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Effects of rare earth treatment on tribological properties of self-lubricating spherical plain bearings
Wear 305 (2013) 274–279
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effects of rare earth treatment on tribological properties of selflubricating spherical plain bearings$ Ming Qiu n, Yingchun Li, Long Chen, Yaoxing Bai Henan University of Science and Technology, Luoyang, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2012 Received in revised form 4 December 2012 Accepted 8 December 2012 Available online 27 December 2012
The effects of the rare earth treatment on the friction, wear, and bonding behavior of polymeric, selflubricating spherical plain bearing materials were investigated. The materials of interest consisted of woven Kevlar (TM) and poly-tetrafluoroethylene (PTFE) fibers. An oscillating wear tester, operating at angles of 7 101, four different frequencies, and a constant load, was used to test assembled bearings at room temperature. Worn surface morphologies of the liners were investigated by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Compared with bearings without rare earth treatment, the bonding qualities between the binder and the surrounding steel housing, and the wear resistance of the treated bearings were both greatly improved. Contact surface morphology indicates that the treated bearing liners experienced only slight amounts of adhesive wear while those without the treatment seemed to wear more by abrasion. Overall, rare earth treatments served both to improve bonding quality of the bearing liner and improve their tribological properties. & 2013 Elsevier B.V. All rights reserved.
Keywords: Rare earth Spherical plain bearing Friction Wear
1. Introduction PTFE fiber can be used as the liner of the spherical plain bearing because of low friction coefficient, excellent corrosion resistance and other advantages. But the application scope of PTFE fiber, as self-lubricating liner, is restricted owing to the huge wear rate [1–4]. To this end, some researchers carried out a large number of research works about hybrid self-lubricating materials made of some functional fibric and PTFE fabric to advance wear property of PTFE [5,6]. The Kevlar fiber with PTFE fabric knitting has been widely used to improve the wear resistance, mechanical properties and dimensional stability due to the high tensile strength and high elastic modulus [7]. However, the woven liner of Kevlar and PTFE fabric exhibits poor adhesion to adhesive resin due to its chemically inert surface, which leads to poor interface binding force between the liner and the adhesive, thus affect the tribological performance of the self-lubricating liner. There are many reports related to the approaches used to modify the fiber surface [8–11]. Among those approaches, rare earth treatment seemed to be a simple and interesting method [12,13]. It is well known that rare earth can be widely used in electronics, metallurgy, and chemical engineering, etc. The rare earth elements have unique chemical properties and strong activity and the rare earth atom shows larger effective nuclear charge because its 4f
$ n
This paper was presented at the WOM 2013 conference, Portland, Oregon USA. Corresponding author. Tel./fax: þ 86 379 64231479. E-mail address: [email protected] (M. Qiu).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.021
electron is weakly bonded for the nuclear, which can affect the properties of the materials [14]. Rare earth atom is a typical hard ion and has strong complex ability and strong ionic bond between the atomic coordination. Therefore, rare earth treatment can improve the interfacial adhesion force of the composite material and its tribological properties [15,16]. In this paper, in order to improve the bonding quality of liner and the tribological properties of the self-lubricating spherical plain bearing, the liners were treated with the rare earth and the influences of the rare earth treatment on bonding quality and tribological properties of selflubricating spherical plain bearing are investigated.
2. Experiments 2.1. Test preparation The basic dimensions of a test spherical plain bearing are shown as follows: outer ring diameter F35 mm, outer ring width 12 mm, inner ring diameter F20 mm, inner ring width 16 mm, ball diameter F29 mm. The materials of inner and outer ring were made of GCrl5 steel and the liner was the mixed woven material of the Kevlar and PTFE fiber. The one rich in PTFE fiber was used as a friction surface while the other rich in Kevlar fiber was used as a binding surface in Fig. 1. The mixed woven liner was treated by the rare earth before bonding, and the detailed steps were shown as follows: (1) the liner was immersed in acetone solution for 10–14 h at room temperature, then was dried in the 60–80 1C oven for 10–20 min. (2) The liner of the acetone treatment was
M. Qiu et al. / Wear 305 (2013) 274–279
275
PTFE fiber
Kevlar fiber
Fig. 1. The structure of woven PTFE and Kevlar fibers. (a) The surface rich in PTFE fibers; (b) the surface rich in Kevlar fibers.
