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Performance of low-friction coatings in helium environments
Surface & Coatings Technology 206 (2012) 4651–4658
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Performance of low-friction coatings in helium environments B. Podgornik a, b,⁎, U. Borovšak c, F. Megušar c, K. Košir c a b c
Institute of Metals and Technology, Ljubljana, Slovenia University of Ljubljana, Centre for Tribology and Technical Diagnostics, Ljubljana, Slovenia LE-Tehnika d.o.o., Kranj, Slovenia
a r t i c l e
i n f o
Article history: Received 9 October 2011 Accepted in revised form 12 May 2012 Available online 20 May 2012 Keywords: DLC coatings Helium atmosphere Friction Wear
a b s t r a c t The aim of this research work was to investigate the possibility of replacing soft PTFE‐based coatings on components operating in helium atmosphere. Focus was on maintaining low friction and improving surface wear resistance. Therefore two commercial DLC coatings (a-C:H and Me-C:H), CrN coating and reference PTFE‐based Rulon and Xylan coatings were included in this investigation. Coatings were deposited on hardened 100Cr6‐bearing steel discs and tested against uncoated steel balls in pin-on-disc contact configuration under dry reciprocating and unidirectional sliding in helium atmosphere. Investigation was concentrated on the effect of running-in, contact pressure, sliding speed and counter-material type and surface treatment on the tribological behaviour of hydrogenated DLC coatings when running‐in in helium. Results show that for PTFE-based self-lubricating coatings low friction is reached through coating removal and formation of thick layer of transferred coating material on the steel counter-surface. As long as coating is not worn through stable coefficient of friction of about 0.2 is maintained. In a similar way DLC coatings can provide even lower friction in helium, at the same time almost eliminating wear of the coated part. However, at the same time hard DLC coatings prolong running-in and provoke wear of the steel counter-surface, which greatly depends on the coating type, counter material and contact conditions. The best tribological behaviour in helium was thus achieved when pairing softer metal doped W-DLC coating with nitrided steel surface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Positioning, guiding and sliding components operating in vacuum and inert atmosphere, i.e. nitrogen, argon, and helium are exposed to very specific and demanding contact conditions. Although normal loads are usually quite low, inability of liquid lubrication and lack of oxide surface layers result in high friction and adhesion between contact surfaces. Commonly MoS2‐, PTFE‐ and Teflon‐based soft coatings are applied on the contact surfaces to obtain low friction and non-sticking behaviour [1]. However, their low wear resistance doesn't provide long‐term effects [2–4]. For many moving mechanical assemblies (MEMS devices, spacemechanism assemblies, or ordinary sliding components) not only low friction but also combined effect of low friction and wear-less sliding is often required but unfortunately seldom achieved [5]. As one could envision, the realization of near friction-less and wear-less surfaces achieved through deposition of DLC coatings [6,7] is enormous, meaning reduced maintenance, increased reliability, and lowered energy consumption along with longer wear lives in mechanical components. ⁎ Corresponding author at: Institute of Metals and Technology, Ljubljana, Slovenia. Tel.: + 386 1 4701930; fax: + 386 1 4701939. E-mail address: [email protected] (B. Podgornik). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.040
In the last decade a lot of carbon‐based low-friction coatings, also known as diamond-like carbon coatings (DLC) were developed, which provide a wide range of exceptional physical, mechanical, optical, electrical and especially tribological properties, combining low friction and high wear resistance [8,9]. It has been reported that DLC films derived from hydrogen-rich CH4 source gas plasmas exhibit a friction coefficient as low as 0.001 and wear rate of 10− 10–10 − 11 mm3/Nm in vacuum or inert-gas environments [10–13]. Unfortunately, such superior tribological behaviour cannot be sustained when exposed to ambient conditions and increased humidity, even if just for a short period of time [5,6,14]. One approach to reduce tribological dependence of DLC films on environment is incorporation of metal or nonmetal elements (such as Ti, W, Ta, Si, F, N, etc.) into the carbon matrix [7,15,16]. However, tribological behaviour of DLC films is also dependent on the contact conditions, including type of motion, load, sliding speed and countermaterial [17–21]. The aim of this research work was to investigate the possibility of replacing soft PTFE‐based coatings on components operating in helium atmosphere, i.e. in gas‐cooled reactors and Stirling engines. Focus was on maintaining low friction and improving surface wear resistance through application of un-doped and doped DLC coatings and to investigate the effect of running-in process, contact pressure, sliding speed and counter-material on their tribological behaviour in helium.
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2. Experimental 2.1. Materials and coatings Two groups of materials and coatings were included in this investigation. First group, evaluated under more severe reciprocating sliding motion was used to identify potential candidate materials and coatings for dry sliding components operating in helium atmosphere. The first group of disc‐type specimens (ϕ 24 × 8 mm) included hardened 100Cr6 steel, Al2O3 and Si3N4 ceramics, three commercial PFTE-based soft coatings; reference Rulon® J, Xylan® 1010 and Xylan® 1054, and three commercial PVD/PACVD hard coatings, provided by Oerlikon Balzers Austria. Hard coatings comprised 3 μm thick CrN ceramic coating, deposited by reactive sputtering at about 450 °C and two ~2 μm thick DLC coatings. DLC coating denoted D1 was a monolayer hydrogenated amorphous carbon coating (a-C:H) deposited by a highfrequency plasma-assisted CVD process (PACVD) at a deposition temperature of ~200 °C. On the other hand, D2 was a multilayer W-doped hydrogenated amorphous carbon coating (a-C:H/WC; W-DLC) deposited by a reactive sputtering process at ~230 °C. Both DLC coatings were deposited on about 2 μm thick CrN support layer in order to provide adequate load carrying capacity of the coated surface. All coatings were deposited on hardened 100Cr6 steel (750 HV0.1), ground and polished to an average surface roughness of 0.1 μm. The materials' and coatings' description, details and properties are given in Table 1. Based on the preliminary test results, obtained under reciprocating sliding motion the second group of specimens was prepared, which included Rulon® J, Xylan® 1010, DLC