- Email: [email protected]
Studies on wear behavior of nano-Y2O3 dispersed ferritic steel developed by mechanical alloying and hot isostatic pressing
Wear 270 (2010) 5–11
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Studies on wear behavior of nano-Y2 O3 dispersed ferritic steel developed by mechanical alloying and hot isostatic pressing S.K. Karak a , C.S. Vishnu a , Z. Witczak b , W. Lojkowski b , J. Dutta Majumdar a , I. Manna a,c,∗ a b c
Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721302, India Institute of High Pressure Physics (Unipress), Polish Academy of Sciences, Sokolowska 29, 01-142 Warsaw, Poland Central Glass and Ceramic Research Institute, Kolkata 70032, India
a r t i c l e
i n f o
Article history: Received 31 January 2010 Received in revised form 21 August 2010 Accepted 26 August 2010 Available online 12 October 2010 Keywords: Metal matrix composite Hardness Electron microscopy Wear testing
a b s t r a c t Resistance to wear is an important factor in design and selection of structural components in motion and in contact with a mating surface. The present work deals with studies on fretting wear behavior of mechanically alloyed 1.0 wt.% nano-Y2 O3 dispersed 71.0Fe–25.5Cr–2.0Al–0.5Ti (wt.%) ferritic steel consolidated by hot isostatic pressing at 600, 800 and 1000 ◦ C under 1.2 GPa uniaxial pressure applied for 1 h. The wear experiments were carried out in gross slip fretting regime against 6 mm diameter tungsten carbide ball at ambient temperature. The highest resistance to fretting wear has been observed in the alloy sintered at 1000 ◦ C. The fretting involves localized plastic deformation and oxidation as evidenced by post wear surface profile and micro-compositional analysis, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanocrystalline materials have attracted great scientific interests in the recent because they possess superior mechanical, physical and chemical properties compared to conventional coarsegrained materials [1,2]. Besides monolithic single phase aggregate, nanostuctured solids with dispersion strengthening offered by uniform distribution of nano-intermetallic or nano-ceramic phases are extremely useful for structural applications at ambient or elevated temperatures [3,4]. Among various ex situ strengthened alloys, ceramic oxide dispersion strengthened iron base ferritic steel (containing significant amounts of Cr, Al and Ti) find wide ranging applications in fabricating discs and other critical parts of jet engines, pump bodies and parts, rocket motors and thrust reversers, nuclear fuel element spacers, hot extrusion tools, high strength bolts and down hole shafts [5]. Nano-Y2 O3 dispersed ferritic steel possesses a unique combination of body centre cubic (BCC) structure with low coefficient thermal of expansion, high thermal conductivity, good creep resistance and high tensile/compressive strength [6–9]. Mechanical alloying is a solid state synthesis process that consists of repeated cold-welding, fracturing, dynamic recrys-
∗ Corresponding author at: Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur, W.B. 721302, India. Tel.: +91 3222 283266; fax: +91 3222 255280. E-mail address: [email protected] (I. Manna). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.08.021
tallization and mechanically activated inter-diffusion among the powder particles in a high energy ball mill [10]. While mechanical alloying is a useful technique to synthesize novel materials including nanostructured and amorphous products in powder form (starting from elemental/alloyed/compound precursors), consolidation of such powders into a bulk component without deteriorating or destroying the as-milled novel microstructural state poses a genuine challenge. Various attempts that have achieved reasonable success in this regard (consolidation of mechanically alloyed powder) are high pressure sintering [11,12], equi-channel angular pressing [13,14], laser sintering [15], pulse plasma sintering [16] and hot isostatic pressing [17]. However, studies on hot isostatic pressing of nanooxide dispersed mechanically alloyed ferritic steel are scarce. Mechanical degradation of structural components in un-lubricated contact with a mating surface at low amplitude oscillatory sliding, known as fretting wear, is of great importance in mechanical joints of vibrating structures ranging from household applications, automobiles, electrical motors and even in human body implants [18]. The displacement amplitudes (5–300 m) encountered in fretting are smaller than those of reciprocating sliding [19]. This means that contact between the counter bodies is maintained over almost the entire period of operation or fretting. As a result, much of the wear debris produced by fretting remains trapped at the interface, which can cause seizure in components such as flexible couplings [20]. Another important aspect of fretting is the development of fatigue cracks in the damaged region, which reduces the fatigue strength of a cyclically loaded component. In general, fretting studies on nano-Y2 O3 dispersed in Fe–Cr
6
S.K. Karak et al. / Wear 270 (2010) 5–11
ferritic steel are limited. In the present work, fretting wear of a new generation of nano-Y2 O3 dispersed in Fe–Cr ferritic steel, synthesized by mechanical alloying and sintered by hot isostatic pressing, against tungsten carbide ball is investigated using a newly designed fretting wear tester. An effort has also been made to understand the operative wear mechanism through a detailed microstructural and micro-compositional analysis.
2. Experimental Elemental powders (>99.9 wt.% purity and 30–80 m size) blended to approximate composition of 1.0 wt.% nano-Y2 O3 dispersed 71.0Fe–25.5Cr–2.0Al–0.5Ti (all in wt.%) were subjected to mechanical alloying by high energy ball milling in a multi vial Fritsch PM400 planetary ball mill, operated at 300 rpm using stainless steel vials and balls (10 mm diameter). Milling was conducted with a ball to powder ratio of 10:1 in wet (toluene) medium to avoid agglomeration of powders, prevent continued oxidation except at early stage and ensure sufficient yield after milling. The mechanically milled or alloyed powders were placed in a steel can or cylinder sealed under vacuum, and placed in the anvil of the hot isostatic pressing (HIP) machine. The pressure was maintained at 1.2 GPa using Ar gas for 1 h in the temperature range 600–1000 ◦ C. Identity of the phases following hot isostatic pressing was determined by X-ray diffraction (XRD) using a Panalytical X’Pert Pro diffractometer with Co-K␣ (0.179 nm) radiation. Density of the sintered samples was measured using Archimedes principle after weighing in air and water separately using an electronic balance with a precision of 0.1 mg. Morphology, size and shape and distribution of the phases in the sintered components and surface damage in worn tracks were studied using a field emission scanning electron microscopy (FESEM, Zeiss, Supra 40 V). Electron transparent thin foils were prepared for transmission electron microscopy (TEM) studies using mechanical polishing followed by argon ion thinning using a GATAN precision ion mill for about 30 min. Selected foils prepared from alloys sintered at 1000 ◦ C were examined under a JEOL JEM 2100 high resolution transmission electron microscope (HRTEM) operated at 200 kV, using both bright and dark field mode as well as high resolution lattice imaging conditions. Selected area diffraction (SAD) analysis was conducted to identify the phases present. Qualitative information on chemical compositions at different locations was obtained using the energy dispersive spectroscopy (EDS) attachment (Oxford, UK) equipped with an ultra-thin window and attached to both the FESEM and the HRTEM. Mechanical properties in terms of hardness, Young’s modulus and projected yield strength were determined from nano-indentation test on selected sintered samples using standard nano-indentation hardness (TriboIndenter with MultiRange NanoProbe, Hysitron) at 200 mN load. Each hardness value was measured from average of 25 point measurement for nanoindentation and repeated 3–5 times on equivalent locations to ensure precision. From these hardness values, the compressive yield strength ( y ) is estimated using the empirical relationship y = Hv /3 [21]. Wear experiments were carried out with the help of a fretting wear tester (Ducom, India) using 6 mm diameter tungsten carbide ball as the counter-body material against the flat specimens of hot isostatic pressed nano-Y2 O3 dispersed Fe–Cr–Al–Ti ferritic steel at ambient temperature (22–25 ◦ C) and humidity (50–55%). A ballon-plate type of tribometer, working on the principle of the mode I fretting (linear relative tangential displacement at constant normal load) was used in the present investigation [22,23]. This computerized fretting tester is equipped with an inductive displacement transducer to monitor the displacement on the flat sample, and a
Fig. 1. XRD patterns of the 71.0Fe–25.5Cr–2.0Al–0.5Ti powder blend with 1.0 nanoY2 O3 (wt.%) dispersion subjected to mechanical alloying for 0 h (manually blended before mechanical alloying) to 40 h.
