more preferably 3-9% by area, most preferably 5-8% by area. The
average crack-opening width is preferably 0.6-2.5 μm, more preferably 0.8-2.1 μm, most
preferably 1.0-2.0 μm.
20 [0034] Furthermore, in order to control the average crack-opening width to 0.6-3.0 μm,
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, Al2O3, AlN, SiO2, SiC,
25 Si3N4, TiO2, 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 a median diameter
30 of 1.2-4.0 μm, and more preferably have a median diameter of 2.0-3.0 μm. The second
hard particles are preferably alumina (Al2O3) particles.
[0035] Fig. 1(a) to Fig. 1(d) show network-shaped micro-cracks formed on a film
7
surface of the composite plating film of chromium with diamond particles of the present
invention further containing alumina particles as the second hard particles. The
secondary electron image of Fig. 1(a) shows white shinny hard particles embedded in the
micro-cracks (1). On the other hand, in the backscattering electron image of Fig. 1(b),
5 black network-shaped micro-cracks (1) are formed in a gray chromium (Cr) matrix. The
hard particles (diamond (C) and Al2O3) composed of lighter element than chromium (Cr)
look to be dark like the micro-cracks. It is recognized (but not so clear) that there are
differences between crack-opening widths of the micro-cracks depending on the size of the
hard particles embedded.
10 [0036] Fig. 1(c) and Fig. 1(d) show the element mapping images of carbon (C) and
aluminum (Al), indicating the presence of diamond particles and Al2O3 particles,
respectively. Fine diamond particles and a little bit larger Al2O3 particles are relatively
uniformly dispersed in the micro-cracks.
[0037] Fig. 2(a) and Fig. 2(b) respectively show a diamond particle (2) containing area
15 of the micro-cracks and a second hard particle containing area of the micro-cracks, i.e.,
Al2O3 particle (3) containing area of the micro-cracks. Closure of the micro-cracks is
restricted due to the presence of diamond particles or Al2O3 particles. An average
crack-opening width of a diamond particle containing area of the micro-cracks is
preferably 0.5-1.1 μm, more preferably 0.6-1.0 μm, most preferably 0.7-1.0 μm. An
20 average crack-opening width of a second hard particle containing area, i.e., Al2O3 particle
containing area, of the micro-cracks, is preferably 1.2-4.5 μm, more preferably 1.4-3.8 μm,
most preferably 2.1-3.4 μm.
[0038] Though the network-shaped micro-cracks are exclusively defined in terms of a
surface-occupying ratio and an average crack-opening width in the present invention,
25 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 600/cm or more. An
upper limit of the distribution density is preferably 1,900/cm or less. The distribution
density of micro-cracks is more preferably 600-1,200/cm, most preferably 800-1,100/cm.
30 [0039] In the composite plating film of chromium of the present invention, the amount
of diamond particles is preferably 0.5-12.0% by mass per 100% by mass of the entire
plating film, more preferably 0.5-5.0% by mass, most preferably 0.5-2.0% by mass.
8
[0040] Further, the amount of second hard particles is preferably 0.5-15.0% 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.
[0041] Since it is difficult to deposit the composite plating film of chromium, in which a
5 surface-occupying ratio and an average crack-opening width of the network-shaped
micro-cracks are controlled within a predetermined range, up to a thickness more than 50
μm, it is preferable to laminate several of the composite plating layer of chromium having
a thickness of about 5-20 μm, forming a multi-layered film, in order to form the composite
plating film of chromium having several hundred μm as a whole.
10 [0042] 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.
[0043] A method for producing a piston ring having the composite plating film of
15 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 diamond particles having a median diameter of 0.5-1.0
20 μm, particles having particle size of more than 0.2 μm and 2.0 μm or less being 95% or
more by mass. Since diamond particles having a larger particle size increase attacking
nature to the mating member at the time of sliding, it is preferable to use diamond particles
having narrow particle size distribution, particularly diamond particles having particle size
of more than 2.0 μm being less than 4% by mass. When the proportion of diamond
25 particles having particle size of more than 2.0 μm is 5% or more, the attacking nature to the
mating member is unfavorably increased. With respect to the second hard particles, it is
preferable to use second hard particles having a median diameter of 1.2-4.0 μm, particles
having particle size of more than 0.5 μm and 8.0 μm or less being 95% or more by mass.