and W-DLC coatings, all deposited on hardened 100Cr6 steel discs, also ground and polished to Ra value of 0.1 μm. During preliminary tribological experiments hardened (~850 HV0.1) and polished (Ra ≈ 0.05 μm) 100Cr6 steel ball bearings (ϕ 10 mm) were used as a counter material. However, in the second set of experiments selected coatings (Table 1 — 2nd gr.) were evaluated against polished (Ra ≈ 0.05 μm) and hardened 100Cr6 steel (~850 HV0.1), hardened 25CrMo4 steel (~500 HV0.1) and nitrided 25CrMo4 steel (~870 HV0.1) pins (ϕ 10 mm), respectively. Nitrided 25CrMo4 steel pins were first tempered to 350 HV0.1, then plasma nitrided at 520 °C in 75%H2– 25%N2 gas mixture for 4 h and finally surface polished, which resulted in 3–4 μm thick γ′ compound layer over 250 μm thick diffusion zone. 2.2. Tribological testing Preliminary tribological tests, aimed at identifying potential candidate materials for dry sliding in helium atmosphere were performed under more severe reciprocating sliding motion using pin-on-disc contact configuration, load of 10 N (pH = 1.0 GPa for reference steel disc), amplitude of 1.5 mm and frequency of 60 Hz (vs = 0.2 m/s). Tests, where hardened 100Cr6 steel ball bearing (ϕ 10 mm) was loaded against a stationary disc of interest (Fig. 1a) were run at room temperature (20 °C) in helium atmosphere, achieved by filling the testing chamber with helium gas prior to the test and then maintaining
constant flow of helium (130 l/h; 1 bar) into the chamber. However, no UHV conditions were established in the testing chamber. Second set of experiments, including Rulon® J, Xylan® 1010, DLC and W-DLC coated discs was again carried out in helium atmosphere at 20 °C, but under milder unidirectional sliding conditions using Pin-Disc machine (Fig. 1b), which allowed use of lower, more representative loads. This time stationary uncoated pin (ϕ 10 mm), made of 100Cr6 or 25CrMo4 steel was loaded against the coated‐rotating disc, using load of 0.5 N (pH = 350 MPa for reference steel disc) and 0.05 N (pH = 150 MPa for reference steel disc), sliding speed of 0.2 m/s and 0.4 m/s, and sliding distance of up to 6.000 m. Finally, by starting experiment in air, interrupting it and separating contact surfaces after 5–10 min of sliding and then continuing sliding in helium atmosphere the effect of running-in in air on tribological properties of selected coatings was investigated. Before each test both contact surfaces were cleaned with ethanol and dried in air. Results were evaluated in terms of coefficient of friction value and stability, and wear volume/wear rate of mating surfaces. Wear volume of the steel ball/pin was estimated from the diameter of the wear scar after wiping the surface and cleaning it in the ultrasound bath, and wear volume of the coated disc was measured using 3D stylus profilometry. 3. Results 3.1. Reciprocating sliding Reciprocating sliding contact of uncoated steel surfaces in helium atmosphere led to very high and unstable coefficient of friction of over 1, caused by initial removal of oxide layers from the contact surfaces and followed by severe adhesive wear. As expected, friction properties of the contact were improved by applying soft selflubricating Rulon® J coating. Under reciprocating sliding in helium Rulon® J coating resulted in low (~0.2) and very stable coefficient of friction, which was maintained for the whole 30,000‐cycle test, as shown in Fig. 2. Analysis of contact surfaces showed that low friction was achieved by flake-like wear particles generation and transfer of coating material from Rulon® J coated disc to uncoated steel ball, forming a low shear strength transfer film [22]. However, the duration of low and stable friction behaviour is limited due to the high wear rate (3.85·10− 5 mm3/Nm), observed for Rulon® J coating (Fig. 3). In the case of reference Rulon® J coating all the wear was concentrated on the coated disc, where after 30,000 sliding cycles coating thickness was reduced below 50 μm, with the steel counter-ball still showing original topography, as shown in Fig. 4a. Low friction behaviour in helium was obtained also with Xylan soft coatings, which on the other hand showed better wear resistance (Fig. 3). However, their tribological behaviour was coating type/ composition dependent. In the case of Xylan® 1010 coating (PTFE/FEP composition) similar low (b0.2) and stable coefficient of friction as for reference Rulon® J coating was observed during 30,000‐cycle test, with the wear rate being reduced to 0.86·10− 5 mm3/Nm. For Xylan® 1054 coating (MoS2/graphite composition) wear rate was further reduced to
Table 1 Materials and coatings included in the investigation.
St Al Si R X1 X2 CN D1 D2 a
Sample
Material
Details
Hardnessa
Thickness
Notes
100Cr6 Al2O3 Si3N4 Rulon® J Xylan® 1010 Xylan® 1054 CrN DLC W-DLC
Steel Ceramic Ceramic PTFE Fluoro-polymer Fluoro-polymer Ceramic a-C:H a-C:H/WC
Hardened 99% grade – PTFE-based soft coating PTFE and FEP composite soft coating MoS2 and graphite composite soft coating Monolayer coating PVD; Tdep. ≈ 450 °C Monolayer coating PACVD; Tdep. ≈ 200 °C Multilayer coating PVD; Tdep. ≈ 230 °C
~ 750 HV ~ 1450 HV ~ 1600 HV 60 Shore D 60–90 Shore D 60–90 Shore D ~ 1750 HV ~ 2000 HV ~ 1500 HV
– – – ~ 200 μm ~ 20 μm ~ 20 μm ~ 3 μm ~ 2 μm ~ 2 μm
Substrate material – – Reference coating; 2nd gr. 2nd gr. – – CrN support layer; 2nd gr. CrN support layer; 2nd gr.
Values provided by coating supplier.
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Fig. 1. Tribological test setup; a) reciprocating sliding and b) unidirectional sliding.
about 0.8·10− 5 mm3/Nm and initial coefficient of friction dropped below 0.2. However, after only 5000–7000 sliding cycles coefficient of friction for Xylan® 1054 coating became more unstable and started to rise, exceeding value of 0.3 at the end of 30,000‐cycle test (Fig. 2). Although both Xylan coatings provided 4–5 times better wear resistance than reference Rulon® J coating they also provoked mild abrasive wear of the steel counter-body, especially Xylan® 1054 coating, as shown in Fig. 4b. Deposition of CrN coating or use of ceramic discs almost eliminated wear of the disc surface in helium atmosphere, reducing disc wear rate after 30,000 reciprocating sliding cycles below 0.1·10− 5 mm3/Nm. However, this also resulted in significant wear of the steel ball counter-surface (≥1.0·10 − 5 mm3/Nm) and high friction (Figs. 2–4c). In the case of Si3N4‐ceramic disc distinctive abrasive wear of the steel counter-surface but no signs of steel transfer were observed, which led to high but still stable coefficient of friction of about 0.6. On the other hand, severe wear of the steel ball and transfer of steel [23] to CrN and especially Al2O3 surface led to even higher and more unstable
Fig. 2. Coefficient of friction curves for investigated materials during dry reciprocating sliding test in helium atmosphere.
