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Mechanical response of gasoline soot nanoparticles under compression: An in situ TEM study
Tribology International 131 (2019) 446–453
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Mechanical response of gasoline soot nanoparticles under compression: An in situ TEM study
T
Istvan Zoltan Jeneia,1, Fabrice Dassenoya,∗, Thierry Epicierb, Arash Khajehc, Ashlie Martinic, Dairene Uyd, Hamed Ghaedniad, Arup Gangopadhyayd a
Ecole Centrale de Lyon, LTDS, 36 avenue Guy de Collongue, 69134, Ecully, France Université de Lyon, INSA-Lyon, MATEIS, UMR5510 CNRS, 7 Avenue Jean Capelle, 69621, Villeurbanne Cedex, France c Department of Mechanical Engineering, University of California Merced, Merced, CA, 95343, USA d Powertrain Research and Advanced Engineering, Ford Motor Company, Dearborn, MI, 48121, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Soot nanoparticle Compression Molecular dynamics In situ TEM
Gasoline soot nanoparticles (SNPs) were studied by performing in situ compression tests on individual nanoparticles inside a transmission electron microscope. After consecutive compressions, the SNPs exhibited an elasto-plastic behavior, and an increasing trend in Young's modulus and hardness values. Molecular dynamics were used to simulate compression cycles, the results of which confirmed the observations made during the experiments. The simulations were used to investigate how the different structural components of the nanoparticles affect their elastic and plastic response. By comparing the behavior of gasoline and diesel SNPs under compression, differences were observed both experimentally and in the simulations: the former were found to be more elastic and less prone to become hard under compression compared to the latter.
1. Introduction Soot nanoparticles (SNPs) are constantly being generated inside internal combustion engines. These nanoparticles are the result of incomplete fuel (hydrocarbon) burning in the cylinders. For this reason, SNPs are mainly composed of carbon. Hydrogen content at approximately 1 wt% is also present, which leads to an empirical formula of C8 H [1]. Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), Raman spectroscopy, Xray absorption near edge structure (XANES) spectroscopy and X-ray diffraction (XRD) characterization techniques have been used to reveal the morphology and composition of the SNPs [2–7]. These studies concluded that SNPs are composed of spherically shaped primary particles, a few tens of nm in diameter, which can agglomerate and form large clusters with a diameter of a few hundred nm. Computer simulations corroborated these experimental findings [8]. The primary particles have a core-shell structure, the shell consisting of carbon atoms forming hexagonal face centered platelets that, are perpendicular to the nanoparticle's radius. Developments, such as gasoline engines with direct injection systems, aimed at increasing efficiency and engine power and lowering fuel consumption, have had the side effect of increased SNP formation
[9]. Most of the generated soot nanoparticles are evacuated with other exhaust gases, but a small fraction can be adsorbed into the thin oil film present on the cylinder walls. SNPs present in the engine oil system can affect the lubrication process in the following ways: the viscosity of the oil can increase [10,11]; large amounts of SNPs can cause pumpability problems [12]; and SNPs can increase wear inside the engine. The following are a few of the proposed wear mechanisms [13–16]: large soot agglomerates in the oil could cause lubricant starvation in the contact zone; the SNPs can hinder the anti-wear additive's ability to protect the surface; and, based on the hardness values reported previously [17–19], an abrasive wear mechanism is also plausible inside the engine. Motamen Salehi et al. has recently demonstrated how SNPs can adsorb on different engine oil additives, and this way it can affect the wear of different engine components [16,20]. Kontou et al. [21] showed that, if carbon black (a soot surrogate) is present in a lubricant consisting of base oil, dispersant and ZDDP (an anti-wear additive), no protective tribofilm is formed on the rubbing counter-surfaces. These demonstrated a corrosive-abrasive mechanism of soot wear. Considering abrasive wear, the mechanical properties of the SNPs, such as hardness and Young's modulus, can play an important role. Lahouij et al. [22] performed in situ indentation experiments in a TEM and made the following observations: diesel SNPs show elasto-plastic
∗
Corresponding author. E-mail address: [email protected] (F. Dassenoy). 1 Present address: Department of Materials and Environmental Chemistry, Stockholm University, 106 91 Stockholm, Sweden. https://doi.org/10.1016/j.triboint.2018.11.001 Received 9 August 2018; Received in revised form 27 October 2018; Accepted 1 November 2018 Available online 10 November 2018 0301-679X/ © 2018 Elsevier Ltd. All rights reserved.
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core of the primary SNPs were approximated with an ellipsoid, where the long and short diameters (d1 and d2 ) were measured manually. The diameter associated with the core was then calculated as the average of these. The thickness of the shell was measured at several radial locations, the mean value being reported. These parameters are illustrated in the supplementary document.
behavior under compression and are resistant to external load. Jao et al. [17] and Pavon [19] reported hardness values for diesel SNPs in the range of ≈7–14 GPa, which were calculated from the low-loss electron energy loss spectra (EELS) based on the method developed by Oleshko et al. [23]. In situ nanoindentation TEM was used by Jenei et al. [18] to measure hardness and the Young's modulus of individual diesel SNPs. The authors performed consecutive compressions on the nanoparticles and observed that the measured hardness values increased with compression cycle: 4.9 GPa during the first compression, and 12.3 GPa during the second compression. These results support the hypothesized abrasive wear mechanism. Uy et al. [2] characterized SNPs originating from diesel and gasoline turbocharged direct injection (GTDI) engines. In both cases, the SNPs were obtained from engine oil. The authors found that gasoline soot contained more non-carbonaceous elements than diesel soot; in particular, Zn, Ca, P and S quantities were several times higher. Zn, P and S originated from a well-known anti-wear additive, ZDDP. The soot containing these elements might indicate that the performance of antiwear additive is altered or hindered. The authors suggested that the increased amount of oil additive elements associated with gasoline soot particles could affect the wear mechanisms inside the engine. The objective of this article is to study the mechanical properties of SNPs originating from gasoline engine oils and compare them with their diesel counterparts. In order to quantify mechanical properties such as hardness and Young's modulus on the nanoscale, consecutive compressions were performed on individual SNPs in situ inside a TEM using a commercially available nanoindentation device. These properties were calculated based on TEM images recorded before and after each compression, video recordings of the compressions and force-displacement data. The methodology is described in more detail in Ref. [18]. Molecular dynamics simulations were performed to identify the contribution of each structural component of the SNPs on the abovementioned mechanical properties. Finally, the behavior of gasoline and diesel SNPs was compared. The mechanical properties, such as Young's modulus and hardness, were already studied previously using this method [18].
