- Email: [email protected]
Characterization of MoN coatings for pistons in a diesel engine
Materials and Design 31 (2010) 624–627
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Characterization of MoN coatings for pistons in a diesel engine Hanbey Hazar * Department of Automotive, Technical Education Faculty, Firat University, Elazig 23119, Turkey
a r t i c l e
i n f o
Article history: Received 25 April 2009 Accepted 6 June 2009 Available online 12 June 2009
a b s t r a c t In this study, the surface of a piston in a diesel engine was coated with molybdenum nitride (MoN) by using the arc PVD method, and its surface behaviour was subsequently analyzed. Analyses of microhardness, SEM, X-ray and surface roughness were carried out in order to examine surface characteristics of pistons. It is found that the hardness of coated piston is 2000 ± 400 HV while hardness uncoated piston 123 HV. The results show less deformation and fewer scratches due to wear on the MoN-coated piston as compared to uncoated one. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In an internal combustion engine, the tribological performance of the piston-ring and cylinder liner system has long been recognized as important in achieving desired engine efficiency and durability in terms of power loss, fuel consumption, oil consumption, blow by, and even harmful exhaust emissions [1]. In existing engines, the thermal constraints set in terms of thermal stability of a particular material also limit the efficiency of the engine. Producing engine and combustion chamber components from ceramic materials both lengthens the lives of these components and supplements the efficiency of the engine [2,3]. For pistons are components which are used in the combustion chambers of an engine. If the tribological features of pistons are improved, energy consumed by friction will decrease, and overall emission and performance values will improve. Coating the piston surface with various ceramic materials improves features of this material and thus enables the production of components with less weight and volume. Typical piston materials are light alloys, cast iron, nodular cast iron, and alloyed steels. The pistons for highspeed engines are primarily made of aluminum silicon alloys [4]. Due to their structural characteristics and excessive surface roughness and inadequate strength, pistons cause too much wear in cylinder surfaces. Thus, lengthening the life of pistons will both lengthen life of the engine and contribute to the economy of the engine. Using the PVD method, various ceramic materials can used to create coatings of specified thicknesses. One of these ceramic materials is MoN, which has recently attracted attention due to its characteristic high resistance to wear and to corrosion with a very thin coating. It can be seen that the studies conducted so far are promising and they can be further developed. Most of these studies are carried out under laboratory test conditions instead * Tel.: +90 424 2370000x4349. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.06.006
of real engine conditions. However, real engine conditions cannot be created in even very well designed laboratory test conditions; because chemical and mechanical events occurring in the combustion chamber are very complex [5]. This is why it was important for us to conduct our experiments in real engines. In this study, piston surface of a diesel engine was coated with MoN using the arc PVD method. The purpose of this study is to analyze tribological effects of surface coating for a piston in frictional mechanism. 2. Experiment In this study, a four-stroke, single cylinder and direct injection diesel engine has been utilized. Table 1 presents technical specification of the experimental engine, Table 2 provides information on the method of arc PVD (physical vapor deposition) [6] and production parameters regarding MoN coating The piston of the engine employed in this experiment has been coated with MoN coating, the surface of which is a ceramic material with a thickness of 2.0 ± 0.3 lm. In the experiments carried out in two phases, uncoated and MoN-coated engines were run for approximately 450 h for each engine under the same load conditions (at 1/2 load). LEO 440 Model was been utilized in SEM analyses and Fisher H100 XYPROG Model appliance in the measurement of hardness. D8 ADVANCED BRUKER AXS model appliance was employed in X-ray analysis and Mitutoyo Surftest-211 appliance for surface roughness in measurement of roughness. In order to place a load on the engines, Cusson P8 160 Model electrical dynamometer was used. At the end of the experiments carried out, pistons in both engines were dismantled and same regions of these components were taken. 3. Results and discussion The surface properties of pistons are one of the most important factors affecting the friction, wear and lubrication of the sliding
