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Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process
Journal Pre-proof Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process Piao Zhou, Tao Sun, Xunda Shi, Jun Li, Yongwei Zhu, Zikun Wang PII:
S0301-679X(19)30650-4
DOI:
https://doi.org/10.1016/j.triboint.2019.106136
Reference:
JTRI 106136
To appear in:
Tribology International
Received Date: 13 October 2019 Revised Date:
7 December 2019
Accepted Date: 22 December 2019
Please cite this article as: Zhou P, Sun T, Shi X, Li J, Zhu Y, Wang Z, Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process, Tribology International (2020), doi: https://doi.org/10.1016/j.triboint.2019.106136. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Author Contribution Statement Piao Zhou:
Conceptualization, Methodology, Software, Validation, Formal analysis,
Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization. Tao Sun: Conceptualization, Methodology, Writing - Review & Editing, Supervision. Xunda Shi: Writing - Review & Editing. Jun Li: Writing - Review & Editing. Yongwei Zhu*:
Conceptualization, Methodology, Resources, Writing - Review & Editing,
Supervision, Funding acquisition. Zikun Wang: Software.
Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process Piao Zhoua, Tao Sunb, d, Xunda Shic, Jun Lia, Yongwei Zhua,*, Zikun Wanga a
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, China b c
Zhuhai Qinsun Innovative Materials Co., Limited, Zhuhai 519000, China
GRINM Semiconductor Materials Co.,Ltd., Beijing 100088, China
d
College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science,
Shanghai 201620, China
Abstract The mechanical properties of mono-crystalline SiC with vacancy defects in fixed abrasive polishing processes are not well known at the nanometric scale. In the molecular dynamic (MD) simulation, the removal mechanism of mono-crystalline SiC substrates with vacancy defects is investigated. So is the wear mechanism of diamond abrasives explored. The simulation result reveals that the increase of vacancy defects in SiC substrates leads to reduced von Mises Stress, however, to increased temperature of SiC substrates during nano-abrading process. More vacancy defects are found to lead higher removal efficiency and less subsurface damage on SiC substrates. Furthermore, the diamond abrasives are worn out through a combination of thermo-chemical wear, graphitization wear and abrasive wear in the simulation. Keywords: Molecular dynamic simulation; SiC removal; Vacancy defect; Abrasive wear
1 Introduction SiC has attracted a lot of attention owing to its exceptional physical and chemical properties. As a promising third-generation wide-band-gap semiconductor material, SiC has found ever increasing applications as power electronic components in green energy industry[1].
Fixed
abrasive polishing has been explored as a high-efficiency planarization technology that affords SiC substrates a high quality surface with an excellent local and global flatness necessary for making electronic components. In a fixed abrasive polishing process for high surface quality, the SiC removal of the damaged surface needs to be conducted at a nano-metric scale. The Molecular Dynamics (MD) simulation has become a powerful tool for nano-metric scale research to
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researchers around the world. The removal mechanism of perfect SiC substrates, relating to abrasion temperature and structural stress, subsurface damage, brittle-ductility transformation, et.al., has been widely researched by using MD simulation. Xu[2] investigated the impact of strain rate and of abrasion heat on the removal behavior of mono-crystalline 3C-SiC by MD simulation, found that abrasion temperatures led to thermal softening of SiC surface, which resulted in lower yield stress and scratch hardness. As nano-abrasion speeds increase, the thermal softening and structural strain became even more pronounced. Also, Xu also reported perfect dislocations: Burgers vector 1/2<110>, Shockely partials with Burgers vector 1/6<112> and stair-rod partials with Burgers vector 1/6<110>, along the [100] direction on the (001) plane of SiC substrates. When MD
simulation was used to investigate the nucleation and propagation of dislocations during nanometric abrasion of brittle materials Si and SiC[3,4], dislocations and stacking faults on the abraded substrate surface were surprisingly found to be anisotropic, and the brittle-ductile transformation was driven by amorphization at the initial stage of the nano-abrasion process. Zhang[5]’ MD simulations concluded that structural transformation and the dislocation migration allowed brittle 6H-SiC substrates to be nano-metrically processed in ductile-regime.
A critical stress for
dislocation nucleation was reported to decrease with abrasion temperature increase.
Luo[6]
performed MD simulation on the deformation of a Si substrate related to abrasion temperatures during a nano-abrasion process, and found that the von Mises stress decreased due to reduced abrasion resistance caused by the higher abrasion temperature. In Fang's report[7], silicon ion implantations on mono-crystalline Si substrates led to lower abrasion force and less shearing stress than those on pristine Si substrates. In fixed abrasive polishing, diamond particles function as abrasives to abrade the super-hard SiC materials, and are worn down during the abrading process. Tang[8] studied the diamond abrasive wear on 3C-SiC crystals by MD simulation, and had, not surprisingly, found that defects on abrasives led to more severe wear of diamond abrasives. Graphitization of diamond abrasives were discovered in Luo's MD simulation reports[9,10] during a single point diamond turning of mono-crystalline SiC and Si through radial distribution function analysis. Graphitization, carbon diffusion and oxidation reaction were attributed to be the major culprits for diamond tool wear in nano-abrasion both by MD simulation and experiments[11]. The above mentioned extensive researches have contributed greatly to our understanding on
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the removal mechanism of substrates and wear mechanism of diamond tool during nano-abrasion processes. During SiC crystal growth and substrate processing, various atomic-scale defects[12-
17], including structural vacancy, atomic dislocations and voids, are generated. Therefore, to understand the impact of atomic-scale defects on the mechanical and chemical behavior of SiC materials becomes critical for future process developments. The mechanical behavior of materials with voids had been widely studied by researchers using MD simulation[18-20]. However, SiC substrates with structural vacancy defects have not been investigated for their processing characteristics during the nano-abrasion process. In this paper, the removal mechanism of monocrystalline 3C-SiC with vacancy defects at 0%, 0.1%, 0.5%, 0.9% and 1.3% in the machining region is subjected to MD simulations in a nano-abrasion process. The impact of vacancy concentration on the abrasion temperature, yield stress, and subsurface damage on monocrystalline SiC is analyzed. A detailed analysis of the brittle-ductility transformation based on dislocation theory in the mono-crystalline SiC is conducted. So do we study the wear mechanism of diamond abrasives for the purposes of improving the life span of fixed diamond abrasive pads.