2.3. Picoindenter The compression tests were performed in situ, using a PI95 picoindenter manufactured by Hysitron, Inc. A schematic representation of the picoindenter and the experimental setup is shown in Fig. 1. The compression tests were performed in situ, inside the TEM with the diamond tip attached to the transducer of the picoindenter (see Fig. 1). The contact surface between the diamond tip and SNP was approximated by the area of a circular disk, the diameter of which could be measured from the compression video (as shown in Fig. 2). Based on the force-displacement curve and the estimated contact surface, the Young's modulus and the hardness could be calculated. A detailed description of these calculations and the compression experiments can be found in Ref. [18], where the method was applied to measure the Young's modulus and hardness of diesel SNPs. 2.4. Simulations Molecular dynamics (MD) simulations complemented the experimental measurements. A model SNP was constructed with an amorphous core and a graphitic shell. Nanoparticle properties, such as size, atomic composition, density etc., were set to resemble SNPs observed experimentally [4,5,22,25]. The model nanoparticle was subject to repeated compression cycles between two virtual walls at the top and the bottom of the particle. The bottom wall was fixed, while the upper wall moved downward with a constant speed of 40 m/s. The interactions between the walls and the atoms in the nanoparticle were modeled using a 9-3 Lenard-Jones potential with parameters ε = 0.043 eV, σ = 0.1 nm, and a 0.2 nm cutoff distance. The interactions between atoms within the SNP were modeled using the reactive force-field ReaxFF [26]. The compressions were simulated the following way: the upper wall was moved down until the SNP height decreased to 40% of the initial value and then unloading was performed until the net force acting on the upper wall became zero. All MD simulations were performed using the Large Atomic/Molecular Massively Parallel Simulation (LAMMPS) software [27]. During each compression cycle, the force-displacement diagram and hardness were recorded. Further details on the model SNP and the simulation protocol can be found in Ref. [18].
2. Material and methods 2.1. Soot nanoparticles SNPs were extracted from GTDI engines, see Table 1. The nanoparticles were collected from the engine oils by washing the oil with hexane through a centrifugation procedure, which is described in more detail in Ref. [7]. 2.2. TEM
3. Results and discussion
The compression tests were performed in situ inside an FEI Cs corrected environmental transmission electron microscope (ETEM) operated at 300 kV accelerating voltage. The TEM images were recorded using Gatan's OneView camera, the videos were recorded using Hysitron's screen capture software. The core-shell structure of primary SNPs was revealed previously in Refs. [2,4,5,7,17,24]. In the present study the core-shell structure of diesel and gasoline SNPs were characterized quantitatively. High resolution TEM images were used to measure the diameter of the core and the thickness of the shell of SNPs that possessed such structures. The
Four GTDI SNPs were selected for the compression experiments: G1A, G1B, G2C and G2D (Table 1). In the following, the results from two consecutive compressions of the G1A GTDI particle are presented. Fig. 3 shows TEM images of the nanoparticle before and after each consecutive compression. The compression videos can be seen at the following links: compression 1 and compression 2. The force-displacement data from the two consecutive compressions on particle G1A is displayed in Fig. 4, which shows that the particle exhibits elasto-plastic behavior. It can be observed that the slope of the curves is increasing with consecutive compressions. This slope is a measure of elasticity and so is subsequently referred to as the “elastic constant”. The size of the particle changed after each compression: the height (y-size) of the particle decreased significantly, while the width (x-size) remained relatively constant (see Table 2). The calculated Young's moduli and hardness values increased with each compression as well.
Table 1 Information about the soot nanoparticles used in the experiments. Sample ref.
Engine type
Soot source
Tested particles
G1 G2
GTDI GTDI
Vehicle test Engine dynamometer test
G1A, G1B G2C, G2D
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Fig. 1. Schematic representation of the experimental setup.
Fig. 2. A frame captured from a compression video showing the diameter of the circular disk, which is being, used for approximating the contact surface. The contour of the SNP is marked with red. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. The force-displacement data on the two consecutive compressions on particle G1A.
Fig. 3. TEM images of particle G1A before (a) and after each consecutive compression (b), (c). 448
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have higher Young's moduli values than diesel SNPs. Also, diesel SNPs appear to be harder than their GTDI counterparts. It should be noted that the Young's moduli and hardness values for the diesel SNPs were reported in Ref. [18]. Uy et al. performed ball-on-flat tribological tests with engine oils drained from diesel and GTDI engines and their supernatants [2]. Drained oil from diesel engine provided a larger wear scar on the ball compared to the wear scar obtained with the GTDI oil. The authors explained the larger wear by the fact that thermogravimetric analysis (TGA) indicated larger SNP content in the diesel drain. In the light of the results presented in this article (see Table 3 and Fig. 7), it can be argued that the hardness of the SNPs could also be an important factor in the wear mechanism: the harder diesel SNPs could contribute to larger wear scar diameters. To understand the origin of the difference between the two types of SNPs, consecutive compressions were simulated with the model SNP. The results of these simulations were similar to the experimental findings: the particle height and relative particle height decreased with consecutive compressions. Additional details are available in the supporting document. The force-displacement diagram obtained from the simulation indicated that the deformation of the model SNP contains both elastic and plastic components. Most importantly, the “elastic constant” of the model SNP increased with each compression cycle (consistent with experimental observations). This indicates that the model SNP and the simulation of the compression process are physically realistic. The simulations were also used to identify the relative contributions of the various components of the soot particle structure to changes in the elastic constant and hardness. To do this, four parts of the soot particle were isolated, as shown in Fig. 8. Model #1 is an amorphous
Table 2 The measured parameters on particle G1A.
