AB INITIO STUDY OF ADHESION TO ALUMINUM

1. AB INITIO STUDY OF ADHESION TO ALUMINUM Newton Ooi: newton.ooi@asu.edu Computational Materials Science Group of Dr. James Adams http://ceaspub.eas.asu.edu/cms/ APS 4 Corners Meeting: October 24-25, 2003

2. ALUMINUM • Properties • Thermal and electrical conductor • Forms stable oxide • Low cost and low weight • Ductile • No magnetic properties • Uses • Microelectronics • Structural materials in vehicles and buildings • Packaging for food and drinks Al sheet rolling Al circuit board

3. PROBLEMS WITH USING AL • Poor surface properties • Soft and low hardness • Abrades and wears easily • Low melting point  friction welding occurs with other materials • Sticks to opposing tool pieces • Requires use of coatings or lubricants in many forming processes • Welding • Cold/hot rolling • Drilling or riveting • Extrusion • Determine adhesion between Al and coating/lubricant

4. ADHESION TO ALUMINUM • Measure with wetting experiments • Oxidation and surface contamination • No insight into atomic bonding • Difficult to quantify results • Examine by computer simulation • No concern about oxidation and contamination • Find ideal work of adhesion  work of separation • Assumes no plastic deformation • Complex interfacial bonding & geometry  need reliable quantum mechanical approaches • We have examined adhesion of Al to following materials: • Common coatings: CrN, VN, WC, diamond… • Native oxide: Al2O3 • Common lubricants: graphite

5.  1 E2 ET E1 A 2 WORK OF SEPARATION = +

6. DENSITY FUNCTIONAL THEORY • Total energy is functional of electron density • Proposed first by Thomas and Fermi in 1920s • Current model proposed by Hohenberg, Kohn and Sham in 1960s and applicable to ground state • Replace many-electron Schrödinger equation with single particle Kohn-Sham (KS) equation Kinetic energy of non-interacting electrons Potential energy of non-interacting electrons Electrostatic energy Exchange correlation energy

7. METHODOLOGY • Software: Vienna Ab initio Software Package (VASP) • Fortran 90 code for Unix / Linux • Born – Oppenheimer approximation • Plane wave basis set to span Hilbert space • Pseudopotentials to represent ion – electron interactions • Super cell method  3D periodic boundary conditions • Variational method with free energy as variational quantity • Exchange – correlation energy: LDA or GGA • VASP website: http://cms.mpi.univie.ac.at/vasp/ • Simulation procedures • Bulk calculations • Surface calculations • Interface calculations • Calculate work of separation • Analyze atomic and electronic structure of interface Aluminum FCC Cell

8. BULK CALCULATIONS • Find irreducible Brillouin zone • Plane wave convergence to minimize basis set • Calculate energy (enthalpy) as function of volume • Fit to equation of state • Determine cohesive energy, bulk modulus and lattice constants • Select pseudopotential for surface calculations

9. Cell Vacuum Slab SURFACE ENERGY CALCULATIONS • Construct surface slabs to make interfaces with • Determine irreducible Brillouin zone • Vacuum convergence to reduce interaction between adjacent slabs • Calculate surface energy via surface thickness convergence • We used equation of Boettger: PRB 49, 23 (1994) 16798

10. INTERFACE CALCULATIONS • Generate periodic interfaces • With or without vacuum? • Sandwich or bi-layer? • Lattice mismatch? • Interface registry? • Determine equilibrium interfacial separation • Relax interface and isolated slabs to minimal energy geometries • Calculate work of separation • Analyze interfacial geometry and structure • Electronic structure analysis • Charge density plots • Electron localization function

11. Vacuum or not? Vacuum allows more room for atoms to relax  increases accuracy Vacuum must be populated by plane waves  increases calculation cost Bi-layer or sandwich? Dipoles must cancel All interfaces must be identical in geometry and composition Mirror/inversion symmetry requirements CREATE INTERFACES

12. Matching up surfaces Minimize lattice mismatch Al(111) – graphite (0001) Interface registry or coherency Fully coherent to fully incoherent C = black and Al = gray INTERFACE GEOMETRY

13. Al – Graphite charge density Abrupt change at interface = negligible Al – graphite bonding

14. Al – Graphite ELF • ELF (Electron Localization Function) measures the Pauli exclusion principle • Different bonding types are differentiated by color • Red areas  bonding pairs  localized bonding  covalent • Blue to green  unpaired electrons or vacuum • Yellow to orange  metallic bonding

15. SUMMARY • Adhesion to aluminum increases with the polarity of opposing material  polarity increases bond formation • Graphite has lowest adhesion to aluminum • Adhesion at interface proportional to the surface energies of contacting surfaces  surface reactivity • DFT adhesion calculations give results in good agreement with available experimental data

16. FUTURE WORK • Aluminum – Diamond-like carbon (DLC) • Influence of surface stresses in carbon • Effect of sp3/sp2 bonding ratio in carbon • Surface termination • Aluminum – BN • Hexagonal or cubic BN • Surface stoichiometry: B or N or BxNy ELF of 64-atom DLC cubic supercell with gray iso-surface of 0.53

17. CREDITS • Acknowledgements • Dr. J. B. Adams • Dr. D. J. Siegel • Dr. L. G. Hector and Dr. Y. Qi at General Motors • Members of my research group • NCSA at UIUC for computational resources • NSF for funding under grant DMR 9619353 • Georg Kresse and authors of VASP • References • Siegel, Hector, Adams. PRB 67 (2003) 092105 • Kittel. Introduction to Solid State Physics: 7th Edition 2000 John Wiley & Sons • Ooi, Adams, Singisetti. Physica Status Solidi B 239 (2003) 44 • Adams et al. Journal of Nuclear Materials 216 (1994) 265 • Landry et al. Mat. Science and Engineering A254 (1998) 99 • www.accelrys.com • www.webelements.com