[0044] In the production method of the present invention, the hard chromium plating
30 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
9
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
5 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/dm2 and a plating bath temperature of
40-70°C. The inverse voltage treatment is preferably conducted on the condition of a
10 current density of 5-70 A/dm2 and a plating bath temperature of 50-70°C. A current
density of the inverse voltage treatment is more preferably 15-65 A/dm2.
EXAMPLES
[0045] Example 1
15 [0046] (1) Chromium-Plating Bath
[0047] As a chromium-plating bath, the following were prepared: chromic acid
(CrO3) : 245 g/L, sulfuric acid (H2SO4) : 1 g/L, sodium hexa-fluorosilicate (Na2SiF6) : 3.7
g/L, methane sulfonic acid (CH3SO3H) : 6 g/L, anionic surfactant (tradename “Ftergent
110”) : 500 ppm (by mass), diamond particles (median diameter of 0.6 μm, the proportion
20 of particles having particle size of more than 0.2 μm and 2.0 μm or less is 97% by mass) :
25 g/L, Al2O3 particles (median diameter of 2.3 μm, the proportion of particles having
particle size of more than 0.5 μm and 6.0 μm or less is 99% by mass) : 160 g/L. Surface
tension of the plating bath was 40.3 mN/m.
[0048] (2) Piston Ring Base Material and Its Preprocessing
25 [0049] A piston ring base material made of spheroidal graphite casting iron for a top
ring was prepared. Outer peripheral surfaces of piston rings were ground. Fifty pieces
of piston rings were stacked in a cylindrical shape, packed with a jig so that a cylindrical
outer surface was constituted of outer peripheral surfaces of the piston rings, and immersed
in the chromium-plating bath mentioned above. Setting the piston rings as an anode and
30 the counter electrode as a cathode, the preprocessing was conducted by applying current on
the condition of the current density of 60 A/dm2.
[0050] (3) Formation of A Base Film
10
[0051] Following the preprocessing of the piston ring base material, by reversing the
polarity (i.e., setting the piston ring as a cathode and the counter electrode as an anode), a
hard chromium plating film as a base film was deposited.
[0052] (4) Formation of Multi-Layered Composite Plating Film of Chromium
5 [0053] After forming the base film, a first layer of the chromium plating film was
deposited. Next, by reversing the polarity again (i.e., setting the piston ring as an anode
and the counter electrode as a cathode), micro-cracks were formed and extended by the
inverse voltage treatment, and hard particles composed of diamond particles and Al2O3
particles, dispersed in the chromium-plating bath were captured in the micro-cracks. As a
10 result of repeating this cycle composed of the plating treatment and the inverse voltage
treatment 6 times in total, a multi-layered composite plating film of chromium having a
thickness of about 60 μm was formed. Each layer had a thickness of about 10 μm.
[0054] (5) Evaluation of Multi-Layered Composite Plating Film of Chromium
[0055] The multi-layered composite plating film of chromium is mainly evaluated in
15 terms of a surface-occupying ratio, an average crack-opening width and a distribution
density of the network-shaped micro-cracks. Paying attention to the hard particles, it is
evaluated in terms of an average crack-opening width of the hard particle (diamond
particles etc.) containing area of the micro-cracks, the amount of the hard particles
(diamond particles etc), etc.
20 [0056] (5-1) Surface-Occupying Ratio and Average Crack-Opening Width of
Micro-Cracks
[0057] The surface-occupying ratio and the average crack-opening width of
micro-cracks were determined by using an image analysis from a scanning electron
photomicrograph of the plating film surface. Here it was determined based on the
25 predetermined view area (128 μm × 16 μm). The average crack-opening width of
micro-cracks was calculated as the surface-occupying area of the micro-cracks divided by
the crack length. The surface-occupying ratio and the average crack-opening width of
Example 1 were respectively 7.3% and 1.2 μm.
