Ceramic-metal Composite Coated Piston Ring and Cylinder Liner of Marine Low Speed Diesel Engine*

Transcription

1 77 Ceramic-metal Composite Coated Piston Ring and Cylinder Liner of Marine Low Speed Diesel Engine* Masaki Tanaka * *, Yoshihisa Kitajima * *, Yasuhisa Endoh * *, Masaoki Watanabe * *, Yutaka Nagita * * This paper describes the fruit of the project by the Technological Research Association of Highly Reliable Marine Propulsion Plant. A future marine diesel engine will be required with superior performance, output, efficiency and reliability using degraded fuel. It has attendant requirements of wear characteristics and strength of cylinder components far exceeding those of conventional engines. Ceramic-metal composite material was developed and spray coated on piston rings and cylinder liners by using a low pressure plasma splay process. They were tested by full scale test engines in the objective "future" conditions. As a result, the wear of plasma coated piston rings and cylinder liners were small enough to prolong overhauling interval to 4 years. Plasma spraying coating process also makes possible to apply strong materials for cylinder liner. 1. Preface Plasma spray coated piston rings and cylinder liners are developed for future engine instead of cast iron components. The operating conditions of the future engine are characterized, production process and selection of altemative material are described. Then engine test results are shown. 1.1 Objective Engine Marine diesel engines in future are characterized as high output, high efficiency, operated with least maintenance and using degraded fuel. Predicted operation conditions of future engine are shown in Table 1. Advanced techniques ofcombustion, structure, lubrication, e.t.c. me inevitable to achieve it. Techniques for piston ring and cylinder liner are summarized as followings. 1) Sliding surface material for pistonring and cylinder liner interface, having superior properties to bear high contact pressure and high sliding speed. 2) Durable cylinder liner having enough strength to bear high cylinder pressure and high temperature. The future engine defined in Table l has similar piston structure and lubricating system as present engine. Its cylinder liner is cooled to keep the sliding surface * Translated from Journal of the MESJ Vo1. 27, No. 3 (Manuscript Received Aug. 5, 1991) Lectured Oct. 17, 1990 ** Mitsui Engineering & Shipbuilding Co., Ltd. ( ,Tama. Tamano-shi) below 250 CG, therefore it is not an adiabatic engine. Cooling loss through cylinder liner is only 3% to 4% of input energy, and gain in thermal efficiency by rejecting cylinder liner cooling is predicted only 2%. On the other hand, lubricating the high temperature cylinder surfaces of adiabatic engine is difficult of solution. Cylinder liner cooling is not able to neglect at the present state. October1993 (19)

2 78 Masaki Tanaka, Yoshihisa Kitajinla, Yasuhisa Endoh, Masaoki Watanabe, Yutaka Nagita 1.2 Cylinder component's materials In this 50 years, maximum cylinder pressure and mean effective pressure of large diesel engines are almost doubled. Pisto1_1 speed increased for about 207o.In spite of these changes, materials of piston rings and cylinder liners have not been changed so much. They are sti1l gray cast iron or chromium plated cast iron. The strengtl-1 of cast iron l-1ave improved to be 25 kgf/mm2 today. But the strength is usually not compatible with sliding ability. Surface treatment and specially structured cylinder liners have been developed. Laser hardening technique was applied to cylinder liner to achieve superior wear protective characteristics.(1) Double layered cylinder liner was developed to bear high stress caused by high pressure and high temperature. (2) But laser hardening is not answer for strength and double layered cylinder liner is not answer for rubbing (tribological) requirements. Both strength and excellent sliding characteristics will be required for the material using to piston rings and cylinder Iiners in future. Cast iron is thought not to meet the demand. Ceramics have characteristics of hardness and chemical stability, which are thought to be better using to cylinder sliding interface than cast iron. But most of the previous works intended ceramics using to adiabatic engines. (3)-(5) In that case, ceramics were used in very high temperature and with poor lubrication. It is not the Condition of the objective engine of this report. In this study, the purpose of ceramics application is concentrated on a strength of cylinder liner and sliding characteristics of cylinder interface. possible quick processing. 2.2 Coating process To obtain the durable and wear protective coatings, peculiar material preparation process was used. Two processes, "composite powder process" and "mixed powder process", are compared and "composite powder process" was adopted. Fig. 1 shows two kinds of production processes of Cr2O3/Mo composite coating which were named as "composite powder process" and as "mixed powder process". In the "composite powder process", Cr2O3 and Mo were once melted with an arc furnace then pulverized and classified to obtain suitable grain size for plasma spraying. In the mixed powder process,cr2o3 and Mo powder are merely mixed. Using the powder made by the "composite powder process", a homogeneous coating structure was formed. "mixed powder process" forms lamellar coating structure. The structure of coatings are shown in Fig.2 al1d Fig. 3. This difference is due to the characteristics of powders. 2. Spray coated materials Detail of selected material is described here. Selection of material is mentioned subsequently. 2.1 Features of low pressure plasma spray coating Low pressure plasma spray (LPPS) coating process was selected for our purpose. It makes possible free selection of materials for coating and substrate. Coating having good sliding properties on the strong base material is able to realize both wear protection and cylinder liner strength. The advantages of LPPS, comparing with atmospheric spraying (APS) are great impact of material Without reducing of speed by surrounded gas and less oxidation of material in controlled gas composition. They are helpful to form dense and strongly adhered coating. Furthermore, plasma spraying is easy applying to big components and is

