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The lubricant-coating interaction in rolling and sliding contacts
ARTICLE IN PRESS Tribology International 42 (2009) 554– 560
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
The lubricant-coating interaction in rolling and sliding contacts Remigiusz Michalczewski , Witold Piekoszewski, Marian Szczerek, Waldemar Tuszynski Tribology Department, Institute for Sustainable Technologies—National Research Institute (ITeE-PIB), ul. K. Pulaskiego 6/10, 26-600 Radom, Poland
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 September 2007 Received in revised form 28 April 2008 Accepted 4 May 2008 Available online 20 June 2008
The research presented in this paper was aimed at elaboration of a new technology for heavy-loaded machine elements, lubricated with ecological oils. The tribological experiments were performed using four-ball tester (scuffing resistance), cone-three balls pitting tester (fatigue life), as well as gear test rig (resistance of lubricated gears to scuffing). The tribosystems were lubricated with various base oil and vegetable-based eco-oil. The tested components were coated with standard single coatings (TiN, CrN) and low-friction coatings (a-C:H:W, MoS2/Ti). The results obtained confirm that low-friction a-C:H:W coating has a great potential for application in heavy-loaded machine components. Under extreme-pressure conditions this coating can take over the functions of anti-wear/extreme-pressure (AW/EP) additives and through this it is possible to minimise the application of toxic lubricating additives and achieve ‘‘ecological lubrication’’. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Coating Scuffing Pitting
1. Introduction Increasing attention to environmental issues and more restrictive environmental regulations are driving the lubricant industry to increase the ecological friendliness of its products. Environmental issues are also of concern for lubricants used in heavy-loaded friction pairs like gears. The durability of heavy-loaded machine components working in non-conformal contacts depends on two phenomena: scuffing of mating elements and rolling contact fatigue—pitting. So, the crucial feature of environmentally friendly lubricants is their effective lubricating action under extreme-pressure working conditions. It is well known that ecolubricants, because of the lack of effective additives, do not exhibit satisfactory anti-wear action in heavy-loaded contacts [1]. To promote the application of lubricants without environmentally hazardous additives the authors postulate a new concept: taking over the function of lubricating additives by thin, hard coatings deposited on sliding elements. There are no guidelines for selection of coatings in the case of machine components subjected to extreme conditions. In spite of successful solution of technological problems, concerning the elaboration of low-temperature processes, surface coating technologies are still in the very first phase of application in heavy-loaded contacts, like gears and rolling bearings. Nowadays, thin hard coatings are widely applied in cutting and forging processes to increase the tool life. In comparison with cutting tools, the number of heavy-loaded mechanical components is used in gigantic, while the proportion that is coated is extremely small. In contrast to
Corresponding author.
E-mail address: [email protected] (R. Michalczewski). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.05.001
cutting and forming tools, none of the surfaces in machine element contacts should be subjected to deformation or wear. A machine component contact should preferably display low friction and wear. When a machine is new the contacting surfaces are rough and the contact pressure at the surface irregularities can be very high. Thus, micro-scale wear and plastic deformations initially take place. After some time in service, the steel surfaces wear to fit each other perfectly and the wear rate is significantly lower. So, the coatings should also be able to perform run-in processes. The other problem is that the coatings used today are known not to interact chemically with lubricants or their additives in the way metals do. This is because lubricants and their active additives were once developed to form protective films, by physical and chemical reactions, not on coatings but on contacting metals (mainly steel). The tribological behaviour of DLC-coated components in DLC/ DCL and steel/DLC contacts lubricated with various ecological oils was a subject of many studies [2–6]. The authors present some results of tribological investigations of coated elements lubricated with various types of base oils and oils with anti-wear/extreme-pressure (AW/EP) additives, subjected to test conditions which cause scuffing and pitting.
2. Experimental set-up 2.1. The coatings Two kinds of thin hard coatings were deposited: standard single coatings and low-friction coatings. The properties of investigated coatings are summarised in Table 1.
