Tribological Performances of Polymer- based Coating Materials

Advances in Science and Technology Vol. 64 (2010) pp 33-42 © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AST.64.33

Tribological Performances of Polymer- based Coating Materials Designed for Compressor Applications Seung Min Yeo1, a, Emerson Escobar Nunez1,b and Andreas A. Polycarpou1,c 1

Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green Street, Urbana, IL 61801, USA a

[email protected], [email protected], [email protected]

Keywords: Tribology, Polymer coatings, PTFE, PEEK, ATSP, Compressors, Friction, Wear.

Abstract. With increasing importance of advanced coating materials for use in interacting parts of air-conditioning compressors, several commercially available polymer-based coatings (PTFE/ Pyrrolidone-1, 2, PTFE/MoS2-1, 2, Fluorocarbon, PEEK/PTFE and PEEK/Ceramic coatings) were tribologically evaluated. Friction and wear behavior of these coatings, deposited on gray cast iron were in-situ measured using a specialized pin-on-disk tribometer. The experiments were performed under compressor specific conditions, namely under oscillatory motion simulating piston-type compressor and unidirectional motion simulating swash plate-type compressor operation. Also, the tribological properties of newly developed ATSP-based coatings deposited on aluminum substrates were evaluated under ball-on-disk, unidirectional sliding experiments. Polymer-based coatings exhibited excellent frictional properties, while their wear resistance was also acceptable, even though lower compared to hard coatings. However, the wear debris generated at the interface acted as a third-body solid lubricant with a beneficial role in their overall performance. ATSP coatings blended with fluoroadditives showed superior frictional behavior than pure ATSP coatings, and their wear rate was extremely low compared to commercially available PTFE-based coatings. Introduction Traditionally liquid-type lubricants have effectively served in reducing the friction and wear of various mechanical devices such as rolling and sliding bearings, gears and cutting tools to prolong their lifetime. However, in compressor components, the liquid-type lubricants have negative effects on their thermodynamic efficiencies, and also the state of lubrication in these components is usually not known and is considered to be in the boundary and mixed lubrication regimes [1]. Recently, research interest and efforts are on oil-less compressor conditions to eliminate the adverse effects of liquid-type lubricants and to further improve the performance of compressors [2]. Consequently, it becomes necessary to develop advanced coating materials that exhibit lower friction and higher wear resistant under compressor- specific conditions (Fig. 1). Coatings can be broadly classified as either hard coatings or soft (one category of them being polymer-based) coatings. Conventionally, hard coatings such as diamond-like carbon (DLC), Ti-N and WC/C synthesized through physical vapor deposition (PVD) techniques are thought to be effective in preventing both abrasive and adhesive wear of metal sliding contacts [3]. DLC is one of the most researched tribological coatings [4], and it is widely found in commercial applications such as magnetic storage hard disk drives (due to its superior tribological properties) [5]. These coatings form a hard film on the surface reducing scratching, and offer good load-carrying capacity. Also, they have the ability to form low shear strength reaction and transfer layers on the top surface and the counterface, resulting in weak shear planes and low friction [6]. Another type of hard coatings, WC/C was also shown to have superior tribological properties not only as far as wear resistance, but also in regards to its friction coefficient values, being as low as 0.05 under dry unidirectional pin-on-disk sliding conditions [7]. However, hard coatings are relatively expensive and exhibit difficulties in

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.126.177.211-27/08/10,17:26:39)

