Plasma sprayed Thermal Barrier Coatings (TBCs) exhibit many interlamellar pores, voids and microcracks. These microstructural features are primarily responsible for the low global stiffnesses and the low thermal conductivities commonly exhibited by such coatings. The pore architecture thus has an important influence on such thermophysical properties. In the present work, the effect of heat treatment (at temperatures up to 1400°C, for times of up to 10 hours) on the pore architecture in detached YSZ top coats has been characterised by Mercury Intrusion Porosimetry (MIP) and gas-sorption techniques. While the overall porosity level remained relatively unaffected (at around 10-12%) after the heat treatments concerned, there were substantial changes in the pore size distribution and the (inter-connected) specific surface area. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high treatment temperatures (~1400°C). These changes are thought to be associated with intra-splat microcrack healing, improved inter-splat bonding and increased contact area, leading to disappearance of much of the fine porosity. These microstructural changes are reflected in sharply increased stiffness and thermal conductivity. Measured thermal conductivity data are compared with predictions from a recently-developed analytical model, using the deduced inter-splat contact area results as input parameters. Good agreement is obtained, suggesting that the model captures the main geometrical effects and the pore size distribution measurements reflect the most significant microstructural changes.

It was found that the content of impurity oxides in 7YSZ, such as SiO 2 and Al 2 O 3 , has a significant effect on the coating sintering resistance and phase stability of 7YSZ thermal barrier coatings (TBCs). The reduction of the impurity content will significantly improve the sintering resistance and phase stability of 7YSZ TBCs and thus allow the 7YSZ TBCs to be used at higher temperatures.

A model of the sintering exhibited by EB-PVD TBCs, based on the principles of free energy minimization, was recently published by Hutchinson et al. In the current paper, this approach is applied to the sintering of plasma-sprayed TBCs and comparisons are made with experimental results. Predictions of through-thickness shrinkage and changing pore surface area are compared with experimental data obtained by dilatometry and BET analysis respectively. The sensitivity of the predictions to initial pore architecture and material properties are assessed. The model can be used to predict the evolution of the contact area between overlying splats. This is in turn related to the through-thickness thermal conductivity, using a previously-developed analytical model.

Superalloy substrates coated with plasma sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats have been subjected to a high heat flux in a controlled atmosphere chamber. The sintering exhibited by the top coat under these conditions has been studied and compared with the behavior observed during isothermal heating, both when attached to the substrate and when detached. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical heat flow model has been used to explore the stress state under isothermal and thermal gradient conditions. Sintering proceeds faster at higher temperature, but is retarded by the presence of tensile stresses (from differential thermal expansion between coating and substrate) within the top coat. Sintering occurs preferentially near the free surface of the top coat under gradient conditions, not only because of the higher temperature, but also because the in-plane stress is more compressive in that region.

A numerical finite difference model has been developed to treat the transfer of heat and momentum between a gas environment and a particle injected into it. The model is based on an explicit solution scheme for the thermal field and explicit treatment of the momentum exchange. The latent heat associated with phase changes is simulated via a post-iterative heat accumulation scheme. Particle-gas heat transfer is represented by a heat transfer coefficient, which is a function of relative gas velocity. The validity of the model is confirmed via comparisons between predicted behaviour and previously-published experimental data for thermal histories and particle trajectories. Comparisons are also presented with predictions from previously-developed models. Results are then presented for the behaviour of hollow zirconia particles, with particular attention being paid to in-flight melting characteristics. It is shown that there is an optimum combination of particle size and wall thickness for the promotion of efficient melting, for a given gas flow and temperature field.

Thermal barrier coatings systems are composed of a zirconium dioxide-(6 to 8 wt.%) yttrium oxide ceramic top coat about 300-500 micrometer in thickness, deposited either by air plasma spraying or electron beam assisted physical vapour deposition, over an MCrAlY (M = Ni, Co or NiCo) bond coat, about 100 micrometer thick, deposited by vacuum plasma spraying. In this paper, the stiffness of as-sprayed zirconia is measured using three different techniques, namely cantilever beam bending, ultrasonic resonance and nanoindentation. The paper explores the effect of post-deposition heat treatment on the value obtained. The results show that the cantilever bend technique, employed with a high precision scanning laser method of displacement measurement, was found to be the most reliable procedure. Paper includes a German-language abstract.

