Julie M. Schoenung

Oxidation behavior of HVOF sprayed nanocrystalline NiCrAlY powder

Abstract This paper describes recent progress on the research into improving the oxidation behavior of the bond coat using a HVOF nanostructured NiCrAlY coating. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed onto Ni-based alloy to form a nanocrystalline bond coat. The powder and coating structure were characterized by XRD, SEM, and TEM. Oxidation experiments were performed on the coating to form the thermally grown oxide layer (TGO). After heat treatment at 1000 °C for 24 and 95 h, a homogeneous α-Al2O3 layer was formed on top of the bond coat. The oxide layer was analyzed and compared to the coating sprayed using the as-received powder. As shown in the results, the nanostructured characteristic of the coating and the presence of Al2O3 within the cryomilled powders (oxidation occurred during cryomilling process) seem to affect the nucleation of the alumina layer on the top of the coating. The formation of a continuous TGO layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions, such as those presented in the coating sprayed using the as-received powder.

Human health and ecological toxicity potentials due to heavy metal content in waste electronic devices with flat panel displays

Display devices such as cathode-ray tube (CRT) televisions and computer monitors are known to contain toxic substances and have consequently been banned from disposal in landfills in the State of California and elsewhere. New types of flat panel display (FPD) devices, millions of which are now purchased each year, also contain toxic substances, but have not previously been systematically studied and compared to assess the potential impact that could result from their ultimate disposal. In the current work, the focus is on the evaluation of end-of-life toxicity potential from the heavy metal content in select FPD devices with the intent to inform material selection and design-for-environment (DfE) decisions. Specifically, the metals antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, selenium, silver, vanadium, and zinc in plasma TVs, LCD (liquid crystal display) TVs, LCD computer monitors and laptop computers are considered. The human health and ecotoxicity potentials are evaluated through a life cycle assessment perspective by combining data on the respective heavy metal contents, the characterization factors in the U.S. EPA Tool for the Reduction and Assessment of Chemical and other environmental Impacts (TRACI), and a pathway and impact model. Principal contributors to the toxicity potentials are lead, arsenic, copper, and mercury. Although the heavy metal content in newer flat panel display devices creates less human health toxicity potential than that in CRTs, for ecological toxicity, the new devices are worse, especially because of the mercury in LCD TVs and the copper in plasma TVs.

A review on nanostructured WC-Co coatings

Abstract Unique mechanical properties of nanostructured materials motivate a worldwide interest to synthesize nanostructured coatings. Nanostructured WC–Co coatings have been extensively investigated because of the importance of WC–Co coatings in industrial applications, usually with respect to wear resistance requirements. The present paper has reviewed the studies in this field, including synthesis of nanostructured WC–Co powder, concern on decomposition of nanostructured WC particles, influence of spraying parameters on microstructure and mechanical properties of nanostructured coatings. Significant achievements in this field are: (1) It is possible to produce nanostructured WC–Co powders without non-WC/Co phases in quantities although a high percentage of non-WC/Co phases is frequently reported in conventional WC–Co powder; (2) By controlling agglomerate size of feedstock powder, fuel chemistry and fuel–oxygen ratio, the decomposition of WC particles can be reasonably eliminated so as to synthesize near nanostructured WC–Co coating with a low amount of non-WC/Co phases; and (3) Increased hardness, toughness and wear-resistance can be obtained in near nanostructured WC–Co coatings that are properly synthesized.

Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.

Influence of Cryomilling on the Morphology and Composition of the Oxide Scales Formed on HVOF CoNiCrAlY Coatings

Commercially available, gas-atomized CoNiCrAlY powder was cryomilled to produce powder with nanocrystalline grains. The cryomilled powder and conventional gas-atomized powder were thermally sprayed using the HVOF process to prepare two coatings with fine-grain (∼15 nm) and coarse-grain (∼1 μm) microstructure, respectively. The two coatings were isothermally oxidized in air at 1000° C for up to 330 hr. The morphology and composition of the oxide scales formed on the two coatings were compared with each other. The results indicate that, while a fine-grain microstructure can promote the formation of a pure alumina layer on the coating by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.