immersed in rare-earth modifier solution for 2–3 h, and then they were dried in the 80–120 1C oven for 20–30 min. The components of the rare earth modifier solution mainly included lanthanum chloride (LaCl3 0.3%), deionized water (H2O 99.2%), ammonium chloride (NH4Cl 0.2%), nitric acid (HNO3 0.1%) and urea (CON2H4 0.2%). The adhesive was bi-component enhanced phenolic adhesive resin (204 phenolic adhesive) purchased from Shanghai Xinguang Chemical Plant of China. The dried liner was glued on the inner surface of outer ring, and then they were placed in calorstat oven at the temperature of 100 1C for 4 h after fixing the inner ring. Finally, the friction and wear test specimens can be obtained. 2.2. Test apparatus Bonding quality was estimated by the liner peel strength. The self-made peel fixtures were used to fix the peel sample which was gripped in the Instron5944 electronic universal material test machine, then the two fixtures were away from the liner at a certain speed and the liner was peeled down from the outer ring of the bearing. The peel test diagram is shown in Fig. 2. The bonding force and displacement information were collected and stored with the sensor and data acquisition system. The peel strength data were calculated with the data processing software. The specific calculation formula of the peel strength s is shown as follows:
Fig. 2. The diagram of peel strength test.
S s¼C LB where C is the load of the unit highly in the peeling curve, N/mm; S is the area of the peeling curve, mm2; L is the length of the bottom line in the peeling curve, mm; B is the width of the peeling sample, mm. The tests of the tribological properties were conducted with the testing machine which was a self-made friction and wear tester for spherical plain bearing shown in Fig. 3. By using a torque sensor, a lever indicator and a thermocouple, changes of the friction torque, wear loss and friction temperature were detected. The test conditions were as follows: (1) oscillating frequencies were 2.0 Hz, 2.5 Hz, 3.0 Hz, 3.5 Hz; (2) the oscillation range was 7101; (3) the applied pressure on the bearing was 52 MPa (applied load 12.5 kN); (4) tests were done at room temperature. After mounting the bearing in the test machine and after a static load was applied for 15 min, the torque sensor and wear testing sensor were activated and the test was run for 25,000 oscillating cycles. Finally, the wear surface morphologies of the liners were analyzed by the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS).
3. Results and discussion 3.1. The liner bonding quality Based on the bonding quality test aerospace standard SAEAS81820 of the spherical plain bearings in USA [17], the test program
Fig. 3. Schematic diagram of a spherical plain bearing testing machine.
is as follows: the peeling angle is 104.32–178.871. The peeling speed is 19 mm/min and the acquisition interval is 2 ms. The adhesion degrees of the liners were examined before peeling and 90% tightly adherent must be ensured. The reliabilities of the liners were examined after peeling and the situation of the unbonded area equal to 25% of the outer ring width should not be allowed. The results are shown in Table 1. It can be seen from the Table 1 that the peel strength of the test bearings is all higher than the value of 0.357 N/mm which is obtained with the formula in the standard SAE-AS81820, so the peel strength of all test bearings can meet the standard requirements. However, the peel strength of liner treated with the rare earth is two times as large as the untreated liner which can be seen obviously from the Table 1. It shows that the bonding quality
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Table1 Results of the liner peeling strength. Sample
Peeling strength (N/mm)
Liner adhesion degree
Bonding integrity test
Untreated Rare earth treated
0.64 1.305
Qualified Qualified
Qualified Qualified
liner are shown in Fig. 5 under the condition of the four oscillating frequencies. It can be seen from Fig. 5(a) that the friction coefficients of the bearings with untreated and rare earth treated liners are increasing with the incremental swing frequency. Besides, the friction coefficients of bearings with the rare earth
Fig. 4. Peeling curves of the two kinds of bearings. (a) Untreated and (b) rare earth treated.
of the liner with rare earth treatment has been greatly improved. The peeling curves of bearings with the two kinds of liner are shown in Fig. 4. It can be seen that the peeling curve for untreated bearing is fluctuating much more obvious, so it indicate that the untreated liner for spherical plain bearings did not bond well with outer ring and uneven situation may exist in the bonding process, which clearly affects the bonding quality of the self-lubricating spherical plain bearing. But the peel curve of the rare earth treated bearing is relatively stable that explains the liner treated with rare earth can adhere evenly to the outer ring and its bonding quality is better. 3.2. Friction and wear properties The curves of friction coefficient, wear loss and friction temperature of the bearings with untreated and rare earth treated
Fig. 5. The characteristic curves of the bearings with two kinds of liners: (a) friction coefficient; (b) wear loss; (c) friction temperature.
