Effect of mesh structure of tetrahedral amorphous carbon (ta-C) coating on friction and wear properties under base-oil lubrication condition☆
Introduction
Diamond-like carbon (DLC) coatings are known as remarkable features applied as the solid lubricants because of low friction, high hardness as well as wear resistance and chemical inertness. Presently, the DLC coating has been widely utilized in various industrial applications to reduce friction and wear, particularly in the automotive industry. As such, this industry continuously is pursuing lower energy loss and low gas emissions. In order to achieve legal regulations in relation to energy efficiency and greenhouse gas emissions, DLC coatings could be a promising surface material applied on extremely stressed components of internal combustion engines to reduce friction and to extend component lifetime [1].
DLC is divided into two major categories, namely the hydrogenated amorphous carbon (a-C:H, ta-C:H) and the hydrogen-free amorphous carbon (a-C, ta-C). Both hydrogenated amorphous carbon (a-C:H) and hydrogen-free tetrahedral amorphous carbon (ta-C) coatings offer low friction performance and excellent wear resistance with diverse characteristics of friction and wear properties owing to their particular hydrogen contents and microstructure [2]. Friction and wear sensitively rely upon system encompassing surface material, lubricants (oil and additive), environment (temperature, humidity) and lubrication condition [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. It should be noted that most engine components are operated in the boundary lubrication regime. Under moderately severe contact conditions in boundary lubrication regime, direct contact occurs between the sliding surfaces.
Numerous studies have conducted the friction test under the boundary lubricated regime for DLC coating [[11], [12], [13], [14]]. Nevertheless, many researchers studied the effect of different lubricating oil and additive to the tribological performance of the DLC coatings. Under appropriate base oil and lubricant additives, the tribological performance of various types of DLC coating provides ultra-low friction and high wear resistance [15]. Oil additives and DLC doping elements demonstrated substantial and valuable impact on the wear behaviour [11,16]. Nonetheless, evidence has shown that the use of oil additives containing phosphorus and sulfur results in substantial harmful effect to the environment upon refinement process [17].
High hardness material commonly provides a higher wear resistance compared to the low hardness material under moderate experimental setup. Nevertheless, with a severe condition, hard and brittle material with lesser crack resistance results in coating damage due to microfracture. Wear by means of fracture-induced, which has been regarded as significant wear mechanism of brittle material can increase approximately ten times compared to abrasive wear [18]. In the case of DLC coatings, wear is accelerated by the through-thickness crack that causes spalling of the coating [19]. Furthermore, low hardness coating that forms of high fraction sp2 carbon atoms also results in high material removal rates under severe condition due to the structural modification such as graphitization of the contact surface [20,21]. The hardness of the DLC coating depends on the fraction of the sp3 and sp2 carbon structure [22].
Many studies have focused on the ta-C and a-C:H coatings, which demonstrated high wear or damage to the coating as the sliding distance, speed, temperature, and load are increased. According to a study by Ronkainen et al. [2], ta-C coating shows high wear resistance in contrast to the a-C:H, however, the ta-C coating created larger wear on the counterpart material. Superhard hydrogen free ta-C coatings are denoted by a high fraction of tetrahedral bonded (sp3) carbon atoms [23]. A study by Al Mahmud et al. [10] revealed that the coefficient of friction (CoF) for both ta-C and a-C:H coatings decreased and largely affected by the coating graphitization that consequently increased the wear rates. This finding is in agreement with a previous study by Tasdemir et al. [9], where the ta-C coating had limited durability causing high wear rates at high temperature. In addition, ta-C performance also degraded due to an increase in the sliding distance and load when ta-C slides against steel in pure base oil lubrication due to fracture-induced wear. These led to coatings worn out due to polishing wear combined with tribo-chemical wear [6]. Notably, several studies have shown that a-C:H coating had larger wear rates under the similar experimental condition with ta-C coating [20,24].
Commonly, DLC is deposited with a layer of homogeneous structure except for multilayer coating design. The current research introduced and investigated a novel Mesh ta-C with as-deposited mesh structure. At present, investigation on the as-deposited coating structure and its influence on the tribological performance have been scant. Therefore, the current study is aimed to investigate the tribological features of the as-deposited mesh-type structure of tetrahedral amorphous carbon coating under base-oil lubrication conditions. Unlike the aforementioned DLC, Mesh ta-C coating properties are unique for its’ mesh-like structure, which is characterized by the hardness controlled DLC in the direction of the coating thickness. This novel DLC consists of a softer topmost surface layer (sp2–rich mesh structure) and hard substrate-side layer (sp3–rich conventional ta-C), proposed to improve both friction and wear resistance of the coating.
Section snippets
Materials and lubricants
Cylindrical pin and disk were made of high carbon chrome steel (SUJ-2). Both forms of tetrahedral amorphous carbon (ta-C) and a type of hydrogenated amorphous carbon (a-C:H) were supplied by the Nippon ITF Inc. These two forms of ta-C were described by the structure, which are conventional tetrahedral amorphous carbon (ta-C) and mesh tetrahedral amorphous carbon (Mesh ta-C) deposited with mesh structure. The a-C:H coating was produced through chemical vapour deposition (CVD), while ta-C and
Friction properties for DLC films under PAO4 boundary lubrication
The coefficient of friction plotted against the load for all three distinct DLC coatings sliding on-to SUJ-2 disk in PAO4 lubrication are illustrated in Fig. 7. The CoF for Mesh ta-C was varied from 0.06 to 0.08. Furthermore, it possesses similar pattern and range as observed in ta-C results for all loads examined regardless of the high average surface roughness of the Mesh ta-C in contrast to ta-C and a-C:H. In addition, the a-C:H coating provides the lowest value of CoF at 20 N loads.
Conclusion
The current study examined the effect of mesh structure DLC coating on tribological performance, particularly with regard to friction and wear in base-oil lubrication conditions. The findings of the friction test revealed that the Mesh ta-C had a similar pattern and value of the friction coefficient to that of ta-C. Apart from that, there were no graphitized transferred-film found on the SUJ-2 disk for Mesh ta-C which detected on SUJ-2 disk for a-C:H and ta-C coatings mating material. Mesh ta-C
Acknowledgment
The author would like to thank Ministry of Higher Education, Malaysia and Universiti Teknikal Malaysia Melaka for their financial support.
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This paper was presented at the ASIATRIB2018.