A physics-based understanding of material aging mechanisms helps to increase reliability when predicting the lifetime of mechanical and electrical components. This report examines in detail the mechanisms of atmospheric copper sulfidation and evaluates new methods of parallel experimentation for high-throughput corrosion analysis. Often our knowledge of aging mechanisms is limited because coupled chemical reactions and physical processes are involved that depend on complex interactions with the environment and component functionality. Atmospheric corrosion is one of the most complex aging phenomena and it has profound consequences for the nation's economy and safety. Therefore, copper sulfidation was used as a test-case to examine the utility of parallel experimentation. Through the use of parallel and conventional experimentation, we measured: (1) the sulfidation rate as a function of humidity, light, temperature and O{sub 2} concentration; (2) the primary moving species in solid state transport; (3) the diffusivity of Cu vacancies through Cu{sub 2}S; (4) the sulfidation activation energies as a function of relative humidity (RH); (5) the sulfidation induction times at low humidities; and (6) the effect of light on the sulfidation rate. Also, the importance of various sulfidation mechanisms was determined as a function of RH and sulfide thickness. Different models for sulfidation-reactor geometries and the sulfidation reaction process are presented.
The relative electronic defect densities and oxide interface potentials were determined for naturally-occurring and synthetic Al oxides on Al. In addition, the effect of electrochemical treatment on the oxide electrical properties was assessed. The measurements revealed (1) that the open circuit potential of Al in aqueous solution is inversely correlated with the oxide electronic defect density (viz., lower oxide conductivities are correlated with higher open circuit potentials), and (2) the electronic defect density within the Al oxide is increased upon exposure to an aqueous electrolyte at open circuit or applied cathodic potentials, while the electronic defect density is reduced upon exposure to slight anodic potentials in solution. This last result, combined with recent theoretical predictions, suggests that hydrogen may be associated with electronic defects within the Al oxide, and that this H may be a mobile species, diffusing as H{sup +}. The potential drop across the oxide layer when immersed in solution at open circuit conditions was also estimated and found to be 0.3 V, with the field direction attracting positive charge towards the Al/oxide interface.
Nanostructural characterization of amorphous diamondlike carbon (a-C) films grown on silicon using pulsed-laser deposition (PLD) is correlated to both growth energetic and film thickness. Raman spectroscopy and x-ray reflectivity probe both the topological nature of 3- and 4-fold coordinated carbon atom bonding and the topographical clustering of their distributions within a given film. In general, increasing the energetic of PLD growth results in films becoming more ``diamondlike'', i.e. increasing mass density and decreasing optical absorbance. However, these same properties decrease appreciably with thickness. The topology of carbon atom bonding is different for material near the substrate interface compared to material within the bulk portion of an a-C film. A simple model balancing the energy of residual stress and the free energies of resulting carbon topologies is proposed to provide an explanation of the evolution of topographical bonding clusters in a growing a-C film.
Al{sub 2}Cu thin films ({approx} 382 nm) are fabricated by melting and resolidifying Al/Cu bilayers in the presence of a {micro} 3 nm Al{sub 2}O{sub 3} passivating layer. X-ray Photoelectron Spectroscopy (XPS) measures a 1.0 eV shift of the Cu2p{sub 3/2} peak and a 1.6 eV shift of the valence band relative to metallic Cu upon Al{sub 2}Cu formation. Scanning Electron microscopy (SEM) and Electron Back-Scattered Diffraction (EBSD) show that the Al{sub 2}Cu film is composed of 30-70 {micro}m wide and 10-25 mm long cellular grains with (110) orientation. The atomic composition of the film as estimated by Energy Dispersive Spectroscopy (EDS) is 67 {+-} 2% Al and 33 {+-} 2% Cu. XPS scans of Al{sub 2}O{sub 3}/Al{sub 2}Cu taken before and after air exposure indicate that the upper Al{sub 2}Cu layers undergo further oxidation to Al{sub 2}O{sub 3} even in the presence of {approx} 5 nm Al{sub 2}O{sub 3}. The majority of Cu produced from oxidation is believed to migrate below the Al{sub 2}O{sub 3} layers, based upon the lack of evidence for metallic Cu in the XPS scans. In contrast to Al/Cu passivated with Al{sub 2}O{sub 3}, melting/resolidifying the Al/Cu bilayer without Al{sub 2}O{sub 3} results in phase-segregated dendritic film growth.