Test pieces were machined from vacuum plasma sprayed (VPS) deposits of the bond coat materials Ni-22Cr-10Al-1Y and Co-32Ni-21Cr-8Al-0.5Y. Changes in gauge length at high temperatures under various applied stresses were measured using a scanning laser extensometer. In this way, isothermal creep data and, using no applied load, expansivity data were obtained. The CoNiCrAlY creeps faster than the NiCrAlY at low stresses, but the reverse is true at high stresses. The CoNiCrAlY has an appreciably higher expansivity than the NiCrAlY. These data were used in a numerical process model to evaluate the effect of bond coat creep on the stress state of a TBC system. Comparisons between measured and predicted curvature histories during deposition were used to evaluate the quenching stress for the two materials. This is considerably higher for the CoNiCrAlY. Although creep generally results in reduced stress levels at service temperatures, it can generate residual stresses which are raised after cooling down to ambient temperature, particularly for the CoNiCrAlY. Evaluation of the strain energy release rates associated with various stress distributions, and comparison with measured interfacial fracture energy values, confirmed that debonding will tend to occur at the top coat / bond coat interface, rather than between the bond coat and the substrate. However, bond coat spallation is more likely with CoNiCrAlY than with NiCrAlY.

The gas permeability of plasma sprayed yttria-stabilised zirconia coatings has been measured over a range of temperature, using hydrogen and oxygen gas. The permeability was found to be greater for coatings produced with longer stand-off distances, higher chamber pressures and lower torch powers. Porosity levels have been measured using densitometry and microstructural features have been examined using SEM. A model has been developed for prediction of the permeability from such microstructural features, based on percolation theory. Agreement between predicted and measured permeabilities is good. Ionic conduction through the coatings has also been briefly explored. It is concluded that transport of oxygen through the top coat in thermal barrier coating (TBC) systems, causing oxidation of the bond coat, occurs primarily by gas permeation rather than ionic conduction, at least up to temperatures of about 1000°C and probably up to higher temperatures. Top coat permeabilities appreciably below those measured will be required if the rate of bond coat oxidation is to be reduced by cutting the supply of oxygen to the interface.

Thermal barrier coating systems have been heat treated in order to study the oxidation kinetics of the bond coat. All the surfaces of Ni superalloy substrates were sprayed with ~100 μm of a NiCrAlY bond coat, with or without ~250 μm of a ZrO 2 top coat. Thermogravimetric analysis (TGA) was used to monitor continuously the mass change as a result of oxidation of the bond coat during heating at 1000°C for 100 hours in flowing air. In addition, some specimens were heated to 1000°C in static air, cooled to room temperature, weighed and re-heated cyclically. The total exposure time was 1000 hours. Rates of weight gain were found to be higher for the cycled specimens, despite the absence of air flow. This is attributed to damage to the oxide film, which was predominantly α-Al 2 O 3 , as a consequence of differential thermal contraction stresses. The changing residual stress state during heat treatment was predicted using a previously-developed numerical model. A thin (1 mm) substrate with ~100 μm bond coat and ~250 μm ZrO 2 top coat was used in these simulations, which incorporated creep of the bond coat and the lateral strain associated with oxidation. It is concluded from these computations that, while high stresses develop in the oxide layer, the associated driving forces for interfacial debonding remain relatively low, as do specimen curvature changes. It seems likely that coating spallation after extensive oxide layer formation arises because the interface is strongly embrittled as the layer thickens.

A simple test procedure, based on steady state flow through a membrane, has been developed for measurement of the gas permeability of specimens over a range of temperature. The reliability of this equipment has been verified by testing solid disks containing single perforations and comparing the measured flow rates with those expected on the basis of laminar flow. Coatings of yttria-stabilised zirconia have been produced by plasma spraying in vacuum and in air. The specific permeability of these coatings has been measured at temperatures ranging up to 600°C, using hydrogen gas. It has been found that permeability is increased for coatings produced with longer stand-off distances and at higher pressures. Porosity levels have been measured using densitometry and microstructural features have been examined using SEM. A model has been developed for prediction of the permeability from such microstructural features, based on percolation theory. Agreement between predicted and measured permeabilities is good, although it is clear that more comprehensive data are needed in order to validate the model systematically.