2 Molecular Dynamic simulation
2.1 MD simulation model In the MD model, mono-crystalline silicon carbide substrates and mono-crystalline diamond particles are taken as the substrate and abrasive respectively. To assess the wearing process of abrasives, the diamond abrasive is considered deformable. The MD simulation models of the substrate and of the abrasive are illustrated in Fig. 1a, where the substrate and abrasive particles are categorized into three layers: a boundary layer, a thermostatic layer and a newtonian layer. To maintain the structural integrity of crystal lattices, the boundary layer is fixed and maintained unchanged throughout the simulation. The thermostatic layer is taken as a heat exchange zone, and the newtonian layer is the abrasion region where a nano-abrasion takes place. The lattice constants of diamond abrasives and of SiC substrates used in this study are 3.57 Å and 4.348 Å respectively[21], shown in Fig. 1c and d. The SiC substrate with zincblende structure oriented X=[100], Y=[010], Z=[001]. Periodic boundary conditions are respectively used in the x, y and z direction. The canonical ensemble (nvt) and micro-canonical ensemble (nve) are applied in the relaxation stage and in the
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abrasion process respectively. ABOP potential was adopted as the potential scheme for Si–Si, C– C and Si–C interactions in the whole MD system[22]. In the simulation system, the nanoscratching depth is fixed at 2.6nm, and the nano-scratching velocity is set at 60m/s. Detailed parameters in the current simulation system are listed in Table 1. The removal process of SiC in nano-metric abrasion is calculated by using a large-scale atomic/molecular massively parallel simulator (LAMMPS) software. The OVITO software is used to visualize and analyze the MD results from LAMMPS, and the dislocation extraction algorithm (DXA) is selected as a aid to investigate the dislocation nucleation and propagation[23,24]. The vacancy defects would have little effect on the material removal mechanism should they exist in the unmachined region. Also, it is not necessary to study the mechanical behavior of SiC substrates with large vacancy concentration due to its high quality requirement for substrate materials and semiconductor industries. So, for our research purposes to be achievable, the vacancy defects with low density (0.1%, 0.5%, 0.9%, and 1.3%) are intentionally constricted in the processing area of SiC substrates by randomly removing Si and C atoms from a defect-free SiC substrate (Fig. 1b). The dimension of processing area is 80×199.052×40Å (X, Y, and Z directions, respectively).
The vacancy defect distributions on SiC substrates at different
concentrations are presented in Fig. 2. It is found that vacancy defects almost scatter the entire machining region of mono-crystalline SiC substrates when the vacancy concentration are higher than 0.5%, which is expected to significant impact on the physical and chemical properties of SiC substrates.
2.2 Analysis methods Intensity variation of radial distribution function of atom-pairs after nano-abrasion has been closely related to structural transformation of SiC substrates and graphitization of diamond abrasives[9,25], and is used in this study to do the same. The subsurface damage is referred to the distance from the top base plane surface to the end of structural transformation and dislocation lines on the SiC substrates. The removed atoms are defined as atoms debris residing half a lattice height higher than the base plane surface of the substrates[26]. The coordination number, which is often selected as an auxiliary means to analyze the crystal structure, referring to the number of atoms in the nearest neighbor around an atom, is 4 and 3 for carbon atoms in diamonds and in graphites, respectively. The abrasion temperature and yield stress of processing area are analyzed
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by matlab, using data derived from the MD simulation. The spacing average method is applied in the temperature and stress distribution of diamond abrasives. In addition, the temperature and von Mises stress is averaged in the stable processing stage to quantitative investigate the effect of vacancy concentration on the variety of stress and temperature of the entire SiC substrates. The structural transformation of the substrates is affected by yield stress. Material dependent yielding criteria of the von Mises stress is a term commonly used to predict the yielding of a material. The stress state of single atoms is calculated by virial stress. The expression of virial stress is[27]:
σ αβ (i ) = where
1 NS
is the mass of the ith atom,
in the area of S,
miν iαν iβ 1 ∑i V + 2V i i
∑ Fij j
xijα xijβ rij
is the volume of the ith atom,
is the distance between the ith and the jth atom,
between the ith and the jth atom,
(1) is the number of atoms
is the direction vector of m
is the direction vector of m between the ith and the jth atom.
Based on the results of virial stress, the von Mises stress can be calculated as follows[28]:
[
(
)]
1 2 2 2 σ vm (i ) = (σ xx (i ) − σ yy (i )) + (σ yy (i ) − σ zz (i )) + (σ zz (i ) − σ xx (i )) + 6 σ xy2 (i ) + σ zy2 (i ) + σ xz2 (i ) 2 (2)
1/ 2
During the abrading process, heat is inevitably generated by the deformation and friction, and leads to the rise of local temperature in the abrasion area. The thickness of a soft layer on the substrates surface is produced by the reaction between slurry and substrates and is affected by the local abrasion temperature in a fixed-abrasived nano-abrasion.
The equation of abrasion
temperature in MD simulation can be expressed as[28]:
1 3 mi vi2 = Nk bT ∑ 2 i 2
(3) (4)
In which N is the number of atoms in the setting area, kinetic energy of atoms, the temperature of atoms.
is the velocity of ith atom,
is the
is the Boltzmann constant, and the value is 1.3806503×1023J/K, T is
3 Results and discussion
3.1 Temperature and stress To quantify the abrasion heat and stress during nano-abrasion in presence crystal vacancy
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defects, the average temperature and von Mises stress of SiC substrates in the stable processing stage are extracted and plotted out in Fig. 3. As the vacancy concentration in SiC substrates increases during the abrasion process, the resulting von Mises stress decreases from abrasion (Fig. 3), which is in a good agreement with results with Luo[29]’reports, who found that the yield strength and the vacancy defect concentration followed a power-law
σ /σ
= (c /c )
relationship, in which σ is the yield strength, c is the vacancy concentration：an increase of vacancy concentration leads to decrease in the yield strength of materials. The temperature on the SiC substrate, on the other hand, steadily rises higher as the vacancy defects are inching up (Fig. 3). The temperature of the processing area is related to the total amount of materials removed. As will be discussed in the later section, the increase of vacancy defects leads to the increase of material removal rate (Fig. 7), which will contribute to a higher temperature on processing defected SiC substrates.