x-size (nm) y-size (nm) “Elastic constant” (μN /nm) Young's modulus (GPa) Hardness (GPa)
Before
Compression 1
Compression 2
85.0 35.4 – – –
84.3 29.8 2.0 19.8 3.7
88.5 17.3 4.2 26.6 6.3
In summary, it can be observed that the particle exhibited elastoplastic behavior upon compression, it suffered permanent size-change and the Young's moduli and hardness values showed an increasing tendency with consecutive compressions. In order to confirm the behavior observed on particle G1A, three other GTDI SNPs were tested. Fig. 5 (a), (b) and (c) show the “elastic constant”, the y-size evolution and the relative y-size changes of consecutively compressed GTDI soot nanoparticles. The “elastic constant” increases with consecutive compressions for all of these particles. They also suffer permanent shape and size change (see Fig. 5 (b) and (c)). The calculated Young's moduli and hardness values can be seen in Fig. 6 (a) and (b). The same increasing tendencies can be observed here. On average, through consecutive compressions, the Young's modulus increased from 17.4 GPa to 27.9 GPa, and the hardness increased from 2.5 GPa to 4.9 GPa. The observed behavior of GTDI SNPs can be compared with their diesel counterparts. The average Young's moduli and hardness values for diesel and GTDI SNPs are shown in Fig. 7 (a) and (b) and in Table 3. Although the spread (the standard deviation) for the displayed values is large, our results indicate that, on average, GTDI soot nanoparticles
Fig. 5. The “elastic constant” (a), the y-size (b) and the relative y-size change (c) of the GTDI soot nanopartricles 449
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Fig. 6. The Young's modulus (a) and hardness (b) values of the GTDI soot nanoparticles.
Fig. 7. Average Young's moduli (a) and hardness (b) values for GTDI and diesel soot nanoparticles. The error bars represent standard deviations. Table 3 Average Young's modulus and hardness values for diesel and GTDI nanoparticles. The data for the diesel SNPs was reported in Ref. [18]. Diesel
Young's modulus (GPa) Hardness (GPa)
GTDI
Comp1
Comp2
Comp3
Comp1
Comp2
Comp3
8.2±1.8 4.9±1.9
20.9± 15.5 12.3±4.4
19.9±8.6 14.3±6.2
17.4± 4.8 2.5±1.0
26.8±3.0 4.8± 2.1
27.9± 13.4 4.9± 2.5
compressions. As can be seen in the upper right figure, there is a negligible gap between loading and unloading diagrams, which shows that the graphene layers arranged perpendicular to the compression direction exhibit a significant amount of elastic deformation. These results are in good agreement with recent experimental studies on lamellar carbon-graphene composites [28] showing a super-elastic behavior of samples composed of graphene layers with this orientation before compression. Further, comparing the results for Model #1 and #2 indicates that adding just a few layers of graphene to the amorphous structure increases the elastic behavior of the structure significantly. This is consistent with the results of previous nanoindentation tests on soot which showed the stored elastic energy increased with the thickness of the outer shell of the soot [29]. Fig. 10 shows the hardness of the four soot component models before compression and after each compression step. This figure indicates that the most significant change is exhibited by Models #1 and #4, which are the amorphous structure and graphene layers arranged parallel to the compression direction, respectively. These two models exhibit the largest amount of densification (plastic deformation) because their sp2 / sp3 ratio decreased continuously after each compression. The
structure, which represents the amorphous core of a soot particle. Model #2 is composed of parallel stacked graphitic layers normal to the compression direction, representative of the graphitic shell at the top or bottom of a particle. These two components are combined in Model #3, which is composed of two graphitic layers at the top and bottom and an amorphous center part with 2 nm height. Finally, Model #4 is comprised of stacked graphitic layers whose direction is perpendicular to that of Model #2 and represents material at the sides of a soot particle. The preparation procedure for these models was the same as that described in the Simulations section for the core-shell soot model or in Ref. [18]. However, to reduce computational cost, smaller models were used; the size of each model was 2.1 nm×2.1 nm×6.8 nm. The compression procedure was the same as described previously for the coreshell model. Fig. 9 shows the force-displacement results for the four soot component models. Models #1 and #4, which are the amorphous structure and the graphene layers parallel to the compression direction, exhibit considerable plastic deformation. However, after each compression step, the gap between the loading and unloading curve decreases, and the modeled structure shows more elastic behavior than in previous 450
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found in the supplementary document. Model #2 exhibits a negligible change in hardness and Model #3 exhibits behavior in between those of Models #1 and #2, as expected since it is comprised of both amorphous carbon and graphene components perpendicular to the compression direction. These results are consistent with previous experiments showing an increase of soot strength as the particle becomes more disordered [29]. In case of GTDI SNPs, those particles, which exhibited both core and shell structure, 22 were characterized quantitatively by analyzing the TEM images. The same measurements were performed on diesel SNPs originating from vehicle test. The results are summarized in Table 4, mean values and standard deviations are reported here. GTDI soot nanoparticles, that were selected for this study, have considerably thicker shells relative to their core than their diesel counterparts do. La Rocca et al. [5,30] have made similar observations. In Ref. [5], the authors describe a diesel SNP with a core diameter of 21 nm surrounded by a 5–9 nm thick shell, the shell being composed of graphitic platelets. In Ref. [30], the authors describe a gasoline SNP having a core of 25 nm diameter and an outer shell thickness of 16–20 nm. The former two observations show that the relative thickness of the shell (compared to the diameter of the core) is significantly larger in the case of gasoline SNPs compared to the diesel SNPs. The simulations suggest that, the graphitic component of the SNP is responsible for the elastic behavior (see the difference between Model #1 and #2). Based on this, it is expected that the GTDI SNPs would provide a more pronounced elastic response, compared to the diesel SNP. This, in fact, is confirmed by the compression experiments (see Fig. 7 and Table 3): on average, the Young's modulus is larger for the GTDI SNPs.
