Friction and wear of diamond-containing polyimide composites in water and air

Friction and wear of diamond-containing polyimide composites in water and air

Wear 257 (2004) 1096–1102 Friction and wear of diamond-containing polyimide composites in water and air Akihiro Tanaka∗ , Kazunori Umeda, Sokichi Tak...

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Wear 257 (2004) 1096–1102

Friction and wear of diamond-containing polyimide composites in water and air Akihiro Tanaka∗ , Kazunori Umeda, Sokichi Takatsu Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, Ibaraki 305-8565, Japan Received 14 November 2003; received in revised form 28 June 2004; accepted 28 June 2004 Available online 6 October 2004

Abstract Polyimide-based composites containing fine diamond powder were fabricated using spark plasma sintering. The based material was polyimide (PI) containing a small amount of polytetrafluoroethylene (PTFE). Two types of diamond powder were used: one synthesized by statically high pressure, i.e., high-pressure diamond (HD), and the other synthesized by shock compression, i.e., shock-compression diamond (SD). We evaluated their tribological properties using a reciprocating friction tester in water and air using an Al2 O3 mating ball. Adding HD to the polyimide–PTFE-based material decreased the composite’s friction in water, but the effect of this addition in air was negligible. The specific wear rate of composites with different HD content was similar to that of the based material alone in water, while the wear of composites decreased with the addition of diamond in air. The effect of diamond powder size on friction and wear of composites was generally low in both water and air. The addition of SD decreased the friction coefficient of composites, but SD content only negligibly affected the friction in water and air. The specific wear rate was minimal at SD content of 5 vol.%, when diamond content was varied. Wear was almost independent of diamond powder size. SD reduced composite friction and wear better than HD; regardless of environment, its friction coefficient was less than 0.1 and the specific wear rate was in the level of 10−7 mm3 /N m in both water and air. © 2004 Elsevier B.V. All rights reserved. Keywords: Diamond; Polyimide; Composite; Friction; Wear; Water environment

1. Introduction The leakage of oil from oil hydraulics widely used to drive machines and instruments pollute plants where they are used, together with soil, rivers, and the sea. A potential alternative, water hydraulics, presents technical problems of controllability, corrosion, reliability, and tribology involving high friction, wear, and seizure. Diamond has attracted attention due to its superior hardness, wear resistance, corrosion resistance, heat conductivity, and its fairly low friction in liquid and air environments [1]. Diamond coatings [2–10] have been targeted for use where machine parts come into contact. Adhesion between diamond film and substrates is critical, and limitations on adhesive ∗

Corresponding author. Tel.: +81 29 861 7069; fax: +81 29 851 5425. E-mail address: [email protected] (A. Tanaka).

0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.06.003

force restrict diamond film applications. In addition to its use in coatings, diamond is used as a component of powdersintering composites. The composite releases material from the restraint of adhesion, but research into composites containing diamond powder has lagged [11,12]. We fabricated polyimide-based composites containing fine diamond powder by spark plasma sintering using two types of differently synthesized diamond, and evaluated their friction and wear properties using a reciprocating friction tester in water and air. 2. Experimental 2.1. Specimens Tested composites were made of polyimide (PI), polytetrafluoroethylene (PTFE), and diamond powder using spark

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Fig. 3. Ball-on-block-type reciprocating friction tester. Fig. 1. Main part of spark plasma sintering process.

plasma sintering (SPS) process. This process is a pressure sintering process utilizing on–off DC pulse energizing (Fig. 1), as detailed elsewhere [13,14]. Kerimid 1010 (Nippon Polyimide Co.) with a powder size of less than 50 ␮m was used for PI, and Aflon G350 (Asahi Glass Co.) with an average powder size of 350 ␮m as PTFE. Two types of diamond powder were used. One was synthesized by statically high pressure, i.e., high-pressure diamond (HD, ‘man-made industrial diamond’; General Electric Co.), and the other was synthesized by shock compression, i.e., shock-compression diamond (SD, ‘scm fine dia’; Izumi Technology Co. Ltd.). HD powders were <1, 1–2, 3–6, and 6–12 ␮m in size and SD powders were <0.1, 0.25–0.75, 1.25–2.25, 3–5, 6–10, and 15–25 ␮m in size (Fig. 2).

