ICMCTF2006 Session H4: Surface Engineering to Address Environmental and Life-Cycle Cost Issues

Tuesday, May 2, 2006 8:50 AM in Room Terrace Pavilion

Tuesday Morning

Start	Invited?	Item
8:50 AM		H4-1 Excluding Hexavalent Chrome from Production of Black Functional Coatings on Aluminium Alloys A. Pilkington, A.L. Yerokhin, A. Leyland, A. Matthews (The University of Sheffield, United Kingdom) Surface engineering of aluminium alloys is frequently carried out with the objective of improving the wear and corrosion resistance . Industrial products additionally require a decorative finish. One of the most requested industrial colourations is achromatic black. Conventional anodising processes commonly utilise Cr (VI) salts with attendant hazards during processsing and end of life pollution issues. Plasma electrolytic oxidation (PEO)is an environmentally friendly process that offers the possibility of producing black oxide ceramic coatings in chrome-free electrolytes. This work invesigates the optical characteristics of PEO coatings formed in alkaline solutions containing additions of non-chrome metal salts. The morphology of the coatings produced was characterised by optical microscopy and SEM. Phase composition was investigated by XRD and chromaticity was measured using colourphotospectrometer. The mechanical properties were characterised by microhardness and scratch adhesion testing. The study demonstrate the feasibility of PEO technology in depositing abrasion resistant black coatings on aluminium alloys in chrome-free electrolyte solutions.
9:10 AM	Invited	H4-3 Growth Processes of Anodic Films on Light Metals P. Skeldon, G.E. Thompson (The University of Manchester, United Kingdom) Anodic films remain of major importance to protection of light metals, where they play key roles in corrosion protection and in adhesion of organic coatings. However, the surface treatment procedures can involve use of chemicals that raise health and environmental concerns, fluorides and chromates being particularly common examples. Consequently, new anodizing processes are being developed that utilize more acceptable ingredients, while at the same time match the performance of existing treatments. For instance, various chromium-free mixed acid anodizing treatments are being pursued as alternatives to chromic acid anodizing. Similarly, environmentally-friendly high speed anodizing treatments are being developed for surface finishing of aluminium coil. New anodizing treatments are also being progressed for magnesium and titanium, with resultant coatings incorporating self-arranging pore morphologies or ceramic-like qualities appropriate for wear resistance and biocompatibility. The present work, concerns findings from investigations of the underlying growth mechanisms of the various films and coatings on aluminium, magnesium and aluminium, which are relevant to understanding and development of improved coating processes and subsequent performance in service conditions. The studies combine the use of markers and tracers, for determining the locations of new oxide and transport processes of reactants, with use of model alloys, for focusing on roles of specific species, and commercial alloys, for evaluating interactions of alloy constituents. Analytical procedures range from relatively broad-beam surface analytical approaches, to high resolution analytical transmission electron microscopy. New insights are gained into the coating processes, which impact upon technologically important aspects of coating development, for instance morphology and composition of coatings and efficiency of coating formation.
9:50 AM		H4-5 Plasma Electrolytic Oxidation Coatings on Ti: Effect of Pulsed Current on Residual Stresses R. Khan, A.L. Yerokhin, A. Pilkington, A. Leyland, A. Matthews (The University of Sheffield, United Kingdom) Plasma Electrolytic Oxidation (PEO) represents significant interest as an ecologically friendly alternative to acid-based anodising processes for Ti alloys. During PEO, the oxide film formation is influenced by multiple heating-cooling cycles initiated by surface plasma microdischarge events; this affects the coating structure, phase composition and stress state. In this work, the effect of pulsed current is studied on the residual stresses in PEO coatings produced on titanium. Residual stresses are evaluated by X-ray diffraction using the Sin²ψ method. The effects of duty cycles and frequencies of the pulsed unipolar current are evaluated. Correlations between internal stress and coating thickness, surface morphology and phase composition are discussed. Regimes of PEO treatment favorable for the production of the coatings with minimal stress level are recommended.
