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a-C:HMe coatings deposited by the cathodic vacuum arc deposition: properties and application potential
Surface and Coatings Technology 120–121 (1999) 709–717 www.elsevier.nl/locate/surfcoat
a-C:HMe coatings deposited by the cathodic vacuum arc deposition: properties and application potential J. Vetter a, *, A. Nevoigt b a Metaplas Ionon Oberfla¨chenveredlungstechnik GmbH, Am Bo¨ttcherberg 30–38, D-51427 Bergisch Gladbach,Germany b Institute of Fluid Power Transmission and Control (IFAS), RWTH Aachen, Steinbachstraße 53, D-52074 Aachen, Germany
Abstract a-C:HMe coatings were deposited by cathodic vacuum arc evaporation. Selected mechanical and tribological properties of the a-C:HMe coatings are presented. It is shown that the coatings exhibit a low solid-state friction, a high seizure load and a high wear resistance. Their functional behaviour was investigated for both dry and lubricated friction couples. Beside the results achieved by laboratory model tests, tests of components from socket joints, hydraulic cylinders and hydraulic piston pumps are also presented. The present investigations indicate that the a-C:HMe coatings have a high application potential in fluid power transmission systems and in dry friction couples. © 1999 Elsevier Science S.A. All rights reserved. Keywords: a-C:HMe coatings; Cathodic vacuum arc evaporation; Friction; Wear
1. Introduction New developments for wear parts used in hydraulic cylinders, pumps and gear parts include lubrication reduction, weight reduction, higher efficiency, higher specific transmission power, higher stability against bending forces, and a longer life time. Traditional contacting surfaces include nitrided or carburized steel surfaces, nickel- and chromium-coated steels, PTFEbronze or bronze. Often, traditional wear couples consist of one treated steel part sliding against a soft counter part made of PTFE-bronze or bronze, in order to reduce the adhesive wear and cold welding in regions of solidstate friction. The softer parts are sensitive to abrasive wear and deformation by bending forces. To overcome the disadvantages of traditional material couples, an increasing number of different types of hard carbon coatings have been applied for contact surfaces of wear couples. In the last two decades, there have been many reports on the excellent friction and wear properties of different types of hard carbon coatings [1– 10]. The most important types of hard carbon coatings are amorphous carbon coatings containing hydrogen (a-C:H ) deposited by plasma decomposition [3,11], pure carbon coatings a-C (i-C ) deposited by the cathodic vacuum arc evaporation [12–14] and metal-containing * Corresponding author. Tel.: +49-02204-299-266; fax: +49-02204-299-261.
carbon coatings (a-C:HMe) deposited by various deposition methods, both on the base of a-C:H and a-C(i-C ) [3,7,15,16 ]. Most of the metal-containing hard carbon coatings are deposited by evaporation or sputtering of the metal component [ Ti, Cr, W, etc.] in a reactive hydrocarbon atmosphere. The aim of this work is to show several ways of applying new material combinations, which have more stiffness against plastic deformations and are more resistant to abrasion than traditional material couples. It is important that the a-C:HMe coatings are deposited on steels with an appropriate heat treatment or on thermochemically heat-treated (nitrided, carburized, etc.) steels. The paper briefly describes the deposition procedure and selected functional related properties of the a-C:HMe coatings deposited by the cathodic vacuum arc evaporation. The main subject of this investigation is to demonstrate the excellent functional performance of wear couples in hydraulic cylinders, hydraulic pumps as well as in dry friction of socket joints if a-C:HMe coatings are deposited on steels.
2. Experimental 2.1. Deposition method The CrN, AlTiN and a-C:HMe coatings were deposited by means of reactive cathodic vacuum arc evapora-
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tion in a machine built by Metaplas Ionon. The samples were chemically cleaned and then heated by electron impact heating. The ion cleaning was carried out by the AEGD (arc-enhanced glow discharge) process in a argon plasma [17]. A brief description of the a-C:HMe deposition process is given in the following. First the cathodic vacuum arc evaporation was switched on, in order to deposit an interfacial chromium film (thickness 150±100 nm). This is necessary for sufficient adherence of the a-C:HMe coatings. Acetylene was used as the hydrocarbon gas. The coating architecture can be characterized as a gradient coating with an increasing carbon content. The deposition parameters were set to obtain a coating temperature during the deposition of the a-C:HMe coatings of about (225±25)°C. The substrates were rotated with a twofold rotation. The deposition rate of the a-C:HMe coatings was about 4 mm/h. The CrN and AlTiN coatings were deposited with coating thicknesses of about 4 mm by commercial coating processes at a temperature of 200°C. Selected properties of the CrN and AlTiN coatings were published previously [18,19]. Typical coating thicknesses of tested parts for hydraulic cylinders and pumps were about 4 mm, whereas the coating thicknesses for the socket joint balls increased to 20 mm. 2.2. Used materials for samples and components The substrates used for the Siebel–Kehl tribometer and for the test of piston guide bushes in hydraulic cylinders were manufactured from ground Ck45 (DIN1.1191) with a hardness of 580 HK (5 N ). The discs for the pin-on-disc tribometer and the inner rings (balls) for the socket joint were made from hardened 100Cr6 (DIN1.3505) with a hardness of 800 HK (5 N ). A free cutting steel 9SMnPb28 (DIN1.0718) with a hardness of 250 HK (5 N ) was used both for the outer ring of the socket joints and for the pin used in the pinon-disc test. The parts of the axial piston pump were made from a nitriding steel, which was gas-nitrided. The hardness was 850 HK (5 N ). The brittle compound zone was ground down, in order to achieve a sufficient coating adhesion. The steel St52 (DIN 1.0570), with a hardness of 150 HK (5 N ), was used for the cylinder tube. For the piston guide bushes in the hydraulic cylinder, bronze, with a hardness of 90 HK (5 N ), and PTFE-bronze were used as reference materials. 2.3. Laboratory test procedures and results 2.3.1. Microhardness and critical load The microhardness was measured with a Knoop indenter. The scratch test was carried out using a scratch test unit produced by CSEM (Revetest Scratch Adhesion
Tester) under standard conditions (diamond stylus R= 0.2 mm, speed dx/dt=10 mm/min, load rate dL/dt= 100 N/min) according to DIN VENV1071/3 [20]. The indentation tests were carried out using a Rockwell C Indenter. The microhardnesses of the a-C:HMe coatings deposited on the HSS samples were in the range of (1750±500) HK (5 N ). The scratch results showed that the destroying mechanism is characterized by a sharp transition from a range with nothing but crack generation to a range that is characterized by a more or less shell-like spallation structure. The critical loads measured on the hardened HSS [hardness: 1100 HK (5 N )] samples were in the range of (35±10) N for the 4 mm thick coating and (45±15) N for the coatings with a thickness of about 20 mm. The interpretation of the Rockwell-C indentation patterns is quite difficult. It seems that the coating reaction is based on a cohesive destroying process of the carbon-rich outer layers of the gradient coating. The classification scale of the Rockwell C indentation test starts with HF 1 (no spallations) and ends with HF 6 (total spallation) [20]. The measured values of the a-C:HMe coatings were in the range of HF2–HF3 for the 4 mm thick coating and in the range of HF3–HF4 for the 20 mm thick coatings. It should be mentioned that the coating–substrate systems had sufficient properties to fulfil the mechanical and tribological requirements for the component tests, as shown below.
