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Manufacturing of surface microstructures for improved tribological efficiency of powertrain components and forming tools
CIRP Journal of Manufacturing Science and Technology 4 (2011) 200–207
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CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj
Manufacturing of surface microstructures for improved tribological efficiency of powertrain components and forming tools A. Schubert a,b, R. Neugebauer a,b, D. Sylla a,*, M. Avila a, M. Hackert a a b
Institute for Machine Tools and Production Processes, Chemnitz University of Technology, Reichenhainer Str. 70, 09107 Chemnitz, Germany Fraunhofer Institute for Machine Tools and Forming Technology IWU, Reichenhainer Str. 88, 09126 Chemnitz, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 26 February 2011
The performance and energy efficiency of mechanical components is strongly influenced by the tribological behaviour of their surfaces. This paper investigates the design and manufacturing of microstructured surfaces for improved wear resistance of forming tools and reduced friction in powertrain components. The potential of microstructures in the improvement of adhesion strength and resistance to delamination of hard coatings under the severe thermomechanical service conditions of hot forging tool surfaces is discussed. In the case of powertrain components, the ability of surface structures to reduce friction between lubricated bearing surfaces was investigated numerically and experimentally. The COMSOL Multiphysics Computational Fluid Dynamics (CFD) package was used to estimate the effect of geometric parameters of patterned spherical-segment cavities on hydrodynamic pressure. Electrochemical Machining by closed-electrolytic-free Jet (Jet-ECM) was used to manufacture the microstructures. Tribological ring-on-disc tests with structured surfaces were performed. It was determined that surface microstructures in the form of patterned spherical-segment cavities generate additional lift pressure which allows the surfaces to reach hydrodynamic lubrication at lower operating speeds. ß 2011 CIRP.
Keywords: Energy efficiency Tribology Surface texturing Microstructures Forging tool Coating Powertrain Bearing Laser ablation Electrochemical machining Jet-ECM
1. Introduction Manufacturing technology and powertrain development in recent years have placed increased emphasis on energy efficiency. This is motivated by rising fuel prices, limited natural resources and CO2 emission abatement policies. Tribological behaviour has a highly significant influence on the energy and resource efficiency of both the manufacture and operation of powertrain components. Pioneering work on energy efficiency in manufacturing by Lange [1] and Berry and Fels [2] showed that the free energy change of basic material production is the most energy-intensive phase in metalworking process chains. Consequently, scrap reduction in the form of worn-out tools and material removal steps for both forming die manufacturing and component series production can yield significant energy and resources savings. Wear-resistant tools reduce the number of tools manufactured per batch and facilitate the implementation of near-net-shape forming. This later process reduces scrap during the forming process as well as the amount of subsequent material removal steps required to reach the final component specifications.
* Corresponding author. E-mail address: [email protected] (D. Sylla). 1755-5817/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirpj.2011.01.010
Coatings have been used to increase the wear resistance of tool steel in hot forging applications. Yet, poor adhesion to the substrate, high residual stresses from the coating deposition process, sharp mismatch between the thermal expansion coefficients of the hard coatings and tool steel substrates, and high manufacturing costs have limited the practicality of coated dies in the hot forging industry. However, substrate surface structures provide an opportunity to improve coating adhesion under hot forging conditions. The potential advantages are threefold: structures can promote the generation of coating-to-substrate bonding during the deposition process, create higher interfacial fracture toughness, and provide gradient material properties in the interface of the coating–substrate composite. Measures to decrease friction in powertrain components are key to the reduction of fuel consumption and emissions of automobiles. Components that show a high potential for efficiency improvements include crankshaft bearings, piston rings, cylinder liners and valve trains. Reducing friction decreases wear, improves efficiency and lengthens the service life. Numerous publications have shown that surface geometry in the form of microstructures has a considerable influence on tribological properties. Reductions in friction and wear of up to 60% have been reported [3–8]. In these studies, microstructuring was generated by laser machining. In the present investigation, Electrochemical Machining by closed-electrolyticfree Jet (Jet-ECM) was used to produce microstructured surfaces.
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Electrochemical processing has the advantage of high level of ablation precision with no mechanical nor thermal damage to the surfaces. This precludes the need for finishing because, unlike laser processing, electrochemical processing does not cause burrs or warping. As an added advantage, in contrast to conventional electrochemical machining, Jet-ECM enables localized material removal by control of the jet position without relying on a solid electrode with the negative form of the feature to be machined [9]. 2. Operating conditions of hot working tools and powertrain components and the role of surface microstructures: state of the art Although surface structuring can improve the tribological properties of hot forming tools and hydrodynamic bearings of powertrain components and achieve higher energy and resource efficiency (Fig. 1), the operating conditions in both applications are different. Surfaces of hot-working tools must withstand severe thermal and mechanical loads that are transient in nature and they typically fail within the regime of low-cycle fatigue (up to 104 cycles). On the other hand, hydrodynamic friction bearings in powertrain components are subjected to relatively low loads, with the surfaces of these bearings lasting considerably longer than 104 cycles before failing. 2.1. Hot forming tools Hot working conditions of ferrous materials require that die surfaces withstand sliding contact with hot metal at 1000–1200 8C under apparent area pressures of 155–310 MPa [10]. In critical die surface locations, however, the combination of contact stresses and steep temperature gradients far exceed the apparent area pressures. Finite Element Method studies [11,12] have estimated that thermal stresses can exceed the mechanical stresses tenfold and that the combined equivalent stress from thermal and mechanical stresses can reach 1360 MPa at die shoulders. In general, die surface temperatures temporarily exceed 600 8C upon contact with the hot workpiece, thus exceeding the tempering temperature of tool steels [13]. Forging die surfaces are also subjected to thermal shock and mechanical impact. Forging cycle frequencies range from 4 to 200 cycles per minute for hydraulic press forging and high-speed forging, respectively [14,15] and dies are usually cooled to 200–300 8C between forging cycles.
Fig. 1. Potential improvements in energy and resource efficiency by surface microstructures in the manufacturing-to-service cycle.