H. Hazar / Materials and Design 31 (2010) 624–627 Table 1 Test engine specifications. Item
Specification
Type of engine Stroke Number of cylinders Bore/stroke (mm) Compression ratio Maximum engine power (kW) Fuel type Lubricating Type of injection Pressure of injection (kg/cm2) Type of coolant Maximum engine speed (1/min) Engine volume (mm3)
Lombardini 6LD 400 4 1 86/68 18:1 6.25 (3600 1/min) Diesel Full pressure Direct injection 200 Air coolant 3600 382 427 491
Table 2 Deposition conditions of arc PVD and MoN coating. Coating thickness (lm) Hardness (HV) Coating temperature (°C) Deposition time per run (min) Cathode current (A) Bias voltage (V) Coating pressure (Pa)
2.0 ± 0.3 2000 ± 400 300 60 125 100 4 10 1
surfaces within the cylinders. There are two factors effective on the surface properties of the piston. One of them is the chemical composition and the other one is the sensitivity during the treating. There is a direct relation between the increase in surface roughness and the wear resistance. Thus, the amount of wear which occurs the most in pistons can be reduced through decrease in surface roughness and rise in matrix microhardness. Increase in the wear system is observed more in rough regions. The application of a thin coating here is important for the improvement of frictional and wear characters. The surface topography, residual stress, microstructure of the coating and thickness of the coating is related to the type of the coating material. Fig. 1 displays the coating layer in the SEM photo taken from the cross section of the piston surface. It can be clearly seen that there is no space or cracks in the interface between the coating layer and the substrate. Surface roughness values of the samples collected from the same corresponding parts of the coated piston and the uncoated piston were compared. Roughness value (Ra) of the coated piston was measured as 3.76 lm, the uncoated piston as 4.14 lm prior to tests. Hardness values of the coated and the uncoated cylinder
Fig. 1. SEM photo of the cross section of the coated piston.
625
liners were measured as 2000 ± 400 and 123 HV (kg/mm2), respectively. Low roughness level in the coated piston is thought to diminish abrasive wear occurring on the piston’s surface. Expansion of the contact area in rough surfaces increases the frictional coefficient in piston surfaces. Surface roughness values of the materials have great effects on the wear and frictional coefficients. Fig. 2 presents the SEM photos taken from the parts of uncoated (a) and the MoN-coated (b) piston. Fig. 2a shows that there are lines of deep wear. Considering that the engine was excessively heated after running under a specific amount of load, the deep wear lines observed in Fig. 2a can be said to result from the creepage. Additionally, Fig. 2a displays abrasive craters in the sliding direction. As can be concluded from Fig. 2b, the MoN-coated piston has an orderly surface structure. This is due to the increased hardness and the lower surface roughness of the coated piston compared to the uncoated one. Hardness of the MoN coating and its contact area have effects on the lower frictional coefficient of the coated piston compared to the uncoated piston. Higher hardness of the ceramic coated piston compared to that of the uncoated piston increases the load capacity of the pistons. Because the oil film between the piston-ring and cylinder breaks from time to time or during the first start of the engine, contact between metals results in metal damage. Fig. 2a shows traces of abrasive wear and free surface breaks resulting from abrasive particles and micro-cracks on the uncoated piston surface. In the micro-structure photos, abrasive craters can be seen to widen in some regions, even though they are narrow and deep. In Fig. 2b it is clearly observed that there are no deep micro-cracks and fewer wear lines on the MoN-coated piston surface than on the uncoated one. It is thought that the higher surface hardness of the MoNcoated surface reduces the traces of wear. High adhesion structure in the coating applied process shows that the piston has high wear and frictional resistance. Wear in pistons occurs as a result of effects of physical