Abstract
The present thesis deals with the microstructural characteristics and tribological properties of metallic and ceramic thermal sprayed coatings. These properties as well as the overall quality of thermal sprayed coatings depend on many factors such as spraying parameters, feedstock materials and adoption of the spraying method. This study brought into focus on five parameters that seem to influence substantially the quality of coatings and need to be taken into account during design the spraying procedure. Firstly, the influence of substrate metallurgy on the properties of coatings was studied, by selecting two stainless steels of different metallurgical and thermal properties. A martensitic and an austenitic stainless steel were chosen as substrates to be coated with a metallic (Mo-based) and a ceramic (ZrO2-24%MgO) coating by using atmospheric plasma spraying technique (APS). When forming the metallic and the ceramic coating, the higher thermal conductivity of the martensitic steel c ...
The present thesis deals with the microstructural characteristics and tribological properties of metallic and ceramic thermal sprayed coatings. These properties as well as the overall quality of thermal sprayed coatings depend on many factors such as spraying parameters, feedstock materials and adoption of the spraying method. This study brought into focus on five parameters that seem to influence substantially the quality of coatings and need to be taken into account during design the spraying procedure. Firstly, the influence of substrate metallurgy on the properties of coatings was studied, by selecting two stainless steels of different metallurgical and thermal properties. A martensitic and an austenitic stainless steel were chosen as substrates to be coated with a metallic (Mo-based) and a ceramic (ZrO2-24%MgO) coating by using atmospheric plasma spraying technique (APS). When forming the metallic and the ceramic coating, the higher thermal conductivity of the martensitic steel compared to austenitic resulted in the rapid dissipation of heat from the surface and hence lower substrate temperature. This led to unsatisfactory deposition and solidification of molten particles of powder and hence the forming layers of the coating, resulting in degradation of microstructural characteristics.Secondly, the effect of a metallic bond coating (NiAl) in the properties of ceramic coating (Cr¬2O3¬¬) is investigated. More precisely, the microstructural, tribological and mechanical properties of three coatings (Cr¬2O3¬¬, NiAl and Cr2O3¬¬–bond coating NiAl), obtained by atmospheric plasma spraying on medium carbon steel was studied focusing on the influence of bond coating. The application of Ni-5% Al, as a bonding layer between the steel and ceramic coating Cr2O3, led to increased microhardness values, reduced percentage of porosity (by ~ 27%), increased the adhesion strength and improve wear behavior.Another factor that was investigated was the feedstock powder. More precisely, the development of Mo and Cr-Mn alloyed steel thermal spray coatings, produced by conventional and nanostructured feedstock was studied. The coatings have been developed by APS and HVOF on AISI 1045 substrates. The purpose was the development of APS and HVOF alloyed steel coatings from nanostructured feedstock materials and their comparison with the respective conventional coatings in terms of microstructure, mechanical properties and tribological behavior. The atmospheric plasma spraying of conventional powder FeCr (Mn) C and nanocrystalline, provided two coatings of different properties as expected. Although the conventional coating showed lower roughness and porosity compared to the nanocrystalline coating, which due to these characteristics showed lower adhesion strength, microhardness was the same while friction and wear behavior was improved. By HVOF spraying technique conventional and nanostructured coatings presented similar microstructure characteristics. However, the nanocrystalline coating offers a reduced friction coefficient by about 30% compared with conventional powder, but with higher wear rate, due to the higher porosity rate presented.Ti coatings produced by in-situ oxidation of Ti powder when air plasma sprayed on stainless steel AISI 316 were investigated using two different preheating temperatures, which induce different conditions of oxidation produced by moderate cooling (via high preheating temperature) and maximum cooling (via low preheating temperature). In this section the effect of the difference in oxidation conditions in the microstructure, tribological behaviour and adhesion of the produced coatings is examined. The coatings had similar characteristics with regard to thickness, surface morphology (porosity and roughness), microstructure and friction behavior. However they showed a significant difference in rates of microhardness due primarily to different preheating temperatures between the two coatings. Moreover, the coating produced by high preheating temperature showed the highest adhesion strength and lower wear.Finely, the tribological behaviour of Al2O3 coatings on AISI 316 SS obtained by the controlled atmosphere plasma spraying (CAPS) is studied. More specifically Atmospheric Plasma Spraying (APS) and High Pressure Plasma Spraying (HPPS) were applied in order to produce these coatings. Also, complete metallographic examination was carried out on the obtained coatings including optical and electronic. The purpose was to investigate how the achieved coating properties depend on the selected spray conditions (pressure and composition of the spraying chamber atmosphere, surrounding gas, temperature, power, etc). The ceramic powder was sprayed by using high pressure plasma spraying methods (HPPS), using argon to create the atmosphere within the system, and atmospheric plasma spray (APS) using air to create the atmosphere within the system. HPPS formed coatings with significantly improved surface properties, suitable for demanding applications. These properties depend heavily on the process conditions, such as pressure and composition of the atmosphere in the chamber, spraying gases, temperature, power, etc.Moreover, a simplified thermal model of atmospheric plasma sprayed coating was developed by using finite element analysis software, aiming to provide a rapid tool for the estimation of temperature distribution of substrate during plasma spraying process.
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