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Abradable seal coatings and claddings for compressor applications


Abradable seal coatings require accurate metallographic treatment to ensure a realistic representation of the inherent microstructure in the resulting micrographs. High level of porosity as well as the combination of metallic, ceramic and/or synthetic material phases are prone to polishing failures due to their different properties and behaviour during the micro preparation process. Detailed mounting and polishing guidelines are essential for a consistent high micrograph quality and a comparative microstructure analysis. Metallograpic analysis are a standard technique for the evaluation of surface coatings. Therefore the here established standardised metallographic preparation techniques are useful for any coating technology, particularly for porous and imhomogenous coating structures. They can be used for (destructive) quality control as well as process optimization or failure assessment.
An innovative route has been developed to model very porous materials, such as abradable deposits. SEM micrographs of coatings sections give information about the different phases present in the structure (matrix, dislocator and porosities) and their morphology. Equivalent images are then produced, replacing particles and porosities by ellipses, which help to: - Simplify the picture and the mesh, - Characterize geometrically each inclusion, allowing statistics on them, - Restore the effective three-dimensional geometry of the coating.
LEG program (LERMPS Ellipses Generator) has been developed to produce images equivalent to microstructures and well adapted to finite element analyses. It can be used in two cases: Case 1 - The material to be modelized exists and micrographs have been analyzed and characterised by a set of ellipses. To eliminate unrealistic overlapping of these ellipses, LEG is used to redistribute them randomly in the picture, saving the initial matrix % by an adjustment of the picture size. Case 2: The material to be modelized does not exist yet, but can be described by a set of ellipses equivalent to the pores and particles of additional phases included in a metallic matrix. Assuming some limits for the areas, shapes and angles of these ellipses, LEG can generate a random set of ellipses and distribute them with a given density to produce the same kind of images as those deduced from real micrographs.
The software OOF (Object-Oriented Finite Element Analysis of Real Material Microstructure Working Group) has been developed by NIST (US National Institute of Standards and Technology). It is used to create a mesh from a picture representing the microstructure of a coating and to simulate mechanical and thermal tests. The meshing process produces elements of decreasing sizes close to the interfaces between two different materials. Virtual tensile and shear tests are conducted on the meshed structure to generate the 5 parameters of an anisotropic linear elastic law of behaviour (2 Young's moduli - 2 Poisson's coefficients - 1 shear modulus). 2 different values of the anisotropic coefficient of thermal expansion are also computed by the same model after the simulation of a temperature step.
LATS (LERMPS Abradability Test Simulator) is an approach developed to estimate the temperatures evolutions during a rig test. The target is to describe the thermal phenomena seen during real rub. To-date, it is available as an Excel file including several specific functions. LATS is a completely analytical approach considering a large number of revolutions comprising each 3 successive steps: - Interaction between the blade and the coating at contact instant, unless a gap was generated previously between both parts. - Heating by rubbing, assuming a very simple generation of a heat flux. - Cooling in air, with the hypothesis of a medium at constant temperature existing around both parts. The results are the variations of the temperatures in the system versus the number of rotor revolutions.
A finite element model of abradable coating rig tests was developed, allowing extensive studies on the influence of coating properties and test conditions on their performances. The FE code was used to simulate a single contact between a blade and a part of the abradable coating. Target and contact elements were used to model the contact between both parts. The coupling of thermal and mechanical analyses is allowed by these elements, where the heat generation due to frictional dissipated energy is computed. The motions of the two parts during the tests were simulated by dynamical displacements. An initial incursion of the blade into the coating was taken into account by an initial distortion of the mesh, resulting from a preliminary static calculation. Relevant algorithms and pertinent parameters were chosen for the contact analysis, the non-linear analysis and the dynamical analysis. The results obtained for reference coatings were dynamical fields of stresses and temperatures. Comparisons between the maximum computed values in each studied case and the corresponding blade wear measurements showed some consistency. Furthermore, these results contribute to explain why different test rigs work sometimes differently.
New developed coatings such as abradables have to be tested before their applications in engines. At MTU a specially designed rub in rig is used to test these new abradables under nearly realistic conditions. However, the test temperature of this worldwide unique rub in rig was limited to a maximum temperature of 650°C. Because engine performance will be improved upon other terms by increasing the pressure ratio higher temperatures occur in compressors. Thus, higher test temperatures up to 750°C are required for MTU´s test rig. To meet this requirement from the design the existing rub in rig was modified with a new heating system, clamping elements for discs as well as for casings and with a new unit to control the heating system. Through this upgrade MTU got the ability to test abradables for future applications for themselves as well as for customers.
The HR15Y hardness test, which is widely used to characterize abradable coatings, is based of the measurement of the permanent residual deformation remaining after the application of a load through a spherical indenter. Thus it is clearly dependent on the plastic properties of the material. Estimates of the yield stress and of the tangent modulus were deduced from the simulation of a hardness test on 2 deposits of the same material, but having different thicknesses. This model was developed using a finite elements code.

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