Fundamental research on deposition of amorphous coatings on surfaces of inner geometries and evaluation of phase stability under tribological loading

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In comparison to crystalline materials, amorphous alloys exhibit unique properties.This type of materials can only be manufactured if the alloy allows high undercooling and if rapid cooling rates can be realised. Required high cooling rates can be achieved by the coating technology "Thermal Spraying", allowing the deposition of amorphous coatings. The applicability of coatings on outer surfaces by means of powder based APS and HVOF as well as by means of wire based WAS is investigated intensively. Thereby the stand-off distance is known to be one of the key parameters influencing the amount of amorphous phases. For WAS coatings, stand-off distances between s = 100 and 200 mm are common. In the context of this project, it will be investigated whether amorphous Fe-based coatings can be deposited on inner geometries exhibiting small diameters. Thereby very small stand-off distance of s<40 mm needs to be realized. Due to the short stand-off distance and the utilization of a cored wire, the time for the formation of the alloy is short as well. Within this project the influence of the short stand-off distance on the formation of the alloy as well as the amorphous phases will be investigated. Thereby the effect of further process parameters will be considered. Besides metallographic analysis, the crystallization energy will be determined. These results will be correlated with the process parameters, particle in-flight properties and results from metallography. Subsequently, suitable samples will be heat treated in a vacuum furnace at crystallization temperature. Due to the heterogeneity especially with regard to the chemical composition, crystallization will occur locally. These areas will be identified and analysed in order to clarify the crystallization mechanisms of amorphous coatings. Afterwards, selected coatings will be characterized with regard to the tribological behaviour by means of a model test. Thereby, it will be investigated under which conditions crystallization might occur. In addition, temperature treated samples exhibiting a partially amorphous and a fully crystalline microstructure will be characterized as well. Knowing the characteristics of the wear track of crystallized coatings, assumptions with regard to the crystallisation process on the samples, which are not heat treated, can be made. In order to confirm these results analysis by means of XRD and TEM will be conducted.