[0058] (5-2) Distribution Density of Micro-Cracks
30 [0059] The distribution density (number/cm) of the network-shaped micro-cracks was
determined by drawing 5 arbitrary straight lines having a length of 10 cm on a
photomicrograph (magnification: 10×10) of the plating film surface, counting the numbers
11
of intersections of the lines and the micro-cracks, and averaging them. The distribution
density of the micro-cracks of Example 1 was 879/cm.
[0060] (5-3) Average Crack-Opening Width of Hard Particle Containing Area of
Micro-Cracks
5 [0061] The average crack-opening width of the hard particle (e.g., diamond particle and
Al2O3 particle) containing area of the micro-cracks was determined by specifying diamond
particles and Al2O3 particles from the element mapping images of carbon (C) and
aluminum (Al) analyzed by using an electron probe micro-analyzer, measuring
crack-opening widths of diamond particle containing area and Al2O3 particle containing
10 area, respectively, based on the predetermined view area (128 μm x 16 μm), and averaging
them. The average crack-opening widths of diamond particle containing area and Al2O3
particle containing area of the micro-cracks were respectively 0.7 μm and 2.4 μm.
[0062] (5-4) The Amount of Hard Particles
[0063] In general, the amount of hard particles can be determined by conducting a
15 quantitative analysis of the film surface using a fluorescent X-ray analyzer, and calculating
the proportion of the hard particles from the proportion of each element quantified.
However, the amount of diamond particles can be determined with high accuracy by
peeling the composite plating film of chromium from the outer peripheral surface of the
piston ring, and measuring carbon content by using a carbon sulfur analyzer. Here it
20 should be corrected by using the carbon content measured in the same manner of the
composite plating film of chromium without diamond particles as the background. The
amount of diamond particles of Example 1 is 0.97%. The amount of Al2O3 particles of
Example 1 is 1.37%.
[0064] (6) Wear Test
25 [0065] 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 (11), and a drum made of FC250 material was prepared as a mating
member (12). The test piece (11) and the mating member (12) were contacted each other
30 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 (11), and rotating the mating member (12) at a peripheral
speed of 0.5 m/sec for 4 hours, while supplying SAE #30 lubricant oil (13) at a rate of 0.15
12
ml/min. Here the mating member (12) 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 wear depth of the plating
5 film and the wear area of the mating member of Example 1 were 1.7 μm and 3.1×10-4 cm2,
respectively.
[0066] (7) Scuffing Test
[0067] Fig. 5 shows a schematic view of a scuffing test machine. A cut piece (21) of
the piston ring, on which the composite plating film of chromium is coated, was prepared
10 as a test piece, and a drum (22) made of SUJ 2 material having a diameter of 80 mm was
prepared as a mating member. Tests were conducted by applying the repeated load of a
sine curve of 50 Hz in the range of 20-50 N to the test piece while supplying base oil once
every 30 seconds (0.1 mL), and increasing a peripheral speed of the drum at a
predetermined acceleration speed until the occurrence of scuffing. Scuffing occurrence
15 point was detected by an abrupt rise of the frictional force measured by using a load cell.
Scuffing resistance was evaluated in terms of a scuffing occurrence peripheral speed and a
sliding distance until the occurrence of scuffing. The scuffing occurrence peripheral
speed and the sliding distance until the occurrence of scuffing of Example 1 were 4.1 m/sec
and 92 m, respectively.
20 [0068] Comparative Example 1
[0069] The piston rings having the composite plating film of chromium were prepared
in the same manner as in Example 1, except for not containing diamond particles.
Namely, the composite plating film of chromium of Comparative Example 1 contains only
Al2O3 particles as hard particles. The plating film evaluation, the wear test and the
25 scuffing test were conducted in the same manner as in Example 1. The results are shown
in Table 1. Though the scuffing resistance of Example 1 was about the same as that of
Comparative Example 1, the wear depth (1.7 μm) of the plating film of Example was
superior to that (3.2μm) of the Comparative Example, demonstrating 47% improvement.