piezoelectric transducer attached to the loading arm to measure the friction force. Each of the alloy specimens was subjected to oscillations under a normal load of 10 N against the tungsten carbide counter-body, with a constant frequency of 10 Hz and a constant displacement stroke of 1 mm, till the completion of 36,000 fretting cycles. The coefficient of friction (COF) after subjecting the specimen to a specific number of cycles is obtained from the instantaneous measurement of tangential force. The reproducibility of the experiments was ensured by cleaning of both the specimen and the tungsten carbide ball in an ultrasonic bath of acetone. The wear scar geometry (width, depth and contour) was traced at specific locations and orientation using a contact type stylus profilometer (Model Dektak 150 Veeco Instruments, USA) at a load of 3 mg force and diameter (both in the sliding and in the transverse direction). These dimensions were verified from relevant micrographs. Accordingly the wear volume was computed using the formulation proposed by Klaffke [24]. The worn surfaces generated due to wear testing were examined under the FESEM at appropriate magnification to study the surface damage and topology and determine micro-composition in the wear track using EDS. 3. Results and discussion Fig. 1 shows the XRD patterns of 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) powder blend following mechanical alloying for different cumulative times. While peaks of almost all the constituents are present in the initial stages, XRD pattern of the milled powder at the final (30–40 h) stage consists of only one set of peaks indicating that the final milled product is a single phase body centre cubic (BCC) phase with the entire amount of Cr and Al dissolved in Fe. Amount of Y2 O3 is too small to produce separate XRD peaks. Furthermore, the increase in full width at half maximum () with milling time suggests that both crystallite size reduction and plastic strain accumulation are significant. Careful analysis of as a function of milling time allows determination of plastic strain and crystallite size using the standard peak broadening analysis procedure based on Scherrer equation after elimination of the contributions from the strain and instrumental error [25]. Perhaps, this reduction of crystallite size aids easy dissolution of solute atoms (Cr and Al) in the BCC-Fe matrix and formation of single
S.K. Karak et al. / Wear 270 (2010) 5–11
7
Fig. 2. TEM image of the mechanical alloyed (for 40 h) powder showing homogeneous microstructure. The SAD pattern (inset) confirms that the matrix is nanocrystalline ferrite with dispersion of Y2 Ti2 O7.
phase extended solid solution (beyond equilibrium solubility limit) due to Gibbs–Thompson effect [26]. Fig. 2 shows a typical TEM image of the microstructure developed after 40 h milling. The powder particles contain nanocrystalline BCC-Fe grains with intermetallic phase dispersed in the matrix. The corresponding selected area diffraction (SAD) pattern (inset) shows diffraction rings that can be indexed due to BCC-Fe and Y2 Ti2 O7 phases. The interplanar spacings (d) calculated from one set of rings of this SAD pattern in Fig. 2 matches with those of Y2 Ti2 O7 . Fig. 3 shows the XRD pattern of the powder alloy sintered at 600, 800 and 1000 ◦ C. It appears that BCC-Fe(Cr) phase is the predominant constituent of the sintered product along with intermetallic phases like Fe11 TiY and Al9.22 Cr2.78 Y in addition to oxides like Y2 Ti2 O7 . It may be noted that the presence of Y2 Ti2 O7 was already noted in the mechanically alloyed product prior to sintering. The other two phases formed during sintering possibly through in situ precipitation during sintering. Fig. 4a–c shows the respective bright field TEM images of the alloy sintered at 600, 800 and 1000 ◦ C, which confirm the presence of 10–20 nm Y2 Ti2 O7
Fig. 3. XRD patterns of the consolidated alloy by isochronal (1 h) hot isostatic pressing at different temperatures.
particles distributed uniformly in the BCC-Fe(Cr) matrix. Possibly, partial substitution of Y with Ti during mechanical alloying and/or sintering leads to formation of Y2 Ti2 O7 (in place of Y2 O3 ). Thus, strengthening in the present alloy may arise due to both precipitation and dispersion hardening. It is observed that porosity content of the sample sintered at 600 and 800 ◦ C is higher than that of the sample sintered at 1000 ◦ C. This can be obviously attributed to incomplete sintering at lower temperatures. At high temperature, the as-milled nanocrystalline matrix has transformed entirely to a coarse crystalline aggregate along with precipitation of intermetallic Fe11 TiY and Al9.22 Cr2.78 Y phases. Furthermore, a large area fraction of grain boundaries and triple points reduces the density, as has been reported in the literature [27]. In addition, it should be noted that a majority of the pores are expected to be closed and located at grain boundaries and interfaces in the materials, and the pressure of the entrapped gas is
Fig. 4. Bright field TEM image of the mechanical alloyed (40 h) product sintered by hot isostatic pressing at (a) 600 ◦ C, (b) 800 ◦ C and (c) 1000 ◦ C.
8
S.K. Karak et al. / Wear 270 (2010) 5–11
Table 1 Summary of the selected physical (density, porosity) surface (hardness and wear) and bulk (Young’s modulus and yield strength) mechanical properties following sintering by hot isostatic pressing at 600, 800 and 1000 ◦ C for 1 h. Sintering temperature (◦ C)
Density (mg/m3 )
Porosity (%)
Hardness (GPa)
Young’s modulus (GPa)
Yield strength (MPa)
Wear volume (×107 m3 )
Wear rate (×10−7 mm3 /mm)
600 800 1000
7.14 7.20 7.24
5.52 1.69 0.12
17.6 18.5 20.8
272 285 293
585 616 685
4.15 3.83 3.58
5.76 5.31 4.98
Fig. 5. Variation of the coefficient of friction (COF) against the number of cycles for samples sintered by hot isostatic pressing at different temperatures.
