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Effects of environment on the sliding tribological behaviors of Zr-based bulk metallic glass
Intermetallics 25 (2012) 115e125
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Effects of environment on the sliding tribological behaviors of Zr-based bulk metallic glass Hong Wu a, b, Ian Baker b, *, Yong Liu a, Xiaolan Wu b, Paul R. Munroe c a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083 Hunan, China Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, USA c Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2011 Received in revised form 7 December 2011 Accepted 26 December 2011 Available online 27 January 2012
The high hardness of bulk metallic glasses (BMGs) makes them promising candidates for high wear applications. This paper focuses on the effects of oxygen on the tribological behavior of a zirconium-based BMG using pin-on-disk wear tests in three different environments, i.e. air, oxygen and argon. It was found that the wear rate of the BMG specimens increased dramatically with increasing oxygen content in the testing environment, i.e. in the order argon, air, oxygen. The pins and disk were examined using X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. A number of cracks and pits were present on the worn surface of the pin tested in oxygen-containing environments, whilst a relatively smooth worn surface and a mixed layer with a thickness of about 2e10 mm were observed in the specimens tested in argon. For the tests in oxygen, abrasive particles induced by oxidation protruded and peeled off from the glassy matrix, resulting in a combination of two-body and three-body abrasion. In an oxygen-free environment, plastic flow took place, presumably accompanied by work-softening, due to frictional heating and local stress concentrations. This led to the formation of the mixed layer on the pin and a material-transfer film on the disk. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: B. Glasses, metallic B. Tribological properties B. Oxidation B. Surface properties G. Wear-resistant applications
1. Introduction In the past decades, metallic glasses have attracted intense research because of their ultrahigh strength, large elastic limit and excellent corrosion resistance [1,2]. Bulk metallic glasses (BMGs), produced at critical cooling rates for glass forming of less than 10 K/ s, with high hardness make them promising candidates for bulk tribological applications [3e6]. However, the broadly held view of the generally better tribological performance of metallic glasses was principally based on the results of a thin, ribbon-shaped Fe-based metallic glass [7]. Recent studies on the wear resistance of BMGs are ambiguous about their potential utility. Some BMGs show excellent wear behavior, sometimes better than conventional structural materials or their crystallized counterparts [5,6]. In contrast, some reports indicate that the wear performance of BMGs is quite inferior to expectations [8,9]. Since BMGs are in a non-equilibrium state, their tribological behaviors are strongly dependent on the test conditions and processing history, and a number of investigations have been
* Corresponding author. E-mail address: [email protected] (I. Baker). 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.12.025
conducted over a wide range of experimental conditions, e.g. different applied loads, sliding speeds, sliding distances, lubrication conditions, wear modes (sliding and rolling) as well as different annealed states of the specimens [5e11]. For industrial applications, it is necessary to understand how BMG materials would behave if the oxygen content in the environment changes. Even though some studies report that the friction and wear of metallic glasses have sensitivity to oxygen in the test environment, the underlying mechanism is not completely understood [12e15]. For instance, Miyoshi and Buckley [13] investigated the tribological properties of three Fe-based metallic glass foils using a pin-on-flat configuration, and noted that the surface oxide layers formed on the as-received amorphous alloys were stable and provided low friction and a protective film against wear in contact with an alumina ball under argon or air at a relatively short sliding distance. Fu et al. [12] found a higher coefficient of friction and lower wear rate caused by the removal of oxygen from the test environment on a fully amorphous Zr-based alloy by performing a range of pin-on-disk sliding tests in both ambient air and vacuum. Recent work by Jin et al. [16] compared the dry sliding wear characteristics of a Zr-based BMG at room temperature in the as-cast, relaxed and crystallized states, and observed that the formation and subsequent peeling-off of oxygen rich tribolayers during wear was the main wear mechanism
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in the as-cast and relaxed specimens. Thus, it can be deduced that test environment also plays an important role on the wear characteristics of BMGs. Further, it is well-known that not only does oxygen have detrimental effects on the glass-forming ability and thermal stability of metallic glass due to the formation of long-range order, submicron- or nano-clusters and even oxides, leading to heterogeneous nucleation [17,18], but also affects the malleability of BMG by changing the failure mode from relatively ductile to catastrophic brittle fracture by suppressing the formation of multiple shear bands [19]. In view of the harmful effects of oxygen on such properties of BMGs, it is of great importance to systematically characterize the tribological behavior of a BMG under different oxygen conditions and determine the relationship between the wear rate and the environmental oxygen content. This paper describes the friction and the wear behavior of a Zr-based BMG during dry sliding in different environments, and examines the effects of oxygen on the wear behavior.
2. Experimental A master alloy with a nominal composition of Zr52.5Cu17.9Ni14.6Al10Ti5 (at. %) was prepared by arc melting a mixture of Zr (99.8 wt. %), Cu (99.9 wt. %), Ni (99.9 wt. %), Al (99.99 wt. %), and Ti (99.9 wt. %) in a Ti-gettered high-purity argon atmosphere. The ingot was melted four times to ensure a homogeneous composition, and then suction cast into a 3 mm diameter and 70 mm length, water-cooled copper mold. Wear pins of 6 mm length were cut from the suction-cast rods using a high speed abrasive saw, cooled with water to avoid possible crystallization. The two ends were polished using 600 grit silicon carbide paper and finished using 0.3 mm alumina powders. Pin-on-disk wear tests were performed against an yttriastabilized zirconia counterface material polished to a surface roughness of w0.01e0.05 mm. The test system is described in detail in Johnson et al. [20]. Cylindrical pin specimens were fixed on a holder and loaded against the rotating disk. The tests were conducted under constant applied normal load of 23 N at a sliding velocity of 1 ms1 and for a sliding distance of 1 km, in either air, oxygen or argon. Three tests were performed in each environment
Fig. 1. Wear loss (mean value of mass loss) of Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass after 1 km sliding tests in oxygen, air and argon. Measured humidity levels in the test chamber showed relative humidity of w6% under oxygen and argon, and a relative humidity of w40% when tests were performed in air. Error bars signify standard deviations.
Fig. 2. XRD patterns of the worn surfaces generated by sliding of BMG in different environments.
for reproducibility. Debris was collected during the wear tests using adhesive tape wrapped around the outside of the zirconia disk. The phases of the specimens both before and after wear testing, and debris collected from the wear tests conducted in oxygen (insufficient debris was available under other test conditions) were analyzed using a Rigaku D/Max 2000 X-ray diffractometer (XRD) with Cu Ka radiation operated at 40 kV and 300 mA. Measurements were performed by step scanning 2q from 10 to 120 with a 0.02 step size. A count time of 1 s per step was used, giving a total scan time of w1.5 h. The worn surfaces were examined using an FEI XL-30 scanning electron microscope (SEM) operated at 15 kV, equipped with an EDAX Li-drifted energy dispersive X-ray spectrometer (EDS). Transmission electron microscope (TEM) specimens of the worn pins were prepared using an FEI Nova 200 Nanolab focused ion beam microscope (FIB) using the lift-out method [21,22]. TEM specimens extracted from the worn pins were characterized in a Philips CM 200 TEM operating at 200 kV to which an EDS system was interfaced. Elemental X-ray maps were collected with this instrument operating in STEM mode. The counterface after wear tests was examined using an optical microscope equipped with a digital camera. The weight loss of the
Fig. 3. XRD pattern of debris collected from the wear test performed in oxygen.