Fig. 8. Illustration of the four model systems used to capture the individual contributions of the various components of the soot particle to the change in elastic constant and hardness with compression.
decrease in sp2 / sp3 ratio and its association with increase in hardness was demonstrated in Ref. [18]. The formation of new bonds and increasing amount of sp3 hybridized carbon atoms can be viewed as an increasing number of diamond-like material, which leads to an increase in hardness. The evolution of the sp2 / sp3 ratio for all four models can be
4. Conclusions In situ compression tests were performed on SNPs originating from GTDI engines. During compression, the nanoparticles exhibited elastoplastic behavior, and they suffered permanent shape and size change. The calculated Young's moduli and hardness of the nanoparticles
Fig. 9. Force displacement data from simulations of compressions of individual components of the soot particle. 451
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Fig. 10. Hardness of the soot particle components as a function of compression cycle.
Appendix A. Supplementary data
Table 4 The quantitative morphological data on diesel and gasoline SNPs: mean values and standard deviation.
Average core diameter (nm) Average shell thickness (nm) Shell thickness/core diameter ratio
Diesel
GTDI
14.7±2.9 5.6±1.6 0.38±0.06
20.8± 7.7 10.9± 3.8 0.53± 0.08
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.triboint.2018.11.001. References [1] Palmer H, Cullis C. The formation of carbon from gases. Chem Phys Carbon 1965;1:265. [2] Uy D, Ford M a, Jayne DT, ONeill AE, Haack LP, Hangas J, Jagner MJ, Sammut A, Gangopadhyay AK. Characterization of gasoline soot and comparison to diesel soot: morphology, chemistry, and wear. Tribol Int 2014;80:198–209. [3] Ferraro G, Fratini E, Rausa R, Baglioni P. Impact of oil aging and composition on the morphology and structure of diesel soot. J Colloid Interface Sci 2018;512:291–9. [4] Patel M, Azanza Ricardo CL, Scardi P, Aswath PB. Morphology, structure and chemistry of extracted diesel sootPart I: transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and synchrotron X-ray diffraction study. Tribol Int 2012;52:29–39. [5] La Rocca A, Di Liberto G, Shayler PJ, Fay MW. The nanostructure of soot-in-oil particles and agglomerates from an automotive diesel engine. Tribol Int 2013;61:80–7. [6] Key S, La Rocca A, Fay M, Rance G, Growney D. Morphological characterisation of diesel soot in oil and the associated extraction dependence, SAE technical paper 2018-01-0935. 2018. [7] Sharma V, Uy D, Gangopadhyay A, O'Neill A, Paxton WA, Sammut A, Ford MA, Aswath PB. Structure and chemistry of crankcase and exhaust soot extracted from diesel engines. Carbon 2016;103:327–38. [8] Violi A, Izvekov S. Soot primary particle formation from multiscale coarse-grained molecular dynamics simulation. Proc Combust Inst 2007;31 I:529–37. [9] Bardasz EA, Arters DC, Schiferl EA, Righi DW. A comparison of gasoline direct injection and port fuel injection vehicles: Part II - lubricant oil performance and engine wear. SAE technical paper, 1999-01-1499. SAE International; 1999. [10] Duncan DA, Reed M, Szabo AL, Williams L. Soot related viscosity increase - a comparison of the mack T-11 engine test to field performance. SAE technical paper, 2004-01-3009. SAE International; 2004. [11] Tang Z, Feng Z, Jin P, Fu X, Chen H. The soot handling ability requirements and how to solve soot related viscosity increases of heavy duty diesel engine oil. Ind Lubric Tribol 2017;69:683–9. [12] George S, Balla S, Gautam M. Effect of diesel soot contaminated oil on engine wear. Wear 2007;262:1113–22. [13] Gautam M, Chitoor K, Durbha M, Summers JC. Effect of diesel soot contaminated oil on engine wear - investigation of novel oil formulations. Tribol Int 1999;32:687. [14] Ratoi M, Castle RC, Bovington CH, Spikes HA. The influence of soot and dispersant on ZDDP film thickness and friction. Lubric Sci 2004;17:25–43. [15] Green DA, Lewis R. The effects of soot-contaminated engine oil on wear and friction: a review. Proc Inst Mech Eng - Part D J Automob Eng 2008;222:1669–89. [16] Motamen Salehi F, Morina A, Neville A. Zinc dialkyldithiophosphate additive adsorption on carbon black particles. Tribol Lett 2018;66:1–7.
increased with consecutive compressions. The measured mechanical parameters of GTDI SNPs were compared with their diesel counterparts, and it was found that they exhibit different hardness and elasticity, which can affect the wear mechanism inside the engine. The hardness values reported here, are in the range of or higher than the hardness of the materials found in the contact zones within the cylinders of an internal combustion engine, which would make the abrasive wear plausible. Molecular dynamics simulations complemented the experimental measurements. The compression of the model SNP also resulted in elasto-plastic behavior, which indicates the model is physically realistic. Simulated consecutive compressions confirmed a permanent size change and an increasing elastic modulus. The contributions of the constituents of the soot model to the mechanical behavior of soot particles were also analyzed. The results showed that the graphene layers perpendicular to the compression direction are the major contributors to the elastic response of the particle. Further, the amorphous core can increase the hardness of the particle when exposed to compressive stress. Future work in this area can investigate the distance between graphene layers, SNPs without shell, primary and agglomerate sizes, and morphology of agglomerates.