34

12th INTERNATIONAL CERAMICS CONGRESS PART C

coating them on substrates with low surface energy [8]. Also, hard coatings sometimes wear out the counterface they slide against due to their relatively high hardness [9]. Due to these issues with hard coatings, recently attention is being focused on soft, thermoplasticbased polymer materials such as polytetrafluoroethylene (PTFE) and polyetherether- ketone (PEEK) which show relatively low friction coefficient and self-lubricating properties [10], and are also inexpensive and easy to fabricate [6]. PTFE has been used extensively since its discovery because of its desirable tribological properties such as chemical inertness and superb lubricity [11]. However, PTFE has fairly poor resistance to wear and creep because it easily yields in shear due to its low intermolecular strength [12]. To alleviate some of these problems and to enhance its tribological performances, PTFE is typically used in the form of composites filled with various fillers and micro/nano particles such as MoS2, graphite, glass and carbon-nano-tubes (CNTs). Significant work has been done in evaluating the tribological behavior of such PTFE composites [13,14]. Another polymer material of great interest is PEEK which shows excellent thermal stability, good friction and wear resistance [15], and PTFE-filled PEEK blends have been a main focus of research to enhance its lubricity along with good wear performance [16-20]. Newly developed aromatic thermosetting polyesters (ATSP) were blended directly with PTFE, and tribotested under conditions simulating air conditioning compressor environments [21,22]. In regards to the wear characteristics of this family of materials, it was demonstrated that these blends display very little wear for the entire compositional range of the ATSP-PTFE blends. However, despite these promising polymer tribological properties, the majority of studies were performed for bulk materials, and there is little information in the open literature about the behavior of polymer-based coatings, especially under compressor-specific conditions. In this work, the tribological performance of commercially available polymer-based coatings was evaluated in environments simulating compressor conditions. These coatings showed promising tribological properties, and under certain conditions surpassed the performance of a commercially deposited DLC coating. Under high contact pressures, the PTFE-based coatings were fully penetrated, however, it was shown that the wear debris generated acted as a third-body solid lubricant preventing premature, catastrophic failure (scuffing). Newly developed ATSP-based coatings were prepared on aluminum disks and ball-on-disk tribological tests were conducted to verify their applicability as a coating material. Development of oil-less type compressors - Adverse effect of lubricant on refrigeration cycle - Not sufficient liquid film at all parts of compressor - Adverse environmental effects BUT,

Oil-less Compressors

Excessive wear & premature failure of conventional tribo-pair materials (ex. Al390-T6 vs. 52100 steel) without surface treatments

Replacement of BULK interacting components with advanced tribological materials such as ceramics and polymeric-based materials - Cost could be an issue - Long term reliability of polymers (creep, thermal instability)

‘COATING’ on top of conventional materials Fig. 1 Motivation and development of advanced coating materials for oil-less compressor applications.

Advances in Science and Technology Vol. 64

35

Experimental Procedure Samples and Contact Geometry. Gray cast iron (Dura-Bar® G2) disks, a common material used in compressors, were coated by an authorized applicator with the polymeric coatings used in this work. The diameter of the disks was 75 mm, and their hardness was 95 HRB (220 VHN). Seven coatings, namely PTFE/Pyrrolidone-1 (DuPont® 958-303), PTFE/Pyrrolidone-2 (DuPont® 958-414), PTFE/ MoS2-1 (Whitford Xylan® 1052), PTFE/MoS2-2 (Fluorolon® 325), Fluorocarbon (Impreglon® 218), PEEK/PTFE (1704 PEEK/PTFE) and PEEK/Ceramic (1707 PEEK/Ceramic) were investigated. The disks had an initial root mean square (RMS) surface roughness of 0.2-0.4 µm. The surface roughness of the coatings was in the range of 1.0-3.5 µm, which is higher than the disk’s roughness (due to the roughening process of the disks before coating deposition). The hardness of these coatings was measured using a micro-Vickers tester at a load of 0.3 N and found to be 375 MPa (38 VHN) for PTFE/Pyrrolidone-1, 245 MPa (25 VHN) for PTFE/Pyrrolidone-2, 315 (32 VHN) MPa for PTFE/MoS2-1, 370 MPa (37 VHN) for PTFE/MoS2-2 and 240-250 (24-25 VHN) for Fluorocarbon, PEEK/PTFE and PEEK/Ceramic. Pure ATSP and ATSP blended with Fluoroadditives (Zonyl® TE-5069AN) were coated on aluminum disks whose diameter was 64 mm The nanoindentation hardness and reduced modulus of the ATSP coatings were 0.13 GPa and 3.23 GPa, respectively [23]. The pins (counterpart) for the oscillatory experiments (Fig. 2(a)) were cut to length and machined out of 52100 steel wrist pins to sit flat. The semi-cylindrical pins were oriented to create a line contact as illustrated in Fig. 2(a), and were 8 mm in diameter and 8 mm long with a 1 mm diameter hole to accept a miniature thermocouple that recorded temperature during tests, 2 mm below the surface. Uniform contact between the pin and the disk was ensured utilizing a self-aligning pin holder. Illustrated in Fig. 2(b) is the contact configuration for the unidirectional tests showing the rotating disk and the crowned pin (or shoe) in contact, simulating a swash-plate type compressor. Note that ATSP-based coatings were tested under different contact geometry, ball-on-disk (point contact).