ARTICLE IN PRESS R. Michalczewski et al. / Tribology International 42 (2009) 554–560
Standard single TiN and CrN coatings were deposited by the arc-vacuum method (PVD—physical vapour deposition), at the Surface Engineering Department of ITeE-PIB. The a-C:H:W (also denoted as WC/C) coating is DLC type representing a-C:H:Me group [7]. The coating consisted of an elemental Cr adhesion layer adjacent to the steel substrate, followed by an intermediate transition region consisting of alternating lamellae of Cr and WC, and an outermost hydrocarbon layer doped with W. The coating was deposited using PVD process by reactive sputtering by Oerlikon Balzers Ltd., Poland. The MoS2/Ti (commercial name MoST) coating was deposited by Teer Coatings Ltd., UK. The MoS2/Ti coating was deposited by DC magnetron sputtering using a CFUBMSIP process (closed field unbalanced magnetron sputter ion plating) [8]. 2.2. Lubricants The tribosystems were lubricated with a mineral base oil (denoted RL-144/4 with viscosity at 100 1C–7.2 mm2/s), synthetic base oils (PAO-8 and PAG-8—viscosity at 100 1C was 8 mm2/s) and vegetable base oils (rapeseed and sunflower oils—viscosity at 100 1C was 7.5 mm2/s). The mineral base oil was also blended with a commercial package of AW additives at 3% (wt.) concentration, and EP additives at 5%. AW additives were based on zinc dialkyldithiophosphate (ZDDP). EP additives contained organic S–P compounds. Also a commercial automotive gear oil of GL-5 performance level (viscosity at 100 1C was 13.9 mm2/s), as well as an eco-oil intended for lubrication of industrial transmissions (ELAS-B) were used (viscosity at 100 1C was 7.9 mm2/s). The automotive gear oil contains toxic AW/EP additives. ELAS-B is fully formulated vegetable-based, environmentally friendly oil, with, especially for the purpose of this project, AW/EP additives removed. This oil has been elaborated at ITeE-PIB. 2.3. Test methods The tribological experiments were performed using T-02 fourball tester, cone-three balls pitting tester (T-03), as well as T-12 gear test rig (FZG type). These tribotesters have been engineered and manufactured by the Institute for Sustainable Technologies National Research Institute (ITeE-PIB). The scuffing resistance was measured using the T-02 four-ball tester using the method described in [9]. Scuffing resistance was characterised by the means of the scuffing load (Pt) defined as the load at which a sudden rise in the friction torque occurs during a run at continuously increasing load. The higher Pt value, the better action of the tested tribosystem. The test balls were chrome alloy bearing steel, with a diameter of 12.7 mm, surface roughness Ra ¼ 0.032 mm and hardness 60–65 HRC. In scuffing tests all balls were coated. The fatigue life (pitting) was characterised by the 10% fatigue life L10. The test method is described in [10]. The value of L10 represents the life at which 10% of a large number of test cones,
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lubricated with the tested lubricant, would be expected to have failed. L10 life was determined using the T-03 tester. The tribosystem presented in Fig. 1 consists of three lower balls (2) freely rotating in a special race (3), and driven by the top cone (1) with the investigated coating. Test conditions were 3924 N (400 kgF) load and 1450 rpm top cone speed. The contact stress is about 3 GPa, about one-third of the average hardness expressed in GPa. Pitting was detected by a special system with a vibration sensor, which automatically stopped the machine. Twenty-four top cone failures were necessary to assess the lubricant performance. The L10 life was obtained by a linear regression analysis of the trend line for each oil (Weibull analysis). The results were verified using T-12 gear test rig (FZG type), employing test conditions according to DIN 51 354 and IP 334 standards, procedure A/8,3/90. The test gears were made of casehardened 20MnCr5 steel. The surface hardness after tempering was 60–62 HRC, roughness Ra ¼ 0.3–0.7 mm. The surface was Maag-Cross hatch ground. In gear tests both gears were coated. The wear scars were analysed using scanning electron microscopy (SEM), X-ray microanalysis (EDS) and atomic force microscopy (AFM).
3. Results 3.1. The effect of coating type on scuffing and pitting The durability of highly loaded friction joints depends on the load-carrying capacity (scuffing resistance) and fatigue life (resistance to pitting). The effect of the type of the coating on the scuffing resistance and fatigue life for coated steel balls lubricated with mineral base oil is presented in Fig. 2. As can be seen, PVD coatings exhibit a satisfactory scuffing resistance. Single coatings (TiN, CrN) show a better scuffing resistance than low-friction coatings (a-C:H:W, MoS2/Ti). It has also been observed that the L10 fatigue life of TiN- and CrN-coated specimens is several times shorter than for uncoated cones. Simple coatings like TiN and CrN, in spite of their excellent behaviour in scuffing conditions, do not exhibit satisfactory resistance to cyclic stress. However, the fatigue life of specimens coated with low-friction multilayer, lamellar a-C:H:W coating, is comparable to uncoated elements.
P n
1 2 3
Fig. 1. Tribosystems employed for testing pitting: 1—cone, 2—balls, 3—race.
Table 1 The properties of the investigated coatings Coating
Name
Interlayer
Thickness (mm)
Nano hardness (GPa)
Scratch-test critical load (N)
TiN CrN a-C:H:W MoS2/Ti
TiN CrN WC/C MoST
– – Cr, WC Ti
2.0 2.0 2.0 1.2
22.3 18.6 10.8 9.8
67 102 106 101
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Scuffing load, Pt [N]
8000 7000 6000 5000 4000 3000 2000 1000 0 uncoated
TiN
uncoated
TiN
CrN
a-C:H:W
MoS2/Ti
700
Life L10 [min.]
600 500 400 300 200 100 0 CrN
a-C:H:W
MoS2/Ti
Fig. 2. Scuffing load for steel–steel and coating–coating contacts (a) and L10 fatigue life for steel–steel and coating–steel contacts (b) (mineral base oil).
Fig. 3. The pitting wear on the a-C:H:W-coated test cone (mineral base oil): (a) SEM image, (b) cross-section and (c) enlarged dashed zone.