Acknowledgments This work was supported by LABEX iMUST (ANR-10-LABX-0064) of Université de Lyon, within the program ‘Investissements d’Avenir’ (ANR-11-IDEX-0007) operated by the French National Research Agency. Thanks are also due to CLYM for access to the ETEM microscope. 452
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[24] Kawamura M, Ishiguro T, Morimoto H. Electron microscopic observation of soots in used diesel engine oils. Lubric Eng 1987;43. [25] Vander Wal RL, Tomasek AJ, Street K, Hull DR, Thompson WK. Carbon nanostructure examined by lattice fringe analysis of high-resolution transmission electron microscopy images. Appl Spectrosc 2004;58:230–7. [26] Shin YK, Gai L, Raman S, Van Duin ACT. Development of a ReaxFF reactive force field for the Pt-Ni alloy catalyst. J Phys Chem 2016;120:8044–55. [27] Plimpton S. Fast parallel algorithms for short-range molecular dynamics. J Comput Phys 1995;117:1–19. [28] Gao H-L, Zhu Y-B, Mao L-B, Wang F-C, Luo X-S, Liu Y-Y, Lu Y, Pan Z, Ge J, Shen W, Zheng Y-R, Xu L, Wang L-J, Xu W-H, Wu H-A, Yu S-H. Super-elastic and fatigue resistant carbon material with lamellar multi-arch microstructure. Nat Commun 2016;7:12920. [29] Bhowmick H, Biswas SK. Relationship between physical structure and tribology of single soot particles generated by burning ethylene. Tribol Lett 2011;44:139–49. [30] La Rocca A, Bonatesta F, Fay MW, Campanella F. Characterisation of soot in oil from a gasoline direct injection engine using Transmission Electron Microscopy. Tribol Int 2015;86:77–84.
[17] Jao T-C, Li S, Yatasumi K, Chen SJ, Csontos AA, Howe JM. Soot characterisation and diesel engine wear. Lubric Sci 2004;16:111–26. [18] Jenei IZ, Dassenoy F, Epicier T, Khajeh A, Martini A, Uy D, Ghaednia H, Gangopadhyay A. Mechanical characterization of diesel soot nanoparticles: in situ compression in a transmission electron microscope and simulations. Nanotechnology 2018;29:085703. [19] Pavon D. Caractérisation des dépôts sur les pistons PhD. thesis, Ecole Centrale de Lyon; 2007. [20] Motamen Salehi F, Morina A, Neville A. The effect of soot and diesel contamination on wear and friction of engine oil pump. Tribol Int 2017;115:285–96. [21] Kontou A, Southby M, Spikes HA. Effect of steel hardness on soot wear. Wear 2017;390–391:236–45. [22] Lahouij I, Dassenoy F, Vacher B, Sinha K, Brass DA, Devine M. Understanding the deformation of soot particles/agglomerates in a dynamic contact: tem in situ compression and shear experiments. Tribol Lett 2014;53:91–9. [23] Oleshko VP, Murayama M, Howe JM. Use of plasmon spectroscopy to evaluate the mechanical properties of materials at the nanoscale. Microsc Microanal 2002;8:350–64.
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Mechanical response of gasoline soot nanoparticles under compression: An in situ TEM study
T
Istvan Zoltan Jeneia,1, Fabrice Dassenoya,∗, Thierry Epicierb, Arash Khajehc, Ashlie Martinic, Dairene Uyd, Hamed Ghaedniad, Arup Gangopadhyayd a
Ecole Centrale de Lyon, LTDS, 36 avenue Guy de Collongue, 69134, Ecully, France Université de Lyon, INSA-Lyon, MATEIS, UMR5510 CNRS, 7 Avenue Jean Capelle, 69621, Villeurbanne Cedex, France c Department of Mechanical Engineering, University of California Merced, Merced, CA, 95343, USA d Powertrain Research and Advanced Engineering, Ford Motor Company, Dearborn, MI, 48121, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Soot nanoparticle Compression Molecular dynamics In situ TEM
Gasoline soot nanoparticles (SNPs) were studied by performing in situ compression tests on individual nanoparticles inside a transmission electron microscope. After consecutive compressions, the SNPs exhibited an elasto-plastic behavior, and an increasing trend in Young's modulus and hardness values. Molecular dynamics were used to simulate compression cycles, the results of which confirmed the observations made during the experiments. The simulations were used to investigate how the different structural components of the nanoparticles affect their elastic and plastic response. By comparing the behavior of gasoline and diesel SNPs under compression, differences were observed both experimentally and in the simulations: the former were found to be more elastic and less prone to become hard under compression compared to the latter.