2.3.2. Siebel–Kehl: lubricated tribological system The Siebel–Kehl tribometer was designed and built at the Institute of Fluid Power Transmission and Control (IFAS ) [21]. Both normal load and friction torque were measured by wire strain gauges. The upper specimen had a height of 25 mm, an outer diameter of 110 mm and an inner diameter of 74 mm and stationary. The coatings were deposited on the upper specimen. Its contact surface was pressed against a rotating base specimen with a height of 25 mm, an outer diameter of 116 mm and an inner diameter of 80 mm. The specimens were placed in a tank filled with mineral oil HLP32. The fluid temperature was 40°C, and the sliding velocity was 0.375 m/s. The value of static friction, the volumetric wear of the base sample and the seizure load were measured. The average wear heights of the base specimen were measured by a profilometer ( Talysurf 120). The wear track width follows from the diameters of the specimen (15 mm in width). The seizure load was determined by a pressure rising test. The contact pressures given in the diagrams were calculated from the applied normal load divided by the contacting area. The initial load was 0.5 N/mm2. This load was continuously increased by 2 N/mm2/h.
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Fig. 1. Friction value versus contact pressure for the couple upper specimen a-C:HMe on Ck45 and base specimen Ck45. A sharp increase in the friction value was defined as the seizure load of the wear couples: sliding velocity v=0.375 m/s, mineral oil HLP 32, temperature 40°C.
An example of the seizure load measurement is shown in Fig. 1. The graph shows the results of the couple uncoated base specimen (Ck45) against the a-C:HMecoated upper specimen (Ck45). This couple enabled a load of about 15 N/mm2, which was the highest seizure load of all investigated couples. Fig. 2 summarizes the
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most important results of the Siebel–Kehl tribometer. Fig. 2(a) shows the seizure loads. The couple hard chromium-plated upper specimen against a bronze base specimen (a standard wear couple) had a much smaller seizure load of about 7 N/mm2. Additionally, the couple CrN-coated upper specimen against an uncoated base specimen (Ck45) was investigated. The seizure load of that couple was only slightly higher than that of the couple bronze/hard chromium. Moreover, Fig. 2(b) and (c) show the value of static friction and the volumetric wear of the base specimen after a 4 h test. It should be mentioned that all coatings deposited on the upper specimen showed almost no abrasion wear: only the uncoated base specimens were worn. The highest abrasive wear was observed for the couple hard chromium sliding against bronze. The couple CrN against Ck45 had a lower wear volume due to the hardness of the steel Ck45 [hardness of 580 HK (5 N )], which is much higher than that of bronze [hardness of 90 HK (5 N )]. The lowest wear volume was measured for the couple a-C:HMe against Ck45. The values of static friction are important for the friction force gradient in the range of mixed friction. It is evident that the couple, a-C:HMe against Ck45, had the lowest static friction value. This indicates a low stick-slip tendency for this couple, which is required for tribological systems of fluid power components.
Fig. 2. Results measured on the Siebel–Kehl tribometer: (a) seizure load after pressure rising test, (b) wear volume after 4 h test, (c) value of static friction; sliding velocity v=0.375 m/s, mineral oil HLP 32, temperature 40°C, initial load 0.5 N/mm2.
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disc, is smaller by four orders of magnitude, and the a-C:HMe coating is partly worn, too. However, the total wear rate is still smaller by three orders of magnitude.
3. Component tests 3.1. Hydraulic cylinders under cross-load
Fig. 3. Friction test curves measured by pin-on-disc for uncoated steel couple and for a couple with an a-C:HMe coating on the disc.
2.3.3. Pin-on-disc: dry friction Friction values and wear rates of dry friction were investigated using a pin-on-disc tribometer. Pins used for the test had a radius of 3 mm and were made from the free cutting steel (9SMnPb28). The hardened 100Cr6 steel was used for the discs. The sliding distances were about 500 m for the uncoated disc, respectively, 1 860 000 m for the a-C:HMe-coated disc. The sliding speed was 1 m/s. A normal load of 5 N was applied. The relative humidity was about (70±10)%. Fig. 3 shows the friction duration curves. The friction value of the steel couple started at 0.83. With increasing test duration, the friction value rises to 0.97 and then drops to about 0.65 up to 500 m. The couple with the a-C:HMe-coated disc (thickness 13 mm) started with a relatively high friction value of about 0.46. With increasing test duration, this value dropped to 0.15. The reason for the relatively high starting friction value might be the high roughness of the thick a-C:H:Me coating [R =(0.5−0.7) mm]. It is clear that the friction value a increased slightly during the friction path of 1860 km. The reason is the mechanical adherence of wear debris from the uncoated pin in microspallations of the coating, as discussed in [6 ]. Fig. 4 shows the wear rate of the two couples. Typical adhesive wear of the pin material was found on the disc wear track, if the pin was sliding against the uncoated disc. The pin wear of the couple, uncoated pin-coated
Fig. 4. Wear rates measured by pin-on-disc for uncoated steel couple and for a couple with an a-C:HMe coating on the disc.
