Robert C. McCune

Characterization of copper layers produced by cold gas-dynamic spraying

The cold gas-dynamic spray method produces coatings or deposits by introducing solid feedstock particles into a supersonic gas stream developed through the use of a converging-diverging (de Laval) nozzle. The particles thus accelerated impact on a substrate surface and develop into a dense deposit through a process believed to be similar to cold compaction. The work reported here explores the internal nature and physical characteristics of copper deposits produced by the cold gas-dynamic spray method using two vastly different starting powders: in one case, a “spongy” copper obtained by a direct-reduction process, and in the second, a denser, more spheroidal particulate produced by gas atomization. Optical and electron microscopies (scanning electron microscopy [SEM] and transmission electron microscopy [TEM]) were used to observe details of microstructure in the feedstock particles and deposits. Young’s modulus and residual stress measurements for the deposits were obtained through mechanical means, and measurements of hardness and electrical conductivity are reported. The internal structure of the cold-spray deposit was influenced by the surface purity of the feedstock material.

Characterization of surface layers produced by ion implantation of nitrogen in bulk aluminium

Abstract Surface layers of overall extent less than 400 nm have been generated by ion implantation of nitrogen as N + or N + 2 at energies of 50 and 100 keV into high-purity aluminum sheet at ambient temperature. These layers have been characterized by Auger electron spectroscopy with concurrent ion etching, scanning electron microscopy of the surface, and transmission electron microscopy in plan and cross-section geometries. Rutherford backscattering spectrometry, particle-induced X-ray emission and nuclear reaction analysis were used to assess cumulative changes in surface composition. The surface composition was found to be determined by the combined effects of sputtering, ablation, and recoil implantation of both existing surface oxides and reactive species incorporated into the surface during the process of ion implantation. Regions of AlN could be identified in the subsurface at an ion dose of 1 × 10 17 N + 2 cm -2 at 50 keV. The apparently non-contiguous regions of AlN formed semi-coherently with parent aluminum grains. The reaction layer, containing AlN, residual aluminum metal and oxygen, was found to grow toward the original passivated metal surface with increasing ion dose.

Cation Solute Segregation to Surfaces of MgO and α-Al2O3

Low energy ion scattering spectroscopy (LEIS) and Auger electron spectroscopy (AES) were used to measure the extent of impurity cation solute segregation to MgO (100) and various surfaces of single and polycrystalline Al 2 O 3 , following equilibration anneals at temperatures above 1000°C in ulfra-high vacuum. Systems studied include Ca/MgO, Ni/MgO, Ca/A1 2 O 3 and Y/Al 2 O 3 . Calcium segregation to MgO (100) is reversible and exhibits monofayer adsorption behavior with an enthalpy of segregation of approximately -55 kJ/mole with maximum occupation of surface cation sites approaching 40%. Nickel segregation to MgO(100) is masked by a preferential segregation of calcium. Calcium segregation to A1 2 O 3 surfaces has been found to be transient with a maximum surface cation site occupation of less than 10% as determined by LEIS and estimated enthalpy of segregation in the range -go to -190 kJ/mole. Yttrium segregation to surfaces of a polycrystalline Al 2 O 3 compact was limited by competing calcium segregation at temperatures be ow 1600°C. An estimated enthalpy of segregation for yttrium to A1 2 O 3 surfaces was in the range -23 to -43 kJ/mole, with maximum cation surface site occupation of about 15%. Practical limitations to free surface measurements of solute segregation in these materials are discussed.

The Use of AC-DC-AC Methods in Assessing Corrosion Resistance Performance of Coating Systems for Magnesium Alloys

The potential utility of AC-DC-AC electrochemical methods in comparative measures of corrosion-resisting coating system performance for magnesium alloys under consideration for the USAMP “Magnesium Front End Research and Development” project was previously shown in this forum [1]. Additional studies of this approach using statistically-designed experiments have been conducted with focus on alloy types, pretreatment, topcoat material and topcoat thickness as the variables. Additionally, sample coupons made for these designed experiments were also subjected to a typical automotive cyclic corrosion test cycle (SAE J2334) as well as ASTM B117 for comparison of relative performance. Results of these studies are presented along with advantages and limitations of the proposed methodology.

