The Sensitivity of Abradable Coating Residual Stresses to Varying Material Properties R.E. Johnston (Submitted January 30, 2009 in revised form May 21, 2009) This paper reports recent research on abradable materials employed for aero-engine applications. Such thermal spray coatings are used extensively within the gas turbine, applied to the inner surface of compressor and turbine shroud sections, coating the periphery of the blade rotation path. The function of an abradable seal is to wear preferentially when rotating blades come into contact with it, while mini- mizing over-tip clearance and improving the efficiency of the engine. Thermal spraying of an abradable coating onto a substrate imparts two components of residual stress rapid quenching stresses as the spray material cools on impact and stresses arising from differential thermal contraction. In-service thermal stresses are superimposed by the differential expansion of these bonded layers. The combination of the production and operation history will lead to thermal-mechanical fatigue damage within the abradable coating. The present paper will describe the numerical modeling and sensitivity analysis of the thermal spray process. The sensitivity of residual stresses (with varying material properties, coating/substrate thickness, Poisson��s ratio, and substrate temperature) predicted by the Tsui and Clyne progressive deposition model enabled identification of performance drivers to coating integrity. Selecting material properties that minimize in-service stresses is a crucial stage in advancing future abradable performance. Keywords applications, APS coatings, coatings for gas tur- bine components, coating-substrate interaction, modeling, properties, spray deposition 1. Introduction Plasma spraying of coatings onto a suitable substrate falls under the umbrella of thermal spray processing. In the production of gas turbines, plasma spraying is used to manufacture abradable seals around the circumference of the compressor and turbine blade sections. The abradable seal lies along the periphery of the blade rotation path, providing reduced clearance between the rotating blade tip and stationary engine casing (termed over-tip clear- ance). The clearance is minimized because the abradable coating is installed and wears preferentially to the blade tip. Blades cut into the abradable as it sweeps a path during rotation, producing a rub track where the blade tips rotate while seated within a groove in the abradable coating. Reducing the over-tip clearance can lead to sig- nificant improvements in the efficiency of the engine and specific fuel consumption (SFC) (Ref 1) because more gas is drawn through the blade area and less escapes over the tips of the blades, therefore imparting more work to blade rotation. A plasma flame is used to rapidly heat the abradable powder, which is accelerated at high velocity onto the surface of the substrate. The coating is formed in layers with successive passes of the plasma spray gun. A coating powder material consisting of an AlSi matrix and hexag- onal boron nitride (hBN) dislocator phase (Metco 320NS), a bond coat of composition 96% Ni 5% Al (Metco 450NS), and substrate material Jethete M152 (RR EAK) were considered for this study. Residual stresses are generated in coatings during their manufacture. Thermal spraying of a coating onto a sub- strate imparts two components of residual stress. The first component is introduced as the molten or thermally softened coating powder impacts with the surface of the substrate or previously deposited coating layer, which is at a lower temperature, forming a ��splat��. These particles are flattened and quenched to the underlying surface tem- perature with very high cooling rates, typically in excess of 106 K/s (Ref 2). During this rapid cooling, the thermal contraction of the splat will be constrained by the under- lying material, therefore introducing a tensile stress into the sprayed material. The second component of residual stress is due to differential thermal contraction during cooling of the sprayed coating and substrate part to room temperature. Stresses are induced when bonded layer materials having different coefficients of thermal expansion, cool and contract at different rates. Normally, the coating will have a greater coefficient of thermal expansion than the substrate and will therefore experience greater contrac- tion upon cooling, and subsequently an induced tensile stress. R.E. Johnston, Materials Research Centre, School of Engineer- ing, Swansea University, Singleton Park, Swansea SA2 8PP, UK. Contact e-mail: r.johnston@swansea.ac.uk. JTTEE5 18:1004���1013 DOI: 10.1007/s11666-009-9378-2 1059-9630/$19.00 �� ASM International 1004���Volume 18(5-6) Mid-December 2009 Journal of Thermal Spray Technology Peer Reviewed

Stoney (Ref 3) derived an equation to predict residual stresses in thin films (film thickness substrate thick- ness). The first analytical model for elastic thermal stresses in a bilayer system was derived by Timoshenko (Ref 4), and was based on classical bending theory. This approach has been widely adopted to analyze thermal stress in multilayer systems (Ref 5-7). Closed-form solutions con- sidering metal/ceramic bonded strips have been derived for elastic loadings (Ref 8, 9) and for the elastic/plastic state, incorporating work hardening into the metallic layer (Ref 5). These models consider only the residual stresses due to differential thermal expansion/contraction of bon- ded multilayer systems. Models have been developed that incorporate both the stresses derived from differential thermal contraction, and the ��quenching�� stresses developed during the thermal spray deposition process. Tsui and Clyne (Ref 10) devel- oped an analytical model for prediction of residual stress distributions in progressively deposited coatings. This paper investigated the effect on predicted residual stress distributions of changing various material properties and deposition parameters. The aim of this work was to devise, for abradable coating materials, a tool that will aid future coating design decisions, based on minimizing the post- production residual stresses. The Tsui and Clyne deposi- tion and contraction model architecture is well suited to a sensitivity study of input parameters, and the processes modeled are the equivalent of those used to produce abradable coatings. 2. Tsui and Clyne Progressively Deposited Coatings Analytical Model Tsui and Clyne (Ref 10) formulated a model, based on planar geometry, to predict residual stresses introduced to a substrate and coating during deposition and also during cooling of the sprayed coating/substrate. The model assumes that the substrate is clamped only at one end, while being free to bend during the process, as shown in Fig. 1. The full derivation of formulae and nomenclature can be found in Tsui and Clyne��s paper entitled ��An Analytical Model for Predicting Residual Stresses in Progressively Deposited Coatings. Part 1: Planar Geometry�� (Ref 10). The Tsui and Clyne model was selected for several rea- sons. Firstly, its suitability for a sensitivity study, and secondly, it does not require consideration of complex heat transfer mechanisms, inelastic material behavior and temperature dependence. Abradable material properties in freestanding form are relatively unknown. Only in recent years have methods (Ref 11) been developed for investigating these properties without the associated sub- strate interactions. Until these properties have been fully investigated, the Tsui and Clyne model is the most suitable method for performing a sensitivity analysis of abradable coating materials. 3. Modifications to Tsui and Clyne��s Model Several modifications were made to the Tsui and Clyne progressively deposited coatings analytical model to pro- duce a model that mimics abradable coating production processes as closely as possible: Firstly, Tsui and Clyne��s model uses one value for DT throughout the chain of calculations. The modified model uses a value of DT (DTdep) during the deposition stress calculation and a separate DT (DTcool) value during the cooling stress calculation. A bond coat is applied to the substrate to assist adherence of coating (keying). An example of Metco 320NS abradable coating applied to a Metco 450NS bond coat and Jethete Steel substrate is shown in Fig. 2. The bond coat (Metco 450NS) was included in the model as the first tier in the discretized coating layer. The coating was divided into ten layers previously for the model this model utilizes the first layer for bond coat material and remaining nine layers are abradable coating material. The bond coat was not discretized into more than one layer as the calculation of the associated residual stress distribution was deemed unnecessary, particularly Fig. 1 Tsui and Clyne beam configuration Fig. 2 Micrograph displaying substrate-bond coat-coating system Journal of Thermal Spray Technology Volume 18(5-6) Mid-December 2009���1005 Peer Reviewed