(a)

Wrist pin/connecting rod interface 60º

52100 Steel Wrist Pin Iron Journal

Gray Cast Iron Disk (coated)

Oscillatory Testing

Normal load

52100 Steel Pin

60º

(b) Gray Cast Iron Disk (coated)

Normal load

Unidirectional Testing 52100 Steel Shoe

Fig. 2 (a) Cylindrical self-aligned pin-on-disk test configuration for oscillatory testing simulating a piston-type compressor, (b) crowned self-aligned pin (or shoe)-on-disk test configuration for unidirectional testing simulating a swash-plate type compressor.

36


A diamond-like-carbon (DLC) coating (Balinit® DLC) under oscillatory and unidirectional motion pin-on-disk tests was also examined and compared with one of the polymeric coatings. The DLC coating was an amorphous a-C:H carbon coating with a thickness of 2.5 µm and was deposited on 4340 quenched and tempered steel. The roughness of the DLC coating was 0.25 µm. The thickness of each coating was measured using cross section scanning electron microscopy (SEM), and a typical cross-section SEM image of the PTFE/Pyrrolidone-2 can be seen in Fig. 3 (showing both the cast iron substrate and the coating). Based on these measurements, the average coating thickness for polymer- based coatings varied from 20-35 µm, and 50 µm for the ATSP coatings, which is much thicker than the DLC coatings.

Coating

~ 20 µm

Cast iron substrate Fig. 3 Typical cross-section SEM image showing the coating thickness of PTFE/Pyrrolidone-2. Tribological Experiments. A custom-build High Pressure Tribometer (HPT) was used to perform the controlled tribological experiments. It is basically a pin-on-disk configuration consisting of a disk mounted to a rotor and a pin placed in a self-aligning fixture (Fig. 2). In-situ friction coefficient and near contact temperature were monitored during testing. A normal load of 445 N (100 lbs) was used for the oscillatory motion experiments at an oscillatory frequency of 4.5 Hz, oscillatory amplitude of 60 degrees, and an average wear track diameter of 47.6 mm in an environment of CO2 refrigerant (25 psi (0.17 MPa), 22 °C). A study of different test durations was also performed for PTFE/Pyrrolidone-1, 2 and PTFE/MoS2-1 coatings to investigate wear performance as a function of run-in time. Specifically, experiments at 2, 5, 10 and 60 min were carried out and wear profilometric measurements were performed in each case. Additionally, under CO2 atmosphere, the results for PTFE/Pyrrolidone-1 were compared with a DLC coating over 20-minute test durations at 60 °C. Scuffing experiments for PTFE/MoS2-1 and DLC coatings were performed under ambient air and unidirectional sliding conditions simulating swash-plate compressor operation. The rotational speed used was 1000 rpm which corresponds to a linear speed of 2.4 m/s, and a step-loading routine of 156 N every 15 s was applied until the point of scuffing failure. Ball-on-disk, unidirectional sliding experiments of ATSP-based coatings were conducted on a different tribometer (designated as HTT) in an ambient environment. Pure ATSP coatings were tested under mild operating conditions of 9.525 mm diameter sphere, 5 N normal load (initial Hertzian contact pressure of 0.075 GPa) and sliding speed of 58 mm/s. ATSP/Fluoroadditive coating was tested under more aggressive contact conditions, with a smaller sphere of 6.35 mm diameter, 10 N normal load (Hertzian contact pressure of 0.124 GPa) at the same sliding speed of 58 mm/s. Results and Discussion Friction and Wear of Polymer-based Coatings under Dry Oscillatory Testing. Experiments of different time durations (run-in) were performed to investigate wear performance as a function of run-in time. Four different experiments using different samples for each test were carried out and stopped at 2, 5, 10 and 60 min for all coatings. Wear profilometric measurement at the end of each test enabled understanding the wear evolution during tribological testing. Representative results for