coefficient of friction of over 1, as shown in Fig. 2. Contrary to ceramic materials and CrN coating application of DLC coatings in helium atmosphere resulted in combined effect of low friction (~0.3) and low wear of both contact surfaces. In agreement with previous investigations [24,25] beneficial tribological properties of investigated DLC coatings were achieved through low-friction transfer film formation, observed on the steel counter-surface. However, as shown in Figs. 2 and 3, un-doped DLC coating was found to provide better disc wear protection, while W-doped coating gave faster running-in. 3.2. Unidirectional sliding Preliminary reciprocating sliding tests at 1 GPa pointed on DLC coatings as the most promising materials to replace soft PTFE-based coatings in sliding systems operating in helium environment. Further evaluation and comparison of un-doped and W-doped DLC coatings against PTFE-based Rulon® J and Xylan® 1010 soft coatings was performed under low-load unidirectional sliding, including the effect of running-in in air, counter-material type and surface treatment, applied load and sliding speed. 3.2.1. Effect of sliding distance After 1500 m of sliding in helium at the load of 0.5 N (350 MPa) and sliding speed of 0.4 m/s both PTFE-based soft coatings exhibited transfer of original coating material to the 100Cr6 steel countersurface (Fig. 5a), which provided low and stable coefficient of friction of about 0.22. In agreement with reciprocating sliding tests almost no measurable wear of the steel counter-pin could be detected for either soft coating. However, continuous process of coating removal and transfer to the steel counter-surface resulted in considerable wear of the coating itself, as shown in Figs. 5a and 6. In the case of Rulon® J coating wear rate after 1500 m of sliding was as high as 6·10 − 5 mm3/Nm, which again was more than 3 times larger than measured for thinner but more wear‐resistant Xylan® 1010 coating. Application of hydrogenated DLC coatings resulted in slightly higher coefficient of friction (~0.25) after 1500 m of sliding in helium, but almost complete protection of the disc surface against wear. At the end of the 1500 m sliding test both DLC coated discs (Ra = 0.1 μm) still displayed basic topography inside the wear track, as shown in Fig. 5b and c. However, although DLC coatings eliminated wear of the disc surface and provided low friction in helium atmosphere through thin graphite-like C-rich tribofilm formation [26], they also increased wear of the steel counter-surface (Fig. 6). In the case of softer W-doped DLC coating, which shows faster running-in and lower initial friction (Fig. 7), steel pin wear rate was still less than 0.03·10 − 5 mm3/Nm. However, longer and more intense running-in process with higher initial friction observed for harder undoped DLC coating (Fig. 7) increased steel pin wear rate to about 0.25·10 − 5 mm3/Nm. Longer sliding tests revealed, that for both DLC coatings wear of the steel counter-surface more or less took place during running-in period. Increasing sliding distance to 6000 m resulted in increased wear volume for both PTFE-based soft coatings and wear rate between 1.5
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Fig. 3. Tribological properties of different materials evaluated under dry reciprocating sliding in helium atmosphere.
and 3.0·10 − 5 mm3/Nm, as well as in mild wear of the steel countersurface. However, as long as the coating was not worn through, low and stable coefficient of friction was maintained for Rulon® J and Xylan® 1010 coatings, as shown in Fig. 7. Also for un-doped DLC coating constant coefficient of friction of about 0.2 was maintained throughout the test as soon as running-in process was finished. However, this can take up to 3000 m of sliding and results in merely smoothening of the coated surface but high wear of the steel counter-surface, being one order of magnitude larger than for other coatings investigated. On the other hand, wear of the coated surface took place for W-doped DLC coating, which after 6000 m of sliding in helium was in the range of 0.05·10− 5 mm3/Nm. However, wear of the W-DLC coating was still way below wear of the PFTE-based soft coatings (Fig. 6), which were almost worn through. Although mild wear of the W-doped DLC coating took place it led to formation of dense tribofilm on the steel countersurface, slowing down wear of the counter-surface and reducing friction in helium down to ~0.1. However, it took about 4000 m of sliding
before this low friction behaviour was obtained, as shown in Fig. 7. As compared to un-doped DLC coating, where very thin and discontinued C-type tribofilm was formed on the steel counter-surface (Fig. 8c), W in the W-doped DLC coating provided thicker, denser and more durable nanocrystalline WC tribofilm [16] which covered almost the whole contact area (Fig. 8d). Furthermore, presence of W was also reported to improve tribofilm adherence to the steel surface [27]. 3.2.2. Running-in in air Many components are assembled, tested or even run-in in air before they are put into operation in inert atmosphere or vacuum. Therefore effect of running-in in air or ambient air irruption into the system on the friction behaviour of the investigated coatings was also included. Performing running-in in ambient air or introducing ambient air into the testing chamber during sliding had no effect on the friction behaviour of PTFE-based soft coatings. However, for DLC coatings runningin in air affected frictional properties when continuing sliding in helium.
Fig. 4. Worn surface of the uncoated steel ball after 30,000 sliding cycles in helium atmosphere against (a) Rulon® J, (b) Xylan® 1054, (c) CrN and (d) DLC‐coated disc.
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Fig. 5. Contact surfaces of coated disc and steel counter-pin after 1500 m of unidirectional sliding in helium (pH = 350 MPa; vs = 0.4 m/s); (a) Rulon® J, (b) W-DLC and (c) DLC‐coated discs.
In the case of W-doped DLC coating already 200 m of sliding in ambient air resulted in steady-state friction of ~0.2 and formation of protective C-rich low-friction tribofilm on the steel counter-surface. Further use of such well run-in surfaces in helium atmosphere showed improved frictional properties, immediately providing low and stable coefficient of friction of about 0.15, as shown in Fig. 9. Furthermore, as soon as low-friction tribofilm was formed on the steel counter-surface, which protected it against environmental effects frictional behaviour of W-DLC/steel contact remained unchanged even if sliding in helium was interrupted and contact surfaces exposed to ambient air. Also for un-doped DLC coating running-in process and friction drop to ~0.2 is much faster in air than in helium. However, this didn't provide any beneficial effect in helium. On contrary, due to the lack of dense durable tribofilms coefficient of friction increased back to high initial values as soon as sliding was continued in helium. Thus every interruption of sliding in helium and exposure of contact surfaces to ambient air required new running-in period of 500–1000 m to retrieve low friction,
as shown in Fig. 9. For both DLC coatings extending running-in time in air had no effect on the coefficient of friction in helium, but it initiated wear of the coated surface. 3.2.3. Counter material Effect of the steel counter material type and surface treatment on the tribological properties of coated surfaces in helium is shown in Fig. 10. For both PTFE-based soft coatings contact with hardened 25CrMo4 steel, with a lower hardness of ~500 HV0.1 resulted in longer running-in process (~700 m) than for 100Cr6 steel, but similar coefficient of friction of ~0.2. This also led to higher initial wear of the coatings, especially Rulon® J coating, and in the case of more wear‐ resistant Xylan® 1010 coating also to 25CrMo4 steel wear, as shown in Fig. 10. Also for W-doped DLC coating running-in process was prolonged to about 1000 m of sliding, but coefficient of friction remained at high level of ~0.3. Analysis of contact surfaces showed no wear of the W-DLC coating, but up to 5 times higher wear rate of the hardened
Fig. 6. Coefficient of friction and wear rate of contact surfaces after 1500 and 6000 m of unidirectional sliding in helium atmosphere (100Cr6 steel pin, pH = 350 MPa, vs = 0.4 m/s).
