INDUSTRIAL MATERIALS

Novel Diamond-Like Carbon Film “HC-DLC” for Engine
Parts That Slide on Soft Metals
Takehiko OSHIRO* and Koji MIYAKE---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------We have developed a new diamond-like carbon film “HC-DLC," which is produced by vacuum arc deposition and containshydrogen. With its appropriate hydrogen content, this film obtained extremely low attackability to soft metals without compensating
its high wear resistance and seizure resistance in engine oil containing MoDTC. The film demonstrates excellent performance inprotecting piston pins that repeatedly slide on soft metals. To meet market needs, the mass production of the film started in the fallof 2019, and now further application to the next-generation high-efficiency engines is under consideration.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: DLC, MoDTC, piston pin, vehicle engine parts, sliding

1. Introduction
Although vehicles will be electrified rapidly in thefuture, the majority of them will be equipped with a combination of an engine and motor, even in the 2050s.
Therefore, the number of engine-mounted vehicles willcontinue to increase. The load exerted on engine piston
pins is increasing, as the engine is downsized and its efficiency is improved. Consequently, higher seizure resistanceand wear resistance are required of them.
As the currently effective method of improving theseizure resistance and wear resistance of piston pins, themethod involves coating the piston pins with a diamondlike carbon (DLC)*1 film. It is known that the hydrogencontaining DLC films used for coating piston pins reactwith molybdenum dithio-carbamate (MoDTC)*2, an engine
oil additive, and cause abnormal wear of the pins.(1)Conversely, hydrogen-free high hardness DLC filmsproduced by the vacuum arc vapor deposition method orother suitable method protect the piston pins fromabnormal wear, even when these pins are used in engine oilcontaining MoDTC. However, hydrogen-free DLC films
are disadvantageous in that they demonstrate high attackability to the mating material and therefore are unsuitable
for sliding with soft metal parts, such as piston pins.
To overcome the above situation, the authors developed
a new DLC film called “HC-DLC.” The new film does notsuffer abnormal wear, even in engine oil containing MoDTCand demonstrates low attackability to soft metals.

2. Outline of HC-DLC Film
A DLC film is an amorphous thin film containing
carbon as a major component, and it comprises a diamondstructure (sp3 structure) and graphite structure (sp2 structure). The features of the new film can be controlled bychanging the sp3 structure/sp2 structure ratio and the ratioof hydrogen taken into the structures. As a conceptual
diagram of a DLC film based on these ratios, A. C. Ferrariand J. Robertson proposed the following ternary phase
diagram (Fig. 1).(2)Among DLC films containing hydrogen, a-C:H*3

sp3

diamondta-C
ta-C:Ha-C

a-C:HHC polymers

graphitic C

no ﬁlmsH

sp2

Fig. 1. Ternary phase diagram of DLC

films are used for piston pins and many other parts. This
type of films are mainly produced by the plasma chemicalvapor deposition (CVD)*4 method or sputtering method.
Among hydrogen-free DLC films, ta-C*5 films
produced by the vacuum arc vapor deposition method aremainly used. When a-C:H films and ta-C films arecompared, both of them are excellent in slidability, but ta-C
films containing a lot of sp3 structure exhibit higher hardness and heat resistance. Meanwhile, a-C:H films are known
to react with MoDTC, which is often used as a friction
modifier for engine oil, causing abnormal wear. Therefore,
depending on applications, a-C:H films are unsuitable forpiston pins. Conversely, ta-C films contain many hardparticles called droplets in the production process.
Although these particles are removed in the lapping process
after film formation, they cannot be completely removed.
Depending on the usage environment, the particles raise aproblem of peering off the ta-C films and damaging thefilms themselves and mating sliding members.
Due to their high attackability to the mating material,
ta-C films are unsuitable for piston pins that slide on
aluminum (Al), copper (Cu), or other soft metal. In addition, MoDTC is often added to engine oil to further reduce
the fuel consumption of vehicles, and this makes using
a-C:H films for piston pins difficult.