One has to keep in mind, the rising
temperature will also lead to lower yield stress, so the observed lower yield in presence of high vacancy defects are a double-dip of vacancy defects and higher substrate temperature. Existence of vacancy defects complicates stress yield of defected substrates in comparison to that of a defect-free crystal. It is explained that vacancy defects slow down the dislocation nucleation, which, in turn, contribute to reduce yield strength (von Mises stress). Cutting force and normal force derived from nano-abrading MD simulation (Fig. 4), decreasing with the increase of vacancy concentration in the machining area of substrates, from another angle, confirm the negative impact of vacancy defects on yield stress.
3.2 Removal To obtain better insights about structural changes of materials under abrasion, intensity of atom pair separation (bond) distances at 1.884, 3.066, 3.628, and 4.359Å, corresponding to Si-C, Si-Si/C-C, Si-C, and Si-Si in mono-crystalline SiC, is calculated and compared in Fig. 5 before and after the vacancy defect filled SiC being abraded. Clearly the peak intensity of atom pairs at 1.884, 3.066, 3.628 and 4.359Å decreases after nanometric abrasion of mono-crystalline SiC, indicating partial lattice atom migration and partial crystal structure deformation during the abrading process, that is the occurrence of structural transformation. It is found from Fig. 6 that amorphous structure is present in the chips and around the surface of substrates, and the structure of SiC substrates in the inner region is unchanged, suggesting that the extrusion of diamond
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abrasives only affect the machining region of SiC substrates during the abrading process. The removed atoms are defined to be the atoms residing half a lattice height higher than the base plane surface of the substrates in our MD simulation. The number of removed atoms with vacancy concentration from 0% to 1.3% in mono-crystalline SiC at the abrasion position of 10.8nm, 12.6nm, 14.4nm, 16.2nm and 18.0nm are presented in Fig. 7. The removal efficiency under the same abrasion conditions is lowest as the SiC substrate is perfect without defects. The tendency is present that the material removal rate increases with the increase of the vacancy concentration, although the relationship between material removal rate and vacancy concentration in some abrading position is non monotonic. The distribution of vacancy defect is random. It is speculative that the concentration of vacancy defect is not monotonic when the SiC substrate is processed in the same scratching distance in all the simulated cases. So, the variety is different in different scratching distance, shown in Fig. 7. However, the removal rate of SiC substrates increases with a larger increase of vacancy concentration from 0% to 0.5% to 1.3%. A large increase of vacancy concentration decreases the non monotonic effect in the same scratching distance. Therefore, the tendency is present that the material removal rate increases with the increase of the vacancy concentration. The presence of high vacancy defects in mono-crystalline SiC increases the removal efficiency because of the low yield stress and high abrading temperature during the abrading process.
3.3 Subsurface damage Snapshots of the subsurface damage and dislocation distribution on SiC substrates from the MD simulation are presented in Fig. 8 at the abrading distance of 18nm with vacancy defects from 0% to 1.3%. There exist two types of dislocations (Fig. 8(a1)-(a3)) on the abraded SiC substrates: perfect dislocations with 1/2<110> Burgers vector and other dislocations.
Both types of
dislocations are lying within the bottom section of the subsurface damage layer, which are consistent with the experimental observations in a nano-abrasion process[30-32]. The dislocation density and length are lower in vacancy-defect SiC substrates than defect-free SiC substrates （Fig. 8), suggesting that the presence of vacancy defects hinders the dislocation nucleation and promotes the dislocation annihilation. To quantify the effect of vacancy concentration on the subsurface damage, the elastic deformation layer of substrates is ignored due to its less thickness than subsurface damage layer. The subsurface damage is defined to be the distance from the top
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plane surface to the end of structural transformation and dislocation lines on the SiC substrate, shown in Fig. 8 (b1)-(b3). The quantitative influence of vacancy concentration on the subsurface damage can be found in Fig. 9 where the abrasive has been advanced by 18nm：the more the vacancy defect, the less subsurface damage in the machining area of mono-crystalline SiC. Such a result can be attributed to that a large number of vacancies are accumulated in the processing area of the substrates, and hinders the dislocation nucleation and promotes the dislocation annihilation. Thus, the defects suppress the propagation of subsurface damages to a deeper region of substrates.
3.4 Wear of diamond abrasive Not only are the removal behaviors affected by the vacancy concentration in the SiC substrates, but also by the wear of abrasives. A better understanding about the wear mechanism of an abrasive will contribute for better quality of processed substrate surface and for an improved life cycle of a diamond polishing pad. It is not difficult to speculate that the abrasive compression between SiC and diamond will lead to deformation and wear of diamond abrasives. In order to investigate the structural change of diamond abrasives, the radial distribution functions (RDF) of carbon atom pairs in diamond abrasive are calculated and overlapped in Fig. 10 before and after nano-metric abrading simulation. In graphites, the bond distance between two SP2 carbon atoms and bond angle of three adjacent atoms on the same graphite layer are 0.142nm and 120°, respectively. The bond length and bond angle of the nearest neighboring carbon atoms are 1.54 Å and 109.5°, respectively, in diamond abrasives. To be noted in the MD simulation, a small deviation of the pair separation (bond) distance with ±0.1 Å and bond angle with ±2.5° at each peak are caused by energy minimization process. Intensities of pair separation distances between two SP3 carbon atoms in diamonds decrease at 0.152nm while increase at 0.146nm after the abrading process (Fig. 10), suggesting a graphitization on diamond surface. To better illustrate the effect of vacancy defects of SiC on the graphitization of diamond during the abrading process, the intensities of pair separation distances at 0.146nm and 0.152nm are extracted from RDF curve of a diamond before and after the abrading process (Fig. 11a). Very clearly shown in Fig. 11a, the intensity of a pair separation distance at 0.146nm basically decreases with the increase of the vacancy concentration in the machining region of SiC substrates, a result of less strain stress being applied to the surface of diamond abrasives. The bond angle distribution profile of diamond abrasives is presented in Fig. 11b. Consistent with the observation on atom pair distance at 0.146
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nm (sign of graphitization), the counts of bond angle decrease at 122.5° as the vacancy concentration increases in the machining region of SiC substrates. In summary, the structural transformation from a perfect diamond structure to graphite structure decreases as the vacancy concentration of SiC substrates increases from 0% to 1.3%. High temperature and high stress have been attributed to be the two main factors that cause the diamond-graphite transformation[33-35]. The graphitization wear is mainly affected by the shear stress as the abrasion temperature in diamond abrasive is lower than the Debye temperature (1910K).