1. Introduction Soot nanoparticles (SNPs) are constantly being generated inside internal combustion engines. These nanoparticles are the result of incomplete fuel (hydrocarbon) burning in the cylinders. For this reason, SNPs are mainly composed of carbon. Hydrogen content at approximately 1 wt% is also present, which leads to an empirical formula of C8 H [1]. Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), Raman spectroscopy, Xray absorption near edge structure (XANES) spectroscopy and X-ray diffraction (XRD) characterization techniques have been used to reveal the morphology and composition of the SNPs [2–7]. These studies concluded that SNPs are composed of spherically shaped primary particles, a few tens of nm in diameter, which can agglomerate and form large clusters with a diameter of a few hundred nm. Computer simulations corroborated these experimental findings [8]. The primary particles have a core-shell structure, the shell consisting of carbon atoms forming hexagonal face centered platelets that, are perpendicular to the nanoparticle's radius. Developments, such as gasoline engines with direct injection systems, aimed at increasing efficiency and engine power and lowering fuel consumption, have had the side effect of increased SNP formation
[9]. Most of the generated soot nanoparticles are evacuated with other exhaust gases, but a small fraction can be adsorbed into the thin oil film present on the cylinder walls. SNPs present in the engine oil system can affect the lubrication process in the following ways: the viscosity of the oil can increase [10,11]; large amounts of SNPs can cause pumpability problems [12]; and SNPs can increase wear inside the engine. The following are a few of the proposed wear mechanisms [13–16]: large soot agglomerates in the oil could cause lubricant starvation in the contact zone; the SNPs can hinder the anti-wear additive's ability to protect the surface; and, based on the hardness values reported previously [17–19], an abrasive wear mechanism is also plausible inside the engine. Motamen Salehi et al. has recently demonstrated how SNPs can adsorb on different engine oil additives, and this way it can affect the wear of different engine components [16,20]. Kontou et al. [21] showed that, if carbon black (a soot surrogate) is present in a lubricant consisting of base oil, dispersant and ZDDP (an anti-wear additive), no protective tribofilm is formed on the rubbing counter-surfaces. These demonstrated a corrosive-abrasive mechanism of soot wear. Considering abrasive wear, the mechanical properties of the SNPs, such as hardness and Young's modulus, can play an important role. Lahouij et al. [22] performed in situ indentation experiments in a TEM and made the following observations: diesel SNPs show elasto-plastic
∗
Corresponding author. E-mail address: [email protected] (F. Dassenoy). 1 Present address: Department of Materials and Environmental Chemistry, Stockholm University, 106 91 Stockholm, Sweden. https://doi.org/10.1016/j.triboint.2018.11.001 Received 9 August 2018; Received in revised form 27 October 2018; Accepted 1 November 2018 Available online 10 November 2018 0301-679X/ © 2018 Elsevier Ltd. All rights reserved.
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core of the primary SNPs were approximated with an ellipsoid, where the long and short diameters (d1 and d2 ) were measured manually. The diameter associated with the core was then calculated as the average of these. The thickness of the shell was measured at several radial locations, the mean value being reported. These parameters are illustrated in the supplementary document.
behavior under compression and are resistant to external load. Jao et al. [17] and Pavon [19] reported hardness values for diesel SNPs in the range of ≈7–14 GPa, which were calculated from the low-loss electron energy loss spectra (EELS) based on the method developed by Oleshko et al. [23]. In situ nanoindentation TEM was used by Jenei et al. [18] to measure hardness and the Young's modulus of individual diesel SNPs. The authors performed consecutive compressions on the nanoparticles and observed that the measured hardness values increased with compression cycle: 4.9 GPa during the first compression, and 12.3 GPa during the second compression. These results support the hypothesized abrasive wear mechanism. Uy et al. [2] characterized SNPs originating from diesel and gasoline turbocharged direct injection (GTDI) engines. In both cases, the SNPs were obtained from engine oil. The authors found that gasoline soot contained more non-carbonaceous elements than diesel soot; in particular, Zn, Ca, P and S quantities were several times higher. Zn, P and S originated from a well-known anti-wear additive, ZDDP. The soot containing these elements might indicate that the performance of antiwear additive is altered or hindered. The authors suggested that the increased amount of oil additive elements associated with gasoline soot particles could affect the wear mechanisms inside the engine. The objective of this article is to study the mechanical properties of SNPs originating from gasoline engine oils and compare them with their diesel counterparts. In order to quantify mechanical properties such as hardness and Young's modulus on the nanoscale, consecutive compressions were performed on individual SNPs in situ inside a TEM using a commercially available nanoindentation device. These properties were calculated based on TEM images recorded before and after each compression, video recordings of the compressions and force-displacement data. The methodology is described in more detail in Ref. [18]. Molecular dynamics simulations were performed to identify the contribution of each structural component of the SNPs on the abovementioned mechanical properties. Finally, the behavior of gasoline and diesel SNPs was compared. The mechanical properties, such as Young's modulus and hardness, were already studied previously using this method [18].
2.3. Picoindenter The compression tests were performed in situ, using a PI95 picoindenter manufactured by Hysitron, Inc. A schematic representation of the picoindenter and the experimental setup is shown in Fig. 1. The compression tests were performed in situ, inside the TEM with the diamond tip attached to the transducer of the picoindenter (see Fig. 1). The contact surface between the diamond tip and SNP was approximated by the area of a circular disk, the diameter of which could be measured from the compression video (as shown in Fig. 2). Based on the force-displacement curve and the estimated contact surface, the Young's modulus and the hardness could be calculated. A detailed description of these calculations and the compression experiments can be found in Ref. [18], where the method was applied to measure the Young's modulus and hardness of diesel SNPs. 2.4. Simulations Molecular dynamics (MD) simulations complemented the experimental measurements. A model SNP was constructed with an amorphous core and a graphitic shell. Nanoparticle properties, such as size, atomic composition, density etc., were set to resemble SNPs observed experimentally [4,5,22,25]. The model nanoparticle was subject to repeated compression cycles between two virtual walls at the top and the bottom of the particle. The bottom wall was fixed, while the upper wall moved downward with a constant speed of 40 m/s. The interactions between the walls and the atoms in the nanoparticle were modeled using a 9-3 Lenard-Jones potential with parameters ε = 0.043 eV, σ = 0.1 nm, and a 0.2 nm cutoff distance. The interactions between atoms within the SNP were modeled using the reactive force-field ReaxFF [26]. The compressions were simulated the following way: the upper wall was moved down until the SNP height decreased to 40% of the initial value and then unloading was performed until the net force acting on the upper wall became zero. All MD simulations were performed using the Large Atomic/Molecular Massively Parallel Simulation (LAMMPS) software [27]. During each compression cycle, the force-displacement diagram and hardness were recorded. Further details on the model SNP and the simulation protocol can be found in Ref. [18].