The powders were blended with a stirrer in ethanol and then dried. Blended powders composed of PI, PTFE, and diamond were laid as diamond-containing surface layer on PI substrate powder in a graphite mold of SPS. Then the composite was sintered at a pressure of 50 MPa and a temperature of 220 ◦ C, which was monitored by a thermocouple inserted into the mold for 5 min. Some composites were sintered at higher temperatures such as 250 and 280 ◦ C, but some cracks were found in sintered composites and their antiwear properties were bad. Composites were 20 mm in diameter and 6-mm thick, with a 1-mm thick diamond-containing surface layer. The diamond-containing layer was composed of the blend of PI–20 vol.% PTFE (PI–PTFE) and diamond; the diamond content was varied from 5 to 30 vol.%. The surfaces of composites were ground by emery papers with different particle sizes, and then polished with alumina slurry (the powder size: <2 ␮m). The average surface roughness Ra of some polished composites was in the range from 0.1 to 0.6 ␮m; the surface roughness seemed to increase with the increase of the diamond content or powder size. The observation of polished composite surface showed that diamond powders were fairly dispersed in PI–PTFEbased material, although some powder cohesion existed. We measured the Vickers hardness of some composites, i.e. PI–PTFE alone; PI–PTFE containing the finest HD contents of 5 or 10%, and PI–PTFE containing the finest SD contents of 5 or 10%, at a load of 4.9 N. Their average hardness was in the range from 0.27 to 0. 30 GPa, irrespective of the type and content of diamond. 2.2. Friction tests

Fig. 2. SEM photographs of diamond powders.

Friction and wear tests were done using a ball-on-blocktype reciprocating friction tester (Fig. 3). The mating was an Al2 O3 ball of 9.2 mm diameter. The reciprocating friction stroke was 10 mm and tests were conducted at a normal spring-driven load of 25 N. Average sliding speed was 20 mm/s (60 cycle/min) and the number of cycles was 14,400. Test environments were deionised water and ambient air. During tests, the friction coefficient was continuously measured using a load cell. The wear volume of the composite was calculated by measuring wear scars with a surface profilometer. For some part of friction tests, two experiments were con-

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Fig. 5. Friction coefficient of composites containing <1 ␮m HD.

Fig. 4. Friction behavior of composites with different diamond content.

ducted to examine the scatter of friction coefficient and wear rate; the repeated friction tests showed that the scatter of friction and wear was fairly small as shown in later figures.

3. Results and discussion 3.1. Friction behavior The friction behavior of composites with different diamond powder and size is shown in Fig. 4. For HD, the friction coefficient decreased during early testing, thereafter approached a steady state, regardless of diamond powder size and the sliding environment. The friction fluctuation in water was larger than that in air. The friction behavior of composites with other diamond content was similar to those shown in Fig. 4. For SD, the friction coefficient was stable from very early sliding, irrespective of the environment. Another composite with different SD diamond size and content showed similar friction behavior.

vol.%, although the friction of the composite with a diamond content of 30% was clearly greater than that of PI–PTFE alone. Specific wear rate in composites with different HD content was similar to that of the PI–PTFE alone in water. Composite wear in air, however, decreased with the addition of diamond, apart from the composite containing 30% diamond. Composites of 5–20% diamond had low wear of 10−7 mm3 /N m, independent of diamond content. No clear difference was seen in the friction coefficient between water and air, apart from the PI–PTFE alone. Wear in water was somewhat less than that in air, regardless of diamond content. Figs. 7 and 8 show the friction and wear of composites with different SD content at the size of less than 0.1 ␮m. In water, the friction coefficient decreased to less than 0.05 when diamond was added to PI–PTFE, but SD content did not seem to affect friction. Friction in air decreased slightly when 5 or 10% SD was added, although the composite with SD content of 15% had higher friction than that of PI–PTFE alone. Dependence of the specific wear rate on diamond content is common to both water and air. When diamond content was increased, wear was initially reduced, reached a minimum, and then increased. The minimum wear rate was in the level of 10−7 mm3 /N m in both water and air. The friction of composites in water was fairly less than that in air, while the difference in wear was unclear regardless of diamond content.

3.2. Effect of diamond content The relationship between friction and wear of composites containing HD with powder size of less than 1 ␮m and content are shown in Figs. 5 and 6. The vertical bars in figures show the scatter ranges of two tests. In water, composites with a diamond content of 5% had a friction coefficient of about 0.08, while PI–PTFE alone had a friction of about 0.13. For composites containing 10 or 15% diamond, friction increased only negligibly. In ambient air, however, adding diamond slightly reduced friction at a diamond content of 5–20

Fig. 6. Specific wear rate of composites containing <1 ␮m HD.

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Fig. 7. Friction coefficient of composites containing <0.1 ␮m SD. Fig. 10. Effect of powder size on specific wear rate of composites containing HD.