The critical shear stress is 95 GPa for diamond-to-graphitization transition[36].
According to the abrading result (Fig. 13b), the abrasion temperature of diamond abrasive is hard to rise up to 1910K.
It is not enough to cause a large area of graphitization by abrasion
temperature in our simulation. It is illustrated in fig. 13d that the highest local shear stress in abrasive edge is higher than the critical shear with 95 GPa for diamond-to-graphitization transition. Fig. 12a graphically elucidates the morphological evolution of a diamond abrasive particle as the diamond particle moves along the abrasion direction, and a severe wear（fall-offs along the trail of abrasions) of a diamond abrasive is observed by nanometric abrading the SiC material. In addition, it can be seen that the size of the diamond abrasive reduced to 7.5 nm from 8.0 nm after nano-abrading process, clearly indicating the passivation of abrasive tip which is the main performance of attrition wear (Fig. 12a). There are a large number of carbon atoms breaking-off from diamond abrasives, which agrees with the characteristics of abrasive wear during the ploughing process[37]. The coordination number of a carbon atom in diamond and graphite is 4 and 3, respectively. As noted in Fig. 12b, graphitized SP2 (three coordinated) atoms can be seen on the distorted diamond particle surface and in the wear-off dusts. Furthermore, the single 6-ring structure which is similar to the 6-ring structure in graphite is found in the diamond abrasive. The interaction between SiC substrates and diamond abrasives which is shown in Fig. 13a leads to the variety of the temperature and stress of the system. The distribution of temperature and of stress on the diamond abrasive is investigated.
As mapped in Fig. 13, the highest
temperature, highest von Mises stress and highest shear stress all are distributed on the abrasion edge of the diamond abrasives, where a diamond-graphite transformation was observed during the abrading process. Thermal-chemical wear is mainly affected by temperature induced by the friction between substrates and abrasives. The temperature at the abrasive edge with about 800 K
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is sufficient to weaken the C-C coherent bond in the diamond abrasive, making atoms to get separated from the tool[38].
This indicates that thermal chemical wear occur in diamond
abrasives during nano-abrading process. In summaries, thermo-chemical wear, graphitized wear and abrasive wear are the basic mechanisms for diamond abrasives in pads during nano-abrading process (Fig. 14).
4 Summary MD simulation analysis was conducted to investigate the influence of vacancy defects on the mechanical behaviors of mono-crystalline SiC and the wear properties of diamond abrasives in a two-body nano-metric abrading process. The detailed results can be summarized as follows: As the vacancy concentration in the machining area of mono-crystalline SiC increases, the temperature increases while the von Mises stress decreases, respectively. The removal efficiency on a defect-free substrate is the lowest. The higher vacancy defect concentration in the machining area, the easier to be removed the surface atoms on the abraded substrate. A decreased yield strength resulting from increased vacancy defects in the monocrystalline SiC reduces the abrasion resistance during nano-metric abrading process. Dislocations produced from nano-abrading are found present in the subsurface damage layer. The depth of subsurface damage layer decreases with the increase of vacancy concentration in the machining region of mono-crystalline SiC substrates. Graphitization of carbon atoms in the diamond abrasive decrease as the vacancy concentration in the machining region of SiC substrates increases from 0% to 1.3%. Graphitized wear, abrasive wear, and thermal-chemical wear of abrasive particles are observed during the abrading process. The higher the vacancy defect concentration in SiC substrates, the less the graphitization wear on the diamond abrasive surface.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 51675276) and Jiangsu Province Key Laboratory of Precision and Micro-manufacturing Technology.
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（c）
（d）
Fig. 1. (a) the MD simulation model (b) vacancy area (c) the cell of diamond (d) the cell of mono-crystalline SiC
Fig. 2. Crystal structure of mono-crystalline SiC with different vacancy concentration of (a) 0% (b) 0.1% (c)0.5% (d)0.9% (e)1.3%
13
Fig. 3. Diagrams of average von Mises stress and average temperature of SiC substrates at different vacancy concentration
Fig. 4. The abrasion force curve at different vacancy concentration
14
Fig. 5. RDF of SiC substrates before after nano-abrasion
Fig. 6.Thestructural transformation of SiC substrates after abrasion
15
Fig. 7. The removed atoms of SiC substrates at different vacancy concentration
Fig. 8. The surface topography and subsurface damage of SiC substrates at different vacancy concentration (a1)-(a3) dislocation distribution (b1)-(b3) subsurface damage 16
Fig. 9. The subsurface damage depth at different vacancy concentration
Fig. 10. RDF of diamond abrasive before after abrading
17
Fig. 11. Graphitization (a) RDF at bond distance of 0.146nm and 0.152nm (b) The Bond angle of diamond at 122.5o for different vacancy concentration after abrading process
(a)
(b) Fig. 12. The deformation and wear of diamond abrasive at different abrading distance (a) passivation (b) graphitization
18
Fig. 13. Distribution of temperature and von Mises stress of diamond abrasive after nano-cutting (a) interaction of SiC substrate and diamond abrasive (b) temperature distribution (c) von Mises stress distribution (d) shear stress distribution
Fig. 14. Wear types of diamond abrasives
19
Table 1 Parameters of molecular dynamics simulation Configuration
Abrading
Substrates
Mono-crystalline Silicon carbide
Abrasive particle
Diamond abrasive
Vacancy concentration in machining area
0% 0.1% 0.5% 0.9% 1.3%
Substrates dimensions
152.18*213.052*91.308 Å
Abrasive radius
4 nm
abrasion depth
2.6 nm
Abrading velocity
60 m/s
Potential function
ABOP
Timestep
0.001ps
20
Highlights The influence of vacancy concentration on removal mechanisms of SiC substrates is explored by MD simulation in nano-abrasion. The wear mechanism of diamond abrasive is studied. The presence of vacancy defects in SiC substrates reduces the von Mises Stress and increses the abrading temperature. Removal efficiency increases and subsurface damage depth decreases, respectively, with increasing the vacancy concentration in the machining area of SiC substrates. The wear of diamond abrasives is induced by thermo-chemical wear, abrasive wear and graphitized wear.
