Fretting wear behavior of superelastic nickel titanium shape memory alloy

Transcription

1 Tribology Letters, Vol. 8, No. 4, April 25 (Ó 25) 463 DOI:.7/s Fretting wear behavior of superelastic nickel titanium shape memory alloy L.M. Qian a, Q.P. Sun b and Z.R. Zhou a, * a Tribology Research Institute, National Traction Power Laboratory, Southwest Jiaotong University, Chengdu, 63, Sichuan Province, P.R. China b Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong Received 4 July 24; accepted 2 January 25 The fretting behavior of superelastic nickel titanium (NiTi) shape memory alloy was studied at various displacement amplitudes on a serve-hydraulic dynamic test machine. The results showed that the superelastic properties of the material played a key role in the observed excellent fretting behavior of NiTi alloy. Due to the low phase transition stress (only /4 the value of its plastic yield stress) and the large recoverable phase transition strain (5%) of NiTi, the friction force of NiTi/GCr5 stainless steel pair is smaller than the value of GCr5/GCr5 pair and at the same time the Rabinowicz wear coefficient of NiTi plate is about /9 the value of GCr5 plate under the same fretting conditions. For NiTi/GCr5 pair, even NiTi has a much lower hardness than GCr5, the superelastic NiTi alloy exhibits superior fretting wear property than GCr5 steel. It was found that the weak ploughing was the main wear mechanism of NiTi alloy in the partial slip regime. While in the mixed regime and gross slip regime, the wear of NiTi was mainly caused by the abrasive wear of the GCr5 debris in the three-body wear mode. KEY WORDS: fretting wear, nickel titanium, shape memory alloy, superelasticity, reversible phase transition. Introduction The outstanding superelastic behavior of nickel titanium (NiTi) shape memory alloy (SMA) has found many important applications in areas such as medical surgery, microelectromechanical systems (MEMS) and even as a material for train wheels or as contact tires on train wheels [ 3]. The high flexibility, excellent biocompatibility and outstanding superelastic behavior of the material at around body temperature have allowed NiTi SMA to perform functions that are impossible with other materials [,2]. Because of the high work output per unit volume, prototype devices made of NiTi thin films have been fabricated and proposed as both sensors as well as large-strain actuators for use in MEMS including volume control, micro-shape and stiffness control, micro-vibration and damping control, etc [3]. However, in all these applications, NiTi components may lose their function by fretting wear, such as those between the surgical implants and bones [,2]. Also, NiTi micro sensors and actuators may break off substrates owing to the cyclic deformation in small displacement amplitudes [3]. Thus, the understanding and control of tribological behaviors, especially the fretting wear property of NiTi SMA, have become an important issue of concern [ 7]. Tan et al. [8] showed that the surface modification by sequential ion implantation with *To whom correspondence should be addressed. zrzhou@home.swjtu.edu.cn argon and oxygen could enhance the fretting wear characteristics of the NiTi alloy. However, most of the previous tribological studies focused on the sliding wear property of NiTi, the fretting wear has not been well addressed in the literatures [8 4]. Due to its unique reversible phase transition between the low-temperature martensite phase and the high-temperature austenite phase, NiTi SMA can exhibit the well-known shape-memory effect and superelastic behavior [,9]. Shape-memory effect refers to the case where the stress induced austenite to martensite transformation can be recovered by the reverse transition to austenite phase upon heating to a critical temperature. Superelastic behavior refers to the case where a reversible austenite martensitic phase transition can be induced by the applied stress and is recoverable as the stress is removed [,9]. For the application of NiTi in the medical community and MEMS, stress-induced phase transition process is unavoidable, and therefore may play an important role in deformation and fretting wear of materials. In previous studies, NiTi alloy was found to exhibit excellent wear resistance to erosion and abrasive wear [,]. These investigations showed that both superelasticity and shape memory effect might be responsible for the high wear resistance of NiTi alloy [ 4]. Nevertheless, these studies were mainly conducted either by a sand-blasting erosion tester or by a traditional pinon-disk type machine with a relatively large sliding distance, the fretting wear properties of NiTi alloy is still not clear [8 4] /5/4 463/ Ó 25 Springer Science+Business Media, Inc.

