Derek D. Hass

Low Thermal Conductivity Vapor Deposited Zirconia Microstructures

Abstract Low thermal conductivity yttria stabilized zirconia (YSZ) coatings have been grown using a low-vacuum (0.20 Torr) electron beam directed vapor deposition process. In this approach, a transsonic helium jet was used to entrain and transport an evaporated YSZ flux to a substrate. The interaction of the helium jet with the coating surface resulted in many of the evaporated species making oblique angles of contact with the substrate. This resulted in the formation of a highly porous, columnar microstructure without substrate rotation. When the substrate was positioned perpendicular to the axis of the jet, coatings with intercolumnar pores normal to the substrate surface were formed. The ambient temperature thermal conductivity of a coating grown in this arrangement was 1.9 Wm/K, comparable to that of conventional, high-vacuum electron beam coatings. When the column and pore orientation was inclined (by tilting the substrate) the thermal conductivity was observed to fall. By alternating the inclination angle as growth progressed, coatings containing zig-zag columns and pores could be synthesized. Using this technique, YSZ coatings with thermal conductivities as low as 0.8 W/m K were obtained. The observed thermal conductivity reduction arises from the longer thermal diffusion path of the zig-zag pore micro-structures.

Thermal conductivity of zirconia coatings with zig-zag pore microstructures

Highly porous zirconia based thermal barrier coatings have recently been synthesised with zig-zag morphology pores which appear to impede heat flow through the thickness of the coating. A combined analytical/numerical study of heat conduction across these microstructures is presented and compared with thermal conductivity measurements. The effects of pore volume fraction, pore type, pore orientation and pore spacing, together with the wave length and the amplitude of zig-zag pore microstructures on overall thermal performance are quantified. The results indicate that even a few volume percent of zig-zag inter-column pores oriented normal to the substrate surface reduce the overall thermal conductivity of the coatings by more than 50%.

Synthesis of open-cell metal foams by templated directed vapor deposition

Low-density, open-cell nickel base superalloy foams have been synthesized by a high-rate, electron beam-directed vapor deposition process and their mechanical properties evaluated. The deposition process uses an open-cell polymer foam template upon which is deposited a metal alloy coating. The electron beam evaporated flux was entrained in a rarefied transonic gas jet and propagated along the flow stream lines through the polymer structure. After vapor deposition, the polymer template was removed by low-temperature thermal decomposition. The resultant ultralightweight metal foams consisted of a three-dimensional open cell, reticulated structure possessing hollow triangular ligaments with relative densities of

Electron beam directed vapor deposition of thermal barrier coatings

Electron beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) are currently applied to gas turbine engine components to improve engine durability but, in the future, may be used to increase engine efficiency. These coatings are costly to apply in part because of the low evaporated materials utilization efficiency (a few percent) and the relatively slow rate of deposition (∼5–10 μm min−1). Here, an emerging electron beam directed vapor deposition (EB-DVD) approach is explored as a method for producing these TBCs. The EB-DVD technique combines low vacuum (10−3–10 Torr) electron beam evaporation with a carrier gas jet to rapidly create and efficiently transport evaporant to a deposition surface. Studies of the deposition of yttria partially stabilized zirconia (YSZ) and reactively formed zirconia coatings have been performed and the composition, microstructure, phase content and crystallographic texture of deposited layers are reported. The results indicate EB-DVD’s ability to form composit...

Gas jet assisted vapor deposition of yttria stabilized zirconia

A gas jet assisted electron beam evaporation process for synthesizing yttria stabilized zirconia (YSZ) coatings has recently been reported. The process uses a rarefied inert gas jet to entrain and transport vapor to a substrate. The gas jet enables the lateral spreading of the flux to be controlled and large fractions of the vapor to be deposited on samples of relatively small size. When the gas pressure is high, coatings grown at 1050°C and below have a columnar structure and a high pore fraction. The total pore volume fraction, the morphology of the inter- and intracolumn pores and the coating texture are all observed to be a strong function of the gas pressure in the chamber with increasing chamber pressure leading to larger intercolumnar pore spacings, wider pores, a higher total pore volume fraction, and a reduction in the coating texture. A direct simulation Monte Carlo simulation approach has been used to investigate vapor transport for the various gas pressures explored in this study. The simulati...

Reactive vapor deposition of metal oxide coatings

The reactive deposition of metal oxide coatings has been achieved using an electron beam directed vapor deposition (DVD) approach. In this approach, a transonic helium carrier gas jet has been combined with electron beam evaporation to create and efficiently transport metal vapor to a substrate. Metal oxide coatings were then produced by adding oxygen to the carrier gas. The synthesis of multi-component oxide coatings could be achieved using either an alloy source or by simultaneous evaporation of metals from two or more sources. In the latter, the reactive carrier gas jet was used to facilitate vapor phase mixing of the evaporated fluxes. The preferred approach was found to be dependent on the vapor pressures of the individual source components. When large differences between the component vapor pressures exist, multi-source evaporation is preferred. Results are reported for the reactive deposition of zirconia from a single metal source and reactive deposition of yttria stabilized zirconia from two metal sources (Y and Zr). The phase and morphology are similar to those found in similar coating compositions created by the evaporation of a pre-compounded oxide. Very good compositional uniformity in the binary metal oxide films has been observed. Vapor transport modeling is used to explore the origin of the composition uniformity.

COMPUTATIONAL INVESTIGATION OF THE EFFECT OF CLUSTER IMPACT ENERGY ON THE MICROSTRUCTURE OF FILMS GROWN BY CLUSTER DEPOSITION

The microstructure of thin film growth during low-energy cluster beam deposition is studied in a series of molecular dynamics simulations. The films are grown by depositing Ni clusters on a Ni (111) substrate at room temperature. The deposition of a single Ni cluster is first studied, followed by a detailed analysis of the effect of the impact velocity of the deposited clusters on the microstructure of the growing film. The observed differences in the microstructure are related to the differences in the impact-induced processes. In the case of the lower incident energy only a partial transient melting of a small contact region between the incoming cluster and the film takes place. Epitaxial growth is seen to occur for the first few layers of the clusters in contact with the substrate, above which the clusters largely retain their crystal structure and orientation. The films grown by deposition of low-energy clusters have a low density (~50% of the density of a perfect crystal) and a porous “foamy” structure with a large number of interconnected voids. The higher-energy impacts lead to the complete melting and recrystallization of the whole cluster and a large region of the film, leading to the epitaxial growth, smaller number of localized voids, and a higher overall density of the growing film.

Electron-beam directed vapor deposition of multifunctional structures for electrochemical storage

Multifunctional structures are those, which combine load- bearing support in addition to additional functions such as mechanical actuation, distributed power supply or thermal management. Electron beam - directed vapor deposition technology has been used to investigate deposition methodologies for two multifunctional battery concepts: a linear/truss base nickel - metal hydride and a fiber based solid-state Li+ ion multifunctional battery. Porous nickel coatings for the cathodes and porous rare earth metal coatings based on La and Ni or Ti and Zr for the anodes are being investigated for the nickel - metal hydride system; where LiV2O5, LiPON, and Sn3N4 are being investigated for the Li+ ion based system. Electron beam - directed vapor deposition is being used for deposition of all cathode/anode structures to provide an economical method for the development of these novel multifunctional structures.