Showing papers in "International Materials Reviews in 2007"

Analysis of solid state phase transformation kinetics: models and recipes

Abstract: The progress of solid-state phase transformations can generally be subdivided into three overlapping mechanisms: nucleation, growth and impingement. These can be modelled separately if hard impingement prevails. On that basis, an overview has been given of recent numerical and analytical methods for determination of the kinetic parameters of a transformation. The treatment focuses on both isothermally and isochronally conducted transformations. To extend the range of transformations that can be described analytically, a number of more or less empirical submodels, which are compatible with experimental results, has been included in the discussion. It has been shown that powerful, flexible, analytical models are possible, once the concept of time or temperature dependent growth exponent and effective activation energy, in agreement with the existing experimental observations, has been adopted. An explicit (numerical) procedure to deduce the operating kinetic processes from experimental transformation-rate data, on the basis of different nucleation, growth and hard impingement mechanisms, has been demonstrated. Without recourse to any specific kinetic model, simple recipes have been given for the determination of the growth exponent and the effective activation energy from the experimental transformation-rate data.

Diamond like carbon coatings for tribology: production techniques, characterisation methods and applications

Abstract: The application of diamond like carbon (DLC) coatings in tribological applications, the range of deposition methods employed and techniques for characterising the structure and properties of the films produced are reviewed. Thus far, DLC coatings have found broad industrial application, particularly in optical and electronic areas. In tribological applications, DLC coatings are used successfully as coatings for ball bearings where they decrease the friction coefficient between the ball and race; in shaving applications where they increase the life of razor blades in wet shaving applications; and increasingly in automotive applications such as racing engines and standard production vehicles. The structure and mechanical properties of DLC coatings are dependent on the deposition method and the incorporation of additional elements such as nitrogen, hydrogen, silicon and metal dopants. These additional elements control the hardness of the resultant film, the level of residual stress and the tribologic...

Consolidation–microstructure–property relationships in bulk nanoceramics and ceramic nanocomposites: a review

Abstract: The recognition that bulk nanoceramic materials, having grain sizes typically 100 nm or finer, possess appealing mechanical, physical and tribological properties has generated considerable recent research activity. A major challenge in the research on bulk nanoceramics and nanoceramic composites is concerned with the aspect of processing, in particular restriction of grain growth during sintering. In this regard novel processing techniques have been developed with the aim of fabricating bulk ceramic nanomaterials. The superior properties exhibited by these bulk nanoceramics and ceramic nanocomposites, compared with conventional ceramic materials, have opened up prospects for their use in applications demanding better mechanical properties. The present review highlights outstanding issues related with the processing of nanoceramics and ceramic nanocomposites, discusses the novel processing techniques currently used to develop these materials, critically analyses the property modifications resulting...

A review of nucleation, growth and low temperature synthesis of diamond thin films

Abstract: Diamond thin films have outstanding optical, electrical, mechanical and thermal properties, which make these attractive for applications in a variety of current and future systems. In particular, t...

Requirements for and use of miniature test specimens to provide mechanical and creep properties of materials : a review

Abstract: The review is concerned with the use of miniature specimens to identify the mechanical/creep properties of metals and alloys. It is not intended to consider the nanoscales, which have been reviewed elsewhere, but focus on the size ranges and forms that are of use to areas such as alloy development, surveillance monitoring, effects of irradiation, properties of weld structures and remaining life. For many of these cases, there are technical advantages in the use of small specimens. Where possible, available reviews in this area are used and identified. The needs for small specimen sizes are considered, and the wide range of sizes of both conventional and more unusual specimen designs is reviewed. The potential effects of specimen size are considered: the microstructural features of the material, the effect of constraint and the actual region of the specimen that is undergoing deformation, and finally statistical approaches. The results of published studies are considered under the general groupings of yield/deformation behaviour, creep properties, toughness and fatigue, with an examination of the current state of interpretation of data from mini-specimens because interpretation is one of the key areas of interest. Finally, the current status of each area is considered, as certain designs of specimen will be better suited to answering specific technical questions. The need for standards for specific designs is also examined.

Application of electron backscatter diffraction to the study of phase transformations

Abstract: The application of the electron backscatter diffraction technique to the investigation of phase transformations is reviewed. The wide variety of results obtained using this technique is illustrated and discussed, focusing on thermodynamics and kinetics of phase transformations, solidification, solid state phase transformations, environmentally assisted reactions and thin film deposition. Emphasis is also placed on two rapidly growing developments: coupling electron backscatter diffraction with advanced experimental techniques and with more and more complex modelling of phase transformations and of resulting material properties.