2 464 L.M. Qian et al./fretting wear behavior of NiTi alloy In this paper, fretting wear tests at various displacement amplitudes were performed on NiTi plates with GCr5 steel balls as counterbody under unlubricated and ambient air conditions. The role of phase transition in the observed unusual fretting behavior of NiTi was emphasized. F n d F n 2. Materials and testing methods Commercial.3-mm thick superelastic NiTi polycrystalline cold-rolled sheets were purchased from Shape Memory Applications, Inc. (San Jose, CA, USA). The nominal alloy composition was Ni-56.4% Ti-43.6% (wt.%). The size of the grains is about 5 nm as observed by transmission electron microscope [5]. With a differential scanning calorimeter (DSC) (DSC 92, SETARAM, France), the characteristic transformation temperatures, namely R s,r f (rhombohedral phase start and finish temperatures on cooling), M s,m f (martensite start and finish temperatures on cooling), A s and A f (austenite start and finish temperatures on heating) were measured at heating and cooling rates of C/min and are listed in table. The result indicates that the material is in the austenite phase at room temperature (22 C) and will exhibit superelasticity under stress. The stress strain curves of the material were obtained by uniaxial tensile tests of the NiTi sheet on a Universal Testing Machine (MTS SINTECH /D). To prepare the samples for fretting tests, a NiTi sheet was cut into mm 2 mm pieces by a wire cutting machine and glued onto mm 2 mm mm foundry iron blocks. Silicon carbide and aluminum oxide sand papers of various grades were used to polish the NiTi sample surfaces until a root-mean-square roughness of about 4 nm was reached. The fretting tests were carried out on a Deltlab Nene-2 serve-hydraulic dynamic test machine [6], as shown in figure. The NiTi plate on the left was stationary and the GCr5 steel ball (4 mm in diameter) on the right can move vertically with small but controlled amplitude. The specimens were cleaned in an ultrasonic bath of alcohol before the tests. In each fretting test, the ball moved up and down for, cycles at a frequency of Hz under a constant normal load F n of N. The imposed displacement of the ball d varied from ±2 lm to ±5 lm in order to study the fretting behavior of NiTi. At each condition, the fretting tests were repeated at least two times to ensure the repetition of data. All fretting experiments were performed under unlubricated Table. Phase transformation temperatures and room temperature structure. A s ( C) A f ( C) R s ( C) R f ( C) M s ( C) M f ( C) )7 6 )2 )68 )88 Austenite Room temperature structure Foundry iron NiTi GCr5 steel ball Figure. The schematic diagram of the Deltlab Nene-2 fretting test machine. The left sample is a NiTi plate glued on a foundry iron block. During fretting tests, the right GCr5 steel ball (4 mm in diameter) moves vertically with an imposed displacement of d under a normal load of F n. condition, ambient temperature of 22 C and relative humidity of 5 6%. After the tests, the morphologies of fretting wear scars were characterized by an optical microscope mounted with a digital camera and laser confocal scanning microscope (Olympus-OLS-, Japan). The chemical compositions in the scar area were determined by a scanning electron microscope (SEM) (CAMSCAN- 4DV, English) with an energy dispersive spectroscope (EDS) (Oxford 58). The fretting behavior of NiTi alloy was studied by analyzing both the frictional logs (variation of the tangential force F t and displacement d with number of fretting cycles N) and fretting scar features, together with the chemical compositions in the scar area. 3. Experimental results 3.. The frictional logs As plotted in figure 2, three typical fretting regimes can be identified from the friction logs under various values of displacement amplitude D. For D<5 lm, all fretting cycles remain quasi-closed after the first cycles, indicating a partial slip regime. While for D 2 lm, all the friction force versus displacement (F t -d) curves are open and finally stable in a parallelepipedic shape, which is the characteristic of the gross slip regime. For D between 5 and 5 lm, the mixed fretting regime appeared, where the stable stage is characterized by elliptic F t -d curves after the first parallelepipedic loops [7]. In spite of the shape of the F t -d curve, the tangential force is also an important indicator of the fretting kinetic behavior. The maximum tangential force F tmax in a cycle versus the number of fretting cycle N (F tmax -N) curves for various values of D are plotted in figure 3(a). All F tmax -N curves show an increase in F tmax in the first cycles, which can be explained as the removal of the contamination on the surface and the increasing contact area of pure metals [8,9]. After that, F tmax reached its maximum values due to the adhesive contact between