Coatings have been produced by HVOF spraying of four different WC-Co powders, using two fuel gases and two oxygen contents in the flame, and characterised in terms of microstructure and resistance to abrasive wear. It is concluded that there is a close correlation between high levels of chemical reaction, occurring during spraying (and possibly during powder production), and poor wear resistance. Good wear resistance is favoured by using low porosity powders, which interact with the atmosphere less readily during spraying, and also by using a flame with a relatively low oxygen content. This probably minimises the degree of reaction by ensuring that conditions are reducing. Use of propylene rather than hydrogen gives coatings with slightly better wear resistance, despite the fact that the flame temperatures are higher. It is concluded that, for this relatively small rise in temperature, the positive effect on inter-splat cohesion seems to outweigh the negative effect of increased decarburisation.

An analytical model has been developed to predict the residual stress distributions in thermal spray coatings on substrates of finite thickness. This is based on the concept of a misfit strain, caused by either the quenching of splats or by differential thermal contraction during cooling. During spraying, the coatings are asssumed to deposit on the substrate in a progressive (layer-by-layer) manner. Although the misfit strain ("the quenching strain") is the same for each successive incremental layer of deposit, this is imposed each time on a "substrate" of changing thickness. The final stress distribution will in general differ from that which would result if the coating were imposed on the substrate (with the same misfit strain) in a single operation. The model is straightforward to apply: for example, it can be implemented using a standard spreadsheet program. The required input data are the quenching strain (or stress), the spraying temperature, material properties and specimen dimensions. Comparisons have been made between the predictions from this model and from a numerical model for two plasma sprayed systems. Good agreement is observed. The effects of varying certain parameters, such as coating thickness, substrate thickness, coating stiffness, etc, are readily explored, so that the model provides a useful tool for controlling residual stress levels. Application of the model to determine the quenching stress, in conjunction with the use of a curvature monitoring technique, is briefly outlined. In addition, an analysis is made of the errors introduced by using Stoney's equation to deduce stress levels from curvature measurements.

Cermet (WC-Co) coatings have been produced on steel substrates by plasma spraying in vacuum and in air. These have been examined microstructurally and characterised in terms of porosity content, stiffness, microhardness and abrasion resistance. Particular attention has been paid to the phase constitution, as revealed by X-ray diffraction and scanning electron microscopy. High precision densitometry has been used to study porosity levels. Coatings with three different metal contents (9, 12 and 17wt.%Co) have been examined. There is a strong tendency for chemical reactions to occur within the plasma plume, particularly for spraying in air. These reactions can result in the formation of various carbides and even of metallic tungsten. Thermodynamic and kinetic aspects of the reactions involved are briefly examined. Such reactions are strongly promoted by the presence of oxygen, and are much less marked during vacuum plasma spraying. Plasma power and substrate temperature have secondary effects on the degree of reaction which occurs. A marked correlation was observed between degree of reaction and resistance to abrasive wear. This is consistent with the reaction products being brittle and causing poor interfacial cohesion. It was also found that wear resistance was greater for the coatings with lower metal contents. This behaviour can be attributed to the wear occurring predominantly by ploughing of the metallic phase and consequent release of ceramic particles. This occurred more readily when the metal content was higher. In coatings which had undergone pronounced chemical reaction, however, metal had been replaced by reaction products which conferred poor cohesive strength, leading to poor wear resistance.

The adhesion of various interfaces in thermal barrier coating systems strongly affects their stability and thermal cycling life. A new spontaneous debonding technique and the four point bend delamination test have been applied to measure the interfacial fracture toughnesses of various interfaces in several thermal barrier coating systems. The spontaneous debonding technique is based on spraying a relatively stiff layer on top of the ZrO 2 coating. This raises the strain energy release rate for debonding, the magnitude of which is monitored via modelling of the stress distribution. The critical strain energy release rate for debonding (interfacial fracture energy) was then determined from the stress states before and after debonding. Thermal barrier coatings (TBCs), consisting of a Ni-22wt.%Cr-10wt.%Al-lwt.%Y bond coat and a ZrO 2 -8%Y 2 O 3 top coat, were deposited on a nickelbased superalloy. Two methods, air (APS) and vacuum (VPS) plasma spraying, were used to produce the bond and the top coats. The corresponding as-sprayed residual stress distributions and the interfacial fracture energies were evaluated. It was found that a VPS bond coat and an APS top coat produced the most mechanically stable structure. A layer of vacuum plasma sprayed AI 2 O 3 was then introduced between the top and the bond coat, designed to act as an oxygen diffusion barrier. The effect on residual stress distributions, and associated crack driving forces for debonding, at different interfaces were determined. The effect of the alumina layer on the oxidation behaviour was also studied. It is shown that the oxidation barrier could significantly enhance the coating life-time.