and chemical factors caused by combustion, lubrication and cooling together with friction on the cylinder’s surface. Rising temperature with the starting of the engine creates different stresses around the cylinder in the upper region in particular. Cylinders change shape with the effect of the above mentioned stress originating from different sources and its cylindricality is a bit deformed. Piston rings on the other hand which are circular, do not fit in with the oval shape of the cylinder, in the first moments of the deformity. Thus, the piston, cylinder and the rings wear jointly until they achieve a mutual harmony. It can be said that refractions create wear particles in these regions and thus abrasive wear initiates wear. Lubrication cannot carry all wear particles to the oil crankcase. These particles adhere to the piston and cylinder’s surface or cause micro scotches and traces on the surface. Wear particles in the oil and degradation of the oil speed up the deformation on the surfaces of the piston and the ring. Hard particles detached during the wear action have negative impact on the total wear characteristics. These particles break through the surface during the wear mechanism and result in abrasive wear. As displayed in Fig. 2a, craters and micro cuts caused by these wear particles on the surface prove that the wear mechanism under the limiting lubricating conditions is abrasive. In an analysis of the SEM photo taken from the same region of the coated piston, Fig. 2b shows that there are not any significant ruptures, refractions or plastic deformations. As can be observed in Fig. 2b, the reason for the formation of droplets on the surface of the modified cylinder can be attributed to the high internal stress created by the magnetic area created by the bias voltage applied between the cathode and anode during coating [7]. The irregular spread of the flame front in the combustion chamber results in negative impact, such as partial flame collisions and knocking. Thus components of the combustion chamber are exposed to thermal stresses and thermal shocks. These negative results cause particles
626
H. Hazar / Materials and Design 31 (2010) 624–627
Fig. 2. SEM photographs of uncoated (a) and the coated (b) piston after test runs.
Fig. 3. SEM photographs of uncoated (a) and the coated (b) pistons after test runs.
ceramic coating. Therefore, it is considered that these parts experience minimum negative effects. 3.1. 1. The Results of X-ray analysis Fig. 4 presents the X-ray analyses of the samples collected from the stand surface regions after coated piston was run. The Al and Si element displayed in Fig. 4 XRD analysis is considered to result from the material transfer on the surface in the wear mechanism of the cylinder, ring and piston while the engine runs. It has a cubic structure, while the MoN was detected to have a hexagonal structure. 4. Conclusions The conclusions derived from this study can be as follows: Fig. 4. XRD Spectrum of the coated piston after test runs.
of the combustion chamber to be subjected to stress in an irregular way and perform under more load than is normal. Coating the piston enables these negative effects to be reduced by the coating, keeping the substrate relatively unexposed to damage and increasing the life of the piston. As can be seen in Fig. 3, SEM photographs of the piston’s top surfaces are shown after test runs. The negative effects occurring during the burning cycle in the combustion chamber such as thermal shock, extreme temperature and irregular thermal tension are countered by the applied
When the piston’s surface is coated with the arc PVD method, pistons can be used without any need to apply any additional process on the surface after the coating. Surface hardness of the coated piston is higher than the uncoated one and this has positive effects on its resistance against wear. The surface of the coated piston is harder than that of the uncoated one and this contributes to the piston’s load capacity. As a result, these engines can be run in higher compression rates.
H. Hazar / Materials and Design 31 (2010) 624–627
Acknowledgement The author would like to thank the Prof. Dr. Mustafa Ürgen (Istanbul Technical University) for his support. References [1] Tung SC, Huang Y. Modeling of abrasive wear in a piston ring and engine cylinder bore system tribology. Transactions 2004;47:17–22. [2] Hazar H. The experimental investigation of wear behaviour of a diesel engine with cylinder having ceramic coated surface. PhD thesis, Fırat University; 2004.