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To withstand such severe working conditions, materials for hot forging dies are required to have: high hot hardness to resist abrasive wear at elevated temperatures, low chill-crack sensitivity to resist thermal shock, temper resistance to avoid thermal softening, bearing capacity to resist plastic deformation, toughness to inhibit mechanically-induced cracking, chemical incompatibility with the workpiece to prevent galling and pick-up, oxidation resistance at high temperatures to prevent corrosioninduced cracking and economical applicability in a mass-production environment. The great majority of hot forming tools in industrial practice are made of hardened tool steel. A range of different surface modification methods are used to increase the wear-resistance of tool steel under hot working conditions: Thermal methods to tailor the tool steel microstructure. Thermochemical methods to alter the composition of the material at the surface. Coatings, whereby material such as hardmetals and hard alloys are deposited with a degree of diffusion bonding with the steel substrate that depends on the coating method and composition of the coating and substrate. Arc welding, thermal spray and laser cladding are the most common deposition methods for coating thicknesses above several tens of micrometers. In comparison to hardened tool steel, hardmetals (carbide phase and metallic binder, e.g. cobalt-based) are more resistant to thermal softening and oxidation. They are also chemically stable when in contact with ferrous workpieces. On the other hand, tool steels are highly resistant to fracture and plastic deformation at high temperatures and cost relatively little. Energy and costintensive monolithic hardmetal tools can be replaced by tool steel with a hardmetal coating to achieve considerable wear resistance improvements over tool steel at reasonable costs. Adequate toughness of the coatings can be attained by adjusting the composition and fraction of the metallic binder phase and by obtaining a homogeneous and pore-free microstructure. There is controversy in the literature, however, regarding the wear resistance improvements of coatings relative to surfacetreated or simply bulk-hardened tool steel at critical die surface areas such as shoulders [16,17]. Thermal-spray coatings have met with limited success in hot forging tools owing to their poor adhesion strength to the substrate. Although arc welding and laser cladding develop excellent coating adhesion strength thanks to diffusion bonding with the substrate [18], it induces – unlike thermal spraying – a heat-affected zone in the substrate. Furthermore, arc welding and laser cladding often generate tensile residual stresses that can lead to cracking and the range of materials that can be deposited by these methods is relatively restricted. To realize the potential advantages of coatings in hot forging tool applications it is important to overcome these limitations of current coating deposition methods. In order reach adequate adhesion strength, substrate surface roughening and cleaning by grit blasting has been the standard practice for preparing substrates prior to thermal spray coating deposition. Substrate surface roughness provides mechanical interlocking – the main adhesion mechanism of thermal spray coatings deposited with state-of-the-art technology [19]. However, mechanical adhesion alone may be insufficient to withstand the severe working conditions of hot forging die surfaces. There is
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opportunity to fabricate surface microstructures on tool steel substrates prior to thermal spray coating deposition that are targeted at influencing the thermal mechanisms of adhesion and the thermal and mechanical properties at the coating–substrate interface. 2.2. Powertrain components The surfaces of powertrain components are subjected to a variety of thermal and mechanical loads that depend on the operating regime. In the case of lubricated bearings, at very low speeds (start-up), surfaces are in direct contact with each other. Consequently, elevated friction and considerable wear rate take place. As speed increases, in the mixed lubrication regime, the bearing surfaces partially separate from each other thanks to pressure build-up by the lubricant. When the relative speed exceeds a certain threshold, the bearing surfaces become completely separated by the lubricant and reach the hydrodynamic lubrication regime. At this point, the only friction at play is fluid friction. Friction coefficient is minimum at the speed corresponding to the onset of the hydrodynamic regime. However, as speed increases further, the friction coefficient rises due to frictional losses within the fluid. This heats up the lubricating oil and may increase its temperature to up to 130 8C [20]. The hydrodynamic pressure increase created by surface microstructures has been studied by several researchers. Wakuda [4] observed that spherical segment cavities reduced friction by up to 20%. Hoppermann [21] showed that surface microstructures reduced the speed range of the mixed lubrication regime and lead to hydrodynamic regime at relatively low rotational speeds. Fig. 2 is an image of a cylinder surface with a saucer-shaped microstructure. This microstructure reduced oil consumption by up to 85% and fuel consumption by up to 6% [22]. Other investigations highlighting the positive effects of spherical-segment cavities on friction and wear can be found in [5,7,23–27]. However, these studies reached no uniform agreement as regards the influence of the different geometric parameters of the surface microstructures on friction and wear.
creating energy and resource-efficient process chains for forming tool manufacturing.
3.2. Methods There is the opportunity to increase the adhesion strength of thermal-spray coatings by optimizing the surface geometry of the substrate. The potential benefits of optimized surface geometries are threefold: development of coating-to-substrate diffusion bonding, increase of the delamination fracture toughness by inhibiting crack propagation along the coating–substrate interface and provide a gradual volume fraction change of the coating and substrate materials along their interface. Investigations on thermal-spray coating adhesion have proposed that surface geometry is the most important parameter in the development of coating adhesion [19,28]. Roughening of substrates prior to coating deposition is conducted in practice because adhesion is poor on smooth surfaces [29]. The improvement is attributed to the effect of surface roughness on the mechanical keying and the thermal mechanisms (i.e. diffusion bonding) of thermal spray coating adhesion [19]. In the case of High-Velocity-Oxy-Fuel (HVOF) spraying of hardmetal coatings on steel substrates, mechanical keying to the substrate is believed to be the main adhesion mechanism. Because of the low deposition temperatures of HVOF sprays in comparison to plasma spray and laser cladding, it is generally assumed that diffusion bonding does not occur. However, Sobolev et al. reported that substrate melted zones with the mentioned materials can reach a depth of 3 mm into grit-blasted steel surfaces [19]. A detrimental effect during thermal spraying is the formation of voids on the underside of splats as a result of the lateral spreading that occurs as the semi-molten particles impinge the substrate and solidify [19,28]. These voids reduce the actual coating–substrate contact area and thus bond strength. Surface asperities with an average distance that is approximately equal to the diameter of spray particles – with a typical size distribution from 10 to 60 mm – reduce the tendency of splats to spread out once they hit the
3. Surface microstructures for wear-resistant forming tools 3.1. Objective The goal is to increase the wear resistance of hot-forging tools in order to improve the energy and resource efficiency of metalworking chains. To overcome the limitations of current coating technologies for forming tools, preliminary investigations are focused on: incorporating deterministic surface structures in substrate preparation in order to influence the thermal mechanisms of thermal-spray coating adhesion and improve their adhesion under the cyclical thermal and mechanical loads encountered by hot-working tool surfaces;
Fig. 2. Cylinder liner with microstructure produced using laser honing [22].
Fig. 3. Effect of substrate roughness on the thermal adhesion mechanisms of thermal spray coatings [17,19].
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4. Surface structures for low friction and wear in powertrain components 4.1. Objective The Stribeck curve in Fig. 6 is used to illustrate the goal of microstructuring of bearing surfaces of powertrain components. In this figure, friction coefficient m is represented over the relative speed v of the sliding partners. The curve can be divided into three regimes: boundary lubrication, mixed lubrication and hydrodynamic lubrication. In the boundary and mixed regimes, friction coefficient is high and a maximum of wear is produced. The preferred regime for powertrain bearings is the hydrodynamic lubrication regime because it generates the least friction and wear on the sliding partners. The aim of surface structuring is twofold:
Fig. 4. Conceptual sketch of gradient material properties at the coating–substrate interface by deterministic surface texturing.
substrate. Sobolev [19] suggested that splats with aspects ratios close to unity may increase the duration of the thermal pulse upon impact onto the substrate and, hence, the opportunity to develop diffusion bonding (Fig. 3). Grit blasting as a substrate preparation method offers a limited range of surface geometries in comparison to deterministic material removal processes. Another drawback of grit blasting is that grit particles may remain embedded on the substrate. Grit contamination has been reported to initiate delimitation cracking of thermal spray coatings [30]. With the exception of high pressure water jets [31], no alternatives to grit blasting have been evaluated in the scientific literature regarding the development of diffusion bonding of thermal spray coatings. Furthermore, no studies exist on the effect of deterministic surface structures on the thermal mechanisms of adhesion of thermal spray coatings. With deterministic material removal processes such as milling and laser ablation, a wide range of patterned geometric features can be generated on the substrate and tailored to improve coating adhesion. Another opportunity from substrate surface structuring is the creation of gradient material properties across the substrate–coating interface that reduce misfit thermal stresses as well as increase the interfacial fracture toughness under hot forging conditions (Fig. 4). The application of milling for substrate texturing would eliminate substrate contamination by grit particles and the equipment and consumables of the grit blasting process. This novel forming die process chain is compared to the conventional approach in Fig. 5.
Fig. 5. Conventional vs. new process chains for hardfaced, hot working tool manufacturing.
Reduce the friction coefficient along the three regimes. Shift the Stribeck curve toward the left in order to overcome the boundary and mixed lubrication regimes at lower operating velocities. 4.2. Methods 4.2.1. Determining a suitable geometry for the surface microstructures The ability of surface microstructures to increase hydrodynamic pressure and the contribution of their geometric parameters to reach this goal were studied using the Computational Fluid Dynamics (CFD) package COMSOL Multiphysics. The Multiphysics model (Fig. 7) is based on the incompressible Navier–Stokes equation (1). A lubrication gap initially set at s = 20 mm exists between the static, microstructured base body and smooth counter body. The relative speed of the parallel surfaces was kept at v = 1 m/s. The intermediary substance is oil with a viscosity h of 0.79 Pa s, a density r of 887.68 kg/m3 and a temperature of 293 K.
rðvrÞv ¼ r½ pI þ hðrv þ ðrvÞT Þ þ f
(1)
where v is the fluid velocity, p is the pressure, I the identity matrix, (5v)T the transpose of (5v) and f the body forces. Condition of incompressibility: 5v = 0. The pressure profiles as a result of different diameters, distances and depths of spherical segments in an array of five
Fig. 6. Stribeck curve.
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A. Schubert et al. / CIRP Journal of Manufacturing Science and Technology 4 (2011) 200–207 Table 1 Simulated geometry conditions.
Fig. 7. Scheme of the simulation model.
cavities were calculated. The combination of geometric parameters evaluated are shown in Table 1. The results show that surface microstructures act as micro pressure chambers. Upon relative motion of the two sliding bodies the lubricant circulates in the microstructures (Fig. 8a). Consequently, a pressure peak is generated over the microstructure (Fig. 8b). The effect of geometric parameters on the hydrodynamic pressure resulting from fluid circulation is presented in Fig. 9. The diagram plots average pressure as a function of diameter d, distance a, and depth h of the cavities. It can be observed that cavity diameter has the largest influence on the pressurisation. The pressure rises linearly between a = 50 and 500 mm and reaches its highest value of 10 bar at 500 mm. The distance between adjacent spherical segments was identified as the second most important influence. In this case, it should be kept as small as possible because a low pattern density causes a decrease in pressure. An average pressure of over 7 bar can be achieved with an inter-cavity distance of 10 mm. The depth of the spherical segment had the least influence. Depths between 0 and 20 mm showed a strong rise in average pressure, of up to 2.4 bar at a depth of 20 mm. As depth is
No.
Diameter d (mm)
Depth h (mm)
Distance a (mm)
1 2 3
50. . .500 200 200
50 2. . .500 50
100 100 10. . .500
further increased, average pressure drops asymptotically to a constant value. The geometric parameters with the maximum pressure values: d = 500 mm, a = 100 mm and h = 20 mm were combined and the simulations showed that an average pressure of up to 125 bar could be reached with a lubrication gap of s = 5 mm. This value represents roughly 12% of the pressure that develops in a bearing under wedge-shaped gap conditions (approx. 1000 bar). Hence, the hydrodynamic condition could likely be reached more rapidly by use of this patterned structure. Patterned spherical-segment microstructures with the above combination of geometric parameters were fabricated and evaluated in a tribometer as described in the following sections. 4.2.2. Test materials and samples In order to measure the hydrodynamic effect of microstructures, a sufficiently large surface area is necessary. For this reason, it was decided to build a tribometric ring-on-disc test rig for the experimental investigations (Fig. 10). Only the ring counterpart featured the surface microstructures. The discs for the tribological tests consisted of heat treatable steel 42CrMo4 (1.7225) and had an outer diameter of dsa = 100 mm and a thickness of hs = 8 mm. The ring counter bodies were produced of bronze (2.1030) with an outer diameter dra = 100 mm, inner diameter dri = 70 mm and thickness hr = 10 mm. Further details are listed in Table 2. 4.2.3. Jet-ECM microstructuring Patterned spherical segments were produced by Jet-ECM. In this process, a high-velocity electrolytic jet is generated from a cathodically polarized nozzle and led toward the anodically polarized workpiece. As positively charged ions are attracted to the nozzle, the negatively charged ions are attracted to the
Fig. 8. CFD simulation of lubricant circulation in a microstructure (a); sketch of a micro-pressure chamber, after [32] (b).
Fig. 9. Effect of spherical-segment-cavity geometric parameters on hydrodynamic pressure.
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Table 2 Hardness and surface precision of the tribometric samples.
Hardness (HV) Roughness Rz (mm) Flatness (mm) Parallelism (mm)
Ring
Disc
160 1 5 10
650 1 5 10
Fig. 10. Ring-on-disc test.
workpiece and material is removed as metal ions are drawn into the electrolytic solution [9]. Initially, the Jet-ECM process parameters were determined for the production of the spherical segments (Fig. 11a). Thereafter, the ring samples were textured (Fig. 11b). Under consideration of the CFD simulation results, which indicated that, for high pressurisation, spherical segments shall be as shallow and as wide as possible, cavities were manufactured with a diameter of 500 mm and a depth of 20 mm. The array of spherical segments is illustrated in Fig. 12. Two offset rows were repeated in a circular pattern with a resulting overlap su¨ = 50 mm. The rings were microstructured with two different pattern densities (expressed as percent surface coverage area): 10% (a = 0.9 mm, a = 2.558, n = 2040 cavities) and 30% (a = 0.9 mm, a ( 0.858, n = 6120 cavities).
Fig. 12. Spherical segment cavity array.
4.2.4. Tribometric characterisation The tribometric measurements were carried out in a Wazau TRM500 tribometer (Fig. 13). The structured ring was fixed to the sample holder. The unstructured disc was affixed to the rotating platen. Ring and disc were loaded and the disc was rotated following the experiment program in Table 3. The tests were conducted under immersion lubrication with engine oil
Fig. 13. Tribometer TRM500 test rig.
Castrol EDGE 5W-30 at a temperature of 60 8C. The kinematic viscosity at this temperature was 32 mm2/s. The friction moment was measured and converted into friction coefficient. An unstructured ring-disc pair served as the reference condition. The sliding speed values were chosen so that all three lubrication regimes (boundary, mixed and hydrodynamic) were included in the measurements. 4.3. Results and discussion
Fig. 11. SEM image of the microstructured surface (a); microstructured bronze ring (b).
The friction coefficient of structured and unstructured sliding pairs as a function of speed is presented in Fig. 14. Under a normal force of 40 N (Fig. 14a), the unstructured pair showed a
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Table 3 Test parameters for the tribometrical characterisation. Motion sequence Normal force FN (N) Sliding speed v (m/s)
Continuous sliding 40; 280 0.3; 1; 6
The same results were obtained under a normal force of 280 N (Fig. 14b). A 10% pattern density results in lower friction compared to 30% pattern density. This indicates that a minimum of smooth bearing surface area is necessary to reduce friction coefficient. 5. Conclusions Energy efficiency is an important and pressing issue. Friction and wear are two factors which have a significant impact on energy requirements. Friction causes energy to be lost in the form of heat. A large amount of energy is lost due to friction-induced wear because it shortens the service life of the worn components, necessitating replacement ahead of schedule. Wear also leads to energy losses in manufacturing process chains. For these reasons, friction and wear reduction is an important goal. To this end, surface structures hold a large potential. Energy efficiency in the manufacture of components can be improved by structuring hot forging tool surfaces to increase the bond strength of wear-resistant coatings. In the service cycle, surface microstructures improve energy efficiency by reducing friction and wear. Additional hydrodynamic pressure generated by surface microstructures can lead to hydrodynamic lubrication regime at lower operating regimes where friction and wear are the lowest. A pattern of spherical-segment cavities was found to be an effective microstructure for this purpose. FEM simulations using COMSOL Multiphysics demonstrated that microstructures generate an additional hydrodynamic pressure of approx. 90 bar. The effectiveness of the structured surface was evaluated using a Wazau TRM500 tribometer in a ring-on-disc configuration. The ring surface microstructures were produced using Electrochemical Machining by closed electrolytic free Jet (Jet-ECM). The subsequent tribometric measurements determined that patterned spherical-segment cavities realize a friction coefficient reduction of up to 27%. Acknowledgements The Cluster of Excellence ‘‘Energy-Efficient Product and Process Innovations in Production Engineering’’ (eniPROD) is funded by the European Union (European Regional Development Fund) and the Free State of Saxony. References
Fig. 14. Comparison of the friction coefficient of unstructured and structured surfaces for a normal force of 40 N (a) and a normal force of 280 N (b).
friction coefficient of 0.08 at the smallest test speed of 0.3 m/s. Friction coefficient increased to 0.2 when speed was increased to 1 m/s. However, upon further increase of test speed to 6 m/s, excessive vibration and a sharp increase of the friction coefficient was detected. In consequence, the test had to be interrupted at this point. Contrariwise, no vibrations were observed with the microstructured samples. In this case, the sliding pairs operated smoothly over the test speed range (0.3– 6 m/s) and the standard deviation of the friction coefficient was approximately half of the deviation of the unstructured sliding pairs. Despite a smoother running, the friction coefficient of the microstructured pair with the highest pattern density (30%) resulted in a 45% increase of the friction coefficient in relation to the unstructured pair. A reduction in friction coefficient of up to 27% was possible with the lowest pattern density tested (10%).
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CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj
Manufacturing of surface microstructures for improved tribological efficiency of powertrain components and forming tools A. Schubert a,b, R. Neugebauer a,b, D. Sylla a,*, M. Avila a, M. Hackert a a b
Institute for Machine Tools and Production Processes, Chemnitz University of Technology, Reichenhainer Str. 70, 09107 Chemnitz, Germany Fraunhofer Institute for Machine Tools and Forming Technology IWU, Reichenhainer Str. 88, 09126 Chemnitz, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 26 February 2011
The performance and energy efficiency of mechanical components is strongly influenced by the tribological behaviour of their surfaces. This paper investigates the design and manufacturing of microstructured surfaces for improved wear resistance of forming tools and reduced friction in powertrain components. The potential of microstructures in the improvement of adhesion strength and resistance to delamination of hard coatings under the severe thermomechanical service conditions of hot forging tool surfaces is discussed. In the case of powertrain components, the ability of surface structures to reduce friction between lubricated bearing surfaces was investigated numerically and experimentally. The COMSOL Multiphysics Computational Fluid Dynamics (CFD) package was used to estimate the effect of geometric parameters of patterned spherical-segment cavities on hydrodynamic pressure. Electrochemical Machining by closed-electrolytic-free Jet (Jet-ECM) was used to manufacture the microstructures. Tribological ring-on-disc tests with structured surfaces were performed. It was determined that surface microstructures in the form of patterned spherical-segment cavities generate additional lift pressure which allows the surfaces to reach hydrodynamic lubrication at lower operating speeds. ß 2011 CIRP.
Keywords: Energy efficiency Tribology Surface texturing Microstructures Forging tool Coating Powertrain Bearing Laser ablation Electrochemical machining Jet-ECM
1. Introduction Manufacturing technology and powertrain development in recent years have placed increased emphasis on energy efficiency. This is motivated by rising fuel prices, limited natural resources and CO2 emission abatement policies. Tribological behaviour has a highly significant influence on the energy and resource efficiency of both the manufacture and operation of powertrain components. Pioneering work on energy efficiency in manufacturing by Lange [1] and Berry and Fels [2] showed that the free energy change of basic material production is the most energy-intensive phase in metalworking process chains. Consequently, scrap reduction in the form of worn-out tools and material removal steps for both forming die manufacturing and component series production can yield significant energy and resources savings. Wear-resistant tools reduce the number of tools manufactured per batch and facilitate the implementation of near-net-shape forming. This later process reduces scrap during the forming process as well as the amount of subsequent material removal steps required to reach the final component specifications.
* Corresponding author. E-mail address: [email protected] (D. Sylla). 1755-5817/$ – see front matter ß 2011 CIRP. doi:10.1016/j.cirpj.2011.01.010
Coatings have been used to increase the wear resistance of tool steel in hot forging applications. Yet, poor adhesion to the substrate, high residual stresses from the coating deposition process, sharp mismatch between the thermal expansion coefficients of the hard coatings and tool steel substrates, and high manufacturing costs have limited the practicality of coated dies in the hot forging industry. However, substrate surface structures provide an opportunity to improve coating adhesion under hot forging conditions. The potential advantages are threefold: structures can promote the generation of coating-to-substrate bonding during the deposition process, create higher interfacial fracture toughness, and provide gradient material properties in the interface of the coating–substrate composite. Measures to decrease friction in powertrain components are key to the reduction of fuel consumption and emissions of automobiles. Components that show a high potential for efficiency improvements include crankshaft bearings, piston rings, cylinder liners and valve trains. Reducing friction decreases wear, improves efficiency and lengthens the service life. Numerous publications have shown that surface geometry in the form of microstructures has a considerable influence on tribological properties. Reductions in friction and wear of up to 60% have been reported [3–8]. In these studies, microstructuring was generated by laser machining. In the present investigation, Electrochemical Machining by closed-electrolyticfree Jet (Jet-ECM) was used to produce microstructured surfaces.
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Electrochemical processing has the advantage of high level of ablation precision with no mechanical nor thermal damage to the surfaces. This precludes the need for finishing because, unlike laser processing, electrochemical processing does not cause burrs or warping. As an added advantage, in contrast to conventional electrochemical machining, Jet-ECM enables localized material removal by control of the jet position without relying on a solid electrode with the negative form of the feature to be machined [9]. 2. Operating conditions of hot working tools and powertrain components and the role of surface microstructures: state of the art Although surface structuring can improve the tribological properties of hot forming tools and hydrodynamic bearings of powertrain components and achieve higher energy and resource efficiency (Fig. 1), the operating conditions in both applications are different. Surfaces of hot-working tools must withstand severe thermal and mechanical loads that are transient in nature and they typically fail within the regime of low-cycle fatigue (up to 104 cycles). On the other hand, hydrodynamic friction bearings in powertrain components are subjected to relatively low loads, with the surfaces of these bearings lasting considerably longer than 104 cycles before failing. 2.1. Hot forming tools Hot working conditions of ferrous materials require that die surfaces withstand sliding contact with hot metal at 1000–1200 8C under apparent area pressures of 155–310 MPa [10]. In critical die surface locations, however, the combination of contact stresses and steep temperature gradients far exceed the apparent area pressures. Finite Element Method studies [11,12] have estimated that thermal stresses can exceed the mechanical stresses tenfold and that the combined equivalent stress from thermal and mechanical stresses can reach 1360 MPa at die shoulders. In general, die surface temperatures temporarily exceed 600 8C upon contact with the hot workpiece, thus exceeding the tempering temperature of tool steels [13]. Forging die surfaces are also subjected to thermal shock and mechanical impact. Forging cycle frequencies range from 4 to 200 cycles per minute for hydraulic press forging and high-speed forging, respectively [14,15] and dies are usually cooled to 200–300 8C between forging cycles.
Fig. 1. Potential improvements in energy and resource efficiency by surface microstructures in the manufacturing-to-service cycle.
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To withstand such severe working conditions, materials for hot forging dies are required to have: high hot hardness to resist abrasive wear at elevated temperatures, low chill-crack sensitivity to resist thermal shock, temper resistance to avoid thermal softening, bearing capacity to resist plastic deformation, toughness to inhibit mechanically-induced cracking, chemical incompatibility with the workpiece to prevent galling and pick-up, oxidation resistance at high temperatures to prevent corrosioninduced cracking and economical applicability in a mass-production environment. The great majority of hot forming tools in industrial practice are made of hardened tool steel. A range of different surface modification methods are used to increase the wear-resistance of tool steel under hot working conditions: Thermal methods to tailor the tool steel microstructure. Thermochemical methods to alter the composition of the material at the surface. Coatings, whereby material such as hardmetals and hard alloys are deposited with a degree of diffusion bonding with the steel substrate that depends on the coating method and composition of the coating and substrate. Arc welding, thermal spray and laser cladding are the most common deposition methods for coating thicknesses above several tens of micrometers. In comparison to hardened tool steel, hardmetals (carbide phase and metallic binder, e.g. cobalt-based) are more resistant to thermal softening and oxidation. They are also chemically stable when in contact with ferrous workpieces. On the other hand, tool steels are highly resistant to fracture and plastic deformation at high temperatures and cost relatively little. Energy and costintensive monolithic hardmetal tools can be replaced by tool steel with a hardmetal coating to achieve considerable wear resistance improvements over tool steel at reasonable costs. Adequate toughness of the coatings can be attained by adjusting the composition and fraction of the metallic binder phase and by obtaining a homogeneous and pore-free microstructure. There is controversy in the literature, however, regarding the wear resistance improvements of coatings relative to surfacetreated or simply bulk-hardened tool steel at critical die surface areas such as shoulders [16,17]. Thermal-spray coatings have met with limited success in hot forging tools owing to their poor adhesion strength to the substrate. Although arc welding and laser cladding develop excellent coating adhesion strength thanks to diffusion bonding with the substrate [18], it induces – unlike thermal spraying – a heat-affected zone in the substrate. Furthermore, arc welding and laser cladding often generate tensile residual stresses that can lead to cracking and the range of materials that can be deposited by these methods is relatively restricted. To realize the potential advantages of coatings in hot forging tool applications it is important to overcome these limitations of current coating deposition methods. In order reach adequate adhesion strength, substrate surface roughening and cleaning by grit blasting has been the standard practice for preparing substrates prior to thermal spray coating deposition. Substrate surface roughness provides mechanical interlocking – the main adhesion mechanism of thermal spray coatings deposited with state-of-the-art technology [19]. However, mechanical adhesion alone may be insufficient to withstand the severe working conditions of hot forging die surfaces. There is
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opportunity to fabricate surface microstructures on tool steel substrates prior to thermal spray coating deposition that are targeted at influencing the thermal mechanisms of adhesion and the thermal and mechanical properties at the coating–substrate interface. 2.2. Powertrain components The surfaces of powertrain components are subjected to a variety of thermal and mechanical loads that depend on the operating regime. In the case of lubricated bearings, at very low speeds (start-up), surfaces are in direct contact with each other. Consequently, elevated friction and considerable wear rate take place. As speed increases, in the mixed lubrication regime, the bearing surfaces partially separate from each other thanks to pressure build-up by the lubricant. When the relative speed exceeds a certain threshold, the bearing surfaces become completely separated by the lubricant and reach the hydrodynamic lubrication regime. At this point, the only friction at play is fluid friction. Friction coefficient is minimum at the speed corresponding to the onset of the hydrodynamic regime. However, as speed increases further, the friction coefficient rises due to frictional losses within the fluid. This heats up the lubricating oil and may increase its temperature to up to 130 8C [20]. The hydrodynamic pressure increase created by surface microstructures has been studied by several researchers. Wakuda [4] observed that spherical segment cavities reduced friction by up to 20%. Hoppermann [21] showed that surface microstructures reduced the speed range of the mixed lubrication regime and lead to hydrodynamic regime at relatively low rotational speeds. Fig. 2 is an image of a cylinder surface with a saucer-shaped microstructure. This microstructure reduced oil consumption by up to 85% and fuel consumption by up to 6% [22]. Other investigations highlighting the positive effects of spherical-segment cavities on friction and wear can be found in [5,7,23–27]. However, these studies reached no uniform agreement as regards the influence of the different geometric parameters of the surface microstructures on friction and wear.
creating energy and resource-efficient process chains for forming tool manufacturing.
3.2. Methods There is the opportunity to increase the adhesion strength of thermal-spray coatings by optimizing the surface geometry of the substrate. The potential benefits of optimized surface geometries are threefold: development of coating-to-substrate diffusion bonding, increase of the delamination fracture toughness by inhibiting crack propagation along the coating–substrate interface and provide a gradual volume fraction change of the coating and substrate materials along their interface. Investigations on thermal-spray coating adhesion have proposed that surface geometry is the most important parameter in the development of coating adhesion [19,28]. Roughening of substrates prior to coating deposition is conducted in practice because adhesion is poor on smooth surfaces [29]. The improvement is attributed to the effect of surface roughness on the mechanical keying and the thermal mechanisms (i.e. diffusion bonding) of thermal spray coating adhesion [19]. In the case of High-Velocity-Oxy-Fuel (HVOF) spraying of hardmetal coatings on steel substrates, mechanical keying to the substrate is believed to be the main adhesion mechanism. Because of the low deposition temperatures of HVOF sprays in comparison to plasma spray and laser cladding, it is generally assumed that diffusion bonding does not occur. However, Sobolev et al. reported that substrate melted zones with the mentioned materials can reach a depth of 3 mm into grit-blasted steel surfaces [19]. A detrimental effect during thermal spraying is the formation of voids on the underside of splats as a result of the lateral spreading that occurs as the semi-molten particles impinge the substrate and solidify [19,28]. These voids reduce the actual coating–substrate contact area and thus bond strength. Surface asperities with an average distance that is approximately equal to the diameter of spray particles – with a typical size distribution from 10 to 60 mm – reduce the tendency of splats to spread out once they hit the
3. Surface microstructures for wear-resistant forming tools 3.1. Objective The goal is to increase the wear resistance of hot-forging tools in order to improve the energy and resource efficiency of metalworking chains. To overcome the limitations of current coating technologies for forming tools, preliminary investigations are focused on: incorporating deterministic surface structures in substrate preparation in order to influence the thermal mechanisms of thermal-spray coating adhesion and improve their adhesion under the cyclical thermal and mechanical loads encountered by hot-working tool surfaces;
Fig. 2. Cylinder liner with microstructure produced using laser honing [22].
Fig. 3. Effect of substrate roughness on the thermal adhesion mechanisms of thermal spray coatings [17,19].
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4. Surface structures for low friction and wear in powertrain components 4.1. Objective The Stribeck curve in Fig. 6 is used to illustrate the goal of microstructuring of bearing surfaces of powertrain components. In this figure, friction coefficient m is represented over the relative speed v of the sliding partners. The curve can be divided into three regimes: boundary lubrication, mixed lubrication and hydrodynamic lubrication. In the boundary and mixed regimes, friction coefficient is high and a maximum of wear is produced. The preferred regime for powertrain bearings is the hydrodynamic lubrication regime because it generates the least friction and wear on the sliding partners. The aim of surface structuring is twofold:
Fig. 4. Conceptual sketch of gradient material properties at the coating–substrate interface by deterministic surface texturing.
substrate. Sobolev [19] suggested that splats with aspects ratios close to unity may increase the duration of the thermal pulse upon impact onto the substrate and, hence, the opportunity to develop diffusion bonding (Fig. 3). Grit blasting as a substrate preparation method offers a limited range of surface geometries in comparison to deterministic material removal processes. Another drawback of grit blasting is that grit particles may remain embedded on the substrate. Grit contamination has been reported to initiate delimitation cracking of thermal spray coatings [30]. With the exception of high pressure water jets [31], no alternatives to grit blasting have been evaluated in the scientific literature regarding the development of diffusion bonding of thermal spray coatings. Furthermore, no studies exist on the effect of deterministic surface structures on the thermal mechanisms of adhesion of thermal spray coatings. With deterministic material removal processes such as milling and laser ablation, a wide range of patterned geometric features can be generated on the substrate and tailored to improve coating adhesion. Another opportunity from substrate surface structuring is the creation of gradient material properties across the substrate–coating interface that reduce misfit thermal stresses as well as increase the interfacial fracture toughness under hot forging conditions (Fig. 4). The application of milling for substrate texturing would eliminate substrate contamination by grit particles and the equipment and consumables of the grit blasting process. This novel forming die process chain is compared to the conventional approach in Fig. 5.
Fig. 5. Conventional vs. new process chains for hardfaced, hot working tool manufacturing.
Reduce the friction coefficient along the three regimes. Shift the Stribeck curve toward the left in order to overcome the boundary and mixed lubrication regimes at lower operating velocities. 4.2. Methods 4.2.1. Determining a suitable geometry for the surface microstructures The ability of surface microstructures to increase hydrodynamic pressure and the contribution of their geometric parameters to reach this goal were studied using the Computational Fluid Dynamics (CFD) package COMSOL Multiphysics. The Multiphysics model (Fig. 7) is based on the incompressible Navier–Stokes equation (1). A lubrication gap initially set at s = 20 mm exists between the static, microstructured base body and smooth counter body. The relative speed of the parallel surfaces was kept at v = 1 m/s. The intermediary substance is oil with a viscosity h of 0.79 Pa s, a density r of 887.68 kg/m3 and a temperature of 293 K.
rðvrÞv ¼ r½ pI þ hðrv þ ðrvÞT Þ þ f
(1)
where v is the fluid velocity, p is the pressure, I the identity matrix, (5v)T the transpose of (5v) and f the body forces. Condition of incompressibility: 5v = 0. The pressure profiles as a result of different diameters, distances and depths of spherical segments in an array of five
Fig. 6. Stribeck curve.
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A. Schubert et al. / CIRP Journal of Manufacturing Science and Technology 4 (2011) 200–207 Table 1 Simulated geometry conditions.
Fig. 7. Scheme of the simulation model.
cavities were calculated. The combination of geometric parameters evaluated are shown in Table 1. The results show that surface microstructures act as micro pressure chambers. Upon relative motion of the two sliding bodies the lubricant circulates in the microstructures (Fig. 8a). Consequently, a pressure peak is generated over the microstructure (Fig. 8b). The effect of geometric parameters on the hydrodynamic pressure resulting from fluid circulation is presented in Fig. 9. The diagram plots average pressure as a function of diameter d, distance a, and depth h of the cavities. It can be observed that cavity diameter has the largest influence on the pressurisation. The pressure rises linearly between a = 50 and 500 mm and reaches its highest value of 10 bar at 500 mm. The distance between adjacent spherical segments was identified as the second most important influence. In this case, it should be kept as small as possible because a low pattern density causes a decrease in pressure. An average pressure of over 7 bar can be achieved with an inter-cavity distance of 10 mm. The depth of the spherical segment had the least influence. Depths between 0 and 20 mm showed a strong rise in average pressure, of up to 2.4 bar at a depth of 20 mm. As depth is
No.