inversely proportional to the radius of pore and temperature. At higher temperature, the pressure of entrapped gas in the closed porosities is higher, which could make it more difficult to close by the application of uniaxial pressure. The influence of sintering temperature of the BCC-Fe(Cr) alloy powders on their tribological behavior during fretting against tungsten carbide ball is illustrated in the plots of the coefficient of friction (COF) against the number of cycles in Fig. 5. Comparison of the wear profile in Fig. 5 leads to the inference that the steady-state COF of the samples sintered at 600 and 800 ◦ C are marginally lower than that sintered at 1000 ◦ C. For the alloy sintered at 600, 800 and 1000 ◦ C, the COF rises rapidly from a low value during the running in period (the first 10,000 cycles) and then reaches the steady-state value. However, in case of the sample sintered at 1000 ◦ C, the COF is found to be almost twice than that of alloys sintered at 600 and 800 ◦ C. Table 1 presents the summary of the selected physical (density, porosity), surface (hardness and wear) and bulk (Young’s modulus and yield strength) mechanical properties determined from nano-
indentation tests on samples sintered by hot isostatic pressing at 600, 800 and 1000 ◦ C for 1 h. It is apparent that the higher the sintering temperature, the greater the density, hardness, modulus and strength of the compact. Furthermore, wear resistance is also the maximum in the sample sintered at 1000 ◦ C. The details of wear related studies are discussed below. It is known that coefficient of friction is directly related to the applied load, surface mechanical properties and surface roughness. For a given alloy with identical prior history, coefficient of friction primarily depends on surface roughness. In the present study, surface roughness marginally increases with sintering temperature, perhaps due to surface oxidation or grain growth at an elevated temperature. Incidentally, the surface roughness amplitude is the highest (=0.5 m) for sample sintered at 1000 ◦ C, which is about 50% higher than that of the sample sintered at lower temperature. This higher roughness likely to enhance coefficient of friction in the sample sintered at 1000 ◦ C than that of lower temperature. However, it may be noted that roughness index does not remain constant in the sample sintered by hipping at any temperature. Thus wear properties of the present alloys correlate better with surface hardness and roughness of a given alloy under specific condition of wear. Measuring width of the wear scar (transverse to the fretting direction) and the span or displacement of the scar parallel to the fretting direction, the wear volume of the worn surface has been calculated using the equation formulated by Klaffke [24]. The average wear volume and wear rate measured in the fretting tests carried out on the mechanically alloyed 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) alloy specimens following hot isostatic pressing at three different temperatures have been summarized in Fig. 6a and b. The error bars in the wear results indicate the scatter in the data (standard deviation) obtained from the results of at least three identical tests. It is evident from Fig. 6a that the wear volume is the least and hence the wear resistance is the highest for the sample sintered by hot isostatic pressing at 1000 ◦ C. Thus, sintering at 1000 ◦ C appears to produce the strongest and wear resistant component in the present study. Fig. 7 shows a typical front view of the worn/damaged region caused by fretting wear (as viewed on the cross-sectional plane perpendicular to both fretting direction and wear/top surface). From the surface profilometer data shown in Fig. 7 it is apparent that the
Fig. 6. Variation of (a) wear volume and (b) wear rate in samples sintered by hot isostatic pressing at different temperatures. Note that wear resistance is highest at 1000 ◦ C.
S.K. Karak et al. / Wear 270 (2010) 5–11
Fig. 7. Surface contour of worn surface across the fretting wear track on the vertical (cross-sectional) plane perpendicular to the top surface and wear direction. The contour was determined at the middle of the wear track using a surface profilometer.
depth of worn surfaces of the alloys sintered at 1000 ◦ C is greater at the centre than that at the periphery. The morphological features on the worn surfaces have been investigated using FESEM and TEM (discussed in Section 2) in an attempt to study the wear mechanism. Fig. 8a and b reveals the extent of wear damage on the worn surface of mechanically alloyed 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) sample compacted and sintered at 1000 ◦ C at low and high magnification, respectively. This difference in the extent of damage appears to be a strong evidence of localized plastic deformation during fretting wear. Examination of the FESEM image in Fig. 8b indicates that micro-cracks are present on the worn surface of the alloy sintered at 1000 ◦ C. The wear mechanism can be better understood from the changes in chemical composition of the worn surface during fretting wear. Fig. 9 presents the compositional microanalysis by EDS line scan over the wear scar developed by fretting wear test under a normal load of 10 N (as revealed in Fig. 8a). The variation in the oxygen-content across (shown in Fig. 10) the boundary of wear scar suggests that a thin oxide film or layer is formed only within
9
Fig. 9. EDS line scan on the worn surface of the sample sintered at 1000 ◦ C (i.e., the wear scar shown in Fig. 8a).
the worn region and not beyond. Small or limited dimension of this oxidized region prevents identification of the phase aggregate in this region by XRD analysis. However, the EDS data suggest that the oxide layer is primarily composed of mixed oxides of Fe and Cr. Thus, it appears that wear of the present alloy primarily occurs through an oxidative wear mechanism. It has been earlier reported [28,29] that Fe2 O3 could be formed during wear due to friction, say, at 200–570 ◦ C, or under a low oxygen partial pressure. In the case of nitrided steel subjected to fretting, the temperature might rise in some regions where the material has undergone high contact pressure. Among the three types of iron oxides likely to form on Fe-bearing components during abrasive, fretting or sliding wear, Sullivan et al. [30] have proposed that ␣-Fe2 O3 is likely to form to at low temperature (less than 450 ◦ C), while Fe3 O4 forms between 450 and 600 ◦ C, and FeO at temperature greater than 600 ◦ C. It may be pointed out that Roy et al. [3] have recently noted identical mechanism of oxidative wear in nano-intermetallic reinforced Al50 Si40 Ti10 in situ composite. Earlier, Karanjai et al. [31] have also
Fig. 8. FESEM image of the worn surface of the alloy sintered at 1000 ◦ C at (a) low and (b) high magnification. Note that (b) reveals surface cracks (marked by arrowheads).
10
S.K. Karak et al. / Wear 270 (2010) 5–11
it may be pointed out that the summary of physical (density, porosity), and bulk (Young’s modulus and yield strength) or surface (hardness and wear volume or rate) mechanical properties of the present nano-Y2 O3 dispersed ferritic steel clearly indicates that sintering by hot isostatic pressing at 1000 ◦ C has been most effective routine to yield a strong and wear resistance product from the mechanically alloyed powder. 4. Conclusion The wear data reveal that the wear volume, estimated principally from the transverse wear scar diameter, decreases considerably with increase in sintering temperature under the present set of fretting wear condition. The wear behavior of the 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) alloy critically depends on the sintering temperature and phase aggregate in the microstructure. The maximum resistance to wear in this alloy was recorded when the mechanical alloyed single phase ferritic alloy with 1.0 wt.% nano-Y2 O3 dispersion was sintered at 1000 ◦ C by hot isostatic pressing. Acknowledgement Fig. 10. EDS X-ray line scans on worn surface of nano-Y2 O3 dispersed ferritic steel sintered at 1000 ◦ C (as shown in Fig. 8b).