Characterization of Coatings on Steel Self-Piercing Rivets for Use with Magnesium Alloys

Incorporation of magnesium alloys in self-pierce rivet (SPR) joints poses several unique challenges among which are the creation of spurious galvanic cells and aggravated corrosion of adjacent magnesium when coated steel rivets are employed. This work firstly reviews efforts on development of coatings to steel fasteners for the diminution of galvanic corrosion when used with magnesium alloys. Secondly, approaches, based on several electrochemical methods, for the measurement of the galvanic-limiting effect of a number of commercially-available coatings to hardened 10B37 steel self-piercing rivets inserted into alloy couples incorporating several grades of magnesium are reported. Electrochemical impedance spectroscopy (EIS), zero-resistance ammeter (ZRA), corrosion potential and potential-mapping visualization methods (e.g. scanning vibrating electrode technique — SVET) are illustrated for the several rivet coatings considered.

Use of an AC/DC/AC Electrochemical Technique to Assess the Durability of Protection Systems for Magnesium Alloys

One task under the U.S. Automotive Materials Partnership (USAMP) “Magnesium Front End Research and Development” (MFERD) Project has been the evaluation of methodologies for the assessment of protective capability for a variety of proposed protection schemes for this hypothesized multi-material, articulated structure. Techniques which consider the entire protection system, including both pretreatments and topcoats are of interest. In recent years, an adaptation of the classical electrochemical impedance spectroscopy (EIS) approach using an intermediate cathodic DC polarization step (viz. AC/DC/AC) has been employed to accelerate breakdown of coating protection, specifically at the polymer-pretreatment interface. This work reports outcomes of studies to employ the AC/DC/AC approach for comparison of protective coatings to various magnesium alloys considered for front end structures. In at least one instance, the protective coating system breakdown could be attributed to the poorer intrinsic corrosion resistance of the sheet material (AZ31) relative to die-cast AM60B.

Automatic digital data collection for ion scattering depth profiles

A means for acquiring elemental depth profiles in digital form using ion scattering spectrometry for materials having well resolved binary elastic scattering peaks is described. The integrated counts for each elemental peak or background region are stored in consecutive channels of a multichannel scaler using the system sweep multiplexer to provide the time base necessary to advance the scaler address. A typical depth profile for a copper-chromium bi-layer sample collected in this manner is presented.

Investigation of Coating and Corrosion Mitigation Strategies in Magnesium/Mixed Metal Assemblies

The US Automotive Materials Partnership through the Magnesium-Intensive Front End Development Project (MFERD) is currently investigating a number of joining, coating and corrosion mitigation strategies to incorporate magnesium components into the automotive body-in-white with the ultimate goal of decreasing vehicle curb weight, thus improving fuel economy. Because Mg is anodic to all other structural metals, this is a key hurdle to Mg component implementation in vehicles. This paper will discuss the results of a study to examine the effectiveness of different corrosion mitigation strategies in joined plate assemblies and provide some insight into the systems challenges of incorporation of Mg parts into a vehicle. Details of a statistically-designed experiment developed to explore the interaction of several materials of construction (magnesium, steel and aluminum), pretreatment and topcoatings, joining methods and standardized test protocols including SAE J-2334 and ASTM B-117 are discussed. A number of avenues have emerged from this study as potential strategies for corrosion mitigation.

Selective Galvanizing Using Kinetic Spraying

General corrosion protection of sheet materials such as steel used in automobile construction has reached a high level of performance, due primarily to the incorporation of mill-applied treatments such as electrogalvanizing, galvannealing and other coil-coating processes developed over the last half century. While such treatments have greatly extended the corrosion resistance of steel and its various body constructs, attention is now focused on aspects of the manufacturing process wherein these intended protections are compromised by such features as weldments, joins, cut edges and extreme metal deformations such as hems. A novel metal deposition process, based on high-velocity impact fusion of solid metal particles, has been used to extend the corrosion resistance of base steel and pre-galvanized sheet, by selectively placing highly controlled depositions of zinc and other sacrificial materials in close proximity to critical manufacturing details. The technology of this process is reviewed and preliminary results are reported for hem flange construction.

Microstructure and Composition of Surface Layers Formed by Ion Implantation of Nitrogen in High-Purity Aluminum

Surface layers with overall thickness + or N 2 + at energies of 50 or 100 keV in 99.99% pure aluminum. These surfaces were characterized by scanning and transmission electron microscopy, Auger electron spectroscopy, Rutherford backscattering, nuclear reaction analysis and particle-induced X-ray analysis. At doses above 2×10 17 N 2 /cm 2 , blistering of the surfaces was observed along with a reduction in the extent of the coulometric dose retained by the material. Oxygen is believed to be introduced into the near-surface region by a process of reaction and ion-beam mixing, as well as possible CO contamination of the beam. A phase, isostructural with AlN, forms semi-coherently with parent aluminum grains, however, some fraction of the metallic aluminum phase remains in the reaction layer, even at overall nitrogen contents which exceed the stoichiometry of AlN.