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treated liners are lower than those of bearings with untreated liners when the swing frequency is from 2 Hz to 3.5 Hz, and the values of friction coefficients of the bearings with rare earth treated liners are stable within the test swing frequency scope. However, the friction coefficients of the bearings with untreated liners are higher and their frictional performances are worse. The curve of wear loss is shown in Fig. 5(b), and it can be seen that the wear losses of the bearings with both kinds of liners are increasing with the increasing of swing frequencies. The wear losses of the bearings with the rare earth treated liners are lower than the bearing with untreated liner under the contact pressure of 52 MPa and the swing frequencies of 2–3.5 Hz. It shows that spherical plain bearings with rare earth treated liner have excellent wear properties in the experimental condition. The curve of friction temperature is shown in Fig. 5(c), it can be seen that the friction temperatures of bearings with both kinds of liners are increasing with the increasing of swing frequency and the friction temperatures of the bearings with rare earth treated liner are lower than those of the bearings with untreated liners. It indicates that the bearings with rare earth treated liner reduce more heat generation than those of the bearings with untreated liners in the experimental condition. This reason is that the lower friction coefficient generates the lower friction heat. After some comprehensive analysis the tribological performance of the bearings with two kinds of liners concludes that the friction coefficient, wear loss and friction temperature of the spherical plain bearings with rare earth treated liner are lower obviously. It shows that the tribological performances of the bearings with rare earth treated liners are improved and increased evidently in the experimental condition. 3.3. SEM and EDS analysis of worn liner surface The SEM morphologies of the worn liners are shown in Fig. 6. It can be seen that the treated liners not only ensure that the adhesive force between the fabric liner and the outer ring of the
277
bearing was greatly improved, but also ensure that the selflubricating function of the PTFE woven liner was fully exerted. In the same condition, the worn surface of the woven liner with the rare earth treatment was lighter than that of the untreated liner. The SEM morphologies of the worn liner untreated are shown in Fig. 6(a) and (b), and it can be perceived that the untreated liners have been seriously damaged. Moreover, the PTFE fibers of the liner had basically been worn completely and the Kevlar fibers had been worn and broken messily. It can be seen that the base material of the untreated liner was worn and broken from the SEM morphologies of 200 times (Fig. 6(a)). It shows that the material of the untreated liner has also been badly damaged. Consequently, in the untreated condition, the material of the liner emerged more serious adhesive wear after suffering heavier loads and higher swing frequency. A portion of the fiber matrix material is torn apart or even fallen off and most of them are changed into the abrasive particle. Another portion of the fiber matrix material is adhered on the outer surface of the bearing inner ring or extruded out along with the rotation direction. The EDS spectrum of the untreated wear liner is shown in Fig. 7(a), and it can be seen clearly that lots of Fe elements were transferred to the untreated liner from the inner ring outside surface of the bearing. This explains that the untreated liner was suffered with severe adhesive wear and leads to the badly friction state. The reason is that the lubrication of PTFE transfer film has not been fully exploited, which makes the friction temperature becomes higher, even leads to the abrasive particle of the inner ring including chrome and iron elements being gradually transferred and embedded in the gap of the fibers. Finally, adhesive wear can be found and the state of friction deterioration exists. With the self-lubricating material PTFE fiber has been worn completely and the Kevlar fiber has been broken, the untreated liner is severely damaged that causes the self-lubricating function of liner to be declined and even failure. Finally, it makes friction coefficient bigger, friction temperature higher and causes the state of friction deterioration, so leads to failure of the bearing.
Fig. 6. SEM morphologies of the worn liners (f¼ 3.5 Hz, P¼52 MPa). (a) Untreated (200 ); (b) untreated (35 ); (c) rare earth treated (200 ); (d) rare earth treated (35 ).