H. Wu et al. / Intermetallics 25 (2012) 115e125
specimens was measured in an electronic balance of 0.1 mg precision before and after the wear tests. The density of the metallic glass was determined according to Archimedes’ method and the weight loss was then converted to a volume loss value. The Vickers microhardness of the as-cast specimen was measured using a Leitz Miniload microhardness tester at a load of 100 g. The thermal diffusivity and thermal conductivity of the as-cast specimen were obtained using a JR-3 Laser Heat Conductive Instrument by means of flash method.
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3. Results 3.1. Wear and friction The mass loss results for the pin-on-disk tests in three different environments, i.e. air, oxygen and argon, are shown in Fig. 1. It can be seen that the wear loss of the pins is significantly reduced by decreasing oxygen in the environment, i.e. in the order of oxygen, air and argon. The mean mass loss of the specimens after 1 km of
Fig. 4. Typical secondary electron images of the surface for the pin worn in air (aec), and back-scattered electron images of the surface (d and e), and the debris (f) for the pin worn in oxygen.
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sliding are 5.07 mg in oxygen, 2.77 mg in air and 1.47 mg in argon, respectively. These correspond to volumetric wear rates of 0.76 mm3/km in oxygen, 0.42 mm3/km in air, and 0.22 mm3/km in argon. This is similar to the result of Fu et al. [12,14], who reported that less wear was obtained in a Zr-based BMG by changing the test environment from air to vacuum. The small error bars indicates both the homogeneous microstructure of the specimens and the reproducibility of the sliding tests. The measured frictional force was somewhat variable during the tests, and decreased from a higher initial value to a lower steady state value in all environments. The measured friction coefficient (steady state) was approximately 0.22 for tests performed in both oxygen and air, but was slightly lower (w0.15) for tests conducted in argon. These are considerably lower than the results of Blau [9], but comparatively close to the values reported by Liu et al. [11], although the same Zr52.5Cu17.9Ni14.6Al10Ti5 BMG was used in both their studies. Since the friction coefficient decreases with increasing load [12,14], the low friction obtained from the sliding tests may be ascribed to the high normal load employed in the present study. 3.2. Worn surface and debris Fig. 2 shows XRD patterns of the worn surfaces of the BMG pins after wear tests performed in different environments. The XRD pattern of the as-cast BMG is included for comparison. A pattern typical of an amorphous structure with only a broad diffraction halo, with no detectable sharp Bragg peaks corresponding to crystalline phases, is present for the as-cast BMG. For the pin wear-tested in oxygen, several sharp Bragg diffraction peaks corresponding to two different polymorphs of ZrO2, i.e. tetragonal ZrO2, and monoclinic ZrO2, are superimposed on a broad diffraction halo that corresponds to the residual glassy matrix. These signify the coexistence of various crystalline phases and the
Fig. 5. Typical secondary electron images of the worn surface of the pin after sliding under argon.
Fig. 6. Longitudinal cross-sections of the specimen tested in air (a) and in oxygen (b), X-ray spectra (c and d) obtained from the corresponding marked region in (a).
H. Wu et al. / Intermetallics 25 (2012) 115e125
residual amorphous phase on the worn surface after sliding. However, there are still a few peaks that are difficult to positively identify due to limited number of diffraction peaks in Fig. 2. Based on the investigation of phases of the same Zr52.5Cu17.9Ni14.6Al10Ti5 BMG after annealing at temperatures ranging from 400 C up to 550 C reported by He et al. [23,24], these could be either Zr2Ni0.67O0.33, Zr2Cu, or ZrAl, but this needs to be confirmed by further study. Similar diffraction peaks were present after testing in air. Interestingly, the Bragg peaks are much weaker in the pattern from the pin tested in argon, which means that the amount of the crystalline phases was probably much less. The XRD pattern of debris collected from the wear test performed in oxygen is shown in Fig. 3. The same zirconium oxides and unknown phases were observed as the phases present on the worn surface of the pins (Fig. 2). The worn surface of the pin tested in air shows the characteristic morphologies that result from abrasive wear, e.g. long parallel grooves generated by plowing (Fig. 4a), cracks (Fig. 4b), and a large pit where material had been pulled out (Fig. 4c). Pins that were
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Fig. 8. Secondary electron image of a cross-section of a triboloayer in a specimen wear-tested in oxygen.
Fig. 7. Longitudinal cross-section of the specimen tested under argon (a and b). X-ray spectra obtained from black area (c), white area (d) in the mixed layer, and BMG matrix (e).
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tested in oxygen presented behavior mostly similar to those of the pins tested in air, but the features were more exaggerated. A typical back-scattered electron (BSE) image of the surface for the pin after tests performed in oxygen evidently exhibits a number of cracks and pits where material has been pulled out (Fig. 4d). Fig. 4e shows a long groove produced by severe plowing, which has roughly the same width as the debris (see a and b in Fig. 4e and f), further confirming the occurrence of the abrasive wear during sliding in the oxygen-containing environments. In contrast, the worn surface of the specimen tested in argon is relatively smooth; smearing of the wear tracks occurred, and the pits where material has been pulled out of the surface for tests conducted in oxygen-containing
environments are not evident (Fig. 5a). Plastic deformation can also be observed at the edge of the worn area (Fig. 5b), as reported by Ishida et al. [4], in which evidence of the plastic deformation along the sliding direction was detected. 3.3. Cross-sections of the worn pins The tribological behavior depends not only on the surface behavior, but also on processes taking place beneath the surface. In order to further explore the underlying mechanisms for the effect of oxygen on the tribological behaviors of the BMGs, the morphologies of the cross-section of worn specimens after sliding
Fig. 9. (a) Bright field TEM image of subsurface region in wear pin for test performed in oxygen; (b) close-up view of the near-surface region in (a); (c) SAD patterns showing the presence of both monoclinic ZrO2 and tetragonal ZrO2; (d) SAD pattern showing the presence of cubic ZrO2. The Pt strip was deposited in the FIB to protect the surface while machining with the FIB. The dark area, as labeled, is BMG matrix. The outmost worn surface consists of monoclinic ZrO2 and tetragonal ZrO2. The discrete gray regions, such as the one marked, are cubic ZrO2.
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Fig. 10. X-ray maps of the constituent elements from the region shown in Fig. 9a.
tests were examined. Fig. 6 shows a longitudinal cross-section of the BMG pin after sliding wear in oxygen-containing environments. It is noted that the cracks that appeared on the cross-section are roughly perpendicular to the worn surface (Fig. 6a). This is distinct from the observations of cracks running parallel to the interface on cross-section after testing of a Zr-based BMG in vacuum reported by Fu et al. [12], suggesting a different wear behavior when tests are performed in oxygen-containing environment. In addition, a dark layer is observable at the outmost worn surface (Fig. 6a), which contains a larger amount of oxygen than that in the substrate material, as confirmed by EDS analysis (Fig. 6c and d), a feature consistent with the results of Fu et al. [12]. In oxygen, surface material peeled off by abrasive particles on the cross-section of the pin (Fig. 6b) indicates an abrasive wear dominated process. Examination of a cross-section of the pin tested in argon shown in Fig. 7 reveals entirely different features compared to those generated in oxygen. A mixed layer with a thickness of about 2e10 mm, without any detectable cracks, is present at the interface (Fig. 7a and b). The X-ray spectra of each region as determined using EDS are shown in Fig. 7c and e. Compared with the similar composition between the white area in the mixed layer and the BMG matrix, the black area has a much higher oxygen content.