627
[3] Merlo AM. The contribution of surface engineering to the product performance in the automotive industry. Surf Coat Technol 2003;174–175:21–6. [4] Andersson P, Tamminen J, Sandström CE. Piston ring tribology. VTT Res Notes 2002;2178:1–105. [5] Truhan JJ, Qu J, Blau PJ. A rig test to measure friction and wear of heavy duty diesel engine piston rings and cylinder liners using realistic lubricants. Tribology 2005;38:211–8. [6] Öner C, Hazar H, Nursoy M. Surface properties of CrN coated engine cylinders. Mater Design 2008;30:914–20. [7] Bozyazı EE. Comparison of lubricated wear behaviours of electrolytic hard chromium and CrN coating produced by cathodic arc physical vapour deposition. Master thesis, Istanbul Technical University; 2002.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Characterization of MoN coatings for pistons in a diesel engine Hanbey Hazar * Department of Automotive, Technical Education Faculty, Firat University, Elazig 23119, Turkey
a r t i c l e
i n f o
Article history: Received 25 April 2009 Accepted 6 June 2009 Available online 12 June 2009
a b s t r a c t In this study, the surface of a piston in a diesel engine was coated with molybdenum nitride (MoN) by using the arc PVD method, and its surface behaviour was subsequently analyzed. Analyses of microhardness, SEM, X-ray and surface roughness were carried out in order to examine surface characteristics of pistons. It is found that the hardness of coated piston is 2000 ± 400 HV while hardness uncoated piston 123 HV. The results show less deformation and fewer scratches due to wear on the MoN-coated piston as compared to uncoated one. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In an internal combustion engine, the tribological performance of the piston-ring and cylinder liner system has long been recognized as important in achieving desired engine efficiency and durability in terms of power loss, fuel consumption, oil consumption, blow by, and even harmful exhaust emissions [1]. In existing engines, the thermal constraints set in terms of thermal stability of a particular material also limit the efficiency of the engine. Producing engine and combustion chamber components from ceramic materials both lengthens the lives of these components and supplements the efficiency of the engine [2,3]. For pistons are components which are used in the combustion chambers of an engine. If the tribological features of pistons are improved, energy consumed by friction will decrease, and overall emission and performance values will improve. Coating the piston surface with various ceramic materials improves features of this material and thus enables the production of components with less weight and volume. Typical piston materials are light alloys, cast iron, nodular cast iron, and alloyed steels. The pistons for highspeed engines are primarily made of aluminum silicon alloys [4]. Due to their structural characteristics and excessive surface roughness and inadequate strength, pistons cause too much wear in cylinder surfaces. Thus, lengthening the life of pistons will both lengthen life of the engine and contribute to the economy of the engine. Using the PVD method, various ceramic materials can used to create coatings of specified thicknesses. One of these ceramic materials is MoN, which has recently attracted attention due to its characteristic high resistance to wear and to corrosion with a very thin coating. It can be seen that the studies conducted so far are promising and they can be further developed. Most of these studies are carried out under laboratory test conditions instead * Tel.: +90 424 2370000x4349. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.06.006
of real engine conditions. However, real engine conditions cannot be created in even very well designed laboratory test conditions; because chemical and mechanical events occurring in the combustion chamber are very complex [5]. This is why it was important for us to conduct our experiments in real engines. In this study, piston surface of a diesel engine was coated with MoN using the arc PVD method. The purpose of this study is to analyze tribological effects of surface coating for a piston in frictional mechanism. 2. Experiment In this study, a four-stroke, single cylinder and direct injection diesel engine has been utilized. Table 1 presents technical specification of the experimental engine, Table 2 provides information on the method of arc PVD (physical vapor deposition) [6] and production parameters regarding MoN coating The piston of the engine employed in this experiment has been coated with MoN coating, the surface of which is a ceramic material with a thickness of 2.0 ± 0.3 lm. In the experiments carried out in two phases, uncoated and MoN-coated engines were run for approximately 450 h for each engine under the same load conditions (at 1/2 load). LEO 440 Model was been utilized in SEM analyses and Fisher H100 XYPROG Model appliance in the measurement of hardness. D8 ADVANCED BRUKER AXS model appliance was employed in X-ray analysis and Mitutoyo Surftest-211 appliance for surface roughness in measurement of roughness. In order to place a load on the engines, Cusson P8 160 Model electrical dynamometer was used. At the end of the experiments carried out, pistons in both engines were dismantled and same regions of these components were taken. 3. Results and discussion The surface properties of pistons are one of the most important factors affecting the friction, wear and lubrication of the sliding