The photographs of pits for a-C:H:W-coated cone is presented in Fig. 3. According to the elastic contact theory the maximum shear stress under the mechanical contact is found at a certain depth under the surface; much deeper than the coating thickness. Due to limited thickness, the vapour-deposited coatings do not affect the deeper located maximum stress caused by the macroscopic contact. This is why pit depths for uncoated and coated elements are similar. The results of AFM observation of worn surfaces for uncoated and a-C:H:W-coated cones are presented in Fig. 4.
It can be seen that when the parts are new, the contacting surfaces are rough, so the contact pressure at the surface irregularities can be very high. Thus, micro-scale wear and plastic deformation initially take place. In steel–steel rolling contact the rubbing surfaces become very smooth as a result of run-in processes. This reduces the stress and in turn extends fatigue life. In comparison, standard single coatings (TiN and CrN) are relatively hard and brittle, so their running-in processes are limited, stress is very high and hence the fatigue life is very short (results not presented here). For a-C:H:W low-friction coating the running-in process is more effective and the fatigue life is longer.
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Fig. 4. AFM analyses from the wear track on the test cones: (a) uncoated cone and (b) a-C:H:W-coated cone (pitting tester, mineral base oil).
3.2. The effect of the type of the oil base on scuffing and pitting The results of testing the effect of base oils (mineral, synthetic and vegetable) on scuffing and pitting of uncoated and a-C:H:Wcoated elements are presented in Fig. 5. For a-C:H:W coating the best scuffing characteristics were obtained for oil bases containing polar components (PAG-8 and vegetable oils). So, it is possible to replace the mineral base with synthetic or vegetable oils with no risk of decreasing the scuffing resistance of the tribosystem. In pitting tests the highest L10 life was obtained for the mineral base oil. For ecologically friendly vegetable bases (sunflower and rapeseed) the L10 lives were shorter than that for the mineral base but longer than that for synthetic bases (PAO-8 and PAG-8). The obtained results indicate that mineral bases can be replaced with vegetable bases in case of heavy-loaded friction joints with a-C:H:W-coated elements.
3.3. The effect of AW and EP additives on scuffing and pitting The results of testing an effect of oil additives (AW and EP) on scuffing resistance and surface fatigue life are presented in Fig. 6. The scuffing resistance of a-C:H:W-coated elements is much higher than obtained for uncoated elements lubricated with oilscontaining reactive additives. The results of reactivity of the additives can be observed in Fig. 7 where the results of SEM/EDS analyses for uncoated balls are presented.
It is well known that good anti-scuffing properties of uncoated tribosystems lubricated with S-containing oils result from e.g. FeS creation in the friction zone. In the case of a-C:H:W-coated tribosystems the coating is removed (Fig. 8). The mechanism of improving anti-scuffing properties by chemical modification of the surface layer is replaced with the more effective anti-scuffing mechanism of preventing the creation of adhesive bonds. However, a beneficial tribochemical action of lubricating additives cannot be excluded. One of the explanations of the beneficial interaction between a-C:H:W coating and oil-containing EP additives is the formation of a new class of a tribofilm. For mineral base oil the fatigue life for uncoated and a-C:H: W-coated cones is similar. AW additive in the base oil extends the time to pitting failure of uncoated tribosystems, but reduces the life of a-C:H:W-coated specimens. EP additives significantly reduce the fatigue life for the steel–steel tribosystem. According to previous study [11] a small concentration of EP additives gives practically no influence on the fatigue life, but their increasing content significantly accelerates pitting. Their deleterious effect may be attributed rather to high corrosion aggressiveness which leads to creation on the lubricated surfaces numerous depressions and micropits, being potential nuclei for ‘‘macropits’, and in this way reducing the fatigue life. For the a-C:H:W-coated specimens their effect is similar to those lubricated with oil-containing AW additives. a-C:H:W-coated specimens, lubricated with the mineral base oil, show a much higher fatigue life than those obtained for the steel–steel tribosystem lubricated with oils-containing EP
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8000
Scuffing load, Pt [N]
7000
uncoated a-C:H:W
6000 5000 4000 3000 2000 1000 0 mineral base
synthetic base (PAO-8)
synthetic base (PAG-8)
vegetable base vegetable base (rapeseed) (sunflower)
800 uncoated a-C:H:W
Life L10 [min.]
700 600 500 400 300 200 100 0 mineral base
synthetic base (PAO-8)
synthetic base (PAG-8)
vegetable base vegetable base (rapeseed) (sunflower)
Fig. 5. Scuffing load for steel–steel and a-C:H:W–a-C:H:W contacts (a) and L10 fatigue life for steel–steel and a-C:H:W–steel contacts (b) for tribosystems lubricated with various base oils.
additives. This confirms the suitability of a-C:H:W coatings for application on heavy-loaded machine components lubricated by an oil without toxic additives.