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

Abstract: The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited...

Protection and surface modification of metals with sol-gel coatings

Abstract: The preparation and properties of protective sol–gel coatings on metallic substrates are reviewed. Control of the coating microstructure is critical to obtain the desired properties and this requires optimisation of processing parameters that include sol composition, synthesis and processing conditions, deposition method and post-deposition sintering or curing. Current knowledge in these areas, e.g. structure–property–processing relationships, and the characterisation of the coatings produced are discussed. SiO2 layers have been found to confer good oxidation resistance, but for electrochemical corrosion resistance, mixed oxide layers or hybrid organic–inorganic layers are required. Incorporation of alkylalkoxides with polymerisable groups attached has allowed synthesis of hybrid nanostructured coatings consisting of interpenetrating, chemically bonded organic and inorganic networks. Doping of hybrid sol–gel coatings with environmentally friendly inhibitors is also a promising approach combining ...

Review of materials and processing methods used in the production of bipolar plates for fuel cells

Abstract: The bipolar plate is by weight, volume and cost one of the most significant components of a fuel cell stack. The existence or creation of materials to produce bipolar plates which meet all the property requirements and can be readily manufactured to impart the fine channels represents the major challenge. Bipolar plates can be made from various materials with the most common being graphite, metal, carbon/carbon and polymer composites. Each type of material has its strengths and weaknesses. The traditional graphite plates are highly conductive and have good corrosion resistance, but are brittle and require costly machining. The metal plate has excellent bulk conductivity but its major drawback is the expense in imparting the channels and the need for a corrosion resistant coating. Carbon/carbon also requires high machining costs and the plates are subject to warping owing to chemical vapor infiltration (CVI). Materials for polymer composites are relatively inexpensive, and channels can be formed by means of a moulding process. The key is to develop a balance between sufficient electrical conductivity, processability of the resin and the mechanical properties.

Review of data for velocity of sound in pure liquid metals and metalloids

Abstract: A review of the experimentally determined values of the velocity of sound c in molten pure liquid metals and metalloids has been carried out. Based on the available data for each element, recommended velocity–temperature relationships have been suggested. A range of thermophysical properties has been derived from these values. Theoretical models for the velocity of sound and its temperature dependence have also been considered. Sound velocities have also been estimated via the inter-relationship of velocity to other thermophysical properties.

Metallic nanostructured materials based sensors

Abstract: In recent years, a significant interest has developed on metallic nanostructures based sensors for the detection of gas, volatile organic compounds (VOCs), and biomolecules in the wake of counter-terrorism threats and environmental pollution. Enhancement in selectivity and overall efficiency of the sensors is achieved by tailoring size, structure and shape of the nanoparticles. In the last decade, numerous studies revealed various sensing properties of these nanomaterials. However, improvements in overall sensor performance such as sensitivity, selectivity, response time, recovery time and robustness demand further improvement. This warrants a proper understanding of the basic principles behind the sensing properties of metallic nanostructures. In this review, principles of few important sensing systems, their sensing mechanisms, merits and demerits along with some of the applications on the detection of gas, VOC and biological molecules are discussed. This review also highlights the importance of...

Synthesis and crystal structure of n-diamond

Abstract: Allotropes of carbon based materials, such as graphite, fullerenes and nanotubes, and diamond are currently a focus of great interest. New diamond (n-diamond) has been proposed as a new carbon allotrope; its electron diffraction pattern matches that of cubic (F d3m) diamond apart from some additional reflections that are forbidden for diamond, indexed as {200}, {222} and {420}. n-Diamond has been found experimentally for more than 10 years, yet its structure and stability are not unambiguously resolved, and some controversies still exist. A comprehensive review of recent developments in the experimental and theoretical knowledge of n-diamond is presented. Synthesis methods reported for n-diamond and transformation mechanisms from other carbon allotropes are discussed. The various crystal structure models proposed for n-diamond are summarised and critically assessed. The stability to aging and other properties of n-diamond are briefly considered. Finally, the potential for technological applicatio...

Polystyrene/clay nanocomposites

Abstract: Polystyrene has been widely studied as a matrix in nanocomposites, because it is a model amorphous polymer. The present article provides an extensive review of studies on polystyrene/layered silica...