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Fig. 7. The untreated and rare earth treated EDS photos of the wear liner (f¼ 3.5 Hz, P¼ 52 MPa). (a) Untreated; (b) rare earth treated.
The SEM morphologies of the worn rare earth treated liner are shown in Fig. 6(c) and (d), and it can be perceived that the treated liner has a smoother surface under the condition of contact pressure 52 MPa and the swing frequencies 3.5 Hz. It can be seen that the rare earth treated liner was slightly worn relative to the untreated liner from the SEM morphology of 35 times (Fig. 6(d)), and the Kevlar fiber material was initially revealed but has not yet been fully revealed, which explains that the rare earth treated liner is almost not damaged after wearing. Although it is evident that the Kevlar fiber material was flattened, the fiber bundles were regular from the SEM morphology of 200 times (Fig. 6(c)), and the treated liner does not turn up the phenomenon that the Kevlar fibers have been broken, and it can be seen that the corrugated and lubricating PTFE transfer film exists from the chart. At the same time, the liner came up slightly adhesive wear in wear process from Fig. 7(b), a small amount of Fe and Cr elements existed on the worn liner. The complete PTFE lubricating film reduced the friction coefficient, wear loss and friction temperature of the bearing, and further improved the tribological property of the bearing. This can be further proved that the rare earth treated liner not only can improve the bonding properties of the bearing liner, but enhance the lubrication function because the rare earth lanthanum chloride will produce lanthanum oxide crystals due to frictional heat and the lanthanum oxide crystals can be effective to antifriction lubricating action at proper temperature [18]. Visible, the liner treated by the rare earth can make the liner give full play to self-lubricating function in the friction process.
4. Conclusions An investigation of the effects of rare earth treatment on the friction, wear, and bonding to the housing of spherical bearings
was conducted on mixed Kevlar and PTFE fiber weaves. A special oscillating test apparatus was used to study friction and wear effects, and peel tests were performed on specially modified fixtures. As a result of this work, the following conclusions can be drawn: (a) The peel strength between the liner, which was higher in Kevlar than PTFE, and the surrounding steel housing can be improved by rare earth treatment. (b) The friction coefficient and wear life of the contact surface materials, that were higher in PTFE than Kevlar, were improved by rare earth treatment. (c) Abrasive wear predominates in untreated liners while mild adhesive wear predominates in rare earth treated liners.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant no. 51275155), Henan Province Funds for Distinguished Young Scientists (No. 114100510002) and Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 2009HASTIT008).
References [1] J.K. Lancaster, Accelerated wear testing of PTFE composite bearing materials, Tribology International 12 (1979) 65–75. ¨ [2] G. Theiler, W. Hubner, T. Gradt, et al., Friction and wear of PTFE composites at cryogenic temperatures, Tribology International 35 (2002) 449–458. [3] N.V. Klaas, K. Marcus, C. Kellock, The tribological behaviour of glass filled polytetrafluoroethylene, Tribology International 38 (2005) 824–833.
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[4] P. Samyn, J. Quintelier, W. Ost, et al., Sliding behaviour of pure polyester and polyester-PTFE filled bulk composites in overload conditions, Polymer Testing 24 (2005) 588–603. [5] F.H. Su, Z.Z. Zhang, W.M. Liu, Tribological behavior of hybrid glass/PTFE fabric composites with phenolic resin binder and nano-TiO2 filler, Wear 264 (2008) 562–570. [6] F.H. Su, Z.Z. Zhang, W.M. Liu, Friction and wear behavior of hybrid glass/PTFE fabric composite reinforced with surface modified nanometer ZnO, Wear 265 (2008) 311–318. [7] Z.Z. Zhang, H.J. Zhang, F. Guo, et al., Enhanced wear resistance of hybrid PTFE/ Kevlar fabric/phenolic composite by cryogenic treatment, Journal of Materials Science 44 (2009) 6199–6205. [8] J. Bijwe, S. Awtade, B.K. Satapathy, et al., Influence of concentration of aramid fabric on abrasive wear performance of polyethersulfone composites, Tribology Letters 17 (2004) 187–194. [9] F. Guo, Z.Z. Zhang, H.J. Zhang, et al., Tribological behavior of spun Kevlar fabric composites filled with fluorinated compounds, Tribology International 43 (2010) 1466–1471. [10] J. Wu, X.H. Cheng, Effect of surface treatment on the mechanical and tribological performance of Kevlar pulp reinforced epoxy composites, Tribology Letters 24 (2006) 195–199.