Only a very brief description of the test system can be given here; for more details, see Ref. [22]. The cylinder tube with an inner diameter of 63 mm was manufactured from low-carbon steel St52. The piston rods, with a length of 450 mm and a diameter of 30 mm, were manufactured from Ck45. The piston bushes had a length of 30 mm and were manufactured from bronze, Ck45 covered with a PTFE-bronze ring (thickness 2 mm) and Ck45 coated with a-C:HMe. Both piston guide bushes and piston rod guide bushes were made from similar materials and were tested separately on different test rigs. The geometry of the guides enabled hydrodynamic lubrication only. Therefore, complete separation of the contact surfaces could only be achieved at high sliding velocities (v>200 mm/s). The cross-load was varied from 0 to 300 N. According to our calculations, the effective cross-loads acting on the guide bushes were about 10 times higher than at the piston rod, where the load was applied and measured. This is a result of the leverage ( length of piston rod 450 mm, length of guide bush 30 mm). To keep this ratio constant, the load system was moved together with the piston. After a running-in period, the sliding velocity was reduced to 50 mm/s in order to operate the system in the range of mixed friction. The sliding distance per stroke was 200 mm. Friction forces, wear profiles and the tendency toward stick-slip movement were measured. Fig. 5 shows the abrasive wear under a cross-load. The piston guide bush with the PTFE-bronze ring allowed a continuous crossload of only 75 N. Higher loads led to creep effects of the soft material. The piston guide bush manufactured from bronze and the a-C:HMe-coated Ck45 were operated at a cross-load of 150 N along the whole sliding distance of 800 m (corresponding to 2000 strokes). The bronze guide bush was strongly worn, whereas the a-C:HMe-coated Ck45 showed no observable wear. It should be noted that profilometric inspections of the contact surface of the cylinder tube (cold rolled St52) sliding against the a-C:HMe coating revealed no wear even after a sliding distance of 2400 m. A similar behaviour for the different materials was also observed for the piston rod guides. Fig. 6 shows the friction force in relation to the sliding velocity at different cross-loads. The friction force results from friction at the contact surfaces of the piston guide bushes and at the rod seals. Due to the
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Fig. 5. Abrasive wear of the piston guide bushes manufactured from different materials, tube material St52.
described creep effects, the PTFE-bronze guide bushes could be operated with a cross-load of 150 N only for a very short time. Bronze piston guide bushes have a high tendency toward stick-slip effects at loads higher than 150 N. Only the piston guide bushes manufactured from Ck45 and covered with a-C:HMe coatings allowed a permanent high cross-load of 300 N. PTFE-bronze rings had the lowest friction force at a cross-load of 150 N, but, as mentioned above, such loads can only be applied temporarily, not permanently. The piston guide with the a-C:HMe coating showed a lower friction force than the bronze piston guide at a cross-load of 150 N. In order to demonstrate the different friction characteristics of the bronze and the a-C:HMe-coated guide bushes, Fig. 7 shows the friction force in relation to the
Fig. 6. Friction force of piston guide bushes under a cross-load manufactured from different materials in relation to the sliding velocity.
sliding distance. The friction force of the a-C:HMecoated guide bushes remained constant over the sliding distance of 800 m, whereas the friction force of the bronze guide bushes increased during the first 250 m and was unstable thereafter. The reason for this can be correlated to the abrasion wear, as shown in Fig. 6. Fig. 8 shows a comparison of the friction force at a cross-load of 150 N in relation to the sliding velocity for the three investigated guide systems. The a-C:HMe guide bushes have a significantly higher friction force than the PTFE-bronze guide bushes. However, the friction force at low velocities (mixed friction) is lower than that of the bronze guide bushes. If we calculate the change in friction force with the sliding velocity (friction gradient), we find that the bronze guide bushes have a much higher gradient up to about 175 mm/s than
Fig. 7. Friction force in relation to the sliding distance for piston guide bushes manufactured from bronze and a-C:HMe-coated Ck45: lubricant mineral oil HLP32, temperature 40°C, tube material St52, crossload 150 N, sliding velocity 50 mm/s, hydraulic pressure 50 bar.
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Fig. 8. Friction force versus sliding velocity and its impact on stickslip effects.
the a-C:HMe guide bushes. The gradients of the PTFEbronze guide bushes and the a-C:HMe guide bushes are very similar. It should be noted that this gradient is a hint of the stick-slip tendency at low sliding velocities. At higher sliding velocities, the hydrodynamic lubrication determines the friction force for all systems. 3.2. Axial piston pump operating with contaminated fluids Fig. 9 shows the schematic design of an axial piston pump. The purpose of this test was to investigate the pump behaviour when the hydraulic fluid is contaminated with solid particles acting as abrasive particles. The test dust was ACFTD (Air Cleaner Fine Test Dust). The particles were added to the hydraulic fluids at determined time intervals. The test procedure was according to ISO/DIS 9632. Fig. 10 describes the test procedure and shows the variation in flow rate during the test. Solid particles with a specific size distribution were added every 30 min. The critical wear couples are
Fig. 9. Schematic design of an axial piston pump showing critical wear couples and types of volumetric leakage.
Fig. 10. Relative flow rates versus operation time during the wear test for the standard pump and the modified pump I: test dust ACFTD, quantity of dust per injection 1.35 mg, hydraulic fluid HLP32, operating pressure difference 210 bar, initial low rate 45 l/min.
highlighted in Fig. 9. In the investigation, the materials of the wear couples cylinder block/valve plate (modified pump I ) and piston/guide bush (modified pump II ) were changed. The materials of the standard pump were nitrided steel for the piston, which slide inside a bronze guide bush, and nitrided steel for the cylinder block coated with bronze (1 mm thickness), which rotated onto a valve plate of nitrided steel. If the contact surfaces were worn by the abrasive medium, both the mechanical efficiency and the volumetric efficiency would be reduced. The total leakage rate, Qt, mainly results from leakage in three different systems: cylinder block/valve plate, Qv, piston/piston guide bush, Qp, and slipper/swash plate, Qs. Fig. 10 shows the reduction in flow rate during the test run, which was caused by abrasive wear of the tribological systems. In order to keep all other conditions constant, only the materials of the couple cylinder block/valve plate were changed (modified pump I ). The new material couple was a cylinder block of nitrided steel without a bronze coating and a plate valve coated with a-C:HMe. This proceeding guaranteed that the
Fig. 11. Wear of the couple cylinder drum/valve plate after the wear test carried out with the standard pump and the modified pump I.
