Tribological properties of twin wire arc spray coated aluminum cylinder liner
Graphical abstract
Introduction
Tribological characteristics of piston ring and cylinder liner (PRCL) system are crucial in dictating the degree of energy loss in internal combustion engines. It has been reported that a large portion of energy loss in engines is caused by friction between PRCL which accounts for 20%–40% of the total energy loss [1]. In this regard, extensive research has been conducted to improve the tribological characteristics of PRCL system with the aim to minimize the energy loss and guarantee the long life of engines [2], [3]. Particularly, much effort has been devoted to identify the effects of material, surface treatment, and surface finishing process on the tribological characteristics of the PRCL system [4], [5], [6], [7], [8].
Gray cast iron has been widely used for PRCL because it has low production costs and acceptable frictional performance. The relatively low frictional characteristic of cast iron has been attributed to the self-lubricating effect of the graphite debris generated from the surface [9]. Also, Qu et al. and others attempted to improve the durability of cylinder liner by coating gray cast iron with materials such as Cr [4], [10], [11]. Furthermore, there were efforts to optimize the contact interface between PRCL by creating patterns on the cylinder liner surface. Ronen et al. verified the wear debris trapping characteristics of cylinder liners with dimple patterns fabricated on the surface [5]. Křupka et al. showed that micro-dents created on the surface of cylinder liner can induce increase in local lubricant film thickness [6]. With regard to surface finishing technology, extensive research has been conducted to optimize the cylinder liner honing process with respect to reduction in friction, wear and oil consumption of engines [12], [13], [14], [15]. It has been reported that oil consumption and friction of PRCL are influenced significantly by the cross hatch honing angle [11], [13], [16].
Concerning further to material development, the continuing demand for increase in the efficiency of new engines requires the development of novel technologies that can significantly increase the wear resistance, decrease the frictional loss, improve the cooling capacity, and reduce the weight by the substitution of gray cast iron cylinder [17], [18], [19], [20], [21], [22], [23], [24]. In this context, aluminum alloy engine blocks have been used instead of the conventional cast iron engine blocks to improve the energy efficiency through weight reduction of engines. However, because of relatively poor tribological properties of aluminum alloys, cast iron cylinder liners have been used in conjunction with the aluminum engine blocks [25], [26]. As an effort to replace even the cast iron cylinder liner with aluminum alloy, thermal spray coating method has been introduced as a viable method that can improve the tribological characteristics of aluminum alloy cylinders with low cost and short processing time [27], [28], [29], [30], [31], [32]. In order to maximize the friction reduction potential of the coating deposited by the thermal spray coating technique, the finishing process needs to be optimized according to the coating process and the engine application. As a first option, the thermal spray coated surface may be smooth honed, in which case the intrinsic microstructure morphology of the coating material serves as the oil retention volume. Also, as the surface is very smooth, the honing angle can be considered to be irrelevant for smooth honed cylinders.
In conventional or in high performance engine applications, the smooth honed surface may not fully satisfy the tribological requirements of the engine, particularly in cases where the quality of fuel is below the European standards and the engine is exposed to extremely low start temperatures. In such engine operating conditions high wear can occur at the top dead center (TDC) of the liner and scuffing may also occur during cold start due to oil dilution or formation of oil carbon residues that close the pores of the surface finish [33].
Another option to hone the cylinder liner is to use the helical structure honing technology. The helical structure honing consists of tailored machining of honing grooves (helical structures), which proportionate extra oil retention volume with a very smooth surface in the plateau areas. It has been suggested that the use of the helical structure honing has a great potential to reduce wear and friction, especially in the critical area of the reversal zones at TDC and bottom dead center (BDC) [34], [35], [36].
As described above, aluminum liners coated by the thermal spray coating technique is promising, but further investigation is needed to assess the effects of the honing technique on the friction and wear characteristics. HSH technique has not been fully investigated and it is a new technique that has not been generally adopted in the industry [34], [35], [36], [37]. Nevertheless, with the synergistic advantages of using aluminum cylinder liners processed by thermal spray coating method and helical structure honing, superior tribological characteristics are expected. In this work, the tribological characteristics of the twin wire arc spray (TWAS) coated liners, using an Fe0.8C wire 1.0616 (DIN 8566), and honed with two different technologies; SH with a honing angle of 40° and HSH with a honing angle of 140°, were investigated. TWAS coating method was utilized because TWAS is one of the most cost effective coating processes and has been proven to be applicable in high volume productions. FeO material was used as the coating since it has the advantages of adequate mechanical properties and relatively low cost compared to other materials such as CrO [31]. A commercial reciprocating type of a tribotester with temperature control capability was used to obtain the frictional behavior of the liner specimens under various conditions. The liner specimens were tested against a pin made from a commercial piston ring under both dry and lubricated conditions. The following sections describe the details of the experimental work.
Section snippets
Manufacture of cylinder liner
The liner specimens were cut from a full size cylinder liner fabricated according to the following procedure: First, the aluminum liners were mechanical roughened in order to obtain sufficient adherence between the coating and the substrate. Second, the aluminum liners were cleaned and pre-heated. Then, the liners were coated using the TWAS process at Heller CBC Technology Center. The final dimensions of the liners were 82.5 mm inner diameter, 100 mm outer diameter, and 130 mm length.
Following the
Friction test results in un-lubricated condition
Sliding tests were conducted to assess the frictional behavior of the liner specimens in un-lubricated condition. As mentioned earlier, the un-lubricated condition was intended to represent a severe sliding condition in which the lubricant has been depleted. The experiments were conducted with respect to varying normal load, temperature, and speed.
Fig. 7 shows the COF values obtained for the un-lubricated sliding tests with respect to various experimental conditions. All the COFs were obtained
Conclusions
In this study, two types of TWAS coated (FeO) aluminum cylinder liners were fabricated to characterize their friction and wear behaviors with respect to the honing process: Type I — SH with ~ 40° honing angle, and Type II — HSH with ~ 140° honing angle. Based on the various experimental results obtained by using a commercial tribotester and measurements performed with SEM, EDS and confocal microscope, the following conclusions may be drawn:
- 1).
In un-lubricated condition, Type II specimen had lower
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grant by the Korea government (MSIP) (No. 2010-0018289).
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