Thermisch gespritzte, mikrostrukturierte Beschichtungen
Final Report Abstract
Different coatings for the application of thermo-mechanically highly stressed surfaces were prepared by means of atmospheric plasma spraying. The focus of these experiments was the investigation of the influence of spray parameters on the formation of different porosities, which can function as micro-dimples. All materials show nearly the same tendencies. The lower the heat input, due to low current or low hydrogen gas flow, the higher the amount of non-melted and resolidified particles which form the starting point for porosities. Additionally, it is shown that by increasing the porosity also the pore depth increases. First, the coatings were prepared with a torch for coatings on outer contours, then analyzed and subsequently transferred to an internal coating process. The coatings which were made with a torch for outer contours exhibit a much lower porosity and a higher hardness than the coatings which were made with a torch for inner coatings. The melting rale of the powder particles is reduced by the lower energy input of the torch for internal coatings, thus the amount of non-melted and re-solidified particles increases. It is observed that the reduction of the coating cohesion and related hardness is associated with increased porosity. For their suitability for thermo-mechanically highly stressed surfaces, a mixture of iron and molybdenum, and chromium steel were used as coating materials. It was found that molybdenum-containing coatings provide a sufficiently high hardness due to the formation of brittle oxide phases. The high melting point of molybdenum also allows its use for applications with high thermal stresses. By mixing ductile phases to the molybdenum, such as the iron, processing such as grinding and polishing is possible. The chromium in the steel coating exhibits a different high oxidation as a function of the used spray parameters. The chromium oxide also leads to an increase in hardness. Especially the coatings which were sprayed with low heat input exhibit a high oxidation of chromium particles because the particles were not strongly melted and accelerated, so that during the flight phase, oxide films can be formed around the particles. The first materials were further developed so that the iron was replaced with the chromium steel in the molybdenum mixture, both to increase the corrosion resistance of the coatings and additionally provide the sufficient hardness of the internal coatings. The Mo/FeCr13 mixture was invesitgaled with respect to the possible control of the porosity and the pore depth. For this material a full factorial plan with different spraying distances and currents were used. It was revealed that depending on the spray parameters, different amount of porosities and pore depths were created. In particular, the current controls the amount of pores. The lower the used current, and therefore the applied energy contribution, the higher amount of pores can be observed. An additional chosen material for the inner coating was a self-fluxing molybdenum/nickel alloy. Similar to the FeCr13/Mo mixture, the ductile phase nickel is added to this material in order to ensure further processing of the coating. The nickel alloy also shows good corrosion characteristics and hard oxide phases. This coating system has also been studied with regard to the formation of porosity as a function of the spray distance and the current. It was shown that the porosity is also strongly dependent of the current and subordinated from the spray distance. For the tribological experiments with cylinder liners of the other project partners, the FeCrl 3/Mo mixture was selected. It was found that due to the differences in hardness between the molybdenum and steel particles, a special attention must been given to post treatments, in this case, the honing process.