H. Hazar / Materials and Design 31 (2010) 624–627 Table 1 Test engine specifications. Item
Specification
Type of engine Stroke Number of cylinders Bore/stroke (mm) Compression ratio Maximum engine power (kW) Fuel type Lubricating Type of injection Pressure of injection (kg/cm2) Type of coolant Maximum engine speed (1/min) Engine volume (mm3)
Lombardini 6LD 400 4 1 86/68 18:1 6.25 (3600 1/min) Diesel Full pressure Direct injection 200 Air coolant 3600 382 427 491
Table 2 Deposition conditions of arc PVD and MoN coating. Coating thickness (lm) Hardness (HV) Coating temperature (°C) Deposition time per run (min) Cathode current (A) Bias voltage (V) Coating pressure (Pa)
2.0 ± 0.3 2000 ± 400 300 60 125 100 4 10 1
surfaces within the cylinders. There are two factors effective on the surface properties of the piston. One of them is the chemical composition and the other one is the sensitivity during the treating. There is a direct relation between the increase in surface roughness and the wear resistance. Thus, the amount of wear which occurs the most in pistons can be reduced through decrease in surface roughness and rise in matrix microhardness. Increase in the wear system is observed more in rough regions. The application of a thin coating here is important for the improvement of frictional and wear characters. The surface topography, residual stress, microstructure of the coating and thickness of the coating is related to the type of the coating material. Fig. 1 displays the coating layer in the SEM photo taken from the cross section of the piston surface. It can be clearly seen that there is no space or cracks in the interface between the coating layer and the substrate. Surface roughness values of the samples collected from the same corresponding parts of the coated piston and the uncoated piston were compared. Roughness value (Ra) of the coated piston was measured as 3.76 lm, the uncoated piston as 4.14 lm prior to tests. Hardness values of the coated and the uncoated cylinder
Fig. 1. SEM photo of the cross section of the coated piston.
625
liners were measured as 2000 ± 400 and 123 HV (kg/mm2), respectively. Low roughness level in the coated piston is thought to diminish abrasive wear occurring on the piston’s surface. Expansion of the contact area in rough surfaces increases the frictional coefficient in piston surfaces. Surface roughness values of the materials have great effects on the wear and frictional coefficients. Fig. 2 presents the SEM photos taken from the parts of uncoated (a) and the MoN-coated (b) piston. Fig. 2a shows that there are lines of deep wear. Considering that the engine was excessively heated after running under a specific amount of load, the deep wear lines observed in Fig. 2a can be said to result from the creepage. Additionally, Fig. 2a displays abrasive craters in the sliding direction. As can be concluded from Fig. 2b, the MoN-coated piston has an orderly surface structure. This is due to the increased hardness and the lower surface roughness of the coated piston compared to the uncoated one. Hardness of the MoN coating and its contact area have effects on the lower frictional coefficient of the coated piston compared to the uncoated piston. Higher hardness of the ceramic coated piston compared to that of the uncoated piston increases the load capacity of the pistons. Because the oil film between the piston-ring and cylinder breaks from time to time or during the first start of the engine, contact between metals results in metal damage. Fig. 2a shows traces of abrasive wear and free surface breaks resulting from abrasive particles and micro-cracks on the uncoated piston surface. In the micro-structure photos, abrasive craters can be seen to widen in some regions, even though they are narrow and deep. In Fig. 2b it is clearly observed that there are no deep micro-cracks and fewer wear lines on the MoN-coated piston surface than on the uncoated one. It is thought that the higher surface hardness of the MoNcoated surface reduces the traces of wear. High adhesion structure in the coating applied process shows that the piston has high wear and frictional resistance. Wear in pistons occurs as a result of effects of physical and chemical factors caused by combustion, lubrication and cooling together with friction on the cylinder’s surface. Rising temperature with the starting of the engine creates different stresses around the cylinder in the upper region in particular. Cylinders change shape with the effect of