64 · Novel Diamond-Like Carbon Film “HC-DLC” for Engine Parts That Slide on Soft Metals

3-1 Properties of DLC
The properties of HC-DLC and conventional DLC arecompared in Table 1.
HC-DLC and ta-C films are produced on the samefilm-forming equipment. In their production process, ahydrocarbon gas is introduced to take hydrogen into thefilms. In the test for determining the properties of the above
films, the test samples were prepared under the same
condition for the adhesion layer with base material, tocompare the difference in the DLC film only. Additionally,
since the DLC film produced by the vacuum arc vapor
deposition method increased surface roughness due to thedroplets, the HC-DLC and ta-C film samples were lapped
under the same condition after formation of the films.
Table 1. Property comparison between HC-DLC and conventional DLC
HC-DLC(ta-C:H)
Production method
Raw material

ta-C

a-C:H

Vacuum arc vapor deposition method
Graphite
Hydrocarbon gas

Graphite

Plasma CVD
methodHydrocarbon gas

Film thickness

1 μm

1 μm

3 μm

Hydrogen content

13 at%-26 at%Hardness

39 GPa56 GPa23 GPaDisk

SCM415 (DLC-coated)

BallSUJ-2, φ3/8 inch, fixedLoad250 N

Turning

1500 rpm, φ15 mm

OilEngine oil (5W-30), 80℃, Presence or absence of MoDTC

Time40 min

50

10-6 mm3/ Nm

3. Performance Evaluation of HC-DLC Film

Table 2. Ball-on-disk test conditions

Speciﬁc wear rate of DLC

The HC-DLC we developed is a ta-C:H*6 film
containing hydrogen. When the film is produced by thevacuum arc vapor deposition method, a hydrocarbon gas is
introduced into the film formation atmosphere to takehydrogen into the film. The DLC film exhibits a structure
between the ta-C film structure and a-C:H film structure. As aresult of structural optimization, we succeeded in developing
a new DLC film exhibiting excellent characteristics requiredfor piston pins. In particular, the new DLC film eliminates
abnormal wear, even in engine oil containing MoDTC, and itexhibits low attackability to Al, Cu, and other soft metals.

Absence of MoDTC

40

Presence of MoDTC

30
20
100
HC-DLCta-C ﬁlm

a-C:H ﬁlm

Fig. 3. Ball-on-disk test results

The HC-DLC and ta-C films were worn at almost thesame rate, irrespective of the presence or absence ofMoDTC, while the a-C:H film was significantly worn
when MoDTC was added. The above results verified that
HC-DLC does not suffer abnormal wear, even in a sliding
environment where an a-C:H film is worn abnormally.
3-3 Attackability of HC-DLC film to mating material
A cylinder-on-disk test (Fig. 4 and Table 3) wascarried out to check the attackability of HC-DLC to soft
metals. The test results are shown in Fig. 5.
LoadCylinder

3-2 Wear resistance of HC-DLC FilmTo check whether the addition of MoDTC causes
abnormal wear of the HC-DLC film, we carried out a ballon-disk test (Fig. 2 and Table 2) for engine oils with andwithout MoDTC, and the test results are shown in Fig. 3.

Reciprocal
motion

Oil

Disk

Oil

Turning

Fig. 4. Outline of cylinder-on-disk testBall

LoadDiskFig. 2. Outline of ball-on-disk test

Table 3. Cylinder-on-disk test conditions
CylinderSCr415 (DLC-coated), φ33 mm × 22 mm

Disk

Al alloy or Cu alloy

Load100 N

Reciprocal motion

Reciprocation frequency: 28 Hz, Amplitude: 3 mm

OilEngine oil (0W-16), 80℃, Presence of MoDTC

Time30 min

SEI TECHNICAL REVIEW · NUMBER 91 · OCTOBER 2020 · 65

removing the droplets easy and lowered the attackability ofthe new film to the mating materials.
3-4 Seizure resistance of HC-DLC film
To check the seizure resistance of HC-DLC, a seizure
test was carried out using engine oil containing MoDTC(Fig. 8 and Table 4). In this test, the load was increased
until the brass pin broke. The load when the brass pin broke
was defined as the seizure load (Fig. 9).

