1. AB INITIO STUDY OF ADHESION TO ALUMINUM Newton Ooi: newton.ooi@asu.edunewton.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 circuit board Al sheet rolling

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: Al 2 O 3 –Common lubricants: graphite

5. WORK OF SEPARATION =+ E2E2 22 E1E1 11 A  ETET

6. DENSITY FUNCTIONAL THEORY Kinetic energy of non-interacting electrons Electrostatic energy Exchange correlation energy Potential energy of non-interacting electrons 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

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/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 Aluminum dataa (Å)E c (eV)V (Å 3 )B o (GPa) LDA : GGA3.971 : 4.039-4.22 : -3.7215.66 : 16.4782.55 : 72.75 Experiment4.045-3.3916.60 72.2

9. 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 SURFACE ENERGY CALCULATIONS Cell Vacuum Slab

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. CREATE INTERFACES 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

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

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 SystemExperiment W s (J/m 2 )Calculated W s (J/m 2 ) Al – Al 2 O 3 1.131.06 Al – graphite0.1 – 0.40.2 – 0.35

16. FUTURE WORK Aluminum – Diamond-like carbon (DLC) –Influence of surface stresses in carbon –Effect of sp 3 /sp 2 bonding ratio in carbon –Surface termination Aluminum – BN –Hexagonal or cubic BN –Surface stoichiometry: B or N or B x N y 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: 7 th 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.comwww.accelrys.com –www.webelements.comwww.webelements.com