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Fig. 7. Coefficient of friction during unidirectional sliding in helium atmosphere (100Cr6 steel pin, pH = 350 MPa, vs = 0.4 m/s).
25CrMo4 steel pin as compared to hardened 100Cr6 steel and absence of low-friction transfer film formation on the steel counter-surface. On the other hand, pairing of the un-doped DLC coating with hardened 25CrMo4 steel had the opposite effect. While still resulting in the
Fig. 9. Effect of (a) running-in in air and (b) exposure of DLC/steel contact to air on coefficient of friction in helium.
highest wear rate of the steel counter-part, being similar to hardened 100Cr6 steel, and steady-state coefficient of friction of ~0.2 (Fig. 10), it reduced running-in to about 750 m.
Fig. 8. Wear track on (a) un-doped and (b) W-doped DLC‐coated disc and corresponding tribofilms (c and d) generated on 100Cr6 steel counter-surface after 6000 m of sliding in helium atmosphere (pH = 350 MPa, vs = 0.4 m/s).
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Fig. 10. Effect of counter-material on tribological properties of investigated coatings in helium (pH = 350 MPa, vs = 0.4 m/s, 1500 m of sliding).
Nitriding of 25CrMo4 steel counter-surface increased its hardens to ~870 HV0.1, which in the case of soft PTFE-based coatings completely eliminated wear of the steel pin but increased intensity of the coating removal and transfer film formation on the counter-surface. On one hand, this resulted in lower coefficient of friction in helium (~0.18) but also in increased wear of the coated surface. For Rulon® J coating wear rate after 1500 m of sliding increased to 13·10 − 5 mm3/Nm and for Xylan® 1010 to 3.5·10 − 5 mm3/Nm (Fig. 9). Also in the case of W-DLC coating nitriding of the counter-surface almost eliminated wear of the steel pin while maintaining non-measurable coating wear, and providing the fastest running-in and lower coefficient of friction of about 0.2. In the case of harder un-doped DLC coating all three steel counter-surfaces being hardened or nitrided showed the same steadystate friction (~0.2) and very similar wear rate after ~1500 m of sliding in helium (0.20–0.25·10− 5 mm3/Nm; Fig. 9). However, the fastest running-in process was observed for the hardened 25CrMo4 steel counter-surface, followed by nitrided one, which although being harder still provided two times faster running-in than hardened 100Cr6 steel. This clearly indicates that for un-doped DLC coating level of the steady-state friction and wear in helium are more or less independent on the steel counter-surface, as soon as running-in process is finished. 3.2.4. Effect of load and sliding speed In the case of hardened 100Cr6 steel pin (Fig. 11) decreasing load from 0.5 N to 0.05 N had no effect on the tribological properties of Rulon® J or W-doped DLC coating in helium. On the other hand, it practically eliminated wear of the Xylan® 1010 coating and through reduced ploughing effect reduced its coefficient of friction to ~ 0.17. Also in the case of un-doped DLC coating lower load of 0.05 N resulted
in faster drop and lower coefficient of friction, related to reduced abrasive wear component. In the case of softer counter-surface (hardened 25CrMo4 steel) both PTFE-based coatings displayed greatly reduced wear and lower friction at lower load. Steady-state coefficient of friction was decreased from 0.23 to 0.20 for Rulon® J coating and from 0.19 to 0.17 for Xylan® 1010 coating. In terms of wear, decrease in load to 0.05 N reduced wear volume of Rulon® J coating after 1500 m of sliding against hardened 25CrMo4 steel pin from 0.09 mm3 to ~0.02 mm3, while for Xylan® 1010 coating no wear could be detected anymore. On the other hand, for both DLC coatings absence of C-rich tribofilm formation prolonged running-in period and increased coefficient of friction in helium when sliding against hardened 25CrMo4 steel to over 1500 m and 0.4, respectively. Sliding against hardened 100Cr6 steel at lower sliding speed of 0.2 m/s again resulted in reduced coefficient of friction for Xylan® 1010 coating, but also in about 30% lower wear for the Rulon® J coating. Furthermore, while showing very similar running-in period, both DLC coatings displayed lower coefficient of friction at the end of 1500 m sliding test in helium if tested against hardened 100Cr6 steel and lower sliding speed of 0.2 m/s. For W-doped DLC coating lower sliding speed also led to about 20% lower wear of the steel counter-surface (Fig. 11). 4. Conclusions • PTFE‐based soft coatings provide low and stable friction in helium atmosphere, achieved through coating removal and formation of transfer film on the steel counter-surface. Harder is the counter-
Fig. 11. Effect of load and sliding speed on tribological properties of investigated coatings after 1500 m of sliding against hardened 100Cr6 steel pin in helium.
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surface, faster is the process of coating removal and transfer film formation. However, period of stable friction is quite limited due to low wear resistance of the coating, especially for Rulon® coating and nitrided counter-surface. Hydrogenated DLC coatings were found to provide similar steadystate coefficient of friction in helium atmosphere as reference PTFEbased coatings showed, at the same time practically eliminating wear of the coated part. However, while protecting coated surface they prolong running-in period and provoke wear of the steel counter-part, which mainly takes place during running-in. Also in helium, tribological properties of DLC/steel contact depend on coating type, counter-material and contact conditions. For different combinations un-doped DLC coating showed steady-state coefficient of friction of about 0.2 and relatively high wear rates of the steel counter-surface. However, running-in period with high initial friction will differ from as short as few hundred meters of sliding to over 3000 m. On the other hand, W-doped DLC coating gave much shorter running-in and up to 10 times lower wear of the steel counter-surface. Furthermore, with the formation of dense W‐ and C-containing tribofilm on the steel counter-surface coefficient of friction in helium may drop even below 0.1. Running-in in air had no effect on the tribological properties of PTFEbased coatings. However, for W-doped DLC coating it accelerated formation of low-friction tribofilm and resulted in low and stable coefficient of friction from the very beginning of sliding in helium. Although running-in in air gave very fast drop in friction also for undoped DLC coating, this had no influence on its tribological behaviour in helium, with each interruption of sliding and exposure of contact surfaces to air requiring new running-in period. In the case of PTFE-based soft coatings decrease in load reduced ploughing effect, which resulted in lower coating wear and lower coefficient of friction. For DLC coatings and harder 100Cr6 steel counter-surface decrease in load or sliding speed also resulted in lower coefficient of friction and lower counter-surface wear. However, by hindering C-rich tribofilm formation it prolonged running-in and increased coefficient of friction of DLC coatings if sliding against softer 25CrMo4 steel counter-surface.
• Results of the present investigation reveal W-doped DLC coatings as the best candidate to replace PFTE-based soft coatings on components operating in helium atmospheres under low-load conditions.