37

PTFE/Pyrrolidone-1 coating are shown in Fig. 4. For the first 2 min, as shown in Fig. 4(a1), the friction coefficient starts at approximately 0.08 and gradually increases. The near contact temperature also increases at the start of the test shown in Fig. 4(a2). The profilometric measurement at the end of the 2-minute test showed significant wear of 15 µm deep, as seen in Fig. 4(a3). The trends are similar for the 5-minute test as shown in Fig. 4(b1) and (b2), with the wear being slightly higher than 15 µm. The friction coefficient reached a steady-state value at approximately 6 minutes, as shown in Fig. 4(c1). The temperature gradually increased, with the rate of increase being very slow (Fig. 4(c2)). The trends are similar for the 60-minute test shown in Fig. 4(d1) where the friction coefficient remains constant for the whole duration of the test. As seen in Fig. 4(d2), steady-state is reached at approximately 20 min. It is interesting to note that after 5 min, the wear does not increase, which means that most of the wear takes place early and the wear debris generated and remained trapped between the interface allows tribological interactions with no further significant increase in wear. This is evident from Fig. 4(d3), which shows a wear measurement very similar to the 5 and 10-minute tests shown in Fig. 4(b3) and (c3), respectively. Similar trends were observed for PTFE/Pyrrolidone-2 and PTFE/MoS2 coatings, but with distinct differences in the friction coefficient, temperature variation and wear measurements. The comparison of friction coefficient and wear rate values for all seven polymeric coatings is discussed in details at the end of this section. 0.2

80

0.1

40

20 40 60

0.1

(c1) 20 40 60

0.1

20 40 60

40

(b2) 0 0 80

20 40 60

40

(c2) 0 0 80

20 40 60

40

(d2)

(d1) 0 0

20 40 60

Time (min)

0 0

20 40 60

Time (min)

Profile (µm)

(b1)

0 0 0.2

0 0 80

20 40 60

0.1 0 0 0.2

(a2)

(a1)

Temperature (o C)

Friction coefficient

0 0 0.2

30 15 0 -15 -30 0 30 15 0 -15 -30 0 30 15 0 -15 -30 0 30 15 0 -15 -30 0

(a3)

4

8

12

(b3)

4

8

12

(c3)

4

8

12

(d3)

4

8

12

Scan Length (mm)

Fig. 4 Friction coefficient, near contact temperature and profilometric wear measurements for run-in periods of 2, 5, 10 and 60 min for PTFE/Pyrrolidone-1 in CO2 atmosphere (25 psi), 22 °C, 445 N and oscillatory conditions [24].

38


30

Average Wear Depth (µm)

28 26 24 22 20 18

C1 ● PTFE/Pyrrolidone-1 C2 ■ PTFE/Pyrrolidone-2 C3 ▲ PTFE/MoS2-1

16 14

0

10

20

30

40

50

60

70

Time (min) Fig. 5 Average wear depth as a function of time for PTFE/Pyrrolidone-1, PTFE/Pyrrolidone-2 and PTFE/MoS2 tested in CO2 (25 psi) and 22 °C at 445 N and oscillatory conditions. The third bodies that are forming from the wear debris and are not ejected away from the interface are having a beneficial role and support the contact load, preventing catastrophic tribological failure. The average wear depths for each of the three coatings tested at the different times of 2, 5, 10 and 60 min is summarized and depicted in Fig. 5. It can be seen that wear does not increase after 5 min, but rather reaches a steady-state value as described above.

PEEK/PTFE

3

Wear Rate (x10 mm /Nm)

PEEK/Ceramic

5

-5

4 3

PTFE/ Pyrrolidone-2

Fluorocarbon

PTFE/MoS2-1

2 PTFE/

Pyrrolidone-1

PTFE/MoS2-2

1 0.08

0.12

0.16

0.20

Coefficient of Friction

Fig. 6 Friction coefficient (x-axis) vs. wear rate (y-axis) of seven commercially available polymeric coatings in CO2 (22 °C, 25 psi) environment at 445 N normal load under dry-oscillatory conditions. The friction and wear performance for seven commercially available polymeric coatings including three coatings already examined in the run-in tests were directly compared under the same testing conditions and compared in Fig. 6. All coatings showed relatively low friction coefficient in the range


