Probing the tribological performances of hydrogenated amorphous carbon film in methane atmosphere based on Hertzian elastic contact model
Graphical abstract
Introduction
Global primary energy consumption changes with the development of economy, science and technology. And natural gas has become one of the mainstays of global energy. Specifically, worldwide consumption of natural gas raised rapidly and contributed over three quarters of the growth in total global energy demand of 2019 with renewables [1]. Meanwhile, the share of natural gas in primary energy increased to record highs. This energy transition was mainly attributed to following two factors. The coal-to-gas transition reduced the amount of CO2 on combustion by more than 50%. On the other hand, the coal-to-gas transition improved efficiency of power plants. Research showed the major component of natural gas, methane, was obtained by electrochemical reduction of CO2 [2], and was produced by microbial activity [3]. Consequently, natural gas is considered as one of the mainstays of global energy for a long time [4]. However, with the production and consumption of energy increasing, the wide application of natural gas is limited by the imperfect deployment of carbon capture, utilization and storage (CCUs) technology. And this state is going to last 30 years [1].
An intractable question regarding natural gas application is the damage of moving parts operating in natural gas, such as pistons and seal packs of reciprocating natural gas compressors in pipelines, refueling stations, and the fuel injectors of natural gas - powered engines [5,6]. The damage of moving parts results in environmental pollution and waste of resources [7,8]. As a result, it is of great significance for the social security and economic development to reduce wear of moving parts. One effective way for this issue is to cover the moving parts with functional coating. Diamond-like carbon (DLC) film, with excellent frictional performances, has been attracting much more attention. It is due to the fact that sliding interfaces of DLC films change easily with the various conditions, resulting in the sensitivity of the tribological properties of films to the preparation and test conditions [9,10]. Based on this fact, the different rehybridization and chemical termination across the sliding interface are explored to give the information of critical factors governing the frictional behaviors of film under some conditions. Achievements in mechanism and performance study provide guidance for the preparation of DLC films used in specific conditions and promote the wide applications of DLC film in industrial field, such as on bearings, piston rings, valves. Ali Erdemir's team demonstrated that tribochemical conversion of methane to carbonaceous materials across the sliding interface of VN coatings, which contained catalytic elements, resulted in 2ā3 orders of magnitude reduction in wear and ~50% reduction in friction compared to those of the uncoated steels [5]. Our recent works on tribological performances of non-hydrogenate amorphous carbon (a-C) films in methane atmosphere proved that groups dissociated from methane molecules passivated the carbon dangling bonds on the sliding interface [11,12]. These results indicate that a-C film is a potential protective material to protect moving parts operating in methane atmosphere. However, to obtain a-C film with low friction and wear under methane atmosphere, it is necessary to further improve its tribological properties. a-C film containing H and N exhibited excellent tribological properties under methane atmosphere with the relative humidity (RH)Ā ~Ā 25% [13], which provided an idea for the preparation of film used in methane atmosphere. Unfortunately, this work has not given a mechanism explanation on the tribological performances of film systematically. Interfacial evolution process needs to be analyzed systematically to know the key factors, which govern the tribological behaviors of a-C:H film in methane atmosphere. Load applied in tribological system plays a major role in rehybridization and chemical termination at the sliding interface, giving the factors governing the tribological performances of film. Therefore, it is necessary to explore the influences of load in the frictional behaviors of DLC film under methane atmosphere.
Here, a-C:H films are regarded as the candidate materials for protection of moving mechanical components during the supply chain and usage of natural gas. Three types of a-C:H films were prepared to find out tribological performances of films in methane under various loads. These ball-on-disk contacts of a-C:H films followed the Hertzian elastic contact model. The first principles calculation was carried out to explore the deeper insight into the load influences on sliding interface of a-C:H film. It demonstrated that adsorbates and carbon dangling bonds across the sliding interface governed frictional performances of a-C:H film, resulting in reduced friction coefficient with load. In addition, the friction coefficient of a-C: H film with defects varied greatly with the load while dense a-C:H film exhibited a low friction coefficient and small variation with various loads. This work reveals the tribological mechanism of a-C:H film under methane and gives guidance to prepare a-C:H film applied under methane atmosphere.
Section snippets
Film preparations
Preparations of three types of a-C:H films were carried out by the physical vapor deposition (PVD) and plasma enhanced chemical vapor deposition (PECVD). Prior to synthesis, 304 stainless steel plates and silicon wafers were ultrasonically cleaned with petroleum ether, acetone and alcohol for 20Ā min, separately. PVD preparing conditions for a-C:H film were given in our previous work [14]. A thickness of ~ 200Ā nm Cr transition layer was deposited before the deposition of a-C:H film with a
Tribological performances
Fig. 1 presents the frictional curves of the prepared a-C:H films rubbing against GCr15 counterpart balls with various loads under methane atmosphere. From Fig. 1(a), (b) and (c), the frictional curves of a-C:H-T film present bigger fluctuation than those of a-C:HāH and a-C:H-L films. Additionally, for a-C films without failure, their friction coefficients reach a highest value and immediately drop to a low value after running-in period. And a-C:H-L film wears out quickly at a load of 2Ā N. Fig.
Discussion
As discuss above, chemical termination and graphitization occur on the sliding interface during sliding. Fig. 10 displays the schematic description. After sliding, the carbon dangling bonds across the sliding interfaces are passivated. And transfer film exhibits obvious sp3-to-sp2 rehybridization. Specifically, the hydrogens release from subsurface during sliding, leading to the loss of hydrogens in the a-C:H films [21]. Groups from dissociation of methane passivate carbon dangling bonds across
Conclusions
In this work, the effects of methane on tribological performances of a-C:H film have been explored with three a-C:H films under various loads based on experiments and first principles calculation. And the conclusions are as follows:
- (1)
With the increasing normal load, the friction coefficient of a-C:H film displays an obvious reduction before the failure of the film, which is mainly attributed to the various interfacial shear strength under different loads.
- (2)
The sliding interface of a-C: H films is
CRediT authorship contribution statement
Lin Chen: Methodology, Data curation, Writing - original draft. Xubing Wei: Validation, Writing - review & editing. Guangan Zhang: Writing - review & editing, Resources. Lunlin Shang: Writing - review & editing. Zhibin Lu: Funding acquisition, Writing - review & editing. Xiangfan Nie: Resources, Writing - review & editing. Qunji Xue: Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National Natural Science Foundation of China [grant number 51775535, 11972344], CAS āLight of West Chinaā Program, China.
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