فهرست مطالب احسان صدری

Thermal fatigue (thermal shock lifetime) of thermal barrier coatings is considered a key factor in performance of aero gas turbines and, thus, there is a need to simulate and perform thermal shock tests with near-actual operation in turbines. By using burner rig tests, for applying the simultaneous cooling from backside of the substrate along with cooling shocks at constant periods, the practical thermal shock conditions in aero turbines were designed and applied. In this research, two-layer conventional TBC with APS NiCoCrAlY bondcoat (BC) and APS YSZ topcoat (TC) were studied. The microstructure of coatings after 10 and 40 thermal shock cycles were evaluated for failure analysis. The interlamellar TGO with planar morpholohgy revealed detrimental effects on protection performance of BCs. Actually, the non-beneficial consumption of aluminum not only limits the oxidation protection of the bondcoat, but also the interlamellar APS-TGO intensified the harmful stresses at the TC/BC interface. The detrimental effects of Ratcheting at BC and TGO surfaces were higher than the stresses from growth of both interlamellar and interfacial TGOs at thermal shock performance of the coating.

In plasma sprayed nanostructured composite coatings with ceramic matrix, the feedstock must consist of nanoparticles of appropriate specifications. In this research, the procedure for production of Cr2O3-Ag agglomerated nanostructured composite powder to produce comosite coatings has been investigated. Nanopowders of Cr2O3 with 0, 2, 5, and 10 volume percentages of silver were dispersed to obtain a homogeneous aqueous dispersion appropriate for spray drying process. In the second stage, Cr2O3-Ag composite powders were produced by agglomeration process. The nanostructured composite powders were, then, used in the atmospheric plasma spray (APS) process to deposit a series of composite coatings for evaluation. The composite powders, with a granulated morphology, had uniform distribution of silver in a ceramic matrix and the coatings were composed of nanoparticles and particles of nano-sized crystallites. Experimental results indicated that presence of nanoparticle zones within the microstructure led to non uniform porosities formed between splats and these zones. Furthermore, use of nanopowders in the feedstock caused a reduction in lamellae thickness of plasma sprayed coatings.