PLASMA SPRAY COATING MATERIALS SELECTION OF PISTON RING AND CYLINDER LINER FOR MARINE DIESEL ENGINE J.-H. HWANG, M.-S. HAN, D.-Y. KIM Hyundai Heavy Industries Co. Ltd., 1 Cheonha-Dong, Dong-Gu, Ulsan, 682-792, KOREA; e-mail: [email protected] SUMMARY Following the trend of higher engine output, the piston ring and cylinder liner of large marine diesel engines suffer various damages from severe lubricating condition. Sometimes this kind of abnormal wear causes much shorter engine service life. From this point of view, the engine designer should consider the tribological characteristics of the ring and liner themselves, as well as their correlations. The goal of this study is to suggest the optimal combination of piston ringcylinder liner and to design the engine system that can ensure the sufficient service life. There are many coating materials to be applied to piston ring and cylinder liner for enhancing their anti-wear properties. To select a proper coating materials is critical to fabricate long lasting engine parts, in which understanding wear characteristics of the coating layer is essential. Thus, wear tests were accomplished using an oscillating high frequency friction machine in order to inspect the effect of the coating materials on their wear characteristics. We also tried to establish the criteria for the selection of the optimal coating material by inspection of the condition that minimizes the wear loss.
Keywords: Plasma spray coating, Grey cast iron, CV cast iron, Abrasive wear, Adhesive wear, ASTM G133
1 INTRODUCTION Recent marine diesel engines have been progressively developed to meet the operating condition of higher power, higher pressure and higher piston speed for obtaining high efficiency and performance. Therefore, marine diesel engines under these harsh operating conditions suffer an excessive wear of reciprocating parts such as cylinder liners and piston ring [1]. Wear of these parts causes blow-by, abnormal consumption of lubricating oil, and other damages. Plasma spray coating method has been widely adopted as piston ring coating applications due to its effectiveness in improving the wear resistance of these parts. Plasma spray coated piston ring was, however, reported to experience excessive wear of the coated piston ring itself and cylinder liner under certain circumstances. Improvement in design of coatings and their combinations are, therefore, needed to improve wear resistance of the piston ring and cylinder liner for marine diesel engine. Thus, in this study, the effect of the coating materials selection on the wear characteristics of piston ring and cylinder liner was studied in order to suggest the optimal combination of piston ring - cylinder liner and the selection criteria of the optimal material.
2 PROCEDURES
2.1 Substrate and coating materials Base materials used in this study were a gray cast iron having tensile strength of 250 MPa and a compacted vermicular cast iron, 300 MPa, widely used for cylinder liner and piston ring, respectively. The powders for coating used in this study are summarized in Table 1. By mixture of these alloy powders, three coating materials were designed as shown in Table 2. Figure 1 shows their powders configurations after mixed, which are designed as PO3, PO30 and PM.
2.2 Plasma spray coating An 80 kW METCOTM plasma-spray apparatus was used to coat the powders onto the substrates. Prior to coating all substrates were prepared through the following procedures; (1) pickling and degreasing for removing surface organic matters, (2) cleaning by ultrasonar, (3) blasting using aluminum oxide grit to enhance the binding strength between substrate and coating layer, and finally (4) re-cleaning in acetone by ultrasonar after removal of the grits by compressed air. All process parameters, e.g. arc current and voltage, gas pressure, specimen rotating speed, gun traveling speed, feed rate, and spraying distance were maintained constant except powder flow rate. Each plasma spray coating was prepared for a coating thickness of about 500 m.
2.3 Wear test Wear tests were carried out using an oscillating high frequency friction machine, PLINT TE77TM, according to ASTM G133 standard [2], as shown in Figure 2. Wear tests were conducted under the condition of 80 N load, oscillating frequency of 20 Hz with stroke 15mm at temperature of 450 C by oscillating the disc specimen (124) against the stationary plate specimen (58384) in order to simulate the reciprocating motion between ring and liner of actual engine system. Prior to wear test, all the specimen were polished by emery paper #400 to give a same surface condition. All the test runs were conducted for 1.5 hours after running-in for 10 minutes without lubrication at the above condition. In order to inspect the effect of the coating materials on the wear characteristics, six combinations of friction pairs were tested as shown in Table 3.