10:10 AM		H4-6 Effect of Combined Shot-Peening and PEO Treatment on Fatigue Life of 2024 Al Alloy D.T. Asquith, A.L. Yerokhin, A. Pilkington, J.R. Yates, A. Matthews (The University of Sheffield, United Kingdom) One of the most important objectives of surface engineering of light-weight alloys is to increase fatigue properties of these materials, allowing both enhanced performance and an extened service life. This can be achieved through a hardened surface layer while introducing a favourable stress state in it. Single surface treatments, for example Plasma Electrolytic Oxidation (PEO), are not always capable of creating optimal combinations of these characteristics, whereas greater durability can be achieved by applying mechanical pre-treatments prior to the coating. In this work, a combination of shot-peening pre-treatment with plasma electrolytic oxidation coating is considered to improve fatigue performance of 2024 T351 Al alloy. The shot-peening was carried out in a compressed air configuration using S110 gauge shot at 200% coverage with an intensity of 20 Almen C. PEO coatings of 50 micron thickness were produced using pulsed bipolar current technology. Fatigue properties were evaluated by a four-point bending technique at a stress ratio of 0.1. Hardness, phase composition and microstructure of the surface layers were studied by Vickers microhardness tests, XRD and SEM analyses, respectively. The effect of the combined shot-peening and PEO treatment was evaluated in comparison with effects of single treatments and the effects observed are discussed.
10:30 AM	Invited	H4-7 Fully Dense Nanostructured Metallic Coatings for Functional Applications F. Gonzalez, G. Palumbo, J.L. McCrea (Integran Technologies, Canada) A nanomaterial or nanostructured material is defined by having a controlling property at the nanometer scale. Grain size is a characteristic of a crystalline material that determines many of its properties. Materials with grain size at the nanometer scale can be produced using a variety of techniques and in many cases the process used to synthesize the material dramatically influences its limiting properties. Metals and alloys in fully dense form can be produced in nanocrystalline form, both as coatings or as self-standing structures, using electrodeposition techniques. The mechanical and tribological properties of these materials can be significantly enhanced by the reduction in grain size. By judicious selection of metallic and non-metallic components, and by properly engineering their microstructure, it is possible to produce materials that have ideal properties for specific applications. An example of this is the Co-P family of materials that, when possessing a nanocrystalline grain size, can be used as an alternative to hexavalent chrome coatings in functional applications in which high hardness and high wear resistance are required. Other possibilities include the production of polymer/nanometal composites that have very attractive strength to mass ratios and can be used for demanding structural applications.
11:10 AM		H4-9 Qualification of HVOF WC/Co Thermal Spray Coatings as a Replacement for Hard Chrome Plating on Aircraft Hydraulic Actuators B.D. Sartwell (Battelle); K.O. Legg (Rowan Technology Group) Electrolytic hard chrome plating is widely used to impart wear and corrosion resistance to components in flight-control and utility actuators on aircraft. However, the process utilizes hexavalent chromium, a known carcinogen, with more stringent worker exposure regulations being promulgated that will greatly increase costs. A project was executed to qualify high-velocity oxygen-fuel (HVOF) thermal spray coatings as an environmentally friendly, cost-effective alternative to hard chrome. Extensive fatigue, corrosion, wear and environmental embrittlement testing was conducted that generally showed superior performance of the HVOF coatings. For tests of WC/Co-coated rods sliding against elastomeric or PTFE seals, the surface finish on the coatings was critical to obtaining good performance. Costs associated with implementation of the HVOF coatings in comparison to hard chrome will be presented.
11:30 AM		H4-11 Dopping Effects on the Photocatalytic Properties of Titanium Dioxide Reactively Sputter-Deposited Coatings E. Aubry, V. Demange, A. Billard (Ecole des Mines, France) Titanium dioxide is a well known material for its photocatalytic properties, but its major defect is its energy activation range. Due to its band gap energy, it can only be activated by ultraviolet light which limits its applications. Moreover the sputtered titanium dioxide coatings often present a lower photocatalytic activity regarding with other processes such as sol-gel method which allows the synthesis of very porous layers. Extensive efforts have been realised by several authors to develop titania coatings that can use solar or indoor light and which present a better quantum yield (or photocatalytic response to the light activation). In this paper, we study the effect of addition elements (Ag, N, Fe, La, Na...) in titania coatings on their chemical, physical and photocatalytic properties. The coatings are synthesised by reactive magnetron sputtering on soda-lime glass. The titania doping is carried out either by using a second metallic target simultaneously sputtered with titanium or by introducing nitrogen in combination with oxygen. In all cases, a diffusion barrier layer is previously deposited to prevent the coatings poisoning by the sodium of glass. In a first part, we investigate the coatings structure by X-ray diffraction, as well as their morphology and microstructure by scanning and transmission electron microscopy, as a function of the doping element content, measured either by energy dispersive spectroscopy or by electron energy loss spectroscopy for the lighter elements. The optical properties of the films are also determined by means of NUV-visible-NIR spectroscopy. The second part is ascribed to the photocatalytic activity measurements under UV irradiation. The tests are been performed in water containing organic compounds considered as model polluants (e.g. orange II). Finally, the photocatalytic properties are discussed in relation with the films composition, microstructure and morphology.