3 L.M. Qian et al./fretting wear behavior of NiTi alloy 465 (a) (b) D=2µm D=3µm -6 (c) (d) D=5µm - D=7µm (e) (f) D=µm D=5µm -2 (g) (h) D=2µm D=5µm Figure 2. Friction logs at various fretting displacement amplitudes D under unlubricated condition. Normal force F n = N. (a) D =2lm, (b) D =3lm, (c) D =5lm, (d) D =7lm, (e) D =lm, (f) D =5lm, (g) D =2lm, (h) D =5lm. -6

4 466 L.M. Qian et al./fretting wear behavior of NiTi alloy (a) 5 F tmax (N) µm 2µm 5µm µm 7µm 5µm 3µm 2µm a stable friction property. The values of F max and F stable at various values of D are shown in figure 3(b). It is clear that with D increasing both F max and F stable increase during the partial slip regime and mixed regime even the wear scar area almost keeps constant (figure 4(a) (d)). As D further increases until the gross slip regime of the contact pairs, F max and F stable begin to decrease due to three-body wear [2]. In summary, with D increasing, both F max and F stable first increase and then decrease, with the maximum values located at the end of the mixed regime. (b) 5 Tangential force (N) F max F stable Displacement amplitude D (µm) Figure 3. (a) The maximum tangential force F tmax in a cycle versus the number of fretting cycles N (F tmax -N) curves at various fretting displacement amplitudes D. F n = N. (b) The maximum and stable values of F tmax during all the cycles as a function of D. the pure NiTi alloy and GCr5 steel. Finally, F tmax either kept constant or decreased depending on the presence of debris. In the gross slip regime, the friction coefficient f in a cycle was obtained as: f =F tmax /F n. For D 7lm, F tmax reached stable values after a monotonic increase in the first few hundred cycles, where F t -d curves are either quasi-closed or of an elliptic shape (figure 2(a) (d)) and no debris appeared in the contact area (figure 4(a) (c)). For values of D between and 5 lm, F tmax remained constant till 2 3 cycles and then showed a slight decrease, where elliptic shaped F t -d curves became a little open (figure 2(e) (f)) as well as a few small size debris were produced in the wear area simultaneously (figure 4(d) (e)). Finally, when D 2lm, after a rapid increase in the first cycles, F tmax decreased in the residual cycles accompanying the formation of many thin debris in the contact area (figure 4(f)), where the shape of the F t -d curve changed gradually either from an ellipse to parallelogram (D =2lm) or from initially a thick parallelogram to a thinner one (D =5lm) (figure 2(g) (h)). In each curve of figure 3(a), the maximum tangential force F max during all the cycles usually occurs in the region corresponding to the elliptic shaped F t -d curves with the maximum static friction force, while the tangential force after, cycles (F stable ) normally exhibit 3.2. Morphology and composition of wear scars To understand the fretting mechanism of NiTi alloy, the morphology of wear scars were characterized by an optical microscope and laser confocal scanning microscope. The full views of the wear scars on NiTi surfaces were examined by optical microscope and are shown in figure 4. Corresponding to the different stages of the frictional logs, the wear scars at various values of D exhibit different morphological features according to the three fretting regimes. For values of D below 5 lm, the wear scars can be clearly divided into two parts, the stick zone in the center and the micro-slip zone at the edge, which is consistent with the typical partial slip regime in the frictional log. As D varies between 5 and lm, no stick zone is observed in the wear scars and almost no debris is found around the wear area. In addition, the entire contact area become rough and the wear scars remain the same size as those in the partial slip regime. This is consistent with the features of the mixed regime. Finally, for values of D above 5 lm, the area of the wear scars becomes much larger than those obtained in the partial slip regime and mixed regime, and plenty of wear debris appear in and around the wear area. The results suggest that serious fretting wear happened in the gross slip regime. The microscopic features in the wear area for various values of D were obtained by laser confocal scanning microscope and are shown in figure 5. As shown in figure 5(a) and (b), no obvious wear occurred in the stick zone for values of D below 5 lm, except a few parallel slip lines for D =2lm and some ploughing lines for D = 3lm. As D increases, obvious adhesion damages appeared in the contact area for D = 5 lm (figure 5(c) and (d)), also some wear debris could be observed in the contact area for D= lm. For values of D above 5 lm, plenty of thin piece wear debris covered all the contact area and the wear became more and more serious with the increase in D, as demonstrated by the increase in both the number and the size of the wear debris in the contact area (figure 5(e) and (f)).

