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A new method is described for the evaluation of the creep properties of as-deposited overlay coatings. The method uses a composite tensile specimen consisting of a core alloy of well-characterised creep properties onto which the overlay coating is deposited. The overall strain/time response of this specimen, tested under constant stress conditions, can then be deconvoluted to obtain the creep characteristics of the coating. The method has the advantage that the coatings are evaluated for the same microstructural conditions as they will be used under, i.e. having the same porosity levels and microstructure. A limited demonstration of the method has been made for an air plasma-sprayed Ni25Cr6AlY coating deposited onto an austenitic steel core and creep tested at 900°C. The creep strength of this coating was surmised to be higher than that of equivalent monolithic samples produced by low pressure plasma spraying.

Alumina-SiC nanocomposites, which offer a better mechanical performance than conventional ceramic materials, are a class of potentially important structural materials. Unfortunately, alumina-SiC nanocomposite is a thermodynamically metastable system at elevated temperatures. The SiC particles, especially those near the sample surface, are susceptible to oxidation at temperatures above 1,000 C, forming silica which may subsequently react with the alumina matrix. The objective of the present work is to investigate the mullitization reaction between the oxidation-led nano-silica particles and alumina matrix in an alumina-5 vol% SiC nanocomposite

Superplastic forming of metal sheet, using gas pressure at elevated temperature at low strain rates, has been performed with ceramic dies formed using lost-wax techniques, which are utilised commonly in the manufacture of investment-casting moulds. In the case of superplastic forming, low die-loading and, commonly, low production volumes would permit the use of such disposable ceramic shell dies. Although care must be taken with these shell dies due to their limited mechanical strength, their use gives added flexibility to the process: this includes the production of re-entrant forms with good surface finish which would normally lead to considerable problems with conventional metal tooling. The evaluation of ceramic die material is reported here, together with the results of forming trials using lost-wax ceramic dies for a simple re-entrant shape.

Nanocomposites are known to exhibit dramatic improvements in both room and high temperature strength. Alumina/SiC nanocomposites have been produced by both sol routes and powder processing. Density measurements and TEM studies have revealed that the sintered density is related to particle morphology and sintering conditions. The addition of ultrafine SiC inhibits densification greatly, preventing the attainment of full density unless the driving force is increased. However, SiC also causes the formation of an intragranular dislocation network, although it is not clear if this acts as a strengthening mechanism.

This paper reports that the microstructure-mechanical property relationships in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) have been well established in the advanced ceramic community. These highly toughened ceramic materials suffer, unfortunately, a thermal instability when they are subjected to a hydrothermal environment at temperatures in the range of 100 to 400[degrees]C, due to the tetragonal to monoclinic transformation occurring on the aged surface. It has been observed that a Y-TZP will be completely disintegrated on ageing at the low temperatures for a long enough period. On the other hand, surface toughening is obtainable when the Y-TZP is thermally treated at the low temperatures in a controlled manner, as a result of the development of compressive stresses on the aged surface. Structural ceramic materials are very sensitive to microstructural defects, which include both the processing-related flaws and the structural damage caused during the period of application. Catastrophic failure of ceramic materials is a consequence of fast crack propagation under loading conditions. It is therefore essential to control both the number and sizes of microstructural defects present in an advanced ceramic material in order to improve its engineering reliability. For many engineering applications, such as those in chemical engineering industries, Y-TZP ceramics aremore » often subjected to a combination of mechanical and hydrothermal environments. Apparently, the presence of any microstructural defects such as cracks and pores may have a large impact on the response of Y-TZP to these mechanical and hydrothermal conditions. The aim of the present work is to investigate the effects of a hydrothermal environment on the post-indentation cracks in a 3 mol% yttria stabilized tetragonal zirconia polycrystals.« less