Declaration of interests √The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0301-679X(19)30650-4
DOI:
https://doi.org/10.1016/j.triboint.2019.106136
Reference:
JTRI 106136
To appear in:
Tribology International
Received Date: 13 October 2019 Revised Date:
7 December 2019
Accepted Date: 22 December 2019
Please cite this article as: Zhou P, Sun T, Shi X, Li J, Zhu Y, Wang Z, Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process, Tribology International (2020), doi: https://doi.org/10.1016/j.triboint.2019.106136. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Author Contribution Statement Piao Zhou:
Conceptualization, Methodology, Software, Validation, Formal analysis,
Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization. Tao Sun: Conceptualization, Methodology, Writing - Review & Editing, Supervision. Xunda Shi: Writing - Review & Editing. Jun Li: Writing - Review & Editing. Yongwei Zhu*:
Conceptualization, Methodology, Resources, Writing - Review & Editing,
Supervision, Funding acquisition. Zikun Wang: Software.
Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process Piao Zhoua, Tao Sunb, d, Xunda Shic, Jun Lia, Yongwei Zhua,*, Zikun Wanga a
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, China b c
Zhuhai Qinsun Innovative Materials Co., Limited, Zhuhai 519000, China
GRINM Semiconductor Materials Co.,Ltd., Beijing 100088, China
d
College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science,
Shanghai 201620, China
Abstract The mechanical properties of mono-crystalline SiC with vacancy defects in fixed abrasive polishing processes are not well known at the nanometric scale. In the molecular dynamic (MD) simulation, the removal mechanism of mono-crystalline SiC substrates with vacancy defects is investigated. So is the wear mechanism of diamond abrasives explored. The simulation result reveals that the increase of vacancy defects in SiC substrates leads to reduced von Mises Stress, however, to increased temperature of SiC substrates during nano-abrading process. More vacancy defects are found to lead higher removal efficiency and less subsurface damage on SiC substrates. Furthermore, the diamond abrasives are worn out through a combination of thermo-chemical wear, graphitization wear and abrasive wear in the simulation. Keywords: Molecular dynamic simulation; SiC removal; Vacancy defect; Abrasive wear
1 Introduction SiC has attracted a lot of attention owing to its exceptional physical and chemical properties. As a promising third-generation wide-band-gap semiconductor material, SiC has found ever increasing applications as power electronic components in green energy industry[1].
Fixed
abrasive polishing has been explored as a high-efficiency planarization technology that affords SiC substrates a high quality surface with an excellent local and global flatness necessary for making electronic components. In a fixed abrasive polishing process for high surface quality, the SiC removal of the damaged surface needs to be conducted at a nano-metric scale. The Molecular Dynamics (MD) simulation has become a powerful tool for nano-metric scale research to
1
researchers around the world. The removal mechanism of perfect SiC substrates, relating to abrasion temperature and structural stress, subsurface damage, brittle-ductility transformation, et.al., has been widely researched by using MD simulation. Xu[2] investigated the impact of strain rate and of abrasion heat on the removal behavior of mono-crystalline 3C-SiC by MD simulation, found that abrasion temperatures led to thermal softening of SiC surface, which resulted in lower yield stress and scratch hardness. As nano-abrasion speeds increase, the thermal softening and structural strain became even more pronounced. Also, Xu also reported perfect dislocations: Burgers vector 1/2<110>, Shockely partials with Burgers vector 1/6<112> and stair-rod partials with Burgers vector 1/6<110>, along the [100] direction on the (001) plane of SiC substrates. When MD
simulation was used to investigate the nucleation and propagation of dislocations during nanometric abrasion of brittle materials Si and SiC[3,4], dislocations and stacking faults on the abraded substrate surface were surprisingly found to be anisotropic, and the brittle-ductile transformation was driven by amorphization at the initial stage of the nano-abrasion process. Zhang[5]’ MD simulations concluded that structural transformation and the dislocation migration allowed brittle 6H-SiC substrates to be nano-metrically processed in ductile-regime.
A critical stress for
dislocation nucleation was reported to decrease with abrasion temperature increase.
Luo[6]
performed MD simulation on the deformation of a Si substrate related to abrasion temperatures during a nano-abrasion process, and found that the von Mises stress decreased due to reduced abrasion resistance caused by the higher abrasion temperature. In Fang's report[7], silicon ion implantations on mono-crystalline Si substrates led to lower abrasion force and less shearing stress than those on pristine Si substrates. In fixed abrasive polishing, diamond particles function as abrasives to abrade the super-hard SiC materials, and are worn down during the abrading process. Tang[8] studied the diamond abrasive wear on 3C-SiC crystals by MD simulation, and had, not surprisingly, found that defects on abrasives led to more severe wear of diamond abrasives. Graphitization of diamond abrasives were discovered in Luo's MD simulation reports[9,10] during a single point diamond turning of mono-crystalline SiC and Si through radial distribution function analysis. Graphitization, carbon diffusion and oxidation reaction were attributed to be the major culprits for diamond tool wear in nano-abrasion both by MD simulation and experiments[11]. The above mentioned extensive researches have contributed greatly to our understanding on
2
the removal mechanism of substrates and wear mechanism of diamond tool during nano-abrasion processes. During SiC crystal growth and substrate processing, various atomic-scale defects[12-
17], including structural vacancy, atomic dislocations and voids, are generated. Therefore, to understand the impact of atomic-scale defects on the mechanical and chemical behavior of SiC materials becomes critical for future process developments. The mechanical behavior of materials with voids had been widely studied by researchers using MD simulation[18-20]. However, SiC substrates with structural vacancy defects have not been investigated for their processing characteristics during the nano-abrasion process. In this paper, the removal mechanism of monocrystalline 3C-SiC with vacancy defects at 0%, 0.1%, 0.5%, 0.9% and 1.3% in the machining region is subjected to MD simulations in a nano-abrasion process. The impact of vacancy concentration on the abrasion temperature, yield stress, and subsurface damage on monocrystalline SiC is analyzed. A detailed analysis of the brittle-ductility transformation based on dislocation theory in the mono-crystalline SiC is conducted. So do we study the wear mechanism of diamond abrasives for the purposes of improving the life span of fixed diamond abrasive pads.