Fig. 8. Specific wear rate of composites containing <0.1 ␮m SD.

3.3. Effect of diamond powder size

of smaller than 3–6 ␮m in water was almost independent of diamond powder size, although that of composite with the coarsest powder was notably greater. In air, wear of composites appeared to decrease slightly with increasing powder size at less than 3–6 ␮m, but wear of composites with the coarsest powders was somewhat greater. Fig. 11 shows the relationship between the specific wear rate of composites containing SD and diamond powder size. The wear rate decreased a little when SD was added, independent of the powder size, except for the composite with the coarsest powders of SD (15–25 ␮m). No clear difference was seen in wear between water and air except for composites with SD powders of <0.1 or 15–25 ␮m. 3.4. Friction surface

Figs. 9 and 10 show the effects of HD powder size on friction and wear in composites with a diamond content of 5%. The friction coefficient of the composite in water appeared to be negligibly affected by powder size. In air, the friction of the composite with the coarsest powders was fairly high, although the effect of powder size was generally very low. The specific wear rate of composites with powder sizes

Friction surfaces on both the composite and Al2 O3 mating ball were observed by optical microscopy and SEM. Fig. 12 shows those on the composite with 5% HD powders of <1 ␮m. In water, numerous abrasion marks appeared on friction surfaces of both the composite and mating ball. The high-magnification SEM photograph showed the exis-

Fig. 9. Effect of powder size on friction coefficient of composites containing HD.

Fig. 11. Effect of powder size on specific wear rate of composites containing SD.

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Fig. 12. Friction surfaces on composite containing <1 ␮m HD and mating ball.

tence of cracks on friction surfaces of composites; the cracks on composite tested in water seemed to be richer than those in air. The friction surface contour on the mating ball was unclear. In air, abrasion marks were fine, although not clear in the photograph, and thin films appeared to form to some degree on the friction surface of the composite. The friction surface contour on the mating ball was clear, and numerous wear fragments accumulated at both the beginning and end of the friction surface. The abrasion marks on both composite and mating ball surfaces must be caused by diamondcontaining wear fragments released from composites. Fig. 13 shows friction surfaces of the composite with 5% SD powders of <0.1 ␮m. In water, the friction surface on the composite was smooth and appeared partially covered with thin film, but partial detachment of the surface layer was noticeable. The friction surface contour on the mating ball was fairly clear. In air, the friction surface on the composite was also smooth, but surface layer detachment less marked. Considerable part of the friction surface on the mating ball was covered with

transferred materials and numerous wear fragments scattered around the friction surface. 3.5. Discussion In both water and air, the friction and wear of HD composites were affected only negligibly by diamond content and powder size, expect for composites with very high diamond content or very large diamond particles. For SD, the effects of diamond content and powder size on friction were also fairly low in both water and air, although the composite with 5% diamond content appeared to have the minimum wear rate. Furthermore, we compared the effect of HD and SD on friction and wear in composites, focusing on composites containing 5% HD powders of <1 ␮m and that containing 5% SD powders of <0.1 ␮m (Fig. 14). We considered the comparison of two composites with powder sizes about one order of magnitude different, which was adequate since the effect of SD powder sizes was small; as shown in Figs. 11 and 14, it

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Fig. 13. Friction surfaces on composite containing <0.1 ␮m SD and mating ball.

was seen that SD was superior in reducing friction and wear in both water and air, i.e., the friction coefficient was <0.1 and the specific wear rate was in the level of 10−7 mm3 /N m. These were similar or superior to conventional antiwear mate-

Fig. 14. Friction and wear of composites with HD or SD diamond content of 5 vol.%.

rials such as metal containing graphite and PTFE containing carbon fiber. The difference between HD and SD may be attributable to the difference in powder shape, i.e., HD powders had sharper edges (Fig. 2) that abraded both the composite and mating material more easily than SD powders. In addition, the fact that HD powders had a significantly different aspect ratio to the more spherical SD powders appeared to affect the difference in friction and wear between HD and SD. We, therefore, concluded that friction and wear in composites containing HD were higher than in composites containing SD — a finding supported by the fact that abrasion marks were clear on friction surfaces of the composite containing HD but negligible on the friction surface of that containing SD (Figs. 12 and 13). In composites containing either HD or SD, the wear rate in water was roughly greater than that in air, although the dependence of friction on the environment appeared to be reversed. In composites containing HD, abrasion was pro-

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moted by diamond powders detached from the composite in water, compared to abrasion in air, i.e., composite wear increased in water (Fig. 12). This may differ for composites containing SD because abrasion marks were negligible even on the friction surface of the composite in water. In composites containing SD, thin surface and noticeable detachment of surface layers were seen on the friction surface in water, although the detachment of the surface layer was less in air. No transferred material was seen on the friction surface of the mating ball in water, although it was clearly present on the friction surface in air. The difference in friction and wear of composites containing SD in water and air thus appears to correlate with the difference in the extent of surface layer detachment and material transfer, although details remain to be clarified.