Mechanisms of microstructure evolution during cryomilling in the presence of hard particles

Abstract The present study was undertaken to provide insight into the mechanisms that govern the evolution of microstructure in Ni powder during cryomilling with nitride particles. The AlN particles are distributed in Ni powder particles after cryomilling, and the particles with initial size of 2 μm are fractured into smaller size, 50∼300 nm, during cryomilling. The distribution of particles is uniform, and some extremely small particles, size range of ∼20 nm, are also observed by TEM after cryomilling. With addition of AlN particles, the Ni powder particle size after cryomilling is reduced, and contamination of iron and gaseous atoms, N and O, is increased. For the grain size of Ni, the present results show that, in the presence of 2 wt% (5 vol%) AlN particles, the Ni grain size is reduced to 37 nm after 8 h of cryomilling. In contrast, the grain size of Ni cryomilled under identical conditions but without particles exceeded 100 nm. In terms of volume fraction, the results show an increase in the rate of grain size refinement with increasing volume fraction of AlN particles for the range studied, i.e. 1.2–5.0 vol%. The grain size is also reduced to 25 nm with increasing impeller speed up to 340 rpm, which provides higher kinetic energy, and longer cryomilling time of 20 h. This observation is rationalized on the basis of a mechanism involving the interactions of dislocations with hard, non-deformable nitride particles, and thermally induced dislocation generation due to the thermal expansion coefficient difference between the Ni matrix and the nitride particles.

Toxicity potentials from waste cellular phones, and a waste management policy integrating consumer, corporate, and government responsibilities

Cellular phones have high environmental impact potentials because of their heavy metal content and current consumer attitudes toward purchasing new phones with higher functionality and neglecting to return waste phones into proper take-back systems. This study evaluates human health and ecological toxicity potentials from waste cellular phones; highlights consumer, corporate, and government responsibilities for effective waste management; and identifies key elements needed for an effective waste management strategy. The toxicity potentials are evaluated by using heavy metal content, respective characterization factors, and a pathway and impact model for heavy metals that considers end-of-life disposal in landfills or by incineration. Cancer potentials derive primarily from Pb and As; non-cancer potentials primarily from Cu and Pb; and ecotoxicity potentials primarily from Cu and Hg. These results are not completely in agreement with previous work in which leachability thresholds were the metric used to establish priority, thereby indicating the need for multiple or revised metrics. The triple bottom line of consumer, corporate, and government responsibilities is emphasized in terms of consumer attitudes, design for environment (DfE), and establishment and implementation of waste management systems including recycling streams, respectively. The key strategic elements for effective waste management include environmental taxation and a deposit-refund system to motivate consumer responsibility, which is linked and integrated with corporate and government responsibilities. The results of this study can contribute to DfE and waste management policy for cellular phones.

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

Thermal Stability of Nanostructured Cr 3 C 2 -NiCr Coatings

The thermal stability behavior of nanostructured Cr3C2-NiCr coatings was investigated. The nanostructured Cr3C2-NiCr coatings, synthesized using mechanical milling and high-velocity oxygen fuel (HVOF) thermal spraying, were thermally exposed in air at 473, 673, 873, and 1073 K for 8 h. The results show that microhardness of the conventional coating increased slightly with increasing temperature, while that of the nanostructured coating drastically increased from 1020 to 1240 HV300 for the same temperature increases. Heat treatment led to increases in scratch resistance and decreases in the coefficient of friction for the nanostructured Cr3C2-NiCr coatings. A high density of Cr2O3 oxide particles with average size of 8.3 nm was found in the nanostructured coatings exposed to high temperatures, which is thought to be responsible for the observed increase in microhardness and scratch resistance and the decrease in the coefficient of friction of the nanostructured coatings.

Adopting Lead-Free Electronics: Policy Differences and Knowledge Gaps

For more than a decade, the use of lead (Pb) in electronics has been controversial: Indeed, its toxic effects are well documented, whereas relatively little is known about proposed alternative materials. As the quantity of electronic and electrical waste (e-waste) increases, legislative initiatives and corporate marketing strategies are driving a reduction in the use of some toxic substances in electronics. This article argues that the primacy of legislation over engineering and economics may result in selecting undesirable replacement materials for Pb because of overlooked knowledge gaps. These gaps include the need for: assessments of the effects of changes in policy on the flow of e-waste across state and national boundaries; further reliability testing of alternative solder alloys; further toxicology and environmental impact studies for high environmental loading of the alternative solders (and their metal components); improved risk assessment methodologies that can capture complexities such as changes in waste management practices, in electronic product design, and in rate of product obsolescence; carefully executed allocation methods when evaluating the impact of raw material extraction; and in-depth risk assessment of alternative end-of-life (EOL) options. The resulting environmental and human health consequences may be exacerbated by policy differences across political boundaries. To address this conundrum, legislation and policies dealing with Pb in electronics are first reviewed. A discussion of the current state of knowledge on alternative solder materials relative to product design, environmental performance, and risk assessment follows. Previous studies are reviewed, and consistent with their results, this analysis finds that there is great uncertainty in the trade-offs between Pb-based solders and proposed replacements. Bridging policy and knowledge gaps will require increased international cooperation on materials use, product market coverage, and e-waste EOL management.