Fig. 11. Secondary electron image from a cross-section produced using a FIB in a specimen wear-tested in argon.
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0w11=2
3.4. Tribolayer In general, the layers underneath a worn surface which are commonly associated with deformation, compaction, and fragmentation when subjected to sliding wear are called mechanically mixed layers [25], or tribolayers [26]. In the present work, TEM specimens were extracted from the worn pins using FIB milling and cross-sections through the surface tribolayers were prepared and examined directly in the FIB. A SE image of a pit from the pin worn in oxygen is shown in Fig. 8. In this cross-section, contrast corresponding to different phases is clearly observable. The specific structure and composition of these phases will be presented later. Fig. 9 shows a bright field TEM image and corresponding selected-area diffraction (SAD) patterns for the subsurface of the pin worn in oxygen, while Fig. 10 shows X-ray elemental maps corresponding to the area in Fig. 9a. A close-up view of the nearsurface region in Fig. 9a is shown in Fig. 9b. The outermost worn surface consisted mainly of monoclinic ZrO2 and tetragonal ZrO2 as confirmed in Fig. 9c. It is worth noting that the zirconia disk has been shown to have a cubic crystal structure [21]. Thus, the cubic ZrO2, as indexed in Fig. 9d, found well below the worn surface of the pin indicates that the particles shed from the disk have been mechanically mixed into the tribolayer. Similar behavior was found in an AleSi alloy worn against zirconia [21]. A secondary electron image from a cross-section produced using a FIB in the specimen tested under argon is shown in Fig. 11. In sharp contrast to the pit shown in Fig. 8 from the test in oxygen, a relatively homogeneous layer (white region) can be seen on the cross-section compared to the presence of multiple phases in the tribolayer of the pin worn in oxygen. In order to reveal the composition and structure of the layer, bright field TEM images and accompanying X-ray elemental maps of the near-surface region of the pin worn in argon are presented in Fig. 12 and Fig. 13. The incorporation of cubic ZrO2 particles and crystals shown in Fig. 12b again indicates that mechanical mixing occurred in the tests performed under argon.
Tf ¼ T a þ
mTc* b@ F A 2
N
w
v
(2)
where Ta is the average temperature, Tf is the flash temperature, T0 is the room temperature, 298 K. T* is the equivalent temperature, T* ¼ aH/Km, in which a represents the thermal diffusivity (2.43 106 m2 s1), H is the surface hardness (5.58 109 N m2), and Km is the thermal conductivity (5.83 W m1 K1). Thus, T* is 2330 K.
3.5. Counterface For comparison, the surface morphologies of the counterface, i.e. zirconia disk, were examined. The wear tracks on the zirconia disk exhibit intense wear and a number of distributed abrasive particles after tests performed in oxygen, as marked in Fig. 14a and b. In contrast, a shiny transfer film formed without any observable abrasive particles on the wear tracks in tests conducted under argon, as shown in Fig. 14c and d. 4. Discussion 4.1. Oxidation and crystallization The frictional heating that occurs during the sliding process, can produce both oxidation and crystallization in a BMG if the temperature rise is sufficiently high, and in some cases, obvious molten characteristics have been observed in the wear tracks [27]. According to the model described by Lim and Ashby [28], the theoretical average temperatures and flash temperature present at the sliding surface in the wear tests arising from frictional heating can be calculated by Eqs. (1) and (2):
Ta ¼ T0 þ
ww mT * b Fv w 1=2 2 þ bðp v =8Þ
(1)
Fig. 12. (a) TEM image of subsurface region in wear pin tested in argon; (b) detailed view of the incorporation of cubic ZrO2 and crystals; (c) corresponding SAD pattern of the phases labeled in (b).
H. Wu et al. / Intermetallics 25 (2012) 115e125
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Fig. 13. X-ray maps of the constituent elements from the region shown in Fig. 12a.
b is the equivalent distance, b ¼ lb/r0, lb is the equivalent thermal w
diffusion distance, r0 is the radius of the pin. F is the equivalent w F normal stress, F ¼ , where F is the exterior force (23 N), An is the A n H0 contact area, H0 is the hardness of the pin. Therefore, it can be deduced w
w
w
that F ¼ 5:8 104 $ v is equivalent sliding velocity, v ¼ vr0 =a, v is the sliding velocity (1 m s1), r0 is the radius of the pin (1.5 103 m), w
a is the thermal diffusivity. It can be calculated that v is 625. Tc* is the actual equivalent temperature, Tc* ¼ aH=Kc , substituting H with H0, Kc is the equivalent thermal conductivity, which is less than Km, and can be substituted with 0.8Km [29]. Thus, Tc* is 2913 K. N is the total w
w
number of asperities, N ¼ ðr0 =ra Þ2 F ð1 F Þ þ 1, r0 is the radius of the pin, ra is the radius of contact units. Then N is 459. m is the friction
w
coefficient. Since friction coefficient is variable and depends on v during sliding, it can be approximately given by w
m ¼ 0:78 0:13lg v
(3)
This is used in Eqs. (1) and (2) to calculate the contact temperature. By substituting Eqs. (1) and (2) with relevant data, it can be approximately calculated that under the load of 23 N, the theoretical average temperature Ta and flash temperature Tf on the friction surface of glassy specimens are 320 K and 2898 K, respectively. Even though the average temperature on the surface of the BMG does not increase greatly during sliding, the flash temperature is elevated significantly. Thus, both oxidation and crystallization could take place during the sliding process due to the sufficiently high flash temperature. Triwikantoro et al. [30] analyzed the oxidation behavior in a series of Zr-based metallic glasses and found that the oxide scales formed during oxidation have a nanocrystalline microstructure consisting mainly of tetragonal ZrO2 and monoclinic ZrO2. Likewise, a lamellar structure consisting of tetragonal ZrO2, embedded in a nanocrystalline solid solution, was found after oxidation of Zr-based metallic glasses [31]. These are quite similar to the XRD results and TEM observations found here (see in Figs. 2 and 9), hence confirming the occurrence of oxidation during wear.
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Fig. 14. Optical micrographs showing morphologies of the disk after sliding performed in oxygen: (a) low magnification and (b) high magnification, and in argon: (c) low magnification and (d) high magnification. Circles mark abrasive particles.
