Hot isostatic pressing (hipping) can be used for upgrading castings, densifying presintered components, consolidating powders, and interfacial bonding. It involves the simultaneous application of a high pressure and elevated temperature in a specially constructed vessel. The pressure is applied with a gas (usually inert) and, so, is isostatic. Under these conditions of heat and pressure, internal pores or defects within a solid body collapse and diffusion bond. Encapsulated powder and sintered components alike are densified to give improved mechanical properties and a reduction in the scatter band of properties. In this article, the basic science of sintering and hipping is summarized and contrasted. The current state of understanding and modeling of hipping is then reviewed. Models can be classified either as microscopic or macroscopic in their approach. In the microscopic approach, the various mechanisms of densification are analyzed in terms of a single particle and its surroundings. In the macroscopic approach, the compact is treated as a continuous medium. In hipping, although the pressure is isostatic, shrinkage is not generally isotropic, particularly if containment is used. However, the shrinkage can now be well predicted, provided that the material and container properties are accurately known.

Fundamental aspects of hot isostatic pressing: An overview

Thermal barrier coatings on combustor liners and on turbine vanes and rotating blades are important for reducing metal temperatures in current and advanced turbine engines. Some coating materials such as zirconia are partially transparent to thermal radiation, and radiation within a coating will increase as temperatures are raised for higher efficiency engines. Hence, it is necessary to determine if radiation effects in a coating are a design consideration. For this purpose, the engine thermal environment is first summarized with regard to factors affecting radiative heat transfer. Radiative and thermal properties of zirconia are then considered, and methods of radiative analysis are briefly discussed. Typical temperature distributions and heat fluxes are given from the analysis of zirconia thermal barrier coatings on vanes and rotating blades, and on a combustor liner where the coating surface is expected to be covered with soot. The effects of various thermal conditions and heat transfer parameters are examined to indicate when radiation effects might be significant within a coating in a turbine engine. The largest effects were found in the combustor where coatings are subjected to large incident radiation. For coatings on turbine blades away from the combustor, and hence without large incident radiation, effects of radiation were found to be very small.

Analysis of thermal radiation effects on temperatures in turbine engine thermal barrier coatings

Finite element analyses and simulations of manufacturing processes of composites and their mechanical properties: a bibliography (1985–2003)

The invention is directed to a ceramic material for use in thermal barriers for high temperature cycling applications and high temperature abradable coatings. The material is an alloy formed predominantly from ultra-pure zirconia (ZrO 2 ) and/or hafnia (HfO 2 ) that has uncharacteristically high sintering resistance to achieve a high service lifetime and low thermal conductivity to achieve high operating temperatures. In the material, oxide impurities such as soda (Na 2 O), silica (SiO 2 ), alumina (Al 2 O 3 ), titania (TiO 2 ), hematite (Fe 2 O 3 ), calcia (CaO), and magnesia (MgO) make up no more than 0.15 weight percent. The invention provides materials to produce a coating structure so that the changes in the coating microstructure over the in-service lifetime are either limited or beneficial.

Ceramic material for high temperature service

An Al2O3–YSZ thermal barrier coating was prepared by co-deposition of Al2O3 and YSZ (8 wt.% Yttria Stabilized Zirconia) onto NiCoCrAlY bond coat by means of electron beam physical vapor deposition (EB-PVD). Analysis of SEM/EDXS showed that the structure and composition distribution across the thickness of the coating changed continuously. Simultaneously, a gradient micro-porous microstructure formed in the coating, which resulted in micro-hardness of the coating increasing gradually towards its surface. Additionally, the Al2O3–YSZ coating exhibited a lower thermal conductivity than traditional YSZ coating, which was caused by the micro-porous microstructure in the coating and the vacancies appearing in the ZrO2 lattice.

Preparation of Al2O3–YSZ composite coating by EB-PVD

Improvements in the efficiency of gas turbine require the highest operating temperatures possible. Because the Ni-base superalloys used as turbine materials rapidly lose strength and oxidize above 1,000 C, a reduction in service temperature is often accomplished by the use of thermal barrier coatings. The temperature dependence of the thermal conductivity of multilayer coatings made by a plasma spray technique as well as some coatings made by physical vapor deposition (PVD) was investigated. The multilayer coatings consisted of a varying number of layers of Al{sub 2}O{sub 3} and ZrO{sub 2} stabilized by 8% Y{sub 2}O{sub 3}. Plasma sprayed coatings exhibited a large reduction in thermal conductivity at all temperatures when compared to the bulk monolithic materials. This reduction was found to be due to porosity as well as thermal resistance brought about by interfaces in the coatings. A comparable reduction in thermal conductivity was achieved in monolithic ZrO{sub 2} as well as in a composite coating deposited by the PVD technique. Microstructural factors that may be responsible for this reduction are discussed.

Microstructure and Thermal Conductivity of Thermal Barrier Coatings Processed by Plasma Spray and Physical Vapor Deposition Techniques

Finite element method (FEM) simulation results were compared to experimental observations to establish the suitability of advanced modeling techniques for the prediction of the consolidation behavior of continuous fiber, metal—matrix composites. Two consolidation techniques were examined: hot isostatic pressing (HIP) of foil—fiber—foil layups and HIP of tapecast monotapes. In both cases, the matrix was the alpha-two titanium aluminide alloy Ti—24Al—11Nb (a/o), and the fibers were silicon carbide. Model predictions and accompanying experimental measurements revealed the important effect of the interface friction—shear factor on consolidation time for foil—fiber—foil layups. In addition, the predicted consolidation times for the foil—fiber—foil method were found to be sensitive to small variations in HIP temperature and material flow properties such as the strain-rate sensitivity, especially for low consolidation temperatures. By contrast, predicted consolidation times for tapecast monotape layups were relatively insensitive to the magnitude of the interface friction—shear factor. The kinetics of densification of the tapecast monotapes were well described using an FEM model incorporating a material-sensitive yield function and associated flow rule.

Validation of computer models for the consolidation of metal—matrix composites

A simple analysis of the hot consolidation of metal-matrix composite panels from tapecast monotape layups

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Microstructure and Thermal Conductivity of Thermal Barrier Coatings Processed by Plasma Spray and Physical Vapor Deposition Techniques

Validation of computer models for the consolidation of metal—matrix composites

A simple analysis of the hot consolidation of metal-matrix composite panels from tapecast monotape layups