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[11] A. Bhattacharya, B.N. Misra, Grafting: a versatile means to modify polymers: techniques, factors and applications, Progress Polymer Science 29 (2004) 767–814. [12] Z.W. Xu, Y.D. Huang, Y.J. Song, et al., Surface characteristics of rare earth treated carbon fibers and interfacial properties of composites, Journal of Rare Earths 25 (2007) 462–468. [13] Q.Q. Shangguan, X.H. Cheng, On the friction and wear behavior of PTFE composite filled with rare earths treated carbon fibers under oil-lubricated condition, Wear 260 (2006) 1243–1247. [14] Y.F. Lian, L.G. Yu, Q.J. Xue, The correlation among the tribological characteristic parameters of rare earth trifluorides and their electron structures, bond parameters, and thermodynamic parameters, Wear 188 (1995) 56–60. [15] J. Li, X.H. Cheng, Effect of rare earth solution on mechanical and tribological properties of carbon fiber reinforced thermoplastic polyimide composite, Tribology Letters 25 (2007) 207–214. [16] X.H. Cheng, Y.J. Xue, C.Y. Xie, Friction and wear of rare-earth modified glassfiber filled PTFE composites in dry reciprocating sliding motion with impact loads, Wear 253 (2002) 869–877. [17] SAE-AS81820. Aerospace Standard in USA, 2008. [18] Y.F. Lian, H.X. Dang, A survey on tribology research of the rare earth profiles, Tribology 13 (1993) 183–190.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Effects of rare earth treatment on tribological properties of selflubricating spherical plain bearings$ Ming Qiu n, Yingchun Li, Long Chen, Yaoxing Bai Henan University of Science and Technology, Luoyang, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2012 Received in revised form 4 December 2012 Accepted 8 December 2012 Available online 27 December 2012
The effects of the rare earth treatment on the friction, wear, and bonding behavior of polymeric, selflubricating spherical plain bearing materials were investigated. The materials of interest consisted of woven Kevlar (TM) and poly-tetrafluoroethylene (PTFE) fibers. An oscillating wear tester, operating at angles of 7 101, four different frequencies, and a constant load, was used to test assembled bearings at room temperature. Worn surface morphologies of the liners were investigated by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Compared with bearings without rare earth treatment, the bonding qualities between the binder and the surrounding steel housing, and the wear resistance of the treated bearings were both greatly improved. Contact surface morphology indicates that the treated bearing liners experienced only slight amounts of adhesive wear while those without the treatment seemed to wear more by abrasion. Overall, rare earth treatments served both to improve bonding quality of the bearing liner and improve their tribological properties. & 2013 Elsevier B.V. All rights reserved.
Keywords: Rare earth Spherical plain bearing Friction Wear
1. Introduction PTFE fiber can be used as the liner of the spherical plain bearing because of low friction coefficient, excellent corrosion resistance and other advantages. But the application scope of PTFE fiber, as self-lubricating liner, is restricted owing to the huge wear rate [1–4]. To this end, some researchers carried out a large number of research works about hybrid self-lubricating materials made of some functional fibric and PTFE fabric to advance wear property of PTFE [5,6]. The Kevlar fiber with PTFE fabric knitting has been widely used to improve the wear resistance, mechanical properties and dimensional stability due to the high tensile strength and high elastic modulus [7]. However, the woven liner of Kevlar and PTFE fabric exhibits poor adhesion to adhesive resin due to its chemically inert surface, which leads to poor interface binding force between the liner and the adhesive, thus affect the tribological performance of the self-lubricating liner. There are many reports related to the approaches used to modify the fiber surface [8–11]. Among those approaches, rare earth treatment seemed to be a simple and interesting method [12,13]. It is well known that rare earth can be widely used in electronics, metallurgy, and chemical engineering, etc. The rare earth elements have unique chemical properties and strong activity and the rare earth atom shows larger effective nuclear charge because its 4f
$ n
This paper was presented at the WOM 2013 conference, Portland, Oregon USA. Corresponding author. Tel./fax: þ 86 379 64231479. E-mail address: [email protected] (M. Qiu).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.021
electron is weakly bonded for the nuclear, which can affect the properties of the materials [14]. Rare earth atom is a typical hard ion and has strong complex ability and strong ionic bond between the atomic coordination. Therefore, rare earth treatment can improve the interfacial adhesion force of the composite material and its tribological properties [15,16]. In this paper, in order to improve the bonding quality of liner and the tribological properties of the self-lubricating spherical plain bearing, the liners were treated with the rare earth and the influences of the rare earth treatment on bonding quality and tribological properties of selflubricating spherical plain bearing are investigated.
