Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
1Sliding Surface Improvement Technology Using LowFriction Surface Treatment to Reduce Friction Lossand Improve Efficiency of Products

NORIHISA HORAGUCHI*1 TAKAO ISHIMOTO*1
KOHEI KURACHI*1

NORITSUGU UMEHARA*2

LEE WOO YOUNG*3

Using ta-CNx coatings deposited by the IBA-FAD (Ion Beam Assisted Filtered ArcDeposition) method, we conducted friction tests. The test results showed that the friction coefficientin dry friction of refrigerant environment was about 0.02 and the friction coefficient in mixedenvironment of refrigerant and refrigerating oil was 0.03 or less. The ta-CNx coatings expressed
low friction coefficients about one-third or less of the friction coefficients of current DLC coatings(Diamond Like Carbon Coatings), which are generally about 0.1. Compared with the DLC
coatings of other companies (which have been used for automobile engines) and no coating, whichexhibits friction coefficients of 0.1 to 0.15, the ta-CNx coatings we developed showed significantly
lower friction coefficients. Furthermore, in the environment of engine oil, under the condition
where an additive agent was additionally mixed with the oil, the ta-CNx coating showed a low
friction coefficient of 0.04. We observed the sliding faces after friction tests and analyzed themwith reflectance spectroscopy. As a result, we found that there was tribo-film formed on thefriction surface and clarified the low friction mechanism of ta-CNx coatings. In addition, as oneexample, we applied the developed ta-CNx coatings to the sliding parts of a compressor of an airconditioner and confirmed that the efficiency was improved by about 1%.

|1. Introduction
Various energy conservation measures of machine products have been undertaken by eachcompany. One such energy conservation measure is the reduction of friction loss of sliding parts. In
recent years, it has been reported that carbonaceous hard coatings expressed low friction
coefficients and among them, ta-CNx (tetrahedral amorphous Carbon Nitride) coatings showedfriction coefficients of 0.01 or less in dry nitrogen environment(1)(2). However, there have been noreports about the friction properties of ta-CNx coatings in refrigerant environment. Therefore, wedeveloped a sliding surface improvement technology using low friction surface treatment to reduce
friction loss and improve the efficiency of machine products. In this report, we evaluate the friction
properties of ta-CNx coatings in refrigerant environment and suggest the low friction expression
mechanism. We also report one example where we applied the developed sliding surface
improvement technology to a compressor of an air conditioner aiming at the further improvementof efficiency.

|2. Method of element sliding test, results and consideration
2.1

Method of sliding test of elements

Figure 1 shows the outline of the IBA-FAD system owned by Nagoya University with which
we have conducted joint research, the deposition conditions of ta-CNx coating deposited using agridless ion beam generation system and the specimen. The ring-on-disk friction tester used infriction tests is shown in Figure 2. The friction tests were conducted in dry friction with refrigerantgas, in mixed environment of refrigerant and refrigerating oil and in engine oil environment, while*1

Machinery Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Professor, School of Engineering/Graduate School of Engineering, Nagoya University*3

Specially Appointed Professor, School of Engineering/Graduate School of Engineering, Nagoya University

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)2

the load was increased at certain intervals. A reflectance spectroscopy analyzer was used for theanalysis of the sliding surfaces after the friction tests.

Figure 1 Outline of IBA-FAD system, coating deposition conditions and ta-CNx coatingspecimen

Figure 2

2.2(1)

Ring-on-disk friction tester

Results of element sliding test

Evaluation and study of application to compressor for air conditioner
The results of the dry friction test in R32 refrigerant environment are indicated inFigure 3. Although the friction coefficient immediately after the test started was high, it fell
below 0.03 in about eight minutes. After that, while the load was increasing, low friction
coefficients were maintained.
Tre results of the friction tests in mixed environment of refrigerant and refrigerating oil
are given in Figure 4. The friction coefficient of no coating was 0.1 or more and although notshown in the figure, the friction coefficient of the current DLC coating was about 0.1, on theother hand, the friction coefficient of ta-CNx coating was one-third or less of 0.1.Theapplication of ta-CNx coatings reduced friction coefficients by about 70% under boundarylubrication.
(2) Evaluation and study of application to engine

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3

The results of the friction tests in engine oil environment are shown in Figure 5. The
friction coefficient in current oil is about 0.06. Under the condition where an additive agent wasadditionally mixed with the current oil, the friction coefficient was reduced to 0.04.

Figure 3 Results of dry friction test in R32 refrigerant environment(friction trend graph)

Figure 4 Results of friction tests in mixed environment ofrefrigerant and refrigerating oil

Figure 5

2.3

Results of friction tests in engine oil environment

Clarification of low friction expression mechanism of ta-CNx coating in
refrigerant environment

The observation results of the sliding surface after the dry friction test in R32 refrigerant
environment are presented in Figure 6. Tribo-film formed on the sliding surface was observed. The
tribo-film was about 270 to 370 nm thick at the thickest portion. The results of the reflectancespectroscopy analysis for the sliding surface are given in Figure 7. The refractive index, n, of thetribo-film was 2 or less and the extinction coefficient, k, was 0.15. According to the classification

Mitsubishi Heavy Industries Technical Review Vol. 58 No. 1 (March 2021)
4(3)(4)reported by Hiratsuka, et al.
, the obtained values are equivalent to those of polymer-like carbon.
Accordingly, we considered that soft tribo-film(5)(6) was formed on the sliding surface and thethin-film solid lubrication with a low shear strength achieved low friction coefficients.

Figure 6 Results of sliding surface observation after dry friction test in R32 refrigerant
environment (state of formation of tribo-film)

Figure 7 Results of reflectance spectroscopy analysis for sliding surface (study of proposed
low friction expression mechanism)|3. Verification results for efficiency improvement of compressor for
air conditioner
The developed ta-CNx coating was applied to the sliding parts of a compressor of an air
conditioner and the performance was evaluated. The results are shown in Figure 8. Compared withcurrent DLC coating, the developed ta-CNx coating improved the average efficiency ratio of theelectricity charge by about 1%. Low friction coatings reduce friction loss and allow for improved
efficiency. Therefore, it is expected that as a result of the reduction in the power consumption ofmachine products with sliding parts, running costs can be reduced and as a result of the downsizingof motors by the degree of improved efficiency, parts costs, etc., can also be lowered. There is also
a possibility that the use of ta-CNx coatings will contribute to environmental protection throughenergy saving, decarbonization, etc. We will clarify the durability and long-term reliability ofta-CNx coatings in the future.

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5Figure 8 Results of performance test of compressor for air
conditioner|4. Conclusion
The ta-CNx coating we developed is a surface treatment using carbon nitride as the maincomponent. Nitrogen is added to conventional DLC coating to soften the coating. By optimizing
the level of nitrogen content, which controls coating hardness, as well as the coating thickness, wederived the specifications of a coating that achieved both low friction coefficients and wear
resistance.
ta-CNx coatings reduce friction loss and improve the efficiency of machine products.
Therefore, ta-CNx coatings are expected to be used for compressors for air conditioner.
In the future, we will promote research about durability, lower costs and the stable supply of
ta-CNx coatings as well as evaluations of extended product applications. Toward application toengines, we will aim to develop a friction/wear sliding surface control technology for low friction
coefficients in oil environment and apply it to the piston pins, piston rings, etc., of engines, thereby
making a contribution to environmental conservation through energy saving and decarbonization inmachine products.
With the increase of electrically operated machine elements, lower noise is required.
Therefore, we will create new value from lower friction toward the reduction of noise andvibration.

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