2.4 Evaluations Surface roughness of all specimens was evaluated before/after test in terms of arithmetic average value (Ra) by a stylus type roughness tester. Weight loss of each tested sample was measured using a automatic weight balance with an accuracy of 10-4g. Finally both topo- and metallo-graphic observation on the worn surface of the coated layer were carried out using a scanning electron microscope and an optical microscope to investigate the wear characteristics, such as wear pattern and surface morphology. 3 RESULTS AND DISCUSSION
3.1 Microstructure of coated layer Figure 3 exhibits a typical microstructure of the coating layers. There are apparent differences in the morphologies, e.g. splats size and configuration, and the porosity content, of the coating layers. The microstructure of the PO3 coated layer is composed of dark phase of Cr2O3 splats and gray phase of NiCr splats, and the PO30 has white phase of Mo, gray phase of NiCr and dark phase of Cr2O3, whereas PM consists of white phase of Mo and gray phase of Cr3C2-NiCr. Of these three coating layers, PM has the largest splat size. This is, of course, due to the original powder size, as previously shown in the Figure 1. Meanwhile, Table 4, indicating the porosity and hardness level of the coating layer shows that the PO30 contains relatively less amount of porosity, but possess higher hardness. These differences in porosity content and hardness level on the coating layers are generally related to the chemical and physical characteristic of the powders used.
3.2 Wear characteristics Figure 4 displays the change of the friction coefficient value on the designed wear pairs as a function of the wear time. Among the tests with the uncoated disc as moving specimen (Run 1, 2, 3), Run 2 revealed the lowest friction coefficient value, about 0.4, but among the tests with the coated one, Run 5 has the lowest one. It is generally known that the friction coefficient depends on both the coating materials and their wear mechanics occurring during wear test. Figure 5 shows the wear tracks according to the designed wear pairs. The wear tracks of bare CV cast iron disc on bare cylinder liner plate (Run 1) shows many deep scratches in both worn surfaces. This is produced mainly by abrasion, which suggest that severe wear of the component will occur for long-term operation. However, the wear tracks in Run 2 show some different wear pattern compared to that of Run 1. That is, the scratches formed on the disc show more finer and shallower than that of Run 1, perhaps due to the use of fine coating powder, while the plate shows not severe wear but smooth wear due to its relative higher hardness, as shown in Table 4. It is reported that the formation of the smooth layer like this in the plasma-sprayed ceramic coatings was related to material transfer as well as plastic deformation of the surface layer [3]. Table 5 shows the characteristics of worn surface after wear test.
Of six combinations of friction pairs, the pairs bearing Mo powder in their coating layer show a relative higher friction coefficient. It is well known that Mo was selected in order to improve the high temperature fracture toughness without sacrificing the wear properties [4], which in turn generally contributes to better wear resistance by decreasing the spalling level of the coating particle due to the effect of solid lubricant. But contrary to this, there was a report that Mo based coating generally gave high friction coefficient [5]. This may be caused by its adhesion wear pattern. It is natural that adhesive friction gets larger total contact area than abrasive one during wear test. Figure 6 shows the weight change according to the friction pairs after wear test. Most plates gained a weight, whereas the weight change of the rings showed almost similar pattern besides the case of Run 3. It can be explained that this phenomenon occurred due to the transfer of materials from the mating surface, possibly including wear debris and some oxide formed at higher friction temperature [3]. On the other hand, there were marked differences in the surface roughness of the worn surface. Surface roughness of the bare CV cast iron disc on bare gray cast iron plate shows much higher surface roughness than that of the coated one. In the bare grey cast iron plate, surface roughness indicated about 4.5 m in Ra, whereas other coated plates ranged 0.4 to 2.8 m in Ra. Increase in surface roughness of the bare gray cast iron was attributed to an increase in the wear rate due to its lower hardness in comparison with other coated one. 4 CONCLUSIONS On the basis of the tribological analysis result on the worn surface of various coating layers, the following conclusions are drawn.
The Cr2O3-NiCr coating showed an abrasive wear pattern and had lower friction coefficient value than that of the coating layer containing Mo.
The coating layer containing Mo yielded increase of the friction