the above mentioned stress originating from different sources and its cylindricality is a bit deformed. Piston rings on the other hand which are circular, do not fit in with the oval shape of the cylinder, in the first moments of the deformity. Thus, the piston, cylinder and the rings wear jointly until they achieve a mutual harmony. It can be said that refractions create wear particles in these regions and thus abrasive wear initiates wear. Lubrication cannot carry all wear particles to the oil crankcase. These particles adhere to the piston and cylinder’s surface or cause micro scotches and traces on the surface. Wear particles in the oil and degradation of the oil speed up the deformation on the surfaces of the piston and the ring. Hard particles detached during the wear action have negative impact on the total wear characteristics. These particles break through the surface during the wear mechanism and result in abrasive wear. As displayed in Fig. 2a, craters and micro cuts caused by these wear particles on the surface prove that the wear mechanism under the limiting lubricating conditions is abrasive. In an analysis of the SEM photo taken from the same region of the coated piston, Fig. 2b shows that there are not any significant ruptures, refractions or plastic deformations. As can be observed in Fig. 2b, the reason for the formation of droplets on the surface of the modified cylinder can be attributed to the high internal stress created by the magnetic area created by the bias voltage applied between the cathode and anode during coating [7]. The irregular spread of the flame front in the combustion chamber results in negative impact, such as partial flame collisions and knocking. Thus components of the combustion chamber are exposed to thermal stresses and thermal shocks. These negative results cause particles
626
H. Hazar / Materials and Design 31 (2010) 624–627
Fig. 2. SEM photographs of uncoated (a) and the coated (b) piston after test runs.
Fig. 3. SEM photographs of uncoated (a) and the coated (b) pistons after test runs.
ceramic coating. Therefore, it is considered that these parts experience minimum negative effects. 3.1. 1. The Results of X-ray analysis Fig. 4 presents the X-ray analyses of the samples collected from the stand surface regions after coated piston was run. The Al and Si element displayed in Fig. 4 XRD analysis is considered to result from the material transfer on the surface in the wear mechanism of the cylinder, ring and piston while the engine runs. It has a cubic structure, while the MoN was detected to have a hexagonal structure. 4. Conclusions The conclusions derived from this study can be as follows: Fig. 4. XRD Spectrum of the coated piston after test runs.
of the combustion chamber to be subjected to stress in an irregular way and perform under more load than is normal. Coating the piston enables these negative effects to be reduced by the coating, keeping the substrate relatively unexposed to damage and increasing the life of the piston. As can be seen in Fig. 3, SEM photographs of the piston’s top surfaces are shown after test runs. The negative effects occurring during the burning cycle in the combustion chamber such as thermal shock, extreme temperature and irregular thermal tension are countered by the applied
When the piston’s surface is coated with the arc PVD method, pistons can be used without any need to apply any additional process on the surface after the coating. Surface hardness of the coated piston is higher than the uncoated one and this has positive effects on its resistance against wear. The surface of the coated piston is harder than that of the uncoated one and this contributes to the piston’s load capacity. As a result, these engines can be run in higher compression rates.
H. Hazar / Materials and Design 31 (2010) 624–627
Acknowledgement The author would like to thank the Prof. Dr. Mustafa Ürgen (Istanbul Technical University) for his support. References [1] Tung SC, Huang Y. Modeling of abrasive wear in a piston ring and engine cylinder bore system tribology. Transactions 2004;47:17–22. [2] Hazar H. The experimental investigation of wear behaviour of a diesel engine with cylinder having ceramic coated surface. PhD thesis, Fırat University; 2004.
627
[3] Merlo AM. The contribution of surface engineering to the product performance in the automotive industry. Surf Coat Technol 2003;174–175:21–6. [4] Andersson P, Tamminen J, Sandström CE. Piston ring tribology. VTT Res Notes 2002;2178:1–105. [5] Truhan JJ, Qu J, Blau PJ. A rig test to measure friction and wear of heavy duty diesel engine piston rings and cylinder liners using realistic lubricants. Tribology 2005;38:211–8. [6] Öner C, Hazar H, Nursoy M. Surface properties of CrN coated engine cylinders. Mater Design 2008;30:914–20. [7] Bozyazı EE. Comparison of lubricated wear behaviours of electrolytic hard chromium and CrN coating produced by cathodic arc physical vapour deposition. Master thesis, Istanbul Technical University; 2002.