Περίληψη σε άλλη γλώσσα
The present thesis is entitled “Study of simple and composite cold sprayed coatings: microstructure, co-deposition mechanism, tribology and corrosion resistance”. A complete study of metallic and composite (metal+ceramic) coatings produced using cold spray, was the aim of this thesis. Cold Gas Dynamic Spray or Cold Spray is the most recent development in the field of spray techniques for the production of coatings. In this process, the gas (nitrogen, helium or air) is introduced into a Laval type nozzle and produces a supersonic gas flow. Spray particles are accelerated to a high velocity (typically 300-1200 m/s) and are deposited at a temperature well below their melting temperature. Initially in this research, three different materials with important industrial applications were cold sprayed: copper on Al2017 substrate, CoNiCrAlY on Hastelloy X substrate and titanium on Ti6Al4V substrate. The microstructure of these coatings was studied in order to determine the most suitable materia ...
The present thesis is entitled “Study of simple and composite cold sprayed coatings: microstructure, co-deposition mechanism, tribology and corrosion resistance”. A complete study of metallic and composite (metal+ceramic) coatings produced using cold spray, was the aim of this thesis. Cold Gas Dynamic Spray or Cold Spray is the most recent development in the field of spray techniques for the production of coatings. In this process, the gas (nitrogen, helium or air) is introduced into a Laval type nozzle and produces a supersonic gas flow. Spray particles are accelerated to a high velocity (typically 300-1200 m/s) and are deposited at a temperature well below their melting temperature. Initially in this research, three different materials with important industrial applications were cold sprayed: copper on Al2017 substrate, CoNiCrAlY on Hastelloy X substrate and titanium on Ti6Al4V substrate. The microstructure of these coatings was studied in order to determine the most suitable material for cold spraying. Also, copper and CoNiCrAlY coatings were compared with coatings obtained using conventional thermal spray techniques (High Velocity Oxy-Fuel and Wire Arc). The most suitable material for cold spray was found to be copper due to the high deformation of copper particles. Cold sprayed copper coatings were qualitatively superior, as they were fully dense, they did not contain oxides and the coating-substrate interface did not present any defects (micro-cracks, porosity or voids). Cold sprayed CoNiCrAlY coatings and Ti coatings contained porosity and voids (4.2% and 14%, respectively). Oxidation was not observed in any of the cold sprayed coatings. Considering that copper was the most suitable material for use in cold spray, composite Cu+Al2O3 coatings on Al2017 substrate were studied. The aim was to study the microstructural characteristics of the coatings as well as to understand the way that copper and alumina particles were co-deposited. A copper powder of 13μm particle size and two alumina powders, a fine with a size range of 2-12 μm and a coarse of 15-45 μm size range, were used in order to prepare the feedstock mixtures. The copper powder was mechanically blended with each one of the alumina powders in various Al2O3 contents: 0, 10, 20, 25 and 30%wt. Porosity of all the composite and pure copper coatings was very low (<1%). Al2O3 particles were uniformly dispersed in the copper matrix of the composite coatings. Spraying with mixtures of 10, 20, 25 and 30%wt. Al2O3 resulted in coatings with 3, 5, 6 and 7% area (and volume) fraction. The %wt. percentage of deposited particles was equal to 10% of the initial percentage in the feedstock mixture. Since alumina particles could not be deformed on impact, some very small gaps (1-2μm) surrounding the hard phase particles were observed.Craters and deformations of angular morphology were observed in the surface of the coatings. They were created from Cu and Al2O3 particles (respectively) that impacted, but rebounded and did not adhere. Also, fragmentation of large Al2O3 particles was observed. As the copper particles impinged on the substrate and on the particles previously deposited, a flow of material took place and metal jetting was formed at some parts of interfaces. The coating-substrate interface was qualitatively very good for all the composite and copper coatings, as no defects were detected (porosity, voids or microcracks). The bonding of coating with the substrate was achieved mainly through the ductile copper particles. However, alumina particles were present in the interface, too. Metallurgical bonding was formed at some interfaces between copper particle and substrate and between copper particles, indicating strong bonding.Al2O3 particles bonded with copper particles mechanically. They were entrapped among the copper particles, which were deformed and surrounded them. The bonding of copper particles was based on the heavy deformation and on the formation of metallurgical bonds at some interfaces. From the results of EDS analysis it was clear that oxidation did not happen during cold spraying, as the oxygen content was negligible. The cold sprayed composite and pure copper coatings presented a relatively high hardness of 163-177 HV0.3 for the various alumina contents. The microhardness values of composite coatings were slightly higher than that of copper coating, due to the reinforcing effect of Al2O3 hard phase. The X-ray diffraction patterns revealed no evidence for the presence of any phase change or oxidation during cold spraying. The results of the total study of Cu+Al2O3 coatings provided the information for the physical model that was suggested. This physical model describes the phenomena that took place during cold spray and how the composite coating was built.The study of corrosion behavior of specimens coated with Ti, Cu+Al2O3 and Cu, was also one of the aims of this research. The electrolyte was 3.5%wt. NaCl solution. Potentiodynamic polarization curves were obtained for cold sprayed Ti coating and Ti6Al4V alloy (substrate). The passive layer (TiO2) that was formed, was stable in a wider range of potential values in the substrate than in the Ti coating. The high porosity (14%) of the coating influenced its corrosion behavior and phenomena that took place in the substrate could have been interfered. However, generally, the corrosion behavior of cold sprayed Ti coating was similar to that of bulk Ti6Al4V. The corrosion behaviour of Cu+Al2O3 and Cu coatings was studied using linear polarization and potentiodynamic polarization. The potentiodynamic polarization curves of all the coatings coincided regardless of the alumina content and the alumina particle size. The cold sprayed copper coatings retained the very good corrosion behavior of bulk copper. All the coatings’ curves presented passivity and the corrosion products were copper oxides together with insoluble hydrated copper chlorides.Moreover, the tribological behavior of cold sprayed pure copper coating and Cu+Al2O3 coatings which contained 3, 5 and 7% (area percentage) of fine (2-12μm) and coarse (15-45μm) Al2O3 was studied. Tribological tests were performed with a “ball-on-disc” tribometer until 20,000 cycles were completed. The values of sliding velocities were 5 and 10 cm/s and the loads used were 2, 5, 7 and 10N. The antagonistic (counter) material was an Al2O3 ball. Copper coating presented the lowest friction coefficient compared to all the composite coatings. Also the coefficient of friction (μ) of Cu coating reached the steady state earlier than that of the Cu+Al2O3 coatings. The specific wear rates (mm3/N•m) of all the composite coatings, for all the Al2O3 contents and both Al2O3 particle size ranges, were lower than the specific wear rate of copper coating. Coatings reinforced with fine alumina (2-12μm) particles exhibited lower wear rates than coatings with coarse alumina particles (15-45μm) for both sliding velocities. The dominant wear mechanism of the coatings was microabrasion – microploughing combined with plastic deformation and oxidation of the copper particles.
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