2 Molecular Dynamic simulation
2.1 MD simulation model In the MD model, mono-crystalline silicon carbide substrates and mono-crystalline diamond particles are taken as the substrate and abrasive respectively. To assess the wearing process of abrasives, the diamond abrasive is considered deformable. The MD simulation models of the substrate and of the abrasive are illustrated in Fig. 1a, where the substrate and abrasive particles are categorized into three layers: a boundary layer, a thermostatic layer and a newtonian layer. To maintain the structural integrity of crystal lattices, the boundary layer is fixed and maintained unchanged throughout the simulation. The thermostatic layer is taken as a heat exchange zone, and the newtonian layer is the abrasion region where a nano-abrasion takes place. The lattice constants of diamond abrasives and of SiC substrates used in this study are 3.57 Å and 4.348 Å respectively[21], shown in Fig. 1c and d. The SiC substrate with zincblende structure oriented X=[100], Y=[010], Z=[001]. Periodic boundary conditions are respectively used in the x, y and z direction. The canonical ensemble (nvt) and micro-canonical ensemble (nve) are applied in the relaxation stage and in the
3
abrasion process respectively. ABOP potential was adopted as the potential scheme for Si–Si, C– C and Si–C interactions in the whole MD system[22]. In the simulation system, the nanoscratching depth is fixed at 2.6nm, and the nano-scratching velocity is set at 60m/s. Detailed parameters in the current simulation system are listed in Table 1. The removal process of SiC in nano-metric abrasion is calculated by using a large-scale atomic/molecular massively parallel simulator (LAMMPS) software. The OVITO software is used to visualize and analyze the MD results from LAMMPS, and the dislocation extraction algorithm (DXA) is selected as a aid to investigate the dislocation nucleation and propagation[23,24]. The vacancy defects would have little effect on the material removal mechanism should they exist in the unmachined region. Also, it is not necessary to study the mechanical behavior of SiC substrates with large vacancy concentration due to its high quality requirement for substrate materials and semiconductor industries. So, for our research purposes to be achievable, the vacancy defects with low density (0.1%, 0.5%, 0.9%, and 1.3%) are intentionally constricted in the processing area of SiC substrates by randomly removing Si and C atoms from a defect-free SiC substrate (Fig. 1b). The dimension of processing area is 80×199.052×40Å (X, Y, and Z directions, respectively).
The vacancy defect distributions on SiC substrates at different
concentrations are presented in Fig. 2. It is found that vacancy defects almost scatter the entire machining region of mono-crystalline SiC substrates when the vacancy concentration are higher than 0.5%, which is expected to significant impact on the physical and chemical properties of SiC substrates.
2.2 Analysis methods Intensity variation of radial distribution function of atom-pairs after nano-abrasion has been closely related to structural transformation of SiC substrates and graphitization of diamond abrasives[9,25], and is used in this study to do the same. The subsurface damage is referred to the distance from the top base plane surface to the end of structural transformation and dislocation lines on the SiC substrates. The removed atoms are defined as atoms debris residing half a lattice height higher than the base plane surface of the substrates[26]. The coordination number, which is often selected as an auxiliary means to analyze the crystal structure, referring to the number of atoms in the nearest neighbor around an atom, is 4 and 3 for carbon atoms in diamonds and in graphites, respectively. The abrasion temperature and yield stress of processing area are analyzed
4
by matlab, using data derived from the MD simulation. The spacing average method is applied in the temperature and stress distribution of diamond abrasives. In addition, the temperature and von Mises stress is averaged in the stable processing stage to quantitative investigate the effect of vacancy concentration on the variety of stress and temperature of the entire SiC substrates. The structural transformation of the substrates is affected by yield stress. Material dependent yielding criteria of the von Mises stress is a term commonly used to predict the yielding of a material. The stress state of single atoms is calculated by virial stress. The expression of virial stress is[27]:
σ αβ (i ) = where
1 NS
is the mass of the ith atom,
in the area of S,
miν iαν iβ 1 ∑i V + 2V i i
∑ Fij j
xijα xijβ rij
is the volume of the ith atom,
is the distance between the ith and the jth atom,
between the ith and the jth atom,
(1) is the number of atoms
is the direction vector of m
is the direction vector of m between the ith and the jth atom.
Based on the results of virial stress, the von Mises stress can be calculated as follows[28]:
[
(
)]
1 2 2 2 σ vm (i ) = (σ xx (i ) − σ yy (i )) + (σ yy (i ) − σ zz (i )) + (σ zz (i ) − σ xx (i )) + 6 σ xy2 (i ) + σ zy2 (i ) + σ xz2 (i ) 2 (2)
1/ 2
During the abrading process, heat is inevitably generated by the deformation and friction, and leads to the rise of local temperature in the abrasion area. The thickness of a soft layer on the substrates surface is produced by the reaction between slurry and substrates and is affected by the local abrasion temperature in a fixed-abrasived nano-abrasion.
The equation of abrasion
temperature in MD simulation can be expressed as[28]:
1 3 mi vi2 = Nk bT ∑ 2 i 2
(3) (4)
In which N is the number of atoms in the setting area, kinetic energy of atoms, the temperature of atoms.
is the velocity of ith atom,
is the
is the Boltzmann constant, and the value is 1.3806503×1023J/K, T is
3 Results and discussion
3.1 Temperature and stress To quantify the abrasion heat and stress during nano-abrasion in presence crystal vacancy
5
defects, the average temperature and von Mises stress of SiC substrates in the stable processing stage are extracted and plotted out in Fig. 3. As the vacancy concentration in SiC substrates increases during the abrasion process, the resulting von Mises stress decreases from abrasion (Fig. 3), which is in a good agreement with results with Luo[29]’reports, who found that the yield strength and the vacancy defect concentration followed a power-law
σ /σ
= (c /c )
relationship, in which σ is the yield strength, c is the vacancy concentration：an increase of vacancy concentration leads to decrease in the yield strength of materials. The temperature on the SiC substrate, on the other hand, steadily rises higher as the vacancy defects are inching up (Fig. 3). The temperature of the processing area is related to the total amount of materials removed. As will be discussed in the later section, the increase of vacancy defects leads to the increase of material removal rate (Fig. 7), which will contribute to a higher temperature on processing defected SiC substrates.