reported similar wear degradation of Ta–Ca–P bio-composite subjected to similar fretting wear under dry condition as well as in contact with simulated body fluid. In order to study the wear mechanism quantitatively, the volume of the worn/fretted surface (V) at a given load has been calculated using Klaffke’s [24] empirical equation: d 8r
V=
d 4a 8
+
3
(mm3 )
(1)
where d is the average transverse diameter of the worn surface (mm), a is half the stroke length (≈40 × 10−3 mm); and r is the radius of the ball/indenter (6 mm). The use of this equation is valid for the present fretting condition as it complies with the governing criteria that (i) the wear scar diameter is larger than twice the Hertzian contact diameter, and (ii) the variation of wear volume in the present samples is less than 5%. From the estimated wear volume, the wear rate (W) in mm3 /mm has been calculated using the following equation: W=
V total sliding distance
mm3 mm
(2)
Furthermore, the approximate loading condition at the contact area can be estimated by using the Hertzian contact theory [32]. For a circular point contact, as in the case of the present study, the contact area A is related to the contact stress by the expression: A=
3NR 1/3 4E
(3)
where N is the applied normal load, R is the radius of the counterbody, and E is the elastic modulus of alloy. The mean ( mean ) and maximum ( max ) values of are given by the relations: (4)mean = N 2 and max = 1.5mean A Accordingly the effective stress operative during fretting wear is the maximum contact stress (650 MPa). The radius of the contact area and the maximum contact stress increase with increasing normal load of the alloy sintered at 1000 ◦ C due to high density, hardness and Young’s modulus compared to the other samples. Recently, similar results were also reported by Roy et al. [3]. Finally,
The authors would like to acknowledge the partial financial support provide for this research work by DST, New Delhi (NSTI grant no. SR/S5/NM/2005). References [1] R.W. Cahn, Nanostructured materials, Nature 348 (1990) 389. [2] R. Bohn, T. Haubopld, R. Birringer, H. Gleiter, Nanocrystalline intermetallic compounds—an approach to ductility? Scr. Metall. Mater. 25 (1991) 811. [3] D. Roy, S.S. Singh, B. Basu, W. Lojkowski, R. Mitra, I. Manna, Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in-situ composite, Wear 266 (2009) 1113–1118. [4] S. Bera, Z. Zuberova, R.J. Helling, Y. Estrin, I. Manna, Synthesis of copper alloys with extended solid solubility and nano-Al2 O3 dispersion by mechanical alloying and equal channel angular pressing, Philos. Mag. 90 (2010) 1465–1483. [5] E.A. Loria, Recent development in the progress of superalloy 718, J. Mater. 44 (1992) 33–36. [6] T. Allen, R.L. Klueh, S. Ukai, Oxidation of 9Cr oxide dispersion strengthened steel exposed in supercritical water, Fuels and Materials for Transmutation, OECD NEA Report Number 5419, 2005, 135. [7] S. Ohtuska, S. Ukai, M. Fujiwara, T. Kaito, T. Narita, Improvement of mechanical properties of 9CrODS martensitic steel by controlling titanium concentration, Mater. Trans. 46 (2005) 493. [8] S. Ukai, S. Mizuta, T. Yoshitake, T. Okuda, M. Fujiwara, S. Hagi, T. Kobayashi, Tube manufacturing and characterization of oxide dispersion strengthened ferritic steel, J. Nucl. Mater. 283–287 (2000) 702. [9] M.K. Miller, D.T. Hoelzer, E.A. Kenik, K.F. Russell, Stability of ferritic MA/ODS alloys at high temperature, Intermetallics 13 (2005) 387–392. [10] C. Suryanarayana, Mechanical alloying and milling, Progress Mater. Sci. 46 (2001) 1–184. [11] S. Bera, W. Lojkowski, I. Manna, Development of wear-resistant Cu–10Cr–3Ag electrical contacts with nano-Al2 O3 dispersion by mechanical alloying and high pressure sintering, Metall. Mater. Trans. A: Phys. Metall. Mater. Sci. 40 (2009) 3276–3283. [12] D. Roy, R. Mitra, T. Chudoba, Z. Witczak, W. Lojkowski, I. Manna, Structure and mechanical properties of Al65 Cu20 Ti15 -based amorphous/nanocrystalline alloys prepared by high pressure sintering, Mater. Sci. Eng. A 497 (2008) 93–100. [13] S.Z. Han, M. Goto, C. Lim, C.J. Kim, S. Kim, Fatigue behavior of nano-grained copper prepared by ECAP, J. Alloys Compd. 434–435 (2007) 304–306. [14] E. Hosseini, M. Kazeminezhad, The effect of ECAP die shape on nano-structure of materials, Comput. Mater. Sci. 44 (2009) 962–967. [15] S.S. Singh, D. Roy, R. Mitra, R.V. Subba Rao, R.K. Dayal, B. Raj, I. Manna, Studies on laser sintering of mechanically alloyed Al50 Ti40 Si10 composite, Mater. Sci. Eng. A 501 (2009) 242–247. [16] A. Michalski, D. Siemiaszko, Nanocrystalline cemented carbides sintered by the pulse plasma method, Int. J. Refractory Met. Hard Mater. 25 (2007) 153–158. [17] C. Cayron, E. Rath, I. Chu, S. Launois, Microstructural evolution of Y2 O3 and MgAl2 O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing, J. Nucl. Mater. 335 (2004) 83–102. [18] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992.
S.K. Karak et al. / Wear 270 (2010) 5–11 [19] J.P. Tu, Y.Z. Yang, Tribological behaviour of Al18 B4 O33 -whisker-reinforced hypoeutectic Al–Si–Mg-matrix composites under dry sliding conditions, Compos. Sci. Technol. 60 (2000) 1801–1809. [20] M. Bai, Q. Xue, X. Wang, Y. Wan, W. Liu, Wear mechanism of SiC whiskerreinforced 2024 aluminum alloy matrix composites in oscillating sliding wear tests, Wear 185 (1995) 197–202. [21] H.S. Chen, J.T. Krause, E.J. Coleman, Elastic constants, hardness and their implications to flow properties of metallic glasses, J. Non-Cryst. Solids 18 (1975) 157–171. [22] R.B. Waterhouse, Fretting wear ASM Handbook, vol. 18, ASM International, 1992, p. 242. [23] S.R. Brown (Ed.), Materials Evaluation under Fretting Conditions, ASTM Special Technical Publication 780, Warminster, PA, 1982, p. 1. [24] D. Klaffke, Fretting wear of ceramics, Tribol. Int. 22 (1989) 89–101. [25] T.H. Keijser, I.L. Langford, E.J. Mittemeijer, B.P. Vogel, Microstrain characterization of metals using high-resolution neutron diffraction, J. Appl. Crystallogr. 15 (1982) 308.
