Stoichiometry and Adhesion of Al/WC
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Stoichiometry and Adhesion of Al/WC Donald J. Siegel Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green St., Urbana, IL, 61801. Louis G. Hector, Jr. GM Research and Development Center, 30500 Mound Road, P.O. Box 9055, Warren, MI 48090 James B. Adams Chemical and Materials Engineering Department, Arizona State University, Tempe, AZ 85287-6006.
ABSTRACT We examine the relative stability and adhesion of nonstoichiometric (polar) Al/WC interfaces and WC(0001) surfaces using Density Functional Theory as implemented in a planewave, pseudopotential formalism. Relaxed atomic geometries and the ideal work of adhesion were calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. Based on the surface and interfacial free energies, we find that both the clean surface and the optimal interface geometry are W-terminated. However, the largest adhesion energies are obtained with the C-termination, consistent with an argument based on surface reactivity. INTRODUCTION Interfaces between metals and ceramics play a vital role in an increasingly large number of industrial applications[1]: heterogeneous catalysis, microelectronics, thermal barriers, corrosion protection and metals processing are but a few representative examples. However, experimental complications associated with the study of a buried interface, and theoretical difficulties arising from complex interfacial bonding interactions have hindered the development of general, analytic models capable of accurately predicting fundamental interfacial quantities. One such quantity, which is key to predicting the mechanical properties of an interface, is the ideal work of adhesion, Wad [1], which is defined as the bond energy needed (per unit area) to reversibly separate an interface into two free surfaces, neglecting plastic and diffusional degrees of freedom. Formally, Wad can be defined in terms of either the surface and interfacial energies (relative to the respective bulk materials) or by the difference in total energy between the interface and its isolated slabs: ;
Wad = 1v + 2v ; 12 = E1tot + E2tot ; E12tot =A:
(1)
Here iv is the surface energy of slab i, 12 is the interface energy, Eitot is the total energy of slab i, and E12tot is the total energy of the interface system. The total interface area is given by A. AA4.25.1
˚ ˚ ˚ 3) Pseudopot. a (A) c (A) c/a V0 (A PAW 2.932 2.849 0.972 21.21 GGA Ultra-soft 2.920 2.840 0.973 20.98 Ultra-soft 2.881 2.802 0.973 20.15 LDA LCAO[17] 2.88 2.81 0.977 20.18 Experiment[17] 2.91 2.84 0.976 20.83 XC
B0 (GPa) Ecoh (eV) 365 16.87 375 16.67 418 19.70 413 17.8 329,577,434,443 16.7
Table 1: Comparison of WC bulk properties: LDA vs. GGA and ultra-soft pseudopotentials vs. the projector augmented wave method. Experimental data and another first-principles calculation (based on a linear combination of atomic orbitals) are also presented. Although there has recently been much activity aimed at understanding metal/oxide interfaces[1, 2, 3], much
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