R.J.K. Wood

Plasma electrolytic oxidation (PEO) for production of anodised coatings on lightweight metal (Al, Mg, Ti) alloys

Abstract The introduction of plasma electrolytic oxidation (PEO) as a surface finishing technique has enabled a range of hard, dense oxide coatings to be produced on aluminium, magnesium, titanium and other lightweight alloy substrates. As with all surface coating technologies, successful development of PEO coatings requires adequate attention to substrate pretreatment together with careful control of electrolyte conditions and process variables. The principles and applications of the PEO coating process are considered, including the fundamentals of oxide deposition, the technology involved and the typical characteristics of the coatings. Industrial applications are considered together with their coating requirements. Plasma electrolytic oxidation coating is a specialised but well developed process. Suitable control of electrolyte and process conditions can realise a novel range of coatings having technologically attractive physical and chemical properties. The development of PEO technology over the last decade has provided coatings having controlled appearance, hardness, corrosion resistance and other tribological properties across an extending range of industrial sectors. Continuing developments are concisely reviewed and the PEO process is illustrated by the characterisation of anodised coatings on an AZ91 magnesium alloy surface.

Tribo-corrosion of coatings: a review

This paper reviews the available literature relating to the emerging research into the performance of coatings under combined wear and corrosion conditions. Understanding how coatings perform under these tribo-corrosion conditions is essential if the service life of equipment is to be predicted and to allow service life to be extended. Therefore, the tribo-corrosion performance of coatings deposited by a variety of techniques is discussed and the main mechanisms associated with their degradation under combined wear and corrosion highlighted. Coating composition, microstructure, defect level, adhesion, cohesion and substrate properties are seen as some of the critical elements in coating performance when subjected to tribo-corrosion contacts. The importance of post-coating deposition treatments such as laser resurfacing and sealing are also discussed. Interactions between wear and corrosion mechanisms are identified along with some models and mapping techniques that aim to inform coating selection and predict performance. Recent investigations into mono-layer as well as multilayered and functionally graded coatings are reviewed as candidates for wear–corrosion resistant surfaces. The review reveals the need for a more considered approach to tribo-corrosion testing and the way in which the results are analysed and presented. For example, the test conditions should be appropriate to the coating system under test; the level of in situ instrumentation deployed and the post-test analysis of in situ electrochemical data should be carefully selected as well as details given of the composition of any surface tribofilms formed and the identification of the degradation mechanisms.

Designing biomimetic antifouling surfaces.

Marine biofouling is the accumulation of biological material on underwater surfaces, which has plagued both commercial and naval fleets. Biomimetic approaches may well provide new insights into designing and developing alternative, non-toxic, surface-active antifouling (AF) technologies. In the marine environment, all submerged surfaces are affected by the attachment of fouling organisms, such as bacteria, diatoms, algae and invertebrates, causing increased hydrodynamic drag, resulting in increased fuel consumption, and decreased speed and operational range. There are also additional expenses of dry-docking, together with increased fuel costs and corrosion, which are all important economic factors that demand the prevention of biofouling. Past solutions to AF have generally used toxic paints or coatings that have had a detrimental effect on marine life worldwide. The prohibited use of these antifoulants has led to the search for biologically inspired AF strategies. This review will explore the natural and biomimetic AF surface strategies for marine systems.

Pseudotumour formation due to tribocorrosion at the taper interface of large diameter metal on polymer modular total hip replacements

We present an in-depth failure analysis of two large diameter bearing metal-on-polymer (MoP) modular total hip replacements, which have required revision surgery due to pseudotumour formation. The failure analysis showed a discrete pattern of material loss from the distal end of the head taper/stem trunnion interface. We postulate that the use of a proximal contacting taper design had provided insufficient mechanical locking between the head and the stem, enabling the head to toggle on the trunnion. In addition, the difference in angle between the taper and the trunnion formed a crevice between the two components. Through a combination of crevice environment, mechanically assisted corrosion, mechanical wear and erosion; debris and metal-ions have been released resulting in the adverse local tissue reactions (ALTR).

Sand erosion performance of detonation-gun applied tungsten carbide/cobalt-chromium coatings

Abstract Sand erosion studies of thermally sprayed WCCoCr (Denotation-Gun LW45) have been undertaken using a sand/water jet impingement rig. Results are presented which show that the erosion rate of sprayed compared to sintered tungsten carbide-cobalt-chrome is similar for low energy impacts but the sintered material outperforms by 4 times the sprayed material for high energy impacts. This reflects the anisotropic microstructure of the thermally sprayed coating with a preferred crack propagation parallel to the coating surface followed by crack interlinking and spalling. This is the dominant erosion mechanism present. A minor erosion mechanism consists of micro-cutting and ploughing at low angles of particle impact. The coatings have a relatively high density of defects including thermal stress induced transverse cracks, voids, oxides, and grit blasting remnants. Such defects are shown to accelerate the erosion process considerably because they aid crack initiation and growth leading to partial, mono or multi-splat spalling of loose material. The influence of slurry jet angle was found to be more pronounced under low energy conditions where maximum erosion occurred at 90° and the minimum at 30° in contrast to the high energy erosion rates which were independent of jet angle. This is a result of the lower levels of fluctuating stresses imparted to the coating during low energy impacts leading to the impact angle having a greater effect on sub critical growth rate than for the high energy conditions.