11
[26] R. Raj, Analysis of the sintering pressure, J. Am. Ceram. Soc. 70 (1987) C210–C211. [27] G. Palumbo, S.J. Thorpe, K.T. Aust, On the contribution of triple junctions to the structure and properties of nanocrystalline materials, Scripta Mater. 24 (1990) 1347–1350. [28] A. Iwabuchi, T. Kayaba, K. Kato, Effect of atmospheric pressure on friction and wear of 0.46% C steel in fretting, Wear 91 (1983) 289. [29] T. Kayaba, A. Iwabuchi, Effect of the hardness of hardened steels and the action of oxides on fretting wear, Wear 74 (1981–1982) 229. [30] J.L. Sullivan, T.F.J. Quinn, D.M. Rowson, Developments in the oxidational theory of mild wear, Tribol. Int. 13 (1980) 153–158. [31] M. Karanjai, B.V. Manoj Kumar, R. Sundaresan, B. Basu, T.R. Rama Mohan, B.P. Kashyap, Fretting wear study on Ti–Ca–P biocomposite in dry and simulated body fluid, Mater. Sci. Eng. A 475 (2008) 299–307. [32] K.L. Johonson, Contact Mechanics, Cambridge University Press, Cambridge, 1985.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Studies on wear behavior of nano-Y2 O3 dispersed ferritic steel developed by mechanical alloying and hot isostatic pressing S.K. Karak a , C.S. Vishnu a , Z. Witczak b , W. Lojkowski b , J. Dutta Majumdar a , I. Manna a,c,∗ a b c
Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721302, India Institute of High Pressure Physics (Unipress), Polish Academy of Sciences, Sokolowska 29, 01-142 Warsaw, Poland Central Glass and Ceramic Research Institute, Kolkata 70032, India
a r t i c l e
i n f o
Article history: Received 31 January 2010 Received in revised form 21 August 2010 Accepted 26 August 2010 Available online 12 October 2010 Keywords: Metal matrix composite Hardness Electron microscopy Wear testing
a b s t r a c t Resistance to wear is an important factor in design and selection of structural components in motion and in contact with a mating surface. The present work deals with studies on fretting wear behavior of mechanically alloyed 1.0 wt.% nano-Y2 O3 dispersed 71.0Fe–25.5Cr–2.0Al–0.5Ti (wt.%) ferritic steel consolidated by hot isostatic pressing at 600, 800 and 1000 ◦ C under 1.2 GPa uniaxial pressure applied for 1 h. The wear experiments were carried out in gross slip fretting regime against 6 mm diameter tungsten carbide ball at ambient temperature. The highest resistance to fretting wear has been observed in the alloy sintered at 1000 ◦ C. The fretting involves localized plastic deformation and oxidation as evidenced by post wear surface profile and micro-compositional analysis, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanocrystalline materials have attracted great scientific interests in the recent because they possess superior mechanical, physical and chemical properties compared to conventional coarsegrained materials [1,2]. Besides monolithic single phase aggregate, nanostuctured solids with dispersion strengthening offered by uniform distribution of nano-intermetallic or nano-ceramic phases are extremely useful for structural applications at ambient or elevated temperatures [3,4]. Among various ex situ strengthened alloys, ceramic oxide dispersion strengthened iron base ferritic steel (containing significant amounts of Cr, Al and Ti) find wide ranging applications in fabricating discs and other critical parts of jet engines, pump bodies and parts, rocket motors and thrust reversers, nuclear fuel element spacers, hot extrusion tools, high strength bolts and down hole shafts [5]. Nano-Y2 O3 dispersed ferritic steel possesses a unique combination of body centre cubic (BCC) structure with low coefficient thermal of expansion, high thermal conductivity, good creep resistance and high tensile/compressive strength [6–9]. Mechanical alloying is a solid state synthesis process that consists of repeated cold-welding, fracturing, dynamic recrys-
∗ Corresponding author at: Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur, W.B. 721302, India. Tel.: +91 3222 283266; fax: +91 3222 255280. E-mail address: [email protected] (I. Manna). 0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.08.021
tallization and mechanically activated inter-diffusion among the powder particles in a high energy ball mill [10]. While mechanical alloying is a useful technique to synthesize novel materials including nanostructured and amorphous products in powder form (starting from elemental/alloyed/compound precursors), consolidation of such powders into a bulk component without deteriorating or destroying the as-milled novel microstructural state poses a genuine challenge. Various attempts that have achieved reasonable success in this regard (consolidation of mechanically alloyed powder) are high pressure sintering [11,12], equi-channel angular pressing [13,14], laser sintering [15], pulse plasma sintering [16] and hot isostatic pressing [17]. However, studies on hot isostatic pressing of nanooxide dispersed mechanically alloyed ferritic steel are scarce. Mechanical degradation of structural components in un-lubricated contact with a mating surface at low amplitude oscillatory sliding, known as fretting wear, is of great importance in mechanical joints of vibrating structures ranging from household applications, automobiles, electrical motors and even in human body implants [18]. The displacement amplitudes (5–300 m) encountered in fretting are smaller than those of reciprocating sliding [19]. This means that contact between the counter bodies is maintained over almost the entire period of operation or fretting. As a result, much of the wear debris produced by fretting remains trapped at the interface, which can cause seizure in components such as flexible couplings [20]. Another important aspect of fretting is the development of fatigue cracks in the damaged region, which reduces the fatigue strength of a cyclically loaded component. In general, fretting studies on nano-Y2 O3 dispersed in Fe–Cr
6
S.K. Karak et al. / Wear 270 (2010) 5–11
ferritic steel are limited. In the present work, fretting wear of a new generation of nano-Y2 O3 dispersed in Fe–Cr ferritic steel, synthesized by mechanical alloying and sintered by hot isostatic pressing, against tungsten carbide ball is investigated using a newly designed fretting wear tester. An effort has also been made to understand the operative wear mechanism through a detailed microstructural and micro-compositional analysis.
2. Experimental Elemental powders (>99.9 wt.% purity and 30–80 m size) blended to approximate composition of 1.0 wt.% nano-Y2 O3 dispersed 71.0Fe–25.5Cr–2.0Al–0.5Ti (all in wt.%) were subjected to mechanical alloying by high energy ball milling in a multi vial Fritsch PM400 planetary ball mill, operated at 300 rpm using stainless steel vials and balls (10 mm diameter). Milling was conducted with a ball to powder ratio of 10:1 in wet (toluene) medium to avoid agglomeration of powders, prevent continued oxidation except at early stage and ensure sufficient yield after milling. The mechanically milled or alloyed powders were placed in a steel can or cylinder sealed under vacuum, and placed in the anvil of the hot isostatic pressing (HIP) machine. The pressure was maintained at 1.2 GPa using Ar gas for 1 h in the temperature range 600–1000 ◦ C. Identity of the phases following hot isostatic pressing was determined by X-ray diffraction (XRD) using a Panalytical X’Pert Pro diffractometer with Co-K␣ (0.179 nm) radiation. Density of the sintered samples was measured using Archimedes principle after weighing in air and water separately using an electronic balance with a precision of 0.1 mg. Morphology, size and shape and distribution of the phases in the sintered components and surface damage in worn tracks were studied using a field emission scanning electron microscopy (FESEM, Zeiss, Supra 40 V). Electron transparent thin foils were prepared for transmission electron microscopy (TEM) studies using mechanical polishing followed by argon ion thinning using a GATAN precision ion mill for about 30 min. Selected foils prepared from alloys sintered at 1000 ◦ C were examined under a JEOL JEM 2100 high resolution transmission electron microscope (HRTEM) operated at 200 kV, using both bright and dark field mode as well as high resolution lattice imaging conditions. Selected area diffraction (SAD) analysis was conducted to identify the phases present. Qualitative information on chemical compositions at different locations was obtained using the energy dispersive spectroscopy (EDS) attachment (Oxford, UK) equipped with an ultra-thin window and attached to both the FESEM and the HRTEM. Mechanical properties in terms of hardness, Young’s modulus and projected yield strength were determined from nano-indentation test on selected sintered samples using standard nano-indentation hardness (TriboIndenter with MultiRange NanoProbe, Hysitron) at 200 mN load. Each hardness value was measured from average of 25 point measurement for nanoindentation and repeated 3–5 times on equivalent locations to ensure precision. From these hardness values, the compressive yield strength ( y ) is estimated using the empirical relationship y = Hv /3 [21]. Wear experiments were carried out with the help of a fretting wear tester (Ducom, India) using 6 mm diameter tungsten carbide ball as the counter-body material against the flat specimens of hot isostatic pressed nano-Y2 O3 dispersed Fe–Cr–Al–Ti ferritic steel at ambient temperature (22–25 ◦ C) and humidity (50–55%). A ballon-plate type of tribometer, working on the principle of the mode I fretting (linear relative tangential displacement at constant normal load) was used in the present investigation [22,23]. This computerized fretting tester is equipped with an inductive displacement transducer to monitor the displacement on the flat sample, and a
Fig. 1. XRD patterns of the 71.0Fe–25.5Cr–2.0Al–0.5Ti powder blend with 1.0 nanoY2 O3 (wt.%) dispersion subjected to mechanical alloying for 0 h (manually blended before mechanical alloying) to 40 h.
