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MCrAlY overlay coatings have been successfully used as a means of improving the oxidation performance of gas turbine blades operating at elevated temperatures. However, depletion of aluminium can limit the ability of such coatings to form a protective oxide layer should spallation of the original α-Al2O3 oxide layer occur under thermal cycling conditions. It is the objective of the current research to evaluate the potential of NiAl3 as a reservoir phase for a NiCrAlY overlay coating on a IN738LC superalloy substrate at 1,100°C in air. The morphologies and microstructures of the conventional NiCrAlY and NiAl3-modified NiCrAlY overlay coatings in the as-sprayed and oxidised conditions were characterised using SEM, EDX and XRD techniques.

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing ...

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.

The oxidation behavior in air of air-plasma sprayed (APS) overlay coatingsof Ni–25Cr–6Al–Y have been studied at 1100°C. Aprotective alumina scale developed after 5- to 10-hr exposure with, initially,parabolic growth kinetics. With protracted exposures (>100 hr),subparabolic behavior developed, associated with aluminum depletion withinthe coating caused, principally, by internal oxidation of the low-densityAPS structure. This depletion caused intrinsic chemical failure, manifestedby the formation of a layer of Cr,Al,Ni-rich oxide beneath the residualalumina layer. Associated with this process of chemical failure was theformation of a layer of porous Ni,Cr-rich oxide above the aluminalayer. Oxide spallation occurred by delamination within this layer duringcooling; the spallation sites tended to lie above protuberances in theunderlying coating. Initial spallation occurred at a critical temperaturedrop, which decreased rapidly with increasing exposure time. A nonrigorousmodel of this spallation process has been developed which envisages thatdelamination occurs by the propagation of an oxide void under the action ofout-of-plane tensile stresses developed during cooling. Agreement with thespallation data is encouraging and shows that the deterioration ofspallation resistance with exposure time arises not only because oxidethickness increases but also because the maximum void size within the porousoxide layer increases.

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

Summary form only given. Substitution of La and/or La-Co into Sr hexaferrite (SrM) has been reported to improve magnetic properties. Meanwhile, we have observed that the substitution of Sm for Sr in SrM produces an improvement in the coercivity similar to that noted for SrM with La additives. In an effort to improve the magnetic properties of SrM with other rare-earth element additives, Nd doped SrM samples with various Nd/sup 3+//Sr/sup 2+/ ratios were prepared by hydrothermal synthesis and subsequently calcined at 1000/spl deg/C to 1250/spl deg/C.

The production of high strength hydroxyapatite ceramics has to date been hindered by the formation of agglomerates during the powder processing stages. In this paper it is shown how the introduction of an emulsion conditioning stage has modified the nature of the agglomerates to produce a “soft” deformable powder leading to improvements in both strength and consistency of sintered HA. On sintering at 1100°C for 2 hours, the conventionally processed and the emulsion refined material produced fracture strengths of 70.4 MPa and 201.1 MPa, respectively. The effect of powder agglomeration on densification during both the initial and intermediate/late stages of sintering is discussed.