No abstract

Compositions in the ZrO2-Y2O3-Ta2O5 system are of interest as low thermal conductivity, fracture resistant oxides for the next generation thermal barrier coatings (TBC). Multiple phases occur in the system offering the opportunity to compare the thermal properties of single, two-phase, and three-phase materials. Despite rather large variations in compositions almost all the solid solution compounds had rather similar thermal conductivities and, furthermore, exhibited only relatively small variations with temperature up to 1000 o C. These characteristics are attributed to the extensive mass disorder in all the compounds and, in turn, small interfacial Kapitza (thermal) resistances. More complicated behavior, associated with the transformation from the tetragonal to monoclinic phase, occurs on long-term annealing in air of some of the compositions. However, the phases in two of the compositional regions do not change with annealing in air and their thermal conductivities remain unchanged suggesting they may be suitable for further exploration as thermally stable TBC overcoats.

The extreme properties of diamond have led its use in a wide variety of applications from drilling for oil to ultra sharp knives for eye surgery. In the nanotechnology realm, diamond is being recognized as a material with great potential. For example, the extremely high modulus suggests that diamond could be used in Nano-Electro-Mechanical systems (NEMS) and quantum NEMS applications since diamond cantilevers will have higher oscillation frequencies than other materials. Furthermore diamond is chemically inert and biocompatible, and the surface can be functionalized which makes it attractive for potential nano-bio

The residual stress distribution in plasma-sprayed zirconia thermal barrier coatings subjected to cyclic thermal gradient testing was evaluated using Raman piezospectroscopy and finite element computation. The thermal gradient testing (approximately 440°C/mm at temperature), consisted of repeated front-side heating with a flame and constant cooling of the back-side of the substrate either with front-side radiative cooling only or with additional forced air cooling between the heating cycles. The coatings exhibited characteristic “mud-cracking” with the average crack spacing dependent on the cooling treatment. This is consistent with finite element calculations and Raman spectroscopy measurements in which the sudden drop in coating surface temperature on initial cooling leads to a large biaxial tension at the surface. The key to proper interpretation of the Raman shifts is that the stress-free Raman peaks need to be corrected for shifts associated with the evolution of the metastable tetragonal phase with aging.

The thermal conductivity of yttria-stabilized zirconia (YSZ) thermal barrier coatings increases with high temperature aging. This common observation has been attributed to the densification of the coatings as porosity sinters out and pores and cracks spheroidize to minimize their surface energy. We show that the thermal conductivity of fully-dense 3 mol% Y 2 O 3 stabilized zirconia (3YSZ) also increases with high temperature aging, indicating that densification and pore shape changes alone are not responsible for all the observed increase in thermal conductivity of coatings. Instead, there are increases due to a combination of phase separation and grain growth. The increase in thermal conductivity can be described by a LarsonMiller parameter. It is also found that the increase in thermal conductivity with aging is greatest when measured at room temperature and decreases with increasing measurement temperature. At 1000 o C, the thermal conductivity is almost temperature independent and the changes in thermal conductivity with aging are less than 15%, even after aging for 50 hours at 1400 o C.