Since metallic glasses are metastable, crystallization may occur unavoidably, caused by thermal activation and energy fluctuation, including temperature increase and deformation. Recent studies showed different crystallization behaviors of BMGs during wear tests. For instance, some reports indicated that there was no crystallization in the friction surface of BMGs [5,27], while others showed crystallization [6,11]. On one hand, crystallization may occur when the contact temperature presenting on the friction surface exceeds the onset crystallization temperature of the glassy pin during sliding; On the other hand, nanocrystallization can also be induced by plastic flow during deformation of BMGs [32,33]. Accordingly, it is evident that the crystallization occurred during the sliding tests in the present study, presumably originating from the localized flash temperature and flow stress, as present in Figs. 2 and 12. However, the crystals were hardly present in TEM diffraction patterns, which may be due to two reasons. First, the temperatures reached at the tip are the most likely to produce crystallization. This is so close to the tip that the material is immediately worn away during sliding. Second, the grain size of the crystals could be too small to be determined in TEM diffraction pattern, since the diffraction pattern of nanocrystals exhibits an amorphous-like halo ring without evident diffraction spots. 4.2. Wear mechanism The wear behavior of the glassy pin appears to be significantly dependent on the environment, which indicates that there are different wear mechanisms occurring in different environments. In the oxygen-containing environments, i.e. air and oxygen, the wear process is mainly abetted by oxidation. At the beginning of the wear process, hard oxides, i.e. monoclinic ZrO2 and tetragonal ZrO2, formed as a consequence of frictional heating on the wear surface of the pin. The cubic ZrO2 (counterface) debris, subsequently, were peeled away by those oxides, embedded in and protruded from the
pin surface. This subsequently acted as an abrasive particle together with the oxides debonded from the pin, exhibiting a combination of two-body abrasion and three-body abrasion. The severe abrasion under both wear mechanisms resulted in the presence of a rough worn surface of the pin, mechanically mixed cubic ZrO2 in the BMG substrate, and significant wear on the counterface. Abrasive wear involves several mechanisms, such as plowing, microcracking and microcutting, that are all damage modes to the specimen surface and give rise to material removal from the specimen. All of those features were observed on the worn surface of the pins (Fig. 4). Consequently, it is suggested that there was an abrasive wearcontrolled process when tests were performed in the oxygencontaining environments. In an oxygen-free environment, i.e. argon, the influence of oxidation on the wear mechanism of the BMG specimens can be ruled out due to the removal of oxygen. The superplastic behavior of BMGs in the supercooled liquid region has been well characterized in previous studies [32e34]. Since the contact temperature of the sliding interface was above the glass transition temperature due to the highly localized frictional heating (flash temperature), the surface material of the pin experienced a large plastic strain, hence exhibiting the features of smearing and plastic flow (Fig. 5). Further, the highly localized shear deformation may induce softening of metallic glasses because of the formation of more free volume [35]. In addition to plastic softening (work-softening) caused by deformation, softening occurred on the worn surface of the metallic glass during the wear process, similar to that noted in a recent report by Liu et al. [11]. Thus, the plastic flow took place presumably accompanied by work-softening due to the frictional heat and local stress concentrations. This led to the formation of the mixed layer on the pin and a material-transfer film on the disk. Simultaneously, the crystallization triggered by the frictional heating and flow stress on the wear surface of the BMG may give rise to an increase of both strength and hardness [11,36]. It has been
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reported in the literature that the presence of a crystalline phase can enhance both the hardness and toughness of a metallic glass, thus improving wear resistance [37,38]. This may lead to the improvement of the wear resistance of the BMG, resulting in a comparatively lower wear rate. Sliding wear occurs when two solids surfaces in contact slide over each other. As the boundary between different types of wear is not rigid, the sliding wear can roughly be distinguished from abrasive wear which involves particles harder than the material being abraded [39]. For conventional materials, the wear behavior follows Archard’s equation [40]:
Vw ¼ K
NS H
(4)
where Vw is the total volume of material removed by wear, N is the normal load, S is the total relative sliding distance, H is the material hardness, and K is the wear coefficient. Eq. (4) provides a valuable means of describing the severity of wear, via the dimensionless, K. Typical values of K are >102e104 for severe wear and 104e106 for mild wear [39]. The wear coefficient K of the pins after tests conducted in argon was obtained using Eq. (4) to be 5.4 105, thus implying that mild sliding wear was dominated. It is worth noting that moisture in the testing environment has been considered to be an important factor affecting the wear behavior of eutectic AleSi [41]. In a recent study, Gwaze et al. [42] characterized the effect of both moisture and oxygen on the wear mechanisms of as-cast eutectic AleSi, and found that the wear rate increased when oxygen was present and further increased when both oxygen and water were present. Nevertheless, the influence of moisture on the wear mechanism of BMG has rarely been reported. Liu et al. [43] reported that, unlike those of intermetallics alloys, e.g. FeAl and Ni3Al, the tensile fracture strength and ductility of Zr-based BMGs are not sensitive to the test environments, and proposed that moisture-induced hydrogen embrittlement might be masked by catastrophic shear fracture. Similarly, in the present study, although the relative humidity in the environment of tests conducted in air (w40%) was considerably higher than that of tests performed in oxygen (w6%), the wear of the pins was increased dramatically by modifying the atmosphere from air to dry oxygen. That is to say, the wear of the BMG specimens was more sensitive to oxygen than moisture in the test environment. The influence of moisture, therefore, may be masked by the severe abrasive wear. 5. Conclusions The tribological behavior of a Zr52.5Cu17.9Ni14.6Al10Ti5 BMG was investigated using pin-on-disk wear tests in three different environments. The wear of the BMG is more sensitive to oxygen than moisture in the environment. Increasing the oxygen content of the sliding environment, i.e. in the order of argon, air and oxygen, decreases the wear resistance significantly. The results show that an abrasive wearcontrolled process occurs in the tests performed in oxygencontaining environments, while mild sliding wear is the main mechanism in the tests conducted in an oxygen-free environment.
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Acknowledgments This research was supported by U.S. Department of Energy (DOE), Office of Basic Energy Science Award DE-FG02-07ER46392. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the DOE or the U.S. Government. We would like to acknowledge the help of Dr. Charles Daghlian and Mr. Michael Gwaze. H. Wu acknowledges the support of China Scholarship Council for studying abroad program.
References [1] Johnson WL. MRS Bull 1999;24:42e56. [2] Inoue A. Acta Mater 2000;48:279e306. [3] Ma MZ, Liu RP, Xiao Y, Lou DC, Liu L, Wang Q, et al. Mater Sci Eng A 2004;386: 326e30. [4] Ishida M, Takeda H, Nishiyama N, Kita K, Shimizu Y, Saotome Y, et al. Mater Sci Eng A 2007;449-451:149e54. [5] Parlar Z, Bakkal M, Shih AJ. Intermetallics 2008;16:34e41. [6] Gloriant TJ. Non-Cryst Solids 2003;316:96e103. [7] Imura T, Hasegawa K, Moori M, Nishiwaki T, Takagi M, Kawamura Y. Mater Sci Eng A 1991;133:332e6. [8] Tam CY, Shek CH. Mater Sci Eng A 2004;384:138e42. [9] Blau PJ. Wear 2001;250:431e4. [10] Bakkal M. Intermetallics 2010;18:1251e3. [11] Liu Y, Yitian Z, Xuekun L, Liu Z. J Alloys Compd 2010;503:138e44. [12] Fu X, Kasai T, Falk ML, Rigney DA. Wear 2001;250:409e19. [13] Miyoshi K, Buckley DH. Wear 1986;110:295e313. [14] Fu X, Rigney DA. Mater Res Soc Symp Proc 1998;554:437e42. [15] Lee DH, Evetts JE. Acta Metall 1984;32:1035e43. [16] Jin HW, Ayer R, Koo JY, Raghavan R, Ramamurty U. J Mater Res 2007;22: 264e73. [17] Liu CT, Chisholm MF, Miller MK. Intermetallics 2002;10:1105e12. [18] Gebert A, Eckert J, Schultz L. Acta Mater 1998;46:5475e82. [19] Lu ZP, Bei H, Wu Y, Chen GL, George EP, Liu CT. Appl Phys Lett 2008;92: 011915. [20] Johnson BJ, Kennedy FE, Baker I. Wear 1996;192:241e7. [21] Baker I, Sun Y, Kennedy FE, Munroe PR. J Mater Sci 2010;45:969e78. [22] Giannuzzi LA, Stevie FA. Micron 1999;30:197e204. [23] He G, Bian Z, Chen GL. Mater Sci Eng A 2000;279:237e43. [24] He G, Bian Z, Chen GL. J Mater Sci Technol 2000;16:357e61. [25] Rigney DA. Wear 2000;245:1e9. [26] Li J, Elmadagli M, Gertsman VY, Lo J, Alpas AT. Mater Sci Eng A 2006;421: 317e27. [27] Tao PJ, Yang YZ, Ru Q. J Alloys Compd 2010;492:L36e9. [28] Lim SC, Ashby MF. Acta Metall 1987;35:1e24. [29] Kong J, Xiong D, Li J, Yuan Q, Tyagi R. Tribol Lett 2009;35:151e8. [30] Triwikantoro, Toma D, Meuris M, Koster U. J Non-Cryst Solids 1999;250-252: 719e23. [31] Koster U, Jastrow L. Mater Sci Eng A 2007;449-451:57e62. [32] Nieh TG, Mukai T, Liu CT, Wadsworth J. Scripta Mater 1999;40:1021e7. [33] Shen J, Wang G, Sun JF, Stachurski ZH, Yan C, Ye L, et al. Intermetallics 2005; 13:79e85. [34] Chan KC, Liu L, Wang JF. J Non-Cryst Solids 2007;353:3758e63. [35] Bhowmick R, Raghavan R, Chattopadhyay K, Ramamurty U. Acta Mater 2006; 54:4221e8. [36] He G, Bian Z, Chen GL. Mater Sci Eng A 1999;270:291e8. [37] Kim YH, Inoue A, Masumoto T. Mater Trans 1991;32:331e8. [38] Chen H, He Y, Shiflet GJ, Poon SJ. Scripta Metall Mater 1991;25:1421e4. [39] Greer AL, Rutherford KL, Hutchings IM. Int Mater Rev 2002;47:87e112. [40] Archard JF. J Appl Phys 1953;24:981e8. [41] Yen BK, Ishihara T. Wear 1996;198:169e75. [42] Gwaze M, Baker I, Kennedy FE, Munroe PR. J Mater Sci 2010;45:6849e52. [43] Liu CT, Heatherly L, Easton DS, Carmichael CA, Schneibel JH, Chen CH, et al. Metall Mater Trans A 1998;29:1811e20.