One has to keep in mind, the rising
temperature will also lead to lower yield stress, so the observed lower yield in presence of high vacancy defects are a double-dip of vacancy defects and higher substrate temperature. Existence of vacancy defects complicates stress yield of defected substrates in comparison to that of a defect-free crystal. It is explained that vacancy defects slow down the dislocation nucleation, which, in turn, contribute to reduce yield strength (von Mises stress). Cutting force and normal force derived from nano-abrading MD simulation (Fig. 4), decreasing with the increase of vacancy concentration in the machining area of substrates, from another angle, confirm the negative impact of vacancy defects on yield stress.
3.2 Removal To obtain better insights about structural changes of materials under abrasion, intensity of atom pair separation (bond) distances at 1.884, 3.066, 3.628, and 4.359Å, corresponding to Si-C, Si-Si/C-C, Si-C, and Si-Si in mono-crystalline SiC, is calculated and compared in Fig. 5 before and after the vacancy defect filled SiC being abraded. Clearly the peak intensity of atom pairs at 1.884, 3.066, 3.628 and 4.359Å decreases after nanometric abrasion of mono-crystalline SiC, indicating partial lattice atom migration and partial crystal structure deformation during the abrading process, that is the occurrence of structural transformation. It is found from Fig. 6 that amorphous structure is present in the chips and around the surface of substrates, and the structure of SiC substrates in the inner region is unchanged, suggesting that the extrusion of diamond
6
abrasives only affect the machining region of SiC substrates during the abrading process. The removed atoms are defined to be the atoms residing half a lattice height higher than the base plane surface of the substrates in our MD simulation. The number of removed atoms with vacancy concentration from 0% to 1.3% in mono-crystalline SiC at the abrasion position of 10.8nm, 12.6nm, 14.4nm, 16.2nm and 18.0nm are presented in Fig. 7. The removal efficiency under the same abrasion conditions is lowest as the SiC substrate is perfect without defects. The tendency is present that the material removal rate increases with the increase of the vacancy concentration, although the relationship between material removal rate and vacancy concentration in some abrading position is non monotonic. The distribution of vacancy defect is random. It is speculative that the concentration of vacancy defect is not monotonic when the SiC substrate is processed in the same scratching distance in all the simulated cases. So, the variety is different in different scratching distance, shown in Fig. 7. However, the removal rate of SiC substrates increases with a larger increase of vacancy concentration from 0% to 0.5% to 1.3%. A large increase of vacancy concentration decreases the non monotonic effect in the same scratching distance. Therefore, the tendency is present that the material removal rate increases with the increase of the vacancy concentration. The presence of high vacancy defects in mono-crystalline SiC increases the removal efficiency because of the low yield stress and high abrading temperature during the abrading process.
3.3 Subsurface damage Snapshots of the subsurface damage and dislocation distribution on SiC substrates from the MD simulation are presented in Fig. 8 at the abrading distance of 18nm with vacancy defects from 0% to 1.3%. There exist two types of dislocations (Fig. 8(a1)-(a3)) on the abraded SiC substrates: perfect dislocations with 1/2<110> Burgers vector and other dislocations.
Both types of
dislocations are lying within the bottom section of the subsurface damage layer, which are consistent with the experimental observations in a nano-abrasion process[30-32]. The dislocation density and length are lower in vacancy-defect SiC substrates than defect-free SiC substrates （Fig. 8), suggesting that the presence of vacancy defects hinders the dislocation nucleation and promotes the dislocation annihilation. To quantify the effect of vacancy concentration on the subsurface damage, the elastic deformation layer of substrates is ignored due to its less thickness than subsurface damage layer. The subsurface damage is defined to be the distance from the top
7
plane surface to the end of structural transformation and dislocation lines on the SiC substrate, shown in Fig. 8 (b1)-(b3). The quantitative influence of vacancy concentration on the subsurface damage can be found in Fig. 9 where the abrasive has been advanced by 18nm：the more the vacancy defect, the less subsurface damage in the machining area of mono-crystalline SiC. Such a result can be attributed to that a large number of vacancies are accumulated in the processing area of the substrates, and hinders the dislocation nucleation and promotes the dislocation annihilation. Thus, the defects suppress the propagation of subsurface damages to a deeper region of substrates.
3.4 Wear of diamond abrasive Not only are the removal behaviors affected by the vacancy concentration in the SiC substrates, but also by the wear of abrasives. A better understanding about the wear mechanism of an abrasive will contribute for better quality of processed substrate surface and for an improved life cycle of a diamond polishing pad. It is not difficult to speculate that the abrasive compression between SiC and diamond will lead to deformation and wear of diamond abrasives. In order to investigate the structural change of diamond abrasives, the radial distribution functions (RDF) of carbon atom pairs in diamond abrasive are calculated and overlapped in Fig. 10 before and after nano-metric abrading simulation. In graphites, the bond distance between two SP2 carbon atoms and bond angle of three adjacent atoms on the same graphite layer are 0.142nm and 120°, respectively. The bond length and bond angle of the nearest neighboring carbon atoms are 1.54 Å and 109.5°, respectively, in diamond abrasives. To be noted in the MD simulation, a small deviation of the pair separation (bond) distance with ±0.1 Å and bond angle with ±2.5° at each peak are caused by energy minimization process. Intensities of pair separation distances between two SP3 carbon atoms in diamonds decrease at 0.152nm while increase at 0.146nm after the abrading process (Fig. 10), suggesting a graphitization on diamond surface. To better illustrate the effect of vacancy defects of SiC on the graphitization of diamond during the abrading process, the intensities of pair separation distances at 0.146nm and 0.152nm are extracted from RDF curve of a diamond before and after the abrading process (Fig. 11a). Very clearly shown in Fig. 11a, the intensity of a pair separation distance at 0.146nm basically decreases with the increase of the vacancy concentration in the machining region of SiC substrates, a result of less strain stress being applied to the surface of diamond abrasives. The bond angle distribution profile of diamond abrasives is presented in Fig. 11b. Consistent with the observation on atom pair distance at 0.146
8
nm (sign of graphitization), the counts of bond angle decrease at 122.5° as the vacancy concentration increases in the machining region of SiC substrates. In summary, the structural transformation from a perfect diamond structure to graphite structure decreases as the vacancy concentration of SiC substrates increases from 0% to 1.3%. High temperature and high stress have been attributed to be the two main factors that cause the diamond-graphite transformation[33-35]. The graphitization wear is mainly affected by the shear stress as the abrasion temperature in diamond abrasive is lower than the Debye temperature (1910K).