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Fig. 12. Relative torque of new and tested pumps versus the relative operation pressure both for the standard pump and for the modified pump I: initial flow rate 47 l/min, rotation speed 1500 rpm.
leakage rates, Qp and Qs, of the modified pump I were similar to those of the standard pump. The test results illustrated in Fig. 10 show that the flow rate of the modified pump I was significantly higher than that of the standard pump (respectively, the leakage rate of the modified pump I was lower). The reason for this difference is shown in Fig. 11. The parts of the modified pump I have less wear at the outlet (pressure) port as well as at the inlet port (not shown here). The soft bronze material enables the embedment of abrasive particles, which results in a grinding effect at the valve plate surface. Due to the higher hardness, this was dramatically reduced by the new material of the wear couple. Fig. 12 demonstrates that the mechanical efficiency of the modified pump I remained constant during the whole wear test. In contrast, the torque of the standard pump increased as a result of wear especially at the system cylinder block/valve plate. Next, the system piston/piston guide bush was investigated (modified pump II ). A typical result for piston wear is shown in Fig. 13. The piston of the standard pump, was strongly worn by abrasion, although this piston was manufactured from nitrided steel and sliding against a soft bronze guide. The a-C:HMe-coated piston (base material: nitrided steel ) showed almost no wear, although the counterpart was hard nitrided steel. The explanation for this great difference might be the same as that for the couple cylinder block/valve plate. The abrasive wear particles are easily embedded in the bronze guide bush. It should be mentioned that the a-C:HMecoated guide did not generate any observable wear of the counterpart. 3.3. Socket joints — dry operation The schematic design of the socket joint is shown in Fig. 14. The inner ring has an outer diameter of 19.05 mm and a bore of 10 mm for connection with other parts. The inner rings are manufactured from hardened 100Cr6. The outer rings of the standard
Fig. 13. Wear of pistons after the wear test both of the standard pump and of the modified pump II.
Fig. 14. Schematic design of the socket joint.
greased socket joints are manufactured from bronze. The outer ring is mounted in a case that can be connected with other parts. The standard socket joint is filled with a lubricating grease. The goal of this investigation was to examine both the possibility of replacing the bronze material of the outer ring by the free cutting steel 9SMnPb28 and the possibility of eliminating any lubricant. This should be achieved by coating the inner ring with an a-C:HMe coating. The number of possible rotations of the socket joints are limited by the wear of the outer ring, which generates a bearing slackness. Different surface states of the inner ring were tested in special test equipment [23] built and operated by ASK
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Fig. 15. Number of rotations until the maximum value of bearing slackness measured for different ball surfaces and ring materials in comparison with a standard socket joint (once-through greasing).
Kugellagerfabrik Arthur Seifert GmbH. Fig. 15 shows the number of possible rotations for different dry operated wear couples in comparison to the standard socket joint once through greased. The wear couple uncoated inner ring rotating against the outer steel ring enabled only 3500 rotations under dry operation. A slightly greater number of rotations were achieved with the wear couple uncoated inner ring rotating against the bronze outer ring. Also, metallic hard coatings ( like AlTiN and others) rotating against a steel outer ring enabled only a low number of rotations. A solid lubricant coating of WS (with a thickness of about 1 mm) rotating against 2 a steel outer ring allowed only slightly more rotations than the metallic hard coatings. The number of rotations of the standard socket joint could be exceeded only by using carbon-based coatings. However, it was found that thick and relatively soft coatings are best suited for that test. The number of rotations was increased by a factor of 2.5 in comparison to the standard socket joint with one time greasing, if coatings with a thickness of about 20 mm were deposited on the inner rings.
couples of nitrided steel against a-C:HMe-coated nitride steel are suitable for operation with fluids that are contaminated with solid particles. Replacement of the soft bronze material by a-C:HMe-coated nitrided steel dramatically increases the wear resistance of the pumps against three-body abrasion. An important effect of the higher surface hardness is the elimination of embedment of abrasive particles in the surface, as is typical of bronze materials. This results in a strong abrasion of both the hard nitrided steel part and the soft bronze part. The reduction in wear intensity by applying a-C:HMe-coated nitrided parts is combined with a much better volumetric and mechanical efficiency after operation with contaminated fluids. The a-C:HMe coatings can not only be applied in lubricated systems as investigated for socket joints: it was found that a-C:HMe coatings deposited on the inner rings of the socket joints act as a solid lubricant coating. Thus, such socket joints can be operated without any lubrication. The results of the three-component test show that new material selections for wear couples are possible to suit the best functional behaviour in different tribosystems. The a-C:HMe coatings can be applied in a variety of tribosystems with and without lubrication. Due to their higher performance, coatings can replace traditional wear couples like steel sliding against bronze or PTFE-bronze in a large number of applications.
5. Conclusions $
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4. Discussion $
It was shown that the replacement of standard wear couples (bronze or PTFE-bronze) used in hydraulic cylinders for piston guide bushes and for piston rod guides by a-C:HMe-coated steels enables higher crossloads on the piston rod. The cross-loads applied onto soft materials like PTFE-bronze or bronze as the standard material for the guide bushes were limited due to deformation processes and wear. It was found that the a-C:HMe coating sliding against the steel tube has a better stick-slip behaviour than bronze. The low friction values and low stick-slip tendency of PTFE-bronze could not be achieved, but as mentioned above, the allowable cross-loads were much higher. The results of the wear test of axial piston pumps show that wear
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Different grades of steel (Ck 45, 100Cr6 and a gasnitrided steel ) were coated with a-C:HMe using the cathodic vacuum arc evaporation and showed excellent functional behaviour in different tribosystems. The seizure load of the couple a-C:HMe/steel, measured by means of the Siebel–Kehl tribometer, is significantly higher than the seizure load of hard chromium with bronze. The maximum thicknesses of the a-C:HMe coating can be as much as 20 mm with sufficient adhesion, as investigated in the component test of the socket joints. The wear of the counter parts is reduced, and the dry friction values are extremely low in comparison to dry friction of steel parts or steel-bronze wear couples. The tribosystems have emergency running properties for a certain time when a-C:HMe coatings are deposited at the surface of wear couples. The a-C:HMe coatings have excellent functional properties in hydraulic cylinders as coatings for piston guide bushes operating under high cross-loads, as coatings for valve plates and pistons in axial piston pumps operating with contaminated fluids and for dry bearings like socket joints.
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Acknowledgements This work was in part supported by BMBF of Germany under contract numbers 13N6223 and 13N6783. The authors are thankful for experimental assistance from Herrn Schmidt-Mauer from Metaplas Ionon, Herrn Liebe and Herrn Krause from ASK Kugellagerfabrik Arthur Seyfert GmbH, KorntalMu¨nchingen, for surface analysis done by the staff of GFE Gemeinschaftslabor fuer Elektronenmikroskopie, Aachen and for technical support from Mr Braeckelmann and Mr Schmitte, Montanhydraulik GmbH, Holzwickede.