piezoelectric transducer attached to the loading arm to measure the friction force. Each of the alloy specimens was subjected to oscillations under a normal load of 10 N against the tungsten carbide counter-body, with a constant frequency of 10 Hz and a constant displacement stroke of 1 mm, till the completion of 36,000 fretting cycles. The coefficient of friction (COF) after subjecting the specimen to a specific number of cycles is obtained from the instantaneous measurement of tangential force. The reproducibility of the experiments was ensured by cleaning of both the specimen and the tungsten carbide ball in an ultrasonic bath of acetone. The wear scar geometry (width, depth and contour) was traced at specific locations and orientation using a contact type stylus profilometer (Model Dektak 150 Veeco Instruments, USA) at a load of 3 mg force and diameter (both in the sliding and in the transverse direction). These dimensions were verified from relevant micrographs. Accordingly the wear volume was computed using the formulation proposed by Klaffke [24]. The worn surfaces generated due to wear testing were examined under the FESEM at appropriate magnification to study the surface damage and topology and determine micro-composition in the wear track using EDS. 3. Results and discussion Fig. 1 shows the XRD patterns of 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) powder blend following mechanical alloying for different cumulative times. While peaks of almost all the constituents are present in the initial stages, XRD pattern of the milled powder at the final (30–40 h) stage consists of only one set of peaks indicating that the final milled product is a single phase body centre cubic (BCC) phase with the entire amount of Cr and Al dissolved in Fe. Amount of Y2 O3 is too small to produce separate XRD peaks. Furthermore, the increase in full width at half maximum () with milling time suggests that both crystallite size reduction and plastic strain accumulation are significant. Careful analysis of as a function of milling time allows determination of plastic strain and crystallite size using the standard peak broadening analysis procedure based on Scherrer equation after elimination of the contributions from the strain and instrumental error [25]. Perhaps, this reduction of crystallite size aids easy dissolution of solute atoms (Cr and Al) in the BCC-Fe matrix and formation of single
S.K. Karak et al. / Wear 270 (2010) 5–11
7
Fig. 2. TEM image of the mechanical alloyed (for 40 h) powder showing homogeneous microstructure. The SAD pattern (inset) confirms that the matrix is nanocrystalline ferrite with dispersion of Y2 Ti2 O7.
phase extended solid solution (beyond equilibrium solubility limit) due to Gibbs–Thompson effect [26]. Fig. 2 shows a typical TEM image of the microstructure developed after 40 h milling. The powder particles contain nanocrystalline BCC-Fe grains with intermetallic phase dispersed in the matrix. The corresponding selected area diffraction (SAD) pattern (inset) shows diffraction rings that can be indexed due to BCC-Fe and Y2 Ti2 O7 phases. The interplanar spacings (d) calculated from one set of rings of this SAD pattern in Fig. 2 matches with those of Y2 Ti2 O7 . Fig. 3 shows the XRD pattern of the powder alloy sintered at 600, 800 and 1000 ◦ C. It appears that BCC-Fe(Cr) phase is the predominant constituent of the sintered product along with intermetallic phases like Fe11 TiY and Al9.22 Cr2.78 Y in addition to oxides like Y2 Ti2 O7 . It may be noted that the presence of Y2 Ti2 O7 was already noted in the mechanically alloyed product prior to sintering. The other two phases formed during sintering possibly through in situ precipitation during sintering. Fig. 4a–c shows the respective bright field TEM images of the alloy sintered at 600, 800 and 1000 ◦ C, which confirm the presence of 10–20 nm Y2 Ti2 O7
Fig. 3. XRD patterns of the consolidated alloy by isochronal (1 h) hot isostatic pressing at different temperatures.
particles distributed uniformly in the BCC-Fe(Cr) matrix. Possibly, partial substitution of Y with Ti during mechanical alloying and/or sintering leads to formation of Y2 Ti2 O7 (in place of Y2 O3 ). Thus, strengthening in the present alloy may arise due to both precipitation and dispersion hardening. It is observed that porosity content of the sample sintered at 600 and 800 ◦ C is higher than that of the sample sintered at 1000 ◦ C. This can be obviously attributed to incomplete sintering at lower temperatures. At high temperature, the as-milled nanocrystalline matrix has transformed entirely to a coarse crystalline aggregate along with precipitation of intermetallic Fe11 TiY and Al9.22 Cr2.78 Y phases. Furthermore, a large area fraction of grain boundaries and triple points reduces the density, as has been reported in the literature [27]. In addition, it should be noted that a majority of the pores are expected to be closed and located at grain boundaries and interfaces in the materials, and the pressure of the entrapped gas is
Fig. 4. Bright field TEM image of the mechanical alloyed (40 h) product sintered by hot isostatic pressing at (a) 600 ◦ C, (b) 800 ◦ C and (c) 1000 ◦ C.
8
S.K. Karak et al. / Wear 270 (2010) 5–11
Table 1 Summary of the selected physical (density, porosity) surface (hardness and wear) and bulk (Young’s modulus and yield strength) mechanical properties following sintering by hot isostatic pressing at 600, 800 and 1000 ◦ C for 1 h. Sintering temperature (◦ C)
Density (mg/m3 )
Porosity (%)
Hardness (GPa)
Young’s modulus (GPa)
Yield strength (MPa)
Wear volume (×107 m3 )
Wear rate (×10−7 mm3 /mm)
600 800 1000
7.14 7.20 7.24
5.52 1.69 0.12
17.6 18.5 20.8
272 285 293
585 616 685
4.15 3.83 3.58
5.76 5.31 4.98
Fig. 5. Variation of the coefficient of friction (COF) against the number of cycles for samples sintered by hot isostatic pressing at different temperatures.