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Performance of low-friction coatings in helium environments B. Podgornik a, b,⁎, U. Borovšak c, F. Megušar c, K. Košir c a b c
Institute of Metals and Technology, Ljubljana, Slovenia University of Ljubljana, Centre for Tribology and Technical Diagnostics, Ljubljana, Slovenia LE-Tehnika d.o.o., Kranj, Slovenia
a r t i c l e
i n f o
Article history: Received 9 October 2011 Accepted in revised form 12 May 2012 Available online 20 May 2012 Keywords: DLC coatings Helium atmosphere Friction Wear
a b s t r a c t The aim of this research work was to investigate the possibility of replacing soft PTFE‐based coatings on components operating in helium atmosphere. Focus was on maintaining low friction and improving surface wear resistance. Therefore two commercial DLC coatings (a-C:H and Me-C:H), CrN coating and reference PTFE‐based Rulon and Xylan coatings were included in this investigation. Coatings were deposited on hardened 100Cr6‐bearing steel discs and tested against uncoated steel balls in pin-on-disc contact configuration under dry reciprocating and unidirectional sliding in helium atmosphere. Investigation was concentrated on the effect of running-in, contact pressure, sliding speed and counter-material type and surface treatment on the tribological behaviour of hydrogenated DLC coatings when running‐in in helium. Results show that for PTFE-based self-lubricating coatings low friction is reached through coating removal and formation of thick layer of transferred coating material on the steel counter-surface. As long as coating is not worn through stable coefficient of friction of about 0.2 is maintained. In a similar way DLC coatings can provide even lower friction in helium, at the same time almost eliminating wear of the coated part. However, at the same time hard DLC coatings prolong running-in and provoke wear of the steel counter-surface, which greatly depends on the coating type, counter material and contact conditions. The best tribological behaviour in helium was thus achieved when pairing softer metal doped W-DLC coating with nitrided steel surface. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Positioning, guiding and sliding components operating in vacuum and inert atmosphere, i.e. nitrogen, argon, and helium are exposed to very specific and demanding contact conditions. Although normal loads are usually quite low, inability of liquid lubrication and lack of oxide surface layers result in high friction and adhesion between contact surfaces. Commonly MoS2‐, PTFE‐ and Teflon‐based soft coatings are applied on the contact surfaces to obtain low friction and non-sticking behaviour [1]. However, their low wear resistance doesn't provide long‐term effects [2–4]. For many moving mechanical assemblies (MEMS devices, spacemechanism assemblies, or ordinary sliding components) not only low friction but also combined effect of low friction and wear-less sliding is often required but unfortunately seldom achieved [5]. As one could envision, the realization of near friction-less and wear-less surfaces achieved through deposition of DLC coatings [6,7] is enormous, meaning reduced maintenance, increased reliability, and lowered energy consumption along with longer wear lives in mechanical components. ⁎ Corresponding author at: Institute of Metals and Technology, Ljubljana, Slovenia. Tel.: + 386 1 4701930; fax: + 386 1 4701939. E-mail address: [email protected] (B. Podgornik). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.05.040
In the last decade a lot of carbon‐based low-friction coatings, also known as diamond-like carbon coatings (DLC) were developed, which provide a wide range of exceptional physical, mechanical, optical, electrical and especially tribological properties, combining low friction and high wear resistance [8,9]. It has been reported that DLC films derived from hydrogen-rich CH4 source gas plasmas exhibit a friction coefficient as low as 0.001 and wear rate of 10− 10–10 − 11 mm3/Nm in vacuum or inert-gas environments [10–13]. Unfortunately, such superior tribological behaviour cannot be sustained when exposed to ambient conditions and increased humidity, even if just for a short period of time [5,6,14]. One approach to reduce tribological dependence of DLC films on environment is incorporation of metal or nonmetal elements (such as Ti, W, Ta, Si, F, N, etc.) into the carbon matrix [7,15,16]. However, tribological behaviour of DLC films is also dependent on the contact conditions, including type of motion, load, sliding speed and countermaterial [17–21]. The aim of this research work was to investigate the possibility of replacing soft PTFE‐based coatings on components operating in helium atmosphere, i.e. in gas‐cooled reactors and Stirling engines. Focus was on maintaining low friction and improving surface wear resistance through application of un-doped and doped DLC coatings and to investigate the effect of running-in process, contact pressure, sliding speed and counter-material on their tribological behaviour in helium.
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2. Experimental 2.1. Materials and coatings Two groups of materials and coatings were included in this investigation. First group, evaluated under more severe reciprocating sliding motion was used to identify potential candidate materials and coatings for dry sliding components operating in helium atmosphere. The first group of disc‐type specimens (ϕ 24 × 8 mm) included hardened 100Cr6 steel, Al2O3 and Si3N4 ceramics, three commercial PFTE-based soft coatings; reference Rulon® J, Xylan® 1010 and Xylan® 1054, and three commercial PVD/PACVD hard coatings, provided by Oerlikon Balzers Austria. Hard coatings comprised 3 μm thick CrN ceramic coating, deposited by reactive sputtering at about 450 °C and two ~2 μm thick DLC coatings. DLC coating denoted D1 was a monolayer hydrogenated amorphous carbon coating (a-C:H) deposited by a highfrequency plasma-assisted CVD process (PACVD) at a deposition temperature of ~200 °C. On the other hand, D2 was a multilayer W-doped hydrogenated amorphous carbon coating (a-C:H/WC; W-DLC) deposited by a reactive sputtering process at ~230 °C. Both DLC coatings were deposited on about 2 μm thick CrN support layer in order to provide adequate load carrying capacity of the coated surface. All coatings were deposited on hardened 100Cr6 steel (750 HV0.1), ground and polished to an average surface roughness of 0.1 μm. The materials' and coatings' description, details and properties are given in Table 1. Based on the preliminary test results, obtained under reciprocating sliding motion the second group of specimens was prepared, which included Rulon® J, Xylan® 1010, DLC and W-DLC coatings, all deposited on hardened 100Cr6 steel discs, also ground