Diameter d (mm)
Depth h (mm)
Distance a (mm)
1 2 3
50. . .500 200 200
50 2. . .500 50
100 100 10. . .500
further increased, average pressure drops asymptotically to a constant value. The geometric parameters with the maximum pressure values: d = 500 mm, a = 100 mm and h = 20 mm were combined and the simulations showed that an average pressure of up to 125 bar could be reached with a lubrication gap of s = 5 mm. This value represents roughly 12% of the pressure that develops in a bearing under wedge-shaped gap conditions (approx. 1000 bar). Hence, the hydrodynamic condition could likely be reached more rapidly by use of this patterned structure. Patterned spherical-segment microstructures with the above combination of geometric parameters were fabricated and evaluated in a tribometer as described in the following sections. 4.2.2. Test materials and samples In order to measure the hydrodynamic effect of microstructures, a sufficiently large surface area is necessary. For this reason, it was decided to build a tribometric ring-on-disc test rig for the experimental investigations (Fig. 10). Only the ring counterpart featured the surface microstructures. The discs for the tribological tests consisted of heat treatable steel 42CrMo4 (1.7225) and had an outer diameter of dsa = 100 mm and a thickness of hs = 8 mm. The ring counter bodies were produced of bronze (2.1030) with an outer diameter dra = 100 mm, inner diameter dri = 70 mm and thickness hr = 10 mm. Further details are listed in Table 2. 4.2.3. Jet-ECM microstructuring Patterned spherical segments were produced by Jet-ECM. In this process, a high-velocity electrolytic jet is generated from a cathodically polarized nozzle and led toward the anodically polarized workpiece. As positively charged ions are attracted to the nozzle, the negatively charged ions are attracted to the
Fig. 8. CFD simulation of lubricant circulation in a microstructure (a); sketch of a micro-pressure chamber, after [32] (b).
Fig. 9. Effect of spherical-segment-cavity geometric parameters on hydrodynamic pressure.
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Table 2 Hardness and surface precision of the tribometric samples.
Hardness (HV) Roughness Rz (mm) Flatness (mm) Parallelism (mm)
Ring
Disc
160 1 5 10
650 1 5 10
Fig. 10. Ring-on-disc test.
workpiece and material is removed as metal ions are drawn into the electrolytic solution [9]. Initially, the Jet-ECM process parameters were determined for the production of the spherical segments (Fig. 11a). Thereafter, the ring samples were textured (Fig. 11b). Under consideration of the CFD simulation results, which indicated that, for high pressurisation, spherical segments shall be as shallow and as wide as possible, cavities were manufactured with a diameter of 500 mm and a depth of 20 mm. The array of spherical segments is illustrated in Fig. 12. Two offset rows were repeated in a circular pattern with a resulting overlap su¨ = 50 mm. The rings were microstructured with two different pattern densities (expressed as percent surface coverage area): 10% (a = 0.9 mm, a = 2.558, n = 2040 cavities) and 30% (a = 0.9 mm, a ( 0.858, n = 6120 cavities).
Fig. 12. Spherical segment cavity array.
4.2.4. Tribometric characterisation The tribometric measurements were carried out in a Wazau TRM500 tribometer (Fig. 13). The structured ring was fixed to the sample holder. The unstructured disc was affixed to the rotating platen. Ring and disc were loaded and the disc was rotated following the experiment program in Table 3. The tests were conducted under immersion lubrication with engine oil
Fig. 13. Tribometer TRM500 test rig.
Castrol EDGE 5W-30 at a temperature of 60 8C. The kinematic viscosity at this temperature was 32 mm2/s. The friction moment was measured and converted into friction coefficient. An unstructured ring-disc pair served as the reference condition. The sliding speed values were chosen so that all three lubrication regimes (boundary, mixed and hydrodynamic) were included in the measurements. 4.3. Results and discussion
Fig. 11. SEM image of the microstructured surface (a); microstructured bronze ring (b).
The friction coefficient of structured and unstructured sliding pairs as a function of speed is presented in Fig. 14. Under a normal force of 40 N (Fig. 14a), the unstructured pair showed a
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Table 3 Test parameters for the tribometrical characterisation. Motion sequence Normal force FN (N) Sliding speed v (m/s)
Continuous sliding 40; 280 0.3; 1; 6
The same results were obtained under a normal force of 280 N (Fig. 14b). A 10% pattern density results in lower friction compared to 30% pattern density. This indicates that a minimum of smooth bearing surface area is necessary to reduce friction coefficient. 5. Conclusions Energy efficiency is an important and pressing issue. Friction and wear are two factors which have a significant impact on energy requirements. Friction causes energy to be lost in the form of heat. A large amount of energy is lost due to friction-induced wear because it shortens the service life of the worn components, necessitating replacement ahead of schedule. Wear also leads to energy losses in manufacturing process chains. For these reasons, friction and wear reduction is an important goal. To this end, surface structures hold a large potential. Energy efficiency in the manufacture of components can be improved by structuring hot forging tool surfaces to increase the bond strength of wear-resistant coatings. In the service cycle, surface microstructures improve energy efficiency by reducing friction and wear. Additional hydrodynamic pressure generated by surface microstructures can lead to hydrodynamic lubrication regime at lower operating regimes where friction and wear are the lowest. A pattern of spherical-segment cavities was found to be an effective microstructure for this purpose. FEM simulations using COMSOL Multiphysics demonstrated that microstructures generate an additional hydrodynamic pressure of approx. 90 bar. The effectiveness of the structured surface was evaluated using a Wazau TRM500 tribometer in a ring-on-disc configuration. The ring surface microstructures were produced using Electrochemical Machining by closed electrolytic free Jet (Jet-ECM). The subsequent tribometric measurements determined that patterned spherical-segment cavities realize a friction coefficient reduction of up to 27%. Acknowledgements The Cluster of Excellence ‘‘Energy-Efficient Product and Process Innovations in Production Engineering’’ (eniPROD) is funded by the European Union (European Regional Development Fund) and the Free State of Saxony. References
Fig. 14. Comparison of the friction coefficient of unstructured and structured surfaces for a normal force of 40 N (a) and a normal force of 280 N (b).
friction coefficient of 0.08 at the smallest test speed of 0.3 m/s. Friction coefficient increased to 0.2 when speed was increased to 1 m/s. However, upon further increase of test speed to 6 m/s, excessive vibration and a sharp increase of the friction coefficient was detected. In consequence, the test had to be interrupted at this point. Contrariwise, no vibrations were observed with the microstructured samples. In this case, the sliding pairs operated smoothly over the test speed range (0.3– 6 m/s) and the standard deviation of the friction coefficient was approximately half of the deviation of the unstructured sliding pairs. Despite a smoother running, the friction coefficient of the microstructured pair with the highest pattern density (30%) resulted in a 45% increase of the friction coefficient in relation to the unstructured pair. A reduction in friction coefficient of up to 27% was possible with the lowest pattern density tested (10%).
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