inversely proportional to the radius of pore and temperature. At higher temperature, the pressure of entrapped gas in the closed porosities is higher, which could make it more difficult to close by the application of uniaxial pressure. The influence of sintering temperature of the BCC-Fe(Cr) alloy powders on their tribological behavior during fretting against tungsten carbide ball is illustrated in the plots of the coefficient of friction (COF) against the number of cycles in Fig. 5. Comparison of the wear profile in Fig. 5 leads to the inference that the steady-state COF of the samples sintered at 600 and 800 ◦ C are marginally lower than that sintered at 1000 ◦ C. For the alloy sintered at 600, 800 and 1000 ◦ C, the COF rises rapidly from a low value during the running in period (the first 10,000 cycles) and then reaches the steady-state value. However, in case of the sample sintered at 1000 ◦ C, the COF is found to be almost twice than that of alloys sintered at 600 and 800 ◦ C. Table 1 presents the summary of the selected physical (density, porosity), surface (hardness and wear) and bulk (Young’s modulus and yield strength) mechanical properties determined from nano-
indentation tests on samples sintered by hot isostatic pressing at 600, 800 and 1000 ◦ C for 1 h. It is apparent that the higher the sintering temperature, the greater the density, hardness, modulus and strength of the compact. Furthermore, wear resistance is also the maximum in the sample sintered at 1000 ◦ C. The details of wear related studies are discussed below. It is known that coefficient of friction is directly related to the applied load, surface mechanical properties and surface roughness. For a given alloy with identical prior history, coefficient of friction primarily depends on surface roughness. In the present study, surface roughness marginally increases with sintering temperature, perhaps due to surface oxidation or grain growth at an elevated temperature. Incidentally, the surface roughness amplitude is the highest (=0.5 m) for sample sintered at 1000 ◦ C, which is about 50% higher than that of the sample sintered at lower temperature. This higher roughness likely to enhance coefficient of friction in the sample sintered at 1000 ◦ C than that of lower temperature. However, it may be noted that roughness index does not remain constant in the sample sintered by hipping at any temperature. Thus wear properties of the present alloys correlate better with surface hardness and roughness of a given alloy under specific condition of wear. Measuring width of the wear scar (transverse to the fretting direction) and the span or displacement of the scar parallel to the fretting direction, the wear volume of the worn surface has been calculated using the equation formulated by Klaffke [24]. The average wear volume and wear rate measured in the fretting tests carried out on the mechanically alloyed 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) alloy specimens following hot isostatic pressing at three different temperatures have been summarized in Fig. 6a and b. The error bars in the wear results indicate the scatter in the data (standard deviation) obtained from the results of at least three identical tests. It is evident from Fig. 6a that the wear volume is the least and hence the wear resistance is the highest for the sample sintered by hot isostatic pressing at 1000 ◦ C. Thus, sintering at 1000 ◦ C appears to produce the strongest and wear resistant component in the present study. Fig. 7 shows a typical front view of the worn/damaged region caused by fretting wear (as viewed on the cross-sectional plane perpendicular to both fretting direction and wear/top surface). From the surface profilometer data shown in Fig. 7 it is apparent that the
Fig. 6. Variation of (a) wear volume and (b) wear rate in samples sintered by hot isostatic pressing at different temperatures. Note that wear resistance is highest at 1000 ◦ C.
S.K. Karak et al. / Wear 270 (2010) 5–11
Fig. 7. Surface contour of worn surface across the fretting wear track on the vertical (cross-sectional) plane perpendicular to the top surface and wear direction. The contour was determined at the middle of the wear track using a surface profilometer.
depth of worn surfaces of the alloys sintered at 1000 ◦ C is greater at the centre than that at the periphery. The morphological features on the worn surfaces have been investigated using FESEM and TEM (discussed in Section 2) in an attempt to study the wear mechanism. Fig. 8a and b reveals the extent of wear damage on the worn surface of mechanically alloyed 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) sample compacted and sintered at 1000 ◦ C at low and high magnification, respectively. This difference in the extent of damage appears to be a strong evidence of localized plastic deformation during fretting wear. Examination of the FESEM image in Fig. 8b indicates that micro-cracks are present on the worn surface of the alloy sintered at 1000 ◦ C. The wear mechanism can be better understood from the changes in chemical composition of the worn surface during fretting wear. Fig. 9 presents the compositional microanalysis by EDS line scan over the wear scar developed by fretting wear test under a normal load of 10 N (as revealed in Fig. 8a). The variation in the oxygen-content across (shown in Fig. 10) the boundary of wear scar suggests that a thin oxide film or layer is formed only within
9
Fig. 9. EDS line scan on the worn surface of the sample sintered at 1000 ◦ C (i.e., the wear scar shown in Fig. 8a).
the worn region and not beyond. Small or limited dimension of this oxidized region prevents identification of the phase aggregate in this region by XRD analysis. However, the EDS data suggest that the oxide layer is primarily composed of mixed oxides of Fe and Cr. Thus, it appears that wear of the present alloy primarily occurs through an oxidative wear mechanism. It has been earlier reported [28,29] that Fe2 O3 could be formed during wear due to friction, say, at 200–570 ◦ C, or under a low oxygen partial pressure. In the case of nitrided steel subjected to fretting, the temperature might rise in some regions where the material has undergone high contact pressure. Among the three types of iron oxides likely to form on Fe-bearing components during abrasive, fretting or sliding wear, Sullivan et al. [30] have proposed that ␣-Fe2 O3 is likely to form to at low temperature (less than 450 ◦ C), while Fe3 O4 forms between 450 and 600 ◦ C, and FeO at temperature greater than 600 ◦ C. It may be pointed out that Roy et al. [3] have recently noted identical mechanism of oxidative wear in nano-intermetallic reinforced Al50 Si40 Ti10 in situ composite. Earlier, Karanjai et al. [31] have also
Fig. 8. FESEM image of the worn surface of the alloy sintered at 1000 ◦ C at (a) low and (b) high magnification. Note that (b) reveals surface cracks (marked by arrowheads).
10
S.K. Karak et al. / Wear 270 (2010) 5–11
it may be pointed out that the summary of physical (density, porosity), and bulk (Young’s modulus and yield strength) or surface (hardness and wear volume or rate) mechanical properties of the present nano-Y2 O3 dispersed ferritic steel clearly indicates that sintering by hot isostatic pressing at 1000 ◦ C has been most effective routine to yield a strong and wear resistance product from the mechanically alloyed powder. 4. Conclusion The wear data reveal that the wear volume, estimated principally from the transverse wear scar diameter, decreases considerably with increase in sintering temperature under the present set of fretting wear condition. The wear behavior of the 71.0Fe–25.5Cr–2.0Al–0.5Ti–1.0Y2 O3 (in wt.%) alloy critically depends on the sintering temperature and phase aggregate in the microstructure. The maximum resistance to wear in this alloy was recorded when the mechanical alloyed single phase ferritic alloy with 1.0 wt.% nano-Y2 O3 dispersion was sintered at 1000 ◦ C by hot isostatic pressing. Acknowledgement Fig. 10. EDS X-ray line scans on worn surface of nano-Y2 O3 dispersed ferritic steel sintered at 1000 ◦ C (as shown in Fig. 8b).
reported similar wear degradation of Ta–Ca–P bio-composite subjected to similar fretting wear under dry condition as well as in contact with simulated body fluid. In order to study the wear mechanism quantitatively, the volume of the worn/fretted surface (V) at a given load has been calculated using Klaffke’s [24] empirical equation: d 8r
V=
d 4a 8
+
3
(mm3 )
(1)
where d is the average transverse diameter of the worn surface (mm), a is half the stroke length (≈40 × 10−3 mm); and r is the radius of the ball/indenter (6 mm). The use of this equation is valid for the present fretting condition as it complies with the governing criteria that (i) the wear scar diameter is larger than twice the Hertzian contact diameter, and (ii) the variation of wear volume in the present samples is less than 5%. From the estimated wear volume, the wear rate (W) in mm3 /mm has been calculated using the following equation: W=
V total sliding distance
mm3 mm
(2)
Furthermore, the approximate loading condition at the contact area can be estimated by using the Hertzian contact theory [32]. For a circular point contact, as in the case of the present study, the contact area A is related to the contact stress by the expression: A=
3NR 1/3 4E
(3)
where N is the applied normal load, R is the radius of the counterbody, and E is the elastic modulus of alloy. The mean ( mean ) and maximum ( max ) values of are given by the relations: (4)mean = N 2 and max = 1.5mean A Accordingly the effective stress operative during fretting wear is the maximum contact stress (650 MPa). The radius of the contact area and the maximum contact stress increase with increasing normal load of the alloy sintered at 1000 ◦ C due to high density, hardness and Young’s modulus compared to the other samples. Recently, similar results were also reported by Roy et al. [3]. Finally,
The authors would like to acknowledge the partial financial support provide for this research work by DST, New Delhi (NSTI grant no. SR/S5/NM/2005). References [1] R.W. Cahn, Nanostructured materials, Nature 348 (1990) 389. [2] R. Bohn, T. Haubopld, R. Birringer, H. Gleiter, Nanocrystalline intermetallic compounds—an approach to ductility? Scr. Metall. Mater. 25 (1991) 811. [3] D. Roy, S.S. Singh, B. Basu, W. Lojkowski, R. Mitra, I. Manna, Studies on wear behavior of nano-intermetallic reinforced Al-base amorphous/nanocrystalline matrix in-situ composite, Wear 266 (2009) 1113–1118. [4] S. Bera, Z. Zuberova, R.J. Helling, Y. Estrin, I. Manna, Synthesis of copper alloys with extended solid solubility and nano-Al2 O3 dispersion by mechanical alloying and equal channel angular pressing, Philos. Mag. 90 (2010) 1465–1483. [5] E.A. Loria, Recent development in the progress of superalloy 718, J. Mater. 44 (1992) 33–36. [6] T. Allen, R.L. Klueh, S. Ukai, Oxidation of 9Cr oxide dispersion strengthened steel exposed in supercritical water, Fuels and Materials for Transmutation, OECD NEA Report Number 5419, 2005, 135. [7] S. Ohtuska, S. Ukai, M. Fujiwara, T. Kaito, T. Narita, Improvement of mechanical properties of 9CrODS martensitic steel by controlling titanium concentration, Mater. Trans. 46 (2005) 493. [8] S. Ukai, S. Mizuta, T. Yoshitake, T. Okuda, M. Fujiwara, S. Hagi, T. Kobayashi, Tube manufacturing and characterization of oxide dispersion strengthened ferritic steel, J. Nucl. Mater. 283–287 (2000) 702. [9] M.K. Miller, D.T. Hoelzer, E.A. Kenik, K.F. Russell, Stability of ferritic MA/ODS alloys at high temperature, Intermetallics 13 (2005) 387–392. [10] C. Suryanarayana, Mechanical alloying and milling, Progress Mater. Sci. 46 (2001) 1–184. [11] S. Bera, W. Lojkowski, I. Manna, Development of wear-resistant Cu–10Cr–3Ag electrical contacts with nano-Al2 O3 dispersion by mechanical alloying and high pressure sintering, Metall. Mater. Trans. A: Phys. Metall. Mater. Sci. 40 (2009) 3276–3283. [12] D. Roy, R. Mitra, T. Chudoba, Z. Witczak, W. Lojkowski, I. Manna, Structure and mechanical properties of Al65 Cu20 Ti15 -based amorphous/nanocrystalline alloys prepared by high pressure sintering, Mater. Sci. Eng. A 497 (2008) 93–100. [13] S.Z. Han, M. Goto, C. Lim, C.J. Kim, S. Kim, Fatigue behavior of nano-grained copper prepared by ECAP, J. Alloys Compd. 434–435 (2007) 304–306. [14] E. Hosseini, M. Kazeminezhad, The effect of ECAP die shape on nano-structure of materials, Comput. Mater. Sci. 44 (2009) 962–967. [15] S.S. Singh, D. Roy, R. Mitra, R.V. Subba Rao, R.K. Dayal, B. Raj, I. Manna, Studies on laser sintering of mechanically alloyed Al50 Ti40 Si10 composite, Mater. Sci. Eng. A 501 (2009) 242–247. [16] A. Michalski, D. Siemiaszko, Nanocrystalline cemented carbides sintered by the pulse plasma method, Int. J. Refractory Met. Hard Mater. 25 (2007) 153–158. [17] C. Cayron, E. Rath, I. Chu, S. Launois, Microstructural evolution of Y2 O3 and MgAl2 O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing, J. Nucl. Mater. 335 (2004) 83–102. [18] I.M. Hutchings, Tribology: Friction and Wear of Engineering Materials, Edward Arnold, London, 1992.
S.K. Karak et al. / Wear 270 (2010) 5–11 [19] J.P. Tu, Y.Z. Yang, Tribological behaviour of Al18 B4 O33 -whisker-reinforced hypoeutectic Al–Si–Mg-matrix composites under dry sliding conditions, Compos. Sci. Technol. 60 (2000) 1801–1809. [20] M. Bai, Q. Xue, X. Wang, Y. Wan, W. Liu, Wear mechanism of SiC whiskerreinforced 2024 aluminum alloy matrix composites in oscillating sliding wear tests, Wear 185 (1995) 197–202. [21] H.S. Chen, J.T. Krause, E.J. Coleman, Elastic constants, hardness and their implications to flow properties of metallic glasses, J. Non-Cryst. Solids 18 (1975) 157–171. [22] R.B. Waterhouse, Fretting wear ASM Handbook, vol. 18, ASM International, 1992, p. 242. [23] S.R. Brown (Ed.), Materials Evaluation under Fretting Conditions, ASTM Special Technical Publication 780, Warminster, PA, 1982, p. 1. [24] D. Klaffke, Fretting wear of ceramics, Tribol. Int. 22 (1989) 89–101. [25] T.H. Keijser, I.L. Langford, E.J. Mittemeijer, B.P. Vogel, Microstrain characterization of metals using high-resolution neutron diffraction, J. Appl. Crystallogr. 15 (1982) 308.
11
[26] R. Raj, Analysis of the sintering pressure, J. Am. Ceram. Soc. 70 (1987) C210–C211. [27] G. Palumbo, S.J. Thorpe, K.T. Aust, On the contribution of triple junctions to the structure and properties of nanocrystalline materials, Scripta Mater. 24 (1990) 1347–1350. [28] A. Iwabuchi, T. Kayaba, K. Kato, Effect of atmospheric pressure on friction and wear of 0.46% C steel in fretting, Wear 91 (1983) 289. [29] T. Kayaba, A. Iwabuchi, Effect of the hardness of hardened steels and the action of oxides on fretting wear, Wear 74 (1981–1982) 229. [30] J.L. Sullivan, T.F.J. Quinn, D.M. Rowson, Developments in the oxidational theory of mild wear, Tribol. Int. 13 (1980) 153–158. [31] M. Karanjai, B.V. Manoj Kumar, R. Sundaresan, B. Basu, T.R. Rama Mohan, B.P. Kashyap, Fretting wear study on Ti–Ca–P biocomposite in dry and simulated body fluid, Mater. Sci. Eng. A 475 (2008) 299–307. [32] K.L. Johonson, Contact Mechanics, Cambridge University Press, Cambridge, 1985.