Modelling of gas permeation through ceramic coatings produced by thermal spraying

An analytical model has been developed for simulation of gas permeation through thermal spray coatings, and for prediction of the permeability. The model is based on a geometrical representation of the coating microstructure. This involves thin splats, with limited inter-splat bridging, and microcracks within the splats. Important variables include the thickness (smallest dimension) of the inter-splat pores and intra-splat microcracks. Uniform values of these parameters are used in the model. Predicted permeabilities are compared with experimental data obtained using plasma-sprayed zirconia coatings, both in the as-sprayed state and after heat treatments sufficient to generate significant microstructural changes via sintering effects. In general, good agreement is obtained and it is concluded that the model constitutes a useful tool for exploration of gas permeation characteristics in coatings or layers produced by thermal sprayin

The Effect of a High Thermal Gradient on Sintering and Stiffening in the Top Coat of a Thermal Barrier Coating (TBC) System

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

Numerical analysis of switching performance evaluators in low-voltage switching devices

An arc modelling is a valuable and useful tool to evaluate the switching performance of low-voltage switching devices (LVSDs) during breaking operation before testing real products. Moreover, it helps improve interruption capability of LVSDs and optimize them. This paper focuses on the numerical simulation of the arc behavior in AC devices before zero current and prediction of the re-ignition after current zero based on the simulated arc voltage. The 3-D arc modelling is based on the conventional magnetohydrodynamics theory and it considers the motion of a contact, arc root, radiation and air properties which vary with the temperature and pressure

Modelling of transient liquid phase bonding in binary systems — A new parametric study

An established mathematical model, describing the rate at which transient liquid phase bonding (TLP bonding) progresses in binary alloy systems, is subjected to careful scrutiny. It is shown that the process can be characterised using just two dimensionless parameters. An advantage of such dimensionless characterisation is demonstrated by analysis of the solution for solidification of semi-infinite systems. It is known that analytical formulae for the rate at which the liquid region solidifies are valid only for certain restricted cases. This is investigated by numerical modelling, and the requirements for the formulae to be applicable are rationalised. Maps presented here can be used to determine whether the semi-infinite solution would provide an acceptable approximation for any given system. Information is also presented concerning optimal combinations of phase diagram characteristics, diffusivities and system dimensions required for rapid TLP solidification, which can be used to identify the best melting point depressant (MPD) materials to use for particular TLP requirements. The analysis reveals that, as a consequence of their higher solubilities, elements forming substitutional solutes in the parent plates may often allow faster TLP solidification than those forming interstitial solutes, despite the fact that the latter group normally exhibits much higher diffusivities

Properties and Performance of High-Purity Thermal Barrier Coatings

It has been found that reducing the level of impurity oxides (particularly SiO2 and Al2O3) in 7YSZ, from about 0.2 wt% to below 0.1 wt% raises the sintering resistance and the phase stability of plasma-sprayed coatings. The implications for the usage of these coatings at elevated temperatures are examined. It is concluded that using relatively high-purity powder of this type is likely to confer substantial benefits in terms of the thermomechanical stability of the coatings under service conditions

Effect of Heat Treatment on Pore Architecture and Associated Property Charges in Plasma Sprayed TBCs

Plasma sprayed 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 1400C, for times of up to 100 hours) and coating purity on the pore architecture in detached YSZ top coats has been characterised by Mercury Intrusion Porosimetry (MIP) and BJH Analysis. While the overall porosity level (measured by densitometry) 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, although these changes occurred less rapidly with coatings produced using high purity powders. Fine pores (<~50 nm) rapidly disappeared, while the specific surface area dropped dramatically, particularly at high treatment temperatures (>~1300C). These changes are thought to be associated with improved inter-splat bonding and increased contact area, leading to disappearance of much of the very fine inter-splat 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 [1], 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 porosity architecture measurements reflect the most significant microstructural changes. REF.1. Golosnoy, IO, Tsipas, SA and Clyne, TW, An Analytical Model For Simulation Of Heat Flow In Plasma Sprayed Thermal Barrier Coating, J. Thermal Spray Techn., 14 (2005) 205-214

Development of an engineering optimization tool for miniature pulsed plasma thrusters

Pulsed Plasma Thrusters (PPT) are an established technology for compact thrust propulsion systems. Although PPT optimization has been performed previously it requires complex numerical codes. Although the scaling laws have been suggested they mainly applicable for large thrusters when edge effects can be neglected. A new 0D pulsed inductive acceleration model has been developed which links together the dynamics of the current sheet with the plasma dimensions and ionization processes. The model novelty is in a self-consistent estimation of the plasma sheet properties (temperature, density, thickness) driven by the magnetic pinch pressure and propellant ablation together with its simplicity. Parametric studies have been performed in an attempt to arrive at modified scaling laws for small PPTs

Development of an engineering optimization tool for miniature Pulsed Plasma Thrusters

Pulsed Plasma Thrusters (PPT) are an established technology for compact thrust propulsion systems. Although PPT optimization has been performed previously it requires complex numerical codes. A 0D pulsed inductive acceleration model has been developed which links together the dynamics of the current sheet with the plasma dimensions and ionization processes. The model novelty is in a self-consistent estimation of the plasma sheet properties (temperature, density, thickness) driven by the magnetic pinch pressure and propellant ablation together with its simplicity. Parametric studies have been performed in an attempt to arrive at optimized design solutions for small PPTs

Investigation of heaterless hollow cathode breakdown

The development of long life high powered (>50A) hollow cathodes is of importance to meet the demand of increasingly powerful Gridded Ion engines and Hall Effect thrusters. High power cathodes typically operate at greater temperature ranges, which poses a significant challenge to maintain heater reliability. The heater component commonly used to raise the insert to emissive temperatures, has inherent reliability issues from thermal fatigue caused by thermal cycling with large temperature variations. A self-heating hollow cathode allows for potentially higher reliability through design simplicity of removing the heater component, and in addition there can be savings in mass, volume, ignition time and power. This study characterizes the initiation of the start-up process for a heaterless hollow cathode. As such the study analyses conditions of the initiation as a function of detailed geometrical and physical parameters. The Paschen curve can be seen to give a qualitative explanation for the breakdown voltage variance. The quantitative variations between the empirical results and Paschen curve are discussed in relation to non-uniform pressure simulations