4. Conclusions We fabricated polyimide-based composites containing two types of diamond powder using spark plasma sintering. Their tribological properties were evaluated using a reciprocating friction tester in water and air. The following results were obtained: (1) Adding HD to a polyimide–PTFE-based material decreased the friction of the composite in water, although this effect was negligible in air. The specific wear of composites with different HD content was similar to that of the polyimide–PTFE alone in water, while the wear of composites decreased with increasing diamond content in air. The effect of diamond powder size on composite friction and wear was generally low in both water and air, except for very coarse powder. (2) Adding SD decreased the friction coefficient of composites, but SD content only negligibly affected friction in water and air. Specific wear rate was minimal at an SD content of 5 vol.% when diamond content was varied. Wear was almost independent of diamond powder size. (3) SD reduced composite friction and wear regardless of the environment, and its friction coefficient and specific

wear rate in both water and air was <0.1 and in the level of 10−7 mm3 /N m, respectively.

References [1] Y. Enomoto, S. Miyake, S. Yazu, Friction and wear of synthetic diamond with and without N-implantation and CVD diamond coating in air, water and methanol, Tribol. Lett. 2 (1996) 241–246. [2] R.L. Wu, A.K. Rai, A. Garscadden, P. Kee, H.D. Desai, K. Miyoshi, Synthesis and characterization of fine-grain diamond films, J. Appl. Phys. 72 (1) (1992) 110–116. [3] W.D. Fan, H. Wu, K. Jagannadham, B.C. Goral, Wear resistant diamond coatings on alumina, Surf. Coat. Technol. 72 (1995) 78–87. [4] A. Erdemir, M. Halter, G.R. Fenske, C. Zuiker, R. Csencsits, A.R. Krauss, D.M. Gruen, Friction and wear mechanisms of smooth diamond films during sliding in air and dry nitrogen, Tribol. Trans. 40 (4) (1997) 667–675. [5] T.L. Huu, D. Paulmier, A. Grabchenko, M. Horvath, I. Meszaros, A.G. Mamalis, Autolubrication of diamond coating at high sliding speed, Surf. Coat. Technol. 108–109 (1998) 431–436. [6] P. Hollman, O. Wanstrand, S. Hogmark, Friction properties of smooth nanocrystalline diamond coatings, Diamond Relat. Mater. 7 (1998) 1471–1477. [7] T. Grogler, E. Zeiler, A. Franz, O. Plewa, S.M. Rosiwal, R.F. Singer, Erosion resistance of CVD diamond-coated titanium alloy for aerospace applications, Surf. Coat. Technol. 112 (1999) 129–132. [8] M.I. De Barros, L. Vandenbulcke, J.J. Blechet, Influence of diamond characteristics on the tribological behavior of metals against diamond-coated Ti-6Al-4V alloy, Wear 249 (2001) 68–78. [9] G. Straffelini, P. Scardi, A. Molinari, R. Polini, Characterization and sliding behavior of HFCVD diamond coatings on WC-Co, Wear 249 (2001) 461–472. [10] S.P. Hong, H. Yoshikawa, K. Wazumi, Y. Koga, Synthesis and tribological characteristics of nanocrystalline diamond film using CH4 /H2 microwave plasmas, Diamond Relat. Mater. 11 (3-6) (2002) 877–881. [11] Q. Ouyang, K. Okada, Friction properties of aluminium-based composites containing cluster diamond, J. Vac. Sci. Technol. A12 (1994) 2577–2580. [12] K. Hanada, N. Nakayama, M. Mayuzumi, T. Sano, H. Takeishi, Tribological properties of Al–Si–Cu–Mg alloy-based composite dispersing diamond nanocluster, Diamond Relat. Mater. 11 (3–6) (2002) 749–752. [13] M. Tokita, Trends in advanced SPS systems and FGM technology, in: Proceedings of the NEDO International Symposium on Functionally Graded Materials, 1999, pp. 23–33. [14] Home page of Sumitomo Coal Mining Co. Ltd., http://www.scmsps.com/e htm/whatsps e htm/whatsps e.htm.