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4 79 Ceramic-metal Composite Coated Piston Ring and Cylinder Liner of Marine Low Speed Diesel Engine For example, if a Cr2O3 particle was supplied with a equivalent size of Mo particle by the same vector into the plasma flow, their flow locus will be different because of the difference of acceleration due to the difference of their density. Fig.4 shows this mechanism. The light particle, such as Cr2O3 particle, accelerated by gas flow more than heavy particle, such as Mo particle. Consequently, they hit different points of substrate. This phenomenon causes the separation of Cr2O3 and Mo, in consequence forming an lamellar structure. 0n the other hand, the density of particles made by "composite powder process" scatters slightly owing to the same composite structure of each particle. That is the reason why the powder made by "mixed powder process" forms an unhomogeneous lalnellar structure and the powder made by the "composite powder process" forms a homogeneous structure of coatings. Another possible method to form homogeneous coating is to control grain size, small particle for dense material and large particle for low density material. But the difference in thermal characteristics of two materials are still difficult to contro1. The other method is to use separated supply pipe for each material. in that case, to aim two supply pipe at one point of substrate is much difficult. Thus they are not worth consideration. Accordingly, the "composite powder process" is thought to be favorable process to form homogeneous coating. The four point bending test was conducted. Testing equipments are shown in Fig. 5. Peeling of coating occurred by bending the coated piece loading compressive stress on the coating side. Consequently, the bending test was considered able to evaluate the bonding strength of coatings. Acoustic emission sensor was adopted for detecting the initiation of peeling. Fig. 6 shows the results. As per the results, the homogeneous coating, made by "composite powder process" has higher bonding strength than the lamellar coating, made by "mixed powder process". October 1993 (21 )

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6 79 Masaki Tanaka, Yoshihisa Kitajima, Yasuhisa Endoh. Masaoki Watanabe. Yutaka Nagita 3. Material selection 3.1 Test procedure Selection of material was made prior to set up the production process. Many kinds of materials were considered possible to use. Also many studies had published about the materials for adiabatic engines. They described valuable knowledge about high tempel. ature sliding surfaces. Although, the guidelines for applying ceramics coating onto low temperature lubricated sliding surface was difficult to fi1'ld. Tl.1e objective engine requires the wear protective material on the sliding surface which conditions are below 250CG of temperature and lubricated by mineral oil. Some tests were inevitable to select the material. Pin-on-disk wear test was introduced to evaluate the materials. Fig. 7 is the test apparatus and Fig. 8 is specimens. Test procedure is shown in Fig. 9. To exclude initial wear, 30 hours of break in was made previously. About half of specimens were abandoned at the end of break in due to abnormal wear, scuffing or peeling of coatings. Then 100 hours of wear test was conducted. Wear was described as the average wear depth by measuring of pin height with micro meter and of cross sectional disk shape with profi1ometer. 3.2 Veryfying wear test method A reference test prior to material selection was made to examine the correlation between actual engine wear and pin-on-disk test results. Two kinds of specimens which were previously tested by actual marine diesel engine were examined. Table 2 shows these materials and Fig. 10 shows actual engine wear. Abnormal cylinder liner wear was observed in case of rubbing the boron doped cast iron cylinder liner by titanium-vanadium doped cast iron piston rings. On the other hand, the wear was normal in case of rubbing the boron doped cast iron cylinder liner by gray cast iron piston rings. (22) Bulletin of the M.E.S.J., Vo1. 21, No.2

7 Ceramic-metal Composite Coated Piston Ring and Cylinder Liner of Marine Low Speed Diesel Engine 81 Pin and disk specimens made of these materials were also tested by pin-on-disk wear tester. Fig. 1l shows the results. They were well correlated with actual engine tests. Consequently, the test method was thought to be appropriate for screening of materials. October 1993 (23 )

8 82 Masaki Tanaka, Yoshihisa Kitajin1a, Yasuhisa Endoh, Masaoki Watanabe, Yutaka Nagita 3.2 Load capacity test Ceramics and ceramic base spray coated speci mens,whose coating processes were not limited to LPPS were tested. Load and sliding speed were automatically controlled as in Fig. 12. Table 3 shows the testing conditions. Table 4 Shows test results. Cast iron and plasma sprayed coatings except Cr2O3 specimen caused severe wear, surface cracking or scuffing. Only in case of Cr2O3, neither the surface damage nor the significant wear was observed. 3.3 Wear test of gray cast iron and plasma spray coated Cr2O3 Gray cast iron and plasma spray coated Cr2O3 were tested. Atmospheric plasma spraying, APS, was tried together with LPPS. Fig. 13 shows test results. As can be seen, the wear resistant property of LPPS coated Cr2O3 was far better than that of cast iron and of APS Cr2O3 coating. (24) Bulletin of the M.E.S.J., Vo1. 21, No.2