5 L.M. Qian et al./fretting wear behavior of NiTi alloy 467 Figure 4. The morphology of the wear scars on NiTi at various fretting displacement amplitudes D by an optical microscope mounted with a digital camera. F n = N, N=,. (a) D =2lm, (b) D =3lm, (c) D =5lm, (d) D =lm, (e) D =5lm, (f) D =5lm. With laser confocal scanning microscope, the wear scar radius, wear depth and volume were measured for various values of D after, fretting cycles. Since it is difficult to obtain the average wear depth of the wear scar, we measured the maximum wear depth in the wear scar area to characterize the wear of the material. As shown in figure 6, the maximum wear depth increases with D. Nevertheless, the wear scar radius and the wear volume was kept almost constant for values of D below lm, which can be explained from the quasi-closed or elliptic F t -d curves in the frictional logs (figure 2) as well as from the weak ploughing/adhesion damage in the contact area (figure 5). With the increase in D ( lm < D< 5lm) and the increase in the parallelogram F t -d cycles in frictional logs (figure 2), the number and size of the wear debris in the contact area increase (figure 5) and at the same time the wear scar radius and the wear volume begin to increase quickly. In order to determine the material being transferred during the wear process, the chemical compositions in the wear scars were characterized by EDS. The EDS spectrums on the original NiTi and GCr5 sample surfaces are shown in figure 7(a) and (b), respectively. The measured value of Ni-54 % Ti-4% O-5% (wt.%) on the NiTi alloy original surface shows the same Ni and Ti weight scale as the values provided by the company (figure 7(a)). Si-.3% Cr-.9% Mn-.4% Fe-97.4% (wt.%) on GCr5 steel original surface is also a typical GCr5 steel composition (figure 7(b)). Figure 7(c) shows the EDS spectrums on the wear debris from the NiTi surface. Figure 7(d) shows the EDS spectrums on the GCr5 wear area for D =5lm. It is found that the main elements on the GCr5 wear scar are Fe and O, which suggests that almost no NiTi is transferred onto the GCr5 wear area. The chemical compositions of the wear debris on the NiTi surface are very similar to those of the GCr5 wear scar, showing rich Fe and O and very small amounts of Ni and Ti. Clearly, the debris on the NiTi surface is in fact mainly from the wear of GCr5 steel. Figure 8 shows the EDS spectrums at the center of the NiTi wear scars corresponding to various values of

6 468 L.M. Qian et al./fretting wear behavior of NiTi alloy Figure 5. The micro features of the wear scars on NiTi alloy at various fretting displacement amplitudes D by laser confocal scanning microscope. F n = N, N=,. (a) D =2lm, (b) D =3lm, (c) D =5lm, (d) D =lm, (e) D =5lm, (f) D =5lm. D. Clearly, with the increase in D, the height of the Ni and Ti peaks decreases, whereas the height of the O and Fe peaks increases quickly. This implies that more and more GCr5 debris is transferred onto the NiTi surface. The wt.% of Ni, Ti, O and Fe at the center of NiTi wear scars are plotted in figure 9 as a function of D. Obviously, in the partial slip regime (for values of D below 5 lm), almost no Fe and O appear on the NiTi wear scars. In the mixed regime (for values of D between 5 and 5 lm), as the wt.% of Ni and Ti decrease, the wt.% of Fe and O increase sharply. In the gross slip regime (for values of D above 5 lm), Fe and O almost cover the entire NiTi wear area. Therefore, the thin wear debris on the NiTi wear area (figure 5) is actually the transferred oxide of GCr5 steel (Fe x O y ). As D increases, the wear of GCr5 increases and more and more GCr5 oxide debris are transferred onto the NiTi alloy. Since these oxide debris are usually trapped in the contact region, the wear gradually transform from the two-body wear in the partial slip regime into the three-body wear in the mixed regime and gross slip regime [2] Comparison of the wear properties of NiTi alloy and GCr5 steel For D=5 lm, the wear scars on the NiTi alloy and on GCr5 steel are put together in figure for comparison. The full views of the wear scars are shown in figure (a) and (b). Figure (c) and (d) show the morphology details in the center of the wear area. Plenty of thin piece wear debris appears on the NiTi wear scar (figure (c)) and many deep ploughs are found on the wear scar of GCr5 steel (figure (d)). The wear scar radius, the maximum wear depth and wear volume were measured by laser confocal scanning microscope. Even

