Investigation of the Subsurface Temperature Effects on Nanocutting Processes via Molecular Dynamics Simulations

At a Glance

Metadata	Details
Publication Date	2020-09-10
Journal	Metals
Authors	Michail Papanikolaou, Francisco Rodríguez, Konstantinos Salonitis
Institutions	Cranfield University
Citations	8
Analysis	Full AI Review Included

Executive Summary

This study utilized three-dimensional Molecular Dynamics (MD) simulations to quantify the effects of initial workpiece subsurface temperature (T_w) on copper nanocutting mechanics using a diamond tool.

Thermal Softening Dominance: Increasing the initial workpiece temperature (from 300 K to 900 K) causes significant thermal softening in the copper, leading to a measurable reduction in both tangential (F_x) and normal (F_y) cutting forces.
Non-Linear Thermal Response: The temperature rise in the cutting area (T_ca) is non-linear with respect to T_w. At higher T_w values, the difference (T_ca - T_w) decreases, indicating a change in material behavior.
Local Thermal Conductivity Effect: The simulations captured, for the first time, the dependency of local thermal conductivity on T_w. Higher T_w results in slower heat dissipation and a delayed cooling response in the subsurface region.
Stress Distribution: At low T_w (300 K), high Equivalent Von Mises Stresses (EVMS) are concentrated primarily in the shear zone. At high T_w (900 K), the stress distribution becomes more uniform across the workpiece due to thermal softening.
Friction Factor: The average friction factor remains approximately constant, fluctuating around 1.7 across the entire tested temperature range (300 K to 900 K).
Subsurface Damage: High stress areas were consistently identified in the vicinity of lattice dislocations (Hexagonal Close-Packed atoms), linking thermal effects directly to dislocation generation and propagation.

Technical Specifications

Parameter	Value	Unit	Context
Simulation Type	3D	-	Molecular Dynamics (MD)
MD Software	LAMMPS (v. 16 May 2018)	-	Large-scale Atomic/Molecular Massively Parallel Simulator
Workpiece Material	Copper (FCC)	-	Lattice constant: 3.597 A
Tool Material	Diamond	-	Lattice constant: 3.57 A
Rake Angle	-45	Degrees	Negative rake angle
Grinding Speed	100	m/s	Constant tool velocity
Depth of Cut (d_c)	20	A	Constant cutting depth (2 nm)
Initial Workpiece Temperatures (T_w)	300, 500, 700, 900	K	Tested range for subsurface temperature
Maximum Observed T_ca	~1100	K	Average temperature at the cutting area (at T_w = 900 K)
Maximum Observed EVMS	15	GPa	Equivalent Von Mises Stress
Maximum Observed F_x (Tangential)	~375	nN	Observed at T_w = 300 K
Maximum Observed F_y (Normal)	~250	nN	Observed at T_w = 300 K
Average Friction Factor (eta_ave)	~1.7	-	Ratio of tangential to normal forces (F_x/F_y)
Workpiece Dimensions (x, y, z)	717, 70, 140	A	Length, Height, Width
Time Step	1.5	fs	Time integration step

Key Methodologies

The investigation relied on a highly controlled 3D MD simulation setup using LAMMPS, focusing on the interaction between a copper workpiece and a diamond tool.