HC-DLC0.15mm3Wear loss of mating material

0.2ta-C ﬁlm

0.1
0.05

Brass pin0Al alloyPinCu alloy

V-blockFig. 5. Cylinder-on-disk test results

Compared with the ta-C film, HC-DLC wore Al andCu alloys less significantly, when these metals were usedas the mating members. This verified that HC-DLC
exhibits lower attackability to the mating material than ata-C film when sliding on a soft metal.
Prior to the test, the surface conditions of HC-DLC
and the ta-C film were examined under a scanning probe
microscope (SPM). Consequently, these films exhibited alarge difference in the quantity of fine protrusions (Figs. 6and 7), despite the fact that these films were lapped under
the same condition. The probable reason is that the additionof a suitable amount of hydrogen to HC-DLC madeLoad

Turning

OilFig. 8. Outline of seizure test

Table 4. Seizure test conditions
PinSAE 3135 (DLC-coated or uncoated)

V-blockAISI 1137

Load

Increased at a rate of 3 kN/min

Turning

500 rpm

OilEngine oil (0W-20), 80℃, Presence of MoDTC

12Seizure load kN

1086420HC-DLC

Fig. 6. SPM image of HC-DLC (surface roughness: Rz = 0.32 μm)

ta-C ﬁlm

Uncoated

Fig. 9. Seizure test results

Fig. 7. SPM image of ta-C film (surface roughness: Rz = 0.70 μm)

We confirmed from this seizure test that HC-DLC
exhibits higher seizure resistance than ta-C films.
To investigate the reason, the test was suspended
when the load reached 5 kN, and the surface condition ofthe DLC film was observed under a microscope. As aresult, we found that, when compared with the ta-C film,
HC-DLC demonstrated an extremely smaller number ofscratch marks created by the droplets removed (Photos 1and 2). The above result suggested that ease of droplet
removal lowers the attackability of HC-DLC to the mating
material and restricts the damage to the DLC film itself,
thereby enhancing the seizure resistance of the new film.

66 · Novel Diamond-Like Carbon Film “HC-DLC” for Engine Parts That Slide on Soft Metals

Technical Terms

100 µｍPhoto 1. Surface of HC-DLC when the application of load was stopped at 5 kN

DLC: An abbreviation for diamond-like carbon, a＊1	general term for amorphous thin films containing
carbon as a major component.
MoDTC: An abbreviation for molybdenum dithio＊2	carbamate, a friction modifier that is widely used inengine oil.
＊3	a-C:H: An abbreviation for hydrogenated amorphous
carbon, a DLC film containing hydrogen. This film iscalled a hydrogenated amorphous carbon film.
＊4	CVD: An abbreviation for chemical vapor deposition, amethod in which a source gas is poured to deposit afilm by a chemical reaction.
ta-C: An abbreviation for tetrahedral amorphous＊5	carbon, a DLC film called a tetrahedral amorphous
carbon film. This DLC film does not contain hydrogenand demonstrates a high diamond structure ratio.
＊6	ta-C:H: An abbreviation for hydrogenated tetrahedral
amorphous carbon, a DLC film containing hydrogenin ta-C. This DLC film is called a hydrogenatedtetrahedral amorphous carbon film.

References

100 µｍPhoto 2. Surface of ta-C film when the application of load was stopped at 5 kN(1)	T. Shinyoshi, Y. Fuwa & Y. Ozaki, “Wear Analysis of DLC Coating in
Oil Containing Mo-DTC,” SAE Technical Paper, 2007-01-1969 (July2007)(2)	A. C. Ferrari and J. Robertson, “Interpretation of Raman spectra ofdisordered and amorphous carbon," PHYSICAL REVIEW B Vol. 61
No. 20, pp. 14095-14107 (May 2000)

4. Conclusions
Newly developed HC-DLC does not suffer abnormal
wear, even in sliding conditions where a-C:H films areworn abnormally in the presence of MoDTC. Therefore,
HC-DLC can be used even in sliding environments whereoil containing MoDTC is used. Due to its lower attackability to soft metals and higher seizure resistance thanta-C films, HC-DLC is suitable as a protective film forpiston pins and other engine parts that slide on soft metals.

Contributors The lead author is indicated by an asterisk (*).

T. OSHIRO*• ‌Ph. D.
Chief, Nippon ITF Inc.

K. MIYAKE• ‌General Manager, Nippon ITF Inc.

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