7 L.M. Qian et al./fretting wear behavior of NiTi alloy 469 Maximum wear depth (µm) (a) Displacement D (µm) Wear scar radius (µm) (b) Displacement D (µm) Wear volume (x 6 µm 3 ) (c) Displacement D (µm) Figure 6. The measurements of the wear scars at various fretting displacement amplitudes D. (a) wear depth, (b) wear scar radius and (c) wear volume were plotted as the function of D, respectively. Figure 7. The energy dispersive spectroscope (EDS) spectrums on the (a) NiTi original surface, (b) GCr5 original surface, (c) wear debris on NiTi for D =5lm and (d) GCr5 wear area for D =5lm. the wear scar radius on NiTi is almost the same as that on GCr5, the wear depth and volume on NiTi are much smaller than those on GCr5 (figure ). Furthermore, to understand the mechanism for the transfer of GCr5 to NiTi, the cross section of the wear scars of GCr5 and NiTi for D =5lm are characterized by laser confocal scanning microscope (figure 2). Some micro cracks are observed on the cross section of GCr5 wear scar, which implies that the transfer of GCr5 may start from crack generation and spread. Nevertheless, the surface of NiTi is quite flat and no cracks are found on the cross section of the wear scar. Combined with EDS analysis which indicates that no NiTi is found transferred to the GCr5 wear surface and the wear debris on NiTi is mainly from GCr5, we can therefore conclude that the NiTi alloy has much better wear resistance than that of GCr5 steel. To do a better comparison, we have made the tests on NiTi and GCr5 plate with the same GCr5 steel ball as counterbody (diameter: 4 mm) under the same fretting conditions (load: N, displacement amplitude: 5 lm, frequency: Hz, cycle number: 5.). Here, the GCr5 plate is cut from a GCr5 steel ball. The results are plotted in figures 3 and 4. Figure 3 shows that the steady friction coefficient (for N>2) of NiTi/ GCr5 pair is smaller than the value of GCr5/GCr5 pair. Figure 4 indicates that the wear area on NiTi plate is about 3/4 the value on GCr5 plate under the

8 47 L.M. Qian et al./fretting wear behavior of NiTi alloy Figure 8. The energy dispersive spectroscope (EDS) spectrums at the center of the NiTi wear scars. (a) D = 2 lm, (b) D = 3 lm, (c) D =lm, (d) D =5lm, (e) D =2lm, (f) D =5lm. Weight (%) Partial slip regime Mixed regime Ni Ti Displacement D (µm) same fretting conditions. At the same time, the wear volume V of NiTi plate (.6 6 lm 3 ) is about 3% the value of GCr5 plate (5.5 6 lm 3 ). Since the Rabinowicz wear coefficient K R of materials can be determined by the following function: K R ¼ HV LFn Gross slip regime Fe Figure 9. The wt.% of elements Ni, Ti, O, and Fe as a function of fretting displacement amplitudes D by EDS analysis at the center of NiTi wear scars. With the increase in D, as the wt.% of Ni and Ti composition decreases, the wt.% of Fe and O increases, indicating that more and more GCr5 debris is transferred to NiTi. O ðþ where the indentation hardness H of NiTi and GCr5 are 5 and 3 GPa, respectively (figure 5); the wear distance L for D =5lm is m and the normal load F n is N. Thus, for N=5, the K R of NiTi is.8 )4 and the K R of GCr5 is 7 )4 from equation (). Similarly, for N=,, the wear volume V of NiTi plate is lm 3 and the K R of NiTi is calculated as.9 )4. Clearly, K R shows weak fretting cycle s dependence and is better than wear volume to be an indicator of the wear properties of materials. Levinson et al. [2] showed that the friction coefficient between a ball and a flat was strongly dependent on the ball diameter, normal load and radius of contact area. Due to the differences in the diameter of ball (6 mm in [22] and 4 mm here) and the normal load (3 N in [22] and N here), the K R of GCr5 in [22] is smaller than the K R of GCr5 here but still four times larger than the K R of NiTi. All these evidences further demonstrate that NiTi alloy exhibits much better fretting behavior than GCr5 steel even with a much lower hardness. 4. Discussion 4.. The hardness and the fretting wear behavior For most metals, wear is resulted from the accumulation of plastic deformation and the final removal of

9 L.M. Qian et al./fretting wear behavior of NiTi alloy 47 Figure. Morphology comparison of the wear scars on NiTi alloy and GCr5 steel for D =5lm. Full view of the wear scar for (a) NiTi and (b) GCr5, micro feature of the wear scar for (c) NiTi and (d) GCr x 6 NiTi GCr5 3.6x Friction coefficient f NiTi GCr5 Depth (µm) Volume (µm 3 ) Radius (µm) Figure. Comparison of wear depth, wear volume and wear scar radius of the wear scars on NiTi alloy and GCr5 steel for D =5lm.. Figure 3. For D =5lm and F n = N, the friction coefficient f of NiTi plate/gcr5 ball and GCr5 plates/gcr5 ball versus the number of fretting cycles N (f)n) curves. Figure 2. The morphology of the cross sections of the wear scars on (a) GCr5 steel and (b) NiTi alloy for D =5lm. A micro crack is found in the cross section of the wear scar on GCr5 steel. material by micro fracture [23]. As hardness is the measure of a material s ability against plastic deformation, materials with a high hardness should have a good wear behavior [23]. Therefore, materials with a high hardness normally exhibit a higher ability to resist fretting wear [24]. The experimental results in Section 3 confirm that NiTi alloy exhibits much better fretting wear property

