Cavitation erosion and wear behaviour of a boron cast iron cylinder liner under bio-fuel conditions

Abstract

The use of renewable bio-fuel in internal combustion engines is the trend for the near future. However, it is easy to affect the cylinder liner due to the cavitation of the fuels. This study shows definite evidence that the cavitation erosion affects the tribological behaviour of the boron cast iron cylinder liner. The results indicate that the amount of both adsorbed lubricating film and tribo-film on the worn surfaces of the boron cast iron decreased with an increase in cavitation time, which is the direct reason for the increase of the friction coefficient and wear loss of the cylinder liner.

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Cavitation is a common phenomenon in engineering equipment involving flowing liquid. In recent years, it has attracted much attention because cavitation could lead to severe cavities on materials and reduce the service life of the equipment.¹ Many works and new techniques have been done on the cavitation behaviour, mechanisms and anti-cavitation of metal materials.^2–8 Murali et al.⁹ found that a shear band can mediate cavitation in brittle metallic glasses. In addition, several mechanisms including shock wave mechanism,^10,11 microfluidization,¹² heating effect¹³ and chemical corrosion¹⁴ were relatively widely accepted views. Although great progress has been made, there is still no universal opinion on the relationship between cavitation and wear especially for a specific material under certain conditions.

The internal combustion (IC) engine is widely used equipment, which also suffers from cavitation. Ohta et al.¹⁵ reported that the cooling water can cause cavitation on the wet cylinder liner surface due to the pressure fluctuation in the cooling water caused by the reciprocating friction of the piston. In fact, besides the cooling water the incompletely burned fuel atomized droplets also have a great cavitation effect on the inside surfaces of the cylinder liner.¹⁶ Great efforts have been exerted to reduce the cavitation of the fuels.^17,18 On the other hand, due to the depletion of the traditional fossil fuel the use of reproducible bio-fuel has become common in IC engines.^19–21 Unfortunately, these bio-fuels usually contain oxygen and a little water, which results in an incomplete combustion in IC engines.¹⁶ Unburned bio-fuel droplets inevitably spray onto the cylinder liner surface, causing cavitation erosion in the cylinder liner due to fluctuation of combustion chamber pressure. To the best of our knowledge, few studies have covered this area.

In order to accelerate the application of bio-fuel in IC engines, it is necessary to understand the effect of cavitation on the wear behaviour of the cylinder liner and piston rings. Because the relationship between cavitation and wear is hard to study in real IC engine combustion chamber, a two-step experimental method was used in this work. A magnetostrictive-induced cavitation facility was employed to simulate the cavitation process under room temperature (25 ± 2 °C). The boron cast iron cylinder liners were thoroughly immersed in bio-oil during the cavitation tests. Then a multifunctional cylinder liner-piston ring was used to do the frictional test under bio-oil drop lubrication. The physicochemical properties of the bio-oil are given in our published paper.²² The detailed frictional method can be seen in a previous work.¹⁶ The worn surfaces of the cylinder liner were characterized by scanning electronic microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to disclose the corresponding mechanisms. The chemical components of the boron cast iron cylinder liner include: 3.22% C, 2.33% Si, 0.25% P, 0.81% Mn, 0.05% B and 0.36% Cr. The piston ring is made from ductile iron. Both of the cylinder liner and piston ring specimen are cut from the commercial products. Sliding conditions are as follows; oil drop speed: 0.42 mL min⁻¹; load: 280 N; testing temperature 90 °C; sliding speed: 0.8 m s⁻¹.

Fig. 1 shows the surface images before and after different cavitation time. As shown in the images, compared with the pristine surface of cylinder liner (Fig. 1a), the surfaces after cavitation changed obviously (Fig. 1b–f). Some cavities appeared after 30 min cavitation (Fig. 1b), and they became bigger after 60 min (Fig. 1c). The cavities connected with each after 90 min (Fig. 1d), and the obvious cracks occurred after 120 min (Fig. 1e). Two big pits can be found after 150 min (Fig. 1f). The similar processes are also observed by other researchers,^23,24 in which the destructive effect on the cylinder liner increases with time. After the surface layer of the material was removed, the cracks in subsurface increased quickly,²⁵ which resulted in extensive exfoliation after cavitation for 150 min.