and polished to Ra value of 0.1 μm. During preliminary tribological experiments hardened (~850 HV0.1) and polished (Ra ≈ 0.05 μm) 100Cr6 steel ball bearings (ϕ 10 mm) were used as a counter material. However, in the second set of experiments selected coatings (Table 1 — 2nd gr.) were evaluated against polished (Ra ≈ 0.05 μm) and hardened 100Cr6 steel (~850 HV0.1), hardened 25CrMo4 steel (~500 HV0.1) and nitrided 25CrMo4 steel (~870 HV0.1) pins (ϕ 10 mm), respectively. Nitrided 25CrMo4 steel pins were first tempered to 350 HV0.1, then plasma nitrided at 520 °C in 75%H2– 25%N2 gas mixture for 4 h and finally surface polished, which resulted in 3–4 μm thick γ′ compound layer over 250 μm thick diffusion zone. 2.2. Tribological testing Preliminary tribological tests, aimed at identifying potential candidate materials for dry sliding in helium atmosphere were performed under more severe reciprocating sliding motion using pin-on-disc contact configuration, load of 10 N (pH = 1.0 GPa for reference steel disc), amplitude of 1.5 mm and frequency of 60 Hz (vs = 0.2 m/s). Tests, where hardened 100Cr6 steel ball bearing (ϕ 10 mm) was loaded against a stationary disc of interest (Fig. 1a) were run at room temperature (20 °C) in helium atmosphere, achieved by filling the testing chamber with helium gas prior to the test and then maintaining
constant flow of helium (130 l/h; 1 bar) into the chamber. However, no UHV conditions were established in the testing chamber. Second set of experiments, including Rulon® J, Xylan® 1010, DLC and W-DLC coated discs was again carried out in helium atmosphere at 20 °C, but under milder unidirectional sliding conditions using Pin-Disc machine (Fig. 1b), which allowed use of lower, more representative loads. This time stationary uncoated pin (ϕ 10 mm), made of 100Cr6 or 25CrMo4 steel was loaded against the coated‐rotating disc, using load of 0.5 N (pH = 350 MPa for reference steel disc) and 0.05 N (pH = 150 MPa for reference steel disc), sliding speed of 0.2 m/s and 0.4 m/s, and sliding distance of up to 6.000 m. Finally, by starting experiment in air, interrupting it and separating contact surfaces after 5–10 min of sliding and then continuing sliding in helium atmosphere the effect of running-in in air on tribological properties of selected coatings was investigated. Before each test both contact surfaces were cleaned with ethanol and dried in air. Results were evaluated in terms of coefficient of friction value and stability, and wear volume/wear rate of mating surfaces. Wear volume of the steel ball/pin was estimated from the diameter of the wear scar after wiping the surface and cleaning it in the ultrasound bath, and wear volume of the coated disc was measured using 3D stylus profilometry. 3. Results 3.1. Reciprocating sliding Reciprocating sliding contact of uncoated steel surfaces in helium atmosphere led to very high and unstable coefficient of friction of over 1, caused by initial removal of oxide layers from the contact surfaces and followed by severe adhesive wear. As expected, friction properties of the contact were improved by applying soft selflubricating Rulon® J coating. Under reciprocating sliding in helium Rulon® J coating resulted in low (~0.2) and very stable coefficient of friction, which was maintained for the whole 30,000‐cycle test, as shown in Fig. 2. Analysis of contact surfaces showed that low friction was achieved by flake-like wear particles generation and transfer of coating material from Rulon® J coated disc to uncoated steel ball, forming a low shear strength transfer film [22]. However, the duration of low and stable friction behaviour is limited due to the high wear rate (3.85·10− 5 mm3/Nm), observed for Rulon® J coating (Fig. 3). In the case of reference Rulon® J coating all the wear was concentrated on the coated disc, where after 30,000 sliding cycles coating thickness was reduced below 50 μm, with the steel counter-ball still showing original topography, as shown in Fig. 4a. Low friction behaviour in helium was obtained also with Xylan soft coatings, which on the other hand showed better wear resistance (Fig. 3). However, their tribological behaviour was coating type/ composition dependent. In the case of Xylan® 1010 coating (PTFE/FEP composition) similar low (b0.2) and stable coefficient of friction as for reference Rulon® J coating was observed during 30,000‐cycle test, with the wear rate being reduced to 0.86·10− 5 mm3/Nm. For Xylan® 1054 coating (MoS2/graphite composition) wear rate was further reduced to
Table 1 Materials and coatings included in the investigation.
St Al Si R X1 X2 CN D1 D2 a
Sample
Material
Details
Hardnessa
Thickness
Notes
100Cr6 Al2O3 Si3N4 Rulon® J Xylan® 1010 Xylan® 1054 CrN DLC W-DLC
Steel Ceramic Ceramic PTFE Fluoro-polymer Fluoro-polymer Ceramic a-C:H a-C:H/WC
Hardened 99% grade – PTFE-based soft coating PTFE and FEP composite soft coating MoS2 and graphite composite soft coating Monolayer coating PVD; Tdep. ≈ 450 °C Monolayer coating PACVD; Tdep. ≈ 200 °C Multilayer coating PVD; Tdep. ≈ 230 °C
~ 750 HV ~ 1450 HV ~ 1600 HV 60 Shore D 60–90 Shore D 60–90 Shore D ~ 1750 HV ~ 2000 HV ~ 1500 HV
– – – ~ 200 μm ~ 20 μm ~ 20 μm ~ 3 μm ~ 2 μm ~ 2 μm
Substrate material – – Reference coating; 2nd gr. 2nd gr. – – CrN support layer; 2nd gr. CrN support layer; 2nd gr.
Values provided by coating supplier.
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Fig. 1. Tribological test setup; a) reciprocating sliding and b) unidirectional sliding.
about 0.8·10− 5 mm3/Nm and initial coefficient of friction dropped below 0.2. However, after only 5000–7000 sliding cycles coefficient of friction for Xylan® 1054 coating became more unstable and started to rise, exceeding value of 0.3 at the end of 30,000‐cycle test (Fig. 2). Although both Xylan coatings provided 4–5 times better wear resistance than reference Rulon® J coating they also provoked mild abrasive wear of the steel counter-body, especially Xylan® 1054 coating, as shown in Fig. 4b. Deposition of CrN coating or use of ceramic discs almost eliminated wear of the disc surface in helium atmosphere, reducing disc wear rate after 30,000 reciprocating sliding cycles below 0.1·10− 5 mm3/Nm. However, this also resulted in significant wear of the steel ball counter-surface (≥1.0·10 − 5 mm3/Nm) and high friction (Figs. 2–4c). In the case of Si3N4‐ceramic disc distinctive abrasive wear of the steel counter-surface but no signs of steel transfer were observed, which led to high but still stable coefficient of friction of about 0.6. On the other hand, severe wear of the steel ball and transfer of steel [23] to CrN and especially Al2O3 surface led to even higher and more unstable
Fig. 2. Coefficient of friction curves for investigated materials during dry reciprocating sliding test in helium atmosphere.