39

of 0.1-0.2 while their wear rate in the range of 10-5 mm3/N·m was higher than that of hard coatings such as DLC and CrN coatings in the range of 10-8-10-9 mm3/N·m [6, 25]. The friction and wear behavior seemed to be significantly affected by additives which polymeric coatings were blended with. Specifically, PTFE coatings blended with pyrrolidone showed the best friction performance with a low friction coefficient of 0.1. Pyrrolidone, usually is referred to as poly (vinyl pyrrolidone) in the literature and has been investigated for medical applications such as articular cartilage replacement due to its excellent low frictional properties [26,27]. The coatings blended with MoS2 (PTFE/MoS2-1, 2) had higher wear resistance than other coatings as seen in Fig. 6. Even though PEEK materials are usually known to be harder than PTFE, and thus expected to have better wear performance, this is not the case in our experiments, where polymers in the form of thin coatings were tested. Further experiments under different operating conditions would shed further light on the behavior of PTFE and PEEK-based coatings. Comparison between PTFE and DLC Coatings. Fig. 7 shows a typical experiment comparing a PTFE-based coating, PTFE/Pyrrolidone-1 and a DLC coating under 20-minute oscillatory conditions in a CO2 refrigerant atmosphere (25 psi and 60 °C, 445 N). The friction coefficient for PTFE/Pyrrolidone-1 started below 0.1 and reached a steady-state value after approximately 5-6 min, as noted earlier. Then, it remained constant for the whole duration of the 20-minute test assuming values around 0.1. The DLC coating had a higher friction coefficient and in contrast with PTFE/Pyrrolidone-1 coating, it exhibited values of approximately 0.4 at the start of the test reaching a steady-state value of around 0.2, after 6 min. Profilometric measurements showed no measurable wear on the DLC coating while the PTFE/ Pyrrolidone-1 coating wore as described previously in Fig. 4(d3). The higher values of the friction coefficient observed for the DLC coating are attributed to the testing environment, as DLC coatings are known to be sensitive to environment conditions [28].


0.5 0.4

DLC

0.3 0.2 0.1

C1 PTFE/Pyrrolidone-1 0 0

5

10

15

20

Time (min) Fig. 7 Friction coefficient measurements during testing in CO2 environment and 60 °C for DLC and PTFE/Pyrrolidone-1 at 445 N normal load under oscillatory conditions. One of the PTFE-based coatings, PTFE/MoS2-1, was also used to perform scuffing experiments (under ambient air and unidirectional sliding conditions). A step-load of 156 N every 15 sec was applied until the point of scuffing and the results were compared with the DLC coating. Fig. 8(a) shows the step-loading routine for PTFE/MoS2-1 coating. The capability of the machine was reached and thus a scuffing load could not be determined. The friction coefficient for the scuffing experiment is shown in Fig. 8(b). The scuffing load for the case of the DLC coating was approximately 454 N, significantly lower than that of PTFE/MoS2-1 coating, as shown in Fig. 8(c). The friction coefficient for the DLC coating is shown in Fig. 8(d). These experiments were performed twice and the results were repeatable (as the case for all the experiments reported in this work). Scuffing of DLC is viewed

40


as the failure of the coating resulting from coating penetration. Once penetration of the coating takes place, metal-to-metal contact occurs, and the coating or the wear debris can no longer provide any protection, resulting in macroscopically large contact areas of bare metal and a strong adhesion between the sliding surfaces [29]. On the other hand, a scuffing load was not reached for PTFE/MoS2-1 due to the presence of the beneficial third-body wear particles. Once again, the differences between the soft polymeric PTFE-based coatings and the hard DLC coatings are seen.