2. Experiments 2.1. Test preparation The basic dimensions of a test spherical plain bearing are shown as follows: outer ring diameter F35 mm, outer ring width 12 mm, inner ring diameter F20 mm, inner ring width 16 mm, ball diameter F29 mm. The materials of inner and outer ring were made of GCrl5 steel and the liner was the mixed woven material of the Kevlar and PTFE fiber. The one rich in PTFE fiber was used as a friction surface while the other rich in Kevlar fiber was used as a binding surface in Fig. 1. The mixed woven liner was treated by the rare earth before bonding, and the detailed steps were shown as follows: (1) the liner was immersed in acetone solution for 10–14 h at room temperature, then was dried in the 60–80 1C oven for 10–20 min. (2) The liner of the acetone treatment was
M. Qiu et al. / Wear 305 (2013) 274–279
275
PTFE fiber
Kevlar fiber
Fig. 1. The structure of woven PTFE and Kevlar fibers. (a) The surface rich in PTFE fibers; (b) the surface rich in Kevlar fibers.
immersed in rare-earth modifier solution for 2–3 h, and then they were dried in the 80–120 1C oven for 20–30 min. The components of the rare earth modifier solution mainly included lanthanum chloride (LaCl3 0.3%), deionized water (H2O 99.2%), ammonium chloride (NH4Cl 0.2%), nitric acid (HNO3 0.1%) and urea (CON2H4 0.2%). The adhesive was bi-component enhanced phenolic adhesive resin (204 phenolic adhesive) purchased from Shanghai Xinguang Chemical Plant of China. The dried liner was glued on the inner surface of outer ring, and then they were placed in calorstat oven at the temperature of 100 1C for 4 h after fixing the inner ring. Finally, the friction and wear test specimens can be obtained. 2.2. Test apparatus Bonding quality was estimated by the liner peel strength. The self-made peel fixtures were used to fix the peel sample which was gripped in the Instron5944 electronic universal material test machine, then the two fixtures were away from the liner at a certain speed and the liner was peeled down from the outer ring of the bearing. The peel test diagram is shown in Fig. 2. The bonding force and displacement information were collected and stored with the sensor and data acquisition system. The peel strength data were calculated with the data processing software. The specific calculation formula of the peel strength s is shown as follows:
Fig. 2. The diagram of peel strength test.
S s¼C LB where C is the load of the unit highly in the peeling curve, N/mm; S is the area of the peeling curve, mm2; L is the length of the bottom line in the peeling curve, mm; B is the width of the peeling sample, mm. The tests of the tribological properties were conducted with the testing machine which was a self-made friction and wear tester for spherical plain bearing shown in Fig. 3. By using a torque sensor, a lever indicator and a thermocouple, changes of the friction torque, wear loss and friction temperature were detected. The test conditions were as follows: (1) oscillating frequencies were 2.0 Hz, 2.5 Hz, 3.0 Hz, 3.5 Hz; (2) the oscillation range was 7101; (3) the applied pressure on the bearing was 52 MPa (applied load 12.5 kN); (4) tests were done at room temperature. After mounting the bearing in the test machine and after a static load was applied for 15 min, the torque sensor and wear testing sensor were activated and the test was run for 25,000 oscillating cycles. Finally, the wear surface morphologies of the liners were analyzed by the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS).