Scuffing load, Pt [N]
8000 7000 6000 5000
base oil base oil + AW base oil + EP
3.4. Results of gear tests
4000 3000 2000 1000 0 uncoated
1200
base oil base oil + AW base oil + EP
1000 Life L10 [min.]
a-C:H:W
800 600 400 200
The idea of taking over the functions of AW/EP additives by a thin hard coating, was verified during gear testing of the eco-oil (ELAS-B) using a-C:H:W-coated test gears. The results were compared with those obtained for uncoated, steel test gears lubricated by the commercial gear oil of GL-5 performance level, which contains toxic AW/EP additives (Fig. 9). The failure load stage obtained for a-C:H:W-coated test gears lubricated by the eco-oil without any AW/EP additives is comparable to that given by the commercial gear oils-containing toxic AW/EP additives. Also the vibration level is kept at a similar level. However, for the coated gears the operating temperature was lower showing the benefits from using coated gears. Similar conclusions were presented in [12]. So, it has been shown here that under extreme-pressure conditions low-friction coatings can take over the functions of AW/EP additives and make it possible to use ecological oils for lubrication.
0 uncoated
a-C:H:W
Fig. 6. Scuffing load for steel–steel and a-C:H:W–a-C:H:W contacts (a) and L10 fatigue life for steel–steel and a-C:H:W–steel contacts (b) for tribosystems lubricated with different additives blended with the mineral base oil.
4. Conclusions Thin coatings deposited on the surface of machine components improve their scuffing resistance. What is more, the presence of
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Fig. 7. SEM images of wear scars as well as Fe, S and P maps for uncoated steel specimens (four-ball apparatus, mineral oil with EP additive).
Fig. 8. SEM images of wear scars as well as Fe, S and P maps for a-C:H:W-coated tribosystem (four-ball apparatus, mineral oil with EP additive).
10
150
8
140
Oil temperature [°C]
Failure stage
12
6 4 2
uncoated (GL-5 oil) a-C:H:W coated (Eco-oil)
130 120 110 100 90 80
0 a-C:H:W coated (Eco-oil)
Vibration amplitude (RMS) [m/s2 ]
uncoated (GL-5 oil)
4
5
6
7
8 9 Load stage
10
11
12
140 120 100 80 60 40 20
uncoated (GL-5 oil) a-C:H:W coated (Eco-oil)
0 5000 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 Frequency [Hz]
Fig. 9. Results from gear tests: (a) failure load stage, (b) oil temperature and (c) vibration amplitude at the 8th stage, for uncoated gears lubricated with GL-5 oil and a-C:H:W-coated gears lubricated with eco-oil.
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thin coatings on the surface of machine components gives the possibility of eliminating or reducing the content of toxic AW/EP additives in lubricating oils, as well as using ecological oils made of renewable resources, without any risk of a scuffing failure. However, a factor still limiting the scope of application of some coatings is their poor performance under conditions of cyclic contact stress, which leads to accelerated fatigue failures (pitting). This concerns typical PVD coatings (like TiN) and radically limits the area of application of such coatings to machine components subjected only to scuffing. In situations where resistance to pitting is an important factor (gears, rolling bearings) the application of these coatings is still limited. A satisfactory fatigue behaviour was observed only for the a-C:H:W coating, making it a strong candidate for application on machine components. References [1] Bartz WJ. Lubricants and the environment. Tribol Int 1998;31:35–47. [2] Kalin M, Vizintin J. A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contacts when lubricated with mineral and biodegradable oils. Wear 2006;261:22–31.
[3] Kalin M, Vizintin J. Tribological performance of titanium doped and pure DLC coatings combined with a synthetic bio-lubricant. Wear 2006;261:9–14. [4] Kalin M, Vizintin J, Vercammen K, Barriga J, Arnsek A. The lubrication of DLC coatings with mineral and biodegradable oils having different polar and saturation characteristics. Wear 2006;200:4515–22. [5] Vercammen K, Van Acker K, Vanhulsel A, Barriga J, Arnsek A, Kalin M, et al. Tribological behaviour of DLC coatings in combination with biodegradable lubricants. Tribol Int 2004;37:983–9. [6] Kalin M, Majdic F, Vizintin J, Pezdirnik J, Velkavrh I. Analyses of the long-term performance and tribological behaviour of an axial piston pump using diamond-like-coated piston shoes and biodegradable oil. J Tribol 2008;130: 011001–014501-4. [7] Jiang JC, Meng WJ, Evans AG, Cooper CV. Surf Coat Technol 2003;176:50–6. [8] Amaro R, Martins R, Seabra J, Renevier NM, Teer DG. Molybdenum disulphide/ titanium low friction coating for gears application. Tribol Int 2005;38:423–34. [9] Piekoszewski W, Szczerek M, Tuszynski W. The action of lubricants under extreme pressure conditions in a modified four-ball tester. Wear 2001;249: 188–93. [10] Michalczewski R, Piekoszewski W. The method for assessment of rolling contact fatigue of PVD/CVD coated elements in lubricated contacts. Tribol Finnish J Tribol 2006;25:34–43. [11] Tuszynski W. An effect of lubricating additives on tribochemical phenomena in a rolling steel four-ball contact. Tribol Lett 2006;24(3):207–15. [12] Kalin M, Vizintin J. The tribological performance of DLC-coated gears lubricated with biodegradable oil in various pinion/gear material combinations. Wear 2005;259:1270–80.