3000

0.3

(a)

0.2

1000 0 0 1 2 3 4 5 3000

(c)

2000 1000


Normal Load (N)

2000

(b)

0.1 0 0 1 2 3 4 5 0.3

(d)

0.2 0.1

0 0 1 2 3 4 5

0 0 1 2 3 4 5

Time (min)

Time (min)

Fig. 8 Scuffing experiments: (a) Normal load and (b) friction coefficient for the PTFE/MoS2-1 coating, (c) Normal load and (b) Friction coefficient for the DLC coating [24]. ATSP-based Coatings. The tribological behavior of pure ATSP coatings were first evaluated using ball-on-disk experiments in ambient environment conditions at 5 N normal load (0.075 GPa) and 58 mm/s sliding speed. A typical cross section of the wear the track after testing is shown in Fig. 9(a). In the case of pure ATSP coatings, significantly high friction coefficient around 1.0 was observed. Even though the coating was not completely penetrated, its wear depth is still relatively high on the order of 12 µm, resulting in a wear rate of 1.62 × 10-4 mm3/N·m. Therefore, the bare ATSP coating with high wear rate and friction coefficient is undesirable for typical low friction tribological applications, as in compressors. The ATSP coatings were then tested with blended fluoroadditive powder lubricant (Zonyl® TE-5069AN). Initially, they were tested under the same conditions as in the case of pure ATSP coatings, but there was no measurable wear observed on the ATSP/Fluoroadditive coatings. Therefore, more aggressive testing conditions of 0.124 GPa were implemented to generate wear. The experiments were performed for a duration of 60 min, corresponding to a sliding distance of 210 meters. The friction coefficient in this case started at 0.2 and increased slightly during the run-in period until stabilizing at an average value of 0.265 which is lower than that of pure ATSP coatings due to the lubricity effects of the fluoroadditives. Also, it was seen that the wear rate was significantly reduced to 7.36 × 10-7 mm3/N·m (note that this wear rate is at higher contact pressure than that of pure ATSP coatings). As can be seen in Fig. 9(b), the wear depth is around 0.34 µm, which is almost unnoticeable compared to the wear depth of pure ATSP coatings (and the PTFE and PEEK-based commercial coatings described earlier). To directly compare the improved tribological performance of ATSP/Fluoroadditive coatings with the state-of-the-art commercially available PTFE-based coatings, PTFE/Pyrrolidone-1 which already showed the best performance in the previous section of oscillatory testing was tested under the exact same conditions as in the case of ATSP/Fluoroadditive coatings. The friction coefficient was stable


41

throughout the whole duration of the test with an average value of 0.2 which was slightly lower than that of ATSP/Fluoroadditive coatings. However, the wear behavior of PTFE/Pyrrolidone-1 coating was very similar to that of pure ATSP coating as seen in Fig. 9(c), and its wear rate was estimated to be 1.23 × 10-4 mm3/N·m, which was over 150 times higher than for ATSP/Fluoroadditive coatings. 10

(a)

~ 0.4 mm

0.2

(b)

~ 0.3 mm

0.1

Profile (µm)

Profile (µm)

5

~5 μm 0 -5

~12 μm

0 -0.1

~ 0.34 μm

-0.2 -0.3

-10

-0.4

Pure ATSP

-15 0

0.5

1

1.5

2

2.5

-0.5 0

ATSP/Fluoroadditive 0.2

0.4

0.6

0.8

1

Scan Length (mm)

Scan Length (mm) 10

(c) Profile (µm)

5

~8 μm

~ 0.5 mm

0 -5

~10 μm -10 -15 0

PTFE/Pyrrolidone-1 0.5

1

1.5

2

Scan Length (mm)

Fig. 9 Typical wear tracks of (a) pure ATSP coating after 20-minute unidirectional sliding test at 5 N normal load, (b) ATSP/Fluoroadditive coating and (c) PTFE/Pyrrolidone-1 coating after 1-hour unidirectional sliding testing at 10 N normal load. Conclusions The tribological performance of commercially available PTFE-, PEEK-based and fluorocarbon coatings was evaluated under realistic compressor operating conditions. It was found that these coatings have excellent frictional characteristics as low as 0.1 friction coefficient. Even though these polymer-based coatings showed relatively weak wear resistance compared to hard coatings, wear debris generated at the tribological interface acted as a beneficial third-body resulting in high wear and scuffing resistance. It was seen that the friction and wear behavior were generally determined by the additives which polymeric coatings were blended with. Also, the polymeric coatings compared well against a DLC coating. Additionally, the tribological properties of newly developed ATSP coatings were evaluated using ball-on-disk experiments. Fluoroadditives added to pure ATSP coatings significantly decreased the friction coefficient from 1.0 to 0.265. ATSP/Fluoroadditive coatings also showed superior wear performance showing an extremely low wear rate of 7.36 × 10-7 mm3/N·m which was almost three orders of magnitude lower than that of the commercially available PTFE-based coating. References [1] S.R. Pergande, A.A. Polycarpou and T.F. Conry: J. Tribol. 126 (2004), P. 573-582. [2] T.A. Solzak and A.A. Polycarpou: Surf. Coat. Technol. 201 (2006), P. 4260-4265.