9 Ceramic-metal Composite Coated Piston Ring and Cylinder Liner of Marine Low Speed Diesel Engine Wear test of Cr2O3 and Cr2OjNlo composites Ceramics-metal composites are an attractive material for wear protective coatings. They have probabilities to improve wear, scuffing protection ability, toughness and strength. Molybdenum, which is often employed as a wear protective coating was selected to be an additive material to Cr2O3. "Composite powder process", described previously was applied. LPPS coated Cr2O3 and LPPS coated Cr2O3/Mo composite was compared. Fig. 14 shows the results that the wear of Cr2O3/Mo composite was far smaller than Cr2O3. Accordingly it proved the best material. liners were finished to be corrugated profi1e. Fig. 15 shows the production process of ceramic coated piston rings and cylinder liners. They were plasma sprayed and. ground. Only No. 2 engine cylinder liner was finished by honing because it was too large for grinding. At the first installment of piston rings and cylinder liners, more than 20 hours of partial load running for breaking was conducted. Then, cylinders were overhauled and prime data was sampled. Then full load running tests were made. Ring wear was measured by micrometer and 4. Engine test Engine tests were conducted by three test diesel engines. Table 5 shows their principal particulars and operating conditions that were set to be as similar to the objective engine as much as possible. Cylinder components made of cast iron and those coated by LPPS were prepared for every engine. Cast iron piston rings and cylinder liners were finished by lathing. Sliding surface of cylinder cylinder wear was measured by profile meter. October 1993 (25)

10 84 Masaki Tanaka, Yoshihisa Kitajima, Yasuhisa Endoh, Masaoki Watanabe, Yutaka Nagita No. l engine is very small as a crosshead type engine and it inheres several unreasonable designs. That is why the wear was large enough to measure. The other test engines (No. 2 and No. 3) are full scale engine, so the wear was too small to measure accurately. 4.l Two cycle small engine test Fig. 16 and Fig. 17 show the results of No. 1 engi1^1e test. Peeling of coating had occurred in the Cr2O3 coated (not composite) cylinder liner after the 60 hours' running. Neither peeling nor scuffing occurred by usil-1g of piston rings and cylinder liners coated witl,1 Cr2O3/Mocompositeby "mixed powder process''. They wore about 1/10 to 1/100 of cast iron. As the result,cr2o3/mo composite coating was proved having better wear resistant property and stronger coating bonding than Cr2O3 coating. 4.2 Two cycle full scale engine test Cylinder components made of cast iron and those coated by plasma spraying were tested by No. 2 engine for about 400 hours. Table 6 shows test results. The wear of plasma coated piston rings and cylinder liners was too small to measure accurately. Wear was supposed to be less than mm/i000 hours in cylinder liner and less than 0.02 mm/1000 hours in top piston ring.

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12 Ceramic-metal Composite Coated Piston Ring and Cylinder Liner of Marine Low Speed Diesel Engine Four cycle full scale engine test Cylinder components made of cast iron and those coated by plasma spraying were tested by No. 3 engine for about 300 hours. Table 6 shows test results. Almost all of piston rings and cylinder liners wore more than those of No. 2 engine. It is because of high temperature (210CG) at the top piston ring groove and frequent cylinder overhauling. The wear of plasma coated piston rings and cylinder liners were far smaller than that of cast iron compo1.lents. 5. Discussion The lifespan of plasma coated piston ring and cylinder liner were estimated based on No. 2 engine test results. Coating thickness which is thought to be wear limit, assumed to be 0.5 mm. As a result, piston ring lifespan are estimated at 4 years, previously being 2 years, and cylinder liner lifespan is 25 years, previously being 12 years. This enables periodic inspection every 4 years instead of every 2 years, consequently reducing the maintel. lance cost for overhaulling. Pro1ongation of piston ring and cylinder liner lifespan enables us to reduce the cost of spare components. Since coated surface bears the whole sliding duty, plasma coated cylinder liner l.1as the advantage of being able to use strong material for substrate instead of gray cast iron. For example, ductile cast iron, whose strength is more than 500 MPa, can be used for the objective engine cylinder liners. 6. Conclusions Engine test results showed that cast iron piston rings and cylinder liners have insufficient wear characteristics for objective "future" engine. Also cast iron cylinder liner has insufficient strength. On the other hand, developed piston rings and cylinder liners, coated with ceramic-metal composite by low pressure plasma spraying, proved excellent results on the wear resistant characteristics. Moreover the process developed in this study makes possible to apply stronger materials for cylinder liners. References 1 ) Anon., Source book on application of lasers in metal working, ASME (198 I ), 227 2) E. Anderson, et al., ISME Kobe D ) T. Shimauchi et al., SAE No ) T. Yoshimitsu, et al., SAE No ) A. Levy, et al., Wear, 101 (1985) P127 October 1993 (27)