The final target of this study was to achieve a better understanding of the behaviour of thermally sprayed abradable seals such as AlSi/polyester composites. These coatings are used as seals between the static and rotating parts in gas turbine applications. The machinability of the composite coatings during the friction of the blades depends on their mechanical and thermal effective properties. In order to predict these properties from micrographs, numerical studies were performed with different software packages such as OOF developed by NIST and TS2C developed at the UTBM. In 2008, differences were reported concerning prediction of effective thermal conductivity obtained with the two codes. In the present paper, it is shown that a particular attention must be paid to the mathematical formulation of the problem. In particular, results obtained with a finite difference method using a cell centre approach or a nodal formulation, allow explaining the discrepancies previously noticed. A comparison of the predictions of computed effective thermal conductivities is thus proposed for different codes and different meshing methods. This study is part of the NEWAC project, funded by the European Commission within the 6th RTD Framework program (FP6).

This paper reports morphological study of coatings microstructure performed on two selected reference abradable materials: AlSi-hBN and NiCrAl-Bentonite. Scanning Electron Microscopy (SEM) was used to obtain adequate micrographs for analysis. The porosity and non-metal particles morphology was assimilated to elliptical inclusions within the metallic matrix. Accordingly, they were described by means of three different geometrical parameters related to the shape, size and orientation distribution, allowing additional statistical analysis on the coatings.

Abradable coatings are located on the stationary parts of gas turbines, in front of blades, which cut a track in them. This has to be achieved with minimum wear of the blades, in order to control the over-tip leakage. These coatings are generally deposited by thermal spraying of composite powders comprising a metal base, a polymer filler generating porosities and a dislocator such as hBN. The very demanding properties are nowadays adjusted using rig tests, where samples are rubbed by the contact of a dummy, simulating actual working conditions in an aircraft engine. Several types of behaviour are usually described, but few numerical data are produced from these tests. Only the blade wear (or metal transfer) is generally measured. As the understanding of contact phenomena is fundamental for the development of predictive models allowing the design of more performing materials, a comprehensive characterization process of the rub path was developed. The study was based on a topological survey made by laser profilometry, giving three-dimensional maps. These maps were then processed by image analysis and several parameters were computed, like surface roughness and parameters giving information on the shape and orientation of the holes or grooves in the rubbed surface of the samples.