10 472 L.M. Qian et al./fretting wear behavior of NiTi alloy (a) 6 A M Stress (MPa) 4 2 s t Austenite Unloading Loading Martensite Nominal strain (%) Figure 4. The morphology of the wear scars on NiTi and GCr5 plates by an optical microscope. D =5lm, F n = N, N=5. Hardness (GPa) GCr5 Load (mn) Figure 5. The indentation hardness as a function of the indentation load on NiTi alloy and GCr5 steel. The hardness of NiTi is about /3 the value of GCr5 steel. than GCr5 steel. The hardness of the two materials was measured by a nanoindenter (CSEM, Instruments SA, Switzerland) using a Berkovich type diamond tip with a face angle of As shown in figure 5, the indentation hardness of GCr5 steel is about three times the value of NiTi alloy. Combined with the fretting results, it is found that the lower hardness actually corresponds to the higher fretting wear resistance. This unusual wear hardness relationship cannot be explained by the existing wear theory that has been supported by much experimental work [23]. A recent investigation on the wear mechanism of NiTi demonstrated that the high wear resistance of NiTi could be attributed to its unique stress-induced phase transition properties involved in the fretting wear process [25]. We will explain it briefly in the following The role of phase transition in the fretting behavior of NiTi Figure 6(a) shows the measured stress strain curve of NiTi during the phase transition process under a tensile test. It exhibits the forward austenite to martensite (A fi M) and reverse (M fi A) transformation NiTi (b) 2 Stress (MPa) 5 5 s y GCr5 s t GCr5 Phase transition zone Austenite elastic zone NiTi Nominal strain (%) process during loading and unloading with a hysteresis. The stress strain curves of NiTi (solid line) and GCr5 (dashed line) are plotted together in figure 6(b) for comparison. During the loading process, GCr5 yielded after reaching the elastic limit. However, the NiTi alloy experienced mainly four stages of deformation: elastic deformation of austenite, phase transition from austenite to martensite, elastic deformation of the martensite and finally plastic deformation of martensite []. Owing to the superelastic nature of NiTi, both the elastic and phase transition deformations will recover during unloading []. Compression test of NiTi gives similar curves [26,27]. It is seen that the phase transition of NiTi plays an important role in the deformation and wear behavior of NiTi alloy [25]. Because of the reversible phase transition, NiTi alloy exhibits a large recoverable deformation (5% recoverable strain, compared with.25% for GCr5 steel) and at the same time with very low elastic modulus (58 GPa, 27 GPa for GCr5 steel). In general, a simple qualitative understanding is that Ôtransformation shielding at the contact region (due to the lower transformation stress plateau in figure 6(a)) together with the lower modulus provide the essential mechanism for the high wear resistance of NiTi. In fact, it is this Ôstress s m NiTi Figure 6. Typical stress versus strain curve from the tension test. (a) Phase transition of NiTi. (b) Comparison of NiTi and GCr5 tensile property. The inset picture illustrates the contact between the GCr5 ball and the NiTi flat surface. The contact region of NiTi consists of the austenite elastic deformation zone (on the edge) and the phase transition zone (in the center). F n

11 L.M. Qian et al./fretting wear behavior of NiTi alloy 473 shielding effect at the contact region that postpones the start of plastic deformation to a high load level, and therefore greatly improves the wear behavior of NiTi [25]. However, such transformation shielding mechanism for GCr5 steel does not exist at all. Thus, the intrinsic phase transition property of NiTi not only influences the friction but also the wear property of NiTi during the fretting process. Firstly, the phase transition of NiTi has a strong effect on the fretting kinetics of NiTi alloy. Due to the large elastic recovery, the F t -d curves can keep an elliptic shape till D = 2lm (figure 2(g)). In the gross slip regime (D =5lm), the steady friction coefficient F stable of NiTi/GCr5 pair is smaller than the value of GCr5/GCr5 pair (figure 3). This is most probably due to the low phase transition stress (38 MPa at 22 C) and the large recoverable phase transition strain (5%) of NiTi, therefore the required force to plough the surface is greatly reduced. Furthermore, since the reverse phase transition can greatly improve NiTi s deformation accommodation and diminish the nucleation of fatigue cracks, the F t -d curves of NiTi alloy remain elliptical except the initial parallelogram loops in mixed regime, which is much different from those of alumina alloy but similar to those of polymers [7,28]. For alumina alloy, because of the competition between the particle detachment and the nucleation of fatigue cracks, the elliptical and parallelogram loops are encountered in the mixed regime [7]. Due to the high polymer compliance, the F t -d curves change gradually from the initial parallelogram to elliptic shape [28]. Secondly, NiTi plate shows much better wear resistance than GCr5 plate under the fretting wear of GCr5 steel ball. As calculated in section 3.3, the Rabinowicz wear coefficient K R of NiTi is only /9 that of GCr5 plate for the same fretting condition. To provide a quantitative understanding of the role of phase transition in the fretting wear process, a simple contact mechanics analysis was performed by the Hertzian contact theory. Here, the radius R of the GCr5 steel ball is.2 m, the elastic modulus of GCr5 E ball and NiTi E NiTi were measured by a nanoindenter as 27 and 58 GPa, respectively. Assuming linear elasticity (no phase transition occurs before NiTi yields), the maximum stress r c in the contact area can be obtained by [29] r 3 c ¼ :48 F ne 2 R 2 : ð2þ Here,the effective elastic modulus E can be calculated by E ball and E NiTi as 5 GPa, and the normal load F n in experiments is set to N. From equation (2), r c is 5 MPa, which is much higher than the critical tensile stress to induce the phase transition at 22 C (38 MPa). For spherical indentation, the representative strain e r under the indenter is proportional to the ratio of the contact radius a over the indenter radius R, i.e. e r =.2a/ R [3]. By taking the contact radius a as the typical wear scar radius of NiTi, we have a = 2 lm for D =lm from figure 4. The representative strain e r can then be evaluated as.2. Since the representative strain e r is larger than the austenite elastic limit (.) and far below the phase transition strain (.5), the deformation region below the contact area of NiTi will only consist of two parts: the austenite elastic deformation region and phase transition region (see the inset picture of figure 6(b)). However, since such shielding mechanism does not exist for GCr5 steel, the maximum contact stress of GCr5/GCr5 pair is 22 MPa at the normal load of N by equation (2), which is about three times the value of NiTi/GCr5 pair. Compared with GCr5, it is seen that the local stress in NiTi is greatly reduced by the shielding effect of phase transition of NiTi. This shielding effect highly suppressed the crack generation and delamination of the NiTi/GCr5 pair. In the next section, a three-body contact analysis will show that NiTi also has much high wear resistance against abrasive wear than GCr5. Therefore, it is not surprise that NiTi plate shows much better wear behavior than GCr5 plate under the same fretting condition The wear mechanism of the NiTi-GCr5 pair The EDS analysis confirmed that the thin wear debris on NiTi surface was mainly from the wear of GCr5. With the increase in D, both Fe and O compositions increased by a similar scale on the wear area of NiTi, implying that oxidation happened together with the wear of GCr5 (figure 7). Combined with the micro cracks in the sub-surface of GCr5 (figure 2 (a)), we could therefore speculate that the wear process of GCr5 steel was delamination and followed by oxidation in the mixed regime and gross sliding regime. Due to the stress induced phase transition, NiTi exhibits a large recoverable deformation with low elastic modulus, and therefore is very easy to deform under stress but is difficult to be removed under wear []. In the partial slip regime, the wear on NiTi was slight ploughing by the asperity of the GCr5 steel ball. In the mixed regime and gross slip regime, because of the shielding effect of the phase transition of NiTi, the damage and final fracture mostly occurred in the subsurface of GCr5. As a result, the detached GCr5 oxide debris adhered to the NiTi surface and finally covered all the wear area. As shown in figure 7, because of a layer of GCr5 oxide debris, the wear now becomes the three-body wear. To estimate the wear of NiTi and GCr5, we can also use the Hertzian contact theory to do a simple contact mechanics analysis. Assuming the GCr5 oxide debris as rigid balls with radius of r (r = lm andr<<r, refer to figures 5 and ), the threshold load P for the plastic yield of NiTi and GCr5 can be estimated as:

12 474 L.M. Qian et al./fretting wear behavior of NiTi alloy NiTi Fe x O y R F n GCr5 r = ~ mm Figure 7. Schematic illustration of the three-body wear mode for the NiTi/GCr5 pair. P ¼ 2:8 r2 r 3 E 2 : For the same debris, with the values of the elastic modulus E and the plastic yield stress r of NiTi and GCr5, the threshold load of NiTi is times the value of GCr5 according to equation (3). It indicates that the GCr5 is much easier to be worn than NiTi by the same debris. This analysis is consistent with the experimental observation that both the wear depth and wear volume of the GCr5 ball are larger than those of the NiTi plate (figure ). In this situation, the wear of NiTi might be caused by the abrasion due to the GCr5 debris, as evidenced by the small amount of Ni and Ti in the GCr5 debris around the NiTi wear area (figure 7(c)). In summary, it seemed that the wear mechanism of NiTi was weak ploughing in partial slip regime, and the dominant abrasive wear in the mixed regime and gross slip regime. The details of the evolution process of the wear mechanism still remains to be investigated. 5. Conclusions The fretting wear tests at various displacement amplitudes were performed on NiTi plates with GCr5 steel balls. The conclusions can be summarized as follows: () Three typical fretting regimes (partial slip regime, mixed regime and gross slip regime) were identified from the frictional logs and the wear scars of NiTi alloy at various displacement amplitudes. Experiments indicate that the weak ploughing was the main wear mechanism of NiTi alloy in the partial slip regime. While in the mixed regime and gross slip regime, the wear of NiTi is mainly caused by the abrasive wear of the GCr5 debris in the three-body wear mode. (2) Even the hardness of NiTi is much lower than that of GCr5 steel, it exhibits much better fretting wear property. Such unusual wear behavior can be attributed to the reversible phase transformation process during fretting wear. (3) The intrinsic superelastic properties of NiTi were believed to be responsible for the excellent fretting behavior of NiTi alloy. Due to the low phase transition stress (compared with its own plastic flow stress) and the large recoverable phase transition strain (5%), the friction of NiTi/GCr5 pair is smaller than that of GCr5/GCr5 pair and the wear volume of NiTi plate were greatly reduced compared to GCr5 plate under the same fretting condition. Acknowledgments The authors are grateful for the financial support from the Natural Science Foundation of China (Project Number: 53529, ), Sichuan Youth Science and Technology Foundation (Project Number: 4ZQ26 4) and the Research Grants Council of Hong Kong SAR of China (Project No. HKUST699/ 3E). References [] T. Duerig, K.N. Melton, D. Stockel and C.M. Wayman, Engineering Aspect of Shape memory Alloys (Butterworth-Heinemann, Boston, 99). [2] S.B. Kang, K.S. Yoon, T.H. Nam and J.S. Kim, Mater. Sci. Forum 449(-4) (24) 37. [3] Y.Q. Fu, H.J. Du, W.M. Huang, S. Zhang and M. Hu, Sensor. Actuat. A Phys. 2(2 3) (24) 395. [4] R. Maboudian, MRS Bull. 23(6) (998) 47. [5] S.M. Spearing, Acta Mater. 48 (2) 79. [6] L.M. Qian, X.D. Xiao and S.Z. Wen, Langmuir 6 (2) 662. [7] D.Y. Li, Wear 255 (23) 67. [8] L. Tan, W.C. Crone and K. Sridharan, J. Mater. Sci. Mater M. 3 (22) 5. [9] L.C. Brinson and B. Moran, (Mechanics of Phase Transformations and Shape Memory Alloys ASME, USA, 994). [] Y. Shida and Y. Sugimoto, Wear 46 (99) 29. [] R.H. Richman, A.S. Rao and D.E. Hodgson, Wear 57 (992) 4. [2] R. Liu and D.Y. Li, Metall. Mater. Trans. A 3A (2) [3] F.T. Cheng, P. Shi and H.C. Man, Scripta Mater. 45 (2) 83. [4] V. Imbeni, C. Martini, D. Prandstraller, G. Poli, C. Trepanier and T.W. Duerig, Wear 254 (23) 299. [5] K.L. NG, Stress-induced Phase Transformation and Reorientation in NiTi Tubes, Master Thesis (The Hong Kong University of Science and Technology, Hong Kong, 22), p. 39. [6] Ch. Colombie, Y. Berthier, A. Floquet, L. Vincent and M. Godet, J. Tribol. ASME 6 (984) 23. [7] Z.R. Zhou and L. Vincent, Wear 8 83 (995) 53. [8] P. Blanchard, C. Colombie, V. Pellerin, S. Fayeulle and L. Vincent, Metall. Trans. A. 22 (99) 535.

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