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a-C:HMe coatings deposited by the cathodic vacuum arc deposition: properties and application potential J. Vetter a, *, A. Nevoigt b a Metaplas Ionon Oberfla¨chenveredlungstechnik GmbH, Am Bo¨ttcherberg 30–38, D-51427 Bergisch Gladbach,Germany b Institute of Fluid Power Transmission and Control (IFAS), RWTH Aachen, Steinbachstraße 53, D-52074 Aachen, Germany
Abstract a-C:HMe coatings were deposited by cathodic vacuum arc evaporation. Selected mechanical and tribological properties of the a-C:HMe coatings are presented. It is shown that the coatings exhibit a low solid-state friction, a high seizure load and a high wear resistance. Their functional behaviour was investigated for both dry and lubricated friction couples. Beside the results achieved by laboratory model tests, tests of components from socket joints, hydraulic cylinders and hydraulic piston pumps are also presented. The present investigations indicate that the a-C:HMe coatings have a high application potential in fluid power transmission systems and in dry friction couples. © 1999 Elsevier Science S.A. All rights reserved. Keywords: a-C:HMe coatings; Cathodic vacuum arc evaporation; Friction; Wear
1. Introduction New developments for wear parts used in hydraulic cylinders, pumps and gear parts include lubrication reduction, weight reduction, higher efficiency, higher specific transmission power, higher stability against bending forces, and a longer life time. Traditional contacting surfaces include nitrided or carburized steel surfaces, nickel- and chromium-coated steels, PTFEbronze or bronze. Often, traditional wear couples consist of one treated steel part sliding against a soft counter part made of PTFE-bronze or bronze, in order to reduce the adhesive wear and cold welding in regions of solidstate friction. The softer parts are sensitive to abrasive wear and deformation by bending forces. To overcome the disadvantages of traditional material couples, an increasing number of different types of hard carbon coatings have been applied for contact surfaces of wear couples. In the last two decades, there have been many reports on the excellent friction and wear properties of different types of hard carbon coatings [1– 10]. The most important types of hard carbon coatings are amorphous carbon coatings containing hydrogen (a-C:H ) deposited by plasma decomposition [3,11], pure carbon coatings a-C (i-C ) deposited by the cathodic vacuum arc evaporation [12–14] and metal-containing * Corresponding author. Tel.: +49-02204-299-266; fax: +49-02204-299-261.
carbon coatings (a-C:HMe) deposited by various deposition methods, both on the base of a-C:H and a-C(i-C ) [3,7,15,16 ]. Most of the metal-containing hard carbon coatings are deposited by evaporation or sputtering of the metal component [ Ti, Cr, W, etc.] in a reactive hydrocarbon atmosphere. The aim of this work is to show several ways of applying new material combinations, which have more stiffness against plastic deformations and are more resistant to abrasion than traditional material couples. It is important that the a-C:HMe coatings are deposited on steels with an appropriate heat treatment or on thermochemically heat-treated (nitrided, carburized, etc.) steels. The paper briefly describes the deposition procedure and selected functional related properties of the a-C:HMe coatings deposited by the cathodic vacuum arc evaporation. The main subject of this investigation is to demonstrate the excellent functional performance of wear couples in hydraulic cylinders, hydraulic pumps as well as in dry friction of socket joints if a-C:HMe coatings are deposited on steels.
2. Experimental 2.1. Deposition method The CrN, AlTiN and a-C:HMe coatings were deposited by means of reactive cathodic vacuum arc evapora-
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tion in a machine built by Metaplas Ionon. The samples were chemically cleaned and then heated by electron impact heating. The ion cleaning was carried out by the AEGD (arc-enhanced glow discharge) process in a argon plasma [17]. A brief description of the a-C:HMe deposition process is given in the following. First the cathodic vacuum arc evaporation was switched on, in order to deposit an interfacial chromium film (thickness 150±100 nm). This is necessary for sufficient adherence of the a-C:HMe coatings. Acetylene was used as the hydrocarbon gas. The coating architecture can be characterized as a gradient coating with an increasing carbon content. The deposition parameters were set to obtain a coating temperature during the deposition of the a-C:HMe coatings of about (225±25)°C. The substrates were rotated with a twofold rotation. The deposition rate of the a-C:HMe coatings was about 4 mm/h. The CrN and AlTiN coatings were deposited with coating thicknesses of about 4 mm by commercial coating processes at a temperature of 200°C. Selected properties of the CrN and AlTiN coatings were published previously [18,19]. Typical coating thicknesses of tested parts for hydraulic cylinders and pumps were about 4 mm, whereas the coating thicknesses for the socket joint balls increased to 20 mm. 2.2. Used materials for samples and components The substrates used for the Siebel–Kehl tribometer and for the test of piston guide bushes in hydraulic cylinders were manufactured from ground Ck45 (DIN1.1191) with a hardness of 580 HK (5 N ). The discs for the pin-on-disc tribometer and the inner rings (balls) for the socket joint were made from hardened 100Cr6 (DIN1.3505) with a hardness of 800 HK (5 N ). A free cutting steel 9SMnPb28 (DIN1.0718) with a hardness of 250 HK (5 N ) was used both for the outer ring of the socket joints and for the pin used in the pinon-disc test. The parts of the axial piston pump were made from a nitriding steel, which was gas-nitrided. The hardness was 850 HK (5 N ). The brittle compound zone was ground down, in order to achieve a sufficient coating adhesion. The steel St52 (DIN 1.0570), with a hardness of 150 HK (5 N ), was used for the cylinder tube. For the piston guide bushes in the hydraulic cylinder, bronze, with a hardness of 90 HK (5 N ), and PTFE-bronze were used as reference materials. 2.3. Laboratory test procedures and results 2.3.1. Microhardness and critical load The microhardness was measured with a Knoop indenter. The scratch test was carried out using a scratch test unit produced by CSEM (Revetest Scratch Adhesion
Tester) under standard conditions (diamond stylus R= 0.2 mm, speed dx/dt=10 mm/min, load rate dL/dt= 100 N/min) according to DIN VENV1071/3 [20]. The indentation tests were carried out using a Rockwell C Indenter. The microhardnesses of the a-C:HMe coatings deposited on the HSS samples were in the range of (1750±500) HK (5 N ). The scratch results showed that the destroying mechanism is characterized by a sharp transition from a range with nothing but crack generation to a range that is characterized by a more or less shell-like spallation structure. The critical loads measured on the hardened HSS [hardness: 1100 HK (5 N )] samples were in the range of (35±10) N for the 4 mm thick coating and (45±15) N for the coatings with a thickness of about 20 mm. The interpretation of the Rockwell-C indentation patterns is quite difficult. It seems that the coating reaction is based on a cohesive destroying process of the carbon-rich outer layers of the gradient coating. The classification scale of the Rockwell C indentation test starts with HF 1 (no spallations) and ends with HF 6 (total spallation) [20]. The measured values of the a-C:HMe coatings were in the range of HF2–HF3 for the 4 mm thick coating and in the range of HF3–HF4 for the 20 mm thick coatings. It should be mentioned that the coating–substrate systems had sufficient properties to fulfil the mechanical and tribological requirements for the component tests, as shown below.
2.3.2. Siebel–Kehl: lubricated tribological system The Siebel–Kehl tribometer was designed and built at the Institute of Fluid Power Transmission and Control (IFAS ) [21]. Both normal load and friction torque were measured by wire strain gauges. The upper specimen had a height of 25 mm, an outer diameter of 110 mm and an inner diameter of 74 mm and stationary. The coatings were deposited on the upper specimen. Its contact surface was pressed against a rotating base specimen with a height of 25 mm, an outer diameter of 116 mm and an inner diameter of 80 mm. The specimens were placed in a tank filled with mineral oil HLP32. The fluid temperature was 40°C, and the sliding velocity was 0.375 m/s. The value of static friction, the volumetric wear of the base sample and the seizure load were measured. The average wear heights of the base specimen were measured by a profilometer ( Talysurf 120). The wear track width follows from the diameters of the specimen (15 mm in width). The seizure load was determined by a pressure rising test. The contact pressures given in the diagrams were calculated from the applied normal load divided by the contacting area. The initial load was 0.5 N/mm2. This load was continuously increased by 2 N/mm2/h.
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Fig. 1. Friction value versus contact pressure for the couple upper specimen a-C:HMe on Ck45 and base specimen Ck45. A sharp increase in the friction value was defined as the seizure load of the wear couples: sliding velocity v=0.375 m/s, mineral oil HLP 32, temperature 40°C.
An example of the seizure load measurement is shown in Fig. 1. The graph shows the results of the couple uncoated base specimen (Ck45) against the a-C:HMecoated upper specimen (Ck45). This couple enabled a load of about 15 N/mm2, which was the highest seizure load of all investigated couples. Fig. 2 summarizes the
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most important results of the Siebel–Kehl tribometer. Fig. 2(a) shows the seizure loads. The couple hard chromium-plated upper specimen against a bronze base specimen (a standard wear couple) had a much smaller seizure load of about 7 N/mm2. Additionally, the couple CrN-coated upper specimen against an uncoated base specimen (Ck45) was investigated. The seizure load of that couple was only slightly higher than that of the couple bronze/hard chromium. Moreover, Fig. 2(b) and (c) show the value of static friction and the volumetric wear of the base specimen after a 4 h test. It should be mentioned that all coatings deposited on the upper specimen showed almost no abrasion wear: only the uncoated base specimens were worn. The highest abrasive wear was observed for the couple hard chromium sliding against bronze. The couple CrN against Ck45 had a lower wear volume due to the hardness of the steel Ck45 [hardness of 580 HK (5 N )], which is much higher than that of bronze [hardness of 90 HK (5 N )]. The lowest wear volume was measured for the couple a-C:HMe against Ck45. The values of static friction are important for the friction force gradient in the range of mixed friction. It is evident that the couple, a-C:HMe against Ck45, had the lowest static friction value. This indicates a low stick-slip tendency for this couple, which is required for tribological systems of fluid power components.
Fig. 2. Results measured on the Siebel–Kehl tribometer: (a) seizure load after pressure rising test, (b) wear volume after 4 h test, (c) value of static friction; sliding velocity v=0.375 m/s, mineral oil HLP 32, temperature 40°C, initial load 0.5 N/mm2.
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disc, is smaller by four orders of magnitude, and the a-C:HMe coating is partly worn, too. However, the total wear rate is still smaller by three orders of magnitude.
3. Component tests 3.1. Hydraulic cylinders under cross-load
Fig. 3. Friction test curves measured by pin-on-disc for uncoated steel couple and for a couple with an a-C:HMe coating on the disc.
2.3.3. Pin-on-disc: dry friction Friction values and wear rates of dry friction were investigated using a pin-on-disc tribometer. Pins used for the test had a radius of 3 mm and were made from the free cutting steel (9SMnPb28). The hardened 100Cr6 steel was used for the discs. The sliding distances were about 500 m for the uncoated disc, respectively, 1 860 000 m for the a-C:HMe-coated disc. The sliding speed was 1 m/s. A normal load of 5 N was applied. The relative humidity was about (70±10)%. Fig. 3 shows the friction duration curves. The friction value of the steel couple started at 0.83. With increasing test duration, the friction value rises to 0.97 and then drops to about 0.65 up to 500 m. The couple with the a-C:HMe-coated disc (thickness 13 mm) started with a relatively high friction value of about 0.46. With increasing test duration, this value dropped to 0.15. The reason for the relatively high starting friction value might be the high roughness of the thick a-C:H:Me coating [R =(0.5−0.7) mm]. It is clear that the friction value a increased slightly during the friction path of 1860 km. The reason is the mechanical adherence of wear debris from the uncoated pin in microspallations of the coating, as discussed in [6 ]. Fig. 4 shows the wear rate of the two couples. Typical adhesive wear of the pin material was found on the disc wear track, if the pin was sliding against the uncoated disc. The pin wear of the couple, uncoated pin-coated
Fig. 4. Wear rates measured by pin-on-disc for uncoated steel couple and for a couple with an a-C:HMe coating on the disc.
