Influence of the Definition of the Representative Volume Element on Effective Thermoelastic Properties of Thermal Barrie
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JTTEE5 18:988–995 DOI: 10.1007/s11666-009-9351-0 1059-9630/$19.00 ASM International
Influence of the Definition of the Representative Volume Element on Effective Thermoelastic Properties of Thermal Barrier Coatings with Random Microstructure K. Bobzin, N. Bagcivan, D. Parkot, T. Kashko, G. Laschet, and J. Scheele (Submitted January 29, 2009; in revised form April 17, 2009) Coatings produced by air plasma spraying (APS) are characterized by a lamellar structure with high porosity, numerous microcracks and present a random microstructure. The process parameters influence not only the porosity but also the form of the microstructure defects. Different methods have been developed and used to investigate the influence of such random microstructures on the effective mechanical properties. Homogenization methods, based on asymptotic expansion of the involved fields, assume the periodicity of the microstructure. For materials with periodic microstructure they predict accurately and cost efficiently their effective properties. But, for non-periodic microstructures like APS thermal barrier coatings (TBC), the geometrical definition of the so-called representative volume element (RVE) plays a crucial role. In fact, these microstructures are approximated by periodic ones, whose characteristics (extension and phase distribution) are obtained by statistical methods. In this work, several approaches were applied to investigate the influence of the RVE position and geometry on the resulting effective properties of the TBC.
Keywords
APS coatings, coatings for gas turbine components, hardness and (visco)elastic properties, influence of process parameters, influence of spray parameters, properties of coatings
1. Introduction Air plasma spraying (APS) is a widely used technology for production of protective coatings against wear, heat and corrosion. The microstructure of APS coatings is defined by the (partially) molten powder particles solidifying on the substrate. Such microstructures are characterized by high porosity and microcracks. The investigation of the polished samples provides the evidence of so-called mesopores with a diameter between 0.5 and 10 lm together with micropores in the order of magnitude of 10-100 nm. These pores are typically connected to some open porous net of finest microcracks starting frequently from a mesopore and having a length between 10 and 100 lm. The characteristics of the microstructure defects (average crack length, pore diameters and shapes, distributions of the defects) as well K. Bobzin, N. Bagcivan, D. Parkot, and T. Kashko, Institut fu¨r Oberfla¨chentechnik (IOT), RWTH Aachen University, Augustinerbach 4-22, 52062 Aachen, Germany; and G. Laschet and J. Scheele, ACCESS e.V, Intzestr. 5, D-52072 Aachen, Germany. Contact e-mail: [email protected].
988—Volume 18(5-6) Mid-December 2009
as porosity are influenced by process parameters of the APS. Mathematical homogenization was applied to take into account the effects of the micro-heterogeneities of the coating in the structural sim
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