A review of experimental techniques to produce a nacre-like structure

The performance of man-made materials can be improved by exploring new structures inspired by the architecture of biological materials. Natural materials, such as nacre (mother-of-pearl), can have outstanding mechanical properties due to their complicated architecture and hierarchical structure at the nano-, micro- and meso-levels which have evolved over millions of years. This review describes the numerous experimental methods explored to date to produce composites with structures and mechanical properties similar to those of natural nacre. The materials produced have sizes ranging from nanometres to centimetres, processing times varying from a few minutes to several months and a different range of mechanical properties that render them suitable for various applications. For the first time, these techniques have been divided into those producing bulk materials, coatings and free-standing films. This is due to the fact that the materials application strongly depends on its dimensions and different results have been reported by applying the same technique to produce materials with different sizes. The limitations and capabilities of these methodologies have been also described.

The sand erosion performance of coatings

This paper reviews studies of the performance of polymeric, metallic and ceramic including diamond coatings on steel and ceramic substrates when subjected to water–sand jet impingement erosion conditions. The coatings tested have been deposited by the thermal spray, electroless plating, PVD and CVD routes. The erosion conditions covered include sand impact velocities between 10 and 30 m/s with sand sizes 60–235 ?m and at jet impingement angles of 30° and 90°. Tests compared the performance of the substrate material carbon steel AISI 1020 with the various polymeric, metallic and ceramic coatings. Coating and substrate erosion rates are plotted against mean particle impact energy, Ek or EkVp0.5 allowing easy surface selection once the erosion conditions of an application are known. The effect of jet impingement on the erosion resistance of selected coatings is presented. As with bulk materials, the slurry jet impingement angle and mean Ek must be considered in coating selection. Of the softer coatings tested the flexible polyurethane coatings seem to have promise for future use in fluid-borne sand particle erosion environments. Pure epoxy coatings show typical brittle erosion behaviour, with fusion-bonded epoxy having mixed ductile and brittle behaviour and glass fibre reinforced epoxy showing strong ductile behaviour. CVD deposited coatings of either boron carbide or diamond are the most resistant surfaces tested and outperform bulk ceramics such as silicon carbide by orders of magnitude for Ek<8 ?J and 90° jet impingement angle. Areas requiring further research to allow accurate prediction of coating life are highlighted.

Diamond coatings on tungsten carbide and their erosive wear properties

Abstract Diamond coatings up to ∼60-μm thick have been grown by microwave plasma CVD (MPCVD) on sintered tungsten carbide (WC) substrates, and their erosive wear properties are investigated under high velocity air–sand erosion testing. Two different sintered tungsten carbide (WC) substrates have been investigated and compared, the binder being either 6% Co or 5% Ni by weight. Significant differences in morphology, residual stress, adhesion and erosion performance are seen as a function of pre-deposition treatment, deposition conditions and the source of the substrates. Adherent coatings could be deposited to a thickness of ∼35 μm. They offer significantly better erosion resistance compared to uncoated substrates, with the erosion rate being lowered by up to a factor between ∼5 and 20 for particle test velocities of 148 and 63 m s−1, respectively. The steady-state erosion rates of the coatings are a function of a gradual micro-chipping mechanism. However, the life of the coating is dependent on the progression of sub-surface damage promoted by sub-surface shear stresses associated with the particle impacts. It is thought that the coating debonding is driven by the shear stresses interacting with the grain boundary porosity at the substrate/coating interface.

Investigation of erosion–corrosion processes using electrochemical noise measurements

This paper presents an example-based discussion of erosion–corrosion and flow corrosion processes that have been identified using electrochemical noise measurements. Various single and dual phase corrosion and erosion–corrosion experiments on austenitic stainless steels and various thermally sprayed coatings using jet impingement and pipe flow rigs are discussed. Localised corrosion events, metastable and propagating pitting, passive and general corrosion processes have been identified under various flow conditions of NaCl solutions. Oscillations in the electrochemical potential noise signals have been related to an erosion-enhanced corrosion synergistic effect. Electrochemical noise measurements show responses to electrolyte permeation of the coating, coating erosion penetration and substrate activity under erosion–corrosion conditions.

Review on the development of truly portable and in-situ capillary electrophoresis systems

Capillary electrophoresis (CE) is a technique which uses an electric field to separate a mixed sample into its constituents. Portable CE systems enable this powerful analysis technique to be used in the field. Many of the challenges for portable systems are similar to those of autonomous in-situ analysis and therefore portable systems may be considered a stepping stone towards autonomous in-situ analysis. CE is widely used for biological and chemical analysis and example applications include: water quality analysis; drug development and quality control; proteomics and DNA analysis; counter-terrorism (explosive material identification) and corrosion monitoring. The technique is often limited to laboratory use, since it requires large electric fields, sensitive detection systems and fluidic control systems. All of these place restrictions in terms of: size, weight, cost, choice of operating solutions, choice of fabrication materials, electrical power and lifetime. In this review we bring together and critique the work by researchers addressing these issues. We emphasize the importance of a holistic approach for portable and in-situ CE systems and discuss all the aspects of the design. We identify gaps in the literature which require attention for the realization of both truly portable and in-situ CE systems.