Only a very brief description of the test system can be given here; for more details, see Ref. [22]. The cylinder tube with an inner diameter of 63 mm was manufactured from low-carbon steel St52. The piston rods, with a length of 450 mm and a diameter of 30 mm, were manufactured from Ck45. The piston bushes had a length of 30 mm and were manufactured from bronze, Ck45 covered with a PTFE-bronze ring (thickness 2 mm) and Ck45 coated with a-C:HMe. Both piston guide bushes and piston rod guide bushes were made from similar materials and were tested separately on different test rigs. The geometry of the guides enabled hydrodynamic lubrication only. Therefore, complete separation of the contact surfaces could only be achieved at high sliding velocities (v>200 mm/s). The cross-load was varied from 0 to 300 N. According to our calculations, the effective cross-loads acting on the guide bushes were about 10 times higher than at the piston rod, where the load was applied and measured. This is a result of the leverage ( length of piston rod 450 mm, length of guide bush 30 mm). To keep this ratio constant, the load system was moved together with the piston. After a running-in period, the sliding velocity was reduced to 50 mm/s in order to operate the system in the range of mixed friction. The sliding distance per stroke was 200 mm. Friction forces, wear profiles and the tendency toward stick-slip movement were measured. Fig. 5 shows the abrasive wear under a cross-load. The piston guide bush with the PTFE-bronze ring allowed a continuous crossload of only 75 N. Higher loads led to creep effects of the soft material. The piston guide bush manufactured from bronze and the a-C:HMe-coated Ck45 were operated at a cross-load of 150 N along the whole sliding distance of 800 m (corresponding to 2000 strokes). The bronze guide bush was strongly worn, whereas the a-C:HMe-coated Ck45 showed no observable wear. It should be noted that profilometric inspections of the contact surface of the cylinder tube (cold rolled St52) sliding against the a-C:HMe coating revealed no wear even after a sliding distance of 2400 m. A similar behaviour for the different materials was also observed for the piston rod guides. Fig. 6 shows the friction force in relation to the sliding velocity at different cross-loads. The friction force results from friction at the contact surfaces of the piston guide bushes and at the rod seals. Due to the
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Fig. 5. Abrasive wear of the piston guide bushes manufactured from different materials, tube material St52.
described creep effects, the PTFE-bronze guide bushes could be operated with a cross-load of 150 N only for a very short time. Bronze piston guide bushes have a high tendency toward stick-slip effects at loads higher than 150 N. Only the piston guide bushes manufactured from Ck45 and covered with a-C:HMe coatings allowed a permanent high cross-load of 300 N. PTFE-bronze rings had the lowest friction force at a cross-load of 150 N, but, as mentioned above, such loads can only be applied temporarily, not permanently. The piston guide with the a-C:HMe coating showed a lower friction force than the bronze piston guide at a cross-load of 150 N. In order to demonstrate the different friction characteristics of the bronze and the a-C:HMe-coated guide bushes, Fig. 7 shows the friction force in relation to the
Fig. 6. Friction force of piston guide bushes under a cross-load manufactured from different materials in relation to the sliding velocity.
sliding distance. The friction force of the a-C:HMecoated guide bushes remained constant over the sliding distance of 800 m, whereas the friction force of the bronze guide bushes increased during the first 250 m and was unstable thereafter. The reason for this can be correlated to the abrasion wear, as shown in Fig. 6. Fig. 8 shows a comparison of the friction force at a cross-load of 150 N in relation to the sliding velocity for the three investigated guide systems. The a-C:HMe guide bushes have a significantly higher friction force than the PTFE-bronze guide bushes. However, the friction force at low velocities (mixed friction) is lower than that of the bronze guide bushes. If we calculate the change in friction force with the sliding velocity (friction gradient), we find that the bronze guide bushes have a much higher gradient up to about 175 mm/s than
Fig. 7. Friction force in relation to the sliding distance for piston guide bushes manufactured from bronze and a-C:HMe-coated Ck45: lubricant mineral oil HLP32, temperature 40°C, tube material St52, crossload 150 N, sliding velocity 50 mm/s, hydraulic pressure 50 bar.
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Fig. 8. Friction force versus sliding velocity and its impact on stickslip effects.
the a-C:HMe guide bushes. The gradients of the PTFEbronze guide bushes and the a-C:HMe guide bushes are very similar. It should be noted that this gradient is a hint of the stick-slip tendency at low sliding velocities. At higher sliding velocities, the hydrodynamic lubrication determines the friction force for all systems. 3.2. Axial piston pump operating with contaminated fluids Fig. 9 shows the schematic design of an axial piston pump. The purpose of this test was to investigate the pump behaviour when the hydraulic fluid is contaminated with solid particles acting as abrasive particles. The test dust was ACFTD (Air Cleaner Fine Test Dust). The particles were added to the hydraulic fluids at determined time intervals. The test procedure was according to ISO/DIS 9632. Fig. 10 describes the test procedure and shows the variation in flow rate during the test. Solid particles with a specific size distribution were added every 30 min. The critical wear couples are
Fig. 9. Schematic design of an axial piston pump showing critical wear couples and types of volumetric leakage.
Fig. 10. Relative flow rates versus operation time during the wear test for the standard pump and the modified pump I: test dust ACFTD, quantity of dust per injection 1.35 mg, hydraulic fluid HLP32, operating pressure difference 210 bar, initial low rate 45 l/min.
highlighted in Fig. 9. In the investigation, the materials of the wear couples cylinder block/valve plate (modified pump I ) and piston/guide bush (modified pump II ) were changed. The materials of the standard pump were nitrided steel for the piston, which slide inside a bronze guide bush, and nitrided steel for the cylinder block coated with bronze (1 mm thickness), which rotated onto a valve plate of nitrided steel. If the contact surfaces were worn by the abrasive medium, both the mechanical efficiency and the volumetric efficiency would be reduced. The total leakage rate, Qt, mainly results from leakage in three different systems: cylinder block/valve plate, Qv, piston/piston guide bush, Qp, and slipper/swash plate, Qs. Fig. 10 shows the reduction in flow rate during the test run, which was caused by abrasive wear of the tribological systems. In order to keep all other conditions constant, only the materials of the couple cylinder block/valve plate were changed (modified pump I ). The new material couple was a cylinder block of nitrided steel without a bronze coating and a plate valve coated with a-C:HMe. This proceeding guaranteed that the
Fig. 11. Wear of the couple cylinder drum/valve plate after the wear test carried out with the standard pump and the modified pump I.