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
The lubricant-coating interaction in rolling and sliding contacts Remigiusz Michalczewski , Witold Piekoszewski, Marian Szczerek, Waldemar Tuszynski Tribology Department, Institute for Sustainable Technologies—National Research Institute (ITeE-PIB), ul. K. Pulaskiego 6/10, 26-600 Radom, Poland
a r t i c l e in f o
a b s t r a c t
Article history: Received 14 September 2007 Received in revised form 28 April 2008 Accepted 4 May 2008 Available online 20 June 2008
The research presented in this paper was aimed at elaboration of a new technology for heavy-loaded machine elements, lubricated with ecological oils. The tribological experiments were performed using four-ball tester (scuffing resistance), cone-three balls pitting tester (fatigue life), as well as gear test rig (resistance of lubricated gears to scuffing). The tribosystems were lubricated with various base oil and vegetable-based eco-oil. The tested components were coated with standard single coatings (TiN, CrN) and low-friction coatings (a-C:H:W, MoS2/Ti). The results obtained confirm that low-friction a-C:H:W coating has a great potential for application in heavy-loaded machine components. Under extreme-pressure conditions this coating can take over the functions of anti-wear/extreme-pressure (AW/EP) additives and through this it is possible to minimise the application of toxic lubricating additives and achieve ‘‘ecological lubrication’’. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Coating Scuffing Pitting
1. Introduction Increasing attention to environmental issues and more restrictive environmental regulations are driving the lubricant industry to increase the ecological friendliness of its products. Environmental issues are also of concern for lubricants used in heavy-loaded friction pairs like gears. The durability of heavy-loaded machine components working in non-conformal contacts depends on two phenomena: scuffing of mating elements and rolling contact fatigue—pitting. So, the crucial feature of environmentally friendly lubricants is their effective lubricating action under extreme-pressure working conditions. It is well known that ecolubricants, because of the lack of effective additives, do not exhibit satisfactory anti-wear action in heavy-loaded contacts [1]. To promote the application of lubricants without environmentally hazardous additives the authors postulate a new concept: taking over the function of lubricating additives by thin, hard coatings deposited on sliding elements. There are no guidelines for selection of coatings in the case of machine components subjected to extreme conditions. In spite of successful solution of technological problems, concerning the elaboration of low-temperature processes, surface coating technologies are still in the very first phase of application in heavy-loaded contacts, like gears and rolling bearings. Nowadays, thin hard coatings are widely applied in cutting and forging processes to increase the tool life. In comparison with cutting tools, the number of heavy-loaded mechanical components is used in gigantic, while the proportion that is coated is extremely small. In contrast to
Corresponding author.
E-mail address: [email protected] (R. Michalczewski). 0301-679X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2008.05.001
cutting and forming tools, none of the surfaces in machine element contacts should be subjected to deformation or wear. A machine component contact should preferably display low friction and wear. When a machine is new the contacting surfaces are rough and the contact pressure at the surface irregularities can be very high. Thus, micro-scale wear and plastic deformations initially take place. After some time in service, the steel surfaces wear to fit each other perfectly and the wear rate is significantly lower. So, the coatings should also be able to perform run-in processes. The other problem is that the coatings used today are known not to interact chemically with lubricants or their additives in the way metals do. This is because lubricants and their active additives were once developed to form protective films, by physical and chemical reactions, not on coatings but on contacting metals (mainly steel). The tribological behaviour of DLC-coated components in DLC/ DCL and steel/DLC contacts lubricated with various ecological oils was a subject of many studies [2–6]. The authors present some results of tribological investigations of coated elements lubricated with various types of base oils and oils with anti-wear/extreme-pressure (AW/EP) additives, subjected to test conditions which cause scuffing and pitting.
2. Experimental set-up 2.1. The coatings Two kinds of thin hard coatings were deposited: standard single coatings and low-friction coatings. The properties of investigated coatings are summarised in Table 1.