2. Material and methods 2.1. Soot nanoparticles SNPs were extracted from GTDI engines, see Table 1. The nanoparticles were collected from the engine oils by washing the oil with hexane through a centrifugation procedure, which is described in more detail in Ref. [7]. 2.2. TEM
3. Results and discussion
The compression tests were performed in situ inside an FEI Cs corrected environmental transmission electron microscope (ETEM) operated at 300 kV accelerating voltage. The TEM images were recorded using Gatan's OneView camera, the videos were recorded using Hysitron's screen capture software. The core-shell structure of primary SNPs was revealed previously in Refs. [2,4,5,7,17,24]. In the present study the core-shell structure of diesel and gasoline SNPs were characterized quantitatively. High resolution TEM images were used to measure the diameter of the core and the thickness of the shell of SNPs that possessed such structures. The
Four GTDI SNPs were selected for the compression experiments: G1A, G1B, G2C and G2D (Table 1). In the following, the results from two consecutive compressions of the G1A GTDI particle are presented. Fig. 3 shows TEM images of the nanoparticle before and after each consecutive compression. The compression videos can be seen at the following links: compression 1 and compression 2. The force-displacement data from the two consecutive compressions on particle G1A is displayed in Fig. 4, which shows that the particle exhibits elasto-plastic behavior. It can be observed that the slope of the curves is increasing with consecutive compressions. This slope is a measure of elasticity and so is subsequently referred to as the “elastic constant”. The size of the particle changed after each compression: the height (y-size) of the particle decreased significantly, while the width (x-size) remained relatively constant (see Table 2). The calculated Young's moduli and hardness values increased with each compression as well.
Table 1 Information about the soot nanoparticles used in the experiments. Sample ref.
Engine type
Soot source
Tested particles
G1 G2
GTDI GTDI
Vehicle test Engine dynamometer test
G1A, G1B G2C, G2D
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Fig. 1. Schematic representation of the experimental setup.
Fig. 2. A frame captured from a compression video showing the diameter of the circular disk, which is being, used for approximating the contact surface. The contour of the SNP is marked with red. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. The force-displacement data on the two consecutive compressions on particle G1A.
Fig. 3. TEM images of particle G1A before (a) and after each consecutive compression (b), (c). 448
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have higher Young's moduli values than diesel SNPs. Also, diesel SNPs appear to be harder than their GTDI counterparts. It should be noted that the Young's moduli and hardness values for the diesel SNPs were reported in Ref. [18]. Uy et al. performed ball-on-flat tribological tests with engine oils drained from diesel and GTDI engines and their supernatants [2]. Drained oil from diesel engine provided a larger wear scar on the ball compared to the wear scar obtained with the GTDI oil. The authors explained the larger wear by the fact that thermogravimetric analysis (TGA) indicated larger SNP content in the diesel drain. In the light of the results presented in this article (see Table 3 and Fig. 7), it can be argued that the hardness of the SNPs could also be an important factor in the wear mechanism: the harder diesel SNPs could contribute to larger wear scar diameters. To understand the origin of the difference between the two types of SNPs, consecutive compressions were simulated with the model SNP. The results of these simulations were similar to the experimental findings: the particle height and relative particle height decreased with consecutive compressions. Additional details are available in the supporting document. The force-displacement diagram obtained from the simulation indicated that the deformation of the model SNP contains both elastic and plastic components. Most importantly, the “elastic constant” of the model SNP increased with each compression cycle (consistent with experimental observations). This indicates that the model SNP and the simulation of the compression process are physically realistic. The simulations were also used to identify the relative contributions of the various components of the soot particle structure to changes in the elastic constant and hardness. To do this, four parts of the soot particle were isolated, as shown in Fig. 8. Model #1 is an amorphous
Table 2 The measured parameters on particle G1A.
x-size (nm) y-size (nm) “Elastic constant” (μN /nm) Young's modulus (GPa) Hardness (GPa)
Before
Compression 1
Compression 2
85.0 35.4 – – –
84.3 29.8 2.0 19.8 3.7
88.5 17.3 4.2 26.6 6.3
In summary, it can be observed that the particle exhibited elastoplastic behavior upon compression, it suffered permanent size-change and the Young's moduli and hardness values showed an increasing tendency with consecutive compressions. In order to confirm the behavior observed on particle G1A, three other GTDI SNPs were tested. Fig. 5 (a), (b) and (c) show the “elastic constant”, the y-size evolution and the relative y-size changes of consecutively compressed GTDI soot nanoparticles. The “elastic constant” increases with consecutive compressions for all of these particles. They also suffer permanent shape and size change (see Fig. 5 (b) and (c)). The calculated Young's moduli and hardness values can be seen in Fig. 6 (a) and (b). The same increasing tendencies can be observed here. On average, through consecutive compressions, the Young's modulus increased from 17.4 GPa to 27.9 GPa, and the hardness increased from 2.5 GPa to 4.9 GPa. The observed behavior of GTDI SNPs can be compared with their diesel counterparts. The average Young's moduli and hardness values for diesel and GTDI SNPs are shown in Fig. 7 (a) and (b) and in Table 3. Although the spread (the standard deviation) for the displayed values is large, our results indicate that, on average, GTDI soot nanoparticles
Fig. 5. The “elastic constant” (a), the y-size (b) and the relative y-size change (c) of the GTDI soot nanopartricles 449
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Fig. 6. The Young's modulus (a) and hardness (b) values of the GTDI soot nanoparticles.