The critical shear stress is 95 GPa for diamond-to-graphitization transition[36].
According to the abrading result (Fig. 13b), the abrasion temperature of diamond abrasive is hard to rise up to 1910K.
It is not enough to cause a large area of graphitization by abrasion
temperature in our simulation. It is illustrated in fig. 13d that the highest local shear stress in abrasive edge is higher than the critical shear with 95 GPa for diamond-to-graphitization transition. Fig. 12a graphically elucidates the morphological evolution of a diamond abrasive particle as the diamond particle moves along the abrasion direction, and a severe wear（fall-offs along the trail of abrasions) of a diamond abrasive is observed by nanometric abrading the SiC material. In addition, it can be seen that the size of the diamond abrasive reduced to 7.5 nm from 8.0 nm after nano-abrading process, clearly indicating the passivation of abrasive tip which is the main performance of attrition wear (Fig. 12a). There are a large number of carbon atoms breaking-off from diamond abrasives, which agrees with the characteristics of abrasive wear during the ploughing process[37]. The coordination number of a carbon atom in diamond and graphite is 4 and 3, respectively. As noted in Fig. 12b, graphitized SP2 (three coordinated) atoms can be seen on the distorted diamond particle surface and in the wear-off dusts. Furthermore, the single 6-ring structure which is similar to the 6-ring structure in graphite is found in the diamond abrasive. The interaction between SiC substrates and diamond abrasives which is shown in Fig. 13a leads to the variety of the temperature and stress of the system. The distribution of temperature and of stress on the diamond abrasive is investigated.
As mapped in Fig. 13, the highest
temperature, highest von Mises stress and highest shear stress all are distributed on the abrasion edge of the diamond abrasives, where a diamond-graphite transformation was observed during the abrading process. Thermal-chemical wear is mainly affected by temperature induced by the friction between substrates and abrasives. The temperature at the abrasive edge with about 800 K
9
is sufficient to weaken the C-C coherent bond in the diamond abrasive, making atoms to get separated from the tool[38].
This indicates that thermal chemical wear occur in diamond
abrasives during nano-abrading process. In summaries, thermo-chemical wear, graphitized wear and abrasive wear are the basic mechanisms for diamond abrasives in pads during nano-abrading process (Fig. 14).
4 Summary MD simulation analysis was conducted to investigate the influence of vacancy defects on the mechanical behaviors of mono-crystalline SiC and the wear properties of diamond abrasives in a two-body nano-metric abrading process. The detailed results can be summarized as follows: As the vacancy concentration in the machining area of mono-crystalline SiC increases, the temperature increases while the von Mises stress decreases, respectively. The removal efficiency on a defect-free substrate is the lowest. The higher vacancy defect concentration in the machining area, the easier to be removed the surface atoms on the abraded substrate. A decreased yield strength resulting from increased vacancy defects in the monocrystalline SiC reduces the abrasion resistance during nano-metric abrading process. Dislocations produced from nano-abrading are found present in the subsurface damage layer. The depth of subsurface damage layer decreases with the increase of vacancy concentration in the machining region of mono-crystalline SiC substrates. Graphitization of carbon atoms in the diamond abrasive decrease as the vacancy concentration in the machining region of SiC substrates increases from 0% to 1.3%. Graphitized wear, abrasive wear, and thermal-chemical wear of abrasive particles are observed during the abrading process. The higher the vacancy defect concentration in SiC substrates, the less the graphitization wear on the diamond abrasive surface.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 51675276) and Jiangsu Province Key Laboratory of Precision and Micro-manufacturing Technology.
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（c）
（d）
Fig. 1. (a) the MD simulation model (b) vacancy area (c) the cell of diamond (d) the cell of mono-crystalline SiC
Fig. 2. Crystal structure of mono-crystalline SiC with different vacancy concentration of (a) 0% (b) 0.1% (c)0.5% (d)0.9% (e)1.3%
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Fig. 3. Diagrams of average von Mises stress and average temperature of SiC substrates at different vacancy concentration
Fig. 4. The abrasion force curve at different vacancy concentration
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Fig. 5. RDF of SiC substrates before after nano-abrasion
Fig. 6.Thestructural transformation of SiC substrates after abrasion
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Fig. 7. The removed atoms of SiC substrates at different vacancy concentration
Fig. 8. The surface topography and subsurface damage of SiC substrates at different vacancy concentration (a1)-(a3) dislocation distribution (b1)-(b3) subsurface damage 16
Fig. 9. The subsurface damage depth at different vacancy concentration
Fig. 10. RDF of diamond abrasive before after abrading
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Fig. 11. Graphitization (a) RDF at bond distance of 0.146nm and 0.152nm (b) The Bond angle of diamond at 122.5o for different vacancy concentration after abrading process
(a)
(b) Fig. 12. The deformation and wear of diamond abrasive at different abrading distance (a) passivation (b) graphitization
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Fig. 13. Distribution of temperature and von Mises stress of diamond abrasive after nano-cutting (a) interaction of SiC substrate and diamond abrasive (b) temperature distribution (c) von Mises stress distribution (d) shear stress distribution
Fig. 14. Wear types of diamond abrasives
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Table 1 Parameters of molecular dynamics simulation Configuration
Abrading
Substrates
Mono-crystalline Silicon carbide
Abrasive particle
Diamond abrasive
Vacancy concentration in machining area
0% 0.1% 0.5% 0.9% 1.3%
Substrates dimensions
152.18*213.052*91.308 Å
Abrasive radius
4 nm
abrasion depth
2.6 nm
Abrading velocity
60 m/s
Potential function
ABOP
Timestep
0.001ps
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Highlights The influence of vacancy concentration on removal mechanisms of SiC substrates is explored by MD simulation in nano-abrasion. The wear mechanism of diamond abrasive is studied. The presence of vacancy defects in SiC substrates reduces the von Mises Stress and increses the abrading temperature. Removal efficiency increases and subsurface damage depth decreases, respectively, with increasing the vacancy concentration in the machining area of SiC substrates. The wear of diamond abrasives is induced by thermo-chemical wear, abrasive wear and graphitized wear.
Declaration of interests √The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: