In plasma spraying coating processes of single-crystal and directional superalloy substrates, the incompatibility between the polycrystalline MCrAlY coating and the substrate leads to the creation of mismatch strains in the interface and, as a result, reduces the thermal-mechanical fatigue resistance. It can be The laser coating process allows the protective coatings to match the substrate by creating depositional coatings. The purpose of the present research is to deposit NiCoCrAlY coating on oriented superalloy substrate by laser cladding method and eva‎luate its effect on coating properties. For this purpose, the effect of the variables of the laser coating process on the sedimentation growth of the coating on the directional CMSX-4 substrate was investigated and the microscopic structure, defects, microhardness, roughness and high temperature oxidation resistance of the samples were eva‎luated. Microstructural studies were done using light microscope and electron microscope. The hardness of the samples was measured with a microhardness tester and roughness measurement with a laser profilometer. The isothermal oxidation test was performed at a temperature of 1000 ℃ for 144 hours in a programmable furnace. The results of microstructural investigations showed that single-pass and multi-pass NiCOCrAlY coatings on the oriented CMSX-4 substrate were grown by Epitaxial in the preferred [001] direction of the substrate. The microstructure of the coatings changed from the interface to the coating surface in the form of cellular dendritic, columnar dendritic and coaxial. The results of the geometry of single-pass coatings and the influence of process parameters on it showed that with increasing power and powder feeding rate and decreasing the speed of laser scanning, the height of the coating increases. On the other hand, coating width and penetration depth increase with increasing power and decreasing laser scanning speed and powder feeding rate. The amount of geometric fusion increases due to the increase in laser scanning power and speed and the decrease in the powder feeding rate. By reducing the laser scanning power and speed and increasing the powder feeding rate, the wetting angle increases. By changing the cooling conditions from air to dry ice, the columnar region of the optimal coating showed an increase in height of about 25% and the distance between the primary and secondary arms decreased. By increasing the speed of laser scanning, the distance between the arms of primary and secondary dendrites decreased from the interface to the coating surface. For the optimal sample, the microhardness of the substrate, interface, and coating was measured as 372, 413, and 516 Vickers, respectively. The surface roughness of the coatings decreased with increasing laser scanning speed. Examination of the samples obtained from the oxidation test showed that two phases α-Al2O3 and harmful spinel Ni(Cr, Al)2O4 were formed on the surface of the coatings. The oxide layer formed on the substrate suffered cracks, porosity and peeling; While the oxide layer of the coatings did not have any porosity, cracks and peeling. By increasing the laser scanning speed and reducing the surface roughness, the penetration of nickel and chromium elements from the coating to the oxide layer increased and more spinel phase was formed. The investigation of the oxidation kinetics of the substrate and NiCoCrAlY coatings showed an initial fast stage and a steady stage after that. The increase in the weight of the substrate was more than that of the coatings, and the oxidation rate increased in the coatings as the speed of the laser method increased and the surface roughness decreased. Cooling the substrate with dry ice slightly increased the oxidation rate.

احمد كرمانپور , فخرالدين اشرفي زاده

بهزاد نيرومند، قاسم عظيمي