Fig. 7. Average Young's moduli (a) and hardness (b) values for GTDI and diesel soot nanoparticles. The error bars represent standard deviations. Table 3 Average Young's modulus and hardness values for diesel and GTDI nanoparticles. The data for the diesel SNPs was reported in Ref. [18]. Diesel
Young's modulus (GPa) Hardness (GPa)
GTDI
Comp1
Comp2
Comp3
Comp1
Comp2
Comp3
8.2±1.8 4.9±1.9
20.9± 15.5 12.3±4.4
19.9±8.6 14.3±6.2
17.4± 4.8 2.5±1.0
26.8±3.0 4.8± 2.1
27.9± 13.4 4.9± 2.5
compressions. As can be seen in the upper right figure, there is a negligible gap between loading and unloading diagrams, which shows that the graphene layers arranged perpendicular to the compression direction exhibit a significant amount of elastic deformation. These results are in good agreement with recent experimental studies on lamellar carbon-graphene composites [28] showing a super-elastic behavior of samples composed of graphene layers with this orientation before compression. Further, comparing the results for Model #1 and #2 indicates that adding just a few layers of graphene to the amorphous structure increases the elastic behavior of the structure significantly. This is consistent with the results of previous nanoindentation tests on soot which showed the stored elastic energy increased with the thickness of the outer shell of the soot [29]. Fig. 10 shows the hardness of the four soot component models before compression and after each compression step. This figure indicates that the most significant change is exhibited by Models #1 and #4, which are the amorphous structure and graphene layers arranged parallel to the compression direction, respectively. These two models exhibit the largest amount of densification (plastic deformation) because their sp2 / sp3 ratio decreased continuously after each compression. The
structure, which represents the amorphous core of a soot particle. Model #2 is composed of parallel stacked graphitic layers normal to the compression direction, representative of the graphitic shell at the top or bottom of a particle. These two components are combined in Model #3, which is composed of two graphitic layers at the top and bottom and an amorphous center part with 2 nm height. Finally, Model #4 is comprised of stacked graphitic layers whose direction is perpendicular to that of Model #2 and represents material at the sides of a soot particle. The preparation procedure for these models was the same as that described in the Simulations section for the core-shell soot model or in Ref. [18]. However, to reduce computational cost, smaller models were used; the size of each model was 2.1 nm×2.1 nm×6.8 nm. The compression procedure was the same as described previously for the coreshell model. Fig. 9 shows the force-displacement results for the four soot component models. Models #1 and #4, which are the amorphous structure and the graphene layers parallel to the compression direction, exhibit considerable plastic deformation. However, after each compression step, the gap between the loading and unloading curve decreases, and the modeled structure shows more elastic behavior than in previous 450
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found in the supplementary document. Model #2 exhibits a negligible change in hardness and Model #3 exhibits behavior in between those of Models #1 and #2, as expected since it is comprised of both amorphous carbon and graphene components perpendicular to the compression direction. These results are consistent with previous experiments showing an increase of soot strength as the particle becomes more disordered [29]. In case of GTDI SNPs, those particles, which exhibited both core and shell structure, 22 were characterized quantitatively by analyzing the TEM images. The same measurements were performed on diesel SNPs originating from vehicle test. The results are summarized in Table 4, mean values and standard deviations are reported here. GTDI soot nanoparticles, that were selected for this study, have considerably thicker shells relative to their core than their diesel counterparts do. La Rocca et al. [5,30] have made similar observations. In Ref. [5], the authors describe a diesel SNP with a core diameter of 21 nm surrounded by a 5–9 nm thick shell, the shell being composed of graphitic platelets. In Ref. [30], the authors describe a gasoline SNP having a core of 25 nm diameter and an outer shell thickness of 16–20 nm. The former two observations show that the relative thickness of the shell (compared to the diameter of the core) is significantly larger in the case of gasoline SNPs compared to the diesel SNPs. The simulations suggest that, the graphitic component of the SNP is responsible for the elastic behavior (see the difference between Model #1 and #2). Based on this, it is expected that the GTDI SNPs would provide a more pronounced elastic response, compared to the diesel SNP. This, in fact, is confirmed by the compression experiments (see Fig. 7 and Table 3): on average, the Young's modulus is larger for the GTDI SNPs.
Fig. 8. Illustration of the four model systems used to capture the individual contributions of the various components of the soot particle to the change in elastic constant and hardness with compression.
decrease in sp2 / sp3 ratio and its association with increase in hardness was demonstrated in Ref. [18]. The formation of new bonds and increasing amount of sp3 hybridized carbon atoms can be viewed as an increasing number of diamond-like material, which leads to an increase in hardness. The evolution of the sp2 / sp3 ratio for all four models can be
4. Conclusions In situ compression tests were performed on SNPs originating from GTDI engines. During compression, the nanoparticles exhibited elastoplastic behavior, and they suffered permanent shape and size change. The calculated Young's moduli and hardness of the nanoparticles
Fig. 9. Force displacement data from simulations of compressions of individual components of the soot particle. 451
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Fig. 10. Hardness of the soot particle components as a function of compression cycle.
Appendix A. Supplementary data
Table 4 The quantitative morphological data on diesel and gasoline SNPs: mean values and standard deviation.
Average core diameter (nm) Average shell thickness (nm) Shell thickness/core diameter ratio
Diesel
GTDI
14.7±2.9 5.6±1.6 0.38±0.06
20.8± 7.7 10.9± 3.8 0.53± 0.08
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increased with consecutive compressions. The measured mechanical parameters of GTDI SNPs were compared with their diesel counterparts, and it was found that they exhibit different hardness and elasticity, which can affect the wear mechanism inside the engine. The hardness values reported here, are in the range of or higher than the hardness of the materials found in the contact zones within the cylinders of an internal combustion engine, which would make the abrasive wear plausible. Molecular dynamics simulations complemented the experimental measurements. The compression of the model SNP also resulted in elasto-plastic behavior, which indicates the model is physically realistic. Simulated consecutive compressions confirmed a permanent size change and an increasing elastic modulus. The contributions of the constituents of the soot model to the mechanical behavior of soot particles were also analyzed. The results showed that the graphene layers perpendicular to the compression direction are the major contributors to the elastic response of the particle. Further, the amorphous core can increase the hardness of the particle when exposed to compressive stress. Future work in this area can investigate the distance between graphene layers, SNPs without shell, primary and agglomerate sizes, and morphology of agglomerates.
Acknowledgments This work was supported by LABEX iMUST (ANR-10-LABX-0064) of Université de Lyon, within the program ‘Investissements d’Avenir’ (ANR-11-IDEX-0007) operated by the French National Research Agency. Thanks are also due to CLYM for access to the ETEM microscope. 452
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