3. Results and discussion 3.1. The liner bonding quality Based on the bonding quality test aerospace standard SAEAS81820 of the spherical plain bearings in USA [17], the test program
Fig. 3. Schematic diagram of a spherical plain bearing testing machine.
is as follows: the peeling angle is 104.32–178.871. The peeling speed is 19 mm/min and the acquisition interval is 2 ms. The adhesion degrees of the liners were examined before peeling and 90% tightly adherent must be ensured. The reliabilities of the liners were examined after peeling and the situation of the unbonded area equal to 25% of the outer ring width should not be allowed. The results are shown in Table 1. It can be seen from the Table 1 that the peel strength of the test bearings is all higher than the value of 0.357 N/mm which is obtained with the formula in the standard SAE-AS81820, so the peel strength of all test bearings can meet the standard requirements. However, the peel strength of liner treated with the rare earth is two times as large as the untreated liner which can be seen obviously from the Table 1. It shows that the bonding quality
276
M. Qiu et al. / Wear 305 (2013) 274–279
Table1 Results of the liner peeling strength. Sample
Peeling strength (N/mm)
Liner adhesion degree
Bonding integrity test
Untreated Rare earth treated
0.64 1.305
Qualified Qualified
Qualified Qualified
liner are shown in Fig. 5 under the condition of the four oscillating frequencies. It can be seen from Fig. 5(a) that the friction coefficients of the bearings with untreated and rare earth treated liners are increasing with the incremental swing frequency. Besides, the friction coefficients of bearings with the rare earth
Fig. 4. Peeling curves of the two kinds of bearings. (a) Untreated and (b) rare earth treated.
of the liner with rare earth treatment has been greatly improved. The peeling curves of bearings with the two kinds of liner are shown in Fig. 4. It can be seen that the peeling curve for untreated bearing is fluctuating much more obvious, so it indicate that the untreated liner for spherical plain bearings did not bond well with outer ring and uneven situation may exist in the bonding process, which clearly affects the bonding quality of the self-lubricating spherical plain bearing. But the peel curve of the rare earth treated bearing is relatively stable that explains the liner treated with rare earth can adhere evenly to the outer ring and its bonding quality is better. 3.2. Friction and wear properties The curves of friction coefficient, wear loss and friction temperature of the bearings with untreated and rare earth treated
Fig. 5. The characteristic curves of the bearings with two kinds of liners: (a) friction coefficient; (b) wear loss; (c) friction temperature.
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treated liners are lower than those of bearings with untreated liners when the swing frequency is from 2 Hz to 3.5 Hz, and the values of friction coefficients of the bearings with rare earth treated liners are stable within the test swing frequency scope. However, the friction coefficients of the bearings with untreated liners are higher and their frictional performances are worse. The curve of wear loss is shown in Fig. 5(b), and it can be seen that the wear losses of the bearings with both kinds of liners are increasing with the increasing of swing frequencies. The wear losses of the bearings with the rare earth treated liners are lower than the bearing with untreated liner under the contact pressure of 52 MPa and the swing frequencies of 2–3.5 Hz. It shows that spherical plain bearings with rare earth treated liner have excellent wear properties in the experimental condition. The curve of friction temperature is shown in Fig. 5(c), it can be seen that the friction temperatures of bearings with both kinds of liners are increasing with the increasing of swing frequency and the friction temperatures of the bearings with rare earth treated liner are lower than those of the bearings with untreated liners. It indicates that the bearings with rare earth treated liner reduce more heat generation than those of the bearings with untreated liners in the experimental condition. This reason is that the lower friction coefficient generates the lower friction heat. After some comprehensive analysis the tribological performance of the bearings with two kinds of liners concludes that the friction coefficient, wear loss and friction temperature of the spherical plain bearings with rare earth treated liner are lower obviously. It shows that the tribological performances of the bearings with rare earth treated liners are improved and increased evidently in the experimental condition. 3.3. SEM and EDS analysis of worn liner surface The SEM morphologies of the worn liners are shown in Fig. 6. It can be seen that the treated liners not only ensure that the adhesive force between the fabric liner and the outer ring of the