[0070] Table 1
30
13
Evaluation items
Micro-cracks
Surface-occupying ratio (%)
Average crack-opening width (μm)
Distribution density (number/cm)
Average crack-opening width of diamond
Particle containing area (μm)
Average crack-opening width of Al2O3
Particle containing area (μm)
Hard particles
The amount of diamond particles (%)
The amount of Al2O3 particles (%)
Wear resistance, Attacking nature to mating member,
Scuffing resistance
Wear depth of plating film (μm)
Wear area of mating member (10-4 cm2)
Scuffing occurrence peripheral speed (m/sec)
Scuffing occurrence sliding distance (m)
Ex. 1
7.3
1.2
879
0.7
2.4
0.97
1.37
1.7
3.1
4.1
92
Comp. Ex. 1
4.6
1.6
792
-
2.7
-
2.55
3.2
2.9
4.1
93
[0071] (8) Engine Test
5 [0072] In a 6 cylinder heavy duty turbo diesel engine for truck application, piston ring
of Example 1 was installed in 3rd cylinder, and piston rings of Comparative Example 1
were installed in 1st cylinder and 2nd Cylinder. The remaining compression rings and the
oil rings intended to the engine were used. An endurance test of 350 hours was conducted
by using an engine dynamometer. The amount of wear of the piston rings were
10 determined by comparing the outer peripheral surface profiles of the same position after
finishing 200 hours and 350 hours with the profile before the testing. Results are shown
in Table 2. The average amount of wear at 200 hours of Example 1 was 1.67 μm, which
was 36% of that of Comparative Example 1, 4.58 μm. The average amount of wear at
14
350 hours of Example 1 was 2.33um, which was 38% of that of Comparative Example 1,
6.17 urn. 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. Fig. 7 shows photographs of outer peripheral surfaces after finishing the
testing. The amounts of wear of the piston rings of Comparative Example 1 (1st cylinder
and 2nd cylinder) were considerably larger than the amounts of wear of the piston ring of
Example 1 (3 cylinder).
[0073] Table 2
Operating Time
200 hours
350 hours
The Amount of Wear (urn)
Ex. 1
1.67
2.33
Comp. Ex. 1
4.58
6.17
REFERENCE SIGNS LIST
[0074]
1 Micro-cracks
2 Diamond particles
3 Alumina particles
11 Test piece
12 Mating member (drum)
13 Lubricating oil
21 Test piece
22 Mating member (drum)
23 Heater
24 Lubricating oil supply hole

WE CLAIM:
1. A composite plating film of chromium with hard particles in network-shaped
micro-cracks, said micro-cracks having a surface-occupying ratio of 1-20% by area and an
average crack-opening width of 0.6-3.0 urn, and said hard particles comprising diamond
particles.
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
diamond particles.
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 diamond particle containing area of said
micro-cracks.
5. The composite plating film of chromium as claimed in claims 4, wherein said
diamond particle containing area of said micro-cracks has an average crack-opening width
of 0.5-1.1 urn.
6. The composite plating film of chromium as claimed in claim 4 or 5, wherein
said second hard particle containing area of said micro-cracks has an average
crack-opening width of 1.2-4.5 urn.
7. The composite plating film of chromium as claimed in any of claims 1-6,
wherein said micro-cracks have a distribution density of 600-1,900/cm.
8. The composite plating film of chromium as claimed in any of claims 1-7,
wherein the amount of said diamond particles is 0.5-12.0% by mass.
9. The composite plating film of chromium as claimed in any of claims 1-8,
wherein the amount of said second hard particles is 0.5-15.0% by mass.
10. The composite plating film of chromium as claimed in any of claims 1-9,
wherein said second hard particles comprise alumina (AI2O3) particles.
11. The composite plating film of chromium as claimed in any of claims 1-10,
wherein said plating film comprises at least two layers.
12. A piston ring having the composite plating film of chromium recited in any of
claims 1-11 formed on at least a sliding surface of a piston ring base material.
13. A method for producing the piston ring recited in claim 12, 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 diamond particles having a median diameter of 0.5-1.0 um, particles having
particle sizes of more than 0.2 um and 2.0 um or less being 95% or more by mass.
14. The method for producing the piston ring according to claim 13, wherein said
chromium-plating bath further comprises the second hard particles having a median
diameter of 1.2-4.0 um, particles having particle sizes of more than 0.5 um and 8.0 um or
less being 95% or more by mass.