Fig. 1 Surface images before (a) and after (b–f) different cavitation time: 30 min (b); 60 min (c); 90 min (d); 120 min (e) and 150 min (f).

Fig. 2 shows the relative weight and surface roughness of the cylinder liner after different cavitation time. It can be found that the relative weight of the cylinder liner (Fig. 2a) decreased linearly with an increase in cavitation time up to 120 min, but decreased severely and amounted to ∼97.6 wt% of the original cylinder liner after 150 min. This may be explained by the fact that large area exfoliation²⁶ on the cylinder liner occurred after cavitation for 150 min. At the same time, the surface roughness of the cylinder liner (Fig. 2b) increased with the increasing cavitation time. Due to the same reason, the surface roughness became much higher and unstable after 150 min cavitation. These results agreed well with those in SEM images. They all suggested that cavitation time has an important effect on the surface morphology and state of the cylinder liner, which might further affect its tribological behaviour.

Fig. 2 Relative weight (a) and surface roughness R_a (b) after different cavitation time.

Fig. 3 presents the schematic diagram of the rubbing pairs (Fig. 3a) and the tribological results (Fig. 3b and c) of the cylinder liner under different cavitation time. Because of the unstable frictional testing with the sample after 150 min cavitation, we only collected the tribological data under the sample before and after 30, 60, 90 and 120 min cavitation. As shown in the Fig. 3b, there are two typical stages in all the frictional process.^20,27 The first stage is the run-in stage within the first 30 min. In this stage, the friction is dominated by the coordination of the cylinder liner and piston ring.²⁸ The second stage is the normal wearing stage after the first 30 min. In this stage, the friction coefficient kept relatively smooth. The average friction coefficient in the second stage and wear loss are shown in Fig. 3c. It can be seen that the average friction coefficient and wear loss both increased with the increasing cavitation time. This may be because with the increase of cavitation time the surface became much rougher, and real contact area increased correspondingly.²⁹ These worn areas with cavities are more easily to be rubbed which leads to higher wear loss with the increasing cavitation time. Furthermore, the presence of the more wear debris on the worn surfaces under longer cavitation time will increase the friction coefficient.³⁰

Fig. 3 Schematic diagram of the cylinder liner-piston ring tribometer (a), friction coefficient versus sliding time (b) and average friction coefficient and wear loss (c) under different cavitation time.

In order to know the tribological mechanisms, EDX on the worn surfaces of cylinder liner were performed and the results are shown Fig. 4. The carbon content decreased but the oxygen increased with the increasing cavitation time, suggesting that the adsorbed film with organics on the worn surfaces decreased while the corrosive components increased due to the possible corrosion of the bio-oil.³¹ Moreover, the elements such as B, Cr and Mn which can form a tribo-oxide film also decreased with the increasing cavitation time.³² This indicates that longer cavitation time will do harm to the formation of the protective tribo-film on the worn surfaces, which might be another reason for the higher friction coefficient and wear loss under longer cavitation time.

Fig. 4 EDX results of the main elements on the worn surfaces after different cavitation time.

In summary, we present a typical process of cavitation enhanced wear on boron cast iron cylinder liner under bio-oil lubricating conditions. Dynamic changes on the surfaces of the cylinder liner were observed. With an increase in cavitation time, to the point of macroscopic view, the increase of the rougher surfaces and more real contact area are ascribed to the increasing friction coefficient and wear of the cylinder liner; however, to the point of microscopic view, the decrease of both the adsorbed lubricating film and tribo-film on the worn surfaces of the boron cast iron is the direct reason for the increasing friction coefficient and wear loss.

Acknowledgements

We appreciate Mr Dustin Olson of University of Wisconsin-Milwaukee (UWM) for his helpful grammar revisions. We thank Mr Yumeng Su for his assistance in the cavitation testing. This work was supported by the National Natural Science Foundation of China (Grant No. 51405124), China Postdoctoral Science Foundation (Grant Nos. 2015T80648 & 2014M560505), Anhui Provincial Natural Science Foundation (Grant No. 1408085ME82) and the Tribology Science Fund of State Key Laboratory of Tribology, Tsinghua University (Grant No. SKLTKF15A05). The support of China Scholarship Council for Dr Yufu Xu as a visiting scholar in Professor Wilfred T. Tysoe's group at UWM is also gratefully acknowledged.

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