First-principles modeling of degradation processes at the coating-metal interface
Abstract
First-principles modeling of degradation processes at the coating-metal interface. The research will focus on the adhesion and degradation of siloxane based coatings on aluminum metal and alloy substrates. By creating atomistic representations of the metal/siloxane and metaloxide/ siloxane interface, the interactions between water molecules and the metal-oxygensiloxane binding environment will be characterized using density functional theory and molecular dynamics. Coatings on structural metals provide physical barriers to the passage of environmental species – predominantly aqueous – that would otherwise induce corrosion. Even while protecting metals against corrosion, coatings themselves undergo deterioration. One of the chief causes of coatings failure is when adhesion at the metal/coatings interface is compromised, allowing direct access of water and aqueous ions to the metal surface. For this reason, coating optimization seeks to maximize adhesion forces between the coating and the metal surface and strengthen resistance to chemical attack. Direct experimental characterization of the metal/coating interface is challenging since the coating is by nature designed to hinder accessibility. For this reason it is proposed that computational modeling provide an alternative means for simulating, and thereby providing a first-principles physics based characterization of, the metal/coating interface and its interactions with water molecules that may ultimately lead to coating decohesion. The research will focus on the adhesion and degradation of siloxane based coatings on aluminum metal and alloy substrates. By creating atomistic representations of the metal/siloxane and metaloxide/ siloxane interface, the interactions between water molecules and the metal-oxygensiloxane binding environment will be characterized using density functional theory and molecular dynamics. Literature suggests that clusters of water can form within the coating, contributing to lateral diffusion within the polymer and multiphase (i.e. binders, pigments, etc.) network, and these clusters can effectively create “triple phase” interfaces when they form at the metal/coating interface. Reaction profile analysis (nudged elastic band or similar) methods of the hydrolysis of the …Al-O-Si… interfacial bond will also be carried out. Activation energies for this process will be obtained that will provide a quantitative measure of the resistance of the bond to attack. Since there will be multiple directions of approach, and multiple possible interface environments determined via the molecular dynamics approach, the activation energies can be used to sort the configurations from most- to least-likely points of interface degradation. The identification of the mechanism of the metal-coating interface degradation and the quantification of activation energies will provide a route for using computation to better design coating systems based on the siloxane (or alternatives) framework. For example, the R- alkyl substituents attached to silicon could be varied in order to increase the activation energy, and thereby slow the rate of metal/coating interface failure. Alternatively, the substrate composition could be varied to provide metallurgical solutions that lead to a stronger interfacial bond.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Nov 23, 2016
- Source ID
- N000141612763
Entities
People
- Christopher Taylor
Organizations
- Office of Naval Research
- Ohio State University
- United States Navy