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effects of environment on the sliding tribological behaviors of Zr-based bulk metallic glass Hong Wu a, b, Ian Baker b, *, Yong Liu a, Xiaolan Wu b, Paul R. Munroe c a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083 Hunan, China Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, USA c Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 September 2011 Received in revised form 7 December 2011 Accepted 26 December 2011 Available online 27 January 2012
The high hardness of bulk metallic glasses (BMGs) makes them promising candidates for high wear applications. This paper focuses on the effects of oxygen on the tribological behavior of a zirconium-based BMG using pin-on-disk wear tests in three different environments, i.e. air, oxygen and argon. It was found that the wear rate of the BMG specimens increased dramatically with increasing oxygen content in the testing environment, i.e. in the order argon, air, oxygen. The pins and disk were examined using X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. A number of cracks and pits were present on the worn surface of the pin tested in oxygen-containing environments, whilst a relatively smooth worn surface and a mixed layer with a thickness of about 2e10 mm were observed in the specimens tested in argon. For the tests in oxygen, abrasive particles induced by oxidation protruded and peeled off from the glassy matrix, resulting in a combination of two-body and three-body abrasion. In an oxygen-free environment, plastic flow took place, presumably accompanied by work-softening, due to frictional heating and local stress concentrations. This led to the formation of the mixed layer on the pin and a material-transfer film on the disk. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: B. Glasses, metallic B. Tribological properties B. Oxidation B. Surface properties G. Wear-resistant applications
1. Introduction In the past decades, metallic glasses have attracted intense research because of their ultrahigh strength, large elastic limit and excellent corrosion resistance [1,2]. Bulk metallic glasses (BMGs), produced at critical cooling rates for glass forming of less than 10 K/ s, with high hardness make them promising candidates for bulk tribological applications [3e6]. However, the broadly held view of the generally better tribological performance of metallic glasses was principally based on the results of a thin, ribbon-shaped Fe-based metallic glass [7]. Recent studies on the wear resistance of BMGs are ambiguous about their potential utility. Some BMGs show excellent wear behavior, sometimes better than conventional structural materials or their crystallized counterparts [5,6]. In contrast, some reports indicate that the wear performance of BMGs is quite inferior to expectations [8,9]. Since BMGs are in a non-equilibrium state, their tribological behaviors are strongly dependent on the test conditions and processing history, and a number of investigations have been
* Corresponding author. E-mail address: [email protected] (I. Baker). 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.12.025
conducted over a wide range of experimental conditions, e.g. different applied loads, sliding speeds, sliding distances, lubrication conditions, wear modes (sliding and rolling) as well as different annealed states of the specimens [5e11]. For industrial applications, it is necessary to understand how BMG materials would behave if the oxygen content in the environment changes. Even though some studies report that the friction and wear of metallic glasses have sensitivity to oxygen in the test environment, the underlying mechanism is not completely understood [12e15]. For instance, Miyoshi and Buckley [13] investigated the tribological properties of three Fe-based metallic glass foils using a pin-on-flat configuration, and noted that the surface oxide layers formed on the as-received amorphous alloys were stable and provided low friction and a protective film against wear in contact with an alumina ball under argon or air at a relatively short sliding distance. Fu et al. [12] found a higher coefficient of friction and lower wear rate caused by the removal of oxygen from the test environment on a fully amorphous Zr-based alloy by performing a range of pin-on-disk sliding tests in both ambient air and vacuum. Recent work by Jin et al. [16] compared the dry sliding wear characteristics of a Zr-based BMG at room temperature in the as-cast, relaxed and crystallized states, and observed that the formation and subsequent peeling-off of oxygen rich tribolayers during wear was the main wear mechanism
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in the as-cast and relaxed specimens. Thus, it can be deduced that test environment also plays an important role on the wear characteristics of BMGs. Further, it is well-known that not only does oxygen have detrimental effects on the glass-forming ability and thermal stability of metallic glass due to the formation of long-range order, submicron- or nano-clusters and even oxides, leading to heterogeneous nucleation [17,18], but also affects the malleability of BMG by changing the failure mode from relatively ductile to catastrophic brittle fracture by suppressing the formation of multiple shear bands [19]. In view of the harmful effects of oxygen on such properties of BMGs, it is of great importance to systematically characterize the tribological behavior of a BMG under different oxygen conditions and determine the relationship between the wear rate and the environmental oxygen content. This paper describes the friction and the wear behavior of a Zr-based BMG during dry sliding in different environments, and examines the effects of oxygen on the wear behavior.
2. Experimental A master alloy with a nominal composition of Zr52.5Cu17.9Ni14.6Al10Ti5 (at. %) was prepared by arc melting a mixture of Zr (99.8 wt. %), Cu (99.9 wt. %), Ni (99.9 wt. %), Al (99.99 wt. %), and Ti (99.9 wt. %) in a Ti-gettered high-purity argon atmosphere. The ingot was melted four times to ensure a homogeneous composition, and then suction cast into a 3 mm diameter and 70 mm length, water-cooled copper mold. Wear pins of 6 mm length were cut from the suction-cast rods using a high speed abrasive saw, cooled with water to avoid possible crystallization. The two ends were polished using 600 grit silicon carbide paper and finished using 0.3 mm alumina powders. Pin-on-disk wear tests were performed against an yttriastabilized zirconia counterface material polished to a surface roughness of w0.01e0.05 mm. The test system is described in detail in Johnson et al. [20]. Cylindrical pin specimens were fixed on a holder and loaded against the rotating disk. The tests were conducted under constant applied normal load of 23 N at a sliding velocity of 1 ms1 and for a sliding distance of 1 km, in either air, oxygen or argon. Three tests were performed in each environment
Fig. 1. Wear loss (mean value of mass loss) of Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass after 1 km sliding tests in oxygen, air and argon. Measured humidity levels in the test chamber showed relative humidity of w6% under oxygen and argon, and a relative humidity of w40% when tests were performed in air. Error bars signify standard deviations.
Fig. 2. XRD patterns of the worn surfaces generated by sliding of BMG in different environments.
for reproducibility. Debris was collected during the wear tests using adhesive tape wrapped around the outside of the zirconia disk. The phases of the specimens both before and after wear testing, and debris collected from the wear tests conducted in oxygen (insufficient debris was available under other test conditions) were analyzed using a Rigaku D/Max 2000 X-ray diffractometer (XRD) with Cu Ka radiation operated at 40 kV and 300 mA. Measurements were performed by step scanning 2q from 10 to 120 with a 0.02 step size. A count time of 1 s per step was used, giving a total scan time of w1.5 h. The worn surfaces were examined using an FEI XL-30 scanning electron microscope (SEM) operated at 15 kV, equipped with an EDAX Li-drifted energy dispersive X-ray spectrometer (EDS). Transmission electron microscope (TEM) specimens of the worn pins were prepared using an FEI Nova 200 Nanolab focused ion beam microscope (FIB) using the lift-out method [21,22]. TEM specimens extracted from the worn pins were characterized in a Philips CM 200 TEM operating at 200 kV to which an EDS system was interfaced. Elemental X-ray maps were collected with this instrument operating in STEM mode. The counterface after wear tests was examined using an optical microscope equipped with a digital camera. The weight loss of the
Fig. 3. XRD pattern of debris collected from the wear test performed in oxygen.
H. Wu et al. / Intermetallics 25 (2012) 115e125
specimens was measured in an electronic balance of 0.1 mg precision before and after the wear tests. The density of the metallic glass was determined according to Archimedes’ method and the weight loss was then converted to a volume loss value. The Vickers microhardness of the as-cast specimen was measured using a Leitz Miniload microhardness tester at a load of 100 g. The thermal diffusivity and thermal conductivity of the as-cast specimen were obtained using a JR-3 Laser Heat Conductive Instrument by means of flash method.
117
3. Results 3.1. Wear and friction The mass loss results for the pin-on-disk tests in three different environments, i.e. air, oxygen and argon, are shown in Fig. 1. It can be seen that the wear loss of the pins is significantly reduced by decreasing oxygen in the environment, i.e. in the order of oxygen, air and argon. The mean mass loss of the specimens after 1 km of
Fig. 4. Typical secondary electron images of the surface for the pin worn in air (aec), and back-scattered electron images of the surface (d and e), and the debris (f) for the pin worn in oxygen.
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sliding are 5.07 mg in oxygen, 2.77 mg in air and 1.47 mg in argon, respectively. These correspond to volumetric wear rates of 0.76 mm3/km in oxygen, 0.42 mm3/km in air, and 0.22 mm3/km in argon. This is similar to the result of Fu et al. [12,14], who reported that less wear was obtained in a Zr-based BMG by changing the test environment from air to vacuum. The small error bars indicates both the homogeneous microstructure of the specimens and the reproducibility of the sliding tests. The measured frictional force was somewhat variable during the tests, and decreased from a higher initial value to a lower steady state value in all environments. The measured friction coefficient (steady state) was approximately 0.22 for tests performed in both oxygen and air, but was slightly lower (w0.15) for tests conducted in argon. These are considerably lower than the results of Blau [9], but comparatively close to the values reported by Liu et al. [11], although the same Zr52.5Cu17.9Ni14.6Al10Ti5 BMG was used in both their studies. Since the friction coefficient decreases with increasing load [12,14], the low friction obtained from the sliding tests may be ascribed to the high normal load employed in the present study. 3.2. Worn surface and debris Fig. 2 shows XRD patterns of the worn surfaces of the BMG pins after wear tests performed in different environments. The XRD pattern of the as-cast BMG is included for comparison. A pattern typical of an amorphous structure with only a broad diffraction halo, with no detectable sharp Bragg peaks corresponding to crystalline phases, is present for the as-cast BMG. For the pin wear-tested in oxygen, several sharp Bragg diffraction peaks corresponding to two different polymorphs of ZrO2, i.e. tetragonal ZrO2, and monoclinic ZrO2, are superimposed on a broad diffraction halo that corresponds to the residual glassy matrix. These signify the coexistence of various crystalline phases and the
Fig. 5. Typical secondary electron images of the worn surface of the pin after sliding under argon.
Fig. 6. Longitudinal cross-sections of the specimen tested in air (a) and in oxygen (b), X-ray spectra (c and d) obtained from the corresponding marked region in (a).
H. Wu et al. / Intermetallics 25 (2012) 115e125
residual amorphous phase on the worn surface after sliding. However, there are still a few peaks that are difficult to positively identify due to limited number of diffraction peaks in Fig. 2. Based on the investigation of phases of the same Zr52.5Cu17.9Ni14.6Al10Ti5 BMG after annealing at temperatures ranging from 400 C up to 550 C reported by He et al. [23,24], these could be either Zr2Ni0.67O0.33, Zr2Cu, or ZrAl, but this needs to be confirmed by further study. Similar diffraction peaks were present after testing in air. Interestingly, the Bragg peaks are much weaker in the pattern from the pin tested in argon, which means that the amount of the crystalline phases was probably much less. The XRD pattern of debris collected from the wear test performed in oxygen is shown in Fig. 3. The same zirconium oxides and unknown phases were observed as the phases present on the worn surface of the pins (Fig. 2). The worn surface of the pin tested in air shows the characteristic morphologies that result from abrasive wear, e.g. long parallel grooves generated by plowing (Fig. 4a), cracks (Fig. 4b), and a large pit where material had been pulled out (Fig. 4c). Pins that were
119
Fig. 8. Secondary electron image of a cross-section of a triboloayer in a specimen wear-tested in oxygen.
Fig. 7. Longitudinal cross-section of the specimen tested under argon (a and b). X-ray spectra obtained from black area (c), white area (d) in the mixed layer, and BMG matrix (e).
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tested in oxygen presented behavior mostly similar to those of the pins tested in air, but the features were more exaggerated. A typical back-scattered electron (BSE) image of the surface for the pin after tests performed in oxygen evidently exhibits a number of cracks and pits where material has been pulled out (Fig. 4d). Fig. 4e shows a long groove produced by severe plowing, which has roughly the same width as the debris (see a and b in Fig. 4e and f), further confirming the occurrence of the abrasive wear during sliding in the oxygen-containing environments. In contrast, the worn surface of the specimen tested in argon is relatively smooth; smearing of the wear tracks occurred, and the pits where material has been pulled out of the surface for tests conducted in oxygen-containing
environments are not evident (Fig. 5a). Plastic deformation can also be observed at the edge of the worn area (Fig. 5b), as reported by Ishida et al. [4], in which evidence of the plastic deformation along the sliding direction was detected. 3.3. Cross-sections of the worn pins The tribological behavior depends not only on the surface behavior, but also on processes taking place beneath the surface. In order to further explore the underlying mechanisms for the effect of oxygen on the tribological behaviors of the BMGs, the morphologies of the cross-section of worn specimens after sliding
Fig. 9. (a) Bright field TEM image of subsurface region in wear pin for test performed in oxygen; (b) close-up view of the near-surface region in (a); (c) SAD patterns showing the presence of both monoclinic ZrO2 and tetragonal ZrO2; (d) SAD pattern showing the presence of cubic ZrO2. The Pt strip was deposited in the FIB to protect the surface while machining with the FIB. The dark area, as labeled, is BMG matrix. The outmost worn surface consists of monoclinic ZrO2 and tetragonal ZrO2. The discrete gray regions, such as the one marked, are cubic ZrO2.
H. Wu et al. / Intermetallics 25 (2012) 115e125
121
Fig. 10. X-ray maps of the constituent elements from the region shown in Fig. 9a.
tests were examined. Fig. 6 shows a longitudinal cross-section of the BMG pin after sliding wear in oxygen-containing environments. It is noted that the cracks that appeared on the cross-section are roughly perpendicular to the worn surface (Fig. 6a). This is distinct from the observations of cracks running parallel to the interface on cross-section after testing of a Zr-based BMG in vacuum reported by Fu et al. [12], suggesting a different wear behavior when tests are performed in oxygen-containing environment. In addition, a dark layer is observable at the outmost worn surface (Fig. 6a), which contains a larger amount of oxygen than that in the substrate material, as confirmed by EDS analysis (Fig. 6c and d), a feature consistent with the results of Fu et al. [12]. In oxygen, surface material peeled off by abrasive particles on the cross-section of the pin (Fig. 6b) indicates an abrasive wear dominated process. Examination of a cross-section of the pin tested in argon shown in Fig. 7 reveals entirely different features compared to those generated in oxygen. A mixed layer with a thickness of about 2e10 mm, without any detectable cracks, is present at the interface (Fig. 7a and b). The X-ray spectra of each region as determined using EDS are shown in Fig. 7c and e. Compared with the similar composition between the white area in the mixed layer and the BMG matrix, the black area has a much higher oxygen content.
Fig. 11. Secondary electron image from a cross-section produced using a FIB in a specimen wear-tested in argon.
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0w11=2
3.4. Tribolayer In general, the layers underneath a worn surface which are commonly associated with deformation, compaction, and fragmentation when subjected to sliding wear are called mechanically mixed layers [25], or tribolayers [26]. In the present work, TEM specimens were extracted from the worn pins using FIB milling and cross-sections through the surface tribolayers were prepared and examined directly in the FIB. A SE image of a pit from the pin worn in oxygen is shown in Fig. 8. In this cross-section, contrast corresponding to different phases is clearly observable. The specific structure and composition of these phases will be presented later. Fig. 9 shows a bright field TEM image and corresponding selected-area diffraction (SAD) patterns for the subsurface of the pin worn in oxygen, while Fig. 10 shows X-ray elemental maps corresponding to the area in Fig. 9a. A close-up view of the nearsurface region in Fig. 9a is shown in Fig. 9b. The outermost worn surface consisted mainly of monoclinic ZrO2 and tetragonal ZrO2 as confirmed in Fig. 9c. It is worth noting that the zirconia disk has been shown to have a cubic crystal structure [21]. Thus, the cubic ZrO2, as indexed in Fig. 9d, found well below the worn surface of the pin indicates that the particles shed from the disk have been mechanically mixed into the tribolayer. Similar behavior was found in an AleSi alloy worn against zirconia [21]. A secondary electron image from a cross-section produced using a FIB in the specimen tested under argon is shown in Fig. 11. In sharp contrast to the pit shown in Fig. 8 from the test in oxygen, a relatively homogeneous layer (white region) can be seen on the cross-section compared to the presence of multiple phases in the tribolayer of the pin worn in oxygen. In order to reveal the composition and structure of the layer, bright field TEM images and accompanying X-ray elemental maps of the near-surface region of the pin worn in argon are presented in Fig. 12 and Fig. 13. The incorporation of cubic ZrO2 particles and crystals shown in Fig. 12b again indicates that mechanical mixing occurred in the tests performed under argon.
Tf ¼ T a þ
mTc* b@ F A 2
N
w
v
(2)
where Ta is the average temperature, Tf is the flash temperature, T0 is the room temperature, 298 K. T* is the equivalent temperature, T* ¼ aH/Km, in which a represents the thermal diffusivity (2.43 106 m2 s1), H is the surface hardness (5.58 109 N m2), and Km is the thermal conductivity (5.83 W m1 K1). Thus, T* is 2330 K.
3.5. Counterface For comparison, the surface morphologies of the counterface, i.e. zirconia disk, were examined. The wear tracks on the zirconia disk exhibit intense wear and a number of distributed abrasive particles after tests performed in oxygen, as marked in Fig. 14a and b. In contrast, a shiny transfer film formed without any observable abrasive particles on the wear tracks in tests conducted under argon, as shown in Fig. 14c and d. 4. Discussion 4.1. Oxidation and crystallization The frictional heating that occurs during the sliding process, can produce both oxidation and crystallization in a BMG if the temperature rise is sufficiently high, and in some cases, obvious molten characteristics have been observed in the wear tracks [27]. According to the model described by Lim and Ashby [28], the theoretical average temperatures and flash temperature present at the sliding surface in the wear tests arising from frictional heating can be calculated by Eqs. (1) and (2):
Ta ¼ T0 þ
ww mT * b Fv w 1=2 2 þ bðp v =8Þ
(1)
Fig. 12. (a) TEM image of subsurface region in wear pin tested in argon; (b) detailed view of the incorporation of cubic ZrO2 and crystals; (c) corresponding SAD pattern of the phases labeled in (b).
H. Wu et al. / Intermetallics 25 (2012) 115e125
123
Fig. 13. X-ray maps of the constituent elements from the region shown in Fig. 12a.
b is the equivalent distance, b ¼ lb/r0, lb is the equivalent thermal w
diffusion distance, r0 is the radius of the pin. F is the equivalent w F normal stress, F ¼ , where F is the exterior force (23 N), An is the A n H0 contact area, H0 is the hardness of the pin. Therefore, it can be deduced w
w
w
that F ¼ 5:8 104 $ v is equivalent sliding velocity, v ¼ vr0 =a, v is the sliding velocity (1 m s1), r0 is the radius of the pin (1.5 103 m), w
a is the thermal diffusivity. It can be calculated that v is 625. Tc* is the actual equivalent temperature, Tc* ¼ aH=Kc , substituting H with H0, Kc is the equivalent thermal conductivity, which is less than Km, and can be substituted with 0.8Km [29]. Thus, Tc* is 2913 K. N is the total w
w
number of asperities, N ¼ ðr0 =ra Þ2 F ð1 F Þ þ 1, r0 is the radius of the pin, ra is the radius of contact units. Then N is 459. m is the friction
w
coefficient. Since friction coefficient is variable and depends on v during sliding, it can be approximately given by w
m ¼ 0:78 0:13lg v
(3)
This is used in Eqs. (1) and (2) to calculate the contact temperature. By substituting Eqs. (1) and (2) with relevant data, it can be approximately calculated that under the load of 23 N, the theoretical average temperature Ta and flash temperature Tf on the friction surface of glassy specimens are 320 K and 2898 K, respectively. Even though the average temperature on the surface of the BMG does not increase greatly during sliding, the flash temperature is elevated significantly. Thus, both oxidation and crystallization could take place during the sliding process due to the sufficiently high flash temperature. Triwikantoro et al. [30] analyzed the oxidation behavior in a series of Zr-based metallic glasses and found that the oxide scales formed during oxidation have a nanocrystalline microstructure consisting mainly of tetragonal ZrO2 and monoclinic ZrO2. Likewise, a lamellar structure consisting of tetragonal ZrO2, embedded in a nanocrystalline solid solution, was found after oxidation of Zr-based metallic glasses [31]. These are quite similar to the XRD results and TEM observations found here (see in Figs. 2 and 9), hence confirming the occurrence of oxidation during wear.
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H. Wu et al. / Intermetallics 25 (2012) 115e125
Fig. 14. Optical micrographs showing morphologies of the disk after sliding performed in oxygen: (a) low magnification and (b) high magnification, and in argon: (c) low magnification and (d) high magnification. Circles mark abrasive particles.
Since metallic glasses are metastable, crystallization may occur unavoidably, caused by thermal activation and energy fluctuation, including temperature increase and deformation. Recent studies showed different crystallization behaviors of BMGs during wear tests. For instance, some reports indicated that there was no crystallization in the friction surface of BMGs [5,27], while others showed crystallization [6,11]. On one hand, crystallization may occur when the contact temperature presenting on the friction surface exceeds the onset crystallization temperature of the glassy pin during sliding; On the other hand, nanocrystallization can also be induced by plastic flow during deformation of BMGs [32,33]. Accordingly, it is evident that the crystallization occurred during the sliding tests in the present study, presumably originating from the localized flash temperature and flow stress, as present in Figs. 2 and 12. However, the crystals were hardly present in TEM diffraction patterns, which may be due to two reasons. First, the temperatures reached at the tip are the most likely to produce crystallization. This is so close to the tip that the material is immediately worn away during sliding. Second, the grain size of the crystals could be too small to be determined in TEM diffraction pattern, since the diffraction pattern of nanocrystals exhibits an amorphous-like halo ring without evident diffraction spots. 4.2. Wear mechanism The wear behavior of the glassy pin appears to be significantly dependent on the environment, which indicates that there are different wear mechanisms occurring in different environments. In the oxygen-containing environments, i.e. air and oxygen, the wear process is mainly abetted by oxidation. At the beginning of the wear process, hard oxides, i.e. monoclinic ZrO2 and tetragonal ZrO2, formed as a consequence of frictional heating on the wear surface of the pin. The cubic ZrO2 (counterface) debris, subsequently, were peeled away by those oxides, embedded in and protruded from the
pin surface. This subsequently acted as an abrasive particle together with the oxides debonded from the pin, exhibiting a combination of two-body abrasion and three-body abrasion. The severe abrasion under both wear mechanisms resulted in the presence of a rough worn surface of the pin, mechanically mixed cubic ZrO2 in the BMG substrate, and significant wear on the counterface. Abrasive wear involves several mechanisms, such as plowing, microcracking and microcutting, that are all damage modes to the specimen surface and give rise to material removal from the specimen. All of those features were observed on the worn surface of the pins (Fig. 4). Consequently, it is suggested that there was an abrasive wearcontrolled process when tests were performed in the oxygencontaining environments. In an oxygen-free environment, i.e. argon, the influence of oxidation on the wear mechanism of the BMG specimens can be ruled out due to the removal of oxygen. The superplastic behavior of BMGs in the supercooled liquid region has been well characterized in previous studies [32e34]. Since the contact temperature of the sliding interface was above the glass transition temperature due to the highly localized frictional heating (flash temperature), the surface material of the pin experienced a large plastic strain, hence exhibiting the features of smearing and plastic flow (Fig. 5). Further, the highly localized shear deformation may induce softening of metallic glasses because of the formation of more free volume [35]. In addition to plastic softening (work-softening) caused by deformation, softening occurred on the worn surface of the metallic glass during the wear process, similar to that noted in a recent report by Liu et al. [11]. Thus, the plastic flow took place presumably accompanied by work-softening due to the frictional heat and local stress concentrations. This led to the formation of the mixed layer on the pin and a material-transfer film on the disk. Simultaneously, the crystallization triggered by the frictional heating and flow stress on the wear surface of the BMG may give rise to an increase of both strength and hardness [11,36]. It has been
H. Wu et al. / Intermetallics 25 (2012) 115e125
reported in the literature that the presence of a crystalline phase can enhance both the hardness and toughness of a metallic glass, thus improving wear resistance [37,38]. This may lead to the improvement of the wear resistance of the BMG, resulting in a comparatively lower wear rate. Sliding wear occurs when two solids surfaces in contact slide over each other. As the boundary between different types of wear is not rigid, the sliding wear can roughly be distinguished from abrasive wear which involves particles harder than the material being abraded [39]. For conventional materials, the wear behavior follows Archard’s equation [40]:
Vw ¼ K
NS H
(4)
where Vw is the total volume of material removed by wear, N is the normal load, S is the total relative sliding distance, H is the material hardness, and K is the wear coefficient. Eq. (4) provides a valuable means of describing the severity of wear, via the dimensionless, K. Typical values of K are >102e104 for severe wear and 104e106 for mild wear [39]. The wear coefficient K of the pins after tests conducted in argon was obtained using Eq. (4) to be 5.4 105, thus implying that mild sliding wear was dominated. It is worth noting that moisture in the testing environment has been considered to be an important factor affecting the wear behavior of eutectic AleSi [41]. In a recent study, Gwaze et al. [42] characterized the effect of both moisture and oxygen on the wear mechanisms of as-cast eutectic AleSi, and found that the wear rate increased when oxygen was present and further increased when both oxygen and water were present. Nevertheless, the influence of moisture on the wear mechanism of BMG has rarely been reported. Liu et al. [43] reported that, unlike those of intermetallics alloys, e.g. FeAl and Ni3Al, the tensile fracture strength and ductility of Zr-based BMGs are not sensitive to the test environments, and proposed that moisture-induced hydrogen embrittlement might be masked by catastrophic shear fracture. Similarly, in the present study, although the relative humidity in the environment of tests conducted in air (w40%) was considerably higher than that of tests performed in oxygen (w6%), the wear of the pins was increased dramatically by modifying the atmosphere from air to dry oxygen. That is to say, the wear of the BMG specimens was more sensitive to oxygen than moisture in the test environment. The influence of moisture, therefore, may be masked by the severe abrasive wear. 5. Conclusions The tribological behavior of a Zr52.5Cu17.9Ni14.6Al10Ti5 BMG was investigated using pin-on-disk wear tests in three different environments. The wear of the BMG is more sensitive to oxygen than moisture in the environment. Increasing the oxygen content of the sliding environment, i.e. in the order of argon, air and oxygen, decreases the wear resistance significantly. The results show that an abrasive wearcontrolled process occurs in the tests performed in oxygencontaining environments, while mild sliding wear is the main mechanism in the tests conducted in an oxygen-free environment.
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Acknowledgments This research was supported by U.S. Department of Energy (DOE), Office of Basic Energy Science Award DE-FG02-07ER46392. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the DOE or the U.S. Government. We would like to acknowledge the help of Dr. Charles Daghlian and Mr. Michael Gwaze. H. Wu acknowledges the support of China Scholarship Council for studying abroad program.
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