This paper contributes to the broadening of knowledge about process combinations in connection with thermally sprayed coatings. It describes the influence of a later annealing, nitriding, and carburizing on different thermal sprayed coatings. The connection between the processes is determined and optimal process parameters is adjusted for both processes. The paper analyzes the coating characteristics and the alterations of the substrate. The results of the metallographic and X-ray analysis as well as of the wear and corrosion tests are discussed. Prospects for possible further applications are given. Paper includes a German-language abstract.

At the present, components which require both nitriding and locally a thermal sprayed coating or nitrided components which should l)e reworked are usually nitrided before spraying and the area to be coated is masked during nitriding or is prepared before spraying by locally removing the nitrided layer by grinding. Seen technically, advantages are to be expected if the nitriding process can be carried out after spraying. Moreover a post-nitriding of thermal sprayed coatings is of interest for improving coating characteristics, mainly wear resistance. Understanding the behaviour of sprayed coatings during nitriding in comparison to bulk materials will help to understand generally the behaviour of such coatings in gas atmospheres at increased temperatures. The objectives of the project are the investigation of the interaction between thermal spraying and nitriding, and the optimisation of both processes to achieve improved bonding, wear and corrosion characteristics respectively to get nitriding of the substrate through the coating without spalling or cracking. Furthermore the behaviour and structural changes of different coatings at increased temperatures are determined. The metallographic, X-ray, wear and corrosion results of the resulting compound coatings and parts are presented. Possible new applications are discussed. The project is funded by the German Research Ministry.

New near-net-shape structures of alloy Inconel 718 processed by HVOF spraying require optimum mechanical properties. Dominant factors defining the material quality are the particle properties velocity and temperature adjusted by the HVOF process parameters. Based on theoretical analysis of the HVOF process, experiments were performed with a defined variation of primary process parameters, producing coating samples of alloy 718 and measuring the particle velocities. Microstructural and X-ray analysis shows that in coatings with a high fraction of molten phase and high velocity, mainly divalent and spinell-type oxides are formed during particle impact on the substrate. Due to severe oxidation of the y'/y''- forming elements Ti, Al and Nb, precipitation-hardening effects of In 718 coatings are low. This leads to merely mediocre mechanical properties. The reduction of the molten phase to nearly zero leads to a drastic decrease of the oxide formation. The hardening γ'/γ'' phases are precipitated homogeneously in the Ni-base matrix. Strength values comparable to cast and wrought alloy In718 are attained by spraying with a low molten-phase fraction and high particle velocity. However, extensive intergranular 8-phase precipitation due to too high an Nb content of the powder causes only mediocre fracture elongation. Coatings up to 10 mm thick have been sprayed. The construction effort and hence the costs and weight of combustion chambers for hypersonic propulsion systems are to be reduced through direct thermal spraying of the loadbearing metallic pressure jacket onto the tubular cooling system. As a semifinished product, the selected Inconel 718 alloy exhibits good mechanical properties in the cryogenic temperature range as well as under higher thermal loads, and is commercially available in powder form. Aging serves to increase the strength up to the range of 1,200 N/mm2. For the sprayed In718 version, coating thicknesses in the centimeter range, a porosity < 1% and mechanical properties comparable with those of the cast version are required. The objective of the research work is to optimize spray-process control so that the resultant structural thick layers meet the design as well as the material requirements with respect to combustion-chamber technology. This necessitates elaborating the dominant microstructural parameters influencing the mechanical properties and the effect on them of the spraying process, and correlating them with the particle-condition parameters and the process parameters [1].