coefficient of friction of over 1, as shown in Fig. 2. Contrary to ceramic materials and CrN coating application of DLC coatings in helium atmosphere resulted in combined effect of low friction (~0.3) and low wear of both contact surfaces. In agreement with previous investigations [24,25] beneficial tribological properties of investigated DLC coatings were achieved through low-friction transfer film formation, observed on the steel counter-surface. However, as shown in Figs. 2 and 3, un-doped DLC coating was found to provide better disc wear protection, while W-doped coating gave faster running-in. 3.2. Unidirectional sliding Preliminary reciprocating sliding tests at 1 GPa pointed on DLC coatings as the most promising materials to replace soft PTFE-based coatings in sliding systems operating in helium environment. Further evaluation and comparison of un-doped and W-doped DLC coatings against PTFE-based Rulon® J and Xylan® 1010 soft coatings was performed under low-load unidirectional sliding, including the effect of running-in in air, counter-material type and surface treatment, applied load and sliding speed. 3.2.1. Effect of sliding distance After 1500 m of sliding in helium at the load of 0.5 N (350 MPa) and sliding speed of 0.4 m/s both PTFE-based soft coatings exhibited transfer of original coating material to the 100Cr6 steel countersurface (Fig. 5a), which provided low and stable coefficient of friction of about 0.22. In agreement with reciprocating sliding tests almost no measurable wear of the steel counter-pin could be detected for either soft coating. However, continuous process of coating removal and transfer to the steel counter-surface resulted in considerable wear of the coating itself, as shown in Figs. 5a and 6. In the case of Rulon® J coating wear rate after 1500 m of sliding was as high as 6·10 − 5 mm3/Nm, which again was more than 3 times larger than measured for thinner but more wear‐resistant Xylan® 1010 coating. Application of hydrogenated DLC coatings resulted in slightly higher coefficient of friction (~0.25) after 1500 m of sliding in helium, but almost complete protection of the disc surface against wear. At the end of the 1500 m sliding test both DLC coated discs (Ra = 0.1 μm) still displayed basic topography inside the wear track, as shown in Fig. 5b and c. However, although DLC coatings eliminated wear of the disc surface and provided low friction in helium atmosphere through thin graphite-like C-rich tribofilm formation [26], they also increased wear of the steel counter-surface (Fig. 6). In the case of softer W-doped DLC coating, which shows faster running-in and lower initial friction (Fig. 7), steel pin wear rate was still less than 0.03·10 − 5 mm3/Nm. However, longer and more intense running-in process with higher initial friction observed for harder undoped DLC coating (Fig. 7) increased steel pin wear rate to about 0.25·10 − 5 mm3/Nm. Longer sliding tests revealed, that for both DLC coatings wear of the steel counter-surface more or less took place during running-in period. Increasing sliding distance to 6000 m resulted in increased wear volume for both PTFE-based soft coatings and wear rate between 1.5
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Fig. 3. Tribological properties of different materials evaluated under dry reciprocating sliding in helium atmosphere.
and 3.0·10 − 5 mm3/Nm, as well as in mild wear of the steel countersurface. However, as long as the coating was not worn through, low and stable coefficient of friction was maintained for Rulon® J and Xylan® 1010 coatings, as shown in Fig. 7. Also for un-doped DLC coating constant coefficient of friction of about 0.2 was maintained throughout the test as soon as running-in process was finished. However, this can take up to 3000 m of sliding and results in merely smoothening of the coated surface but high wear of the steel counter-surface, being one order of magnitude larger than for other coatings investigated. On the other hand, wear of the coated surface took place for W-doped DLC coating, which after 6000 m of sliding in helium was in the range of 0.05·10− 5 mm3/Nm. However, wear of the W-DLC coating was still way below wear of the PFTE-based soft coatings (Fig. 6), which were almost worn through. Although mild wear of the W-doped DLC coating took place it led to formation of dense tribofilm on the steel countersurface, slowing down wear of the counter-surface and reducing friction in helium down to ~0.1. However, it took about 4000 m of sliding
before this low friction behaviour was obtained, as shown in Fig. 7. As compared to un-doped DLC coating, where very thin and discontinued C-type tribofilm was formed on the steel counter-surface (Fig. 8c), W in the W-doped DLC coating provided thicker, denser and more durable nanocrystalline WC tribofilm [16] which covered almost the whole contact area (Fig. 8d). Furthermore, presence of W was also reported to improve tribofilm adherence to the steel surface [27]. 3.2.2. Running-in in air Many components are assembled, tested or even run-in in air before they are put into operation in inert atmosphere or vacuum. Therefore effect of running-in in air or ambient air irruption into the system on the friction behaviour of the investigated coatings was also included. Performing running-in in ambient air or introducing ambient air into the testing chamber during sliding had no effect on the friction behaviour of PTFE-based soft coatings. However, for DLC coatings runningin in air affected frictional properties when continuing sliding in helium.
Fig. 4. Worn surface of the uncoated steel ball after 30,000 sliding cycles in helium atmosphere against (a) Rulon® J, (b) Xylan® 1054, (c) CrN and (d) DLC‐coated disc.
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Fig. 5. Contact surfaces of coated disc and steel counter-pin after 1500 m of unidirectional sliding in helium (pH = 350 MPa; vs = 0.4 m/s); (a) Rulon® J, (b) W-DLC and (c) DLC‐coated discs.
In the case of W-doped DLC coating already 200 m of sliding in ambient air resulted in steady-state friction of ~0.2 and formation of protective C-rich low-friction tribofilm on the steel counter-surface. Further use of such well run-in surfaces in helium atmosphere showed improved frictional properties, immediately providing low and stable coefficient of friction of about 0.15, as shown in Fig. 9. Furthermore, as soon as low-friction tribofilm was formed on the steel counter-surface, which protected it against environmental effects frictional behaviour of W-DLC/steel contact remained unchanged even if sliding in helium was interrupted and contact surfaces exposed to ambient air. Also for un-doped DLC coating running-in process and friction drop to ~0.2 is much faster in air than in helium. However, this didn't provide any beneficial effect in helium. On contrary, due to the lack of dense durable tribofilms coefficient of friction increased back to high initial values as soon as sliding was continued in helium. Thus every interruption of sliding in helium and exposure of contact surfaces to ambient air required new running-in period of 500–1000 m to retrieve low friction,
as shown in Fig. 9. For both DLC coatings extending running-in time in air had no effect on the coefficient of friction in helium, but it initiated wear of the coated surface. 3.2.3. Counter material Effect of the steel counter material type and surface treatment on the tribological properties of coated surfaces in helium is shown in Fig. 10. For both PTFE-based soft coatings contact with hardened 25CrMo4 steel, with a lower hardness of ~500 HV0.1 resulted in longer running-in process (~700 m) than for 100Cr6 steel, but similar coefficient of friction of ~0.2. This also led to higher initial wear of the coatings, especially Rulon® J coating, and in the case of more wear‐ resistant Xylan® 1010 coating also to 25CrMo4 steel wear, as shown in Fig. 10. Also for W-doped DLC coating running-in process was prolonged to about 1000 m of sliding, but coefficient of friction remained at high level of ~0.3. Analysis of contact surfaces showed no wear of the W-DLC coating, but up to 5 times higher wear rate of the hardened
Fig. 6. Coefficient of friction and wear rate of contact surfaces after 1500 and 6000 m of unidirectional sliding in helium atmosphere (100Cr6 steel pin, pH = 350 MPa, vs = 0.4 m/s).