42


[3] A. Bloyce: World Pumps 2000(400) (2000), P. 43-45. [4] J. Robertson: Materials Science and Engineering R 37 (2002), P. 129-281. [5] A.Y. Suh and A.A. Polycarpou: J. Appl. Phys. 99, 08N111 (2006). [6] K. Holmberg and A. Matthews, in: Coatings Tribology: Properties, Mechanisms, Techniques and Applications in Surface Engineering, 2nd edition, edited by B.J. Briscoe, Elsevier B.V., Oxford, UK (2009) [7] T.A. Solzak and A.A. Polycarpou, in: Proceedings of the 9th Biennial ASME Conference on Engineering Systems Design and Analysis, ESDA2008-59380, Haifa, Israel (2008). [8] Q. Zhao, Y. Liu, H. Müller-Steinhagen and G. Liu: Surf. Coat. Technol. 155 (2002), P. 279-284. [9] H.C. Sung: Wear 221 (1998), P. 77-85. [10] R.L. Fusaro: Tribology International 23 (1990), P. 105-122. [11] M. Yamane, T.A. Stolarski and S. Tobe: Wear 263 (2007), P. 1364-1374. [12] N.P. Suh, in: Tribophysics, Prentice-Hall, Englewood Cliffs, NJ (1986). [13] W.X. Chen, F. Li, G. Han, J.B. Xia, L.Y. Wang, J.P. Tu and Z.D. Xu: Tribol. Lett. 15(3) (2003), P. 275-278. [14] W.G. Sawyer, K.D. Freudenberg, P. Bhimaraj and L.S. Schadler: Wear 254 (2003), P. 573-580. [15] G. Zhang, W.-Y. Li, M. Cherigui, C. Zhang, H. Liao, J.-M. Bordes and C. Coddet: Progress in Organic Coatings 60 (2007), P. 39-44. [16] W. Hufenbach, K. Kunze and J. Bijwe: J. Synthetic Lubrication 20(3) (2003), P. 227-240. [17] J. Bijwe, S. Sen and A. Ghosh: Wear 258 (2005), P. 1536-1542. [18] B. Lal, S. Alam and G.N. Mathur: Tribol. Lett. 25(1) (2007), P. 71-77. [19] D.L. Burris and W.G. Sawyer: Wear 261 (2006), P. 410-418. [20] D.L. Burris and W.G. Sawyer: Wear 262 (2007), P. 220-224. [21] J. Zhang, N.G. Demas, A.A. Polycarpou and J. Economy: Polym. Adv. Technol. 19 (2008), P. 1105-1112. [22] N.G. Demas, J. Zhang, A.A. Polycarpou and J. Economy: Tribol. Lett. 29 (2008), P. 253-258. [23] J. Zhang, A.A. Polycarpou and J. Economy: Tribol. Lett. In press. [24] N.G. Demas and A.A. Polycarpou: Surf. Coat. Technol. 203 (2008), P. 307-316. [25] S. Yang, K.E. Cooke, X. Li, F. McIntosh and D.G. Teer: J. Phys. D: Appl. Phys. 42, 104001 (2009), P. 1-8. [26] S. Nagaoka and R. Akashi: Biomaterials 11(6) (1990), P. 419-424. [27] J.K. Katta, M. Marcolongo, A. Lowman and K.A. Mansmann: J. Biomed. Mater. Res. A 83A(2) (2007), P. 471-479. [28] A. Grill: Wear 168 (1993), P. 143-153. [29] K.C. Ludema: Wear 100 (1984), P. 315-331.