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bearing was greatly improved, but also ensure that the selflubricating function of the PTFE woven liner was fully exerted. In the same condition, the worn surface of the woven liner with the rare earth treatment was lighter than that of the untreated liner. The SEM morphologies of the worn liner untreated are shown in Fig. 6(a) and (b), and it can be perceived that the untreated liners have been seriously damaged. Moreover, the PTFE fibers of the liner had basically been worn completely and the Kevlar fibers had been worn and broken messily. It can be seen that the base material of the untreated liner was worn and broken from the SEM morphologies of 200 times (Fig. 6(a)). It shows that the material of the untreated liner has also been badly damaged. Consequently, in the untreated condition, the material of the liner emerged more serious adhesive wear after suffering heavier loads and higher swing frequency. A portion of the fiber matrix material is torn apart or even fallen off and most of them are changed into the abrasive particle. Another portion of the fiber matrix material is adhered on the outer surface of the bearing inner ring or extruded out along with the rotation direction. The EDS spectrum of the untreated wear liner is shown in Fig. 7(a), and it can be seen clearly that lots of Fe elements were transferred to the untreated liner from the inner ring outside surface of the bearing. This explains that the untreated liner was suffered with severe adhesive wear and leads to the badly friction state. The reason is that the lubrication of PTFE transfer film has not been fully exploited, which makes the friction temperature becomes higher, even leads to the abrasive particle of the inner ring including chrome and iron elements being gradually transferred and embedded in the gap of the fibers. Finally, adhesive wear can be found and the state of friction deterioration exists. With the self-lubricating material PTFE fiber has been worn completely and the Kevlar fiber has been broken, the untreated liner is severely damaged that causes the self-lubricating function of liner to be declined and even failure. Finally, it makes friction coefficient bigger, friction temperature higher and causes the state of friction deterioration, so leads to failure of the bearing.
Fig. 6. SEM morphologies of the worn liners (f¼ 3.5 Hz, P¼52 MPa). (a) Untreated (200 ); (b) untreated (35 ); (c) rare earth treated (200 ); (d) rare earth treated (35 ).
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Fig. 7. The untreated and rare earth treated EDS photos of the wear liner (f¼ 3.5 Hz, P¼ 52 MPa). (a) Untreated; (b) rare earth treated.
The SEM morphologies of the worn rare earth treated liner are shown in Fig. 6(c) and (d), and it can be perceived that the treated liner has a smoother surface under the condition of contact pressure 52 MPa and the swing frequencies 3.5 Hz. It can be seen that the rare earth treated liner was slightly worn relative to the untreated liner from the SEM morphology of 35 times (Fig. 6(d)), and the Kevlar fiber material was initially revealed but has not yet been fully revealed, which explains that the rare earth treated liner is almost not damaged after wearing. Although it is evident that the Kevlar fiber material was flattened, the fiber bundles were regular from the SEM morphology of 200 times (Fig. 6(c)), and the treated liner does not turn up the phenomenon that the Kevlar fibers have been broken, and it can be seen that the corrugated and lubricating PTFE transfer film exists from the chart. At the same time, the liner came up slightly adhesive wear in wear process from Fig. 7(b), a small amount of Fe and Cr elements existed on the worn liner. The complete PTFE lubricating film reduced the friction coefficient, wear loss and friction temperature of the bearing, and further improved the tribological property of the bearing. This can be further proved that the rare earth treated liner not only can improve the bonding properties of the bearing liner, but enhance the lubrication function because the rare earth lanthanum chloride will produce lanthanum oxide crystals due to frictional heat and the lanthanum oxide crystals can be effective to antifriction lubricating action at proper temperature [18]. Visible, the liner treated by the rare earth can make the liner give full play to self-lubricating function in the friction process.
4. Conclusions An investigation of the effects of rare earth treatment on the friction, wear, and bonding to the housing of spherical bearings
was conducted on mixed Kevlar and PTFE fiber weaves. A special oscillating test apparatus was used to study friction and wear effects, and peel tests were performed on specially modified fixtures. As a result of this work, the following conclusions can be drawn: (a) The peel strength between the liner, which was higher in Kevlar than PTFE, and the surrounding steel housing can be improved by rare earth treatment. (b) The friction coefficient and wear life of the contact surface materials, that were higher in PTFE than Kevlar, were improved by rare earth treatment. (c) Abrasive wear predominates in untreated liners while mild adhesive wear predominates in rare earth treated liners.
Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant no. 51275155), Henan Province Funds for Distinguished Young Scientists (No. 114100510002) and Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 2009HASTIT008).
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