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Fig. 12. Relative torque of new and tested pumps versus the relative operation pressure both for the standard pump and for the modified pump I: initial flow rate 47 l/min, rotation speed 1500 rpm.
leakage rates, Qp and Qs, of the modified pump I were similar to those of the standard pump. The test results illustrated in Fig. 10 show that the flow rate of the modified pump I was significantly higher than that of the standard pump (respectively, the leakage rate of the modified pump I was lower). The reason for this difference is shown in Fig. 11. The parts of the modified pump I have less wear at the outlet (pressure) port as well as at the inlet port (not shown here). The soft bronze material enables the embedment of abrasive particles, which results in a grinding effect at the valve plate surface. Due to the higher hardness, this was dramatically reduced by the new material of the wear couple. Fig. 12 demonstrates that the mechanical efficiency of the modified pump I remained constant during the whole wear test. In contrast, the torque of the standard pump increased as a result of wear especially at the system cylinder block/valve plate. Next, the system piston/piston guide bush was investigated (modified pump II ). A typical result for piston wear is shown in Fig. 13. The piston of the standard pump, was strongly worn by abrasion, although this piston was manufactured from nitrided steel and sliding against a soft bronze guide. The a-C:HMe-coated piston (base material: nitrided steel ) showed almost no wear, although the counterpart was hard nitrided steel. The explanation for this great difference might be the same as that for the couple cylinder block/valve plate. The abrasive wear particles are easily embedded in the bronze guide bush. It should be mentioned that the a-C:HMecoated guide did not generate any observable wear of the counterpart. 3.3. Socket joints — dry operation The schematic design of the socket joint is shown in Fig. 14. The inner ring has an outer diameter of 19.05 mm and a bore of 10 mm for connection with other parts. The inner rings are manufactured from hardened 100Cr6. The outer rings of the standard
Fig. 13. Wear of pistons after the wear test both of the standard pump and of the modified pump II.
Fig. 14. Schematic design of the socket joint.
greased socket joints are manufactured from bronze. The outer ring is mounted in a case that can be connected with other parts. The standard socket joint is filled with a lubricating grease. The goal of this investigation was to examine both the possibility of replacing the bronze material of the outer ring by the free cutting steel 9SMnPb28 and the possibility of eliminating any lubricant. This should be achieved by coating the inner ring with an a-C:HMe coating. The number of possible rotations of the socket joints are limited by the wear of the outer ring, which generates a bearing slackness. Different surface states of the inner ring were tested in special test equipment [23] built and operated by ASK
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Fig. 15. Number of rotations until the maximum value of bearing slackness measured for different ball surfaces and ring materials in comparison with a standard socket joint (once-through greasing).
Kugellagerfabrik Arthur Seifert GmbH. Fig. 15 shows the number of possible rotations for different dry operated wear couples in comparison to the standard socket joint once through greased. The wear couple uncoated inner ring rotating against the outer steel ring enabled only 3500 rotations under dry operation. A slightly greater number of rotations were achieved with the wear couple uncoated inner ring rotating against the bronze outer ring. Also, metallic hard coatings ( like AlTiN and others) rotating against a steel outer ring enabled only a low number of rotations. A solid lubricant coating of WS (with a thickness of about 1 mm) rotating against 2 a steel outer ring allowed only slightly more rotations than the metallic hard coatings. The number of rotations of the standard socket joint could be exceeded only by using carbon-based coatings. However, it was found that thick and relatively soft coatings are best suited for that test. The number of rotations was increased by a factor of 2.5 in comparison to the standard socket joint with one time greasing, if coatings with a thickness of about 20 mm were deposited on the inner rings.
couples of nitrided steel against a-C:HMe-coated nitride steel are suitable for operation with fluids that are contaminated with solid particles. Replacement of the soft bronze material by a-C:HMe-coated nitrided steel dramatically increases the wear resistance of the pumps against three-body abrasion. An important effect of the higher surface hardness is the elimination of embedment of abrasive particles in the surface, as is typical of bronze materials. This results in a strong abrasion of both the hard nitrided steel part and the soft bronze part. The reduction in wear intensity by applying a-C:HMe-coated nitrided parts is combined with a much better volumetric and mechanical efficiency after operation with contaminated fluids. The a-C:HMe coatings can not only be applied in lubricated systems as investigated for socket joints: it was found that a-C:HMe coatings deposited on the inner rings of the socket joints act as a solid lubricant coating. Thus, such socket joints can be operated without any lubrication. The results of the three-component test show that new material selections for wear couples are possible to suit the best functional behaviour in different tribosystems. The a-C:HMe coatings can be applied in a variety of tribosystems with and without lubrication. Due to their higher performance, coatings can replace traditional wear couples like steel sliding against bronze or PTFE-bronze in a large number of applications.
5. Conclusions $
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4. Discussion $
It was shown that the replacement of standard wear couples (bronze or PTFE-bronze) used in hydraulic cylinders for piston guide bushes and for piston rod guides by a-C:HMe-coated steels enables higher crossloads on the piston rod. The cross-loads applied onto soft materials like PTFE-bronze or bronze as the standard material for the guide bushes were limited due to deformation processes and wear. It was found that the a-C:HMe coating sliding against the steel tube has a better stick-slip behaviour than bronze. The low friction values and low stick-slip tendency of PTFE-bronze could not be achieved, but as mentioned above, the allowable cross-loads were much higher. The results of the wear test of axial piston pumps show that wear
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Different grades of steel (Ck 45, 100Cr6 and a gasnitrided steel ) were coated with a-C:HMe using the cathodic vacuum arc evaporation and showed excellent functional behaviour in different tribosystems. The seizure load of the couple a-C:HMe/steel, measured by means of the Siebel–Kehl tribometer, is significantly higher than the seizure load of hard chromium with bronze. The maximum thicknesses of the a-C:HMe coating can be as much as 20 mm with sufficient adhesion, as investigated in the component test of the socket joints. The wear of the counter parts is reduced, and the dry friction values are extremely low in comparison to dry friction of steel parts or steel-bronze wear couples. The tribosystems have emergency running properties for a certain time when a-C:HMe coatings are deposited at the surface of wear couples. The a-C:HMe coatings have excellent functional properties in hydraulic cylinders as coatings for piston guide bushes operating under high cross-loads, as coatings for valve plates and pistons in axial piston pumps operating with contaminated fluids and for dry bearings like socket joints.
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Acknowledgements This work was in part supported by BMBF of Germany under contract numbers 13N6223 and 13N6783. The authors are thankful for experimental assistance from Herrn Schmidt-Mauer from Metaplas Ionon, Herrn Liebe and Herrn Krause from ASK Kugellagerfabrik Arthur Seyfert GmbH, KorntalMu¨nchingen, for surface analysis done by the staff of GFE Gemeinschaftslabor fuer Elektronenmikroskopie, Aachen and for technical support from Mr Braeckelmann and Mr Schmitte, Montanhydraulik GmbH, Holzwickede.
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