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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 ...

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

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.

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.