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Fig. 7. Coefficient of friction during unidirectional sliding in helium atmosphere (100Cr6 steel pin, pH = 350 MPa, vs = 0.4 m/s).
25CrMo4 steel pin as compared to hardened 100Cr6 steel and absence of low-friction transfer film formation on the steel counter-surface. On the other hand, pairing of the un-doped DLC coating with hardened 25CrMo4 steel had the opposite effect. While still resulting in the
Fig. 9. Effect of (a) running-in in air and (b) exposure of DLC/steel contact to air on coefficient of friction in helium.
highest wear rate of the steel counter-part, being similar to hardened 100Cr6 steel, and steady-state coefficient of friction of ~0.2 (Fig. 10), it reduced running-in to about 750 m.
Fig. 8. Wear track on (a) un-doped and (b) W-doped DLC‐coated disc and corresponding tribofilms (c and d) generated on 100Cr6 steel counter-surface after 6000 m of sliding in helium atmosphere (pH = 350 MPa, vs = 0.4 m/s).
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Fig. 10. Effect of counter-material on tribological properties of investigated coatings in helium (pH = 350 MPa, vs = 0.4 m/s, 1500 m of sliding).
Nitriding of 25CrMo4 steel counter-surface increased its hardens to ~870 HV0.1, which in the case of soft PTFE-based coatings completely eliminated wear of the steel pin but increased intensity of the coating removal and transfer film formation on the counter-surface. On one hand, this resulted in lower coefficient of friction in helium (~0.18) but also in increased wear of the coated surface. For Rulon® J coating wear rate after 1500 m of sliding increased to 13·10 − 5 mm3/Nm and for Xylan® 1010 to 3.5·10 − 5 mm3/Nm (Fig. 9). Also in the case of W-DLC coating nitriding of the counter-surface almost eliminated wear of the steel pin while maintaining non-measurable coating wear, and providing the fastest running-in and lower coefficient of friction of about 0.2. In the case of harder un-doped DLC coating all three steel counter-surfaces being hardened or nitrided showed the same steadystate friction (~0.2) and very similar wear rate after ~1500 m of sliding in helium (0.20–0.25·10− 5 mm3/Nm; Fig. 9). However, the fastest running-in process was observed for the hardened 25CrMo4 steel counter-surface, followed by nitrided one, which although being harder still provided two times faster running-in than hardened 100Cr6 steel. This clearly indicates that for un-doped DLC coating level of the steady-state friction and wear in helium are more or less independent on the steel counter-surface, as soon as running-in process is finished. 3.2.4. Effect of load and sliding speed In the case of hardened 100Cr6 steel pin (Fig. 11) decreasing load from 0.5 N to 0.05 N had no effect on the tribological properties of Rulon® J or W-doped DLC coating in helium. On the other hand, it practically eliminated wear of the Xylan® 1010 coating and through reduced ploughing effect reduced its coefficient of friction to ~ 0.17. Also in the case of un-doped DLC coating lower load of 0.05 N resulted
in faster drop and lower coefficient of friction, related to reduced abrasive wear component. In the case of softer counter-surface (hardened 25CrMo4 steel) both PTFE-based coatings displayed greatly reduced wear and lower friction at lower load. Steady-state coefficient of friction was decreased from 0.23 to 0.20 for Rulon® J coating and from 0.19 to 0.17 for Xylan® 1010 coating. In terms of wear, decrease in load to 0.05 N reduced wear volume of Rulon® J coating after 1500 m of sliding against hardened 25CrMo4 steel pin from 0.09 mm3 to ~0.02 mm3, while for Xylan® 1010 coating no wear could be detected anymore. On the other hand, for both DLC coatings absence of C-rich tribofilm formation prolonged running-in period and increased coefficient of friction in helium when sliding against hardened 25CrMo4 steel to over 1500 m and 0.4, respectively. Sliding against hardened 100Cr6 steel at lower sliding speed of 0.2 m/s again resulted in reduced coefficient of friction for Xylan® 1010 coating, but also in about 30% lower wear for the Rulon® J coating. Furthermore, while showing very similar running-in period, both DLC coatings displayed lower coefficient of friction at the end of 1500 m sliding test in helium if tested against hardened 100Cr6 steel and lower sliding speed of 0.2 m/s. For W-doped DLC coating lower sliding speed also led to about 20% lower wear of the steel counter-surface (Fig. 11). 4. Conclusions • PTFE‐based soft coatings provide low and stable friction in helium atmosphere, achieved through coating removal and formation of transfer film on the steel counter-surface. Harder is the counter-
Fig. 11. Effect of load and sliding speed on tribological properties of investigated coatings after 1500 m of sliding against hardened 100Cr6 steel pin in helium.
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surface, faster is the process of coating removal and transfer film formation. However, period of stable friction is quite limited due to low wear resistance of the coating, especially for Rulon® coating and nitrided counter-surface. Hydrogenated DLC coatings were found to provide similar steadystate coefficient of friction in helium atmosphere as reference PTFEbased coatings showed, at the same time practically eliminating wear of the coated part. However, while protecting coated surface they prolong running-in period and provoke wear of the steel counter-part, which mainly takes place during running-in. Also in helium, tribological properties of DLC/steel contact depend on coating type, counter-material and contact conditions. For different combinations un-doped DLC coating showed steady-state coefficient of friction of about 0.2 and relatively high wear rates of the steel counter-surface. However, running-in period with high initial friction will differ from as short as few hundred meters of sliding to over 3000 m. On the other hand, W-doped DLC coating gave much shorter running-in and up to 10 times lower wear of the steel counter-surface. Furthermore, with the formation of dense W‐ and C-containing tribofilm on the steel counter-surface coefficient of friction in helium may drop even below 0.1. Running-in in air had no effect on the tribological properties of PTFEbased coatings. However, for W-doped DLC coating it accelerated formation of low-friction tribofilm and resulted in low and stable coefficient of friction from the very beginning of sliding in helium. Although running-in in air gave very fast drop in friction also for undoped DLC coating, this had no influence on its tribological behaviour in helium, with each interruption of sliding and exposure of contact surfaces to air requiring new running-in period. In the case of PTFE-based soft coatings decrease in load reduced ploughing effect, which resulted in lower coating wear and lower coefficient of friction. For DLC coatings and harder 100Cr6 steel counter-surface decrease in load or sliding speed also resulted in lower coefficient of friction and lower counter-surface wear. However, by hindering C-rich tribofilm formation it prolonged running-in and increased coefficient of friction of DLC coatings if sliding against softer 25CrMo4 steel counter-surface.
• Results of the present investigation reveal W-doped DLC coatings as the best candidate to replace PFTE-based soft coatings on components operating in helium atmospheres under low-load conditions.
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