System Definition and Zoning:
- The simulation domain contained 723,118 atoms, divided into three functional zones for both the tool and the workpiece:
  - Boundary Atoms (Dark Blue): Fixed layers used to ensure stability and prevent rigid body motion.
  - Thermostat Atoms (Red): Controlled by the Langevin thermostat to maintain the desired initial temperature (T_w).
  - Newtonian Atoms (Light Blue): The deformable zone where material removal and stress calculations occur, governed by Newton’s second law.
Interatomic Potentials:
- Cu-Cu Interaction: Modeled using the Embedded Atom Model (EAM) potential, suitable for metallic systems.
- C-C Interaction: Modeled using the Tersoff potential, suitable for covalent materials like diamond.
- Cu-C Interaction: Modeled using the Morse potential to describe the interaction between the tool and workpiece atoms.
Simulation Procedure:
- Equilibration: The system was relaxed over 20,000 timesteps to achieve constant pressure and temperature (NVE ensemble control).
- Cutting Phase: The tool boundary atoms were moved at a constant speed of 100 m/s along the x-[100] direction, maintaining a constant depth of cut (20 A).
- Temperature Variation: Four distinct initial workpiece temperatures (300 K, 500 K, 700 K, 900 K) were tested, with three simulations performed for each case to minimize statistical errors.
Data Extraction and Analysis:
- Cutting Forces: Tangential (F_x) and normal (F_y) forces were averaged over the stable cutting phase (100 ps to 700 ps).
- Subsurface Temperature (T_ca): Calculated using the kinetic energy of atoms within a 15 A spherical region fixed at the tool-workpiece interface, considering only the z-component of velocity (v_z) to isolate abrasive temperature.
- Stress Analysis: Equivalent Von Mises Stresses (EVMS) were calculated across the simulation domain, and Common Neighbor Analysis (CNA) was used to identify lattice defects (FCC, HCP, Amorphous).

Commercial Applications

The fundamental understanding of thermal effects on material removal at the nanoscale is critical for optimizing manufacturing processes where surface integrity and ultra-precision are paramount.

Semiconductor and Microelectronics Fabrication:
- Application: Minimizing subsurface damage and residual stress during the grinding and polishing of silicon, copper interconnects, and other substrate materials.
- Relevance: The findings on thermal softening and stress uniformity directly inform process control to achieve atomic-scale surface finish required for high-performance chips.
Ultra-Precision Machining (UPM) and Optics:
- Application: Deterministic mechanical nanometric machining (DMNM) of complex 3D shapes, especially for high-quality optical components and molds.
- Relevance: Predicting the reduction in cutting forces due to localized heating allows engineers to adjust tool geometry and speed to maintain dimensional accuracy and minimize thermal distortion.
Micro-Electro-Mechanical Systems (MEMS) Manufacturing:
- Application: High-speed machining of micro-components where thermal damage can compromise device functionality.
- Relevance: Understanding the relationship between T_w and local thermal conductivity helps in designing effective cooling strategies (e.g., grinding fluid application) to manage heat dissipation and prevent localized melting or phase transformation.
Advanced Tool Design and Material Selection:
- Application: Developing predictive models for tool wear and material removal rates in high-speed nano-grinding.
- Relevance: The data on stress concentration near dislocations provides insight into the fundamental mechanisms of material failure and chip formation, aiding in the selection of optimal tool materials and geometries for specific workpiece temperatures.

View Original Abstract

In this investigation, three-dimensional molecular dynamics simulations have been performed in order to investigate the effects of the workpiece subsurface temperature on various nanocutting process parameters including cutting forces, friction coefficient, as well as the distribution of temperature and equivalent Von Mises stress at the subsurface. The simulation domain consists of a tool with a negative rake angle made of diamond and a workpiece made of copper. The grinding speed was considered equal to 100 m/s, while the depth of cut was set to 2 nm. The obtained results suggest that the subsurface temperature significantly affects all of the aforementioned nanocutting process parameters. More specifically, it has been numerically validated that, for high subsurface temperature values, thermal softening becomes dominant and this results in the reduction of the cutting forces. Finally, the dependency of local properties of the workpiece material, such as thermal conductivity and residual stresses on the subsurface temperature has been captured using numerical simulations for the first time to the authors’ best knowledge.

Tech Support

Original Source

DOI: https://doi.org/10.3390/met10091220

References

2008 - Grinding wheel effect in the grind-hardening process [Crossref]
2013 - Prediction of grinding force in microgrinding of ceramic materials by cohesive zone-based finite element method [Crossref]
2020 - Analytical and experimental investigations on the mechanisms of surface generation in micro grinding [Crossref]