- Email: [email protected]
Insights into the static friction behavior of Ni-based superalloys
Surface & Coatings Technology 352 (2018) 634–641
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Insights into the static friction behavior of Ni-based superalloys a,⁎
a
a
b
Pantcho Stoyanov , Lesley Dawag , William J. Joost , Daniel G. Goberman , Steven Ivory a b
T a
Pratt & Whitney, United Technologies Corporation, East Hartford, CT, United States of America United Technology Research Center, United Technologies Corporation, East Hartford, CT, United States of America
A R T I C LE I N FO
A B S T R A C T
Keywords: Single crystal alloys Tribofilm Lubricious oxides
The purpose of this study was to provide a better understanding of the static friction behavior of Ni-based superalloys over a wide range of temperatures. The static friction coefficient was investigated using a custombuilt high-temperature apparatus in a flat-on-flat contact configuration. Measurements were performed on Inconel 718, using a variety of counterface materials (i.e. Ni-Cr coating, ME16, Ti-6246 and Inconel 718). The static friction coefficient was overall higher for the experiments at elevated temperatures for all counterface materials against Inconel 718. Ex situ analysis by means of FIB/SEM and XPS revealed the formation of metal oxide layers on the surfaces of the different alloys after testing at elevated temperature. In addition, atomistic simulations were performed in order to elucidate the interfacial mechanism leading to the differences in static friction. The observations of the simulations correlated well with the experiments, where the shear strength of the frictional interfaces was evidently higher for the oxide-on-oxide systems compared to the nickelon-nickel.
1. Introduction Nickel based superalloys are widely used as structural components in gas turbine engines due to their excellent resistance to mechanical and chemical degradation at elevated temperatures [1–10]. Inconel 718, in particular, is well known for high-temperature strength, and superior resistance to creep, oxidation, and low cycle fatigue. Thus, Inconel 718 is commonly used in hot sections of gas turbine engines, as well as in other extreme environment applications. While intensive research on the microstructure characteristics and process development activities of Inconel 718 has been performed over the last few decades, the tribological behavior (i.e. friction and wear) has received relatively little attention. In particular, no systematic studies can be found on the static friction behavior at high temperature and contact stresses of Inconel 718, despite its importance in the design and performance of engines. A few studies have attempted to understand the underlying mechanisms of static friction [11–16]. For instance, Rabinowicz [11] investigated the compatibility of metals through static friction tests and interpreted the results in terms of the surface energy model of friction. Galligan et al. [12] performed a series of tests for different material combinations (i.e. brass on brass and copper on copper) at a wide range of temperatures. The authors showed that the static friction has two different processes governing the static friction; at low temperatures,
⁎
the mechanism of static friction is primarily by creep of asperities, while the dominant mode at high temperatures is microwelding or sintering of asperities and subsequent sliding after the welding bridges are broken. Similarly, Jeremic et al. [13] showed an increase in the static coefficient friction at elevated temperatures (i.e. above 120 °C) using low contact pressures. The static friction coefficient displayed an unexpected relationship to the contact pressures. Etsion et al. [14] studied the effect of normal loads on the static friction coefficient between smooth metallic surfaces. The static friction evidently increased with the reduction in the normal load. The increase in friction was attributed to more pronounced adhesive forces at low normal loads and smooth surfaces [14]. While some valuable information has been gained on the static friction behavior of metals, the underlying mechanisms of static friction in Ni-based alloys at elevated temperature remains unclear. The major objectives of this study were to evaluate the static friction behavior of Ni-based alloys at elevated temperatures as well as determine the governing interfacial mechanisms, including the effects of oxide layers. In addition, to better interrogate the underlying mechanisms responsible for the observed behavior, we pursued a series of molecular dynamics simulations of static friction in non-oxidized and oxidized states, emulating the surface condition of the as-produced and heat treated samples, respectively.
Corresponding author. E-mail address: [email protected] (P. Stoyanov).
https://doi.org/10.1016/j.surfcoat.2018.05.094 Received 23 March 2018; Received in revised form 16 May 2018; Accepted 17 May 2018 Available online 07 July 2018 0257-8972/ © 2018 Published by Elsevier B.V.
Surface & Coatings Technology 352 (2018) 634–641
P. Stoyanov et al.
2. Methods 2.1. Experimental procedure Static friction coefficient experiments were performed using a custom-built high-temperature apparatus in a flat-on-flat configuration. Briefly, load cells aligned in the direction of motion, located above and below the wear pins and plates, measured the friction forces, while normal load cells maintained the static load as the plate displaced relative to the pin. The load cells measuring the frictional forces also monitored the flat-on-flat contact. A mismatch among the load cells indicated a derivation from flat-on-flat contact. The tests were performed at room temperature and elevated temperatures of 430 °C and 665 °C using normal stresses of 117 MPa, for a total displacement of 2.5 mm at a rate of 5.1 mm/min. More specifically, the testing was performed in load control at a rate of 445 N/s/. The static coefficient of friction breakaway load was determined by finding the maximum load prior to a change in load and displacement. The static friction numbers presented in this paper are normalized, such that each coefficient of friction is divided by the lowest common denominator. The static friction coefficient of Inconel 718 was investigated when in contact against itself, ME16, Ti-6246, and a Ni-Cr based coating deposited by thermal spray. The nominal chemistries of the superalloys can be found elsewhere [1]. All material couples were tested at room temperature and elevated temperature. The elevated temperature test of the titanium alloy counterface was performed at 430 °C, while all other couples were tested at 665 °C. X-ray Photoelectron Spectroscopy (XPS) (PHI 5000 VersaProbe, Physical Electronics Inc.) analysis was performed on unworn surfaces in order to provide a better understanding of the chemical changes. Prior to the analysis, the samples are cleaned using isopropanol followed by cyclohexane. This data provided high depth resolution elemental concentration information from the surface down to ~220 nm below the surface. Focused Ion Beam Scanning Electron Microscopy (FIB/SEM) (FEI Helios 600 NanoLab DualBeam FIB) was used to cut site-specific cross-sections of the surfaces (i.e. in the worn and unworn state) and images / elemental maps of those locations were acquired. While utilization of the FIB/SEM offered lower resolution elemental distribution information than the XPS, the information covered a much larger depth, from the surface down to ~8 μm into the wear surface.
Fig. 1. Example of atomistic structure for simulating static friction in Ni. (a) The blue and red blocks are both Ni oriented with the [001] direction parallel to z. (b) The blue atoms are rotated about the [001] direction by 37°. (c) Following the annealing and relaxation process described in the text, friction simulations are conducted using constant velocity boundary conditions. A vacuum gap in the z direction is not shown, but was included in all friction simulations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
physical forces at the start of the friction simulation. To accomplish this, we built structures in three steps, as summarized in Fig. 1. First, we generated atomistic structures for the system of interest with the desired surface orientation and several rotations about the surface normal; these structures were fully relaxed using periodic boundary conditions to achieve the bulk lattice parameters, and to ensure that the rotation process had not introduced structural defects. Second, we assembled friction couples using pairs of the relaxed, rotated cells with some fraction of atoms from the four layers near the interface randomly removed to introduce varying levels of surface damage. We also included a large vacuum gap in the z direction to avoid interaction with periodic images. All three components of velocity of the bottom atoms were all held constant at zero, the x and y components of velocity of the top atoms were held constant at zero, and displacement of the top atoms in the z direction was allowed. Under these constraints, we annealed the system at 600 K for 15 ps, quenched from 600 K to 200 K in 15 ps, and then fully relaxed the system. Finally, we conducted the friction simulations, holding all velocities of the bottom atoms and the y and z components of velocity for the top atoms equal to zero. The x component of velocity for the top atoms was set to a constant value, shearing the system in the x direction. We allowed all other atoms to evolve via constant number/volume/energy (NVE) molecular dynamics from a starting temperature of 300 K, noting that the temperature of the system stays below 300 K for the duration of the simulation. The simulations that produced the results reported here utilized a constant x velocity of 10 m/s, following convergence testing
2.2. Computational methods There is sparse literature describing atomistic simulations of static friction, most notably work by Qi et al. describing static friction in pure Ni [17] and Zhang et al. describing static friction in Al/Al2O3 systems [18]. While these reports provide some insight into simulation techniques, they use constant force boundary conditions which confuse the results by introducing inertial effects and noticeable “ringing” (e.g. rapid oscillations) in the system. To avoid these issues, we implemented a velocity-controlled approach; the key feature in our approach is preconditioning the system to minimize initial forces, and then using a low enough velocity to simulate meaningful interactions. Our molecular dynamics simulations employed the Large-scale Atomic/Molecular Massively Parallel Simulation (LAMMPS) code [19] made available by Sandia National Laboratories (http://lammps.sandia. gov/). We used a ReaxFF style interatomic potential for Ni/NiO [20] and Cr/Fe/O [21]; ReaxFF is particularly appropriate for comparing across metallic (e.g. Ni), mixed (e.g. Ni-O), and covalent (e.g. Cr2O3) systems. Further, both of these potentials were developed to simulate surface behavior. Our objective was to expose various Ni and Cr2O3 interfaces to constant velocity shear loading, and to measure the peak shear stress that each system can sustain before transitioning from sticking behavior to sliding behavior. In order to produce meaningful results, it is essential that the atomistic structures are built in such a way as to minimize spurious or non635
Surface & Coatings Technology 352 (2018) 634–641
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Fig. 3. Example results showing force on the top atoms in the x direction (Fx) (blue) and position offset (d) as defined in the text versus displacement of the top atoms (orange) versus displacement of the top atoms. Lines are 10 point moving averages of the individual measurements. A star indicates the point at which the system begins to slip, relieving force and reducing the position offset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Schematic sequence of simulation events showing a progression from the initial conditions (a), elastic shearing during the “sticking” phase (b), followed by “slipping” after a critical force is exceeded (c). The center of mass of the two layers of atoms near the interface is indicated with a yellow circle, while the center of mass of the two layers of atoms near the top of the cell is indicated with a pink circle. The “position offset” d provides an indication of the transition between sticking and slipping. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at 1 m/s showing nearly identical results. Similarly, the peak shear stress for transition from sticking to sliding was converged with respect to cell height and width in all cases. Visualization, including the atomistic structures presented as figures in this report, was conducted using OVITO [22]. In order to quantify the results of our simulations, we calculate the center of mass of the top two layers of atoms, shown as a pink circle in Fig. 2, and the center of mass of the two interface layers of atoms, shown as a yellow circle in Fig. 2. The difference between these locations, which we call the position offset d, provides an indication of the degree of elastic shearing in the system. An increasing value of d shows that the interface is “sticking”; in other words, the top layers of atoms are moving under the applied velocity, but interface layers of atoms remain bonded to the bottom slab. As the value of d increases, the force required to move the top layers of atoms increases until a critical force is achieved. At this point (indicated by a yellow star in Fig. 3), the interface atoms begin to slip, the value of d decreases (e.g. the interface atoms slip forward and begin to catch up with the top layer atoms), and the force on the top atoms is relieved. This peak force measured during this interaction fundamentally describes the resistance of the system to initial slip. We further convert from force/displacement to shear stress/shear strain to facilitate comparison of cells of different sizes.
Fig. 4. Static coefficient of friction (normalized) of Inconel 718 vs. Ni-based and Ti-based alloys tested at room temperature and at elevated temperatures.
material couples is shown in Fig. 4. The static coefficient of friction is evidently higher for the tests performed at elevated temperature (i.e. 430 °C and 665 °C). In addition, the scatter for the static friction values at elevated temperatures is greater than the scatter of the room temperature values. Interestingly, no significant difference is observed in the static friction coefficient values between the different counterfaces against Inconel 718 when tested at room temperature. Similarly, the static friction was similar for the different counterfaces at elevated temperatures. In order to better understand the influence of the oxidation behavior on the interfacial processes, the static friction was evaluated for Inconel 718 against itself at room temperature after exposure at 665 °C for up to 24 h. The average value static friction value is shown in Fig. 4. Clearly,
3. Results 3.1. Static coefficient of friction The normalized static friction coefficient behavior for the various 636
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Fig. 5. (a). Cross-sectional SEM images produced by FIB of Inconel 718 samples tested at (a) room temperature. (b). Cross-sectional SEM images produced by FIB of Inconel 718 samples tested at (b) elevated temperatures.
3.2. Ex situ analysis
the static friction is higher compared to all other values tested at room temperature. Interestingly, the static friction value after high temperature exposure is also on average higher compared to all other measurements at elevated temperature.
Cross-sectional SEM images of the Inconel 718 surface are shown in Fig. 5 for samples tested at room temperature and elevated 637
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Fig. 6. XPS analysis of the Inconel 718 near surface region, following elevated temperature testing, reveals that the material has been largely oxidized. The photoelectron data from the major elements detected on the surface of Inconel 718 are presented, above, and deconvolution of the peak envelops reveal the likely bonding states of each element. Iron (top left) was found to have at least two binding energy states consistent with Fe3O4 and Fe2O3 or a similar oxy-hydroxide state. Manganese (top center) revealed a broad peak centered around a binding energy consistent with a manganese oxy-hydroxide. Chromium (top right) was deconvoluted into three peaks representing at least three separate bonding states including both oxide and hydroxide states. Oxygen (bottom left) was deconvoluted into three states including metal oxides, metal hydroxides and organic states. Carbon (bottom right) shows deconvoluted states consistent with atmospheric carbon states adsorbed to the surface.
test is shown in Fig. 6. Similarly to the cross-sectional SEM images, the XPS analysis revealed a high concentration of metal oxide in the surface near region. The metal oxide was mainly in form of iron oxides (i.e. Fe3O4, Fe2O3) and chromium oxides (i.e. Cr2O3, CrO3). In addition, some amount of manganese-based oxides were also observed in the form of Mn(OH)O and MnCr2O4. Cross-sectional SEM images for the titanium samples are shown in Fig. 7. Similarly to the Inconel 718, the titanium showed nearly no oxide on the surface of the samples tested at room temperature. However, an oxygen rich layer was observed after testing at elevated temperatures, possibly in the form of aluminum oxide.
temperatures. As expected, the oxidation behavior of the unworn surfaces was different between the samples tested at room temperature and high temperature. The elemental mapping of the Inconel 718 tested at high temperature revealed the formation of a thin oxide layer on the surface, as shown in Fig. 5(b). In addition, a chromium layer is visible on the surface, suggesting the possibility of chromium oxide. The crosssectional images on the coupons tested at room temperature, on the other hand, did not show any visible oxide layer (see Fig. 5a) or metallic rich layers. XPS was performed in order to provide a better understanding of the oxidation behavior for the tests at elevated temperatures. The XPS analysis of the unworn Inconel 718 surface after the high temperature
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Fig. 7. (a). Cross-sectional SEM images produced by FIB of titanium samples tested at room temperature. (b). Cross-sectional SEM images produced by FIB of titanium samples tested at elevated temperatures.
for Ni and Cr2O3 as described in the computational methods section. To ensure representative results, we constructed cells with Ni(100)/Ni (100) interfaces, Ni(111)/Ni(111) interfaces, and Cr2O3(0001)/(0001) interfaces. The energies of the Ni(100) and Ni(111) are quite similar (experimentally measured as 113.67 meV/square-Ang [22] and 115.48 meV/square-Ang [23], respectively) and we therefore expect that the real surfaces will be largely comprised of these planes,
3.3. Molecular dynamics simulations Our experimental results indicated that heat treatment causes oxidation of the superalloy surface, converting from a surface rich in Ni to a surface rich in Cr2O3. We hypothesized that the oxidation of the surfaces contributed significantly to the observed change in coefficient of static friction, and constructed molecular dynamics simulation cells 639
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temperatures is by creep of the asperities. At higher temperatures, on the other hand, the mechanism appeared to be sintering or welding of the asperities, and sliding was initiated when welding bridges were broken. These differences in the interfacial processes between low and high temperature ranges might be also active in our experiments, leading to different static friction values of Inconel 718. However, it should be noted that Ni-based superalloys have a complex oxidation behavior, and thus, it is likely that the difference in friction between room and elevated temperatures is due to the metal-on-metal and oxide-on-oxide contacting interfaces. In order to provide a better understanding of the oxidation influence, we tested pre-oxidized samples at room temperature, which showed even higher static friction compared to those tested at elevated temperatures. Macroscopically, the real surfaces are comprised of asperities and roughness with a length scale greatly exceeding the size of our simulation cells. However, where asperities contact and form welds, we expect interaction of low energy surfaces, and ultimately contribution to static friction through adhesion. Hence our simulations interrogate only the adhesive contributions to static friction occurring at the interfaces between asperities, while other contributions such as plasticity and macroscopic roughness are not considered. Similarly, the real surfaces are heterogeneous combinations of metals, oxides, and impurities from the environment; by comparing only Ni and Cr2O3 we isolate the influence of oxide interactions on the static friction behavior. In this way we assess the degree to which changes in the adhesive strength due to the local chemistry affect static friction, without the confounding influence of other characteristics. However, because we do not explicitly capture the full chemical and geometric complexities of interacting surfaces, we refrain from reporting a coefficient of static friction from the molecular dynamics results, recognizing the hierarchical and macroscopic interactions that contribute to the observed response. In any case, our molecular dynamics simulations indicate that the average critical shear stress for transition from sticking to slipping for Cr2O3 is 5.2 times higher than for Ni. This compares quite favorably with our experimental results, where the measured coefficient of static friction for the pre-exposed (i.e. heat treated) samples tested at room temperature was approximately 4.6 times higher than for the non-heat treated samples. The presence of chromium oxide is associated with an increased coefficient of static friction as measured experimentally. Our simulations reproduce the magnitude of this increase, and suggest that the formation of surface oxide and the corresponding increase in the shear strength of the static interfaces contributes to the observed behavior.
Fig. 8. Critical shear stress at the transition between sticking and slipping for Ni and Cr2O3 interfaces. The range of results for each material is due to changes in relative orientation of the layers, sliding direction of the top layer, and damage introduced at the surface.
separated by steps to accommodate the macroscopic surface directions. For the case of Cr2O3, the (0001) surface energy is approximately half of the next lowest surface energy (that of (10−12)) based on DFT results [21], and we therefore focused only on (0001) simulations. Further, we tested three different sliding directions in Ni, and two different sliding directions in Cr2O3. Finally, for each simulation cell, we conducted three variations where 0%, 5%, or 25% of the atoms in the four layers near the interface are randomly removed prior to annealing. In total, the combinations of rotation angles, surfaces, sliding directions, and surface damage required nine Ni simulations and six Cr2O3 simulations. Exploring a wide range of conditions ensured that we could better represent the overall behavior of Ni and Cr2O3 static friction, rather than producing results uniquely weighted to a specific configuration. A summary of the peak shear stress achieved at the transition between sticking and slipping across all 15 simulations is shown in Fig. 8; the results are normalized to the lowest observed value to simplify comparison with the experimental measurements of static friction. It is important to note that the range of results within each material (due to changes in orientation, sliding direction, and surface damage) are much smaller than the difference between each material.
5. Conclusion In this study, the static friction behavior of superalloys was investigated at room and elevated temperatures. Static friction experiments were performed on conventionally heat-treated Inconel 718, ME16, Ti 6246, and Ni-Cr coating using a conventionally heat-treated Inconel 718 counterface for all experiments. In addition, surface and sub-surface characterization by means of FIB/SEM and XPS analysis was performed in order to capture the underlying mechanisms leading to the static friction behavior at the different temperatures. The static friction coefficient was overall higher for the experiments at elevated temperatures for all counterface materials against Inconel 718. The elemental analysis revealed the formation of metal oxide layers on the surfaces of the different alloys after testing at elevated temperature. In addition, atomistic simulations were performed in order to elucidate the interfacial mechanism leading to the differences in static friction. The observations of the simulations correlated well with the experiments, where the shear stress was approximately five times higher for the oxide-on-oxide systems compared to the nickel-on‑nickel.
4. Discussion Our elemental analysis revealed the formation of a metallic oxide layer on the Inconel 718 surface, mainly comprised of Cr- and Feoxides. Similarly, an oxide layer was observed on titanium indicating an oxide-on-oxide contact for the tests at elevated temperatures. Consistently, the static coefficient of friction is approximately 3 to 4 times higher for the experiments carried out at elevated temperatures compared to the ones at room temperature. The static friction behavior of IN718 presented in this study is similar to one observed by Jeremic et al. [13] showing an increase in the static coefficient friction at elevated temperatures (i.e. above 120 °C) using low contact pressures. Gilligan et al. [12] showed two different mechanisms being operative in different temperature ranges for copperon-copper and brass-on-brass. For lower temperatures, the authors showed a decrease in static friction with increasing temperature, indicating that the dominant mechanism of static friction at lower 640
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Acknowledgment
friction tests, ASLE Trans. 14 (3) (1971) 198–205. [12] J.M. Galligan, P. McCullough, On the nature of static friction, Wear 105 (4) (1985) 337–340. [13] B. Jeremic, D. Vukelic, P.M. Todorovic, I. Macuzic, M. Pantic, D. Dzunic, B. Tadic, Static friction at high contact temperatures and low contact pressure, J. Fric. Wear 34 (2) (2013) 114–119. [14] I. Etsion, M. Amit, The effect of small normal loads on the static friction coefficient for very smooth surfaces, J. Tribol. 115 (3) (1993) 406–410. [15] B. Lorenz, B.N.J. Persson, On the origin of why static or break loose friction is larger than kinetic friction, and how to reduce it: the role of aging, elasticity and sequential interfacial slip, J. Phys. Condens. Matter 24 (2012) 225008. [16] Satoru Maegawa, Fumihiro Itoigawa, Takashi Nakamura, New insight into the mechanism of static friction: a theoretical prediction of the effect of loading history on static friction force based on the static friction model proposed by Lorenz and Persson, Tribol. Int. 102 (2016) 532–539 (ISSN 0301-679X). [17] Y. Qi, Y.-T. Cheng, T. Çağin, W.A. Goddard, Friction anisotropy at Ni(100)/(100) interfaces: molecular dynamics studies, Phys. Rev. B 66 (2002) 085420. [18] Q. Zhang, Y. Qi, L.G. Hector Jr., T. Cagin, W.A. Goddard, Origin of static friction and its relationship to adhesion at the atomic scale, Phys. Rev. B 75 (2007) 144114. [19] S. Plimpton, Fast parallel algorithms for short-range molecular dynamics, J. Comput. Phys. 117 (1995) 1–19. [20] C. Zou, Y.K. Shin, A.C.T. van Duin, H. Fang, Z.-K. Liu, Molecular dynamics simulations of the effects of vacancies on nickel self-diffusion, oxygen diffusion and oxidation initiation in nickel, using the ReaxFF reactive force field, Acta Mater. 83 (2015) 102–112. [21] Y.K. Shin, H. Kwak, A.V. Vasenkov, D. Sengupta, A.C.T. van Duin, Development of a ReaxFF reactive force field for Fe/Cr/O/S and application to oxidation of butane over a pyrite-covered Cr2O3 catalyst, ACS Catal. 5 (2015) 7226–7236. [22] P.S. Maiya, J.M. Blakely, Surface self-diffusion and surface energy of nickel, J. Appl. Phys. 38 (1967) 698. [23] S.H. Overbury, P.A. Bertrand, G.A. Somorjai, Surface composition of binary systems. Prediction of surface phase diagrams of solid solutions, Chem. Rev. 75 (1975) 547–560.
The authors thank Margaret Fiore for the valuable discussions and suggestions on improving the final version of this manuscript. References [1] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, 2006. [2] H. Fecht, D. Furrer, Processing of nickel-base superalloys for turbine engine disc applications, Adv. Eng. Mater. 2 (12) (2000) 777–787. [3] Z. Huda, P. Edi, Materials selection in design of structures and engines of supersonic aircrafts: a review, Mater. Des. 46 (2013) 552–560. [4] G.W. Meetham, High-temperature materials—a general review, J. Mater. Sci. 26 (4) (1991 Jan 1) 853–860. [5] R.W. Kozar, A. Suzuki, W.W. Milligan, J.J. Schirra, M.F. Savage, T.M. Pollock, Strengthening mechanisms in polycrystalline multimodal nickel-base superalloys, Metall. Mater. Trans. A 40 (7) (2009 Jul 1) 1588–1603. [6] C.T. Sims, N.S. Stoloff, W.C. Hagel, Superalloys II: high-temperature materials for aerospace and industrial power, Technology & Engineering, 2nd edition, Wiley, 1987(615 pp.). [7] T.M. Pollock, S. Tin, Nickel-based superalloys for advanced turbine engines: chemistry, microstructure, and properties, J. Propuls. Power 22 (2) (March–April 2006). [8] W. Betteridge, S.W.K. Shaw, Development of superalloys, Mater. Sci. Technol. 3 (9) (1987) 682–694. [9] Y. Koizumi, T. Kobayashi, T. Yokokawa, J. Zhang, M. Osawa, H. Harada, Y. Aoki, M. Arai, Development of next-generation Ni-base single crystal superalloys, Superalloys 2004 (2004) 35–43. [10] P. Caron, K. Tasadduq, Evolution of Ni-based superalloys for single crystal gas turbine blade applications, Aerosp. Sci. Technol. 3 (8) (1999) 513–523. [11] E. Rabinowicz, The determination of the compatibility of metals through static
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Insights into the static friction behavior of Ni-based superalloys a,⁎
a
a
b
Pantcho Stoyanov , Lesley Dawag , William J. Joost , Daniel G. Goberman , Steven Ivory a b
T a
Pratt & Whitney, United Technologies Corporation, East Hartford, CT, United States of America United Technology Research Center, United Technologies Corporation, East Hartford, CT, United States of America
A R T I C LE I N FO
A B S T R A C T
Keywords: Single crystal alloys Tribofilm Lubricious oxides
The purpose of this study was to provide a better understanding of the static friction behavior of Ni-based superalloys over a wide range of temperatures. The static friction coefficient was investigated using a custombuilt high-temperature apparatus in a flat-on-flat contact configuration. Measurements were performed on Inconel 718, using a variety of counterface materials (i.e. Ni-Cr coating, ME16, Ti-6246 and Inconel 718). The static friction coefficient was overall higher for the experiments at elevated temperatures for all counterface materials against Inconel 718. Ex situ analysis by means of FIB/SEM and XPS revealed the formation of metal oxide layers on the surfaces of the different alloys after testing at elevated temperature. In addition, atomistic simulations were performed in order to elucidate the interfacial mechanism leading to the differences in static friction. The observations of the simulations correlated well with the experiments, where the shear strength of the frictional interfaces was evidently higher for the oxide-on-oxide systems compared to the nickelon-nickel.
1. Introduction Nickel based superalloys are widely used as structural components in gas turbine engines due to their excellent resistance to mechanical and chemical degradation at elevated temperatures [1–10]. Inconel 718, in particular, is well known for high-temperature strength, and superior resistance to creep, oxidation, and low cycle fatigue. Thus, Inconel 718 is commonly used in hot sections of gas turbine engines, as well as in other extreme environment applications. While intensive research on the microstructure characteristics and process development activities of Inconel 718 has been performed over the last few decades, the tribological behavior (i.e. friction and wear) has received relatively little attention. In particular, no systematic studies can be found on the static friction behavior at high temperature and contact stresses of Inconel 718, despite its importance in the design and performance of engines. A few studies have attempted to understand the underlying mechanisms of static friction [11–16]. For instance, Rabinowicz [11] investigated the compatibility of metals through static friction tests and interpreted the results in terms of the surface energy model of friction. Galligan et al. [12] performed a series of tests for different material combinations (i.e. brass on brass and copper on copper) at a wide range of temperatures. The authors showed that the static friction has two different processes governing the static friction; at low temperatures,
⁎
the mechanism of static friction is primarily by creep of asperities, while the dominant mode at high temperatures is microwelding or sintering of asperities and subsequent sliding after the welding bridges are broken. Similarly, Jeremic et al. [13] showed an increase in the static coefficient friction at elevated temperatures (i.e. above 120 °C) using low contact pressures. The static friction coefficient displayed an unexpected relationship to the contact pressures. Etsion et al. [14] studied the effect of normal loads on the static friction coefficient between smooth metallic surfaces. The static friction evidently increased with the reduction in the normal load. The increase in friction was attributed to more pronounced adhesive forces at low normal loads and smooth surfaces [14]. While some valuable information has been gained on the static friction behavior of metals, the underlying mechanisms of static friction in Ni-based alloys at elevated temperature remains unclear. The major objectives of this study were to evaluate the static friction behavior of Ni-based alloys at elevated temperatures as well as determine the governing interfacial mechanisms, including the effects of oxide layers. In addition, to better interrogate the underlying mechanisms responsible for the observed behavior, we pursued a series of molecular dynamics simulations of static friction in non-oxidized and oxidized states, emulating the surface condition of the as-produced and heat treated samples, respectively.
Corresponding author. E-mail address: [email protected] (P. Stoyanov).
https://doi.org/10.1016/j.surfcoat.2018.05.094 Received 23 March 2018; Received in revised form 16 May 2018; Accepted 17 May 2018 Available online 07 July 2018 0257-8972/ © 2018 Published by Elsevier B.V.
Surface & Coatings Technology 352 (2018) 634–641
P. Stoyanov et al.
2. Methods 2.1. Experimental procedure Static friction coefficient experiments were performed using a custom-built high-temperature apparatus in a flat-on-flat configuration. Briefly, load cells aligned in the direction of motion, located above and below the wear pins and plates, measured the friction forces, while normal load cells maintained the static load as the plate displaced relative to the pin. The load cells measuring the frictional forces also monitored the flat-on-flat contact. A mismatch among the load cells indicated a derivation from flat-on-flat contact. The tests were performed at room temperature and elevated temperatures of 430 °C and 665 °C using normal stresses of 117 MPa, for a total displacement of 2.5 mm at a rate of 5.1 mm/min. More specifically, the testing was performed in load control at a rate of 445 N/s/. The static coefficient of friction breakaway load was determined by finding the maximum load prior to a change in load and displacement. The static friction numbers presented in this paper are normalized, such that each coefficient of friction is divided by the lowest common denominator. The static friction coefficient of Inconel 718 was investigated when in contact against itself, ME16, Ti-6246, and a Ni-Cr based coating deposited by thermal spray. The nominal chemistries of the superalloys can be found elsewhere [1]. All material couples were tested at room temperature and elevated temperature. The elevated temperature test of the titanium alloy counterface was performed at 430 °C, while all other couples were tested at 665 °C. X-ray Photoelectron Spectroscopy (XPS) (PHI 5000 VersaProbe, Physical Electronics Inc.) analysis was performed on unworn surfaces in order to provide a better understanding of the chemical changes. Prior to the analysis, the samples are cleaned using isopropanol followed by cyclohexane. This data provided high depth resolution elemental concentration information from the surface down to ~220 nm below the surface. Focused Ion Beam Scanning Electron Microscopy (FIB/SEM) (FEI Helios 600 NanoLab DualBeam FIB) was used to cut site-specific cross-sections of the surfaces (i.e. in the worn and unworn state) and images / elemental maps of those locations were acquired. While utilization of the FIB/SEM offered lower resolution elemental distribution information than the XPS, the information covered a much larger depth, from the surface down to ~8 μm into the wear surface.
Fig. 1. Example of atomistic structure for simulating static friction in Ni. (a) The blue and red blocks are both Ni oriented with the [001] direction parallel to z. (b) The blue atoms are rotated about the [001] direction by 37°. (c) Following the annealing and relaxation process described in the text, friction simulations are conducted using constant velocity boundary conditions. A vacuum gap in the z direction is not shown, but was included in all friction simulations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
physical forces at the start of the friction simulation. To accomplish this, we built structures in three steps, as summarized in Fig. 1. First, we generated atomistic structures for the system of interest with the desired surface orientation and several rotations about the surface normal; these structures were fully relaxed using periodic boundary conditions to achieve the bulk lattice parameters, and to ensure that the rotation process had not introduced structural defects. Second, we assembled friction couples using pairs of the relaxed, rotated cells with some fraction of atoms from the four layers near the interface randomly removed to introduce varying levels of surface damage. We also included a large vacuum gap in the z direction to avoid interaction with periodic images. All three components of velocity of the bottom atoms were all held constant at zero, the x and y components of velocity of the top atoms were held constant at zero, and displacement of the top atoms in the z direction was allowed. Under these constraints, we annealed the system at 600 K for 15 ps, quenched from 600 K to 200 K in 15 ps, and then fully relaxed the system. Finally, we conducted the friction simulations, holding all velocities of the bottom atoms and the y and z components of velocity for the top atoms equal to zero. The x component of velocity for the top atoms was set to a constant value, shearing the system in the x direction. We allowed all other atoms to evolve via constant number/volume/energy (NVE) molecular dynamics from a starting temperature of 300 K, noting that the temperature of the system stays below 300 K for the duration of the simulation. The simulations that produced the results reported here utilized a constant x velocity of 10 m/s, following convergence testing
2.2. Computational methods There is sparse literature describing atomistic simulations of static friction, most notably work by Qi et al. describing static friction in pure Ni [17] and Zhang et al. describing static friction in Al/Al2O3 systems [18]. While these reports provide some insight into simulation techniques, they use constant force boundary conditions which confuse the results by introducing inertial effects and noticeable “ringing” (e.g. rapid oscillations) in the system. To avoid these issues, we implemented a velocity-controlled approach; the key feature in our approach is preconditioning the system to minimize initial forces, and then using a low enough velocity to simulate meaningful interactions. Our molecular dynamics simulations employed the Large-scale Atomic/Molecular Massively Parallel Simulation (LAMMPS) code [19] made available by Sandia National Laboratories (http://lammps.sandia. gov/). We used a ReaxFF style interatomic potential for Ni/NiO [20] and Cr/Fe/O [21]; ReaxFF is particularly appropriate for comparing across metallic (e.g. Ni), mixed (e.g. Ni-O), and covalent (e.g. Cr2O3) systems. Further, both of these potentials were developed to simulate surface behavior. Our objective was to expose various Ni and Cr2O3 interfaces to constant velocity shear loading, and to measure the peak shear stress that each system can sustain before transitioning from sticking behavior to sliding behavior. In order to produce meaningful results, it is essential that the atomistic structures are built in such a way as to minimize spurious or non635
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Fig. 3. Example results showing force on the top atoms in the x direction (Fx) (blue) and position offset (d) as defined in the text versus displacement of the top atoms (orange) versus displacement of the top atoms. Lines are 10 point moving averages of the individual measurements. A star indicates the point at which the system begins to slip, relieving force and reducing the position offset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Schematic sequence of simulation events showing a progression from the initial conditions (a), elastic shearing during the “sticking” phase (b), followed by “slipping” after a critical force is exceeded (c). The center of mass of the two layers of atoms near the interface is indicated with a yellow circle, while the center of mass of the two layers of atoms near the top of the cell is indicated with a pink circle. The “position offset” d provides an indication of the transition between sticking and slipping. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
at 1 m/s showing nearly identical results. Similarly, the peak shear stress for transition from sticking to sliding was converged with respect to cell height and width in all cases. Visualization, including the atomistic structures presented as figures in this report, was conducted using OVITO [22]. In order to quantify the results of our simulations, we calculate the center of mass of the top two layers of atoms, shown as a pink circle in Fig. 2, and the center of mass of the two interface layers of atoms, shown as a yellow circle in Fig. 2. The difference between these locations, which we call the position offset d, provides an indication of the degree of elastic shearing in the system. An increasing value of d shows that the interface is “sticking”; in other words, the top layers of atoms are moving under the applied velocity, but interface layers of atoms remain bonded to the bottom slab. As the value of d increases, the force required to move the top layers of atoms increases until a critical force is achieved. At this point (indicated by a yellow star in Fig. 3), the interface atoms begin to slip, the value of d decreases (e.g. the interface atoms slip forward and begin to catch up with the top layer atoms), and the force on the top atoms is relieved. This peak force measured during this interaction fundamentally describes the resistance of the system to initial slip. We further convert from force/displacement to shear stress/shear strain to facilitate comparison of cells of different sizes.
Fig. 4. Static coefficient of friction (normalized) of Inconel 718 vs. Ni-based and Ti-based alloys tested at room temperature and at elevated temperatures.
material couples is shown in Fig. 4. The static coefficient of friction is evidently higher for the tests performed at elevated temperature (i.e. 430 °C and 665 °C). In addition, the scatter for the static friction values at elevated temperatures is greater than the scatter of the room temperature values. Interestingly, no significant difference is observed in the static friction coefficient values between the different counterfaces against Inconel 718 when tested at room temperature. Similarly, the static friction was similar for the different counterfaces at elevated temperatures. In order to better understand the influence of the oxidation behavior on the interfacial processes, the static friction was evaluated for Inconel 718 against itself at room temperature after exposure at 665 °C for up to 24 h. The average value static friction value is shown in Fig. 4. Clearly,
3. Results 3.1. Static coefficient of friction The normalized static friction coefficient behavior for the various 636
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Fig. 5. (a). Cross-sectional SEM images produced by FIB of Inconel 718 samples tested at (a) room temperature. (b). Cross-sectional SEM images produced by FIB of Inconel 718 samples tested at (b) elevated temperatures.
3.2. Ex situ analysis
the static friction is higher compared to all other values tested at room temperature. Interestingly, the static friction value after high temperature exposure is also on average higher compared to all other measurements at elevated temperature.
Cross-sectional SEM images of the Inconel 718 surface are shown in Fig. 5 for samples tested at room temperature and elevated 637
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Fig. 6. XPS analysis of the Inconel 718 near surface region, following elevated temperature testing, reveals that the material has been largely oxidized. The photoelectron data from the major elements detected on the surface of Inconel 718 are presented, above, and deconvolution of the peak envelops reveal the likely bonding states of each element. Iron (top left) was found to have at least two binding energy states consistent with Fe3O4 and Fe2O3 or a similar oxy-hydroxide state. Manganese (top center) revealed a broad peak centered around a binding energy consistent with a manganese oxy-hydroxide. Chromium (top right) was deconvoluted into three peaks representing at least three separate bonding states including both oxide and hydroxide states. Oxygen (bottom left) was deconvoluted into three states including metal oxides, metal hydroxides and organic states. Carbon (bottom right) shows deconvoluted states consistent with atmospheric carbon states adsorbed to the surface.
test is shown in Fig. 6. Similarly to the cross-sectional SEM images, the XPS analysis revealed a high concentration of metal oxide in the surface near region. The metal oxide was mainly in form of iron oxides (i.e. Fe3O4, Fe2O3) and chromium oxides (i.e. Cr2O3, CrO3). In addition, some amount of manganese-based oxides were also observed in the form of Mn(OH)O and MnCr2O4. Cross-sectional SEM images for the titanium samples are shown in Fig. 7. Similarly to the Inconel 718, the titanium showed nearly no oxide on the surface of the samples tested at room temperature. However, an oxygen rich layer was observed after testing at elevated temperatures, possibly in the form of aluminum oxide.
temperatures. As expected, the oxidation behavior of the unworn surfaces was different between the samples tested at room temperature and high temperature. The elemental mapping of the Inconel 718 tested at high temperature revealed the formation of a thin oxide layer on the surface, as shown in Fig. 5(b). In addition, a chromium layer is visible on the surface, suggesting the possibility of chromium oxide. The crosssectional images on the coupons tested at room temperature, on the other hand, did not show any visible oxide layer (see Fig. 5a) or metallic rich layers. XPS was performed in order to provide a better understanding of the oxidation behavior for the tests at elevated temperatures. The XPS analysis of the unworn Inconel 718 surface after the high temperature
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Fig. 7. (a). Cross-sectional SEM images produced by FIB of titanium samples tested at room temperature. (b). Cross-sectional SEM images produced by FIB of titanium samples tested at elevated temperatures.
for Ni and Cr2O3 as described in the computational methods section. To ensure representative results, we constructed cells with Ni(100)/Ni (100) interfaces, Ni(111)/Ni(111) interfaces, and Cr2O3(0001)/(0001) interfaces. The energies of the Ni(100) and Ni(111) are quite similar (experimentally measured as 113.67 meV/square-Ang [22] and 115.48 meV/square-Ang [23], respectively) and we therefore expect that the real surfaces will be largely comprised of these planes,
3.3. Molecular dynamics simulations Our experimental results indicated that heat treatment causes oxidation of the superalloy surface, converting from a surface rich in Ni to a surface rich in Cr2O3. We hypothesized that the oxidation of the surfaces contributed significantly to the observed change in coefficient of static friction, and constructed molecular dynamics simulation cells 639
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temperatures is by creep of the asperities. At higher temperatures, on the other hand, the mechanism appeared to be sintering or welding of the asperities, and sliding was initiated when welding bridges were broken. These differences in the interfacial processes between low and high temperature ranges might be also active in our experiments, leading to different static friction values of Inconel 718. However, it should be noted that Ni-based superalloys have a complex oxidation behavior, and thus, it is likely that the difference in friction between room and elevated temperatures is due to the metal-on-metal and oxide-on-oxide contacting interfaces. In order to provide a better understanding of the oxidation influence, we tested pre-oxidized samples at room temperature, which showed even higher static friction compared to those tested at elevated temperatures. Macroscopically, the real surfaces are comprised of asperities and roughness with a length scale greatly exceeding the size of our simulation cells. However, where asperities contact and form welds, we expect interaction of low energy surfaces, and ultimately contribution to static friction through adhesion. Hence our simulations interrogate only the adhesive contributions to static friction occurring at the interfaces between asperities, while other contributions such as plasticity and macroscopic roughness are not considered. Similarly, the real surfaces are heterogeneous combinations of metals, oxides, and impurities from the environment; by comparing only Ni and Cr2O3 we isolate the influence of oxide interactions on the static friction behavior. In this way we assess the degree to which changes in the adhesive strength due to the local chemistry affect static friction, without the confounding influence of other characteristics. However, because we do not explicitly capture the full chemical and geometric complexities of interacting surfaces, we refrain from reporting a coefficient of static friction from the molecular dynamics results, recognizing the hierarchical and macroscopic interactions that contribute to the observed response. In any case, our molecular dynamics simulations indicate that the average critical shear stress for transition from sticking to slipping for Cr2O3 is 5.2 times higher than for Ni. This compares quite favorably with our experimental results, where the measured coefficient of static friction for the pre-exposed (i.e. heat treated) samples tested at room temperature was approximately 4.6 times higher than for the non-heat treated samples. The presence of chromium oxide is associated with an increased coefficient of static friction as measured experimentally. Our simulations reproduce the magnitude of this increase, and suggest that the formation of surface oxide and the corresponding increase in the shear strength of the static interfaces contributes to the observed behavior.
Fig. 8. Critical shear stress at the transition between sticking and slipping for Ni and Cr2O3 interfaces. The range of results for each material is due to changes in relative orientation of the layers, sliding direction of the top layer, and damage introduced at the surface.
separated by steps to accommodate the macroscopic surface directions. For the case of Cr2O3, the (0001) surface energy is approximately half of the next lowest surface energy (that of (10−12)) based on DFT results [21], and we therefore focused only on (0001) simulations. Further, we tested three different sliding directions in Ni, and two different sliding directions in Cr2O3. Finally, for each simulation cell, we conducted three variations where 0%, 5%, or 25% of the atoms in the four layers near the interface are randomly removed prior to annealing. In total, the combinations of rotation angles, surfaces, sliding directions, and surface damage required nine Ni simulations and six Cr2O3 simulations. Exploring a wide range of conditions ensured that we could better represent the overall behavior of Ni and Cr2O3 static friction, rather than producing results uniquely weighted to a specific configuration. A summary of the peak shear stress achieved at the transition between sticking and slipping across all 15 simulations is shown in Fig. 8; the results are normalized to the lowest observed value to simplify comparison with the experimental measurements of static friction. It is important to note that the range of results within each material (due to changes in orientation, sliding direction, and surface damage) are much smaller than the difference between each material.
5. Conclusion In this study, the static friction behavior of superalloys was investigated at room and elevated temperatures. Static friction experiments were performed on conventionally heat-treated Inconel 718, ME16, Ti 6246, and Ni-Cr coating using a conventionally heat-treated Inconel 718 counterface for all experiments. In addition, surface and sub-surface characterization by means of FIB/SEM and XPS analysis was performed in order to capture the underlying mechanisms leading to the static friction behavior at the different temperatures. The static friction coefficient was overall higher for the experiments at elevated temperatures for all counterface materials against Inconel 718. The elemental analysis revealed the formation of metal oxide layers on the surfaces of the different alloys after testing at elevated temperature. In addition, atomistic simulations were performed in order to elucidate the interfacial mechanism leading to the differences in static friction. The observations of the simulations correlated well with the experiments, where the shear stress was approximately five times higher for the oxide-on-oxide systems compared to the nickel-on‑nickel.
4. Discussion Our elemental analysis revealed the formation of a metallic oxide layer on the Inconel 718 surface, mainly comprised of Cr- and Feoxides. Similarly, an oxide layer was observed on titanium indicating an oxide-on-oxide contact for the tests at elevated temperatures. Consistently, the static coefficient of friction is approximately 3 to 4 times higher for the experiments carried out at elevated temperatures compared to the ones at room temperature. The static friction behavior of IN718 presented in this study is similar to one observed by Jeremic et al. [13] showing an increase in the static coefficient friction at elevated temperatures (i.e. above 120 °C) using low contact pressures. Gilligan et al. [12] showed two different mechanisms being operative in different temperature ranges for copperon-copper and brass-on-brass. For lower temperatures, the authors showed a decrease in static friction with increasing temperature, indicating that the dominant mechanism of static friction at lower 640
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Acknowledgment
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