ARTICLE IN PRESS R. Michalczewski et al. / Tribology International 42 (2009) 554–560
Standard single TiN and CrN coatings were deposited by the arc-vacuum method (PVD—physical vapour deposition), at the Surface Engineering Department of ITeE-PIB. The a-C:H:W (also denoted as WC/C) coating is DLC type representing a-C:H:Me group [7]. The coating consisted of an elemental Cr adhesion layer adjacent to the steel substrate, followed by an intermediate transition region consisting of alternating lamellae of Cr and WC, and an outermost hydrocarbon layer doped with W. The coating was deposited using PVD process by reactive sputtering by Oerlikon Balzers Ltd., Poland. The MoS2/Ti (commercial name MoST) coating was deposited by Teer Coatings Ltd., UK. The MoS2/Ti coating was deposited by DC magnetron sputtering using a CFUBMSIP process (closed field unbalanced magnetron sputter ion plating) [8]. 2.2. Lubricants The tribosystems were lubricated with a mineral base oil (denoted RL-144/4 with viscosity at 100 1C–7.2 mm2/s), synthetic base oils (PAO-8 and PAG-8—viscosity at 100 1C was 8 mm2/s) and vegetable base oils (rapeseed and sunflower oils—viscosity at 100 1C was 7.5 mm2/s). The mineral base oil was also blended with a commercial package of AW additives at 3% (wt.) concentration, and EP additives at 5%. AW additives were based on zinc dialkyldithiophosphate (ZDDP). EP additives contained organic S–P compounds. Also a commercial automotive gear oil of GL-5 performance level (viscosity at 100 1C was 13.9 mm2/s), as well as an eco-oil intended for lubrication of industrial transmissions (ELAS-B) were used (viscosity at 100 1C was 7.9 mm2/s). The automotive gear oil contains toxic AW/EP additives. ELAS-B is fully formulated vegetable-based, environmentally friendly oil, with, especially for the purpose of this project, AW/EP additives removed. This oil has been elaborated at ITeE-PIB. 2.3. Test methods The tribological experiments were performed using T-02 fourball tester, cone-three balls pitting tester (T-03), as well as T-12 gear test rig (FZG type). These tribotesters have been engineered and manufactured by the Institute for Sustainable Technologies National Research Institute (ITeE-PIB). The scuffing resistance was measured using the T-02 four-ball tester using the method described in [9]. Scuffing resistance was characterised by the means of the scuffing load (Pt) defined as the load at which a sudden rise in the friction torque occurs during a run at continuously increasing load. The higher Pt value, the better action of the tested tribosystem. The test balls were chrome alloy bearing steel, with a diameter of 12.7 mm, surface roughness Ra ¼ 0.032 mm and hardness 60–65 HRC. In scuffing tests all balls were coated. The fatigue life (pitting) was characterised by the 10% fatigue life L10. The test method is described in [10]. The value of L10 represents the life at which 10% of a large number of test cones,
555
lubricated with the tested lubricant, would be expected to have failed. L10 life was determined using the T-03 tester. The tribosystem presented in Fig. 1 consists of three lower balls (2) freely rotating in a special race (3), and driven by the top cone (1) with the investigated coating. Test conditions were 3924 N (400 kgF) load and 1450 rpm top cone speed. The contact stress is about 3 GPa, about one-third of the average hardness expressed in GPa. Pitting was detected by a special system with a vibration sensor, which automatically stopped the machine. Twenty-four top cone failures were necessary to assess the lubricant performance. The L10 life was obtained by a linear regression analysis of the trend line for each oil (Weibull analysis). The results were verified using T-12 gear test rig (FZG type), employing test conditions according to DIN 51 354 and IP 334 standards, procedure A/8,3/90. The test gears were made of casehardened 20MnCr5 steel. The surface hardness after tempering was 60–62 HRC, roughness Ra ¼ 0.3–0.7 mm. The surface was Maag-Cross hatch ground. In gear tests both gears were coated. The wear scars were analysed using scanning electron microscopy (SEM), X-ray microanalysis (EDS) and atomic force microscopy (AFM).
3. Results 3.1. The effect of coating type on scuffing and pitting The durability of highly loaded friction joints depends on the load-carrying capacity (scuffing resistance) and fatigue life (resistance to pitting). The effect of the type of the coating on the scuffing resistance and fatigue life for coated steel balls lubricated with mineral base oil is presented in Fig. 2. As can be seen, PVD coatings exhibit a satisfactory scuffing resistance. Single coatings (TiN, CrN) show a better scuffing resistance than low-friction coatings (a-C:H:W, MoS2/Ti). It has also been observed that the L10 fatigue life of TiN- and CrN-coated specimens is several times shorter than for uncoated cones. Simple coatings like TiN and CrN, in spite of their excellent behaviour in scuffing conditions, do not exhibit satisfactory resistance to cyclic stress. However, the fatigue life of specimens coated with low-friction multilayer, lamellar a-C:H:W coating, is comparable to uncoated elements.
P n
1 2 3
Fig. 1. Tribosystems employed for testing pitting: 1—cone, 2—balls, 3—race.
Table 1 The properties of the investigated coatings Coating
Name
Interlayer
Thickness (mm)
Nano hardness (GPa)
Scratch-test critical load (N)
TiN CrN a-C:H:W MoS2/Ti
TiN CrN WC/C MoST
– – Cr, WC Ti
2.0 2.0 2.0 1.2
22.3 18.6 10.8 9.8
67 102 106 101
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Scuffing load, Pt [N]
8000 7000 6000 5000 4000 3000 2000 1000 0 uncoated
TiN
uncoated
TiN
CrN
a-C:H:W
MoS2/Ti
700
Life L10 [min.]
600 500 400 300 200 100 0 CrN
a-C:H:W
MoS2/Ti
Fig. 2. Scuffing load for steel–steel and coating–coating contacts (a) and L10 fatigue life for steel–steel and coating–steel contacts (b) (mineral base oil).
Fig. 3. The pitting wear on the a-C:H:W-coated test cone (mineral base oil): (a) SEM image, (b) cross-section and (c) enlarged dashed zone.
The photographs of pits for a-C:H:W-coated cone is presented in Fig. 3. According to the elastic contact theory the maximum shear stress under the mechanical contact is found at a certain depth under the surface; much deeper than the coating thickness. Due to limited thickness, the vapour-deposited coatings do not affect the deeper located maximum stress caused by the macroscopic contact. This is why pit depths for uncoated and coated elements are similar. The results of AFM observation of worn surfaces for uncoated and a-C:H:W-coated cones are presented in Fig. 4.
It can be seen that when the parts are new, the contacting surfaces are rough, so the contact pressure at the surface irregularities can be very high. Thus, micro-scale wear and plastic deformation initially take place. In steel–steel rolling contact the rubbing surfaces become very smooth as a result of run-in processes. This reduces the stress and in turn extends fatigue life. In comparison, standard single coatings (TiN and CrN) are relatively hard and brittle, so their running-in processes are limited, stress is very high and hence the fatigue life is very short (results not presented here). For a-C:H:W low-friction coating the running-in process is more effective and the fatigue life is longer.
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Fig. 4. AFM analyses from the wear track on the test cones: (a) uncoated cone and (b) a-C:H:W-coated cone (pitting tester, mineral base oil).
3.2. The effect of the type of the oil base on scuffing and pitting The results of testing the effect of base oils (mineral, synthetic and vegetable) on scuffing and pitting of uncoated and a-C:H:Wcoated elements are presented in Fig. 5. For a-C:H:W coating the best scuffing characteristics were obtained for oil bases containing polar components (PAG-8 and vegetable oils). So, it is possible to replace the mineral base with synthetic or vegetable oils with no risk of decreasing the scuffing resistance of the tribosystem. In pitting tests the highest L10 life was obtained for the mineral base oil. For ecologically friendly vegetable bases (sunflower and rapeseed) the L10 lives were shorter than that for the mineral base but longer than that for synthetic bases (PAO-8 and PAG-8). The obtained results indicate that mineral bases can be replaced with vegetable bases in case of heavy-loaded friction joints with a-C:H:W-coated elements.
3.3. The effect of AW and EP additives on scuffing and pitting The results of testing an effect of oil additives (AW and EP) on scuffing resistance and surface fatigue life are presented in Fig. 6. The scuffing resistance of a-C:H:W-coated elements is much higher than obtained for uncoated elements lubricated with oilscontaining reactive additives. The results of reactivity of the additives can be observed in Fig. 7 where the results of SEM/EDS analyses for uncoated balls are presented.
It is well known that good anti-scuffing properties of uncoated tribosystems lubricated with S-containing oils result from e.g. FeS creation in the friction zone. In the case of a-C:H:W-coated tribosystems the coating is removed (Fig. 8). The mechanism of improving anti-scuffing properties by chemical modification of the surface layer is replaced with the more effective anti-scuffing mechanism of preventing the creation of adhesive bonds. However, a beneficial tribochemical action of lubricating additives cannot be excluded. One of the explanations of the beneficial interaction between a-C:H:W coating and oil-containing EP additives is the formation of a new class of a tribofilm. For mineral base oil the fatigue life for uncoated and a-C:H: W-coated cones is similar. AW additive in the base oil extends the time to pitting failure of uncoated tribosystems, but reduces the life of a-C:H:W-coated specimens. EP additives significantly reduce the fatigue life for the steel–steel tribosystem. According to previous study [11] a small concentration of EP additives gives practically no influence on the fatigue life, but their increasing content significantly accelerates pitting. Their deleterious effect may be attributed rather to high corrosion aggressiveness which leads to creation on the lubricated surfaces numerous depressions and micropits, being potential nuclei for ‘‘macropits’, and in this way reducing the fatigue life. For the a-C:H:W-coated specimens their effect is similar to those lubricated with oil-containing AW additives. a-C:H:W-coated specimens, lubricated with the mineral base oil, show a much higher fatigue life than those obtained for the steel–steel tribosystem lubricated with oils-containing EP
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8000
Scuffing load, Pt [N]
7000
uncoated a-C:H:W
6000 5000 4000 3000 2000 1000 0 mineral base
synthetic base (PAO-8)
synthetic base (PAG-8)
vegetable base vegetable base (rapeseed) (sunflower)
800 uncoated a-C:H:W
Life L10 [min.]
700 600 500 400 300 200 100 0 mineral base
synthetic base (PAO-8)
synthetic base (PAG-8)
vegetable base vegetable base (rapeseed) (sunflower)
Fig. 5. Scuffing load for steel–steel and a-C:H:W–a-C:H:W contacts (a) and L10 fatigue life for steel–steel and a-C:H:W–steel contacts (b) for tribosystems lubricated with various base oils.
additives. This confirms the suitability of a-C:H:W coatings for application on heavy-loaded machine components lubricated by an oil without toxic additives.
Scuffing load, Pt [N]
8000 7000 6000 5000
base oil base oil + AW base oil + EP
3.4. Results of gear tests
4000 3000 2000 1000 0 uncoated
1200
base oil base oil + AW base oil + EP
1000 Life L10 [min.]
a-C:H:W
800 600 400 200
The idea of taking over the functions of AW/EP additives by a thin hard coating, was verified during gear testing of the eco-oil (ELAS-B) using a-C:H:W-coated test gears. The results were compared with those obtained for uncoated, steel test gears lubricated by the commercial gear oil of GL-5 performance level, which contains toxic AW/EP additives (Fig. 9). The failure load stage obtained for a-C:H:W-coated test gears lubricated by the eco-oil without any AW/EP additives is comparable to that given by the commercial gear oils-containing toxic AW/EP additives. Also the vibration level is kept at a similar level. However, for the coated gears the operating temperature was lower showing the benefits from using coated gears. Similar conclusions were presented in [12]. So, it has been shown here that under extreme-pressure conditions low-friction coatings can take over the functions of AW/EP additives and make it possible to use ecological oils for lubrication.
0 uncoated
a-C:H:W
Fig. 6. Scuffing load for steel–steel and a-C:H:W–a-C:H:W contacts (a) and L10 fatigue life for steel–steel and a-C:H:W–steel contacts (b) for tribosystems lubricated with different additives blended with the mineral base oil.
4. Conclusions Thin coatings deposited on the surface of machine components improve their scuffing resistance. What is more, the presence of
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Fig. 7. SEM images of wear scars as well as Fe, S and P maps for uncoated steel specimens (four-ball apparatus, mineral oil with EP additive).
Fig. 8. SEM images of wear scars as well as Fe, S and P maps for a-C:H:W-coated tribosystem (four-ball apparatus, mineral oil with EP additive).
10
150
8
140
Oil temperature [°C]
Failure stage
12
6 4 2
uncoated (GL-5 oil) a-C:H:W coated (Eco-oil)
130 120 110 100 90 80
0 a-C:H:W coated (Eco-oil)
Vibration amplitude (RMS) [m/s2 ]
uncoated (GL-5 oil)
4
5
6
7
8 9 Load stage
10
11
12
140 120 100 80 60 40 20
uncoated (GL-5 oil) a-C:H:W coated (Eco-oil)
0 5000 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 Frequency [Hz]
Fig. 9. Results from gear tests: (a) failure load stage, (b) oil temperature and (c) vibration amplitude at the 8th stage, for uncoated gears lubricated with GL-5 oil and a-C:H:W-coated gears lubricated with eco-oil.
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thin coatings on the surface of machine components gives the possibility of eliminating or reducing the content of toxic AW/EP additives in lubricating oils, as well as using ecological oils made of renewable resources, without any risk of a scuffing failure. However, a factor still limiting the scope of application of some coatings is their poor performance under conditions of cyclic contact stress, which leads to accelerated fatigue failures (pitting). This concerns typical PVD coatings (like TiN) and radically limits the area of application of such coatings to machine components subjected only to scuffing. In situations where resistance to pitting is an important factor (gears, rolling bearings) the application of these coatings is still limited. A satisfactory fatigue behaviour was observed only for the a-C:H:W coating, making it a strong candidate for application on machine components. References [1] Bartz WJ. Lubricants and the environment. Tribol Int 1998;31:35–47. [2] Kalin M, Vizintin J. A comparison of the tribological behaviour of steel/steel, steel/DLC and DLC/DLC contacts when lubricated with mineral and biodegradable oils. Wear 2006;261:22–31.
[3] Kalin M, Vizintin J. Tribological performance of titanium doped and pure DLC coatings combined with a synthetic bio-lubricant. Wear 2006;261:9–14. [4] Kalin M, Vizintin J, Vercammen K, Barriga J, Arnsek A. The lubrication of DLC coatings with mineral and biodegradable oils having different polar and saturation characteristics. Wear 2006;200:4515–22. [5] Vercammen K, Van Acker K, Vanhulsel A, Barriga J, Arnsek A, Kalin M, et al. Tribological behaviour of DLC coatings in combination with biodegradable lubricants. Tribol Int 2004;37:983–9. [6] Kalin M, Majdic F, Vizintin J, Pezdirnik J, Velkavrh I. Analyses of the long-term performance and tribological behaviour of an axial piston pump using diamond-like-coated piston shoes and biodegradable oil. J Tribol 2008;130: 011001–014501-4. [7] Jiang JC, Meng WJ, Evans AG, Cooper CV. Surf Coat Technol 2003;176:50–6. [8] Amaro R, Martins R, Seabra J, Renevier NM, Teer DG. Molybdenum disulphide/ titanium low friction coating for gears application. Tribol Int 2005;38:423–34. [9] Piekoszewski W, Szczerek M, Tuszynski W. The action of lubricants under extreme pressure conditions in a modified four-ball tester. Wear 2001;249: 188–93. [10] Michalczewski R, Piekoszewski W. The method for assessment of rolling contact fatigue of PVD/CVD coated elements in lubricated contacts. Tribol Finnish J Tribol 2006;25:34–43. [11] Tuszynski W. An effect of lubricating additives on tribochemical phenomena in a rolling steel four-ball contact. Tribol Lett 2006;24(3):207–15. [12] Kalin M, Vizintin J. The tribological performance of DLC-coated gears lubricated with biodegradable oil in various pinion/gear material combinations. Wear 2005;259:1270–80.